Colorimetric speciation of Cr on paper-based analytical devices based on field amplified stacking

In this work, a paper-based analytical device (PAD) for sensitive speciation detection of chromium (Cr) by smartphone camera was introduced. In anion electrokinetic stacking mode, Cr (VI) in the form of CrO4^2- was firstly enriched into a narrow band on a paper fluidic channel, and then the band was visualized by 1,5-diphenylcarbazide (DPC). Colorimetric detection of Cr (VI) was based on the gray intensity of stacking bands. Detection of Cr (III) was achieved by subtraction of the detection results of Cr (VI) before and after the sample was oxidized. Under optimized conditions, the detection limit of Cr (VI) and Cr (III) of 0.20 μM (~10.4 μg L^-1) and 0.30 μM (~14.6 μg L^-1) were achieved with a linear response in the range of 0.67-5.00 μM and 0.93-6.00 μM, respectively. The detection performance of PAD is close to that obtained by desktop spectrophotometry, thanks to the online stacking effect. Meanwhile, this PAD also exhibited a good selectivity over coexisting cations and anions, thanks to the charge selectivity of the stacking mode and the selective visualization reaction of DPC. Speciation detection of Cr from tap water, river water, surface water, and electroplating wastewater was demonstrated. This work shows the potential of PADs for on-site detection of Cr (VI) in environmental water samples.