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 (Tyr^Ox) 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, Tyr^Ox 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, Tyr^Ox@AgNPs were selective to interact with Cu²⁺ ions through formation of highly stable Cu²⁺-Tyr^Ox@AgNPs complexes. The evidence in formation of Cu²⁺-Tyr^Ox@AgNPs complex could be obtained through the red shift of the surface plasmonic resonance (SPR) band of Tyr^Ox@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.