Green synthesis of silver nanoparticles and their application for colorimetric detection of L-cysteine: a spectroscopic investigation

In this study, we report an eco-friendly synthesis of silver nanoparticles (AgNPs) using banana peel extract (BPE), along with their use for colorimetric detection of L-cysteine and investigation of the mechanisms involved in these processes. Transmission electron microscopy (TEM) and UV-Vis spectroscopy were used to confirm the formation of silver particles at the nanoscale (similar to 20 nm). TEM images also showed the presence of BPE-derived organic material enveloping the nanoparticles. The nature and role of this capping material were investigated by Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis/differential scanning calorimetry (TGA/DSC). Visual inspection and UV-Vis spectroscopy revealed that the insertion of L-cysteine into the AgNPs solutions induced discernible color changes. pH variations in the solutions and FTIR spectroscopy were also employed to study the molecular dynamics underlying the formation of AgNPs, the capping material and the detection of L-cysteine. Studies concerning different amino acids in AgNPs solutions confirmed the selectivity for L-cysteine. The overall spectroscopic results, particularly those from FTIR spectroscopy, shed light on the crucial role of carboxylate ions (COO-) of BPE in the formation of AgNPs and their interaction with L-cysteine. Finally, the importance of the thiol group of L-cysteine in the colorimetric detection process was also reinforced by this study.