Full shell coating or cation exchange enhances luminescence

Lanthanide-Doped Upconversion Nanoparticles for Single-Particle Imaging

Lanthanide-doped upconversion nanoparticles (UCNPs) have recently demonstrated great promise in single-particle imaging (SPI) due to their exceptional photostability and minimal background fluorescence. However, their limited brightness has posed a significant barrier to wider adoption in SPI applications. This review highlights recent advances in applying UCNPs for SPI, focusing on strategies to enhance their brightness and reduce quenching effects in aqueous environments. Additionally, it summarizes the latest progress in using UCNPs for single-particle tracking and super-resolution imaging, underscoring their potential in biomedical research. Finally, the review outlines current challenges and future directions in this field.

Anomalously Large Luminescence Modulation Induced by Trace Lanthanide Impurities in Alloyed Upconversion Nanocrystals

Achieving precise control over emission characteristics, such as wavelength and lifetime, is critical to unlocking the full potential of luminescent nanomaterials for diverse applications. In this work, we present a strategy for fine-tuning the optical properties of upconversion nanocrystals by engineering parts-per-million (ppm)-doping-level lanthanide impurities. We show that even trace impurities (∼10 ppm, fewer than 10 atoms per nanocrystal), which are only a hundredth of the conventionally studied doping levels and were previously considered negligible, serve as efficient energy traps in energy migration-based upconversion processes. By introducing controlled trapping centers via minimal impurity doping, we successfully regulate the upconversion emission colors and lifetimes with high precision. Moreover, we find that high-purity nanocrystals exhibit significantly greater lifetime changes in response to surface interactions, enabling an energy-transfer-based ultrasensitive spectrum and lifetime sensing. This approach facilitates the development of upconversion-based DNA sensors with detection limits over an order of magnitude lower than those of conventional methods, highlighting the potential of these nanocrystals as highly effective nanoprobes for interference-resistant biosensing in complex environments.

Heterogenous Core-Shell Persistent Luminescent Nanoparticles with Enhanced Afterglow Luminescence

Persistent luminescent nanoparticles (PLNPs) are promising for many bioapplications due to their unique afterglow luminescence following the stoppage of light excitation. However, PLNPs are prone to surface quenching that results in weak afterglow luminescence. Although some efforts have been made to reduce surface quenching through designing homogeneous core-shell PLNPs, the enhancement in afterglow luminescence was insignificant. We hypothesize that the independent absorption and emission of the shell caused less energy to reach the activator ions in the core. Hence, a heterogeneous core-shell PLNP where the shell has a higher band gap than the core would reduce the absorption and emission of the shell. In this work, ZnGa2O4 and Zn2GeO4 were coated on Zn1.2Ga1.6Ge0.2O4:Cr and Zn3Ga2Ge2O10:Eu nanocrystals, respectively, to form heterogeneous core-shell PLNPs and significant luminescence enhancement was achieved compared to their traditional homogeneous core-shell nanostructures.

Motor-Cargo Structured Nanotractors for Augmented NIR Phototherapy via Gas-Boosted Tumor Penetration and Respiration-Impaired Mitochondrial Dysfunction

Tumor microenvironment, characterized by dense extracellular matrix and severe hypoxia, has caused pronounced resistance to photodynamic therapy (PDT). Herein, it has designed an artificial nitric oxide (NO) nanotractor with a unique "motor-cargo" structure, where a photoswitching upconversion nanoparticle (UCNP) core serves as the optical engine to harvest NIR light and asymmetrically coated mesoporous silica (SiO2) shell acts as a cargo unit to load nitric oxide (NO) fuel molecule (RBS, Roussin's black salt) and PDT photosensitizer (ZnPc, zinc phthalocyanine). Upon illumination by 980 nm light, the UCNP emits blue light to excite RBS salt and release NO gas. On one hand, NO is used as the driving force to propel the particle with a high speed of ≈194 µm s-1 that generates significant rupture stress (over 0.95 kPa) on cell membrane to promote cellular endocytosis and intratumoral penetration. On the other hand, NO enables to alleviate tumor hypoxia by inhibiting cellular respiration as an oxygen conserver. When the excitation is subsequently switched to 808 nm light, the UCNP emits red light, triggering ZnPc to produce large amount of reactive oxygen species for PDT treatment. This study explores Janus-typed nanostructures for cell-particle interaction and gas-assisted phototherapy, opening avenues for versatile bioapplications.

Full-course NIR-II imaging-navigated fractionated photodynamic therapy of bladder tumours with X-ray-activated nanotransducers

The poor 5-year survival rate for bladder cancers is associated with the lack of efficient diagnostic and treatment techniques. Despite cystoscopy-assisted photomedicine and external radiation being promising modalities to supplement or replace surgery, they remain invasive or fail to provide real-time navigation. Here, we report non-invasive fractionated photodynamic therapy of bladder cancer with full-course real-time near-infrared-II imaging based on engineered X-ray-activated nanotransducers that contain lanthanide-doped nanoscintillators with concurrent emissions in visible and the second near-infrared regions and conjugated photosensitizers. Following intravesical instillation in mice with carcinogen-induced autochthonous bladder tumours, tumour-homing peptide-labelled nanotransducers realize enhanced tumour regression, robust recurrence inhibition, improved survival rates, and restored immune homeostasis under X-ray irradiation with accompanied near-infrared-II imaging. On-demand fractionated photodynamic therapy with customized doses is further achieved based on quantifiable near-infrared-II imaging signal-to-background ratios. Our study presents a promising non-invasive strategy to confront the current bladder cancer dilemma from diagnosis to treatment and prognosis.

Fluoride Hafnium/Zirconium-Softened Nanoprobes for Near-Infrared-IIb and CT Dual-Mode Bioimaging

Fluoride-based lanthanide-doped nanoparticles (LDNPs) featuring second near-infrared (NIR-II, 1000-1700 nm) downconversion emission for bioimaging have attracted extensive attention. However, conventional LDNPs cannot be degraded and eliminated from organisms because of an inert lattice, which obstructs bioimaging applications. Herein, the core-shell LDNPs of Na3HfF7:Yb,Er@CaF2:Ce,Zr(Hf) [labeled as Zr(Hf)Ce-HC] with pH-selective and tunable degradability were synthesized for dual-modal bioimaging. Notably, the "softening" lattice of the Na3HfF7 matrix and different Zr4+(Hf4+) doping amounts in the shell enable Zr(Hf)Ce-HC with acidity-dependent and tunable degradability. After coating of an optimized Ce3+-doped CaF2:Zr shell, the near-infrared-IIb (NIR-IIb, 1500-1700 nm) luminescence intensity of ZrCe-HC is enhanced by 5.2 times compared with that of Na3HfF7:Yb,Er. The Hf element with high X-ray attenuation allows ZrCe-HC as the contrast agent for computed tomography (CT) bioimaging. The modification of oxidized sodium alginate endows ZrCe-HC with satisfying biocompatibility for NIR-IIb/CT dual-modal bioimaging. These findings would benefit the bioimaging applications of degradable fluoride-based LDNPs.

Upconversion Nanoparticles Based Sensing: From Design to Point-of-Care Testing

Rare earth-doped upconversion nanoparticles (UCNPs) have achieved a wide range of applications in the sensing field due to their unique anti-Stokes luminescence property, minimized background interference, excellent biocompatibility, and stable physicochemical properties. However, UCNPs-based sensing platforms still face several challenges, including inherent limitations from UCNPs such as low quantum yields and narrow absorption cross-sections, as well as constraints related to energy transfer efficiencies in sensing systems. Therefore, the construction of high-performance UCNPs-based sensing platforms is an important cornerstone for conducting relevant research. This work begins by providing a brief overview of the upconversion luminescence mechanism in UCNPs. Subsequently, it offers a comprehensive summary of the sensors' types, design principles, and optimized design strategies for UCNPs sensing platforms. More cost-effective and promising point-of-care testing applications implemented based on UCNPs sensing systems are also summarized. Finally, this work addresses the future challenges and prospects for UCNPs-based sensing platforms.

Recent progress of UCNPs-MoS2 nanocomposites as a platform for biological applications

Composite materials can take advantages of the functional benefits of multiple pure nanomaterials to a greater degree than single nanomaterials alone. The UCNPs-MoS2 composite is a nano-application platform that combines upconversion luminescence and photothermal properties. Upconversion nanoparticles (UCNPs) are inorganic nanomaterials with long-wavelength excitation and short-wavelength tunable emission capabilities, and are able to effectively convert near-infrared (NIR) light into visible light for increased photostability. However, UCNPs have a low capacity for absorbing visible light, whereas MoS2 shows better absorption in the ultraviolet and visible regions. By integrating the benefits of UCNPs and MoS2, UCNPs-MoS2 nanocomposites can convert NIR light with a higher depth of detection into visible light for application with MoS2 through fluorescence resonance energy transfer (FRET), which compensates for the issues of MoS2's low tissue penetration light-absorbing wavelengths and expands its potential biological applications. Therefore, starting from the construction of UCNPs-MoS2 nanoplatforms, herein, we review the research progress in biological applications, including biosensing, phototherapy, bioimaging, and targeted drug delivery. Additionally, the current challenges and future development trends of UCNPs-MoS2 nanocomposites for biological applications are also discussed.

Raspberry-like Nanoheterostructures Comprising Glutathione-Capped Gold Nanoclusters Grown on the Lanthanide Nanoparticle Surface

Bare lanthanide-doped nanoparticles (LnNPs), in particular, NaYF4:Yb3+,Tm3+ NPs (UCTm), have been seeded in situ with gold cations to be used in the subsequent growth of gold nanoclusters (AuNCs) in the presence of glutathione (GSH) to obtain a novel UCTm@AuNC nanoheterostructure (NHS) with a raspberry-like morphology. UCTm@AuNC displays unique optical properties (multiple absorption and emission wavelengths). Specifically, upon 350 nm excitation, it exhibits AuNC photoluminescence (PL) (500-1200 nm, λmax 650 nm) and Yb emission (λmax 980 nm); this is the first example of Yb sensitization in a UCTm@AuNC NHS. Moreover, under 980 nm excitation, it displays (i) upconverting PL of the UCTm (at the blue, red and NIR-I, ca. 800 nm, regions); (ii) two-photon PL of AuNC; and (iii) down-shifting PL of thulium (around 1470 nm). The occurrence of energy transfer from UCTm to AuNCs in the UCTm@AuNC NHS was evidenced by the drastic lengthening of the AuNC PL lifetime (τPL) (from few hundred nanoseconds to more than one hundred microseconds). Initial biological assessment of UCTm@AuNC NHSs in vitro revealed high biocompatibility and bioimaging capabilities upon near-infrared excitation.

Ternary heterostructure-driven photoinduced electron-hole separation enhanced oxidative stress for triple-negative breast cancer therapy

Zinc oxide nanoparticles (ZnO NPs) stand as among the most significant metal oxide nanoparticles in trigger the formation of reactive oxygen species (ROS) and induce apoptosis. Nevertheless, the utilization of ZnO NPs has been limited by the shallowness of short-wavelength light and the constrained production of ROS. To overcome these limitations, a strategy involves achieving a red shift towards the near-infrared (NIR) light spectrum, promoting the separation and restraining the recombination of electron-hole (e--h+) pairs. Herein, the hybrid plasmonic system Au@ZnO (AZ) with graphene quantum dots (GQDs) doping (AZG) nano heterostructures is rationally designed for optimal NIR-driven cancer treatment. Significantly, a multifold increase in ROS generation can be achieved through the following creative initiatives: (i) plasmonic Au nanorods expands the photocatalytic capabilities of AZG into the NIR domain, offering a foundation for NIR-induced ROS generation for clinical utilization; (ii) elaborate design of mesoporous core-shell AZ structures facilitates the redistribution of electron-hole pairs; (iii) the incorporation GQDs in mesoporous structure could efficiently restrain the recombination of the e--h+ pairs; (iv) Modification of hyaluronic acid (HA) can enhance CD44 receptor mediated targeted triple-negative breast cancer (TNBC). In addition, the introduced Au NRs present as catalysts for enhancing photothermal therapy (PTT), effectively inducing apoptosis in tumor cells. The resulting HA-modified AZG (AZGH) exhibits efficient hot electron injection and e--h+ separation, affording unparalleled convenience for ROS production and enabling NIR-induced PDT for the cancer treanment. As a result, our well-designed mesoporous core-shell AZGH hybrid as photosensitizers can exhibit excellent PDT efficacy.

Janus Micromotors for Photophoretic Motion and Photon Upconversion Applications Using a Single Near-Infrared Wavelength

External stimuli can trigger changes in temperature, concentration, and momentum between micromotors and the medium, causing their propulsion and enabling them to perform different tasks with improved kinetic efficiencies. Light-activated micromotors are attractive systems that achieve improved motion and have the potential for high spatiotemporal control. Photophoretic swarming motion represents an attractive means to induce micromotor movement through the generation of temperature gradients in the medium, enabling the micromotors to move from cold to hot regions. The micromotors studied herein are assembled with Fe3O4 nanoparticles, and NaGdF4:Yb3+,Er3+/NaGdF4:Yb3+ and LiYF4:Yb3+,Tm3+ upconverting nanoparticles. The Fe3O4 nanoparticles were localized to one hemisphere to produce a Janus architecture that facilitates improved upconversion luminescence with the upconverting nanoparticles distributed throughout. Under 976 nm excitation, Fe3O4 nanoparticles generate the temperature gradient, while the upconverting nanoparticles produce visible light that is used for micromotor motion tracking and triggering of reactive oxygen species generation. As such, the motion and application of the micromotors are achieved using a single excitation wavelength. To demonstrate the practicality of this system, curcumin was adsorbed to the micromotor surface and degradation of Rhodamine B was achieved with kinetic rates that were over twice as fast as the static micromotors. The upconversion luminescence was also used to track the motion of the micromotors from a single image frame, providing a convenient means to understand the trajectory of these systems. Together, this system provides a versatile approach to achieving light-driven motion while taking advantage of the potential applications of upconversion luminescence such as tracking and detection, sensing, nanothermometry, particle velocimetry, photodynamic therapy, and pollutant degradation.

Upconversion Luminescence in Mononuclear Yb/Sm Co-crystal Assemblies at Room Temperature

Metal-based upconversion luminescence transforming high-energy photons into low-energy photons is an attractive anti-Stokes shift process for fundamental research and promising applications. In this work, we developed the upconversion luminescence in co-crystal assemblies consisting of discrete mononuclear Yb and Sm complexes. The characteristic visible emissions of Sm3+ were observed under the excitation of absorption band of Yb3+ at 980 nm. A series of co-crystal assemblies were investigated based on mononuclear Yb and Sm complexes, and the strongest luminescence was obtained when the molar concentration between Yb3+ and Sm3+ is equivalent. The crystal structure was fully characterized by the single crystal X-ray diffraction and upconverting energy transfer mechanisms were verified as cooperative sensitization upconversion and energy transfer upconversion. This is the first example of Sm3+ -based upconverting luminescence in discrete lanthanide complexes which present as co-crystal assemblies at room temperature.

Photoswitchable upconversion nanoparticles with excitation-dependent emission for programmed stepwise NIR phototherapy

Programmable control over therapeutic processes in phototherapy, like photodynamic therapy (PDT), is promising but challenging. This study uses an energy segmentation-based strategy to synthesize core-multi-shell upconversion nanoparticles (UCNPs), which can release three different colors (red, green, and blue) upon exposure to different near-infrared light (1550 nm, 808 nm, and 980 nm). By combining these UCNPs with photosensitizers and nitric oxide (NO) donors, a smart "off-on" PDT nanoplatform is developed. UCNPs enable independent activation of imaging, release of NO, and generation of reactive oxygen species using specific light wavelengths. The results show that sequential NO release before PDT can greatly alleviate tumor hypoxia by reducing oxygen consumption. This stepwise approach shows potential for precise NIR light-activated and imaging-guided phototherapy.

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.

Suppression of Cation Intermixing Highly Boosts the Performance of Core-Shell Lanthanide Upconversion Nanoparticles

Lanthanide upconversion nanoparticles (UCNPs) have been extensively explored as biomarkers, energy transducers, and information carriers in wide-ranging applications in areas from healthcare and energy to information technology. In promoting the brightness and enriching the functionalities of UCNPs, core-shell structural engineering has been well-established as an important approach. Despite its importance, a strong limiting issue has been identified, namely, cation intermixing in the interfacial region of the synthesized core-shell nanoparticles. Currently, there still exists confusion regarding this destructive phenomenon and there is a lack of facile means to reach a delicate control of it. By means of a new set of experiments, we identify and provide in this work a comprehensive picture for the major physical mechanism of cation intermixing occurring in synthesis of core-shell UCNPs, i.e., partial or substantial core nanoparticle dissolution followed by epitaxial growth of the outer layer and ripening of the entire particle. Based on this picture, we provide an easy but effective approach to tackle this issue that enables us to produce UCNPs with highly boosted optical properties.

Achieving Photon Upconversion in Mononuclear Lanthanide Molecular Complexes at Room Temperature

Photon upconversion luminescence at the molecule scale is a rarely observed phenomenon despite possessing colossal potential for basic research and reality applications. Here we show that the eight-coordinate erbium molecular complex composed of Er³⁺ ion, dibenzoylmethane, and 2,2'-bipyridine exhibits upconversion emission. Under direct excitation at the absorption band of Er³⁺ ion at 980 nm, the complex shows upconverted green emissions of Er³⁺ ion at 525 and 545 nm at room temperature. Noticeably, upon the introduction of fluoride ions into this complex, an additional upconverted red emission at 667 nm appears as well, and the luminescence intensities of both the green and red emissions increase by a factor of 13 at most. This study not only provides a strategy to adjust the green and red emissions in mononuclear erbium complexes but also broadens the horizons of designing lanthanide-based molecular upconversion systems.

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.

Glycosylated phospholipid-coated upconversion nanoparticles for bioimaging of non-muscle invasive bladder cancers

Detection of non-muscle invasive bladder cancer (NMIBC) is crucial to facilitate complete tumor resection, thus improving the survival rate as well as reducing the recurrence frequency and treatment expense. Fluorescence imaging cystoscopy is an effective method for the detection of NMIBC. However, its application is limited as the commonly applied fluorescent agents such as dyes and photosensitizers usually lack specific tumor accumulation and are vulnerable to photobleaching. Furthermore, the broad emission band of conventional fluorescent agents limits their imaging and detection efficacy. To overcome these limitations, upconversion nanoparticles (UCNPs) have been selected as the fluorescent agent, due to their resistance to photobleaching, less background auto-fluorescence, and narrow emission bands. In order to achieve active tumor targeting, the UCNPs are coated with a glycosylated phospholipid layer. The glycosylated phospholipid-coated UCNPs exhibited high selective accumulation in cancer cells over normal cells and enhanced the upconversion luminescence (UCL) (at 540 nm and 660 nm) from bladder cancer cells under 980 nm laser irradiation. Glycosylated phospholipid coating that promotes uptake of UCNPs by cancer cells, and UCL emitted from UCNPs under NIR (980 nm) laser irradiation for cancer cell imaging.

Fluorescent Nanoparticles for Super-Resolution Imaging

Super-resolution imaging techniques that overcome the diffraction limit of light have gained wide popularity for visualizing cellular structures with nanometric resolution. Following the pace of hardware developments, the availability of new fluorescent probes with superior properties is becoming ever more important. In this context, fluorescent nanoparticles (NPs) have attracted increasing attention as bright and photostable probes that address many shortcomings of traditional fluorescent probes. The use of NPs for super-resolution imaging is a recent development and this provides the focus for the current review. We give an overview of different super-resolution methods and discuss their demands on the properties of fluorescent NPs. We then review in detail the features, strengths, and weaknesses of each NP class to support these applications and provide examples from their utilization in various biological systems. Moreover, we provide an outlook on the future of the field and opportunities in material science for the development of probes for multiplexed subcellular imaging with nanometric resolution.

Shedding Light on Luminescent Janus Nanoparticles: From Synthesis to Photoluminescence and Applications

Luminescent Janus nanoparticles refer to a special category of Janus-based nanomaterials that not only exhibit dual-asymmetric surface nature but also attractive optical properties. The introduction of luminescence has endowed conventional Janus nanoparticles with many alluring light-responsive functionalities and broadens their applications in imaging, sensing, nanomotors, photo-based therapy, etc. The past few decades have witnessed significant achievements in this field. This review first summarizes well-established strategies to design and prepare luminescent Janus nanoparticles and then discusses optical properties of luminescent Janus nanoparticles based on downconversion and upconversion photoluminescence mechanisms. Various emerging applications of luminescent Janus nanoparticles are also introduced. Finally, opportunities and future challenges are highlighted with respect to the development of next-generation luminescent Janus nanoparticles with diverse applications.

Near-infrared mediated orthogonal bioimaging and intracellular tracking of upconversion nanophotosensitizers

Although upconversion photodynamic therapy (PDT) has gained extensive interests in disease treatment, the intracellular migration pathway of upconversion photosensitizers and underlying cell-particle interaction mechanism is still largely unexplored. In this work photoswitchable upconversion nanoparticles (UCNPs) are reported  that can release orthogonal emissions excited by two near-infrared lights, i.e., red color of 980-nm and green color of 808-nm light excitation. Taking advantage of the dual-emissive property, a methodology based on Pearson's correlation analysis is proposed to verify the accuracy of upconversion luminescence signals under different excitation lights, which has been previously neglected. Meanwhile, we have designed a near-infrared mediated bioimaging nanoplatform that can generate reactive oxygen species (ROS) using one light and simultaneously track the location of upconversion photosensitizers using another excitation light. Our study not only depicts the migration pathway of upconversion photosensitizers, but also demonstrates the organelle escape of these upconversion nanoparticles via PCI (photochemical internalization) process. It is believed that our results inspire more efficient synergistic therapy by combining PDT with other modalities in a programmable manner.