Terbium Oxalatophosphonate as Efficient Multiresponsive Luminescent Sensors for Chromate Anions and Tryptophan Molecules

Abstract

A stable 2D terbium oxalatophosphonate with green luminescence, namely, [Tb₂(H₃L)(C₂O₄)₃(H₂O)₄]·2H₂O (1), has been hydrothermally obtained by using (4-carboxypiperidyl)-N-methylenephosphonic acid (H₃L) and oxalate ligand. The luminescent investigation indicates that the emission behavior of compound 1 shows high water and pH stabilities. It can be applied as a multiresponsive luminescent probe with high selectivity, high sensitivity, recycling capability, and fast sensing of CrO₄ ^2-, Cr₂O₇ ^2- anions and tryptophan (Trp) molecules in aqueous solution through the luminescence quenching effect. Moreover, the sensing results can be distinguished by the naked eye under the irradiation of UV light of 254 nm. In addition, the probable mechanisms for the quenching behavior are also discussed, which can be mainly attributed to the competitive absorption of excitation energy between compound 1 and the analytes.

Figure 1

(a) Coordination environment of Tb^III ions in compound 1 (symmetry codes: −x + 1/2, y + 1/2, −z + 3/2 for A; −x – 1/2, y + 1/2, −z + 3/2 for B). (b) Polyhedral representation of 2D layered structure of compound 1 in the ab-plane. Turquoise polyhedra: {TbO₈} and {TbO₉}. (c) 24-atom rings in compound 1. (d) Topology of compound 1.

Figure 2

Luminescence spectrum of compound 1 in the solid state at room temperature. Inset: Fluorescent images under UV irradiation (λ_ex = 254 nm).

Figure 3

(a) PXRD patterns of compound 1; the simulated, as-synthesized product, stayed in air for 1 month and immersed in water for 10 days. (b) Fluorescence measurements of compound 1 after treatment with different pH values in aqueous solutions.

Figure 4

(a) Relative intensities at 547 nm for compound 1 dispersed in different anion aqueous solutions upon excitation at 254 nm. (b) Corresponding photographs for different anion aqueous solutions under irradiation of 254 nm UV light. (c) Relative intensities at 547 nm for compound 1, blank, add other mixed anions, and CrO₄^2– and Cr₂O₇^2– anions. Inset: Corresponding photographs under irradiation of 254 nm UV light.

Figure 5

Emission spectra of compound 1 with CrO₄^2– (a) and Cr₂O₇^2– (c) anions at different concentrations in aqueous solutions (excited at 254 nm), and the Stern–Volmer plots of CrO₄^2– (b) and Cr₂O₇^2– (d) anions.

Figure 6

(a) Relative intensities at 547 nm for compound 1 dispersed in different amino acid solutions upon excitation at 254 nm. (b) Corresponding photographs for different amino acid aqueous solutions under irradiation of 254 nm UV light. (c) Relative intensities at 547 nm for compound 1, blank, add other mixed amino acids, and Trp. Inset: Corresponding photographs under irradiation of 254 nm UV light. Emission spectra of compound 1 with Trp (d) at different concentrations in aqueous solution (excited at 254 nm), and the Stern–Volmer plots of Trp (e).