Achieving high-efficiency emission depletion nanoscopy by employing cross relaxation in upconversion nanoparticles

Natural and polyanionic heparin polysaccharide functionalized upconversion nanoparticles for highly sensitive and selective ratiometric detection of pesticide

Pesticide contamination is a global concern, threatening human health and food safety. Herein, we developed heparin (HEP) functionalized upconversion nanoparticles (UCNPs)-based ratiometric nanosensor for the sensitive detection of 2,6-dichloro-4-nitroaniline (DCN) pesticide via inner filter effect. The strategy for HEP functionalization of UCNPs is based on adjusting the surface potentials of UCNPs with polyanionic HEP through the electrostatic interaction. UCNPs (NaYbF4:Gd/Y/Tm@NaYbF4@NaYF4) was designed with core-shell-shell structure and extra sensitizer layer for efficient and strong upconversion luminescence (UCL) in the range of UV to NIR. After incorporation of DCN, the upconverted UV emission of UCNPs-HEP ratiometric nanosensor was considerably quenched with the NIR UCL at 800 nm remaining unchanged as internal standard. The UCNPs-HEP ratiometric nanosensor can achieve outstandingly selective and sensitive detection of DCN at the wide linear range of 5-300 μM with a detection limit of 0.41 μM. The remarkable applicability of the UCNPs-HEP ratiometric nanosensor was verified in apple, cucumber and grapes samples. The developed UCNPs-HEP ratiometric nanosensor with excellent biocompatibility and water dispersion capability, is promising for convenient, selective and sensitive sensing of DCN towards food and aqueous samples.

Optical super-resolution nanothermometry via stimulated emission depletion imaging of upconverting nanoparticles

From engineering improved device performance to unraveling the breakdown of classical heat transfer laws, far-field optical temperature mapping with nanoscale spatial resolution would benefit diverse areas. However, these attributes are traditionally in opposition because conventional far-field optical temperature mapping techniques are inherently diffraction limited. Optical super-resolution imaging techniques revolutionized biological imaging, but such approaches have yet to be applied to thermometry. Here, we demonstrate a super-resolution nanothermometry technique based on highly doped upconverting nanoparticles (UCNPs) that enable stimulated emission depletion (STED) super-resolution imaging. We identify a ratiometric thermometry signal and maintain imaging resolution better than ~120 nm for the relevant spectral bands. We also form self-assembled UCNP monolayers and multilayers and implement a detection scheme with scan times >0.25 μm2/min. We further show that STED nanothermometry reveals a temperature gradient across a joule-heated microstructure that is undetectable with diffraction limited thermometry, indicating the potential of this technique to uncover local temperature variation in wide-ranging practical applications.

Recent Advances in Fluorescent Nanoparticles for Stimulated Emission Depletion Imaging

Stimulated emission depletion (STED) microscopy, as a popular super-resolution imaging technique, has been widely used in bio-structure analysis and resolving the dynamics of biological processes beyond the diffraction limit. The performance of STED critically depends on the optical properties of the fluorescent probes. Ideally, the probe should process high brightness and good photostability, and exhibit a sensitive response to the depletion beam. Organic dyes and fluorescent proteins, as the most widely used STED probes, suffer from low brightness and exhibit rapid photobleaching under a high excitation power. Recently, luminescent nanoparticles (NPs) have emerged as promising fluorescent probes in biological imaging due to their high brightness and good photostability. STED imaging using various kinds of NPs, including quantum dots, polymer dots, carbon dots, aggregation-induced emission dots, etc., has been demonstrated. This review will comprehensively review recent advances in fluorescent NP-based STED probes, discuss their advantages and pitfalls, and outline the directions for future development.

Large-Area Near-Infrared Emission Enhancement on Single Upconversion Nanoparticles by Metal Nanohole Array

Single lanthanide (Ln) ion doped upconversion nanoparticles (UCNPs) exhibit great potential for biomolecule sensing and counting. Plasmonic structures can improve the emission efficiency of single UCNPs by modulating the energy transferring process. Yet, achieving robust and large-area single UCNP emission modulation remains a challenge, which obstructs investigation and application of single UCNPs. Here, we present a strategy using metal nanohole arrays (NHAs) to achieve energy-transfer modulation on single UCNPs simultaneously within large-area plasmonic structures. By coupling surface plasmon polaritons (SPPs) with higher-intermediate state (1D2 → 3F3, 1D2 → 3H4) transitions, we achieved a remarkable up to 10-fold enhancement in 800 nm emission, surpassing the conventional approach of coupling SPPs with an intermediate ground state (3H4 → 3H6). We numerically simulate the electrical field distribution and reveal that luminescent enhancement is robust and insensitive to the exact location of particles. It is anticipated that the strategy provides a platform for widely exploring applications in single-particle quantitative biosensing.

A ratiometric upconversion nanoprobe enables super-resolution imaging sensing of biothiols in living cells

We have developed an upconversion luminescent ratiometric nanoprobe, specifically designed for detection of biothiols with high sensitivity (∼25 nM) at the single-particle level. Using a single-particle localization and rendering method, this nanoprobe enables super-resolution imaging sensing of biothiols within a confined 22 nm space in living cells.

Carbon Dots: Synthesis, Characterizations, and Recent Advancements in Biomedical, Optoelectronics, Sensing, and Catalysis Applications

Carbon nanodots (CNDs), a fascinating carbon-based nanomaterial (typical size 2-10 nm) owing to their superior optical properties, high biocompatibility, and cell penetrability, have tremendous applications in different interdisciplinary fields. Here, in this Review, we first explore the superiority of CNDs over other nanomaterials in the biomedical, optoelectronics, analytical sensing, and photocatalysis domains. Beginning with synthesis, characterization, and purification techniques, we even address fundamental questions surrounding CNDs such as emission origin and excitation-dependent behavior. Then we explore recent advancements in their applications, focusing on biological/biomedical uses like specific organelle bioimaging, drug/gene delivery, biosensing, and photothermal therapy. In optoelectronics, we cover CND-based solar cells, perovskite solar cells, and their role in LEDs and WLEDs. Analytical sensing applications include the detection of metals, hazardous chemicals, and proteins. In catalysis, we examine roles in photocatalysis, CO2 reduction, water splitting, stereospecific synthesis, and pollutant degradation. With this Review, we intend to further spark interest in CNDs and CND-based composites by highlighting their many benefits across a wide range of applications.

Three-dimensional, dual-color nanoscopy enabled by migrating photon avalanches with one single low-power CW beam

The development of super-resolution fluorescence microscopy is very essential for understanding the physical and biological fundamentals at nanometer scale. However, to date most super-resolution modalities require either complicated/costly purpose-built systems such as multiple-beam architectures or complex post-processing procedures with intrinsic artifacts. Achieving three-dimensional (3D) or multi-channel sub-diffraction microscopic imaging using a simple method remains a challenging and struggling task. Herein, we proposed 3D highly-nonlinear super-resolution microscopy using a single-beam excitation strategy, and the microscopy principle was modelled and studied based on the ultrahigh nonlinearity enabled by photon avalanches. According to the simulation, the point spread function of highly nonlinear microscopy is switchable among different modes and can shrink three-dimensionally to sub-diffraction scale at the photon avalanche mode. Experimentally, we demonstrated 3D optical nanoscopy assisted with huge optical nonlinearities in a simple laser scanning configuration, achieving a lateral resolution down to 58 nm (λ/14) and an axial resolution down to 185 nm (λ/5) with one single beam of low-power, continuous-wave, near-infrared laser. We further extended the photon avalanche effect to many other emitters to develop multi-color photon avalanching nanoprobes based on migrating photon avalanche mechanism, which enables us to implement single-beam dual-color sub-diffraction super-resolution microscopic imaging.

Luminescence Thermometry Beyond the Biological Realm

As the field of luminescence thermometry has matured, practical applications of luminescence thermometry techniques have grown in both frequency and scope. Due to the biocompatibility of most luminescent thermometers, many of these applications fall within the realm of biology. However, luminescence thermometry is increasingly employed beyond the biological realm, with expanding applications in areas such as thermal characterization of microelectronics, catalysis, and plasmonics. Here, we review the motivations, methodologies, and advances linked to nonbiological applications of luminescence thermometry. We begin with a brief overview of luminescence thermometry probes and techniques, focusing on those most commonly used for nonbiological applications. We then address measurement capabilities that are particularly relevant for these applications and provide a detailed survey of results across various application categories. Throughout the review, we highlight measurement challenges and requirements that are distinct from those of biological applications. Finally, we discuss emerging areas and future directions that present opportunities for continued research.

Recent Advances in Functional Materials for Optical Data Storage

In the current data age, the fundamental research related to optical applications has been rapidly developed. Countless new-born materials equipped with distinct optical properties have been widely explored, exhibiting tremendous values in practical applications. The optical data storage technique is one of the most significant topics of the optical applications, which is considered as the prominent solution for conquering the challenge of the explosive increase in mass data, to achieve the long-life, low-energy, and super high-capacity data storage. On this basis, our review outlines the representative reports for mainly introducing the functional systems based on the newly established materials applied in the optical storage field. According to the material categories, the representative functional systems are divided into rare-earth doped nanoparticles, graphene, and diarylethene. In terms of the difference of structural features and delicate properties among the three materials, the application in optical storage is comprehensively illustrated in the review. Meanwhile, the potential opportunities and critical challenges of optical storage are also discussed in detail.

Over 104 -fold amplified upconversion luminescence of lanthanide nanocrystals through optical oscillator-like system

Recently, lanthanide (Ln) luminescent nanocrystals have attracted increasing attention in various fields such as biomedical imaging, lasers, and anticounterfeiting. However, due to the forbidden 4f-4f transition of lanthanide ions, the absorption cross-section and luminescence brightness of lanthanide nanocrystals are limited. To address the challenge, we constructed an optical oscillator-like system to repeatedly simulate lanthanide nanocrystals to enhance the absorption efficiency of lanthanide ions on excitation photons. In this optical system, the upconversion luminescence (UCL) of Tm3+ emission of ~450 nm excited by a 980 nm laser can be amplified by a factor beyond 104 . The corresponding downshifting luminescence of Tm3+ at 1460 nm was enhanced by three orders of magnitude. We also demonstrated that the significant luminescence enhancement in the designed optical oscillator-like system was general for various lanthanide nanocrystals including NaYF4 :Yb3+ /Ln3+ , NaErF4 @NaYF4 and NaYF4 :Yb3+ /Ln3+ @NaYF4 :Yb3+ @NaYF4 (Ln = Er, Tm, Ho) regardless of the wavelengths of excitation sources (808 and 980 nm). The mechanism study revealed that both elevated laser power in the optical system and multiple excitations on lanthanide nanocrystals were the main reason for the luminescence amplification. Our findings may benefit the future development of low-threshold upconversion and downshifting luminescence of lanthanide nanocrystals and expand their applications.

Tandem Photon Avalanches for Various Nanoscale Emitters with Optical Nonlinearity up to 41st-Order through Interfacial Energy Transfer

Photon avalanche has received continuous attention owing to its superior nonlinear dynamics and promising advanced applications. However, its impact is limited due to the intrinsic energy levels as well as the harsh requirements for the composites and sizes of doped materials. Here, with a universal mechanism named tandem photon avalanche (TPA), giant optical nonlinear response up to 41st-order in erbium ions, one of the most important lanthanide emitters, has been achieved on the nanoscale through interfacial energy transfer process. After capturing energy directly from the avalanched energy state 3 H4 of Tm3+ (800-nm emission), erbium ions also exhibit bright green and red PA emissions with intensities comparable to that of Tm3+ at a low excitation threshold (7.1 kWcm-2 ). Using the same strategy, effective PA looping cycles are successfully activated in Ce3+ and Ho3+ . Additionally, Yb3+ -mediated networks are constructed to further propagate PA effects to lowly-doped Tm3+ , enabling 475-nm PA emission. The newly proposed TPA strategy provides a facile route for generating photon avalanche not only from erbium ions but also from various emitters in multilayered core-shell nanoparticles.

An Extended NIR-II Superior Imaging Window from 1500 to 1900 nm for High-Resolution In Vivo Multiplexed Imaging Based on Lanthanide Nanocrystals

High-resolution in vivo optical multiplexing in second near-infrared window (NIR-II, 1000-1700 nm) is vital to biomedical research. Presently, limited by bio-tissue scattering, only luminescent probes located at NIR-IIb (1500-1700 nm) window can provide high-resolution in vivo multiplexed imaging. However, the number of available luminescent probes in this narrow NIR-IIb region is limited, which hampers the available multiplexed channels of in vivo imaging. To overcome the above challenges, through theoretical simulation we expanded the conventional NIR-IIb window to NIR-II long-wavelength (NIR-II-L, 1500-1900 nm) window on the basis of photon-scattering and water-absorption. We developed a series of novel lanthanide luminescent nanoprobes with emission wavelengths from 1852 nm to 2842 nm. NIR-II-L nanoprobes enabled high-resolution in vivo dynamic multiplexed imaging on blood vessels and intestines, and provided multi-channels imaging on lymph tubes, tumors and intestines. The proposed NIR-II-L probes without mutual interference are powerful tools for high-contrast in vivo multiplexed detection, which holds promise for revealing physiological process in living body.

A novel broadband Ba3Ca4(BO3)3(SiO4)Cl:Mn4+ near-infrared phosphor with a special pseudo-octahedral Mn4+ coordination structure

A pseudo-octahedral coordination structure of Mn4+ has been innovatively designed, which has realized the maximum red shift and the widest full width at half-maximum (FWHM) of Mn4+ emission so far, not only extending the emission wavelength of Mn4+ to the near-infrared (NIR) region, but also effectively broadening its bandwidth. In the Ba3Ca4(BO3)3(SiO4)Cl:Mn4+ (BCBSC:Mn4+) phosphor, the [Mn/Ca1O9] polyhedron contains one [Mn/Ca1O6] octahedron, which constitutes the pseudo-octahedral coordination structure of Mn4+. The BCBSC:Mn4+ phosphor can be excited at 362 nm and 470 nm and exhibits a broadband NIR emission centered at ∼756 nm with a super-wide range from 650 nm to 1100 nm. The FWHM can reach ∼90 nm. In addition, the internal quantum efficiency (IQE) of the BCBSC:0.01Mn4+ phosphor is 69.7%. The unique luminescence characteristics of BCBSC:Mn4+ phosphors are explored using experimental data and first principles calculation. The significant redshift, the abnormal broadband emission, and the high luminous efficiency are closely related to the special highly distorted [Mn/Ca1O6] pseudo-octahedral coordination environment. The results contribute to comprehending the mechanism of the broadband NIR emission of Mn4+ activated phosphors and broaden the research ideas of developing high-performance Mn4+ doped phosphors for NIR phosphor-converted light-emission diode 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.

Lanthanide-doped Na2MgScF7 exhibiting downshifting and upconversion emissions for multicolor anti-counterfeiting

Na₂MgScF₇ (NMSF) was experimentally obtained for the first time by combining hydrothermal and high-temperature solid-state reactions. X-ray powder diffraction (XRD) combined with Rietveld refinement confirms that NMSF is crystallized in the space group Imma with the cell parameters a = 10.40860(18), b = 7.32804(12) and c = 7.52879(11) Å, α = β = γ = 90° and V = 574.256(24) ų. Through doping with Tb³⁺ or Eu³⁺ ions, downshifting yellow-green or red emission could be achieved in NMSF-based phosphors, respectively. Upconversion emission could also be designed by doping with Yb³⁺-Er³⁺, Yb³⁺-Tm³⁺, Yb³⁺-Ho³⁺ or Er³⁺. Moreover, the NMSF:Er³⁺ phosphor exhibited green upconversion emission upon excitation at 980 nm, and it exhibited red emission upon excitation at 1532 nm. Finally, recognizable patterns were obtained under excitation at 254, 365 and 980 nm, indicating that the as-prepared phosphors can be applied to multicolor anti-counterfeiting. Moreover, our synthesis strategy opens up new avenues for the synthesis of novel fluorides.

Superenhancement Photon Upconversion Nanoparticles for Photoactivated Nanocryometer

Highly doped lanthanide luminescent nanoparticles exhibit unique optical properties, providing exciting opportunities for many ground-breaking applications, such as super-resolution microscopy, deep-tissue bioimaging, confidentiality, and anticounterfeiting. However, the concentration-quenching effect compromises their luminescence efficiency/brightness, hindering their wide range of applications. Herein, we developed a low-temperature suppression cross-relaxation strategy, which drastically enhanced upconversion luminescence (up to 2150-fold of green emission) in Er³⁺-rich nanosystems. The cryogenic field opens the energy transport channel of Er³⁺ multiphoton upconversion by further suppressing phonon-assisted cross-relaxation. Our results provide direct evidence for understanding the energy loss mechanism of photon upconversion, deepening a fundamental understanding of the upconversion process in highly doped nanosystems. Furthermore, it also suggests the potential applications of upconversion nanoparticles for extreme ambient-temperature detection and anticounterfeiting.

Phase-Transition-Driven Regional Distribution of Rare-Earth Ions for Multiplexed Upconversion Emissions

Phase transition of the polymorphs is critical for controlled synthesis and property modulation of functional materials. Upconversion emissions from an efficient hexagonal sodium rare-earth (RE) fluoride compound, β-NaREF₄, which is generally obtained from the phase transition of the cubic (α-) phase counterpart, are attractive for photonic applications. However, the investigation of the α → β phase transition of NaREF₄ and its effect on the composition and architecture is still preliminary. Herein, we investigated the phase transition with two kinds of α-NaREF₄ particles. Instead of a uniform composition, the β-NaREF₄ microcrystals exhibited regionally distributed RE³⁺ ions, in which the RE³⁺ with a smaller ionic radius (smaller RE³⁺) sandwiched the RE³⁺ with a larger ionic radius (larger RE³⁺). We unravel that the α-NaREF₄ particles transformed to β-NaREF₄ nuclei with no controversial dissolution, and the α → β phase transition toward NaREF₄ microcrystals included nucleation and growth steps. The component-dependent phase transition is corroborated with RE³⁺ ions from Ho³⁺ to Lu³⁺ and multiple sandwiched microcrystals were obtained, in which up to five kinds of RE components were distributed regionally. Moreover, with rational integration of luminescent RE³⁺ ions, a single particle with multiplexed upconversion emissions in wavelength and lifetime domains is demonstrated, which provides a unique platform for optical multiplexing applications.

Upconversion nanoscopy revealing surface heterogeneity of tumor-secreted extracellular vesicles

Super-resolution microscopic imaging employing upconversion nanoparticles is applied to reveal the surface heterogeneity of tumor cell-derived small extracellular vesicles, i.e., exosome. The number of surface antigens of every extracellular vesicles can be quantified by both the high imaging resolution and stable brightness of upconversion nanoparticles. This method proves its great potential in nanoscale biological studies.

Solid optical tissue phantom tools based on upconverting nanoparticles for biomedical applications

SIGNIFICANCE

Phantoms play a critical role in the development of biophotonics techniques. There is a lack of novel phantom tools in the emerging field of upconverting nanoparticles (UCNPs) for biophotonics application. This work provides a range of UCNP-based phantom tools and a manufacturing recipe to bridge the gap and accelerate the development of UCNP-based biophotonics applications.

AIM

The study aims to provide a well-characterized UCNP-based solid phantom recipe and set of phantom tools to address a wide range of UCNP-based biophotonics applications.

APPROACH

A solid phantom recipe based on silicone matrix was developed to manufacture UCNP-based phantoms. A lab built UCNP imaging system was used to characterize upconverted fluorescence emission of phantoms for linearity, homogeneity, and long-term stability. A photon time-of-flight spectroscopy technique was used to characterize the optical properties of the phantoms.

RESULTS

In total, 24 phantoms classified into 4 types, namely homogeneous, multilayer, inclusion, and base phantoms, were manufactured. The phantoms exhibit linear behavior over the dosage range of UCNPs. The phantoms were found to be stable over a limited observed period of 4 months with a coefficient of variation of < 4 % . The deep tissue imaging case showed that increasing the thickness of tissue reduced the UCNP emission.

CONCLUSIONS

A first-of-its-kind UCNP-based solid phantom recipe was developed, and four types of UCNP phantom tools to explore biophotonics applications were presented. The UCNP phantoms exhibited a linear behavior with dosage and were stable over time. An example case showed the potential use of the phantom for deep tissue imaging applications. With recent advance in the use of UCNPs for biophotonics, we believe our recipe and tools will play a pivotal role in the growth of the UCNPs for biophotonics applications.

Significant Enhancement of the Upconversion Emission in Highly Er3+ -Doped Nanoparticles at Cryogenic Temperatures

Relatively low efficiency is the bottleneck for the application of lanthanide-doped upconversion nanoparticles (UCNPs). The high-level doping strategy realized in recent years has not improved the efficiency as much as expected. It is argued that cross relaxation (CR) is not detrimental to upconversion. Here we combine theoretical simulation and spectroscopy to elucidate the role of CR in upconversion process of Er³⁺ highly doped (HD) UCNPs. It is found that if CR is purposively suppressed, upconversion efficiency can be significantly improved. Specifically, we demonstrate experimentally that inhibition of CR by introducing cryogenic environment (40 K) enhances upconversion emission by more than two orders of magnitude. This work not only elucidates the nature of CR and its non-negligible adverse effects, but also provides a new perspective for improving upconversion efficiency. The result can be directly applied to cryogenic imaging and wide range temperature sensing.

Lanthanide-Doped Upconversion Nanoparticles: Exploring A Treasure Trove of NIR-Mediated Emerging Applications

Lanthanide-doped upconversion nanoparticles (UCNPs) possess the remarkable ability to convert multiple near-infrared (NIR) photons into higher energy ultraviolet-visible (UV-vis) photons, making them a prime candidate for several advanced applications within the realm of nanotechnology. Compared to traditional organic fluorophores and quantum dots (QDs), UCNPs possess narrower emission bands (fwhm of 10-50 nm), large anti-Stokes shifts, low toxicity, high chemical stability, and resistance to photobleaching and blinking. In addition, unlike UV-vis excitation, NIR excitation is nondestructive at lower power intensities and has high tissue penetration depths (up to 2 mm) with low autofluorescence and scattering. Together, these properties make UCNPs exceedingly favored for advanced bioanalytical and theranostic applications, where these systems have been well-explored. UCNPs are also well-suited for bioimaging, optically modulating chemistries, forensic science, and other state-of-the-art research applications. In this review, an up-to-date account of emerging applications in UCNP research, beyond bioanalytical and theranostics, are presented including optogenetics, super-resolution imaging, encoded barcodes, fingerprinting, NIR vision, UCNP-assisted photochemical manipulations, optical tweezers, 3D printing, lasing, NIR-II imaging, UCNP-molecule nanohybrids, and UCNP-based persistent luminescent nanocrystals.

Universal Emission Characteristics of Upconverting Nanoparticles Revealed by Single-Particle Spectroscopy

Upconverting nanoparticles (UCNPs) have been extensively investigated for nanophotonics and biomedical applications. However, establishing a unified view of their emission characteristics to elucidate the underlying photophysics and expand the application fields of these materials is a great challenge due to their sophisticated internal energy transfer and lack of standardized single-particle spectroscopy (SPS) platform. Here, we present an SPS technique called multiband single-particle irradiance-dependent imaging (multiband SPIDI). We demonstrate that the emission characteristics of Yb³⁺,Tm³⁺-doped UCNPs are universal for three emission bands over a wide range of irradiance and dependent on the Tm³⁺ doping concentration, indicating that the number of emitted photons of each band is proportional to the number of activator ions and is dependent on the number of absorbed photons and the activator interionic distance. We also suggest a cooperative energy transfer upconversion (CETU) mechanism for transition to a higher-energy state through photon accumulation. For a single UCNP, the emission at 800 nm is detectable at an ultralow irradiance of 4.9 W cm^-2; moreover, that at 450 nm is measurable at 98 W cm^-2, based on the optimal concentration. These findings based on the multiband SPIDI platform can provide insights into the interionic energy transfer by studying irradiance-dependent steady-state dynamics to achieve brighter UCNPs and their broader applications.

Nanoparticles for super-resolution microscopy: intracellular delivery and molecular targeting

Following an overview of the approaches and techniques used to acheive super-resolution microscopy, this review presents the advantages supplied by nanoparticle based probes for these applications. The various clases of nanoparticles that have been developed toward these goals are then critically described and these discussions are illustrated with a variety of examples from the recent literature.

Excitation Pulse Duration Response of Upconversion Nanoparticles and Its Applications

Lanthanide-doped upconversion nanoparticles (UCNPs) have rich photophysics exhibiting complex luminescence kinetics. In this work, we thoroughly investigated the luminescence response of UCNPs to excitation pulse durations. Analyzing this response opens new opportunities in optical encoding/decoding and the assignment of transitions to emission peaks and provides advantages in applications of UCNPs, e.g., for better optical sectioning and improved luminescence nanothermometry. Our work shows that monitoring the UCNP luminescence response to excitation pulse durations (while keeping the duty cycle constant) by recording the average luminescence intensity using a low-time resolution detector such as a spectrometer offers a powerful approach for significantly extending the utility of UCNPs.

Bioimaging with Upconversion Nanoparticles

Bioimaging enables the spatiotemporal visualization of biological processes at various scales empowered by a range of different imaging modalities and contrast agents. Upconversion nanoparticles (UCNPs) represent a distinct type of such contrast agents with the potential to transform bioimaging due to their unique optical properties and functional design flexibilities. This review explores and discusses the opportunities, challenges, and limitations that UCNPs exhibit as bioimaging probes and highlights applications with spatial dimensions ranging from the single nanoparticle level to cellular, tissue, and whole animal imaging. We further summarized recent advancements in bioimaging applications enabled by UCNPs, including super-resolution techniques and multimodal imaging methods, and provide a perspective on the future potential of UCNP-based technologies in bioimaging research and clinical translation. This review may provide a valuable resource for researchers interested in exploring and applying UCNP-based bioimaging technologies.

Super-resolution microscopy enabled by high-efficiency surface-migration emission depletion

Nonlinear depletion of fluorescence states by stimulated emission constitutes the basis of stimulated emission depletion (STED) microscopy. Despite significant efforts over the past decade, achieving super-resolution at low saturation intensities by STED remains a major technical challenge. By harnessing the surface quenching effect in NaGdF4:Yb/Tm nanocrystals, we report here high-efficiency emission depletion through surface migration. Using a dual-beam, continuous-wave laser manipulation scheme (975-nm excitation and 730-nm de-excitation), we achieved an emission depletion efficiency of over 95% and a low saturation intensity of 18.3 kW cm-2. Emission depletion by surface migration through gadolinium sublattices enables super-resolution imaging with sub-20 nm lateral resolution. Our approach circumvents the fundamental limitation of high-intensity STED microscopy, providing autofluorescence-free, re-excitation-background-free imaging with a saturation intensity over three orders of magnitude lower than conventional fluorophores. We also demonstrated super-resolution imaging of actin filaments in Hela cells labeled with 8-nm nanoparticles. Combined with the highly photostable lanthanide luminescence, surface-migration emission depletion (SMED) could provide a powerful mechanism for low-power, super-resolution imaging or biological tracking as well as super-resolved optical sensing/writing and lithography.

Lanthanide Ion Resonance-Driven Rayleigh Scattering of Nanoparticles for Dual-Modality Interferometric Scattering Microscopy

Light scattering from nanoparticles is significant in nanoscale imaging, photon confinement. and biosensing. However, engineering the scattering spectrum, traditionally by modifying the geometric feature of particles, requires synthesis and fabrication with nanometre accuracy. Here it is reported that doping lanthanide ions can engineer the scattering properties of low-refractive-index nanoparticles. When the excitation wavelength matches the ion resonance frequency of lanthanide ions, the polarizability and the resulted scattering cross-section of nanoparticles are dramatically enhanced. It is demonstrated that these purposely engineered nanoparticles can be used for interferometric scattering (iSCAT) microscopy. Conceptually, a dual-modality iSCAT microscopy is further developed to identify different nanoparticle types in living HeLa cells. The work provides insight into engineering the scattering features by doping elements in nanomaterials, further inspiring exploration of the geometry-independent scattering modulation strategy.

Towards highly efficient NIR II response up-conversion phosphor enabled by long lifetimes of Er3

The second near-infrared (NIR II) response photon up-conversion (UC) materials show great application prospects in the fields of biology and optical communication. However, it is still an enormous challenge to obtain efficient NIR II response materials. Herein, we develop a series of Er³⁺ doped ternary sulfides phosphors with highly efficient UC emissions under 1532 nm irradiation. β-NaYS₂:Er³⁺ achieves a visible UC efficiency as high as 2.6%, along with high brightness, spectral stability of lights illumination and temperature. Such efficient UC is dominated by excited state absorption, accompanied by the advantage of long lifetimes (⁴I_9/2, 9.24 ms; ⁴I_13/2, 30.27 ms) of excited state levels of Er³⁺, instead of the well-recognized energy transfer UC between sensitizer and activator. NaYS₂:Er³⁺ phosphors are further developed for high-performance underwater communication and narrowband NIR photodetectors. Our findings suggest a novel approach for developing NIR II response UC materials, and simulate new applications, eg., simultaneous NIR and visible optical communication.

Photoswitching the injected energy flux via core-sensitized energy migration upconversion for emission-varying STED microscopy

Stimulated emission depletion (STED) microscopy achieved with lanthanide-doped upconversion nanoparticles (UCNPs) exhibits many outstanding advantages such as low-power illumination, near-infrared (NIR) excitation, and high photostability. However, the available types of UCNP-STED probes are very limited and rely greatly on the specific depletion mechanism. Here, by combining the STED and the energy migration upconversion processes, emissions of Tb³⁺, Eu³⁺, Dy³⁺, and Sm³⁺ distributed in the shell can all be depleted by interrupting the injected energy flux from the Tm³⁺-doped core nanoparticles. With the merit of the proposed strategy, new types of UCNP-STED probes are demonstrated to perform emission-varying STED imaging with one single, fixed pair of low-power NIR continuous wave lasers.

Exploiting Dynamic Nonlinearity in Upconversion Nanoparticles for Super-Resolution Imaging

Single-beam super-resolution microscopy, also known as superlinear microscopy, exploits the nonlinear response of fluorescent probes in confocal microscopy. The technique requires no complex purpose-built system, light field modulation, or beam shaping. Here, we present a strategy to enhance this technique's spatial resolution by modulating excitation intensity during image acquisition. This modulation induces dynamic optical nonlinearity in upconversion nanoparticles (UCNPs), resulting in variations of nonlinear fluorescence response in the obtained images. The higher orders of fluorescence response can be extracted with a proposed weighted finite difference imaging algorithm from raw fluorescence images to generate an image with higher resolution than superlinear microscopy images. We apply this approach to resolve single nanoparticles in a large area, improving the resolution to 132 nm. This work suggests a new scope for the development of dynamic nonlinear fluorescent probes in super-resolution nanoscopy.

Protein-Mimicking Nanoparticles in Biosystems

Proteins are essential elements for almost all life activities. The emergence of nanotechnology offers innovative strategies to create a diversity of nanoparticles (NPs) with intrinsic capacities of mimicking the functions of proteins. These artificial mimics are produced in a cost-efficient and controllable manner, with their protein-mimicking performances comparable or superior to those of natural proteins. Moreover, they can be endowed with additional functionalities that are absent in natural proteins, such as cargo loading, active targeting, membrane penetrating, and multistimuli responding. Therefore, protein-mimicking NPs have been utilized more and more often in biosystems for a wide range of applications including detection, imaging, diagnosis, and therapy. To highlight recent progress in this broad field, herein, representative protein-mimicking NPs that fall into one of the four distinct categories are summarized: mimics of enzymes (nanozymes), mimics of fluorescent proteins, NPs with high affinity binding to specific proteins or DNA sequences, and mimics of protein scaffolds. This review covers their subclassifications, characteristic features, functioning mechanisms, as well as the extensive exploitation of their great potential for biological and biomedical purposes. Finally, the challenges and prospects in future development of protein-mimicking NPs are discussed.

Excitation energy mediated cross-relaxation for tunable upconversion luminescence from a single lanthanide ion

Precise control of energy migration between sensitizer ions and activator ions in lanthanide-doped upconversion nanoparticles (UCNPs) nowadays has been extensively investigated to achieve efficient photon upconversion. However, these UCNPs generally emit blue, green or red light only under fixed excitation conditions. In this work, regulation of the photon transition process between different energy levels of a single activator ion to obtain tunable upconversion fluorescence under different excitation conditions is achieved by introducing a modulator ion. The cross-relaxation process between modulator ion and activator ion can be controlled to generate tunable luminescence from the same lanthanide activator ion under excitation at different wavelengths or with different laser power density and pulse frequency. This strategy has been tested and proven effective in two different nanocrystal systems and its usefulness has been demonstrated for high-level optical encryption.

Here, the authors report tunable luminescence from a single lanthanide ion upon changing excitation conditions through co-doping an energy-modulator ion, thus adjusting the photon transition process of the lanthanide activator ion. Optical encryption has also been demonstrated as an application of this universal strategy.

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.

Recent progress in upconversion nanomaterials for emerging optical biological applications

The recent advances of upconversion nanoparticles (UCNPs) have made them the ideal "partner" for a variety of biological applications. In this review, we describe the emerging biological optical applications of UCNPs, focus on their potential therapeutic advantages. Firstly, we briefly review the development and mechanisms of upconversion luminescence, including organic and inorganic UCNPs. Next, in the section on UCNPs for imaging and detection, we list the development of UCNPs in visualization, temperature sensing, and detection. In the section on therapy, recent results are described concerning optogenetics and neurotherapy. Tumor therapy is another major part of this section, including the synergistic application of phototherapy such as photoimmunotherapy. In a special section, we briefly cover the integration of UCNPs in therapeutics. Finally, we present our understanding of the limitations and prospects of applications of UCNPs in biological fields, hoping to provide a more comprehensive understanding of UCNPs and attract more attention.

Organelle Interaction and Drug Discovery: Towards Correlative Nanoscopy and Molecular Dynamics Simulation

The inter-organelle interactions, including the cytomembrane, endoplasmic reticulum, mitochondrion, lysosome, dictyosome, and nucleus, play the important roles in maintaining the normal function and homeostasis of cells. Organelle dysfunction can lead to a range of diseases (e.g., Alzheimer's disease (AD), Parkinson's disease (PD), and cancer), and provide a new perspective for drug discovery. With the development of imaging techniques and functional fluorescent probes, a variety of algorithms and strategies have been developed for the ever-improving estimation of subcellular structures, organelle interaction, and organelle-related drug discovery with accounting for the dynamic structures of organelles, such as the nanoscopy technology and molecular dynamics (MD) simulations. Accordingly, this work summarizes a series of state-of-the-art examples of the recent progress in this rapidly changing field and uncovering the drug screening based on the structures and interactions of organelles. Finally, we propose the future outlook for exciting applications of organelle-related drug discovery, with the cooperation of nanoscopy and MD simulations.

Power-Dependent Optimal Concentrations of Tm3+ and Yb3+ in Upconversion Nanoparticles

Lanthanide-doped upconversion nanoparticles (UCNPs) have enabled a broad range of emerging nanophotonics and biophotonics applications. Here, we provide a quantitative guide to the optimum concentrations of Yb3+ sensitizer and Tm3+ emitter ions, highly dependent on the excitation power densities. To achieve this, we fabricate the inert-core@active-shell@inert-shell architecture to sandwich the same volume of the optically active section. Our results show that highly doped UCNPs enable an approximately 18-fold enhancement in brightness over that of conventional ones. Increasing the Tm3+ concentration improves the brightness by 6 times and increases the NIR/blue ratio by 11 times, while the increase of Yb3+ concentration enhances the brightness by 3 times and only slightly affects the NIR/blue ratio. Moreover, the optimal doping concentration of Tm3+ varies from 2% to 16%, which is highly dependent on the excitation power density ranging from 102 to 107 W/cm2. This work provides a guideline for designing bright UCNPs under different excitation conditions.

Rare nanoparticles shine colors with low-power STED

The effect of cascade amplified depletion in lanthanide upconversion systems boosts their own emission bands inhibition, which facilitates multi-color nanoscopy with only one pair of low-power NIR CW lasers.

Achieving low-power single-wavelength-pair nanoscopy with NIR-II continuous-wave laser for multi-chromatic probes

Stimulated emission depletion (STED) microscopy is a powerful diffraction-unlimited technique for fluorescence imaging. Despite its rapid evolution, STED fundamentally suffers from high-intensity light illumination, sophisticated probe-defined laser schemes, and limited photon budget of the probes. Here, we demonstrate a versatile strategy, stimulated-emission induced excitation depletion (STExD), to deplete the emission of multi-chromatic probes using a single pair of low-power, near-infrared (NIR), continuous-wave (CW) lasers with fixed wavelengths. With the effect of cascade amplified depletion in lanthanide upconversion systems, we achieve emission inhibition for a wide range of emitters (e.g., Nd3+, Yb3+, Er3+, Ho3+, Pr3+, Eu3+, Tm3+, Gd3+, and Tb3+) by manipulating their common sensitizer, i.e., Nd3+ ions, using a 1064-nm laser. With NaYF4:Nd nanoparticles, we demonstrate an ultrahigh depletion efficiency of 99.3 ± 0.3% for the 450 nm emission with a low saturation intensity of 23.8 ± 0.4 kW cm-2. We further demonstrate nanoscopic imaging with a series of multi-chromatic nanoprobes with a lateral resolution down to 34 nm, two-color STExD imaging, and subcellular imaging of the immunolabelled actin filaments. The strategy expounded here promotes single wavelength-pair nanoscopy for multi-chromatic probes and for multi-color imaging under low-intensity-level NIR-II CW laser depletion.

Migrating photon avalanche in different emitters at the nanoscale enables 46th-order optical nonlinearity

A photon avalanche (PA) effect that occurs in lanthanide-doped solids gives rise to a giant nonlinear response in the luminescence intensity to the excitation light intensity. As a result, much weaker lasers are needed to evoke such PAs than for other nonlinear optical processes. Photon avalanches are mostly restricted to bulk materials and conventionally rely on sophisticated excitation schemes, specific for each individual system. Here we show a universal strategy, based on a migrating photon avalanche (MPA) mechanism, to generate huge optical nonlinearities from various lanthanide emitters located in multilayer core/shell nanostructrues. The core of the MPA nanoparticle, composed of Yb³⁺ and Pr³⁺ ions, activates avalanche looping cycles, where PAs are synchronously achieved for both Yb³⁺ and Pr³⁺ ions under 852 nm laser excitation. These nanocrystals exhibit a 26th-order nonlinearity and a clear pumping threshold of 60 kW cm^-2. In addition, we demonstrate that the avalanching Yb³⁺ ions can migrate their optical nonlinear response to other emitters (for example, Ho³⁺ and Tm³⁺) located in the outer shell layer, resulting in an even higher-order nonlinearity (up to the 46th for Tm³⁺) due to further cascading multiplicative effects. Our strategy therefore provides a facile route to achieve giant optical nonlinearity in different emitters. Finally, we also demonstrate applicability of MPA emitters to bioimaging, achieving a lateral resolution of ~62 nm using one low-power 852 nm continuous-wave laser beam.

3D Upconversion Barcodes for Combinatory Wireless Neuromodulation in Behaving Animals

Upconversion techniques offer all-optical wireless alternatives to modulate targeted neurons in behaving animals, but most existing upconversion-based optogenetic devices show prefixed emission that is used to excite just one channelrhodopsin at a restricted brain region. Here, a hierarchical upconversion device is reported to enable spatially selective and combinatory optogenetics in behaving rodent animals. The device assumes a multiarrayed optrode format containing engineered upconversion nanoparticles (UCNPs) to deliver dynamic light palettes as a function of excitation wavelength. Three primary emissions at 477, 540, and 654 nm are selected to match the absorption of different channelrhodopsins. The UCNPs are barcode assembled to multiple nanomachined optical pinholes in a microscale pipette device to allow remotely addressable, spectrum programmable, and spatially selective optical interrogation of complex brain circuits. Using the unique device, the basolateral amygdala and caudoputamen circuits are selectively modulated and the associated fear or anxiety behavior in freely behaving rodents is successfully differentiated. It is believed that the 3D barcode upconversion device would be a great supplement to current optogenetic toolsets and opens up new possibilities for sophisticated neural control.

Expanding the toolbox of photon upconversion for emerging frontier applications

Photon upconversion in lanthanide-based materials has recently shown compelling advantages in a wide range of fields due to their exceptional anti-Stokes luminescence performances and physicochemical properties. In particular, the latest breakthroughs in the optical manipulation of photon upconversion, such as the precise tuning of switchable emission profiles and lifetimes, open up new opportunities for diverse frontier applications from biological imaging to therapy, nanophotonics and three-dimensional displays. A summary and discussion on the recent progress can provide new insights into the fundamental understanding of luminescence mechanisms and also help to inspire new upconversion concepts and promote their frontier applications. Herein, we present a review on the state-of-the-art progress of lanthanide-based upconversion materials, focusing on the newly emerging approaches to the smart control of upconversion in aspects of light intensity, colors, and lifetimes, as well as new concepts. The emerging scientific and technological discoveries based on the well-designed upconversion materials are highlighted and discussed, along with the challenges and future perspectives. This review will contribute to the understanding of the fundamental research of photon upconversion and further promote the development of new classes of efficient upconversion materials towards diversities of frontier applications in the future.

Near-infrared excitation/emission microscopy with lanthanide-based nanoparticles

Near-infrared optical imaging offers some advantages over conventional imaging, such as deeper tissue penetration, low or no autofluorescence, and reduced tissue scattering. Lanthanide-doped nanoparticles (LnNPs) have become a trend in the field of photoactive nanomaterials for optical imaging due to their unique optical features and because they can use NIR light as excitation and/or emission light. This review is focused on NaREF₄ NPs and offers an overview of the state-of-the-art investigation in their use as luminophores in optical microscopy, time-resolved imaging, and super-resolution nanoscopy based on, or applied to, LnNPs. Secondly, whenever LnNPs are combined with other nanomaterial or nanoparticle to afford nanohybrids, the characterization of their physical and chemical properties is of current interest. In this context, the latest trends in optical microscopy and their future perspectives are discussed.

Bleaching-Resistant Super-Resolution Fluorescence Microscopy

Photobleaching is the permanent loss of fluorescence after extended exposure to light and is a major limiting factor in super-resolution microscopy (SRM) that restricts spatiotemporal resolution and observation time. Strategies for preventing or overcoming photobleaching in SRM are reviewed developing new probes and chemical environments. Photostabilization strategies are introduced first, which are borrowed from conventional fluorescence microscopy, that are employed in SRM. SRM-specific strategies are then highlighted that exploit the on-off transitions of fluorescence, which is the key mechanism for achieving super-resolution, which are becoming new routes to address photobleaching in SRM. Off states can serve as a shelter from excitation by light or an exit to release a damaged probe and replace it with a fresh one. Such efforts in overcoming the photobleaching limits are anticipated to enhance resolution to molecular scales and to extend the observation time to physiological lifespans.

Bioconjugates of photon-upconversion nanoparticles for cancer biomarker detection and imaging

The detection of cancer biomarkers in histological samples and blood is of paramount importance for clinical diagnosis. Current methods are limited in terms of sensitivity, hindering early detection of disease. We have overcome the shortcomings of currently available staining and fluorescence labeling methods by taking an integrative approach to establish photon-upconversion nanoparticles (UCNP) as a powerful platform for cancer detection. These nanoparticles are readily synthesized in different sizes to yield efficient and tunable short-wavelength light emission under near-infrared excitation, which eliminates optical background interference of the specimen. Here we present a protocol for the synthesis of UCNPs by high-temperature co-precipitation or seed-mediated growth by thermal decomposition, surface modification by silica or poly(ethylene glycol) that renders the particles resistant to nonspecific binding, and the conjugation of streptavidin or antibodies for biological detection. To detect blood-based biomarkers, we present an upconversion-linked immunosorbent assay for the analog and digital detection of the cancer marker prostate-specific antigen. When applied to immunocytochemistry analysis, UCNPs enable the detection of the breast cancer marker human epidermal growth factor receptor 2 with a signal-to-background ratio 50-fold higher than conventional fluorescent labels. UCNP synthesis takes 4.5 d, the preparation of the antibody-silica-UCNP conjugate takes 3 d, the streptavidin-poly(ethylene glycol)-UCNP conjugate takes 2-3 weeks, upconversion-linked immunosorbent assay takes 2-4 d and immunocytochemistry takes 8-10 h. The procedures can be performed after standard laboratory training in nanomaterials research.

Surface-Engineered Gold Nanoclusters for Stimulated Emission Depletion and Correlated Light and Electron Microscopy Imaging

Stimulated emission depletion (STED) nanoscopy is an emerging super-resolution imaging platform for the study of the cellular structure. Developing suitable fluorescent probes of small size, good photostability, and easy functionalization is still in demand. Herein, we introduce a new type of surface-engineered gold nanoclusters (Au NCs) that are ultrasmall (1.7 nm) and ultrabright (QY = 60%) for STED bioimaging. A rigid shell formed by l-arginine (l-Arg) and 6-aza-2-thiothymine (ATT) on the Au NC surface enables not only its strong fluorescence in aqueous solution but also its easy chemical modification for specific biomolecule labeling. Au NCs show remarkable performance as STED nanoprobes, including high depletion efficiency, good photobleaching resistance, and low saturation intensity. Super-resolution imaging has been achieved with these Au NCs, and targeted nanoscopic imaging of cellular tubulin has been demonstrated. Moreover, the circular structure of lysosomes in live cells has been revealed. As a Au NC is also an ideal probe for electron microscopy, dual imaging of Aβ₄₂ aggregates with the single labeling probe of Au NCs has been realized in correlative light and electron microscopy (CLEM). This work reports, for the first time, the application of Au NCs as a novel probe in STED and CLEM imaging. With their excellent properties, Au NCs show promising potential for nanoscale bioimaging.

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.

Fluorescent Probes for STED Optical Nanoscopy

Progress in developing fluorescent probes, such as fluorescent proteins, organic dyes, and fluorescent nanoparticles, is inseparable from the advancement in optical fluorescence microscopy. Super-resolution microscopy, or optical nanoscopy, overcame the far-field optical resolution limit, known as Abbe's diffraction limit, by taking advantage of the photophysical properties of fluorescent probes. Therefore, fluorescent probes for super-resolution microscopy should meet the new requirements in the probes' photophysical and photochemical properties. STED optical nanoscopy achieves super-resolution by depleting excited fluorophores at the periphery of an excitation laser beam using a depletion beam with a hollow core. An ideal fluorescent probe for STED nanoscopy must meet specific photophysical and photochemical properties, including high photostability, depletability at the depletion wavelength, low adverse excitability, and biocompatibility. This review introduces the requirements of fluorescent probes for STED nanoscopy and discusses the recent progress in the development of fluorescent probes, such as fluorescent proteins, organic dyes, and fluorescent nanoparticles, for the STED nanoscopy. The strengths and the limitations of the fluorescent probes are analyzed in detail.

Plasmon-Triggered Upconversion Emissions and Hot Carrier Injection for Combinatorial Photothermal and Photodynamic Cancer Therapy

Despite the unique ability of lanthanide-doped upconversion nanoparticles (UCNPs) to convert near-infrared (NIR) light to high-energy UV-vis radiation, low quantum efficiency has rendered their application unpractical in biomedical fields. Here, we report anatase titania-coated plasmonic gold nanorods decorated with UCNPs (Au NR@aTiO₂@UCNPs) for combinational photothermal and photodynamic therapy to treat cancer. Our novel architecture employs the incorporation of an anatase titanium dioxide (aTiO₂) photosensitizer as a spacer and exploits the localized surface plasmon resonance (LSPR) properties of the Au core. The LSPR-derived near-field enhancement induces a threefold boost of upconversion emissions, which are re-absorbed by neighboring aTiO₂ and Au nanocomponents. Photocatalytic experiments strongly infer that LSPR-induced hot electrons are injected into the conduction band of aTiO₂, generating reactive oxygen species. As phototherapeutic agents, our hybrid nanostructures show remarkable in vitro anticancer effect under NIR light [28.0% cancer cell viability against Au NR@aTiO₂ (77.3%) and UCNP@aTiO₂ (98.8%)] ascribed to the efficient radical formation and LSPR-induced heat generation, with cancer cell death primarily following an apoptotic pathway. In vivo animal studies further confirm the tumor suppression ability of Au NR@aTiO₂@UCNPs through combinatorial photothermal and photodynamic effect. Our hybrid nanomaterials emerge as excellent multifunctional phototherapy agents, providing a valuable addition to light-triggered cancer treatments in deep tissue.