Tunable luminescent lanthanide cluster-based hydrogel probe for ratiometric detection of ofloxacin, anti-counterfeiting, and magnetocaloric applications

Abstract

Misuse and overuse of antibiotics, especially Ofloxacin (OFX), have become significant global health concerns, emphasizing the urgent need for efficient and accessible detection methods. To address this issue, a series of Ln₁₄ clusters, formulated as [Ln₁₄(acac)₂₄(μ₄-OH)₂(μ₃-OH)₁₆]·C₆H₁₄ (Ln^III = Gd^III (Gd₁₄), Tb^III (Tb₁₄), Eu^III (Eu₁₄) and Sm^III (Sm₁₄)) (Hacac = acetylacetone), were successfully synthesized using the Hacac ligand and lanthanide salts in this work. Among these, Gd₁₄ exhibits typical magnetocaloric effect behavior with the −ΔS^max_m of 39.58 J kg⁻¹ K⁻¹ and more importantly, a ratiometric luminescent probe based on Tb₉Eu₅ by co-doping Tb^III and Eu^III ions was found enabling visual detection and reusable sensing of OFX in tap water. This marks the first application of lanthanide clusters in ratiometric luminescent detection of OFX. Further to expand the practical applications of this material, a composite film sensor was prepared by incorporating the cluster with carrageenan hydrogel, facilitating visual detection of OFX and anti-counterfeiting. The results demonstrate that the sensor possesses excellent sensitivity for OFX detection and can achieve multi-layered anti-counterfeiting through simple cutting and assembly.

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Introduction

At the dawn of the 20th century, antibiotics revolutionized medicine, saving countless lives from bacterial infections. However, their misuse and overuse have led to significant health risks and escalated antibiotic resistance, turning it into a major global public health threat.^1,2 Ofloxacin, a fluoroquinolone antibiotic, is widely used to treat respiratory, urinary, and soft tissue infections due to its broad-spectrum efficacy. However, its widespread use leads to environmental and health risks, as unmetabolized OFX contaminates water sources, affects aquatic life, and promotes the spread of antibiotic resistance genes, which may impact human health through the food chain.^3,4 Current OFX detection methods, such as high-performance liquid chromatography (HPLC) and mass spectrometry (LC-MS), require expensive equipment and complex procedures, limiting their ability for real-time, high-sensitivity detection.^5–7 Therefore, developing a simple, rapid, and highly sensitive method to detect low concentrations of OFX in environmental samples is crucial.

Ratiometric luminescent probes, due to their high sensitivity, excellent selectivity, and capability for real-time monitoring, are expected to be an alternative method for the detection of ofloxacin and other antibiotics.^8–10 Their main advantage lies in the ability to generate multiple luminescence signals at different wavelengths simultaneously, allowing for intensity ratio analysis, eliminating background interference, and enhancing detection accuracy and sensitivity. Lanthanide clusters have attracted considerable attention in probe design due to their unique optical properties.^11–15 Lanthanide ions exhibit high luminescence efficiency, long luminescence lifetimes, and narrow emission bands, all of which improve probe performance. Furthermore, the emission spectra of lanthanide clusters possess strong tunability, enabling the generation of dual-wavelength emission signals that improve detection accuracy in complex systems. By modulating these luminescent properties, multi-color emission can be generated, providing distinctive optical markers for anti-counterfeiting applications.¹⁶ Moreover, integrating lanthanide clusters with flexible materials like carrageenan hydrogels allows the development of functional composites that preserve optical properties while enhancing mechanical strength and environmental stability, broadening their applicability in real-time detection and anti-counterfeiting.^17–20 Additionally, lanthanide clusters, especially those with Gd^III ions, exhibit significant magnetocaloric effects, making them strong candidates for efficient and environmentally friendly cooling devices.^21–24

Drawing on the unique optical properties of lanthanide clusters in probe design, acetylacetone (Hacac) has shown promise as an effective ligand for enhancing the stability and luminescent performance of lanthanide luminescent complexes.^25,26 As a typical β-diketone ligand, Hacac forms stable chelates with lanthanide ions through its two carbonyl oxygen atoms, improving complex stability. Additionally, Hacac exhibits an “antenna effect”, efficiently transferring light energy to lanthanide luminescent centers, boosting luminescence efficiency, especially in Eu^III and Tb^III complexes. It also reduces non-radiative transition losses, extending the luminescence lifetime, which is beneficial for applications requiring long-term stable signals. Furthermore, the lightweight and multidentate Hacac ligand may potentially generate lanthanide complexes with low M_W/N_M (M_W = molecular weight, N_M = number of metal ions), exhibiting pronounced magnetocaloric effects, thus offering potential applications in magnetic refrigeration.^27,28

Building upon the research context, this paper reports a series of tetradecanuclear lanthanide clusters with highly symmetrical three-layer double sandwich structure. Among these, Tb₁₄ exhibits high stability and long luminescence lifetime, with high selectivity, low detection limit, and rapid response toward OFX in tap water. Leveraging the comparable ionic radii and coordination chemistry inherent to lanthanide ions, a series of bimetallic materials, Tb_XEu_14−X, has been strategically designed to enable tunable multicolor emissions. Remarkably, Tb₉Eu₅, as a distinguished representative of this series, marks the first lanthanide cluster-based ratiometric luminescent probe for the detection of OFX, demonstrating a detection limit of 4.0 μM, which is competitive compared to other reported probes. To validate real-time detection capability, the multicolor emission properties of Tb_XEu_14−X were combined with the flexibility of carrageenan hydrogel, resulting in Tb_XEu_14−X@CRG functional composite hydrogel sensors with high sensitivity for OFX. Additionally, two anti-counterfeiting strategies were proposed, further demonstrating the broad application prospects of this material. Notably, Gd₁₄ shows a magnetic entropy change of 39.58 J kg⁻¹ K⁻¹ in magnetic studies, highlighting the potential of lanthanide clusters in multifunctional materials.

Experimental section

Materials and methods

All complexes were synthesized using readily available chemicals and solvents, without the need for further purification. A PerkinElmer 2400 analyzer was used to determine the elemental composition (C, N, and H) during the experiments. The complexes’ magnetic properties were investigated using a quantum-designed VSM SQUID magnetometer. Diamagnetic corrections were applied to the measured data, utilizing Pascal's constants and accounting for the contribution of the sample holder.²⁹ The luminescence properties of the complexes were investigated at room temperature using an FLS1000 spectrophotometer (Edinburgh Instruments, UK). The crystal structure of Gd₁₄ was analyzed at 273.15 K using an Xcalibur Eos diffractometer with a wavelength of 0.71073 Å. The structure was refined using the F² full-matrix least-squares method with the software packages SHELXTL-2015 and Olex2.³⁰ The infrared spectra of the complexes in KBr particles were recorded in the region of 4000–500 cm⁻¹ using a Nicolet Impact 410 spectrometer (Thermo Fisher, USA).

Synthesis of [Ln₁₄(acac)₂₄(μ₄-OH)₂(μ₃-OH)₁₆]·C₆H₁₄ (Ln^III = Gd^III (Gd₁₄), Tb^III (Tb₁₄), Eu^III (Eu₁₄) and Sm^III (Sm₁₄))

The synthesis method of the complexes was modified based on previously reported procedures.²⁶ In this study, we synthesized a series of lanthanide clusters, including the new clusters Gd₁₄ and Sm₁₄, with reference to the previously reported Eu₁₄ and Tb₁₄ clusters. The newly synthesized Gd₁₄ and Sm₁₄ clusters are isostructural with the previously reported Eu₁₄ and Tb₁₄ clusters. As an example for Gd₁₄, 0.2 mmol of NH₃·H₂O (0.007 g) and 0.2 mmol of Hacac (0.020 g) were sequentially added to 2 mL of water and stirred for 10 minutes. The resulting mixture was gradually introduced into 10 mL of cold water containing 0.1 mmol of GdCl₃·6H₂O (0.037 g), while maintaining the pH between 7.0 and 7.5. A white precipitate formed, and the reaction mixture was stirred for an additional 5 hours to ensure completeness. The precipitate was then filtered, washed thoroughly with water to remove residual impurities, and dried in a desiccator. Finally, recrystallization using n-hexane yielded colorless Gd₁₄ crystals with a yield of 0.13 g (37% based on Gd). IR data (KBr, cm⁻¹): 3480 (m), 2917 (w) 1616 (s), 1517 (s), 1385 (s), 1260 (s), 1015 (s), 920 (s), 761 (m). Elemental analyses calc (%) for C₁₂₆H₂₀₀Gd₁₄O₆₆: C 30.44, H 4.05; found: C 30.52, H 3.96.

Results and discussion

Description of crystal structure

Analysis from single-crystal X-ray diffraction reveals that the crystal structure of Gd₁₄ comprises a centrosymmetric octahedral Gd₆ unit and two symmetric tetrahedral Gd₅ units, with the Gd₆ octahedron sharing two opposite vertices with the Gd₅ tetrahedra, forming a highly symmetrical three-layer double sandwich structure³¹ (Fig. 1 and Fig. S3†). This vertex-sharing coordination leads to a stable polynuclear structure. In each Gd₅ unit, the Gd₄ square plane is rotated by 45° relative to the Gd₄ square plane in the central Gd₆ unit (Fig. 1d). The asymmetric unit comprises three deprotonated acetylacetone ligands, three Gd^III ions, one μ₄-OH group and three μ₃-OH groups (Fig. S4†). All Gd^III ions are eight-coordinated, with Gd1 exhibiting a D_4d geometry and Gd2 and Gd3 adopting a C_2v geometry (Fig. S5† and Table S4†). The ligands exhibit two distinct coordination modes: 16 ligands are terminally chelated, while 8 acts as bridging chelates (Fig. S6†). The Gd–O bond lengths range from 2.317(5) to 2.651(11) Å, and the distances between adjacent Gd^III atoms vary from 3.609 to 3.991 Å. Notably, the Gd–Gd distances within the octahedral Gd₆ unit are longer than those in the tetrahedral Gd₅ units. Table S1† summarizes the crystallographic experimental data of Gd₁₄, while the selected bond lengths and bond angles of Gd₁₄ are listed in Tables S2 and S3,† respectively. The infrared and XRD spectra of clusters are shown in Fig. S1 and S2.†

Fig. 1 (a) and (b) molecular structure, (c) and (d) metal core topology for Gd₁₄. Color code: Gd (plum), O (red), C (grey). H atoms were omitted for clarity.

Magnetic properties

Gadolinium ion, with its half-filled 4f electronic configuration, high-spin ground state (S = 7/2), and nearly isotropic magnetic properties, is an ideal candidate for efficient magnetocaloric refrigeration materials. The magnetocaloric effect of the polynuclear Gd₁₄ structure has been thoroughly investigated to evaluate its potential applications in magnetic refrigeration.

Initially, the direct current (dc) magnetic susceptibility of Gd₁₄ was measured under a 500 Oe dc magnetic field over a temperature range of 2 to 300 K. At 300 K, the χ_MT value of Gd₁₄ is 108.91 cm³ K mol⁻¹, which is in close agreement with the theoretical value of 110.18 cm³ K mol⁻¹ for 14 isolated Gd^III ions (S = 7/2, L = 0, ⁸S_7/2, g = 2) (Fig. S7†). With the decrease in temperature, the χ_MT value of Gd₁₄ gradually decreases in the range of 300 to 50 K, followed by a sharp decline to 48.19 cm³ K mol⁻¹ at 2 K. This pronounced decline suggests the presence of weak intramolecular antiferromagnetic interactions, which dominate the magnetic behavior at low temperatures.^32,33 The χ_M⁻¹vs. T plot was fitted using the Curie–Weiss law, yielding two key parameters: θ = −3.17 K and C = 110.01 cm³ K mol⁻¹ (Fig. S8†). The negative value of θ further confirms the presence of antiferromagnetic coupling between the Gd^III ions within Gd₁₄.³⁴

Subsequently, the relationship between the magnetization (M) and the applied magnetic field (H) for Gd₁₄ was investigated, as depicted in Fig. S9.† The results indicate that in the low-field region, the magnetization increases rapidly with the applied field, followed by a gradual reduction in the rate of increase. At 7 T, the magnetization reaches saturation, exhibiting a maximum value of 97.13Nβ. The magnetization of Gd₁₄ is slightly lower than the theoretical spin-only value for fourteen non-interacting Gd^III ions, indicating the presence of weak antiferromagnetic interactions between the metal ions. On the other hand, the non-overlapping nature of the M–H/T curves for Gd₁₄ may be attributed to the pronounced magnetic isotropy of the Gd^III ions (Fig. S10†).

To explore the potential magnetocaloric effect behavior of Gd₁₄, the Maxwell equation can be applied to determine the maximum −ΔS_m values, expressed as:^35,36

As the temperature decreases and the applied magnetic field increases, the −ΔS_m value of Gd₁₄ gradually rises, reaching a maximum of 39.58 J kg⁻¹ K⁻¹ at T = 2 K and ΔH = 7 T (Fig. 2). This value is lower than the theoretical value of 48.67 J kg⁻¹ K⁻¹, calculated using the formula ΔS_m = nR ln(2S + 1).³⁷ This discrepancy may be attributed to the presence of antiferromagnetic interactions between the Gd^III centers. Although Gd₁₄ possesses a lower nuclearity compared to Gd₂₇,³⁸ Gd₂₈,³⁹ and Gd₃₆,⁴⁰ its −ΔS_m value is comparable, underscoring its remarkable potential for application in magnetic refrigeration.

Fig. 2 Plot of experimental magnetic entropy change (−ΔS_m) versus T for Gd₁₄.

Luminescence properties

Given the unique luminescence properties of lanthanide elements, the photoluminescence characteristics of Tb₁₄, Eu₁₄ and Sm₁₄ were evaluated at room temperature (Fig. 3a and Fig. S11–S13†). Upon excitation at 335 nm, Tb₁₄ exhibits characteristic sharp emission peaks at 487, 542, 580, and 615 nm, corresponding to the ⁵D₄ → ⁷F_J (J = 6–3) transitions of the Tb^III ions. Similarly, in the emission spectrum of Eu₁₄, the characteristic peaks of Eu^III corresponding to the ⁵D₀ → ⁷F_J (J = 0–4) transitions are observed at 574, 587, 608, 648, and 698 nm, respectively. In the emission spectrum of Sm₁₄, the characteristic peaks of Sm^III corresponding to the ⁴G_5/2 → ⁶H_J (J = 5/2–11/2) transitions are observed at 563, 603, 645, and 709 nm, respectively.

Fig. 3 (a) Excitation and emission spectra of Ln₁₄; (b) luminescence images of potential fingerprints on glass after sprinkling the solid powders of Tb₁₄ under a 365 nm UV lamp; (c) refills loaded with Tb₁₄ under daylight (top) and 365 nm UV lamp (bottom); (d) paper after writing under daylight (left) and 365 nm UV lamp (right).

The strong luminescence properties of Tb₁₄ make it a potential candidate for applications in forensic science. By first depositing a volunteer's fingerprint on a glass surface and evenly sprinkling Tb₁₄ solid powder over it, followed by the gentle removal of excess powder using a rubber bulb, the fingerprint's luminescent image becomes clearly visible under 365 nm ultraviolet light (Fig. 3b). Additionally, Tb₁₄ suspension in a glycerol–water mixture remains stable for at least one month, making it suitable for use as capillary ink (Fig. 3c). Under ultraviolet light, it emits a bright luminescence (Fig. 3d), demonstrating its potential applications in writing.

To investigate the energy transfer between metal ions within the cluster, heterometallic clusters Tb_XEu_14−X were synthesized by doping Eu^III into Tb₁₄ at various molar ratios, with the resulting emission spectra depicted in Fig. 4. The PXRD and IR results indicate that Tb_XEu_14−X and Tb₁₄ are isostructural (Fig. S14 and S15†). In addition, ICP-OES analysis confirms that the metal ratio is consistent with the lanthanide ratio used for synthesis (Table S5†). Furthermore, the EDX mapping confirms the uniform distribution of Eu^III and Tb^III ions within the bimetallic cluster particles (Fig. S16†). As the Tb^III/Eu^III ratio decreases, the emission of Tb_XEu_14−X gradually shifts from the green emission of Tb^III to the red emission of Eu^III, thereby achieving tunable luminescence. The corresponding CIE chromaticity diagram is shown in Fig. S17.† To verify the energy transfer process from Tb^III to Eu^III in the co-doped samples, the luminescence lifetime decay curves of the Tb^III ions were measured. Fig. S18 and S19† show that, upon the introduction of Eu^III, the luminescence lifetime of Tb^III decreases, confirming efficient energy transfer from the excited state of Tb^III to Eu^III. To verify the generality of this mechanism, we further synthesized two heterometallic clusters containing Sm^III and Tb^III ions: Tb₁₀Sm₄ and Tb₄Sm₁₀. Given the similar emission levels of Sm^III and Eu^III ions (⁴G_5/2 and 18 000 cm⁻¹ for Sm^III, ⁵D₀ and 17 200 cm⁻¹ for Eu^III), energy transfer from Tb^III to Sm^III is also expected to occur. As shown in Fig. S20,† this phenomenon further demonstrates the versatility of this system.¹⁶

Fig. 4 Emission spectra (λ_ex = 335 nm) of Tb₁₄ (a), Tb₁₃Eu₁ (b), Tb₁₂Eu₂ (c), Tb₁₁Eu₃ (d), Tb₁₀Eu₄ (e), Tb₉Eu₅ (f), Tb₈Eu₆ (g), and Tb₇Eu₇ (h). Inset: photographs of the in Tb_XEu_14−X excited with a UV lamp.

Luminescence detection of OFX

Considering the excellent stability and strong luminescence performance of Ln₁₄, we initially selected Tb₁₄ to explore its sensing potential for OFX in tap water suspensions (Fig. 5). Under excitation at 335 nm, Tb₁₄ exhibited a significant quenching effect in the presence of OFX, whereas other antibiotics (Scheme S1†), including fluoroquinolone compounds like ciprofloxacin (CIP), resulted in a quenching efficiency of no more than 50%. Even in complex background solutions containing multiple antibiotics, the quenching effect of OFX on Tb₁₄'s luminescence remained consistent with that observed in tap water, demonstrating its exceptional selectivity and anti-interference capability (Fig. S21†). Within the low concentration range of 5 to 40 μM, the luminescence intensity showed a linear relationship with the OFX concentration, consistent with the Stern–Volmer equation, yielding a high K_SV value of 68 657 M⁻¹ and a detection limit as low as 0.44 × 10⁻⁶ M.^41,42

Fig. 5 (a) Luminescence sensing of Tb₁₄ in diverse antibiotic solutions in tap water (λ_ex = 335 nm); (b) luminescence sensing of Tb₁₄ to OFX in tap water (λ_ex = 335 nm). Inset: photographs of Tb₁₄ in tap-water under the irradiation of a UV lamp (365 nm) before and after addition of 40 μM OFX; (c) CIE coordinates of Tb₁₄ after addition of different concentrations of OFX; (d) the shape of the relationship between the luminescence intensities of Tb₁₄ and the concentrations of OFX.

To evaluate the sensing potential of a ratiometric luminescent probe for OFX, we conducted luminescence titration experiments using Tb₉Eu₅. The reason for selecting Tb₉Eu₅ is that for Tb₁₃Eu₁, Tb₁₂Eu₂, and Tb₇Eu₇, the color change observed with the naked eye during OFX detection is not very pronounced, which may limit their practical application (Fig. S22†). In contrast, for Tb₁₁Eu₃, Tb₁₀Eu₄, Tb₉Eu₅, and Tb₈Eu₆, after adding 100 μM OFX, Tb₉Eu₅ shows the largest change, with the I_608nm/I_542nm ratio increasing from an initial 0.85 to 2.34, corresponding to a relative change of 175% (Table S6†). Therefore, Tb₉Eu₅ is selected as the ratiometric luminescence probe for detecting OFX. Upon excitation at 335 nm, Tb₉Eu₅ exhibited a characteristic ratiometric luminescence response to OFX in tap water. With the increasing concentration of OFX, the emission of Tb^III at 542 nm gradually diminished, while the emission of Eu^III at 608 nm became progressively stronger (Fig. 6). This shift led to a distinct color change from yellow to bright red, clearly visible to the naked eye. Within the concentration range of 10 μM to 100 μM, the ratio of intensities (I_608nm/I_542nm) displayed a linear correlation with OFX concentration, yielding a K_SV value of 7565 M⁻¹ and a detection limit as low as 4.0 μM (Fig. S23†). Furthermore, selectivity and anti-interference tests confirmed that Tb₉Eu₅ functions as a rapid and reliable ratiometric luminescent probe for OFX (Fig. S24†). The test results indicate that the detection limit of the ratio luminescent probe Tb₉Eu₅ is higher than that of the single luminescent probe Tb₁₄. This may be attributed to the reliance of the ratio luminescent probe on the energy transfer mechanism between distinct luminescent centers, where the energy transfer efficiency from Tb^III to Eu^III in the Tb₉Eu₅ system is likely to be lower, which leads to reduced signal intensity and, consequently, lower sensitivity.

Fig. 6 Luminescence sensing of different concentrations of OFX by Tb₉Eu₅ in tap water (λ_ex = 335 nm). Inset: photographs of Tb₉Eu₅ in tap-water under the irradiation of a UV lamp (365 nm) before and after addition of 100 μM OFX.

The stability of Tb₁₄ and Tb₉Eu₅ in the presence of OFX was further confirmed (Fig. S25†). After exposure to OFX for 3 days, the emission spectra of Tb₁₄ showed no significant changes, indicating its long-term stability as an OFX sensing material. For Tb₉Eu₅, a slight variation was observed in the emission wavelengths between 560 and 600 nm, which could be attributed to environmental factors such as temperature, humidity, or interactions between the ligand and the environment. However, since the comparison was based on the emission intensity ratio of the two peaks at 608 nm and 542 nm for Tb₉Eu₅, which remained unchanged, the overall stability of the sample was minimally affected. In addition, Tb₁₄ and Tb₉Eu₅ demonstrated excellent recyclability (Fig. S26†). Following the sensing and detection process, the luminescence intensity could be effectively restored through ultrasonic washing and centrifugation, allowing the materials to be reused at least two times. This highlights their potential for sustainable application in real-world scenarios.

To evaluate the detection performance of these clusters in real samples, we selected antibiotics purchased from a pharmacy for testing (Fig. S27†). Upon the addition of 20 mg L⁻¹ of ofloxacin, the luminescence intensity of Tb₁₄ at 542 nm decreased by 79%, while the emission intensity ratio of Tb₉Eu₅ at 608 nm to 542 nm (I_608nm/I_542nm) increased from 0.87 to 2.04. These findings confirm the effective detection capabilities of Tb₁₄ and Tb₉Eu₅ for OFX in complex matrices, further confirming their potential as reliable sensing materials for practical applications.

To comprehensively investigate the sensing mechanism of Tb₁₄ towards OFX, we considered the following aspects. Firstly, the PXRD and IR spectra of the complexes post-response showed excellent consistency with the initial spectra (Fig. 7a and b). These results not only indicate that the structural integrity of the complexes remains intact after the identification of the target analyte but also confirm the absence of new coordination or hydrogen bonds between the complexes and OFX. Therefore, the luminescence response is not attributable to structural collapse or coordination and hydrogen-bonding interactions.⁴³

Fig. 7 (a) Infrared spectrum (b) and PXRD patterns of Tb₁₄ after OFX treatment; (c) UV–vis absorption spectrum of antibiotics (C = 10⁻⁵ M) and excitation and emission spectra of Tb₁₄ in water suspension; (d) HOMO and LUMO energies and isosurfaces for Tb₁₄ and antibiotics.

Furthermore, as shown in Fig. 7c, there was no overlap between the emission spectrum of Tb₁₄ and the UV absorption spectrum of OFX, effectively ruling out the possibility of luminescence quenching via the Förster resonance energy transfer (FRET) mechanism. However, among the antibiotics tested, the excitation spectrum of Tb₁₄ overlapped most effectively with the UV absorption spectrum of OFX, suggesting that OFX competes with Tb^III ions in Tb₁₄ for the absorption of excitation light, leading to luminescence quenching, a phenomenon attributed to the internal filtration effect.^18,44

The possibility of charge transfer was evaluated by comparing the frontier orbital energy levels of Tb₁₄ and OFX using density functional theory (DFT) calculations (Fig. 7d). The results show that the LUMO level of Tb₁₄ is higher than the HOMO level of OFX, and the HOMO level of Tb₁₄ is lower than the LUMO level of OFX, indicating that charge transfer (CT) in the ground state is not feasible. However, since the HOMO level of OFX is higher than that of Tb₁₄, photoinduced electron transfer (PET) may occur after the excitation of Tb₁₄. Therefore, the luminescence quenching response of Tb₁₄ to OFX may be attributed to the combined effects of the IFE and PET.

As for the possible sensing mechanism of Tb₉Eu₅ towards OFX, the PXRD, IR, luminescence and UV-Vis absorption spectra of the complexes after the response show excellent consistency with the initial spectra, indicating that the structural integrity of Tb₉Eu₅ is maintained after the identification of the target analyte (Fig. 7a, b and Fig. S28†). No new peaks were observed in all of these spectra, confirming that no new coordination bonds or hydrogen bonds were formed between Tb₉Eu₅ and OFX, and ruling out the possibility of free Eu^III ions leaching from Tb₉Eu₅ and forming a new complex with OFX that would enhance the red luminescence. Furthermore, as shown in the luminescence lifetime curve in Fig. S18,† the energy transfer efficiency from Tb^III to Eu^III is significantly enhanced after the addition of OFX. As the emission intensity of Tb^III weakens and that of Eu^III increases, a change in the emission color of Tb₉Eu₅ is observed, shifting from yellow to red, which can easily be seen by the naked eye. Additionally, theoretical calculations revealed that the triplet energy level of OFX is 23 870 cm⁻¹, which is highly suitable for sensitizing the luminescence of Eu^III ions.⁴⁵ Therefore, the enhancement of red emission observed in the Tb₉Eu₅ complex upon interaction with OFX can be partly attributed to the “antenna effect”, where OFX acts as an energy donor, transferring energy to the Eu^III ions and thus amplifying their luminescence.

To further expand the application of Ln₁₄ in OFX sensing and enhance the visualization and portability of the detection process, we developed a composite film sensor based on the combination of clusters and hydrogel. Specifically, we integrated Ln₁₄ with an inexpensive, edible carrageenan hydrogel, taking advantage of its excellent water stability and luminescent properties. This approach led to the successful fabrication of a series of Tb_XEu_14−X@CRG composite films that display various colors under UV light (Fig. 8a). Taking the Tb₉Eu₅@CRG film as an example, as shown in Fig. 9b–d, the film demonstrates a clear visual response to varying concentrations of OFX under 365 nm UV light. Initially emitting yellow in the absence of OFX, the film undergoes a noticeable shift to bright red luminescence after immersion in a 50 ppm OFX aqueous solution for 5 seconds. When exposed to a lower concentration of 20 ppm OFX for the same duration, the emission transitions to an orange-yellow hue, reflecting its sensitivity to different OFX concentrations. Notably, the Tb₉Eu₅@CRG hydrogel membrane exhibits a detection limit for OFX as low as 20 ppm, which is significantly below the maximum residue limit of 90 ppm for quinolone drugs set by the European Union.¹⁸

Fig. 8 (a) Advantages of composite film materials Tb_XEu_14−X@CRG; (b) luminescence changes of Tb9Eu5@CRG under 365 nm UV light after immersion in different concentrations of OFX aqueous solution; (c) color images of Tb9Eu5@CRG after immersion in OFX solutions of varying concentrations under 365 nm UV light; (d) linear relationship between color change (R/(G + B)) and OFX concentration; (e) schematic showing the detection of OFX using a smartphone device.

Fig. 9 (a) and (b) Photographs of an anti-counterfeiting pattern “Flower” taken before and after turning on the 365 nm UV light; (c) partial enlarged view of “Flower”; (d) photographs of “Flower” after immersion in 50 ppm OFX solution under 365 nm UV light; (e) pictures of the letter “H” stitched together using different colored hydrogel films in daylight and under 365 nm UV light.

Subsequently, Tb₉Eu₅@CRG was separately immersed in OFX solutions of varying concentrations, and its color change was observed under 365 nm UV light irradiation. As the OFX concentration fluctuated within the range of 20 ppm to 70 ppm, the luminescence emission intensity of Tb₉Eu₅@CRG undergoes corresponding changes. It is noteworthy that the luminescence emission photographs could be conveniently captured using a smartphone, and the corresponding RGB values were readily obtained via a commonly available application (as illustrated in Fig. 8c). Within the concentration range of 20 ppm to 70 ppm, a linear relationship was established between the (R/(G + B)) ratio and the OFX concentration, which can be utilized to accurately determine the OFX concentration in unknown samples (as depicted in Fig. 8d and e).

Anti-counterfeiting application

To harness the luminescent properties of the co-doped materials and composite films for practical applications, we devised two innovative anti-counterfeiting strategies leveraging their unique characteristics.

Strategy 1: by cutting films with varying emission colors, a variety of anti-counterfeiting patterns can be assembled. For example, a three-dimensional “Flower” was created through this approach, forming the first layer of security (Fig. 9a). The films, with different Tb^III/Eu^III ratios, emit distinct colors under 365 nm UV light, adding a second layer of protection (Fig. 9b). This method demonstrates an effective dual-layer anti-counterfeiting labeling strategy. Furthermore, immersing the “Flower” in a 50 ppm OFX solution for 5 seconds and then exposing it to 365 nm UV light caused the green luminescent areas to be quenched, resulting in a red emission overall (Fig. 9c and d).

Furthermore, an “H” character was designed using the same cutting and assembly technique. This pattern remained invisible under natural light but emitted green and orange luminescence under UV irradiation (Fig. 9e). Leveraging the luminescence response characteristics of Ln₁₄ to OFX, OFX solution was sprayed onto the “H” pattern. The results demonstrated that, after application, the left side of the “H” experienced luminescence quenching, turning blue, while the right side exhibited enhanced luminescence, shifting to red. The distinct contrast in luminescence before and after the application of OFX was pronounced, and this localized variation further strengthened its utility as an anti-counterfeiting feature.

Strategy 2: by using the variations in the relative intensities of the distinct emission spectra of Tb_XEu_14−X clusters, optical barcode outputs can be created (Fig. S29†). From left to right, these bars correspond to the following transitions: ⁵D₄ → ⁷F₆ (487 nm), ⁵D₄ → ⁷F₅ (542 nm), ⁵D₀ → ⁷F₂ (608 nm), and ⁵D₀ → ⁷F₄ (698 nm). In these barcodes, green bars represent the emissions of Tb^III, while red bars indicate the emissions of Eu^III. And the inset displays photographs of the heterometallic clusters under UV lamp excitation, illustrating their respective emission colors. This tunability offers potential applications in constructing high-fidelity optical barcodes through color encoding.¹⁶

Conclusions

In summary, this study highlights the multifunctionality of lanthanide clusters in magnetic refrigeration, luminescence sensing, and anti-counterfeiting. Magnetic measurements reveal the weak antiferromagnetic interactions between Gd^III ions and the significant magnetocaloric effect of Gd₁₄. Photoluminescence experiments indicate that Tb₁₄, Eu₁₄, and Sm₁₄ complexes exhibit characteristic emission properties, with tunable luminescence achieved through heterometallic doping. Most importantly, the study demonstrates that Tb₉Eu₅ represents the first example of Ln-based cluster as a ratiometric luminescent probe for the detection of OFX. Furthermore, the composite film sensor demonstrates exceptional performance in visual detection and anti-counterfeiting applications. This work not only expands the application potential of lanthanide clusters but also provide innovative methodologies for material design and performance optimization in related technologies.

Author contributions

Tian-Tian Wang: synthesis, measurement, analysis and writing; Hao Wang: measurement and check writing; Wen-Bin Sun: supervision, discussion, writing – reviewing and editing.

Data availability

The authors confirm that the data supporting the findings of this article is given within this article and the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the NSFC (No. 22071047) and Natural Science Foundation of Heilongjiang Province (PL2024B013).

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Footnote

† Electronic supplementary information (ESI) available: Selected bond lengths (Å) and angles (°), the Gd^III geometry analysis, IR, PXRD patterns, analysis of frontier molecular orbitals, magnetic data and luminescence properties for complexes. CCDC 2403575. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi03133d

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