A selective and sensitive fluorescent chemosensor for Hg2+, which was composed of two aminonaphthalimide fluorophores and a receptor of 2,6-bis(aminomethyl)pyridine, was synthesized through the reaction of 2,6-bis(chloromethyl)pyridine and N-[2-(2-hydroxyethoxy)ethyl]-4-piperazino-1,8-naphthalimide. The chemosensor showed an about 17-fold increase in fluorescence quantum yield upon addition of 1 equiv of Hg2+ in neutral buffer aqueous solution, and the other common metal ions did not notably disturb the detection of Hg2+.

In this feature article, we report our recent progresses in fluorescent sensors of biological dyes from the viewpoint of supramolecular and bioorganic chemistry. For signalling fluorophores, we extended or created naphthalene-based ICT systems, e.g. amino-1,8-naphthalimides, amino-1,8-dicyanonaphthalenes and acenaphthopyrrol-9-carbonitriles. We also developed BODIPY derivatives with large Stokes shifts and high fluorescence quantum yields in polar solvents, and a rhodamine analogue working in strong competitive aqueous solution as well as its silaanthracene analogue with a bathochromic shift as large as 90 nm. For sensing mechanisms, we extended or developed the following methods to improve sensing: e.g. PET in a photogenerated electronic field, TICT promoted PET derived from aminoalkyl or piperazino aminonaphthalimides, and the translation/amplification effect of surfactant micelles or aggregation on fluorescent sensing. We also successfully designed deprotonation strengthened ICT, FRET-chemodosimeter sensing systems. For non-cyclic recognition receptors, naphthalimides with two or more side chains at their 4,5- or 3,4-positions, as a convenient and simple platform for ratiometric sensors, were created for the recognition of heavy and transition metallic cations; multi-armed polyamides with more side chains were innovated as a versatile platform for the sensing of metal ions with high affinity, selectivity and positive homotropic allosteric effects. We designed V-shape sensors of the bis(aminomethyl)pyridine receptor with two fluorophores to show high performance. Finally, the intracellular applications of the above sensors and dyes, e.g. imaging heavy and transition metal ions in cells, fluorescent marking of hypoxia of tumour cells, are also reviewed.

A novel "naked-eye" and ratiometric fluorescent zinc sensor (AQZ) of carboxamidoquinoline with an alkoxyethylamino chain as receptor was designed and synthesized. AQZ shows good water solubility and high selectivity for sensing; about an 8-fold increase in fluorescence quantum yield and a 75 nm red-shift of fluorescence emission upon binding Zn2+ in buffer aqueous solution are observed. Moreover, AQZ can enter yeast cells and signal the presence of Zn2+.