Dongmei Wanga,
Bingxin Liub,
Jianhua Lüa and
Changli Lü*a
aKey Laboratory of Nanobiosensing and Nanobioanalysis at Universities of Jilin Province, College of Chemistry, Northeast Normal University, Changchun 130024, P. R. China. E-mail: lucl055@nenu.edu.cn; Fax: +86 431 85098768; Tel: +86 431 85099236
bSchool of Mechanical Engineering, Qinghai University, Xining 810016, P. R. China
First published on 5th April 2016
We herein report the synthesis of well-defined novel thermo-responsive diblock copolymers, poly(N-isopropylacrylamide-co-2,3-epithiopropyl methacrylate)-block-poly(poly(ethylene glycol) methyl ether methacrylate) P(NIPAM-co-ETMA)-b-P(PEGMA), bearing an episulfide moiety as a ligand via reversible addition fragmentation chain transfer (RAFT) polymerization, followed by assembly into micelles in aqueous solution. The diblock copolymer-stabilized gold nanoparticles (Au NPs@P) could be further obtained by a Au3+-induced episulfide opening reaction and in situ reduction of HAuCl4. The block copolymer played a great role in the stabilization of Au NPs. These Au NPs@P were employed as an efficient catalyst and presented excellent catalytic activity to reduce 4-nitrophenol to 4-aminophenol. The content of PETMA segments, different ratios of Au atoms to sulfur atoms (MRs) of Au NPs@P1 and the dosage of catalytic carriers and reductants had varying degrees of influence on the catalytic activity. It was found that the catalytic activity of Au NPs@P increased with the decreasing content of PETMA segments. The size of the diblock copolymer-stabilized Au NPs also increased and the catalytic activity became higher with the increase of the MRs. The Au NPs@block copolymer with PNIPAM blocks also exhibited excellent temperature-responsive behavior for the catalytic reduction of 4-nitrophenol.
The preparation of Au NPs in polymeric micelles in an aqueous medium has been reported by the direct self-assembly approach.23–25 The majority of block copolymer self-assembly methods depend on the application of amphiphilic block copolymers, including polystyrene-block-poly(D,L-lactide) (PS-b-PLA),4 polystyrene-b-poly(acrylic acid) (PS-b-PAA),26 poly boc-L-tryptophan acryloyloxyethyl ester-b-poly(ethylene glycol) monomethyl ether acrylate [P(L-Trp-HEA)-b-P(PEGMA)],27 poly(2-(methacryloyloxy)ethyl phosphorylcholine)-b-poly(N-isopropylacrylamide-co-2-(N,N-dimethylamino)ethyl methacrylate) (PMPC-b-P(NIPAM-co-DMA)),28 polystyrene-b-polyethylene oxide (PS-b-PEO)29 and so on. Another method is to directly use the double-hydrophilic block copolymer (DHBC) and metal compounds in aqueous solutions to form hybrid micelles. For example, the use of poly(ethylene oxide)-b-poly(acrylic acid) (PEO-b-PAA),18 poly(2-vinylpyridine)-poly(ethylene oxide) (P2VP-b-PEO),23 di-alkylated polyethylenimines (PEI-1R),30 and polyethyleneoxide-polyethyleneimine (PEO)n-b-PEI31 has been reported. The block copolymers with the above examples can improve the stability of Au NPs. Among these polymers, P(PEGMA) or PEO-based block copolymers as a stabilizer are the most attractive representatives because they can endow Au NPs with good water solubility and high colloidal stability. Taking into account biological applications and the protection of the environment, water is by far the most environmentally friendly medium, and it is essential to develop polymer templates in water medium to synthesize and stabilize nanoparticles.
Stimuli-responsive polymers, also known as smart polymers, have attracted considerable attention as they can exhibit discontinuous changes in physical properties upon receiving environmental stimuli such as solvent exchange, temperature, pH, light, a magnetic or electric field, ion factors, chemical or biological molecules, and mechanical stress. These responses are expressed as the nature of the material.32 Among the stimuli-responsive polymers, poly(N-isopropylacrylamide) (PNIPAM) is the most popularly studied thermo-responsive polymer, which can undergo a reversible coil-to-globule phase transition in water around its lower critical solution temperature (LCST) due to a fine hydrophobic–hydrophilic balance in its structure.33,34 Wu et al. found that the hydrophobic and hydrophilic interaction will have different degrees of impact on the LCST in the block copolymer system.35 Smart materials can be found in several application areas such as active delivery in the biomedical sciences, sensors, catalysis and nanoelectronics.17,36 Over the last decade, thermo-responsive block copolymer-stabilized Au NPs have received much attention,37 for example, they can be used as a catalyst in thermo-responsive catalytic systems and the catalytic activity can be regulated by the temperature.38,39
Block copolymers containing atoms or groups capable of coordinating with gold can be attached to the surface of gold nanoparticles to improve the stabilization of the nanoparticles. A lot of electron-rich atoms or groups including carboxylate,18 thiol,40 sulfur,28 nitrogen41 and amino42 can coordinate with gold. In these groups, thiol is widely employed as an anchoring group via a Au-to-thiol bond. However, in theoretical analysis it is very easy to form two sulfur bonds which may enhance the crosslinking of the polymers. It is well known that episulfide compounds have interesting characteristics for chelating heavy metal ions via a ring-opening reaction under acidic or basic conditions.43 The episulfide monomer of 2,3-epithiopropyl methacrylate (ETMA) as a ligand is very stable and has a strong coordination ability for metal. In previous studies the copolymers of ETMA and vinyl monomers have been synthesized and used as reversible reducing and chelating agents and ion exchange membranes.44–46 Until now, no work has been reported for using the new techniques in the polymerization of ETMA.
Herein, we firstly reported a new type of episulfide group-containing thermo-responsive block copolymer, which can be used as effective ligands for the synthesis of Au NPs with enhanced catalytic activity for the reduction of 4-nitrophenol (4-NP). As shown in Scheme 1, the novel block copolymer ligands composed of a biocompatible PEGMA block and a thermo-responsive block consisting of a random copolymer of NIPAM and episulfide monomer of ETMA were synthesized via reversible addition fragmentation chain transfer (RAFT) polymerization. RAFT polymerization was selected due to the fact that this controlled living radical polymerization technique can be performed under mild reaction conditions, and has a high tolerance towards functional groups.47 We synthesized three block copolymers of P(NIPAM-co-ETMA)-b-P(PEGMA) with different contents of PETMA segments, P(NIPAM53-co-ETMA10)63-b-P(PEGMA)111 (P1), P(NIPAM48-co-ETMA27)75-b-P(PEGMA)113 (P2) and P(NIPAM42-co-ETMA35)77-b-P(PEGMA)114 (P3), respectively. The resulting block copolymers can be dispersed in water and self-assembled to form polymer micelles because the block polymer contains hydrophobic PETMA segments, and a stable micelle system is formed because the hydrophilic block is larger than the hydrophobic block. The PETMA segments in P(NIPAM-co-ETMA)-b-P(PEGMA) interact with the Au precursor, whereas the PPEGMA block stabilizes the Au NPs in aqueous solution. As shown in Scheme 2, the diblock copolymer-stabilized Au NPs were obtained through the in situ reduction of gold precursors with NaBH4 in polymer micelle solution because strong acidic HAuCl4 can induce episulfide group ring-opening to form a complex of Au3+–S–P(NIPAM-co-ETMA)-b-P(PEGMA). Through the study we found that the catalytic activity of the diblock copolymer-stabilized Au NPs for 4-NP was dependent on the content of PETMA segments due to the limitation of the reaction diffusion. Especially, the size of the Au NPs could be controlled by adjusting the molar feed ratios (MRs, the ratios of Au atoms to sulfur atoms in the copolymers). Another advantage of our design is that the Au hybrid particles can be stabilized by the water-soluble PEGMA block as the shell, ensuring that the current system has excellent colloidal stability in aqueous solution under the reaction conditions. The resulting Au NPs obtained using P(NIPAM-co-ETMA)-b-P(PEGMA) as a stabilizer had a superior colloidal stability and their potential as catalysts for the reduction of 4-NP in aqueous solutions was investigated. Furthermore, the temperature-dependent catalytic behavior for the reduction of 4-NP based on the Au NPs@block polymers with PNIPAM chains as catalysts was also studied in this work.
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Scheme 2 Schematic illustration of the self-assembled structures of block copolymers and Au NPs@block copolymers formed in situ in aqueous solution. |
[NIPAM]![]() ![]() ![]() ![]() ![]() ![]() |
[PEGMA]![]() ![]() ![]() ![]() |
Time (h) | GPCa | 1H NMR | |||||
---|---|---|---|---|---|---|---|---|---|
Mn,GPCa (g mol−1) | PDI | Mn,NMRb (g mol−1) | NNIPAMc | NETMAc | NPEGMAc | ||||
a Measured by GPC in DMF.b Obtained from 1H NMR.c Number of P(NIPAM-co-ETMA)-CTA and PPEGMA repeated units obtained from 1H NMR. | |||||||||
A1-CTA | 660![]() ![]() ![]() ![]() ![]() ![]() |
15 | 7900 | 1.17 | 7600 | 53 | 10 | ||
A2-CTA | 630![]() ![]() ![]() ![]() ![]() ![]() |
15 | 8300 | 1.13 | 9700 | 48 | 27 | ||
A3-CTA | 560![]() ![]() ![]() ![]() ![]() ![]() |
15 | 9000 | 1.21 | 10![]() |
42 | 35 | ||
P1 | 400![]() ![]() ![]() ![]() |
20 | 63![]() |
1.45 | 63![]() |
111 | |||
P2 | 400![]() ![]() ![]() ![]() |
20 | 65![]() |
1.24 | 65![]() |
113 | |||
P3 | 400![]() ![]() ![]() ![]() |
20 | 66![]() |
1.31 | 66![]() |
114 |
Fig. 1 shows the FT-IR spectra of A1-CTA, P1 and the Au precursor-containing P1 (Au3+@P1). A1-CTA displays a strong carbonyl absorption at 1653 cm−1 (amide I) and 1547 cm−1 (amide II) of the PNIPAM segments.55 The small peak at 1728 cm−1 is ascribed to the stretching vibration of carbonyl groups of PETMA segments, while the stretching vibration of the episulfide three-membered ring of the PETMA units can be observed at 615 cm−1.56 This observation indicates that A1-CTA has been successfully synthesized by RAFT copolymerization of NIPAM and ETMA. For P1 as shown in Fig. 1b, the characteristic peaks of carbonyl groups become stronger due to the stretching vibrations for CO in the ester groups of PPEGMA blocks. Clearly, the new peak that appeared at about 1112 cm−1 is assigned to the stretching vibrations for C–O–C in the ether groups of PPEGMA blocks, indicating the successful conjugation of PEGMA into P(NIPAM-co-ETMA)-CTA. In addition, the interaction between the episulfide groups in the PETMA segments and the Au precursors can be carefully confirmed by Fig. 1c. After the addition of the Au precursor, the peak at 615 cm−1 is considerably diminished due to the strong coordination interaction between the Au precursor and the episulfide group. This interaction can induce an episulfide opening reaction to form a sulfur–gold bond (S–Au3+) (Scheme S1, ESI†). Further evidence will be provided below.
The chemical structure of P(NIPAM-co-ETMA)-CTA was ascertained by 1H NMR analysis as shown in Fig. 2a and the ESI (Fig. S1a and S2a†). The characteristic signals of the PNIPAM and PETMA segments are clearly observed at 6.4 (a), 4.0 (b) and 3.18 ppm (d). By comparing the integral area ratio of peaks d (the methine proton on the episulfide ring, –CH2CH(CH2)–S–) and e (the methine proton in the PNIPAM block, CO–N–CH(CH3)2), the repeating units of the PNIPAM block can be calculated to be 53. The repeating unit of PETMA is determined to be 10 for each initiate site. For the typical 1H NMR spectra (Fig. 2) of A1-CTA and P1, it can be seen that several new peaks appear in the spectrum of P1 (Fig. 2b) (for P2 and P3, see Fig. S1b and S2b, ESI†), except for the peaks ascribed to A1-CTA. The chemical shifts around 3.67 (j), 3.4 (k) and 0.78–0.93 ppm (l) are the characteristic signals of the PPEGMA blocks. Hence, the appearance of these new peaks demonstrates that the P(NIPAM-co-ETMA)-PPEGMA diblock copolymers have been successfully synthesized. To testify the occurrence of the episulfide opening reaction induced by gold ions in aqueous solution, the 1H NMR spectra of A1-CTA and Au3+@A1-CTA using D2O as a solvent are presented in the ESI (Fig. S3†). It can be clearly seen that there are characteristic signals of episulfide groups (d, 1H, SCH–; c, 2H, SCH–CH2) in Fig. S3a,† while the signals at these chemical shifts disappear and shift to other locations after the episulfide opening reaction induced by gold ions as shown in Fig. S3b,† which is direct evidence for the gold ion-induced episulfide opening reaction to form a sulfur–gold bond (S–Au3+).
The molecular weights of P1, P2 and P3 can be determined by GPC and 1H NMR, respectively. The molecular weight obtained from GPC testing is a relative value, which was calibrated with linear polystyrene standards. The molecular weight of the polymers and the lengths of the P(NIPAM-co-ETMA)-CTA and PPEGMA blocks were also determined on the basis of the 1H NMR integral area ratio of typical protons attributed to the P(NIPAM-co-ETMA)-CTA and P(PEGMA) blocks. The results of the molecular weights of the three block copolymers with different unit numbers of ETMA are summarized in Table 1. The measurements of molecular weights are in good agreement with the theoretical values predicted from the ratio of monomer-to-initiating sites, indicating the successful synthesis of P(NIPAM-co-ETMA)m-b-P(PEGMA)n (m ≈ 63, 75, 77, n ≈ 111, 113, 114). From the summary of Table 1, we can see that the composition of the P(NIPAM-co-ETMA) block in the diblock polymers with a similar length of PPEGMA blocks can be controlled by adjusting the molar ratio of monomers. Moreover, the polydispersity of the diblock polymers is relatively low. These results show that the RAFT polymerization is controlled well and the block copolymers are well defined.
The lower critical solution temperature (LCST) of P1-CTA is around 25 °C, determined from the temperature-dependent optical transmissions at 550 nm (Fig. 3), while the LCST of PNIPAM is around 33 °C.34 The LCST of P1-CTA is obviously lower than that of the pure PNIPAM, which can be explained by the hydrophobic interactions of the PNIPAM and PETMA segments. Moreover, the LCST of P1 is higher (about 30 °C) than that of P1-CTA, which should be attributed to the increased hydrophilicity of the polymer component with the incorporation of PEGMA blocks.35 As shown in Fig. S4 of the ESI,† the aqueous solution of P1 appears to be semi-transparent white after the phase transition with a higher temperature above 30 °C.
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Fig. 3 Temperature dependence of optical transmittance at 550 nm for A1-CTA (a) and P1 (b) aqueous solutions (sample concentration: 12 mg mL−1). |
TEM was used to investigate the morphology of the self-assembling aggregates when P1, P2 and P3 were dispersed in aqueous solution. It is clearly seen from Fig. 4a–c that polymer micelles (M1, M2 and M3) were formed by the direct self-assembly approach in aqueous solution because the block copolymers contain a certain amount of PETMA hydrophobic segments. For the P(NIPAM-co-ETMA)-b-P(PEGMA) diblock polymer, the PEGMA block can dissolve fully in water, while the PETMA segment has bad solubility. In this way, water becomes a solvent of selectivity to the diblock polymer. So the differences in the solubility of different segments become the main factor in judging whether there micelles are forming in the aqueous solution.57 In addition, these diblock copolymer self-assembled micelles are spherical in shape. Moreover, a relatively narrow size distribution was observed for M1, M2 and M3 with an average diameter of 20.8 ± 2.7, 25.6 ± 4.5 and 38.0 ± 8 nm, respectively (Fig. 4d–f). The micelles were found to have different sizes depending on the ratio of segments of PNIPAM and PETMA. When the ratio of segments of PNIPAM and PETMA is relatively small, the formed micelle has a large hydrophobic core. This can be explained by the fact that the P(NIPAM-co-ETMA) block with hydrophobic PETMA segments has bad solubility in water, which results in the P(NIPAM-co-ETMA) chains being entangled with each other to form large micelles,58 while there is a smaller effect of the external hydrophilic shell with a similar block length on the morphology of the micelles. So the micelle size is mainly related to the ratio of segments of PNIPAM and PETMA in the diblock copolymers.
The aqueous solution of the Au NPs@P1 systems containing HAuCl4 and P1 (MR = 1/2) was kept under magnetic stirring at room temperature to collect the changes in the UV-vis absorption spectra of different time periods (Fig. 5). NaBH4 is a strong reducing agent that leads to a fast nucleation rate and smaller gold nanoparticles. Fig. 5 shows the typical surface plasmon resonance at about 518 nm, indicating the formation of Au NPs.15 Moreover, the UV-vis absorption spectra (Fig. 5) collected from the solution have no obvious change in a typical position of the SPR maximum by increasing the reaction time. This result indicates that 1 h duration is enough for the reduction of NaBH4 in the mixture of HAuCl4 and block copolymer P1. Fig. 6A-a shows the photographs of the mixed aqueous solutions of HAuCl4 and the block copolymer P1 with different MRs left at room temperature for 0 h. When NaBH4 is added to Au@P1 systems, the solution immediately turns to a red wine color (Fig. 6A-b) and after magnetic stirring for 24 h at room temperature, further chemical reduction has occurred (Fig. 6A-c). Fig. 6B shows that different MRs of the Au@P1 systems correspond to different absorption wavelengths, suggesting that the gold nanoparticles have different sizes.11,60 In addition, we found that there is no obvious absorption at 520 nm for Au@P1 with MR = 1/30, which may be attributed to the formation of smaller gold nanoclusters.61
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Fig. 5 UV-vis absorption spectra of Au@P1 (MR = 1/2) aqueous solution prepared at different time periods with added NaBH4 to the Au@P1 system. |
The specific size and morphology of Au NPs@P1 with different MRs can be obtained from the TEM images in Fig. 7. The clear spherical micelle morphology is observed for the sample with MR = 1/30, and the micelle size is about 42 ± 21 nm (Fig. S5a, ESI†). It can be seen that the size of the micelle is obviously bigger than that of the pure M1 micelle because HAuCl4 is absorbed on the core of the P(NIPAM-co-ETMA) blocks and made the structure of the micelles become dense. The P(NIPAM-co-ETMA) block with episulfide ligands gradually forms the core due to the induction of HAuCl4, while the PPEGMA block remains hydrophilic and forms the hydrophilic shell. This indicates that the gold nanoparticles are well dispersed in the micelles and the size is very small. In all cases, the gold nanoparticles formed by NaBH4 reduction are uniformly distributed in the core of the micelles. Although single Au NPs cannot be seen, we can observe the crystalline lattice fringes of gold nanoparticles from the high resolution TEM image (yellow mark in the inset of Fig. 7a). The TEM images show that Au NPs@P1 forms different morphologies on further increasing the MR (Fig. 7b–d). With the MR increasing from 1/10 and 1/5 to 1/2, the content of HAuCl4 in all the solutions is the same but the block copolymers coated with Au NPs are gradually reduced. The micelle morphology of the Au NPs encapsulated by polymeric micelles becomes less obvious. This result indicates that the micelles of the polymers were disassembled when the concentration of the Au precursors increases (or MRs increase) in the micelle solution. So the morphology of the thin polymer layer-coated Au NPs becomes apparent. One can see from Fig. S5b–d in the ESI† that the Au NPs have an average particle diameter of 1.3 ± 0.4 nm, 1.9 ± 0.5 nm and 6.5 ± 2 nm, respectively. The Au NPs show an increased size from 1.3 to 6.5 nm with the increasing MRs. The high resolution TEM images in the inset of Fig. 7a and d further reveal that the Au NPs have a crystalline lattice fringe of 0.206 nm (yellow mark in Fig. 7a) and 0.238 nm (yellow mark in Fig. 7d), corresponding to the primary reflection of the (200) and (111) lattice of Au, respectively. Therefore, it is easy to infer that the higher the P1 concentration is, the more effective the capping function of P1 is, and the more nucleation sites P1 provides to result in smaller Au NPs finally. Similar results have also been reported in previous work.62 These results, on the other hand, further reveal the important role of the diblock copolymers in the in situ generation of gold nanoparticles.
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Fig. 7 Representative TEM images of Au NPs@P1 prepared at the different MRs of 1/30 (a), 1/10 (b), 1/5 (c) and 1/2 (d). |
Fig. 8 shows the TEM images of Au NPs@P1, Au NPs@P2 and Au NPs@P3. We can see that there is no large change in the size and the size distribution (Fig. S6, ESI†) of the gold nanoparticles. The TEM image and diameter distribution histogram of the control sample of Au NPs@citrate are also shown in Fig. S7b of the ESI.† The average diameter of the Au NPs is 10.4 ± 1.1 nm. Au NPs@citrate exhibits signs of aggregation or clumping in Fig. S7a,† whereas Au NPs@P1, Au NPs@P2 and Au NPs@P3 present a well-dispersed fashion regardless of the content of PETMA segments as shown in Fig. 8. Furthermore, we attempted to prepare the Au NPs without addition of P1. It was found that a lot of precipitation soon appeared in the aqueous solution of gold ions after the fresh aqueous solution of NaBH4 was added. Therefore, we believe that gold nanoparticles should be present within the polymers for the block copolymer-stabilized Au NPs. No precipitate and obvious color change is observed for a long period of three months, indicating the good stability of the block copolymer-stabilized Au NPs, and the hydrophobic PETMA domains still endowed the nanoparticles with a high colloidal stability.
The composition of the formed Au NPs@P1 hybrid was checked further using XPS. Fig. 9 represents the XPS spectrum of the P1-stabilized Au NPs (MR = 1/2), and only Au(0) peaks are observed at binding energies of 87.3 eV (Au-4f5/2) and 83.7 eV (Au-4f7/2).63 Therefore, Au(III) can be reduced to Au(0) when the strong reducing agent NaBH4 is added to the mixture solution of HAuCl4 and the block copolymer P1. The XPS spectra of S 2p of P1 and Au NPs@P1 are shown in Fig. S8 of the ESI.† According to the literature report, the peak at 164 eV corresponds to non-oxidized sulfur (episulfide) and the 2p profiles can be fitted using a splitting value of 1.2 eV.64,65 The peaks of S 2p3/2 and S 2p1/2 for the episulfide groups of the PETMA segments are observed at binding energies of 163.1 and 164.3 eV, respectively (Fig. S8a, ESI†). For Au NPs@P1 (MR = 1/2), two sets of peaks corresponding to S 2p3/2 and S 2p1/2 are observed at the binding energies of 162.9 and 164.1 eV, respectively (Fig. S8b, ESI†).65,66 It seems that there is no obvious change in the XPS spectrum, but the peaks of S 2p slightly shift to a low binding energy on the whole. The above discussions about the FT-IR and 1H NMR spectra have demonstrated the formation of a sulfur–gold bond (S–Au3+) by the gold ion-induced episulfide opening reaction. So, this observation should be attributed to the fact that the sulfur–gold bonds formed by the episulfide opening reaction also have XPS peaks which are very similar to that of S 2p of the episulfide groups at binding energies between 162 and 164 eV.64–66
We choose Au NPs@P1 (MR = 1/2) as the catalyst to conduct several control experiments to find out the optimum catalytic reduction reaction conditions of 4-NP. Furthermore, the effect of the concentration of catalyst and reductant on the reduction rate was studied when keeping the reaction temperature constant (Fig. 11). From the linear relations of ln(Ct/C0) with time, we can determine the rate constant (k) at 27 °C to be 0.185, 1.002, 1.084 min−1 with different concentrations of reductant (Fig. 11a) and k is 0.424, 0.807, 1.084 and 1.604 min−1 with different concentrations of catalysts (Fig. 11b). It can be found that the value of k shows a linear increase with the increasing concentration of catalyst and reductant. Similar results have also been found in other research systems.18,26,69,70 These results are comparable or superior to those reported previously. For example, Au NPs on the surface of poly(2-(dimethylamino)ethyl methacrylate) grafted onto a solid polystyrene core showed a k value of 0.192 min−1 at 436 μM of Au NPs.71 β-D-Glucose-stabilized Au NPs presented a k value of 0.392 min−1 using 200 μM of Au NPs.72 Poly(amidoamine)-based hollow capsule-stabilized Au NPs presented a rate constant of 0.12 min−1.73 Although k is lower in all of our samples for the reduction system containing 20 μL of Au NPs@P1 (MR = 1/2) with 400 μL of NaBH4, this Au NPs@P1 system still shows a high catalytic activity for the reduction of 4-NP as compared to the above reports. In addition, it is noted that Au NPs@P1 as a catalyst displays a superior catalytic activity (k = 0.424 min−1) in the reduction of 4-NP as compared to that of the Au NPs prepared by citrate reduction (k = 0.134 min−1) with an average diameter of 10.4 ± 1.1 nm (Fig. S7c, ESI†). This observation shows that not only the Au NPs but also the surrounding P1 play a critical role in the enhancement of the reduction efficiency.
Here, the catalytic efficiency of Au NPs@P1 was also tested using four samples with different MRs of 1/30, 1/10, 1/5 and 1/2 (Fig. 12). The different catalytic times are obtained when their gold content is the same, indicating that the catalytic efficiency is different with the different MRs of Au NPs@P1. It is found that the catalytic activity of Au NPs@P1 increases with the increase of the MRs. In addition, the reduction time of the catalyst is shorter with the increasing size of the Au NPs (Fig. 7), and the Au NPs with a smaller size have a slower reduction rate. Similar results have also been found in other research systems.41,62,74 As is known, the metal NPs catalyze this reaction by facilitating electron relay from the donor BH4− to acceptor 4-NP to overcome the kinetic barrier.62 The rate of electron transfer at the metal surface can be influenced by two aspects: (1) diffusion of 4-NP to the metal surfaces, and (2) interfacial electron transfer and diffusion of 4-AP away from the surface.62 Thus, the diffusion of 4-NP and the particle size should mainly determine the rate of the reduction. The adsorbing polymer chains would affect the diffusion of 4-NP to the surface of the metal nanoparticles. Thus, from this point, the high catalytic efficiency of Au NPs@P1 (MR = 1/2) should be attributed to the least coating amount of P1 on the surface of Au NPs as compared with the samples with the low MRs of 1/30, 1/10 and 1/5. In addition, the Au NPs for Au NPs@P1 (MR = 1/2) have an average particle diameter of 6.5 ± 1 nm. Previous studies have shown that the gold nanoparticles of about 8 nm in diameter exhibited a relatively high catalytic efficiency.41,62,74,75 Therefore, the block copolymer P1 can be used as a stabilizer in the preparation of Au NPs@P1 (MR = 1/2) with a high catalytic activity for the reduction of 4-NP by NaBH4.
The PNIPAM chains in the block copolymers can change their position with the shrinking and extension of the PNIPAM chains under thermal stimuli.40 So the thermo-responsive catalytic behavior of Au NPs@P1 as a catalyst was also investigated by UV-vis absorption spectroscopy. The catalytic systems containing Au NPs@P1 (MR = 1/2) were subjected to an increasing temperature from 20 to 40 °C. Fig. 13 shows the values of k at different temperatures. Clearly, the value of k increases with the increasing temperature in the range from 20 to 27 °C, indicating that the rate of the reduction reaction can be enhanced by increasing the temperature. This tendency follows the typical dependence of the rate constant on temperature described by the Arrhenius equation and is similar to a general catalyst. It has been reported that the reduction rate and therefore the value of k increases with the increase in temperature76–78 when the reduction is performed at a temperature below the LCST of PNIPAM. As shown in Scheme 3, the possible reason is that the corona-forming PNIPAM chains are hydrophilic and therefore the reactants of 4-NP and NaBH4 can easily diffuse through the corona layer of PNIPAM to reach the surface of the gold nanoparticles to arouse the reduction when the reduction is performed at a temperature below the LCST of the thermo-responsive polymers.22 Whereas, when temperature is further increased to above the LCST, the value of k decreased with the increase in temperature until reaching a constant at about 40 °C. The abnormal decrease in the value of k is possibly due to the fact that the enhancement of temperature would lead to the hydrophobicity of PNIPAM and the PNIPAM chains collapsing to form a hydrophobic barrier on the gold nanoparticles at this temperature, which decelerates the access of the reactants to the gold nanoparticles and therefore decreases the reaction rate (Scheme 3). Thus, the thermo-responsive PNIPAM chains are just like a switch which controls the reaction by adjusting the temperature.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02885c |
This journal is © The Royal Society of Chemistry 2016 |