A Microdroplet Fluorescence Method for the Determination of Nitrite in Water Using a Hydrophobic Cellulose Substrate Platform

Abstract In this work, a miniaturized and sustainable method was developed, based on the fluorimetric reaction with 2,3-diaminonaphthalene (DAN) in an acid medium, for the determination of trace amounts of nitrite in different water samples using a hydrophobic cellulose substrate platform. The innovation consists of the use of a nonwettable cellulose substrate that suspends stable microdroplets, enabling direct measurement of fluorescence on the front surface and mitigating the signal attenuation inherent in traditional hydrophilic paper devices. The cellulose substrate acted both as a solid support for the reaction and as a holder for microdroplet suspension during fluorescence signal acquisition. The method was designed to reduce reagent consumption and waste generation by operating at the microliter scale, using a total reaction volume of 15 μL. The procedure showed great tolerance to the presence of 11 potentially interfering ions, evaluated at concentrations relevant to water samples. The values obtained for the limit of detection were 0.35 μg L–1, the limit of quantification was 1.17 μg L–1. The precision, based on the relative standard deviation (RSD%), was 3.5% (N = 7). The method was successfully applied to various natural waters, with recoveries ranging from 83 to 115%. With an AGREEprep score of 0.77, the procedure significantly reduces reagent consumption and waste, offering an environmentally friendly and low-cost alternative to conventional laboratory assays and providing a simple and sensitive alternative for laboratory-based environmental monitoring of nitrite.