Smartphone-assisted colorimetric determination of uranyl ions in aqueous solutions

Abstract

A convenient and portable smartphone-assisted method for the colorimetric detection of uranyl ions (UO₂²⁺) in aqueous solutions is proposed using rifampicin and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (Br-PADAP) as chromogenic agents. The addition of UO₂²⁺ would cause reaction with chromogenic agents to form complexes, changing the colour from orange to dark red in the rifampicin system and from yellow to light pink in the Br-PADAP system. To achieve the on-site estimation of the trace UO₂²⁺ concentration range, digital images of colour changes captured by the smartphone under fixed lighting conditions were imported into the built-in application named “Detect” to obtain HSV (hue, saturation, and value) and HSL (hue, saturation, lightness) information, which could be associated with the concentration of uranyl ions. For more precise laboratory-level detection, ultraviolet-visible (UV-vis) spectroscopy was employed as well. The absorbance of the rifampicin system exhibited strong linearity with the UO₂²⁺ concentration in the range from 4 to 50 μM at 375 nm with a method detection limit (MDL) of 3.17 μM by the UV-vis-based method, while that of Br-PADAP system had a linear relationship with the concentration from 0.9 to 7 μM at 562 nm with an MDL of 0.89 μM. The two strategies may be combined to detect UO₂²⁺ across a larger concentration range, and this method has been successfully used to analyze real samples including soil around uranium mines.