Self-assembled gold nanoclusters for fluorescence turn-on and colorimetric dual-readout detection of alkaline phosphatase activity via DCIP-mediated fluorescence resonance energy transfer

We present a fluorescence turn-on and colorimetric dual-readout sensing system for the sensitive detection of alkaline phosphatase (ALP) activity via self-assembled gold nanoclusters (AuNCs) based on fluorescence resonance energy transfer (FRET). The positively charged polyallylamine hydrochloride (PAH)-crosslinked AuNCs (PAH-AuNCs) with aggregation-induced enhancement (AIE) characteristics can electrostatically adsorb the negatively charged 2, 6-dichlorophenolindophenol (DCIP). Thus, the fluorescence of PAH-AuNCs can be significantly quenched by the occurrence of FRET from PAH-AuNCs to DCIP. However, the reduction reaction of DCIP from blue to colourless by L-ascorbic acid (AA) which is generated by the ALP catalyse hydrolysis of 2-Phospho-L-ascorbic acid (AAP) disturbs the FRET between PAH-AuNCs to DCIP. The quenched PAH-AuNCs fluorescence can be recovered efficiently. The strategy of first creating AIE-enhanced PAH-AuNCs, followed by the effective FRET manipulation, is an important contribution to the sensitive detection of ALP. More importantly, the distinct colorimetric signal change can be used to visually distinguish the presence of ALP. More importantly, the distinct colorimetric signal change can be used to visually distinguish the presence of ALP. Good linear relationships of fluorescence and colorimetric sensing towards ALP were obtained in the range from 0.5 to 100 U/L, and the detection limits were 0.2 U/L and 0.5 U/L, respectively. In addition, the proposed FRET sensing system was applied to the detection of ALP in human serum samples with satisfactory results. The simple and efficient sensing approach proposed here has the potential to promote the development of chemo/biodetection methods using fluorescence and colorimetric dual-readout.