A barcode-enabled distance-based paper sensor using silver chloranilate for semi-quantitative determination of chloride in various water sources

Chloride monitoring is important for drinking-water quality assessment, corrosion control, and environmental analysis. In this work, a dual-mode distance-based paper analytical device (d-PAD) was developed for chloride determination using silver chloranilate as a chromate-free colorimetric indicator. An in situ silver chloranilate sensing layer was prepared on the paper substrate through sequential deposition of silver nitrate and chloranilic acid. In the presence of chloride ions, AgCl formation was accompanied by the release of violet chloranilate species, producing a concentration-dependent migration zone along the paper channel. The device enabled quantitative determination based on migration distance and complementary semi-quantitative classification using a six-character Code 39 barcode mask and a smartphone barcode-scanning application. Under the optimized conditions, the PEG-functionalized d-PAD exhibited a linear working range of 100-800 mg L^-1 chloride with a coefficient of determination of r² = 0.9980. PEG treatment provided a more clearly defined migration front with improved contrast against the brown background. The barcode-assisted readout classified chloride concentrations into five intervals: <100, 100 to <250, 250 to <400, 400 to <600, and ≥600 mg L^-1. The method exhibited adequate selectivity for the coexisting ions evaluated and yielded recoveries of 98.98-105.39% with RSD values not exceeding 5% in spiked water samples. Results obtained using the d-PAD were comparable to those obtained by Mohr titration, with relative errors ranging from 0.64% to 4.31%. Overall, the combined distance-based and barcode-assisted readouts provide a simple and portable platform for point-of-need chloride analysis in water samples.