Spiral frameworks constructed by 1,2-phenylene-dioxydiacetic acid as highly sensitive and selective luminescent probes to detect PO₄³⁻ ions in aqueous solutions

Abstract

Three novel clubbed photo-luminescent lanthanide–organic frameworks 1D coordination polymers, [Ln(L)(NO₃)(H₂O)₂]_n (1–3) (where Ln is La, Ce, and Pr, respectively, and H₂L = 1,2-phenylenedioxydiacetic acid), have been synthesized in a concise and reproducible manner under hydrothermal conditions. It is worth noting that the H₂L ligand is used for the first time in synthesis complexes. 1, 2 and 3 possess same structure with the same topology of [3^3; 4^2; 5] built using Ln-L 1D helical chains. Fluorescence properties of the obtained complexes have been studied in detail, revealing that 3 can sensitively and selectively detect pollutant PO₄³⁻ among various anions.

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Introduction

The accumulation of PO₄³⁻ results in serious pollution in aquatic ecosystems, for example, excessive algal growth accompanied by red tide, depletion of dissolved oxygen and decrease in water quality.¹ Owing to the extremely toxic effect of PO₄³⁻ ion in the environment and living systems, designing chemosensors for its recognition and sensing has received significant attention.^2,3 Therefore, there is an increasing demand to develop sensitive and selective assays to detect PO₄³⁻ ion, which will be suitable for rapid determination in drinking water and aquatic community sample.^4,5 Chemosensors that can induce changes in fluorescence appear to be particularly attractive owing to their simplicity and high detection limit of fluorescence.^6–9 In this regard, rare earth ion-selective fluorescent chemosensors serve as useful tools for the detection of ions, and thus have been widely exploited to detect environmentally relevant PO₄³⁻.⁴ Moreover, it also has appealing potential applications in modern areas of materials science such as photoluminescence, probes in biosciences, and molecular adsorption.^10,11

In this paper, we report a facile strategy of crafting three new helix complexes, 1, 2 and 3, consisting of three different metal ions and 1,2-phenylenedioxydiacetic acid, which was for synthesis of single crystal to use for the first time through the hydrothermal method (Fig. 1). Detailed studies, including IR, elemental analysis, characterization of structural characteristics, ions recognition ability and binding mechanism, have been conducted using fluorescence spectral measurements, emission spectroscopy, and PXRD and density functional theory calculations.^12–16 The outstanding performance of this novel material is highlighted by the complexes used as the fluorescence probe, which have high sensitivity and high selectivity to PO₄³⁻. It can be noted that helix receptors often show higher affinity and selectivity in ions complexation for their high degree of preorganization and rigidity.¹⁷ In addition, fluorescent probes for PO₄³⁻ recognition are known, but reusable helix chemo-sensors still rare. Hence, it still remains a challenge to develop new chemosensors, particularly helix receptors, for the rapid detection of PO₄³⁻ with high selectivity and sensitivity.

Fig. 1 Construction of the structure types of complexes.

Results and discussion

Crystal structure of [Ln(L)(NO₃)(H₂O)₂]_n

X-ray analysis reveals that the structures of complexes 1, 2, and 3 are very similar. All the complex crystallizes in the monoclinic crystal system space group of P2(1)/c. These complexes are self-assembled from H₂L with La(III), Ce(III), or Pr(III) salts, which are further connected into a 3D network. Hence, only the crystal structure of complex 1 is described. The asymmetric unit contains one La(III) ion, one NO₃⁻ anion, one H₂L ligand, and two coordinated water. As shown in Fig. 2, one La(III) center is ten-coordinated irregular geometry by six O atoms from three L⁻ anions, two O atoms from one NO₃⁻ anions, and two O atoms from two coordinated water. The La–O lengths are in the range of 2.443(18)–2.775(18) Å, which are similar to those found in other La(III) complex.^18–20

Fig. 2 The monomer of La(III) in complex 1.

In addition, C–H⋯O hydrogen bonds were observed between oxygen of the coordinated L⁻ and H₂O of the spiral linkages, which were located inside the central cavity of the complexes. The bond lengths of the C–H⋯O hydrogen bonds were distributed from 2.640 to 2.920 Å. Forming an edge-sharing di-nuclear subunit,²¹ the adjacent La⋯La distance is about 6.211 Å. The hydrogen bonds would be strengthened by the electrostatic effect arising from the electron-deficient H₂L ligand and the anionic charged oxygen of complex.^22,23

Complex 1 has an unusual network, considering the metal center as a node. Interestingly, unlike most of the 1D network, it is not straight but spiral, resulting from an interesting self-assembly of ligand molecules while coordinating to the metal center. As shown in Fig. 3, the grid network can also be described in terms of the self-assembly to two intersecting 1D networks, parallel strands of vertically expanded helices of metal coordinated H₂L molecules and a linear chain of complexes molecules expanding in the horizontal direction. Furthermore, these wavelike sheets undergo offset stacking, sustained by several π–π stacking.²⁴

Fig. 3 (a) The 1D structure of complex 1; (b) 2D layer of complex 1; (c) another 2D layer of complex 1; (d) a perspectives of 3D frame work of complex 1.

XRD powder X-ray diffraction analysis

The bulk phase purity of the complexes was examined by X-ray powder diffraction measurement. Fig. S1† shows the observed powder diffraction patterns acquired from the as-prepared compounds together with the calculated patterns generated from the single-crystal X-ray diffraction data. There is a good agreement between the experimental and theoretical vibration data for this study, confirming that a single phase (more than 96% purity) is formed for each complex. The dissimilarities in intensity may be due to the preferred orientation of the crystalline powder samples.^25–27

Solid state fluorescence spectroscopy

It is well known that the luminescence of lanthanide(III) ions has low molar absorptivity and the f–f transitions are usually generated via the “antenna effect”, because of the f–f transition is spin- and parity-forbidden.^11,28,29 The excitation spectra of 1, 2, 3 are shown in Fig. S2† (b, c, d, respectively). The emission spectra of complex 3 exhibit a very wide band ranging from 520 to 600 nm, which indicates that the π*–π transition of the ligand is dominant throughout the emissions.³⁰ The characteristic emission peaks of Pr³⁺ ions can also be found at the wavelength of 487 nm and 535 nm, which can be attributed to the transition from ⁵D₄ → ⁷F₅ and ⁴F_9/2 → ⁶H_13/2. This indicates that not only the ions occupy the sites of the non-inversion center, which agrees with the structural analyses,³¹ but also provides reference for the part behind the fluorescence wavelength selection.

Fluorescence spectroscopic studies

Sensing properties. To examine the potential applications of these complexes in sensing small molecules, samples of complexes 1–3 were immersed in different pure salt solvents for two days to form inclusions prior to fluorescence studies.³² The luminescence investigations were conducted to explore the influence of various anions on the luminescence of complexes.³³ 4 mg sample of 1–3 was dispersed in 3.6 mL of aqueous solution to form a suspension by ultrasound methods, and 0.4 mL of NaX solution (1 × 10⁻² mol L⁻¹) (X = PO₄³⁻, F⁻, Cl⁻, Br⁻, I⁻, NO₃⁻, HCO₃⁻, OAc⁻, CO₃²⁻, WO₄²⁻, CrO₄²⁻, MoO₄²⁻, C₂O₄²⁻, or P₂O₇⁴⁻) was slowly dropped into the abovementioned solutions to form N–X suspension (1 × 10⁻³ mol L⁻¹). We also tested H₂PO₄⁻ and HPO₄²⁻, quenching degree is not PO₄³⁻ strong on them, the effect is not obvious, and the test results in Fig. S3.†

Under the perturbation of various anions, the resultant suspensions were monitored using a fluorescence spectrophotometer, and only dominant emission peaks (487 nm, complex 2 and 3; 531 nm, complex 1) are recorded in Fig. 4, 5, S4 and S5.† As shown in the figures, majority of the anions have a negligible effect on the luminescence of 1–3, except in the case of P₂O₇⁴⁻, which can reduce the luminescence to some extent, whereas PO₄³⁻ exhibits a drastic quenching effect on emission. Comparatively speaking, complex 1 and 2 also exhibited a good fluorescence quenching effect (about 90% quenching) to PO₄³⁻. However, the quenching effect of 3 is better than that of 1 and 2. This result indicates that 3 shows a better selectivity for PO₄³⁻ than 1 and 2 owing to its high degree of pre-organization and rigidity.

Fig. 4 Complex 3 comparison of the luminescence intensity of the ⁵D₄ → ⁷F₅ transitions (487 nm) of 3–X in 10⁻³ mol L⁻¹ different anions.

Fig. 5 Comparison of the luminescence intensity of N–X in 10⁻³ mol L⁻¹ different anions for complex 1 (a) and 2 (b).

Moreover, to explore the detection limit of 3 as a luminescent probe for detecting PO₄³⁻, a series suspension of 1–PO₄³⁻ (10⁻⁶ to 10⁻³ mol L⁻¹) was prepared by dropping different concentrations of PO₄³⁻ solutions into the suspension of 3.³⁴ The luminescence intensity of 3 gradually decreases with increasing the concentration of PO₄³⁻. As shown in Fig. 6, the luminescence intensity decreases with the concentration of PO₄³⁻ ranging from 10⁻⁶ to 10⁻³ mol L⁻¹. The decrease of luminescence intensity is still clearly observed when 3 was immersed in 10⁻⁶ mol L⁻¹ PO₄³⁻ solution (Fig. 6), comparing with some other literature data, slightly better than them.^35–37 Obviously, the probe is more sensitive and accurate, which is supported by the calculated results based on the equation:³⁸ detection limit = 3δ/k (δ is the standard deviation of blank measurement; k is the slope between the luminescence intensity vs. log[PO₄³⁻], as shown in Fig. S6†). To explore the acid–base stability of complex 3, the complex was immersed in a series of solutions with pH values ranging from 1.0 to 14.0 for four hours, as shown in Fig. 7, which suggest that complex 3 in an alkaline condition is relatively stable.

Fig. 6 The liquid luminescence spectra of 3 under different concentrations of PO₄³⁻ aqueous solutions, and the luminescence intensity vs. PO₄³⁻ concentration plot.

Fig. 7 Comparison of the dominated emission peaks (487 nm) of 3 after exposure to various aqueous solutions with pH values from 1.0 to 14.0.

To further explore the relationship between the quenching effect and PO₄³⁻ concentration, the luminescence intensity vs. PO₄³⁻ concentration plot was made (Fig. 6), which can be linearly fitted into I₀/I = 0.624 + K_sv[PO₄³⁻] (I₀ and I represent the luminescence intensity of 3 before and after adding PO₄³⁻, respectively; [PO₄³⁻] represents the concentration of PO₄³⁻, and K_sv represents the quenching rate constant), close to the Stern–Volmer equation:³⁹ I₀/I = 1 + K_sv[PO₄³⁻]. The K_sv value is calculated to be 4.48 × 10³ L mol⁻¹ compared with other literature data, slightly better than the data,⁴⁰ indicating the selectivity of the probe and the high quenching efficiency of PO₄³⁻ in the emission of complex 3. Through the experiment results, we conclude that the matching degree of complex 3 with PO₄³⁻ anions is determined by the PO₄³⁻ coordination with metal instead of the ligand, thereby leading to fluorescence quenching. In addition, we also calculated quantum yields for complex 3. Eventually, the quantum yield for complex 3 is Q = 0.147.

Antitumor activity study

KB cells under usual conditions were seeded in a 24-well culture plate at 2 × 10⁵ cells per well in a 1 mL culture medium and 24 h after the complexes was added. The concentration of complex was 50 μmol L⁻¹. The cells in a usable condition were seeded in a 6-well culture plate at 1 × 10⁶ cells per well in a 3 mL culture medium. Respectively, the tested complexes with serial concentrations were added to the medium. After incubation, cells were gathered, wash cells twice with cold phosphate-buffered saline (PBS) and then resuspended in 1× binding buffer at a concentration of 1 × 10⁶ cells per mL. 100 μL of the solution (1 × 10⁵ cells) was transferred to a 5 mL culture tube. Briefly, KB cells were first grown to 80% confluence on cover slips followed by incubation with concentrations (50 μmol L⁻¹) of the complex for 24 h. The complex treated cells were gently washed twice with cold phosphate-buffered saline (PBS: 137 mmol L⁻¹ NaCl, 2.7 mmol L⁻¹ KCl, 10 mmol L⁻¹ Na₂HPO₄, 1.76 mmol L⁻¹ KH₂PO₄, pH 7.4) and then fixed with 4% paraformaldehyde in PBS for 15 min. The cells were washed three times with PBS, permeabilized with 0.2% Triton X-100 in PBS for 5 min, and washed three times with cold PBS followed by incubation with Annexin-V/propidium iodide at 37 °C for 1 h. Annexin-V/propidium iodide stained cells were washed three times with cold PBS, mounted on a confocal fluorescence microscope slide, and visualized under a light microscope fitted with a photometric camera.

The mode of cell death was studied by Annexin-V/propidium iodide (PI) binding assay with complex 1, 2, 3 and the ligand in KB cells. The shape of apoptotic and viable KB cells stained with Annexin V-FITC and propidium iodide (PI), the experiment is repeated by reproducing the conditions as nearly as possible, under confocal fluorescence images of KB cells, the result is depicted in Fig. 8. The results suggest that complexes 1, 2 and 3 have different degrees of antitumor activity.

Fig. 8 The complexes 1, 2, 3 and the ligand effects on cancer cells.

Experimental

Materials and methods

All materials were reagent grade and obtained from commercial sources and used without further purification; solvents were dried using standard procedures. KB cells (human oral epithelial carcinoma cells) were obtained from the Institute of Biochemistry and Cell Biology, SIBS, CAS. Element analysis (C, H, and N) was performed with a model Finnigan EA 1112 instrument.⁴¹ The IR spectra are obtained on a Nicolet IR 470 spectrophotometer in KBr table in the range of 400–4000 cm⁻¹.⁴² Solid state and liquid fluorescence spectra were obtained at room temperature on a Perkin-Elmer LS55 fluorescence spectrophotometer fluorometer.⁴³

X-ray powder diffraction

PXRD data were collected using a PANalytical X'Pert Pro X-ray powder diffractometer⁴⁴ (PANalytical B.V., Almelo, the Netherlands) equipped with an X'Celerator Real Time Multi-Strip detector. Cu Kα radiation was used at 45 kV and 40 mA.⁴⁵ Samples were wrapped in two pieces of a Mylar film and scanned in the transmission mode. The scan range, step size, and time per step were 2θ = 3.0° to 40.0°, 0.0167113°, and 30 s, respectively.

Elemental analysis

Elemental analyses were performed with a GKF-VI elemental analyser. We take the mineralized method by dry digestion. Complexes step into dry, carbonization, ashing and dissolved ash several processes.

Synthesis of the complex 1–3

Preparation of [La(L)(NO₃)(H₂O)₂]_n 1. A mixture of H₂L (0.005 mmol), (NH₄)₂Ce(IV)(NO₃)₆ (0.03 mmol), and 3.5 mL of ET/water (3 : 1) solution was sealed in a Teflon-lined stainless vessel (5 mL) and 0.1 mol L⁻¹ KOH was used to adjust the pH to 8–9, and then heated at 85 °C for 72 h, and then the vessel was cooled slowly to room temperature at 1.5 °C h⁻¹, affording the products as flavescences transparent needle clubbed crystals. The yield was 70% based on Tb. Elemental analysis (%) calcd: C, 26.04; H, 2.62; O, 38.17; N, 3.04. Found: C, 26.08; H, 2.60; O, 38.13; N, 3.09. IR (cm⁻¹, s, strong; m, medium; b, broad; d, double): 3440(s), 1623(m), 1541(m), 1431(d), 1384(m), 1048(b), 584(s).

Preparation of [Ce(L)(NO₃)(H₂O)₂]_n 2. The complex was synthesized in an identical manner as that described for 1, finally affording the products as colourless transparent needle clubbed crystals. The yield was 74% based on Tb. Elemental analysis (%) calcd: C, 25.98; H, 2.62; O, 38.07; N, 3.03. Found: C, 26.00; H, 2.67; O, 38.10; N, 3.06. IR (KBr; ν, cm⁻¹, s, strong; m, medium; b, broad; d, double): 3407(s), 2065(m), 1763(m), 1634(m), 1384(b), 1043(m), 597(s).

Preparation of [Pr(L)(NO₃)(H₂O)₂]_n 3. The complex was synthesized in an identical manner as that described for 1, finally affording the products as green transparent needle clubbed crystals. The yield was 78% based on Tb. Elemental analysis (%) calcd: C, 25.93; H, 2.61; O, 38.00; N, 3.02. Found: C, 25.89; H, 2.65; O, 38.09; N, 3.05. IR (KBr, cm⁻¹, s, strong; m, medim, b, broad; d, double): 3447(s), 1635(m), 1541(d), 1384(s), 1252(m), 1120(b), 1043(m), 583(s).

X-ray crystallographic studies

Single-crystal X-ray data of all the complex were collected on a BRUKER SMART 1000 CCD diffractometer with Mo Kα (λ = 0.71073 Å) radiation at room temperature, and the intensity data were obtained in the range of 3.40° < θ < 27.53° at 273 K using an ω-scan technique. The structures were solved by direct methods and refined by full-matrix least-squares on F² using the SHELXL-97 program. Corrections for the least-squares factor and an empirical absorption correction were applied.^46–48 All the metal atoms were located first, and then oxygen and carbon atoms of the compounds were subsequently found in difference Fourier maps. The hydrogen atoms of the ligand were placed geometrically. The final formulae of complexes 1–3 were derived from crystallographic data combined with elemental analysis data. Crystal data and detailed data collection and refinement and selected bond lengths and angles of complexes 1–3 are summarized in Tables S1 and S2.†

Conclusions

Three one-dimensional spiral lanthanide coordination polymers with H₂L are synthesized and characterized. First, the PXRD and fluorescent studies in solid and solution states reveal that it is potential probe materials. Moreover, the selective sensing of 3 toward PO₄³⁻ in solutions is a very rare example to our knowledge, which is associated with the electronic and structure of the complex, configuration of PO₄³⁻, and the external environment. Moreover, such a rigid ligand (H₂L) is a key to the strong emission of 3 and further ensures its subsequent applications in probing PO₄³⁻ in solutions. Complex 3 may be used as a useful dual-purpose luminescent probe for the real-time monitoring of excessive algal growth and water quality. The detection of PO₄³⁻ ions in water has provided the foundation for the sensing of trace amount of phosphate ions in biological system. We expect that by modifying the H₂L ligand, more complexes with new structures and higher sensitivity could be generated for the multi-targeted detection of trace hazardous substance in aquatic ecosystems. Further studies are under way in our lab.

Acknowledgements

We gratefully acknowledge the Natural Science Foundation of China (No. 21171118 and 21671138) and the Distinguished Professor Project of Liaoning province.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1472875–1472877. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra13361d

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