Highly sensitive and selective label-free detection of dopamine in human serum based on nitrogen-doped graphene quantum dots decorated on Au nanoparticles: Mechanistic insights through microscopic and spectroscopic studies

A rapid, facile and label-free sensing strategy is developed for the detection of dopamine (DA) in the real samples by exploiting nitrogen-doped graphene quantum dots (N-GQDs) decorated on Au nanoparticles (Au@N-GQD). The as-grown Au@N-GQD exhibits strong blue fluorescence at room temperature and the fluorescence intensity is drastically quenched in presence of DA in neutral medium. The mechanistic insight into the DA sensing by Au@N-GQDs is explored here by careful monitoring of the evolution of the interaction of Au NPs and N-GQDs with DA under different conditions through electron microscopic and spectroscopic studies. The highly sensitive and selective detection of DA over a wide range is attributed to the unique core-shell structure formation with Au@N-GQD hybrids. The quenching mechanism involves the ground state complex formation as well as electron transfer from N-GQDs. The presence of Au NPs in Au@N-GQD hybrids accelerates the quenching process (similar to 14 fold higher than bare N-GQDs) by the formation of stable dopamine-o-quinone (DQ) in the present detection scheme. The fluorescence quenching follows the linear Stern-Volmer plot in the range 0-100 mu M, establishing its efficacy as a fluorescence-based DA sensor with a limit of detection (LOD) 430 nM. Further, based on the systematic change in the intensity of absorption peak of Au@N-GQD with DA concentration, the well-known Hill equation is introduced for the sensing of DA in the range 0-10 mu M with detection limit 40 nM. The proposed sensing method has a high selectivity towards DA over a wide range of common biological molecules as well as metal ions. The quenching in Au@N-GQD fluorescence intensity makes it possible to determine the spiked DA in human serum in the linear range from 0.0 to 80.0 mu M with the limit of detection (LOD) 590 nM, which is similar to 27 fold lower than the lowest abnormal concentration of DA in serum (16 mu M). This sensing scheme is also successively applied to trace DA in Brahmaputra river water sample with LOD 480 nM including its satisfactory recovery (95-112%). Our studies reveal a novel sensing pathway for DA through the core-shell structure formation and it is highly promising for the design of efficient biological and environmental sensor.