Dual-nanocluster of copper and silver as a ratiometric-based smartphone-assisted visual detection of biothiols

Ratiometric-based probes improve detection precision and accuracy by minimizing the influence of various experimental conditions. Here, we present a unique dual-emission fluorescence probe constructed by a dual-nanocluster. This sensing platform is composed of copper nanoclusters (CuNCs) as a reference signal and silver nanoclusters (AgNCs) as a response signal. Uracil was used as a capping agent to improve the stability of AgNCs by forming a complex with silver metal. Ratiometrically, the composite of copper/silver bimetallic nanoclusters (Cu@Ag NCs) was used to measure the amount of cysteine as a typical biothiol. The Cu@Ag NCs showed a reddish fluorescence that originated from AgNCs and CuNCs when excited by a UV lamp. Under UV illumination, the addition of cysteine caused a noticeable fluorescence color shift from red to blue. Due to the color tonality, a smartphone and the Color Grab app were used to analyze red, green, and blue (RGB) values from the images of the dual-nanocluster solutions before and after the addition of cysteine. A satisfactory linear correlation between the ratio of red to blue values as a function of cysteine concentration was established for the quantitative measurement of cysteine from 20 to 180 μM with a limit of detection of 7.0 μM. The proposed probe was applied to assay cysteine in human serum with satisfactory accuracy and precision.