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

•The fluorescence test of Cr(VI) with g-C3N4 nanosheet has been improved by secondary calcination and HNO3 treatment.•The limit of detection is 0.3 μM by the widely-accepted Stern-Volmer equation.•The limit of detection is further obtained to 0.06 μM by using the double logarithmic plot.•Secondary calcination efficiently increases the surface area of g-C3N4 nanosheet, thereby promoting adsorption of Cr(VI).•HNO3 treatment increases surface positive charge sites of g-C3N4 nanosheet, thus also improving adsorption of Cr(VI).