Kinetics and Reaction Mechanisms of Thiophenol Adsorption on Gold Studied by Surface-Enhanced Raman Spectroscopy

Abstract

Thiophenol is commonly used as a model system for surface-enhanced Raman scattering (SERS) of molecules due to the strong affinity of the −SH group toward noble metal surfaces. By performing time-, temperature-, and pH-dependent measurements of thiophenol adsorption on commercial nanostructured gold SERS substrates, we have observed both physisorption and chemisorption processes. These two distinct adsorption regimes were found dependent on the pH which controlled the ionization state of thiophenol in an aqueous medium. At low pH the sulfhydryl proton remains bound, and the kinetic adsorption profile obtained from the SERS intensity follows a sigmoid-shaped curve with an initially slow adsorption rate that deviates from a Langmuir profile. In addition, from temperature-dependent measurements, a near zero value for the activation energy is obtained, indicating that physisorption is the rate-limiting step. At high pH, where the sulfhydryl proton becomes detached, the kinetic adsorption profile follows a classical Langmuir profile, and the activation energy is significantly higher than at low pH, indicating that chemisorption is the rate-limiting step.

Gold Nanoparticles from Vegetable Extracts Using Different Plants from the Market: A Study on Stability, Shape and Toxicity

“…The resulting SERS spectra of thiophenol are in agreement with the literature . In spite of its high affinity for Au surfaces and relatively high concentration, thiophenol could displace the adsorbed species only to a limited extent (< 20%), confirming the strong interactions between capping agents and the NPs, which determined their long‐term stability. However, we were not able to discriminate whether the different SERS enhancement came from different thiophenol concentration at the surface or from structural properties of the AuNPs.…”

SERS and DFT investigation of the adsorption behavior of 4-mercaptobenzoic acid on silver colloids

“…More detailed analysis shows that there is a shoulder band partially covered by the 1417 cm −1 v s (COO − ) band. The shoulder band at 1403 cm −1 belongs to the surface-bonded COO − group whereas the stronger band at 1417 cm −1 is due to non-bonded COO − group in 4-MBA [7]. The intensity ratios of the 1403 cm −1 band to the 1417 cm −1 band change from 0.55, 0.46, to 0.43, with pH values change from 13, 10, to 7, respectively.…”

“…Applications using SERS as the research tool can be found in the fields of analytical chemistry, surface science, and biomedical studies [4][5][6][7][8][9]. Most of the SERS studies have been carried out with molecules adsorbed on the surfaces of transition metals [10][11][12][13], particularly Ag and Au.…”

“…Model compounds such as pyridine [1], thiophenol [7], 4-mercaptobenzoic acid (4-MBA) [10], 4-mercaptopyridine (4-MPY) [20], and azo dyes [21,22] have been used to probe the adsorbatesubstrate interactions using SERS. All studies have revealed that molecules adsorbed on the substrates through certain functional groups from SERS spectroscopic interpretation.…”

Role of Surface Adsorption in the Surface-Enhanced Raman Scattering and Electrochemical Detection of Neurotransmitters

“…1, 7-9 Examining the SERS signal of these systems with respect to pH changes 3, 10-12 , electrochemical potential 10, 13 , and temperature 3 allows for the elucidation of the analyte-substrate interaction. Molecules adsorbed at surfaces have been used to evaluate the distance dependence of the SERS effect.…”