Surface-enhanced Raman spectroscopy of one and a few molecules of acid 4-mercaptobenzoic in AgNP enabled by hot spots generated by hydrogen bonding

Abstract

The present study reports a direct approach to generate efficient hot spots using a nonresonant molecule bound to the inner part or hot spots that can increase the SERS sensitivity to obtain spectra of one and a few molecules. The 4-Mercaptobenzoic acid (4-MBA) adsorbate, connected to the Ag surface by a thiolate–Ag bonding, was able to trigger a self-assembly process of AgNP, which occurred by cooperative hydrogen bonds between the carboxylic groups of 4-MBA located in different nanoparticles when the pH was adjusted to 4. The self-assembly structure was characterized by UV-Vis spectroscopy and SERS (Surface Enhancement Raman Scattering), and DFT-based calculation of the model complex [AgNP–(4-MBA)₂–AgNP] was employed to improve the understanding of the self-assembled complex formation through the comparison of calculated and experimental SERS spectra. The SERS signal of 4-MBA on AgNP above the pK_a of the carboxyl group was not observed below 1 × 10⁻⁶ mol L⁻¹ in any condition. Additionally, the SERS spectra of 4-bromobezenothiol (4-BrBT) at 5.0 × 10⁻⁷ mol L⁻¹ had no bands assignable to 4-BrBT, reinforcing the hypothesis that the SERS intensification for 4-MBA in low concentrations (below 1 × 10⁻⁶ mol L⁻¹) is due to the hydrogen bonding triggered self-assembly of AgNP below 4-MBA pK_a. The average SERS of the 4-MBA in low surface coverage shows a mixture of structures, such as protonated and deprotonated 4-MBA, as well as a small amount of benzenethiol coming from decarboxylation of part of 4-MBA molecules. The few molecules SERS detection of 4-MBA was demonstrated experimentally and the experimental results were associated with a greater number of hot spots formed, being befitting with the generalized Mie theory simulations.