Fast upconversion super-resolution microscopy with 10 μs per pixel dwell times

Lanthanide ion-doped upconversion nanoparticles for low-energy super-resolution applications

Energy-intensive technologies and high-precision research require energy-efficient techniques and materials. Lens-based optical microscopy technology is useful for low-energy applications in the life sciences and other fields of technology, but standard techniques cannot achieve applications at the nanoscale because of light diffraction. Far-field super-resolution techniques have broken beyond the light diffraction limit, enabling 3D applications down to the molecular scale and striving to reduce energy use. Typically targeted super-resolution techniques have achieved high resolution, but the high light intensity needed to outperform competing optical transitions in nanomaterials may result in photo-damage and high energy consumption. Great efforts have been made in the development of nanomaterials to improve the resolution and efficiency of these techniques toward low-energy super-resolution applications. Lanthanide ion-doped upconversion nanoparticles that exhibit multiple long-lived excited energy states and emit upconversion luminescence have enabled the development of targeted super-resolution techniques that need low-intensity light. The use of lanthanide ion-doped upconversion nanoparticles in these techniques for emerging low-energy super-resolution applications will have a significant impact on life sciences and other areas of technology. In this review, we describe the dynamics of lanthanide ion-doped upconversion nanoparticles for super-resolution under low-intensity light and their use in targeted super-resolution techniques. We highlight low-energy super-resolution applications of lanthanide ion-doped upconversion nanoparticles, as well as the related research directions and challenges. Our aim is to analyze targeted super-resolution techniques using lanthanide ion-doped upconversion nanoparticles, emphasizing fundamental mechanisms governing transitions in lanthanide ions to surpass the diffraction limit with low-intensity light, and exploring their implications for low-energy nanoscale 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.

Lanthanide molecular cluster-aggregates as the next generation of optical materials

In this perspective, we provide an overview of the recent achievements in luminescent lanthanide-based molecular cluster-aggregates (MCAs) and illustrate why MCAs can be seen as the next generation of highly efficient optical materials. MCAs are high nuclearity compounds composed of rigid multinuclear metal cores encapsulated by organic ligands. The combination of high nuclearity and molecular structure makes MCAs an ideal class of compounds that can unify the properties of traditional nanoparticles and small molecules. By bridging the gap between both domains, MCAs intrinsically retain unique features with tremendous impacts on their optical properties. Although homometallic luminescent MCAs have been extensively studied since the late 1990s, it was only recently that heterometallic luminescent MCAs were pioneered as tunable luminescent materials. These heterometallic systems have shown tremendous impacts in areas such as anti-counterfeiting materials, luminescent thermometry, and molecular upconversion, thus representing a new generation of lanthanide-based optical materials.

Frequency-Domain Method for Characterization of Upconversion Luminescence Kinetics

The frequency-domain (FD) method provides an alternative to the commonly used time-domain (TD) approach in characterizing the luminescence kinetics of luminophores, with its own strengths, e.g., the capability to decouple multiple lifetime components with higher reliability and accuracy. While extensively explored for characterizing luminophores with down-shifted emission, this method has not been investigated for studying nonlinear luminescent materials such as lanthanide-doped upconversion nanoparticles (UCNPs), featuring more complicated kinetics. In this work, employing a simplified rate-equation model representing a standard two-photon energy-transfer upconversion process, we thoroughly analyzed the response of the luminescence of UCNPs in the FD method. We found that the FD method can potentially obtain from a single experiment the effective decay rates of three critical energy states of the sensitizer/activator ions involved in the upconversion process. The validity of the FD method is demonstrated by experimental data, agreeing reasonably well with the results obtained by TD methods.

Far Red-Shifted CdTe Quantum Dots for Multicolour Stimulated Emission Depletion Nanoscopy

Stimulated emission depletion (STED) nanoscopy is a widely used nanoscopy technique. Two-colour STED imaging in fixed and living cells is standardised today utilising both fluorescent dyes and fluorescent proteins. Solutions to image additional colours have been demonstrated using spectral unmixing, photobleaching steps, or long-Stokes-shift dyes. However, these approaches often compromise speed, spatial resolution, and image quality, and increase complexity. Here, we present multicolour STED nanoscopy with far red-shifted semiconductor CdTe quantum dots (QDs). STED imaging of the QDs is optimized to minimize blinking effects and maximize the number of detected photons. The far-red and compact emission spectra of the investigated QDs free spectral space for the simultaneous use of fluorescent dyes, enabling straightforward three-colour STED imaging with a single depletion beam. We use our method to study the internalization of QDs in cells, opening up the way for future super-resolution studies of particle uptake and internalization.

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.

Realization of multiple luminescence manipulation in tungsten bronze oxides based on photochromism toward real-time, reversible, and fast processes

Multiple luminescence manipulation in tungsten bronze oxides based on photochromism.

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.

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.

Enhanced up-conversion photoluminescence in fluoride-oxyfluoride nanophosphor films by embedding gold nanoparticles

Owing to their unique non-linear optical character, lanthanide-based up-converting materials are potentially interesting for a wide variety of fields ranging from biomedicine to light harvesting. However, their poor luminescent efficiency challenges the development of technological applications. In this context, localized surface plasmon resonances (LSPRs) have been demonstrated as a valuable strategy to improve light conversion. Herein, we utilize LSPR induced by gold nanoparticles (NPs) to enhance up-conversion photoluminescence (UCPL) in transparent, i.e. scattering-free, films made of nanophosphors formed by fluoride-oxyfluoride host matrix that feature high thermal stability. Transparency allows excitation by an external source without extinction losses caused by unwanted diffuse reflection. We provide a simple method to embed gold NPs in films made of YF/YOF:Yb³⁺,Er³⁺ UC nanophosphors, via preparation of a viscous paste composed of both UC nanophosphors and colloidal gold NPs, reducing complexity in sample fabrication. The dimensions of gold NPs are such that their associated LSPR matches spectrally with the green emission band of the Er³⁺ doped nanophosphors. In order to demonstrate the benefits of plasmonic nanoparticles for UCPL in nanophosphor films, we provide a careful analysis of the structural properties of the composite thin films along with precise characterization of the impact of the gold NPs on the photophysical properties of UC nanophosphors.

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.

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.

Local Structure Engineering in Lanthanide-Doped Nanocrystals for Tunable Upconversion Emissions

Upconversion emissions from lanthanide-doped nanocrystals have sparked extensive research interests in nanophotonics, biomedicine, photovoltaics, photocatalysis, etc. Rational modulation of upconversion emissions is highly desirable to meet the requirements of specific applications. Among the diverse developed methods, local structure engineering is fundamentally feasible, through which the upconversion emission intensity, selectivity, wavelength shift, and lifetime can be tuned effectively. The underlying mechanism of the local-structure-dependent upconversion emissions lies in the degree of parity hybridization and energy level splitting of lanthanide ions as well as the interionic energy transfer efficiency. Over the past few years, there has been significant progress in local-structure-engineered upconversion emissions. In this Perspective, we first introduce the principles of upconversion emissions and typical characterization methods for local structure. Subsequently, we summarize recent achievements in tuning of upconversion emissions through local structure engineering, including host composition adjustment, external field regulation, and interfacial strain management. Finally, we propose a few perspectives that should tackle the current bottlenecks. This Perspective is expected to deepen the understanding of local-structure-dependent upconversion emissions and arouse adequate attention to the engineering of local structure for desired properties of inorganic nanocrystals.

Super-Resolution Imaging With Lanthanide Luminescent Nanocrystals: Progress and Prospect

Stimulated emission depletion (STED) nanoscopy has overcome a serious diffraction barrier on the optical resolution and facilitated new discoveries on detailed nanostructures in cell biology. Traditional fluorescence probes employed in the super-resolution imaging approach include organic dyes and fluorescent proteins. However, some limitations of these probes, such as photobleaching, short emission wavelengths, and high saturation intensity, still hamper the promotion of optical resolution and bio-applications. Recently, lanthanide luminescent probes with unique optical properties of non-photobleaching and sharp emissions have been applied in super-resolution imaging. In this mini-review, we will introduce several different mechanisms for lanthanide ions to achieve super-resolution imaging based on an STED-like setup. Then, several lanthanide ions used in super-resolution imaging will be described in detail and discussed. Last but not least, we will emphasize the future challenges and outlooks in hope of advancing the next-generation lanthanide fluorescent probes for super-resolution optical imaging.

Highly Versatile Upconverting Oxyfluoride-Based Nanophosphor Films

Fluoride-based compounds doped with rare-earth cations are the preferred choice of materials to achieve efficient upconversion, of interest for a plethora of applications ranging from bioimaging to energy harvesting. Herein, we demonstrate a simple route to fabricate bright upconverting films that are transparent, self-standing, flexible, and emit different colors. Starting from the solvothermal synthesis of uniform and colloidally stable yttrium fluoride nanoparticles doped with Yb3+ and Er3+, Ho3+, or Tm3+, we find the experimental conditions to process the nanophosphors as optical quality films of controlled thickness between few hundreds of nanometers and several micrometers. A thorough analysis of both structural and photophysical properties of films annealed at different temperatures reveals a tradeoff between the oxidation of the matrix, which transitions through an oxyfluoride crystal phase, and the efficiency of the upconversion photoluminescence process. It represents a significant step forward in the understanding of the fundamental properties of upconverting materials and can be leveraged for the optimization of upconversion systems in general. We prove bright multicolor upconversion photoluminescence in oxyfluoride-based phosphor transparent films upon excitation with a 980 nm laser for both rigid and flexible versions of the layers, being possible to use the latter to coat surfaces of arbitrary shape. Our results pave the way toward the development of upconverting coatings that can be conveniently integrated in applications that demand a large degree of versatility.

NIR laser scanning microscopy for photophysical characterization of upconversion nanoparticles and nanohybrids

Photophysical characterization of upconversion nanoparticles (UCNPs) and nanohybrids (UCNHs) is more challenging than that of down-conversion nanomaterials. Moreover, it is still difficult to gain knowledge about the homogeneity of the sample and colocalization of emissive chromophores and nanoparticles in nanohybrids. Near infrared laser scanning microscopy (NIR-LSM) is a well-known and useful imaging technique, which enables excitation in the NIR region and has been extensively applied to optical fluorescence imaging of organic fluorophores and nanomaterials, such as quantum dots, which exhibit a short-lived emission. NIR-LSM has recently been used to determine the empirical emission lifetime of UCNPs, thus extending its application range to nanomaterials with a long lifetime emission. Here, we review our previous findings and include new measurements and samples to fully address the potential of this technique. NIR-LSM has proved to be extraordinarily useful not only for photophysical characterization of UCNHs consisting of UCNPs capped with a fluorophore to easily visualize the occurrence of the resonance energy transfer process between the UCNH constituents and their homogeneity, but also to assess the colocalization of the fluorophore and the UCNP in the UCNH; all this information can be acquired on the micro-/nano-meter scale by just taking one image.

The Spectroscopic Properties and Microscopic Imaging of Thulium-Doped Upconversion Nanoparticles Excited at Different NIR-II Light

Lanthanide-doped upconversion nanoparticles (UCNPs) are promising bioimaging nanoprobes due to their excellent photostability. As one of the most commonly used lanthanide activators, Tm³⁺ ions have perfect ladder-type electron configuration and can be directly excited by bio-friendly near-infrared-II (NIR-II) wavelengths. Here, the emission characteristics of Tm³⁺-doped nanoparticles under laser excitations of different near-infrared-II wavelengths were systematically investigated. The 1064 nm, 1150 nm, and 1208 nm lasers are proposed to be three excitation strategies with different response spectra of Tm³⁺ ions. In particular, we found that 1150 nm laser excitation enables intense three-photon 475 nm emission, which is nearly 100 times stronger than that excited by 1064 nm excitation. We further optimized the luminescence brightness after investigating the luminescence quenching mechanism of bare NaYF₄: Tm (1.75%) core. After growing an inert shell, a ten-fold increase of emission intensity was achieved. Combining the advantages of NIR-II wavelength and the higher-order nonlinear excitation, a promising facile excitation strategy was developed for the application of thulium-doped upconversion nanoparticles in nanoparticles imaging and cancer cell microscopic imaging.

Shedding New Lights Into STED Microscopy: Emerging Nanoprobes for Imaging

First reported in 1994, stimulated emission depletion (STED) microscopy has long been regarded as a powerful tool for real-time superresolved bioimaging . However, high STED light power (101∼3 MW/cm2) is often required to achieve significant resolution improvement, which inevitably introduces phototoxicity and severe photobleaching, damaging the imaging quality, especially for long-term cases. Recently, the employment of nanoprobes (quantum dots, upconversion nanoparticles, carbon dots, polymer dots, AIE dots, etc.) in STED imaging has brought opportunities to overcoming such long-existing issues. These nanomaterials designed for STED imaging show not only lower STED power requirements but also more efficient photoluminescence (PL) and enhanced photostability than organic molecular probes. Herein, we review the recent progress in the development of nanoprobes for STED imaging, to highlight their potential in improving the long-term imaging quality of STED microscopy and broadening its application scope. We also discuss the pros and cons for specific classes of nanoprobes for STED bioimaging in detail to provide practical references for biological researchers seeking suitable imaging kits, promoting the development of relative research field.

Lanthanide-Doped Upconversion Nanoparticles for Super-Resolution Microscopy

Super-resolution microscopy offers a non-invasive and real-time tool for probing the subcellular structures and activities on nanometer precision. Exploring adequate luminescent probes is a great concern for acquiring higher-resolution image. Benefiting from the atomic-like transitions among real energy levels, lanthanide-doped upconversion nanoparticles are featured by unique optical properties including excellent photostability, large anti-Stokes shifts, multicolor narrowband emissions, tunable emission lifetimes, etc. The past few years have witnessed the development of upconversion nanoparticles as probes for super-resolution imaging studies. To date, the optimal resolution reached 28 nm (λ/36) for single nanoparticles and 82 nm (λ/12) for cytoskeleton structures with upconversion nanoparticles. Compared with conventional probes such as organic dyes and quantum dots, upconversion nanoparticle-related super-resolution microscopy is still in the preliminary stage, and both opportunities and challenges exist. In this perspective article, we summarized the recent advances of upconversion nanoparticles for super-resolution microscopy and projected the future directions of this emerging field. This perspective article should be enlightening for designing efficient upconversion nanoprobes for super-resolution imaging and promote the development of upconversion nanoprobes for cell biology applications.

A New Method on Super Pixel Reducing Stereo Matching Time of Integrated Imaging

Stereo vision and 3D reconstruction technologies are increasingly concerned in many fields. Stereo matching algorithm is the core of stereo vision and also a technical difficulty. A novel method based on super pixels is mentioned in this paper to reduce the calculating amount and the time. Stereo images from University of Tsukuba are used to test our method. The proposed method spends only 1% of the time spent by the conventional method. Through a two-step super-pixel matching optimization, it takes 6.72 s to match a picture, which is 12.96% of the pre-optimization.

Controlling the non-linear emission of upconversion nanoparticles to enhance super-resolution imaging performance

Upconversion nanoparticles (UCNPs) exhibit unique optical properties such as photo-emission stability, large anti-Stokes shift, and long excited-state lifetimes, allowing significant advances in a broad range of applications from biomedical sensing to super-resolution microscopy. In recent years, progress on nanoparticle synthesis led to the development of many strategies for enhancing their upconversion luminescence, focused in particular on heavy doping of lanthanide ions and core-shell structures. In this article, we investigate the non-linear emission properties of fully Yb-based core-shell UCNPs and their impact on the super-resolution performance of stimulated excitation-depletion (STED) microscopy and super-linear excitation-emission (uSEE) microscopy. Controlling the power-dependent emission curve enables us to relax constraints on the doping concentrations and to reduce the excitation power required for accessing sub-diffraction regimes. We take advantage of this feature to implement multiplexed super-resolution imaging of a two-sample mixture.

Video-rate upconversion display from optimized lanthanide ion doped upconversion nanoparticles

Volumetric displays that create bright image points within a transparent bulk are one of the most attractive technologies in everyday life. Lanthanide ion doped upconversion nanoparticles (UCNPs) are promising luminescent nanomaterials for background free, full-colour volumetric displays of transparent bulk materials. However, video-rate display using UCNPs has been limited by their low emission intensity. Herein, we developed a video-rate upconversion display system with much enhanced brightness. The integral emission intensity of the single UCNPs was fully employed for video-rate display. It was maximized by optimizing the emitter concentration and, more importantly, by temporally synchronizing the scanning time of the excitation light to the the raised emission time of the single UCNPs. The excitation power dependent emission response and emission time decay curves were systematically characterized for the single UCNPs with various emitter concentrations from 0.5% to 6%. 1%Tm³⁺ doped UCNPs presented the highest integral emission intensity. By embedding this UCNPs into a polyvinyl acetate (PVA) film, we achieved a two-dimensional (2D) upconversion display with a frame rate of 29 Hz for 35 by 50 pixels. This work demonstrates that the temporal response as well as the integral emission intensity enable video-rate upconversion display.

Simultaneous super-linear excitation-emission and emission depletion allows imaging of upconversion nanoparticles with higher sub-diffraction resolution

Upconversion nanoparticles (UCNPs) are becoming increasingly popular as biological markers as they offer photo-stable imaging in the near-infrared (NIR) biological transparency window. Imaging at NIR wavelengths benefits from low auto-fluorescence background and minimal photo-damage. However, as the diffraction limit increases with the wavelength, the imaging resolution deteriorates. To address this limitation, recently two independent approaches have been proposed for imaging UCNPs with sub-diffraction resolution, namely stimulated emission-depletion (STED) microscopy and super linear excitation-emission (uSEE) microscopy. Both methods are very sensitive to the UCNP composition and the imaging conditions, i.e. to the excitation and depletion power. Here, we demonstrate that the imaging conditions can be chosen in a way that activates both super-resolution regimes simultaneously when imaging NaYF₄:Yb,Tm UCNPs. The combined uSEE-STED mode benefits from the advantages of both techniques, allowing for imaging with lateral resolution about six times better than the diffraction limit due to STED and simultaneous improvement of the axial resolution about twice over the diffraction limit due to uSEE. Conveniently, at certain imaging conditions, the uSEE-STED modality can achieve better resolution at four times lower laser power compared to STED mode, making the method appealing for biological applications. We illustrate this by imaging UCNPs functionalized by colominic acid in fixed neuronal phenotype cells.

Upconversion Nonlinear Structured Illumination Microscopy

Video-rate super-resolution imaging through biological tissue can visualize and track biomolecule interplays and transportations inside cellular organisms. Structured illumination microscopy allows for wide-field super resolution observation of biological samples but is limited by the strong extinction of light by biological tissues, which restricts the imaging depth and degrades its imaging resolution. Here we report a photon upconversion scheme using lanthanide-doped nanoparticles for wide-field super-resolution imaging through the biological transparent window, featured by near-infrared and low-irradiance nonlinear structured illumination. We demonstrate that the 976 nm excitation and 800 nm upconverted emission can mitigate the aberration. We found that the nonlinear response of upconversion emissions from single nanoparticles can effectively generate the required high spatial frequency components in the Fourier domain. These strategies lead to a new modality in microscopy with a resolution below 131 nm, 1/7th of the excitation wavelength, and an imaging rate of 1 Hz.