SERS detection and DFT calculation of 2-naphthalene thiol adsorbed on Ag and Au probes

Two different surface enhanced Raman scattering (SERS) sensors are described, tested and compared against the detection of 2-naphthalenethiol (2NPT, a volatile compound) in both solution state as well as vapor phase. The first sensor is based on an optical fiber properly modeled to induce the adhesion of colloidal Ag nanoparticles on its surface. Excitation and detection of the Raman signal is performed through the optical fiber that can be used as in situ probe for the detection of molecules adsorbed on the SERS sensitized surface. The second SERS sensor is based on nanostructured substrates consisting of Au nanoparticles produced by pulsed laser deposition in presence of a controlled Ar atmosphere. Details at the nanometer scale were observed by SEM and TEM imaging to understand the size and structure of the islands formed as a function of deposition parameters that were selected in order to maximize their SERS response. The sensitivity of the substrates to volatile species was tested by letting evaporate controlled drops of a methanol solution of 2NPT in a chamber of known volume, where the substrate was placed. After complete evaporation of the drops, this provided an in-situ environment suitable for vapor phase measurements at known concentration. SERS spectra were collected after exposing the substrates to the environment within the chamber (vapor phase measurements) or dipping them in a solution for condensed state measurements. The complete absence of the SH stretching peak in the SERS spectra proves the covalent bonding of 2NPT to the metal substrates via the sulfur atom. DFT calculations, including metal-sulfur interaction, provide a good description of the observed SERS spectra. The reported data allow concluding that our SERS substrates are suitable for detection of volatile compounds.