One-pot preparation of pomegranate-like polydopamine stabilized small gold nanoparticles with superior stability for recyclable nanocatalysts

Abstract

A pre-mixing and post-polymerization strategy has been developed for one-pot preparation of pomegranate-like polydopamine (PDA) stabilized small gold nanoparticles (AuNPs) with sizes smaller than 5 nm in aqueous media. Firstly, the pre-mixing of dopamine (DA) and HAuCl₄ was used to generate the Au clusters as nucleating seeds. Secondly, the rapid self-polymerization of DA into polydopamine (PDA) and simultaneous Au redox reaction were initiated when the pH of the reaction system was tuned to 8 by the injection of a NaOH solution into the mixture, leading to the formation of the pomegranate-like PDA–AuNPs nanocomposites with small AuNPs well dispersed inside. The as-prepared nanocomposites exhibited superior stability in high saline conditions (500 mM NaCl) and were able to be coated onto a filter paper to form paper catalysts, which were used as recyclable catalysts for the reduction of high concentrated (50 mM) 4-nitrophenol (4-NP) with a TOF of 1006 h⁻¹.

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1. Introduction

Gold nanoparticles (AuNPs) have attracted considerable interests due to their amazing physicochemical properties and wide applications in catalysis, bioengineering, optics, etc., and a considerable variety of synthetic methods have been developed.¹ Size is one of the key parameters to determine the property of AuNPs.² The small AuNPs (generally less than 5 nm in diameter) can actively catalyze broad reactions such as hydrogenation, oxidation, epoxidation in mild conditions, and are very promising and attractive in catalysis applications.³ However, small AuNPs usually tend to aggregate together to form larger particles due to the high surface energy, which will certainly reduce the catalysis activities.⁴ To prevent the aggregation, various stabilizers have been developed to stabilize small AuNPs, such as silica,⁵ metal oxide,⁶ carbon,⁷ and etc.

Polymers have been widely used as stabilizing agents for small AuNPs.⁸ Generally, the fabrication process involves the mixing of polymers together with the Au precursors like HAuCl₄ and then followed with the in situ reduction into AuNPs. These polymers are designed to have strong interactions with the Au precursors and with the reduced AuNPs, and thus the polymer-coated small AuNPs are prepared. Up to now, many kinds of polymers, including the synthetic polymers with thiol,⁹ pyridine,¹⁰ carbazole¹¹ or amino¹² groups, and the biopolymers like DNAs,¹³ proteins¹⁴ have been intriguingly used as the stabilizing agents to get AuNPs smaller than 5 nm. However, these methods usually require the preparation of specific polymers in advance, which generally involves time-consuming synthesis steps. In addition, besides classical thiol-containing polymer stabilized small AuNPs,¹⁵ the stability of the synthetic polymer-stabilized small AuNPs in electrolyte solutions, especially in high concentrated electrolyte solutions like the buffers with many salts is still a challenge, and these small AuNPs tend to aggregate together due to the disability of the protective polymers on the shells in high saline conditions.

Polydopamine (PDA), a bio-inspired polymer synthesized by the oxidation of the neurotransmitter dopamine (DA), has received great interests because of its strong adhesive ability.¹⁶ In addition, PDA is also capable to reduce many noble metals (such as Au, Ag, Pt) and form metal NPs. Thus, PDA-functionalized AuNPs (PDA–AuNPs) can also be prepared by mixing PDA and HAuCl₄ together, followed with the reduction of AuNPs and the adhesion between PDA and AuNPs.¹⁷ As a pioneering work,¹⁸ Messersmith et al. even got PDA–AuNPs by simple mixing the DA derivatives like the PEGylated DAs with HAuCl₄, which provides a facile way to fabricate polymer-functionalized AuNPs with no need of synthesizing polymers in advance. Liu and coworkers also used the graphene/PDA–AuNP nanocomposites to stabilize the AuNPs.¹⁹ PDA has shown great potentials in the preparation of stable AuNPs, however, to our knowledge the reported PDA-stabilized AuNPs are usually equal or larger than 5 nm.

Herein, we have developed a pre-mixing and post-polymerization strategy based on catechol redox chemistry for one-pot preparation of PDA stabilized AuNPs smaller than 5 nm in aqueous media. As illustrated in Scheme 1, DA and Au precursor were first mixed in water for a few minutes at ambient condition, then sodium hydroxide (NaOH) solution was injected into the mixture to tune the pH value of system to 8 and initiate the self-polymerization and redox reaction to form nanocomposites (PDA–AuNPs). The AuNPs inside the nanocomposites are smaller than 5 nm, and exhibit superior stability in high saline condition (500 mM NaCl). In addition, the nanocomposites can be loaded on the filter paper for recyclable reduction of highly concentrated 4-NP (50 mM) to 4-aminophenol (4-AP). Compared with the literatures, the present work shows a novel and facile way to prepare polymer-stabilized AuNPs with a size smaller than 5 nm and with good catalysis properties.

Scheme 1 Schematic illustration of the preparation of PDA–AuNPs for recyclable catalysis of the reduction of 50 mM 4-NP.

2. Experimental section

Materials

3-Hydroxytyramine hydrochloride (dopamine) was purchased from Acros and used without purification. Tetrachloroauric(III) acid hydrate (HAuCl₄), sodium hydroxide (NaOH), and sodium borohydride (NaBH₄) were purchased from Sinopharm Chemical Reagent Co., Ltd. 4-Nitrophenol was purchased from Aladdin Industrial Inc. The glassware used in this paper was cleaned with freshly prepared aqua regia (3 : 1 of HCl : HNO₃) and rinsed thoroughly by water prior to use. The water used in all experiments was prepared in Milli-Q purification system and had a resistivity greater than 18.2 MΩ cm.

Preparation of PDA–AuNPs

Typically, 1.92 mL of dopamine hydrochloride (1 mg mL⁻¹) and 4.12 mL of tetrachloroauric(III) acid hydrate (1 mg mL⁻¹) were mixed in a flask and kept stirring for 2 min. Then, the NaOH solution (1 M) was injected to adjust the pH of the mixture to a proper value. Unless otherwise specified, the reaction was carried out at ambient condition for 1 day. The kinetics of the preparation of PDA–AuNPs was measured by UV-visible spectrophotometer and TEM. For UV-vis, the dilute reaction solution at a 10 : 1 ratio was added to a 1 cm quartz cuvette for the measurements, deionized ultrapure water standard was used for the reference beam. For TEM, the reaction solution with different reaction times was directly used for TEM observation without any purification.

Anti saline study

Typically, 29.22 mg NaCl were added to 1 mL suspensions of PDA–AuNPs and mixed to reach final [NaCl] = 500 mM. Spectral scans were performed over the 400–800 nm range of wavelengths in the UV-visible region of the electromagnetic spectrum.

Preparation of PDA–AuNPs coated filter paper

The filter paper was cut into small piece with 1.0 cm wide, 1.3 cm long. With the stirring for better dispersion, 10 μL of PDA–AuNPs was gently dropped on the small piece of filter paper for further usage.

Catalytic study

Typically, 4-nitrophenol (1.0 mL, 100 mM) was mixed with fresh NaBH₄ solution (1.0 mL, 7500 mM) in a 5 mL flask with gentle shaking at room temperature, then two pieces of catalysts coated paper was carefully immersed in order to obtain a fully accessible surface. The molar ratio of 4-NP/NaBH₄/Au was 6038/452898/1. The reduction reaction was carried out at room temperature and determined by UV-vis spectroscopy. The absorbance change between 280 nm and 550 nm was recorded. The recyclable experiments were also carried out under the same condition.

Material characterization

Transmission electron microscopy (TEM) imaging was implemented using a Tecnai G2 spirit Biotwin (FEI, USA) with a Gatan 832 microscope operated at an acceleration voltage of 120 kV and JEM 2100F (JEOL, Japan) with a Gatan detector at an acceleration voltage of 200 kV. SEM the scanning electron microscopy (SEM) imaging were obtained using a JSM-7401F (JEOL Ltd., Japan) field emission scanning electron microscope operated at an acceleration voltage of 5 kV. The Fourier transform infrared spectroscopy (FTIR) spectra were obtained using performed on PerkinElmer Spectrum 100 FITR Spectrometer at room temperature. X-ray diffraction (XRD) patterns were obtained using a Bruker D8 Advance instrument with Cu Kα radiation operated at 40 kV and 40 mA. The scan range was 6° min⁻¹ from 20° to 80° (2θ). The UV-vis absorption spectra were obtained with a UV/V-16/18 (Mapada, China). The atomic force microscopy (AFM) imaging was obtained using a Nanonavi E-Sweep (SII, Japan). The thermogravimertic analysis (TGA) experiments were performed on PerkinElmer TGA 7 at a heating rate of 10 °C min⁻¹ under a dry air flow. The Au content of the nanocatalysts was analysed by using inductively coupled Plasma Optical Emission Spectrometer (ICP-OES, iCAP6300, Thermo). DLS and zeta potential measurements were performed on aqueous solutions with a Malvern Zetasizer Nano S (Malvern Instruments, Ltd.) equipped with a 4 mW He–Ne laser light operating at =633 nm. The pH value was obtained using a digital display pH meter (PHS-3TC), made by Shanghai reaches instrument limited company.

3. Results and discussion

At first, the pre-mixing of DA and HAuCl₄ was performed at a solution pH of 2. As shown in Fig. S1 in the ESI,† the absorption peaks around 390 nm and 670 nm appeared and kept stable after DA and HAuCl₄ were mixed together within 1–6 min according to the UV-vis spectra, which indicated the formation of very small Au clusters.²⁰ As at such an acidic condition (pH = 2), the self-polymerization of DA would be almost inhibited¹⁶ and only a small amount of quinone moieties would be formed, which leaded to the formation of small Au clusters.²¹ It was proposed that the Au cluster could be used as nucleating seeds to synthesize small AuNPs. Thus, we selected a pre-mixing time of two minutes for post-polymerization to make sure that the mixture was stopped at the level of Au clusters.

It is well known that the self-polymerization of dopamine can be realized by increase the solution pH.¹⁶ Herein, NaOH solution was employed to adjust the pH of the reaction system of DA and HAuCl₄ into 8 after premixing for 2 min. As shown in Movie S1 in the ESI,† the color of system turned to dark purple within seconds after the injection of NaOH solution, suggesting the reaction was initiated rapidly. The reaction process was monitored by using UV-vis spectrometer. The pronounced adsorption around 500 nm (less than 525 nm) appeared after reacted for 5 min (Fig. 1a), which indicated the formation of AuNPs smaller than 5 nm.⁹ Meanwhile, self-polymerization of PDA also happened quickly. According to the literatures,²² the absorption intensity at 400 nm with time was generally used to evaluate the kinetics of polymerization, which was also shown in Fig. 1b. The absorbance sharply grew within 30 min and increased slowly after 180 min (3 h), then kept the same absorbance from 1 day (24 h) to 1 month, which indicated that the PDA self-polymerization process in our reaction system mostly occurred within 3 h. In the present work, the reaction was performed for 24 h in order to make sure the completion of the self-polymerization. It should be noted that the adsorption of AuNPs around 500 nm seemed to be weaker and weaker after reacted for 30 min, which was probably due to the overlap of it with the adsorption of the newly-formed PDA through self-polymerization. Nevertheless, there was no sharp plasmon absorption bands appearing near 525 nm in the spectra of the reactants even reacted for 1 month, which indicated that the average size of the as-prepared AuNPs was smaller than 5 nm.⁹

Fig. 1 (a) UV-vis spectra of the reaction system of DA and HAuCl₄ after reacted for different time at pH = 8. (b) Time evolution of the absorbance at 400 nm.

The solution after reacted for 1 day looked homogeneous and was directly measured by dynamic light scattering (DLS) after diluted 50 times. The results showed the formation of colloids in this reaction system with the hydrodynamic diameter (D_h) of 25 nm (Fig. S2a, ESI†) and zeta potential of −27 mV. After the reactants were kept for 1 month in ambient condition, the solution was still homogeneous with no precipitates, and the colloids with the similar size (Fig. S2b, ESI†) and zeta potential were still observed. These results indicated the stable colloidal particles were formed in our reaction system, which might be attributed to the electrostatic repulsion between them. It should be noted that some flocculations would be formed if we still extended the storage time. However, the flocculation could be re-dispersed in water after stirred strongly.

The colloidal particles were further characterized by SEM, AFM and TEM. The SEM and AFM images (Fig. 2a and b) showed the particles were spherical and around 25 ± 4 nm by statistical calculation of 100 particles from the SEM image (Fig. 2a) and 26 nm from AFM measurement (Fig. 2b). Furthermore, the morphology and size of the particles were kept after stored for 1 month according to the SEM measurements (Fig. S4a, ESI†). These data were well consistent to the DLS results as mentioned above (Fig. S2, ESI†). The TEM image showed that the colloidal particles obtained after reacted for 1 day were pomegranate-like nanocomposites with AuNPs enveloped and well dispersed inside the PDAs (Fig. 2c). The AuNPs inside the nanocomposites were around 2.8 ± 0.3 nm and 2.8 ± 0.4 nm in diameter by the statistical calculation of 100 particles after reacted for 1 day and kept for 1 month in ambient condition respectively (Fig. 2d and e). In addition, the HRTEM image of AuNPs inside the PDA–AuNP nanocomposites was taken (Fig. S3, ESI†), which showed small AuNPs below 5 nm and with good crystallinity were formed based on the visible lattice fringes in the AuNPs. To our knowledge, the pomegranate-like PDA stabilized small AuNPs nanostructure as shown in Fig. 2c has seldom been reported before. It should be noted that the PDA–AuNP nanocomposites looked to be aggregated together to some extent during the SEM, AFM and TEM measurements, which might be caused by the solvent-evaporation induced aggregation during the sample preparation procedures since the colloidal particles are stable in solution according to the DLS results (Fig. S2, ESI†).

Fig. 2 The SEM (a), AFM (b) and TEM (c) images of PDA–AuNP nanocomposites obtained via 1 day's reaction at pH = 8. (d) and (e) The size distributions of small AuNPs inside PDA–AuNP nanocomposites obtained at different reaction time of 1 day (d) and 1 month (e) through the statistical analyses of 100 particles from the TEM images.

The as-prepared PDA–AuNP nanocomposites were further characterized by XRD, FTIR and TGA. The XRD pattern (Fig. S4b, ESI†) of PDA–AuNPs showed characteristic peaks of metallic gold (38.4°, 44.4°, 64.8° and 77.8°, JCPDS, card no. 04-0784), which confirmed the successful reduction of Au(III) to Au(0). The FTIR spectrum of PDA–AuNPs was similar to that of pure PDAs (Fig. S4c, ESI†), which supported the formation of PDA in the PDA–AuNP nanocomposites through self-polymerization. Based on the feed ratio, the content of Au should be 56% if all DAs were reacted with all Au moieties. The TGA curve (Fig. S4d, ESI†) revealed that the PDA–AuNP nanocomposites contained around 57% of metallic Au or 43% of PDA, and it agreed well with the ICP-OES analysis which showed the Au content of 56.58 ± 0.13 (wt%) in the nanocomposites (calibration line in Fig. S5, ESI†). Both of them were almost equal to the Au loading as expected from the stoichiometry of the reaction. Hence, the reaction of PDA–AuNPs system is highly efficient. These evidences together with the abovementioned morphology characterizations indicate the successful preparation of pomegranate-like PDA–AuNP nanocomposites with the size of AuNPs smaller than 5 nm.

The stability of the AuNPs is very important for the final applications. To track the stability, herein, the PDA–AuNPs nanocomposites prepared with different reaction time were measured by TEM. As shown in Fig. S6 in the ESI,† the pomegranate-like PDA–AuNPs nanocomposites were formed and kept at different reaction time in the range from 5 min to 1 month. The AuNPs were effectively separated from each other by PDAs in the nanocomposites, and no clear aggregation of AuNPs was observed. Meanwhile, we also calculated the size of the AuNPs by statistical analysis of 100 particles from each TEM image in Fig. S6 in the ESI.† The mean size of AuNPs was 2.8 ± 0.5 nm when reacted for 5 min. While with the increasing of reaction time from 15 min, 30 min, 1.5 h, 3 h, 6 h, 1 day to even 1 month, the mean size of AuNPs was still around 2.5–3.0 nm (Fig. S7, ESI†). The size statistical results of the AuNPs in the nanocomposites obtained after reacted with 1 day and 1 month were shown in Fig. 2d and e. These data agreed well with the UV-vis spectra as shown in Fig. 1b. Since the AuNPs were smaller than 5 nm in the PDA–AuNP nanocomposites, there was no sharp plasmon absorption band appearing near 525 nm in the UV-vis spectra⁹ when reacted for 1 day and 1 month. In other words, the AuNPs prepared in this work are very stable, and can be kept without aggregation inside the nanocomposites at least for one month.

For the formation of PDA–AuNP nanocomposites with small AuNPs, the premixing process DA and HAuCl₄ at pH = 2 is very important. If DA and HAuCl₄ were reacted directly at pH = 7.2 without a premixing process, the obtained AuNPs were around tens of nanometers in size as mentioned in the literatures.²³ As shown in the work, the premixing process could generate very small Au nanoclusters, and these nanoclusters were supposed to be the nucleating seeds for the subsequent growth of AuNPs. As we know, the smaller is the nucleating seeds, the smaller is the crystals. These Au nanoclusters formed during the premixing process were useful to obtain the small size of final AuNPs.

Besides, the rapid self-polymerization of PDA is also very important for the formation of final PDA–AuNPs nanocomposites. As shown in Fig. 1b, almost 40% of self-polymerization of DA was completed after reacted for 5 min when the solution pH was adjusted to 8. The rapidly formed PDA would be attached onto the AuNPs and prevented the further aggregation of AuNPs. That was the reason why the small AuNPs below 5 nm were formed in the PDA–AuNPs nanocomposites when reacted for 5 min at pH = 8, and such a small-sized AuNPs were kept even after reacted for 1 month. As a further support, a control experiment by the injection of NaOH solution into the DA and HAuCl₄ mixture (after premixing at PH = 2 for 2 min) to tune the pH of the mixture to 4 was performed. In such an acidic condition, the self-polymerization of DA is very slow.¹⁶ After reacted for one day, the TEM image (Fig. S8a, ESI†) showed the formation of PDA–AuNPs nanocomposites with the size of AuNPs around 6.6 ± 0.8 nm. The nanocomposites were different from the pomegranate-like structure and looked more like a core–shell structure. The TGA measurement showed there were 28% of PDA in the obtained PDA–AuNPs (Fig. S8d, ESI†), which was lower than 44% of PDA calculated from the feed ratio. In other words, only a part of DAs were transferred into PDA and then attached into AuNPs after reacted for 1 day at pH = 4 due to the low self-polymerization process. In addition, the stability of AuNPs was not good, and the as-prepared AuNPs would be further aggregated together after reacted for 1 month at pH = 4, and some larger AuNPs with the size around 30–40 nm were found (Fig. S8b, ESI†). Additionally, the UV-vis spectra also showed the new plasmon absorption peak appeared around 564 nm (Fig. S8c, ESI†) after kept for 1 month in ambient condition, which also proved the aggregation of AuNPs. This relatively weak stability was similar with small molecular which was not effective for the stabilization of AuNPs.²⁴ Possibly, it was attributed to the poor polymerization degree of dopamine at acidic condition, which cannot construct a robust shell to prevent the growth and aggregation between AuNPs. Evidently, according to the control experiments, the rapid self-polymerization of PDA at pH = 8 is an important precondition for the formation of pomegranate-like PDA–AuNP nanocomposites with the AuNPs smaller than 5 nm.

In general, metal nanoparticles are unstable in electrolyte solutions,²⁵ which will impede their applications, such as in the buffer and high saline condition. Thus, the stability of the PDA–AuNP nanocomposites in electrolyte solution was further evaluated by adding salts into them. The UV-vis spectrum was employed to characterize the anti-saline ability.²⁶ As shown in Fig. S9a in the ESI,† there was no significant changes in the spectra when the NaCl concentration in the system was adjusted up to 500 mM after stabilized for 30 min. According to literatures,²⁶ the ratio of absorbance at 510 nm and 700 nm (A₅₁₀/A₇₀₀) can be used to evaluate the stability of AuNPs against time. Herein, it was found that no clear change of A₅₁₀/A₇₀₀ value was observed after adding 500 mM of NaCl into the PDA–AuNP nanocomposite dispersion within 2 h (Fig. S9b, ESI†). These results indicated the pomegranate-like PDA–AuNP nanocomposites prepared in this work showed excellent salinity stability, which was probably attributed to the fact that the small AuNPs were well capped and separated in the cross-linked network of PDA polymers.¹⁶ This special property makes it possible to effectively catalyze the reaction in high concentrated reactant system.

Nanoparticle-based heterogeneous catalysts in liquid phase reaction have been made significant achievement in recent years,^1c but many reactions are performed in diluted systems. Obviously, a liquid reaction system with high concentrated reactants will increase the production efficiency and reduce waste production. To our knowledge, till now, there are few reports on the construction of recyclable nanocatalysts with effective activity for high concentrated reactant system. Herein, the as-prepared pomegranate-like PDA–AuNP nanocomposites might be possible to address this challenge because of the good stability in high saline aqueous media. In addition, the cross-linking PDA polymers with plenty of hydrophilic groups in the PDA–AuNP nanocomposites might have a porous hydrogel structure,²⁷ which is useful for the mass transformation during the catalysis. Benefiting from the excellent adhesive property of PDA, the cellulose filter paper was chosen as model scaffold for loading PDA–AuNP nanocomposites in present work. The preparation of catalytic paper was very easy just involving dropping 0.01 mL nanocomposite dispersion (Fig. 3a) onto the paper (typically 1.0 cm wide and 1.2 cm long), and a circular black layer of catalysts was formed on the paper after the evaporation of the solvent (Fig. 3b). The PDA–AuNPs loaded paper was further measured by TEM. The TEM image of the cross section of catalytic paper showed the nanocomposites were fully covered on the surface of the paper (Fig. 3c) and small AuNPs with good dispersity could be cleared observed from the inset of Fig. 3c. Above data indicates the successful preparation of PDA–AuNPs loaded catalytic paper.

Fig. 3 (a) The digital photograph of the aqueous PDA–AuNP dispersion (prepared at pH = 8). (b) Photographs of PDA–AuNPs loaded catalytic paper. (c) The TEM image of the cross section of the PDA–AuNPs loaded catalytic paper; the inset is the magnification of the red boxed site in image (c).

The reaction of 4-nitrophenol (4-NP) reduced by NaBH₄ into 4-aminophenol (4-AP) is widely used to evaluate the catalytic activity of nanocatalysts. Usually, the concentration of 4-NP used in the previous reports is not more than 0.5 mM (Table S1, ESI†). Herein, the catalytic performance of the as-prepared PDA–AuNPs loaded paper was examined through the reduction of 50 mM 4-NP to 4-AP, which is 100 times higher than the commonly used one in catalysis studies. The absorbance at 295 nm increased and absorbance at 400 nm decreased when the catalytic paper was immersed into the reaction system, corresponding to the formation of 4-AP at 295 nm and the reduction of 4-nitrophenolate ion at 400 nm in the presence of NaBH₄ respectively (Fig. 4a and b). Certainly, the reduction did not occur without the catalytic paper (Fig. 4b). This reduction reaction can be analyzed as a pseudo-first-order reaction for 4-NP because the concentration of NaBH₄ are greatly excessive (75 times). The linear relationship (coefficient of determination R² = 0.99, Table S2, ESI†) between ln(C_t/C₀) and reaction time showed in Fig. 4c was well in accordance with the first-order reaction kinetics. The turnover frequency (TOF) of 4-NP reduction was used to evaluate the efficiency of as-prepared catalysts. The TOF in this system is 1006 h⁻¹ when the conversion reaches 90%, which indicates the as-prepared catalytic paper has a good catalytic activity in a high 4-NP concentration (50 mM). Generally, the molar ratio of 4-NP/Au in catalytic systems (C_4-NP < 0.5 mM) reported in the literatures was less than 100 : 1. However, it reached 6038/1 in this work, which was 60 times higher than the usual one and indicated that the Au atoms in our catalytic system were economically utilized. The kinetic constant K of our catalysis system was calculated to be 0.057 min⁻¹, which was smaller than those reported in the literature (Table S1, ESI†). It should be probably ascribed to the fact that the AuNPs were encapsulated inside PDA network in our nanocatalysts and the reactants should diffuse through PDA network to the surface of AuNPs, which slowed down the diffusion of reactants of 4-NP ions.

Fig. 4 Catalytic performance of PDA–AuNPs (prepared at pH = 8) loaded catalytic paper. (a) Time dependent UV-vis spectra of the reaction system catalyzed by the catalytic paper. (b) The variation of absorbance at 400 nm with the reaction time in the presence or absence of the catalytic paper. (c) The plot of ln(C_t/C₀) versus reaction time for the catalytic reaction. (d) The conversion efficiency of 4-NP to 4-AP within 3 h for 10 consecutive reaction cycles with the catalytic paper.

Furthermore, the recyclable performance was tested by repeatedly using the same catalytic paper 10 times for the reduction reaction. Between each cycle, the catalytic paper was just removed from the finished reaction system and immersed into fresh reaction system without any other treatments. Almost identical conversion efficiency of 100% from 4-NP to 4-AP was obtained in ten successive cycles (Fig. 4d). Meanwhile, the kinetic constant K and R² for each catalysis cycle were calculated during the revision, and the results were summarized in Table S2 (ESI†). With the increase of catalysis cycle from 1 to 10, K gradually decreased from 0.057 to 0.030 min⁻¹. We believed the decrease of K originated from the partial collapse of PDA gels in the PDA–AuNP nanocomposites in each catalysis cycle since a high osmotic pressure would be generated under such a high concentrated electrolyte solutions (50 mM 4-NP, 3.75 M NaBH₄), which could certainly slow down the diffusion of the reactants of 4-NP ions. Nevertheless, the AuNPs prepared in this work maintained the catalytic activity well in ten successive cycles in high concentrated reactants condition. In addition, the R² of the ten catalysis cycle was close to 1 to support the first-order reaction kinetics. All of these results support the super stability of our AuNP catalysts.

4. Conclusions

In summary, a pre-mixing and post-polymerization strategy has been successfully developed for one-pot preparation of PDA stabilized small AuNPs in aqueous media. Compared with literatures, the preparation process of AuNPs reported here is quite simple just by mixing the commercially available dopamine with HAuCl₄ and NaOH, and no complex synthesis of polymer precursors is needed. In addition, the as-prepared PDA–AuNPs has the advantages of small size (<5 nm), superior stability without aggregation during the storage in ambient condition even at high saline condition (500 mM NaCl), and effectively recyclable catalysis of high concentrated reactants (50 mM 4-NP) in liquid system. We believed the method presented here might be extended to prepare other kinds of metal nanoparticles with small size. Furthermore, the strong adhesive property of PDA suggests the PDA–AuNP nanocomposites as-prepared in this work could be loaded onto many types of substrates, which might further extend their application in gold-catalyzed reactions.

Acknowledgements

We thank the National Basic Research Program (2013CB834506), China National Funds for Distinguished Young Scholar (21225420), National Natural Science Foundation of China (21474062, 21374062, 91527304) and “Shu Guang” project supported by Shanghai Municipal Education commission and Shanghai Education Development Foundation (13SG14) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional figures, table and movie. See DOI: 10.1039/c6ra05902c

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