Smartphone-assisted fluorescence and colorimetric dual-mode sensor for visual quantitative detection of nitrite and nitrate in real samples.

Nitrite (NO2-) and nitrate (NO3-) play important roles in the global nitrogen cycle and are closely associated with human health and environmental safety, necessitating the development of sensitive and accurate detection methods. Herein, we develop a novel and rapid fluorescence/colorimetric dual-mode sensor for the visual and quantitative detection of NO2- by utilizing a specific reaction of nitrite with p-aminobenzoic acid (PABA) and N-phenyl-1-naphthylamine (NPN) under acidic condition. Specifically, nitrite could react with PABA/NPN/HCl probe solution via diazotization and azo coupling reaction to generate azo dye, therefore leading to an obvious colorimetric variation from colorless transparent to purple under white light with increasing the NO2- concentration. While the blue fluorescence of NPN at 452 nm could be quenched gradually after introducing NO2- (from 0 to 150 μM) into PABA/NPN/HCl probe solution, accompanied by the fluorescence color change from strong blue to light blue under 365 nm UV light. By the fluorescence and colorimetric signal variations, the proposed method enables the quantitative detection of nitrite within a linear range of 0.1 to 120 μM, with the limit of detection (LOD) down to 32 nM in fluorescent mode and 53 nM in colorimetric mode. Meanwhile, determination of nitrate can be achieved by reducing nitrate to nitrite using zinc powder/CdCl2 L-(+)-tartaric acid (L-TA) as reductants, and then reacting with PABA/NPN/HCl based sensor in the same way. Furthermore, integrated with the test paper that was previously impregnated in the probe solution and dried, the portable smartphone platform was successfully used for nitrite and nitrate detection in real samples with good anti-interference ability and high reliability, demonstrating the great application potential for food safety monitoring and soil nutrient determination.