Fluorescent “On-Off-On” Probe for Cascade Detection of Ag(I) and Ascorbic Acid Using Folic Acid Protected Copper Nanoclusters

Folic acid (FA) protected copper nanoclusters (FA-CuNCs, λ_ex=350 nm, λ_em=445 nm) were synthesized and used as a fluorescent “on-off-on” probe for the cascade detection of Ag⁺ and ascorbic acid (AA). The fluorescence emission of FA-CuNCs at 445 nm was quenched significantly by Ag⁺ due to the analyte-induced aggregation followed by the formation of larger nanoparticles. However, when AA was added to the in-situ generated Ag@FA-CuNCs, the fluorescence emission of FA-CuNCs was recovered at 445 nm because of the AA-directed reduction of Ag⁺ to Ag⁰. The experimental conditions, such as pH, incubation time, and quencher Ag⁺ concentrations, were optimized to achieve improved sensitivity. The detection limit for Ag⁺ and AA was estimated as 37.1 nM and 0.27 μM with a linearity range of 2.49–22 μM and 4.71–81.53 μM, respectively. In real sample analysis, the recoveries of Ag⁺ ions in river water and AA in orange juice samples were found between 99–93 % and 97–94 % using the probes FA-CuNCs and Ag@FA-CuNCs, respectively. Overall, this work offers a viable method for the sequential detection of Ag⁺ and AA using FA-CuNCs via a fluorescence “on-off-on” switch mechanism.