A smartphone-assisted ratiometric colorimetric and fluorescent probe for triple-mode determination of nitrite based on MnO2 nanoparticles and carbon quantum dots

A triple-mode colorimetric and fluorescent sensing scheme based on manganese dioxide nanoparticles (MnO2NPs) and carbon quantum dots (CQDs) were developed to determine nitrite. MnO2NPs can oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into a blue oxidation product (TMBox), which is further oxidized into a yellow diimine derivative by nitrite. The ratio of absorbance at 652 nm to 452 nm was monitored as signal response for UV-vis detection mode. A "turn-off" CQDs fluorescence probe was also constructed for fluorescent detection mode. Smartphone tool kit was used to capture the color of sample for smartphone-based measurement. Various analytical performance under different detection modes were obtained and compared. The proposed methods were applied to food samples with satisfactory recoveries (83.3-106 %). The results were validated with AOAC standard spectrophotometric method. The current triple-mode detection were accurate, convenient, low-cost and fast for analyzing nitrite in foods and water samples on-site.