Detection of acid protease using the fluorescent probe 3,3′-dipentylthiacarbocyanine iodide-BSA

In this work, an ‘on-off’ fluorescent probe was developed for detection of acid protease (ACP) based on bovine serum albumin (BSA) as the substrate. 3,3′-Dipentylthiacarbocyanine iodide, DiSC5(3), exhibits very weak fluorescence emission in aqueous solution, but following the addition of BSA, due to hydrophobic interactions between them, DiSC5(3) forms an aggregate with BSA and emits strong red fluorescence. When ACP is added, the fluorescence emission of the DiSC5(3)-BSA system is quenched because BSA is hydrolysed into peptide fragments and amino acids by ACP, destroying the DiSC5(3)-BSA system. The degree of fluorescence quenching of the DiSC5(3)-BSA system is proportional to the concentration of ACP over a range of 10–40 μg/mL, with a linear equation y = −4.8270x + 509.7066, a correlation coefficient R2 = 0.9986, and a detection limit of 1.44 μg/mL. The method has advantages including simple operation, high sensitivity and good selectivity, and can be used for detection of ACP in real samples.