On the construction and performance of pre-oxidized probe inserted into charged MOF for the in vitro vitamin C detection: A highly selective emission turn-on sensing.

Owing to the critical role of ascorbic acid (vitamin C, VC) in human bio-activities as an antioxidant and free radical scavenger, the development for an easy and fast VC detection has always been pushed. In this work, a traditional ligand bipyridine (bpy) was firstly oxidized (denoted as bpy-O) and then coordinated to Eu(III) (denoted as Eu-bpy-O). Due to the oxidization of bpy N atoms, which led to improper triplet ligand energy level and increased ligand-to-Eu distance, the efficient ligand energy transfer (ET) to Eu(III) was suspended, resulting in a low emission quantum yield of 1.6%. Upon being deoxidized by VC, the ligand ET to Eu(III) became efficient, with emission quantum yield recovered to 22.4%, showing emission turn-on effect for VC. To allow smooth analyte dispersion and uniform probe distribution, Eu-bpy-O was loaded into the micropores of bio-MOF-1 (Eu-bpy-O@MOF) with various doping levels. Good sensing selectivity was observed owing to the well-designed probe and bio-MOF-1 sieving effect. Linear fitting equations were obtained for the optimal sample named 5Eu-bpy-O@MOF within VC concentration region of 0-100 μM, showing LOD of 1.7 μM, sensitivity of 0.191 μM-1, and response time of ∼240 s at 35 °C. The practical sensing performance of 5Eu-bpy-O@MOF was confirmed by its sensing plates upon human serum samples. The novelty of this work was the application of a pre-oxidized probe and a porous host, which offered emission "turn-on" effect for VC with promising performance.