Tracking chemical interactions of caspofungin on silver nanoparticles through SERS spectroscopy for antifungal applications

Silver nanoparticles (AgNP) act as efficient drug delivery carriers due to particular surface chemistry and high surface area, and even though such may present toxicity in function of the size distribution and molecular ion coverage, have been receiving special attention for their inhibitory potential against bacteria and fungi. This study explored the use of AgNP to deliver the antifungal caspofungin (CFG) for in vitro combating Candida albicans. CFG molecules were coadsorbed with low-weight chitosan (LWC) on AgNP surfaces, in the presence and absence of 2-mercaptoethanol (ME) surface modifier. Surface-enhanced Raman scattering (SERS) spectroscopy was used both to monitor the adsorption process and to assess the synergistic interaction between the components in actions against fungal cultures, observed by checkerboard assays. Results showed significant lowering of minimum inhibitory concentration (MIC) values for the CFG-LWC-AgNP ternary mixture in comparison with CFG alone. Transmission Electron Microscopy (TEM) images provided insights about the mechanism of action of components of the ternary mixture. Spectral analysis of CFG by using FT-Raman, FTIR, and SERS spectroscopies identified distinct spectral patterns, indicating effective chemical interactions of CFG on AgNP surfaces. The vibrational assignment of such spectra was done based on Density Functional Theory (DFT) calculations. Such findings suggest that AgNP could be a promising platform for enhancing the efficacy of CFG drug against fungal infections.