Manipulating energy transfer in lanthanide-doped single nanoparticles for highly enhanced upconverting luminescence

Nonstoichiometric Nanocubes with a Controllable Morphology and Persistent Luminescence for Autofluorescence-Free Biosensing

Persistent luminescence nanoparticles (PLNPs) have shown special advantages in areas such as bioimaging, cancer therapy, stress sensing, and photo-biocatalysis. However, the lack of methods for controllable synthesis of PLNPs with uniform morphologies and strong persistent luminescence has seriously hindered the applications of PLNPs. Herein, we reported that modifying the electronic structures of zinc gallogermanate (ZGGO) PLNPs by nonstoichiometric reactions can produce highly uniform nanocubes with controllable size and persistent luminescence. By nonstoichiometric increase of the Ge/Ga ratio in ZGGO, the ZGGO PLNPs were transformed from a mixture of nanocubes and small nanospheres into highly symmetrical and uniform large nanocubes, accompanied by the enhancement of persistent luminescence intensity by about 3.7 times. Moreover, we found that ZGGO PLNPs were responsive to reactive oxygen species (ROS), that is, the persistent luminescence of ZGGO can be quenched by ROS. Autofluorescence-free serum ROS detection was achieved with the developed PLNPs. Further, a biosensing assay for glucose oxidase (GOx) was designed based on the responsiveness of ZGGO PLNPs to H2O2. This study may pave a new way for better control of PLNPs' size, morphology, and persistent luminescence, and it can further promote the applications of PLNPs in areas ranging from theranostics to solar energy utilization.

Dye-sensitized lanthanide containing nanoparticles for luminescence based applications

Due to their exceptional luminescent properties, lanthanide (Ln) complexes represent a unique palette of probes in the spectroscopic toolkit. Their extremely weak brightness due to forbidden Ln electronic transitions can be overcome by indirect dye-sensitization from the antenna effect brought by organic ligands. Despite the improvement brought by the antenna effect, (bio)analytical applications with discrete Ln complexes as luminescent markers still suffers from low sensitivity as they are limited by the complex brightness. Thus, there is a need to develop nano-objects that cumulate the spectroscopic properties of multiple Ln ions. This review firstly gives a brief introduction of the spectral properties of lanthanides both in complexes and in nanoparticles (NPs). Then, the research progress of the design of Ln-doped inorganic NPs with capping antennas, Ln-complex encapsulated NPs and Ln-complex surface functionalized NPs is presented along with a summary of the various photosensitizing ligands and of the spectroscopic properties (excited-state lifetime, brightness, quantum yield). The review also emphasizes the problems and limitations encountered over the years and the solutions provided to address them. Finally, a comparison of the advantages and drawbacks of the three types of NP is provided as well as a conclusion about the remaining challenges both in the design of brighter NPs and in the luminescence based applications.

Controlling upconversion in emerging multilayer core-shell nanostructures: from fundamentals to frontier applications

Lanthanide-based upconversion nanomaterials have recently attracted considerable attention in both fundamental research and various frontier applications owing to their excellent photon upconversion performance and favourable physicochemical properties. In particular, the emergence of multi-layer core-shell (MLCS) nanostructures offers a versatile and powerful tool to realize well-defined matrix compositions and spatial distributions of the dopant on the nanometer length scale. In contrast to the conventional nanomaterials and commonly investigated core-shell nanoparticles, the rational design of MLCS nanostructures allows us to deliberately introduce more functional properties into an upconversion system, thus providing unprecedented opportunities for the precise manipulation of energy transfer channels, the dynamic control of upconversion processes, the fine tuning of switchable emission colours and new functional integration at a single-particle level. In this review, we present a summary and discussion on the key aspects of the recent progress in lanthanide-based MLCS nanoparticles, including the manipulation of emission and lifetime, the switchable multicolour output and the lanthanide ionic interactions on the nanoscale. Benefitting from the multifunctional and versatile luminescence properties, the MLCS nanostructures exhibit great potential in diversities of frontier applications such as three-dimensional display, upconversion laser, optical memory, anti-counterfeiting, thermometry, bioimaging, and therapy. The outlook and challenges as well as perspectives for the research in MLCS nanostructure materials are also provided. This review would be greatly helpful in exploring new structural designs of lanthanide-based materials to further manipulate the upconversion phenomenon and expand their application boundaries.

Photon upconversion through triplet exciton-mediated energy relay

Exploration of upconversion luminescence from lanthanide emitters through energy migration has profound implications for fundamental research and technology development. However, energy migration-mediated upconversion requires stringent experimental conditions, such as high power excitation and special migratory ions in the host lattice, imposing selection constraints on lanthanide emitters. Here we demonstrate photon upconversion of diverse lanthanide emitters by harnessing triplet exciton-mediated energy relay. Compared with gadolinium-based systems, this energy relay is less dependent on excitation power and enhances the emission intensity of Tb3+ by 158-fold. Mechanistic investigations reveal that emission enhancement is attributable to strong coupling between lanthanides and surface molecules, which enables fast triplet generation (<100 ps) and subsequent near-unity triplet transfer efficiency from surface ligands to lanthanides. Moreover, the energy relay approach supports long-distance energy transfer and allows upconversion modulation in microstructures. These findings enhance fundamental understanding of energy transfer at molecule-nanoparticle interfaces and open exciting avenues for developing hybrid, high-performance optical materials.

Local-structure-dependent luminescence in lanthanide-doped inorganic nanocrystals for biological applications

This feature article overviews the recent advances in the local-structure-dependent luminescence in lanthanide-doped inorganic nanocrystals for various biological applications.

Structure, luminescence of Eu2+ and Eu3+ in CaMgSi2O6 and their co-existence for the excitation-wavelength/temperature driven colour evolution

Luminescent materials with controllable colour evolution features are demanded for the development of multi-level anti-counterfeiting technologies. Here we report the structural and luminescence properties of CaMgSi2O6:Ln (Ln = Eu2+, Eu3+, Eu2+/3+) samples in detail and reveal their excitation-wavelength/temperature driven colour evolution characteristics. By tuning either the excitation-wavelength (276, 304, 343, 394 nm) or temperature (in the 330-505 K range), the designed samples with co-existing Eu2+/Eu3+ ions can achieve diverse and controllable colour evolution from red, to pink, purple and blue. This shows their potential application in anti-counterfeiting with the help of sophisticated pattern design. In addition, the underlying mechanism of the Stokes shift of the Eu2+ emission and valence stability of both Eu2+/Eu3+ ions in CaMgSi2O6 are also studied in depth. These results are valuable for designing colour-controllable luminescent materials based on the co-existence of the Eu2+/Eu3+ ions for anti-counterfeiting applications.

Temperature-dependent upconversion luminescence multicolor tuning and temperature sensing of multifunctional β-NaYF4:Yb/Er@β-NaYF4:Yb/Tm microcrystals

Multifunctional β-NaYF₄:Yb/Er@β-NaYF₄:Yb/Tm microcrystals with simultaneous white-light emission, temperature-dependent upconversion luminescence multicolor tuning and good temperature sensitivity.

Energy transfer designing in lanthanide-doped upconversion nanoparticles

Lanthanide (Ln3+)-doped upconversion nanoparticles (UCNPs), exhibiting excellent optical properties such as long photoluminescence lifetime, narrow emission bandwidth, and low autofluorescence background, have been applied in many fields, especially in biological analysis and medical diagnostics. Despite the exciting progress, the applications of Ln3+-doped UCNPs are hindered by the small absorption cross-section and low upconversion luminescence efficiency of Ln3+. To this regard, several effective strategies associated with energy transfer designing have been proposed to modulate the upconversion luminescence properties of Ln3+ in the past few decades. In this feature article, we focus on the most recent development of optical property designing in Ln3+-doped UCNPs on the basis of energy transfer between Ln3+-Ln3+, Ln3+-dyes, and Ln3+-quantum dots. Some future efforts towards the energy transfer designing in Ln3+-doped UCNPs are also proposed.

Metal Ions Doping for Boosting Luminescence of Lanthanide-Doped Nanocrystals

With the developing need for luminous materials with better performance, lanthanide-doped nanocrystals have been widely studied for their unique luminescence properties such as their narrow bandwidth emission, excellent chemical stability, and photostability, adjustable emission color, high signal-to-background ratio, deeper tissue penetration with less photo-damage, and low toxicity, etc., which triggered enthusiasm for research on the broad applications of lanthanide-doped nanocrystals in bioimaging, anti-counterfeiting, biosensing, and cancer diagnosis and treatment. Considerable progress has been made in the past few decades, but low upconversion luminescence efficiency has been a hindrance in achieving further progress. It is necessary to summarize the recently relevant literature and find solutions to improve the efficiency. The latest experimental and theoretical studies related to the deliberate design of rare earth luminescent nanocrystals have, however, shown the development of metal ion-doped approaches to enhance the luminescent intensity. Host lattice manipulation can enhance the luminescence through increasing the asymmetry, which improves the probability of electric dipole transition; and the energy transfer modulation offers a reduced cross-relaxation pathway to improve the efficiency of the energy transfer. Based on the mechanisms of host lattice manipulation and energy transfer modulation, a wide range of enhancements at all wavelengths or even within a particular wavelength have been accomplished with an enhancement of up to a hundred times. In this mini review, we present the strategy of metal ion-doped lanthanide nanocrystals to cope with the issue of enhancing luminescence, overview the advantages and tricky challenges in boosting the luminescence, and provide a potential trend of future study in this field.

Peasecod-Like Hollow Upconversion Nanocrystals with Excellent Optical Thermometric Performance

Trivalent lanthanide (Ln3+)-doped hollow upconversion nanocrystals (UCNCs) usually exhibit unique optical performance that cannot be realized in their solid counterparts, and thus have been receiving tremendous interest from their fundamentals to diverse applications. However, all currently available Ln3+-doped UCNCs are solid in appearance, the preparation of hollow UCNCs remains nearly untouched hitherto. Herein, a class of UCNCs based on Yb3+/Er3+-doped tetralithium zirconium octafluoride (Li4ZrF8:Yb/Er) featuring 2D layered crystal lattice is reported, which makes the fabrication of hollow UCNCs with a peasecod-like shape possible after Ln3+ doping. By employing the first-principle calculations, the unique peasecod-like hollow nanoarchitecture primarily associated with the hetero-valence Yb3+/Er3+ doping into the 2D layered crystal lattice of Li4ZrF8 matrix is revealed. Benefiting from this hollow nanoarchitecture, the resulting Li4ZrF8:Yb/Er UCNCs exhibit an abnormal green upconversion luminescence in terms of the population ratio between two thermally coupled states (2H11/2 and 4S3/2) of Er3+ relative to their solid Li2ZrF6:Yb/Er counterparts, thereby allowing to prepare the first family of hollow Ln3+-doped UCNCs as supersensitive luminescent nanothermometer with almost the widest temperature sensing range (123-800 K). These findings described here unambiguously pave a new way to fabricate hollow Ln3+-doped UCNCs for numerous applications.

Single-particle spectroscopy for functional nanomaterials

Tremendous progress in nanotechnology has enabled advances in the use of luminescent nanomaterials in imaging, sensing and photonic devices. This translational process relies on controlling the photophysical properties of the building block, that is, single luminescent nanoparticles. In this Review, we highlight the importance of single-particle spectroscopy in revealing the diverse optical properties and functionalities of nanomaterials, and compare it with ensemble fluorescence spectroscopy. The information provided by this technique has guided materials science in tailoring the synthesis of nanomaterials to achieve optical uniformity and to develop novel applications. We discuss the opportunities and challenges that arise from pushing the resolution limit, integrating measurement and manipulation modalities, and establishing the relationship between the structure and functionality of single nanoparticles.

Efficient modulation of upconversion luminescence in NaErF4-based core–shell nanocrystals

Efficient modulation of upconversion luminescence in heavily-doped core–shell nanocrystals by the tuning of [F]/[RE] ratio during synthesis.

Fabrication of NaYF4:Yb3+,Tm3+-modified Ag nanocubes with upconversion luminescence and photothermal conversion properties

Herein, uniform Ag nanocube@NaYF₄:Yb³⁺,Tm³⁺ multifunctional nanocomposites were constructed by a facile synthetic strategy. Following the successful synthesis of Ag nanocubes, cubic phase NaYF₄:Yb³⁺,Tm³⁺ upconversion luminescent nanocrystals were modified on the surface of the Ag nanocubes, enhanced the mutual assistance of energy and versatility of the materials. Upon irradiation with a near-infrared laser, the nanocomposites exhibited excellent upconversion fluorescence and good photothermal conversion capability. Furthermore, the potential biological applications and biosafety of the Ag@NaYF₄:Yb³⁺,Tm³⁺ nanocomposites were also demonstrated; the obtained nanocomposites could achieve a good bactericidal performance and photothermal treatment of cancer cells, and they could be potentially applied in the biomedical field as a promising agent. Furthermore, this method provides a facile approach for synthesizing a multifunctional nanocomposite with different properties.

Ag nanocubes@NaYF₄:Yb³⁺,Tm³⁺ multifunctional nanocomposites with florescence and photothermal ability are fabricated by a facile strategy, which enhance the mutual assistance of energy and biological application.

Energy-Transfer Editing in Lanthanide-Activated Upconversion Nanocrystals: A Toolbox for Emerging Applications

Advanced nanoscale synthetic techniques provide a versatile platform for programmable control over the size, morphology, and composition of nanocrystals doped with lanthanide ions. Characteristic upconversion luminescence features originating from the 4f-4f optical transitions of lanthanides can be achieved through predesigned energy transfer pathways, enabling wide applications ranging from ultrasensitive biological detection to advanced spectroscopic instrumentation with high spatiotemporal resolution. Here, we review recent scientific and technological discoveries that have prompted the realization of these peculiar functions of lanthanide-doped upconversion nanocrystals and discuss the mechanistic studies of energy transfer involved in upconversion processes. These advanced schemes include cross relaxation-mediated depletion, multipulse sequential pumping, and nanostructural configuration design. Our emphasis is placed on disruptive technologies such as super-resolution microscopy, optogenetics, nanolasing, and optical anticounterfeiting, which take full advantage of the upconversion nanophenomena in relation to lanthanide-doped nanocrystals.

Simultaneous Dual-mode Emission and Tunable Multicolor in the Time Domain from Lanthanide-doped Core-shell Microcrystals

The dual-mode emission and multicolor outputs in the time domain from core-shell microcrystals are presented. The core-shell microcrystals, with NaYF₄:Yb/Er as the core and NaYF₄:Ce/Tb/Eu as the shell, were successfully fabricated by employing the hydrothermal method, which confines the activator ions into a separate region and minimizes the effect of surface quenching. The material is capable of both upconversion and downshifting emission, and their multicolor outputs in response to 980 nm near-infrared (NIR) excitation laser and 252 nm, and 395 nm ultraviolet (UV) excitation light have been investigated. Furthermore, the tunable color emissions by controlling the Tb³⁺- Eu³⁺ ratio in shells and the energy transfer of Ce³⁺→Tb³⁺→ Eu³⁺ were discussed in details. In addition, color tuning of core-shell-structured microrods from green to red region in the time domain could be obtained by setting suitable delay time. Due to downshifting multicolor outputs (time-resolved and pump-wavelength-induced downshifting) coupled with the upconversion mode, the core-shell microrods can be potentially applied to displays and high-level security.

Paving Metal-Organic Frameworks with Upconversion Nanoparticles via Self-Assembly

The combination of metal-organic frameworks (MOFs) and luminescent nanomaterials with upconversion characteristics could enable the development of new nanomaterials and applications in information security, optical sensing, and theranostics. However, currently available methods are not ideally suitable for fabricating composites of MOF and upconversion nanomaterial, and incorporating upconversion nanomaterials with MOFs in a controllable manner remains challenging. Here, we demonstrate an in situ self-assembly route to the nanocomposites in which MOFs are homogeneously paved with upconversion nanoparticles. Without additional assistance, this strategy, mainly driven by electrostatic interactions, can be used to incorporate different upconversion nanoparticles with diverse MOFs. The as-synthesized composites can be further used to construct composites with unique structures, such as MOF@upconversion nanoparticles@MOF sandwiched nanocomposites, and would be useful for applications including luminescence-monitored drug delivery, anticounterfeiting, and photodynamic therapy. These findings should shed light on new avenues for fabricating multifunctional composites of MOF and upconversion nanomaterials for varied applications.

Misconceptions in electronic energy transfer: bridging the gap between chemistry and physics

Many treatments of energy transfer (ET) phenomena in current literature employ incorrect arguments and formulae and are not quantitative enough. This is unfortunate because we witness important breakthroughs from ET experiments in nanoscience. This review aims to clarify basic principles by focusing upon Förster-Dexter electric dipole-electric dipole (ED-ED) ET. The roles of ET in upconversion, downconversion and the antenna effect are described and the clichés and simple formulae to be avoided in ET studies are highlighted with alternative treatments provided.

Advances in highly doped upconversion nanoparticles

Lanthanide-doped upconversion nanoparticles (UCNPs) are capable of converting near-infra-red excitation into visible and ultraviolet emission. Their unique optical properties have advanced a broad range of applications, such as fluorescent microscopy, deep-tissue bioimaging, nanomedicine, optogenetics, security labelling and volumetric display. However, the constraint of concentration quenching on upconversion luminescence has hampered the nanoscience community to develop bright UCNPs with a large number of dopants. This review surveys recent advances in developing highly doped UCNPs, highlights the strategies that bypass the concentration quenching effect, and discusses new optical properties as well as emerging applications enabled by these nanoparticles.

A high performance Sc-based nanoprobe for through-skull fluorescence imaging of brain vessels beyond 1500 nm

Optical bioimaging that works in the second near infrared region (NIR-II, 1000-1700 nm) has emerged as a next generation imaging technique with superior imaging sensitivity and spatial resolution compared to traditional optical imaging utilizing visible and near-infrared lights (below 900 nm). Herein, a new Sc-based NIR-II probe was explored for high performance NIR-II in vivo bioimaging and optical imaging-guided non-invasive brain blood vessel visualization. The lanthanide doped Sc-based probes (KSc2F7:Yb3+/Er3+) possess a pure orthorhombic phase structure with size control by adjusting the F- ion content. These probes present a dominant red upconversion (UC) emission, which is significantly different from the traditional NaYF4:Yb/Er host, which usually has a green UC emission. More importantly, apart from the dominant red UC emission, these probes also possess a strong NIR-II downconversion (DC) emission centered at 1525 nm, which is usually ignored for bioimaging applications. In vivo NIR-II imaging reveals that our explored Sc-based nanorods are promising probes for highly sensitive optical imaging. Moreover, non-invasive through-skull fluorescence bioimaging of brain vessels with high spatial resolution was demonstrated. Therefore, it is expected that Sc-based nanomaterials with unique dominant red UC and DC NIR-II emissions beyond 1500 nm are ideal probes for bio-applications.

A general strategy for tailoring upconversion luminescence in lanthanide-doped inorganic nanocrystals through local structure engineering

A local structure around lanthanide (Ln3+) emitters in Ln3+-doped upconversion nanocrystals (UCNCs) is of fundamental importance in tailoring their upconversion luminescence (UCL) features. However, a general strategy responsible for the local-structure-dependent UCL in Ln3+-doped UCNCs has not been conclusively established to date. Herein, we report a new class of alkaline zirconium fluoride-based Yb3+/Er3+ co-doped UCNCs featuring a diversity of crystallographic structures for Ln3+ ion doping, which thereby allow us to thoroughly understand the origin underlying the local-structure-dependent UCL of the Er3+ ion for the first time. We reveal that the high-symmetry crystal lattice of Yb3+/Er3+ co-doped UCNCs may incur the large UCL red-to-green intensity ratio of Er3+ regardless of their identical elemental compositions. In combination with the first-principles calculations, we show that such local-structure-dependent UCL of Er3+ is primarily due to the varied electronic band structures induced by the Yb3+/Er3+ doping in different crystallographic structures of alkaline zirconium fluorides. These findings may open up a new avenue for constructing high-quality UCNCs with a tailored UCL profile and lifetime for diverse applications.

Interconversion between KSc2F7:Yb/Er and K2NaScF6:Yb/Er nanocrystals: the role of chemistry

Interconversion between nanocrystals of orthorhombic KSc₂F₇:Yb/Er and cubic K₂NaScF₆:Yb/Er was realized by adjusting the according chemical reaction conditions.