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Determination of Thiourea by On–Off Fluorescence Using Nitrogen-Doped Graphene Quantum Dots

Abstract A thiourea-detecting fluorescence sensor with Hg2+ as a switch was developed using nitrogen-doped graphene quantum dots (N-GQDs). The surface of N-GQDs had many organic functional groups on which Hg2+ was effectively bound and turned off the fluorescence of the N-GQDs. The fluorescence of N-GQDs was turned on by the thiol functional group of thiourea that bound strongly with Hg2+ and formed Hg2+/thiourea complexes. After constructing the sensor, the experimental conditions and parameters, such as the pH and Hg2+ concentration, were investigated and optimized. Under the optimum conditions, the constructed fluorescence sensor showed high sensitivity to thiourea at concentrations from 0.5 to 14 µM with a low detection limit of 41.7 nM. The sensor also exhibited high specificity, excellent stability, and good reproducibility so that the determination of thiourea in various samples had acceptable values with good recoveries from 99% to 106%. The relative standard deviation was less than 4.1% (n = 3).