SERS activity of silver nanoparticles and silver-modified 2D graphitic carbon nitride towards ciprofloxacin drug

Herein, silver nanoparticles (AgNPs) and silver-loaded graphitic carbon nitride (Ag@g-C₃N₄) nanocomposites have been synthesized and used as an effective surface-enhanced Raman scattering (SERS) substrates for the detection of low concentrations (10^-14 M) of ciprofloxacin (CIP), a commonly bioactive medication used to treat bacterial illnesses. A combined approach of vibrational spectroscopy and density functional theory (DFT) has been developed to understand the possible modes of analyte (CIP) and SERS substrate (AgNPs and Ag@g-C₃N₄) interactions. Furthermore, it has been noticed that the behavior of drug molecules in terms of SERS response and energetics of interaction changed significantly when interacted with the noble metal AgNPs decorated onto the g-C₃N₄ framework in comparison to only AgNPs as substrate. The most prominent interaction scenario between AgNPs and CIP is likely to be through the -NH moiety of drug molecule with an interaction energy of -306 kcal/mol. Whereas, the CIP molecules adsorbed onto Ag@g-C₃N₄ nanocomposite were more flexible with interaction energy of -107 kcal/mol, suggesting a greater association of analyte with the skeletal modes of substrate leading to Raman enhancements in the low wavenumber region i.e. below 600 cm^-1. Hence, the Ag@g-C₃N₄ nanocomposite-based SERS substrates investigated served two distinct spectral ranges, making them complementary of each other in terms of SERS detection of CIP. The characteristics of the computed frontier molecular orbitals indicated a pronounced amount of charge transfer between the drug and the substrate, highlighting the significance of the chemical mechanism of the overall process. These results represent a successful approach to have an extended spectral range that covers lower wavenumber shifts by applying simple and meaningful modifications to the normally utilized noble metal-based nanoparticles, which can lead to more effective and reliable detection of bioactive drugs.