Radiation mediated oxidation processes for the conversion of 4-aminothiophenol in to 4, 4′-dimercaptoazobenzene

Gold (Au) nanoparticles of average size ∼ 4 nm were in-situ synthesized and simultaneously embedded on SiO2 nanospheres in a single step process by irradiating a solution (HAuCl4, PVP and SiO2 nanospheres) with 6 MeV electrons at a fluence of 1.0 × 1015 e−/cm2. Substrates were prepared by coating silicon wafers with the solution of SiO2 nanospheres embedded with Au nanoparticles and used for recording Surface-enhanced Raman scattering (SERS) spectra of 4-aminothiophenol (4-ATP) and 4-ATP along with NaOH with a Raman spectrometer. The appearance of characteristic SERS peaks of both 4-ATP and 4, 4′-DMAB in Raman spectra confirmed conversion of a fraction of 4-ATP into 4, 4′-DMAB in the presence of Au nanoparticles, and the intensity of SERS peaks increased in the presence of NaOH. In the recorded SERS spectra, the prominent peaks at around 1140 cm−1, 1390 cm−1 and 1438 cm−1 were observed and assigned respectively to β(C–H), ν(NN) and ν(NN) chemical bonds, which are called b2 modes. The results of the present work revealed that in the presence of Au nanoparticles, the OH− radicals produced from NaOH play an important role in the conversion of 4-ATP into 4, 4′-DMAB, and enhancing the intensities of b2 peaks. The Au nanoparticles of size ≤5 nm have inherent nanoscale defects and therefore surface with nanoscale roughness. All these together greatly enhanced electric field which brought out the strong Raman intensity and catalytic oxidation reaction leading to conversion of 4-ATP into 4,4′-DMAB on the Au–SiO2 substrates.