Graphene quantum dot coupled with gold nanoparticle based "off-on" fluorescent probe for sensitive and selective detection of L-cysteine

The authors report that graphene quantum dots (GQDs) coupled to gold nanoparticles (AuNPs) are viable "off-on" fluorescent probes for L-cysteine. The fluorescence of GQDs is quenched by AuNPs through fluorescence resonance energy transfer (FRET), and this shifts the blue fluorescence of the GQDs to an "off" status. Fluorescence intensity (measured under 345 nm excitation) is inversely proportional to the concentration of AuNPs in the 0.15 to 2.16 nM range, with a 15 pM detection limit. The fluorescence of the GQDs is converted back to an "on" status by adding L-cysteine which binds to the AuNPs and thereby suppresses quenching. The intensity of the recovered fluorescence is directly proportional to the concentration of L-cysteine in the 1.0 to 4000 nM range, and the limit of detection is 0.32 nM. The assay was optimized with respect to the concentration of GQDs and AuNPs, pH value, and reaction time. The method is selective in that many other thiol-containing biomolecules do not interfere. It was applied to the detection of L-cysteine in human urine and human plasma samples and gave satisfactory results.