Nitrogen‐Doped Carbon Dots‐Eu ³⁺ ‐Tetracycline as a Ratiometric Fluorescence Probe for Turn “Off–On” Determination of Pyrophosphate and Alkaline Phosphatase Activity

The monitoring of alkaline phosphatase (ALP) activity and pyrophosphate (PPi) concentration is critical for assessing phosphorus dynamics and eutrophication in aquatic ecosystems. However, on-site, rapid detection of these dual targets in complex water matrices remains a challenge. In this work, a nitrogen-doped carbon dots (N-CDs)-Eu³⁺-tetracycline (TC)-based ratiometric fluorescence probe was constructed for turn "off-on" determination of PPi and ALP based on the strong binding interaction between PPi and Eu³⁺ and PPi hydrolyzed by ALP. In the N-CDs-Eu³⁺-TC system, with the addition of PPi, PPi could replace the N-CDs to form the PPi-Eu³⁺-TC multicomplex, and N-CDs were released, resulting in the fluorescence quenching of Eu³⁺ and fluorescence recovery of N-CDs. ALP could hydrolyze PPi to PO₄ ^3-, which could destroy the PPi-Eu³⁺-TC multicomplex and form the N-CDs-Eu³⁺-TC multicomplex again, and the fluorescence of Eu³⁺ at 617 nm was recovered. The N-CDs-Eu³⁺-TC-based ratiometric fluorescent probe possesses a quick fluorescence response, obvious fluorescence color variance, and excellent selectivity and sensitivity with LOD of 40 nM for PPi and 0.03 U/L for ALP. The proposed probe was successfully applied for the detection of PPi and ALP in the actual environmental water samples or serum samples.