pH modulated luminescent switching and discriminative detection of amino acid based on metal-organic framework

Abstract

The detection of glutamic (Glu) or aspartic (Asp) acids is vital for human nutrition and diagnosis of disease. Herein, the dht ligand containing hydroxy group (-OH) is used to design and synthesize a 2D luminescent [Cd₂(idc)(dht)(H₂O)₄] (1); H₂idc = 4,5-imidazoledicarboxylic acid and H₂dht = 2,5-dihydroxyterephthalic acid for sensing amino acids. The compound 1 can discriminatively detect Asp and Glu among other amino acids through blue-shifted emission (yellow → green). The dual sensing mechanism may be attributed to the intermolecular excited-state proton transfer between MOF and water to produce keto form along with the subsequent switching of keto form to enol form by protonation causing the increased band gap energy. This material can serve several benefits in terms of high selectivity, fast response (30s), good reproducibility and low LOD value of 11.34 μM which is less than the harmful concentration of Glu for human health (>400 μM). In addition, 1 shows the broad range detection of Glu covering in safe and unsafe levels. For on-site detection of Glu, MOF-based paper is devised and can be applied through color-scanning application in smartphone. Besides, this sensor can serve to detect Glu in real samples with good recovery.

Keywords: Amino acid; Colorimetric detection; Excited-state proton transfer; Metal-organic framework; pH sensor.