A simple colorimetric sensor based on a new deep eutectic solvent for the detection of chromium†

A selective and sensitive method was developed for the detection of Cr⁶⁺. This method was based on a combination of colorimetry and liquid–liquid extraction using a new deep eutectic solvent (DES). A DES/EDTA (ethylenediaminetetraacetic acid) colorimetric probe was prepared using tetrabutylammonium bromide (TBABr) as a hydrogen-bond acceptor, ascorbic acid (AA) as a hydrogen-bond donor, and EDTA as a chelating agent. Following the reduction of Cr⁶⁺ to Cr³⁺ by AA present in the DES/EDTA probe, a purple complex was formed with EDTA, which was immediately extracted into the DES phase. The enriched complex was subsequently quantified by performing absorbance measurements at 550 nm using an ultraviolet-visible spectrophotometer and a smartphone equipped with ImageJ software to detect the color intensity. Under the optimized conditions, the limit of detection was determined to be 0.056 μM (2.08 μg L⁻¹) and the linear range was 0.5–100 μM with a correlation coefficient of 0.9962. In addition, the intra- and inter-day relative standard deviations were <6%. Notably, this simple and affordable colorimetric sensor was successfully employed in the detection of chromium in water and soil samples with satisfactory outcomes. The developed approach could therefore be highly useful for the determination of chromium in other environmental and biological samples.