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Boron and nitrogen codoped carbon dots functionalized with cyclodextrin (β-CD-N/B-C-dots) were obtained from β-cyclodextrin. The material displays strong fluorescence (with excitation/emission peak wavelengths of 400/500 nm) and was characterized by UV-vis, transmission electron microscopy and FTIR. If the substrate p-nitrophenylphosphate is enzymatically cleaved by alkaline phosphatase (ALP), a yellow product is formed whose absorption overlaps the excitation spectrum of the β-CD-N/B-C-dots. Hence, fluorescence is reduced due to an inner filter effect. In additon, the β-CD cavity offers a pocket for substrate recognition. The findings were used to design a method for the determination of the activity of ALP. It has a working range that extends from 0.003 to 5.5 U·L -1 , with a 0.3 mU·L -1 detection limit. The method is fast, simple, inexpensive, and highly sensitive and selective. Graphical abstract Schematic of an inner filter effect based probe for alkaline phosphatase based on the use boron and nitrogen co-doped carbon dots (N/B-C-dots) modified with β-cyclodextrin (β-CD). PNPP: p-Nitrophenylphosphate; PNP: p-Nitrophenol anion.