Sodium hexametaphosphate modulated fluorescence responsive biosensor based on self-assembly / disassembly mode of reduced-graphene quantum dots / chitosan system for alkaline phosphatase

Herein, a sodium hexametaphosphate ((NaPO₃)₆) modulated fluorescence responsive probe based on the integration of reduced graphene quantum dots (rGQDs) and chitosan (CS) via self-assembly/disassembly for label-free alkaline phosphatase assay was constructed. The cationic-charged CS can couple with anionic rGQDs and quench their fluorescence intensity through electrostatic attraction and structure transformation. This self-assembly system above could be decomposed when (NaPO₃)₆ present, because (NaPO₃)₆ could competes with rGQDs for the binding sites on the CS, leading to the disassembly of the rGQDs/CS system, as well as to the system exhibiting a turn-on fluorescence signal. By introducing alkaline phosphatase (ALP) into the system, (NaPO₃)₆ can be hydrolyzed to give phosphate anions. The decomposition effect of enzymatic products on the rGQDs/CS system is weakened. Thus the self-assembling system shows a decreasing photoluminescence (PL) signal compared with the rGQDs/CS-(NaPO₃)₆ disassembling system. The concentration of ALP can be reflected by the variation of the PL intensity of rGQDs/CS system mixed with the enzymatic hydrolysis products. The dynamic detection range for ALP is 20-500 mU mL^-1, with a detection limit (LOD) of 7.8 mU mL^-1. The present fluorescence probe based on the rGQDs/CS system for ALP has excellent selectivity and strong anti-interference capability. When applied to real samples analysis, the present strategy exhibits satisfactory results. In addition, the rGQDs/CS system was used to fabricate paper-based test strips for visual detection of ALP activity, validating its great potential in the application of on-site ALP assays.