Understanding the selective-sensing mechanism of lysine by fluorescent nanosensors based on graphene quantum dots.

The selectivity of single-amino acid nanosensors is still not well understood. Herein, the factors that govern graphene-based nanomaterials for the selective detection of lysine are reported to guide the design of single-amino acid nanosensors. Graphene quantum dots (GQDs), nitrogen-doped GQDs (N-GQDs), and nitrogen/sulfur co-doped GQDs (N,S-GQDs) were used to sense lysine. The interaction mode and mechanism adjusted selectivity of the zero-dimensional graphene-based quantum dots to lysine ascribe to the solution behavior, molecular size, number of atoms as electron donors in graphene, and driving force. Being a basic amino acid, lysine is protonated with a positive charge below solution pH of 9. It adsorbed on the graphene-based quantum dots via electrostatic attraction, which blocked the internal charge transfer pathway inducing fluorescence enhancement at 420 nm. The protonated ɛ-amine side of lysine is responsible for the course. The small diameter of the lysine of ɛ-amine (<0.35 nm) favored its approach to the quantum dots, resulting in a fluorescence change, which could not be achieved with the larger arginine. The activated sites for interaction with lysine located at the edges of the layers of graphene to reach high selectivity. The N-GQDs and N,S-GQDs are much more sensitive to lysine than the GQDs because they contain nitrogen atoms as electron donors. They had similar linear detection ranges and detection limits, which suggested that the contribution of sulfur for lysine detection was minor. The results of this study provide new insights into the design of GQDs-based single-analyte nanosensors with high selectivity.