Highly selective and sensitive rhodamine based chemosensors for Al3+ and their applications in living cell imaging

Abstract

To address limitations like susceptibility to hydrolysis of an imine bond and low selectivity of rhodamine based chemosensors, we report here two rhodamine dyes, namely, L-Et i.e. 3′,6′-bis(ethylamino)-2′,7′-dimethyl-2-(2-morpholinoethyl)spiro[isoindoline-1,9′-xanthen]-3-one and L-Pr i.e. 3′,6′-bis(ethylamino)-2′,7′-dimethyl-2-(3-morpholinopropyl)spiro[isoindoline-1,9′-xanthen]-3-one for detection of Al3+ in 10 mM HEPES buffer in H2O/EtOH = 1:9 (v/v) (pH 7.4). Both the probes detect exclusively Al3+ among 16 different metal ions. The sensing ability has been achieved by noting color change (colorless to pink), increment in absorption intensity at 530 nm and fluorescence intensity at 555 nm. Opening of spirolactam ring of the chemosensors followed by complexation with Al3+ is responsible for such changes. A 1:1 complex is formed by both the probes with high association constant (1.14 × 105 M−1 for L-Et and 1.17 × 105 M−1 for L-Pr). The limit of detection values of L-Et and L-Pr towards Al3+ have been determined as 1.7 nM and 1.1 nM, respectively signifying very high sensitivity. These have been successfully applied in cell imaging studies in C6 cells. Introduction of a methylene group does not influence spectral properties vividly. As both the probes do not contain any imine bond, the possibility of hydrolysis in the presence of Al3+ has been avoided.