Preparation of Carbon Quantum Dots with Tobacco Leaf and Their Application for Determination of Nitrite in Water

Commercially available tobacco leaves were dried and cut, and then an aliquot (1 g) was placed into a 100 mL-reaction kettle. After 60 mL of water was added, the mixture was heated at 180℃ for 8 h, and filtrated, and the acidity of the filtrate was adjusted to neutral. The solution was centrifuged for 15 min, and passed through 0.45, 0.22 μm aqueous filtrate membranes in turn. The filtrate was diluted to 100 mL with water, and tobacco leaf CQDs were obtained, which were characterized by transmission electron microscopy, ultraviolet-visible (UV-Vis) spectrophotometer and fluorescence spectrometer. The Weihe water sample was centrifuged for 10 min after settling overnight, and 2 mL of the supernatant was placed into a colorimetric tube, in which 100 μL of tobacco leaf CQDs and 3 mL of B-R buffer solution (pH 1.81) were added. The mixed solution was diluted to 10 mL with water, settled for 4 h, placed in the fluorescence spectrophotometer and detected at excitation wavelength of 440 nm and emission wavelength of 510 nm, respectively. It was shown that the tobacco leaf CQDs were regular spherical in shape, uniform in distribution and size, with UV obvious absorption peaks at 260, 280 nm and quantum yield of 4.88%, and the fluorescence could be quenched by nitrite. Linear relationship between the concentrations of nitrite and quenching values of fluorescence intensity was kept in the range of 2.00×10^-4-2.50×10^-1μmol·L^-1, and the detection limit (3s) was 0.66×10^-4μmol·L^-1. The recoveries obtained by the spiked recovery test on the Weihe river water ranged from 97.5% to 108%, with RSDs (n=5) of the determined values in the range of 1.6%-3.3%.