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

[Display omitted] •Wax-modified paper as a platform for executing in-situ droplet colorimetric assay.•Demonstrated sensitive determination of Fe3+ in environmental water samples.•Excellent LOD (0.01 ppm) and LDR (0.1–100 ppm) was achieved.•LOD improved by 3-order of magnitude as compared to unmodified paper. 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.