In-situ droplet assay on wax-modified paper for rapid and trace determination of Fe³⁺ in water

The present study is focused on developing a colorimetric spot assay for direct determination of Fe³⁺ in water. The established method was based upon the use of wax modified paper as a low-cost platform onto which a droplet-based colorimetric reaction between aqueous Fe³⁺ and 2-(5-Bromo-2-pyridylazo)-5-(diethylamino) phenol (5-Br-PADAP) was executed. The wax-modified paper was characterized by goniometry that clearly showed a substantial increase in the water contact angle (θ) from 0° to ~123°. Morphological differences in the cellulose fibers of unmodified and modified paper were further confirmed via SEM and AFM analysis. The proposed assay was simple to execute and the resulting color change was readable via naked eyes with total assay time not exceedingly more than 10 min. The limit of detection and linear dynamic range for Fe³⁺ assay executed on the wax-modified paper were 0.01 ppm and 0.1–100 ppm, respectively. The attractive features of the established assay mainly include its simplicity, fast response time, inexpensive modification/platform, low reagent/analyte volume requirement and instrument-free readout. Hence, the proposed paper-based chemosensor can serve as a promising tool for point-of-use application in resource limiting regions especially dealing with iron contamination.