Mixed Tb/Dy coordination ladders based on tetra(carboxymethyl)thiacalix[4]arene: a new avenue towards luminescent molecular nanomagnets

The macrocyclic ligand calix[4]arene (L1) and its sulphur-containing analogue thia[4]calixarene (L2) are promising precursors for functional molecular materials as they offer rational functionalization with various organic groups. Here, we present the first example of lanthanide-based coordination polymers built from the macrocyclic thiacalix[4]arene backbone bearing four carboxylic moieties, namely, ligand H₄L3. The combination of H₄L3 with the Tb³⁺ and Dy³⁺ cations led to the formation of 1D ladder-type coordination polymers with the formula [Ln^IIIHL3DMF₃]·(DMF) (where DMF = dimethylformamide and Ln = Tb or Dy, denoted as HL3–Tb and HL3–Dy), which resulted from the coordination of the lanthanide cations with the partially deprotonated ligand HL3³⁻ that behaved as a T-shape connector. The coordination sphere around the metal was completed by the coordinated DMF solvent molecules. By combining both Tb³⁺ and Dy³⁺ cations, isostructural heterobimetallic solid solutions HL3–Tb_1−xDy_x were also prepared. HL3–Tb and HL3–Dy showed visible light photoluminescence originating from the f–f electronic transitions of pale green emissive Tb³⁺ and pale yellow emissive Dy³⁺ with efficient sensitization by the functionalized thia[4]calixarene ligand HL3. In the HL3–Tb_1−xDy_x solid solutions, the Tb/Dy ratio governed both the emission colour as well as the emission quantum yield, which reached even 28% at room temperature for HL3–Tb. Moreover, HL3–Dy exhibited a slow magnetic relaxation effect related to the magnetic anisotropy of the dodecahedral Dy³⁺ complexes, which were well isolated in the crystal lattice by expanded organic spacers.

The single crystals of the two isostructural Tb³⁺- and Dy³⁺-based coordination polymers (HL3–Tb and HL3–Dy) were structurally characterized, and their photophysical properties were investigated, together with their corresponding solid solutions.     

Color tuning and white light emission by codoping in isostructural homochiral lanthanide metal–organic frameworks

Four lanthanide-based homochiral metal–organic frameworks (Ln-HMOFs), {[Ln₂(HL)₂(H₂O)₄]·2Cl·5H₂O}_n [Ln = Gd (1), Eu (2), Tb (3) and Dy (4)], have been synthesized through solvothermal reactions of chiral ligand (S)-5-(((1-carboxyethyl)amino)methyl)isophthalic acid (H₃L) with corresponding LnCl₃·6H₂O. They are binodal (3,6)-connected frameworks with kgd nets based on binuclear cluster units and zwitterionic (HL)²⁻ linkers. Considering the isostructuralism of these Ln-HMOFs as well as the blue emission of compound 1 and the strong typical Eu³⁺ and Tb³⁺ emissions of compounds 2 and 3, single-phase mixed-lanthanide HMOFs have been prepared by doping of Ln³⁺ into the Ln-HMOFs to modulate light-emitting color. Interestingly, the bimetallic doped Eu/Tb-HMOFs [(Eu_xTb_1−x)₂(HL)₂(H₂O)₄]·2Cl·5H₂O display a fluent change of light-emitting color among green, yellow, orange, orange-red, and red by adjusting the doping concentration of Eu³⁺ ions into the Tb-HMOF. Very importantly, the trimetallic doped Eu/Gd/Tb-HMOF [(Eu_0.1388Gd_0.6108Tb_0.2504)₂(HL)₂(H₂O)₄]·2Cl·5H₂O emits white light upon excitation at 355 nm, whose emission can also be switched between different colors when excited with different ultraviolet light. Furthermore, the fluorescence response of Tb-HMOF to various usual metal ions, and especially fluorescent sensing behaviours to Fe³⁺, Cr³⁺ and Al³⁺ have been preliminarily investigated.

Pure white-light emission and fluent light-emitting color change can be facilely obtained by codoping isostructural homochiral lanthanide metal–organic frameworks.     

Unraveling the site-specific energy transfer driven tunable emission characteristics of Eu3+ & Tb3+ co-doped Ca10(PO4)6F2 phosphors

In this study we have explored Ca₁₀(PO₄)₆F₂ as host to develop a variety of phosphor materials with tunable emission and lifetime characteristics based on Eu³⁺ and Tb³⁺ as co-dopant ions and the energy transfer process involved with them. The energy transfer from the excited state of Tb³⁺ ion to the ⁵D₀ state of Eu³⁺ makes it possible to tune the colour characteristics from yellow to orange to red. Further, such energy transfer process is highly dependent on the concentration of Eu³⁺ and Tb³⁺ ions and their site-selective distribution among the two different Ca-sites (CaO₉ and CaO₆F) available. We have carried out DFT based theoretical calculation for both Eu³⁺ and Tb³⁺ ions in order to understand their distribution. It was observed that in cases of co-doped sample, Tb³⁺ ions prefer to occupy the Ca2 site in the CaO₆F network while Eu³⁺ ions prefer Ca1 site in the CaO₉ network. This distribution has significant impact on the lifetime values and the energy transfer process as observed in the experimental photoluminescence lifetime values. We have observed that for the 1^st series of compounds, wherein the concentration Tb³⁺ ions are fixed, the energy transfer from Tb³⁺ ion at Ca2 site to Eu³⁺ ion at Ca1 site is dominating (Tb³⁺@Ca2 → Eu³⁺@Ca1). However, for the 2^nd series of compounds, wherein the concentration Eu³⁺ ions are fixed, the energy transfer process was found to occur from the excited Tb³⁺ ion at Ca1 site to Eu³⁺ ions at both Ca1 and Ca2 (Tb³⁺@Ca1 → Eu³⁺@Ca1 and Tb³⁺@Ca1 → Eu³⁺@Ca2). This is the first reports of its kind on site-specific energy transfer driven colour tunable emission characteristics in Eu³⁺ and Tb³⁺ co-doped Ca₁₀(PO₄)₆F₂ phosphor and it will pave the way for the future development of effective colour tunable phosphor materials based on a single host and same co-dopant ions.

Various site specific energy transfer (ET) process such as Tb³⁺@Ca2 → Eu³⁺@Ca1, Tb³⁺@Ca1 → Eu³⁺@Ca2 and Tb³⁺@Ca1 → Eu³⁺@Ca1 were explored in Eu³⁺ and Tb³⁺ co-doped Ca₁₀(PO₄)₆F₂ phosphor, which are responsible for tunable colour characteristics.     

Structure,tunable luminescence and energy transfer in Tb3+ and Eu3+ codoped Ba3InB9O18 phosphors

The borate Ba₃InB₉O₁₈ (BIBO) is a promising host material for phosphors. A series of Tb³⁺ and Eu³⁺ codoped Ba₃InB₉O₁₈ phosphors were synthesized. Based on the Rietveld method, structure refinement of the codoped BIBO phosphor was carried out. Then, the luminescence properties of BIBO:Tb³⁺, Eu³⁺ phosphors were extensively investigated under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation. The measured PL spectra and decay times evidenced that energy transfer occurs between the Tb³⁺ and Eu³⁺ ions. The energy-transfer mechanism from Tb³⁺ to Eu³⁺ in Ba₃InB₉O₁₈ is dominated by electric multipolar interactions, with the critical distance calculated to be 10.97 Å. The temperature sensitivity of the Tb³⁺ and Eu³⁺ codoped sample under VUV was also investigated at the low temperature range from 25 K to 298 K. The emission color could be tuned from green to the red region by adjusting the concentration of codoped ions. The results indicate that the BIBO-based phosphors are valuable candidates for applications in the display and lighting fields.

The emission colors of Ba₃InB₉O₁₈:Tb³⁺, Eu³⁺ can be adjusted from yellowish green to orange by tuning the content of Eu³⁺ ions due to the energy transfer from Tb³⁺ to Eu³⁺, thus showing a great potential for display and lighting fields applications.     

Luminescence properties and energy transfer investigations of Ba2La2.85−xTb0.15Eux(SiO4)3F multicolor phosphor

The Ba₂La_2.85−xTb_0.15Eu_x(SiO₄)₃F (BLSOF:0.15Tb³⁺, xEu³⁺) multicolor phosphors with apatite structure were synthesized via the solid-state pathway. The crystal structure and luminescence properties of the phosphors were investigated by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), Rietveld refinement, photoluminescence excitation (PLE) and photoluminescence (PL). The luminescence performance of the phosphor was optimum when the concentration of Tb³⁺ was set to be 0.15 mol and the concentration of Eu³⁺ was set to be 0.22 mol. Under the accurate excitation of 373 nm near ultraviolet (n-UV) light, the emitting color of the phosphors can be tuned from green to red with increasing Eu³⁺/Tb³⁺ ratio. It was further proved that the quadrupole–quadrupole (q–q) interaction is responsible for the energy transfer (ET) in the BLSOF:0.15Tb³⁺, 0.22Eu³⁺ phosphor. Owing to the excellent thermal quenching luminescence property, the BLSOF:0.15Tb³⁺, xEu³⁺ phosphor can be applied in n-UV white light emitting diodes (w-LEDs) and serve as the warm part of warm white light.

The developed phosphors can be accurately excited by 373 nm (n-UV) light and produce a gradient of colors.     

Tunable emission from green to red in the GdSr2AlO5:Tb3+,Eu3+ phosphor via efficient energy transfer

Herein, a series of GdSr₂AlO₅:Tb³⁺,Eu³⁺ phosphors were successfully synthesized through a high temperature solid-state reaction, and their crystal structures as well as photoluminescence properties were investigated in detail. Compared to the intense emission of ⁵D₀ → ⁷F₁ or ⁵D₀ → ⁷F₂ transition of Eu³⁺, another strong emission corresponding to ⁵D₀ → ⁷F₄ was observed. Concentration quenching is not obvious in Tb³⁺ or Eu³⁺-doped GdSr₂AlO₅ because structure isolation and energy transfer (ET) of Gd³⁺ → Eu³⁺ and Gd³⁺ → Tb³⁺ were found. Moreover, the energy transfer process from Tb³⁺ to Eu³⁺ was verified by the overlap of luminescence spectra and the variation of lifetime. Energy transfer mechanism was determined to be a dipole–dipole interaction, and ET efficiency as well as quantum efficiency were also obtained. Moreover, the emission color of GdSr₂AlO₅:Tb³⁺,Eu³⁺ can be tuned from green to red by altering the ratio of Tb³⁺/Eu³⁺. These results indicate that the GdSr₂AlO₅:Tb³⁺,Eu³⁺ phosphor is a promising single-component white light-emitting phosphor.

The emission spectra of GdSr₂AlO₅:2%Tb³⁺,x%Eu³⁺ (x = 0, 0.5, 1, 2, 3, and 5) under 275 nm excitation. (b) Variation tendency of the green emission of Tb³⁺ and the red emission of Eu³⁺.     

Size dependent optical temperature sensing properties of Y2O3: Tb3+, Eu3+ nanophosphors

Using urea as a precipitation agent, Tb³⁺, Eu³⁺ co-doped Y₂O₃ nanophosphors were synthesized by a homogeneous precipitation method. The sizes of the sample particles were controlled by changing the molar ratio of the urea and rare earth ions. The microstructure and crystallographic structure of the sample were determined through powder X-ray diffraction (PXRD) and field emission scanning electron microscopy (FE-SEM). The test results show that the sample is body centered cubic. As the molar ratio of urea to rare earth ions increases, the size of the sample particles decreases. The temperature-dependent emission spectra of Tb³⁺, Eu³⁺ co-doped Y₂O₃ phosphors with different particle sizes were measured. The results showed that because the fluorescence intensity ratio (FIR) of Tb³⁺ and Eu³⁺ varies with temperature, it can be used to visually reflect changes in temperature. In addition, the temperature sensing sensitivity of Tb³⁺ and Eu³⁺ co-doped Y₂O₃ phosphors increased upon a decrease in the particle size, but the relative sensitivity decreased with a decrease in the particle size. The physical mechanism of the sensitivity and relative sensitivity changes with the size of the sample particles was also explained.

Using urea as a precipitation agent, Tb³⁺, Eu³⁺ co-doped Y₂O₃ nanophosphors were synthesized by a homogeneous precipitation method. The size dependence-optical temperature sensing properties of nanophosphors have been studied.     

Three resorcin[4]arene-based lanthanide-coordination polymers with multifunctional photoluminescence sensing properties

By utilizing a novel octacarboxylate-functionalized resorcin[4]arene as organic linkers, three lanthanide-coordination polymers, namely, [(CH₃)₂NH₂][Ln₂(HL)(H₂O)₇]·2H₂O (Ln = Tb (1), Eu (2) and Gd (3), H₈L = 2,8,14,20-tetra-pentyl-4,6,10,12,16,18,22,24-octa-carboxymethoxy-resorcin[4]arene) have been solvothermally synthesized and structurally characterized. Isostructural 1–3 display unique two dimensional sandwich-based layers built with Ln³⁺ cations and bowl-shaped HL⁷⁻ anions. Remarkably, 1 and 2 produce intensive green and red emissions respectively and long lifetimes thanks to the antenna effect of HL⁷⁻ anions. The energy level testing of 3 indicates that the newly designed ligand H₈L has a very efficient intersystem crossing process. More importantly, luminescent investigations reveal that 1 and 2 can selectively detect N,N′-dimethylformamide and Fe³⁺ ions with turn-on-type and turn-off-type responses, respectively.

We report three lanthanide-coordination polymers assembled with a resorcin[4]arene ligand, where 1 and 2 could be applied as fluorescent sensors for N,N′-dimethylformamide and Fe³⁺ ion.     

Temperature sensing in Tb3+/Eu3+-based tetranuclear silsesquioxane cages with tunable emission

New luminescent cage-like tetranuclear silsesquioxanes [NEt₄][(Ph₄Si₄O₈)₂(Tb₃Eu)(NO₃)₄(OH)(EtOH)₃(H₂O)]·4(EtOH) (1) and [NEt₄]₂[(Ph₄Si₄O₈)₂(Tb₂Eu₂)(NO₃)₆(EtOH)₂(MeCN)₂]·4(MeCN) (2) present a tunable thermosensitive Tb³⁺-to-Eu³⁺ energy transfer driven by Tb³⁺ and Eu³⁺ emission and may be used as temperature sensors operating in the range 41–100 °C with excellent linearity (R² = 0.9990) and repeatability (>95%). The thermometer performance was evidenced by the maximum relative sensitivity of 0.63% °C⁻¹ achieved at 68 °C.

Tetranuclear silsesquioxane cages with tunable thermosensitive Tb³⁺-to-Eu³⁺ energy transfer were used for temperature sensing based on the Tb³⁺-to-Eu³⁺ emission intensity ratio (LIR) with excellent linearity and sensitivity.     

Di-functional luminescent sensors based on Y3+ doped Eu3+ and Tb3+ coordination polymers: fast response and visible detection of Cr3+, Fe3+ ions in aqueous solutions and acetone

With the careful modulation of the relative ratio of Y³⁺/Eu³⁺and Y³⁺/Tb³⁺, two series of bimetallic RE-CPs (Eu_xY_1−x and Tb_xY_1−x) were successfully obtained through the isomorphous substitution method. Interestingly, the introduction of Y³⁺ ions does not change the fluorescence characteristic peak of 1-Eu and 1-Tb, but enhances its fluorescence lifetime and quantum yield. Experimental and theoretical simulation results show the co-doping process changes the intramolecular energy transfer process and reduces the non-radiative transition resulting from concentration quenching. Eu_0.1Y_0.9 and Tb_0.1Y_0.9 with the largest luminescence lifetime were selected as the representative research objects, their potential application for the detection of toxic metal ions and organic molecules was further investigated. Interestingly, Eu_0.1Y_0.9 and Tb_0.1Y_0.9 demonstrate high sensitivity and good selectivity towards Fe³⁺, Cr³⁺ and acetone. Besides, fine fluorescence visibility provides the necessary conditions for the preparation of simple and fast response fluorescent test papers in order to achieve real-time and convenient detection of these toxic materials.

Two series of doped coordination polymers (Eu_xY_1−x and Tb_xY_1−x) through isomorphous substitution method utilizing Y³⁺ in place of partial Eu³⁺/Tb³⁺ were obtained. The doped materials could detect Fe³⁺, Cr³⁺, and acetone selectively and sensitively.     

Energy transfer between rare earths in layered rare-earth hydroxides

Energy transfer between rare earths in layered rare-earth hydroxides (LRHs) is worth the intensive study because the hydroxyls that act as the bridge connecting the neighbouring rare earths would generate non-radiative transitions. This study focuses on the energy transfer in the intralayer and the adjacent layers of LRHs. A series of LEu_xTb_1−xHs (x = 0, 0.05, 0.2, 0.5, 0.8, and 0.95) was synthesized, the basal spacing (d_basal) was adjusted from 8.3 to 46 Å through ion-exchange process, and unilamellar nanosheets were prepared through a delamination process. The luminescence behaviours of the samples demonstrated the following: (1) for the delaminated nanosheets, the quenching effect of both Eu³⁺ and Tb³⁺ was hardly observed. This implies that in the intralayer, the efficiency of energy transfer is extremely low, so that highly-concentrated co-doping does not influence the luminescence and by controlling the Eu/Tb molar ratio, white light can be obtained. (2) For small d_basal, e.g., 27 Å, the fluorescence quenching of Tb³⁺ and Eu³⁺ was remarkable, while for large d_basal, e.g., 46 Å, the emission of Tb³⁺ emerged and the self-quenching between Eu³⁺ ions weakened. (3) The energy transfer efficiency deceased with an increase in the distance between adjacent layers. In other words, either the energy transfer between Eu³⁺ and Tb³⁺ or the energy migration between Eu³⁺ ions was more efficient when they were located in adjacent layers than in intralayers even when they were the nearest neighbours.

In LRHs, the energy transfer between rare earths in adjacent layers was more efficient than that between neighboring rare earths in intralayers.     

Synthesis and characterizations of YZ-BDC:Eu3+,Tb3+ nanothermometers for luminescence-based temperature sensing

In the present work, nanothermometers based on amorphous zirconium metal–organic frameworks co-doped with rare-earth ions (YZ-BDC:Eu³⁺,Tb³⁺ nanothermometers) with sizes of about 10–30 nm were successfully synthesized via a microwave-assisted hydrothermal method at 120 °C for 15 min. The determined BET surfaces area, total pore volume and average pore diameter were ∼530 m² g⁻¹, 0.45 cm³ g⁻¹ and 3.4 nm, respectively. Based on Fourier transform infrared spectroscopy (FTIR) and simultaneous thermal analysis (STA) results, the formation process of carboxylic acid salts and the molecular formula of the samples have been proposed. The thermometric properties of Zr-BDC:Eu³⁺,Tb³⁺ nanothermometers and their Y³⁺ ion co-doped counterparts (YZ-BDC:Eu³⁺,Tb³⁺) measured in the 133–573 K temperature range were compared. Moreover, the temperature-dependent CIE(x, y) chromaticity coordinates and emission color of the samples were also determined. As the temperature increased from 133 to 573 K, the emission color of Zr-BDC:Eu³⁺,Tb³⁺ nanothermometers without the presence of Y³⁺ ions changed from orange to red, while for YZ-BDC:Eu³⁺,Tb³⁺ nanothermometers, the emission color changed from yellow to orange, due to the strong effect of the presence of Y³⁺ ions on the luminescence intensity of Eu³⁺ and Tb³⁺ ions. The maximum relative sensitivity (S_Rmax) in both materials was close to 0.5%/K, however, the temperature range of their occurrence was significantly shifted toward higher temperatures due to doping with Y³⁺ ions. The obtained results showed that doping with Y³⁺ ions not only enables the modulation of the useful temperature range with high relative sensitivity, but also provides improved thermal stability.

In the present work, nanothermometers based on amorphous zirconium metal–organic frameworks co-doped with rare-earth ions (YZ-BDC:Eu³⁺,Tb³⁺) with sizes of about 10–30 nm were successfully synthesized via a microwave-assisted hydrothermal method at 120 °C for 15 min.     

Construction of a luminescent sensor based on a lanthanide complex for the highly efficient detection of methyl parathion

A highly sensitive and selective luminescent sensor for the detection of methyl parathion (MP) pesticide was described in this study. The target molecule HL was synthesized by modifying the structure of 4-hydroxybenzlidene imidazolinone (HBI) with nitrogen-containing heterocyclic 1,10-phenanthroline. In the presence of Eu³⁺, a HL–Eu³⁺ complex was formed which could emit strong red fluorescence due to the removal of coordinated water molecules and an intramolecular energy transfer from HL to Eu³⁺. Addition of MP into the strongly fluorescent solution of HL–Eu³⁺ induced quenching of the complex''s fluorescence, and this quenching behavior occurred because of the competition coordination of MP and HL for Eu³⁺. A calibration curve was developed that related the extent of fluorescence quenching to MP concentration, making the HL–Eu³⁺ system a sensitive and selective fluorescent sensor for MP. Under the experimental conditions, the detection limit for MP was down to 95 nM based on LOD = 3σ/S. Moreover, the fluorescence assay developed here allowed the detection of MP in two different types of real samples including pond water and pear juice, and satisfactory results demonstrate that this fluorescent sensor based on HL–Eu³⁺ has potential application in environment and food analysis.

A lanthanide complex sensor HL–Eu³⁺ based on an aromatic cyclic polyamine ligand was constructed for MP detection.     

Luminescence properties and energy transfer of K3LuF6:Tb3+,Eu3+ multicolor phosphors with a cryolite structure

In recent years, compounds with a cryolite structure have become excellent hosts for luminescent materials. In this paper, Tb³⁺ doped and Tb³⁺/Eu³⁺ co-doped K₃LuF₆ phosphors were prepared via a high temperature solid phase sintering method. The XRD, SEM, as well as photoluminescence excitation (PLE) and emission (PL) spectra were measured to investigate the structure and luminescence properties of the as-prepared samples. In the Tb³⁺/Eu³⁺ co-doped K₃LuF₆ samples, both characteristic emission spectra of Tb³⁺ and Eu³⁺ could be observed and the emission color of the K₃LuF₆:0.12Tb³⁺,xEu³⁺ phosphors could be adjusted from green to yellowish pink and the corresponding CIE values could be regulated from (0.2781, 0.5407) in the green area to (0.4331, 0.3556) in the yellowish pink area by controlling the concentration ratio of Eu³⁺/Tb³⁺. In addition, the energy transfer mechanism in Tb³⁺/Eu³⁺ co-doped K₃LuF₆ was calculated to be a quadrupole–quadrupole interaction from Tb³⁺ to Eu³⁺ based on the Dexter''s equation.

Single-phase multicolor phosphors with a cryolite structure were obtained via energy transfer from Tb³⁺ to Eu³⁺.     

Ratiometric rapid distinction of two structurally similar fluoroquinolone antibiotics by a Tb/Eu hydrogel

Norfloxacin and ofloxacin are two frequently prescribed second-generation fluoroquinolone antibiotics with an identical 4-quinolone chromophore and hence, are difficult to distinguish by conventional methods (UV or fluorescence). We have designed a Tb³⁺/Eu³⁺/cholate cocktail that enabled us to differentiate these two drugs and rapidly measure their concentrations when present together. Additionally, a Tb³⁺-cholate gel-based paper sensor was developed to detect and quantify them in a single drug containing system with a limit of detection (LOD) well below 100 nM.

Rapid distinction of structurally similar fluoroquinolone antibiotics norfloxacin and ofloxacin, and their quantification in a mixture, were achieved using a supramolecular Tb/Eu gel cocktail.

Fluoroquinolone antibiotics belong to one of the most important classes of human and veterinary medicines owing to their broad-spectrum activity and effective oral absorption. Norfloxacin and ofloxacin (Fig. 1) are among the most prescribed second generation fluoroquinolone antibiotics^1,2 used for the treatment of respiratory and urinary tract infections, ocular and skin infections, pelvic inflammatory disease, gonococcal urethritis, infectious diarrhoea, etc. However, potential adverse reactions such as tendinitis, tendon rupture, arthralgia, myalgia, peripheral neuropathy, hematuria, and central nervous system effects due to their excess intake have been well documented in the literature.³ Extensive use and misuse of the antibiotics as veterinary medicines have led to their appearance in milk,^4,5 chicken⁶ and fish⁷ imposing adverse side effects for the consumer. Contamination of municipal wastewater, surface water and even groundwater results from their extensive usage, and from their excretion from human and animals unchanged.⁸ All these can cause the propagation of antibiotic-resistant micro-organisms⁹ leading to a disturbed ecosystem. Therefore, the development of a robust, rapid and straightforward analytical protocol for screening of Norfloxacin (NFLX) and Ofloxacin (OFLX) residues in biological fluids, sewage water, edible tissues and foodstuff is highly desirable for the control of their appropriate dosage, maintaining their pharmacokinetics and avoid environmental crisis.¹⁰ Various analytical methods for this purpose have been reported in the literature including spectrophotometry,¹¹ spectrofluorimetry,¹² voltammetry,^13,14 capillary electrophoresis,¹⁵ chemiluminescence,^16,17 and high performance liquid chromatography (HPLC).¹⁸ Several fluorescent probes have utilized molecular imprinting chemiluminescence,¹⁹ Neodymium-modified micellar media,²⁰ carbon dots etc.^21,22 Although fluorescence-based reporters provide greater intrinsic sensitivity, many of them are, however, incompatible with biological samples because of complications arising from background scattering and autofluorescence from endogenous components.²³ Trivalent lanthanide ions have long radiative lifetimes allowing background-free measurement using time-gating. They also have sharp, well-defined long-wavelength emission fingerprints²⁴ ideal for biological applications and can therefore serve as a strong sensing platform. They can be sensitized by proximate chromophores (‘‘antennae’’) and may act as attractive fluorogenic (turn on) and fluorolytic (turn-off) bio-probes. This group has extensively investigated photoluminescent lanthanide cholate supramolecular hydrogels^25–32 by utilizing their in-built hydrophobic pockets to encapsulate small organic ‘antennas’. The self-assembly brings the antenna in close proximity to the acceptor lanthanide leading to energy transfer to the lanthanide ion. This strategy thus eliminates the need for the synthesis of antenna-linker-ligand type of probes to achieve lanthanide sensitization. A variety of lanthanide-luminescence based fluoroquinolone-assays^17,33–38 have been reported in the literature. However, tedious synthesis of the probe, complex sample treatments, non-aqueous detection media, strict maintenance of pH have restricted the practical applicability of many of them.^34–37 We have found that when doped in micromolar concentrations in Tb-Ch (5/15 mM) or Eu-Ch (5/15 mM) gels, both Norfloxacin and Ofloxacin significantly enhanced the luminescence intensities. Based on this finding we explored cholate-hydrogel based detection and quantification of these two fluoroquinolones. Initial experiments were performed in Tb (5 mM) cholate (15 mM) hydrogel by doping it with increasing concentrations of norfloxacin (NFLX). The emission intensity of Tb³⁺ showed a linear relationship with concentration (Fig. S1 in ESI^†). Even 4 μM of NFLX resulted in 30-fold enhancement of sensitization of Tb³⁺ luminescence. Atomic force microscopy showed that the fibrous network of Tb³⁺-cholate gel remained unaltered (Fig. S2 in ESI^†) when NFLX was doped in micromolar concentrations, proving the robustness and stability of the sensing gel system. Fluorescence microscopic image of semi-dried NFLX doped Tb-cholate gel (Fig. S2c in ESI^†) showed uniform green luminescent gel fibres, indicating that the NFLX was distributed and embedded on the hydrophobic fibres, promoting the energy transfer process making the fibres green luminescent.Open in a separate windowFig. 1Structures of Norfloxacin and Ofloxacin.In order to simplify the sensing method, we chose a paper-based system. Such sensors^39–41 are affordable, biodegradable, sensitive, specific, user-friendly, rapid, robust, and deliverable to end-users. For this purpose, Tb-cholate (5/15 mM) gel doped with NFLX (4 μM) was prepared, sonicated for 5–6 s to reduce the viscosity, and then applied (20 μL) on a 3 mm paper disc (Fig. 2). SEM images showed dried flakes of gel on the cellulose fibre network (Fig. S3 in ESI^†). The thickness of the gel layer coated on the paper surface was about 8 μm, as determined by tilt-SEM (Fig. S3c in ESI^†). Time gated fluorescence (TRF) was measured on the coated discs using a plate reader. After 5 min of drying, a significantly higher Tb³⁺ luminescence output was observed, compared to that measured in wet gels (Fig. 3b, 8 vs. 7). This increase may be attributed to the efficient wicking of water molecules from the gel in the hydrophilic cellulose frameworks of paper which in turn helps reduce the OH-mediated luminescence quenching.⁴² Increased sensitization on gel-coated paper discs was observed for both NFLX and OFLX. We also confirmed that the extent of sensitization of Tb³⁺ remained constant as a function of time (Fig. S4 in ESI^†). The limit of detection (LOD) for NFLX and OFLX were measured based on Tb³⁺-luminescence output as a function of the drug concentration and were found to be 13.6 nM (Fig. S5a in ESI^†) and 67 nM (Fig. S5b in ESI^†), respectively. The paper-based method was used to quantify the NFLX spiked in milk samples (cow raw milk and a commercial homogenized toned milk) and in human blood serum. The LOD values measured were 68 nM, 100 nM and 100 nM, respectively (Fig. S12 and S13 in SI^†). The time-gated detection method eliminated any interference from the matrix (serum or milk). This method is therefore simple, rapid, sensitive, autofluorescence and background emission free, and may thus serve as a practical tool for analytical screening.Open in a separate windowFig. 2Design of paper-based sensors (paper disc cutting, placing them on plate groove, gel dropcasting).Open in a separate windowFig. 3Tb³⁺ -luminescence (λ_em 545 nm, λ_ex 330 nm) enhancement by NFLX (4 μM) (Ac, Ch, D: acetate solution, cholate matrix and coated paper disc, respectively).Distinguishing two structurally similar compounds of the same class is a fundamental objective of analytical research.Dennany et al. recently reported a [Ru(bpy)₃]²⁺ based ECL sensor⁴³ to distinguish Atropine and Scopolamine, two similar alkaloids. Encouraged by this report, we focused on the differentiation of norfloxacin and ofloxacin which have comparable 4-quinolone chromophores with similar functional groups attached (Fig. 1). With both drugs, the luminescence enhancement for Tb³⁺ was greater than that for Eu³⁺ (Fig. S8 & S9 in ESI^†). Therefore, single lanthanide (Tb³⁺ or Eu³⁺) derived gels cannot differentiate them. For this purpose, heterobimetallic ensemble strategy was explored by adopting the ratiometric detection technique.^44,45 In order to identify optimum conditions, a set of five cholate gels samples containing varying ratios of Tb³⁺ & Eu³⁺ (maintaining total concentration of 5 mM) was prepared. This set was doped with 1 μM NFLX. An identical set was prepared with 1 μM OFLX as the dopant. After drop casting the gels on paper discs, luminescence intensities were quantified on a plate reader (Fig. 4a and b). From these data, the sensitization bias I_Tb/I_Eu, as measured by I₅₄₅/I₆₁₇ (λ_ex 330 nm), was calculated and plotted against the concentration of Tb³⁺ in the mixed gel. Tb³⁺/Eu³⁺ cocktail with [Tb³⁺] ≥ 2.5 mM doped with NFLX showed sensitization bias (I₅₄₅/I₆₁₇) values greater than 1, whereas with OFLX the bias was always less than 1 (Fig. 4c). Based on these observations we chose Tb³⁺/Eu³⁺/cholate (4.5 mM/0.5 mM/15 mM) cocktail as the probe to differentiate these two drugs over a wider concentration range (Fig. S15 in ESI^†). Another set of measurements were made at low μM concentrations of the drugs (Fig. S14 in ESI^†). The Terbium sensitization bias (I_Tb/I_Eu) was calculated and plotted against the concentration of FLXs and found to be greater than 1 for NFLX but less than 1 for OFLX (Fig. S14c in ESI^†). In the Tb³⁺/Eu³⁺ mixed cholate hydrogel matrix three concurrent energy transfer processes can occur–energy transfer from sensitizer (i) to Tb³⁺, (ii) to Eu³⁺, and (iii) energy transfer from Tb³⁺ to Eu³⁺. While one of the emission bands of Tb³⁺ has overlap with the most intense 617 nm peak of Eu³⁺, the Eu³⁺ emission peak at 690 nm is solely contributed by Eu³⁺. The 690 nm emission intensity in 15 μM NFLX doped mixed gels was measured and found to linearly increase with the Tb³⁺ concentration (even though Eu³⁺ concentration is decreasing) indicating Tb³⁺– Eu³⁺ energy transfer (Fig. S19 in ESI^†). Significant reduction of Tb³⁺-luminescence lifetime from 1.53 ms in NFLX doped Tb³⁺-Ch (4.5 mM/15 mM) gel to 0.55 ms in NFLX doped Tb³⁺/Eu³⁺-Ch (4.5 mM/0.5 mM/15 mM) gel provided additional evidence. In the Tb³⁺/Eu³⁺/cholate (4.5 mM/0.5 mM/15 mM) cocktail, the three concurrent energy transfer processes are optimized to show Tb3+/Eu3+ emission intensity ratio exactly reversed for the two drugs (Fig. S14, S16 in ESI^†). The optimized cocktail was subsequently used to determine the ratio of the two drugs in a mixture. The drug mixtures (total concentration 20 μM) were doped in the cocktail and the photophysical behavior was investigated. The excitation spectra corresponding to Eu³⁺ 617 nm emission showed a unique pattern depending on the ratio of NFLX and OFLX in the mixture. The intensity (Fig. 5a) at 280 nm reflects NFLX excitation, whereas the intensity at 300 nm reflects OFLX excitation. Based on these findings, the Eu³⁺ emission intensity at 617 nm from the drug mixtures at two different excitation wavelengths 280 nm and 300 nm were evaluated. Ratiometric response I₆₁₇ (λ_ex 280)/I₆₁₇(λ_ex 300) was found to linearly increase with the ratio of NFLX concentration to the total concentrations of both the drugs in the mixture (Fig. 5c). To validate the need for using the cocktail in this study, the excitation spectra corresponding to Eu³⁺ at 617 nm in (NFLX + OFLX) doped Eu³⁺-Ch gel (no Tb) was recorded in which no such NFLX & OFLX ratio-dependent pattern was observed (Fig. S17 in ESI^†). For further exploration of the cocktail, Terbium emission sensitization bias was calculated and found to be directly proportional to the ratio of NFLX concentration to the total concentrations of both the drugs in the mixture (Fig. 6b). This protocol can thus be used as a platform to determine the ratio and individual concentration of the two drugs when present in a mixture.Open in a separate windowFig. 4Emission intensities from Tb³⁺ at 545 nm and Eu³⁺ at 617 nm (λex 330 nm of 1 μM (a) NFLX and (b) OFLX doped mixed cholate {[Tb³⁺] + [Eu³⁺] = 5 mM} gel coated discs for varying ratios of Tb³⁺& Eu³⁺ (c) Terbium sensitization bias I₅₄₅/I₆₁₇ (λ_ex 330 nm) as a function of Tb³⁺ concentration.Open in a separate windowFig. 5(a) Excitation spectra for Eu³⁺ emission (λ_em 617 nm) with respect to the ratio of two drugs (b) Intensity of Eu³⁺ emission at 617 nm for two excitation wavelengths (280 nm, 300 nm) in Tb³⁺/Eu³⁺/cholate (4.5 mM/0.5 mM/15 mM) gel coated discs with increasing concentrations of NFLX in a mixture of OFLX & NFLX, (c) intensity ratio plotted against the concentration of NFLX in the mixture.Open in a separate windowFig. 6(a) Tb³⁺ emission at 545 nm and Eu³⁺ emission 617 nm (b) Tb³⁺ sensitization bias in Tb³⁺/Eu³⁺/cholate (4.5 mM/0.5 mM/15 mM) gel coated discs with the ratio of NFLX concentration to the total drug concentration in the mixture (λ_ex 330 nm).In conclusion, a hetero bimetallic ensemble Tb³⁺/Eu³⁺/cholate was designed for the differentiation of structurally similar NFLX and OFLX. Till date, only physical separation by HPLC is known to quantify norfloxacin and ofloxacin concentrations in a mixture. Additionally, a paper-based sensor has also been developed for the detection and quantification of them in single drug containing system with a lower nanomolar range. The method allowed autofluorescence free detection with biological samples without the need of deproteinization and sample processing. Nonetheless, our preliminary investigations employed to utilize ratiometric bis-lanthanide strategy will serve as the foundation for the fundamental need to differentiate structurally similar molecules. Further studies to utilize this supramolecular paper-based methodology in real diagnostic fields is underway in our lab.     

Ce3+ and Tb3+ doped Ca3Gd(AlO)3(BO3)4 phosphors: synthesis,tunable photoluminescence,thermal stability,and potential application in white LEDs

Novel blue-green-emitting Ca₃Gd(AlO)₃(BO₃)₄:Ce³⁺,Tb³⁺ phosphors were successfully synthesized via traditional high temperature solid reaction method. X-ray diffraction, luminescence spectroscopy, fluorescence decay time and fluorescent thermal stability tests have been used to characterize the as-prepared samples. The energy transfer from Ce³⁺ to Tb³⁺ ions in the Ca₃Gd(AlO)₃(BO₃)₄ host has been demonstrated to be by dipole–dipole interaction, and the energy transfer efficiency reached as high as 83.6% for Ca₃Gd_0.39(AlO)₃(BO₃)₄:0.01Ce³⁺,0.6Tb³⁺. The critical distance was calculated to be 9.44 Å according to the concentration quenching method. The emission colour of the obtained phosphors can be tuned appropriately from deep blue (0.169, 0.067) to green (0.347, 0.494) through increasing the doping concentrations of Tb³⁺. Moreover, the Ca₃Gd_0.39(AlO)₃(BO₃)₄:0.01Ce³⁺,0.6Tb³⁺ phosphor possessed excellent thermal stability at high temperature, and the emission intensity at 423 K was about 87% of that at 303 K. Finally, the fabricated prototype LED device with a BaMgAl₁₀O₇:Eu²⁺ blue phosphor, CaAlSiN₃:Eu²⁺ red phosphor, Ca₃Gd_0.39(AlO)₃(BO₃)₄:0.01Ce³⁺,0.6Tb³⁺ green phosphor and 365 nm-emitting InGaN chip exhibited bright warm white light. The current study shows that Ca₃Gd_0.39(AlO)₃(BO₃)₄:0.01Ce³⁺,0.6Tb³⁺ can be used as a potential green phosphor for white LEDs.

Novel thermal-stable blue-green-emitting Ca₃Gd(AlO)₃(BO₃)₄:Ce³⁺,Tb³⁺ phosphors were developed for near-ultraviolet-excited white LEDs.     

Luminescence characteristics of rare-earth-doped barium hexafluorogermanate BaGeF6 nanowires: fast subnanosecond decay time and high sensitivity in H2O2 detection

Fluorides are promising host materials for optical applications. This paper reports the photoluminescent (PL) and cathodoluminescent (CL) characteristics of barium hexafluorogermanate BaGeF₆ nanowires codoped with Ce³⁺, Tb³⁺ and Sm³⁺ rare earth ions, produced by a solvothermal route. The synthesized BaGeF₆ nanowires exhibit uniform morphology and size distribution. X-ray diffraction divulges the one-dimensional growth of crystalline BaGeF₆ structure, with the absence of any impurity phases. Visible luminescence is recorded from the nanowires in green and red regions, when the nanowires are codoped with Ce³⁺/Tb³⁺, and Ce³⁺/Tb³⁺/Sm³⁺, respectively, under a UV excitation source. The PL emission from the codoped BaGeF₆ nanowires, when excited by a 254 nm source, originates from the efficient energy transfer bridges between Ce³⁺, Tb³⁺ and Sm³⁺ ions. The decay time of the visible luminescent emission from the nanowires is in the order of subnanoseconds, being one of the shortest decay time records from inorganic scintillators. The CL emission from the BaGeF₆ nanowires in the tunable visible range reveals their potential use for the detection of high-energy radiation. The PL emissions are sensitive to H₂O₂ at low concentrations, enabling their high-sensitivity detection of H₂O₂ using BaGeF₆ nanowires. A comparison with BaSiF₆ nanowires is made in terms of decay time and its sensitivity towards H₂O₂.

The decay time of BaGeF₆ nanowires codoped with rare earths is found in the order of subnanoseconds, being one of the shortest decay time records from inorganic scintillators. Their luminescence emissions are highly sensitive for H₂O₂ detection.     

Tunable luminescence evolution and energy transfer behavior of Na3Sc2(PO4)3:Ce3+/Tb3+/Eu3+ phosphors

A series of color-tunable emitting Na₃Sc₂(PO₄)₃:Ce³⁺/Tb³⁺/Eu³⁺ (NSPO) phosphors were prepared by a combination of hydrothermal synthesis and low temperature calcination. The phase structure, photoluminescence and energy transfer properties of the samples were studied in detail. The tunable colors were obtained by co-doping the Tb³⁺ ions into the NSPO:Ce³⁺ or NSPO:Eu³⁺ phosphors with varying concentrations. Under UV excitation, the energy transfers from Tb³⁺ to Eu³⁺ in the NSPO host occurred mainly via a dipole–dipole mechanism, and the critical distances of the ion pairs (R_c) was calculated to be 17.94 Å by the quenching concentration method. And that, the emission colors of the NSPO:Tb³⁺,Eu³⁺ phosphors could be adjusted from green through yellow to red because of the energy transfer from Tb³⁺ to Eu³⁺. Based on its good photoluminescence properties and abundant emission colors, the NSPO:Ce³⁺/Tb³⁺/Eu³⁺ phosphors might be promising as potential candidates for solid-state lighting and display fields.

A series of color-tunable emitting Na₃Sc₂(PO₄)₃:Ce³⁺/Tb³⁺/Eu³⁺ (NSPO) phosphors were prepared by a combination of hydrothermal synthesis and low temperature calcination.     

The coordination chemistry of benzhydrazide with lanthanide(iii) ions: hydrothermal in situ ligand formation,structures, magnetic and photoluminescence sensing properties

The influence of synthetic conditions on the solid-state structural formation of lanthanide(iii) complexes based on a hydrazide ligand have been investigated and reported. Depending on the solvents and reaction temperatures, the reactions of hydrated Ln(NO₃)₃ with a benzohydrazide (bzz) ligand afforded three classes of lanthanide(iii) coordination complexes viz. [Ln(bzz)(NO₃)](NO₃)₂ (1Ln; Ln = Sm (1), Eu (2), Gd (3), Tb (4), Dy (5)), [Ln(bzz)(ben)₃(H₂O)]·H₂O (2Ln; Ln = Pr (6), Nd (7), Sm (8), Eu (9), Gd (10), Tb (11), Dy (12), Er (13)), and [Ln₃(ben)₃] (3Ln; Ln = Eu (14), Gd (15), Tb (16), Dy (17), Er (18), Tm (19), Yb (20), Lu (21)). Complexes 1–5 in series 1Ln were isolated by slow evaporation of their isopropanol solutions at ambient temperature, and the complexes display similar discrete structures bearing distinct intermolecular N–H⋯O hydrogen bonds to generate a three-dimensional (3D) supramolecular architecture. Complexes 6–13 in series 2Ln were obtained under hydrothermal conditions at 110 °C where the in situ generated benzoate (ben) ligands participated in the formation of one-dimensional (1D) coordination polymers (CPs) with the bzz ligands. At a temperature of 145 °C the hydrothermal conditions result in the formation of the thermodynamically more stable products of 14–21 in series 3Ln, in which the bzz ligand underwent complete in situ hydrolysis to create the ben ligand. These coordination assemblies feature 1D zigzag chains that are formed by unusual low coordination numbers of the six- and seven-fold coordinated Ln³⁺ centers bridged by the ben ligands in μ₂- and μ₃-coordination modes. Notably, the chain structures of 2Ln can be transformed into the zigzag tape-like structures of 3Ln upon heating the crystalline samples to 400 °C in air. In the solid state at room temperature, the Eu- (2, 9, 14) and Tb- (4, 11, 16) containing complexes emit red and green light, respectively. The luminescence investigations show that the Eu- (9, 14) and Tb-(11, 16) based CPs could be used as fluorescent probes for acetone and Co²⁺ ions via an energy competition mechanism. Meanwhile, the Gd- (10, 15) and Dy- (12, 17) based CPs show typical antiferromagnetic interactions.

The influence of synthetic conditions on the solid-state structural formation of lanthanide(iii) complexes based on a hydrazide ligand have been investigated and reported.     

A new family of decanuclear Ln7Cr3 clusters exhibiting a magnetocaloric effect

Two dimeric Ln–Cr clusters with formula {Ln(H₂O)₈[Ln₆Cr₃(L)₆(CH₃COO)₆(μ₃-OH)₁₂(H₂O)₁₂]}·(ClO₄)₆·xH₂O (Ln = Gd, x = 35 for 1 and Ln = Dy, x = 45 for 2, HL = 2-pyrazinecarboxylic acid) were obtained by a ligand-controlled hydrolytic method with a mixed ligand system (2-pyrazinecarboxylic acid and acetate). Single crystal structure analysis showed that two trigonal bipyramids of [Gd₃Cr₂(μ₃-OH)₆]⁹⁺ worked as building blocks in constructing the metal-oxo cluster core of [Gd₆Cr₃(μ₃-OH)₁₂]¹⁵⁺ by sharing a common top – a Cr³⁺ ion. Additionally, compound 1 forms a three-dimensional framework with a one-dimensional nanopore channel along the a-axis through a hydrogen-bond interaction between the cationic cluster core and the free mononuclear cation [Gd(H₂O)₈]³⁺ and the π-bond interactions of the pyrazine groups on the two cationic cluster cores. Magnetic calculations indicated a weak ferromagnetic coupling interaction for Gd⋯Gd and Gd⋯Cr in compound 1, with its magnetic entropy change (−ΔS_m) reaching 21.1 J kg⁻¹ K⁻¹ at 5 K, 7 T, while compound 2 displayed an obvious frequency-dependency at H_dc = 2000 Oe.

Two decanuclear Ln–Cr clusters Ln₇Cr₃ were obtained, which formed a three-dimensional framework with one-dimensional nanopore channel through hydrogen-bond and π-bond interactions. Gd₇Cr₃ had a magnetic entropy change of 21.1 J kg⁻¹ K⁻¹ at 5 K, 7 T.