A "turn off-on" fluorescent nanoprobe consisting of CuInS2 quantum dots for determination of the activity of β-glucosidase and for inhibitor screening.

A fluorescent "turn off-on" nanoprobe is described for highly sensitive and selective determination of the activity of the enzyme β-glucosidase (β-Glu). Firstly, cysteine modified CuInS2 quantum dots (Cys-CuInS2 QDs) were prepared from indium(III) and copper(II) salts and the presence of thiourea. The red fluorescence of the Cys-CuInS2 QDs, with excitation/emission maxima at 590/656 nm, is quenched by Cu(II). However, in the presence of β-Glu and the cyanogenic glycoside, enzymatic hydrolysis leads to the formation of cyanide. The latter competitively binds to Cu(II) owing to its high affinity for cyanide. This restores the fluorescence of the Cys-CuInS2 QDs. Under the optimum conditions, fluorescence increases linearly in the 0.5-700 U·L-1 β-Glu activity range. The detection limit is 0.2 U·L-1. The nanoprobe was applied to analyze spiked soil samples, and satisfactory results were obtained. The average recoveries of β-Glu were in the range of 96-103%, and the RSD was lower than 4.0%. The fluorescent probe can also be used to screen for β-Glu inhibitors as demonstrated for castanospermine as an example. Graphical abstractSchematic representation of the fluorescent nanoprobe for β-glucosidase activity detection and inhibitor screening by taking advantage of the fluorescence (FL) "turn-off" and "turn-on" feature of cysteine capped CuInS2 quantum dots (Cys-CuInS2 QDs).