A Redox-induced Dual-mode Colorimetric and Fluorometric Method based on N-CDs and MnO₂ for Determination of Isoniazid...

We develop a simple hydrothermal method to prepare a novel nitrogen-doped carbon dots (N-CDs) originated from green carbon source Liu-bao tea and ethylene diamine. The N-CDs emits strong and stable blue fluorescence (E_m = 440 nm) under the excitation wavelength of 350 nm with a quantum yield of 35%. And it is used as an excellent fluorescent output for the sensitive and visual dual-mode determination of isoniazid. The fluorescence of N-CDs is "turned off" first by manganese dioxide (MnO₂) nanosheets due to inner filter effect, MnO₂ nanosheets can also oxidize TMB (3,3',5,5'- tetramethylbenzidine) to blue oxTMB. Isoniazid, however, can reduce MnO₂ nanosheets to Mn²⁺, turning on the fluorescence again. The color of the solution fades from blue to colorless because less TMB can be oxidized. Under the optimal conditions, the dual-mode method has a satisfying linear relationship ranging from 2.0 to 120.0 μM with a limit of detection of 0.7 μM (S/N = 3). And it has been applied successfully to colorimetric and fluorescent determination of isoniazid in tablets and clinical plasma samples, with recoveries ranging from 94.0% to 102.4%. The properties of N-CDs and MnO₂ nanosheets were thoroughly characterized using TEM, FT-IR, XPS, AFM and fluorescence spectrophotometer, the quenching mechanism was also discussed.