Dual-modified gold nanoparticles with nitrogen-doped carbon quantum dots and aspartic acid as response system: A paper-based fluorescence sensing platform for selective cysteine identification

A rapid, selective, and sensitive paper-based fluorescence sensing method was developed for cysteine (Cys) detection. The nitrogen-doped carbon quantum dots (CNQDs) and aspartic acid (Asp) reacted with gold nanoparticles (AuNPs) to synthesize the probe AuNPs@CNQDs@Asp, resulting in an efficient quenching of CNQDs fluorescence. Owing to the strong interaction between Cys and AuNPs, as well as the specific interaction between Cys and Asp, triggering the detachment of CNQDs and the recovery of fluorescence. The images of fluorescence signal were collected within 6 min using a portable fluorescence detector and analyzed via a smartphone-based application. The RGB value increased linearly in the 180 to 260 μM Cys concentration range (R = 0.9990), with a low limit of detection (LOD) of 4 μM. Interference deviations of 12 substances to Cys were in the range of −3.1% to +1.0%. The method was applied to determine Cys in human plasma, showing a recovery between 98.2% and 102.3% and demonstrating its superior reliability. The work highlights the potential of the method to detect Cys in real samples with low cost and high efficiency, providing an effective tool for diseases surveillance and control in resource-limited regions.