Dynamic Nature of Thiolate Monolayer in Au25(SR)18 Nanoclusters

Nov 22, 2017 - (2) Among them, thiolate-protected gold nanoclusters (AuNC) have raised a lot of interest in the past decade triggered by the ground br...
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Dynamic Nature of Thiolate Monolayer in Au (SR) Nanoclusters. Giovanni Salassa, Annelies Sels, Fabrizio Mancin, and Thomas Burgi ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06999 • Publication Date (Web): 22 Nov 2017 Downloaded from http://pubs.acs.org on November 23, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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ACS Nano

Dynamic Nature of Thiolate Monolayer in Au25(SR)18 Nanoclusters. Giovanni Salassa1,*, Annelies Sels1, Fabrizio Mancin2, Thomas Bürgi1,* 1 Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland. 2-Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, Padova, Italy.

ABSTRACT: Thiolate monolayer, protecting gold nanocluster, is the principal responsible of their chemical behavior and their interaction with the environment. Understanding the parameters that influence the stability and reactivity of the monolayer will enable its precise and controlled functionalization. Here we present a protocol for the investigation of the monolayer reactivity in Au25(SR)18 based on MALDI mass spectrometry and NMR spectroscopy. Thiol exchange reaction between cluster and thiol molecules has been investigated showing how this reaction is affected by several factors (stability of the thiols in solution, the affinity of the sulfur to the gold cluster, intermolecular interactions within the ligand layer, etc.). Furthermore, inter-cluster thiol exchange has been clarified to occur during collisions between particles without thiol-release to the solution. In this reaction the stability of the thiols in solution and the affinity of the sulfur to the gold for the two thiols does not affect the equilibrium position because for both thiols one S-Au bond is broken and one is formed within the cycle. Importantly, the rate of direct thiol exchange between clusters is comparable to the one for ligand exchange with free thiols. However, the thermodynamic driving force of the two reactions is different, since only the latter involves free thiol species.

KEYWORDS: monolayer protected cluster, thiol exchange, inter-cluster thiol exchange, collision, Au25(SR)18, cluster science The gold-sulfur bond has been a solid base for more than 20 years research in gold surfaces and nanostructures.1 Its strength allowed the creation of several hybrid compounds made of an inorganic gold core which is stabilized by an organic thiol monolayer.2 Among them, thiolate protected gold nanoclusters (AuNC) have risen a lot of interest in the last decade triggered by the ground breaking determination of their structures by x-ray crystallography.3–5 The structure of the gold−sulfur interface of thiolateprotected clusters is characterized by monomeric RS-Au-SR and dimeric RS-Au-SR-Au-SR units, usually called “staples”. This discovery set the basis for the design and interpretation of surface reactivity on thiolate-protected gold clusters and particles. Thiol exchange reaction is the major example of such reactivity.6 Thiols bound to staples can be substituted by other thiols present in solution. This reaction is employed for modifying the solubility7 of AuNCs, inserting the desired functionalities8 and inducing selfassembly.9 Several techniques have been employed for the investigation of thiol exchange reaction such as thermogravimetric techniques,8 MALDI,8 NMR,10 flourescence,11–13 x-ray crystallography,14 XAS,15 normal and chiral HPLC.16–19 Theoretical structural investigations20,21 suggested a SN2 type mechanisms for thiol exchange with some position preferred depending on the

cluster structure. Focusing the attention to small clusters, recent findings by Ackerson and co-workers suggest the possibility of selective exchange of a single position.11 This would enable selective functionalization of AuNC. However, initial rate kinetic studies of the exchange reaction by Pengo and co-workers indicate that all the positions may undergo simultaneous substitution, albeit at different rates.10 The composition of the ligand shell once the exchange reaction reached the equilibrium has scarcely been investigated, and this leaves open the possibility of selective cluster functionalization. The whole picture is further complicated by interclusters reactions, a barely explored aspect of AuNC chemistry. Pradeep and coworkers recently summarized the initial findings in the field.22 Gold and silver clusters have shown the ability to exchange metal atoms and ligands in solution. As a result either a cluster transformation23 or simply a modification of the composition of metal atoms and thiols has been observed.24 These events appear to involve a collision mechanism in which two clusters get in contact during reaction as we recently demonstrated for the case of silver atoms transfer between Au38 and AgxAu38-x.25 These recent discoveries on AuNCs and related systems are changing the general view on their nature. The ability to communicate between each other, exchanging metal atoms and/or thiols while maintaining their integrity, is an exciting characteristic of those systems, full of questions to address.

Scheme 1. Thiol exchange reaction of Au25(SR)18 and thiols used in this work.

In the present work, we investigate the reactivity of thiolate monolayer in Au25 cluster (neutral form) focusing our attention on mainly two thiols: phenyl ethane thiol (2PET) and butane thiol (ButSH). Not only they are among the most commonly used thiols to stabilize small gold clusters but they feature quite different chemical structure and bulkiness. We choose Au25(SR)18 among all the AuNCs due to its simple and highly symmetric structure (icosahedral Au13 core covered with 6 dimeric RS-Au-SR-Au-SR staples).3

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Results and Discussions Firstly, we studied the different affinity of 2PET and ButSH for the Au25 through thiol exchange reactions monitored by 1H-NMR and MALDI. The reaction was performed inside a NMR tube and 1 H-NMR spectra were collected every hour till the reaction reached equilibrium. Afterwards MALDI spectra were recorded to determine the distribution of all the substituted species. Figure 1a shows the time evolution of 1H-NMR for the thiol exchange reaction between Au25(2PET)18 and ButSH. 18 equivalents of ButSH were added to have the same amount of thiol in solution compared to the one bound to the clusters. From the NMR analysis, it was possible to observe how the signal of free ButSH is decreasing, and simultaneously the signal of free 2PET is appearing during the exchange reaction. The evolution over time of the integrals of these signals was fitted (Figure 1b) with a three parameters exponential function from which we could extrapolate the values of signal integrals at equilibrium and consequently the concentration of both free ButSH and 2PET. Furthermore, we observed that the reaction reaches equilibrium within less than a day, optimal time scale for NMR measurements. It is important to highlight that the signal of SH proton is present for both the incoming and leaving thiol. This confirms the proton transfer between the two thiols during the reaction as predicted by DFT calculation.20,21 Positive ion MALDI spectrometry was used to determine all the exchange products obtained during the reaction. Figure 1c shows the Gaussian shape distribution with the most abundant cluster formed being Au25(2PET)12(SBut)6. No clear peak of the starting material is observed, suggesting that all the initial clusters were consumed. If we consider a binomial distribution of a system made of 18 possible sites where each thiol has a probability of 50% (18 over 36) to occupy one of those 18 sites on the cluster,

the distribution of substituted species would be equal to the one reported in Figure 1d. Note that such a distribution would be expected for a system in equilibrium, where all sites on the cluster are identical and where the adsorption probability for both thiols is identical. In such a case the distribution only depends on the ratio of the two thiols in the system. In the case discussed above (Figure 1) the ratios is 1:1 and the most probable species is the one composed by 9 thiols of one type and 9 of the other. If one thiol is preferred on the cluster the distribution is shifted with respect to the statistical distribution towards this thiol. This is in fact the case for the example shown in Figure 1, as appears when Figure 1c and 1d are compared. While the number of species detected (12) is close to that expected for a binomial distribution, the most abundant species is Au25(2PET)12(SBut)6. Such a distribution, shifted towards 2PET enriched species, indicates that 2PET is preferred on the cluster. When we studied the opposite case where Au25(SBut)18 reacts with 18 equivalents of 2PET, the distribution at equilibrium was again in disagreement with the statistical one. As expected, the most abundant cluster is Au25(2PET)12(SBut)6 (see supporting info Figure S3), confirming that the equilibrium distribution is reached in the experimental conditions. The experiments described above indicate that the affinity of 2PET for Au25 is higher than the one of ButSH. A third thiol was used to corroborate this finding. Thiophenol (PhenSH) was selected since it is structurally different from the other two. Thiol exchange reaction with 18 equivalents of PhenSH with both Au25(SBut)18 and Au25(2PET)18 are reported in the Supporting Info. In the case of Au25(SBut)18, PhenSH was able to exchange butane thiol easily and almost statistical distribution was achieved (major species with 8 PhenS-bound, see Figure S9).

Figure 1. a) Evolution of the 1H-NMR peaks relative to ButSH entering thiol (2.53 ppm) and the 2PET leaving thiol (2.92 ppm) during the thiol exchange reaction between Au25(2PET)18 and 18 equivalents of ButSH. 1H-NMR is recorded every hour in DCM-2d. b) Exponential fitting of the integral of the entering ButSH (red crosses, 2.53 ppm) and leaving 2PET (blue circles, 2.92 ppm). c) Positive ion MALDI mass spectra of the thiol exchange reaction after 48 hours from the beginning. The exchange products have the general formula Au25(2PET)18-x(SBut)x, where x is the number of entering thiol. d) Binomial probability distribution for n = 18 and p = 0. 501409 (calculated from the ButSH relative percentage in the system).

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ACS Nano On the other hand, for the case Au25(2PET)18 reacting with PhenSH, a distribution centered around five exchanges only was obtained (see Figure S12). The affinity of the different thiols for the cluster staples is hence 2PET >> ButSH ≈ PhenSH. This gives us an indication why 2PET ligand was largely employed in the synthesis of gold clusters. Apparently, this thiol provides very good stabilization of the nanoparticle. Note that the charge state of Au25(SR)18 influences the NMR spectra.26,27 In our experiments there is no indication from the NMR spectra that the charge state changes during the reaction. Thiol exchange reaction can be pushed towards the extreme situation where all the ligands are substituted. Recently Dass and coworkers. have reported the complete exchange of Au38(2PET)24 with PhenSH.28 Their procedure implies two consecutive exchange reactions in presence of large excess of thiol. Mild conditions (room temperature for short time) were applied to prevent the simultaneous core-size transformation (from Au38(2PET)24 to Au36(SPhen)24) to happen. We employed the same approach to perform a complete thiol exchange of Au25(SBut)18 with 2PET and from the Au25(2PET)18 obtained we re-exchanged completely with ButSH to reform Au25(SBut)18. After every exchange, the reaction mixture was purified by size exclusion chromatography (SEC) to remove all the free thiol. As shown in Figure 2b, already after the first exchange with 2PET, the NMR spectra is mainly characterized by the Au25(2PET)18 peaks although some signals corresponding to mixed cluster are present. With the second exchange a pure Au25(2PET)18 NMR spectrum is obtained (Figure 2c). MALDI spectra confirmed the formation of Au25(2PET)18 (see Figure S14) and only a very small peak relative to Au25(2PET)17(SBut)1 was detected.

to Au25(2PET)18 proved how 2PET has a higher affinity for Au25 cluster than ButSH. The reverse reaction induced partially cluster transformation. This is in line with the recent literature.29,30 Some thiols may induce a rearrangement of the structure in particular conditions like high temperature or large thiol excess. Understanding in depth the influence of the thiol structure on the stabilization of cluster will enable us to have a complete control in the fabrication and functionalization of gold nanoclusters. Furthermore, we wondered if ligands could exchange between clusters without addition of free thiols via inter-clusters reactions. Negishi and coworkers have shown that after mixing Au25(SC10H21)18 with Au25(SC12H25)18 for 10 minutes, thiol exchange occurred between them (1-3 exchanges).16 In their work, the authors attributed this exchange of thiols to a detachment of ligand or gold-ligand species from the surface due to degradation of the cluster itself. Inspired by those results we mixed Au25(SBut)18 and Au25(2PET)18 in DCM (ratio 1:1) and we let them react for 48 hours, collecting MALDI spectra at regular time (see Figure 3 and supporting info). The spectra at 15 minutes showed a similar exchange as the one reported by Negishi and coworkers. After only 1 hour the extent of exchange was so large that initial clusters were not observable anymore. Up to the end of the 48 hours the MALDI spectra did not change significantly, suggesting that equilibrium was already reached. The distribution of the exchange species agreed well with the binomial distribution (Figure S17C).

Figure 3. Positive ion MALDI mass spectra collected at different time after the mixing of Au25(SBut)18 (orange) and Au25(2PET)18 (purple) with a ration 1/1 in DCM.

Figure 2. NMR of complete thiol exchange reaction in DCM-d2. a) Spectra of Au25(SBut)18 starting cluster, b) spectra of the purified material after first exchange with 2PET and c) after the second. d) and e) NMR spectra of the purified cluster after the first and second exchange with ButSH. The circles indicate the peaks relative to Au25(SBut)18 (orange) and Au25(2PET)18 (purple). The Au25(2PET)18 obtained from complete exchange, after purification, underwent two other consecutive exchanges with ButSH. The protocol successfully led to the reformation of the Au25(SBut)18. However, part of Au25 were transformed into bigger sized clusters as confirmed by the broader peaks underneath Au25(SBut)18 signals (Figure 2d-e) and the MALDI spectra (Figure S16). The almost quantitative conversion from Au25(SBut)18

We performed a second experiment in which we mixed Au25(SBut)18 and Au25(2PET)18 in a ratio 1:0.8 in a deuterated DCM and monitored the reaction for 48 hours with NMR. The first spectrum measured at 30 minutes was similar to the superimposition of the NMR spectra of pure Au25(SBut)18 and Au25(2PET)18. During the evolution of the exchange reaction the peaks became broader, less defined and new broad peaks appeared in only few hours. This resulted from the loss of symmetry due to the formation of a mixed monolayer and the concomitant presence of multiple species. MALDI spectra showed a distribution centered at Au25(SBut)10(2PET)8 and agreeing with the statistical distribution expected from 1:0.8 ratio (slight deficiency of 2PET, Figure S20). This experiment indicates that under these conditions there is no preferred composition of the cluster coating nor neighbor preference for the two thiols.

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As a control experiment, we added to the previous exchange reaction pure ButSH and 2PET (ratio 1:0.8, thus keeping the ratio of the two thiols). After 48 hours, at the equilibrium, MALDI showed that the distribution shifted towards the 2PET side (see Figure S21). The presence of free thiol then allows the cluster to rearrange the monolayer composition based on the affinity for each thiol. The mechanism of thiol exchange between clusters seems to occur without the release of low molecular weight species (thiol or gold-thiolate) in solution. In situ NMR investigation showed no clear signal of free ButSH or 2PET during the reaction time. Cluster degradation (reason for the release of the thiolate species) occurs after a week in solution, in stark contrast with the hour time scale of the reaction observed by MALDI and NMR. In order to shed light on the matter we envisioned an experiment employing a dialysis membrane similar as we have recently reported to study the metal exchange in small clusters.25 The membrane cutoff is 1 kDa allowing only small molecules to pass through. A solution of Au25(SBut)18 in DCM was placed inside the dialysis bag, afterward the bag was immersed in a beaker containing a solution of Au25(2PET)18 in DCM and let stirring for 5 days. MALDI spectra of the solutions inside and outside the dialysis membrane showed that the original cluster did not undergo any exchange of thiol during the 5 days (Figure 4). This demonstrates that the thiol exchange occurred through collision between clusters. In addition, we did not detect any significant degradation of the clusters, suggesting that in those conditions they are stable for more than 5 days.

reaction can be described formally by a two-step process, where one ligand is removed from the cluster to the solution and another one adsorbs in its place. Note that Scheme 2 does not refer to a reaction mechanism but rather to formal decomposition of the reaction to illustrate the contributions to the thermodynamic of the reaction. Evidently several factors contribute to the thermodynamic balance of this reaction including the stability of the thiols in solution (solvation, S-H bond energy etc.), the affinity of the sulfur to the gold cluster (which could be different for the two thiols due to for example electronic effects), and intermolecular interactions within the ligand layer (e.g. bulkiness, π−π interactions etc.). In contrast, direct exchange via collisions can be decomposed in two steps, where a thiol is removed from each of the two involved clusters, re-adsorbing on the respective other cluster. In this reaction the stability of the thiols in solution does not matter. Also, the affinity of the sulfur to the gold for the two thiols does not affect the equilibrium position because for both thiols one S-Au bond is broken and one is formed within the cycle. The relevant parameters are essentially the overall entropy of the system and the intermolecular interactions within the ligand layer, because the exchanged ligands exhibit different environment on the two involved clusters. The absence of any compositional preference we found here suggests that with the thiols used interligand interactions are quite weak, and that the reason of the greater affinity of 2PET for the Au25 cluster should be searched elsewhere. This insight provides an interesting method to probe and dissect intra-monolayer interactions more specifically by performing both types of reactions (with and without free ligand).

Scheme 2. Thermodynamic cycles for the thiol exchange between clusters, via free thiol (a) and via collision mechanism (b).

Figure 4. Positive ion MALDI mass spectra of a) Au25(SBut)18 solution put inside the dialysis membrane and b) Au25(2PET)18 solution in which the dialysis membrane was immersed. After five days of stirring, MALDI mass spectra of the inside c) and outside d) solution were recorded, showing no change. There are hence at least two different mechanisms for exchange of thiols between clusters, the first via free thiols (ligand exchange reactions) and the second without free thiols (through collisions between clusters). Even by using the same thiols, we found that the two mechanisms result in different equilibrium distributions. This is likely the result of the different system composition in the two cases as demonstrated by Scheme 2. The ligand exchange

Conclusion In conclusion, we demonstrated that thiolate monolayers of Au25 clusters have a dynamic nature. A thiol bound to the surface can be easily exchanged by another thiol present in the solution or by a thiol on another cluster in only few hours. These two types of exchange have a different mechanism as suggested by experimental data. Furthermore, the thermodynamic driving force for the two reactions is different leading to different equilibrium distributions. Direct exchange without free thiol is sensitive to intermolecular interactions within the ligand layer, whereas for

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ACS Nano ligand exchange with free thiol the stability of the two thiols in solutions as well as the respective Au-S interactions have to be considered as well. Surprisingly and importantly, the time-scale of these two reactions has the same order of magnitude (with the inter-cluster exchange even faster than the thiol exchange) demonstrating how reactive the monolayer is but also indicating that selective cluster functionalization may be difficult. The intercluster thiol exchange was demonstrated to occur via collisions rather than desorption of low molecular weight species in solution. However, we are still working to elucidate the parameters that influence this reaction.

over the total thiol concentration of the system. Note that strict binomial distribution would be expected if each binding site is equal with the other and no affinity for particular thiol is present.

Material and Methods

AUTHOR INFORMATION

Chemicals: Tetrachloroauric acid trihydrate (Sigma-Aldrich, 99.9%), sodium borohydride (Fluka, > 96 %), 2-phenylethanethiol (2PET, Sigma-Aldrich, 98 %), 1-butanethiol (ButSH, SigmaAldrich, 99%), thiophenol (PhenSH, Alfa Aesar, 99+%), methanol (VWR, > 99.8 %), acetone (Fluka, > 99.5 %), methylene chloride (DCM, Sigma-Aldrich, > 99.9 %), toluene (SigmaAldrich, 99.9%), PTFE syringe filters (0.2 µm, Carl Roth, Karlsruhe/Germany), and BioBeads S-X1 (BioRad) were used as purchased if not mentioned otherwise. Nanopure water (> 18 MΩ) was used. All other chemicals were commercial products and were used as received. Characterization: UV−vis spectra were recorded on a Varian Cary 50 spectrometer. Quartz cuvette of 10 mm path length was used (solvents: methylene chloride and toluene). Mass spectra were obtained using a Bruker Autoflex mass spectrometer equipped with a nitrogen laser at near threshold laser fluence in positive linear mode. Trans-2-[3-(4-tert-Butylphenyl)-2-methyl-2propenylidene]malononitrile was used as the matrix with a 1:1.000 analyte:matrix ratio. A volume of 2 µl of the analyte/matrix mixture was applied to the target and air-dried. NMR spectra were recorded on Bruker Avance 400 MHz spectrometer. 1 H NMR chemical shifts are given in ppm relative to SiMe4, with the solvent resonance used as internal reference. Synthesis of Au25(SR)18: Au25(2PET)18‒ and Au25(SBut)18‒ were prepared according to previously reported method.26,27 Both clusters were oxidized by passage through silica column in DCM under aerobic conditions.27 NMR-monitored thiol exchange reaction: In a screw-cup NMR tube Au25(2PET)18 were dissolved in DCM-d2. After addition of free thiol (2PET, ButSH or PhenSH) from stock solution (10 µL of pure thiol in 0.5 mL of DCM) freshly prepared, proton spectra were acquired every hour for 14 hours. Delay time between the thiol addition and the end of the first NMR-spectra was measured. The evolution in time of the peak integrals of free thiol were fitted with a three parameters exponential function included in the MestReNova 11.0.1:

Corresponding Author

‫ ܤ = ݕ‬+ ‫ି(݌ݔ݁ܨ‬௫ீ) B is the value at which peak integral tends towards at infinite time. We use it to calculate the concentration of free thiols at the equilibrium that together with the distribution obtain by MALDI spectra give allow to obtain a quantification of all the species at the equilibrium. Binomial distribution:

f ( x) =

n! p x (1 − p)n − x x !(n − x)!

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Details on the thiol exchange experiments are provided. All the NMR spectra and MALDI-TOF spectra are reported together with every single experiment condition.

[email protected] [email protected]

ACKNOWLEDGMENT Financial support from the University of Geneva and the Swiss National Science Foundation (grant number 200020_152596) is kindly acknowledged.

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X is a random variable which denotes the number of successes in n trials each with probability p. In the distributions reported below, x represents the number of binding sites on the Au25 cluster (x= 18). “p” was chosen as the percentage of entering thiol

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