Direct photoluminescence tuning via enzyme/nano-substrate interaction: A mechanism study and analytical applications

While the facile photoluminescence (PL) tuning of carbon quantum dots (CQDs) permits their versatile analytical applications, including enzyme activity assays, the reliance on indirect CQDs-enzyme interaction mediated by catalytic reaction substrates or products compromises the assay reliability. Herein, understanding the urgency for developing a direct CQDs/enzyme interaction dyad, we synthesized phosphate-functionalized CQDs (CQDs-P) and characterized their chemical and optical responses to alkaline phosphatase (AKP). The PL of CQDs-P is composed of a weak edge-state emission and a dominant core-state emission. However, an edge-to-core electron injection process significantly promotes the core-state emission. Acting as a nano-substrate, CQDs-P undergo AKP-catalyzed dephosphorylation, which suppresses the edge-to-core electron injection and quenches the PL of CQDs-P. Taking advantage of the direct PL tuning of CQDs-P, a sensitive AKP activity assay is achieved. The limit of detection (LOD) is calculated to be 0.07 mU/mL. Furthermore, the CQDs-P/AKP system was employed as a biosensor for detecting organophosphorus compounds (OPs), competitive inhibitors of AKP, demonstrating excellent sensitivity and selectivity. Benefited from the excellent anti-interference capability of CQDs-P/interaction, accurate AKP activity assay in serum, as well as reliable OPs detection in practical food samples are achieved. This study paves the way for an efficient and robust analysis of biologically active substances using a nano-indicator.