Low-cost colorimetric assay of biothiols based on the photochemical reduction of silver halides and consumer electronic imaging devices.

This work describes a new approach for the determination of free biothiols in biological fluids that exploits some of the basic principles of early photographic chemistry - that was based on silver-halide recording materials - and uses broadly-available imaging devices (i.e. flatbed scanners) as detectors. Specifically, the proposed approach relies on the ability of biothiols to bind to silver ions and dissociate the silver halide crystals thus changing the photosensitivity of silver halide crystal suspension. The changes induced by biothiols on the light intensity transmitted through the silver halide suspension, after photochemical reduction, were measured with a simplified photometric approach that employs a flatbed scanner operating in transmittance mode. The overall analytical procedure for the determination of biothiols was easily executable, fast and could be applied with inexpensive and commercially available materials and reagents. What is more, physiologically relevant biothiol levels could be inspected even by the unattended eye. The developed assay was successfully applied to the determination of biothiols in urine and blood plasma samples with detection limits as low as 10μM, satisfactory recoveries (92-97%), good reproducibility (6.7-8.8%) and high selectivity against other major components of biological fluids. The utility of the method to the determination of reduced/oxidized thiol ratio's as well as its application under natural light illumination, without external energy sources, was also demonstrated and is discussed with regard to point-of need applications in facility-limited settings.