Upconverting Near-Infrared Light through Energy Management in Core-Shell-Shell Nanoparticles

Multi-Wavelength Excitable Multicolor Upconversion and Ratiometric Luminescence Thermometry of Yb3+/Er3+ Co-Doped NaYGeO4 Microcrystals

Excitation wavelength controllable lanthanide upconversion allows for real-time manipulation of luminescent color in a composition-fixed material, which has been proven to be conducive to a variety of applications, such as optical anti-counterfeiting and information security. However, current available materials highly rely on the elaborate core-shell structure in order to ensure efficient excitation-dependent energy transfer routes. Herein, multicolor upconversion luminescence in response to both near-infrared I and near-infrared II (NIR-I and NIR-II) excitations is realized in a novel but simple NaYGeO4:Yb3+/Er3+ phosphor. The remarkably enhanced red emission ratio under 1532 nm excitation, compared with that under 980 nm excitation, could be attributed to the Yb3+-mediated cross-relaxation energy transfers. Moreover, multi-wavelength excitable temperature-dependent (295-823 K) upconversion luminescence realizes a ratiometric thermometry relying on the thermally coupled levels (TCLs) of Er3+. Detailed investigations demonstrate that changing excitation wavelength makes little difference for the performances of TCL-based ratiometric thermometry of NaYGeO4:Yb3+/Er3+. These findings gain more insights to manipulate cross-relaxations for excitation controllable upconversion in single activator doped materials and benefit the cognition of the effect of excitation wavelength on ratiometric luminescence thermometry.

Strategy to Achieve a Pure Red/Green/Blue-Emitting Upconversion Luminescence for Full-Color Displays

Multicolor tunable upconversion nanoparticles (UCNPs) have garnered attention owing to their diverse applications such as displays, imaging, and security. Typically, achieving multicolor emission from UCNPs requires complicated core/multishell nanostructures comprising a core with at least five shells. Here, we propose a strategy to achieve bright and orthogonal red (R), green (G), and blue (B) upconversion (UC) luminescence without synthesizing complicated core/quintuple-shell or core/sextuple-shell nanostructures. For achieving bright and orthogonal RGB triprimary color UC luminescence, orthogonal bicolor-emitting core/shell-structured UCNPs are synthesized and blended. Orthogonal RB, RG, and GB luminescence are achieved through photon blocking. The combination of two orthogonal bicolor-emitting UCNPs exhibits pure RGB UC luminescence and full-color tunability via manipulation of excitation laser conditions. Furthermore, we present color displays achieved with transparent UCNP-polymer composites utilizing three distinct near-infrared light wavelengths, implying that the proposed strategy for attaining RGB UC luminescence may facilitate advancements in the development of full-color volumetric displays.

Highly water-soluble and biocompatible hyaluronic acid functionalized upconversion nanoparticles as ratiometric nanoprobes for label-free detection of nitrofuran and doxorubicin

Antibiotic detection is crucial and challenging because the widespread consumption of antibiotics has shown extensive harmful effects on food, environment and human health. Here, we propose highly water-soluble and biocompatible hyaluronic acid (HYA) functionalized upconversion nanoparticles (UCNPs) for ratiometric detection of multiple antibiotics. The ultraviolet upconversion luminescence (UCL) from UCNPs was significantly quenched by nitrofurazone (NFZ)/nitrofurantoin (NFT), and blue UCL was quenched by doxorubicin (DOX), while red UCL remained unchanged for internal reference. The UCNPs-HYA nanoprobes exhibit excellently sensitive and selective NFZ, NFT and DOX detection in linear range of 2.5-100 μM, 2.5-80 μM, and 2.5-200 μM with the LOD at 0.28 μM (55 μg/kg), 0.20 μM (48 μg/kg) and 0.17 μM (97 μg/kg), respectively. The nanoprobes achieved detecting real samples of NFZ in lake water, liquid milk and chicken meat with satisfactory results, and UCL bioimaging of DOX in HeLa cells. The UCNPs-HYA ratiometric nanoprobes are promising for food samples detection and potential biosensing in the cellular environment.

Achieving Orthogonal Upconversion Luminescence of a Single Lanthanide Ion in Crystals for Optical Encryption

Optical encryption shows great potential in meeting the growing demand for advanced anti-counterfeiting in the information age. The development of upconversion luminescence (UCL) materials capable of emitting different colors of light in response to different external stimuli holds great promise in this field. However, the effective realization of multicolor UCL materials usually requires complex structural designs. In this work, orthogonal UCL is achieved in crystals with a simple structure simply by introducing modulator Tm3+ ions to control the photon transition processes between different energy levels of activator Er3+ ions. The obtained crystals emit red and green UCL when excited by 980 nm and 808 nm lasers, respectively. The orthogonal excitation-emission properties of crystals are shown to be very suitable for high-level optical encryption, which is important for information security and anti-counterfeiting. This work provides an effective strategy for obtaining orthogonal UCL in simple structural materials, which will encourage researchers to further explore novel orthogonal UCL materials and their applications, and has important implications for the development of the frontier photonic upconversion fields.

In Vivo High-Contrast Biomedical Imaging in the Second Near-Infrared Window Using Ultrabright Rare-Earth Nanoparticles

Intravital luminescence imaging in the second near-infrared window (NIR-II) enables noninvasive deep-tissue imaging with high spatiotemporal resolution of live mammals because of the properties of suppressed light scattering and diminished autofluorescence in the long-wavelength region. Herein, we present the synthesis of a downconversion luminescence rare-earth nanocrystal with a core-shell-shell structure (NaYF4@NaYbF4:Er,Ce@NaYF4:Ca). The structure efficiently maximized the doping concentration of the sensitizers and increased Er3+ luminescence while preventing cross relaxation. Furthermore, Ce3+ doping in the middle layer efficiently limited the upconversion pathway and increased downconversion by 24-fold to produce bright 1550 nm luminescence under 975 nm excitation. Finally, optimizing the inert shell coating of NaYF4:Ca and liposome encapsulation reduced the luminescence quenching impact by water and improved biological metabolism. Thus, our synthesized biocompatible, ultrabright NIR-II probes provide high contrast and resolution for through-scalp and through-skull luminescence imaging of mice cerebral vasculature without craniotomy as well as imaging of mouse hindlimb microvessels.

Selectively Manipulating Interactions between Lanthanide Sublattices in Nanostructure toward Orthogonal Upconversion

Smart control of ionic interactions is a key factor to manipulate the luminescence dynamics of lanthanides and tune their emission colors. However, it remains challenging to gain a deep insight into the physics involving the interactions between heavily doped lanthanide ions and in particular between the lanthanide sublattices for luminescent materials. Here we report a conceptual model to selectively manipulate the spatial interactions between erbium and ytterbium sublattices by designing a multilayer core-shell nanostructure. The interfacial cross-relaxation is found to be a leading process to quench the green emission of Er³⁺, and red-to-green color-switchable upconversion is realized by fine manipulation of the interfacial energy transfer on the nanoscale. Moreover, the temporal control of up-transition dynamics can also lead to an observation of green emission due to its fast rise time. Our results demonstrate a new strategy to achieve orthogonal upconversion, showing great promise in frontier photonic applications.

Probing the evolutionary mechanism of the hydrogen bond network of cellulose nanofibrils using three DESs

Complex interactions between cellulose molecules and small molecules in Deep Eutectic Solvent (DES) systems can lead to dramatic changes in the structure of the hydrogen bond network in cellulose. However, the mechanism of interaction between cellulose and solvent molecules and the mechanism of evolution of hydrogen bond network are still unclear. In this study, cellulose nanofibrils (CNFs) were treated with DESs based on oxalic acid as hydrogen bond donors (HBD), and choline chloride, betaine, and N-methylmorpholine-N-oxide (NMMO) as hydrogen bond acceptors (HBA). The changes in the properties and microstructure of CNFs during treatment with the three types of solvents were investigated by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The results showed that the crystal structures of CNFs were not changed during the process, but the hydrogen bond network evolved, increasing the crystallinity and crystallite size. Further analysis of the fitted peaks of FTIR and generalized two-dimensional correlation spectra (2DCOS) revealed that all three hydrogen bonds were disrupted to different degrees, the relative content changed, and evolved strictly in a certain order. These findings indicate that the evolution of hydrogen bond networks in nanocellulose has certain regularity.

Elemental-Migration-Assisted Full-Color-Tunable Upconversion Nanoparticles for Video-Rate Three-Dimensional Volumetric Displays

Herein, we demonstrate video-rate color three-dimensional (3D) volumetric displays using elemental-migration-assisted full-color-tunable upconversion nanoparticles (UCNPs). In the heavily doped NaErF₄:Tm-based core@multishell UCNPs, erbium migration was observed. By tailoring this migration through adjustment of the intermediate shell thickness between the core and the sensitizer-doped second shell, red-green orthogonal upconversion luminescence (UCL) was achieved. Furthermore, highly efficient red-green-blue orthogonal UCL and full-color tunability were achieved in the UCNPs through a combination of elemental-migration-assisted color tuning and selective photon blocking. Finally, 3D volumetric displays were fabricated using a UCNP-polydimethylsiloxane composite. More specifically, 3D color images were created and motion pictures based on the expansion, rotation, and up/down movement of the displayed images were realized in the display matrix. Overall, our study provides new insights into upconversion color tuning and the achievement of motion pictures in the UCNP-polydimethylsiloxane composite is expected to accelerate the further development of solid-state full-color 3D volumetric displays.

Defect-assisted dynamic multicolor modulation in KLu3F10:Tb crystals for anti-counterfeiting

Excitation-dependent dynamic multicolor luminescent materials show potential for promising applications in the field of anti-counterfeiting. However, for most ultraviolet (UV)-excited lanthanide-doped materials, more than two types of activators are incorporated to realize multicolors. In this study, for the first time, KLu₃F₁₀:Tb crystals were used to realize excitation-dependent multicolor emissions. The morphology was modified by tuning the surface-coated citric acid (CA) content. During hydrothermal reactions, fluorine vacancy defects are formed on the crystal surface, and carboxyl groups (-COOH) are coated on the crystal surface to maintain the charge balance. Under 254 nm UV excitation, typical Tb³⁺ green emissions were observed, while a strong broadband emission peaking at 442 nm appeared in addition to these Tb³⁺ emissions under 365 nm excitation. The energy transfer (ET) process between the defect state and Tb³⁺ is clarified. This work may promote the development of single-type activator-doped multicolor luminescent materials for high-level anti-counterfeiting.

Plasmonic Chiral Metasurface-Induced Upconverted Circularly Polarized Luminescence from Achiral Upconversion Nanoparticles

Chirality induction, transfer, and manipulation have aroused great interest in achiral nanomaterials. Here, we demonstrate strong upconverted circularly polarized luminescence from achiral core-shell upconversion nanoparticles (UCNPs) via a plasmonic chiral metasurface-induced optical chirality transfer. The Yb³⁺-sensitized core-shell UCNPs with good dispersity exhibit intense upconversion luminescence of Tm³⁺ and Nd³⁺ through the energy transfer process. By spin-coating the core-shell UCNPs on this chiral metasurface, strong enhancement and circular polarization modulation of upconversion luminescence can be achieved due to resonant coupling between surface plasmons and upconversion nanoparticles. In the UCNPs-on-metasurface composite, a significant upconversion luminescence enhancement can be achieved with a maximum enhancement factor of 32.63 at 878 nm and an overall enhancement factor of 11.61. The luminescence dissymmetry factor of the induced upconverted circularly polarized luminescence can reach 0.95 at the emission wavelength of 895 nm. The UCNPs-on-metasurface composite yields efficient modulation for the emission intensity and polarization of UCNPs, paving new pathways to many potential applications in imaging, sensing, and anticounterfeiting fields.

Recent Advances in Light-Controlled Activation of Pt(IV) Prodrugs

Pt(IV) prodrugs remain one of the most promising alternatives to conventional Pt(II) therapy due to their versatility in axial ligand choice and delayed mode of action. Selective activation from an external source is especially attractive due to the opportunity to control the activity of an antitumor drug in space and time and avoid damage to normal tissues. In this review, we discuss recent advances in photoabsorber-mediated photocontrollable activation of Pt(IV) prodrugs. Two main approaches developed are the focus of the review. The first one is the photocatalytic strategy based on the flavin derivatives that are not covalently bound to the Pt(IV) substrate. The second one is the conjugation of photoactive molecules with the Pt(II) drug via axial position, yielding dual-action Pt(IV) molecules capable of the controllable release of Pt(II) cytotoxic agents. Thus, Pt(IV) prodrugs with a light-controlled mode of activation are non-toxic in the absence of light, but show high antiproliferative activity when irradiated. The susceptibility of Pt(IV) prodrugs to photoreduction, photoactivation mechanisms, and biological activity is considered in this review.

Recent Development in Sensitizers for Lanthanide-Doped Upconversion Luminescence

The attractive features of lanthanide-doped upconversion luminescence (UCL), such as high photostability, nonphotobleaching or photoblinking, and large anti-Stokes shift, have shown great potentials in life science, information technology, and energy materials. Therefore, UCL modulation is highly demanded toward expected emission wavelength, lifetime, and relative intensity in order to satisfy stringent requirements raised from a wide variety of areas. Unfortunately, the majority of efforts have been devoted to either simple codoping of multiple activators or variation of hosts, while very little attention has been paid to the critical role that sensitizers have been playing. In fact, different sensitizers possess different excitation wavelengths and different energy transfer pathways (to different activators), which will lead to different UCL features. Thus, rational design of sensitizers shall provide extra opportunities for UCL tuning, particularly from the excitation side. In this review, we specifically focus on advances in sensitizers, including the current status, working mechanisms, design principles, as well as future challenges and endeavor directions.

Visible-light-promoted S-trifluoromethylation of thiophenols with trifluoromethyl phenyl sulfone

Trifluoromethyl phenyl sulfone is traditionally a nucleophilic trifluoromethylating agent. Herein, we report the first example of the use of trifluoromethyl phenyl sulfone as a trifluoromethyl radical precursor. Arylthiolate anions can form electron donor-acceptor (EDA) complexes with trifluoromethyl phenyl sulfone, which can undergo an intramolecular single electron transfer (SET) reaction under visible light irradiation, thus realizing the S-trifluoromethylation of thiophenols under photoredox catalyst-free conditions. Similar S-perfluoroethylation and S-perfluoro-iso-propylation of thiophenols are also achieved using the corresponding perfluoroalkyl phenyl sulfones.

Upconversion/Downshifting Multimode Luminescence of Lanthanide-doped Nanocrystals for Multidimensional Information Encoding Security

Information encoding security has always been a research hotspot in the optical field. Although many studies focused on luminescent materials and techniques for information security, the optical information encoding is limited by low information capacity and security. Herein, we present new core-shell-shell (CSS) lanthanide-doped nanocrystals which display multi-stimuli-responsive and multimode emission. In the designed CSS nanostructure, the Stokes and anti-Stokes processes can be both achieved in the same nanocrystals under the excitation of 1532, 980, and 254 nm via self-excited Er³⁺ and Ce³⁺ -sensitized mechanisms. Subsequently, a group of unique multimode emission CSS nanocrystals were designed as optical modules and successfully utilized in multidimensional information encoding, which demonstrates high-level information encoding capability and security. This work brings a powerful idea for information encoding security designs based on multimode luminesce materials.

Switching the NIR upconversion of nanoparticles for the orthogonal activation of photoacoustic imaging and phototherapy

Phototheranostics based on upconversion nanoparticles (UCNPs) offer the integration of imaging diagnostics and phototherapeutics. However, the programmable control of the photoactivation of imaging and therapy with minimum side effects is challenging due to the lack of ideal switchable UCNPs agents. Here we demonstrate a facile strategy to switch the near infrared emission at 800 nm from rationally designed UCNPs by modulating the irradiation laser into pulse output. We further synthesize a theranostic nanoagent by combining with a photosensitizer and a photoabsorbing agent assembled on the UCNPs. The orthogonal activation of in vivo photoacoustic imaging and photodynamic therapy can be achieved by altering the excitation modes from pulse to continuous-wave output upon a single 980 nm laser. No obvious harmful effects during photoexcitation was identified, suggesting their use for long-term imaging-guidance and phototherapy. This work provides an approach to the orthogonal activation of imaging diagnostics and photodynamic therapeutics.

Designing Next Generation of Persistent Luminescence: Recent Advances in Uniform Persistent Luminescence Nanoparticles

Persistent luminescence is a unique optical process where long-lasting afterglow persists after the cessation of excitation. Nanoscale persistent luminescent materials are getting increased research interest from various fields due to their unique optical property. In recent years, inspiring achievements have been made to produce uniform persistent luminescence nanoparticles (PLNPs) in a controllable manner, unleashing their fascinating potential, surpassing other types of luminescent materials in a wide variety of application such as high-contrast bioimaging and high-resolution X-ray detection. In this review, the evolution of uniform PLNPs, from their bulk phosphor counterparts, to the "top-down" preparation of nanoscale persistent luminescent materials, to the recent "bottom-up" synthesis of uniform PLNPs is first summarized. The respective milestones of uniform PLNPs prepared by templated synthesis, aqueous synthesis, and colloidal synthesis are highlighted. The key optical properties that can be enhanced in uniform PLNPs, including increasing the persistent luminescence intensity, tuning the excitation irradiance, as well as the emission wavelengths are then analyzed. Detailed strategies to enhance each optical property are also discussed in various sections. Finally, future challenges are highlighted with respect to the perspectives on the development of next-generation PLNPs with novel 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.

Advances in fluorescence sensing enabled by lanthanide-doped upconversion nanophosphors

Lanthanide-doped upconversion nanoparticles (UCNPs), characterized by converting low-energy excitation to high-energy emission, have attracted considerable interest due to their inherent advantages of large anti-Stokes shifts, sharp and narrow multicolor emissions, negligible autofluorescence background interference, and excellent chemical- and photo-stability. These features make them promising luminophores for sensing applications. In this review, we give a comprehensive overview of lanthanide-doped upconversion nanophosphors including the fundamental principle for the construction of UCNPs with efficient upconversion luminescence (UCL), followed by state-of-the-art strategies for the synthesis and surface modification of UCNPs, and finally describing current advances in the sensing application of upconversion-based probes for the quantitative analysis of various analytes including pH, ions, molecules, bacteria, reactive species, temperature, and pressure. In addition, emerging sensing applications like photodetection, velocimetry, electromagnetic field, and voltage sensing are highlighted.

Rare-Earth Doping in Nanostructured Inorganic Materials

Impurity doping is a promising method to impart new properties to various materials. Due to their unique optical, magnetic, and electrical properties, rare-earth ions have been extensively explored as active dopants in inorganic crystal lattices since the 18th century. Rare-earth doping can alter the crystallographic phase, morphology, and size, leading to tunable optical responses of doped nanomaterials. Moreover, rare-earth doping can control the ultimate electronic and catalytic performance of doped nanomaterials in a tunable and scalable manner, enabling significant improvements in energy harvesting and conversion. A better understanding of the critical role of rare-earth doping is a prerequisite for the development of an extensive repertoire of functional nanomaterials for practical applications. In this review, we highlight recent advances in rare-earth doping in inorganic nanomaterials and the associated applications in many fields. This review covers the key criteria for rare-earth doping, including basic electronic structures, lattice environments, and doping strategies, as well as fundamental design principles that enhance the electrical, optical, catalytic, and magnetic properties of the material. We also discuss future research directions and challenges in controlling rare-earth doping for new applications.

An erythrocyte-delivered photoactivatable oxaliplatin nanoprodrug for enhanced antitumor efficacy and immune response

The outcome of conventional platinum (Pt)-based chemotherapy is limited by reduced circulation, failure to accumulate in the tumor, and dose-limiting toxicity arising from non-controllable activation. To address these limitations, we present an erythrocyte-delivered and near-infrared (NIR) photoactivatable Pt^IV nanoprodrug for advanced cancer treatment. Compared with small molecule Pt^IV prodrugs, this nanoprodrug exhibits significantly enhanced stability, prolonged circulation in the blood, and minimized side effects. The hitchhiking of the nanoprodrug on erythrocytes dramatically increases Pt accumulation in the tumor. Upon irradiation, the nanoprodrug releases oxaliplatin in a controllable manner, resulting in significant antitumor activity against breast tumors in vivo, as evidenced by the complete elimination of tumors from a single-dose injection. Additionally, this nanoprodrug is associated with remarkably enhanced immunopotentiation. Our study highlights an efficient strategy to overcome the shortcomings of traditional Pt-based chemotherapy via the erythrocyte-mediated delivery of an NIR-activatable nanoprodrug of oxaliplatin, a clinically used anticancer drug.

Strategic illustration of an erythrocyte-delivered and near-infrared photoactivatable oxaliplatin nanoprodrug for enhanced antitumor efficacy and immune response.

Interface Pattern Engineering in Core-Shell Upconverting Nanocrystals: Shedding Light on Critical Parameters and Consequences for the Photoluminescence Properties

Advances in controlling energy migration pathways in core-shell lanthanide (Ln)-based hetero-nanocrystals (HNCs) have relied heavily on assumptions about how optically active centers are distributed within individual HNCs. In this article, it is demonstrated that different types of interface patterns can be formed depending on shell growth conditions. Such interface patterns are not only identified but also characterized with spatial resolution ranging from the nanometer- to the atomic-scale. In the most favorable cases, atomic-scale resolved maps of individual particles are obtained. It is also demonstrated that, for the same type of core-shell architecture, the interface pattern can be engineered with thicknesses of just 1 nm up to several tens of nanometers. Total alloying between the core and shell domains is also possible when using ultra-small particles as seeds. Finally, with different types of interface patterns (same architecture and chemical composition of the core and shell domains) it is possible to modify the output color (yellow, red, and green-yellow) or change (improvement or degradation) the absolute upconversion quantum yield. The results presented in this article introduce an important paradigm shift and pave the way toward the emergence of a new generation of core-shell Ln-based HNCs with better control over their atomic-scale organization.

Mechanisms of Upconversion Luminescence of Er3+-Doped NaYF4 via 980 and 1530 nm Excitation

To date, the mechanisms of Er³⁺ upconversion luminescence via 980 and 1530 nm excitation have been extensively investigated; however, based on discussions, they either suffer from the lack of convincing evidence or require elaborated and time-consuming numerical simulations. In this work, the steady-state and time-resolved upconversion luminescence data of Er³⁺-doped NaYF₄ were measured; we therefore investigated the upconversion mechanisms of Er³⁺ on the basis of the spectroscopic observations and the simplified rate equation modeling. This work provides a relatively simple strategy to reveal the UCL mechanisms of Er³⁺ upon excitation with various wavelengths, which may also be used in other lanthanide ion-doped systems.

Six-photon upconverted excitation energy lock-in for ultraviolet-C enhancement

Photon upconversion of near-infrared (NIR) irradiation into ultraviolet-C (UVC) emission offers many exciting opportunities for drug release in deep tissues, photodynamic therapy, solid-state lasing, energy storage, and photocatalysis. However, NIR-to-UVC upconversion remains a daunting challenge due to low quantum efficiency. Here, we report an unusual six-photon upconversion process in Gd³⁺/Tm³⁺-codoped nanoparticles following a heterogeneous core-multishell architecture. This design efficiently suppresses energy consumption induced by interior energy traps, maximizes cascade sensitizations of the NIR excitation, and promotes upconverted UVC emission from high-lying excited states. We realized the intense six-photon-upconverted UV emissions at 253 nm under 808 nm excitation. This work provides insight into mechanistic understanding of the upconversion process within the heterogeneous architecture, while offering exciting opportunities for developing nanoscale UVC emitters that can be remotely controlled through deep tissues upon NIR illumination.

Photon upconversion with near-infrared excitation and ultraviolet emission has many applications, but suffers from low quantum efficiency. Here, the authors report a six-photon upconversion process in nanoparticles with heterogeneous core-multishell structure, that regulate the energy transfer pathway.

Upconversion luminescent nanomaterials: A promising new platform for food safety analysis

Foodborne diseases have become a significant threat to public health worldwide. Development of analytical techniques that enable fast and accurate detection of foodborne pathogens is significant for food science and safety research. Assays based on lanthanide (Ln) ion-doped upconversion nanoparticles (UCNPs) show up as a cutting edge platform in biomedical fields because of the superior physicochemical features of UCNPs, including negligible autofluorescence, large signal-to-noise ratio, minimum photodamage to biological samples, high penetration depth, and attractive optical and chemical features. In recent decades, this novel and promising technology has been gradually introduced to food safety research. Herein, we have reviewed the recent progress of Ln³⁺-doped UCNPs in food safety research with emphasis on the following aspects: 1) the upconversion mechanism and detection principles; 2) the history of UCNPs development in analytical chemistry; 3) the in-depth state-of-the-art synthesis strategies, including synthesis protocols for UCNPs, luminescence, structure, morphology, and surface engineering; 4) applications of UCNPs in foodborne pathogens detection, including mycotoxins, heavy metal ions, pesticide residue, antibiotics, estrogen residue, and pathogenic bacteria; and 5) the challenging and future perspectives of using UCNPs in food safety research. Considering the diversity and complexity of the foodborne harmful substances, developing novel detections and quantification techniques and the rigorous investigations about the effect of the harmful substances on human health should be accelerated.

Orthogonal R/G/B Upconversion Luminescence-based Full-Color Tunable Upconversion Nanophosphors for Transparent Displays

Here, excitation orthogonalized red/green/blue upconversion luminescence (UCL)-based full-color tunable rare-earth (RE) ion-doped upconversion nanophosphors (UCNPs) are reported. The LiREF₄-based core/sextuple-shell (C/6S) UCNPs are synthesized, and they consist of a blue-emitting core, green-emitting inner shell, and red-emitting outer shell, with inert intermediate and outermost shells. The synthesized C/6S UCNPs emit blue, green, and red light under 980, 800, and 1532 nm, respectively. Importantly, by combining incident near-infrared (NIR) light with various wavelengths (800, 980, and 1532 nm), full-color UCL including blue, cyan, green, yellow, orange, red, purple, and white UCL is achieved from the single C/6S UCNP composition. The color gamut obtained from the C/6S UCNPs shows 101.6% of the sRGB standard color gamut. Furthermore, transparent C/6S UCNP-polydimethylsiloxane (PDMS) composite is prepared. Full-color display realized in the transparent C/6S UCNP-PDMS composite indicates the feasibility of constructing the C/6S UCNP-based three-dimensional volumetric displays with wide color gamut.

Lanthanide doped luminescence nanothermometers in the biological windows: strategies and applications

The development of lanthanide-doped non-contact luminescent nanothermometers with accuracy, efficiency and fast diagnostic tools attributed to their versatility, stability and narrow emission band profiles has spurred the replacement of conventional contact thermal probes. The application of lanthanide-doped materials as temperature nanosensors, excited by ultraviolet, visible or near infrared light, and the generation of emissions lying in the biological window regions, I-BW (650 nm-950 nm), II-BW (1000 nm-1350 nm), III-BW (1400 nm-2000 nm) and IV-BW (centered at 2200 nm), are notably growing due to the advantages they present, including reduced phototoxicity and photobleaching, better image contrast and deeper penetration depths into biological tissues. Here, the different mechanisms used in lanthanide ion-doped nanomaterials to sense temperature in these biological windows for biomedical and other applications are summarized, focusing on factors that affect their thermal sensitivity, and consequently their temperature resolution. Comparing the thermometric performance of these nanomaterials in each biological window, we identified the strategies that allow boosting of their sensing properties.

Tunable upconversion of holmium sublattice through interfacial energy transfer for anti-counterfeiting

Photon upconversion is a fascinating phenomenon that can convert low-energy photons to high-energy photons efficiently. However, most previous relevant research has been focused on upconversion systems with a sufficiently low lanthanide emitter concentration, such as 2 mol% for Er3+ in an Er-Yb coupled system. Realizing the upconversion from lanthanide heavily doped systems in particular, the emitter sublattice is still a challenge. Here, we report a mechanistic strategy to achieve the intense upconversion of the holmium sublattice in a core-shell-based nanostructure design through interfacial energy transfer channels. This design allowed a spatial separation of Ho3+ and sensitizers (e.g., Yb3+) into different regions and unwanted back energy transfers between them could then be minimized. By taking advantage of the dual roles of Yb3+ as both a migrator and energy trapper, a gradual color change from red to yellowish green was achievable upon 808 nm excitation, which could be further markedly enhanced by surface attaching indocyanine green dyes to facilitate the harvesting of the incident excitation energy. Moreover, emission colors could be tuned by applying non-steady state excitation. Such a fine-tunable color behavior holds great promise in anti-counterfeiting. Our results present a facile but effective conceptual model for the upconversion of the holmuim sublattice, which is helpful for the development of a new class of luminescent materials toward frontier applications.

Orthogonal Emissive Upconversion Nanoparticles: Material Design and Applications

Upconversion nanoparticles (UCNPs) have gone beyond traditional fluorophores in a lot of fields due to the outstanding features such as sharp excitation and emission bands, chemical and photo stability of high quality, low auto fluorescence, and high tissue permeation depth of the near-infrared irradiation light used for excitation. Conventional UCNPs carrying single/multiple emissions under a single excitation wavelength can be only employed in concurrent activation, orthogonal emissive upconversion nanoparticles (OUCNPs) with the emissions, a kind of luminescence reliant on excitation, in which by switching the external excitation different lanthanide activators can adopt independent way to control the emission, is more like an ideal UCNPs nanoplatform which can switch their activated emissions depending upon the different application for which it is used at the right time when necessary. This review summaries what has been achieved on the synthesis optimization of designed OUCNPs in recent years and sums up various applications including bioimaging, photo-switching, and programmable control process. And also, the limitations OUCNPs face, and the efforts that have been made to overcome these limitations are discussed.

High Color-Purity Red, Green, and Blue-Emissive Core-Shell Upconversion Nanoparticles Using Ternary Near-Infrared Quadrature Excitations

Development of multicolor-emitting upconversion nanoparticles (UCNPs) is of significant importance for applications in optical encoding, anti-counterfeiting, display, and bioimaging. However, realizing the orthogonal three-primary color (TPC) upconversion luminescence in a single nanoparticle remains a huge challenge. Herein, we have rationally designed core-multishell-structured NaYF₄ UCNPs through regulating the dopant concentration, composition of luminescent layers, and shell position and thickness, which are capable of emitting red, green, and blue luminescence with high color purity in response to ternary near-infrared quadrature excitations (1560/808/980 nm). Moreover, their high color purity is well retained with varying excitation power densities. This orthogonal TPC emissions property of such UCNPs endows them with great promise in the field of security. As a proof-of-concept, we have demonstrated the feasibility of combining such UCNPs with MnO₂ nanosheets for information encryption and decryption. This work not only offers a new way to achieve TPC upconversion luminescence at a single nanoparticle level but also broadens the scope of application for security protection.

PAA Modified Upconversion Nanoparticles for Highly Selective and Sensitive Detection of Cu2+ Ions

Detection of the Cu²⁺ ions is crucial because of its environmental and biological implications. The fluorescent-based organic sensors are not suitable for Cu²⁺ detection due to their short penetration depth caused by the UV/visible excitation source. Therefore, we have demonstrated a highly sensitive and selective near-infrared (NIR) excitable poly(acrylic acid) (PAA) coated upconversion nanoparticles (UCNPs) based sensor for Cu²⁺ detection. We construct the PAA modified Na(Yb, Nd)F₄@Na(Yb, Gd)F₄:Tm@NaGdF₄ core-shell-shell structured UCNPs based sensor via a co-precipitation route. The upconversion emission intensity of the PAA-UCNPs decreases linearly with the increase in the Cu²⁺ concentration from 0.125 to 3.125 μM due to the copper carboxylate complex formation between Cu²⁺ and PAA-UCNPs. The calculated detection limit of the PAA-UCNPs based sensor is 0.1 μM. The PAA-UCNPs based sensor is very sensitive and selective toward detecting the Cu²⁺ ions, even when the Cu²⁺ co-exist with other metal ions. The EDTA addition has significantly reversed the upconversion emission quenching by forming the EDTA-Cu²⁺ complex based on their greater affinity toward the Cu²⁺. Therefore, the PAA-UCNPs based sensor can be a promising candidate for Cu²⁺ detection because of their higher sensitivity and selectivity under 980 nm NIR excitation.

Nano-assemblies of a soluble conjugated organic polymer and an inorganic semiconductor for sacrificial photocatalytic hydrogen production from water

Nanostructured materials have interesting optical and electronic properties that are often drastically different from those of their bulk counterparts. While bulk organic/inorganic semiconductor composites have attracted much attention in the past decade, the preparation of organic/inorganic semiconductor nanocomposites (OISNs) still remains challenging. This work presents an assembly method for the co-encapsulation of titanium dioxide dots (TDs) with a cyano-substituted soluble conjugated polymer (CSCP) into a particular nanoparticle. The as-prepared CSCP/TD semiconductor nanocomposites (CSCP/TD NCs) exhibit different particle surfaces and morphologies depending on the mass ratio of the CSCP to TDs. We then tested them as photocatalysts for sacrificial hydrogen production from water. We found that nanocomposites outperformed nanoparticles of the individual components and physical mixtures thereof. The most active CSCP/TD NC had a catalytic H2 production rate that was 4.25 times higher than that of pure polymer nanoparticles prepared under the same conditions. We ascribe this to energy transfer between the semiconductors, where direct phase contact is essential, highlighting a potential avenue for using soluble, visible light-absorbing conjugated organic polymers to build Z-schemes for overall water splitting in the future.

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.

Up-Conversion Luminescent Nanoparticles for Molecular Imaging, Cancer Diagnosis and Treatment

In the past few years, we have witnessed great development and application potential of various up-conversion luminescent nanoparticles (UCNPs) in the nanomedicine field. Based on the unique luminescent mechanism of UCNPs and the distinguishable features of cancer biomarkers and the microenvironment, an increasing number of smart UCNPs nanoprobes have been designed and widely applied to molecular imaging, cancer diagnosis, and treatment. Considerable technological success has been achieved, but the main obstacles to oncology nanomedicine is becoming an incomplete understanding of nano-bio interactions, the challenges regarding chemistry manufacturing and controls required for clinical translation and so on. This review highlights the progress of the design principles, synthesis and surface functionalization preparation, underlying applications and challenges of UCNPs-based probes for cancer bioimaging, diagnosis and treatment that capitalize on our growing understanding of tumor biology and smart nano-devices for accelerating the commercialization of UCNPs.

Highly Red Light-Emitting Erbium- and Lutetium-Doped Core-Shell Upconverting Nanoparticles Surface-Modified with PEG-Folic Acid/TCPP for Suppressing Cervical Cancer HeLa Cells

Photodynamic therapy (PDT) combined with upconverting nanoparticles (UCNPs) are viewed together as an effective method of ablating tumors. After absorbing highly tissue-penetrating near-infrared (NIR) light, UCNPs emit a shorter wavelength light (~660 nm) suitable for PDT. In this study, we designed and prepared highly red fluorescence-emitting silica-coated core-shell upconverting nanoparticles modified with polyethylene glycol (PEG5k)-folic acid and tetrakis(4-carboxyphenyl)porphyrin (TCPP) (UCNPs@SiO2-NH2@FA/PEG/TCPP) as an efficient photodynamic agent for killing tumor cells. The UCNPs consisted of two simple lanthanides, erbium and lutetium, as the core and shell, respectively. The unique core-shell combination enabled the UCNPs to emit red light without green light. TCPP, folic acid, and PEG were conjugated to the outer silica layer of UCNPs as a photosensitizing agent, a ligand for tumor attachment, and a dispersing stabilizer, respectively. The prepared UCNPs of ~50 nm diameter and -34.5 mV surface potential absorbed 808 nm light and emitted ~660 nm red light. Most notably, these UCNPs were physically well dispersed and stable in the aqueous phase due to PEG attachment and were able to generate singlet oxygen (1O2) with a high efficacy. The HeLa cells were treated with each UCNP sample (0, 1, 5, 10, 20, 30 μg/mL as a free TCPP). The results showed that the combination of UCNPs@SiO2-NH2@FA/PEG/TCPP and the 808 nm laser was significantly cytotoxic to HeLa cells, almost to the same degree as naïve TCPP plus the 660 nm laser based on MTT and Live/Dead assays. Furthermore, the UCNPs@SiO2-NH2@FA/PEG/TCPP was well internalized into HeLa cells and three-dimensional HeLa spheroids, presumably due to the surface folic acid and small size in conjunction with endocytosis and the nonspecific uptake. We believe that our UCNPs@SiO2-NH2@FA/PEG/TCPP will serve as a new platform for highly efficient and deep-penetrating photodynamic agents suitable for various tumor treatments.

Infrared neuromodulation:a neuroengineering perspective

Infrared neuromodulation (INM) is a branch of photobiomodulation that offers direct or indirect control of cellular activity through elevation of temperature in a spatially confined region of the target tissue. Research on INM started about 15 ago and is gradually attracting the attention of the neuroscience community, as numerous experimental studies have provided firm evidence on the safe and reproducible excitation and inhibition of neuronal firing in both in vitro and in vivo conditions. However, its biophysical mechanism is not fully understood and several engineered interfaces have been created to investigate infrared stimulation in both the peripheral and central nervous system. In this review, recent applications and present knowledge on the effects of INM on cellular activity are summarized, and an overview of the technical approaches to deliver infrared light to cells and to interrogate the optically evoked response is provided. The micro- and nanoengineered interfaces used to investigate the influence of INM are described in detail.

Tuning epitaxial growth on NaYbF4 upconversion nanoparticles by strain management

Core-shell structural engineering is a common strategy for tuning upconversion luminescence in lanthanide-doped nanoparticles. However, epitaxial growth on hexagonal phase NaYbF₄ nanoparticles typically suffers from incomplete shell coverage due to the large and anisotropic interfacial strain. Herein, we explore the effects of core particle size and morphology as well as reaction temperature on controlling the epitaxial growth of NaGdF₄ shells on NaYbF₄ nanoparticles with misfit parameters of f_a = 1.58% and f_l = 2.24% for axial and lateral growth, respectively. Rod-like core particles with a long length and a large diameter are found to promote shell growth with high surface coverage by facilitating the relaxation of lattice strains. Furthermore, the primary NaGdF₄ shell can serve as a transition layer to mediate the growth of additional NaNdF₄ coating layers that display an even larger lattice misfit with the core (f_a = 2.98%; f_l = 4.32%). The resultant NaYbF₄@Na(Gd/Nd)F₄ core-shell nanostructures simultaneously show strong multiphoton upconversion luminescence and superior magnetic resonance T₁ ionic relaxivity. Our findings are important for the rational design of core-shell upconversion nanoparticles with optimized properties and functionality for technological applications.

Near-infrared-light regulated angiogenesis in a 4D hydrogel

Light-responsive hydrogels are useful platforms to study cellular responses. Current photosensitive motifs need UV light to be activated, which is intrinsically cytotoxic and has a low penetration depth in tissues. Herein we describe a strategy for near-infrared (NIR) controlled activation of cellular processes (3D cell spreading and angiogenesis) by embedding upconverting nanoparticles (UCNPs) in a hydrogel modified with light-activatable cell adhesive motifs. The UCNPs can convert NIR light (974 nm) into local UV emission and activate photochemical reactions on-demand. Such optoregulation is spatially controllable, dose-dependent and can be performed at different timepoints of the cell culture without appreciable photodamage of the cells. HUVEC cells embedded in this hydrogel can form vascular networks at predefined geometries determined by the irradiation pattern. The penetration depth of NIR light enabled activation of the angiogenesis response through skin tissue with a thickness of 2.5 mm. Our strategy opens a new avenue for 4D cell cultures, with the potential to be extended to dynamically manipulate cell-matrix interactions and derived cellular processes in vivo.

808 nm-activable core@multishell upconverting nanoparticles with enhanced stability for efficient photodynamic therapy

BACKGROUND

The unique upconversion properties of rare-earth-doped nanoparticles offers exciting opportunities for biomedical applications, in which near-IR remote activation of biological processes is desired, including in vivo bioimaging, optogenetics, and light-based therapies. Tuning of upconversion in purposely designed core-shell nanoparticles gives access to biological windows in biological tissue. In recent years there have been several reports on NIR-excitable upconverting nanoparticles capable of working in biological mixtures and cellular settings. Unfortunately, most of these nanosystems are based on ytterbium's upconversion at 980 nm, concurrent with water's absorption within the first biological window. Thus, methods to produce robust upconverting nanoplatforms that can be efficiently excited with other than 980 nm NIR sources, such as 808 nm and 1064 nm, are required for biomedical applications.

RESULTS

Herein, we report a synthetic method to produce aqueous stable upconverting nanoparticles that can be activated with 808 nm excitation sources, thus avoiding unwanted heating processes due to water absorbance at 980 nm. Importantly, these nanoparticles, once transferred to an aqueous environment using an amphiphilic polymer, remain colloidally stable for long periods of time in relevant biological media, while keeping their photoluminescence properties. The selected polymer was covalently modified by click chemistry with two FDA-approved photosensitizers (Rose Bengal and Chlorin e6), which can be efficiently and simultaneously excited by the light emission of our upconverting nanoparticles. Thus, our polymer-functionalization strategy allows producing an 808 nm-activable photodynamic nanoplatform. These upconverting nanocomposites are preferentially stored in acidic lysosomal compartments, which does not negatively affect their performance as photodynamic agents. Upon 808 nm excitation, the production of reactive oxidative species (ROS) and their effect in mitochondrial integrity were demonstrated.

CONCLUSIONS

In summary, we have demonstrated the feasibility of using photosensitizer-polymer-modified upconverting nanoplatforms that can be activated by 808 nm light excitation sources for application in photodynamic therapy. Our nanoplatforms remain photoactive after internalization by living cells, allowing for 808 nm-activated ROS generation. The versatility of our polymer-stabilization strategy promises a straightforward access to other derivatizations (for instance, by integrating other photosensitizers or homing ligands), which could synergistically operate as multifunctional photodynamic platforms nanoreactors for in vivo applications.

Combating Concentration Quenching in Upconversion Nanoparticles

Lanthanide-doped upconversion nanoparticles (UCNPs) are a special class of luminescent nanomaterials that convert multiwavelength near-infrared (NIR) excitation into tunable emissions spanning the deep ultraviolet (UV) to NIR regions. In addition to large anti-Stokes shift, UCNPs also feature a sharp emission bandwidth, long excited-state lifetime, as well as high resistance to optical blinking and photobleaching. Therefore, UCNPs have been identified as promising candidates to solve many challenging problems in fields ranging from biological imaging and therapeutics to photovoltaics and photonics. Nevertheless, the progress of utilizing an upconversion process is being hindered by the limited emission intensity, principally due to low oscillator strength in these nanoparticles. UCNPs essentially resemble the optical characteristics of their bulk counterparts, which take advantage of electronic transition within the 4f configuration of the lanthanide dopants to realize photon energy conversions. In general, a high dopant concentration promotes upconversion luminescence by providing a high density of optical centers to collect and to sustain the energy of the excitation light. However, an increase in dopant concentration induces self-quenching processes that offset the emission gain and may eventually result in attenuation of the overall emission intensity. This phenomenon known as concentration quenching represents a major obstacle to constructing bright UCNPs. In recent years, advances in nanoparticle research have led to the emergence of several strategies for mitigating energy loss at elevated dopant concentrations. In consequence, doping high levels of lanthanide ions in UCNPs has become a viable solution to boosting the emission intensity of photon upconversion. On account of extensive energy exchange interaction in heavily doped UCNPs, the spectrum tunability of photon upconversion is also greatly enhanced. These advances have largely expanded the scope of upconversion research. To provide guidelines for enhancing upconversion through heavy doping, we attempt to review recent advances in the understanding and control of concentration quenching in UCNPs. With significant advancements made in the chemical synthesis, we are now able to exquisitely control the doping of lanthanide ions in various nanoparticles of well-defined size, morphology, and core-shell structure. We show that, by confining energy transfer in nanostructured host materials in conjunction with innovative excitation schemes, concentration quenching of upconversion luminescence is largely alleviated. As a result, unusually high dopant concentrations can be used to construct UCNPs displaying high brightness and large anti-Stokes shift. We demonstrate that the development of heavily doped UCNPs enables advanced bioimaging and photonic applications that can hardly be fulfilled by conventional UCNPs comprising low concentrations of lanthanide dopants.

Upconversion Nanoparticles-Based Multiplex Protein Activation to Neuron Ablation for Locomotion Regulation

The optogenetic neuron ablation approach enables noninvasive remote decoding of specific neuron function within a complex living organism in high spatiotemporal resolution. However, it suffers from shallow tissue penetration of visible light with low ablation efficiency. This study reports a upconversion nanoparticle (UCNP)-based multiplex proteins activation tool to ablate deep-tissue neurons for locomotion modulation. By optimizing the dopant contents and nanoarchitecure, over 300-fold enhancement of blue (450-470 nm) and red (590-610 nm) emissions from UCNPs is achieved upon 808 nm irradiation. Such emissions simultaneously activate mini singlet oxygen generator and Chrimson, leading to boosted near infrared (NIR) light-induced neuronal ablation efficiency due to the synergism between singlet oxygen generation and intracellular Ca²⁺ elevation. The loss of neurons severely inhibits reverse locomotion, revealing the instructive role of neurons in controlling motor activity. The deep penetrance NIR light makes the current system feasible for in vivo deep-tissue neuron elimination. The results not only provide a rapidly adoptable platform to efficient photoablate single- and multiple-cells, but also define the neural circuits underlying behavior, with potential for development of remote therapy in diseases.

Plasmonically Enhanced Upconversion Luminescence via Holographically Formed Silver Nanogratings

Greatly enhanced upconversion luminescence was demonstrated by integrating the core-shell upconversion nanorods with the Ag nanogratings. Both the Ag nanogratings and upconversion nanorods were fabricated/synthesized in a facile, cost-effective, high-throughput way. Experimental results showed that the upconversion luminescence intensity of Er³⁺ in the core-shell upconversion nanorods can be well tuned and enhanced by changing the shell thickness and the period of the Ag nanograting. The underlying physical mechanism for the upconversion luminescence enhancement was attributed to the plasmonically enhanced near infrared broadband absorption of the periodic Ag nanograting and the localized surface plasmon resonance of Ag nanocrystals. The maximum enhanced factors of 523 nm, 544 nm (green emission), and 658 nm (red emission) of Er³⁺ ions excited at 980 nm are 3.8-, 5.5-, and 4.6-folds, respectively. Our fabrication approach and results suggest that such a simple integration is potentially useful for biosensing/imaging and anti-counterfeiting applications.

Recent advances in the synthesis and application of Yb-based fluoride upconversion nanoparticles

This review focuses on recent progress in the development of Yb-based upconversion nanoparticles and their emerging technological applications.

Facile synthesis of sub-10 nm-sized bright red-emitting upconversion nanophosphors via tetrahedral YOF:Yb,Er seed-mediated growth

Ultrasmall and uniform tetrahedral-shaped YOF:Yb,Er upconversion nanophosphors (UCNs) are synthesized and sub-10 nm YOF:Yb,Er/YOF core/shell UCNs are formed via YOF:Yb,Er seed-mediated synthesis. The ultrasmall YOF:Yb,Er/YOF core/shell UCNs realize intense red emission under near infrared light (λ_ex = 980 nm).

Manipulating energy migration within single lanthanide activator for switchable upconversion emissions towards bidirectional photoactivation

Reliance on low tissue penetrating UV or visible light limits clinical applicability of phototherapy, necessitating use of deep tissue penetrating near-infrared (NIR) to visible light transducers like upconversion nanoparticles (UCNPs). While typical UCNPs produce multiple simultaneous emissions for unidirectional control of biological processes, programmable control requires orthogonal non-overlapping light emissions. These can be obtained through doping nanocrystals with multiple activator ions. However, this requires tedious synthesis and produces complicated multi-shell nanoparticles with a lack of control over emission profiles due to activator crosstalk. Herein, we explore cross-relaxation (CR), a non-radiative recombination pathway typically perceived as deleterious, to manipulate energy migration within the same lanthanide activator ion (Er³⁺) towards orthogonal red and green emissions, simply by adjusting excitation wavelength from 980 to 808 nm. These UCNPs allow programmable activation of two synergistic light-gated ion channels VChR1 and Jaws in the same cell to manipulate membrane polarization, demonstrated here for cardiac pacing.

Orthogonal light based control of biology is of interest, yet the synthesis of materials capable of this is complex. Here, the authors report on the synthesis of simpler upconversion nanoparticles which used cross-relaxation to change emission spectra from red to green light with a change in NIR wavelength.