The molecular design of fluorescent sensors for ionic analytes

Molecular fluorescent sensors can be synthesized by covalently linking a photoactive fragment (e.g., anthracene) to a receptor subunit displaying affinity toward the envisaged substrate. The electron transfer process is the privileged signal transduction mechanism: redox active substrates (e.g., transition metals) typically release/uptake an electron to/from the proximate photoexcited fluorophore, the recognition being signaled through fluorescence quenching; redox inactive substrates (d(0) and d(10) metals, H+) deactivate an existing quenching relay (e.g., a tertiary nitrogen atom close to the fluorophore) and their recognition is signaled through fluorescence enhancement. Anionic substrates can be conveniently recognized on the basis of the metal-ligand interaction: polyamine receptors containing the photophysically inactive Zn-II ion bind the carboxylate group. In the case of amino acids, NH3+-CH(R)-COO-, selectivity is improved when the receptor platform bears additional groups capable to interact specifically with the R substituent. If R is capable of transferring an electron to the nearby photoexcited fluorophore, the recognition is signaled through fluorescence quenching.