Distance-Based Colorimetric µPADs for Nickel Detection: Effect of Cover Materials, Reagent Loading, and Data Normalization

Distance-based microfluidic paper-based analytical devices (pPADs) provide instrument-free quantification for metal ions, yet cross-study comparisons are hindered by differences in geometry, paper type, reagent loading, and sample volume. In this work, transparent-sheet-covered pPADs were fabricated on Whatman Grade 42 using a simple impregnation method, and the selective Ni–dimethylglyoxime (Ni–DMG) reaction at pH 10.2 generated a reddish-pink precipitate whose migration length represented the analyte concentration. Four Ni(II) levels (20 − 80 ppm) and three DMG loadings (20, 40, and 60 mM) were tested (???? = 10, 5 pL per sample). Band length increased with Ni(II) concentration but decreased with higher DMG, revealing a trade-off between signal and reagent excess. To compare with wax-printed pPADs in the literature, a channel-width correction and dimensionless length normalization were applied, while data were expressed in terms of Ni(II) amount (nmol). The normalized curves showed close agreement in the overlapping range despite differences in paper grade and channel width. The Wilairat model fitted the data well (????2 > 0.95), with increasing DMG elevating the reagent parameter and lowering the asymptotic distance. Overall, the reproducible, low-cost platform and the introduced normalization framework offer a practical route to harmonize distance-based colorimetric Ni(II) detection across various pPAD designs.