An origami colorimetric paper-based sensor for sustainable on-site and instrument-free analysis of nitrite†

Paper-based sensors have been widely used thanks to their potential for creating simple, low-cost, and sustainable analytical devices, making them particularly suitable for environmental monitoring. The aim of this work is to develop a ready-to-use colorimetric paper sensor, based on the Griess reaction, for nitrite on-site monitoring. We here address the requirement for a sustainable, sensitive, and low-cost nitrite sensor that combines, for the first time i) the use of paper as a support, ii) the immobilization of Griess reagents, iii) the origami strategy for triggering chemical reactions without the need for handling chemicals, and iv) a smartphone as a detector for quantitative measurements. While previous sensors for nitrite detection rely on a complex assay workflow and require separate instrumentation, our paper sensor simply needs a smartphone or, for qualitative results, the naked eye for instrument-free detection. The paper sensor showed satisfactory analytical performance for analysis of drinking water with recoveries from 87 to 110% and limits of detection and quantification for NO₂⁻ of 0.27 mg L⁻¹ and 1.11 mg L⁻¹, respectively. The sustainability of the sensor was also evaluated supporting its potential use for rapid monitoring of nitrites across a range of applications, including water quality assessment in agricultural runoff, wastewater treatment, and surface water monitoring.