Hand drawn paper-based optical assay plate for rapid and trace level determination of Ag^sup +^ in water

Patterning of hydrophobic barriers is duly important for designing paper-based analytical devices. Hydrophobic ink composing of solubilized poly(methyl methacrylate) was re-filled into the commercially available correction pen that was used to draw microzones/hydrophobic barriers on filter paper surface. The stability of hydrophobic barriers was strong enough to withstand elevated surrounding temperatures. The hand-drawn optical assay plate (OAP) was subsequently used for rapid and precise determination of Ag+ ions using 6-hydroxy-3-(2-oxoindolin-3-ylideneamino)-2-thioxo-2H-1,3-thiazin-4(3H)-one (HOTT) as a chromogenic reagent. Within the microzones, the developed reddish brown colored Ag-HOTT chelate was easily visualized by naked eyes at trace level (10−1 μM) of Ag+ in water. The assay sensitivity on the proposed platform was 3-orders of magnitude lower to the similar assay executed on pristine filter paper. Quantitative analysis of Ag-HOTT complex by image analysis software showed a linear dynamic range between (101–104 μM) with a regression coefficient (R2) of 0.995. The proposed Ag+ assay offers easy colorimetric read-out and flexibility in point-of-use testing. The versatility of the in-house fabricated OAP can be easily extended towards other analytes as well especially in resource limited areas.