Networking State of Ytterbium Ions Probing the Origin of Luminescence Quenching and Activation in Nanocrystals

Thermally Prolonged NIR-II Luminescence Lifetimes by Cross-Relaxation

Temperature regulates nonradiative processes in luminescent materials, fundamental to luminescence nanothermometry. However, elevated temperatures often suppress the radiative process, limiting the sensitivity of thermometers. Here, we introduce an approach to populating the excited state of lanthanides at elevated temperatures, resulting in a sizable lifetime lengthening and intensity increase of the near-infrared (NIR)-II emission. The key is to create a five-energy-level system and use a pair of lanthanides to leverage the cross-relaxation process. We observed the lifetime of NIR-II emission of Er3+ has been remarkably increased from 3.85 to 7.54 ms by codoping only 0.5 mol % Ce3+ at 20 °C and further increased to 7.80 ms when increasing the temperature to 40 °C. Moreover, this concept is universal across four ion pairs and remains stable within aqueous nanoparticles. Our findings emphasize the need to design energy transfer systems that overcome the constraint of thermal quenching, enabling efficient imaging and sensing.

Ceria nanomaterials containing ytterbium: low and high concentration - luminescence analyzed in the near infrared region

Lanthanide based ceria nanomaterials are important practical materials due to the redox properties that are useful in the avenues pertaining to technology and life sciences. Sub 10 nm spherical and highly monodisperse Ce1-xYbxO2-y (0.04 ≤ x ≤ 0.22) nanoparticles were synthesized by thermal decomposition, annealed separately at 773 K and 1273 K for 2 hours and characterized. Elemental mapping for Yb3+ doped ceria nanoparticles shows homogeneous distribution of Yb3+ atoms in the ceria with low Yb3+ content annealed at 773 K and 1273 K for 2 hours. However, clusters are observed for 773 K annealed ceria samples with high concentration of Yb3+. These clusters are not detected in 1273 K annealed nanomaterials. Introducing small amounts of Yb3+ ions into the ceria lattice as spectroscopic probes can provide detailed information about the atomic structure and local environments allowing the monitoring of small structural changes, such as clustering. The emission spectra observed at room temperature and at 4 K have a manifold of bands that corresponds to the 2F5/2 → 2F7/2 transition of Yb3+ ions. Some small shifts are observed in the Stark splitting pattern depending on the sample and the annealing conditions. The deconvolution by PARAFAC analysis yielded luminescence decay kinetics as well as the associated luminescence spectra of three species for each of the low Yb3+ doped ceria samples annealed at 773 K and one species for the 1273 K annealed samples. However, the ceria samples with high concentration of Yb3+ annealed at the two temperatures showed only one species with lower decay times as compared to the low Yb3+ doped ceria samples.

Lanthanide-Doped KMgF3 Upconversion Nanoparticles for Photon Avalanche Luminescence with Giant Nonlinearities

Lanthanide (Ln3+)-doped photon avalanche (PA) upconversion nanoparticles (UCNPs) have great prospects in many advanced technologies; however, realizing efficient PA luminescence in Ln3+-doped UCNPs remains challenging due to the deleterious surface and lattice quenching effect. Herein, we report a unique strategy based on the pyrolysis of KHF2 for the controlled synthesis of aliovalent Ln3+-doped KMgF3 UCNPs, which can effectively protect Ln3+ from luminescence quenching by surface and internal OH- defects and thereby boost upconversion luminescence. This enables us to realize efficient PA luminescence from Tm3+ at 802 nm in KMgF3: Tm3+ UCNPs upon 1064 nm excitation, with a giant nonlinearity of ∼27, a PA response time of 281 ms, and an excitation threshold of 16.6 kW cm-2. This work may open up a new avenue for exploring highly nonlinear PA luminescence through aliovalent Ln3+ doping and crystal lattice engineering toward diverse emerging applications.

Volumetric Nanocrystal Lattice Reconstruction through Dynamic Metal Complex Docking

Vacancies pose a major challenge in the production of high-quality crystals, particularly at the nanoscale. To address this problem, we report a convenient strategy that involves volumetric lattice reconstruction and dynamic metal complex docking to produce ultrasmall (10 nm) and bright core-shell upconversion nanoparticles (UCNPs). This strategy involves the formation of lanthanide ion-oleic acid complexes during postannealing in solution, which effectively removes vacancies in nanocrystals. The removal of vacancies restricts the diffusion of lanthanide sensitizers and emitters within the core, thus minimizing surface quenching. Our volumetric lattice reconstruction strategy provides fundamental insights into lattice engineering and presents a general strategy for purifying functional nanocrystals for applications in fields such as single-molecule tracking, quantum optics, energy conversion, and others.

Engineering highly efficient NIR-II FRET platform for Background-Free homogeneous detection of SARS-CoV-2 neutralizing antibodies in whole blood

Förster or fluorescence resonance energy transfer (FRET) enables to probe biomolecular interactions, thus playing a vital role in bioassays. However, conventional FRET platforms suffer from limited sensitivity due to the low FRET efficiency and poor anti-interference of existing FRET pairs. Here we report a NIR-II (1000-1700 nm) FRET platform with extremely high FRET efficiency and exceptional anti-interference capability. This NIR-II FRET platform is established based on a pair of lanthanides downshifting nanoparticles (DSNPs) by employing Nd³⁺ doped DSNPs as an energy donor and Yb³⁺ doped DSNPs as an energy acceptor. The maximum FRET efficiency of this well-engineered NIR-II FRET platform reaches up to 92.2%, which is much higher than most commonly used ones. Owing to the all-NIR advantage (λ_ex = 808 nm, λ_em = 1064 nm), this highly efficient NIR-II FRET platform exhibits extraordinary anti-interference in whole blood, and thus enabling background-free homogeneous detection of SARS-CoV-2 neutralizing antibodies in clinical whole blood sample with high sensitivity (limit of detection = 0.5 μg/mL) and specificity. This work opens up new opportunities for realizing highly sensitive detection of various biomarkers in biological samples with severe background interference.

Understanding and Preventing Photoluminescence Quenching to Achieve Unity Photoluminescence Quantum Yield in Yb:YLF Nanocrystals

Ytterbium-doped LiYF4 (Yb:YLF) is a commonly used material for laser applications, as a photon upconversion medium, and for optical refrigeration. As nanocrystals (NCs), the material is also of interest for biological and physical applications. Unfortunately, as with most phosphors, with the reduction in size comes a large reduction of the photoluminescence quantum yield (PLQY), which is typically associated with an increase in surface-related PL quenching. Here, we report the synthesis of bipyramidal Yb:YLF NCs with a short axis of ∼60 nm. We systematically study and remove all sources of PL quenching in these NCs. By chemically removing all traces of water from the reaction mixture, we obtain NCs that exhibit a near-unity PLQY for an Yb3+ concentration below 20%. At higher Yb3+ concentrations, efficient concentration quenching occurs. The surface PL quenching is mitigated by growing an undoped YLF shell around the NC core, resulting in near-unity PLQY values even for fully Yb3+-based LiYbF4 cores. This unambiguously shows that the only remaining quenching sites in core-only Yb:YLF NCs reside on the surface and that concentration quenching is due to energy transfer to the surface. Monte Carlo simulations can reproduce the concentration dependence of the PLQY. Surprisingly, Förster resonance energy transfer does not give satisfactory agreement with the experimental data, whereas nearest-neighbor energy transfer does. This work demonstrates that Yb3+-based nanophosphors can be synthesized with a quality close to that of bulk single crystals. The high Yb3+ concentration in the LiYbF4/LiYF4 core/shell nanocrystals increases the weak Yb3+ absorption, making these materials highly promising for fundamental studies and increasing their effectiveness in bioapplications and optical refrigeration.

Activating Ultrahigh Thermoresponsive Upconversion in an Erbium Sublattice for Nanothermometry and Information Security

Thermal activation of upconversion luminescence in nanocrystals opens up new opportunities in biotechnology and nanophotonics. However, it remains a daunting challenge to achieve a smart control of luminescence behavior in the thermal field with remarkable enhancement and ultrahigh sensitivity. Moreover, the physical picture involved is also debatable. Here we report a novel mechanistic design to realize an ultrasensitive thermally activated upconversion in an erbium sublattice core-shell nanostructure. By enabling a thermosensitive property into the intermediate ⁴I_11/2 level of Er³⁺ through an energy-migration-mediated surface interaction, the upconverted luminescence was markedly enhanced in the thermal field together with a striking thermochromic feature under 1530 nm irradiation. Importantly, the use of non thermally coupled red and green emissions contributes to the thermal sensitivity up to 5.27% K^-1, 3 times higher than that obtained by using conventional thermally coupled green emissions. We further demonstrate that the controllable surface interaction is a general approach to the thermal enhancement of upconversion for a series of lanthanide-based nanomaterials. Our findings pave a new way for the development of smart luminescent materials toward emerging applications such as noncontact nanothermometry, information security, and anticounterfeiting.

Engineering the Compositional Architecture of Core-Shell Upconverting Lanthanide-Doped Nanoparticles for Optimal Luminescent Donor in Resonance Energy Transfer: The Effects of Energy Migration and Storage

Förster Resonance Energy Transfer (FRET) between single molecule donor (D) and acceptor (A) is well understood from a fundamental perspective and is widely applied in biology, biotechnology, medical diagnostics, and bio-imaging. Lanthanide doped upconverting nanoparticles (UCNPs) have demonstrated their suitability as alternative donor species. Nevertheless, while they solve most disadvantageous features of organic donor molecules, such as photo-bleaching, spectral cross-excitation, and emission bleed-through, the fundamental understanding and practical realizations of bioassays with UCNP donors remain challenging. Among others, the interaction between many donor ions (in donor UCNP) and many acceptors anchored on the NP surface and the upconversion itself within UCNPs, complicate the decay-based analysis of D-A interaction. In this work, the assessment of designed virtual core-shell NP (VNP) models leads to the new designs of UCNPs, such as …@Er, Yb@Er, Yb@YbEr, which are experimentally evaluated as donor NPs and compared to the simulations. Moreover, the luminescence rise and decay kinetics in UCNP donors upon RET is discussed in newly proposed disparity measurements. The presented studies help to understand the role of energy-transfer and energy migration between lanthanide ion dopants and how the architecture of core-shell UCNPs affects their performance as FRET donors to organic acceptor dyes.

Ultrasensitive Thermochromic Upconversion in Core-Shell-Shell Nanoparticles for Nanothermometry and Anticounterfeiting

Upconversion nanoparticle based ratiometric nanothermometry has shown many advantages including high relative sensitivity, fast temperature response, and high spatial resolution. However, most of the existing designs are on the basis of thermally coupled upconversion emissions, and it remains a challenge to improve the thermo-sensitivity. Here, we report a new nanoplatform of NaYF₄:Yb/Er/Ce@NaYF₄@NaYF₄:Yb/Tm core-shell-shell nanostructure to improve the thermal sensitivity through the nonthermally coupled upconversion emissions. With the increase of temperature, the green upconversion of Er³⁺ shows a decline while the blue upconversion of Tm³⁺ exhibits a rapid increase, leading to a huge contrast in both intensity ratio and emission colors. The maximum relative sensitivity can reach up to 9.86% K^-1 at 303 K. It is further found that introducing Ce³⁺ is able to improve the sensitivity and expand the thermochromic green-to-blue gamut greatly. These results show great potential in ultrasensitive lanthanide-based nanothermometry and anticounterfeiting.

An overview of boosting lanthanide upconversion luminescence through chemical methods and physical strategies

Lanthanide-doped upconversion nanoparticles have attracted extensive research interest due to their promising applications in various fields.

Hexagonal-phase NaREF4 upconversion nanocrystals: the matter of crystal structure

The hexagonal-phase (β) of NaREF₄ upconversion nanocrystals (RE = rare earth elements) has been widely employed because of the outstanding luminescence performance, yet less is known about the essence of this superior property. The current understanding of this issue is raised from the advantage of weak electron-vibration interactions in fluoride systems, while the interpretability of this statement is controversial and contradictory results are commonly reported. One feasible way to solve this puzzle is from the aspect of "structure-property" relationship, yet even after decades of investigation, the structural details of β-NaREF₄ are still under debate. Herein, the reported results relevant to this topic are reviewed, and the conflicting viewpoints are summarized. The similarities and differences between different lattice templates are assessed, and the reasons underlying the divergence are analysed. Based on these discussions, it is realized that the crystal structure of β-NaREF₄ should be more reliably depicted as one flexible lattice framework with complex characteristics, and the structural disorder induced by atom displacements in the lattice is probably the key to supporting the superior luminescence properties of β-NaREF₄ nanocrystals.

First-principles calculations of strain engineering in NaYF4-based nanocrystals with hydroxyl impurities

Lanthanide-based nanocrystals with heterogeneous core-shell structures possess elastic strain due to lattice mismatch and volumetric expansion or shrinkage. Strain relaxation is usually accompanied by lattice defects, especially those point defects and small defect clusters. However, the influence of strain on the formation of lattice defects remains unclear. Using OH^- ions as a representative lattice impurity, first-principles calculations can be used to address the correlation between the thermodynamic stability of OH-based substitutional defects and elastic strain. Moreover, the concentration of OH^- impurities in both strained and relaxed sodium yttrium fluoride lattices can be greatly reduced by increasing the concentration of fluoride-containing precursors. These findings suggest that minimal incorporation of OH^- ions effectively suppresses multiphonon nonradiative relaxation and thus boost the efficiency of upconversion conversion.

Enhancing multiphoton upconversion emissions through confined energy migration in lanthanide-doped Cs2NaYF6 nanoplatelets

Lanthanide (Ln3+)-doped upconversion (UC) nanocrystals have drawn tremendous attention because of their intriguing optical properties. Currently, it is highly desired but remains challenging to achieve efficient multiphoton UC emissions. Herein, we report the controlled synthesis of a new class of UC nanocrystals based on Cs2NaYF6:Yb/Tm nanoplatelets (NPs), which can effectively convert the 980 nm light to five-photon and four-photon UC emissions of Tm3+ without the fabrication of a complicated core/multishell structure required in traditional nanocrystals. Particularly, the as-prepared Cs2NaYF6:Yb/Tm NPs exhibit a maximal UV-to-NIR emission intensity ratio of 1.2, which is the highest among Tm3+-doped core-only UC nanocrystals. We reveal that the enhanced multiphoton UC emissions may benefit from the confined energy migration of Ln3+ dopants in the unique two-dimensional-like structure of Cs2NaYF6 NPs. As such, intense red and green UC emissions of Eu3+ and Tb3+ can further be generated via the cascade sensitization of Tm3+ and Gd3+ in Cs2NaYF6:Yb/Tm/Gd/Eu and Cs2NaYF6:Yb/Tm/Gd/Tb NPs, respectively. These results validate the superiority of Cs2NaYF6 for the future design of efficient UC nanocrystals towards versatile applications.