Impact of Gold Nanoparticle Stabilizing Ligands on the Colloidal

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The Impact of Gold Nanoparticle Stabilizing Ligand on Colloidal Catalytic Reduction of 4-Nitrophenol Siyam M. Ansar, and Christopher L. Kitchens ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00635 • Publication Date (Web): 27 May 2016 Downloaded from http://pubs.acs.org on May 30, 2016

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The Impact of Gold Nanoparticle Stabilizing Ligand on Colloidal Catalytic Reduction of 4-Nitrophenol Siyam M. Ansar and Christopher L. Kitchens*

Department of Chemical and Biomolecular Engineering, Clemson University, Clemson, SC 29634, United States

*

Corresponding author. Email: [email protected]

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ABSTRACT: Gold nanoparticles (AuNPs) have received considerable interest owing to their unique properties and applications in catalysis. One of the major challenges for colloidal nanoparticles in catalysis is the limited stability and resulting aggregation. Nanoparticle functionalization with ligands or polymers is a common strategy to improve the colloidal stability, which in turn blocks the reactive surface sites and eliminates catalytic activity. Here, we investigate thiolated polyethylene glycol (HS-PEG) as a stabilizing ligand during AuNP catalytic reduction of 4-nitrophenol. We show a direct relationship between the chain length and packing density of HS-PEG with respect to AuNP catalytic activity. High surface coverage of low molecular weight HS-PEG (1 kDa) completely inhibited the catalytic activity of AuNPs. Increasing HS-PEG molecular weight and decreasing surface coverage was found to correlate directly with increasing rate constants and decreasing induction time. Time-resolved UV-Vis absorbance spectroscopy of 2-mercaptobenzimidazole (2-MIB) adsorption on AuNPs was used to study the ligand adsorption kinetics and to quantify the free active sites available for catalysis as a function of HS-PEG molecular weight and packing density. HS-PEG packing density and estimation of free active sites, coupled with the kinetics of 2-MBI adsorption onto AuNP ruled out the possibility of an educt diffusion barrier as the main cause of reduced catalytic activity and induction time for HS-PEG functionalized AuNPs (molecular weight ≥ 1 kDa). Instead, selective blocking of more active sites by adsorbed thiol functionality is attributed to the induction period and reduced catalytic activity. It is also noticed that H- induced desorption/mobility of thiols regenerates the catalytic activity.

KEYWORDS: Gold nanoparticles, PEG, catalytic activity, induction time, packing density


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INTRODUCTION Metal nanoparticles of different sizes, shapes, and composition have many important catalytic applications for petroleum processing, chemical conversion, and pollution remediation. Due to their exceedingly high surface-to-volume ratios and abundance of edge and corner atoms, metal nanoparticles have superior or novel catalytic properties which are not revealed in bulk forms. During the past few decades, gold nanoparticles (AuNPs) have attracted enormous attention in the field of chemical synthesis due to their unique stability, selectivity, and catalytic activities for transformation reactions. The past 30+ years have seen an explosion in the number of nanomaterials synthesized with different sizes, shapes, crystallinity, and atomic composition; all with unique properties, which offer a plethora of opportunity for new catalysts.1-6 The dilemma is that a majority of the synthesis methods require solution based methods and colloidal chemistry to obtain the desired materials. Thus, surface bound ligands are required to preserve the nanostructures, which in turn acts to block the reactive surface sites and eliminate catalytic activity. For example, organosulfur compounds are excellent nanoparticle capping ligand but are well-known poisons of AuNP catalysis.7-9 To circumvent this dilemma, the nanomaterials can be deposited onto a support and treated to remove the bound ligand, but this results in potential changes in the surface properties, significant decrease in the available surface area, and influence of the support material on the catalyst activity, which can be advantageous or detrimental.10 Therefore, there is a tradeoff between the stability of colloidal nanoparticles in solution and their catalytic activity due to passivation of active surface sites.11,12 While understood phenomenologically, an in-depth study of the impact of stabilizing ligand on NP catalytic activity is an area requiring exploration.


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Quantitative kinetic analysis is imperative to understanding the effect of stabilizing ligands on the catalytic activity of nanoparticles. For this study, the catalytic activity of thiolated polyethylene glycol functionalized AuNP (AuNP-SPEG) is probed with a model reaction; the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by sodium borohydride (NaBH4). It is well documented that this reaction can be catalyzed by AuNPs, and the reaction kinetics can be simply monitored by the change in intensity of 4-nitrophenolate peak at 400 nm using UV-Vis spectroscopy.13-15 An important feature of most reactions catalyzed by ligand functionalized nanoparticles is the induction time, t0, which can take seconds to several minutes. As reported for a number of nanoparticle catalysts, the catalytic reaction starts only after this induction time, and several models have been offered to explain this phenomena.15-19 In this work, the AuNP catalyzed reduction of 4-nitrophenol is investigated as a function of thiolated polyethylene glycol (HS-PEG) chain length and packing density. Investigating the correlation between the molecular structures of different PEG molecular weights and the percent surface coverage with the catalytic activity of AuNPs is important to our fundamental understanding of the mechanism and rate of catalytic activity for other redox reactions. The effect of HS-PEG on catalytic activity of the AuNP was studied using a combination of dynamic light scattering (DLS), time-revolved UV-Vis spectroscopy, quantitative ligand adsorption method (Thermogravimetic analysis, TGA) and free surface area binding site availability using quantitative 2-MBI adsorption method. EXPERIMENTAL SECTION Chemicals and Methods. All chemicals were purchased from Sigma-Aldrich except the thiolated PEG ligands, which were purchased from Laysan Bio, Inc., AL, USA. All chemicals


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were used without further purification. Water was deionized and filtered by a Milli-Q water system. UV-Vis spectra were taken using a Varian Cary 50 UV-Vis-NIR spectrophotometer. HS-PEG Functionalized AuNP Synthesis. Citrate-stabilized AuNPs were first synthesized by the classic citrate reduction methods.20 In brief, 250 µL of 0.05 M HAuCl4 aqueous solutions were heated while gently stirring. Upon boiling, 2.0 mL of 0.05 M citrate in H2O was added and the resulting solution was stirred at 400 rpm for 15 min as the color of the solution changed from colorless to red. PEG-stabilized AuNPs (AuNPs-SPEG) were prepared by ligand exchange reactions between citrate-stabilized AuNPs and the thiolated PEG. In brief, 10 mL of 3 mM HS-PEG in H2O was added to 20 mL of as-synthesized AuNPs and incubated for 24 h before use. Free citrate and HS-PEG in the AuNP-SPEG was removed by centrifuagal precipitation and re-dispertion in neat H2O. This purification process was repeated 4 times. The packing density of HS-PEG1K on AuNPs was varied by mixing citrate stabilized AuNPs with 0, 2.5, 5.0, 25.0, and 150 µM nominal concentations of HS-PEG1K. In brief, 10 mL of 0, 7.5, 15.0, 75.0, and 450 µM HS-PEG1K in H2O was added to 20 mL of as-synthesized AuNPs and incubated for 24 h before use. Free citrate and HS-PEG1K in the AuNP-SPEG1K was removed by centrifuagal precipitation and re-dispertion in neat H2O. This purification process was repeated 4 times. Thermogravimetric (TGA) Analysis. TGA experiments were performed on a TA instruments SDT Q600 to quantify the amount of PEG grafted onto AuNPs. In short. 50 mL of purified AuNP-SPEG was concentrated into 60 µL by centrifugation at 14500 rpm for 1 hr and 50 µL of concentrated AuNP-SPEG was deposited into a clean alumina TGA pan. The temperature was ramped to 100 °C at 10 °C/min and held for 10 min to remove solvent. The temperature was then ramped to 600 °C at 10 °C/min under a N2 purge of 20 mL/min.


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4-Nitrophenol Reduction Catalysis. The 4-NP reduction kinetics was performed in a 4 mL quartz cell using a Varian Cary 50 UV-Vis-NIR spectrophotometer. In brief, 0.25 mL of 8.4 nM AuNP-SPEG solution, 1.45 mL of H2O, and 1.00 mL of 0.2 mM 4-NP were mixed in a 4 mL quartz cell. Time-resolved UV-Vis spectra were taken immediately after addition of freshly prepared 0.30 mL of 0.1 M NaBH4 in water. The progress of the reaction was tracked by monitoring the change in intensity of 4-NP peak at 400 nm as function of time. 2-MBI Adsorption onto AuNP-SPEG Composites. Time-resolved UV-Vis spectra were taken immediately after addition of 1 mL of 20 µM 2-MBI in water into 1 mL of purified AuNP-SPEG composites. The 2-MBI adsorption kinetics was tracked by monitoring the change in intensity of 2-MBI peak at 300 nm. Transmission Electron Microscopy (TEM) Analysis. The particle size and morphological structure of AuNPs were examined with TEM. TEM samples were prepared by drop casting ̴ 5 µL of as-synthesized citrate capped AuNPs dispersion onto a 300 mesh Cu grids covered with a formvar carbon film, followed by solvent evaporation. TEM images were obtained using a Hitachi 7600 with an accelerating voltage of 120 kV. The size distributions were obtained by image analysis with the ImageJ software package. Dynamic Light Scattering (DLS) Measurements. The purified AuNP-SPEG solutions were diluted 5 times prior to DLS measurements. The measurements were obtained using a Malvern instrument (Zeta sizer Nano series) at 25 °C. Hydrodynamic diameter (number weighted) were calculated with four independent measurements.


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RESULTS AND DISCUSSION Citrate-stabilized AuNPs were first synthesized by the classic citrate reduction method.20 TEM analysis shows the average size of as-synthesized AuNPs is 13.7 ± 1.4 nm in diameter (Supporting information Figure, S1). Thiol head group containing poly(ethylene glycol), (HSPEG) was used to attach polymeric PEG chains onto the surface of AuNPs by ligand exchange reaction between citrate-stabilized AuNPs and the HS-PEG. Thiols are widely employed as an anchoring group for metal nanoparticles due to the strong Au-thiol bond.21 HS-PEG with nominal molecular weights of 1, 2, 5, 10, and 30 kDa were used in this work and are referred to hereafter as HS-PEG1K, HS-PEG2K, HS-PEG5K, HS-PEG10K, and HS-PEG30K. Dynamic Light Scattering (DLS) measurements of the HS-PEG stabilized AuNPs are shown in Table 1. The hydrodynamic diameter of NPs is increased with increasing PEG chain length on AuNPs, indicates the stability of PEG functionalized AuNPs in solution. The PEG layer thickness on the nanoparticle surface is a one way to determine the structure (mushroom-like or brush-like) of the PEG on nanoparticle surface. The structure of PEG mushroom or brush caps might affect the surface area passivated by the PEG stems on AuNPs which is significantly effects the catalytic rate of the reaction. The hydrodynamic diameter of citrate-stabilized AuNPs increased by 3.4, 7.2, 15.4, 31.8, and 51.0 nm after increasing molecular weight HS-PEG functionalization (Table 1) which are comparable to literature reported PEG thickness values for mushroom-like conformation.22-23 UV-Vis spectra also confirmed the stability of AuNPs functionalized with HSPEG of different chain lengths in solution (Supporting information, Figure S2). The quantity of HS-PEG adsorbed on AuNPs was determined by thermogravimetric analysis (TGA) (Supporting information, Figure S3). The percentage weight loss of HS-PEG adsorbed onto AuNPs (Table 1) was used to calculate the polymer packing density by modeling the AuNPs as 13.7 nm diameter spheres (Supporting information Figure, S1). The HS-PEG packing densities


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on AuNPs as a function of chain length were calculated from the weight fraction of HS-PEG with respect to the AuNP core determined by TGA (Table 1) and the AuNP surface area. The HS-PEG packing density reported in Table 1 shows an excellent correlation with chain length. As the chain length increases the HS-PEG packing density on AuNP decreases, which is consistent with literature reported trends.24 Our packing density values for HS-PEG2K (2.15 molecules/nm2) and HS-PEG5K (0.82 molecules/nm2) samples are lower than packing density of HS-PEG2K (3.93 molecules/nm2) and HS-PEG5K (2.40 molecules/nm2) on 15 nm AuNP reported by Rahme et al.24 however our values are higher than the packing density values for HS-PEG2K (0.28 molecules/nm2) and HS-PEG5K (0.09 molecules/nm2) on 214 nm gold nanoshells reported by Levine et al.25 It must be noted that the HS-PEG1K sample yielded a packing density of 2.61 molecules/nm2 which is about two times higher than the reported HS-PEG1K monolayer packing densities on 60 nm AuNPs (1.39 molecules/nm2).26

Table 1. The hydrodynamic diameter by DLS, percent weight lost by TGA and calculated packing density for the AuNP-SPEG materials. Hydrodynamic PEG layer Weight Packing density AuNP composite diameter (nm) thickness (nm) loss % (molecules/nm2) AuNP-citrate 17.7± 1.9 N/A N/A N/A AuNP-SPEG1K 21.1 ± 1.8 1.7 ±1.3 9.0 2.61 AuNP-SPEG2K 24.9 ± 2.2 3.6 ± 1.5 14.0 2.15 AuNP-SPEG5K 33.1 ± 2.1 7.7 ± 1.4 13.5 0.82 AuNP-SPEG10K 49.7 ± 5.0 15.9 ± 2.7 18.8 0.61 AuNP-SPEG30K 68.7 ± 6.6 25.5 ± 3.4 35.0 0.47 Error bars represent the standard deviation of 3 measurements

Several chemical reactions can be catalyzed by colloidal gold nanoparticles, including alcohol oxidation,27 nitroarene reduction,28 and carbon-carbon cross coupling reactions.29 In this work, AuNPs catalyzed reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by sodium


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borohydride (NaBH4) in water was a model reaction chosen to study the impact of ligand on AuNPs catalysis. The reaction kinetics can be easily and rapidly monitored in situ by the change in intensity of the 4-nitrophenolate absorbance peak at 400 nm using time-resolved UV-Vis spectroscopy (Supporting Information, Figure S4).13-15, 30 Figure 1A shows the effect of HSPEG chain length on the intensity of 4-nitrophenol (4-NP) peak at 400 nm as a function of reaction time. The rate constant is obtained by fitting the data from Figure 1A to pseudo-firstorder reaction kinetics with respect to 4-NP concentration and the rate constant is indicative of catalytic activity. The linear relationship for -ln(Ct/C0) versus time (Figure 1B) supports the pseudo-first-order kinetic assumption. The calculated reaction rate constant and induction time from three independent measurements for different HS-PEG functionalized AuNPs are averaged and presented in Figure 1.

Figure 1: The effect of HS-PEG chain length of AuNP-SPEG on catalytic activity. (A) The progress of the reaction tracked by the change in 4-NP absorbance peak at 400 nm over the time, (B) Fitting the absorbance data to pseudo-first-order reaction kinetics with respect to 4-NP, and (C) Summary of catalytic reaction rate and reaction induction time as a function of HS-PEG chain length.

The rate constants for the AuNP-SPEG1K, AuNP-SPEG2K, AuNP-SPEG5K, AuNPSPEG10K, and AuNP-SPEG30K are 0, 0.06, 0.15, 0.28, and 2.1 min-1, respectively (Figure 1).


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These rate constants are lower than that of the citrate-capped AuNPs which is 2.3 min-1 (See SI). Complete binding of PEG-SH1K on AuNPs completely inhibited the catalytic activity as indicated by no changes in 4-NP peak intensity. When the HS-PEG molecular weight is 2 kDa and above, the AuNP-SPEG showed excellent catalytic activity, which increased with increasing HS-PEG chain length. It has been reported that polymer and dendrimer stabilized AuNPs have shown a reduction in catalytic activity and emergence of an induction time which can take up to several minutes.5, 31-32 The induction phenomenon is observed in colloidal catalysis as well as some heterogeneous catalytic reactions and is attributed to either a diffusion barrier caused by ligand on the NP surface15-16 or NP surface restructuring induced by the adsorbed reactant molecules.17-19 Regardless, the cause or exact mechanism is elusive and an issue of debate. The catalytic reaction on the AuNP surface has been hypothesized to follow a LangmuirHinshelwood mechanism, where both BH4- and 4-NP initially adsorb on the nanoparticle surface where the BH4- adsorption leads to a surface-hydrogen species.18, 33 Thus, the rate of the reaction is governed by the free surface area available for the reaction and diffusion through the HS-PEG layer. The structure of the HS-PEG on AuNP surface is governed by the HS-PEG surface coverage and chain length, as well as solvent interactions. Previous researchers have proposed “mushroom-like” or “brush-like” conformation of thiolated PEG on AuNPs in water depending on the chain length, surface coverage of PEG, and the curvature of nanoparticles.34-35 However, detailed information of the size, mobility and permeability of PEG mushroom or brush caps are unavailable. Also the surface area passivated by the HS-PEG mushroom or brush caps on AuNPs, which have potential to significantly affect the reaction rate, are not reported.


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Figure 2: 2-MBI adsorption kinetics onto AuNP functionalized with HS-PEG1K, HS-PEG2K, HS-PEG5K, HS-PEG10K, and HS-PEG30K. The kinetics of the 2-MBI adsorption was tracked by monitoring the change in absorbance of 2-MBI peak at 300 nm over the time in 2-MBI/AuNPSPEG mixture (Detailed information can be found in supporting information).

In this work, the available surface area not passivated by HS-PEG molecules on AuNPs was calculated using a ligand absorption experiment introduced by Zhang et. al.22 Here, 2mercaptobenzimidazole (2-MBI) was used as a probe molecule to determine the ligand adsorption kinetics and free surface area (catalytic active sites) available on HS-PEG functionalized AuNPs. 2-MBI adsorption onto different molecular weight HS-PEG functionalized AuNPs as a function of time are shown in Figure 2. More detailed information about the 2-MBI adsorption experiment can be found in Supporting Information S5 and under the experimental section. Figure 2 shows that there is no detectable 2-MBI adsorption observed onto the AuNP-SPEG1K particles. This is attributed to the high packing density of HS-PEG1K (2.61 molecules/nm2). The well-ordered closed packing may cause a high diffusion barrier and/or leaves no space under the HS-PEG1K layer on AuNP surface to adsorb small molecules. This adsorption experiment explains why no catalytic activity was observed for AuNP-SPEG1K. The


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amount of 2-MBI adsorption onto the AuNP surface increases as the HS-PEG molecular weight increases from 2 kDa to 30 kDa. It is also observed that there is no induction time, t0, for 2-MBI adsorption, suggesting that an educt diffusion barrier is not limiting the catalytic reaction rate and t0 for larger molecular weight HS-PEG (2-30 kDa) functionalized AuNPs. It must be noted that the ligand adsorption experiment shows more than 50 % of 2-MBI adsorbed onto AuNPSPEG (2-30 kDa) in less than a few seconds. For example, the t0 for AuNP-SPEG10K is 2.3 min (Figure 1) however more than 80 % of 2-MBI is adsorbed onto AuNP-SPEG10K surface within 0.4 min (Figure 2). Therefore, we can rule out the possibility of diffusion barrier as a significant influence in the catalysis for the studied nanoparticles. By deduction, reconfiguration of the AuNP surface and adsorbed HS-PEG is likely the cause of the induction time and reduction in catalytic activity. It is reported that most catalytic nanoparticle active sites are located mostly on the edges and corners.36 Thiols preferentially bind to the most active sites on AuNPs which leads to a specific site blocking and causes a deactivation of AuNP catalytic activity even when small amounts of thiols are added.36-37 We observed inhibition of catalytic activity for all HS-PEG functionalized AuNPs of at least several minutes before the reaction starts, compared to the citrate-capped AuNPs. Even low amounts of HS-PEG on AuNPs (0.47 molecules/nm2 in case of HS-PEG30K) inhibit the catalytic activity for at least few seconds due to the selective binding of thiols to most active sites on AuNPs. More detailed discussion of the induction phenomenon and catalytic activity regeneration after the induction time is discussed in later sections. After HS-PEG selectively binds to the most active sites, the available free active sites on AuNP-SPEG are estimated from 2-MBI adsorption data (Figure 2). From the saturation 2-MBI adsorption data, the nominal 2-MBI saturation packing densities on AuNP-SPEG2K, AuNPSPEG5K, AuNP-SPEG10K, and AuNP-SPEG30K were determined to be 0.28, 0.90, 1.79, and 2.77


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molecules/nm2, respectively (See SI). The estimated saturation packing density of 2-MBI on the citrate-capped AuNPs is 3.31 molecules/nm2, which is consistent with literature.21 Citrate is a weakly binding ligand to AuNPs surfaces, which is easily replaced by educts (BH4- and 4aminophenol).38 Thus, assuming that the entire surface (100%) of citrate capped AuNPs is available for 2-MBI binding and 4-NP reduction reaction, we can estimate the fraction of surface area available for catalytic reaction on AuNP-SPEG from 2-MBI packing density on AuNPcitrate and AuNP-SPEG. The estimated surface area available for catalysis on HS-PEG2K, HSPEG5K, HS-PEG10K, and HS-PEG30K compared to citrate capped AuNP were 9, 27, 54, and 84 %, respectively (See SI). The 4-NP and 4-AP product are of similar size as the 2-MBI molecule. Furthermore, 2-MBI preferentially binds to the free active sites on AuNPs, so it is reasonable to assume that the packing density of 2-MBI adsorbed onto AuNP-SPEG is equal or proportional to the free active sites for the reduction reaction. Therefore the estimated free active site density on AuNP-SPEG2K, AuNP-SPEG5K, AuNP-SPEG10K, AuNP-SPEG30K, and citrate capped AuNP were 0.28, 0.90, 1.79, 2.77, and 3.31 molecules/nm2, respectively. As the HS-PEG chain length increases the free active sites on the AuNP-SPEG increase. Since the catalytic reaction is a surface mediated reaction, it is reasonable to assume that the catalytic rate is proportional to the free active sites.


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Figure 3: The correlation between (A) 4-NP reduction reaction rate and free catalytic site density on AuNP-citrate, AuNP-SPEG1K, AuNP-SPEG2K, AuNP-SPEG5K, AuNP-SPEG10K, and AuNP-SPEG30K composites that was calculated from 2-MBI adsorption experiment, (B) 4-NP reduction reaction rate and HS-PEG packing density on AuNP that was calculated from TGA analysis, and (C) Induction time of 4-NP reduction reaction and HS-PEG packing density on AuNP-SPEG2K, AuNP-SPEG5K, AuNP-SPEG10K, and AuNP-SPEG30K composites. Error bars represent the standard deviation of 3 runs.

The correlation between the reaction rate constant and free active site density on AuNPSPEG estimated by ligand adsorption method is shown in Figure 3A. It clearly shows an increase in rate constant, k, with increasing available free active sites on AuNP-SPEG. It should be noted that the correlation in Figure 3A is not a linear relationship, particularly with inclusion of the SPEG30K data. Pal et. al. reported that the rate of the 4-nitrophenol reaction is proportional to the surface area and the amount of citrate capped AuNPs with 20 nm.39 Ballauff et. al. also observed that rate of the 4-NP reaction is proportional to the surface area of platinum nanoparticles with diameter of 2 nm and silver nanoparticles with diameters of ranging from 6.5 to 8.5 nm.40-41 Pal et. al. and Ballauff et. al. observed a linear correlation between the catalytic reaction rate and the estimated surface area.

39-41

They estimated the theoretical specific surface area of nanoparticles


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from TGA results and the TEM particle sizes. However, we have estimated the free surface area of HS-PEG functionalized AuNPs using MBI adsorption method. Another way of looking at the impact of nanoparticle stabilizing ligand on the catalytic activity of AuNPs is by correlating the 4-NP reduction rate with the HS-PEG packing density on AuNPs determined by TGA analysis (Figure 3B). The HS-PEG packing density on AuNP surface is decreased as the size of HS-PEG increases (Table 1). It is obvious from Figure 3B that as the HS-PEG packing density increases, 4-NP reduction rate decreases due to the reduction in active sites, but again not in a linear relationship. The combination of 2-MBI adsorption data and HS-PEG packing density from TGA indicates that HS-PEG selectively binds to active sites on AuNPs and decreases the free active sites as the HS-PEG chain length decreases due to steric effects. The ultimate result from the selective binding of the HS-PEG to AuNPs is reduced reaction rate as the HS-PEG chain length decreases. The correlation between the induction time and the HS-PEG packing density determined from TGA analysis (Figure 3C) shows a linear relationship. The quantitative ligand adsorption kinetics data (Figure 2) rules out the diffusion barrier limitation as the main cause of the induction time for 2-30 kDa HS-PEG functionalized AuNPs. The linear relationship between the induction time and the degree of surface binding favors a hypothesis of AuNPs surface restructuring phenomena. Catalytic activity of HS-PEG1K functionalized AuNPs with maximum passivation is completely inhibited, as supported by negligible 2-MBI adsorption data and negligible 4-NP reduction.

However, it is possible to reduce the degree of passivation by restricting the

concentration of HS-PEG1K available during the synthesis ligand exchange, thus controlling the packing density of HS-PEG1K on AuNPs.


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Figure 4: The effect of HS-PEG1K packing density on AuNP catalytic activity (A) The reaction was tracked by monitoring the change in absorbance of 4-NP peak at 400 nm over time for AuNPs functionalized with nominal concentrations of 0, 2.5, 5.0, 25, and 150 µM of HS-PEG1K, (B) Correlation between 4-NP reduction rate and free catalytic site density on AuNP-SPEG1K determined from 2-MBI adsorption method, (C) TGA curves of HS-PEG1K on AuNPs functionalized with nominal concentrations of 2.5, 5.0, 25, and 150 µM of HS-PEG1K, (D) Correlation between free catalytic site density on AuNP-SPEG1K determined from 2-MBI adsorption method and HS-PEG density on AuNP-SPEG1K determined from TGA analysis, and (E) Induction time of 4-NP reduction reaction and HS-PEG1K packing density on AuNP determined from TGA analysis.

Figure 4A shows the effect of HS-PEG1K packing density on the 4-NP absorbance peak intensity at 400 nm as a function of reaction time. The rate constants are obtained by fitting the data from Figure 4A to pseudo-first-order reaction kinetics with respect to 4-NP and the rate


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constant is indicative of catalytic activity (Supporting Information, Figure S6). The calculated reaction rate constants for AuNP functionalized with nominal concentrations of 0, 2.5, 5.0, 25, and 150 µM of HS-PEG1K are 2.30, 1.50, 1.20, 0.80, and 0 min-1, respectively and the induction times for those samples are 0, 0.2, 3.0, 6.7, and > 60 min, respectively. The catalytic activity of AuNPs functionalized with 150 µM HS-PEG1K was completely inhibited. As the concentration of HS-PEG1K is decreased, the catalytic reaction rate of AuNP-SPEG1K increases and the induction time decreases. TGA analysis (Figure 4C) and 2-MBI adsorption experiments (See SI) were employed to estimate the packing density and available free active sites, respectively. The free active site density on AuNP-SPEG1K estimated by 2-MBI adsorption (Supporting Information, Figure S7) 3.31, 2.98, 1.65, 0.33, and 0 molecules/nm2 for 0 µM, 2.5 µM, 5 µM, 25 µM, and 150 µM of HS-PEG1K nominal concentrations, respectively. As the HSPEG1K concentrations increase the free active sites on AuNPs decrease. Figure 4B shows the correlation between the reaction rate constant and free active site density on AuNP-SPEG1K calculated by 2-MBI adsorption method. Interestingly, the relationship is linear neglecting the two extreme points where zero and complete passivation occur. The nominal packing density of HS-PEG1K measured by TGA also shows a direct relationship with values of 1.13, 1.74, 2.45, and 2.74 molecules/nm2 for the increasing concentrations (Figure 4C). The nominal packing density value of HS-PEG1K on the AuNPs in 150 µM (nominal concentration) HS-PEG1K sample is similar to the 1 mM (nominal concentration) HS-PEG1K

packing density value (2.61

molecules/nm2) observed in the HS-PEG chain length study (Figures 1, 2, and 3). This indicates that 150 µM of HS-PEG1K produces a maximum surface coverage with a full monolayer of HSPEG1K, at our experimental conditions. Again the free active sites are inversely proportional to the HS-PEG1K packing density on AuNP-SPEG1K surface estimated from TGA analysis (Figure


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4D). The theoretical free active site density at 100 % surface coverage of HS-PEG1K (2.74 molecules/nm2) on AuNP surface was estimated by fitting the experimental data with linear regression and the estimated free active sites density was 0.03 molecules/nm2. This data indicates that the reason for no detectable 2-MBI adsorption onto fully monolayer (100%) of HS-PEG1K covered AuNPs is due to unavailable free active sites available for 2-MBI adsorption AuNPSPEG1K. This further confirms complete inhibition of catalytic activity of AuNP-SPEG1K due to unavailable free active sites available on AuNP-SPEG1K rather than an educt adsorption diffusion barrier. Again, the linear correlation between the induction time and the HS-PEG1K packing density is present in Figure 4E, suggesting a surface restructuring phenomena as the limiting mechanism. 2-MBI adsorption kinetic results clearly prove that educt diffusion barrier is not the main factor in determining the t0 for HS-PEG ≥ 2 kDa. To further confirm, we conducted experiments changing the reactant and catalyst mixing sequences. The order of addition influence for AuNPSPEG5K, 4-NP, and NaBH4 on catalytic activity and induction time was investigated (Supporting Information, Figure S8). The two components inside the parenthesis were mixed together and incubated for 4 min before the addition of the third component. Immediately after addition of the third component, time-resolved UV-Vis measurements were carried out. The data in the Supporting Information S8 clearly show that (AuNP-SPEG5K + 4-NP) + NaBH4 and (NaBH4 + 4-NP) + AuNP-SPEG5K sequences have little change in induction times which are 3.9 and 4.3 min, respectively. If the educt diffusion barrier was the primary reason for the induction time, then the induction time should be significantly reduced for the (AuNP-SPEG5K + 4-NP) + NaBH4 sample since the AuNP-SPEG5K and 4-NP were incubated for 4 min before addition of the NaBH4 to this sample. Alternatively, the induction time for (NaBH4 + AuNP-SPEG5K) + 4-


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NP sequence was significantly reduced to 0.4 min. This observation can be explained by NaBH4 induced surface reorganization or organothiol desorption from the AuNP surface.38 Ansar et. al recently reported that organothiols adsorbed onto AuNPs are completely desorbed by NaBH4 in water.38 Thiol adsorbed onto AuNPs through formation of strong Au-S bond which is 35-100 kcal/mol.21, 42 However, H- ions from NaBH4 have stronger binging affinity to the AuNPs than that of thiols.38 Given this difference in binding, we hypothesize two different potential mechanisms: one where H- adsorption promotes HS-PEG desorption and another that induces HS-PEG migration from active sites to less active sites thus freeing the active surface for catalysis.

Figure 5: Absorbance of 4-NP peak at 400 nm (□) and AuNP LSPR peak wavelength (○) as a function of time after addition of NaBH4 into the (A) AuNP-citrate/4-NP and (B) AuNPSPEG5K/4-NP mixtures. The blue line indicates the onset of reaction at t0.

HS-PEG desorption/migration by NaBH4 was studied during the 4-NP reduction reaction by AuNP-SPEG5K and compared with a control AuNP-citrate (Figure 5). HS-PEG5K desorption/migration kinetics was indirectly studied by monitoring the AuNP localized surface plasmon resonance (LSPR) peak wavelength change over the reaction time. We observed that the


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LSPR peak wavelength of citrate-AuNPs (520 nm) red shifted to 525, 524, 522, 521, and 520 nm after AuNP functionalized with HS-PEG1K, HS-PEG2K, HS-PEG5K, HS-PEG10K, and HSPEG30K, respectively (Supporting Information, Figure S2), where a red-shift of the LSPR peak is indication of HS-PEG binding to AuNPs.43-44 Similarly, a blue shift would be indicative of potential desorption, suggesting a desorption mechanism based on the observed blue shift of the AuNP LSPR peak after the addition of NaBH4 into AuNP/4-NP mixture. H- electron pumping into AuNP and very fast desorption of citrate from AuNP surface is evident by AuNP LSPR peak wavelength sudden jump from 520 nm to 513 nm immediately after NaBH4 addition into AuNPcitrate/4-NP mixture (Figure 5A).38 Citrate desorption kinetics is well correlated with the kinetics of 4-NP reduction reaction in the same reaction mixture. Immediate citrate desorption from AuNP surface likely explains why there is no detectable induction time for AuNP-citrate catalyst in 4-NP reduction reaction. Compared to citrate desorption, HS-PEG5K desorption is much slower (Figure 5B) due to the relative strength of covalent Au-S bonding and potentially, desorption occurs in two steps. It is possible that first desorbs weekly bound thiols to nonactive (or low activity) sites and followed by desorption of thiols bound to more active sites. Hpumping electron into AuNP and HS-PEG5K desorption from AuNP surface is evident by AuNP LSPR peak wavelength shifting from 522 nm to 519 nm immediately after NaBH4 addition into AuNP-SPEG5K/4-NP mixture. The AuNP LSPR peak wavelength shift from 522 nm to 519 nm may be due to the HS-PEG5K desorption from nonactive (or low activity) sites. This process is energetically more favorable than removing thiols from the most active sites where the thiols are strongly bound.36-37 Additional H- adsorption may induce HS-PEG5K mobility from more active sites to lower active sites and followed by desorption, which is evidenced by the catalytic reaction coordinating with the onset of the AuNP LSPR peak wavelength shift from 519 nm to


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516 nm. It is also noted that complete thiol desorption is not necessary for the AuNP catalytic reactions to start and some fractions of active sites are regenerated. Desorption of HS-PEG5K from the AuNP surface is suggested by the LSPR peak wavelength shift to 513 nm which is equal to LSPR peak wavelength of AuNP-citrate in 4-NP/NaBH4 mixture. A similar correlation is also observed for the AuNP LSPR wavelength blue-shift and absorbance of 4-NP changing over time for other molecular weights of HS-PEG functionalized AuNPs (data not shown here). The NaBH4 induced HS-PEG desorption can also explain the non-linear correlation between the rate constant and the free active sites calculated by 2-MBI adsorption method in Figures 3 and 4. The available surface area calculations were conducted for AuNP-SPEG samples in the absence of NaBH4 in the 2-MBI/AuNP-SPEG mixture. This means the 2-MBI adsorption method gives the available surface area for AuNP-SPEG before the reaction starts. However, in the actual catalytic reaction mixture, NaBH4 induced HS-PEG desorption from the AuNP surface will increase the available active sites. Thus, the available free active sites on AuNP-SPEG during the 4-NP reduction reaction would be higher than that estimated by the 2MBI adsorption method. In-situ determination of the available active sites on AuNP is not easily achieved. Thiolate modification of the AuNP surface can be further explained by understanding the electrochemical properties of AuNPs. AuNPs have collectively oscillating low-lying conduction band electrons that are delocalized around the NPs which makes them good candidates for redox catalysts. Thiols have a preference in binding to most active sites on AuNP via strong Au-S bonding and depletes the electron density on the AuNP surfaces.36-37 X-ray photoelectron spectroscopy (XPS) studies have shown that the surface gold atoms are slightly oxidized after thiol binding to AuNP surface,45-46 indicating slight electron movement from AuNP surface to


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the sulfur atoms. Density functional theory (DFT) calculations have also shown that surface electrons on AuNPs are donated to the thiolate, forming a strong Au-S bond.47 Altering the electronic property of AuNPs may alter the thiol capped AuNPs catalytic activity, rendering it inactive. The linear correlation between the induction time and the surface coverage (Figure 3 and 4) shows that higher thiolate coverage increases the induction time, which may be attributed to the larger electron deficiency on the NP surface. Addition of BH4- induces migration of thiols from the active sites and followed by desorption of thioled PEG from the AuNPs surface.38 It is also believed that H- transfer electrons to the AuNPs surface and the AuNPs act as “electron reservoirs”.38 Desorbing the thiols from most active sites on AuNP and H- ions pumping electron into AuNP lift the Fermi energy level of AuNP.38, 48-49 Altering the Fermi level changes the redox potential of the AuNP to initiate the 4-NP reduction reaction.48 In this model, we envision the Hplaying three roles, temporary removal of catalytic poisons (such as thiols) from the active sites on AuNPs surface, activating the AuNP surface for catalysis by pumping electron into AuNP, and stabilizing the AuNP surface against the NP aggregation in solution.

Conclusions

Our results show that increasing HS-PEG chain length and decreasing the HS-PEG surface coverage on AuNPs leads to increased catalytic activity. Functionalization of AuNPs with HSPEG1K at maximum coverage completely inhibits the catalytic activity; however, larger molecular weight ligands lead to increased available surface area and improved catalytic activity as a result of steric interactions. Combination of ligand adsorption experiments and TGA ligand


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quantification reveal that educt diffusion is not a limiting factor in the catalytic induction time of HS-PEG1K, HS-PEG2K, HS-PEG5K, HS-PEG10K, and HS-PEG30K functionalized AuNPs. Selective binding of thiols to the active sites is the likely cause of induction time and H- induced mobilization and desorption of thiols regenerates the catalytic activity. This work revealed that smaller PEG thiols are more prone to poison the catalytic activity of gold nanoparticles and larger molecular weight PEG thiols are better capping agents for gold nanoparticles used in aqueous catalytic applications. Study of the effects of ligand molecular structure and available free active sites on the nanoparticle surface is important for our fundamental understanding of the mechanisms and the catalytic activity of colloidal nanoparticles for different redox reactions.

ASSOCIATED CONTENT

Supporting Information: This material is available free of charge via the internet at http://pubs.acs.org. TEM image of citrate capped AuNPs, UV-Vis spectra of HS-PEG functionalized AuNPs, TGA curves of neat HS-PEG and HS-PEG capped AuNPs, packing density of different chain length of PEG on AuNPs, AuNP-SPEG catalyzed 4-nitrophenol reduction reactions, and available surface area measurements. Corresponding Author *Email: [email protected] ACKNOWLEDGMENT: This work was sponsored by the National Science Foundation grant No. CBET-1057633.


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TOC GRAPHICS


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Impact of Gold Nanoparticle Stabilizing Ligands on the Colloidal

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