Electrochemical Evaluation of the Number of Au Atoms in Polymeric

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Letter

Electrochemical Evaluation of the Number of Au Atoms in Polymeric Gold Thiolates by Single Particle Collisions Min Zhou, Dengchao Wang, and Michael V. Mirkin Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b05333 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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Analytical Chemistry

Electrochemical Evaluation of the Number of Au Atoms in Polymeric Gold Thiolates by Single Particle Collisions

Min Zhou, Dengchao Wang, and Michael V. Mirkin* Department of Chemistry and Biochemistry, Queens College, City University of New York, Flushing, New York 11367

*Corresponding Author E-mail: [email protected] FAX: 718-997-5531

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ABSTRACT Polymeric gold thiolates, [Au(I)SR]n, are common synthetic intermediate precursors of gold nanoclusters and larger nanoparticles. The size and dispersity of the precursors strongly influence the properties of the synthesis products. Evaluating the size of the precursors is not straightforward because they are irregularly shaped (non-spherical) and hard to isolate from solution. Herein we propose an effective method for determining the number of Au atoms in polymeric thiolate particles from current transients resulting from single precursor collisions, where individual [Au(I)SR]n species are electrochemically reduced at the collector ultramicroelectrode. The developed approach can lead to a better control over the mean size and dispersity of colloidal metal nanoclusters and nanoparticles.


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Gold nanoclusters and nanoparticles (Au NC and Au NP) have opened new avenues in catalysis, energy storage, and sensors due to their unique physicochemical properties.1-5 These properties depend strongly on the particle size,6,7 and the effective control of the NC size and dispersity is essential for catalytic8 and biomedical9 applications, and electron-transfer studies.10 A widely used synthesis of Au NCs consists of two major steps: the preparation of polymeric gold thiolate precursors, [Au(I)SR]n and their post-reductive decomposition into NCs.6,11-18 The synthetic

Au(I)SR [Au(I)SR]n , where Au(III) and route to [Au(I)SR]n precursors is Au(III)+RSH  REDUCTION

POLYMERIZATION

Au(I) represent trivalent and monovalent gold in the chloroauric acid and the intermediate precursors, respectively, and RSH is the thiol ligand. The single-phase synthesis of Au NCs has largely replaced the original two-phase (Brust-Schiffrin method11) due to its relative simplicity, the high yield, and better control over the properties of synthesis products. Extensive literature dedicated to the synthesis of Au nanoparticles reveals that the size and dispersity of polymeric precursors largely determine the properties of the prepared NPs.17,19,20 For instance, size-controlled water soluble Au NPs capped with glutathione (GSH) can be synthesized by tuning the solution pH.6,19 The process includes the formation of polymeric precursors, [Au(I)SG]n whose size and density depend on the pH. Higher pH favors smaller and less dense polymeric species, which, in turn, produce smaller NPs (i.e. NCs); while larger Au NPs are obtained at lower pH. To obtain small, low dispersity particles, Au NPs capped with pH-sensitive ligands are usually prepared in strongly basic solutions.21,22 For nonaqueous syntheses, Liu et al.17 reported strong dependence of the NC size on the nature of the solvent, e.g., Au25 was the main product in tetrahydrofuran (THF) and acetone, while Au144 primarily formed in methanol and acetonitrile. The effects of other synthetic parameters such as the nature of capping thiols, thiol-to-Au ratio, presence of oxygen, and co-dissolving reagents on



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the product size were relatively minor.16,17,23,24 These findings suggest that the solvation medium strongly influences the polymerization (i.e., size and dispersion) of gold precursors and in this way, determines the size of the produced NCs.16,17,20 To attain mechanistic understanding and standardization of the NC synthesis, one needs reliable methods for charactering reaction precursors. Evaluating the precursor size by currently available techniques, such as dynamic light scattering (DLS) and size exclusion chromatography,19 is not straightforward because the polymeric particles are generally nonspherical. These coil-shaped or multilayered structures19,25 cannot be easily isolated from solution for ex-situ characterization.17 Although polymeric precursors can be reduced electrochemically,26 the signal produced by reduction of a single Au thiolate species, from which the number of Au atoms in a precursor could be extracted, has not yet been measured. Herein we employ the electrochemical nanoimpact technique27,28 for characterizing synthetic precursors (Figure 1). The total amount of electrochemically active material in a nanometer-sized single entity (e.g., a metal NP,29-31 or a vesicle32) is determined from the current transient (Figure 1b) recorded during its collision with the collector electrode (Figure 1a). Using a micrometer-sized carbon disk electrode as a collector,33 we obtained a sufficiently high frequency of the collision events at sub-nM concentrations of polymeric thiolates and pA-range noise that did not obscure current spikes produced by discrete collisions of single species. a b Au particles isi on

Current

[Au(I)SR]n

ll co

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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reduction

UME

Time

Figure 1. Schematic representation of the collision of a single polymeric precursor with the collector UME (a) and the resulting current transient (b). The integration of the current under the peak yields the charge corresponding to the reduction of Au(I) contained in the precursor particle.


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Unlike catalytically amplified collisions, where a NP sticking to the collector surface produces a step in the current recording,27,33 a peak-shaped current spike (Figure 1b) should result from complete reduction of Au(I) species within a polymer particle.29 To characterize the electrochemical behavior of aqueous [Au(I)SG]n precursors prepared at pH 8.3, a thin film was formed by drop casting these polymeric species on the surface of the mm-sized glassy carbon electrode. Figure 2a shows a completely irreversible voltammetric peak of [Au(I)SG]n reduction at -0.6 V vs. Ag/AgCl obtained in degassed 10 mM KCl solution (adjusted to pH 8.3). No similar peak appeared in controlled experiments performed at the unmodified glassy carbon electrode in the same solution (data not shown). The reduction peak is greatly diminished in the second voltammetric cycle and essentially undetectable in subsequent cycles, indicating that most Au(I) moieties in a relatively thick film were efficiently reduced during the first cathodic sweep. These observations are consistent with the fast reduction of polymeric Au(I) reported in ref. 26 that can be related to its chemical reduction by NaBH4.16,17 Collisions of polymeric Au glutathionates with a 30 µm carbon UME were monitored in the same 10 mM KCl electrolyte (pH 8.3; volume 5 mL) to which different volumes of the precursor stock solution were added (Figure 2b). The collector electrode potential, E = -0.9 V vs. Ag/AgCl, corresponded to the transport-limited reduction of polymeric species (cf. Figure 2a). Unlike the featureless current vs. time curve obtained in the blank solution, a number of reduction spikes appeared in the 10 s long current recordings following the addition of the precursors. A zoom in of a representative current transient (Figure 2c) shows an asymmetric peak with the relatively short rise time (a few ms) and much longer (tens of ms) decay of the current to the background level. The shape of this spike is quite similar to the nanoimpact transients produced by complete oxidation of relatively large metal NPs.29 The average time



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between the current spikes is much longer than the transient time (Figs. 2b and 3b), so that the probability of two collision events occurring simultaneously is very low. No features attributable to multiple impact events34 produced by the same particle via partial reductions of a polymeric precursor (i.e., small spikes separated from each other by a few ms30,35) have been observed, pointing to strong adsorption of polymers on the UME. b

0 -2 -4

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4 2

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0

2 4 Injected volume, µL

6

0 0.0

0.5

1.0

1.5

2.0

2.5

Charge, pC

3.0

3.5

Figure 2. Electrochemistry of aqueous [Au(I)SG]n polymers prepared at pH 8.3. a) Three consecutive voltammetric cycles of polymer film drop casted onto the 3-mm-diameter glassy carbon electrode. b) Current transients produced by collisions of [Au(I)SG]n particles with the 30-µm-diameter carbon disk electrode. Five 10 s long current recordings were obtained with different injected volumes of the as-prepared polymer solution. E = -0.9 V vs. Ag/AgCl. The recordings are shifted vertically for better clarity. c) Zoom in of a single current transient from panel b (1 µL of [Au(I)SG]n solution). d) Dependence of the frequency of transient events on the injected volume. The precursor concentration was ca. 2 pM for 1 µL added volume. e) Charge distribution obtained by integrating the reduction current for individual collision events in panel b and the corresponding log-normal distribution (solid curve). Solution contained 10 mM KCl at pH 8.3. To exclude the possibility that the proton reduction at the electrodeposited gold contributes to the measured current, we obtained voltammograms at 25-µm-diameter Au UME in the same background solution (i.e. without Au precursors) that showed the onset potential of the hydrogen evolution reaction more negative than -0.9 V. Since the reduced gold species can only be glutathionate capped particles, the proton reduction at their surfaces should not occur at more


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positive potentials than that at the bulk gold,36,37 and no interference from this reaction at E = -0.9 V vs. Ag/AgCl can be expected. While the average peak height has not changed significantly with the added stock volume, the frequency of collision events was directly proportional to the precursor concentration (Figure 2d). The current spikes produced by electrochemical reduction of individual polymeric glutathionates (Figure 2b) showed relatively wide distributions of the peak half-width (t1/2 from 11 to 20 ms) and height (ip from 38 to 225 pA). The much narrower distribution of charge calculated by integrating the current under the peak (0.54 ± 0.03 pC in Figure 2e, where the uncertainty value is the standard deviation) suggests that the precursor size is narrowly distributed, and the larger variations in t1/2 and ip are due to the differences in collision dynamics. The blue curve in Figure 2e shows the log-normal distribution of the collision charge for all recorded impact events. Using the relationship N = QNA/F, where N is the number of Au atoms in a precursor species, Q is the collision charge, NA is the Avogadro constant, and F is the Faraday constant, one can calculate N from the determined average charge value. The results obtained in this way for gold glutathionates at different pH values (5.1, 6.5, 8.3, and 10.5) and summarized in Table 1 show that the aqueous precursor size depends strongly on solution pH. Three functional groups in a GSH ligand have different pKa values, 3.53, 8.66, and 9.62, and its net charge varies with pH up to ca. pH 9.19 The higher pH results in stronger electrostatic repulsion between the neighboring ligands and, consequently, the lower number of Au atoms in a polymeric structure (Table 1). Because the ionization of the thiol group (pKa = 9.62) can be ignored,19 the net charges of GSH ligand at pH= 8.3 and 10.5 are comparable, and there is no significant difference in the size of precursors formed at these pH values. The determined number of Au atoms in the precursors can be compared to that estimated



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from DLS experiments. A broad size distribution for the polymer precursors synthesized at pH 8.3 (Supporting Information) is centered at 230 nm, which corresponds to the effective average hydrodynamic radius, rh = 115 nm. Assuming the linear chain of [Au(I)SR]n, the gyration radius of the polymer is rg = 𝑎 𝑁/6, where N is total polymerization number, and a is the effective length of the monomer.39 Neglecting the difference between rh and rg (it is small compared to the uncertainly of the evaluated rh value given by the 40 nm standard deviation), one can estimate the number of Au atoms in the precursor as N = 6rh2/a2. Based on the Au-S bond length (0.23 nm) and S-Au-S angle (170o),39 a ≈ 0.45 nm, and the number of Au atoms in a ~115-nmradus polymeric gold thiolate is ~4x105. This number is somewhat smaller than those obtained from collision experiments (Table 1), suggesting that most Au(I) species in a precursor were reduced during its collision with the UME. However, this value can only be taken as a rough estimate because DLS cannot accurately measure the size of non-spherical, coil-shaped or multilayered precursor particles. The DLS measurements at pH 6.5 and 5.1 (data not shown) yielded the mean rh values similar to that measured at pH 8.3, and no pH dependence of the precursor size is apparent at this level of precision. Table 1. Dependence of the size of aqueous [Au(I)SG]n polymeric precursors on solution pH pH

Charge, pC

No. of Au atoms, x106

Molecular mass, x109

5.1

1.20 ± 0.06a

7.5 ± 0.4

3.8 ± 0.2

6.5

0.89 ± 0.04

5.6 ± 0.3

2.8 ± 0.2

8.3

0.54 ± 0.03

3.4 ± 0.2

1.7 ± 0.1

10.5

0.51 ± 0.04

3.2 ± 0.3

1.6 ± 0.2

The data shown in Table 1 can be compared to the number of Au atoms in the NPs synthesized in ref. 19 at different pH values. From the reported NP diameters (Fig. 3 in ref. 19), the number of Au atoms varies between ~3x103 at pH8 and ~6x104 at pH5. The numbers in Table 1 are 2-3 orders of magnitude larger, indicating that many NPs are formed from each precursor. THF has been widely used as a solvent suitable for precise control over the size and


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dispersity of clusters prepared by non-aqueous Au NC synthesis.40,41 To investigate the electrochemical reduction of gold thiolate precursors in THF, [Au(I)SC2H4Ph]n polymer solution was drop casted on a glassy carbon macroelectrode. Figure 3a shows three consecutive voltammetric cycles of the polymer-modified electrode, with a single reduction peak observed around -0.8 V. The absence of the oxidation wave on the anodic scan and greatly diminished reduction peak in the second and third cycles point to essentially complete and chemically irreversible reduction of the film. Control voltammograms at the unmodified electrode did not show a reduction peak. These results are indicative of the electrochemical reduction of Au(I) active sites in the polymers.26 a

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Charge, pC

Figure 3. Electrochemistry of [Au(I)SC2H4Ph]n polymers in THF. a) Three consecutive voltammetric cycles of the polymer film drop casted onto the 3-mm glassy carbon electrode. b) Current recordings obtained at the 30-µm-diameter carbon UME poised at -1.0 V in the absence (1) and presence (2) [Au(I)SC2H4Ph]n of polymers in solution. The precursor concentration was ca. 2 pM. The recordings are shifted vertically for better clarity. c) Representative current transient produced by a collision of a gold thiolate precursor with the carbon UME (from curve 2 in b). d) Charge distribution obtained by integrating the reduction current for individual collision events in curve 2 (panel b) and the corresponding log-normal distribution (solid curve). Deaerated THF solution contained 50 mM TBAPF6. To detect single polymer impacts (Figure 3b), a 30 µm-diameter carbon UME was placed



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in the degassed THF solution. The applied electrode potential, -1.0 V vs. Ag/AgCl, corresponded to the voltammetric peak of [Au(I)SC2H4Ph]n reduction in Figure 3a. Well-defined reduction spikes from the transient impacts were recorded in the presence of gold thiolate precursors (curve 2 in Figure 3b), but no such spikes occurred in the blank solution (curve 1 in Figure 3b) and in THF solutions containing either HAuCl4 or hexanethiol (Figure S1; Supporting Information). The average spike height was found to be potential dependent, and no spikes could be observed at potentials more positive than -0.55 V. These findings suggest that current spikes in curve 2 (Figure 3b) are due to the gold polymer impacts. The shape of a representative current transient (Figure 3c) is similar to that produced by [Au(I)SG]n reduction (Figure 2c) except for somewhat larger charge and longer duration pointing to a larger precursor size. The average charge values along with the corresponding numbers of Au atoms and molecular weights of the precursors in Table 2 were measured for two solvents (THF and acetone), two ligands (PhC2H4SH and C6H13SH), and also in the presence and absence of the TOABr surfactant. From Table 2, one can see that the variation of the last two factors resulted in no statistically significant difference in the precursor size. All polymeric precursors prepared in THF were sized similarly to those synthesized in pH 5 aqueous solution and larger than those prepared at higher pH values. The [Au(I)SR]n precursors prepared in acetone were slightly larger than those produced in THF under the same experimental conditions. Table 2. Size Distribution of Non-Aqueous [Au(I)SR]n Polymeric Precursors No. of Au atoms, x106 Molecular weight, x109

Solvent

Charge, pC

THF 1

1.18 ± 0.05a

7.3 ± 0.3

2.4 ± 0.1

THF 2

1.22 ± 0.07

7.6 ± 0.4

2.5 ± 0.1

3

1.16 ± 0.03

7.3 ± 0.2

2.3 ± 0.1

1.36 ± 0.05

8.5 ± 0.3

2.7 ± 0.1

THF

acetone 4 1

HAuCl4/TOABr/ PhC2H4SH molar ratio: 1/1.2/6; HAuCl4/PhC2H4SH molar ratio: 1/6; 3 HAuCl4/TOABr/C6H13SH molar ratio: 1/1.2/6; 4 HAuCl4/TOABr/PhC2H4SH molar ratio: 1/1.2/6. 2


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These findings suggest that comparable capacities of THF and acetone for solvating [Au(I)SR]n result in the similar size distributions of the polymeric precursors and synthesized NCs, e.g., in size-selective formation of Au25 NCs in both solvents.16,17 Because the NCs synthesized in methanol and acetonitrile were larger than those prepared in THF or acetone,17 one can hypothesize that the polymeric precursors produced in the former two solvents should contain more Au atoms. Unfortunately, collision experiments with the precursor intermediates prepared in methanol or acetonitrile could not be carried out because those synthetic thiolates quickly form large aggregates and precipitate.17 The formed precipitates could neither be redispersed nor dissolved in a different solvent (e.g., THF and acetone) for collision studies. In summary, we demonstrated for the first time that the number of Au atoms in synthetic precursors can be determined by analyzing current transients produced by collisions of single polymer species with an UME and obtained the first direct evidence of the pH effect on the precursor size. This method does not rely on any assumption about the products of the electrochemical reduction reaction. Chemical reduction of the same polymeric precursors produced Au NCs and NPs, but the nature of the products of the electrochemical reduction has yet to be investigated. The developed approach is rapid, precise, and requires relatively inexpensive instrumentation. The number of Au atoms in a polymeric precursor evaluated from DLS measurements agreed within an order of magnitude with the results of collision experiments. Our attempt to use DLS for accuracy verification was not successful because it is not really suitable for measuring non-spherical, irregularly shaped polymer particles. Thus, it was only possible to evaluate the precision of our results in terms of the reported standard deviation values, but not their absolute accuracy. The reduction of polymeric precursors is a general way to prepare NPs and NCs from



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metals other than Au as well as bimetal NCs.2 The nanoimpact analysis developed here has potential to improve the mechanistic understanding of and control over the synthesis of wide range of metal NCs, nanowires and nanosheets.1,42,43 ASSOCIATED CONTENT Supporting Information. Experimental methods, DLS characterization of [Au(I)SG]n polymers and control collision experiments. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The support of this work by the National Science Foundation (CHE-1763337) is gratefully acknowledged. We thank Prof. Uri Samuni and Dr. Jorge Ramos for assistance with DLS experiments and Dr. Je Hyun Bae for helpful discussions. REFERENCES (1) Carducci T. M.; Murray R. W. in Nanoelectrochemistry (Eds.: Mirkin M. V.; Amemiya S.), CRC Press, 2015, pp 73-124. (2) Jin, R.; Zeng, C.; Zhou, M.; Chen, Y. Chem. Rev 2016, 116, 10346-10413. (3) Qian, H.; Zhu, M.; Wu, Z.; Jin, R. Acc. Chem. Res. 2012, 45, 1470-1479. (4) Zamborini, F. P.; Bao, L.; Dasari, R. Anal. Chem. 2011, 84, 541-576. (5) Yamazoe, S.; Koyasu, K.; Tsukuda, T. Acc. Chem. Res. 2013, 47, 816-824.


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(36) Ly, L. S.; Batchelor-McAuley, C.; Tschulik, K.; Kätelhön, E.; Compton, R. G. J. Phys. Chem. C 2014, 118, 17756-17763. (37) Xiao, X.; Pan, S.; Jang, J. S.; Fan, F.-R. F.; Bard, A. J., J. Phys. Chem. C 2009, 113, 14978. (38) Flory, P. J. Principles of polymer chemistry; Cornell University Press, 1953, p. 428-429. (39) Bürgi, T. Nanoscale 2015, 7, 15553-15567. (40) Wu, Z.; Suhan, J.; Jin, R. J. Mater. Chem. 2009, 19, 622-626. (41) Dharmaratne, A. C.; Krick, T.; Dass, A. J. Am. Chem. Soc. 2009, 131, 13604-13605. (42) Hu, L.; de la Rama, L. P.; Efremov, M. Y.; Anahory, Y.; Schiettekatte, F.; Allen, L. H. J. Am. Chem. Soc. 2011, 133, 4367-4376. (43) Chen, L.; Chen, Y.-B.; Wu, L.-M. J. Am. Chem. Soc. 2004, 126, 16334-16335.



16

For TOC only

[Au(I)SR]n

ion llis co

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Page 16 of 16

Au particles

reduction

UME


Electrochemical Evaluation of the Number of Au Atoms in Polymeric

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