Widely pH-Stable Fluorescent Metal-Organic Frameworks for Selective Detection of Amino Acids

The detection and sensing of amino acids through turn-on and ratiometric fluorescence signals are always challenging due to different shapes, sizes, acidities, basicities, and structural arrangement of amino acids. In general, amino acids are important biomarkers for identifying human health, providing a rapid, sensitive, and easy way to disease prevention and early diagnosis. Considering this, two highly fluorescent nitro-functionalized metal-organic frameworks (MOFs) have been designed using a mixed-ligands strategy, following a slow-diffusion technique. Both the compounds have a two-dimensional structure with the molecular formula {[Zn(4,4'-dps)(5-nip)](solvent)_x}_n (1) and {[Cd(4,4'-dps)(5-nip)(EtOH)]}_n (2) (where 4,4'-dps = 4,4'-dipyridyl sulfide and 5-nip = 5-nitroisophthalate) and have been characterized thoroughly. The desolvated forms of 1 and 2 (1' and 2') are stable in different solvents and also within the wide pH range of 2-12. Interestingly, the fluorescence spectra of 1' and 2' showed photoinduced electron transfer (PET) driven turn-on fluorescence in the presence of tryptophan (Trp) with a low limit of detection. However, both the compounds involved in excited-state intermolecular proton transfer (ESPT) with aspartic acid (Asp), resulting in the selective ratiometric signal in fluorescence behavior. These observations allow the convenient method for the detection of aforesaid amino acids in water, through a diverse fluorescence mechanistic pathway.