Activation of Thiols at a Silver Nanoparticle Surface

Abstract

Adsorption of organosulfur (thiol) molecules on noble metal surfaces has been widely used to fabricate self-assembled monolayers or modify surface properties in a broad range of novel technological applications. Adsorption of thiols on the surface of noble metals, such as gold or silver, has been intensively studied to gain fundamental understandings on the adsorption process, including its mechanism and rates. It is generally accepted that thiol adsorption onto the gold/ silver surfaces consists of many steps, starting with physical adsorption, followed by the sulfur–metal bonding reaction, the reorientation of the adsorbed thiol molecules, and the formation of a compact self-assembled layer. On the other hand, both diffusion-limited and non-diffusion-limited adsorptions have been reported, though the conditions for the two scenarios have not been consistently defined. 13–16] Thiol adsorption has been widely used to influence the properties of metallic nanoparticles, such as controlling their biomedical functions, tuning optical properties, changing the attached chromophores, and the synthesis of nanoparticles/nanocages of different shapes and size. The adsorption of thiol molecules on metallic nanoparticles have been studied mostly using optical techniques. For example, a sum frequency vibrational spectroscopy study has revealed that long-chain thiol molecules on gold nanoparticles of varying sizes have different conformations. The adsorption free energies of several thiol molecules on gold and silver nanoparticles have been determined using fluorescence or second harmonic generation (SHG) measurements. More than a decade ago, hyper Rayleigh scattering from gold nanoparticles was pursued and first reported as an incoherent second harmonic method for examining nanoparticles, as it was thought that there is an inherent size restriction in SHG such that the method could not be detected from particles with diameters much smaller than the optical wavelength. We have subsequently demonstrated that SHG from the surface of nanometer size particles can be detected at specific scattering angles where phase matching conditions are satisfied. This advancement has led to the detection of SHG from silver nanoparticles (AgNPs). In principle, the SHG signal may arise from the surface and/or bulk of the nanoparticles. In the case of AgNPs, the surface contribution is identified by the response in the SHG signal to the formation of surface bonding that would diminish the polarizability of the free electrons at the surface. The surface portion of the SHG signal can then be used for probing processes occurring at the surface. It was illustrated in our work that the adsorption of thiols onto the AgNP surface can be directly monitored with time using SHG. The SHG intensity generated at the surface layer of the silver particle is sensitive to the formation of the S Ag bonds at the nanoparticle surface. It was found that the majority of the SHG signal from AgNPs of 80 nm diameter can be quenched by the adsorption of thiol molecules as the formation of the S Ag bonds localizes the Ag electrons that are responsible for the nonlinear susceptibility. The change of the SHG intensity can be quantitatively related to the thiol coverage on the surface, and used for the determination of the adsorption and desorption rates as well as the free energy change of the adsorptive reaction process. If there is a barrier associated with the ratelimiting process in the adsorption, temperature dependence of the adsorption rate should enable the determination of the activation energy and reveal which is the activation process. Herein we present experimental observations indicating that the thiol reactions at the silver nanoparticle surface is an activated process and that temperature can be used to control the reaction rates. For the case of 1,2-benzenedithiol adsorption onto a silver nanoparticle, the activation energy was determined as 8.4 kcalmol . This energy barrier is likely associated with the formation of the transition state during the formation of sulfur–silver bond and not from the diffusion-limited process. Figure 1 shows that as 1,2-benzenedithiol at different concentrations was added into Ag colloids at 293 K, the intensity of the SHG light scattered from silver nanoparticles decreased. The decrease is resulted from the reduction of the nonlinear susceptibility of the Ag surface atoms as S Ag bonds form. As indicated by the rate of the SHG intensity decay, which is proportional to the square of the thiol coverage on the surface, the rate of adsorption of thiol molecules is faster when thiol concentration is higher. Figure 1 shows three of the five experimental measurements conducted with added thiol concentrations varying from 0.1 to 0.5 mm. The Langmuir model can be used to describe the adsorption kinetics with adsorption and desorption rate constants ka and kd: