Choline chloride–lactic acid deep eutectic solvent as a functional material for polyphenol extraction and gold nanoparticle synthesis with colorimetric activity for environmental pollutant detection

A natural deep eutectic solvent (NADES) based on choline chloride and lactic acid was used to extract phenolic compounds from banana peels. The extracts served as reducing and stabilizing agents for the green synthesis of gold nanoparticles from hydrogen tetrachloroaurate(III). UV–Vis spectroscopy confirmed AuNP formation through a surface plasmon resonance band at 530–560 nm, with an average particle size of ~55 nm. The synthesized AuNPs were successfully applied as a colorimetric probe for rapid Cr(VI) detection in water. The environmental sustainability of the NADES-based process was also evaluated using the DES Ecoscale metric.