Aliphatic amide salt, a new type of luminogen: Characterization, emission and biological applications

Luminescent materials, particularly those with tunable wavelengths, are gaining considerable attention owing to their advanced applications in optoelectronic devices, fluorescent probes and sensors etc. Among all species studied, luminogens without typical chromophores have been gaining increasing attention. The related studies have been mainly focused on polymers with functional groups, such as aliphatic tertiary amine, carbonyl, ester, hydroxyl, amide, imide etc. Here we report a new type of nonconventional fluorescent emitter, fully water-soluble aliphatic amide salt. By simply blending methanol solution of ethylene diamine with that of succinic acid, the aliphatic amide salt (PA24S) was precipitated out, and identified through a series of advanced testing. Both the salt PA24S and its corresponding polyamide showed fluorescent emission as solid powders and in their solutions. The emission behavior of PA24S was studied with regard to concentration, pH, temperature and non-solvent fraction. It was concluded that the emission was owing to the formation of molecule clusters, and confirmed by UV-vis absorption, cluster size measurement and NMR test. It was also demonstrated that Cu2+ was an effective quenching agent with good selectivity. The cytotoxicity assay demonstrated that PA24S was non-toxic to HeLa cell, and suitable for cell imaging. Combined with selective quenching of Cu2+, PA24S is successfully used for Cu2+ detection and monitoring. This work provides therefore a novel type of luminogen of great potential applications in biological fields, featured by its easiness to make, high water solubility and broad emission in dilute solution and at solid state.