Enhanced electrochemiluminescence of mixed-ligand metal-organic framework with suppressed non-radiative transitions for “signal-off” biosensing of β-galactosidase

Organic molecular emitters usually suffer from the aggregation-caused quenching (ACQ) effect, which significantly decreases their electrochemiluminescence (ECL) efficiency. This work designed a straightforward strategy to alleviate the ACQ effect and thus improve the ECL efficiency by employing a donor-acceptor (D-A) type ligand containing benzothiadiazole group and another ligand with identical connectivity to assemble a mixed-ligand zirconium-based metal organic framework (m-Zr-MOF). Upon the formation of a reticular structure and the distance increase between two ligands, the m-Zr-MOF exhibited alleviating ACQ effect due to the suppressed non-radiative transitions, which was confirmed by the improvements of both quantum yield and fluorescence lifetime. At the molar ratio of 3:1 for two ligands the obtained m-Zr-MOFs displayed the optimal ECL performance, and thus an ECL imaging method was developed for “signal-off” detection of β-galactosidase (β-Gal) by combining its enzymatic property to catalyze the hydrolysis of p-nitrophenyl β-D-galactopyranoside, which generated p-nitrophenol to quench the ECL emission through resonance energy transfer. The proposed method showed a detectable range of 5.0 to 2 × 10⁴ mU/L with a detection limit of 1.92 mU/L, much lower than those of reported fluorescence and electrochemical methods. The designed m-Zr-MOF introduced an innovative concept for the development of mixed-ligand MOFs and their application in ECL imaging.