Synthesis and characterization of a multi-functional on-off-on fluorescent oxidized graphitic carbon nitride nanosensor for iodide, chromium(vi), and ascorbic acid

A novel multi-functional fluorescence nanosensor was developed by introducing carboxyl groups into graphitic carbon nitride. This oxidized graphitic carbon nitride nanoparticles (nano-CNO), which display enhanced fluorescence and water-solubility, exhibited sensitive and selective recognition of iodides and chromium(vi), with a limit of detection (LOD) as 12.4 and 9.6 nM, respectively. In the nano-CNO system containing iodides, the reversible quenching effect, switched by an acid-base reaction, was attributed to the hydrogen bonds established between the I- ions and the carboxyl groups of the nano-CNO, in conjunction with the effect of the heavy I- ion. This on-off-on nanosensor for I- (created by introducing no metal ions at all) is promising for potential use in applications in the field of biosensors. The mechanism of highly sensitive probe for Cr(vi) depends on inner filter effect (IFE) and collision quenching, which can also be turned on though the reduction of Cr(vi) by ascorbic acid (AA). Density functional theory (DFT) calculations shed light on the mechanism of the enhanced fluorescence intensity of nano-CNO and on the mechanism for detecting iodides.