A turn-on fluorescence chemosensor based on a tripodal amine [tris(pyrrolyl--methyl)amine]-rhodamine conjugate for the selective detection of zinc ions

A novel tetradendate ligand derived from a tris(pyrrolyl--methyl)amine ( H 3 tpa ) and rhodamine-based conjugate ( PR ) has been designed for use as a sensor, synthesized and characterized spectroscopically. PR {(tris(5-rhodamineiminopyrrol-2-ylmethyl)amine)} serves as a selective colorimetric as well as a fluorescent chemosensor for Zn 2+ in acetonitrile/water (1:1, v/v). In the presence of Zn 2+ , PR exhibited obvious absorption (558 nm) and emission (577 nm) peaks whose intensity increased along with increasing Zn 2+ concentrations. Titration experiments revealed that a large excess of Zn 2+ was required to saturate the absorption ( max ) and emission intensities. Upon the addition of 1000 equivalents of Zn 2+ , the fluorescence intensity of the PR underwent an 500-fold increase ( f = 0.34) with the emission maximum at 580 nm. These kinetics studies demonstrated that the absorption and emission changes were proportional to the Zn 2+ concentration. The color of the solution changed from colorless to a dark pink color. The fluorescence of the PR -Zn 2+ complex can be reversibly restored by using ammonium water or by heating. Competitive ion tests revealed that the intensity of PR -Zn 2+ was not suppressed by excess amounts of other metal ions. The counter anions did not exert obvious influences on the absorption and emission profiles. 1 H-NMR and FT-IR spectroscopic investigations of PR and PR -Zn 2+ revealed that the pyrrole motifs, C&z.dbd;N groups and spirolactam of rhodamine B are capable of coordinating cation guest species. Because each arm of the tripodal ligand tautomerizes independently, only moderate fluorescence enhancement could be seen until all three C&z.dbd;N groups were coordinated by zinc, which may be due to the spirolactam ring opening mechanism of the rhodamine unit. Once all three C&z.dbd;N groups were locked by coordinating with excess of Zn 2+ , the isomerization was arrested, and PR exhibited highly enhanced fluorescence. In addition, energy optimized structures of PR were found to be cage-like by Gaussian 09, further supporting that it can access a large excess of Zn 2+ . Intriguingly, imaging of HeLa cells by using a confocal microscope revealed that this PR probe could be used for biological applications. A tris(pyrrolyl--methyl)amine ( H 3 tpa ) and rhodamine-based conjugate (PR) served as a sensor for the selective detection of Zn 2+ and their application of imaging living cells were studied.