DOI: 10.1002/chem.201303659
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& Janus Nanoparticles
Unconventional Assembly of Bimetallic Au–Ni Janus Nanoparticles on Chemically Modified Silica Spheres Lei Jia,[a, b] Xiaowei Pei,*[a] Feng Zhou,*[a] and Weimin Liu[a]
Abstract: This paper reports that Janus Au Ni nanoparticles (JANNPs) can self-assemble onto silica spheres in a novel way, which is different from that of single-component isotropic nanoparticles. JANNPs modified with octadecylamine (ODA) assemble onto catechol-modified silica spheres (SiO2 OH) to form a very special core–loop complex structure and finally the core–loop assemblies link each other to form large assemblies through capillary force and the hydrophobic interaction of the alkyl chains of ODA. The nanocomposites disassemble in the presence of vanillin and oleic acid because of the breakage of the catechol–metal link. Vanillin-
Introduction Colloidal nanoparticles (NPs) are attractive building blocks for creating complex materials that have potential applications in chemistry, applied optics, biology, sensing, and catalysis.[1] The key to this feature of colloidal NPs is their controllable self-assembly into nanostructured aggregates. The assembly may be driven by different driving forces such as electrostatic,[2, 3] covalent,[4] van der Waals,[5, 6] and dipole interactions.[7] Generally, the self-assembly process is well directed when a template is used. For instance, Kim et al.[8] showed that silica spheres containing iron oxide (Fe3O4) NPs, gold (Au) NPs, or quantum dots (QDs; for example, CdSe/ZnS) have combined properties of magnetism and surface plasmon resonance or luminescence. Other templates such as polymer single crystal, titanium dioxide NPs, glass substrates, and DNA have also been used as templates to construct complex hierarchical NP structures.[9–12] However, previous studies focused on the assembly of singlecomponent NPs on template materials, and isotropy makes directional assembly difficult. The emergence of anisotropic particles comprising two surface chemistry regions,[13–17] recently
[a] Dr. L. Jia, Dr. X. Pei, Prof. F. Zhou, Prof. W. Liu State Key Laboratory of Solid Lubrication Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences Lanzhou 730000 (China) E-mail: [email protected] [email protected] [b] Dr. L. Jia Department of Physics and Chemistry Henan Polytechnic University, Jiaozuo 454000 (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303659. Chem. Eur. J. 2014, 20, 2065 – 2070
induced disassembly enables the JANNPs to reassemble into a core–loop structure upon ODA addition. The assembly of SiO2 OH and isotropic Ni or Fe3O4 particles generates traditional core–satellite structures. This unconventional self-assembly can be attributed to the synergistic effect of Janus specificity and capillary force, which is also confirmed by the assembly of thiol-terminated silica spheres (SH SiO2) with anisotropic JANNPs, isotropic Au, and Ni nanoparticles. These results can guide the development of novel composite materials using Janus nanoparticles as the primary building blocks.
called Janus NPs (JNPs),[18, 19] offers great opportunity for directional assembly.[20, 21] The anisotropic properties of JNPs can be a potential driving force for one-dimensional assembly into sophisticated assemblies.[22, 23] Meanwhile, the manipulation of the assembly/disassembly process is very important for fabricating intelligent systems but is still challenging. Despite the significant progress that has been made in the organization of nanomaterials, the disassembly of assemblies is often difficult and has not been extensively investigated. Usually, the manipulation of assemblies is not possible once they are constructed in solution for all above mentioned examples, and their further manipulation is even more impossible. The isotropic assembly/disassembly process can be induced by DNA strand pairing,[24, 25] pH,[26] salt,[27] block copolymers,[28] and UV irradiation.[29] However, reports on anisotropic assembly/disassembly are limited. Herein, we report that anisotropic Au Ni JNPs can self-assemble onto silica spheres in a novel way, which is different from the assembly of the isotropic single-component NPs and silica spheres with the same surface chemistry. The novel selfassembly results in very special morphology and composites that can be prepared. The special assembly is supposed to be due to the asymmetric structure and the capillary force created during drying of the chloroform dispersion. As shown in Scheme 1, catechol-terminated colloidal organosilica (OH SiO2) NPs were first synthesized as adsorbent hosts for JANNPs to be chemically immobilized through the catechol–metal link. Surprisingly, core–loop OH SiO2@Au Ni and core–satellite OH SiO2@Ni or OH SiO2@Fe3O4 assemblies were formed. To further confirm this special phenomenon, thiol-terminated colloidal organosilica (SH SiO2) NPs were then synthesized, and the as-
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Scheme 1. Formation of core–loop and core–satellite assemblies of Janus Au Ni nanoparticles and pure Ni or Fe3O4 nanoparticles on functionalized silica spheres modified with a catechol group. Janus Au Ni NP, Ni, and Fe3O4 NP surfaces are coated with octadecylamine or oleylamine.
sembly of SH SiO2@Au Ni, SH SiO2@Au or SH SiO2@Ni were also investigated.
Results and Discussion Silica nanospheres of approximately 128 nm in diameter were synthesized by the Stçber synthesis method[51] (see Experimental Section), and TEM was used to characterize the size of the silica colloids. Based on the size measurements obtained with Image Tool, the size distribution histogram was plotted and the average size of the silica colloids was calculated. Figure 1 a shows a representative TEM image, wherein the size distribution histogram is plotted as percentage silica colloids versus silica colloid size. The average size of the silica colloids is 128 8 nm, and the silica colloids are relatively monodisperse. The silica colloids were functionalized with two different types of
Figure 1. TEM images of a) as-synthesized silica and b) Janus Au Ni nanoparticles. c–f) Core–loop assemblies of OH SiO2@Au Ni clusters with increased concentration of Janus Au Ni nanoparticles. The inset images are the corresponding STEM images. Chem. Eur. J. 2014, 20, 2065 – 2070
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linker: APTES (3-aminopropyl triethoxysilane) and MPTMS (3mercaptopropyl trimethoxysilane). The former was further modified with 3,4-dihydroxybenzaldehyde to generate a catechol layer on the silica surface.[30] The abundant dihydroxyl and mercaptopropyl groups made the silica spheres dispersible in nonpolar solvents such as chloroform or toluene, thereby facilitating the approach of silica spheres and hydrophobic nanocrystals during the assembly process. Monodispersed JANNPs were synthesized according to a previous report using ODA as a stabilizer,[47] as shown in Figure 1 b. The JANNPs were covalently attached onto the functionalized silica colloids through the Ni O bond in the case of silica colloids functionalized with catechol groups and through the metal S bond in the case of silica colloids functionalized with MPTMS. Following simple blending at ambient temperature with a chloroform solution of JANNPs and OH SiO2, the catechol groups of silica spheres may have partially replaced the original capping ligands on the JANNPs surface to reach the metal sites on the particular surface. The morphology of the assemblies depended on the relative concentration of JANNPs and OH SiO2. The concentration of OH SiO2 was fixed at 1 mg mL 1 for all experiments. At a low JANNPs concentration (1 mg mL 1), all particles formed single-loop nanocomposites (Figure 1 c) and no free particles existed in solution. Apparently, the JANNPs assembled in one direction around the silica spheres. By taking advantage of catecholic chemistry, most JANNPs adhered onto silica surface through the Ni domains (STEM images in Figure 1 c) probably because of their relative stronger interaction with the catechol group than the Au domains.[31] This finding agreed well with our recent result.[22] With increased JANNP concentration (1.5, 3, and 8 mg mL 1, respectively; Figure 1 d–f), the morphology of the assemblies transformed from a single loop into multiple loops. Eventually, all the core–loop assemblies connected each other in a large area, as shown in Figure 1 f. Even with the excessive amount, JANNPs still grow around the edge of the first Au Ni loop, while left the most of the silica surface unloaded. On the basis of these images, we assume that the Janus specificity may play an important role in the forming process of the core–loop structure. While after the first chemically attached loop was formed, other loops subsequently formed probably through the hydrophobic interaction of the alkyl chains of ODA, which may be driven by the capillary force created during drying of the chloroform dispersion,[32] as the diameter of the JANNPs and core–loop nanocomposites are obviously less than the deepness of chloroform which were casted on the carboncoated TEM grid. After blending the core–loop OH SiO2@Au Ni clusters with excess JANNPs, firstly, chaotic motion of the core–loop assemblies and excess JANNPs in a thick layer appeared; secondly, the capillary forces appeared and give rise to aggregation after the tops of particles or clusters protrude from the liquid layer. Notably, the approximately particle-toparticle distance between JANNPs in Figure 1 b was 2.3 nm, and this value remained the same throughout the entire assembly process (Figure 1 d–f). This interesting assembly scenario has never been found with symmetric NPs,[33–37] and was further investigated with more experiments.
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Figure 2. TEM images of the assemblies after blending the OH SiO2 and JANNPs for a) 1 h, b) 3 h, c) 6 h, and d) 18 h.
To further illustrate the important effect of Janus specificity in the self-assembly of this unconventional assembly, the forming process of the OH SiO2@Au Ni assemblies (Figure 1 d) was observed through the TEM images in Figure 2. After mixing for 1 and 3 h, only a few JANNPs stuck onto the surface of silica spheres, and most of them remained in solution (Figure 2 a and b), which illustrated that the exchange process of hydrophobic ODA chains on the silica surfaces by the catechol groups might take a relatively long time. After blending for 6 h, the JANNPs became close to the silica spheres (Figure 2 c), and the core–loop OH SiO2@Au Ni assemblies finally formed (Figure 2 d). It is incredible that JANNPs tended to assemble around the fringe of the silica spheres while left the most of the other surface blank. This finding confirmed that the Janus specificity played an important role in the above assembly. And this control experiment can also explain that how core– loop structure formed when blending silica spheres with a small amount of JANNPs. Actually, the formation of the first loop is a slow process, which is due to the hydrophobic or hydrophilic nature of the Janus Au Ni nanoparticles and the silica surfaces. When the original JANNPs attached to the silica spheres, the nature of the hydrophilic silica surface seems to be changed to hydrophobic surface. And the redundant JANNPs tended to link with each other along the first loop through the hydrophobic interaction of the alkyl chains of ODA, rather than to be assembled on the other region of the silica spheres, the schematic of the detailed formation of core– loop structure is shown in Scheme 2. To gain insight into the underlying special mechanisms, the isotropic Au, Ni, or Fe3O4 NPs modified by ODA or oleylamine were synthesized by standard high-temperature thermolysis reaction, as shown in Figure 3 a–c. These NPs were induced to be assembled onto the surface of OH SiO2 spheres by simply mixing them in nonpolar solvents such as chloroform. Generally, for the assembly of core–satellites OH SiO2@Ni and OH SiO2@Fe3O4 nanocomposites, excessive NPs were used to ensure dense coverage on the silica spheres, and the NPscapped silica spheres were collected after washing several times with nonpolar solvent to remove free NPs. Figure 3 d Chem. Eur. J. 2014, 20, 2065 – 2070
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Scheme 2. Detailed formation of core–loop structures.
Figure 3. TEM images of a) Au, b) Ni, and c) Fe3O4 nanoparticles, as well as the assemblies of d) OH SiO2@Au, e) OH SiO2@Ni, and f) OH SiO2@Fe3O4.
shows the TEM image of Au NPs (1 mg mL 1) assembled onto the silica colloids, which was not effective and in accordance with a published modification method.[38] Meanwhile, Figure 3 e and f show a layer of Ni or Fe3O4 (Fe3O4 was tested for analogy with Ni NPs because of its highly similar interaction strength to Ni oxides) NPs can be clearly observed on the originally smooth surface of OH SiO2 spheres. And there were no free NPs around the silica NPs, which suggested the strong attraction between the NPs and silica spheres. During the self-assembly process, the catechol groups may have partially replaced the original surface ligands (ODA or oleylamine) and co-
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Figure 4. Top: reversible and irreversible disassembly process of OH SiO2@Au Ni nanocomposites induced by various ligands. Bottom: TEM images of a) disassembly of OH SiO2@Au Ni clusters in the presence of vanillin, b) disassembly of OH SiO2@Au Ni in the presence of oleic acid, and c) reassembly of OH SiO2@Au Ni clusters in the presence of octadecylamine. d) IR spectra of the assemblies in Figure 2 b. e) UV/Vis spectra and digital photos of the above assemblies and disassemblies; from top to bottom and from left to right in inset: Janus Au Ni nanoparticles (JANNPs); OH SiO2@Au-Ni nanocomposite; OH SiO2@Au-Ni nanocomposite and vanillin; OH SiO2@Au-Ni nanocomposite, vanillin, and ODA.
ordinated to the metal sites on the NP surface. Even the excessive amount of these isotropic NPs cannot form core–loop assemblies. These results indicated that: 1) the anisotropic geometry of JANNPs played a significant role in the above unconventional assembly of the OH SiO2@Au Ni clusters, which is in accordance with the above deduction, 2) the catechol groups had stronger interaction with the Ni or Fe3O4 domains than with the Au domains. Any reagent that can break the ODA–metal or catecholic– metal links would make the above nanocomposites disassemble, as illustrated in Figure 4 (top). ODA can react with vanillin (0.1 mmol) to form a Schiff base, and the depletion of surfactants led to the aggregation of the JANNPs (Figure 4 a). While after adding a small amount of OA (0.185 mmol), all JANNPs dissociated from the silica spheres and redispersed in CHCl3 solution (Figure 4 b). This phenomenon was due to the ligand exchange between the catechol groups and the OA surfactants, which broke the JANNPs away from the silica surface (Figure 4 d), as confirmed by the IR spectrum of the JANNPs. The peak of the carboxyl group at around 1654 cm 1 and 1323 cm 1 proved the presence of OA on JANNPs. The TEM image of the composites collected using a magnet also did not show any silica sphere, indicating that the JANNPs were successfully desquamated from the silica surface by OA. However, the reassembly of the JANNPs onto silica spheres by adding ODA was difficult because of the different chemical affinities and the irreversibility of the process. Furthermore, for the disassemblies caused by vanillin in Figure 4 a, reassembly Chem. Eur. J. 2014, 20, 2065 – 2070
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occurred by rapidly adding ODA surfactant (0.2 mmol) to the assembly system. Figure 4 c shows that the JANNPs tended to be assembled onto the silica surface again to form a core–loop structure. Figure 4 e shows the UV/Vis spectra of the disassembly and reassembly system of OH SiO2 and JANNP solution. The lower wavelength peak at around 536 nm can be attributed to the pure JANNPs.[39] Successful immobilization of JANNPs onto the silica surface did not lead to significant changes in the absorption curve. However, after disassembly by vanillin, the plasmon band shifted to 568 nm with an obvious shoulder because of interparticle association (aggregation).[40–43] After reassembly by adding ODA, the plasmon band reverted to 536 nm, similar to the original OH SiO2@Au Ni aggregates. The unconventional assembly of the above JANNPs on chemically modified silica spheres was further understood by studying the assembly of JANNPs onto thiolated silica prepared by modifying silica NPs with MPTMS and named as SH SiO2. After self-assembly, the ODA-stabilized JANNPs rapidly anchored onto the silica surface in a single loop at a low concentration, which resulted in a core–loop structure (Figure 5 a). The JANNPs assembled onto SH SiO2 NPs either through the Au side or Ni side, and thus the silica spheres tended to share the sandwiched JANNPs,[44] which slightly differed from the assemblies in Figure 1 c. With the increase of JANNPs concentration, the excessive amount of JANNPs assembled around the original JANNP loop which is also due to the capillary force, as shown in Figure 5 b and c. The Janus specificity and the hydrophobic interaction of the alkyl chains of ODA also contributed to the assembly of SH SiO2@Au Ni composites, which is consistent with the previous speculation.[22] For the same reason, any reagent that can break the metal–thiol bond would change the morphology of the as-prepared assemblies. Interestingly, in the presence of OA molecules (0.05 mmol), the JANNPs and SH SiO2 formed core–satellite structures, as depicted in Figure 5 d. The morphology transformation can be due to the combination affinity which was based on the acid/ base properties or Pearson hardness of the incoming NPs.[45]
Figure 5. a–c) Typical TEM images of two-dimensional core–loop assemblies of SH SiO2@Au Ni clusters with increased concentration of Janus Au Ni nanoparticles. d) Core–satellite SH SiO2@Au Ni clusters in the presence of oleic acid. e–f) Core–satellite SH SiO2@Ni and SH-SiO2@Au symmetric clusters. The inset images are the corresponding STEM images.
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Full Paper The Ni domains of the JANNPs tended to link with the OA molecules that dissociated the core–loop assemblies, whereas the thiol layer of silica spheres tended to bind to the Au domain, which is also accordance with the recent report.[46] As expected, the isotropic Ni and Au NPs randomly assembled onto SH SiO2 on the whole surface and formed core–satellite composites (Figure 5 e and f). This result indicated that the surfactant also played an important role in this unconventional self-assembly. To date, many technologically important NPs (such as Fe3O4, Au Ni, Au Fe3O4, Au ZnO, and so on) have been mainly obtained using organic solvents stabilized by hydrophobic surfactants and insoluble in water. Recently, Yin reported a general process for producing multifunctional composite particles by direct self-assembly of hydrophobic NPs on host silica colloids containing inherent surface thiol groups through ligand exchange. While the direct assembly of isotropic Janus NPs onto substrate (such as SiO2, TiO2, etc.) is still rare and the asymmetric structure will lead to the appearance of novel assembled structures.
Conclusion The self-assembly of JANNPs and asymmetric Au, Ni, and Fe3O4 onto chemically modified silica spheres was investigated. JANNPs assembled onto catecholic silica and thiolated silica spheres to form core–loop assemblies and eventually the assemblies were connected in a large area through the capillary force, whereas the homogeneous NPs assembled to a core– satellite structure. The unconventional self-assembly can be attributed to the synergistic effect of Janus specificity and the capillary force created during the drying process and the hydrophobic interaction of the alkyl chains of ODA finally led to the uncommon core–loop assemblies. The results can guide the development of novel composite materials using Janus NPs as the primary building blocks.
Experimental Section Materials and measurement 3-Hydroxytyramine hydrochloride, octadecylamine (ODA), 3-aminopropyl triethoxysilane (APTES), tetraethoxy-silicone, and 3-mercaptopropyl trimethoxysilane (MPTMS) were purchased from Aldrich. p-Toluenesulfonic acid, oleic acid (OA), vanillin, HAuCl4·4H2O, Fe(acac)3, and Ni(NO3)2·6H2O were obtain from the Chemical Reagent Co. of Shanghai (Shanghai, China). Chloroform and other solvents were analytical grade from Beijing Chemical Factory (China) and were used without further purification. Ultrapure water used in all experiments was obtained from a NANO Pure Infinity System (Barnstead/Thermolyne Corp.). Transmission electron microscopy (TEM) was carried out using a JEOL 2100FX and Tecnai-G2-F30 at acceleration voltages of 200 and 300 kV. One drop of suspension was drop casted onto a carbon-coated copper TEM grid. Upon solvent evaporation, the sample was used for TEM observation without further treatment. UV/Vis absorption spectra were recorded using a Varian Cary 100 spectrophotometer. Infrared spectra (KBr pellet) were recorded using an FTS165 Bio-Rad FTIR spectrophotometer within the range of 4000–400 cm 1. Chem. Eur. J. 2014, 20, 2065 – 2070
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Synthesis of Janus Au Ni NPs, asymmetric Au, Ni, and Fe3O4 NPs JANNPs were synthesized according to literature.[47] In a typical procedure, 0.2 g of HAuCl4 and 1.05 g of Ni(NO3)2·6H2O were added to 30 mL of ODA at 120 8C, and the system was heated to 200 8C. After 1 h of magnetic stirring, the products were collected and washed several times with ethanol. In a typical synthesis of Au nanocrystals,[48] 10 mL of ODA was added to a 50 mL beaker in air. The resulting solution was heated to 160 8C, and then 0.05 g of HAuCl4 was added and magnetic stirring was performed for 10 min. After reaction, particles were collected at the bottom of the beaker, washed several times with ethanol, and dispersed in a nonpolar solvent such as cyclohexane. Ni NPs and Fe3O4 NPs were synthesized by the established method.[49, 50]
Surface modification of catechol-terminated OH SiO2 NPs Silica colloids were synthesized by the Stçber synthesis method.[51] About 200 mL of APTES was added to 30 mL of toluene solution containing 100 mg of as-synthesized SiO2 NPs, and the mixture were refluxed for 8 h. The resulting NPs were collected by centrifugation and washed several times with ethanol. The dried SiO2 NH2 NPs were dispersed in 20 mL of ethanol and degassed with dry argon. 3,4-Dihydroxybenzaldehyde (0.1 g) and catalytic p-toluenesulfonic acid were then added to the degassed solution, and the mixture was refluxed overnight. The resulting catechol-terminated SiO2 NPs were centrifuged, washed several times with ethanol, and dried overnight in a vacuum at 50 8C.
Surface modification of thiol-functionalized SH SiO2 NPs 200 mL of MPTMS was added to 30 mL of toluene solution containing 100 mg of SiO2 NPs, and the mixture was refluxed for 8 h. The resulting thiol-terminated SiO2 NPs were centrifuged, washed several times with ethanol, and dried overnight in vacuum at 50 8C.
Fabrication of various nanocomposites Core–loop OH SiO2@Au Ni nanocomposites were prepared as follows. The as-functionalized silica nanospheres were suspended in 1 mL of chloroform at a concentration of 1 mg mL 1 and purged with argon for 5 min. Then, 1 mL of degassed chloroform solution containing JANNPs was added (1, 1.5, 3, and 8 mg mL 1; Figure 1 c– f, respectively), and the solution was reacted for 18 h in a shaker. After reaction completion, the resulting samples were collected using a magnet without washing and investigated by TEM. Core–satellites OH SiO2@Ni, OH SiO2@Fe3O4, and OH SiO2@Au nanocomposites were prepared by a procedure similar to the above immobilization of JANNPs onto OH SiO2, but 1 mL of Ni, Fe3O4, or Au NPs (1 mg mL 1) was used instead of JANNPs. Core–loop SH SiO2@Au Ni nanocomposites were prepared as follows. The as-functionalized silica nanospheres were suspended in 1 mL of chloroform at a concentration of 1 mg mL 1 and purged with argon for 5 min. Then, 1 mL of degassed chloroform containing JANNPs was added (1, 2, and 8 mg mL 1 for Figure 5 a–c, respectively), and the solution was reacted for 6 h in a shaker. After reaction completion, the resulting samples were collected using a magnet without washing and investigated by TEM. Core–satellites SH SiO2@Au and SH SiO2@Ni nanocomposites were prepared by a procedure similar to the above immobilization of JANNPs onto SH SiO2, but 1 mL of Au or Ni NPs (1 mg mL 1) was used instead of JANNPs.
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Full Paper Core–satellites SH SiO2@Au Ni nanocomposites were prepared as follows. The as-functionalized SH SiO2 NPs (1 mg) were suspended in 1 mL of chloroform at a concentration of 1 mg mL 1 and purged with argon for 5 min. Then, 1 mL of degassed chloroform containing 1 mg of JANNPs and 50 mL of OA (1 mol/L) was added, and the solution was reacted for 6 h in a shaker.
Disassembly and reassembly of OH SiO2@Au Ni nanocomposites The as-fabricated nanocomposites (2 mg) in Figure 1 e were suspended in 4 mL of chloroform. Then, 100 mL of vanillin (1 mol/L) or 185 mL of OA (1 mol/L) was added, and the solution was shaken for 6 h. The resulting samples were collected by centrifugation and finally dispersed in chloroform for further characterization. For the reassembly of the disassemblies formed in Figure 4 a, 200 mL of ODA (1 mol/L) was rapidly added to the chloroform solution of fresh prepared aggregates, and the mixture was shaken for 6 h. All reactions were carried out under a positive pressure of argon.
Catalytic reaction Reduction of 4-nitrophenol: typically, in a standard quartz cuvette, 4-nitrophenol (0.025 mL, 0.01 m), NaBH4 (0.1 mL, 0.1 m), and deionized water (2.2 mL) were added. After addition of 1.5 mg OH SiO2@Au Ni or SH SiO2@Au Ni catalysts, the bright yellow solution gradually fades as the reaction progresses. UV/Vis spectra were monitored in the sequence of time. After the catalytic reaction, the catalyst was collected by a magnet and washed with ethanol. Then, the recovered catalyst was reused to initiate another cycle of reaction. The same procedures were conducted for at least 10 cycles.
Acknowledgements The authors gratefully acknowledge financial support from NSFC(21125316), 973 project (2013CB632300) and CAS (KJZDEW-M01). Keywords: nanoparticles · reassembly · self-assembly · silica nanospheres · synergistic effects [1] [2] [3] [4] [5] [6] [7] [8]
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Received: September 17, 2013 Published online on January 15, 2014
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& Janus Nanoparticles
Unconventional Assembly of Bimetallic Au–Ni Janus Nanoparticles on Chemically Modified Silica Spheres Lei Jia,[a, b] Xiaowei Pei,*[a] Feng Zhou,*[a] and Weimin Liu[a]
Abstract: This paper reports that Janus Au Ni nanoparticles (JANNPs) can self-assemble onto silica spheres in a novel way, which is different from that of single-component isotropic nanoparticles. JANNPs modified with octadecylamine (ODA) assemble onto catechol-modified silica spheres (SiO2 OH) to form a very special core–loop complex structure and finally the core–loop assemblies link each other to form large assemblies through capillary force and the hydrophobic interaction of the alkyl chains of ODA. The nanocomposites disassemble in the presence of vanillin and oleic acid because of the breakage of the catechol–metal link. Vanillin-
Introduction Colloidal nanoparticles (NPs) are attractive building blocks for creating complex materials that have potential applications in chemistry, applied optics, biology, sensing, and catalysis.[1] The key to this feature of colloidal NPs is their controllable self-assembly into nanostructured aggregates. The assembly may be driven by different driving forces such as electrostatic,[2, 3] covalent,[4] van der Waals,[5, 6] and dipole interactions.[7] Generally, the self-assembly process is well directed when a template is used. For instance, Kim et al.[8] showed that silica spheres containing iron oxide (Fe3O4) NPs, gold (Au) NPs, or quantum dots (QDs; for example, CdSe/ZnS) have combined properties of magnetism and surface plasmon resonance or luminescence. Other templates such as polymer single crystal, titanium dioxide NPs, glass substrates, and DNA have also been used as templates to construct complex hierarchical NP structures.[9–12] However, previous studies focused on the assembly of singlecomponent NPs on template materials, and isotropy makes directional assembly difficult. The emergence of anisotropic particles comprising two surface chemistry regions,[13–17] recently
[a] Dr. L. Jia, Dr. X. Pei, Prof. F. Zhou, Prof. W. Liu State Key Laboratory of Solid Lubrication Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences Lanzhou 730000 (China) E-mail: [email protected] [email protected] [b] Dr. L. Jia Department of Physics and Chemistry Henan Polytechnic University, Jiaozuo 454000 (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303659. Chem. Eur. J. 2014, 20, 2065 – 2070
induced disassembly enables the JANNPs to reassemble into a core–loop structure upon ODA addition. The assembly of SiO2 OH and isotropic Ni or Fe3O4 particles generates traditional core–satellite structures. This unconventional self-assembly can be attributed to the synergistic effect of Janus specificity and capillary force, which is also confirmed by the assembly of thiol-terminated silica spheres (SH SiO2) with anisotropic JANNPs, isotropic Au, and Ni nanoparticles. These results can guide the development of novel composite materials using Janus nanoparticles as the primary building blocks.
called Janus NPs (JNPs),[18, 19] offers great opportunity for directional assembly.[20, 21] The anisotropic properties of JNPs can be a potential driving force for one-dimensional assembly into sophisticated assemblies.[22, 23] Meanwhile, the manipulation of the assembly/disassembly process is very important for fabricating intelligent systems but is still challenging. Despite the significant progress that has been made in the organization of nanomaterials, the disassembly of assemblies is often difficult and has not been extensively investigated. Usually, the manipulation of assemblies is not possible once they are constructed in solution for all above mentioned examples, and their further manipulation is even more impossible. The isotropic assembly/disassembly process can be induced by DNA strand pairing,[24, 25] pH,[26] salt,[27] block copolymers,[28] and UV irradiation.[29] However, reports on anisotropic assembly/disassembly are limited. Herein, we report that anisotropic Au Ni JNPs can self-assemble onto silica spheres in a novel way, which is different from the assembly of the isotropic single-component NPs and silica spheres with the same surface chemistry. The novel selfassembly results in very special morphology and composites that can be prepared. The special assembly is supposed to be due to the asymmetric structure and the capillary force created during drying of the chloroform dispersion. As shown in Scheme 1, catechol-terminated colloidal organosilica (OH SiO2) NPs were first synthesized as adsorbent hosts for JANNPs to be chemically immobilized through the catechol–metal link. Surprisingly, core–loop OH SiO2@Au Ni and core–satellite OH SiO2@Ni or OH SiO2@Fe3O4 assemblies were formed. To further confirm this special phenomenon, thiol-terminated colloidal organosilica (SH SiO2) NPs were then synthesized, and the as-
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Scheme 1. Formation of core–loop and core–satellite assemblies of Janus Au Ni nanoparticles and pure Ni or Fe3O4 nanoparticles on functionalized silica spheres modified with a catechol group. Janus Au Ni NP, Ni, and Fe3O4 NP surfaces are coated with octadecylamine or oleylamine.
sembly of SH SiO2@Au Ni, SH SiO2@Au or SH SiO2@Ni were also investigated.
Results and Discussion Silica nanospheres of approximately 128 nm in diameter were synthesized by the Stçber synthesis method[51] (see Experimental Section), and TEM was used to characterize the size of the silica colloids. Based on the size measurements obtained with Image Tool, the size distribution histogram was plotted and the average size of the silica colloids was calculated. Figure 1 a shows a representative TEM image, wherein the size distribution histogram is plotted as percentage silica colloids versus silica colloid size. The average size of the silica colloids is 128 8 nm, and the silica colloids are relatively monodisperse. The silica colloids were functionalized with two different types of
Figure 1. TEM images of a) as-synthesized silica and b) Janus Au Ni nanoparticles. c–f) Core–loop assemblies of OH SiO2@Au Ni clusters with increased concentration of Janus Au Ni nanoparticles. The inset images are the corresponding STEM images. Chem. Eur. J. 2014, 20, 2065 – 2070
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linker: APTES (3-aminopropyl triethoxysilane) and MPTMS (3mercaptopropyl trimethoxysilane). The former was further modified with 3,4-dihydroxybenzaldehyde to generate a catechol layer on the silica surface.[30] The abundant dihydroxyl and mercaptopropyl groups made the silica spheres dispersible in nonpolar solvents such as chloroform or toluene, thereby facilitating the approach of silica spheres and hydrophobic nanocrystals during the assembly process. Monodispersed JANNPs were synthesized according to a previous report using ODA as a stabilizer,[47] as shown in Figure 1 b. The JANNPs were covalently attached onto the functionalized silica colloids through the Ni O bond in the case of silica colloids functionalized with catechol groups and through the metal S bond in the case of silica colloids functionalized with MPTMS. Following simple blending at ambient temperature with a chloroform solution of JANNPs and OH SiO2, the catechol groups of silica spheres may have partially replaced the original capping ligands on the JANNPs surface to reach the metal sites on the particular surface. The morphology of the assemblies depended on the relative concentration of JANNPs and OH SiO2. The concentration of OH SiO2 was fixed at 1 mg mL 1 for all experiments. At a low JANNPs concentration (1 mg mL 1), all particles formed single-loop nanocomposites (Figure 1 c) and no free particles existed in solution. Apparently, the JANNPs assembled in one direction around the silica spheres. By taking advantage of catecholic chemistry, most JANNPs adhered onto silica surface through the Ni domains (STEM images in Figure 1 c) probably because of their relative stronger interaction with the catechol group than the Au domains.[31] This finding agreed well with our recent result.[22] With increased JANNP concentration (1.5, 3, and 8 mg mL 1, respectively; Figure 1 d–f), the morphology of the assemblies transformed from a single loop into multiple loops. Eventually, all the core–loop assemblies connected each other in a large area, as shown in Figure 1 f. Even with the excessive amount, JANNPs still grow around the edge of the first Au Ni loop, while left the most of the silica surface unloaded. On the basis of these images, we assume that the Janus specificity may play an important role in the forming process of the core–loop structure. While after the first chemically attached loop was formed, other loops subsequently formed probably through the hydrophobic interaction of the alkyl chains of ODA, which may be driven by the capillary force created during drying of the chloroform dispersion,[32] as the diameter of the JANNPs and core–loop nanocomposites are obviously less than the deepness of chloroform which were casted on the carboncoated TEM grid. After blending the core–loop OH SiO2@Au Ni clusters with excess JANNPs, firstly, chaotic motion of the core–loop assemblies and excess JANNPs in a thick layer appeared; secondly, the capillary forces appeared and give rise to aggregation after the tops of particles or clusters protrude from the liquid layer. Notably, the approximately particle-toparticle distance between JANNPs in Figure 1 b was 2.3 nm, and this value remained the same throughout the entire assembly process (Figure 1 d–f). This interesting assembly scenario has never been found with symmetric NPs,[33–37] and was further investigated with more experiments.
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Figure 2. TEM images of the assemblies after blending the OH SiO2 and JANNPs for a) 1 h, b) 3 h, c) 6 h, and d) 18 h.
To further illustrate the important effect of Janus specificity in the self-assembly of this unconventional assembly, the forming process of the OH SiO2@Au Ni assemblies (Figure 1 d) was observed through the TEM images in Figure 2. After mixing for 1 and 3 h, only a few JANNPs stuck onto the surface of silica spheres, and most of them remained in solution (Figure 2 a and b), which illustrated that the exchange process of hydrophobic ODA chains on the silica surfaces by the catechol groups might take a relatively long time. After blending for 6 h, the JANNPs became close to the silica spheres (Figure 2 c), and the core–loop OH SiO2@Au Ni assemblies finally formed (Figure 2 d). It is incredible that JANNPs tended to assemble around the fringe of the silica spheres while left the most of the other surface blank. This finding confirmed that the Janus specificity played an important role in the above assembly. And this control experiment can also explain that how core– loop structure formed when blending silica spheres with a small amount of JANNPs. Actually, the formation of the first loop is a slow process, which is due to the hydrophobic or hydrophilic nature of the Janus Au Ni nanoparticles and the silica surfaces. When the original JANNPs attached to the silica spheres, the nature of the hydrophilic silica surface seems to be changed to hydrophobic surface. And the redundant JANNPs tended to link with each other along the first loop through the hydrophobic interaction of the alkyl chains of ODA, rather than to be assembled on the other region of the silica spheres, the schematic of the detailed formation of core– loop structure is shown in Scheme 2. To gain insight into the underlying special mechanisms, the isotropic Au, Ni, or Fe3O4 NPs modified by ODA or oleylamine were synthesized by standard high-temperature thermolysis reaction, as shown in Figure 3 a–c. These NPs were induced to be assembled onto the surface of OH SiO2 spheres by simply mixing them in nonpolar solvents such as chloroform. Generally, for the assembly of core–satellites OH SiO2@Ni and OH SiO2@Fe3O4 nanocomposites, excessive NPs were used to ensure dense coverage on the silica spheres, and the NPscapped silica spheres were collected after washing several times with nonpolar solvent to remove free NPs. Figure 3 d Chem. Eur. J. 2014, 20, 2065 – 2070
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Scheme 2. Detailed formation of core–loop structures.
Figure 3. TEM images of a) Au, b) Ni, and c) Fe3O4 nanoparticles, as well as the assemblies of d) OH SiO2@Au, e) OH SiO2@Ni, and f) OH SiO2@Fe3O4.
shows the TEM image of Au NPs (1 mg mL 1) assembled onto the silica colloids, which was not effective and in accordance with a published modification method.[38] Meanwhile, Figure 3 e and f show a layer of Ni or Fe3O4 (Fe3O4 was tested for analogy with Ni NPs because of its highly similar interaction strength to Ni oxides) NPs can be clearly observed on the originally smooth surface of OH SiO2 spheres. And there were no free NPs around the silica NPs, which suggested the strong attraction between the NPs and silica spheres. During the self-assembly process, the catechol groups may have partially replaced the original surface ligands (ODA or oleylamine) and co-
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Figure 4. Top: reversible and irreversible disassembly process of OH SiO2@Au Ni nanocomposites induced by various ligands. Bottom: TEM images of a) disassembly of OH SiO2@Au Ni clusters in the presence of vanillin, b) disassembly of OH SiO2@Au Ni in the presence of oleic acid, and c) reassembly of OH SiO2@Au Ni clusters in the presence of octadecylamine. d) IR spectra of the assemblies in Figure 2 b. e) UV/Vis spectra and digital photos of the above assemblies and disassemblies; from top to bottom and from left to right in inset: Janus Au Ni nanoparticles (JANNPs); OH SiO2@Au-Ni nanocomposite; OH SiO2@Au-Ni nanocomposite and vanillin; OH SiO2@Au-Ni nanocomposite, vanillin, and ODA.
ordinated to the metal sites on the NP surface. Even the excessive amount of these isotropic NPs cannot form core–loop assemblies. These results indicated that: 1) the anisotropic geometry of JANNPs played a significant role in the above unconventional assembly of the OH SiO2@Au Ni clusters, which is in accordance with the above deduction, 2) the catechol groups had stronger interaction with the Ni or Fe3O4 domains than with the Au domains. Any reagent that can break the ODA–metal or catecholic– metal links would make the above nanocomposites disassemble, as illustrated in Figure 4 (top). ODA can react with vanillin (0.1 mmol) to form a Schiff base, and the depletion of surfactants led to the aggregation of the JANNPs (Figure 4 a). While after adding a small amount of OA (0.185 mmol), all JANNPs dissociated from the silica spheres and redispersed in CHCl3 solution (Figure 4 b). This phenomenon was due to the ligand exchange between the catechol groups and the OA surfactants, which broke the JANNPs away from the silica surface (Figure 4 d), as confirmed by the IR spectrum of the JANNPs. The peak of the carboxyl group at around 1654 cm 1 and 1323 cm 1 proved the presence of OA on JANNPs. The TEM image of the composites collected using a magnet also did not show any silica sphere, indicating that the JANNPs were successfully desquamated from the silica surface by OA. However, the reassembly of the JANNPs onto silica spheres by adding ODA was difficult because of the different chemical affinities and the irreversibility of the process. Furthermore, for the disassemblies caused by vanillin in Figure 4 a, reassembly Chem. Eur. J. 2014, 20, 2065 – 2070
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occurred by rapidly adding ODA surfactant (0.2 mmol) to the assembly system. Figure 4 c shows that the JANNPs tended to be assembled onto the silica surface again to form a core–loop structure. Figure 4 e shows the UV/Vis spectra of the disassembly and reassembly system of OH SiO2 and JANNP solution. The lower wavelength peak at around 536 nm can be attributed to the pure JANNPs.[39] Successful immobilization of JANNPs onto the silica surface did not lead to significant changes in the absorption curve. However, after disassembly by vanillin, the plasmon band shifted to 568 nm with an obvious shoulder because of interparticle association (aggregation).[40–43] After reassembly by adding ODA, the plasmon band reverted to 536 nm, similar to the original OH SiO2@Au Ni aggregates. The unconventional assembly of the above JANNPs on chemically modified silica spheres was further understood by studying the assembly of JANNPs onto thiolated silica prepared by modifying silica NPs with MPTMS and named as SH SiO2. After self-assembly, the ODA-stabilized JANNPs rapidly anchored onto the silica surface in a single loop at a low concentration, which resulted in a core–loop structure (Figure 5 a). The JANNPs assembled onto SH SiO2 NPs either through the Au side or Ni side, and thus the silica spheres tended to share the sandwiched JANNPs,[44] which slightly differed from the assemblies in Figure 1 c. With the increase of JANNPs concentration, the excessive amount of JANNPs assembled around the original JANNP loop which is also due to the capillary force, as shown in Figure 5 b and c. The Janus specificity and the hydrophobic interaction of the alkyl chains of ODA also contributed to the assembly of SH SiO2@Au Ni composites, which is consistent with the previous speculation.[22] For the same reason, any reagent that can break the metal–thiol bond would change the morphology of the as-prepared assemblies. Interestingly, in the presence of OA molecules (0.05 mmol), the JANNPs and SH SiO2 formed core–satellite structures, as depicted in Figure 5 d. The morphology transformation can be due to the combination affinity which was based on the acid/ base properties or Pearson hardness of the incoming NPs.[45]
Figure 5. a–c) Typical TEM images of two-dimensional core–loop assemblies of SH SiO2@Au Ni clusters with increased concentration of Janus Au Ni nanoparticles. d) Core–satellite SH SiO2@Au Ni clusters in the presence of oleic acid. e–f) Core–satellite SH SiO2@Ni and SH-SiO2@Au symmetric clusters. The inset images are the corresponding STEM images.
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Full Paper The Ni domains of the JANNPs tended to link with the OA molecules that dissociated the core–loop assemblies, whereas the thiol layer of silica spheres tended to bind to the Au domain, which is also accordance with the recent report.[46] As expected, the isotropic Ni and Au NPs randomly assembled onto SH SiO2 on the whole surface and formed core–satellite composites (Figure 5 e and f). This result indicated that the surfactant also played an important role in this unconventional self-assembly. To date, many technologically important NPs (such as Fe3O4, Au Ni, Au Fe3O4, Au ZnO, and so on) have been mainly obtained using organic solvents stabilized by hydrophobic surfactants and insoluble in water. Recently, Yin reported a general process for producing multifunctional composite particles by direct self-assembly of hydrophobic NPs on host silica colloids containing inherent surface thiol groups through ligand exchange. While the direct assembly of isotropic Janus NPs onto substrate (such as SiO2, TiO2, etc.) is still rare and the asymmetric structure will lead to the appearance of novel assembled structures.
Conclusion The self-assembly of JANNPs and asymmetric Au, Ni, and Fe3O4 onto chemically modified silica spheres was investigated. JANNPs assembled onto catecholic silica and thiolated silica spheres to form core–loop assemblies and eventually the assemblies were connected in a large area through the capillary force, whereas the homogeneous NPs assembled to a core– satellite structure. The unconventional self-assembly can be attributed to the synergistic effect of Janus specificity and the capillary force created during the drying process and the hydrophobic interaction of the alkyl chains of ODA finally led to the uncommon core–loop assemblies. The results can guide the development of novel composite materials using Janus NPs as the primary building blocks.
Experimental Section Materials and measurement 3-Hydroxytyramine hydrochloride, octadecylamine (ODA), 3-aminopropyl triethoxysilane (APTES), tetraethoxy-silicone, and 3-mercaptopropyl trimethoxysilane (MPTMS) were purchased from Aldrich. p-Toluenesulfonic acid, oleic acid (OA), vanillin, HAuCl4·4H2O, Fe(acac)3, and Ni(NO3)2·6H2O were obtain from the Chemical Reagent Co. of Shanghai (Shanghai, China). Chloroform and other solvents were analytical grade from Beijing Chemical Factory (China) and were used without further purification. Ultrapure water used in all experiments was obtained from a NANO Pure Infinity System (Barnstead/Thermolyne Corp.). Transmission electron microscopy (TEM) was carried out using a JEOL 2100FX and Tecnai-G2-F30 at acceleration voltages of 200 and 300 kV. One drop of suspension was drop casted onto a carbon-coated copper TEM grid. Upon solvent evaporation, the sample was used for TEM observation without further treatment. UV/Vis absorption spectra were recorded using a Varian Cary 100 spectrophotometer. Infrared spectra (KBr pellet) were recorded using an FTS165 Bio-Rad FTIR spectrophotometer within the range of 4000–400 cm 1. Chem. Eur. J. 2014, 20, 2065 – 2070
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Synthesis of Janus Au Ni NPs, asymmetric Au, Ni, and Fe3O4 NPs JANNPs were synthesized according to literature.[47] In a typical procedure, 0.2 g of HAuCl4 and 1.05 g of Ni(NO3)2·6H2O were added to 30 mL of ODA at 120 8C, and the system was heated to 200 8C. After 1 h of magnetic stirring, the products were collected and washed several times with ethanol. In a typical synthesis of Au nanocrystals,[48] 10 mL of ODA was added to a 50 mL beaker in air. The resulting solution was heated to 160 8C, and then 0.05 g of HAuCl4 was added and magnetic stirring was performed for 10 min. After reaction, particles were collected at the bottom of the beaker, washed several times with ethanol, and dispersed in a nonpolar solvent such as cyclohexane. Ni NPs and Fe3O4 NPs were synthesized by the established method.[49, 50]
Surface modification of catechol-terminated OH SiO2 NPs Silica colloids were synthesized by the Stçber synthesis method.[51] About 200 mL of APTES was added to 30 mL of toluene solution containing 100 mg of as-synthesized SiO2 NPs, and the mixture were refluxed for 8 h. The resulting NPs were collected by centrifugation and washed several times with ethanol. The dried SiO2 NH2 NPs were dispersed in 20 mL of ethanol and degassed with dry argon. 3,4-Dihydroxybenzaldehyde (0.1 g) and catalytic p-toluenesulfonic acid were then added to the degassed solution, and the mixture was refluxed overnight. The resulting catechol-terminated SiO2 NPs were centrifuged, washed several times with ethanol, and dried overnight in a vacuum at 50 8C.
Surface modification of thiol-functionalized SH SiO2 NPs 200 mL of MPTMS was added to 30 mL of toluene solution containing 100 mg of SiO2 NPs, and the mixture was refluxed for 8 h. The resulting thiol-terminated SiO2 NPs were centrifuged, washed several times with ethanol, and dried overnight in vacuum at 50 8C.
Fabrication of various nanocomposites Core–loop OH SiO2@Au Ni nanocomposites were prepared as follows. The as-functionalized silica nanospheres were suspended in 1 mL of chloroform at a concentration of 1 mg mL 1 and purged with argon for 5 min. Then, 1 mL of degassed chloroform solution containing JANNPs was added (1, 1.5, 3, and 8 mg mL 1; Figure 1 c– f, respectively), and the solution was reacted for 18 h in a shaker. After reaction completion, the resulting samples were collected using a magnet without washing and investigated by TEM. Core–satellites OH SiO2@Ni, OH SiO2@Fe3O4, and OH SiO2@Au nanocomposites were prepared by a procedure similar to the above immobilization of JANNPs onto OH SiO2, but 1 mL of Ni, Fe3O4, or Au NPs (1 mg mL 1) was used instead of JANNPs. Core–loop SH SiO2@Au Ni nanocomposites were prepared as follows. The as-functionalized silica nanospheres were suspended in 1 mL of chloroform at a concentration of 1 mg mL 1 and purged with argon for 5 min. Then, 1 mL of degassed chloroform containing JANNPs was added (1, 2, and 8 mg mL 1 for Figure 5 a–c, respectively), and the solution was reacted for 6 h in a shaker. After reaction completion, the resulting samples were collected using a magnet without washing and investigated by TEM. Core–satellites SH SiO2@Au and SH SiO2@Ni nanocomposites were prepared by a procedure similar to the above immobilization of JANNPs onto SH SiO2, but 1 mL of Au or Ni NPs (1 mg mL 1) was used instead of JANNPs.
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Full Paper Core–satellites SH SiO2@Au Ni nanocomposites were prepared as follows. The as-functionalized SH SiO2 NPs (1 mg) were suspended in 1 mL of chloroform at a concentration of 1 mg mL 1 and purged with argon for 5 min. Then, 1 mL of degassed chloroform containing 1 mg of JANNPs and 50 mL of OA (1 mol/L) was added, and the solution was reacted for 6 h in a shaker.
Disassembly and reassembly of OH SiO2@Au Ni nanocomposites The as-fabricated nanocomposites (2 mg) in Figure 1 e were suspended in 4 mL of chloroform. Then, 100 mL of vanillin (1 mol/L) or 185 mL of OA (1 mol/L) was added, and the solution was shaken for 6 h. The resulting samples were collected by centrifugation and finally dispersed in chloroform for further characterization. For the reassembly of the disassemblies formed in Figure 4 a, 200 mL of ODA (1 mol/L) was rapidly added to the chloroform solution of fresh prepared aggregates, and the mixture was shaken for 6 h. All reactions were carried out under a positive pressure of argon.
Catalytic reaction Reduction of 4-nitrophenol: typically, in a standard quartz cuvette, 4-nitrophenol (0.025 mL, 0.01 m), NaBH4 (0.1 mL, 0.1 m), and deionized water (2.2 mL) were added. After addition of 1.5 mg OH SiO2@Au Ni or SH SiO2@Au Ni catalysts, the bright yellow solution gradually fades as the reaction progresses. UV/Vis spectra were monitored in the sequence of time. After the catalytic reaction, the catalyst was collected by a magnet and washed with ethanol. Then, the recovered catalyst was reused to initiate another cycle of reaction. The same procedures were conducted for at least 10 cycles.
Acknowledgements The authors gratefully acknowledge financial support from NSFC(21125316), 973 project (2013CB632300) and CAS (KJZDEW-M01). Keywords: nanoparticles · reassembly · self-assembly · silica nanospheres · synergistic effects [1] [2] [3] [4] [5] [6] [7] [8]
[9] [10] [11] [12] [13]
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Received: September 17, 2013 Published online on January 15, 2014
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