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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. [ABSTRACT FROM AUTHOR]

The authors describe a nitrocellulose membrane based paper matrix array for detection of mercury(II) with high throughput and repeatability. A thymine-rich signal reporting ssDNA probe labeled with gold nanoparticles (AuNPs) was designed, and detection is based on the strong T-Hg-T interaction which leads to a significant color change. In addition, catalytic signal amplification is accomplished by silver staining in order to enhance coloration. The method has a detection limit as low 0.5 ppt. This is at least 500-fold better than that of existing methods. By using an array, multiple sets of duplicate detections can be carried out simultaneously and in parallel. This improves the efficiency and accuracy of the assay. The method shows remarkable specificity since the signal of other competing metal ions is negligible compared to that of mercury(II) even at 100-fold higher concentration. The protocol was successfully applied to the determination of Hg(II) in spiked tap water and lake water samples. In our perception, this assay is a most promising tool for optical determination f ultra-low levels of Hg(II). [ABSTRACT FROM AUTHOR]