Ratiometric Fluorescent Hydrogel Test Kit for On-Spot Visual Detection of Nitrite.

In this work, we proposed a new method based on carbon dots (named m-CDs) for selective and efficient detection of nitrite (NO2-), which was based on the interaction between the amine group of m-CDs and NO2- via a diazo reaction that produced diazonium salts and induced the fluorescence quenching of m-CDs. The concentration of NO2- shows a good linear relationship with a quenched fluorescence intensity from 0.063 to 2.0 μM ( R2 = 0.996) with a detection limit of 0.018 μM. In addition, a ratiometric fluorescence probe ( m-CDs@[Ru(bpy)3]2+) was constructed via electrostatic interaction by introducing Ru(bpy)3Cl2·6H2O as an internal reference fluorescent reagent. Interestingly, a transition of the fluorescent color of the ratiometric probe from cyan to red could be visually observed upon increasing the concentration of NO2-. Based on these findings, a ratiometric fluorescent-based portable agarose hydrogel test kit was fabricated and applied for on-spot assessment of NO2- content within 10 min. As far as we know, this is the first ratiometric fluorescent sensor for visual detection of NO2-. It has broad application prospects in environmental monitoring and food safety assessment.