Synthesis of tetra-layer polymer composite microspheres and the corresponding hollow polymer microspheres with Au nanoparticles functionalized movable cores

Abstract

Hollow poly(ethyleneglycol dimethacrylate) (PEGDMA) microspheres with Au nanoparticle functionalized movable cores were prepared by selective removal of the sandwiched silica mid-layer from the corresponding poly(ethyleneglycol dimethacrylate-co-methacrylic acid)/poly(ethyleneglycol dimethacrylate-co-vinylpyridine)@gold/silica/PEGDMA (P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂/PEGDMA) tetra-layer composite microspheres, in which the Au nanoparticles were stabilized by the pyridyl groups on the surface and gel-layer of P(EGDMA-co-MAA)/P(EGDMA-co-VPy) particles during the reduction of HAuCl₄ with sodium borohydride. The tetra-layer polymer composite microspheres were synthesized via the combination of distillation precipitation polymerization for the preparation of the polymer core and outer shell layer together with the modified Stöber sol–gel process for the formation of the sandwiched silica mid-layer. The primary results indicated that the Au nanoparticle functionalized movable cores enabled the hollow polymer microspheres to act as efficient catalysts for the reduction of 4-nitrophenol to 4-aminophenol with sodium borohydride as the reductant.

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Introduction

Recently, hollow polymer microspheres have attracted increasing attention due to their special advantages including low density, surface functionality, high specific area, controlled permeability and good flow ability, which enable the hollow polymer microspheres to be used in a variety of applications in chemistry, biotechnology, medical and material science.^1–4 Many efforts have been devoted to prepare these hollow structures, such as the encapsulation of a hydrocarbon non-solvent,⁵ micelle formation of block copolymers with subsequent shell-crosslinking and degradation of the core,⁶ surface-initiated atom transfer radical polymerization (ATRP),⁷ polycondensation interfacial polymerization,^8,9 and layer-by-layer deposition of polyelectrolyte onto sacrificial templates.^10–12

An important concern for hollow microspheres is the encapsulation of guest materials into the cavities, which would provide them with novel properties compared with the original host hollow microspheres and the guest materials. There have been two common synthetic strategies to acquire hollow polymer microspheres with movable cores. One is the so-called bottom-up approach, in which the core and shell are fabricated in order followed by the selective removal of a sacrificial mid-layer to leave a hollow space between the inner core and the shell-layer. Different nanoparticles such as gold,^13,14 silver,¹⁵ tin,¹⁶ silica,¹⁷ polymer beads,¹⁸ and iron oxides^19,20 have been encapsulated in the interior of hollow particles. The other conceptual strategy is the so-called top-down approach, in which the hollow microspheres are constructed at first followed by the in situ formation of the movable cores inside the cavities. Cu²¹ and Ag²² nanoparticles have been prepared inside the hollow microspheres by the reduction of Cu²⁺ and Ag⁺ respectively. Despite diverse frameworks of hollow polymer microspheres with inner cores were fabricated, most of the movable cores were constructed by a single kind of material. Thus, synthesis of more complex structures with functional shells and decorated inner cores is still a challenge for researchers.

In this work, we provide an effective strategy for the fabrication of hollow polymer microspheres containing metal-functionalized movable cores. Tetra-layer composite microspheres were prepared via the combination of distillation precipitation polymerization and a sol–gel process. Subsequently, the sacrificial silica mid-layers were selectively removed to afford the hollow polymer microspheres with Au nanoparticles functionalized movable cores, which could be utilized as a micro-reactor where catalytic reaction would be performed exclusively inside the hollow spaces.

Experimental

Chemicals

Ethyleneglycol dimethacrylate (EGDMA) was purchased from Aldrich Chemical Co. and used without further treatment. 4-Vinylpyridine (VPy) and methacrylic acid (MAA) were provided by Acros and Tianjin Chemical Reagent II Co., respectively, and were purified by vacuum distillation. 2,2′-Azobisisobutyronitrile (AIBN) was obtained from Chemical Factory of Nankai University and recrystallized from methanol. Acetonitrile (analytical grade, Tianjin Chemical Reagent II Co.) was dried over calcium hydride and purified by distillation. Tetrachloroauric acid trihydrate (HAuCl₄·3H₂O) and sodium citrate were obtained from Shenyang Research Institute of Nonferrous Metals, China. Sodium borohydride (NaBH₄) was purchased as analytical grade from Tianjin Chemical Reagent III Co. 4-Nitrophenol (4-NP, Tianjin Chemical Reagent Factory) was recrystallized from petroleum ether and ethyl acetate. Tetraethyl orthosilicate (Si(OEt)₄), TEOS) amd 3-(methacryloxy)propyl trimethoxysilane (MPS) were bought from Aldrich and used without any further purification. Hydrofluoric acid (HF, containing 40 wt% of HF) was available from Tianjin Chemical Reagent Institute and diluted to the concentration of 10 wt% for utilization. All the other reagents were of analytical grade and used without any further treatment.

Preparation of P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au microspheres

Monodisperse poly(ethyleneglycol dimethacrylate-co-methacrylic acid)/poly(ethyleneglycol dimethacrylate-co-4-vinylpyridine) (P(EGDMA-co-MAA)/P(EGDMA-co-VPy)) core-shell microspheres were prepared by a two-stage distillation precipitation polymerization according to our previous work.²³ Polymer microsphere-stabilized Au nanoparticles (P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au) were prepared by in situ reduction of aqueous HAuCl₄ solution with NaBH₄ as a reductant in the presence of functional core-shell polymer microspheres as a stabilizer. A typical procedure was as follows: 0.05 g of P(EGDMA-co-MAA)/P(EGDMA-co-VPy) core-shell polymer microspheres (containing 1 × 10⁻⁶ mol pyridyl groups) were suspended in 8 mL of acetonitrile in a 25 mL round flask with ultrasonic irradiation. Then 0.8 mL of 6.27 mM (5.0 × 10⁻⁶ mol Au atoms) HAuCl₄ solution was introduced into the suspension. After the mixture was stirred at room temperature for 24 h, 1.4 mL of 0.14 M NaBH₄ (2.0 × 10⁻⁴ mol) aqueous solution was rapidly poured into the reaction flask with magnetic stirring. The suspended mixture turned purple immediately, indicating the formation of the Au metallic nanoparticles. Then the reaction system was stirred for further 12 h at room temperature. The resultant polymer microsphere-stabilized Au nanoparticles were purified by five cycles of centrifugation, decanting and resuspension in ethanol with ultrasonic irradiation and then redispersed in 20 mL of ethanol for further utilization.

Preparation of MPS-modified P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂ composite microspheres

For silica coating of polymer microsphere-stabilized Au nanoparticles, 10 mL of water and 1.2 mL of ammonium (NH₄OH) aqueous solution were added into the above suspension with magnetic stirring. The solution of TEOS in ethanol (0.5 mL of TEOS dissolved in 10 mL of ethanol) was added dropwise into the above suspension during 10 h. The mixture was continued stirring further for 12 h. Then 0.5 mL of MPS solution was introduced into the above solution and stirred for 12 h at room temperature for the modification of P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂ tri-layer microspheres. The resultant MPS-modified tri-layer composite microspheres were purified by three cycles of centrifugation, decantation, and resuspension in ethanol and acetonitrile with ultrasonic irradiation and were finally suspended in 40 mL of acetonitrile.

Preparation of P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂/PEGDMA tetra-layer microspheres

The above MPS-modified tri-layer composite microspheres were used as seeds for the further stage distillation precipitation polymerization of EGDMA to result in P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂/PEGDMA tetra-layer microspheres. In a typical procedure, 0.008 g of AIBN and 0.40 mL of EGDMA were dissolved in the above 40 mL of acetonitrile suspension containing MPS-modified tri-layer composite seeds in a 50 mL of two-necked flask attaching with a fractioning column, Liebig condenser and a receiver. The reaction mixture was heated from ambient temperature to the boiling state within 10 min in a heating mantle and the reaction mixture was kept under refluxing state for further 10 min. The color of the mixture became white during the heating process and the solvent was distilled from the reaction system. The polymerization was stopped after 20 mL of acetonitrile was distilled off from the reaction system within 50 min. After the polymerization, the resultant tetra-layer microspheres were purified by repeating centrifugation, decantation, and resuspension in acetonitrile with ultrasonic irradiation for three times and finally dried in a vacuum oven at 50 °C until a constant weight was reached.

Preparation of hollow PEGDMA microspheres containing movable Au nanoparticles functionalized cores

The hollow PEGDMA microspheres containing movable Au nanoparticles functionalized cores were achieved by submerging the resultant P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂/PEGDMA tetra-layer microspheres in 10 wt% HF aqueous solution for 2 h. Then the excess HF and the newly formed SiF₄ gas were expelled out of the resultant hollow microspheres by several centrifugation, decantation cycles in water till the pH of the suspension at 7. The resultant hollow PEGDMA microspheres with Au nanoparticles functionalized cores were dried in a vacuum oven at 50 °C until a constant weight was reached.

Catalytic reduction of 4-nitrophenol to 4-aminophenol in an aqueous medium

Catalytic properties of hollow PEGDMA microspheres with Au nanoparticles functionalized cores were investigated via the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AnP) with NaBH₄ as the reductant under ambient temperature in aqueous solution as a model reaction. A typical experiment was carried out as follows: 0.048 g hollow PEGDMA microspheres with Au nanoparticles functionalized cores powder were dispersed in 10 mL water under ultrasonic irradiation. Then three drops of such suspension were mixed with 2 mL of 4-NP aqueous solution (0.1 mM, 2 × 10⁻⁷ mol) in a 10 mL of plastic pipe under ultrasonic irradiation. After 10 min, 4 mL of NaBH₄ (0.1 M, 4 × 10⁻⁴ mol) aqueous solution was introduced into the mixture with gentle shaking. The bright yellow solution faded gradually as the catalytic reaction proceeded. The catalytic activity was determined by a UV-vis spectrophotometer with a decrease at 399 nm in UV-vis absorption and a simultaneous increase in absorption at 297 nm, indicating the formation of 4-AnP.

To compare the catalytic properties of the hollow structures with pure Au nanoparticles, an aqueous solution of sodium citrate stabilized Au nanoparticles with a diameter of 23 nm was prepared according to the literature.²⁴ Typically, 100 mL of 0.25 mM HAuH₄ aqueous solution was heated to boiling state with mechanical stirring. Then, 1 mL of 5 wt% sodium citrate aqueous solution was quickly added. The reaction was maintained until the color of the reaction mixture changed into wine red. The wine red colored mixture was cooled to room temperature and centrifuged for 40 min at 8000 rpm to obtain concentrated Au nanoparticles aqueous solution. Then, 75% of the colorless supernatant was removed and the remaining solution was redispersed under gentle shaking. To study the catalytic activity of the as-obtained Au nanoparticles, three drops of this concentrated aqueous solution (containing 3.6 × 10⁻⁸ mol of Au atoms) were added into 2 mL of 0.1 mM 4-NP (2 × 10⁻⁷ mol) aqueous solution with gentle shaking. Then, 4 mL of 0.1 M NaBH₄ (4 × 10⁻⁴ mol) aqueous solution was introduced into the mixture with gentle shaking. The catalytic reaction was detected by UV-vis spectroscopy.

To determine the catalytic recycling properties of the hollow PEGDMA microspheres with movable Au nanoparticles functionalized cores within several minutes, both the concentration of 4-NP and catalyst were increased by 20 times compared with the above typical reduction. After complete reduction of 4-NP, the catalysts were separated by centrifugation (1.2 × 10⁴ rpm for 5 min) and redispersed in a new reaction system. The recovery of the catalyst was proven further by recycling it four times.

Characterization

The size and morphology of the resultant polymer microspheres were determined by transmission electron microscopy (TEM, FEI Technai-20 Germany). UV-vis spectroscopy was performed on a JASCO V-570 spectrometer ranging from 250 to 600 nm. Fourier-transform infrared spectra (FT-IR) were scanned over the range 400–4000 cm⁻¹ with a potassium bromide slide on a Bio-Rad FTS 135 FT-IR spectrometer. The zeta-potential was determined with Zeta Pals (Brookhaven Instrument Cooperation) by measuring the electrophoretic mobility of the particles using distilled water as the electrolyte.

Results and discussion

Scheme 1 illustrates the procedures for the preparation of P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂/PEGDMA tetra-layer microspheres and the corresponding hollow PEGDMA microspheres with movable Au nanoparticle functionalized cores. These procedures mainly included the distillation precipitation polymerization to afford the inner P(EGDMA-co-MAA)/P(EGDMA-co-VPy) core and outer PEGDMA shell-layer, the in situ reduction of HAuCl₄ with NaBH₄ as the reductant in the presence of P(EGDMA-co-MAA)/P(EGDMA-co-VPy) microspheres as the stabilizer, the controlled hydrolysis of TEOS in a water–ethanol solvent to get the sacrificial silica mid-layer, and the final formation of hollow PEGDMA microspheres with movable Au nanoparticles functionalized cores via the selective removal of the sandwiched silica layer with hydrofluoric acid.

Scheme 1 Synthesis of hollow PEGDMA microspheres with movable Au nanoparticles functionalized cores.

Synthesis of P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au microspheres

Pyridyl-functionalized microspheres are highly effective for the stabilization of Au nanoparticles without aggregation through the interaction between the functional groups and the gold atoms.²⁵ To get monodisperse polymer microspheres with pyridyl groups on the surface, two-stage distillation precipitation polymerization was used and the typical TEM micrographs of the resultant polymer microspheres were shown in Fig. 1A and 1B, respectively. The results indicated that P(EGDMA-co-MAA) (Fig. 1A) and P(EGDMA-co-MAA)/P(EGDMA-co-VPy) core-shell (Fig. 1B) microspheres had a spherical shape and smooth surface. The average diameter of P(EGDMA-co-MAA) microsphere was 225 nm and a monodispersity index (U) of 1.015 as summarized in Table 1. The loading capacity of the accessible carboxyl groups on the surface of P(EGDMA-co-MAA) microspheres was 4.00 mmol g⁻¹ with crosslinking degree of 0.50,²⁶ which were used as seeds for the second-stage distillation precipitation polymerization to afford monodisperse P(EGDMA-co-MAA)/P(EGDMA-co-VPy) core-shell microspheres via the efficient hydrogen bonding interaction between the carboxyl groups and the pyridyl groups²³ as illustrated in Scheme 1. Our previous work has demonstrated that the nature of the efficient interaction between carboxylic group and pyridyl group for the construction of core-corona composite is hydrogen bonding interaction rather than electrostatic interaction, which has been investigated by FT-IR spectra and the influence of pH on the stability of the raspberry-like polymer composite.²⁷ For the hydrogen bonding between these two groups, carboxylic acid group acted as the hydrogen donor, while the lone pair electron of pyridine behaved as the electron donor. The size of the P(EGDMA-co-MAA)/P(EGDMA-co-VPy) core-shell microspheres as tabulated in Table 1 was 237 nm with a narrow-dispersity index (U) of 1.013. This meant that the thickness of P(EGDMA-co-VPy) shell-layer was around 6 nm, which was calculated as half of the difference between the diameter of P(EGDMA-co-MAA) cores and that of the core-shell microspheres.

Table 1 Size and size distribution of the microspheres

Entry	D n /nm	D w /nm	U
P(EGDMA-co-MAA)	225	228	1.015
P(EGDMA-co-MAA)/P(EGDMA-co-VPy)	237	240	1.013
P(EGDMA-co-MAA)/P(EGDMA-co-VPy) @Au/silica	352	357	1.013
P(EGDMA-co-MAA)/P(EGDMA-co-VPy) @Au/silica/PEGDMA	451	457	1.013

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Fig. 1 TEM micrographs: (A) P(EGDMA-co-MAA); (B) P(EGDMA-co-MAA)/P(EGDMA-co-VPy); (C) P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au.

The formation of P(EGDMA-co-MAA)/P(EGDMA-co-VPy) core-shell microspheres were proven by FT-IR spectra as shown in Fig. 2a and 2b, in which the FT-IR spectra had a strong peak at 1733 cm⁻¹ corresponding to the characteristic stretching vibration of the carbonyl component of the ester groups for both P(EGDMA-co-MAA) and core-shell microspheres. Compared to the FT-IR spectrum of P(EGDMA-co-MAA) core in Fig. 2a, the FT-IR spectrum of the core-shell particles (Fig. 2b) showed an absorption peak at 1598 cm⁻¹, assigned to the typical vibration of the pyridyl groups.

Fig. 2 FT-IR spectra: (a) P(EGDMA-co-MAA); (b) P(EGDMA-co-MAA)/P(EGDMA-co-VPy); (c) P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂; (d) MPS-modified P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂; (e) P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂/PEGDMA.

The zeta potentials of the P(EGDMA-co-MAA) and P(EGDMA-co-MAA)/P(EGDMA-co-VPy) microspheres were further measured to confirm the core-shell structure of the latter. The results indicated that the zeta potentials were −41.4 mV for P(EGDMA-co-VPy) microspheres and 3.00 mV for P(EGDMA-co-MAA)/P(EGDMA-co-VPy) microspheres, respectively. The significant changes of the zeta potentials for these two microspheres further proved the successful encapsulation of the P(EGDMA-co-MAA) microspheres by the P(EGDMA-co-VPy) shell layer. All these results demonstrated the successful synthesis of monodisperse pyridyl-functionalized core-shell microspheres and the further possibility for the in situ reduction of HAuCl₄ with NaBH₄ as the reductant with functional core-shell microsphere as stabilizer to get P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au microspheres.

In our previous work, Au nanoparticles were prepared in situ by reduction of HAuCl₄ with NaBH₄ as the reductant via the interaction between Au nanoparticles and functional pyridyl groups on the surface of the polymer microsphere stabilizer.^25,28 Consequently, P(EGDMA-co-MAA)/P(EGDMA-co-VPy) core-shell microspheres with pyridyl groups on the outer shell-layer were utilized as stabilizers for in situ synthesis of polymer microsphere-stabilized Au nanoparticles in the present work. A TEM micrograph of core-shell microsphere-stabilized Au nanoparticles (P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au) is shown in Fig. 1C, in which the uniform distribution of the Au nanoparticles is clearly observed on the surface of core-shell microspheres in the absence of any aggregation. Nanometer-sized gold particles with a number average diameter of 4.5 nm were formed via the in situ reduction of HAuCl₄ with NaBH₄ as reductant. Careful inspection of the microsphere-stabilized Au nanoparticles in Fig. 1C indicated that some Au nanoparticles were buried under the surface of the polymer shell-layer, which may be due to the efficient stabilization effect of the pyridyl groups on the gel-layer of the functional core-shell polymer microspheres. This was very similar to the results in our previous works,^25,28,29 which implied that the pyridyl functional groups on both the surface and the soft gel-layer of the resultant core-shell microspheres were highly accessible for the efficient stabilization for the formation of Au metallic nanoparticles. The efficient stabilization of the pyridyl groups on the surface of P(EGDMA-co-MAA)/P(EGDMA-co-VPy) to Au nanoparticles was also proven by the extremely rapid formation of homogeneous Au metallic nanoparticles upon addition of NaBH₄ as the reductant without agglomeration.

Preparation of MPS-modified P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂ tri-layer microspheres

One of the most important problems in fabrication of organic/inorganic composite materials is the inherent incompatibility between the surface of organic and inorganic components. To solve such a problem, the encapsulation of the functional P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au microspheres by a silica outer-layer was achieved by a modified sol–gel hydrolysis of TEOS in an ethanol–water mixture via the acid–base interaction between the surface hydroxyl groups (acidic) of the silica component and the surface pyridyl groups (basic) of the functional P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au inner core in the present work.³⁰ A typical TEM micrograph of the resultant P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂ tri-layer microspheres is shown in Fig. 3A, which indicates that the tri-layer microspheres had a spherical shape with a cauliflower-like surface. The TEM micrograph in Fig. 3B with a much higher magnification proves the typical core-shell structure of the resultant composite microspheres, in which the Au nanoparticles were uniformly distributed on the junction of the polymer inner core and silica mid-layer. These results demonstrated that the complex interaction between the pyridyl groups and the Au nanoparticles was strong enough to stabilize the resultant Au nanoparticles during the further growth of the silica outer-layer by the controlled hydrolysis of TEOS in ethanol–water. Furthermore, the Au nanoparticles were clearly observed to fix on the surface and gel-layer of P(EGDMA-co-MAA)/P(EGDMA-co-VPy) inner core without any aggregation and mobility due to the efficient stabilization effect of the pyridyl groups to Au nanoparticles, which would ensure the good catalytic properties of Au nanoparticle functionalized cores inside the corresponding hollow polymer microspheres.

Fig. 3 TEM micrographs: (A) P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂; (B) P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂ with higher magnification; (C) P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂/PEGDMA; (D) hollow PEGDMA microspheres containing movable Au nanoparticle functionalized cores; (E) hollow PEGDMA microsphere containing movable Au nanoparticles functionalized core with higher magnification.

The average-diameter of the resultant P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂ tri-layer microspheres was considerably increased from 237 nm of the polymer inner core to 352 nm with a narrow-dispersity index of 1.013 as summarized in Table 1. This meant that the thickness of the silica shell-layer was 58 nm formed during the controlled hydrolysis of TEOS in the absence of any secondary-initiated small particles with a little rough surface morphology. The rough cauliflower-like morphology of the tri-layer composite microspheres was probably due to the low loading capacity of the pyridyl groups on the surface of P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au cores, which was proven by the significant effect of 4-VPy on the morphology of the resultant poly(styrene-co-4-VPy)/SiO₂ microspheres in the literature.³⁰ The coating of silica onto the P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au microspheres was also proven by the FT-IR spectrum as showed in Fig. 2c displaying a wide band at around 1094 cm⁻¹ corresponding to the typical stretching vibration of Si–O bond from silica component. All these results demonstrated that silica shell-layer were successfully encapsulated over P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au cores during the hydrolysis of TEOS via the acid–base interaction of the pyridyl groups on the surface of polymer inner cores and the hydroxyl groups of the silica component despite the presence of the coated-Au nanoparticles on the surface of polymer inner cores. In other words, the Au nanoparticles did not interfere with the controlled hydrolysis of TEOS for the formation of silica shell-layer for the tri-layer composite microspheres, although the complex interaction occurred between the Au nanoparticles and pyridyl groups for the stabilization of Au nanoparticles. At the same time, the hydrolysis of TEOS for the silica coating onto the gold functionalized polymer cores did not affect the efficient stabilization of the pyridyl groups to Au nanoparticles either.

For the further growth of the polymer outer shell-layer onto the silica surface of tri-layer composite microspheres, reactive vinyl groups should be incorporated onto the surface. The surface of P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂ was modified with reactive vinyl groups by MPS via a sol–gel self-condensation process of the hydroxyl groups from the silica components, which was similar to the modification of the silica particles to afford silica/polymer core-shell hybrid in our previous work.³¹ The formation of MPS-modified P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂ particles was confirmed by FT-IR spectrum as shown in Fig. 2d, which displays an absorption peak at 1652 cm⁻¹, assigned to the typical stretching vibration of the vinyl groups from the MPS component. The successful modification of the tri-layer composite microspheres having reactive vinyl groups provided the possibility for the further growth of outer polymer shell-layer via distillation precipitation polymerization.

Preparation of P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂/PEGDMA tetra-layer microspheres

In our previous work, core-shell silica/polymer hybrid microspheres with various functional shell-layers were synthesized by distillation precipitation polymerization in the presence of MPS-modified silica particles with reactive vinyl groups as seeds.³² In the present work, EGDMA was chosen as the monomer for the synthesis of polymer outer shell-layer over the MPS-modified P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂ seeds via distillation precipitation polymerization technique. A typical TEM micrograph of the resultant P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂/PEGDMA tetra-layer microspheres is shown in Fig. 3C, which indicated that the tetra-layer microspheres had a spherical shape with a smooth surface compared to the cauliflower-like morphology of P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂ tri-layer microspheres in Fig. 3A. The successful formation of a uniform PEGDMA outer shell-layer on the surface of the MPS-modified P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂ tri-layer microspheres was clearly observed from TEM characterization in Fig. 3C with a strong difference in contrast between the inorganic silica mid-layer (deep color), the P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au inner core (light color), and PEGDMA outer shell-layer (light color). In other words, an inorganic silica mid-layer was sandwiched between the polymer functionalized inner core and PEGDMA outer shell-layer via a four-stage reaction in the absence of any secondary particles, which implied that the reactive vinyl groups from MPS modification captured all the newly formed oligomers during the forth-stage distillation precipitation polymerization.

The average diameter of P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂/PEGDMA tetra-layer microspheres was significantly increased from 352 nm of tri-layer composite seeds to 451 nm with narrow-dispersity index (U) of 1.013. These results indicated that PEGDMA outer shell-layer with thickness of 50 nm was encapsulated onto the MPS-modified P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂ seeds through the forth-stage polymerization.

The successful encapsulation of PEGDMA onto the surface of MPS-modified tri-layer seeds was proven further by FT-IR spectrum as shown in Fig. 2e. Compared to Fig. 2d which shows MPS-modified P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂ microspheres, the wide band around at 1094 cm⁻¹ corresponding to the Si–O bond disappeared, which indicated the successful encapsulation of PEGDMA onto the MPS modified silica surface.

Preparation of hollow PEGDMA microspheres with movable Au nanoparticle functionalized cores

The sandwiched silica layers of the resultant P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂/PEGDMA tetra-layer microspheres were selectively removed by an etching process in HF aqueous solution to afford hollow PEGDMA microspheres with movable Au nanoparticles functionalized cores. The driving force for such removal was due to the formation of SiF₄ gas, which was given off from the microspheres during the etching process.

A typical TEM micrograph of hollow PEGDMA microspheres with Au nanoparticle functionalized movable cores is shown in Fig. 3D, which showed that the corresponding hollow PEGDMA microspheres maintained the original spherical shape without collapse, indicating that the PEGDMA with a thickness of 50 nm was thick enough to support the cavity formed during the selective etching of the silica mid-layer. The Au nanoparticle functionalized movable cores were clearly observed away from the center of the composite microspheres in Fig. 3D, whereas the functionalized P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au inner cores in Fig. 3C was exactly located in the center of the P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂/PEGDMA tetra-layer microspheres. In short, the Au nanoparticle functionalized inner cores were believed to be movable inside the cavity of the PEGDMA microspheres.

The Au nanoparticles were uniformly distributed on the surface of the movable inner cores without any aggregation in hollow PEGDMA microspheres from TEM observation with a much higher magnification in Fig. 3E. Furthermore, the colorless nature of the discarded solution after centrifugation during the selective removal silica mid-layer for the formation of hollow PEGDMA microspheres with Au nanoparticles functionalized movable cores indicated that the Au nanoparticles were not washed away, even in HF solution. These results demonstrated that only silica mid-layer was completely removed from the P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂/PEGDMA tetra-layer microspheres by an etching process in HF solution, while the Au nanoparticles were remained on the surface of the movable inner cores with uniform distribution with the aid of efficient stabilization effect of pyridyl groups on the surface and gel-layer of the functional P(EGDMA-co-MAA)/P(EGDMA-co-VPy) cores. In other words, the pyridyl groups on the surface of P(EGDMA-co-MAA)/P(EGDMA-co-VPy) cores played dual roles in the present work, i.e., the acid–base interaction for the construction of P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂ tri-layer composite microspheres and the efficient stabilization of the resultant Au nanoparticles during the whole synthetic procedures. Furthermore, the weight percent of Au nanoparticles in such hollow structures was estimated as 0.54 wt%, i.e. 2.77 × 10⁻⁵ mol Au atom per gram of hollow microsphere catalysts. These polymer stabilized Au nanoparticles could endow the hollow structures as catalyst discussed in the following.

Catalytic reduction of 4-nitrophenol to 4-aminophenol in aqueous solution

To study the catalytic activity of hollow PEGDMA microspheres with movable Au nanoparticle functionalized cores, the reduction of 4-NP to 4-AnP in aqueous medium with NaBH₄ as the reductant was selected as a model reaction. In the present work, addition of aqueous NaBH₄ solution to aqueous 4-NP solution in the presence of hollow PEGDMA microspheres with movable Au nanoparticle functionalized cores as a catalyst led to a significant decrease of the absorption peak at 399 nm in UV-vis spectra corresponding to 4-NP. During the reduction, the yellow color faded with the simultaneous formation of a new peak at 297 nm assigned to 4-AnP in UV-vis spectra in Fig. 4. The complete disappearance of the UV-vis absorption peak at 399 nm of 4-NP occurred at 780 s, which meant the complete reduction of 4-NP to 4-AnP. However, the reduction of 4-NP did not perform under the condition even with large excess amount of NaBH₄ in absence of the hollow PEGDMA microspheres with movable Au nanoparticles functionalized cores as catalyst, which was proven by the unchanged UV-vis absorption peak at 399 nm. Therefore, hollow PEGDMA microspheres with movable Au nanoparticles functionalized cores acted as catalytic micro-reactor sites to transfer the electrons from BH₄⁻ to 4-NP for the formation of 4-AnP in aqueous medium, where the catalytic reaction proceeded confined inside the hollow PEGDMA microspheres. Thus, this functional framework could behave as an effective micro-reactor, where the specific reactions could be carried out exclusively inside the cavities.

Fig. 4 UV-vis spectra of the catalytic reduction of 4-nitrophenol to 4-aminophenol developed by different reaction times.

To compare the catalytic properties of the hollow structures with pure Au nanoparticles, sodium citrate stabilized Au nanoparticles with diameter of 23 nm were chosen as the comparative catalyst. It should be pointed out that the total surface area of the sodium citrate stabilized gold particles in the catalytic reaction approximately equals that of the Au nanoparticles stabilized on the surface of P(EGDMA-co-MAA)/P(EGDMA-co-VPy) movable cores used above. Three parallel reactions were processed and the mean reaction time was 420 s, which indicated that the catalytic activity of the sodium citrate stabilized 23 nm Au nanoparticles is better than that (760 s) of the hollow microspheres with Au nanoparticle functionalized movable cores in the first catalytic cycle of the reduction. This is probably because the outer shell-layer blocks the permeation of the 4-NP molecule to the Au nanoparticles on the surface of movable cores. Despite this, the hollow polymer microsphere-stabilized Au nanoparticles still can be facilely separated from the reaction system by centrifugation while the pure Au nanoparticles can hardly be recovered. The hollow polymer microsphere-stabilized Au nanoparticles also showed better recycle-catalytic property after several reaction times as discussed in the following, while the pure Au nanoparticles would severely aggregate after one catalytic reaction according to the results from Yin's group.³³ Such aggregation would cause sharp decrease in the catalytic properties.

The catalytic recycling properties of the hollow PEGDMA microspheres with movable Au nanoparticle functionalized cores with higher levels of reactant and catalyst was quantitatively determined by the reaction time for the complete disappearance of UV-vis absorption peak at 399 nm corresponding to 4-NP as summarized in Table 2. These results demonstrated that the catalytic properties decreased slightly from 98 s to 165 s after the catalyst had been recycled four times via ultra-centrifugation from the original reaction system and used for the next catalytic reaction. Such decrease in activity originated from the slight agglomeration of Au nanoparticles on the surface of the movable cores during the catalytic reduction.²⁹ This may be due to the competitive coordination of the amino-groups in 4-AnP products with gold atoms, which decreased the stabilization effect of the pyridyl groups on the surface and gel-layer of the movable P(EGDMA-co-MAA)/P(EGDMA-co-VPy) cores.

Table 2 The reaction time for complete disappearance of the peak at 399 nm in UV-vis spectrum of 4-NP after the catalyst was recycled

Number of times the catalyst was recycled	Time for complete reaction/s
0	98
1	105
2	135
3	150
4	165

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Conclusion

Hollow PEGDMA microspheres with Au nanoparticle functionalized movable cores were prepared by the selective removal of the sandwiched silica-layer in aqueous HF solution from the corresponding P(EGDMA-co-MAA)/P(EGDMA-co-VPy)@Au/SiO₂/PEGDMA tetra-layer microspheres, which were synthesized by a combination of distillation precipitation polymerization for the preparation of the functional P(EGDMA-co-MAA)/P(EGDMA-co-VPy) inner core and PEGDMA outer shell-layer, the in situ reduction of HAuCl₄ with NaBH₄ as the reductant in the presence of pyridyl-functionalized microspheres as the stabilizer, and the controlled hydrolysis of TEOS for the formation of a sandwiched silica-layer. The pyridyl groups on the surface and gel-layer of P(EGDMA-co-MAA)/P(EGDMA-co-VPy) microspheres played dual roles during the synthesis of hollow PEGDMA microspheres with movable Au nanoparticles functionalized cores, i.e., the acid–base interaction with the hydroxyl groups of the silica component for the uniform coating of silica-layer and the efficient stabilization of Au nanoparticles via coordination interactions with Au atoms during the whole of the synthetic procedure. The Au nanoparticles stabilized by the pyridyl groups of the inner cores maintained their uniform distribution and catalytic properties through the hydrolysis of TEOS for the formation of silica mid-layer and the selective etching by HF solution, which enabled the Au nanoparticle functionalized movable cores in hollow PEGDMA microspheres to act as confined catalytic micro-reactor sites for the reduction of 4-NP to 4-AnP with NaBH₄ as reductant in aqueous medium. Since the pyridyl group could stabilize other metal nanoparticles such as Ag, Pt, Pd, Cu, and others,^34–37 this work provides a model strategy for the construction of micro-reactors composed of hollow polymer microspheres containing metal nanoparticles functionalized polymer movable cores as confined reaction sites.

Acknowledgements

This work was supported by the National Natural Science Foundation of China with contract project No.: 20874049.

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