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

Abstract

A novel tetradendate ligand derived from a tris(pyrrolyl-α-methyl)amine (H₃tpa) 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 Zn²⁺ in acetonitrile/water (1 : 1, v/v). In the presence of Zn²⁺, PR exhibited obvious absorption (558 nm) and emission (577 nm) peaks whose intensity increased along with increasing Zn²⁺ concentrations. Titration experiments revealed that a large excess of Zn²⁺ was required to saturate the absorption (λ_max) and emission intensities. Upon the addition of 1000 equivalents of Zn²⁺, the fluorescence intensity of the PR underwent an ∼500-fold increase (Φ_f = 0.34) with the emission maximum at 580 nm. These kinetics studies demonstrated that the absorption and emission changes were proportional to the Zn²⁺ concentration. The color of the solution changed from colorless to a dark pink color. The fluorescence of the PR-Zn²⁺ complex can be reversibly restored by using ammonium water or by heating. Competitive ion tests revealed that the intensity of PR-Zn²⁺ was not suppressed by excess amounts of other metal ions. The counter anions did not exert obvious influences on the absorption and emission profiles. ¹H-NMR and FT-IR spectroscopic investigations of PR and PR-Zn²⁺ revealed that the pyrrole motifs, –CN– 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 –CN– groups were coordinated by zinc, which may be due to the spirolactam ring opening mechanism of the rhodamine unit. Once all three –CN– groups were locked by coordinating with excess of Zn²⁺, 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 Zn²⁺. Intriguingly, imaging of HeLa cells by using a confocal microscope revealed that this PR probe could be used for biological applications.