A paper-based colorimetric assay system for sensitive cysteine detection using a fluorescent probe

A fluorescent probe for the colorimetric detection of cysteine (Cys) was synthesized by combining resorufin with 7-nitrobenzofurazan. The resultant probe was characterized by FT-IR spectroscopy, fluorescence detection, mass spectrometry and NMR spectroscopy. After reacting with Cys, resorufin is released and the color changes from light yellow to red on paper-based analytical devices. The results were recorded using a common cell phone and subsequently analyzed using Photoshop software. The color intensity of the RGB channel increased linearly in the 0.04 to 70.04 μM Cys concentration range when the probe concentration was 2.6 mM (R = 0.9965), and the limit of detection was 16 nM. The effects of detection conditions have been investigated and are discussed. The interference deviations of 13 substances to Cys were in the range of −2% to +2%. The Cys concentration was determined as 270.8 ± 19.3 μM in human plasma, and the recovery was from 99.7% to 100.4%. The work demonstrates the potential of the method to detect Cys in real samples with low cost and high sensitivity.