In-situ tunable working ranges on microfluidic paper-based analytical devices for cysteine analysis in food

Colorimetric microfluidic paper-based analytical devices (µPADs) offer a promising platform for point-of-care diagnostics due to their simplicity, portability, and low cost. However, their limited dynamic range, typically restricted by a single reagent concentration, often necessitates sample dilution, increasing analysis time and complexity. This work presents a novel strategy to expand the dynamic range of µPADs by integrating varying reagent concentrations within a single device. This approach was demonstrated for the detection of cysteine (Cys), a biomolecule with a wide concentration range in biological and food samples. Indirect Cys quantification was performed by reacting it with Ag(I) prior to the determination of the remaining Ag(I) using K₂CrO₄. Despite the inherent interference from halides in this assay, an integrated online halide removal layer was incorporated into the sensor. Moreover, the interference from other amino acids was not found, indicating high selectivity. A wide linear range of six orders of magnitude (0.0001-10 mg mL^− 1) with a limit of detection of 0.03 µg mL^− 1 was achieved using the multi-concentration approach. The developed µPADs were successfully applied to the analysis of Cys in food samples, achieving high accuracy with recoveries ranging from 87.0 to 107.8%. This innovative approach holds significant potential for enhancing the analytical capabilities of µPADs for various point-of-care applications.