A naked-eye liquid-phase colorimetric assay of simultaneous detect cysteine and lysine

In this work, we proposed a naked-eye colorimetric strategy for simultaneous detection of cysteine and lysine based on 3,3′,5,5′-tetramethylbenzidine (TMB) system in a neutral condition (pH 7.0). Based of the fact that Ag + could oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) to oxidized TMB (oxTMB) and induce a blue color solution corresponding to an absorption peak centered at 652 nm, thus the adding of cysteine or lysine to formation of different product results in the blue color fading to colorless quickly or change to scarlet, respectively. With this strategy, the limit of detection in experimental measurement for cysteine and lysine is 2.46 μM and 3.87 μM, respectively. And the selectivity can also be guaranteed among the other amino acid. The cost is quite low since the use of TMB and Ag + . More importantly, the colorimetric detection operation is very simple without any further modification process and the reaction under physiological conditions is more meaningful for detection of amino acid.