Kojic acid capped gold nanoclusters with aggregation-induced emission for fluorometric screening of the activity of alkaline phosphatase.

ABSTRACT

The authors describe the preparation of gold nanoclusters (AuNCs) capped with kojic acid. The capped AuNCs exhibit bright green fluorescence (peaking at 500 nm upon excitation at 375 nm), a nanosecond lifetime (0.37 ns), and a quantum yield (QY) of 22% in aqueous solution. This is higher than most of the previously reported AuNCs. The QY increases to 58% due to aggregation-induced emission in ethanol solution, and the lifetime is prolonged to 1.3 ns. The fluorescence of the KA-AuNCs is quenched by Eu(III) ion but is recovered by addition of phosphate due to its stronger affinity for Eu(III). Under the catalytic action of alkaline phosphatase (ALP), ascorbic acid phosphate (AAP) is transformed to free phosphate. On this basis, a fluorogenic assay for ALP was established. Response is linear in the 0.2 to 20 U·L-1 activity range, and the detection limit is 0.04 U·L-1 (at S/N = 3). The assay was successfully applied to the determination of the activity of ALP in spiked human serum and also to screen for its inhibitors. Graphical abstractHighly luminescent and stable gold nanoclusters (AuNCs) with aggregation-induced emission property were synthesized through non-thiolate ligand kojic acid (KA) and demonstrated as an efficient probe for screening for alkaline phosphatase (ALP) activity and its inhibitors.