Hollow-structured mesoporous materials: chemical synthesis, functionalization and applications.

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Hollow-Structured Mesoporous Materials: Chemical Synthesis, Functionalization and Applications Yongsheng Li* and Jianlin Shi* teristics of mesoporous materials such as extraordinarily large specific surface area and pore volume, well-defined ordered mesostructure, tunable pore size, varieties of the framework, and so on, have attracted great attention worldwide and opened up broad spaces for their applications in various fields, such as catalysis, adsorption and separation, drug storage and delivery, nanofabrication, etc.[2] Amongst the various architectures of mesoporous materials, hollow-structured mesoporous materials (HMMs), which integrate hollow interior or voids with mesoporous shells of various dimensions into one nanostructure, have attracted even more attention owing to their outstanding features of low density, the extensive presence of mesoporous channels on the shell, and the resultant high permeability, etc.[3] The hollow void within the mesoporous spheres can be used as a nanoreactor for loading catalytically active species for catalytic reactions, or as a nanocontainer for drug storage and delivery for biomedical applications after suitable surface modifications. On the other hand, mesoporous shells with a well-tuned thickness within the nanometer scale of HMMs are highly favorable as the pathway for the mass transfer of reactants and products into/out of the voids, mostly for liquid-phase reactions, or for the loading and release of drugs or other guest molecules for drug delivery. As the pore channels are much shortened, and thus the diffusion blockage become less significant, the inner surface of the mesopores in the shell structure can be utilized more efficiently when these hollow spheres are used as catalyst supports.[4] Therefore, HMMs have demonstrated great potential in the fields of guest encapsulation, controlled drug release and delivery, confined-space catalysis, storage, adsorption and separation, and so on.[3] The first example of hollow mesoporous materials could track back to the porous lamellar silica with a vesicular morphology (Figure 1a,b), which was synthesized by Pinnavaia et al. in 1996.[5] The approach was based on the hydrolysis and cross-linking of a neutral inorganic alkoxide precursor in the interlayered regions of multilamellar vesicles formed from neutral bora-amphiphile surfactant molecules containing two polar head groups linked by a hydrophobic alkyl chain. Unlike other surfactant-templating methods, this approach produced porous vesicle-like lamellar silicas (designated MSU-V), which possess

Hollow-structured mesoporous materials (HMMs), as a kind of mesoporous material with unique morphology, have been of great interest in the past decade because of the subtle combination of the hollow architecture with the mesoporous nanostructure. Benefitting from the merits of low density, large void space, large specific surface area, and, especially, the good biocompatibility, HMMs present promising application prospects in various fields, such as adsorption and storage, confined catalysis when catalytically active species are incorporated in the core and/or shell, controlled drug release, targeted drug delivery, and simultaneous diagnosis and therapy of cancers when the surface and/or core of the HMMs are functionalized with functional ligands and/or nanoparticles, and so on. In this review, recent progress in the design, synthesis, functionalization, and applications of hollow mesoporous materials are discussed. Two main synthetic strategies, soft-templating and hard-templating routes, are broadly sorted and described in detail. Progress in the main application aspects of HMMs, such as adsorption and storage, catalysis, and biomedicine, are also discussed in detail in this article, in terms of the unique features of the combined large void space in the core and the mesoporous network in the shell. Functionalization of the core and pore/ outer surfaces with functional organic groups and/or nanoparticles, and their performance, are summarized in this article. Finally, an outlook of their prospects and challenges in terms of their controlled synthesis and scaled application is presented.

1. Introduction The past 20 years have witnessed great advances regarding the synthesis, characterization, modification/functionalization and application of mesoporous materials since the discovery of M41S-type ones by Mobil researchers.[1] Particular charac-

Prof. Y. Li, Prof. J. Shi Lab of Low-Dimensional Materials Chemistry School of Materials Science and Engineering Key Laboratory for Ultrafine Materials of Ministry of Education East China University of Science and Technology 130 Meilong Road, Shanghai 200237, China E-mail: [email protected]; [email protected] Prof. J. Shi The State Key Laboratory of High Performance Ceramics and Superfine Microstructures Shanghai Institute of Ceramics Chinese Academy of Sciences 1295 Dingxi Road, Shanghai 200050, China

DOI: 10.1002/adma.201305319

Adv. Mater. 2014, DOI: 10.1002/adma.201305319

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 1. Proposed formation mechenisms (a and c) and TEM (B) and SEM (D) images of the first HMM sample. (1) Mixed lamellar-hexagonal membrane phase; (2) Membrane curvature formation by acidification; (3) Membrane bending into tubule through neutralization; (4) The membrane consists of a hexagonalarray of cylindrical micelles. a,b) Reproduced with permission.[5] Copyright 1996, American Association for the Advancement of Science. c,d) Reproduced with permission.[6] Copyright 1996, American Association for the Advancement of Science.

high specific surface area and pore volume. More importantly, the step of separate pillaring became unnecessary in creating porosity into a lamellar host structure. Almost at the same time, Mou et al.[6] designed another route for synthesizing hollow tubular MCM-41 (Figure 1c,d) based on the sequential separation of the self-organization of template silicates and the subsequent condensation of silicates. By delaying the formation of the rigid structure of the silicates, hierarchically ordered “tubules-within-a-tubule” of MCM-41 were obtained through a liquid-crystal phase-transformation mechanism. In the same year, Schüth et al.[7] reported the synthesis of hollow mesoporous spheres via interfacial reactions in oil-in-water emulsion, which was created by adding tetraethyl orthosilicate (TEOS) dissolved in an organic solvent (such as n-hexane, benzene, toluene, mesitylene, and others) to an acidic solution containing surfactant cetyltrimethylammonium bromide (CTAB) under stirring. Following these, diverse morphologies of hollow mesoporous materials, such as spheres,[8] vesicles,[9] helicoids,[10] fibers,[11] rattles,[12] and so on, have been fabricated by using a variety of chemical routes. The most intensively investigated hollow mesoporous materials are silica-based owing to the special features of silica such as controllable sol-gel processing, easy functionalization, and facile regulation of silica frameworks. However, the relatively low acidity and stability of silica-based hollow mesoporous materials have greatly limited their applications in varied fields.[13] In order to enhance the diversity and to facilitate their practical applications, various components, such as heteroatom-doped silicas (Ti, Zr),[14] metal oxides (TiO2, ZrO2, CeO2, ZnO),[15] metals (Pd),[16] polymers (PCL, PZS),[17] carbon[18] and others[19] have been designed and prepared. The applications of HMMs have also been widely investigated in many fields[20] especially as catalyst supports in the field of catalysis, in which the hollow cores provide big enough spaces for locating the active species, and the mesoporous shell layer

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Yongsheng Li received his B.Sc. (1994) from Zhengzhou Institute of Technology, and obtained his Ph.D. (2001) from Dalian University of Technology. As a post-doctoral research fellow, he has worked in Shanghai Institute of Ceramics, CAS and Institut de Recherches sur la Catalyse et l’environnement de Lyon, CNRS, France, respectively. He is now a professor in East China University of Science and Technology. His recent research interest focuses on the design and controlled synthesis of porous materials and hybrid nanocomposites and their biomedical and catalytic applications. Jianlin Shi received his Bachelor degree from Nanjing University of Technology in 1983, and obtained his Ph.D. degree in 1989 at Shanghai Institute of Ceramics, Chinese Academy of Sciences. Since then, he has been working at the same institute. His current research mainly focuses the structural design and synthesis of mesoporous materials and mesostructured nanocomposites, and the catalytic and biomedical performances of the materials for applications in environmental protection and nanomedicine.

offers a short diffusion pathway for both reactants and products and the location for anchoring active species. On the other hand, the large encapsulation capacity of the hollow cores and the easy functionalization of the mesoporous shells of HMMs create excellent opportunities for exploring their application potentials in biomedical field, including drug loading and controlled release for serious disease chemotherapy, molecular bioimaging, and theranostics for simultaneous diagnosis and therapy.[21] Several recent articles have reviewed the research progress on conventional mesoporous materials without the special hollow structure.[22] Most recently, several reviews[3,20] have discussed, rather briefly, the progress in some specific aspects of the chemical syntheses and applications of hollow mesoporous silicas. Alternatively in this review, we make efforts to give a comprehensive overview of the varioussynthetic strategies for hollow-structured materials, both with silica and non-siliceous components, structural regulations and modifications, and their potential applications mainly in the fields of catalysis, adsorption and drug delivery. In the first part, the design and synthesis of various kinds of hollow mesoporous materials via




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soft-templating or hard-templating routes will be summarized in detail; in the second, the applications of hollow mesoporous materials will be overviewed in detail mainly in the following areas: catalysis, storage, adsorption and separation, and drug delivery/biomedical applications; the final part is a summary and outlook for hollow mesoporous materials. To try to facilitate the understanding of each synthetic strategy and the applications of HMMs, we present a number of schematic illustrations in the following main text.

2. Design and Chemical Synthesis of HMMs To create hollow voids during the mesostructure construction, a number of techniques Scheme 1. Schematic illustration of the soft-templating route for preparing HMMs. Route have been developed so that hollow inte- 1: heterogenerous soft-templating route; Route 2: self-generated soft-templating route. Step (1–1): nucleation of the precursor/surfactant mixture at the interface between the heterogriors and mesoporous shells can be com- enous core and liquid medium containing surfactant and precursor molecules before selfbined together well. In general, the routes assembly; step (1–2): shell growth by the precursor hydrolysis around the core template and the for fabricating hollow mesoporous materials self-assembly with surfactants leading to the mesostructure formation in the shell; step (1–3): can be sorted as soft-templating and hard- removal of both core and mesopore templates via calcination or extraction. Step (2–1): nucleatemplating approaches, according to the tion of the precursor/surfactant mixture at the interface between the precursor core and liquid types of templates employed for the crea- medium containing surfactant molecules before self-assembly; step (2–2): shell growth by the self-assembly between the surfactants and the precursors leading to the gradual consumption tion of the hollow interiors. The soft-temof core template molecules and the synchroneous mesotructure formation in the shell; step plating route refers to the direct generation (2–3): surfactant as mesopore template removal via calcination or extraction. of both mesopores and the hollow structure almost simultaneously via the self-assembly between precursor molecules and organic surfactant temsuch as extraction or calcination for hollow structure formation plates/other organic additives. Herein, the core templates are when the templates are the additional organics to the HMM usually droplets of “soft” precursor molecules, surfactants, or precursors. In the case of using precursor molecule droplets some organic additives, and will be consumed or removed in themselves as the core template, which is also called “self-temthe later procedures of the HMMs formation. In the meantime, plating” (route 2), these molecules will be later utilized as the the mesopore channels are created by the surfactant templates/ building units of the hollow structures, and therefore are gradmicelles after being removed in the later procedures. As for ually consumed in the following construction/formation of the the hard-templating route, some specially prepared rigid solid mesostructured shells; thus, a core template removal process, particles are employed as “hard” core templates, and sacrificed such as calcination or extraction, becomes unnecessary.[24] after the formation of the mesoporous shell on the core for the construction of a hollow structure within mesoporous particles, 2.1.1. Emulsion Templating Approach in which mesopores are usually similarly generated by the self[ 23 ] assembly among the precursors and surfactant micelles. It is well known that emulsion chemistry deals with multiphase liquid systems in which one isolated heterogeneous liquid phase (e.g., organic droplets) is dispersed in the other liquid 2.1. Soft-Templating Route matrix (e.g., water).[25] Usually polymeric particles can be synthesized by polymerizing the organic droplets and later separating them from the liquid matrix. If inorganic components Scheme 1 illustrates the general strategy of the soft-templating are included in the isolated liquid droplets, macro- or nanoscale methodologies for preparing hollow mesoporous materials inorganic particles with or without porous structures, such as with detailed processing steps. The construction of HMMs silica and other metal oxides, can be synthesized; these inherit starts from the transient formation of the core templates, which the topology and/or morphology of the droplets. With the oilmay take place almost simultaneously with the subsequent in-water/water-in-oil emulsion chemistry, hollow-structured generation of mesoporosity on the shell. The core templates calcium carbonate and mesostructured silicas have been can be heterogeneous but nevertheless flexible liquid particles fabricated.[9a,19c] The basic idea is to use sol-gel processing to in an aqueous liquid matrix, such as emulsion droplets, vesicles, or gas bubbles, which usually originate from, but are not deposit inorganic materials, such as silica or metal oxides at the limited to, precursor molecules, templates, or some additives interface between the droplets and media in a dispersed emuloriginally added for constructing the mesostructures (route 1). sion, where the favorable features of the oil droplets being both Usually the core template should be removed by various routes highly deformable and easily removable are employed.[25]




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easily in-and-out of the spheres, which differs from the reported HMMs with hexagonally or circumferentially arranged mesopore channels in the shells. On the basis of this, mesoporous zeolite with a hollow spherical/ ellipsoidal capsule structure was further obtained under the similar process.[29] In order to promote the formation of the emulsion system or stabilize the emulFigure 2. SEM (a) and TEM (b,c) images of hollow mesoporous silicas templated by TEOS sion droplets, some organic additives (cosolvents), such as N,N-dimethylformide formed emulsion droplets. a) Reproduced with permission.[8] Copyright 2001, Royal Scoiety of Chemistry; b) Reproduced with permission.[27] Copyright 2010, Elsevier; c) Reproduced with (DMF),[30] n-octane[31] or dodecyl amine permission.[28a] Copyright 2003, ACS Publications. (DDA),[32] have been used to cooperate with the TEOS. It has been shown that DMF TEOS is commonly used as the silica source in preparing added in the solution as a bridging co-solvent can both promote mesoporous silica materials. More importantly, it can also serve the dispersion of TEOS in the aqueous solution to form small as the “void” template via forming an emulsion in aqueous droplets, and speed up the hydrolysis of TEOS at the intersolution/sol for fabricating hollow mesoporous materials. face.[30] In this way, ordered mesostructure was formed at the [ 6,26 ] Inspired by the idea of Mou et al. interface of the water-TEOS droplets, and hollow mesoporous for preparing hollow silica spheres (Figure 2c) were finally obtained once the TEOS mesoporous materials by a delayed neutralization procedure, droplets were consumed. Furthermore, the pore structure, hollow microspheres of silica with ordered mesoporous walls wall thickness, and the sphere dimension can be well tuned by (Figure 2a) were synthesized by Mann et al.[8] by a simple promeans of varying the amount of DMF. cess involving dilution and neutralization of an aqueous reacIt is widely believed that surfactant molecules are responsible tion mixture of TEOS/CTAB under ambient conditions. The for the construction of mesostructures in preparing mesoporous critical step was demonstrated to be the formation of TEOS materials by self-assembling with precursors of the shell comemulsion droplets in the reaction mixture resulting from the ponents. Interestingly, it has been found that surfactants themdilution. Therefore, it is of utmost importance to control the selves can also serve as core templates in certain cases for reaction conditions precisely, including the stirring rate, inducforming the hollow voids. Chen et al.[33] successfully prepared tion time, and the time delays between the dilution and neutralization steps during the synthesis, so that the formation hollow mesoporous silica spheres with varied morphologies by of TEOS droplets and the interfacial reaction could take place utilizing the sodium salt of the anionic surfactant N-lauroylsarsuccessively and independently.[15c,27] Noticeably, the mesopore cosine (Sar-Na) as a template. With the addition of hydrochloric acid, a part of the Sar anions was converted into Sar-H, which channels in the shells of these materials, though with ordered acts as droplets owing to the amphiphilic property. Then, the porous structures sometimes, are usually neither stable enough condensation between the added silica precursors (3-aminonor penetrating across the shells. propyltrimethoxysilane (APTMS) and TEOS) and the emulsion Later, hollow spheres of mesoporous aluminosilicate with system took place, resulting in hollow mesostructured spheres. a highly ordered three-dimensional and consequently penDifferently from the previously reported emulsion-templating etrating pore network (Figure 2b) were successfully developed routes, the anionic surfactant Sar-Na was used as both a surby Shi et al.[28] by employing the self-templating approach. In factant template for mesopores and an oil phase (droplets) for the synthesis, the precursor for synthesizing zeolite ZSM-5 was interior hollow-core formation after acidification (Figure 3a,b). used as the starting material, which was proposed to form an Afterwards, Han et al.[34] further verified the dual functions oil(TEOS)-in-water emulsion in solvent. The parameters, such as the temperature at which the precursor was prepared, the of emulsion and micelle formation of the anionic surfactants. mixing order of the reactants and the aging duration, were They employed a variety of anionic surfactants, including palfound to be important for the formation of the emulsion, which mitic acid (C16AA), N-acyl-L-phenylalanine (C18Phe), N-palmiaffects the final hollow particle structure. In this approach, toyl-L-alaine (C16AlaA) and oleic acid (OA) as templates and emulsion was formed by adding TEOS to the aqueous solu3-aminopropyl-triethoxysilane (APTES) and TEOS as silica tion containing Al2(SO4)3·18H2O under vigorous stirring, and sources to obtain silica hollow spheres with mesoporous shells. It was found that ethanol, which acts as a co-surfactant, plays a was further stabilized via the quick hydrolysis of the outer layer key role in the porosity generation of the shell. TEOS molecules around the TEOS droplets in strong basic conSimilarly, cationic surfactants have also been demonstrated ditions. This is different from that of mesostructured silica vesto have the dual functions of forming emulsion and micelles. icles, which is created through minimizing the surface energy Accordingly, a micellar aggregate templating route has been by hydrogen bonding between electrically neutral gemini surdeveloped by Shi et al.[35] to synthesize hollow mesoporous silica factants and silica precursors.[5] The TEOS droplets as the core templates were later used/consumed in the generation of the nanospheres (Figure 3c,d) with tunable sizes of both the sphere mesostructured shell via their assembly with surfactant moldiameter and the shell thickness by utilizing (−)-N-dodecyl-Necules (CTAB), consequently leaving hollow voids. Accordmethylephedrinium bromide (DMEB) as the core template. It is ingly, the resultant materials possess ordered 3D penetrating known that DMEB can easily form small micellar aggregates in mesopores in the shell, allowing guest molecules to diffuse water at a relatively low critical aggregation concentration, so that

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it could be used as a “dual” template to assist the construction of the hollow core and the mesostructure simultaneously. The structure of the micellar aggregates can be further stabilized by the addition of carboxyethylsilanetriol sodium salt (CSS), owing to the electrostatic interaction between CSS and DMEB. Under basic conditions, TEOS hydrolysis is rather quick compared with that under neutral conditions, and the assembly between condensed TEOS and DMEB around the micellar aggregates occurs in the meantime, which subsequently results in the formation of hollow mesoprous silica spheres after template removal. By adjusting the pH value of the precursor, the condensation rate of TEOS and the growth rate of the mesostructured shell can be altered. Accordingly, the sphere diameter and the thickness of the shell and even the morphology of the resultant product can be tuned. In view of employing the micelle aggregates as void templates, the diameter of the final mesoporous silica spheres can be easily controlled to be under 100 nm, which is especially favorable for targeted drug-delivery applications. Apart from organic silica sources and surfactants, the cooperative principle between organic compounds and surfactants has been frequently used to assist the formation of the emulsion system or stabilize the emulsion droplets.[10,36] He et al.[37] attempted to control the morphology and structure of mesoporous silica by using various auxiliary compounds, including sodium bis(2-ethylhexyl) sulfosuccinate, ethyl ether, and dodecanethiol as a co-solvent or co-template, which was found to be helpful in forming a stable microemulsion via incorporation with CTAB. By changing the mass ratio of dodecanethiol (C12-SH)/CTAB, mesoporous silicas with different morphologies, such as solid spheres, hierarchical spheres, and hollow-structured spheres were obtained. Furthermore, the diameter of the hollow mesoporous spheres could be tuned and decreased to less than 100 nm by adding different amounts of trimethylbenzene (TMB) to the solutions.[38] Utilizing the aggregation property between poly(vinylpyrrolidone) (PVP) and CTAB mixture, Zhu et al.[39] successfully prepared hollow mesoporous silica spheres with a uniform size and morphology at room temperature. It was found that most of the hexagonally


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Figure 3. TEM images of hollow mesoporous silica spheres prepared via anionic (a,b) and cationic (c,d) surfactant emulsion routes. a,b) Reproduced with permission.[33] Copyright 2006 Wiley; c,d) Reproduced with permission.[35] 2008 Royal Scoiety of Chemistry.

ordered mesoporous channels were penetrating across the shell. Furthermore, much higher capability in storing guest molecules, in comparison with conventional MCM-41, has been proved for these hollow mesoporous silica spheres, and more than half of the guest molecules were located in the hollow cores. Interestingly, based on the above mechanism, the first example of hollow non-siliceous oxide spheres, lead titanate, was obtained via an oil-water interface templating route at the micrometer scale by using acetylacetone and 1-butanol as organic reagents and amphiphilic dodecylamine as the surfactant, through a neutral supramolecular templating route.[19c] In the system involving non-ionic surfactants, organic compounds also play a role in controlling the morphology of the final mesoporous materials. Li et al.[40] used kerosene to assist sorbitan monooleate (C24H44O6, Span 80) in creating a water-in-oil emulsion. Following the sol-gel process of TEOS, stable hollow silica microspheres were prepared. With the same process, hollow mesoporous titanium-silica bicomponent microspheres with excellent catalytic performance in the epoxidation of cyclohexene by tert-butylhydroperoxide were successfully fabricated.[14a,41] TMB is widely used to expand the pores of mesoporous materials, it can also form emulsion droplets in water.[42] Under stabilization by a triblock copolymer (EO76-PO29-EO76), silica hollow spheres with multilamellar or worm-like porous shell structures and uniform sizes could be obtained.[43] Based on this, Guo et al.[44] recently prepared hollow spheres with mesoporous channels perpendicularly arranging on the shells, and the hollow core was generated by the removable template of a benzene-in-water emulsion. With a similar aqueous emulsion co-assembly approach, followed by hydrothermal treatment, a series of mesoporous carbon and carbon-silica nanocomposite vesicles has been synthesized. It was proposed that the silicate oligomer and soluble resol precursors would interact and co-assemble with the triblock copolymer EO97-PO69-EO97 (F127) template through hydrogen bonding. Then, uniform lamellar mesostructure was formed via the co-assembly on the oil/water emulsion interface, which were generated by adding TMB as a co-solvent.[9b] Although the above emulsion templating approaches have been widely used in preparing hollow spherical porous silica or other non-siliceous materials, researchers have been trying to find nontoxic substitutes to replace the commonly used environmentunfriendly and water-immiscible organic additives to generate emulsion systems. Supercritical carbon dioxide (scCO2), which is nontoxic and non-flammable, and can offer several attractive features such as high diffusivity, low viscosity, high biocompatibility, and non-cohesiveness, has been demonstrated to have promising potential in the preparation of hollow mesoporous materials (Figure 4).[45] Mokaya et al.[46] found that hollow silica spheres of large-sized mesopores on the shell could be synthesized through a CO2-in-water emulsion templating route using PEO-PPO-PEO as a template under supercritical fluid conditions. The mesoporosity and morphology of the hollow silica spheres could be tuned by varying the operating CO2 pressure. Moreover, compressed CO2 gas flow could improve the monodisperity and uniformity of the hollow silica spheres.[15i,47] Table 1 lists the physico-chemical properties of several representative HMMs prepared via emulsion routes. It can be found



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Figure 4. Schematic diagram (left) and TEM images (right, a, b) of hollow mesoporous silica spheres prepared via emulsion technology assisted by CO2 gas bubbles. Reproduced with permission.[45b] Copyright 2009, ACS Publications.

that highly ordered HMMs can be obtained if the emulsions are created in the presence of a silica source (TEOS), however, the synthesized HMMs show a broadened distribution in particle size,[7,8,28a] and, the dimension and uniformity of TEOScreated emulsion droplets are highly dependent on various experimental parameters, such as stirring rate, temperature, and additives. In comparison, HMMs of smaller particle sizes and disordered or wormhole-like mesostructured shells are usually obtained where surfactants act as both emulsion droplets and templates for mesostructures,[33,35,37b] but the dispersivity of HMMs can be improved with the increasing particle size.[8,26,40,48] In this case the self-assembly properties (e.g., aggregate or vesicle formation) influence the emulsion property directly, resulting in relatively small emulsion droplets and final smaller particle sizes where surfactants are employed for producing emulsions. Nevertheless, in both cases, the properties of resultant HMMs are largely dependent upon the nature of the emulsion droplets, and the uniformity and dispersity are rather hard to get under fine control. 2.1.2 Vesicle Templating Route Due to the diverse chemistry of surfactants, various shapes and morphologies of surfactant aggregates, including micelles,

vesicles, and liquid-crystal phases, are formed in surfactant solutions under different conditions.[49] As shown in Scheme 2, vesicles are the self-organized structures from surfactants and have fragile bilayer shells and are therefore subject to dissolving and collapsing when the solution properties, such as concentration and temperature, significantly change. Inorganic components can be introduced into the shells of the surfactant vesicles, which will result in organic/inorganic hybrid vesicles or even inorganic vesicles if the organics can be removed without destroying the vesicle structure. When inorganic components such as silica are used as building units for constructing solid vesicle structures, the resulting materials are highly stable and more-biocompatible replacements for the original surfactant components, which will be useful for biomedical applications as, for example, drug-delivery vehicles or for other applications such as high-capacity gas adsorption.[50] Rankin et al.[50,51] reported a new type of vesicle-like hollow silica with single-walled and ordered mesoporous shells, which was generated through the co-assembly between silica and a vesicle template of fluorinated surfactant. By changing the stirring/shearing rate during synthesis, they showed that elongated silica particles with multiple hollow chambers were prepared. By mixing the fluorocarbon surfactant (FC4) with CTAB or F127, periodic mesoporous organosilica hollow spheres with

Table 1. Properties of representative HMMs prepared via emulsion routes. Additives

Framework

Dispersivitya)

Particle size [µm]

Uniformitya)

Periodicitya)

Mesostructure

Pore orientation

Shell thickness [nm]

Ref.

TEOS

-

SiO2

h

1.0

m

h

P6mm

Parallel to shell surface

20

S. Mann[8]

TEOS

-

aluminosilicate

-

0.6

m

h

Ia-3d

3D

200

J. L. Shi[28a]

TEOS

mesitylene

SiO2

-

1–100

-

h

P6mm, p63/ mmc, pm3n

radially

F. Schüth[7]

TEOS

butanol

aluminosilicate

h

2.5–7.6

h

h

Radially/ latitudinally

C. Y. Mou[26,48]

Sar

-

SiO2

m

3

-

-

Wormhole-like

DMEB

-

SiO2

m

0.1

h

m

-

CTAB

C12-SH

SiO2

m

12, to obtain hollow structured mesoporous silica spheres. It was demonstrated that the surfactant directed the assembly of the silica species into a mesoporous 2D hexagonal arrangement, whereas the PS beads played a role in forming the hollow core in the interior of the particulate materials. After the removal of the PS beads and surfactant molecules via high-temperature calcination, hollow silica spheres with mesostructured shells were obtained. The modification of the PS beads with other functional molecules, such as acrylic acid[61] or polystyrene-copoly(4-vinylpyridine) (PS-co-PVP),[62] will facilitate the assembly of a mesoporous shell around the beads due to the introduction of a favorable chemical environment for electrostatic attraction, or a pendent catalyst on the latex surface to initiate the sol–gel




REVIEW Scheme 3. Schematic illustrations of hard-templating routes: (1) Heterogeneous hard-templating and (2,3) homogeneous hard-templating for synthesizing hollow-structured mesoporous materials (route 2: structural difference-based selective etching for synthesizing hollow mesoporous silicas; route 3: etching and calcination for hollow mesoporous non-silica synthesis). Step (1–1, 2–1, 3–1): formation of HMMs around the solid core template; step (1–2): removal of the templates from both the core and mesopores via calcination, etching, or extraction procedures; step (2–2): removal of SiO2 cores via structural difference-based selective etching; (step 3–2, 2–3): removal of surfactants from the mesopores via calcination or extraction; step (3–3): filling the mesopores with non-siliceous components; step (3–4): removal of SiO2 core via etching by HF or NaOH.

process of TEOS. However, the mesoporous shells are usually disordered. In order to improve the periodicity of the mesopore channel arrangement in the shells of the hollow spheres templated by the dual PS latex/surfactant templating route, Rankin et al.[59] employed an unmodified polystyrene latex and CTAB in a concentrated aqueous ammonia solution to promote aggregation among small particles (CTAB/silica aggregates) and large particles (Sukuca-coated latex), so that mesoporous shells of an ordered pore structure around the hard cores could be generated. The shell thickness and core diameter could be tuned independently; however, irreversible particle aggregation would take place during the synthesis, leading to micrometric and disordered aggregates, which will thus give rise to difficulties in applications demanding a high dispersion of particles, in the cases of, for example, inorganic fillers in rubbers and bloodinjectable drug-delivery vehicles. By precisely controlling the experimental parameters, such as the charging properties of the PS-sphere surface, the weight ratio of TEOS to PS, the precursor components, monodisperse hollow silica spheres with ordered mesoporous channels on the shells, which are perpendicular to the core surface, were synthesized for the first time.[63] The particle diameter (less than 200 nm), the shell thickness, and the mesopore orientation could be tuned independently. Recently, a facile and scalable methodology was reported by Giannelis et al.[64] to synthesize monodisperse hollow mesoporous silica (HMS) capsules (Figure 7a,b) with a relatively concentrated latex template. It was verified that the hydrolysis rate of TEOS could be precisely controlled by tuning the ratio of ethanol to water under weak basic conditions. Together with the high latex concentration,


Figure 7. TEM images of representative hollow mesoporous silica nanospheres prepared via PS-templating route. a,b) Reproduced with permission.[64] Copyright 2010, ACS Publications; c,d) Reproduced with permission.[65] Copyright 2010, ACS Publications.

the possibility of forming solid mesoporous particles, as well as fusing them together was significantly reduced. The resultant HMSs possess smooth, uniform, and ordered mesoporous silica shells. They found that both the particle size and the shell thickness of HMSs could be finely tuned by changing the amounts of latex and silica source. It has also been demonstrated that the synthetic conditions, including the concentration of surfactant, the volume fraction of organic solvent, and others, determine the size, periodicity, and monodisperity of



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the final products (Figure 7c,d).[65] Furthermore, monodisperse and uniform hollow mesoporous organosilica nanospheres can be prepared by using PS latex spheres as the hard core template and 1,2-bis(trimethoxysilyl)ethane (BTME) as the organosilica source through an NH4OH-catalyzed sol–gel process. The resultant products have been demonstrated to be good supports for creating pH-responsive supramolecular nanovalve systems that can be alternatively triggered by acid or base for the controlled release of the entrapped guest molecules.[66] Not only the silica-based hollow mesoporous spheres, but also non-silica based hollow mesoporous spheres can be facilely fabricated by employing PS latex as a hard template.[67] To promote the condensation of the non-siliceous precursors onto the surface of PS spheres, it is necessary to modify the surface with various functional groups, such as –NH2, –COOH and –OH. Tang et al.[68] found that magnetite could be easily coated on PS spheres with their surface being carboxyl-functionalized to obtain mesoporous magnetite hollow spheres. By plasma treatment, it is easy to introduce hydroxyl groups onto monodisperse PS spheres, so that titanium (IV) isopropoxide or tetra-nbutoxygermane could condense with hydroxyl groups to form TiO2 or GeO2 coating around the PS spheres, which will be turned into hollow spherical particles upon the core dissolution with tetrahydrofuran (THF).[69] Moreover, cross-linked poly(methacrylic acid) (PMAA) spheres can also be employed as hard cores to fabricate microcapsules with hollow core and mesoporous shell structure. After the treatment by ionic liquid salts, the surface of PMAA was modified by adsorbed double layers, which promoted the subsequent deposition of inorganic salts, so that mesoporous SiO2, Al2O3, and TiO2 microcapsules with different cavity sizes were fabricated successfully. After loading and entrapping chiral catalysts inside the silica microcapsules, the resultant heterogeneous catalysts presented high activity and enantioselectivity in the synthesis of chiral β-blockers.[15d,70]

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2.2.3. SiO2 Spheres as Hard Core Templates

Figure 8. Schematic diagram (a), SEM (b) and TEM (inset of b) images of hollow mesoporous carbon spheres prepared via SiO2-template route. Reproduced with permission.[18a] Copyright 2002, Wiley.

The synthesis of monodisperse silica spheres with different sizes has been widely investigated, and the Stöber method has been verified to be one of the most efficient routes for obtaining uniform silica spheres for decades.[71] With or without surface functionalization, silica spheres have been demonstrated to be a suitable candidate for being utilized as a hard core template. By combining the Sto˝ber approach, the Giesche growth process, and the Kaiser approach, core-shelled monodisperse silica spheres on the nanometer scale, composed of a non-porous solid silica core and a thin mesoporous silica shell (SCMS) were obtained by the simultaneous sol– gel polymerization of TEOS and octadecyltrimethoxysilane (C18TMS) on the previously prepared nonporous silica spheres, followed by the removal of the organic groups.[72] On the basis of this, Heyon et al. employed silica/aluminosilicate spheres with SCMS structures as template materials and in situ polymerized phenol-resin or poly(divinylbenzene) as the carbon source, as shown in Figure 8a, to fabricate carbon capsules with a hollow macroporous core/mesoporous shell (HCMS). It was demonstrated that strong acidic catalytic sites, which are helpful for the polymerization of phenol and formaldehyde,

were generated owing to the aluminum incorporation into the silicate framework. Then, with the introduction of phenol and formaldehyde into the mesopores of SCMS aluminosilicate, further carbonization took place, which yielded the silica core/ carbon-aluminosilicate shell nanocomposites. The dissolution of the silica core and aluminosilicate shell (i.e., SCMS template) using either NaOH- or HF-solution generated HCMS carbon capsules. In this approach, the dimension of the hollow core and the mesoporous shell thickness of the HCSM carbon capsules can be well tuned by employing SCMS silica sphere templates of appropriate core diameter and corresponding shell thickness. Later, gold nanoparticles contained in hollow spherical carbon/polymer capsules were fabricated by using core-shelled composite silica spheres composed of sub-micrometer-sized Au-NP-containing solid cores and a mesoporous shell as a hard core template.[18a,73a,b] The above-mentioned synthetic methodology for preparing hollow mesoporous carbon spheres includes multiple synthetic steps: 1) preparation of core-shell structured SCMS templates by coating mesoporous silica layers on the silica core; 2) infiltration into the mesopores of calcined





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precursor.[75] It was demonstrated that the aluminum species incorporated into the wall of the HMASs played a key role in the formation of the HMCSs, which could both determine the hollow morphology of HMASs, and generate acidic catalytic sites to catalyze the polymerization of the carbon precursor introduced into the pore channels during the replicating process. Consequently, the replicating process was simplified and the conFigure 9. SEM and TEM images of hollow mesoporous aluminosilicate spheres prepared by ventional catalyst loading step or additional acidic catalysis was no longer needed. the nanocasting method. Reproduced with permission.[75] Copyright 2008, Elsevier. Although it is convenient to fabricate hollow mesoporous carbon spheres based on the SCMS hard templating route, there are very few reports on (i.e., surfactant-removed) SCMS with carbon precursors, the synthesis of hollow mesoporous silica spheres by the same such as the mixture of phenol-formaldehyde, furfuryl alcohol, route.[76] Recently, Shi et al. developed a novel strategy, “strucand glucose; 3) polymerization of the precursor within the mesopores in the shell, 4) in situ carbonization of the polymer, tural difference-based selective etching”, to construct monoand 5) removal of the silica core and silica framework from the disperse hollow/rattle-type mesoporous silica spheres. Such a shell.[73] Nevertheless, as can be clearly seen in Figure 8b,[18a] strategy features taking advantage of the structure differences, rather than traditional compositional differences, between the the incorporation of the carbon precursor into mesoporous core and the shell of a silica core/mesoporous silica shell strucchannels of SCMS through infiltration process will most probture to create hollow voids (Figure 10).[77] It was found that ably result in sphere aggregation. Interestingly, Ikeda et al. successfully fabricated hollow the silicate condensation degree in the meosopore framework mesoporous carbon spheres via a hydrothermal treatment of the shell layer is significantly higher than that in the solid route rather than high-temperature calcination to carbonize silica core, which was generated by the self-assembly between the carbon precursor, which effectively prevented aggregation C18TMS and TEOS, and the Stöber method. Consequently it is among the carbon particles. Selective deposition of the prepossible to employ an appropriate etching agent, Na2CO3 solucursor into the pore systems of the template was achieved by tion, to selectively remove the hard solid core, while keeping employing an effective electrostatic attraction between the solid the outer mesoporous shell mostly intact. On the basis of this, template and the carbon precursor. The template was positively highly disperse hollow mesoporous silica spheres with controlcharged by amino-modification so that its mesopore surface lable particle/pore sizes could be synthesized, which showed was then uniformly covered by negatively charged polysacchahigh loading capacity (1222 mg g−1) for an anticancer drug ride, which resulted from the hydrothermal treatment of glu(doxorubicin).[78] Comparatively, this strategy is rather simple, cose, leading to the formation of isolated hollow mesoporous controllable, and scalable.[77,78] Very recently, Zheng et al.[79] [ 74 ] carbon spheres that inversely replicated the templates. further simplified the silica-based etching strategy. They introduced cationic surfactant of CTAB into the system of Na2CO3 Moreover, we have found that hollow mesoporous carbon spheres (HMCSs) with highly ordered, 3D cubic mesostrucsolution to etch the solid spheres, and found that high-quality tured mesopore networks in the shells (Figure 9) could be hollow mesoporous silica spheres with either a wormhole-like directly replicated from hollow mesoporous aluminosilior an oriented mesoporous shell could be facilely prepared. It cate spheres (HMASs) by employing a simple incipient-wetwas proposed that the cationic surfactant plays a critical role in ness impregnation route with furfuryl alcohol as a carbon the formation of HMSS from solid silica spheres: 1) as a soft

Figure 10. Schematic diagram (left) and TEM images (right) of hollow mesoporous spheres prepared via “structural difference-based selective etching” approach (Route A: in Na2CO3 solution and route B: in ammonia solution). Reproduced with permission.[77] Copyright 2010, ACS Publications.




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template to direct the construction of the mesoporous structure in the shell, 2) as an assistant to accelerate the etching of sSiO2 (i.e., hard-template), 3) as a stabilizer to protect the silicateCTAB shell from alkaline etching. Based on the same approach, aqueous colloidal hollow periodic mesoporous oganosilicas (HPMOs) with tailored compositions and nanostructures were further synthesized;[80] they exhibit unique biological behaviors both in vitro and in vivo. Another simplified and representative approach for the fabrication of hollow mesoporous materials by nanocasting route was reported by Fuertes et al.[18b,81] This approach is to impregnate sub-micrometer-sized SCMS with a preceramic polymer-polycarbomethylsilane (PCMS). After the pyrolysis and following removal of the silica template, mesoporous silicon oxycabide capsules were obtained. Further calcination of the mesoporous silicon oxycabide capsules gave rise to the capsules with silica framework. It should be noted that, the organic carbon chains (–(CH2)17–CH3) in the organosilicon compound (C18TMS) used in the synthesis SCMS silica spheres functioned as both the carbon precursor and mesoporogen agent. Therefore, the synthetic procedure for obtaining uniform and nonaggregated spherical carbon capsules was significantly simplified. To achieve this, sulphuric acid was chosen as a catalyst to convert the organic moiety into carbon with a considerably increased carbon yield via dehydration and sulphonation reactions. The advantage of this approach is that neither the incorporation step of polymeric carbon precursors nor an infiltration step is required, thereby ensuring the facile synthesis of isolated hollow mesoporous carbon particles.[18b,82] With an incipient wetness impregnation technology, controlled amounts of a variety of inorganic precursors, such as Fe, Co, Ni, and Cr could be incorporated into the hollow core of the carbon capsules, resulting in core/shell structured nanocomposites with the inorganic cores confined within the mesoporous carbon capsules. A rather unique characteristic of these core/shell structured nanocomposites is that, though the inner hollow core can be almost completely filled by the nanoparticles, the mesopore channels on the carbon shell remain open and few nanoparticle depositions can be found in the shell. This feature makes these nanocomposites promising candidates in many fields, such as high-performance catalyst supports, materials for energy storage application (i.e., Li-ion batteries), or advanced adsorbents with novel functionalities (magnetism, etc.).[82] Liquid impregnation is known to have several drawbacks, including shrinkage of the silica network during synthesis and the formation of additional (micro)porosity during carbonization.[83] In addition to the liquid impregnation technique, chemical vapor deposition (CVD) was also adopted by Xia et al. to nanocast hollow mesoporous carbon and other spheres with heterogeneous components.[84] The carbon sources are usually styrene, acetonitrile, benzene or ethylene. It has been demonstrated that hollow mesoporous carbon spheres containing highly ordered CMK-3 can be successfully fabricated by employing conventional SBA-15 as the template through CVD route, during which the pyrolysis/carbonization temperature of styrene is crucial to the formation of hollow carbon spheres.[82,84a] Recently, Chen et al. presented a rather simple and controllable CVD nanocasting method to prepare highly ordered hollow carbon spheres with graphitic shell structure.

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It was found that surfactant-CTAB confined in the mesopores could function as both the carbon precursor and a promoter to accelerate carbon deposition during the CVD process. Therefore, it was not necessary to remove the surfactant from the mesopore network and then to introduce additional carbon precursors into it (i.e., only one CVD process was needed for the production of carbon-filled silica spheres). After the removal of silica with HF treatment, hollow mesoporous carbon spheres were obtained. More importantly, graphitic mesoporous carbon spheres with hollow structure were produced by using ethylene as the carbon precursor during the CVD process.[85] 2.2.4. Carbon Spheres as Hard Core Templates Monodisperse carbon spheres are another kind of interesting sacrificial template. They can be easily prepared from the aqueous solutions of glucose and polysaccharedies under hydrothermal treating conditions.[86] The as prepared carbon spheres are found to possess abundant functional groups such as –OH and –C=O on the surface, inherited from the precursors, which provided favorable chemical environment for adsorbing other precursors and/or nanoparticles in the following processes. Based on this, a facile route was proposed by Zhu et al. to prepare hollow mesoporous silica spheres and rattle-type Fe3O4@SiO2 hollow mesoprous spheres with large void spaces by using the colloidal carbon spheres as the templates (Figure 11).[87] The fabrication involved the one-pot hydrothermal synthesis of colloidal carbon spheres adsorbed with iron precursors, the simultaneous sol-gel polymerization of TEOS and C18TMS to deposite the organosilicate-incorporated silica shells on the colloidal carbon spheres, and the removal of the carbon templates and the organic groups of C18-TMS by heat treatment, leaving iron species within the core as Fe2O3 nanoparticles, and finally the reduction of Fe2O3 core

Figure 11. SEM and TEM images of rattle-type Fe3O4@SiO2 hollow mesoporous spheres by using colloidal carbon spheres as core templates. Reproduced with permission.[87] Copyright 2009, ACS Publications.




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Table 2. Properties of some representative HMMs prepared via hard core templating routes Template

Framework

Dispersivity

Hollow core diameter [nm]

Shell thickness [nm]

Uniformity

Periodicity

Ref.

PS

SiO2

low

62–138

320–800

+

+

Y. Wei[60]

PS

SiO2

high

60–90

5–40

+

+

B. Charleux[63]

PS

SiO2

high

140–1500

30–60

+

-

E. Giannelis[64]

SiO2@m SiO2

Carbon

high

220

60

+

SiO2

Carbon

high

168

27–49

+

-

M. Matsumura[74]

SiO2

SiO2

high

35–470

5–100

+

-

J. L. Shi[77,79]

SiO2/silicon oxycarbide

high

270–320

40

+

-

A. B. Fuertes[81]

SiO2

high

700

100

+

-

S. Kaskel[87]

SiO2@m SiO2 C

into Fe3O4 nanoparticles under hydrogen atmosphere. On the other hand, owing to the abundant –OH groups on the surface of the carbon nanospheres, Pd nanoparticles (ca. 5 nm) were expected to anchor on the outer surface of them with uniform distribution. Upon forming mesoporous silica layer on the Pd/C spheres and the following removal of the hard carbon cores and CTAB surfactant, Pd nanoparticles residing inside the hollow spheres were obtained, which exhibited extremely high activity for Suzuki cross-coupling reactions.[88] Interestingly, C-doped hollow TiO2 mesoporous microspheres with superior visible light photocatalytic activity for the degradation of toluene have also been achieved by choosing carbonaceous polysaccharide microspheres as hard cores via rapid combustion process.[89] Moreover, hollow spheres of crystalline porous metal oxides, such as γ-Al2O3, MgO-Al2O3 and binary MgTiO3 with relatively high specific surface area have been prepared by using hollow mesoporous carbon spheres as hard templates.[90] The metal oxides were fabricated within the pore channels of the carbon templates after the following removal of the carbon shell. Table 2 summarizes the physico-chemical properties of HMMs prepared via hard-templating routes. With solid polymer, SiO2 or carbon spheres as core templates, it has been found that highly dispersed HMMs with uniform particle sizes and tunable and controllable hollow core dimensions and shell thicknesses can be readily prepared. Moreover, the framework components of HMMs can be varied from SiO2 to the nonsiliceous, such as carbon components. These templated HMMs

T. Hyeon[18a]

benefited from the high-quality hard templates, which are featured with uniform sizes, regular morphologies, high dispersions and easy surface modifications. Though an extra step is needed for the preparation of the hard templates, it is just the separate syntheses of the templates and the mesoporous shells in different media that makes the preparation of monodispersed and uniform HMMS possible, if compared to those by the soft-templating routes. However, it is also worth noting that the periodicity of the mesopore arrangement in these resultant HMMs is usually poor or sometimes hardly controllable, which is probably due to the fact that C18TMS rather than CTAB was more frequently employed to create the mesoporous shell. 2.2.5. Other Hard Core Templates In addition to the above-mentioned hard templates, some other inorganic or metal oxides can also be used as hard core templates.[91] In particular, hematite has been verified to be a useful hard template to fabricate hollow silica particles. The synthetic strategy involves coating an organic-incorporated silicate shell with a desired thickness by the co-polymerization of TEOS and C18TMS mixture on a pre-formed hematite particle of desired size and shape, subsequent removal of the organic groups by calcination to form hematite core-mesoporous silica shell composite particles (as-HMPs), and finally etching away of the cores with hydrochloric acid solution. With this strategy, uniform and well-dispersed hollow mesoporous silica particles (Figure 12a) could be facilely synthesized. Moreover, the HMS shape, such

Figure 12. TEM images of hollow mesoporous silicas with various morphologies by using hematite as core templates. Reproduced with permission.[91] Copyright 2009, Royal Society of Chemistry.




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as ellipsoids, can be tuned according to that of the hematite cores (Figure 12b,c). Moreover, hollow structured phenylenebridged periodic mesoporous organosilica (PMO) spheres with a uniform particle size of 100–200 nm could also be obtained by using α-Fe2O3 as a hard template and 1,4-Bis(triethoxysilyl) benzene (BTEB) as silica source, respectively.[91,92] By adopting hydroxyapatite nanoparticles as an etching-removable core material, hollow mesoporous silica could be prepared as well. Interestingly, the morphology of HMS can be well controlled and tailored according to the nature of HA template.[93] It is of great interest to fabricate organic-inorganic hybrid or even pure organic hollow mesoporous sphere materials, which are expected to possess improved dispersivity, biodegradability and other properties that pure inorganic materials do not have. Huang et al. developed a facile strategy for preparing hollow mesoporous submicrospheres of poly[(cyclotriphosphazeneco-4,4′-sulfonayldiphenol)] (PSZ) by choosing CaCO3 spheres as templates. The fabrication includes the polycondensation between hexachlorocyclotriphosphazene (HCCP) and 4,4′-sulfonyldiphenol (BPS) via a very simple precipitation polymerization procedure on the hard core, and the removal of templates. This method needs a single ultrasonic radiation treatment at room temperature. The as-synthesized hollow mesoporous submicrometer-sized spheres possess excellent biocompatibility and dispersity in both aqueous and organic media. Moreover, these crossliniked polyphosphazene hollow mesoporous submicrometric spheres manifested a relatively high drug storage capacity of 380 mg per g doxorubicin hydrochloride, and extremely sustained release property (up to 15 d).[17b] 2.3. Aerosel-Templating and Template-Free Routes In addition to the common soft- or hard- templates employed for HMMs syntheses, alternatively, aerosols may be also created during synthesis by, for example, evaporation, thermal spraying and salt decomposition, and can thus be used as the template for HMMs preparation. For example, evaporation-induced self assembly (EISA) process of amphiphilic molecules in droplets containing alkoxysilane precursors, which has been shown to be a versatile approach for synthesizing ordered spherical mesoporous or mesostructured silica particles or mesoporous films, can generate aerosols, and under the assistance of the aerosol, hierarchical or hollow mesoporous materials could be thus fabricated.[94] On this basis, Linden et al. and Sanchez and co-workers reported the aerosol-based syntheses of core-shell structured mesoporous nanoshperes with bimodal porosity and large-pore amorphous mesostructured aluminosilicates by using a mixture of two surfactants as structure-directing agents,[95a-c] which presents a highly effective pathway for the fabrication of HMMs. Afterwards, Ward et al. demonstrated that surfactant-directed synthesis of mesoporous silica could be combined with polymer poly(acrylic acid) phase separation during evaporating droplets in a one step synthesis, leading to hierarchical porosity after subsequent calcination.[95d] Owing to the macroscopic phase separation of the sorbitan monooleate surfactant (Span 80) during aerosol assisted spraying, hollow spheres of phenolic resin/silica composite could be obtained. Both the size and number of the hollow cavity were found to

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be tunable by changing the using amount of Span 80 to a certain extent.[96] Recently, Brinker et al. reported that macroscopic phase separation driven by controlled salt ((NH4)2SO4) nucleation and surfactant-directed self-assembly within aerosol droplets can produce hollow spherical silica particles with ordered mesoporous shells in a simple process. Salt decomposition created a spherical void in the particle interior and catalyzed silica condensation which stabilized the hollow particle.[97] Furthermore, with an aerosol-spraying approach, hollow mesoporous spherical BiFeO3 with enhanced activity and durability, could be designed and prepared.[19d] Template-free hydrothermal synthesis, during which Ostwald ripening plays a crucial role, was also adopted to prepare CeO2, or ZnO-based hollow mesoporous spheres.[15c,15f ] As neither surfactants nor templates were used in the reaction system, the mesoporous shell was built from the aggregation of nanoparticles, while the void core was a result of the solid evacuation driven by an Ostwald ripening process. Generally speaking, though few examples of template-free synthesis of HMMs have been reported, a template is usually necessary in fabricating hollow-structured materials, which can be either externally introduced (solid particles for hard-templating) or self-generated (liquid droplets/micelles for soft-templating). As summarized above, soft-templating (including seldom used aerosol template) is usually a one-pot or one-step process which can yield HMMs with ordered mesoporous structure in the shell; unfortunately the particle dimension, morphology and dispersity are usually hard to get under fine control. Alternatively, when employing silica core-mesoporous silica shell (SCMS) or hollow mesoporous silica templates for the replication of hollow mesoporous non-siliceous particles,[18a,73a,73b] the aggregation is frequently unavoidable during the hard-templating process due to the undesired but sometimes inevitable deposition of the precursor on the outer surface of the template particles accompanying the precursor deposition into the pore channels of the hard template, which will bind with each other and finally result in aggregation of the templated particles. One key to prevent the aggregation during hard-templating is to minimize the precursor deposition on the outer surface by either casting all precursors into the pore system by external pressure or vacuumization, or by removing the deposited precursor from the outer surface without affecting those within the pore system.[107c] However, more frequently, if one uses solid nanoparticle (e.g., PS Latex, silica) as the core template, and the porous structured shells are generated by the self-assembly between the mesoporogens and precursors on the surface of the solid core, usually the aggregation can be easily prevented.[62,63,75,82,87] Therefore hardtemplating strategy is becoming more prevailing due to the fact that monodispersed HMMs with highly controllable hollow core diameter, shell thickness, and the mesoporosity in the shell as well, can be obtained with the hard-templating, which is highly important or even crucial in the applications in, for example, catalysis, and especially in biomedical fields.

3. Functionalization and Applications of HMMs Owing to the special core-shell structure, HMMs can be functionalized in the hollow core, on the inner and outer surface of the shell, or in the mesopore channels in the shell. As the




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strategies of functionalization are similar to those for the conventional mesoporous materials, we will not discuss these specifically, but will describe them in detail in the following section along with their applications. Hollow mesoporous materials combine both macroporous and mesoporous features into one unit, where the hollow cavity can function as a storage reservoir or a microreactor, and the permeable mesoporous shell allows the guest species to diffuse through the mesopore channels between the macroporous core and the exterior of the spheres, and in addition, the mesopore channels in the shell layer can also be used to either incorporate, adsorb, or immobilize guest substances. Therefore, HMMs are believed to have great application potential in a variety of fields, such as energy storage, separation, sensors, catalysis and drug storage and delivery.[98] 3.1. Catalysis Mesoporous materials have shown attractive performance in catalysis field.[99] Hollow mesoporous materials have an interconnected bimodal pore system composed of a hollow core and a mesoporous shell and have much higher pore volume compared to the conventional mesoporous materials. Therefore, they are potentially important as a catalyst support, in which the hollow core and sometimes the mesopores in the shell as well can be used for loading/dispersing catalyst nanoparticles/ species, meanwhile the open mesopore network in the shell connecting the hollow core in the interior and the exterior can provide a free highway network around the active catalyst for the free diffusions of reactant molecules accessing active sites and products leaving the catalysts. Based on the uniform hierarchical nanostructure of hollow mesoporous materials, active catalyst nanoparticles can locate either in the mesopore network on the shell or within the hollow core.[100] Scheme 4 illustrates the advantages of catalytic nanoparticles-encapsulated HMMs in catalysis: the catalyst is well protected from the environment and the catalytic particles are well separated from each other as well, thus no catalytic particle growth and/or aggregation will happen during the catalytic reactions especially at elevated temperatures. In the meantime, the mesopore channels function as the gate for shape-selective catalysis where only the reactant/product molecules of smaller than the pore size can diffuse in/out of the composite system.

Scheme 4. A comparison of the catalytic processes between using catalyst nanoparticles-loaded HMMs (1) and conventional nanoparticlate catalysts (2). No catalytic species aggregation will happen among those loaded in HMMs during catalytic reaction, while it will most probably do in traditional nanoparticulate catalysts.

catalysts (Pt@hmC) by encapsulating Pt nanoparticles into the hollow core of HCMS can work as a robust and reusable heterogeneous catalyst for a number of reactions, such as hydrogenation, where the carbon shell functions as a barrier to prevent the possible coalescence of Pt nanoparticles between each other and also provide a void space for organic transforming on the surface of the ligand-free Pt nanoparticle.[103a] Compared with the original Pt-PVP, a commercial Pt catalyst supported on activated carbon (Pt/AC), and Pt@SiO2-mSiO2, remarkable activity for hydrogenation of nitrobenzene has

3.1.1. Catalytically Active Nanoparticles Encapsulated into the Cores of HMMs Nanometer-sized noble metal particles have shown great potentials as advanced catalysts owing to their large surface area and size-dependent properties different from bulk metals.[101] However, the difficulties in the recovery of the fine particulate catalysts from the reaction mixture, as well as the instability of the nanometer-sized particles under different conditions, such as high pressure or high temperature, have strongly hindered their scalable applications. One of the most straightforward approaches to circumvent these problems is to immobilize the metal nanoparticles on/in solid supports.[102] It has been proved that “rattle-type” nanostructured


Figure 13. Liquid-phase hydrogenation of nitrobenzene into aniline by various Pt catalysts at 303 K. Reproduced with permission.[103a] Copyright 2006, Wiley.



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www.MaterialsViews.com Table 3. Hydrogenation of various olefins by pt@hmC, Pt/AC and Pt-PVP catalysts. Reproduced with permission.[103a] Copyright 2006, Wiley. Substrate

tb) [h]

Conv.c) [%]

Pt@hmC

2

>99

Pt-PVP

2

91

Pt/AC

2

7

Pt@hmC

2

96

Pt-PVP

2

70

Pt/AC

2

16

Pt@hmC

1

91

Pt-PVP

1

42

Pt/AC

1

3

Pt@hmC

15

72

Pt-PVP

15

46

Pt/AC

15

16

Catalyst

Product

a)All

reactions were carried out with 0.1 µmol of catalyst (Pt) and 0.5 mmol of substrate under H2 (0.2 MPa in absolute pressure) at 348 K; b)Reaction time; c)Conversion of substrate.

been achieved (Figure 13): nitrobenzene was completely converted into aniline on Pt@hmC, as compared to only a portion of nitrobenzene convertion on Pt-PVP and Pt/AC, and almost no nitrobenzene convertion on Pt@SiO2-mSiO2. These indicate that the presence of the hollow void in the particles Pt@hmC plays a crucial role in enhancing the catalytic activity of Pt@hmC. On the other hand, though Pt-PVP exhibited higher activity than Pt/AC in the first run, it cannot be recovered efficiently for further reactions. Comparatively, the Pt@hmC catalyst could be simply recovered by centrifugation and recycled for further reactions. Moreover, in contrast to Pt-PVP and Pt/AC catalysts, Pt@hmC also showed higher catalytic activity for other reactions, such as the hydrogenation of primary, secondary and cyclic olefins, as presented in Table 3. These demonstrate the importance of the hollow core in maintaining the activity and stability of noble nanoparticles encapsulated into it. Based on the same idea, core-shell structured Pt@mesoporous silica configuration (Pt@mSiO2) was designed and prepared. The outer layer isolated the catalytically active nanoparticles from each other and prevented them from sintering during catalytic processes at elevated temperatures. Furthermore, it is considered that the synergistic effects at the metal-support interfaces may be maximized, which are very important for their catalytic performances. Consequently, the Pt@mSiO2 catalyst could maintain its coreshell structure up to 750 °C, so that high-temperature CO oxidation could take place and high catalytic activity could be obtained. In comparison, it was not possible for bare Pt nanoparticles to stand with such a high tempereature due to their sever deformation or aggregation (Figure 14).[103b] Apart from

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Figure 14. CO oxidation activity of TTAB-capped Pt and Pt@mSiO2 nanoparticles (inset: TEM image of Pt@mSiO2 nanoparticles after calcination at 550 °C). Reproduced with permission.[103b] Copyright 2009, Nature Publishing Group.

single metal nanoparticles, multiple dispersed inorganic oxide nanoparticles (TiO2 and Fe2O3) could also be encapsulated into the hollow cores of hollow mesoporous silica capsules (HMSC) with incipient wetness technique. A large amount of TiO2-encapsulated silica capsules photo-catalyzed much faster decomposition of methyl orange, especially at the initial reaction stage, than that by other mesoporous silica-based TiO2/ SiO2 composites.[100a] Recently, Zheng et al. successfully prepared hollow mesoporous aluminosilicate spheres (HMASs) with pore channels prependicular to the surface simply by treating the solid silica spheres in a hot alkaline solution of sodium aluminate and CTAB.[104a] It is demonstrated that the highly permeable prependicular pore channels could effectively prevent the catalytically active Au nanoparticles incorporated into the cores from aggregation in the reduction reaction of 4-nitrophenol. Forthermore, owing to the accessible acidity introduced by Al incorporation in the framework of the shells, the york-shell structured HMASs with Pd nanopartilces in the cores presented high catalytic performance and recyclability in the one-pot two-step reaction involving an initial acid catalysis and subsequent catalytic hydrogenation for desired benzimidazole derivatives.[104a] On the other hand, by employing polystyrene spheres as both carriers and templates, Pd or Au nanocrystal-embedded hollow mesoporous TiO2 and ZrO2 microspheres were facilely synthesized by Wang et al.[104b] Owing to the unique core-shell structure, Pd nanocubeembedded ZrO2 microspheres exhibited a much higher catalytic activity and reaction rate, and a superior recyclability, in comparision with the commercial Pd/C catalyst, in the reduction of 4-nitrophenol (Figure 15). More interestingly, in addition to the advantages such as fast diffusion of reactants and products, and inhibited growth of the confined active nanoparticles within the mesopore channels in the shells, a synergistic catalytic effect between the encapsulated Pd nanoparticles and the mesoporous CeO2 shells was believed to speed up the charge transfer and effectively inhibit the catalyst poisoning, so that the reduction of 4-nitrophenol could be significantly accelerated.[104c]




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Figure 15. a) TEM image of the ZrO2 microsphere after five cycles for 4-nitrophenol reduction reaction. b) Plot of C(t)/C(0) against the reaction time in five successive cycles of the reduction reaction with the Pd nanocube-embedded hollow mesoporous ZrO2 microspheres as the catalyst. Reproduced with permission.[104b] Copyright 2013, Wiley.

3.1.2. Catalyticaly Active Components Loaded into the Shells of HMMs In as early as 2004, Yu et al. found that carbon capsules with hollow core and mesoporous shell (HCMS) were excellent supports for electrode catalysis in the direct methanol fuel cell (DMFC) (Figure 16).[105] After loading Pt50-Ru50 catalyst onto the mesoporous shell, such a HCMS presented much higher catalytical activity for methanol oxidation than the commercial E-TEK catalyst by about 80%, which was attributed solely to the excellent structure properties (i.e., high specific surface area and well-interconnected bimodal porosities of HMCS.)[105] Later, researchers further demonstrated that Pt/Ru- or Ptloaded HCMS, as anode catalysts, also exhibited significantly enhanced electrocatalytic activities in direct formic acid fuel cell (DFAFC)[106] or proton-exchange membrane fuel cell (PEMFC), which were probably due to the combined contributions by the uniformly dispersed catalyst nanoparticles and fast mass transport network around the active catalyst nanoparticles,


Figure 16. Polarization and power density plots at 30 °C (a) and 60 °C (b) for various hollow mesoporous carbon-supported Pt50Ru50 (60 wt%) anode catalysts. Reproduced with permission.[105] Copyright 2004, Royal Society of Chemistry.

resulting from the large surface area and mesopore volumes and the 3D interconnected hierarchical nanoporous structure of HCMS.[107a,b] Furthermore, the possibility of HCMS as a cathode catalyst support in H2-fueled PEMFC was evidenced by loading Pt (20 wt%) in HCMS, which exhibited an excellent catalytic activity to oxygen reduction reaction (ORR) comparable to the state-of-the-art cathode catalyst.[107c] In order to further improve the stability of the electrocatalysts, Schüth et al.[107d] prepared hollow graphitic carbon spheres for loading Pt nanoparticles in the mesopores of the shells. Testing results showed that the electrochemical stability was enhanced while the high activity was retained, which are benefited from the mesoporous carbon network. These prove that the mesoporous shell of HCMS could effectively maintaine or enhance the catalytic activity/stability of the active nanoparticles by reducing their detachment and agglometration due to the confiment and good separation of them in the mesopore system. In addition, by using glycine as both carbon and nitrogen precursors, nitrogen-doped HMCSs with broken graphene in the wall were



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Table 4. Comparison of catalytic performances of the MacMillan catalyst on different phPMO supports.a) Reproduced with permission.[92] Copyright 2011, Wiley.

Ph

CHO

+

Cat. (20 mol%), TFA Solvent, RT

+

Ph CHO

Entry 1

CHO Ph

Catalyst

t [h]

Solvent

Yieldc) [%]

endo/exod)

endo eee) [%]

exo eee) [%]

1

24

CH3CN/H2Ob)

97

1:1.1

95

92

2

H-PhPMO-Mac

24

CH3CN/H2Ob)

58

1:1.1

89

86

3

PhPMO-Mac

24

CH3CN/H2Ob)

42

1:1.1

87

85

4

H-PhPMO-Mac-G

24

CH3CN/H2Ob)

46

1:1.0

72

69

5

PhPMO-Mac-G

24

CH3CN/H2Ob)

38

1:1.0

70

68

b)

6

1

24

80

1:1.4

93

91

7

H- PhPMO-Mac

12

CH3CN/H2O H2b

98

1:1.1

81

81

8

PhPMO-Mac

12

H2O

84

1:1.1

79

78

9

H-PhPMO-Mac-G

12

H2O

86

1:1.2

63

62

10

PhPMO-Mac-G

12

H2O

80

1:1.1

59

62

TFA = trifluoroacetic acid.; b)CH3CN/H2O, 95:5(v/v); c)Yield of the isolated product; d)Determined by 1H NMR spectroscopy; e)Determined by HPLC.

a)

prepared.[107e] Owing to the absence of active metals, such a material exhibited excellent methanol tolerantce in the ORR in alkaline solution, though the ORR activity was inferior to the commercial Pt/C catalyst. Very recently, a novel sulfur-impregnated hollow mesoporous TiO2 spheres were obtained and studied as cathode material in Li-S batteries.[107f ] With a 68 wt% loading amount of S, an intriguing capacity retentation of 71% and a high coulombic efficiency of 93% over 100 cycles, at a 1C rate, were achieved. By using hollow mesoporous organosilica spheres as support, MacMillan catalyst (H-PhPMO-Mac) could be easily grafted by a co-condensation process and a “click chemistry” post-modification. Due to the hydrophobic properties of PMO surface and the accessible mesopores in the shell, the mass-transport process was accelerated, which endowed the H-PhPMO-Mac catalyst with higher catalytic activity than solid (non-hollow) PhPMO-Mac catalyst in asymmetric Diels–Alder reactions (Table 4).[92] In order to understand the effect of shell thickness on the catalytic activity, hollow mesoporous carbon spheres with different

shell thicknesses (0, 25, 50 to 100 nm) were prepared and the electrochemical capacitance and ionic transport performance were examined. It was found that the shell thickness affected the overall porosity and relative porosities of the shell, core and interstitial regions (Figure 17). A performance-structure relationship is established from the electrochemical tests on a set of hierarchical structures with the stepwise increased thicknesses of the mesoporous shell. At low scan rates and low currents, capacitance depends on the surface area which increases with the increase in the mesoporous shell thickness. At high scan rates and high current loadings, ion transport become hindered in thicker shells.[108] 3.1.3. Enzyme-Encapsulated HMMs for Building Biosensors Owing to the rigid structure, large interior space inside the shell and the isolated catalytic environment, HMMs can be employed as an excellent support to immobilize enzymes for establishing biosensors.[109a,b] For example, a novel rigid artificial

Figure 17. CV curves (a,b) of various carbons recorded at 5 mV s−1 and 200 5 mV s−1; Charge–discharge curves (c) of C-CS0, C-CS50, C-CS80 and C-CS150 obtained at a constant current density of 10 A g−1. Reproduced with permission.[108] Copyright 2011, Royal Society of Chemistry.

18





superoxide dismutase (SOD) has been constructed by immobilizing Mn-(bis(salicylaldehyde)-3,4-diaminobenzoic acid) (Mn-CSalen), a well-known active center for the design of artificial superoxide dismutase (SOD), into HMMs. Thanks to the rigid structure, free diffusion pathways for product molecules, biological fitness and resistance to extreme environmental conditions offered by HMMs, this novel rigid artificial SOD can be easily immobilized as a receptor and be combined with various measuring methods to establish biomimetic sensors (Figure 18). More importantly, this novel rigid artificial SOD possesses higher activity than the unsupported active centers.[109c]

3.2. Electrochemical Energy Storage With the unique hollow core-shell structural features, HMMs have shown great application potentials in mass storage and diffusion, especially in electrochemical energy storage. Specifically, the macro-hollow core can serve as an efficient mass storage and buffer reservoir, which greatly facilitates the reduction of the volume change during the charge–discharge cycling especially at high rates. On the other hand, the mesoporous shell around the hollow core would shorten the mass diffusion path and accelerate the mass diffusion.[110] Consequently, HMMs show ultra-high mass storage capacity and excellent cycling

REVIEW

Figure 18. Schematic representation of the artificial SOD. Reproduced with permission.[109c] Copyright 2011, Elsevier.

performance and rate capability, as demonstrated by Yu et al. in storing electrochemical hydrogen or Li by using hollow core mesoporous shell carbon (HCMSC) as supports. It has been found that the charge capacity of HCMSC is up to 586 mAh/g, corresponding to 2.17% hydrogen uptake, in 6 M KOH at a dischage rate of 25 mA g−1. Furthermore, excellent cycling capacity retainability and rate capability were obtained (Figure 19).[110a,b] Alternatively, the test of Li storage was perfomed by choosing HCMSC as an electrode material for electrochemical double layer capacitor. A very high specific capacitance of 162 F g−1 at 0.3 A g−1 curret density in a more practical two electrode symmetric system was achieved by employing organic electrolytee. At an elevated current density of 1.0 A g−1, the specific capacitance maintained as 148 F g−1, about 91% of the initial capacitance at 0.3 A g−1, which is about 2 times higher than the commercial activated carbon. Moreover, very high cyclic performance with about 88% retention of the initial capacitance value after up to 2000 charge–discharge cycles at 1.0 A g−1 was obtained.[110c] Additionally, hollow mesoporous TiO2 microspheres were investigated as an anode material for lithium-ion batteries. A capacity as high as 200 mAh/g, excellent cycle life and rate capability were achieved. Endowed with decreased crystallite size and enlarged specific surface area, hollow mesoporous TiO2 spheres showed satisfactory electrical contacts and tolerance to the strain occurred during the charge– discharge process, thus almost free lithium-ion diffusion and superior electrochemical performance were achieved.[111a,b] Recently, Lou et al. developed a facile hard-templating strategy for the synthesis of hollow Li4Ti5O12 spheres for the use as anode materials for high-rate lithium-ion batteries (LIBs), which exhibits a remarkable rate capability up to 20C and longterm capacity retention for over 300 cycles.[111c] 3.3. Adsorption and Separation HMMs have exhibited much more advantages in mass transport compared with conventional mesoporous materials due to their large pore/cavity volumes and spherical morphology, which are anticipated to be of great importance in the field of adsorption and separation.[112] Bilirubin is a pathogenic substance and one of the common metabolites of hemoglobin, and is released into blood due to

Figure 19. A) Galvanostatic charge–discharge curves at 100 mA h g−1; B) Cycling performance and coulombic efficiency at a specific current of 100 mA h g−1; C) Rate performances at different current densities from 100 to 1000 mA h g−1 and then back to 100 mA h g−1 using commercial graphite, CMK-3 and HCMSC capsules as anode. Reproduced with permission.[110b] Copyright 2011, Royal Society of Chemistry.




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the hollow cores of HMCSs are good reservoirs for storing guest molecules such as bilirubin and the much shorter channels in the thin mesoporous shell is beneficial for the bilirubin access to all mesoporous channels of HMCSs; the hydrophobicity of HMCSs prefers much higher adsorption capacity of the hydrophobic bilirubin.[115a] Furthermore, when loaded with magnetic hematite into the cores, HMCSs become magnetically separable absorbents while the higher adsorption capability can be roughly maintained.[115b] Moreover rattle-type magnetic mesoporous carbon spheres (RTMMCSs) with a hollow magnetite core, perpendicularly aligned mesopore channels in the shell and a cavity in between were demonstrated to be useful as a magnetically separatable and reusable absorbent for fast, convenient and highly efficient removal of microcystins, which was mainly attributed to the high surface area of the numerous accessible mesopores, as well as the presence of cavities between the core and shell.[116] Similarly, hollow mesoporous silica spheres could be used to remove organic molecules from aqueous solution after certain modifications.[117] Zhou et al. reported that hexadecyltrimethylammonium bromide (HDTMAB)-immobilized hollow mesoporous silica spheres could be used to efficiently adsorb perfluorooctane sulfonate (PFOS) even at low solution pH and ionic strength values through hydrophobic interaction, so that more than 99% PFOS can be removed from water due to the strong adsorption affinity of the modified spheres with PFOS.[117b] Similarly, after hydrophobic modification, hollow porous silica nanospheres could be applied to quickly remove 4-Nonylphenol from water with enhanced saturation adsorption amounts.[117c] Figure 20. Bilirubin equilibrium adsorption isotherms of active carbon (a), CMK-3 and HCMSs (b). Reproduced with permission.[115a] Copyright 2009, Royal Scoiety of Chemistry.

the normal or abnormal destruction of red blood cells.[113] It is necessary to separate the excessive bilirubin from blood, which is believed to the main cause of a characteristic form of crippling known as athetoid cerebral palsy or even death.[114] A recent report shows that hollow mesoporous carbon spheres present considerably higher bilirubin adsorption capacity (304 mg g−1) and rate (bilurubin concentration from 250 mg L−1 down to 0.3 mg L−1 accomplished in 4 min) in PBS solution, as compared with commercial activated carbon (Figure 20 & Table 5). More importantly, HMCSs show relatively high bilirubin adsorption selectivity against albumin at normal albumin concentration and negligible hemolytic activity. It is believed that the excellent adsorption performance of HMCSs is related to the structural features and the surface property: Table 5. Comparison of the samples on the adsorption capacity of Bilirubin. Reproduced with permission.[115] Copyright 2009, Royal Society of Chemistry. Sample

20

Initial conc. [mg L−1]

Balanced conc. [mg L−1]

Balanced period [min]

Adsorption capacity [mg g−1]

Activated carbon

250

110.3

300

70

CMK-3

250

0.3

10

198

HMCSs

250

0.3

4

304


3.4. Biomedical Applications Due to the high surface-to-volume ratio, high pore volume and also the ease of modification of the outer surface, hollow mesoporous silica nanospheres (HMSNs) have attracted increasing attention for the encapsulation and delivery of chemical drugs in biomedical applications to meet the requirements of themo-therapeutic drug vehicles in terms of its high biocompatibility, biodegradability, high loading efficiency and controlled drug release properties. Especially, sophisticatedly engineered multifunctional HMSMs with diverse functions may accomplish (e.g., molecularly/magnetically targeted drug delivery, simultaneous multi-modality bioimaging and therapeutic capability, in vivo stimuli-responsive drug release/therapy, multicomponent synergistic therapy, etc.). The distinctive functions, unique structures, convenience in multifunctionalization and the related excellent biomedical performances endow the multifunctionalized HMSNs with much more application potentials than traditional HMMs, and have attracted intensive attentions among chemists, material scientists, biologists, pharmaceutical companies and even doctors.[118] 3.4.1. Drug Carriers It has been demonstrated that the drug (such as aspirin or i-buprofen) storage capacity of HMSNs is remarkably higher than that of conventional mesoporous silicas, MCM-41 and




SBET [m2 g−1]

Vp [cm3 g−1)

IBUa) [mg g−1]

HMSC-IBU

590

0.596

969

N(0.3)-HMSC-IBU

478

0.439

768

N(0.6)-HMSC-IBU

353

0.319

681

N(1.0)-HMSC-IBU

242

0.195

569

NN(0.3)-HMSC-IBU

445

0.405

742

NN(0.6)-HMSC-IBU

326

0.295

659

NN(1.0)-HMSC-IBU

165

0.123

534

NNN(0.3)-HMSC-IBU

406

0.411

709

NNN(0.6)-HMSC-IBU

347

0.274

615

NNN(1.0)-HMSC-IBU

174

0.104

523

Samples

a)

Calculated from UV absorbance analysis according to Lambert–Beer’s law.

Figure 21. The release percentages of ibuprofen from as-prepared HMSC and modified HMSCs systems. Reproduced with permission.[120] Copyright 2011, Elsevier.

MCM-48 with sustained-release properties due to the presence of the large hollow voids in the interior and/or the nanosized mesopore channels in the shell.[119] With HMSNs as the drug carrier, as given in Table 6, the loading amount of ibuprofen could reach as high as 969 mg g−1. More importantly, the release rate of the loaded ibuprofen molecules could be tuned by modifying the pore surface with functional –NH2

groups (Figure 21).[120] However, as most of above mentioned HMSNs have a poor dispersity and stability in aqueous solution due to the strong aggregation among particles, which would greatly prevent them from being used as carriers in drug delivery systems due to the impossibility of these aggregated HMSNs in circulating in blood stream and subsequently in reaching lesions by the enhanced permeability and retention (EPR) effect, therefore it is of great importance to substantially improve its dispersity and stability in aqueous solutions for possible applications in drug delivery systems for enhanced therapeutic efficacy. Many studies have shown that the PEGylation of nanoparticles is one of the most efficient ways to enhance the blood circulation and EPR effect.[121] Recently, PEGylated HMSNs (HMSNs-PEG) have been successfully fabricated by covalently grafting poly(oxyethylene)bis(amine) (PEG2000-NH2) on HMSNs-NH2 with p-phenylene diisothiocyanate (DITC) as a cross linker. The stability and dispersity of HMSNs were found to be significantly improved by the introduction of PEG barriers (Figure 22). Furthermore, compared to HMSNs, much lower in vitro cytotoxicity to HeLa and NIH3T3 cells up to a concentration of 150 µg mg−1, and near 2 times higher uptake amounts were obtained on HMSNs-PEG. With doxorubicin (DOX) as a model drug, DOX-loaded HMSNs-PEG exhibited noticeably higher cytotoxicity than that of DOX-loaded HMSNs against Hela and NIH3T3 cells (Figure 23).[121] Based on the unique hollow and mesoporous structure, HMSNs can serve as an alternative to traditional liposomes, where the large hollow interior functions as a reservoir for storing hydrophobic agents, while the mesoporous silica shell with controllable thickness, hydrophilic inner/outer surface, tunable pore size can guarantee efficient encapsulation of hydrophilic agents.[78a] This has been successfully demonstrated by the simultaneous co-loading and co-delivery of hydrophobic camptothecin (CPT) and hydrophilic DOX anticancer drugs, which brought forth the enhanced chemotherapeutic effect against DOX-resistant MCM-7/ADR cancer cells. Scheme 5 gives an illustration of drug delivery using HMMs. Drug molecules are mostly loaded in the hollow cavities, and the drug-loaded HMMs can pass through the vasculature and uptaken by the cells via EPR and other targeting effects. Drug molecules will release in the cell plasma and diffuse into the nucleus to damage the DNA double chain, meanwhile the HMMs will either degrade into small fragments and/or molecules

REVIEW

Table 6. The structure parameters and ibuprofen storage capacities of as-prepared HMSC and modified HMSCs after the interaction with the hexane solutions of ibuprofen. Reproduced with permission.[120] Copyright 2005, Elsevier.

Figure 22. CLSM images of Hela (a and b) and NIH3T3 (c and d) cells after 4 h incubation with RBITC/HMS and RBITC/HMS-PEG nanoparticles. Reproduced with permission.[121] Copyright 2011, Elsevier.




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Scheme 5. Schematic illustration of HMMs as a drug/gene delivery system. Step 1: drug-loaded HMMs passing through vascular wall and uptaken by the tumor cells via EPR and other targeting effects; step 2: endocytosis of drug-loaded HMMs by cell; step 3: drug release from HMMs; step 4: self-degration or exocytosis of HMMs.

Figure 23. Cell viabilities against free DOX, DOX-loaded HMS and HMSPEG nanoparticles at a DOX concentration of 2 µg/ml. Reproduced with permission.[121] Copyright 2011, Elsevier.

for the later excretions via urine and feces, or directly excreted via feces.[22c] By an efficient surfactant-directed alkaline-etching strategy based on a reversible alkoxide dissolution/recondensation chemical process, mesopores in the shell of HMSNs could be tuned from 3.2 to larger than 10 nm, which provides opportunities of loading large entities such as biomolecules (siRNA) and nanoparticles (Fe3O4 NPs). It was showed that the resulted large-sized and surface-functionalized HMSNs could effectively encapsulate siRNA molecules and high siRNA transfection efficiency could be achieved.[122] The Fe3O4 NPs-loaded HMSNs exhibited superparamagnetic properties, and their use as T2-weighted MRI contrast agent was demonstrated by giving a significant signal-decreasing effect. Especially, the biological roles of HMSNs with different shell-pore sizes were evaluated in killing multidrug-resistant (MDR) cancer cells.[123] It was revealed that drug release and the MDR-overcoming behaviors of DOX-loaded HMSNs (DMSNs) were pore-size-dependent, and substantial contribution of DMSNs to the anticancer activity against MCF-7/ADR cells was demonstrated. It was found that the larger the pore size of DMSNs was, the more

22


efficient cellular uptake of DOX and the faster intracellular drug release the DMSNs would exhibit, and consequently, the quicker intracellular drug accumulation and stronger MDRreversal effects were resulted. Based on these, the MDR-overcoming mechanism was proposed to be due to the efficient cellular uptake, P-gp inhibition and ATP depletion. Recent strategies for drug encapsulations are mostly focused on loading drugs and genes on the pre-synthesized HMSNs. The main reason is that incompatible techniques/procedures (e.g., strong chemical corrosion, high-temperature calcination or the use of organic solvent or reactive metals) were usually employed in synthesizing HMSNs, which may lead to the deactivation of therapeutic molecules when being loaded in HMSNs during the carrier synthesis. Alternatively, Yu et al have designed a distinctive approach of “preloading” for drug encapsulation, which was accomplished by encapsulating biomedicines during the synthesis of the host HMSNs. Specifically, doxorubicin (DOX) preloaded porous CaCO3 nanospheres were chosen as hard templates to prepare hollow mesoporous silica@DOX nanospheres. The key step was to remove the CaCO3 core template via mild erosion with ethylic acid solution (PH = 4), which could enable the preloaded DOX to be reserved. With this approach, it is more convenient to utilize the interior voids to load cargoes and protect them within the hollow structured materials from external attacks. Moreover, enhanced therapeutic effects of DOX@HMSNs over the free drug itself were exhibited towards tumor cells.[124] In addition to the high drug storage capability and stimuliresponsive release properties of HMSNs, the biological effects including biocompatibility, biodistribution and clearance are the other major concerns from the viewpoint of clinical applications. It has been verified by several studies that the biocompatibility of HMSNs on a variety of cell types in vitro is fairly high.[66,125] Recently, Shi et al. have demonstrated the low in vitro cytotoxicity,[125a] in vivo blood circulation, biodistribution




REVIEW

Figure 24. ICP-OES analysis result of silicon levels in liver, spleen, lung, kidney and brain of animals treated with MHSNs. Reproduced with permission.[125d] Copyright 2011, Elsevier.

and clearance behaviors, and their size effects, of MSNs without hollow interior, especially after PEGylation.[125b] It was showed that most MSNs were distributed in RES organs such as liver and spleen, as many other nanoparticles did, and could be mostly excreted via, for example, urine, in one month of vein-injection due to the degradation of MSNs into small siliceous acid molecules.[66,125c] No significant pathological injury to varied organs and death of mice tested were found. Not surprisingly, Tang et al.[125d] recently reported the high biocompatibility of HMSNs by using rattle-type HMSNs as drug carriers. Systematic studies on mortality, clinical features, pathological examinations as well as blood biochemical indexes demonstrated the low in vivo toxicity of HMSNs (Figure 24). It was found that, similar to the situations of MSNs, HMSNs accumulated mainly in mononuclear phagocytic cells in liver or spleen and could be excreted from the body in or beyond 4 weeks. However, liver injury caused by HMSNs at high doses was observed. Therefore, more extensive and long-term toxicity evaluations are needed to confirm these results. Compared with the sustained-release system, the stimuliresponsive controlled-release system is more favored for achieving the site-selective and on-demand drug release pattern to effectively enhance the therapeutic efficacy. In order to achieve this, it is necessary to create nanovalves, which are designed to perform valve-like functions to encapsulate and release drug molecules within the mesopores on demand, on the surface and/or at the pore openings of hollow mesoporous materials. In the meantime, the incorporated nanovalves can be manipulated by external stimuli including redox, light, enzyme, competitive binding, and pH activation. Amongst these, pH-responsive activation represents a feasible and convenient approach to provide a specific control through the perturbation of protons into the drug-loaded mesopore channels by readily available acidic and/or basic micro-environments because monitoring the pH value’s changing in solution is so simple and swift.[126] It is well known that the properties and structure of polyelectrolyte multilayers (PEM) formed by sodium polystyrene sulfonate (PSS)/polycation poly(allylamine hydrochloride) (PAH), are sensitive to the external conditions, including pH value and specific ion concentration of the surrounding medium. Therefore, by coating HMSNs with PSS/PAH multilayers via electrostatic interaction, a pH stimuli-responsive controlled drug-release system was fabricated.[121] As shown in Figure 25,


Figure 25. Cumulative drug release from the two systems in release media of different pH values. 䊏: pH 1.4 from IBU-HMS, 䊉: pH 1.4 from IBU-HMS@PEM, 䉱: pH 8.0 from IBU-HMS, 䉲: pH 8.0 from IBU-HMS@ PEM. Reproduced with permission.[126b] Copyright 2005, Wiley.

the sustained-release patterns of loaded IBU molecules were monitored for the system of PAH/PSS coated HMS loaded with hydrophobic ibuprofen (IBU-HMS@PEM) at varied pH values from weak basicity to strong acidity, and the release amount from which reached around 80% in 48 h at pH 1.4, as compared to 10% over the same period at pH 8.0. These demonstrated that pH-responsive controlled drug-release pattern could be achieved in the IBU-HMS@PEM system by changing the pH value of the release medium. Moreover, it has been found that the drug releasing rate from IBU-HMS@PEM system could be well tuned by changing the salt concentration of the release medium. Obviously, such a system possesses the features of both high storage capacity for either hydrophobic[126b] or hydrophilic[127] drugs and the stimuli-responsive controlled drug release, which may find possible biomedical application as potential drug delivery systems.[128] On the other hand, covalent bonding could also be used to construct controlled drug release systems. Jin et al. proved that curcumin molecules, which were selectively attached to the inside surface of HMSNs by covalent bonding, could be effectively released by the cleavage of the amide bond by the selected base.[129] Thus, it is believed that this system, based on the amide bond, would be very promising for fabricating custom-made controlled-delivery devices triggered by specific target molecules. Enzymes are known to possess many advantages (e.g., they are not biologically disruptive, can function under mild conditions, and are highly selective). Thus, they have been recently employed for triggering a responsive-controlled drug release from a gated HMSNs-based delivery system.[130] By combining the drug-loaded HMSNs with enzyme degradable poly(L-lysine) (PLL) polymer (positively charged) and the following gene (negatively charged) loading via electrostatic interaction, an enzymetriggered drug and gene co-delivery system was developed. In vitro release results showed that the fluorescein and cytosinephosphodiester-guanine oligodeoxynucleotide (CpG ODN) loaded HMSNs/PLL particles exhibited an enzyme-triggered controlled release pattern of fluorescein and CpG ODN



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simultaneously in the α-chymotrypsin solution. Moreover, the release rates of fluorescein and CpG ODN could be efficiently tuned by adjusting the enzyme concentration.[131] Moreover, the targeted drug delivery is another important issue for anticancer drug delivery system because most of the commonly used anticancer drugs have serious side-effects owing to their unspecific actions on healthy cells and tissues. Moreover, therapeutic efficacy could be significantly enhanced by the targeted drug delivery. Therefore, great effort has been made in designing the targeting anticancer drug delivery vehicles based on HMSNs. One alternative approach is to prepare magnetic hollow mesoporous silica spheres as targeting vehicles for delivering anticancer drugs to cancer tissues under an external magnetic field.[87,132] These magnetic hollow mesoporous silica spheres could not only reach the targeted organs or tissues at accelerated rate,[133] but also exhibited enhanced cellulose tissue penetration behavior under applied external magnetic field, which is promising for delivery applications to plant cells.[134] The magnetic targeting protocol can be further combined with chemical donator-receptor type of targeting, such as the specific mutual recognition between folic acid (FA) grafted on HMSNs and intrinsic folate receptor (FR) on the membrane of certain kinds of cancer cells.[135] Significant magnetic/FA dual targeting effects, though not very distinguished, can be observed based on the FA/PEG-grafted and magnetic NPs-loaded HMSNs. 3.4.2. Multifunctionalized Bioimaging and Therapeutic Agents The most recent research interests in HMSNs have revealed a significant trend of transforming them from single function to double functions, and even to multifunctions. Integrating various functions into HMSNs can produce remarkable multifunctional nanocarriers with 3F (finding-, fighting- and following) capabilities. The designed HMSNs could be efficiently delivered into the lesion locations (finding), killing the abnormal cells (fighting) and monitoring the evolution of the diseases (following). Especially, the integration of fluorescent and magnetic functions could yield bimodal bioimaging probes, which combine the merits of high sensitivity of fluorescent imaging, the noninvasive and high spatial resolution of MR imaging as well as for real-time monitoring the evolution of diseases into one unit. Furthermore, the introduction of bifunctional materials into mesoporous silica will make the construction of multifunctional platforms possible for simultaneous bimodal bioimaging and drug delivery. For instance, by encapsulating diverse functional inorganic nanostructures (such as Fe3O4 magnetic nanoparticles, Au nanocrystals, upconversion nanoparticles and quantum dots) into the cavities or the mesopores of HMSNs, multifunctional HMSNs can be fabricated, which are potential nanotheranostic platforms for bioimaging diagnosis and simultaneous therapy for lesion sites such as cancers.[136] For the labeling and magnetic resonance imaging (MRI) tracking of adipose-derived mesenchymal stem cells (MSCs), mesoporous silica-coated hollow manganese oxide nanoparticles as positive T1 contrast agents were fabricated by Kim et al.[137] Contributed by the free access of water molecules to the magnetic core through the mesoporous silica shell, an effective longitudinal (R1) relaxation enhancement of water protons of 0.99 (mM−1 s−1)

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was obtained at 11.7 T. Consequently, MSCs were efficiently labeled using electroporation with significantly intensified T1-weighted MR images in vitro. Moreover, it was found that the MRI signal could be durably available for over 14 days for monitoring the intracranial grafting process of HMnO@ mSiO2-labeled MSCs.[137] Recently, Shi et al.[138] developed a facile strategy to fabricate hollow nanostructures with magnetic and mesoporous double-shell (HMMNSs) through coating an organosilicate-incorporated silica-shell on β-FeOOH nanorod core followed by in-situ decomposition and reduction of β-FeOOH. Due to the presence of magnetic Fe3O4 nanocapsule as the inner shell converted from β-FeOOH, the MR signal in mouse liver and spleen decreased significantly in 30 min postintravenous administration, indicating HMMNSs a promising candidate as MRI contrast agent. Moreover, after modification of HMMNSs with rhodamine B isothiocyanate (RBITC) and poly(ethylene glycol) (PEG), HMMNS-R/P showed both high loading capacity for water-insoluble anticancer drugs (docetaxel or camptothecin) and enhanced cytotoxicity in comparison with the corresponding free drugs. These confirm that the synthesized HMMNS-R/P is a potential candidate for simultaneous bioimaging and drug delivery. High-intensity focused ultrasound (HIFU) has been widely applied for the non-invasive surgery clinically due to their unique merits of high therapeutic efficiency and low side-effects to patients. Typically, the ultrasound waves generated by outer transducers can be focused on the in vivo malignant tissues due to the intrinsic tissue-penetrating nature of ultrasound. The introduced ultrasonic energy can induce the mechanical, thermal, and cavitation effects to destroy the abnormal cells and tissues. However, the relatively low therapeutic efficiency of HIFU on some deep tissues results in very long treatment durations of up to 10 h, which needs to be greatly enhanced to substantially shorten the treatment durations to less than one hour or even minutes. The clinically adopted method is to increase the therapeutic power to enhance the energy deposition. The increased ultrasound power, however, can damage the normal tissues in the acoustic propagation channels. To overcome these problems, monodispersed hollow mesoporous silica nanocapsules (HMSNs) were designed and used as the carriers to load perfluorohexane (PFH) therein to change the acoustic microenvironment of tissues and enhance the HIFU therapeutic efficacy accordingly.[139] PFH is a biocompatible compound with the phase transition temperature of ca. 56 °C, which can be gasified by the HIFU hyperthermal effect. The ex vivo results showed that MSNC-PFH could induce the substantially larger ablated tissue volume than either MSNC or phosphate buffered saline (PBS). The in vivo assessment further demonstrated the high synergistic effect of MSNC-PFH.[139a] Furthermore, multifunctional mesoporous composite nanocapsules for highly efficient MRI-guided HIFU cancer surgery were constructed by dispersing manganese oxide species within the mesopores for efficient T1-weighted MRI, together with the encapsulation of PFH molecules within large hollow cavities and their synchronous delivery to targets for active HIFU therapy (Figure 26). It was demonstrated that the precise location on the targeted tumor site and greatly enhanced synergistic therapeutic effect could be achieved under the assistance of such a system.[139b]




REVIEW Figure 27. a) Schematic illustration of radiosensitization by CDDP-loaded UCNP-HMSNs nanotheranostics. b) Tumor growth curves of Hela tumor xenografts following the treatments with different modes, control groups received PBS. c) Relative tumor volumes of mice in different treatment groups in half a month of post-injection. Reproduced with permission.[140] Copyright 2013, ACS Publications.

Figure 26. a) Technical principal of MRI-guided HIFU for the surgery of hepatic neoplasm in rabbits. b) In vivo coagulated necrotic-tumor volume by MRI-guided HIFU exposure under the irradiation power of 150 W cm−2 and duration of 5 s in rabbit liver tumors after different agents were applied through the ear vein (inset: digital pictures of tumor tissue after HIFU exposure). Reproduced with permission.[139b] Copyright 2011, Wiley.

core-shell structured UCNP@HMSNs are found to be capable of regulating the release of anticancer drug doxorubicin from the pore channels of mesoporous silica shells by making use of the photoisomerization effect of trans to cis isomers of azobenzene, which was modified in the pore surface of the silica shells, under the irradiation by the upconverted UV–vis light by the UCNP cores. During this process, the azobenzene moiety acts as a molecular stirrer driven by the UV–vis light and the UCNP cores upconvert the external NIR radiation to the UV–vis light. This is an interesting example of using the upconversion properties of UCNPs not only in the fluorescent bioimaging, but also in the drug release control by the in situ upconverted UV–vis light from NIR irradiation by the UCNP cores.[141]

4. Conclusions and Outlook Most recently, a type of multi-functionalized thernostics, Gd-doped upconversion nanoparticles (UCNPs)-functionalized HMSNs, has been constructured by successively coating dense and mesoporous silica layers on UCNPs followed by the etchingaway of the middle dense silica layer. The Gd-doped UCNPs are able to emit visible and UV light under the irradiation of extrnal NIR and act as T1-weighted contrast agent in magnetic resonance imaging as well due to the doping of Gd ions (Figure 27a). More importantly, such a composite was used to deliver a kind of anticancer drug CDDP to tumors for synergetic chemo-/radiotherapy by CDDP-radiosensitization and magnetic/luminescent dual-modal imaging. This CDDP-loaded UCNP-functionalized composites showed more effective in vitro radiasensitization effect that free CDDP as a radiosensitizer, and unambiguously enhanced radiotherapy efficacy in vivo via synergetic chemo-/ radiotherapy (Figure 27b,c), along with the simultaneous dualmodal imaging functions.[140] More interestingly, the similar


Over the past decade, hollow-structured mesoporous materials have been extensively developed toward the control over the hollow mesostructures, morphology and dimensions, multifunctionalization and the applications in the storage, adsorption and separation, confined catalysis and biomedical applications as drug carriers and theranostic agents. They are powerful tools to study the catalytic activity enhancement of confined catalysts, targeted delivery of drug molecules with largely enhanced therapeutic efficacy, simultaneous imaging and therapy on cancers, etc. This review summarizes the major progress of HMMs in term of the chemical synthesis and, more importantly, the applications in biomedical and catalysis fields. The synthesis strategies for hollow mesoporous materials have been broadly categorized into two groups: i) soft-templating and ii) hard-templating approaches. The synthetic process via soft-templating approach is relatively simple and the periodicity of the resultant materials can be well controlled. However, the precise control over the



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www.MaterialsViews.com Table 7. Corresponding roles of the hollow core, mesopore network and the mesoporous shell in HMMs when applied in catalysis, adsorption, drug delivery, and theranostics. Application

Mesopore network in the shell

The shell layer

Catalysis

Encapsulation of Catalytic nanoparticles

The hollow core

Pathways for the catalysts loading, reactant and product molecules’ diffusion in/out; Shape-selective catalysis

Separation, stablization and protection of catalytic species form the environment and self-aggregation

Adsorption/storage

Space for holding guest species with high capacity

Pathways for the storage of guests

Framework supporting the loaded guests

Drug delivery

Encapsulation of drug molecules with enhanced capacity

Pathways for the drug loading and sustained release from the core; Encapsulation of the other type of drugs

Protection of the loaded drugs from the environmental attack; Surface modification for enhanced biocompatibility, dispersity and/or molecular targeting, and controlled/sustained drug releases

Theranostics

Space for loading drugs and functional nanoparticles such as magnetite

Pathways for the drug loading and sustained release from the core; Grafting of functional species/nanoparticles for bioimaging

Protection of the loaded drugs; Surface modification for enhanced biocompatibility, dispersity and/or molecular targeting, and controlled drug release; Surface nanoparticle linkage for multi-mode therapy and imaging

reaction conditions is usually not easy owing to the fact that the structure features and/or chemical properties of soft templates are rather sensitive to the reaction environments. Moreover, hollow mesoporous materials synthesized via soft-templating route are usually non-uniform in size and morphology, and polydispersed and sometimes highly aggregated. Compared with soft-templating, hard-templating is more effective in synthesizing mesoporous particles with defined and highly tunable particle size, controllable morphology and good monodispersivity, though the synthetic process usually involves multi-steps, and the resultant HMMs frequently show less ordered pore arrangement in their shells as compared to those by soft-templating routes. The successes in synthesizing hollow mesoporous materials have provided great opportunities to tune their mechanical, chemical and other properties, which are beneficial to both fundamental researches and practical applications. These advances have in turn catalyzed the quick expansion of the application lists in catalysis, storage, separation and drug delivery. Different from the conventional mesoporous materials, HMMs are mainly featured with the presence of large hollow core in the particle interior and usually spherical morphology. Table 7 summarizes the roles of the hollow core, mesopore network in the shell and the shell layer when applied in the fields of catalysis, adsorption and biomedicine. One can see that it is the hollow cores that endow the materials with special and very useful characters distinguishing from common mesoporous materials. Especially, catalytically active nanoparticles encapsulated into the core or loaded in the mesoporous channel are prevented from self-aggregation during reactions, so that the catalytic acitivity is significantly enhanced. On the other hand, in the construction of nanotheranostics by multifunctionalizing the HMMs, both the core, mesopore channel and outer shell surface can be effectively employed for loading/grafting/linking functional nanoparticles and/or ligands for the multimode therapy, and single or even multimode bioimaging diagnosis, in addition to those for the single purpose of drug delivery. However, there is still great challenges in preparing highquality hollow mesoporous materials (e.g., uniformity, composition, monodispersity, controlled particle size, adjustable mesopore size and shell thickness) with facile and controllable approaches.

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Figure 28 outlined, but not limited to, some of basic requirements to HMMs for their practical cataytic and biomedial applications in future, and the possible tasks and/or mission researches are facing in promoting future fundamental researches and applications as well. For example, it is highly desirable to get facile control over the monodisperity and dimensions of HMMs with simple softtemplating approach, and also, monodispersed HMMs of under 100 nm, especially under 50 nm in particle diameter, meanwhile of larger than 5 nm in pore diameters, are still unavailable in literature reports, which is important for further enhanced intracelluar uptake, prolonged blood circulation and intranuclear drug/ RNA delivery. From the viewpoint of future clinic applications, the currently available strategies, or emerging new strategies for HMMs synthesis, need to be scaled up to produce HMMs with commercial-scale quantities. Challenges also remain in the biocompatibility enhancement of HMMs by finding more suitable biocompatible/biodegradable organic molecules for surface modification, in addition to the commonly adopted methodology of PEGylation, and/or more biocompatible/biodegradable framework composition such as organic-inorganic constitutes. Especially the long term in vivo toxicity information of HMMS in not only mice or rabbit, also large animals like dogs or pigs, is still unavailable which may cost much longer time period and higher expense to carry out than the short term toxicity measurements on small animals. The successful engineering of hollow mesoporous materials would be of particular interest for the development of multifunctional nanospheres as diagnostic and therapeutic agents on cancers, which is under quick development currently, and various types of combinations among different bioimaging modalities (optical, magnetic resonance, X-ray CT, etc) and therapeutic protocols (chemical, radiation, thermal, acoustic, etc.) have been very recently reported and more examples are expected. However, multifunctionalization also brings about difficulties in the careful control over the morphology, size and size distribution, dispersivity of composite particles during synthesis, and also importantly, the bio-cytotoxicity assessment, both in vitro and in vivo, will become much more complicated because of the multi-components involved and unknown interactions between the components and their biological effect in vivo.




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Catalytic

Biomedical

Defined hollow, meso-structures and tunable dimensions in cavity and mesopore sizes;

HMMs Basic requirements

Active guest loading in mespores for applications

Defined hollow, meso-structures and tunable dimensions in cavity and mesopore sizes; Monodispersity & suspensibility;

Framework crystallization;

Nontoxicity & Biocompatibility; Biodegradability & Excretions;

High stability and reusability.

Multifunctionality.

and cavities;

Framework-crystallized HMMs;

Possible tasks/

HMMs Non-siliceous crystalline framework;

mission to be

with

Homogeneous loading of active guests in mesopores and/or

done/accomplished

larger than 5 nm in pore

in future

diameters;

cavities; Synergistic

catalytic

Monodispersed HMMs of under 100 nm in particle diameter and

effects

between loaded guest molecules and frameworks

Novel strategies for easy scaleup; molecules for Suitable modification or frameworks for enhanced biodegradability; Easy multifunctionalization; Long

term

in

vivo

toxicity,

distribution and excretion. Figure 28. Current and future research status of HMMs.

In the respect of catalytic application of HMMS, both the high catalytic performance and superior stability are necessary. One solution to this requirement is to prepare HMMs with highly crystalline framework, which may endow the hollow mesoporous materials with catalytic activity on their framework and enhanced thermal and hydrothermal stability. In fact, HMMs themselves can be made catalytically active by the framework composition modulation (e.g., transient metal oxide instead of inert silica or carbon) and crystallization meanwhile keeping the mesopore network open, thus the guest catalytic component loading might become unnecessary. However, such intrinsically catalytically active HMMs of well-defined and tunable mesoporosity have been rarely reported. Furthermore, additional catalysts can be equally loaded into the HMMs of catalytically active framework in either hollow cores or in the mesoporous channels, some kinds of synergetic catalytic effects between the introduced catalytic species and original active sites on the HMMs framework may be generated. With the possible synergetic effect and the special hollow and penetrating mesoporous structure of HMMs, extraordinary catalytic performance, such as greatly enhanced catalytic activity, high reactant conversions and product selectivities, and favorable reusability, can be in expectation.


In all, many aspects of HMMs should be carefully designed and modulated to meet the requirements of varied applications. Morphologically, the shape, size and size distribution of HMMs, cavity size and mesoporous size are very important in determining their applicability in biomedicines, and also the diffusion of molecules in catlaysis; Compositionally, the inert and biodegradable compositions such as silica, silica based organic/inorgainc hybrides or polymeric frameworks are the prerequisites for biomedical applications, in contrast catalytically active and highly thermal/hydrothermal stability of the HMMs are greatly favored for the applications in catalysis, adsorption/separation, and/or chemical sensing. Detailed and special attentions should be paid to the unique compositional/ morphological/structural requirements for HMMs aiming at every specific application.

Acknowledgements The authors gratefully acknowledge financial support by the 973 Program (Grant Nos. 2012CB933602 and 2013CB933200), the NSFC (Grant Nos. 51172070, 51132009, 51202068); the Program for New Century Excellent



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www.MaterialsViews.com Talents in University (NCET-10–0379); the Shu Guang project (Grant No. 11SG30); and the Fundamental Research Funds for the Central Universities (Grant Nos. WD1114002 and WD1124010). Received: October 27, 2013 Revised: December 18, 2013 Published online:

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Proline functionalization of the mesoporous metal-organic framework DUT-32.

Synthesis of mixed-sequence oligonucleotides on mesoporous silicon: chemical strategies and material stability.

Synthesis of mesoporous silica materials from municipal solid waste incinerator bottom ash.

The development of chiral nematic mesoporous materials.

Synthesis of mesoporous platinum-copper films by electrochemical micelle assembly and their electrochemical applications.

Pore Narrowing of Mesoporous Silica Materials.

Immobilization of Methyltrioxorhenium on Mesoporous Aluminosilicate Materials.

Mesoporous materials for clean energy technologies.

Heat-Initiated Chemical Functionalization of Graphene.

Synthesis and regioselective functionalization of perhalogenated BODIPYs.

Polydopamine and its derivative materials: synthesis and promising applications in energy, environmental, and biomedical fields.

1,3,4-Thiadiazole: synthesis, reactions, and applications in medicinal, agricultural, and materials chemistry.

Synthesis, toxicity, biocompatibility, and biomedical applications of graphene and graphene-related materials.

Preparation and functionalization of graphene nanocomposites for biomedical applications.