Au–polymer hybrid microgels easily prepared by thermo-induced self-crosslinking and in situ reduction

Abstract

This work presents a facile method to prepare Au–polymer hybrid microgels through thermo-induced self-crosslinking and in situ reduction of a gold precursor. Self-assembly of poly(2-dimethylaminoethyl methacrylate-co-3-(trimethoxysilyl)propyl methacrylate) (P(DMAEMA-co-TMSPMA)) produced particles by heating the P(DMAEMA-co-TMSPMA) solution above the lower critical solution temperature (LCST), which were subsequently cross-linked via the hydrolysis and condensation of the methoxysilyl groups to form microgels. The polymer microgels can be used as a reducing agent for in situ reduction of a gold precursor and a stabilizing agent of gold nanoparticles, leading to the formation of Au–polymer hybrid microgels. The size of the Au–polymer hybrid microgels can be adjusted by varying the concentration of the polymer solution. Furthermore, Au–polymer hybrid microgels were used as a catalyst for the reduction of 4-nitrophenol, which exhibited catalytic performance and reusability towards the reaction.

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Introduction

Recently, Au nanoparticles (NPs) have attracted extensive attention due to their applications in catalysis, biosensors, photothermal therapy and drug delivery, etc.^1–7 However, Au NPs tend to aggregate because of their high surface energy, resulting in a remarkable reduction in properties, such as catalytic activity.^8,9 To address this problem, it is necessary to stabilize Au NPs. Organic and inorganic matrixes have been used to stabilize Au NPs, such as polymer nanoparticles, polydopamine, silica nanoparticles and dendrimers.^10–13 As an ideal carrier system of Au NPs, microgels provide attractive features, such as permeability and mechanical properties.¹⁴

A conventional method for preparing Au–polymer hybrid microgels is to modify gold nanoparticles via precipitation polymerization. It has been reported that encapsulation of gold nanoparticles in a microgel was realized by the growth of a thin polystyrene (PS) shell on gold nanoparticles and precipitation polymerization of N-isopropylacrylamide (NIPAM) on the PS shell in the presence of a cross-linker.¹⁵ Au nanoparticle can be encapsulated in a hollow microgel through the precipitation polymerization of NIPAM on Au–SiO₂ particle and etch the silica midlayer.¹⁶ Gold nanoparticles have also been embedded in phenylboronic acid-containing polymer microgels for catalytic applications.¹⁷ However, the gold nanoparticles need to be pre-prepared and sometimes need to be pre-modified before precipitation polymerization in this method, which is time-consuming. Thus, in situ reduction of Au³⁺ in preformed microgels has attracted increasing attention. Lu et al.¹⁸ synthesized gold–microgel nanocomposite by the chemical reduction of gold salt–microgel mixture with sodium borohydride. Akamatsu et al.¹⁹ demonstrated that gold nanoparticles could be deposited on surface of poly(2-vinylpyridine)-based microgels through chemical reduction. A large number of Au NPs were homogeneously incorporated into the thiol-functionalized microgels through in situ reduction of the Au precursor.²⁰ Despite the benefit of omitting the process of pre-preparing Au NPs, microgels still need to be pre-prepared. Our group previously reported thermo-responsive Au–polymer hybrid microgels prepared by a temperature-induced co-aggregation and self-crosslinking (TICASC) method.²¹ No complex operations such as preforming of microgels or prior modification of gold nanoparticles are required, however, the pre-preparation of gold nanoparticles is still indispensable.

Herein, we propose a simple and feasible method to prepare Au–polymer hybrid microgels through temperature-induced self-crosslinking and in situ reduction of gold precursor. The thermo-responsive poly(2-dimethylaminoethyl methacrylate-co-3-(trimethoxysilyl)propylmethacrylate) (P(DMAEMA-co-TMSPMA)) was synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization. While the polymer solution was heated above the lower critical solution temperature (LCST) of P(DMAEMA-co-TMSPMA), the aggregation of polymer chains induced the formation of colloidal particles and then the methoxysilyl groups hydrolyzed and condensed to form microgels. Au NPs were facilely loaded into polymer microgels through in situ reduction of HAuCl₄ using polymer microgels as both reducing agent of gold precursor and stabilizing agent of gold nanoparticles. Compared with conventional methods for preparing Au–polymer hybrid microgels, polymer microgels were prepared by a facile thermo-induced self-assembly and self-crosslinking method without need of additional crosslinking agent; Au NPs were obtained through in situ reduction of Au³⁺ in microgels, and no additional reductant was required. In addition, the obtained Au–polymer hybrid microgels exhibited catalytic activity for the reduction of 4-nitrophenol by NaBH₄ at room temperature.

Experimental

Materials

2-Dimethylaminoethyl methacrylate (DMAEMA, Aldrich, 99%) was purified by passing through Al₂O₃ column and then distilled under reduced pressure prior to use. 3-(Trimethoxysilyl) propyl methacrylate (TMSPMA) was purified by distillation under reduced pressure. Azobis(isobutyronitrile) (AIBN) was purified by recrystallization from ethanol, and 4-(4-cyanopentanoic acid) dithiobenzoate (CPADB) was synthesized according to the literature.²² All other reagents were of analytical grade and used as received.

Instruments and characterization

The ¹H NMR measurements were performed on a BrukerAV300 NMR spectrometer instrument with CDCl₃ as solvent and tetramethylsilane (TMS) as an internal reference. GPC was used to determine the molecular weight and molecular weight distribution on a Waters 515 instrument equipped with two microstyragel columns and a refractive index detector (RI 2414) at 35 °C. The monodispersed polystyrene standards were used in the calibration of molecular weight and molecular weight distribution, DMF was used as eluent at a flow rate of 1.0 mL min⁻¹. Transmission electron microscope (TEM) observations were performed on a Hitachi H-7650 transmission electron microscope at an accelerating voltage of 100 kV. For preparing the TEM samples, a drop of the dispersion was dripped on a copper grid and then dried at room temperature. Dynamic light scattering (DLS) measurements were conducted on a DynaPro light scattering instrument (DynaPro-99E) at 25 °C with 824.3 nm laser. The scattered light was measured at the angle of 90° and collected by the autocorrelator after 2 min. UV-Vis measurements were performed on a Unico UV-vis 2802PCS spectrophotometer. Thermogravimetric analyses (TGA) were conducted on a TA TGA-2950 apparatus under nitrogen at a heating rate of 10 °C min⁻¹. X-ray photoelectron spectroscopy (XPS) measurement was performed on an X-ray photoelectron spectrometer (Thermo ESCALAB 250).

Synthesis of P(DMAEMA-co-TMSPMA)

DMAEMA (2.0 g, 13 mmol), TMSPMA (340 mg, 0.13 mmol), AIBN (6 mg, 0.036 mmol), CPADB (72 mg, 0.26 mmol) and THF (2 mL) were added into a 10 mL glass tube. The tube was degassed by three freeze–pump–thaw cycles and sealed under vacuum. The sealed tube was placed in an oil bath at 70 °C for 8 h, and the polymerization was subsequently quenched by immersion in liquid nitrogen. The polymer solution was diluted with THF and then poured into excess petroleum ether while stirring. The precipitation was dissolved in THF, and the dissolving–precipitation procedure was repeated three times. The resulting product was dried under vacuum overnight at room temperature. The chemical structure of the product was determined to be P(DMAEMA₄₆-co-TMSPMA₅) based on ¹H NMR analysis.

Preparation of polymer microgels and Au–polymer hybrid microgels

P(DMAEMA-co-TMSPMA) (8.0 mg) was dissolved in deionized water (2.0 mL) and then added into a glass tube. The P(DMAEMA-co-TMSPMA) solution was heated to 70 °C to induce self-assembly. After 40 min, HAuCl₄ aqueous solution (16.0 μL, 100 mg mL⁻¹) was added to the solution. The mixed solution was allowed to stir at 70 °C for 0.5 h, and then cooled to room temperature.

Catalytic performance and reusability of Au–polymer hybrid microgels

To study the catalytic properties of the Au–polymer hybrid microgels, the reduction of the 4-nitrophenol (4-NP) by NaBH₄ was chosen as a model reaction. A typical experiment was carried out as follows: 4-NP aqueous solution (0.1 mM, 2.0 mL) and 0.5 mL NaBH₄ aqueous solution (30 mM) were added into a quartz cell and the mixture was incubated for 5 min. The suspension of Au–polymer hybrid microgels (0.05 mL, 2.0 mg mL⁻¹, containing 4.86 × 10⁻⁵ mmol Au) was introduced into the mixture. The UV-Vis spectroscopy was used to monitor the reaction. Au–polymer hybrid microgels were recovered by centrifugation (10 000 rpm for 10 min) after the reaction was finished. The reusability of Au–polymer hybrid microgels was investigated by repeating the reaction under the same condition.

Results and discussion

Synthesis and characterization of P(DMAEMA-co-TMSPMA)

Thermo-responsive poly(2-dimethylaminoethyl methacrylate-co-3-(trimethoxysilyl) propylmethacrylate) (P(DMAEMA-co-TMSPMA)) was synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization using as chain transfer agent (Scheme 1). The ¹H NMR spectrum of P(DMAEMA-co-TMSPMA) is shown in Fig. S1,† and the signals at δ = 7.3–7.9 ppm (a–c) are assigned to the phenyl protons in CPADB, the proton signals of the ester methylene appeared at 3.8–4.2 ppm (d and e). The degree of polymerization (DP) of P(DMAEMA-co-TMSPMA) is 51, which was calculated based on the integral values of the signals appeared at 3.8–4.2 and 7.3–7.9 ppm. According to the integral values of the signals of methyl next to nitrogen appeared at 2.2–2.4 ppm (g) and ester methylene appeared at 3.8–4.2 ppm, the molar ratio of TMSPMA and DMAEMA units in the resultant polymer is about 5 : 46 and M_n,NMR is 9100 g mol⁻¹. Fig. 1A shows that the GPC trace of P(DMAEMA-co-TMSPMA) is monomodal, and M_n,GPC and polydispersity index are 4950 g mol⁻¹ and 1.24, respectively. It is noted that M_n,NMR of P(DMAEMA-co-TMSPMA) is higher than M_n,GPC, the difference between M_n,NMR and M_n,GPC is possibly ascribed to adsorption of the nitrogen containing polymer onto the GPC columns during the GPC analysis.²³

Scheme 1 Synthesis of P(DMAEMA-co-TMSPMA) through RAFT polymerization.

Fig. 1 (A) GPC trace of P(DMAEMA-co-TMSPMA), (B) the temperature-dependent transmittance of P(DMAEMA-co-TMSPMA) solution (4.0 mg mL⁻¹).

The thermo-responsive behaviour of P(DMAEMA-co-TMSPMA) was investigated using turbidity experiments by measuring the optical transparency at 600 nm. The temperature dependence of the transmittance for the aqueous solution of P(DMAEMA-co-TMSPMA) (4.0 mg mL⁻¹) was used to determine the LCST of polymer. As shown in Fig. 1B, the decrease of transmittance indicates the transformation from dissolved polymer chains to colloid particles. The LCST of P(DMAEMA-co-TMSPMA) was 42 °C, which was the temperature at which the transmittance dropped to 50% of its original value, and the thermo-induced phase transition was presumably attribute to the fact that the DMAEMA units undergo the hydrophilic to hydrophobic transition.²⁴

Preparation of polymer microgels and Au–polymer hybrid microgels

Stimuli-responsive polymers are capable of exhibiting reversible or irreversible changes in physical properties and/or chemical structures to small changes in external environment.²⁵ When aqueous solution of P(DMAEMA-co-TMSPMA) (2.0 mL, 4.0 mg mL⁻¹) was placed in a water bath at 70 °C, the transparent solution became opaque quickly as shown in Fig. 2B(b). As mentioned above, the hydrophilicity/hydrophobicity of P(DMAEMA-co-TMSPMA) was affected by the temperature. While the polymer solution was heated above the LCST of P(DMAEMA-co-TMSPMA), the aggregation of polymer chains induced the formation of colloidal particles. After 40 min, the opaque solution cannot return to transparent solution after cooling the solution to room temperature. It indicates that P(DMAEMA-co-TMSPMA) can easily self-crosslinked upon heating, because of the hydrolysis–condensation of methoxysilyl groups. Generally, the hydrolysis and condensation of the methoxysilyl groups need additional catalyst such as triethylamine,²⁶ while in the present work, the cross-linked microgels were obtained quickly without additional catalyst, which may be ascribed to heating and the weak electrolyte PDMAEMA. Heating can increase the condensation rate of methoxysilyl groups, moreover, the weak polyelectrolyte PDMAEMA can be used as a catalyst for the hydrolysis–condensation of methoxysilyl groups.^27,28

Fig. 2 (A) Mechanism of forming Au–polymer hybrid microgels through thermo-induced self-assembly, self-crosslinking and in situ reduction. (B) Images of P(DMAEMA-co-TMSPMA) solution (2.0 mL, 4.0 mg mL⁻¹): (a) at room temperature, (b) after being heated at 70 °C for 40 min, (c) heating at 70 °C for 30 min after addition of HAuCl₄ solution (16.0 μL, 100 mg mL⁻¹).

Transmission electron microscopy (TEM) image of the microgels assembled from aqueous solution of P(DMAEMA-co-TMSPMA) (2.0 mL, 4.0 mg mL⁻¹) is shown in Fig. S2.† The microgels were spherical in shape, and the average diameter is about 650 nm.

Recently, there have been numerous reports utilizing amine-containing polymers as reducing agent to prepare Au NPs.^29,30 PDMAEMA can be utilized to reduce Au³⁺ and stabilize the resulting gold nanoparticles.³¹ In the present work, Au–polymer hybrid microgels were prepared by using the preformed polymer microgels as both reducing agent of gold precursor and stabilizing agent of Au NPs. While HAuCl₄ aqueous solution (16.0 μL, 100 mg mL⁻¹) was added to the suspension of polymer microgels at 70 °C, the color of the mixture became wine red after 30 min (Fig. 2B(c)). The UV-Vis spectroscopy was used to monitor this procedure, and the result is shown in Fig. 3. Absorbance in 450–600 nm appeared after 5 min, which is assigned to the surface plasmon resonance (SPR) of gold NPs, indicating the formation of Au NPs.³¹ The absorption is blue-shifted with increasing reaction time. After 30 min of the reaction, no further change of λ_max value can be observed, indicating that the reduction of HAuCl₄ was completed. It was noted that at room temperature, longer time is needed for the reduction of HAuCl₄ to Au NPs by PDMAEMA.³² In this work, 30 min is taken for the complete reduction of HAuCl₄ by microgels at 70 °C because heating can increase the reaction rate as previous reports.³³ Transmission electron microscope (TEM) images of Au–polymer hybrid microgels were shown in Fig. 4, and it can be observed that Au NPs are homogeneously embedded in the polymer microgels.

Fig. 3 Time-dependent UV-visible absorption spectra of mixture of HAuCl₄ solution (16.0 μL, 100 mg mL⁻¹) and polymer microgels prepared from P(DMAEMA-co-TMSPMA) solution (2.0 mL, 4.0 mg mL⁻¹).

Fig. 4 TEM images of the Au–polymer hybrid microgels prepared via the thermo-induced self-crosslinking and in situ reduction of gold precursor in aqueous solution: (A) P(DMAEMA-co-TMSPMA) (2.0 mL, 1.0 mg mL⁻¹) and HAuCl₄ (4.0 μL, 100 mg mL⁻¹), (B) P(DMAEMA-co-TMSPMA) (2.0 mL, 2.0 mg mL⁻¹) and HAuCl₄ (8.0 μL, 100 mg mL⁻¹), (C) P(DMAEMA-co-TMSPMA) (2.0 mL, 4.0 mg mL⁻¹) and HAuCl₄ (16.0 μL, 100 mg mL⁻¹), (D–F) show the magnified TEM images of (A–C), respectively. DLS curves of the above mentioned Au–polymer hybrid microgels: (a) D_h = 282 nm, (b) D_h = 555 nm and (c) D_h = 761 nm.

In a control experiment, the mixture of HAuCl₄ (16.0 μL, 100 mg mL⁻¹) and P(DMAEMA-co-TMSPMA) (2.0 mL, 4.0 mg mL⁻¹) (the pH of the mixture is about 2.7) was heated at 70 °C. The solution became wine red after 30 min, however, no suspension was found in the process. As shown in Fig. S3,† only Au NPs were obtained, and no microgels can be observed. This can be attributed to protonation of the tertiary amine in the DMAEMA units after the addition of HAuCl₄, which increase the hydrophilicity of P(DMAEMA-co-TMSPMA), so that it cannot self-assemble into nanoparticles at 70 °C.³⁴ The result indicates that HAuCl₄ need to be added after the formation of microgels in order to obtain Au–polymer hybrid microgels in this method.

X-ray photoelectron spectroscopy (XPS) analysis has been used to investigate the elemental composition of materials and determine the interaction at the interface.³⁵ Fig. S4† exhibited the XPS spectra of polymer microgels (N 1s) and Au–polymer hybrid microgels (Au 4f and N 1s). The Au 4f peaks for Au–polymer hybrid microgels are shown in Fig. S4(A),† and the peaks were attributed to Au 4f_7/2 (at around 81.9 eV) and Au 4f_5/2 (at around 85.9 eV). It exhibited negative shifts in the Au 4f binding energies (BE) compared to that of bulk Au (83.9 and 87.8 eV).³⁵ The N 1s peaks of polymer microgels (curve a) and Au–polymer hybrid microgels (curve b) are illustrated in Fig. S4(B),† the BE of N 1s partly shifted from 398.7 eV to 401.2 eV (curve b). The shift in Au 4f BE and N 1s BE may be due to the interaction between the DMAEMA units and Au NPs,³⁶ and it possibly plays an active role in stabilizing Au NPs in the microgels. We also investigated the influence of P(DMAEMA-co-TMSPMA) concentration on the formation of Au–polymer hybrid microgels. The concentrations of P(DMAEMA-co-TMSPMA) solution (2.0 mL) were 1.0, 2.0 and 4.0 mg mL⁻¹, respectively, and the volume of HAuCl₄ aqueous solution (100 mg mL⁻¹) was set at 0, 8.0 and 16.0 μL, correspondingly. The feed molar ratio of HAuCl₄/P(DMAEMA-co-TMSPMA) is constant for avoiding its influence on the size of the Au–polymer hybrid microgels. As shown in Fig. 4A–F, the average diameters of the Au–polymer hybrid microgels are about 230, 440 and 640 nm, respectively, which increased with the increasing concentration of P(DMAEMA-co-TMSPMA). The diameter of gold nanoparticles in the Au–polymer hybrid microgels prepared under these conditions is about 3–8 nm. DLS results (Fig. 4a–c) also demonstrated that the size of the Au–polymer hybrid microgels can be adjusted by varying the concentration of P(DMAEMA-co-TMSPMA) solution. The hydrodynamic diameters of the Au–polymer microgels are larger than those determined by TEM, because the samples for TEM characterization are in the collapsed state while those for DLS are in the swollen state.

The effect of ratio of HAuCl₄/P(DMAEMA-co-TMSPMA) on the content of Au NPs in Au–polymer hybrid microgels was investigated by XRD, TGA, and TEM measurements. Au–polymer hybrid microgels were prepared from the mixture of P(DMAEMA-co-TMSPMA) solution (2.0 mL, 2.0 mg mL⁻¹) and HAuCl₄ aqueous solution (100 mg mL⁻¹) (the volume was 0, 8.0 and 16.0 μL, respectively). The powder XRD patterns of Au–polymer hybrid microgels are shown in Fig. 5A. The 2θ diffraction peaks appearing at 38°, 45°, 65° and 78° correspond to Au (111), Au (200), Au (220) and Au (311) crystal planes (JCPDS 4-0783). The observation of the diffraction pattern demonstrates the formation of crystalline gold. Moreover, the diffraction intensity of Au–polymer hybrid microgels increases with the increasing ratio of HAuCl₄/P(DMAEMA-co-TMSPMA). Au–polymer hybrid microgels were further studied by TGA, and the results are shown in Fig. 5B. The Au–polymer hybrid microgels remained 9%, 18% and 24% of the original weight as the volume of HAuCl₄ aqueous solution (100 mg mL⁻¹) increase from 0, 8.0 to 16.0 μL, respectively. TEM images of Au–polymer hybrid microgels, which were prepared from P(DMAEMA-co-TMSPMA) (2.0 mL, 2.0 mg mL⁻¹) and HAuCl₄ (16.0 μL, 100 mg mL⁻¹), are shown in Fig. 6. Compared with the TEM image shown in Fig. 4E, the content of Au NPs in hybrid microgels is higher (Fig. 6B), indicating that the content of Au NPs in Au–polymer hybrid microgels can be regulated by varying the ratio of HAuCl₄/P(DMAEMA-co-TMSPMA).

Fig. 5 XRD patterns (A) and TGA curves (B) of Au–polymer hybrid microgels prepared from P(DMAEMA-co-TMSPMA) (2.0 mL, 2.0 mg mL⁻¹) and HAuCl₄ aqueous solution (100 mg mL⁻¹): (a) 0 μL, (b) 8.0 μL, (c) 16.0 μL.

Fig. 6 (A) TEM images of the Au–polymer hybrid microgels prepared from P(DMAEMA-co-TMSPMA) (2.0 mL, 2.0 mg mL⁻¹) and HAuCl₄ solution (16.0 μL, 100 mg mL⁻¹). (B) A magnified TEM image of the Au–polymer hybrid microgels and a HRTEM image of Au NP.

Catalytic performance of the Au–polymer hybrid microgels

The catalytic performance of the Au–polymer microgels was studied via the reduction of 4-nitrophenol (4-NP) by NaBH₄ at room temperature. After addition of NaBH₄ into the 4-NP aqueous solution in the absence of a catalyst, a strong absorption peak at 400 nm corresponding to 4-NP was observed in the UV-Vis spectrum.³⁷ The color of the mixture did not change and the absorption peak of 4-aminophenol (4-AP) did not appear after 24 h. Once a certain amount of Au–polymer hybrid microgels was added to the reaction solution, the color of the solution changed from bright yellow to colorless gradually. UV-Vis spectroscopy was used to monitor the reaction. As shown in Fig. 7, the absorption peak at ∼400 nm decreases gradually and a new absorption peak at ∼300 nm appears representing the production of 4-aminophenol (4-AP). In addition, the plot of the ln(C_t/C₀) versus reaction time is shown in Fig. S5.† The linear relationship indicates that the reaction follows the first-order kinetics. The reusability of the Au–polymer hybrid microgels was investigated by repeating the catalytic reduction. The Au–hybrid microgels were separated by centrifugation (10 000 rpm, 3 min) after the reaction was finished, and then utilized for the next cycle of catalysis. Even after 6 times, the Au–polymer hybrid microgels exhibited similar catalytic performance (Fig. S6†). It indicates that Au–polymer hybrid microgels exhibit activity and reusability as catalyst in the reduction of 4-NP to 4-AP.

Fig. 7 Time-dependent UV-Vis spectra for the reduction of 4-nitrophenol by NaBH₄ in the presence of Au–polymer hybrid microgels.

Conclusions

In summary, Au–polymer hybrid microgels were prepared by the thermo-induced self-crosslinking and in situ reduction of gold precursor. Self-assembly of P(DMAEMA-co-TMSPMA) induced colloid particles and the hydrolysis–condensation of methoxysilyl groups leaded to formation of microgels. Au–polymer hybrid microgels were prepared by the in situ reduction of gold precursor in the presence of microgels. The size of Au–polymer microgels and the content of Au NPs in Au–polymer microgels can be adjusted by varying the concentrations of P(DMAEMA-co-TMSPMA) and the ratio of HAuCl₄/P(DMAEMA-co-TMSPMA), respectively. The Au–polymer hybrid microgels can be used as catalyst for the reduction of 4-nitrophenol. Considering their catalytic activity and reusability, the Au–hybrid microgels have potential applications in catalysis.

Acknowledgements

This work was support by the National Natural Science Foundation of China (no. 21525420 and 21374107) and the Fundamental Research Funds for the Central Universities (WK 2060200012).

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR spectrum of P(DMAEMA-co-TMSPMA), TEM images, XPS spectra, plots of ln(C_t/C₀) versus time. See DOI: 10.1039/c6ra07864h

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