Fluorescence sensing of ionic analytes in water: from transition metal ions to vitamin B13.

The fluorescence chemosensor ATMCA has been realised by appending an anthrylmethyl group to an amino nitrogen of TMCA (2,4,6-triamino-1,3,5-trimethoxycyclohexane), a tripodal ligand selective for divalent first-row transition metal ions in water. The ATMCA ligand can act as a versatile sensor for ZnII and CuII ions. Its sensing ability can be switched by simply tuning the operating conditions. At pH 5, ATMCA detects copper(II) ions in aqueous solutions by the complexation-induced quenching of the anthracene emission. Metal ion concentrations < 1 microM can be readily detected and very little interference is exerted by other metal ions. At pH 7, ATMCA signals the presence of ZnII ions at concentrations < 1 microM by a complexation-induced enhancement of the fluorescence. Again the sensor is selective for ZnII over several divalent metal ions, with the exception of CuII, CoII and HgII. Most interestingly, the [ZnII(atmca)]2+ complex can act as a fluorescence sensor for specific organic species, notably selected dicarboxylic acids and nucleotides, by the formation of ternary ligand/zinc/substrate complexes. The oxalate anion is detected in concentrations <0.1 mM; however, no effects on the system's fluorescence is observed in the presence of monocarboxylic acids and long-chain dicarboxylic acids. Among the nucleotides, those containing an imide or amide function are readily detected and an unprecedented high sensitivity for guanine derivatives allows the determination of this nucleotide for 0.05-0.5 mM solutions. Moreover, [ZnII(atmca)]2+ is a very effective and selective sensor in the case of vitamin B13 (orotic acid) in sub-micromolar concentrations. The operative features of the systems investigated are also clearly suitable for intracellular analyses. The factors at the source of organic substrate recognition, here briefly discussed, are of paramount importance for further developments in the applicability of these sensing systems.