A dual-readout paper-based analytical device for the simultaneous determination of hexavalent Cr and total Cr.

A paper-based analytical device (PAD) is presented with colorimetric/electrochemical dual readouts for the simultaneous sensing of total chromium (Cr) and hexavalent chromium (Cr(VI)). This device consists of a homemade three-electrode system and a patterned paper chip, integrating multiple functions including electrochemical detection, fluid driving, online oxidation, and colorimetric detection. The fiberglass filter paper with a hydrophilic microchannel was used to achieve self-driving fluidics without external equipment. One end of the microchannel was integrated with a homemade three-electrode system to achieve sample loading and electrochemical detection. The middle region on the microchannel was modified with oxidizing reagents to perform online pretreatment, and the yield of Cr(III) oxidation can reach 97.9%, ensuring reliable colorimetric detection of total Cr at another end of the microchannel modified with chromogenic agents. With this device, the signals of Cr(VI) (the signal peak at 0.29 V vs. Ag/AgCl) and total Cr can be obtained in one single injection. After optimization, the limit of detection (LOD) of Cr(VI) and total Cr were 0.01 mg L^-1 and 0.06 mg L^-1 and the linear ranges were 0.05-3.0 mg L^-1 and 0.2-3.0 mg L^-1, respectively. The relative standard deviations (RSD) of the electrochemical testing of Cr(VI) results were in a range 1.3%-8.7% (n = 3), and the RSD values of the colorimetric testing of total Cr were between 0.7-9.2% (n = 3). The device's reliability was demonstrated by performing the practical speciation of Cr in tap water, river water, and electroplating wastewater while the recoveries obtained using the present method were in the range 93.5-106%. Overall, the proposed device provides high application prospect in the on-site rapid Cr speciation.