Solid-State Fluorescent Carbon Dots for Fluorimetric Sensing of Hg2+

This article examines how nitrogen speciation, graphitization, and surface functionality of carbon dots (CDs) can influence the structure–optical property relationship in solution and solid-state. Despite the fact that thousands of reports have already been published, research on carbon dots is still emerging due to its questionable key factors in structure and photoluminescence properties. Here, a simple approach in the synthesis has been established to tune the structural–optical property relationship in CDs using well-known precursors citric acid and m-phenylenediamine. Three distinctive N-doped CDs (CD1:1, CD1:0.5, and CD1:0.1) were synthesized via a one-pot microwave-assisted hydrothermal method. Amphiphilic and solid-state yellow luminescent CDs were revealed, which is attributable to the diverse speciations of the nitrogen content (graphitic N, pyridinic N, pyrrolic N, and amino N) and more disorder in the graphitic carbon core. Interestingly, a simple tuning of the stoichiometric ratio of precursors resulted in solid-state yellow luminescent and dual-emissive CDs. An in-depth investigation was performed using steady-state and time-resolved fluorescence studies to unravel the photoluminescence mechanism and the structure of CDs. In addition, due to the unique optical properties of CDs, we developed fluorescent strips and used them as an ON–OFF fluorescent sensor toward highly sensitive and selective detection of toxic Hg2+ metal ions in an aqueous medium. These findings would provide a strategy to tune the fluorescence and structural properties of CDs and toward the development of a simple strip-based probe for sensor applications.