Simultaneous Production and Surface Functionalization of Silver Nanoparticles for Label-free Colorimetric Detection of Copper Ion

In this work, a simple method was developed to simultaneously fabricate silver nanoparticles (AgNPs) and modify their surfaces with recognition functional groups for colorimetric detection of Cu2+ ions. To prepare the AgNPs with proper functional group on their surface for detection of Cu2+ ions, photochemical reaction was employed and a photoactive species of tyrosine (Tyr) was used to trigger the photoreduction of AgNPs, while the oxidized Tyr (TyrOx) was used to functionalize the AgNPs surface at the same time. To understand the behaviors, the prepared color AgNPs colloidal solution was characterized by UV-visible spectrometer, FT-IR spectrometer, dynamic light scattering (DLS), X-ray photoelectron spectrometer (XPS) and density functional theory (DFT). Based on DFT calculation results, TyrOx was adsorbed on the surface of AgNPs by the quinone ring and its functional group of amino acid was freely exposed to the aqueous media for rapid interaction of Cu2+ ions. Based on detection of different metal ions, TyrOx@AgNPs were selective to interact with Cu2+ ions through formation of highly stable Cu2+-TyrOx@AgNPs complexes. The evidence in formation of Cu2+-TyrOx@AgNPs complex could be obtained through the red shift of the surface plasmonic resonance (SPR) band of TyrOx@AgNPs located at 557 nm, which gives a color change from light yellow to brown color allowing visual identification of Cu2+ ions for rapid screening purposes. For quantitative analysis, a band intensity ratio of A557/(A404–A557) was constructed to correlate with the concentration of Cu2+ ions. A linear range up to 10 μM with a detection limit close to 150 nM was found.