The Molecular Design of Fluorescent Sensors for Ionic Analytes

Abstract

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 (d⁰ and d¹⁰ 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. An-ionic substrates can be conveniently recognized on the basis of the metal–ligand interaction: polyamine receptors containing the photophysically inactive Zn^IIion bind the carboxylate group. In the case of amino acids,${\text{NH}}_3^ + {\text{ - CH(R) - COO}}^ -$, 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.