Tubular porous coordination polymer for the selective sensing of Cu²⁺ ions and cyclohexane in mixed suspensions of metal ions via fluorescence quenching

Abstract

A luminescent coordination polymer with tubular channels was synthesized and characterized. The polymer was able to sense small solvent molecules and metal ions via fluorescence quenching. In particular, the polymer was able to rapidly and selectively sense Cu²⁺ ions in a suspension of mixed metal ions without any interference effects.

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The rational design and construction of coordination polymers is an important field of research. Coordination polymers are used in applications that require the inclusion of guest molecules, in gas storage and separation, for chemical sensing and in heterogeneous catalysis.¹ An important method of tuning the properties of coordination polymers is by controlling their architecture; this depends on the selection of a suitable structure and self-assembly reactions involving metal ions and organic linkers. Coordination polymers have become increasingly important in materials science and are emerging as an important group of porous materials, not only because of their intriguing network topologies, but also because they can potentially be used in novel applications.²

2-Aminoterephthalic acid (2-aip) has several interesting characteristics. As a result of its multiple bridging fragments, it can show a wide variety of coordination modes with metal ions and is an excellent candidate for the construction of structural architectures. In addition, the solubility and biocompatibility of 2-aip can be modified after synthesis as a result of the presence of the amino group. It can act as a hydrogen bond donor, depending on the type of amino group present. There has been little reported work on coordination polymers constructed from the 2-aip ligand.³

As it is impossible to predict accurately the final structure of coordination polymers, we used different experimental conditions to investigate the various possible structures. [Cd(2-aip)(bpy)]·2DMF (1) was obtained by a solvothermal technique. We report here the synthesis and structural characterization of this coordination polymer. Compound 1 was then successfully used as a fluorescent sensor to detect Cu²⁺ ions and small molecules and was able to selectively sense Cu²⁺ ions via fluorescence quenching without interference from other mixed metal ions.⁴

Under solvothermal conditions, 2-aip and bpy were co-assembled with Cd(NO₃)₂·4H₂O to generate a 3D supramolecular structure. The hexacoordinated Cd^II metal centre Cd1 was chelated by a bpy unit and the other coordination sites of Cd1 were occupied by two carboxylate groups from two 2-aip subunits. The Cd^II centres were connected by 2-aip linkers along the a direction to generate a 1D zigzag chain (Fig. 1a). These chains were aligned parallel to each other through N3–H⋯O1 (ca. 2.695 Å) interactions between the 2-aip linkers. These interactions facilitated the generation of a 2D corrugated layer standing in the crystallographic bc plane (Fig. 1b). These packed 2D sheets protruded into the groove of another similar 2D sheet through C–H⋯π interactions (approximately 3.409–3.566 Å) between the bpy units and resulted in the generation of a 3D supramolecular architecture with distorted rhombus-shaped 1D channels along the c-axis (cross-section 17.177 × 8.050 Å without taking the van der Waals radius into consideration), which were within the pore size range of micropores (Fig. 1c). These channels were filled with guest DMF molecules. A void space of 21% of the total cell volume was calculated on the removal of these guest molecules.⁵ The bpy unit from each 1D coordination chain was positioned in a 2D sheet in a periodic manner, resulting in the formation of a column of coordinated bpy subunits along the c-axis (Fig. 1d).

Fig. 1 Crystal structure and packing diagrams of [Cd(2-aip)(bpy)]·(DMF)₂ (1). (a) 1D zigzag chains of compound 1. (b) 2D corrugated sheet. (c) View of the rectangular channels along the c-axis formed by the supramolecular organization of the 1D chains. (d) Columnar stacking of the [Cd(bpy)]_n subunit.

Although many analytical techniques (e.g. spectrophotometry, voltammetry and atomic absorption spectrometry) have been developed for the determination of ions,⁶ these techniques often suffer from interference by other metal ions, which results in the need for complicated pre-treatment techniques. Fluorescent sensors are often used to sense metal ions and small molecules.⁷ Compound 1, formed by d¹⁰ metal ions (Cd²⁺) and organic linkers, is a potential candidate for use in luminescent probes and its properties inspired us to systematically explore its applications in this field. The luminescent spectrum of 2-aip was recorded in methanol (CH₃OH) at room temperature (Fig. S2†). To examine the potential application of 1 in detecting common toxic metal ions, it was immersed in DMF solutions containing various metal ions (M^x+ = K⁺, Cd²⁺, Cu²⁺, Co²⁺, Ag⁺, Ni²⁺ or Zn²⁺). Fig. 2 shows that the luminescence intensity of samples containing metal ions varied with the type of metal ion and that the maximum luminescent intensity decreased in the order Cd²⁺ > Zn²⁺ > DMF > K⁺ > Ag⁺ > Ni²⁺ > Co²⁺ > Cu²⁺. Cd²⁺ ions, which have a closed-shell electronic configuration, enhanced the luminescence intensity. In a similar way, Zn²⁺ ions also enhanced the luminescence intensity of 1, whereas the other ions showed various degrees of quenching, especially Cu²⁺ ions (Fig. S3†). The luminescence intensities of 1–Cd²⁺and 1–Zn²⁺ are approximately eight and five times, respectively, that of the luminescence intensities of 1 free of metal ions. The luminescence intensities of the samples containing K⁺, Ag⁺ and Ni²⁺ ions were about half that of the original sample. Co²⁺ and Cu²⁺ ions showed the most significant quenching effect. The emitted visible blue light of a suspension of 1 containing Cu²⁺ ions was significantly less than that of the original suspension under UV light (Fig. 2), which allowed us to easily identify the existence of a small amount of Cu²⁺ ions in dilute solution. Interestingly, the luminescence intensity of 1–Cu²⁺ mainly depended on the concentration of Cu²⁺ ions (Fig S4 and S5†). Ground samples of 1 were suspended in DMF solutions containing different concentrations of Cu²⁺ ions and the luminescence was measured. The luminescence intensity of the suspension containing 1–Cu²⁺ rapidly decreased with increasing concentrations of Cu²⁺ ions in DMF solution (Fig. S4†). No luminescence was detected when the concentration of Cu²⁺ ions reached 10⁻² mol L⁻¹.

Fig. 2 Comparison of the luminescence intensity of 1 immersed in 10⁻² mol L⁻¹ DMF solutions of metal ions (excited at 350 nm). Inset: optical images of the samples under irradiation with 365 nm UV light.

To investigate the selectivity towards Cu²⁺ ions in the presence of other metal ions, the sample was immersed in 5 mL of a solution of DMF containing mixed metal ions (5 × 10⁻⁵ mol each of K⁺, Zn²⁺ and Cd²⁺) and sonicated for 30 min. Under irradiation with 365 nm UV light the emission spectrum of the sample loaded with mixed ions increased significantly compared with the original sample when viewed with the naked eye (Fig. 3). When a 0.1 mL volume of solution containing 5 × 10⁻⁵ mol Cu²⁺ ions was introduced into the solvent containing mixed metal ions, the solvent became darker within 10 s and it was easy to distinguish this difference in colour with the naked eye. The time-dependent emission spectrum of this mixed solvent was also measured. The luminescence was rapidly quenched, indicating that the selectivity for Cu²⁺ ions was not affected by the presence of K⁺, Zn²⁺ and Cd²⁺ ions. Ni²⁺ and Co²⁺ ions also had a significant quenching effect. To evaluate the interference of Ni²⁺ and Co²⁺ ions, a series of experiments was carried out (see ESI†). The experimental results confirmed that 1 could be used for the highly selective and rapid detection of Cu²⁺ ions in dilute solution through fluorescence quenching in a solution of mixed metal ions.

Fig. 3 Time-dependent photoluminescence intensities of 1–M^x+ and 1–M^x+ + Cu²⁺ (M^x− = K⁺, Cd²⁺ and Zn²⁺). Inset: optical images of the samples under irradiation with 365 nm UV light (left to right: 1–DMF, 1–M^x+ and 1–M^x+ + Cu²⁺).

The UV-visible spectra of 1 and 1–M^x+ (Fig. S6†) in DMF solution suggested that the strong absorption of 1–Cu²⁺ at the excitation wavelength (350 nm) may be responsible for the observed luminescence quenching, which effectively suppressed the transfer of the excitation energy and therefore the population of the emissive states.⁸ This explains why other ions have lower quenching effects. The higher the absorbance of 1–M^x+, the more significant the quenching effect on the luminescence of the framework; the lower the absorbance of 1–M^x+, the more significant the enhancing effect of 1–M^x+ on the luminescence of the framework. The skeletal structure of compound 1 remained unchanged after the encapsulation of the metal ions, as demonstrated by powder X-ray diffraction (Fig. S9†).

The luminescent properties of 1 in different solvent emulsions resulted in excellent fluorescence sensing of small molecules. A finely ground sample of 1 (3 mg) was immersed in various organic solvents (5 mL), ultrasonicated for 90 min and then aged for 6 h to generate a stable suspension before the fluorescence study. The solvents used were CH₃OH, ethanol (CH₃CH₂OH), 1,3-propanediol, cyclohexane, tetrahydrofuran (THF), N,N-dimethylformamide, dichloromethane, acetic ether, isopropanol, acetone and acetonitrile (CH₃CN). The luminescence spectrum depended on the solvent. Subtle shifts in the emission peaks were observed with different solvents and the maximum luminescence intensity decreased in the order DMF > CH₃CH₂OH > 1,3-propanediol > THF > isopropanol > acetone > CH₃OH > acetic ether > CH₂Cl₂ > CH₃CN > cyclohexane (Fig. 4). All the solvents were fluorescence quenchers and had varying degrees of quenching on the luminescence intensity. Cyclohexane completely quenched the emission and lower quenching efficiencies (97.6 and 96.7%) were observed for CH₃CN and CH₂Cl₂, respectively. The decreasing trend of the luminescence intensities of 1 in different oxygen-containing solvent emulsions was weaker than in solvents that did not contain oxygen. Acetic ether showed the highest quenching efficiency and the luminescence intensity of 1 in acetic ether emulsion was approximately one-tenth that of 1–DMF. It was assumed that the different binding interactions between the functional NH₂ or carboxyl groups and the guest solvent molecules played an important role in the quenching effects of small solvent molecules.⁹ The powder X-ray diffraction pattern further confirmed that the framework was retained in various solvents (Fig. S11†).

Fig. 4 Comparisons of the luminescent intensity of 1–solvent at room temperature. Inset: corresponding emission spectra.

We selected four different solvents (cyclohexane, CH₃CN, CH₂Cl₂ and acetic ether) to evaluate the reproducibility of the quenching ability of 1. The experimental results showed that 1 could be regenerated and reused for a number of cycles by centrifuging the solution after use and washing several times with DMF. The luminescent intensity of each cycle was observed to be unchanged (Fig. S12†) by monitoring the emission spectra of 1 dispersed in DMF and various different solvents.

Such solvent-dependent luminescent properties are of interest for the sensing of cyclohexane molecules. To examine the sensitivity towards cyclohexane, cyclohexane was gradually introduced into an emulsion of 1–DMF. The fluorescence intensity of the emulsion gradually decreased as the concentration of cyclohexane gradually increased (Fig. S13†) before an equilibrium state was reached.

These results suggest that 1 could be used as a promising luminescent probe for the detection of small molecules such as cyclohexane.

Conclusions

A luminescent coordination polymer with tubular channels was synthesized and characterized. The experimental results showed that compound 1 could selectively sense Cu²⁺ ions and cyclohexane molecules via fluorescence quenching. Mixed metal ions did not interfere in the selectivity of 1 to Cu²⁺ ions, which suggests that 1 may have potential application as a fluorescence probe.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21303012).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic details, PXRD, TGA plot, and experimental details and additional figures of sensing. CCDC 1062871. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra10336c

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