Silver colloid and film substrates in surface-enhanced Raman scattering for 2-thiouracil detection

Surface-enhanced Raman scattering-based silver substrates were designed and fabricated for the detection of 2-thiouracil (2-TU). Two types of silver nanoparticles (Ag/NPs) were prepared using reducing agents: sodium borohydride (type I) and trisodium citrate dehydrate (type II) on a substrate. Their morphology and structure were characterized using Fourier transformed infrared spectroscopy, Raman spectroscopy, and high-resolution transmission electron microscopy. The average size of the Ag/NPs (type I), and Ag/NPs (type II) was about 15 nm and 60 nm, respectively. The UV spectrum showed a maximum absorbance of silver nanoparticles at about 400-430 nm. Raman and Surface-Enhanced Raman Scattering (SERS) spectra of 2-TU were recorded, and the vibrational bands were assigned by means of Density Functional Theory (DFT) based calculations. The 2-TU molecules are adsorbed through the sulfur and nitrogen atoms to the silver surface, due to a high affinity. SERS spectra showed that the interaction between 2-TU and Ag/NPs led to enhanced SERS bands at 1164 cm^-1 for ν (CS), 1052 cm^-1 assigned to ν (N4-C7), 921 cm^-1 for ν (N4-C5), 649 cm^-1 related to γ (N-H), and 572 attributed to γ (S-C5-N). The correlation between the concentrations of 2-thiouracil and the SERS signal intensity was investigated with silver colloid and film substrates. The film mode showed better correlation coefficients and improved detection limits due to the protection against agglomeration.