Article pubs.acs.org/Langmuir
Spectroscopic Identification of the Au−C Bond Formation upon Electroreduction of an Aryl Diazonium Salt on Gold Limin Guo,†,§ Lipo Ma,† Yelong Zhang,†,§ Xun Cheng,∥ Ye Xu,∥ Jin Wang,†,‡ Erkang Wang,*,† and Zhangquan Peng*,† †
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China ‡ College of Physics, Jilin University, Changchun, Jilin 130012, China § University of Chinese Academy of Sciences, Beijing 100039, China ∥ Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States S Supporting Information *
ABSTRACT: Electroreduction of aryl diazonium salts on gold can produce organic films that are more robust than their analogous selfassembled monolayers formed from chemical adsorption of organic thiols on gold. However, whether the enhanced stability is due to the Au−C bond formation remains debated. In this work, we report the electroreduction of an aryl diazonium salt of 4,4′-disulfanediyldibenzenediazonium on gold forming a multilayer of Au−(Ar−S−S−Ar)n, which can be further degraded to a monolayer of Au−Ar−S− by electrochemical cleavage of the S−S moieties within the multilayer. By conducting an in situ surface-enhanced Raman spectroscopic study of both the multilayer formation/degradation and the monolayer reduction/oxidation processes, coupled to density functional theory calculations, we provide compelling evidence that an Au−C bond does form upon electroreduction of aryl diazonium salts on gold and that the enhanced stability of the electrografted organic films is due to the Au−C bond being intrinsically stronger than the Au−S bond for a given phenylthiolate compound by ca. 0.4 eV.
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Liu et al.8 prepared a stable monolayer by electroreduction of 4carboxylphenyl diazonium salt on gold and used the obtained carboxyl-terminated monolayer as a sensing platform, in which the presumed Au−C bond formation was invoked to account for the improved stability of the film. Lehr et al.9 reported that spontaneous reduction of diazonium salts by gold substrates in aqueous acidic solution at open-circuit potential could lead to the formation of an organic film on gold, which was less stable than its counterparts formed via electrografting in nonaqueous medium (e.g., acetonitrile). Shewchuk et al.10 conducted a comparison study of diazonium salt-derived and thiol-derived nitrobenzene films on gold and concluded that aryl films formed from the reduction of diazonium salts were more strongly bonded to gold surfaces than the organic thiol analogues, but no evidence of the Au−C bond formation was provided. Mirkhalaf et al.11 reported the simultaneous reduction of Au salt and long-chain alkane-terminated aryl diazonium salt by NaBH4, leading to the formation of isolated gold nanoparticles with narrow size distribution, and claimed that the gold nanoparticles were stabilized by Au−C bonds. In all of the above works involving the chemical or electrochemical reduction of aryl diazonium salts on gold, direct evidence of the
INTRODUCTION Organic films on solid substrates have generated a great deal of interest because they can impart novel surface properties to the underlying substrates and thus find a wide range of applications in fundamental research, biosensors, molecular electronics, and surface patterning.1 A particularly interesting and popular example of such systems is the self-assembled monolayers (SAMs) formed from the spontaneous adsorption of organic thiols on gold.2 However, one drawback of the SAMs is their limited adhesion under certain conditions due to the weak Au− S interaction.3 To improve the adhesion of the SAMs on gold, chemical and electrochemical reduction of aryl diazonium salts has been devised, in which highly reactive aryl radicals are generated upon reduction and are presumed to react with gold-surfaceforming Au−C bond.4 Bernard et al.5 were the first ones to electroreduce aryl diazonium salts on gold and found that the obtained films were robust and difficult to desorb. Laforgue et al.6 electrografted aryl diazonium salts on gold and found that the obtained film adhered well to gold surfaces and even could hinder gold oxide formation in acidic medium. However, an Xray photoelectron spectroscopic study by the same authors could not conclusively prove the formation of Au−C linkage. Ricci et al.7 reported an ex situ FTIR study of nitrophenyl mono- and multilayers electrografted on gold but did not provide any spectroscopic evidence of Au−C bond formation. © XXXX American Chemical Society
Received: August 31, 2016 Revised: October 11, 2016
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DOI: 10.1021/acs.langmuir.6b03206 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
Scheme 1. Electroreduction of DSBD on Gold Forming a Multilayer That Can Be Degraded to a Monolayer by Electrochemical Cleavage of the S−S Bonds (i.e. Disulfide Bridge) within the Multilayera
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S−S bonds can be reversibly formed within the monolayer. (TBAClO4, 99.99%) was dried by heating under vacuum at 120 °C for 24 h. Bis(4-aminophenyl) disulfide (98%), HCl (35%), NaNO2 (99%), and NaBF4(98%) were used as received without further purification. 4,4′-Disulfanediyldibenzenediazonium (DSBD) was synthesized according to a previously reported procedure.14 In a typical synthesis, a total of 200 mg of bis(4-aminophenyl) disulfide was dissolved in 2 mL of HCl (35%). After cooling the solution at 0 °C with ice, a concentrated water solution of NaNO2 (140 mg) was added for 20 min reaction; then, 220 mg NaBF4 was added to precipitate the obtained diazonium salt. Finally, the product was filtered and washed with cold water and ether, then dried and kept in a freezer at −18 °C. DSBD was confirmed with NMR and ESI−MS characterizations (see Supporting Information Figures S1 and S2). A gold disk electrode (2 mm diameter) was mechanically polished by using 0.05 μm alumina paste, then roughened electrochemically in O2 free 0.1 M KCl electrolyte according to a reported procedure.15 Specifically, the potential was first held at −1.16 V versus SCE for 10 min and stepped to −0.06 V for 2 min. The potential was then swept from −0.06 to 1.44 V and back for 20 oxidation−reduction cycles at 0.75 V s−1. During each cycle, the potential was held at −0.06 V for 30 s and then at 1.44 V for 2 s. Finally, the potential was held at −0.36 V for 2 min to desorb adsorbed Cl−. After potential cycling, the roughened Au was rinsed with large amounts ethanol and dried with Ar gas flow and then immersed in dry DMSO before transferring to the working electrolytes. Electrochemical measurements were performed with a CHI 760E electrochemical workstation. A multiple-necked, water-jacketed, and airtight glass cell equipped with inlet and outlet valves for gas control was used throughout. The roughened Au electrode was used as the working electrode, Ag wire as the reference electrode, and Pt wire as the counter electrode. Both of the counter and reference electrodes were separated from the working electrode compartment by glass frits. In situ surface-enhanced Raman spectroscopy (SERS) setup and the Raman cell design have previously been reported.16 In brief, in situ SERS was carried out with an airtight three-compartment spectroelectrochemical cell. An electrochemically roughened Au working electrode was placed behind a 1 mm thick sapphire window. Raman spectra were recorded using a customized LabRAM HR800 confocal Raman microscope (Horiba JobinYvon). The spectrometer was equipped with a 200 mW 785 nm laser source for excitation, a 600 lines/mm grating to disperse the scattering light, and a long working distance objective lens (Nikon 50× 0.45 NA) to focus laser beam on and collect the scattering light from the electrode surface. The power delivered to the electrode surface was estimated to be 2.5 mW. Computational Methods. Periodic DFT calculations were performed using the Vienna Ab initio Simulations Package (VASP, v.5.3)17,18 with the RPBE generalized gradient approximation functional19 and with the optB88 van der Waals (vdW) functional.20
anticipated or claimed Au−C bond formation, however, is still missing. Only recently, a few works devoted to addressing the Au−C bond formation upon reduction of aryl diazonium salt on gold began to appear. For example, Laurentius et al.12 reported the spontaneous chemical reduction of diazonium salts on gold nanoparticles, in which the formation of Au−C bond was supported by a Raman spectroscopic study that identified a new band at 412 cm−1 that was assigned to the Au−C stretch vibration. Ahmad et al.13 reported that the surface chemistry of gold nanorods can be tailored by a spontaneous reaction with nitrophenyl diazonium salts, where the formation of Au−C bond is probed by the ToF-SIMS that showed a few Au−C containing fragments including Au−C6H4NO2+ and [Au− C6H3(NO2)−NNC6H4(NO2) + H]+. However, direct evidence of the Au−C bond formation from the electroreduction of aryl diazonium salt on bulk gold surface is scarce.12 Here we report the electroreduction of a diazonium salt, 4,4′disulfanediyldibenzenediazonium (DSBD), on a surface-roughened gold that makes an excellent substrate for surfaceenhanced Raman spectroscopy (SERS). The initially electrografted film has a multilayer structure of Au−(Ar−S−S−Ar)n and can be further degraded to a monolayer of Au−Ar−S− by electrochemical cleavage of the S−S moieties within the multilayer; see Scheme 1. The thinned monolayer of Au− Ar−S− is very beneficial to the probing of the Au−C bond formation with SERS, which is a surface-sensitive technique that can effectively detect the species in the immediate vicinity of the SERS-active substrates, without the complication from moieties away from the gold surface. SERS examination of both the multilayer and the monolayer under different conditions, coupled to density functional theory (DFT) calculations, provides compelling evidence that the Au−C bond does form upon electroreduction of diazonium salt on gold and that the enhanced stability of the electrografted organic films is due to a stronger Au−C bond than Au−S bond (with a differential in bond energy of ca. 0.4 eV).
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EXPERIMENTAL SECTION
Chemicals and Procedures. All chemicals were purchased from Sigma-Aldrich unless otherwise noted. Dimethyl sulfoxide (DMSO, 99.9%) was distilled under vacuum over NaNH2(98%) and then dried for at least 3 days over freshly activated 4 Å molecular sieve, resulting in a final water content of
