Article pubs.acs.org/Langmuir

Preparation of Novel Porphyrin Nanomaterials Based on the pHResponsive Shape Evolution of Porphyrin Microspheres Wenbo Zhang,† Lingbo Xing,† Haisheng Wang,† Xiujun Liu,‡ Yaqing Feng,*,‡ and Changyou Gao*,† †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China S Supporting Information *

ABSTRACT: The shapes and properties of self-assembled materials can be adjusted easily using environmental stimuli. Yet, the stimulus-triggered shape evolution of organic microspheres in aqueous solution has rarely been reported so far. Here, a novel type of poly(allylamine hydrochloride)-gporphyrin microspheres (PAH-g-Por MPs) was prepared by a Schiff base reaction between 2-formyl-5,10,15,20-tetraphenylporphyrin (Por-CHO) and PAH doped in 3.5-μm CaCO3 microparticles, followed by template removal. The PAH-g-Por MPs had an average diameter of 2.5 μm and could be transformed into one-dimensional nanorods (NRs) and wormlike nanostructures (WSs) after being incubated for different times in pH 1−4 HCl solutions. The rate and degree of hydrolysis had a significant effect on the formation and morphologies of the nanorods. The NRs@pH1, NRs@pH2, and NRs@pH3 were all composed of the released Por-CHO and the unhydrolyzed PAH-g-Por because of the incomplete hydrolysis of the Schiff base. However, the WSs@pH4 were formed by a pure physical shape transformation, because they had the same composition as the PAH-g-Por MPs and the Schiff base bonds were not hydrolyzed. The self-assembled NRs and WSs exhibited good colloidal stability and could emit stable red fluorescence over a relatively long period of time.

1. INTRODUCTION Self-assembly has been widely employed to design and prepare functional materials. In particular, environment-responsive selfassembly triggered by external stimuli, in which a material’s shape and/or macroscopic properties can be modulated to a great extent, has attracted much attention.1 So far, various types of responsiveness to enzymes,2 light,3 and solvents4 have been used to control the self-assembly process and to produce smart materials. In some cases, the responsiveness is integrated with other characteristics in the biological or photoelectrical field, where the morphology plays an important role in determining the properties and functions of the self-assembled materials.5 Microscale self-assembled microspheres and microcapsules are widely applied in drug delivery, biological imaging, and biosensing because they have the ability to control the release of cargos or unique fluorescence properties.6,7 In particular, the stimulus-triggered transformation of morphology in selfassemblies is of great importance and can be achieved by different strategies such as changing the quality of solvents and applying light.4,8,9 For instance, mannitol/Langmuir−Blodget agar microparticles with controlled surface roughness were prepared by self-assembly using hexane as the polymorphic transformation reagent.10 Moreover, microsized vesicles composed of amphiphilic poly(methacrylic acid)-b-poly(methyl methacrylate-random-methacrylic acid) were transformed into micelles by a pH-triggered method.11 © 2015 American Chemical Society

In our previous work, grafting of 1-pyrenecarboxaldehyde (Py-CHO) onto poly(allylamine hydrochloride) (PAH) by Schiff base linking yielded PAH-g-Py microcapsules (MCs), which have the ability to transform into one-dimensional nanorods (1D NRs) or nanotubes (1D NTs) after being incubated at low pH for a certain amount of time.12 The formed nanostructures were composed of released Py-CHO self-assembled through π−π stacking. All of the processes can be conducted under environmentally friendly conditions, such as in aqueous medium, which is much more preferable for biological applications after appropriate optimization of the triggered conditions.13−15 Moreover, this stimulus-responsive self-assembly method can be generalized to achieve more interesting transformations of morphology and to develop novel nanostructures by changing the assembly units and external conditions. Porphyrins have received much attention because of their excellent spectroscopic, photophysical, photochemical, and assembly properties, such as high absorption coefficients and remarkable chemical stability.16,17 Owing to their unique planar and rigid molecular geometry and easily modified periphery, porphyrins are widely utilized in supramolecular chemistry. Received: January 26, 2015 Revised: March 20, 2015 Published: March 23, 2015 4330

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Langmuir Driven by noncovalent interactions such as π−π stacking,18,19 hydrogen bonding,20 electrostatic interactions,21 and metal coordination,22 porphyrins can self-assemble into various nanostructures, enriching their photophysical and photochemical properties.23−31 In this work, 2-formyl-5,10,15,20-tetraphenylporphyrin (PorCHO) was synthesized and reacted with PAH-doped CaCO3 microparticles, yielding a novel type of porphyrin microspheres (PAH-g-Por MPs) after removal of the sacrificial CaCO3. Incubation of the PAH-g-Por MPs in acid solutions with different pH values resulted in different types of onedimensional assembled structures with stable red fluorescence. Although fluorescent microspheres have been widely used in biological and clinical detection,32,33 environment-responsive shape evolutions of organic microspheres in aqueous solution have rarely been reported so far.

the resultant residue was purified by column chromatography using dichloromethane/petroleum ether (1:1) as the eluent (Rf,product = 0.5). The major band yielded 2-formyl-5,10,15,20-tetraphenylporphyrin (Por-CHO) (0.8 g, 69%) as a purple amorphous solid. 1H NMR (500 MHz, CDCl3; Figure S1a, Supporting Information): δ = −2.54 (br s, 2H, inner NH), 7.72−7.83 (m, 12H, m- and p-PhH), 8.17−8.23 (m, 8H, o-PhH), 8.76−8.78 (m, 2H, β-pyrrolic H), 8.84−8.92 (m, 4H, β-pyrrolic H), 9.22 (br s, 1H, H-3), 9.41 (s, 1H, CHO). MS (ESI) (C45H30N4O, exact mass = 642.8): calcd m/z for [M + H+], 643.8; found, 643.3 (Figure S1b, Supporting Information). 2.3. Fabrication of PAH-g-Por Microspheres. Porous CaCO3 particles doped with polymers such as PAH are widely used as templates for microcapsule fabrication because they can be thoroughly dissolved/decomposed with very simple procedures, for example, by incubation in ethylenediamine tetraacetic acid (EDTA) or HCl solution.12 Previous studies have shown that the dissolution process has no significant influence on the resulting products, assuming that the CaCO3 particles can be removed completely.35 In this study, PAHdoped CaCO3 microparticles were prepared according to the method reported previously.12 Briefly, PAH (10 mg) was dissolved in 5 mL of 0.22 M calcium nitrate solution in a 20 mL glass spawn bottle under magnetic agitation (∼1200 rpm), into which an equal volume of 0.33 M sodium carbonate was rapidly poured at room temperature. After 20 min, the PAH-doped CaCO3 particles were centrifuged and washed three times with water and ethanol. The as-prepared PAH-doped CaCO3 microparticles were dispersed in methanol (20 mg/mL) and mixed with the same volume of 0.9 mg/ mL Por-CHO/tetrahydrofuran (THF) solution. The mixture was allowed to react at room temperature for 2 days, and then the solid microspheres were washed with tetrahydrofuran several times to remove the excess Por-CHO. After being washed with water to remove the tetrahydrofuran, the resultant microparticles were incubated in 0.2 M EDTA solution overnight to obtain the PAH-g-Por microspheres. They were washed with water three times using a membrane filter with a pore size of 0.65 μm. 2.4. Decomposition-Assembly of PAH-g-Por Microspheres. The as-prepared PAH-g-Por microspheres were incubated and dispersed in HCl solutions with pH values of 1, 2, 3, and 4 for different times. At the desired time intervals, one portion of the solution was removed for characterization. For the scanning electron microscopy (SEM), transmission electron microscopy (TEM), confocal laser scanning microscopy (CLSM), optical spectroscopy, and zeta potential measurements, the sample solution was used directly. For the Fourier transform infrared (FTIR) spectroscopy and elemental analysis measurements, the sample solution was washed three times with the corresponding HCl solution and then dried by freeze-drying. 2.5. Characterizations. SEM images were recorded on a fieldemission SEM instrument (Hitachi S-4800) at an acceleration voltage of 3 kV. Samples were prepared by placing a drop of the sample suspension onto a clean glass or silicon wafer and then allowing it to dry naturally. TEM images were recorded on a JEM-1230 TEM instrument at an acceleration voltage of 120 kV. Samples were prepared by placing a drop of the sample suspension onto a carbonfilm-coated copper grid and then allowing it to dry naturally. FTIR spectra were measured on a Bruker Equinox 55/S instrument using KBr pellets. Elemental analysis was performed on a Vario MICRO cube instrument from Elementar Analysensysteme GmbH. CLSM images were recorded on a Leica TCS SP5 system (100× oil immersion using commercial software). Samples were prepared by casting the sample suspensions onto glass slides and then allowing them to dry naturally or keeping them in solution. Fluorescence (FL) emission spectra were recorded with an LS55 instrument (PerkinElmer) at an excitation wavelength of 420 nm. Zeta potentials were measured on a zeta potential/submicron size analyzer (DelsaNano C, Beckman Coulter, Brea, CA). Each value was averaged from five parallel measurements.

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(allylamine hydrochloride (PAH, 58 kDa) was purchased from Sigma-Aldrich. Pyrrole was purchased from Aladdin. Phosphorus oxychloride was purchased from Reagent No. 1 Factory of Shanghai Chemical Reagent Co., Ltd. (Shanghai, China). Copper(II) acetate monohydrate was purchased from TCI Development Co., Ltd. (Shanghai, China). Benzaldehyde, propionic acid, calcium nitrate terahydrate, sodium carbonate, HCl solution (10 mol L−1), and other organic solvents were purchased from Sinopharm Chemical Reagent Company (Beijing, China). HCl solution (10 mol L−1) was diluted to the desired concentrations. Other chemicals were used as received. The water used in all experiments was prepared using a Millipore Milli-Q purification system. 2.2. Synthesis of Por-CHO. Por-CHO was synthesized according to a reported approach with some optimizations.34 2.2.1. Synthesis of 5,10,15,20-Tetraphenylporphyrin. Benzaldehyde (16.4 g, 155 mmol) was added to propionic acid (320 mL) in a 500 mL three-neck, round-bottom flask, and the mixture was heated under magnetic stirring. When the propionic acid began to reflux, newly distilled pyrrole (8 mL, 120 mmol) diluted with propionic acid (80 mL) was added dropwise over 30 min. The reaction was maintained for 45 min under agitation. After the mixture had been cooled to room temperature, 100 mL of methanol was added, and the mixture was maintained in an ice bath overnight. The product was filtered, washed with ethanol and hot water three times, and then dried to give the crude product of 5,10,15,20-tetraphenylporphyrin (4.0 g, 22.5%). 2.2.2. Synthesis of 5,10,15,20-Tetraphenylporphyrinato Copper(II). 5,10,15,20-Tetraphenylporphyrin (2.4 g, 3.9 mmol) and Cu(Ac)2· H2O (0.8 g, 4.0 mmol) were added to a mixed solvent of methanol (120 mL) and toluene (400 mL) in a 1000 mL three-neck, roundbottom flask. Then, the mixture was heated and refluxed for 4 h. The reaction liquid was filtered, and the solvent was removed under reduced pressure to give the crude product of 5,10,15,20tetraphenylporphyrinato copper(II) (2.50 g, 92%). 2.2.3. Synthesis of 2-Formyl-5,10,15,20-tetraphenylporphyrin (Por-CHO). Dimethylformamide (14 mL, 0.18 mol) was added to a 250 mL three-neck, round-bottom flask that was placed in an ice bath. Then, phosphorus oxychloride (11 mL, 0.12 mol) was added dropwise with magnetic stirring under a nitrogen atmosphere to obtain the Vilsmeier complex. 5,10,15,20-Tetraphenylporphyrinato copper(II) (1.25 g, 1.8 mmol) was dissolved in 125 mL of 1,2-dichloroethane, and the solution was then added dropwise to the Vilsmeier complex over 40 min. The mixture was refluxed for 5 h. After it had been cooled to room temperature, 24 mL of concentrated sulfuric acid was added, and the mixture solution was stirred for several minutes. The mixture was poured onto an ice-cold solution of sodium hydroxide (36 g) in water (1250 mL). The organic layer was washed with water and a saturated solution of sodium bicarbonate and was finally extracted with dichloromethane. The solution was dried over anhydrous magnesium sulfate. After the solvent had been removed under reduced pressure, 4331

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Figure 1. (a) Molecular structures showing the synthesis and decomposition of PAH-g-Por. (b) Structure evolution and formation of various nanostructures of PAH-g-Por microspheres at different pH values.

3. RESULTS AND DISCUSSION 3.1. Results. The doped PAH in the CaCO3 particles with a weight ratio of 4.0% reacted with 2-formyl-5,10,15,20tetraphenylporphyrin (Por-CHO) by Schiff base formation (Figure 1a). Removal of the CaCO3 template with EDTA yielded MPs composed of Por-modified PAH (PAH-g-Por) (Figure 1b). Figure 2a shows that the as-prepared PAH-doped CaCO3 particles were spherical and had an average diameter of 3.5 μm. They were porous with a rough surface morphology (Figure 2a, inset), enabling diffusion of Por-CHO and reaction with PAH. The PAH-g-Por maintained the macroscopic shape of the CaCO3 particles, forming MPs after CaCO3 removal as a result of aggregation of the hydrophobic Por and rearrangement of the hydrophilic PAH chains. Shrinking occurred during this process, and therefore, the average diameter of the MPs (2.5 μm) was smaller than that of the CaCO3 particles. The MPs had a rugged surface (Figure 2b) and a solid interior with small cavities in some cases (Figure 2d; Figure S2a, Supporting Information). However, the CLSM (Figure 2c) and TEM images reveal that the periphery of the MPs was denser than the interior, which is attributed to the additional physical adsorption of PAH on the surface of the rough CaCO3 particles and the diffusion gradient of Por. Because the PAH component had been labeled with fluorescein isothiocyanate (FITC), the emissions of both Por and FITC were recorded by CLSM (Figure S2c−e, Supporting Information), demonstrating the presence of both Por and PAH in the MPs. FTIR spectra

Figure 2. SEM images of (a) PAH-doped CaCO3 particles (inset, higher magnification) and (b) PAH-g-Por microspheres (inset, higher magnification). (c) CLSM image (in solution) and (d) cross-sectional (ultramicrotomy) TEM image of PAH-g-Por MPs.

(Figure 3a) indicated the disappearance of the aldehyde group at 1668 cm−1 and the formation of Schiff base bond at 1631 cm−1 in the PAH-g-Por MPs. Moreover, the peak at 3425 cm−1 is assigned to the stretching vibration of NH in the pyrrole 4332

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times. In pH 1 HCl solution, the MPs were decomposed and self-assembled gradually (Figure 4a−e). The MPs were intact and dark in the initial stage (Figure 4a). After incubation for 1 h, nanoparticles were protruded on the periphery of the MPs (Figure 4b). After 1 day, short and uniform nanorods (NRs@ pH1, ∼700 nm in length and ∼200 nm in width) appeared on the edge of the MPs (Figure 4c). The NRs@pH1 kept growing, and the MPs faded and expanded gradually as the incubation time was prolonged (Figure 4d,e). Eventually, the MPs disappeared completely after 7 days (Figure 4e), forming a network of NRs@pH1 (∼2 μm in length and 200−300 nm in width). The NRs@pH1 were needlelike at both ends, and their middle and end sections had thicknesses of 100−120 and 50− 60 nm, respectively (Figure S3a,e,i, Supporting Information). Without disturbance, most of the NRs@pH1 aggregated in a format of clusters representing the contours of the original MPs (Figure 4e,f). They emitted red fluorescence (Figure 4g, inset), which is a sign of Por-containing products. FTIR spectra were recorded to analyze the change in chemical structure at different points during the incubation. An inconspicuous aldehyde peak of Por-CHO appeared at 1668 cm−1 (almost hidden in the Schiff bond peak at 1631 cm−1) after 1 h and became more obvious after 1 day, confirming the hydrolysis of CN in the MPs (Figure 5a). At 4 days, the  CHO peak of Por and the CN peak showed the same strength. After 7 days, the relative strengths of the two peaks remained almost unchanged. Elemental analysis found that the ratio of Por to PAH repeat units ([PAH]) increased from the original 8.9% to 21.2% with the prolongation of the incubation time to 7 days (Table 1), and the ratio remained at this value until 30 days. According to these data, the release ratio of [PAH] (compared to the original ratio) was found to be 58.0% after 30 days (Table 1). At pH 2, the MPs grew into smaller nanorods (NRs@pH2, ∼700 nm in length and ∼150 nm in width) after 7 days,

Figure 3. FTIR spectra of Por-CHO, PAH, and PAH-g-Por MPs.

ring of Por and NH2 in PAH. The stretching vibration of  CH at 3040 cm−1 is characteristic of Por. The bending vibration of CH2 at 1456 cm−1 is indicative of PAH.36 No calcium was detected within the MPs by energy-dispersive X-ray spectroscopy (EDS; Figure S2b, Supporting Information), demonstrating the complete removal of CaCO3. According to elemental analysis, the substitution degree of Por on PAH in the MPs was 8.9%. The PAH-g-Por MPs were stabilized by the physical association between the hydrophobic domains of Por and the hydrophilic protection of PAH, which could be preserved for more than 3 months without destruction. The PAH-g-Por MPs showed different decompositionassembly and shape-evolution behaviors after being incubated in hydrochloric acid of different pH values at 20 °C for different

Figure 4. TEM images showing the process of shape evolution of PAH-g-Por MPs into nanorods in pH 1 HCl for (a) 0 h, (b) 1 h, (c) 1 day, (d) 4 days, and (e) 7 days. (f) SEM and (g) optical microscopy images (inset, CLSM image) of nanorods formed at pH 1 after 7 days. 4333

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Figure 5. FTIR spectra of PAH-g-Por MPs after being incubated at (a) pH 1 and (b) pH 4 HCl for different times, as indicated.

Table 1. Molar Ratio of Por to PAH Repeat Units ([PAH]) and Release Ratio of [PAH] Calculated According to the Elemental Analysis Results pH 1 Por/[PAH] (%) [PAH] release ratio (%)

pH 2

pH 3

pH 4

0 days

1 day

4 days

7 days

30 days

30 days

7 days

30 days

7 days

40 days

8.9

14.3 37.5

16.6 46.2

21.2 58

21.5 58.6

16.2 45

11.9 24.9

14.4 38.3

8.9 0

9 1.1

Figure 6. (a,d) TEM, (b,e) SEM (inset in e, higher magnification), and (c,f) optical microscopy images (inset, CLSM images) of decompositionassembled structures of PAH-g-Por MPs after treatment at (a−c) pH 2 for 7 days and (d−f) pH 3 for 30 days.

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Figure 7. (a−d) TEM images (inset in d, higher magnification) showing the process of shape evolution of PAH-g-Por MPs into wormlike structures after incubation in pH 4 HCl for (a) 1, (b) 7, (c) 20, and (d) 40 days. (e,f) SEM (inset in f, higher magnification) and (g) optical microscopy images (inset, CLSM image) of the evolved structures of PAH-g-Por MPs after treatment in pH 4 HCl for 14, 40, and 60 days, respectively.

judging from the more curved structure in the SEM image37 (Figure 6e). The change in components over time was monitored by FTIR spectroscopy (Figure S5g, Supporting Information) and elemental analysis (Table 1). The CHO peak was not obvious within 1 day and exhibited an intensity comparable to that of the CN peak after 7 days. At 30 days, the ratio of Por to [PAH] in the NRs@pH3 was 14.4% (Table 1), which was smaller than the values for the NRs formed at lower pH values. At pH 4, the phenomenon was quite different from that at the other three pH values. After 1 day, the MPs had expanded and faded, and their size was enlarged to 4−5 μm with a mean value of 4.7 μm (Figure 7a). Interestingly, some sleek microscale structures (2−4 μm in length and ∼1 μm in width) appeared around the MPs. They had a wormlike morphology and tended to stick to each other. TEM images (Figure 7a) revealed that the wormlike structures (WSs@pH4) had the same contrast as the faded MPs. The WSs@pH4 were likely to bud and grow from the MPs. With increasing incubation time, more MPs became incomplete, and the number of WSs@pH4 increased (Figure 7b,c). Finally, after 40 days, the MPs disappeared, and there were only WSs@pH4 with a uniform size (Figure 7d). In the early stage, there was no obvious change in the size of the WSs@pH4, but the number increased. In the later stage, when only WSs@pH4 existed, the WSs@pH4 became longer, whereas the width hardly changed. Some WSs@pH4 became more than 10 μm in length after 60 days (Figure 7g). Apart from the shape features, the WSs@pH4 also had other typical characteristics: the surface was smooth (Figure 7e), and the whole structure appeared to be flexible (curving)37,38 (Figure 7f). Specially, dark dots inside some WSs@pH4 were revealed by TEM (Figure 7d), suggesting local aggregation with a denser structure than at the periphery. The “white dots” observed for some WSs@pH4 (Figure 7c) might

accompanied by some nanoparticles with a size of dozens of nanometers (Figure 6a−c). TEM images (Figure 6a; Figure S4d, Supporting Information) revealed dark-colored NRs@ pH2 embedded in the obvious light-colored vestige of the MPs, and the whole structures also retained the contours of the expanded MPs. The general decomposition-assembly process at pH 2 was similar to that at pH 1, with MPs fading and NRs@ pH2 growing gradually at a comparable transformation rate (Figure S4a−d, Supporting Information). With the number of nanoparticles decreasing, the NRs@pH2 increased in size from ∼400 nm in length and ∼80 nm in width after 2 days to ∼700 nm in length, ∼150 nm in width, and 50−60 nm in thickness (Figure S3b,f,j, Supporting Information) after 7 days. The ratio of Por to [PAH] and the corresponding release ratio of [PAH] in the NRs@pH2 formed at 30 days were 16.2% and 45%, respectively (Table 1), which were smaller than the corresponding values for the NRs@pH1 formed at the same incubation time. At pH 3, the decomposition-assembly process was much slower. In the early stage, the MPs expanded and dissolved gradually into the acid solution, and nanoparticles with sizes ranging from dozens of nanometers to 200 nm were produced (Figure S5ab, Supporting Information). After 14 days, small and light nanorods (NRs@pH3) (500−700 nm in length and 50−100 nm in width) appeared, whereas the MPs disappeared (Figure S5c, Supporting Information). Along with a reduction in the number of nanoparticles, the number of NRs@pH3 increased, and their size became larger (Figure S5d, Supporting Information). After 30 days, the nanoparticles almost disappeared, and the NRs@pH3 became 1−1.2 μm in length, 80−120 nm in width (Figure 6d,e; Figure S5e,f, Supporting Information), and 30−40 nm in thickness (Figure S3c,g,k, Supporting Information). Moreover, the NRs@pH3 appeared to be more flexible than those formed at lower pH values, 4335

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Figure 8. Fluorescence spectra of PAH-g-Por MPs after being incubated in (a) pH 1 and (b) pH 4 HCl for different times, as indicated. The excitation wavelength was 420 nm.

(Figure S6b, Supporting Information). At pH 4 (Figure 8b), the change in fluorescence spectrum was similar to that at pH 3. 3.2. Discussion. The above results confirm that PAH-g-Por MPs can undergo a shape transformation to form nanorods or wormlike structures depending on the incubation pH value. These nanostructures formed based on decomposition-assisted assembly and/or physical rearrangement of the PAH-g-Por component in the MPs. According to the FTIR and elemental analysis results, the formation of the NRs@pH1, NRs@pH2, and NRs@pH3 should be attributed to the hydrolysis of partial Schiff base bonds triggered by low pH and the assembly of released PorCHO under the assistance of PAH. Meanwhile, protonation of PAH,41 which is indicated by the obvious symmetric bending vibration of NH3+ at 1500 cm−1,42 also plays a role in the disassociation of PAH-g-Por MPs and the formation of the newly self-assembled structures through electrostatic repulsion and increasing hydrophilicity. At pH 1, the decompositionassembly of MPs took place gradually (Figure 5a), accompanied by budding and the eventual formation of NRs@pH1 (Figure 4). The relative peak strength of the Schiff base bonds decreased, whereas the peak of aldehyde groups intensified as the incubation time was prolonged to 7 days, demonstrating that the hydrolysis degree was improved and, thereby, more Por-CHO molecules were released. The hydrolysis process reached equilibrium after around 7 days, and the relative quantity of Por-CHO molecules to Schiff base structures inside the NRs@pH1 remained unchanged (Table 1). The released Por-CHO molecules should assemble with each other and with those grafted on the PAH (PAH-g-Por), so that the resultamt NRs@pH1 have a higher quantity of Por-CHO component than the parent polymers. For the Por NRs formed below pH 3, they all exhibited similar chemical structures with variable contents of PAH (Figure 5a; Figure S4e, Supporting Information; Table 1), and their lengths and morphologies were substantially mediated by the solution pH. According to the FTIR spectra at 1 h (Figure 5a; Figure S4e, Supporting Information), the relative strength of the CHO peak compared to the CN peak at pH 2 (0.973) was stronger than that at pH 1 (0.858), indicating a higher hydrolysis rate in pH 2 solution at the initial stage. This

represent dissolution holes. AFM images (Figure S3d,h,l, Supporting Information) showed that their thickness was 100−130 nm, which was much smaller than the width. The WSs@pH4 could also emit the typical red fluorescence of Por39,40 (Figure 7g, inset). Interestingly, no CHO peak of Por appeared in the FTIR spectrum regardless of the incubation time even after 40 days (Figure 5b). According to elemental analysis, the Por/[PAH] ratio hardly changed (Table 1), suggesting that the WSs@pH4 should have a chemical composition similar to that of the MPs. As a result of the presence of fluorophore Por molecules, the PAH-g-Por MPs and the decomposition-assembled structures emitted red fluorescence (Figure 2c; insets of Figures 4g, 6c,f, and 7g). The fluorescence of these structures had good stability and was not easy to quench, as confirmed by a time-resolved photobleaching experiment with the xyt mode of CLSM (Figure S8b, Supporting Information). After continuous laser scanning for 20 min, the fluorescence intensities of PAH-g-Por MPs, NRs@pH1, NRs@pH2, NRs@pH3, and WSs@pH4 were maintained at 60.7%, 92.1%, 88.5%, 55.5%, and 88.1%, respectively. To confirm the decomposition-assembly process of the MPs at different pH values, fluorescence spectra were recorded at different time points (Figure 8a,b; Figure S6a,b, Supporting Information). Before addition of the HCl, the MPs in water had a strong emission peak at around 676 nm (peak I) and a weak peak at around 735 nm (peak II), which was similar to that of Por in THF. Upon incubation in HCl, the fluorescence intensity decreased, and the two emission peaks red-shifted. As the incubation time was prolonged, peak II became stronger and red-shifted further. The change was in accordance with the TEM and FTIR results. At pH 1, there was no obvious change in peak position for peak I, but its strength decreased within 14 days and no longer changed thereafter (Figure 8a). At pH 2, the situation was similar, and only the original and final fluorescence spectra were recorded (Figure S6a, Supporting Information). At pH 3, within 7 days, the fluorescence strength reduced slightly, and the two peaks redshifted slightly, but the peak shape did not change obviously. After 7 days, peak I was weakened further, whereas peak II was enhanced and red-shifted with increasing incubation time 4336

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as found for similar amphiphilic assemblies.46 Theoretically, the diameter of WSs@pH4 is much larger than the estimated chain length of extended PAH (95 nm), suggesting a hollow nature as well. It is assumed that the inside dark dots are hydrophobic Por aggregates and that the light periphery is the hydrophilic PAH, enabling the suspension stability. The wormlike micelles are common in the self-assembly of amphiphilic polymers in aqueous solutions.47,48 Hubbel and co-workers also reported that increasing the hydrophilicity of the amphiphilic polymers can achieve conversion from vesicles to wormlike micelles.49 UV−vis spectra were measured to explore the characteristic features of Por when interacting with neighboring molecules (Figure S7a, Supporting Information). All of the structures exhibited a relatively strong Soret band in the range of 435− 455 nm and four Q bands from 500 to 700 nm. There was a shoulder band close to the Soret bands at about 420 nm. By contrast, the Por monomer in tetrahydrofuran exhibited a narrow, strong Soret band at 428 nm and four Q bands at 523, 563, 605, and 661 nm (Figure S7b, Supporting Information), which is typical for the freebase porphyrin.39 It is known that pophyrins tend to self-assemble by balancing π−π interactions of their hydrophobic porphyrin rings and charged substituent groups by electrostatic forces.50 The UV−vis spectra revealed the aggregation states of these structures. Compared with the spectrum of the homogeneous solution of Por monomer, the Soret and Q bands of the MPs, NRs, and WSs all red-shifted to different degrees (Table S2, Supporting Information), and the Soret bands were obviously broadened. The red shifts of the absorption bands indicate the existence of Por J-aggregates.51,52 The broadening of the Soret bands is normally caused by the inhomogeneity and variable sizes of aggregate structures.53 Therefore, during the process of Por aggregation, the system involves different numbers and sizes of aggregates, including nonspecific aggregates and J-aggregates. The shoulder band at around 420 nm should be assigned to the Por “monomers” (free or linked to PAH). As a reference, the absorption spectra of Por-CHO at different pH values (Figure S7b, Supporting Information) were basically consistent with those of the selfassembled structures at the same pH values. However, the shoulder peak at about 420 nm was not obvious, suggesting that almost all of the Por-CHO formed aggregates in aqueous solution without the presence of PAH. Fluorescence emission spectroscopy further confirmed the formation process of NRs@pH1, NRs@pH2, NRs@pH3, and WSs@pH4. Based on the UV−vis spectra, J-aggregates and nonspecific aggregates were formed in the NRs and WSs (Figure S7a, Supporting Information), which weakened the fluorescence emission.54,55 Therefore, at pH 1−3, a gradual increase in Por aggregation (driven by hydrophobic and π−π interactions) resulted in a decrease of fluorescence intensity and a red shift of the emission peaks in the early stage when the Schiff base bonds were hydrolyzed. At the equilibrium state, no further change took place (Figure 8; Figure S6, Supporting Information). At pH 4, the PAH-g-Por molecules were not hydrolyzed. The weakened and red-shifted emission might have resulted from intensified aggregation of the porphyrin as a result of the molecular rearrangements in the shape evolution process. To support this deduction, the emission spectra of Por-CHO at different pH values (Figure S8a, Supporting Information) were characterized. The results are similar to those of the corresponding MPs, NRs, and WSs. The good photostability of the nanostructures (Figure S8b, Supporting

is consistent with our previous study of PAH-g-Py molecules, whose highest decomposition rate appeared at about pH 2.12,43 The released hydrophobic Por-CHO molecules in the aqueous solution or on the MP surface can self-assemble into seedlike structures because of the hydrophobic force and π−π stacking and act as growth points for the subsequently released PorCHO molecules. A higher hydrolysis rate at pH 2 will form a larger number of growth points, leading to shorter NRs@pH2 compared with those formed at pH 1 and 3. This is consistent with the previous observation that a lower decomposition rate is favorable for the formation of long and regular onedimensional nanostructures such as nanotubes.12 Nevertheless, the final ratio of Por to [PAH] in the NRs@pH2 was lower than that in the NRs@pH1 (Table 1), suggesting that the final hydrolysis degree of PAH-g-Por at pH 2 was lower than that at pH 1, so that it was likely determined by thermodynamics rather than kinetics. The obtained decomposition-assembled structures at pH 2 had an obvious vestige of faded MPs (Figure 6a; Figure S4d, Supporting Information) that was not found for those obtained at pH 1 (Figure 4e). At pH 3, the MPs expanded and collapsed in the early stage, such as within 1 day (Figure S5a, Supporting Information), even though the Schiff base bonds of PAH-g-Por were hardly decomposed in this period of time (Figure S5g, Supporting Information). The expansion of the MPs is attributed to the higher degree of protonation of PAH and CN,44,45 as evidenced by the larger zeta potential of MPs (53 mV) in pH 3 HCl (aq) than in water (32 mV). The charge repulsion and enhanced hydrophilicity enable the MPs to swell and to transform into nanoparticles partially (Figure S5a,b, Supporting Information). At this time, there was not enough released Por to form one-dimensional nanorods. With increasing time, the Schiff base bonds began to decompose, and more Por molecules were released (Figure S5g, Supporting Information) and self-assembled into NRs@pH3 (Figure S5c−f, Supporting Information). Obviously, the hydrolysis was much slower than that at pH 1 and 2. The ratio of Por to [PAH] in the NRs@ pH3 was also relatively low (14.4%) (Table 1), demonstrating the limited hydrolysis degree of Schiff base bonds until the equilibrium state. The lower content of the rigid building block Por and the higher content of the flexible PAH explains why the NRs@pH3 were thinner and appeared to be more flexible than the NRs@pH1 and [email protected],38 Unlike situations at the other three pH values, no CHO was found in the FTIR spectra (Figure 5b), and the Por/[PAH] ratio was hardly changed (Table 1) within 60 days at pH 4, revealing that the decomposition of the Schiff base bonds was negligible.43,45 Nonetheless, wormlike WSs@pH4 indeed formed, accompanied by the disappearance of the MPs. This is a real physical shape evolution process without the breakage of any chemical bonds. This phenomenon might be caused by the enthalpic change, and the WSs@pH4 are more thermodynamically stable than the MPs because the separated WSs@pH4 particles (zeta potential, 42 mV) cause less charge repulsion between the PAH-g-Por molecules41,44 Figure 7a−d shows that there were local aggregation structures inside the WSs@pH4, which were likely formed by the hydrophobic Por groups during the self-assembly. Therefore, the WSs@pH4 likely represent a type of wormlike micelles with hollow cavities (bright domains in Figure 7a−c). This was verified by the significantly smaller height (100−130 nm) than the width (1 μm) in the collapsed state (Figure S3d,h,l Supporting Information), which is generally attributed to a hollow structure 4337

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Figure 9. Proposed mechanism for the formation of decomposition-assembled structures of PAH-g-Por MPs at pH 1−4.

transformation, forming wormlike nanostructures (Figure 9, bottom) as a result of higher protonation of PAH and CN and the local self-assembly of PAH-g-Por components. Unlike the typical self-assembly of organic crystals of porphyrin, these four types of novel one-dimensional structures not only are composed of the self-assembled unit Por, but also contain a certain proportion of protonated hydrophilic PAH located at the periphery. Hence, they can be well suspended in water and exhibit very good colloidal stability without aggregation after being stored for at least 6 months at room temperature. (Neutralization of the solution yields obvious precipitates.) Moreover, these structures have relatively longlived fluorescence. The generalization of this stimulusresponsive method offers chances to prepare novel nanomaterials that could find applications in different fields, for example, intracellular assembly and in situ fluorescent imaging.

Information) is very promising for future applications, although the exact mechanism still needs to be further investigated.

4. CONCLUSIONS Porphyrin-containing microspheres (PAH-g-Por MPs) were prepared by a Schiff base reaction between Por-CHO and PAH doped in CaCO3 microparticles, followed by template removal. The size of the PAH-g-Por MPs was smaller than that of their template because of the shrinking of the MPs. The pH-responsive shape evolution of PAH-g-Por MPs mainly comes from the decomposition of the Schiff base and the protonation of PAH. The self-assembly of Por through π−π and hydrophobic interactions plays a major role in the formation of assembled structures such as NRs@pH1, NRs@ pH2, NRs@pH3, and WSs@pH4, and the hydrolysis rate and degree of Schiff base were strongly influenced by the pH value. The decomposition-assembly of PAH-g-Por MPs is both kinetically and thermodynamically controlled, and the hydrolysis rate and hydrolysis degree have significant impacts on the formation and morphology of the nanostructures. At pH 1−3, the PAH-g-Por MPs are hydrolyzed gradually to release Por molecules, which self-assemble under the assistance of PAH-gPor and serve as growth points for the later-formed nanostructures (Figure 9, top). Unlike the complete hydrolysis found for other systems,12,56 the PAH-g-Por molecules are only partially decomposed to release the PAH, likely because of the stereohindrance effect of large pendant Por, and therefore, the formed nanostructures are composed of Por and PAH-g-Por in different proportions. A higher hydrolysis rate will produce more growth points in the initial stage, leading to nanorods with a smaller size such as at pH 2. A lower hydrolysis degree will result in a larger proportion of the PAH hydrophilic periphery, enabling the formation of more flexible nanostructures such as at pH 3. Unlike the hydrolysis-driven assembly below pH 3, at pH 4, the MPs undergo a purely physical shape

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ASSOCIATED CONTENT

* Supporting Information S

1

H NMR and mass spectra, CLSM images, and EDS analysis of PAH-g-Por MPs. SEM image of a broken PAH-g-Por MPs. AFM images of the formed nanostructures. TEM and SEM images of PAH-g-Por MPs with corresponding FTIR spectra after incubation at pH 3 and 2 for different times. Fluorescence and UV−vis spectra of PAH-g-Por MPs and Por-CHO after incubation at different pH values for different times. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.G.). *E-mail: [email protected] (Y.F.). Notes

The authors declare no competing financial interest. 4338

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ACKNOWLEDGMENTS This work was financially supported by the Ph.D. Programs Foundation of the Ministry of Education of China (20110101130005) and the Natural Science Foundation of China (51120135001).

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