Smartphone-based colorimetric detection of glutathione

Glutathione belongs to the family of small-molecular weight antioxidants like ascorbic acid, cysteine, α-tocopherol, uric acid, etc. These molecules play important role in the neutralization of free radicals and reactive oxygen species (ROS). Oxidative stress may lead to ageing and the development of large scale of pathological states of organism. This low molecular weight antioxidant´s level can alter under pathological conditions from reduced (GSH, thiols) to oxidized (oxidized glutathione -GSSG, disulfides) form. A GSSG-to-GSH ratio is indicative marker of oxidative stress. There is a large scale of methods how to determine this biomarker. The trend of the analysis is to minimalize the instrument equipment, sample application volume and analysis cost.

Reduced glutathione (GSH) solutions were prepared in water in the concentration 0-16 mmol/L. Other small-molecular weight antioxidants like 0.25 mmol/L ascorbic acid, 0.15 mmol/L TROLOX and 0.02 mmol/L N-acetyl-cysteine (NAcCys) were studied as possible interferents. The samples were mixed with 5,5´-dithiobis-(2-nitrobenzoic) acid (DTNB) resulting in yellow colored drops forming. Coloration was assayed using camera integrated in a smartphone and color channels analysis. The total volume of 10 µl of sample was applied for one analysis. The smartphone-based data were compared with the reference Ellman assay.

The calibration of glutathione was evaluated. The blue channel intensity data were decreasing according to the increasing glutathione concentration. Red and green channel intensities were stagnating during the whole concentration scale of glutathione. Limits of detection were 0.4 mmol/l for glutathione. Addition of 0.25 mmol/L of ascorbic acid, 0.15 mmol/L of TROLOX and 0.02mmol/L of N-acetylcysteine to GSH in final concentration 0-16 mmol/L had minimal influence on the assay. The results from smartphone-based analysis correlate with the standard Ellman method. The detection limit for GSH was 0.03 mmol/L.

The smartphone-based assay seems to be promising because of simplicity, reliability, robustness and low cost. In spite of the fact that there is a large scale for approaches for the glutathione determination, the main advantage of our colorimetric method is portability and easibility to perform the assay in the field and publically availability of smartphones for home applications.