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

Dynamic Nature of Thiolate Monolayer in Au25(SR)18 Nanoclusters Giovanni Salassa,*,† Annelies Sels,† Fabrizio Mancin,‡ and Thomas Bürgi*,† †

Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35122 Padova, Italy

‡

S Supporting Information *

ABSTRACT: Thiolate monolayer, protecting a gold nanocluster, is responsible for its chemical behavior and 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, intercluster 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 do 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 that of the 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, intercluster thiol exchange, collision, Au25(SR)18, cluster science

T

selective functionalization of AuNC. However, initial rate kinetic studies of the exchange reaction by Pengo and coworkers indicate that all of 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 co-workers 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

he gold−sulfur bond has been a solid base for more than 20 years of 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 raised a lot of interest in the past 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 a 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 self-assembly.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 © 2017 American Chemical Society

Received: October 2, 2017 Accepted: November 22, 2017 Published: November 22, 2017 12609

DOI: 10.1021/acsnano.7b06999 ACS Nano 2017, 11, 12609−12614

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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. Eighteen equiv 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 decreases, and simultaneously the signal of free 2PET appears during the exchange reaction. The evolution over time of the integrals of these signals was fitted (Figure 1b) with a three parameter 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 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. A 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-2-propenylidene]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. 1H 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 methods.26,27 Both clusters were oxidized by passage through a silica column in DCM under aerobic conditions.27 NMR-Monitored Thiol Exchange Reaction. In a screw-cup NMR tube, Au25(2PET)18 was dissolved in DCM-d2. After addition of free thiol (2PET, ButSH, or PhenSH) from a stock solution (10 μL of pure thiol in 0.6 mL of DCM) freshly prepared, proton spectra were acquired every hour for 14 h. 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 was fitted with a three-parameter exponential function included in the MestReNova 11.0.1:

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, e.g., 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, readsorbing 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 interligand interactions are quite weak with the thiols used and that the reason for the greater affinity of 2PET for the Au25 cluster should be searched elsewhere. This insight provides an interesting method to probe and dissect intramonolayer interactions more specifically by performing both types of reactions (with and without free ligand).

y = B + Fexp(−xG) where B is the value at which peak integral tends toward an infinite time. We use it to calculate the concentration of free thiols at the equilibrium that together with the distribution obtained by MALDI spectra allow to obtain a quantification of all the species at the equilibrium. The binomial distribution:

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 a 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 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. Surprisingly and importantly, the time-scale of these two reactions has the same order of magnitude (with the intercluster exchange even faster than the thiol exchange), demonstrating how reactive the monolayer is and 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.

f (x) =

n! px (1 − p)n − x x! (n − x)!

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

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b06999. Details on the thiol exchange experiments are provided. All of the NMR spectra and MALDI-TOF spectra are reported together with every single experiment condition (PDF)

AUTHOR INFORMATION Corresponding Authors

MATERIAL AND METHODS

*E-mail: [email protected]. *E-mail: [email protected].

Chemicals. Tetrachloroauric acid trihydrate (Sigma-Aldrich, 99.9%), sodium borohydride (Fluka, >96%), 2-phenylethanethiol (2PET, Sigma-Aldrich, 98%), 1-butanethiol (ButSH, Sigma-Aldrich, 99%), thiophenol (PhenSH, Alfa Aesar, 99+%), methanol (VWR, >99.8%), acetone (Fluka, >99.5%), methylene chloride (DCM, SigmaAldrich, >99.9%), toluene (Sigma-Aldrich, 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.

ORCID

Giovanni Salassa: 0000-0002-2396-3884 Thomas Bürgi: 0000-0003-0906-082X Notes

The authors declare no competing financial interest. 12613

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DOI: 10.1021/acsnano.7b06999 ACS Nano 2017, 11, 12609−12614