A cost-effective handheld device and smartphone application: Simplifying precise and rapid on-spot chromium(VI) detection in water.

Chromium(VI), at levels as low as 50 ppb, is difficult to accurately detect on-field, due to interference from co-existing ions. A handheld device has been developed in-house, to rapidly monitor even traces of Cr(VI) in water, using 1,5 diphenylcarbazide (DPC) as a chromogenic probe. The device has a customized 3D-printed container, comprising of a green LED-phototransistor and a microcontroller, for accurately measuring the analyte absorbance at 450-600 nm. At 15:1 volume ratio, the Cr(VI)-DPC complex forms in 5 min and with the reagent remaining effective for 28 days, it enables portable, on-spot measurement, with consistent results over a long period. The operating linear detection range with the device is 10 ppb to 2.5 ppm of Cr(VI). The lower limit of detection (LOD) and limit of quantitation (LOQ) are 6.8 and 20.5 ppb, respectively, much lower than the WHO specified permissible level of 50 ppb Cr(VI), in drinking water. Cr(VI) sensing is highly selective, even with high concentrations (10 ppm) of any of the other ions (Na+, Ni2+,K+,Zn2+,Cu2+,Mg2+,Cr3+,Mn2+,Fe2+,Co2+, F-,Cl-,NO3-, SO42- and PO43-) or their mixtures, if present in water; although the latter have 200 times higher concentration than that of Cr(VI). Only Fe3+ ions (at > 4 ppm) exhibit some interference. The device's results are validated using UV-visible spectrophotometer and ICP-AES, with spiked environmental samples. The device operates reliably under diverse environmental conditions of temperature and humidity. It is cost-effective (manufacturing cost: US$12.5), compact (10 × 12.5 × 10 cm), lightweight (708 g), reagent-efficient, provides rapid (5 min) detection, rechargeable with batteries and supports app-based data transmission.