A novel overtone peak self-referencing fluorescent sensor based on a bipyridine-linked covalent organic framework for highly sensitive copper ion detection†

This study reports a novel ratiometric fluorescence sensor based on a tetraphenylethylene-bipyridine covalent organic framework (TPE-Bpy-COF) for the sensitive detection of Cu²⁺, leveraging the unique coordination properties of the bipyridine moieties. The interaction between Cu²⁺ and the nitrogen atoms in the bipyridine units induces fluorescence quenching at 500 nm through an efficient host–guest electron transfer mechanism, where excited-state electrons from the COF framework are transferred to the vacant orbitals of Cu²⁺. Upon excitation at 410 nm, the sensor exhibits a primary emission peak at 500 nm, which is quenched in the presence of Cu²⁺, while an overtone peak at 820 nm remains stable, serving as an internal reference for ratiometric measurements and significantly enhancing the accuracy and reliability of the sensor. The detection limit for Cu²⁺ is 0.1 μM, with the dual-emission system and the strong affinity of the bipyridine units for Cu²⁺, further improving the sensor's sensitivity and selectivity. Additionally, the sensor demonstrated excellent recovery rates in real water samples, confirming its practical applicability in environmental monitoring.