Improved fluorescence test of chromium (VI) in aqueous solution with g-C3N4 nanosheet and mechanisms

g-C3N4 nanosheet has been modified by secondary calcination at 550 degrees C and HNO3 treatment for effectively testing Cr(VI) in aqueous solution via the fluorescence decay according to the inner filter effects proved by the nearly-unchanged time-resolved decay process. The improved fluorescence test performance is attributed to the promoted adsorption of Cr(VI) on resulting g-C3N4 by increasing surface area and the surface positive charge sites. The limit of detection (LOD) is 0.3 mu M, which is confirmed by the Stern-Volmer equation with the linear range of 1.0-100.0 mu M versus the fluorescence intensity on the optimized conditions of pH = 3 and excitation wavelength of 350 nm. Moreover, at the low-concentration range of 0.2-5.0 mu M, it is well linear to the fluorescence intensity by the double logarithmic plot, with the LOD of 0.06 mu M. This work provides a feasible route to synthesize g-C3N4-based nanomaterials for effectively detecting Cr(VI) in the solution.