A paper-based device for the colorimetric determination of ammonia and carbon dioxide using thiomalic acid and maltol functionalized silver nanoparticles: application to the enzymatic determination of urea in saliva and blood.

A colorimetric assay was developed which has the capability of determining urea in biological samples. It is an origami paper-based sensor consisting of silver nanoparticles that were synthesized by using two different capping agents: thiomalic acid and maltol. The function of the assay relied on hydrolysis of urea to ammonia and carbon dioxide in the presence of urease. The products interacted with nanoparticles which caused aggregation. Interestingly, thiomalic acid capped with silver nanoparticles were selective to ammonia, and the other nanoparticles synthesized by maltol responded to carbon dioxide. These interactions turned the color of nanoparticles from yellow to brown and red, respectively. The resulting colorations were captured by a floatable scanner. A routine image analysis software was utilized to provide the response of the assays. The method was applied to individually determine ammonia, carbon dioxide, and urea. The linear range was 0.06 mg.dL-170.0 mg.dL for ammonia, 0.08 mg.dL-220.0 mg.dL for carbon dioxide, and 0.5 mg.dL-200.0 mg.dL for urea. The respective limits of detection were 0.03 mg.dL, 0.06 mg.dL, and 0.18 mg.dL. No interferences were found in the detremination of urea. The method demonstrates a reliable performance for determination of urea in both saliva and blood samples. Graphical Abstract Schematic representation of paper based colorimetric sensor based on silver nanoparticles for both qualitative and quantitative analyses of urea in biological samples.