A rhodamine-based fluorescent chemosensor for Al3+: is it possible to control the metal ion selectivity of a rhodamine-6G based chemosensor?

Abstract

A rhodamine derivative, 3′,6′-bis(ethylamino)-2-(2-(2-hydroxy-5-methylbenzylideneamino)ethyl)-2′,7′-dimethylspiro[isoindoline-1,9′-xanthen]-3-one (HL-Me), has been found to be a highly selective and sensitive fluorescent chemosensor for Al³⁺. It has been synthesized by condensation between N-(rhodamine-6G)lactam-ethylenediamine (L1) and 5-methylsalicylaldehyde (1 : 1) in acetonitrile and characterized by standard methods including single crystal X-ray diffraction analysis. Absorption and emission spectra have been obtained in 10 mM HEPES buffer at pH 7.4 in H₂O/MeOH = 1 : 9 (v/v) at 25 °C. In the absorption spectra, HL-Me shows bands at 529 nm in the presence of Al³⁺. This metal ion turns the colorless solution of the probe to pink. Other metal ions fail to produce any color change. HL-Me is almost non-fluorescent when it is excited at 500 nm. But in the presence of Al³⁺, the emission intensity at 552 nm increases massively. No other metal ion is able to cause significant enhancement of the emission intensity. HL-Me is colorless and non-fluorescent due to the presence of the spirolactam ring. Al³⁺ induces the opening of the ring, leading to pink coloration and high fluorescence. Ring opening and the subsequent complex formation with the metal ion have been confirmed by ¹H, ¹³C NMR, IR and other spectral studies. The LOD is determined to be 2.8 nM. The quantum yield and life-time of the probe increase significantly upon complex formation with Al³⁺. HL-Me has been used for live cell imaging studies with different prokaryotic and eukaryotic cells. Reactions of L1 with salicylaldehyde with varying derivatives produce different rhodamine-based probes for different analytes. An attempt has been made to categorize the probe based on the substituent present in it with metal ion selectivity.