A bimetal MOF by solvent-exchanged: Converting pH sensing to “Turn-on” fluorescent receptors for amino acid

Precise detection of amino acids plays an essential role in human health. Herein, a 3D bimetal-organic framework (MOF 1) with strong fluorescence emission was synthesized from Zn2+ and K+ with organic ligands under solvothermal conditions, which undergoes solvent exchange into MOF 1a after immersion in water and the fluorescence disappears. MOF 1a exhibits fluorescence enhancement phenomenon as it has a good solubility in alkaline environment. The dissolution-enhanced emission (DEF) feature of MOF 1a can be used for the trace detection of polar amino acids. It was found that the sensor exhibited interesting fluorescence "turn-on" characteristics for L-arginine (Arg) and L-lysine (Lys) with detection limits of 1.23 and 1.41 μM, respectively. In addition, to facilitate the semi-quantitative detection of amino acids with different pKa, a yellow-green-emitting MOF luminescent membrane (MOF 1a@PVA) was further prepared, which showed significant fluorescence enhancement for Arg and Lys showing a distinct fluorescence "turn-on", while showing fluorescence "turn-off" for Asp, Glu and His. The strategy proposed in this work may broaden the range of applications of MOF fluorescent films in optical fields.