Three robust Cd(II) coordination polymers as bifunctional luminescent probes for efficient detection of pefloxacin and Cr₂O₇²⁻ in water

Abstract

The accurate and rapid detection of antibiotics and heavy-metal-based toxic oxo-anions in water media employing coordination polymers (CPs) as luminescent probes has attracted a lot of attention. Three new Cd(II)-based ternary CPs derived from first-presented L ligands, including [Cd(DCTP)(L)(OH)]_n (1), [Cd(TBTA)(L)(OH)]_n (2), and [Cd(NPHT)(L)(H₂O)]_n (3) (L = 2-((1H-imidazol-1-yl)methyl)-5,6-dimethyl-1H-benzo[d]imidazole, H₂DCTP = 2,5-dichloroterephthalic acid, H₂TBTA = tetrabromoterephthalic acid and H₂NPHT = 3-nitrophthalic acid), were successfully assembled and characterized. 1 and 2 show 2D hcb layers, which can be further extended into a 3D supramolecular framework via classic hydrogen bonding interactions. 3 features a 1D double chain that ultimately spreads into a 2D network through weak hydrogen bonding interactions. With the advantages of high stability and excellent luminescent properties, the three CPs display high sensitivity, selectivity, and good anti-interference for the sensing of pefloxacin (PEF) and Cr₂O₇²⁻ ions (LOD values toward PEF: 3.82 × 10⁻⁷ mol L⁻¹ for 1, 4.06 × 10⁻⁷ mol L⁻¹ for 2, and 1.36 × 10⁻⁸ mol L⁻¹ for 3, and toward Cr₂O₇²⁻ ions: 5.97 × 10⁻⁷ mol L⁻¹ for 1, 5.87 × 10⁻⁷ mol L⁻¹ for 2, and 8.21 × 10⁻⁸ mol L⁻¹ for 3). These CPs are the first examples of bifunctional luminescent sensors to detect PEF and Cr₂O₇²⁻ in aqueous solutions. The luminescence quenching mechanisms are explored in detail.

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1. Introduction

Pefloxacin (PEF) is a second-generation fluoroquinolone antimicrobial agent, which shows excellent activity against Gram-positive and Gram-negative bacteria, and it is used primarily in veterinary medicine.^1,2 The structural formula of PEF is shown in Fig. S1.† However, the presence of PEF residues in animal products, such as milk, meat and eggs, may cause allergies in humans and increase the risk of bacterial resistance. The Ministry of Agriculture of China has imposed a ban on administering PEF veterinary drugs to food animals.^3,4

Cr(VI) ions were listed as a tremendously toxic pollutant by the United States Environmental Protection Agency (U.S.EPA), and the World Health Organization (WHO) delimited the threshold limit of Cr(VI) concentration to be 9.6 × 10⁻⁴ mol L⁻¹ in groundwater. Cr₂O₇²⁻ serves as an oxidizing agent in industrial production. Its exceptional water solubility has resulted in its widespread occurrence as an inorganic heavy metal contaminant in industrial effluents, constituting notable threats to human health, such as cancer, pulmonary fibrosis, and genetic mutations.^5–7 Thus, the detection of traces of Cr₂O₇²⁻ and PEF in aqueous media is of vast significance. Technologies for the quantification of PEF and Cr₂O₇²⁻ include high-performance liquid chromatography, atomic absorption spectrophotometry, surface-enhanced Raman spectroscopy, inductively coupled plasma atomic emission spectrometry, and cyclic voltammetry.^8,9 However, these methods suffer from time-consuming techniques, expensive instruments, and cumbersome sample pre-treatment.

Coordination polymers (CPs) as luminescent sensors have gained widespread attention and development due to their merits of high sensitivity, selectivity, operability and anti-interference ability.^10–15 Nevertheless, achieving the rational design and synthesis of CP-based luminescent probes is still a long-term challenge. Herein, we selected Cd(II) ions, a new N-containing co-ligand L (2-((1H-imidazol-1-yl)methyl)-5,6-dimethyl-1H-benzo[d]imidazole), and an O-containing aromatic dicarboxylic acid as the main ligand, including 2,5-dichloroterephthalic acid (H₂DCTP), tetrabromoterephthalic acid (H₂TBTA), and 3-nitrophthalic acid (H₂NPHT), to construct CP-based luminescent sensors using a mixed-ligand strategy. Three key aspects define the rationale behind our design: (1) a Cd(II) center, with its d¹⁰ electronic configuration, stands out as an excellent choice for developing CP-based photoluminescent materials.¹⁶ (2) L is a new multidentate flexible ligand (complexes bearing L ligands are found to have 0 matches in CCDC (CSD ver. 5.45 updates Nov. 2023)), which can bend and rotate freely so as to satisfy the coordination requirement of the metal center with the N atoms of imidazole and benzimidazole groups. (3) H₂DCTP, H₂TBTA, and H₂NPHT are versatile linkers to build higher multidimensional frameworks according to their different coordination modes and supramolecular interactions.^17–19

On the basis of the above discussion, three mixed-ligand ternary Cd(II) CPs, namely, [Cd(DCTP)(L)(OH)]_n (1), [Cd(TBTA)(L)(OH)]_n (2), and [Cd(NPHT)(L)(H₂O)]_n (3), were assembled and characterized. CPs 1–3 can serve as promising bifunctional sensors to detect Cr₂O₇²⁻ and PEF with exceptional stability, selectivity, immunity to interference, and recyclability. To the best of our knowledge, this represents the first example of a CP-based bifunctional luminescent sensor for the detection of Cr₂O₇²⁻ and PEF in water media.

2. Results and discussion

2.1. Description of [Cd(DCTP)(L)(OH)]_n (1)

1 crystallizes in the triclinic crystal system with the P1̄ space group. The asymmetric unit is composed of one Cd(II) center, one L ligand, one fully deprotonated DCTP²⁻ ligand, and a hydroxyl group (Fig. 1a). The Cd(II) center with a six-coordinated configuration center is situated in a slightly distorted octahedral environment denoted {CdN₂O₄}, in which there are three O atoms (O1, O2, and O3) from two different DCTP²⁻ ligands, another oxygen atom (O1W) from a hydroxyl oxygen atom, and two N atoms (N1, N4A; symmetry code: A = 1 − x, 2 − y, and 1 − z) from two different L ligands. The lengths of the Cd–O/N bond vary from 0.222(2) to 0.249(2) nm, and coordination bond angles surrounding Cd(II) centers range from 54.06(8) to 168.02(8)°, consistent with previously reported Cd(II) CPs.^20,21

Fig. 1 (a) Coordination environment centered on the metal Cd(II) in 1. Hydrogen atoms are omitted for clarity. Symmetry codes: A: 1 − x, 2 − y, 1 − z; B: −x, 1 − y, −z; C: −x, 2 − y, and 2 − z. (b) View of the 2D framework in 1. (c) Schematic depiction of the 3-connected hcb network in 1. (d) 3D supramolecular structure generated by hydrogen bond interactions.

The L ligand uses a (κ¹)–(κ¹)–μ₂ coordination mode, attaching to the neighboring Cd(II) centers to form a [Cd₂(L)₂] unit, with a distance of 0.708(4) nm between the Cd(II) centers (Fig. S2†). Fully deprotonated DCTP²⁻ ligands interact with the Cd(II) centers in two different coordination patterns, namely (κ¹–κ¹)–(κ¹–κ¹)–μ₂ and (κ¹–κ⁰)–(κ¹–κ⁰)–μ₂ patterns. These interactions result in the formation of a 1D zigzag chain [Cd(DCTP)]_n, where Cd⋯Cd distances measure 1.130(1) and 1.171(8) nm (Fig. S3†). By sharing Cd(II) centers, [Cd₂(L)₂] rings are connected to 1D infinite chains to form a 2D layer, which contains large hexagonal [Cd₆(DCTP)₄(L)₄] units (Fig. 1b). From the topology, the L ligand and DCTP²⁻ ligands act as a 2-connected linker, and the Cd(II) centers are simplified to 3-connected nodes. As a result, the 2D structure of 1 can be described as an hcb network with the point symbol {6³} (Fig. 1c).²² Furthermore, 2D layers are interconnected into a 3D supramolecular framework by classic hydrogen bonding interactions between hydrogen atoms (H2/H1W) from the L ligand and hydroxyl group and neighboring oxygen atoms (O1W/O4) from the hydroxyl group and DCTP²⁻ ligand (N2⋯O1W = 0.292(3) nm, ∠N2–H2⋯O1W = 167°; O1W⋯O4 = 0.262(5) nm, ∠O1W–H1W⋯O4 = 149°) (Fig. 1d).

2.2. Description of [Cd(TBTA)(L)(OH)]_n (2)

2 belongs to the triclinic crystal system, with P1̄ space group. There are one Cd(II) ion, one L ligand, one TPTA²⁻ ligand, and a hydroxyl group in the asymmetric unit of 2. As depicted in Fig. 2a, the Cd(II) center shows a slightly distorted octahedral coordination geometry {CdN₂O₄}, in which it is six-coordinated by three O atoms (O1, O2, and O3) from two distinct TBTA²⁻ ligands, one oxygen atom (O1W) from a hydroxyl oxygen atom, and two N atoms (N1, N4A; symmetry code: A = 1 − x, 1 − y, and 1 − z) from two different L ligands. The Cd–O/N bonds lengths range from 0.225(3) to 0.252(4) nm, and M–Cd–M (M = O and N) bond angles extend from 53.68(1) to 155.51(1)°, which are well-matched to other Cd(II) CPs.^20,21

Fig. 2 (a) Coordination environment centered on the metal Cd(II) in 2. Hydrogen atoms are omitted for clarity. Symmetry codes: A: 1 − x, 1 − y, 1 − z; B: −x, 2 − y, 1 − z; C: 1 − x, 2 − y, and 2 − z. (b) View of the 2D framework in 2. (c) Schematic depiction of the 3-connected hcb network in 2. (d) 3D supramolecular structure generated by hydrogen bond interactions.

The TPTA²⁻ ligands display (κ¹–κ¹)–(κ¹–κ¹)–μ₂ and (κ¹–κ⁰)–(κ¹–κ⁰)–μ₂ coordination modes, linking adjacent Cd(II) centers to shape a zigzag 1D [Cd(TBTA)]_n infinite chain with Cd⋯Cd distances of 1.171(7) and 1.132(8) nm (Fig. S5†). The 1D chains mentioned above are further connected by the imidazole N and benzimidazole N of L ligands to create an infinite 2D layer (Fig. 2b). In this layer, the L ligand adopts a coordination pattern of (κ¹)–(κ¹)–μ₂, and the length Cd⋯Cd is 0.683(6) nm (Fig. S4†). Similarly, 2 can be described as an hcb network (Fig. 2c), which is further extended into a 3D framework through classic hydrogen bonding interactions between the hydrogen atoms (H2) of the L ligand and neighboring oxygen atoms (O4) of a carboxyl group from TPTA²⁻ anions, the hydrogen atoms (H1W) of the hydroxyl group and neighboring oxygen atoms (O1) of TPTA²⁻ anions (N2⋯O4 = 0.271(5) nm, ∠N2–H2⋯O4 = 174°; O1W⋯O1 = 0.267(5) nm, ∠O1W–H1W⋯O1 = 150°) (Fig. 2d).

2.3. Description of [Cd(NPHT)(L)(H₂O)]_n (3)

By single-crystal diffraction analyses, 3 also crystallizes in the triclinic space group of P1̄. Its asymmetric unit includes one Cd(II) center, one L ligand, one NPHT²⁻ ligand, and a lattice water molecule (Fig. 3a). The Cd(II) center is six-coordinated by three oxygen atoms (O1, O3A, and O4A; symmetry code: A = 1 − x, 1 − y, and 1 − z) derived from two different NPHT²⁻ ligands, while another oxygen atom (O1W) is derived from the coordinated water molecule, and two nitrogen atoms (N2, N5B; symmetry code: B = 1 − x, 2 − y, 1 − z) are provided by two different L ligands. The bond lengths and bond angles of Cd–N/O range from 0.226(3) to 0.260(3) nm and from 51.83(9) to 169.25(1)°, respectively. These values are comparable to those of previously reported Cd(II) CPs.^20,21

Fig. 3 (a) Coordination environment centered on the metal Cd(II) in 3. Hydrogen atoms are omitted for clarity. Symmetry codes: A: 1 − x, 1 − y, 1 − z; B: 1 − x, 2 − y, and 1 − z. (b) View of the 1D chain of 3. (c) 2D framework extended by hydrogen bond interactions.

The carboxylic acid groups of the NPHT²⁻ ligands are used in monodentate and chelated coordination modes, and the L ligands show a (κ¹)–(κ¹)–μ₂ coordination mode to link neighboring Cd(II) ions, resulting in the construction of two types of unit, {Cd₂(NPHT²⁻)}₂ and {Cd₂(L)₂} (Fig. S6 and S7†). The separations between Cd atoms in {Cd₂(NPHT²⁻)₂} and {Cd₂(L)₂} are determined to be 0.533(6) and 0.668(6) nm, respectively. These two types of unit create a 1D double chain structure by alternating through metal knots (Fig. 3b). Further linkage into a 2D structure is achieved through weak hydrogen bonding interactions between the carboxyl oxygen atom (O5) of the NPHT²⁻ anion and hydrogen atom (H10) of the L ligand (C10⋯O5 = 0.331(6) nm, ∠C10–H10⋯O5 = 149°) (Fig. 3c).

2.4. Comparison of the three ternary Cd(II) CPs

CPs 1–3 crystallize in the P1̄ space group in which the Cd(II) centers show slightly distorted octahedral coordination geometry, and the L ligands show (κ¹)–(κ¹)–μ₂ coordination mode. The three Cd(II) CPs bearing the L ligand are structurally altered by dicarboxylate main ligands. In 1 and 2, DCTP²⁻ and TPTA²⁻ adopt alternating (κ¹–κ¹)–(κ¹–κ¹)–μ₂ and (κ¹–κ⁰)–(κ¹–κ⁰)–μ₂ coordination modes to link the adjacent Cd(II) centers, forming a 1D zigzag chain. The 1D chains are further extended into 2D hcb layers through the L ligands. In 3, the NPHT²⁻ ligands connecting Cd(II) centers construct a {Cd₂(NPHT²⁻)}₂ unit by taking a (κ¹–κ¹)–(κ¹–κ⁰)–μ₂ coordination mode. Finally, the 1D double-chain structure is obtained by sharing metal centers with the L ligands. The coordination modes of DCTP²⁻ and TPTA²⁻are similar, but differ from that of NPHT²⁻ in the corresponding Cd(II) CPs, suggesting that these variations may be associated with differences in substituents or the positioning of carboxylate groups. This emphasizes the crucial role played by dicarboxylate main ligands in the self-assembly process of ternary Cd(II) CPs containing L ligands.

2.5. Thermogravimetric analysis (TGA) and powder X-ray diffraction (PXRD)

Stability assessment of CPs 1–3 was conducted using TGA under a nitrogen flow, spanning temperatures from 25 to 800 °C (Fig. S10†). The TGA curves of 1 and 2 exhibited a two-step weight loss. Initially, 1 and 2 experienced a weight loss of approximately 2.98% (calcd. 2.88%) and 1.95% (calcd. 2.03%) in the temperature ranges 155–192 °C and 91–116 °C, respectively, attributed to the removal of the ligand hydroxyl group. Subsequently, following a short stabilization period, the weight losses of 78.39% (calcd. 78.37%) for 1 and 84.81% (calcd. 84.75%) for 2 corresponded to the decomposition of the organic skeleton at 308–621 °C and 186–760 °C, respectively. For 3, the onset of weight loss started at about 61 °C and continued up to 80 °C (observed: 3.18%; calculated: 3.21%), indicative of the loss of the bound water molecule. After that, a plateau was observed until 206 °C. Then, with increasing temperature, the organic skeleton started collapsing, resulting in a weight loss of 77.5% (calculated: 78.20%). It is evident from the TGA curves that CPs 1–3 demonstrate high thermal stability.

As presented in Fig. S11,† by comparison, PXRD peaks align with the simulated ones, which shows the purity of CPs 1–3. Experimental observations following 24-hour immersion in solutions with different pH levels indicated that the structural integrity of CPs 1–3 remained unaltered across a broad range of pH values (pH 3–13 for 1, pH 2–13 for 2, and pH 3–12 for 3) (Fig. S12†). This consistent stability under diverse pH conditions underscores the potential utility of these CPs.

2.6. Solid-state luminescence

At room temperature, the solid-state luminescence spectra of the L ligand and CPs 1–3 were recorded (Fig. 4a). The free L ligand reveals a strong emission peak at 392 nm (λ_ex = 253 nm), which can be ascribed to π* → π or π* → n electron transitions.²³ CPs 1–3 present maximum emission bands at 307 nm (λ_ex = 291 nm), 310 nm (λ_ex = 293 nm), and 309 nm (λ_ex = 270 nm), respectively. In the CIE chromaticity diagram representing the pure emission color, it was noted that the free L ligand and CPs 1–3 occupied distinct regions: L at (0.1475, 0.0428), 1 at (0.1529, 0.0317), 2 at (0.1747, 0.0052), and 3 at (0.1737, 0.0049) (Fig. 4b). The emission spectra of the three CPs displayed blue shifts of 85, 82, and 83 nm relative to the L ligand, possibly due to charge transfer within the CPs.²⁴ To further elucidate the charge transfer process, the energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for Cd(II), free L ligands, H₂DCTP, H₂TBTA, and H₂NPHT ligands, were calculated using density functional theory (DFT).^25–27 As depicted in Fig. 5, the LUMO of the L ligand (−0.55 eV) surpasses that of H₂DCTP (−1.35 eV), H₂TBTA (−1.72 eV), and H₂NPHT (−2.88 eV), yet falls below the LUMO level of Cd(II) (−0.47 eV). Since Cd(II) ions which adopt a d¹⁰ configuration are not easily oxidized or reduced, the blue-shifts can be assigned to ligand-to-ligand charge transfer (LLCT).^28–30

Fig. 4 (a) Solid luminescence spectra of the L ligand and 1–3 at room temperature. (b) CIE diagram of the L ligand and 1–3.

Fig. 5 HOMO and LUMO energies (eV) for ligands, coordinating metal ions, and 1–3.

CPs 1–3 were dispersed in water and their luminescence intensities were measured every 15 min. The experimental results illustrate that time has almost no effect on the luminescence intensity of the CPs, suggesting that 1–3 have excellent dispersibility and stability in aqueous solutions (Fig. S14†). Moreover, the luminescence intensity of CPs 1–3 remains almost unchanged in the ranges of pH 3–13, pH 2–13 and pH 3–12, respectively (Fig. S13†). Additionally, luminescence decay curves were fitted biexponentially, resulting in emission decay lifetimes for 1–3 of 2.03, 2.46 and 2.33 μs, respectively (Fig. S15†).

2.7. Sensing of antibiotics

Considering their impressive luminescence properties and excellent water stability, the application of CPs 1–3 as luminescence sensors for antibiotics in water was investigated. Obviously, CPs 1–3 exhibited significant quenching in aqueous medium with the addition of PEF (4.76 × 10⁻⁵ mol L⁻¹), achieving quenching efficiencies of 90.21%, 85.83% and 99.85%, respectively (Fig. 6). Meanwhile, other available antibiotics had almost no effect on the luminescence intensity. Moreover, a series of tests were performed to confirm their resistance to interference.³¹ From Fig. S16,† it can be seen that the luminescence intensities of CPs 1–3 remained unchanged when other antibiotics were added. Conversely, the addition of PEF to a mixed solution of 1–3 and other antibiotics resulted in a rapid decrease in luminescence intensity, indicating high selectivity of 1–3 for the detection of PEF.

Fig. 6 Luminescence spectra of 1 (a), 2 (b) and 3 (c) in various antibiotic solutions.

The pronounced quenching phenomenon motivated us to conduct further research on the quenching behavior of PEF via titration experiments. As the concentration of PEF increased, the luminescence intensities of 1–3 were gradually quenched (Fig. 7). The quenching efficiency between CPs 1–3 and PEF can be quantified using the Stern–Volmer (S–V) equation: I₀/I − 1 = K_SV·[M], where I₀ and I reflect the luminescence intensities of 1–3 before and after adding analytes, respectively, K_SV is the quenching constant, and [M] is the concentration of PEF.^32,33Fig. 8 illustrates a linear S–V plot at a low concentration of PEF, but shows an upward-bending trend at higher concentrations, which may be caused by self-absorption or an energy-transfer process.³⁴ At lower concentrations (0–5.22 × 10⁻⁶ mol L⁻¹ for 1, 0–6.95 × 10⁻⁶ mol L⁻¹ for 2, and 0–1.25 × 10⁻⁶ mol L⁻¹ for 3), the K_SV values of 1–3 go up to 87 595 L mol⁻¹ (R² = 0.9962), 66 821 L mol⁻¹ (R² = 0.9958), and 478 220 L mol⁻¹ (R² = 0.9969), respectively. In addition, limits of detection (LODs) for PEF are calculated as 3.82 × 10⁻⁷ mol L⁻¹ for 1, 4.06 × 10⁻⁷ mol L⁻¹ for 2, and 1.36 × 10⁻⁸ mol L⁻¹ for 3 by 3δ/k (where δ represents the standard deviation and k refers to the slope of the calibration curve within the low-concentration range).^35,36 The LODs of 1–3 for PEF are lower than those reported in most of the literature (Table 1), and it is imperative to note that there are almost no reports of CP-based probes for the trace detection of PEF.^37–39

Fig. 7 Emission spectra of 1 (a), 2 (b) and 3 (c) dispersed in aqueous solution in the presence of various amounts of PEF solvent.

Fig. 8 Relationship between I₀/I − 1 and the concentration of PEF for 1 (a), 2 (b) and 3 (c). Inset: Linear plot of I₀/I − 1 at low PEF concentration.

Table 1 Comparison of luminescent properties of 1–3 for sensing PEF

Detection methods	Types	LOD (mol L^−1)	Ref.
R-CDs = red-emissive carbon dots.
Spectrophotometry	PEF	1.10 × 10^−5	37
R–CDs	PEF	8.50 × 10^−7	37
Terbium sensitized fluorescence	PEF	4.40 × 10^−6	38
Two-step liquid–liquid extraction and separation using an Adsorbosphere SAX column	PEF	5.20 × 10^−7	39
1	PEF	3.82 × 10^−7	This work
2	PEF	4.06 × 10^−7	This work
3	PEF	1.36 × 10^−8	This work

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2.8. Sensing of metal ions

The ability of CPs 1–3 to probe metal ions in aqueous solution was examined further, inspired by their use in detecting antibiotics. In light of Fig. 9, slight variations in luminescence intensity were observed in CP suspensions upon exposure to different metal ions and, notably, the introduction of Cr₂O₇²⁻ ions induced a pronounced quenching behavior. Moreover, the sensing of Cr₂O₇²⁻ ions by CPs 1–3 remained unaffected by the coexistence of other metal ions (Fig. S17†). Titration experiments proved that the luminescence intensity of 1–3 continuously declined with a growing concentration of Cr₂O₇²⁻ ions (Fig. 10). At Cr₂O₇²⁻ concentrations of 1.69 × 10⁻⁴, 1.73 × 10⁻⁴, and 1.39 × 10⁻⁴ mol L⁻¹, CPs 1–3 displayed quenching ratios of 93.14%, 93.64%, and 94.37%, respectively. It is worth noting that a robust linear relationship was identified between the ratio I₀/I − 1 and lower Cr₂O₇²⁻ concentration, ranging from 0–3.00 × 10⁻⁵ mol L⁻¹ for 1, to 0–2.00 × 10⁻⁵ mol L⁻¹ for 2, and 0–2.00 × 10⁻⁵ mol L⁻¹ for 3 (Fig. 11). The average K_SV values were evaluated as 56 021 L mol⁻¹ (R² = 0.9935) for 1, 46 218 L mol⁻¹ (R² = 0.9952) for 2, and 78 948 L mol⁻¹ (R² = 0.9967) for 3. Concurrently, the LODs for CPs 1–3 were subsequently determined from the corresponding data, resulting in values of 5.97 × 10⁻⁷, 5.87 × 10⁻⁷, and 8.21 × 10⁻⁸ mol L⁻¹, respectively. The trace detection ability of CPs 1–3 for Cr₂O₇²⁻ ions is compared with those of related CPs in Table 2, indicating their potential as highly selective and sensitive sensors for detecting Cr₂O₇²⁻ ions.^40–43 Importantly, the LODs are significantly lower than the internationally recommended maximum contamination limit (9.6 × 10⁻⁷ mol L⁻¹) for drinking water.⁴⁴

Fig. 9 Emission spectra of 1 (a), 2 (b) and 3 (c) in the presence of different metal cations.

Fig. 10 Luminescence spectra of 1 (a), 2 (b) and 3 (c) with different concentrations of Cr₂O₇²⁻ ions.

Fig. 11 Graphs of I₀/I − 1 versus Cr₂O₇²⁻ concentration for 1 (a), 2 (b) and 3 (c). Inset: I₀/I − 1 is proportional to the low concentration of Cr₂O₇²⁻.

Table 2 Comparison of luminescent properties of 1–3 for sensing Cr₂O₇²⁻

Luminescent sensing materials	Types	K SV (L mol^−1)	LOD (mol L^−1)	Ref.
L = 4,4′-(2,5-bis(methylthio)-1,4-phenylene)dipyridine; H2bdc = 1,4-dicarboxybenzene; H3BTC = 1,3,5-benzenetricarboxylic acid; H3BTBA = 4,4′,4′′-(1H-benzo[d]imidazole-2,4,7-triyl)tribenzoic acid; L = 1,4-bis(benzimidazole-2-yl)butane; and H2TBTA = tetrabromoterephthalic acid.
{[Cd(L)(bdc)·2H2O]·2DMF}n	Cr2O7^2−	42 480	1.05 × 10^−5	40
[Bi(BTC)(H2O)]·H2O	Cr2O7^2−	19 500	1.64 × 10^−6	41
Zr6O4(OH)7(H2O)3(BTBA)3	Cr2O7^2−	15 700	1.50 × 10^−6	42
{[Cd(TBTA)(L)]·H2O}n	Cr2O7^2−	30 300	3.14 × 10^−5	43
1	Cr2O7^2−	56 021	5.97 × 10^−7	This work
2	Cr2O7^2−	46 218	5.87 × 10^−7	This work
3	Cr2O7^2−	78 948	8.21 × 10^−8	This work

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2.9. Reusability

Reversible trials were implemented to examine the recoverability of CPs 1–3 as luminescent probes for sensing PEF and Cr₂O₇²⁻. It is observed that the luminescence intensity of CPs 1–3 returned to its initial level after three sensing cycles for PEF and Cr₂O₇²⁻ ions (Fig. S18–S20†). Meanwhile, PXRD spectra demonstrated that CPs 1–3 maintained their structures, exhibiting remarkable reusability and stability (Fig. S21†).

2.10. Discussion of the mechanism

To delve into the luminescence quenching of CPs 1–3 by PEF/Cr₂O₇²⁻, thorough investigations were conducted to elucidate quenching mechanisms. Generally, the commonly acknowledged mechanisms include: (a) the structural collapse of CPs upon the introduction of analytes, (b) resonance energy transfer (RET) and (c) photoinduced electron transfer (PET).^45,46 The PXRD patterns of CPs 1–3 after exposure to PEF/Cr₂O₇²⁻ correspond to the simulated ones, confirming the structural integrity of CPs 1–3 (Fig. S19†).⁴⁷ Next, RET is considered a probable factor contributing to the luminescence quenching.^48,49 The UV–vis absorption spectra of PEF and Cr₂O₇²⁻ were found to overlap significantly with the emission spectra of 1–3, and their degree of overlap is in the order: 3 > 1 > 2, which is consistent with the experimental result (Fig. 12). This observation suggests the presence of competitive energy absorption between CPs 1–3 and PEF/Cr₂O₇²⁻, resulting in luminescence quenching.⁵⁰

Fig. 12 Spectral overlap between the absorption spectra of analytes and emission spectra of 1–3.

Mott–Schottky experiments were executed at 1500, 2000, and 2500 Hz for CPs 1–3 to obtain the flat band potential (E_FB).^51,52 The C⁻²–V curves show positive slopes, confirming their inherent characteristics as n-type semiconductors (Fig. 13).⁵³ Cd(II) is known to have narrow energy bands in the d¹⁰ configuration, with highly localized electronic states. Therefore, for CPs 1–3, the valence band (VB) and conduction band (CB) levels can be considered as the HOMO and LUMO, respectively.^54,55 By referring to Fig. 13, it is evident that the E_FB values for CPs 1–3 are −0.80, −1.00, and −1.17 eV, respectively, in comparison to Ag/AgCl. Following the conversion process, the CB values for CPs 1–3 are −0.58, −0.78, and −0.95 eV, respectively, relative to the normal hydrogen electrode (NHE). Based on the solid state UV–Vis DRS, the energy band gaps (E_g) of CPs 1–3 were obtained as 3.83, 4.02 and 3.22 eV, respectively, and the VB values of CPs 1–3 were further found to be −4.41, −4.80, and −4.17 eV, respectively, calculated using the equation E_VB = E_CB − E_g (Fig. 14).⁵⁶ The LUMO energy level of Cr₂O₇²⁻ is lower than those of CPs 1–3, which means that the LUMO of Cr₂O₇²⁻ more readily accepts excited electrons from CPs 1–3 (Fig. 15).⁵⁷ Therefore, PET theory can contribute to the luminescence quenching by Cr₂O₇²⁻ of CPs 1–3. Conversely, the LUMO energy level of PEF is higher than those of CPs 1–3, substantiating that there is no PET mechanism for the luminescence quenching caused by PEF. Furthermore, the LUMO values for CPs 1–3, determined using DFT as −1.72, −1.16, and −2.51 eV, respectively, continue to support previously mentioned results (Fig. 5).

Fig. 13 Mott–Schottky plots for as-prepared 1 (a), 2 (b) and 3 (c) in 0.1 mol L⁻¹ Na₂SO₄ aqueous solution; the frequency is 1000, 1500 and 2000 Hz, respectively.

Fig. 14 Diffuse reflectance spectra of Kubelka–Munk function versus energy of 1–3.

Fig. 15 HOMO and LUMO energy levels for 1–3, PEF and Cr₂O₇²⁻ ions.

The above discussion demonstrates that the capability of CPs 1–3 to probe PEF selectively and sensitively is ascribed to the RET mechanism, and their ability to sense Cr₂O₇²⁻ is due to the coexistence of the RET and PET mechanisms. The appropriate reason for differences in sensing performances between 1, 2 and 3 may be due to their structures, electronic properties, and surface chemistry.

3. Conclusion

Three new ternary Cd(II)–CPs containing the L ligands were successfully synthesized and characterized. The diverse architectures of CPs 1–3 can be regulated by dicarboxylate main ligands during the assembly process. CPs 1–3 can effectively detect PEF through the RET mechanism, and Cr₂O₇²⁻-induced luminescence quenching occurs through a combination of RET and PET mechanisms. This study presents a significant contribution as the first instance of utilizing CPs for the determination of PEF and Cr₂O₇²⁻. These insights advance the development of luminescent probes for dual-sensing applications.

Author contributions

Xiao-Fei Fan: Methodology, formal analysis, investigation, writing-original draft, data curation. Lianshe Fu: Software, visualization, validation, resources. Guang-Hua Cui: Conceptualization, writing-review & editing, supervision, project administration, funding acquisition.

Conflicts of interest

The authors have declared no conflict of interest.

Acknowledgements

This work was developed within the scope of Hebei Natural Science Foundation (B2021209020) and the project CICECO-Aveiro Institute of Materials (Portugal), IDB/50011/2020, UIDP/50011/2020 and LA/P/0006/2020 and financed by national funds through the FCT/MEC (PIDDAC).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2308927–2308929 for CPs 1–3. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt00128a

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