Remarkably selective and enantiodifferentiating sensing of histidine by a fluorescent homochiral Zn-MOF based on pyrene-tetralactic acid

Abstract

A highly luminescent and water-stable homochiral Zn-MOF, <i>i.e.</i>, <b>Zn-PLA</b>, has been developed based on a pyrene-tetralactic acid, which inherently features concave shapes for guest inclusion, to explore sensing of amino acids by fluorescence quenching; the solid-state fluorescence quantum yield of the MOF was found to be 46%. The fluorescence of an aqueous suspension of <b>Zn-PLA</b> was shown to be quenched specifically by histidine amongst all the other amino acids. Selective sensing of histidine is of prime importance due to its relevance in a variety of biological functions. The lack of quenching of fluorescence of <b>Zn-PLA</b> by all the amino acids other than histidine has been rationalized based on the exchange of the cationic dimethylammonium species in the MOF crystals with histidine that is protonated in water; the latter is envisaged to quench the fluorescence <i>via</i> charge transfer in the excited state. Furthermore, the homochiral crystals of <b>Zn-PLA</b> were found to permit enantiodiscrimination in the quenching by the d- and l-forms such that the ratio of enantioselectivity, <i>i.e.</i>, <i>K</i>_d/<i>K</i>_l, is 1.8, as determined by Stern-Volmer quenching plots. The highly selective as well as enantiodifferentiating sensing of amino acids by MOFs is unprecedented for any sensor type.