Construction of a Stable Lanthanide Metal-Organic Framework as a Luminescent Probe for Rapid Naked-Eye Recognition of Fe3+ and Acetone

Abstract

Four lanthanide metal-organic frameworks (Ln-MOFs), namely {[Me₂NH₂][LnL]·2H₂O}_n (Ln = Eu 1, Tb 2, Dy 3, Gd 4), have been constructed from a new tetradentate ligand 1-(3,5-dicarboxylatobenzyl)-3,5-pyrazole dicarboxylic acid (H₄L). These isostructural Ln-MOFs, crystallizing in the monoclinic P2₁/c space group, feature a 3D structure with 7.5 Å × 9.8 Å channels along the b axis and the point symbol of {4¹⁰.6¹⁴.8⁴} {4⁵.6}₂. The framework shows high air and hydrolytic stability, which can keep stable after exposed to humid air for 30 days or immersed in water for seven days. Four MOFs with different lanthanide ions (Eu³⁺, Tb³⁺, Dy³⁺, and Gd³⁺) ions exhibit red, green, yellow, and blue emissions, respectively. The Tb-MOF emitting bright green luminescence can selectively and rapidly (<40 s) detect Fe³⁺ in aqueous media via a fluorescence quenching effect. The detection shows excellent anti-inference ability toward many other cations and can be easily recognized by naked eyes. In addition, it can also be utilized as a rapid fluorescent sensor to detect acetone solvent as well as acetone vapor. Similar results of sensing experiments were observed from Eu-MOF. The sensing mechanism are further discussed.

Keywords: Fe3+ sensors; acetone sensors; lanthanides; luminescence; metal-organic frameworks.

Scheme 1

1-(3,5-dicarboxylatobenzyl)-3,5-pyrazole dicarboxylic acid (H₄L).

Figure 1

(a) Coordination environment of the Gd³⁺ ion in Gd-metal-organic frameworks (MOF) (Gd, green, O, red, N, blue, C, gray). (b) The binuclear metal cluster unit [Gd₂(COO)₄]. (c) The coordination mode of the [L]⁴⁻ ligand. (d) The coordination mode of the binuclear metal cluster unit [Gd₂(COO)₄]. (e) The 3D framework viewed along the b-axis.

Figure 2

(a) The simplified eight-connected node of the binuclear unit. (b) The simplified 4-connected linker. (c) The (4, 8)-connected topological 3D net with the Schläfli symbol {4¹⁰.6¹⁴.8⁴} {4⁵.6}₂.

Figure 3

(a) The excitation (λ_em = 614 nm) and emission spectra (λ_ex = 297 nm) of 1. (b) The excitation (λ_em = 543 nm) and emission spectra (λ_ex = 299 nm) of compound 2. (c) The excitation (λ_em = 574 nm) and emission spectra (λ_ex = 287 nm) of compound 3. (d) CIE-1931 chromaticity diagram shows the luminescence colors of compounds 1 (0.66, 0.33), 2 (0.32,0.61), 3 (0.38, 0.43), and 4 (0.19, 0.21).

Figure 4

(a) Comparison of the ⁵D₄ → ⁷F₅ photoluminescence intensity of 2 in water and aqueous solutions containing different metal ions when excited at 299 nm. (b) Photoluminescence spectra, (c) quenching efficiency, and (d) a Stern-Volmer plot of 2 with a gradual increase of the Fe³⁺ concentration in aqueous solution.

Figure 5

(a) Time-dependent quenching efficiency of 2 in the presence of Fe³⁺ solutions (3 × 10⁻⁴ M). (b) Comparison of the ⁵D₄ → ⁷F₅ photoluminescence intensity of compound 2 in the pure individual metal ions and mixed metal ions containing 1 mM of Fe³⁺ ion. (c) The image of 2 paper-coated sensor treated with 0.01 M Fe³⁺ solution.

Figure 6

(a) Comparison of the ⁵D₄ → ⁷F₅ photoluminescence intensity of 2 in different organic molecules when excited at 299 nm. (b) Photoluminescence spectra and (c) quenching efficiency of 2 with a gradual increase in the concentration of acetone in N,N′-dimethylformamide (DMF). (d) Time-dependent quenching efficiency of 2 suspension in the presence of acetone in DMF (0.1 vol.%). (e) Photographs of 2 suspension before and after adding acetone. (f) Time-dependent solid photoluminescence spectra of solid-state 2 upon exposure to acetone vapor.