EuIII and TbIII upconversion intermediated by interparticle energy transfer in functionalized NaLnF4 nanoparticles

Lanthanide (LnIII)-doped sodium gadolinium tetrafluoride (NaGdF4) nanoparticles have been excelled as attractive upconversion systems for anti-counterfeiting or energy conversion for instance, with a special interest in the visible upconversion of EuIII and TbIII. The core@shell architecture has enabled the bright upconversion of EuIII and TbIII in this matrix by interfacial energy transfer sensibilized by the TmIII/YbIII pair. Another approach to enable EuIII and TbIII upconversion could be the interparticle energy transfer (IPET) between LnIII-doped sensitizer and acceptor nanoparticles. Yet, the low molar absorptivity of the LnIII through 4f ↔ 4f electronic transitions and the large distance between the nanoparticles are shortcomings that should decrease the energy transfer efficiency. On the other hand, it is feasible to predict that the association of organic ligands displaying large molar absorptivity on the acceptor nanoparticle surface could help to overcome the absorption limitation. Inspired by this exciting possibility, herein, we present the EuIII/TbIII upconversion intermediated by IPET between the donor TmIII, YbIII-doped NaGdF4 nanoparticle and the acceptor LnIII-doped NaGdF4 (Ln = Eu and/or Tb) nanoparticles functionalized with a series organic ligands on the surface (tta- = thenoyltrifluoroacetonate, acac- = acetylacetonate, or 3,5-bbza- = 3,5-dibromebenzoate). Either in solid state or in suspension, upon excitation at 980 nm, visible EuIII/TbIII upconversion could be observed. This emission comes from the absorption of the TmIII, YbIII pair in the donor nanoparticle, followed by IPET from the TmIII excited levels to the ligand singlet/triplet states on the acceptor nanoparticle surface, ligand-to-EuIII/TbIII energy transfer, and upconversion emission. Spectroscopic evidences from the analysis of the donor level lifetimes indicate the contribution of non-radiative energy transfer for the IPET mechanism; the radiative mechanism also contributes for the IPET. Moreover, the design herein introduced enables the development of luminescence temperature probes with relative thermal sensitivity as high as 1.67% K-1 at 373 K. Therefore, this new upconversion pathway opens an avenue of possibilities in an uncharted territory to tune the visible upconversion of LnIII ions.

Lanthanide molecular cluster-aggregates as the next generation of optical materials

In this perspective, we provide an overview of the recent achievements in luminescent lanthanide-based molecular cluster-aggregates (MCAs) and illustrate why MCAs can be seen as the next generation of highly efficient optical materials. MCAs are high nuclearity compounds composed of rigid multinuclear metal cores encapsulated by organic ligands. The combination of high nuclearity and molecular structure makes MCAs an ideal class of compounds that can unify the properties of traditional nanoparticles and small molecules. By bridging the gap between both domains, MCAs intrinsically retain unique features with tremendous impacts on their optical properties. Although homometallic luminescent MCAs have been extensively studied since the late 1990s, it was only recently that heterometallic luminescent MCAs were pioneered as tunable luminescent materials. These heterometallic systems have shown tremendous impacts in areas such as anti-counterfeiting materials, luminescent thermometry, and molecular upconversion, thus representing a new generation of lanthanide-based optical materials.

Tuning the Thermometric Features in 1D Luminescent EuIII and TbIII Coordination Polymers through Different Bridge Phosphine Oxide Ligands

Tb^III and Eu^III systems have been investigated as ratiometric luminescent temperature probes in luminescent coordination polymers due to Tb^III → Eu^III energy transfer (ET). To help understand how ion-ion separation, chain conformation as well as excitation channel impact their thermometric properties, herein, [Eu(tfaa)₃(μ-L)Tb(tfaa)₃]_n one-dimensional (1D) coordination polymers (tfaa^- = trifluoroacetylacetonate, and L = [(diphenylphosphoryl)R](diphenyl)phosphine oxide, R = ethyl - dppeo - or butyl - dppbo) were synthesized. The short μ-dppeo bridge ligand leads to a more linear 1D polymeric chain, while the longer μ-dppbo bridge leads to tighter packed chains. As the temperature rises from 80 K, upon direct Tb^III excitation at 488 nm, the Tb^III emission intensity decreases, while the Eu^III emission intensity increases after 160 and 200 K when L = dppeo or dppbo, respectively. The temperature-dependent emission intensities, due to Tb^III → Eu^III ET, enable the development of ratiometric luminescent temperature probes featuring maximum relative thermal sensitivity up to 3.8% K^-1 (250 K, L = dppbo, excitation at 488 nm). On the other hand, the same system displays maximum thermal sensitivity up to 3.5% K^-1 (323 K) upon ligand excitation at 300 nm. Thus, by changing the excitation channel and bridge ligand that leads to modification of the polymer conformations, the maximum relative thermal sensitivity can be tuned.

Designing Optical Thermometers Using Down/Upconversion Ca14Al10Zn6O35: Ti4+, Eu3+/Yb3+, Er3+ Thermosensitive Phosphors

Down/upconversion Ca₁₄Al₁₀Zn₆O₃₅ inorganic phosphors codoped with Ti⁴⁺/Eu³⁺ or Yb³⁺/Er³⁺ were prepared. The crystal structure and downconversion luminescence properties of Ca₁₄Al₁₀Zn₆O₃₅:Ti⁴⁺, Eu³⁺ phosphors were studied in detail. Ti⁴⁺ and Eu³⁺ occupied Al³⁺ and Ca²⁺ sites in the host lattice, respectively. Under the excitation of 273 nm, the emission peak in the 300-570 nm band originated from the ²T₂ → O^2- transition of Ti⁴⁺. The f-f transition of Eu³⁺ ions generated multiple peaks in the 570-800 nm range. The emission intensity of Ti⁴⁺ and Eu³⁺ ions can be used as a fluorescence intensity ratio (FIR) signal. Based on the FIR technology, the maximum relative sensitivity (S_r) and the minimum temperature uncertainty (δT) reached 1.41% K^-1 and 0.07 K, respectively. Meanwhile, the temperature-sensing behaviors were explored by the temperature-dependent upconversion spectra of Er³⁺- and Yb³⁺-codoped Ca₁₄Al₁₀Zn₆O₃₅ phosphors. Based on the fluorescence intensity ratio of thermal coupling levels (Er³⁺:²H_11/2/⁴S_3/2), the maximum S_r and minimum δT of upconversion phosphors reached 1.28% K^-1 and 0.08 K in the temperature range of 293-473 K, respectively. Ca₁₄Al₁₀Zn₆O₃₅:Ti⁴⁺/Eu³⁺ (Yb³⁺/Er³⁺) phosphors realize temperature sensors with higher relative sensitivity, and it is a good candidate material for optical temperature measurement.

Phonon-assisted molecular upconversion in a holmium(III)-based molecular cluster-aggregate

Upconversion (UC) is a fascinating process in which higher energy photons can be emitted from excitation by lower energy photons. The current challenge remains in downscaling and effectively achieving upconversion with lanthanide ions at the molecular scale. Here, using a rationally designed molecular cluster-aggregate (MCA), we demonstrate for the first time Ho^III ion molecular upconversion. The synthesized MCA exhibits identifiable Ho^III green and red UC emissions with a uniquely enhanced red to green ratio as well as a conventional near-infrared (NIR) emission. A combined rigid spherical cluster core with reduced molecular vibrations, ideally matched donor and acceptor excited levels via a phonon-assisted mechanism, led to an upconversion quantum yield of 5.24 × 10^-6%.

Phytoglycogen Encapsulation of Lanthanide-Based Nanoparticles as an Optical Imaging Platform with Therapeutic Potential

Lanthanide-based upconverting nanoparticles (UCNPs) are largely sought-after for biomedical applications ranging from bioimaging to therapy. A straightforward strategy is proposed here using the naturally sourced polymer phytoglycogen to coencapsulate UCNPs with hydrophobic photosensitizers as an optical imaging platform and light-induced therapeutic agents. The resulting multifunctional sub-micrometer-sized luminescent beads are shown to be cytocompatible as carrier materials, which encourages the assessment of their potential in biomedical applications. The loading of UCNPs of various elemental compositions enables multicolor hyperspectral imaging of the UCNP-loaded beads, endowing these materials with the potential to serve as luminescent tags for multiplexed imaging or simultaneous detection of different moieties under near-infrared (NIR) excitation. Coencapsulation of UCNPs and Rose Bengal opens the door for potential application of these microcarriers for collagen crosslinking. Alternatively, coloading UCNPs with Chlorin e6 enables NIR-light triggered generation of reactive oxygen species. Overall, the developed encapsulation methodology offers a straightforward and noncytotoxic strategy yielding water-dispersible UCNPs while preserving their bright and color-tunable upconversion emission that would allow them to fulfill their potential as multifunctional platforms for biomedical applications.

Synthesis of ZrO2:Pr3+,Gd3+ nanocrystals for optical thermometry with a thermal sensitivity above 2.32% K-1 over 270 K of sensing range

Nowadays, there is enthusiastic effort to develop luminescent thermometers used for remote and high-sensitivity temperature readout over a wide sensing range. Herein, Pr³⁺ and Gd³⁺ co-doped ZrO₂ nanocrystals are designed, prepared and investigated by XRD, Raman spectroscopy, XPS, TEM, EDS, DRS, PLE and PL spectroscopy. Upon 275 nm irradiation, the PL spectrum of ZrO₂:Pr³⁺,Gd³⁺ is found to be composed of a narrow emission peak at 314 nm (Gd^{3+ 6}P_7/2-⁸S_7/2), a broad defect-related emission band at 400 nm, and several emission peaks in the wavelength region of 585-700 nm (Pr^{3+ 1}D₂-³H₄, ³P₀-³H₆, and ³P₀-³F₂), which exhibit different thermal responses owing to the effects of the various non-radiative relaxation processes and trap energy levels. Accordingly, the luminescence intensity ratio (LIR) between the Pr^{3+ 1}D₂-³H₄ and Gd^{3+ 6}P_7/2-⁸S_7/2 transitions demonstrates excellent relative sensing sensitivity values ((2.32 ± 0.01)% K^-1-(8.32 ± 0.05)% K^-1) and low temperature uncertainties (0.08 K-0.28 K) over a wide temperature sensing range of 303 K to 573 K, which are remarkably better than those of many other luminescence thermometers. What is discussed in the present study may be conducive to broadening the research region of RE³⁺ doped luminescence thermometric phosphors, especially for materials with rich 4f-4f transition lines and defect-related luminescence.

Dual magnetic field and temperature optical probes of controlled crystalline phases in lanthanide-doped multi-shell nanoparticles

The engineering of core@multi-shell nanoparticles containing heterogeneous crystalline phases in different layers constitutes an important strategy for obtaining optical probes. The possibility of obtaining an opto-magnetic core@multi-shell nanoparticle capable of emitting in the visible and near-infrared ranges by upconversion and downshifting processes is highly desirable, especially when its optical responses are dependent on temperature and magnetic field variations. This work proposes the synthesis of hierarchically structured core@multi-shell nanoparticles of heterogeneous crystalline phases: a cubic core containing Dy^III ions responsible for magnetic properties and optically active hexagonal shells, where Er^III, Yb^III, and Nd^III ions were distributed. This system shows at least three excitation energies located at different biological windows, and its emission intensities are sensitive to temperature and external magnetic field variations. The selected crystalline phases of the core@multi-shell nanoparticles obtained in this work is fundamental to the development of multifunctional materials with potential applications as temperature and magnetic field optical probes.

Room-Temperature Upconversion in a Nanosized {Ln15} Molecular Cluster-Aggregate

The successive absorption of low-energy photons to the accumulation of the intermediate excited states leading to higher energy emission is still a challenge in molecular architectures. Contrary to low-phonon solids and nanoparticles, the rational construction of molecular systems containing an excess of donor atoms in relation to acceptor ones is far from trivial. Moreover, the vibrations caused by high-energy oscillators commonly present on coordination compounds result in serious drawbacks on molecular upconversion. To overcome these limitations, we demonstrate that upconversion can be achieved even at room temperatures through the use of molecular cluster-aggregates (MCAs). To achieve the upconverted emission, we synthesized a MCA containing 15 lanthanide ions, {Er₂Yb₁₃}, ensuring an excess of donor atoms. With the excitation on the ytterbium ion, the characteristic green and red emissions from erbium were obtained at room temperature. To prove the mechanism behind the upconversion process, four other compositions were synthesized and studied, namely, {Y₁₃Er₂}, {Y₁₀Er₅}, {Er₁₀Yb₅}, and {Y₁₀Er₁Yb₄}. Upconversion quantum yield values on the order of 10^-3% were obtained, values 100000 times higher than for previously reported lanthanide-based molecular upconverting systems. The presented methodology is an interesting approach to address a fine composition control and harness the upconversion properties of nanoscale molecular materials.

Lanthanide-Based Molecular Cluster-Aggregates: Optical Barcoding and White-Light Emission with Nanosized {Ln20 } Compounds

Counterfeit goods represent a major problem to companies, governments, and customers, affecting the global economy. In order to protect the authenticity of products and documents, optical anti-counterfeit technologies have widely been employed via the use of discrete molecular species, extended metal-organic frameworks (MOFs), and nanoparticles. Herein, for the first time we demonstrate the potential use of molecular cluster-aggregates (MCA) as optical barcodes via composition and energy transfer control. The tuneable optical properties for the [Ln₂₀ (chp)₃₀ (CO₃ )₁₂ (NO₃ )₆ (H₂ O)₆ ], where chp^- =deprotonated 6-chloro-2-pyridinol, allow the fine control of the emission colour output, resulting in high-security level optical labelling with a precise read-out. Moreover, a unique tri-doped composition of Gd^III , Tb^III , and Eu^III led to MCAs with white-light emission. The presented methodology is a unique approach to probe the effect of composition control on the luminescent properties of nanosized molecular material.

Water dispersible ligand-free rare earth fluoride nanoparticles: water transfer versus NaREF4-to-REF3 phase transformation

The chemical stability of oleate-capped sub-10 nm α- and β-NaREF4 NPs (RE = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu for α- and RE = Pr, Nd, Sm, Eu, Gd, Tb, Dy for β-phase NPs) was evaluated under the acidic conditions used for ligand removal towards water dispersibility. It was found that for such small NPs, a pH lower than 3 was necessary for the water transfer to be efficient and to yield well-dispersed ligand-free NPs. In stark contrast to the generally considered good chemical stability of NaREF4, these conditions were observed to pose a risk to phase transformation of the NaREF4 NPs into much larger, hexagonal- or orthorhombic-phase REF3, depending on the NP composition. A correlation between the thermodynamic stability of the α/β-NaREF4 and the hexagonal/orthorhombic REF3 phases - dictated by the RE ion choice - and the chemical stability of the NPs was found. For instance, β-NaGdF4 NPs remained stable, while α-NaGdF4 NPs underwent phase transformation into hexagonal GdF3. More general, NaREF4 NPs based on lighter RE ions were more prone towards phase transformation, while those based on heavier RE ions exhibited stability. Herein, within the RE series, the borderline for phase transformation was identified as Tb/Dy for α-NaREF4 NPs and Sm/Eu for β-NaREF4 NPs, respectively. Also, given the large interest in luminescent NPs for, e.g. biomedical applications, optically active Ln3+ ions (Ln = Nd, Eu, Tb, Er/Yb) were doped into α/β-NaGdF4 host NPs, and the dopant influence on the chemical stability was evaluated. Steady state and time-resolved spectroscopy unveiled spectral features characteristic for Ln3+ f-f transitions, i.e. downshifting and upconversion, before and after ligand removal. Overall, the results herein described emphasise the importance of minding the chemical procedure used for ligand removal of NaREF4 NPs of different crystalline phases and RE compositions.

Dual-Mode Upconversion Nanoprobe Enables Broad-Range Thermometry from Cryogenic to Room Temperature

Noncontact optical thermometers based on the luminescence intensity ratio of two thermally coupled energy levels, exhibiting high sensitivity, excellent accuracy, fast response, and low environment dependence, have attracted great interests in scientific research, life activities, and industrial manufacturing processes. However, the use of optical thermometers in extreme atmospheres (below 150 K) is usually limited by the required large temperature activation because of the relatively big energy difference (200 cm^-1 ≤ ΔE ≤ 2000 cm^-1). Here, we propose a strategy to alleviate the ultralow temperature-sensing problem by exploiting and utilizing the near-infrared (NIR) thermally coupled Stark sublevels of Tm³⁺ (³H₄|0 → ³H₆/³H₄|1 → ³H₆, ΔE ≈ 300 cm^-1) that is much sensitive to minimal temperature variation, especially at ultralow temperatures because of the tiny energy difference. The integration of ultralow temperature-sensitive Tm³⁺ ions and room-temperature-sensitive Er³⁺ ions in an ultrasmall α-NaYbF₄:Tm³⁺@CaF₂@NaYF₄:Yb³⁺/Er³⁺@CaF₂ core/multishell nanoparticle (∼15 nm) as a dual-mode upconversion luminescent nanoprobe enables the broad-range temperature detection from 10 to 295 K. This structure induces ∼14 times NIR emission and ∼sixfold green upconversion luminescence output in comparison with the α-NaYbF₄:Tm³⁺ core and α-NaYbF₄:Tm³⁺@CaF₂@NaYF₄:Yb³⁺/Er³⁺ core/shell/shell nanoparticles. The maximum absolute and relative sensitivities of this dual-mode temperature sensor reach 0.67% and 3.06% K^-1, respectively, showing the advantage of the concurrent utilization of the Tm³⁺ NIR 801/820 nm band ratio and the typical Er³⁺ visible 521/538 nm band ratio for a wide-range temperature-sensing purpose. This work provides a promising strategy to develop accurate and effective, contactless broad-range/ultralow temperature sensors.

Cubic versus hexagonal - effect of host crystallinity on the T1 shortening behaviour of NaGdF4 nanoparticles

Sodium gadolinium fluoride (NaGdF4) nanoparticles are promising candidates as T1 shortening magnetic resonance imaging (MRI) contrast agents due to the paramagnetic properties of the Gd3+ ion. Effects of size and surface modification of these nanoparticles on proton relaxation times have been widely studied. However, to date, there has been no report on how T1 relaxivity (r1) is affected by the different polymorphs in which NaGdF4 crystallizes: cubic (α) and hexagonal (β). Here, a microwave-assisted thermal decomposition method was developed that grants selective access to NaGdF4 nanoparticles of either phase in the same size range, allowing the influence of host crystallinity on r1 to be investigated. It was found that at 3 T cubic NaGdF4 nanoparticles exhibit larger r1 values than their hexagonal analogues. This result was interpreted based on Solomon-Bloembergen-Morgan theory, suggesting that the inner sphere contribution to r1 is more pronounced for cubic NaGdF4 nanoparticles as compared to their hexagonal counterparts. This holds true irrespective of the chosen surface modification, i.e. small citrate groups or longer chain poly(acrylic acid). Key aspects were found to be a polymorph-induced larger hydrodynamic diameter and the higher magnetization possessed by cubic nanoparticles.

A highly sensitive luminescent ratiometric thermometer based on europium(iii) and terbium(iii) benzoylacetonate complexes chemically bonded to ethyldiphenylphosphine oxide functionalized polydimethylsiloxane

A terbium(iii) and europium(iii) ratiometric optical temperature probe with a high relative thermal sensitivity of 11.05% K⁻¹.