A facile water-stable MOF-based “off–on” fluorescent switch for label-free detection of dopamine in biological fluid

Abstract

As an emerging class of fluorescent probes, metal–organic frameworks (MOFs) have recently received great interest owing to their fascinating functional properties, intriguing tunable structures, high selectivity and good sensitivity. Herein, we present a novel strategy based on the application of a facile water-stable MOF {[Cd(μ₃-abtz)·2I]}_n (Abtz–CdI₂–MOF, abtz = 1-(4-aminobenzyl)-1,2,4-triazole) using powerful solvo-thermal synthetic techniques, which can serve as an “off–on” fluorescent switch for the label-free detection of dopamine (DA) without any additional surface modification and functionalization. The fluorescence signal of Abtz–CdI₂–MOF can be efficiently quenched by KMnO₄, and then restored by DA in an “off–on” mode. The possible quenching mechanism should be static quenching resulting from the ground state recombination between Abtz–CdI₂–MOF and MnO₄⁻. Further when DA is added, MnO₄⁻ can be gradually released from the hybrid system, and bring about ground state complex decomposition, which is triggered by competitive interactions between Abtz–CdI₂–MOF and the reduced iodine ions. Therefore, the fluorescence signal can be gradually turned “on”. The relative intensity of restored fluorescence is proportional to the concentration of DA in the wide linear range of 0.25–50 μM with a low detection limit of 57 nM (S/N = 3). Additionally, some potentially foreign interfering substances, such as ascorbic acid (AA) and uric acid (UA), glucide, amino acids and metal ions did not affect DA determination significantly. The proposed strategy was also successfully applied for DA determination in biological samples with satisfactory recoveries from 94.5% to 102%.