Quantification of the Kynurenine Biomarker in Saliva Using Smatphone-Based Fluorescence Digital Imaging

L-tryptophan (Trp) is an essential amino acid whose metabolism produces biologically relevant compounds such as serotonin, melatonin, kynurenine (Kyn), kynurenic acid (Kyna), and nicotinamide adenine dinucleotide (NAD+). Alterations in kynurenine pathway metabolism have been associated with several neurological and neurodegenerative disorders. Due to this importance, this study presents a fluorescence digital image-based (FDIB) analytical method for the quantification of kynurenine (Kyn) using a smartphone. The method is based on the derivatization reaction of Kyn with dansyl chloride (DNS-Cl), since the Kyn-DNS product exhibits high fluorescence when excited at 365 ± 50 nm. Thus, a UV-LED chamber was fabricated on a 3D printer, and it was used for excitation, as well as for capturing the fluorescence of Kyn-DNS using a smartphone, leading to the Kyn-DNS/3Dchamber/FDIB analytical method. The proposed method showed a linear range from 0.50 to 7.00 μmol L^-1, limit of detection (LOD) of 0.05 μmol L^-1 and precision below 2.68%, making it suitable for quantifying Kyn in biological samples such as saliva and urine. The accuracy of the method was evaluated by recovery tests, and a recovery rate from 92.1% to 106% was achieved for synthetic saliva samples. A good agreement at a 95% confidence level with n = 3 was obtained when comparing the FDIB results with those supplied by spectrofluorometry and HPLC-UV methods, which certify the accuracy of the method. The Kyn-DNS/3Dchamber/FDIB method proposed reduces several analytical steps normally required in conventional approaches, such as removal of excess derivatizing agent, purification, solvent evaporation, analyte preconcentration, and filtration. This simplification makes the Kyn-DNS/3Dchamber/FDIB method faster, more cost-effective, less waste-generating, and environmentally friendlier compared to spectrofluorometry and HPLC methods.