Vitamin B6 cofactor-directed fluorescent “turn-on” detection of alkaline-phosphatase activity using bovine serum albumin-functionalized Mn–ZnS quantum dots

Abstract

We introduced a simple approach for the fluorescent “turn-on” detection of alkaline phosphatase (ALP) activity. We employed bovine serum albumin (BSA)-functionalized manganese-doped zinc sulfide (Mn–ZnS) quantum dots (QDs) and the phosphate substrate was the vitamin-B₆ cofactor pyridoxal 5′-phosphate (PLP). The emission of Mn–ZnS QDs at 604 nm was quenched in the presence of PLP due to formation of an imine linkage between the free amine groups present in functionalized BSA and aldehyde group of PLP. Such fluorescence quenching was not observed with other cofactors of vitamin B₆, such as pyridoxal, pyridoxamine, or pyridoxine. ALP catalyzes a dephosphorylation reaction in biological processes, where it can convert albumin-bound PLP into pyridoxal. Mimicking this biological process, the PLP-directed quenched fluorescence of QDs was restored upon ALP addition. The developed nanoprobe showed a limit of detection of ALP of 0.003 U L⁻¹. We applied this nanoprobe for the monitoring of ALP activity in biological (human serum and plasma) samples.