Revealing the Role of Electrostatics in Gold-Nanoparticle-Catalyzed

Sep 19, 2017 - Au NPs with varying surface charges were used as catalysts for the reduction of PNP by BH4– to demonstrate the role of electrostatics...
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Revealing the Role of Electrostatics in Gold Nanoparticle Catalyzed Reduction of Charged Substrates Soumendu Roy, Anish Rao, Gayathri Devatha, and Pramod P. Pillai ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02292 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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Revealing the Role of Electrostatics in Gold Nanoparticle Catalyzed Reduction of Charged Substrates Soumendu Roy, Anish Rao, Gayathri Devatha and Pramod P. Pillai* Department of Chemistry, Indian Institute of Science Education and Research (IISER), Dr. Homi Bhabha Road, Pashan, Pune – 411008, India. ABSTRACT: The potency of electrostatic effects arising from nanoparticle (NP) surface in Au NP catalyzed reduction of charged substrates are presented. The electrostatic potential around Au NPs is controlled by varying the nature of ligands and ionic strength of the medium. Favorable interactions arising from the attraction between oppositely charged Au NP and substrates results in the channeling of substrates to the NP surface, which in turn enhances the catalytic reduction. The positively charged ([+]) Au NP outperformed other NP systems despite having comparable or even lower surface area for adsorption, proving the exclusivity of electrostatics in catalysis. At least an order of magnitude higher concentration of negatively charged ([-]) Au NP is required to compete with the catalytic activity of [+] Au NP.

KEYWORDS. Catalytic reduction • Electrostatics • Nanoparticles • Surface chemistry • Substrate channeling The ‘sea of electrons’ contained on the surface of metal nanoparticles (NPs) has been actively used for various catalytic processes.1-9 The metal NP catalyzed reduction of nitro phenols to amino phenols by borohydride ions is often considered as a model reaction to test various hypotheses and catalytic properties of NPs.10-12 The metal NPs help to overcome the kinetic barrier for the reduction process by offering its surface for adsorption and mediating transfer of electron between the substrates.13 From a fundamental point, one of the challenges here is to identify and precisely control the factors influencing the diffusion of substrates to the NP surface. Factors such as NP crystallinity, ligand packing density, hydrophobicity and available NP surface area have all been well documented.14-22 Equally or even more important is the role of forces/and interactions between the NP surface and substrates on adsorption, which is scarcely studied. The long range forces, mainly electrostatics, have already been proved to play a pivotal role in a wide range of NP applications like self-assembly,23-28 sensing,29,30 light harvesting,31,32 nanobiotechnology33,34 etc. In catalysis too, the electrostatic forces should be able to channel the oppositely charged substrates onto the NP surface, thereby improving the catalytic efficiency and selectivity. In this regard, a complete and conclusive evidence – in terms of both attractive (favorable) and repulsive (unfavorable) forces – is required to ascertain the potency of electrostatic effect in metal NP catalyzed reductions. The successful demonstration of such an effect will be a conceptual breakthrough that can be adapted to other NP catalyzed reactions as well. Attempts have been made previously to study the effect of electrostatics on metal NP catalyzed reductions either in the presence of loosely bound charged surfactants or polymers.35-38 The surface area available for the substrate adsorption was high in such NP systems, which will strongly assist the catalysis under both favorable and unfavorable conditions.14 Thus, the observed catalytic properties will be an outcome of the combined effects from surface area and electrostatics. A better approach to study the sole effect of electrostatics in Au NP catalysis will be to use charged small molecules that are tightly bound on the NP surface. Such NPs have been used to study the effect of

donor-acceptor distance on the electron hoping from [-] Au NPs to substrates in the photocatalytic reduction of ferricyanides.39 Recently, we have employed Au NP functionalized with charged thiolates to demonstrate the power of electrostatics in achieving an unprecedented phenomenon of controlled aggregation.29 Similar sets of Au NPs have been used in the present work to provide the much awaited conclusive proof for the potency of electrostatic effect in Au NP catalyzed reduction of charged substrates. The catalytic reduction of 4-nitro phenol (PNP) by borohydride ions (BH4 ) in presence of Au NPs was selected as the model reaction. The effect of electrostatics on catalysis was studied by controlling the surface potential (Ψ) and screening length (κ-1) around the NPs via varying the ligands and ionic strengths, respectively (Figure 1). The range of electrostatic field experienced by the substrates (in terms of κ-1) will vary as a function of ionic strength of the medium.40 We have estimated that the surface potential (both favorable and unfavorable) arising from charged Au NPs of both polarities can spread up to ~ 3 nm from the surface of ligands (κ-1~ 3 nm), in the presence of 10 mM BH4 . Thus, the concentration of [-] substrates (here BH4 and phenolate anions) around the Au NP surface can be controlled by varying the surface potential, and this precisely forms the basis for our current work (Figure 1). The favorable interactions between [+] Au NP and [-] substrates helped in the channeling of the substrates towards the NP surface (Figure 1). Accordingly, the local concentration of the substrates increased around the ligand surface, which in turn enhances the substrate adsorption on NP surface. This resulted in the catalytic reduction of nitro arenes by [+] Au NP within 20 min, even when the catalyst concentration was as low as 0.1 mol % (25 pM in terms of NPs).41 Interestingly, no measurable catalysis was observed when unfavorable conditions for substrate binding were created using [-] Au NP as the catalyst (even up to ~ 1 mol %). At least an order of magnitude higher concentration of [-] Au NP was required to compete with the catalytic activity of [+] Au NP. Similarly, the [+] Au NP outperformed other NP systems despite having comparable or even lower surface area for adsorption, proving the

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exclusivity of electrostatics in Au NP catalysis. The effects of favorable and unfavorable interactions were successfully extended to the catalytic reduction of ferricyanides, ascertaining the scope of electrostatic effects beyond organic substrates. Moreover, the assistance of long range electrostatic forces helped in lowering the use of Au NP catalyst to picomolar level.

Figure 1. Electrostatically assisted channeling of substrates. (a) Schematics of various Au NP systems with varying surface charges under study. (b) The graph on the left shows the exponential decay of the surface potential around Au NPs of both polarities. Blue and red colors indicate positive and negative surface potentials arising from [+] and [-] Au NPs, respectively. The potentials of both polarities extend in the range of ~3.0 – 0.5 nm from the ligand surface, depending on the ionic strengths. The favorable interactions between [+] Au NP and [-] substrates assist in the channeling and adsorption of substrates onto the NP surface (top right), which is responsible for the dominance of [+] Au NP over [-] Au NP (bottom right) in the catalytic reductions.

Au NPs with varying surface charges were used as catalysts for the reduction of PNP by BH4 to demonstrate the role of electrostatics in NP catalysis. [+] and [-] Au NPs (5.4 ± 0.7 nm) were prepared by functionalizing the Au NP surface with N,N,N-trimethyl(11-mercaptoundecyl)ammonium chloride (TMA, [+]) and 11-mercaptoundecanoic acid (MUA, [−]) ligands, respectively.29,42 Details on the synthesis and characterization of AuNPs are provided in the Supporting Information (Figures S1 & S2). No noticeable reduction of PNP (100 µM) by BH4 (10 mM) was observed even after two days in the absence of Au NP (Figure 2a), as reported previously.10-22 However, a rapid reduction of PNP by BH4 was observed within 20 min when 0.2 mol % of [+] Au NP (50 pM) was used as the catalyst (Figures 2a & S3). The reduction process was accompanied by a decrease in the absorption of 4nitrophenolate ion with a concomitant formation of a new band around ~ 300 nm (Figure 2b). The presence of two isosbestic points around ~ 320 and ~ 280 nm confirm the formation of only one product, namely 4-amino phenol,12 which was well characterized using various analytical techniques

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(Figures S4-S6). The absence of a plasmon band around ~ 520 nm signifies the marginal use of Au NPs in catalytic reduction, which will boost its role in industrial applications. Interestingly, no appreciable catalytic reduction by BH4 was observed up to 2 days in the presence of 0.2 mol % [-] Au NPs (Figures 2a & S3). The PNP reduction by BH4 ions in the presence of Au NPs is reported to follow the Langmuir-Hinshelwood mechanism.16,43 This was verified for the present system by performing [+] Au NP catalyzed reduction as a function of PNP concentration (Section 4 in Supporting Information). A decrease in the catalytic activity of [+] Au NP (with lower rate constant and higher induction time) was observed with an increase in the PNP concentration. This along with a nonlinear correlation of rate constant with PNP concentration confirms that the catalytic reduction by [+] Au NPs is following the LangmuirHinshelwood mechanism (Figure S7).43 Accordingly, all the rate constants in the present work are estimated by assuming that the PNP reduction by Au NPs of both polarities is following the Langmuir-Hinshelwood mechanism.

Figure 2: Au NP catalyzed PNP reduction. (a) A complete dampening of PNP absorption peak at ~400 nm was observed in the presence of 10 mM BH4 and 0.2 mol % [+] Au NP. No appreciable spectral changes were observed up to 2 days in the presence of BH4 alone or BH4 and 0.2 mol % [-] Au NP. (b) A gradual decrease in the 4-nitrophenolate absorption with a concomitant formation of a new band ~300 nm was observed in the presence of 0.2 mol % [+] Au NP. (c) Progress of PNP reduction and (d) corresponding linearized data for the first order analysis by tracking the absorption changes of PNP peak at ~400 nm in the presence of varying concentrations of Au NPs.

The initial results clearly indicated that the favorable and unfavorable interactions arising from the electrostatics between the substrates and catalysts were crucial, which demands for a detailed investigation. Accordingly, timedependent absorption experiments were performed to study the progress of PNP reduction. An induction time of 0.7 - 1.5 min with a rate constant of 0.27 min-1 was observed for the PNP reduction in the presence 0.2 mol % [+] Au NPs, which improved upon increasing the mol % of [+] Au NP catalyst (Figures 2c,d). For instance, the induction time was lowered from 1.3 min to 0.4 min and the rate constant increased from 0.27 min-1 to 0.71 min-1 as the [+] Au NP concentration was increased from 0.2 mol % to 1 mol %, respectively (Figure 2c,d and TableS3). On the other hand, even 1 mol % [-] Au NP failed to catalyze the PNP reduction for up to 8 h (Figures

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2c,d and S8). Appreciable catalysis with [-] Au NP was observed only when its concentration was increased to 1.2 mol % (Induction time = ~ 30 min and rate constant = 0.1 min-1; Figure S8). At least an order of magnitude higher concentration of [-] Au NP was required to compete with catalytic performance of [+] Au NP. For e.g., an induction time of ~4 min was observed in the presence of 12 mol% of [-] Au NP (3 nM), whereas the reaction was spontaneous even with 1.2 mol % of [+] Au NP (300 pM; Figure S8 and Table S3, S4). The dominance of [+] Au NP over [-] Au NP was clearly seen at higher reaction volumes too (Figure S9). Recently it has been reported that the presence of dissolved oxygen and incubation of Au NP with BH4 has a strong influence on the induction time and rate constant in the NP catalyzed reduction of PNP.14,44 Accordingly, detailed catalytic studies were performed by removing the oxygen from the reaction medium to validate the effect of electrostatics under different experimental conditions (Details are provided in section 4 in Supporting Information). This was achieved by purging the substrates and NP catalysts separately with argon, followed by monitoring the reduction under argon atmosphere as reported previously.44 Alternately, the catalysis experiments were performed by first incubating Au NPs with BH4 for 15 min under ambient conditions before the addition of PNP, as reported previously.14 The catalysis with 50 pM [+] Au NPs proceeded without any induction time under both the above mentioned reaction conditions (Figures S10 and S11), as reported for BH4 44 and PEG stabilized14 Au NPs. Interestingly, no noticeable reduction of PNP was observed for even up to ~1 day when 50 pM [-] Au NP was used as the catalyst, under the same conditions (Figures S10 and S11). A clear dominance of [+] Au NP over [-] Au NP was even observed for higher catalyst concentrations (300 pM, 1.2 mol %), where NPs of both polarities are capable of catalyzing the PNP reduction (Figures S10 & S11). Thus, the effect of electrostatics on Au NP catalyzed reduction of PNP is consistently observed both in the presence and absence of dissolved oxygen. Consequently, all the further experiments were performed under ambient conditions that are most likely to be followed for practical applications. Next, our focus was to ascertain the role of electrostatics in the observed differences of catalytic efficiencies between [+] and [-] Au NPs. Our hypothesis is based on the favorable interactions arising from the electrostatic attraction between [+] Au NP and [-] substrates. A reduction in this favorable interaction, in principle, should lower the catalytic efficiency of [+] Au NPs. For this, first we decreased the favorable interactions through screening the charges on [+] Au NP surface by performing the reaction in high ionic concentrations (like Phosphate Buffered Saline, PBS). An increase of induction time from ~ 0.7 min to ~ 3 min and lowering of rate constants from 0.25 min-1 to 0.14 min-1 was observed when the reaction medium was changed from water to 1X PBS (Figures 3a, S12 and Table S5). The favorable interactions declined further when the salt concentration was increased by changing the reaction medium to 2X PBS, and no noticeable catalysis was observed for 2 days (Figures 3a, S12 and Table S5). The effect of ions on the screening of charges was supported by screening length calculations. The screening length decreased from ~ 3.0 nm to ~0.7 nm and further to ~0.5 nm as the medium was changed from water to 1X PBS to 2X PBS (Correlation of ionic strength and screening length with reaction kinetics is given in Table S5). Secondly, the favorable interactions were lowered

by diluting the [+] charges on Au NPs with neutral ligands (Figures 3b and S13). In one instance, the dilution was achieved by introducing dodecane thiol (DDT) onto the NP surface. This resulted in the formation of [+/DDT]1.5 Au NPs having ~70% TMA and ~30% DDT on Au NPs (as calculated from previous NMR studies).29,45 The decrease in the zeta potential of [+/DDT]1.5 Au NP to +12.3 ± 0.6 mV confirmed the reduction in the magnitude of surface potential (Figures 1 and S2). The induction time was increased to ~4 min and the rate constant was lowered to 0.16 min-1 when [DDT/+]1.5 Au NPs was used as the catalyst (Figures 3b, S13 and Table S3). Similarly, the use of neutral EG3SH functionalized Au NPs as the catalyst further decreased the favorable interactions (ζ = -1.2 mV ± 1 mV; Figures 1 and S2). This was evident from the increase in the induction time to ~18 min and lowering of rate constant to 0.10 min-1, as shown in Figures 3b and S13.

Figure 3: Proof for electrostatic effects in Au NP catalyzed PNP reduction. Kinetic traces for PNP reduction by 10 mM BH4 in presence of 0.2 mol % of [+] Au NP in (a) water (blue), 1X PBS (green) and 2X PBS (black) reaction medium. (b) Progress of PNP reduction by 10 mM BH4 in presence of 0.2 mol % [+] (blue), [+/DDT]1.5 (green) and neutral EG3SH (magenta) Au NPs in water. Insets of (a) and (b) shows the corresponding linearized data for the first order analysis assuming that the reduction follows the Langmuir–Hinshelwood mechanism. (c) Plot showing the variation in the rate constant of PNP reduction by 10 mM BH4 in the presence of different Au NP catalysts at different temperatures. (d) Plot of lnk vs 1/T for the PNP reduction by 10 mM BH4 in the presence of 1.2 mol % [+] and [-] Au NP. The corresponding individual kinetic decays at different temperatures are shown in Figure S18. Error bars corresponds to standard deviations based on at least three experiments.

The role of surface area available on the NP should be taken into account before confirming the effect of electrostatics in Au NP catalyzed reduction. The calculations based on two independent techniques, namely Thermo Gravimetric Analysis (TGA; Figure S15) and Inductively Coupled Plasma - Mass Spectrometry (ICP-MS), confirmed similar ligand density on both [+] and [-] Au NPs (Section 5 in Supporting Information). Remarkably, the catalysis by [+] Au NP was superior to even citrate stabilized Au NP (Figure S14). The latter being often considered as a benchmark catalyst as it provides ~ 100 % of its surface for catalysis.14 Thus, the [+] Au NP exhibited the highest catalytic activity among other NP systems despite having comparable or even lower surface area available for

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adsorption, confirming the exclusivity of electrostatics in the present studies (Table S3 & S6). Temperature dependent catalytic studies too proved the dominance of [+] Au NP over [+/DDT]1.5 and [-] Au NPs in the PNP reduction (Figures 3c, S16 and S17). Interestingly, 0.2 mol % of [-] Au NP was unable to catalyze the reduction even at 45 oC. Further, the effect of electrostatics on activation energy was studied. For this, 300 pM (1.2 mol %) of Au NP were used, since [-] Au NP failed to catalyze the reaction for ~ 8 h below this concentration. The distinction in the rate constant and induction time was maintained between [+] and [-] Au NPs even at 1.2 mol % and at higher temperatures (Figures 3d and S18). The apparent activation energy for PNP reduction was lower in the presence of [+] Au NP compared to [-] Au NP (Ea[+] Au NP ~ 30 kJ/mol and Ea[-] Au NP ~ 45 kJ/mol). The surface potential calculations of NPs with varying surface charges corroborate the role of electrostatics for the dominance of [+] Au NP in PNP reduction (see Section 4 and Table S6 in Supporting Information for details). The concept of electrostatics in catalysis was verified in other molecular systems too. The catalytic dominance of [+] Au NP over [-] Au NP was clearly observed in the reduction of 4-nitro aniline (PNA) by BH4 . The PNA reduction was completed within 20 min in the presence of ~ 0.1 mol % [+] Au NP, with an induction time of ~ 4 min and rate constant of 0.25 min-1 (Figure 4a,b). The production of diamino arene with merely 0.1 mol % of [+] Au NP (25 pM) will be significant in diazotization chemistry, especially in the dye industry. On the other hand, [-] Au NP failed to catalyze the PNA reduction even up to 2 days.

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A similar trend was observed in the reduction of ferricyanides by BH4 in the presence of Au NPs (Figures 4c,d). Here too, the effect of favorable interactions between [+] Au NP and [-] substrates (Fe(CN)63- complex and BH4 ) was evident. A complete reduction of ferricyanide absorption at ~ 420 nm was observed in presence of 0.2 mol % Au NP within 5 min, confirming the reduction of Fe(III) to Fe(II) complex46 (rate constant of 0.59 min-1; Figure 4c,d). However, the reduction of Fe(III) was not complete by BH4 even up to 12 h using 0.2 mol % of [-] Au NP (Figure 4d). Thus, the last example expands the scope of electrostatics in the catalytic reduction of inorganic complexes as well. In summary, the effects of electrostatics in terms of both attractive (favorable) and repulsive (unfavorable) interactions on Au NP catalyzed reduction of charged substrates was successfully demonstrated. The interactions between the substrates and Au NP surface was made favorable or unfavorable by fine tuning the NP surface potential. An electrostatically assisted channeling of substrates due to the attraction between oppositely charged substrates and Au NP surface was responsible for the dominance of [+] Au NP over other NP systems. A series of experiments with Au NPs with varying surface charges confirmed the exclusivity of electrostatics in catalytic reductions. The potency of electrostatics was established in [+] Au NP despite having a lower or comparable surface area for catalysis. The assistance of electrostatics helped in lowering the use of Au NP catalyst to picomolar level, which can boost its use in industrial applications. The concept of electrostatic effects was extended towards the catalytic reduction of ferricyanides, validating our hypothesis beyond organics substrates. A plausible future direction could be to take the assistance of such electrostatic interactions to develop smart catalysts comparable to enzyme catalysts, especially in terms of substrate selectivity. This can have long lasting implications in biocatalysis and NP based drug discovery research.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *[email protected]

Notes The authors declare no competing financial interests.

SUPPORTING INFORMATION Figure 4: Role of electrostatics in Au NP catalyzed reduction of PNA and ferricyanides. (a) A complete reduction of PNA absorption peak at ~420 nm was observed in the presence of 10 mM BH4 and 0.1 mol % [+] Au NP. No appreciable spectral changes were observed up to 2 days when 0.1 mol % [-] Au NP was used as the catalyst. (b) Kinetic traces for PNA reduction in presence of 0.1 mol % [+] and [-] Au NPs. Inset show the corresponding linearized data for the first order analysis. (c) Spectral changes confirming the reduction of ferricyanides by 10 mM BH4in presence of 0.2 mol % [+] Au NP. (d) Kinetic traces of absorbance at ~420 nm during the reduction of ferricyanide using 0.2 mol % [+] and [-] Au NPs. The inset shows the linearized data for the first order analysis for ferricyanide reduction by [+] Au NPs.

The Supporting Information is available free of charge on the ACS Publications website. Experimental details, characterization and additional kinetic data.

ACKNOWLEDGMENT The authors acknowledge the financial support from DST-SERB India Grant No. EMR/2015/001561. S. R. thanks UGC, and G. D. and A. R. thank MHRD for PhD fellowships. The authors thank SAIF IIT Bombay for ICP analysis.

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