Sensitivity improvement of Au-nanoparticle-based colorimetric probes via surface decoration of carbon quantum dots

Visually colorimetric sensing of hazardous metallic ions based on gold nanoparticles (AuNPs) has been considered a facile, on-site, and reliable monitoring route, yet the improvement of detection limit, as well as environmental pH tolerance, is still needed. Herein, we present an integrated probe design based on functionalizing AuNP cores with carbon quantum dots (CQD) as colorimetric probes for tracing Cu 2+ ions with a limit of detection as low as 18.7 nM. Improved detection selectivity was elucidated with the reduction in thiol groups bound between CQD decorations and AuNP probes initiated by oxidation of Cu 2+ ion analytes, thus separating crossly connected probes, depending on tracing Cu 2+ concentrations. Moreover, distributed CQDs with negative potentials allowed the stabilization of dispersed AuNPs against re-aggregation, which caused the gradual morphological transition from aggregated features to dispersed states in microscopic view, and eventually, reflected the gradual color change from red to purple-red. The correlated practicality of colorimetric detection of the Cu 2+ ion was further validated with sound-sensing selectivity, environmental pH resistance and tests on real samples.