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Thermo-Responsive Amphiphilic Block Copolymers Stablilized Gold Nanoparticles: Synthesis and High Catalytic Properties Jianhua Lü, Yu Yang, Junfang Gao, Haichao Duan, and Changli Lü Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00414 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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Thermo-Responsive Amphiphilic Block Copolymers Stablilized Gold Nanoparticles: Synthesis and High Catalytic Properties †

†

‡

†

†

Jianhua Lü, Yu Yang, Junfang Gao, Haichao Duan, and Changli Lü*, †

Institute of Chemistry, Northeast Normal University, Changchun 130024, P. R. China

‡

Department of Chemistry, Baotou Teachers College, Baotou 014030, P. R. China

* Corresponding author：

Tel: +86 431 85099236.

E-mail addresses: [email protected].

1

ACS Paragon Plus Environment

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ABSTRACT:

A

series

of

novel

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well-defined

8-hydroxyquinoline

(HQ)-containing

thermo-responsive amphiphilic diblock copolymers poly(styrene-co-5-(2-methacryloylethyloxymethyl)-8-quinolinol)-b-poly(N-isopropylacrylamide) P(St-co-MQ)-b-PNIPAm (P1,2), P(NIPAmco-MQ)-b-PSt

(P3,4) and

triblock copolymer

poly(N-isopropylacrylamide)-b-poly(methyl-

methacrylate-co-5-(2-methacryloylethyloxymethyl)-8-quinolinol)-b-polystyrene

PNIPAm-b-

P(MMA-co-MQ)-b-PSt (P5) were prepared by reversible addition fragmentation chain transfer (RAFT) polymerization, and their self-assembly behaviors were studied. The block copolymers P1-P5 stabilized gold nanoparticles (Au@P1-Au@P5) with small size and narrow distribution were obtained through in situ reduction of gold precursors in aqueous solution of polymer micelles with HQ as the coordination groups. The resulting Au@P nanohybrids possessed excellent catalytic activities for the reduction of nitrophenols using NaBH4. The size, morphology and surface chemistry of Au NPs could be controlled by adjusting the structure of block polymers with HQ in different block positions, which plays an important role in the catalytic properties. It was found that longer chain length of hydrophilic or hydrophobic segments of block copolymers were beneficial to elevate the catalytic activity of Au NPs for the reduction of nitrophenols, and the spherical nanoparticles (Au@P5) stabilized with triblock copolymer exhibit higher catalytic performance. Surprisingly, the gold nanowires (Au@P4) produced with P4 have a highest catalytic activity due to large abundance of grain boundaries. Excellent thermo-responsive behaviors for catalytic reaction make the as-prepared Au@P hybrids become an environmentally responsive nano-catalytic materials.

KEYWORDS: block copolymers, 8-hydroxyquinoline, gold nanoparticles, thermo-responsive, catalytic reduction 2


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1. INTRODUCTION Gold nanoparticles (Au NPs), which are different from their bulk counterparts, have received considerable interest owing to their unique photoelectric properties and applications in catalysis, such as coupling,1,2 hydration,3 oxidation4-6 and reduction reaction7-9 of organic compounds. One of the major challenges for Au NPs in the preparation is the limited stability and resulting aggregation due to their high surface energy and vander Waals. To address this problem, many supporting materials such as metal-organic framework,10 functionalized polymers,11 DNA,12 and organic small molecules have been used to improve the stability of gold nanoparticles and prevent aggregation of particles. Metal nanoparticleʼs shape, size, surface chemistry and morphology often affect their performance in catalysis. Among them, the surface chemistry is greatly affected by the ligand. Polymer protected Au NPs with many repetitive functional groups on the surface show special surface chemistry, which often affects the catalytic performance of metal NPs. Polymers are the most widely used stabilizers for metal particles include nonionic polymers, dendrimers,13 polyelectrolytes,14 double-hydrophilic polymers,15 and amphiphilic polymers. Su et al. synthesized bimetallic AuPt alloy nanoparticles in polyelectrolyte multilayers (PEMs) via an ion-exchange and co-reduction process with enhanced catalytic activity.16 Wan et al. reported one-pot synthesis of porous monolith-supported AuNPs as an effective recyclable catalyst.17 Zhao et al. synthesized palladium nanoparticles with superior catalytic property used porous organic polymers for alkene hydrogenation. Multifunctional polymer as a kind of common used ligands not only can stabilize the metal NPs, but also create more value to form nanohybrid system with very high potential in catalysis.18 Zhou and co-workers developed photo-responsive reversible assembly of Au NPs via host-guest 3


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interaction between the ligands for the repeated usage of Au NPs in catalyst reaction.19 Liu et al. have synthesized Au NPs onto thermo-sensitive magnetic core-shell microgels via electrostatic self-assembly for thermally tunable and magnetically recyclable catalysis.20 It was observed that the catalytic activity of Au NPs can be modulated by changing temperature because the conformation of polymer chains could swell or collapse on the surface of Au NPs with the temperature. Unlike reported nanoreactors which usually involve the thermal phase transition of PNIPAm, Zhou et al. have reported a novel polymer nanoreactor containing Pt nanoparticles which adopted the “self-healing” properties of supramolecular building blocks.21 It is important to investigate the effect of polymer structure on the stabilization and catalysis of metal nanoparticles. Previous studies showed that the block copolymers are more favorable for the synthesis of stable metal nanoparticles with high catalytic properties compared with random copolymers.22,23 It was also found that compared with AB diblock copolymer PEG-PEI, ABC triblock copolymer PEG-PEI-PCL as the stabilizer of Au NPs has significant advantages in stability and catalytic activity.24 Ansar et al. proved a direct relationship between the chain length and packing density of HS-PEG on the AuNPs catalytic activity.25 They found high surface coverage of low molecular weight HS-PEG completely inhibited the catalytic activity of Au NPs. Increasing the molecular weight and decreasing surface coverage of HS-PEG was found to correlate directly with increasing rate constants and decreasing induction time. Que et al. reported that the catalytic activity of block copolymer stabilized Au NPs increased with the decrease in the chain length of PS block.26 However, to the best of our knowledge, there are no reports on the influence of position of coordination group in polymer chains on the catalytic properties of metal nanoparticles. And previous studies on the effect of polymer chain length on the catalytic properties of Au NPs were

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focused on the polymers with end group coordination. To get better understanding of the important role of the length of hydrophobic and hydrophilic blocks in the synthesis and catalytic properties of Au NPs, it is necessary to extend the study on the influence of the block polymers with segment doping coordination sites on the catalytic properties of Au NPs.

In addition, although much polymer ligands have been used to stabilize metal nanoparticles, in these polymers, -SH, -NH2 and -COOH are the most commonly used coordination groups. It is significant to develop the novel stabilizers to functionalize metal nanoparticles and achieve effective catalytic properties. Because of its superior coordination ability and photoelectric properties, electron-riched 8-hydroxyquinoline and its derivatives are often used to construct the metal nanoparticles and semiconductor nanoparticles complex, and the resulting composites exhibited excellent stability and photoelectric properties.27,28 However, there are little attentions have been devoted to the study the roles of 8-hydroxyquinoline ligand in the preparation of metal nanoparticles and application for catalysis.29,30

In the present work, we for the first time report the synthesis of a series of novel thermo-responsive amphiphilic diblock copolymers P1-P4 and triblock copolymer P5 with different chain structures, and their self-assembling behaviors were studied in detail. The above block copolymers as stabilizers were used to fabricate small and uniform sized Au NPs (Au@P) which exhibited excellent catalytic properties. We studied the effect of different block polymers with HQ on the formation of Au NPs, and the difference in the catalytic properties of the resulting Au@P nanoparticles. As shown in Scheme 1, we designed and synthesized the diblock copolymers P(St-co-MQ)-b-PNIPAm with different lengths of hydrophilic PNIPAm chains (P1, P2),

5


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P(NIPAm-co-MQ)-b-PSt with different lengths of hydrophobic PSt chains (P3, P4) and triblock copolymer PNIPAm-b-P(MMA-co-MQ)-b-PSt (P5) via reversible addition fragmentation chain transfer (RAFT) polymerization. The block copolymers can be self-assembled into micelles in water, and MQ units as coordination groups were located at the core, shell and the interface of the core and shell of the self-assembled micelles. The HQ ligand-containing block copolymers (P1-P5) stabilized AuNPs (Au@P) can be prepared by in situ reduction process (Scheme 2). The resulting Au@P nanohybrids exhibited high catalytic activities towards the reduction of nitrophenols in the presence of NaBH4.

Scheme 1. Synthetic routes of different block copolymers of P1-P5.

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Scheme 2. Schematic illustration of the formation of P1-P5 stabilized Au NPs and their thermo-responsive behivors for the catalytic reduction of p-NP.

2. RESULTS AND DISCUSSION Synthesis and Characterization of Block Copolymers. Amphiphilic block copolymers as effective nanocarriers of metal NPs can self-assemble into regular micelles in aqueous media with hydrophobic core and hydrophilic coronal.31-33 In this work, the diblock copolymers P1-4 and triblock copolymer P5 were synthesized via RAFT polymerization (Scheme 1). And MQ unit as coordination group of Au NPs was introduced in hydrophobic segments (P1 and P2), hydrophilic segments (P3 and P4) and the interface between hydrophilic segments and hydrophobic segments (P5) by copolymerization process, respectively. We have successfully prepared the required block copolymers, and their structures were characterized by 1H NMR and FTIR spectra (see Figure S1-S10). As shown in 1H NMR spectra (Figure S1-S9), the peaks i and j are attributed to the protons of -CH3 in the end of RAFT reagent and in the MMA unit, respectively. And the protons of -CH in the NIPAm unit is observed at 4 ppm, which is labeled as the peak 2. The peak f is 7


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correspond to -CH2 in the MQ unit. The protons of aromatic ring of PSt block appear in the range of 6.00-7.50 ppm. The ratio of the monomer units in the BCPs can be obtained by integrating the above peaks and the result was shown in Table 1. Additionally, GPC results showed that the as-prepared block copolymers have high molecular weight (Mn) and narrow PDI (see Table 1). Table 1. Synthesis of block copolymers P1-5 and the corresponding macro-RAFT agent. 1

H-NMR

P1 P2 P3 P4

P5

a

GPC

a

Samples

Mn (g/mol)

NSt

NMQ

NNIPAm

P(St48-co-MQ3)CTA P(St0.94-co-MQ0.06)51-b-PNIPAm72 P(St0.94-co-MQ0.06)51-b-PNIPAm221 P(NIPAm115-co-MQ4)CTA P(NIPAm0.97-co-MQ0.03)118-b-St18 P(NIPAm0.97-co-MQ0.03)118-b-St67 PNIPAm67CTA PNIPAm67-b-P(MMA0.93-co-MQ0.07)28 CTA PNIPAm67-b-P(MMA0.93-co-MQ0.07)28b-St30

5860 14100 31100 14160 16040 21100 8600 11800

48 48 48 / 18 67 / /

3 3 3 4 4 4 / 2

/ 67 221 115 115 115 67 67

14900

30

2

67

Mnb (g/mol)

PDI

/ / / / / / / 26

5450 13410 26500 9690 11350 16600 7480 8420

1.10 1.13 1.12 1.11 1.27 1.15 1.10 1.09

26

10000

1.07

NMMA

Determined by 1H NMR data in CDCl3. b Obtained from GPC measurement.

The FTIR spectra of the block copolymers P1-P3 was shown in Figure S10. The absorption band appeared at 699 cm-1 is assigned to the C-H bending of aromatic ring of PSt blocks. The characteristic peaks at 1651 and 1540 cm-1 are attributed to the C=O stretching vibration and N-H stretching vibration in the PNIPAm segments. The C=O stretching vibration in MQ units can be observed at 1718 cm-1. Obvious changes were observed for the characteristic adsorption intensity of PNIPAm and PSt blocks after the block polymerization, which indicates the successful RAFT polymerization.

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Self-Assembly of Block Copolymers. Block copolymer micelles M1-M5 with coordination groups (MQ) in the core, shell and interface between core and shell were prepared via the self-assembly of diblock copolymers P1-P4 and triblock copolymer P5 in a mixture solution of THF/water. Block copolymers were assembled by adding water to the THF solution containing the block copolymers until homogeneous solutions with blue-opalescent were obtained. The self-assembly structures can be observed from the TEM images (Figure 1 A-E). It can be seen that the block copolymers formed uniform spherical structures in water with small sizes around 30 nm. Particle hydrodynamic diameters (Dh) were directly obtained to be about 80 nm from DLS measurements with a little change in sizes with the increasing molecular weights of block copolymers (Table S1). The critical micelle concentration (CMC) has been determined by fluorescence spectroscopy using pyrene as a fluorescence probe. As shown in Figure 1F, the CMC values of M1 and M2 are about 5.62 mg L-1 and 11.2 mg L-1, respectively. So it can be seen that with the increase in the length of hydrophilic PNIPAm blocks, the tendency to form micelles for the block copolymer P2 decreased, resulting in a greater increase in critical micelle concentration. For CMC values of diblock copolymers P3 and P4 (Figure 1G), as the length of the PSt blocks increased, block copolymer P4 was more likely to gather in water to form micelle, so the CMC decreased from 0.55 mg L-1 to 0.21 mg L-1. In addition, from the CMC data we can see that the tendency to form micelles of the block copolymers with MQ units located in the PSt blocks are weaker than that of the polymers with MQ units positioned in the PNIPAm blocks.

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Figure 1. TEM images of the micelles (A) M1, (B) M2, (C) M3, (D) M4 and (E) M5, and the corresponding CMC measurements of (F) M1 and M2, (G) M3 and M4, (H) M5.

Synthesis of Au NPs from Different Molar Feed Ratio of HAuCl4 and P1. The size and structure of metal NPs prepared by the reduction of the metal ions in the presence of polymers normally depends on the loading of the metal precursor.34 Different sized Au NPs stabilized with diblock copolymer P1 were prepared through in situ reduction route and used to catalyze the

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reduction of p-nitrophenol (p-NP) into p-aminophenol (p-AP) in the presence of NaBH4 aqueous solution. The block copolymer P1 with MQ units as the ligand can effectively stabilize the Au NPs. After adding NaBH4, the color of the solution immediately became reddish brown, indicating the formation of Au NPs. And the color didn’t change in two months, which reflected their good stability due to the presence of P1. As shown in Figure 2, the obtained samples of Au@P1 (10/1), Au@P1 (5/1) and Au@P1 (0.1/1) (named Au@P1 (x/y), where x/y is the initial molar feed ratio between HAuCl4 and MQ units in P1) were well dispersed with a narrow size distribution, and the average diameters are 4.08 nm for Au@P1 (10/1) and 3.25 nm for Au@P1 (5/1). However, only Au NPs can be observed in the TEM images, and the self-assembled micelles of block copolymers in the samples of Au@P1 (10/1) and Au@P1 (5/1) solutions disappeared after the formation of Au NPs (Figure 2A-D). Without P1, gold nanoparticles can not stably exist in water solution, so P1 plays an important role in stabilizing Au NPs. In the past literature, the similar situation also was observed.24 So the Au NPs in the solution of Au@P1 (10/1) and Au@P1 (5/1) make the micelles disassemble on the surface of Au NPs to stabilize them. For the sample of Au@P1 (0.1/1) (Figure 2E-F), some aggregates with the size of 15-30 nm were observed. If the aggregate is single gold nanoparticle, there should be distinct lattice under this scale and significant peak in the absorption spectrum. However, in addition to the small black particles, we did not see any obvious lattice, and there was also no significant absorption in the UV-vis spectrum (Figure 3). Therefore the Au NPs did not destroy the structure of micelles, but were generated in the core of micelles by the coordination with MQ units in the P1, and the diameter of Au@P1 (0.1/1) should be very small (< 2 nm) (Figure 2E and F). The above results showed that with the increase of the initial molar feed ratio between HAuCl4 and MQ units in P1, the formed Au NPs gradually became larger, and

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gradually broke the structure of micelles. Figure 3 shows the UV-vis absorption spectra of the samples of Au@P1 (10/1), Au@P1 (5/1), Au@P1 (0.1/1) and P1. An obvious absorption peak at 505 nm attributed to the surface plasma resonance absorption peak of Au NPs was observed for the sample of Au@P1 (10/1) as compared to P1. Samples of Au@P1 (5/1) and Au@P1 (0.1/1) did not exhibit significant surface plasma resonance absorption peaks, which may be due to the fact that the size of Au NPs was too small and the concentration was too low to detect.

Figure 2. TEM and HRTEM images of the samples of Au@P1 (10/1) (A and B), Au@P1 (5/1) (C and D), and Au@P1 (0.1/1) (E and F).

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Figure 3. UV-vis absorption spectra of P1, Au@P1 (0.1/1), Au@P1 (5/1) and Au@P1 (10/1).

Synthesis of Au NPs Stabilized with Different Block Copolymers.

We prepared Au@P1, Au@P2, Au@P3, Au@P4 and Au@P5 from five amphiphilic block copolymers P1-P5 with HQ ligands in the hydrophobic blocks for P1 and P2, in the hydrophilic blocks for P3 and P4, and in the middle blocks of triblock copolymer P5. In order to investigate the effect of hydrophilic/hydrophobic blocks and the position of HQ ligand in block copolymers on the formation of Au NPs, we synthesized Au@P1 and Au@P2 stabilized with different lengths of PNIPAm chains, Au@P3 and Au@P4 stabilized with different lengths of PSt chains and Au@P5 stabilized with triblock copolymer by reducing gold ions in different micelle solutions with the same mass ratio of Au@P1 (5/1). The UV–vis absorption spectra of the samples of Au@P1-Au@P5 showed the similar surface plasma resonance peaks at 536 nm, which proved that the formation of Au NPs (Figure 4). The XRD patterns of Au@P1-Au@P5 in Figure S11 also confirmed the formation of crystalline Au NPs. The characteristic peaks at 2θ = 38.1, 44.2, 66.7 and 77.4° are corresponding to the (111), (200), (220) and (311) lattice planes of face-centered cubic

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(fcc) gold particles, respectively.35 The strong Au characteristic peaks in XPS spectra (Figure S12) also clearly suggested the formation of Au NPs. The Au 4f7/2 and Au 4f5/2 for Au@P2, Au@P4 and Au@P5 can be observed at the binding energies of 87.0 and 83.4 eV, 87.3 and 83.7 eV, 86.9 and 83.3 eV, respectively. These binding energies belong to the surface binding energy of Au0, indicating all of the gold ions have been reduced to Au0, and there are no Au+ adsorbed on the surface of Au NPs.

Figure 4. UV-vis absorption spectra of Au@P1, Au@P2, Au@P3, Au@P4 and Au@P5 (the inset shows the magnified absorbance intensity).

The TEM images of Au@P2-Au@P5 are presented in Figure 5. The well-dispersed morphologies of Au@P2, Au@P3 and Au@P5 with narrow size distribution are observed from the TEM images. The sizes of Au@P2, Au@P3 and Au@P5 all are around 3.0 nm. The TEM images of Au@P2 in Figure 5A and B displayed that the micelle structures were partially destroyed and cross-linked to stabilize the Au NPs. In addition, we can see that the Au NPs mainly dispersed in the micelles, and there are very few particles that are separately. This phenomenon of Au@P2 was

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also reported in the previous work.23 The Au particles are just like a cross-linker, which joint the discrete micelles into seaweed-like aggregates. From the structures of Au@P3 and Au@P5 as shown in Figure 5C and D and Figure 5G and H, the micelle structures are completely absent and the gold nanoparticles are stably present due to the stability of P3 and P5. From Figure 5D and H, it can be seen that the lattice spacing of the Au NPs is 0.24 nm, which is assigned to (111) planes of cubic gold particles. Surprisingly, the TEM images of Au@P4 in Figure 5E and F showed chain-like nanowire structure (Au NWs). The crystal lattice of Au NWs (Au@P4) in Figure 5F is visible and the lattice fringe spacing is 0.235 nm and 0.207 nm, which can be identified as (111) facets and (200) facets, while the other Au NPs showed only (111) facets. We also prepared the contrast sample of citrate stabilized Au NPs (Au@Citrate) with a small size of about 4.5 nm, and the TEM image and the diameter distribution are shown in Figure S13.

As observed in TEM images (Figure 5C-G), the micelle morphologies of Au@P3-Au@P5 have been destroyed, while the micelle of Au@P2 was partially destroyed. The nanoparticles were dispersed separately in Au@P3 and Au@P5, while a number of Au NPs were located in a single micelle in the samples of Au@P2. This may be due to the fact that P2 has a longer hydrophilic chain than P1 and the coordination units of P2 are located in the core of micelles, and the coordination units of P3-P5 are not located in the core, so the external long hydrophilic PNIPAm blocks well protected the morphology of the micelles in Au@P2 without destruction, which would lead the different morphology from Au@P3-Au@P5. For Au@P3 and Au@P4, the length of PSt blocks in Au@P4 is longer than that of Au@P3, so a nanowire shape in Au@P4 was generated in order to reduce the surface area and the entropy of P4 without being precipitated from the water.

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But in Au@P5, the morphology of the micelles also disappeared. This may be because that in addition to PSt chains, the outer layer of gold nanoparticles also has PNIPAm block that makes it

Figure 5. TEM and HRTEM images for the samples of (A, B) Au@P2, (C, D) Au@P3, (E, F) Au@P4 and (G, H) Au@P5.

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stable in water, maintaining a spherical structure. We also prepared the Au nanoparticles stabilized by PSt-b-PNIPAm without HQ ligands. From Figure S14, there were still many micelles in the TEM image of Au@PSt-b-PNIPAm, and Au NPs did not distribute in the micelles of PSt-b-PNIPAm. This result indicates that PNIPAm chains have weak coordination ability, so the morphology of the micelles was not destroyed. However, in our sample, due to the strong coordination interaction between gold and HQ ligands of polymers, the morphology of the micelles was destroyed. And Au@PSt-b-PNIPAm showed broad size distribution between 1.0 and 7.0 nm in the TEM image. Actually, the morphology of the as-prepared Au NPs from PSt-b-PNIPAm was inhomogeneous and the aggregation happened. However, the Au NPs in Au@P1-Au@P5 exhibited the narrow size distributions and were clearly distributed. This shows that our designed block copolymers with HQ ligands have a much stronger coordination ability than PNIPAm, which plays a critical role in the morphology control of Au NPs.

Catalytic Activities of Au NPs Stabilized with Block Copolymers P1. The catalytic activities of Au NPs prepared from different molar feed ratio of HAuCl4 and P1 were firstly evaluated using the reduction of p-NP into p-AP by excessive amount of NaBH4 as a model reaction, because the reactant of p-nitrophenolate anion (λmax=400 nm) and the product (p-AP) (λmax=300 nm) were easy monitored through UV-vis spectroscopy. This reduction reaction is a thermodynamically feasible process involving E0 for p-NP/p-AP=-0.76 V and H3BO3/BH4- = -1.33 V, but kinetically restricted in the absence of a catalyst.36 After catalyst solutions of Au NPs were added into 3 mL aqueous solutions containing NaBH4 and p-NP, the reduction of p-NP after different waiting periods were determined by UV-vis measurements (Figure S15). The color of the solution after adding Au NPs changed from bright yellow to colorless, which proved the reduction of p-NP. The 17


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reduction rate usually is considered to follow pseudo-first-order kinetics as the concentration of NaBH4 far exceeds to the concentration of p-NP.16 The ratio of absorbance (At/A0) at time (t) directly corresponds to the concentration ratio (Ct/C0) for p-NP, thus the kinetic equation of reduction reaction can be shown as eqn (1):

r = ln

Ct A = ln t = −k app t C0 A0

(1)

The apparent rate constant (kapp) can be obtained to be 0.48, 0.30 and 8.3×10-4 min-1 for Au@P1 (10/1), Au@P1 (5/1) and Au@P1 (0.1/1) in Table 3, and the slope of the fitting lines is shown in Figure S15D. But because the gold content is different in the samples after dialysis, in order to evaluate the catalytic performance, the turnover frequency (TOF) values of the corresponding reactions were also calculated according to eqn (2):

TOF =

[ NP ] × Conversion [ Au ] × t

(2)

Where [NP] and [Au] are the molar concentrations of substrate and catalyst, the concentration of p-NP was fixed to be 0.1 mM and the [Au] was determined by ICP-AES (see Table 2). We estimated the TOF values for all the samples with the conversion of p-NP at 50%. As shown in Table 2, among these Au-catalyzed systems, the catalytic activity of catalyst Au@P1 (5/1) is the highest with a TOF value of 189 h-1, while the lowest TOF value was observed for Au@P1 (0.1/1) with a smallest size < 2 nm. The metal NPs catalyze this reaction by facilitating electron relay from the donor BH4- to acceptor NP to overcome the kinetic barrier. There are two processes which influences the catalytic activities: (1) the diffusion of p-NP and BH4- to the metal surfaces and the interfacial electron transfer; (2) p-AP diffuses away from metal NP surface. The high catalytic activity of Au@P1 (5/1) is understandable attributed to the smaller size caused higher surface area

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and a larger number of surface atoms available for electron transfer than that of Au@P1 (10/1).17,36,38 And lowest catalytic activity of Au@P1 (0.1/1) is possibly because the small gold nanoparticles were encapsulated in the polymer micelles, which hinders the substrate of p-nitrophenolate anion and BH4- from contacting the catalyst of Au NPs. Table 2. The catalytic activity for the reduction of p-NP with the catalysts of Au@P1 (10/1), Au@P1 (5/1) and Au@P1 (0.1/1). a,b Catalyst Au@P1 (10/1) Au@P1 (5/1) Au@P1(0.1/1) a

Added volume of catalyst 50 µL 50 µL 500 µL

nAu (mol) c 5.19×10-9 -9

2.5×10 4.94×10-10

kapp (min-1)

TOF (h-1)

0.48

178

0.30 8.3×10-4

189 125

At 25oC. b Cp-NP=0.1 mM, CNaBH4=0.1 M. c The added nAu were determined by ICP-AES.

Catalytic Activities of Au NPs Stabilized with Different Block Copolymers. The reduction reaction of nitrophenols was used to determine the catalytic activities of the Au@P1-Au@P5 and Au@Citrate as catalysts. Figure 6A presents the plots of ln(Ct/C0) as a function of time (t) and each catalyst stock was added by 50 µL. As desired, good linear correction of ln(Ct/C0) vs. t were gained without induction period t0. As shown in Table 3, the kinetic reaction rate constants (kapp) of Au@P1-Au@P5 were calculated to be 0.3018, 0.7122, 0.3654, 0.2634 and 0.8742 min-1, while the apparent rate constants (kapp) of Au@Citrate was determined to be 0.18 min-1 with an induction period t0. This shows that our samples Au@P1-Au@P5 have better activities than Au@Citrate. The TOF values for Au@P1-Au@P5 catalysts were calculated to be 189, 565, 226, 1634 and 927 h-1 based on the content of Au in the catalysts determined by ICP-AES (Table 3), which were higher than that of Au@Citrate with a lower TOF value of 87 h-1 for the reduction of p-NP. It can be seen that Au@P2 and Au@P4 stabilized longer hydrophilic segments and hydrophobic segments at the ends, respectively, exhibit better catalytic activities than Au@P1 and Au@P3 with short

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hydrophilic segments or hydrophobic segments. The catalytic activity of Au@P5 was higher than that of the diblock copolymer-coated nanoparticles in the case of similar particle morphology (Au@P1-Au@P3). In addition, the as-prepared Au@P catalysts in our work showed a higher catalytic efficiency than most of the previous reported Au based catalyst systems for the reduction of p-NP (Table S2).

Table 3. Catalytic activities for the reduction of p-NP catalyzed by Au@P1-Au@P5, Au@Citrate and Au@PSt-b-PNIPAm as catalysts.a,b Catalyst Au@P1

a

AuNP size (nm) 3.25

nAuc (mol)

kapp (min-1)

TOF (h-1)

2.50×10-9

0.3018

189

-9

Au@P2 Au@P3 Au@P4 Au@P5

3 2.63 none 2.77

1.67×10 2.25×10-9 2.10×10-10 2.08×10-9

0.7122 0.3654 0.2634 0.8742

565 226 1634 927

Au@Citrate Au@PSt-b-PNIPAm

4.5 3.89

1.13×10-9 1.36×10-9

0.18 0.146

87 156

At 25oC.

b

Cp-NP=0.1 mM, CNaBH4=0.1 M, added catalyst volume: 50 µL. cThe added nAu were determined by

ICP-AES.

It is known that the catalytic activity of polymer stabilized metal nanoparticles for the reduction of nitroaromatic compounds is significantly influenced by the hydrophobicity and hydrophilicity of capping agents.39, 40 In our study, the bolck copolymers P1-P5 with different structures could lead to different size, morphology and surface chemistry of Au@P1- Au@P5, thus affecting its catalytic properties. The samples of Au@P1 and Au@P2 have similar particle size, shape and composition, differing only in terms of the chain length of hydrophilic PNIPAm block and the number of Au NPs in a single micelle. But the catalytic activity of Au@P2 with longer hydrophilic PNIPAm block was higher than that of Au@P1, indicating that the longer hydrophilic chains of PNIPAm blocks lead to 20


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more stable structure for Au@P2 than Au@P1, which may result in better catalytic properties. And the catalytic efficiency is also dependent on the course of adsorption and desorption of reactant on the surface areas of Au NPs. Therefore, the higher catalytic activity of Au@P2 than that of Au@P1 may result from its lower packing density, surface coverage and better hydrophilicity, which allowed the excellent contact between the catalyst and the substrate during the reaction.25,40,41 Surprisingly, the sample of Au@P4 exhibited the highest catalytic activity among all the catalysts of Au@P1-Au@P5, which may be related to its linear morphology resulted from the ligand of P4 with longer PSt blocks. It has been reported that the catalytic activity of Pt nanowires cross-linked by monodisperse Pt near-spherical NPs is significantly higher than that of monodisperse Pt NPs in reduction of p-NP, which could be caused by large abundance of grain boundaries in nanowires.42 As shown in Figure 5F, the grain boundary in Au@P4 with (111) and (200) facets generated by the nanowire structure may be responsible for a relatively high catalytic activity. Compared with the samples of Au@P1 and Au@P2 stabilized with P1 and P2 which positioned the MQ units in the hydrophobic PSt block and Au@P3 stabilized with P3 which positioned the MQ units in the hydrophilic PNIPAm block, the sample of Au@P5 stabilized with P5 positioned the MQ units between the PSt block and the PNIPAm block hold the highest catalytic activity. It can be seen from the TOF values in Table 3, compared with diblock polymer stabilized spherical Au NPs, the triblock copolymer stabilized spherical Au@P5 exhibited higher catalytic activity. This result may be attributed to that the Au NPs were decorated at the sites between the hydrophobic and hydrophilic blocks of P5 (PNIPAm-b-P(MMA-co-MQ)-b-PSt), and both reactants (p-NP, BH4-) and product (p-AP) could faster diffuse and desorb away from Au NPs, which resulted in a higher catalytic activity.24,43

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Figure 6. (A) Plots of ln(Ct/C0) versus time during the reduction of p-NP catalyzed by Au@P1-Au@P5 and Au@Citrate (the volume of Au@P1-Au@P5 and Au@Citrate are the same). Plots of ln(Ct/C0) versus time during the reduction of p-NP, o-NP and m-NP catalyzed by (B) Au@P1, (C) Au@P2, (D) Au@P3, (E) Au@P4 and (F) Au@P5 (the amount of Au and the concentration of p-NP, o-NP and m-NP are the same).

The catalytic performance of Au@PSt-b-PNIPAm as catalyst was also evaluated as shown in Figure S16. From Table 2, the catalytic rate and TOF value of Au@PSt-b-PNIPAm were determined to be 0.146 min-1 and 156 h-1 respectively, which is much lower than that of Au@P1-Au@P5 with HQ ligands. This result demonstrated that our designed block copolymers had better stabilizing effect for Au NPs with uniform size and morphology for high catalytic activity. 22


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We also investigated the catalytic activities of Au@P2, Au@P4 and Au@P5 for the reduction of other nitrophenol isomers of m-NP and o-NP. The absorption peaks of m-NP and o-NP at 396 and 415 nm decreased rapidly after the addition of Au NPs. And new peaks at about 290 nm appeared due to the formation of m-AP and o-AP, respectively (Figure S17). According to the previous reported catalysts, the catalytic activities of the reduction of m-NP is less efficient than those of o-NP and p-NP, in which the negative charges on nitroxides are delocalized throughout the benzene rings due to both inductive effect and conjugative effect.44-46 Also some of the literatures reported such a sequence of catalytic activity: m-NP > p-NP > o-NP.47 In our work, it was found from the kinetic reaction rate constants (kapp) that the catalytic activities for the reduction of different nitrophenol isomers by Au@P1, Au@P2, Au@P4 and Au@P5 followed the order: m-NP > p-NP > o-NP (Figure 6B, C, E and F), which is consistent with previous studies. Especially, the catalytic activity of the Au@P3 catalyst for the reduction of o-NP was superior than that for m-NP and p-NP (Figure 6D), which was rarely reported in the previous literatures. The long-term catalytic stabilities of the as-prepared Au NPs were also evaluated. All the solutions of Au@P2, Au@P4 and Au@P5 were freeze-dried to give solid samples, and retested their catalytic properties after stored for 8 months. As shown in Figure S18, although the catalytic performance of these catalysts obviously decreased, the solid catalysts still maintained relatively higher catalytic activities than that reported in some literatures (Table S2). Thermo-responsive Catalytic Behavior of Au NPs. The block copolymer coated on the outer layer of the metal nanoparticles not only acts as a stabilizer for the nanoparticles, but also gives the metal nanoparticles more special properties, such as optical properties27, thermally triggered assembly48 and excellent catalytic activity.20,29 Of all the stimuli-responsive polymers, the thermo-

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Figure 7. Plots of the apparent rate constant kapp for the reduction reaction of p-NP catalyzed with (A) Au@P2, (B) Au@P4 and (C) Au@P5 catalysts at different temperatures.

responsive polymer of poly(N-isopropylacrylamide) (PNIPAm) is the most commonly studied, which undergoes a reversible phase-transition in water with the low critical solution temperature (LCST). The block copolymers (P1-P5) with PNIPAm blocks possess the special behavior of shrinking and extension under thermal stimuli with a LCST around 32 oC in aqueous solution. Figure 7 showed the changes of the apparent rate constant kapp of the reduction reaction for p-NP catalyzed by Au@P2, Au@P4 and Au@P5 with different temperature. Clearly, with the increase of temperature from room temperature (about 25 oC), the values of kapp increased first, then decreased as the temperature increased to more than above 32 oC (LCST of copolymers). With further increase in temperature, the kapp increased again for Au@P2 and Au@P5 (Figure 7A and C). However, the kapp of Au@P4 did not increase again (Figure 7B). In order to explore the cause of rate constant variation with temperature, the temperature dependent DLS curves of Au@P2, Au@P4 and Au@P5 were tested (see Figure S19). For Au@P2, the rule of DLS curve is consistent with that of kapp curve. Both kapp and hydrodynamic size (Dh) of Au@P2 increased with the increasing of temperature before 29 oC. The increase tendency of kapp is consistent with the representation of Arrhenius equation, and the stretching of PNIPAm resulted in the increase of Dh. When the temperature was

24


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higher than 29 oC, the Dh decreased due to that the PNIPAm segments shrunk and wrapped on the surface of gold nanoparticles, which resulted in the decrease of diffusion rate of reactants towards Au NPs, thus the kapp decreased.49, 50 When temperature increased to more than 33 oC, both Dh and kapp increased again. It may be that the PNIPAm chains shrunk to form large aggregates above this temperature, and DLS reflected the size of the large aggregates. Actually, the temperature is beginning to play a dominant role again in the increase of kapp when the PNIPAm shrunk to a certain extent. 49, 50 For Au@P4 with special chain-structures, although the kapp exhibited a similar law as that of Au@P2 before 38 oC, there was no obvious change in Dh. It may be because the PNIPAm blocks in Au@P2 are located in the outermost layer of the nanoparticles and sensitive to the change in temperature, but the PNIPAm blocks containing HQ ligands in Au@P4 are mainly anchored to Au NPs. So the PNIPAm blocks in Au@P4 can not freely stretch in water, and the Dh of Au@P4 did not exhibit obvious change. However, the kapp of Au@P4 exhibited a similar law as Au@P2 in this range, which may be due to the microscopic shrinkage of PNIPAm segments, and this cannot be displayed on the hydrodynamic size. However, when the temperature was higher than 38 oC, kapp decreased without rise again with the increase of Dh. The phenomenon that kapp is no longer rising after 38 oC probably because the PSt blocks on Au@P4 hindered the contact of reactants and Au NPs. The Dh increase for Au@P4 may be caused by the further aggregation between the special chain-structures with hydrophobic PSt blocks. The kapp for Au@P5 also exhibited similar tendency as Au@P2 (Figure 7B). But we did not observe the same change pattern in Dh, which may be due to the fact that the outer layer of the Au NPs stabilized with PMMA block with HQ ligands of P5 has PSt blocks in addition to PNIPAm. The free hydrophobic PSt chains may hinder the shrinkage of hydrophilic PNIPAM chains, so there is no obvious dependence of Dh

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of Au@P5 hybrid system on the increasing temperature. But the micro-aggregation within a smaller scale may occur on the surface of Au NPs due to the small volumes shrinkage of free PNIPAM chains with temperature, which resulted in the similar variation rule of kapp to that of Au@P2 system.

4. CONCLUSION A facile RAFT polymerization strategy was developed to prepare a series of novel thermo-responsive block copolymers P1-P5 containing 8-hydroxyquinoline coordination groups for fabricating temperature-responsive Au NPs with excellent catalytic activity for the reduction of nitroarene isomers. The as-prepared block copolymers can well stabilize gold nanoparticles, and the obtained Au NPs possessed a uniform size about 3 nm and a normal distribution. It was found that block copolymer P2 with longer chain length of PNIPAm than that of P1 was beneficial for the catalytic activity of Au NPs with similar uniform size. We are also surprised to find that Au nanowire (Au NWs) linked by Au NPs exhibited a higher catalytic activity than the monodisperse Au NPs with a similar mean size due to the grain boundaries. More importantly, the as-prepared Au@P5 stabilized by triblock copolymer P5 exhibited a higher catalytic performance than that of Au@P1-Au@P4 stabilized by diblock copolymers P1-P4. In addition, the catalytic activity of the as-prepared Au NPs could be accelerated and decelerated when the temperature changed in the range below and above the LCST of the thermo-responsive copolymers. The novel block copolymers synthesized in this study open a new approach for the construction of stable thermo-responsive metal nanoparticles with potential catalytic applications.

ACKNOWLEDGEMENTS 26


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This work was supported by the National Natural Science Foundation of China (21574017 and 21364007).

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected].

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Table of Contents (TOC) Graphic

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Thermoresponsive Amphiphilic Block Copolymer ... - ACS Publications

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