A self-calibrating dual-channel MOF-based fluorescent sensor for rapid and visual phosphate detection

Rapid and reliable phosphate (Pi; PO₄^3-) sensing is of critical importance for environmental monitoring, yet remains challenging due to signal instability and limited field deployability of existing methods. Herein, we report a self-calibrating dual-channel fluorescent sensor based on a metal-organic framework (MOF), denoted as 9-AA@Eu-ZIF-8@TTA, for rapid and visual Pi detection. The sensor is rationally constructed through a modular host-guest and coordination assembly strategy, in which a Pi-insensitive blue-emissive dye (acridin-9-amine, 9-AA) is encapsulated within Eu³⁺-doped ZIF-8 as an internal reference, while a red-emissive Eu³⁺-TTA coordination complex serves as the responsive channel. Pi detection is governed by a competitive coordination mechanism, where Pi preferentially binds to Eu³⁺, displacing the antenna ligand TTA and selectively quenching the Eu³⁺-centered emission at 617 nm without affecting the reference signal. This dual-channel response enables reliable ratiometric quantification of Pi over a linear range of 0-80 μM with a detection limit of 0.48 μM and a rapid response time of 0.5 min. The sensor exhibits excellent stability over a wide pH range (3-11) and high selectivity against competing ions. Importantly, the distinct red-to-blue fluorescence transition under UV excitation allows direct visual discrimination of Pi levels, while smartphone-based RGB analysis enables instrument-free quantitative readout. Accurate Pi determination in real water samples further demonstrates the practical applicability of this sensor. This work provides a versatile MOF-based design strategy for constructing robust ratiometric fluorescent sensors through the integration of host-guest chemistry and analyte-triggered coordination interactions.