Control and Switching of Charge-Selective Catalysis on Nanoparticles

Jul 2, 2018 - It is known that when catalytic nanoparticles are functionalized with charged ligands, the polarity of these ligands can selectively con...
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Control and switching of charge-selective catalysis on nanoparticles by counterions Qiang Zhuang, Zhijie Yang, Yaroslav Sobolev, Wiktor Beker, Jie Kong, and Bartosz A. Grzybowski ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01323 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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ACS Catalysis

Control and Switching of Charge-Selective Catalysis on Nanoparticles by Counterions Qiang Zhuang1,3, Zhijie Yang1, Yaroslav I. Sobolev1, Wiktor Beker4, Jie Kong*3, Bartosz A. Grzybowski*1,2,4 1

IBS Center for Soft and Living Matter, UNIST, 50, UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 44919, South Korea

2

Department of Chemistry, UNIST, 50, UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 44919, South Korea

3

Department of Applied Chemistry, School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi’an, 710072, P. R. China

4

Institute of Organic Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224, Warsaw, Poland

ABSTRACT: It is known that when catalytic nanoparticles are functionalized with charged ligands, the polarity of these ligands can selectively control the approach of either (+) or (-) charged substrates, effectively rendering the particles’ catalytic activity charge-selective. In such experiments, however, the role of the counterions surrounding the charged ligands is generally not considered. The present work demonstrates that counterions – despite being only loosely bound – can have dramatic effect on the on-particle catalysis. In particular, with the same charged ligands but with counterions of different sizes, catalysis can be allowed or completely blocked. Moreover, when counterions are exchanged, the same particles can be reversibly toggled between catalytically active and inactive states.

KEYWORDS: nanoparticle catalysis, charge selectivity, counterions, switchable catalysis, stepwise catalysis

INTRODUCTION Gold nanoparticles, AuNPs, covered with amine,1 thiol,2 polymer,3-4 or dendrimer5 ligands are known to catalyze reactions such as hydrogenation,6 hydrosilylation,7 or phosphorylation.8 Traditionally, the role of these ligands has been to stabilize the particles – in recent years, however, there has also been growing interest in using the ligand shell as a layer controlling catalytic activity. For example, we showed7 that AuNPs decorated with a mixed self-assembled monolayer of dodecylamine and photoswitchable azobenzene-terminated alkane thiols can assemble/disassemble upon light irradiation resulting in the modulation of the NPs’ catalytic activity in the hydrosilylation reaction of 4-methoxybenzaldehyde. More recently, Pillai’s group demonstrated another elegant control system based on AuNPs covered with alkyl thiolates terminated with charged endgroups.9 The role of these charged moieties was to “gate” the transport of a charged substrate, 4-nitrophenol, towards the particle’s surface where it was reduced to 4-aminophenol.10-12 When the endgroups were positively charged (-CH2-N(CH3)3+), the negatively charged nitrophenol molecules could pass freely and the catalysis was “on”; when, however, the endgroups were negatively charged (COO-), the approach of nitrophenol was hindered by electrostatic repulsion and the catalysis was “off”. Here, we build on these results and show another control effect – namely, the dependence of

AuNPs’ catalysis not only on the properties of the charged ligand shell but also on the sizes of the counterions surrounding the charged endgroups. In particular, we show that larger counterions can decrease or completely hinder the particles’ catalytic activity over periods of several days – that is, longer than the covalently attached ligands. Remarkably, by keeping the NPs intact but exchanging the surrounding counterions, it is possible to “dynamically” toggle between the catalytically active and inactive states of the particles. Whereas the role of counterions proximal to the metal center of a homogeneous catalyst has been documented before13-17, their use as “steric hindrance” controlling and “switching” heterogeneous catalysis (on a NP surface that is over a nm away from the counterions) is, to our knowledge, unprecedented. EXPERIMENTAL SECTION Synthesis of AuNPs. Gold nanoparticles, AuNPs, 4.2 ± 0.2 nm in diameter (see Figure S1 for TEM images) and stabilized with oleylamine were synthesized as described by Sun18. Briefly, a precursor solution of 50 mg HAuCl4∙3H2O and 5 mL oleylamine in 5 mL toluene was prepared and vigorously stirred at room temperature for 10 min. A reducing solution of 21.7 mg tert-butylamineborane complex and 0.5 mL oleylamine in 0.5 mL toluene

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was prepared and injected rapidly into the precursor solution, and the mixed solution was stirred violently for 1 hr. Then, gold nanoparticles were washed with ethanol and redissolved in toluene for further ligand exchange. Functionalization of AuNPs. Positively or negatively charged gold nanoparticles were prepared by ligand exchange with, respectively, N,N,N-trimethyl(11mercaptoundecyl) ammonium chloride (TMA) or 11mercaptoundecanoic acid (MUA) according to previously described protocols.19-21 After ligand exchange, both TMA and MUA nanoparticles were dissolved in water for further catalytic reactions. To dissolve MUA nanoparticles, the pH of nanoparticles’ suspension was adjusted to ~11 by addition of tetramethylammonium (TMAOH). TEM images (see Figure S1) evidenced the two types of charged nanoparticles thus prepared had nearly the same diameters: 4.2±0.1 nm for TMA-functionalized AuNPs and 4.2±0.2 nm for MUA-functionalized AuNP. At pH~11, the zeta potential recorded on Malvern Zetasizer Nano ZSP instrument was +37.7 mV for AuTMA NPs and -36.8 mV for AuMUA NP. On-nanoparticle catalysis. In all reactions, the substrates were dissolved in water. Before reactions, 0.2 mM of 4-nitrophenol, 100 mM of NaBH4 and 172.8 nM of AuNPs (concentration is in terms of nanoparticles22) were prepared separately as stock solutions. In a typical procedure, 1 mL of 4-nitrophenol stock solution was diluted with 1.4 mL DI water. Next, 100 μL of NaBH4 solution (freshly prepared before each experiment) was added into diluted 4-nitrophenol solution. Afterwards, 10 μL of AuNP stock solution was injected into the solution of substrates, and the reduction of 4-nitrophenol commenced. The progress of 4-nitrophenol’s reduction into 4-aminophenol was monitored by UV-Vis on a Cary Series UV-Vis-NIR spectrometer, by the decrease of absorption at 400 nm known to be proportional to the concentration of 4nitrophenol23 (note: the surface plasmon resonance, SPR, band of the AuNPs was centered at ~ 520 nm but, because the concentration of the NPs used was very low, its intensity in the catalysis experiments was negligible, ~ 0.4% of the intensity of the 4-niotrophenol’s band). In addition, the apparent size and surface potential of AuNPs before and during the reactions were monitored by Malvern Zetasizer Nano ZSP.

ic salts differing in the sizes of their cationic and/or anionic parts. Unless otherwise stated, the experiments were performed under ambient conditions, although a separate set of experiments in the absence of oxygen was also performed (cf. below and Figure S5). We first calibrated our system – against that described by Pillai and co-workers9 – in the experiments in which equal amounts as-prepared AuTMA NPs or AuMUA NPs were added to the solutions of 4-nitrophenol. Figures 1a and 1b give the raw UV-Vis spectra for the two systems; these raw data are quantified in the semi-logarithmic kinetic plots in Figures 1c and 1d. As seen, for the AuTMA NPs, catalysis commences immediately after adding the particles. The initial linearity of the plot on the semilogarithmic scale evidences the expected9, 24-25 first-order kinetics with the apparent rate constant of 0.00349 s-1. In contrast, for the AuMUA NPs, no concentration changes are initially observed and the reaction kicks-in (with apparent rate constant of 0.00342 s-1) only after an induction period of ~ 12 min. Induction periods of seconds to minutes have been observed before24, 26, 27 including data reported by Pillai’s group9 for similar concentrations of NPs (~15 min for 1000 pM NP concentration vs. 690 pM NP concentration in our experiments; for induction time increase with decreasing NP concentration, see SI, Figure S3). As verified by experiments summarized in Figure S8, the induction times scale with the content of oxygen in water and disappear completely when water is thoroughly purged with nitrogen – at the same time, the modulation of catalysis by different-size counterions we describe below is not affected by oxygen content (see Figure S5, and further discussion in SI, Section 3.4).

RESULTS AND DISCUSSION All experiments were based on 4.2 ± 0.2 nm gold nanoparticles, AuNPs, functionalized with either positively charged N,N,N-trimethyl(11-mercaptoundecyl) ammonium chloride (TMA; CH2-N(CH3)3+ endgroups surrounded by Cl- counterions) or negatively charged, fully deprotonated (at pH = 11) 11-mercaptoundecanoic acid (MUA; COO- endgroups surrounded with N(CH3)4+ counterions). On-particle catalysis was initiated by mixing solutions of either TMA or MUA AuNPs with the solutions of 4nitrophenol and NaBH4 substrates. Effects of counterions on the reduction of 4-nitrophenol into 4-aminophenol were studied by the addition of various amounts of organ-

Figure 1. UV-Vis spectra taken during reduction of 4-nitrophenol on a) TMA AuNPs (the negatively charged substrates, 4-nitrophenol and BH4-, can migrate unhindered into the oppositely-charged ligand shell) and b) on MUA AuNPs (the substrates experience electrostatic repulsion from the like-charged ligand shell). Notice different time scales on which a significant decrease in the intensity of 4nitrophenol’s adsorption peak at 400 nm is observed. Same data but quantified as kinetic plots of –ln(Ai/A0) against time for c) AuTMA NPs and d) AuMUA NP. Ai stands for 4-nitrophenol’s absorption (at 400 nm) at time t and A0 is the initial adsorption at this wavelength.

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ACS Catalysis With these calibrating experiments in place, we investigated whether and how the catalytic activity of the NPs could be influenced by changing the sizes of counterions surrounding them. To this end, we tested three organic salts: tetramethylammonium chloride (N(CH3)4+Cl-; TMACl), tetrabutylammonium chloride (N(C4H9)4+Cl; TBACl), and sodium dodecyl sulfate (CH3(CH2)11OSO3Na+; SDS). Therefore, the counterions around AuTMA NPs could be Cl- or CH3(CH2)11OSO3- while those around AuMUA NPs could be N(CH3)4+, N(C4H9)4+, or Na+ (Figure 2).

Figure 2. Structures of the a) negatively-charged MUA and b) positively-charged TMA thiols. c) Molecular cartoon of MUA thiols tethered onto a gold nanoparticle illustrates the sizes of these ligands, the inter-thiol spacing of ca. 4.5 Angstroms and the sizes of counterions used in various experiments. These counterions are Na+, TMA+ = N(CH3)4+, and TBA+ =N(C4H9)4+. d) An equivalent cartoon for TMA thiols tethered onto a gold nanoparticle. The counterions used in various experiments are SDS-= CH3(CH2)11OSO3- and Cl-.

analogous experiments with TMACl (days vs. ~ 10 minutes in Figure 1). We note that upon addition of TBACl, the AuMUA NPs do not aggregate (see DLS data in Figure S4) and so the lack of catalytic activity cannot be attributed to such aggregation and to the concomitant decrease in effective catalytic surface area. The situation is reversed when SDS is used. Here, the AuTMA NPs surrounded by large, negatively charged CH3(CH2)11OSO3- counterions show no catalysis at all SDS concentrations tested (Figure 3e). Since the size of the NPs measured by DLS (Figure S7) does not increase markedly, this effect is not due to NP aggregation. On the other hand, Molecular Dynamics simulations described in the SI, Section 2, and additional TEM imaging experiments in Figure S8 both suggest that SDS intercalates between the TMAs, in effect physically obstructing the passage of reaction substrates through a “denser” ligand shell and hindering catalysis. An interesting situation arises when AuMUA particles are used (Figure 3f). Here, the particles do aggregate up to ~ 50 nm (Figure S6) but in low to intermediate SDS concentrations they remain catalytic (rates 0.00323 s-1 for 0.40 mM vs. 0.00292 s-1 for 1.95 mM), though there is an induction period (14 vs. 25 min, see black vs. red curves in Figure 3f). Only when concentration of SDS reaches 3.8 mM, catalysis ceases (blue line in Figure 3f), but this is not due to the MUA/SDS intercalation (see simulations in SI, Section 2.2) but rather due to the micellization of the reaction mixture (see conductivity measurements and the determination of the critical micelle concentration in Figure S9).

With TMACl at concentrations ranging from 0.40 mM to 3.83 mM, the small Cl- and TMA+ counterions have only marginal effect on the catalytic activity of either AuTMA (Figure 3a) or AuMUA (Figure 3b) NPs. Fits to the linear parts of the semi-logarithmic kinetic plots give the rate constants for AuTMA NPs at 0.00215 s-1, 0.00227 s-1, and 0.00306 s-1 for TMACl concentrations of, respectively, 0.40, 1.95, and 3.83 mM. For the AuMUA NPs, the rate constants at these TMACl concentration are 0.00293 s-1, 0.00396 s-1, and 0.00336 s-1 and are observed after induction times of ca. 12-18 min. The situation is drastically different when TBACl is used. For the AuTMA NPs, the small Cl- counterions still have only small effect on the catalytic activity. Even though dynamic light scattering, DLS, evidences aggregation of the particles with time (either due SAM restructuring or “bridging” of the NPs by monovalent counterions;28 see Figure S6), the rate constants do not show any systematic concentration trend (0.00500 s-1, 0.00405 s-1, and 0.00469 s-1 for, respectively, 0.40, 1.95, and 3.83 mM TBACl; Figure 3c). In sharp contrast, when AuMUA NPs are used as catalysts, the large N(C4H9)4+ counterions surrounding the COO- endgroups29 form a dense layer (SI, Section 3.1) that hinders the approach of the substrates to the NPs’ surfaces and no catalysis is observed (Figure 3d). This lack of catalytic activity persists for much longer than in

Figure 3. Kinetic plots and pseudo-first-order linear fits for a) AuTMA NPs and b) AuMUA NPs at different TMACl concentrations, c)

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AuTMA NPs and d) AuMUA NPs at different TBACl concentrations, e) AuTMA NPs and f) AuMUA NPs at different SDS concentrations.

Next, based on these differences in catalytic activity, we demonstrated “switchable” control of catalysis by counterions. First, we prepared AuTMA NPs surrounded by SDS- counterions (50 μL of 100 mM SDS mixed with 2.51 mL of 0.69 nM NPs solution). As described above and confirmed by data in Figure 4a, such particles show no catalytic activity. However, when an equivalent amount of CTAB (hexadecyltrimethylammonium bromide) is added, the positively charged CTAB and negatively charged SDS aggregate into cationic micelles30 (or less regular aggregates31), as evidenced by the solution turning hazy and by DLS measurements (Figure S10). Upon the addition of CTAB, the AuNPs are stripped of the surrounding SDS counterions (replaced by much smaller Br- coming from CTAB) and become catalytically active, as evidenced by the raw data in Figure 4b and kinetic plot in Figure 4c. Remarkably, catalysis can be again switched “off” by the addition of fresh SDS and then “on” by the addition of CTAB (Figures 4d,e). After two cycles, the 4-nitrophenol ran out but, in principle, more cycles could be performed with increased initial concentration of this substrate.

approach to particles comprised of different catalytic domains (e.g., Au-Fe3O4 dimers32-34) covered with ligands of opposite charges. In such constructs, the catalytic activity of each of the parts could be regulated by different counterions (provided the salts are used below the solubility product) making them interesting candidates for catalytic “logic gates” – work in this direction is currently under way in our laboratory.

AUTHOR INFORMATION Corresponding Author * Correspondence to [email protected] or [email protected]

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Nanoparticle characterization data, additional kinetic plots, and DLS measurements.

ACKNOWLEDGMENT This work was supported by Korea’s Institute for Basic Science (Grant No. IBS-R020-D1), and National Natural Science Foundation of China (Grant No. 21703172).

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Figure 4. Changing the catalytic activity of AuTMA NPs by the addition of SDS or CTAB. a) UV-Vis spectrum evidencing lack of 4nitrophenol’s reduction by AuTMA NPs surrounded by SDS (1.95 mM) at times 0-51.4 min. b) At 51.4 min, 50 μL CTAB (100 mM) is added and the intensity of the peak at 400 nm starts decreasing, indicating reduction of 4-nitrophenol. The transition from noncatalytic (SDS) to catalytic (CTAB) “states” is quantified in the kinetic plot in c). d) UV-Vis spectra recorded during the sequence of SDSCTAB-SDS-CTAB “steps”. The catalytic activity of the AuTMA NPs during these steps is quantified in the kinetic plot in e).

CONCLUSION To summarize, the results we described evidence that the approach of charged substrates to the surface of catalytic NPs can be controlled by the size of the counterions surrounding charged on-particle ligands. In addition, exchange of these counterions allows for “dynamic” switching of the catalytic activity. We envision extensions of this

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