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

Frequency Encoding of Upconversion Nanoparticle Emission for Multiplexed Imaging of Spectrally and Spatially Overlapping Lanthanide Ions

We present frequency encoded upconversion (FE-UPCON) widefield microscopy, an imaging approach that allows for multiplexed signal recovery based on frequency encoding of selected upconverted lanthanide ion emission rather than separation based on energy or time. FE-UPCON allows for the separation of luminescence from spectrally and spatially overlapping trivalent lanthanide ions (Ln³⁺) in upconversion nanoparticles (UCNPs). Utilizing the numerous electronic energy levels of Ln³⁺, one can generate a frequency encoded signal by periodic coexcitation with a secondary light source (modulated at a chosen frequency) that, for a particular wavelength, enhances the luminescence of the Ln³⁺ of interest. We demonstrate that it is possible to selectively image spectrally overlapping UCNPs co-doped with Yb³⁺/Ho³⁺ or Yb³⁺/Er³⁺ by FE-UPCON in cells up to 10 frames per second on a conventional widefield microscope with the simple extension of an additional secondary light source and a chopper wheel for modulation. Additionally, we show that FE-UPCON does not compromise sensitivity and that single UCNP detection is obtainable. FE-UPCON adds a new dimension (frequency space) for multiplexed imaging with UCNPs.

In Situ Femtosecond-Laser-Induced Fluorophores on Surface of Polyvinyl Alcohol for H2O/Co2+ Sensing and Data Security

In situ fluorophores were induced on polyvinyl alcohol (PVA) bulk materials by direct femtosecond laser writing. The generation of fluorophores was ascribed to localized laser-assisted carbonization. The carbonization of PVA polymers was confirmed through X-ray photoelectron spectroscopy analysis. The distinct fluorescence responses of fluorophores in various solutions were harnessed for implementing in situ reagent sensors, metal ion sensors, data encryption, and data security applications. The demonstrated water detection sensor in acetone exhibited a sensitivity of 3%. Meanwhile, a data encryption scheme and a "burn after reading" technique were demonstrated by taking advantage of the respective reversible and irreversible switching properties of the in situ laser-induced fluorophores. Taking a step further, a quantitative cobalt ion measurement was demonstrated based on the concentration-dependent fluorescence recovery. Combined with a laser-induced hydrophilic modification, our scheme could enable "lab-on-a-chip" microfluidics sensors with arbitrary shape, varied flow delay, designed reaction zones, and targeted functionalities in the future.

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.

Upconversion nanoparticle-assisted single-molecule assay for detecting circulating antigens of aggressive prostate cancer

Sensitive and quantitative detection of molecular biomarkers is crucial for the early diagnosis of diseases like metabolic syndrome and cancer. Here we present a single-molecule sandwich immunoassay by imaging the number of single nanoparticles to diagnose aggressive prostate cancer. Our assay employed the photo-stable upconversion nanoparticles (UCNPs) as labels to detect the four types of circulating antigens in blood circulation, including glypican-1 (GPC-1), leptin, osteopontin (OPN), and vascular endothelial growth factor (VEGF), as their serum concentrations indicate aggressive prostate cancer. Under a wide-field microscope, a single UCNP doped with thousands of lanthanide ions can emit sufficiently bright anti-Stokes' luminescence to become quantitatively detectable. By counting every single streptavidin-functionalized UCNP which specifically labeled on each sandwich immune complex across multiple fields of views, we achieved the Limit of Detection (LOD) of 0.0123 ng/ml, 0.2711 ng/ml, 0.1238 ng/ml, and 0.0158 ng/ml for GPC-1, leptin, OPN and VEGF, respectively. The serum circulating level of GPC-1, leptin, OPN, and VEGF in a mixture of 10 healthy normal human serum was 25.17 ng/ml, 18.04 ng/ml, 11.34 ng/ml, and 1.55 ng/ml, which was within the assay dynamic detection range for each analyte. Moreover, a 20% increase of GPC-1 and OPN was observed by spiking the normal human serum with recombinant antigens to confirm the accuracy of the assay. We observed no cross-reactivity among the four biomarker analytes, which eliminates the false positives and enhances the detection accuracy. The developed single upconversion nanoparticle-assisted single-molecule assay suggests its potential in clinical usage for prostate cancer detection by monitoring tiny concentration differences in a panel of serum biomarkers.

Recent Advances of Upconversion Nanomaterials in the Biological Field

Rare Earth Upconversion nanoparticles (UCNPs) are a type of material that emits high-energy photons by absorbing two or more low-energy photons caused by the anti-stokes process. It can emit ultraviolet (UV) visible light or near-infrared (NIR) luminescence upon NIR light excitation. Due to its excellent physical and chemical properties, including exceptional optical stability, narrow emission band, enormous Anti-Stokes spectral shift, high light penetration in biological tissues, long luminescent lifetime, and a high signal-to-noise ratio, it shows a prodigious application potential for bio-imaging and photodynamic therapy. This paper will briefly introduce the physical mechanism of upconversion luminescence (UCL) and focus on their research progress and achievements in bio-imaging, bio-detection, and photodynamic therapy.

Cryogenic enabled multicolor upconversion luminescence of KLa(MoO4)2:Yb3+/Ho3+ for dual-mode anti-counterfeiting

The rational development of multicolor upconversion (UC) luminescent materials is particularly promising for achieving high-tech anti-counterfeiting and security applications. Here, an Ho³⁺ and Yb³⁺ ion co-doped KLa(MoO₄)₂ material can achieve multicolored UC luminescence by thermally manipulating the electron transition process, which could be developed to execute advanced optical anti-counterfeiting applications. The emission color of this material turns from bright green to deep orange with the temperature controlled from 85 K to 240 K in a cryogenic environment. The maximum absolute sensitivity and relative sensitivity of this temperature-sensing material based on non-thermally coupled levels of Ho³⁺ ions reached 0.049 K^-1 and 4.6% K^-1. And utilizing the thermochromic luminescence properties and high sensitivity for low temperature of the KLa(MoO₄)₂:Yb³⁺/Ho³⁺ UC material, we created KLa(MoO₄)₂:Yb³⁺/Ho³⁺ fluorescent security inks and UC photonic barcodes to realize novel visual reading and digital recognition dual-mode anti-counterfeiting in a secure manner. These results may provide useful enlightenment for the design and modulation of high-sensitivity temperature-sensing materials for high-level anti-counterfeiting applications.

Graphitic Dots Combining Photophysical Characteristics of Organic Molecular Fluorophores and Inorganic Quantum Dots

Thanks to their photophysical properties, both organic molecular fluorophores (MFs) and inorganic quantum dots (QDs) are extensively used for bioimaging applications. However, limitations such as photobleaching for the former or blinking, size, and toxicity for the latter still constitute a challenge for numerous applications. We report here that embedding MFs in graphitic carbon dots (GDs) results in fluorophores which entirely tackle this challenge. Characterized by ultranarrow, bright, and excitation-independent emission devoid of blinking and photobleaching, these hybrid-featured nanoparticles also demonstrate their unique photophysical performances at the single-nanoparticle scale, making them appealing candidates for bioimaging applications.

Heterochromatic Nonlinear Optical Responses in Upconversion Nanoparticles for Super-Resolution Nanoscopy

Point spread function (PSF) engineering by an emitter's response can code higher-spatial-frequency information of an image for microscopy to achieve super-resolution. However, complexed excitation optics or repetitive scans are needed, which explains the issues of low speed, poor stability, and operational complexity associated with the current laser scanning microscopy approaches. Here, the diverse emission responses of upconversion nanoparticles (UCNPs) are reported for super-resolution nanoscopy to improve the imaging quality and speed. The method only needs a doughnut-shaped scanning excitation beam at an appropriate power density. By collecting the four-photon emission of single UCNPs, the high-frequency information of a super-resolution image can be resolved through the doughnut-emission PSF. Meanwhile, the two-photon state of the same nanoparticle is oversaturated, so that the complementary lower-frequency information of the super-resolution image can be simultaneously collected by the Gaussian-like emission PSF. This leads to a method of Fourier-domain heterochromatic fusion, which allows the extended capability of the engineered PSFs to cover both low- and high-frequency information to yield optimized image quality. This approach achieves a spatial resolution of 40 nm, 1/24th of the excitation wavelength. This work suggests a new scope for developing nonlinear multi-color emitting probes in super-resolution nanoscopy.

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.

Near-infrared light excited photodynamic anticancer therapy based on UCNP@AIEgen nanocomposite

Photodynamic therapy (PDT), a clinically approved cancer treatment strategy, features non-invasiveness, few side-effects, high spatial resolution, etc. The advancement of PDT has been significantly restricted by the penetration depth of the excitation light. Herein, an effective fluorogen, TBD, with aggregation-induced emission characteristics (AIEgen) and high reactive-oxygen-species (ROS) generation efficiency was reported and integrated with a near infrared (NIR) light excitable upconversion nanoparticle (UCNP) to construct NIR light excitable UCNP@TBD nanocomposites. The formed nanocomposite has excellent photostability, good biocompatibility, and efficient ROS generation under NIR light excitation via Förster resonance energy transfer (FRET), enabling NIR light excited PDT. Moreover, the proposed NIR light excited PDT can break the impasse between the penetration depth and excitation volume in conventional PDT, effectively improving the anticancer therapeutic outcome. In vitro cancer cell ablation and in vivo tumor growth inhibition validated that the proposed UCNP@TBD nanocomposite is a promising NIR light excitable PDT agent with great potential for future translational research.

Modulated Luminescence of Lanthanide Materials by Local Surface Plasmon Resonance Effect

Lanthanide materials have great applications in optical communication, biological fluorescence imaging, laser, and so on, due to their narrow emission bandwidths, large Stokes' shifts, long emission lifetimes, and excellent photo-stability. However, the photon absorption cross-section of lanthanide ions is generally small, and the luminescence efficiency is relatively low. The effective improvement of the lanthanide-doped materials has been a challenge in the implementation of many applications. The local surface plasmon resonance (LSPR) effect of plasmonic nanoparticles (NPs) can improve the luminescence in different aspects: excitation enhancement induced by enhanced local field, emission enhancement induced by increased radiative decay, and quenching induced by increased non-radiative decay. In addition, plasmonic NPs can also regulate the energy transfer between two close lanthanide ions. In this review, the properties of the nanocomposite systems of lanthanide material and plasmonic NPs are presented, respectively. The mechanism of lanthanide materials regulated by plasmonic NPs and the scientific and technological discoveries of the luminescence technology are elaborated. Due to the large gap between the reported enhancement and the theoretical enhancement, some new strategies applied in lanthanide materials and related development in the plasmonic enhancing luminescence are presented.

Axial localization and tracking of self-interference nanoparticles by lateral point spread functions

Sub-diffraction limited localization of fluorescent emitters is a key goal of microscopy imaging. Here, we report that single upconversion nanoparticles, containing multiple emission centres with random orientations, can generate a series of unique, bright and position-sensitive patterns in the spatial domain when placed on top of a mirror. Supported by our numerical simulation, we attribute this effect to the sum of each single emitter’s interference with its own mirror image. As a result, this configuration generates a series of sophisticated far-field point spread functions (PSFs), e.g. in Gaussian, doughnut and archery target shapes, strongly dependent on the phase difference between the emitter and its image. In this way, the axial locations of nanoparticles are transferred into far-field patterns. We demonstrate a real-time distance sensing technology with a localization accuracy of 2.8 nm, according to the atomic force microscope (AFM) characterization values, smaller than 1/350 of the excitation wavelength.

Here, the authors show that single upconversion nanoparticles can generate position-sensitive patterns in the spatial domain when placed on a mirror. They attribute this to the single emitter’s interference with its own mirror image and show how this can be used to obtain axial localisation of the particle.

Quantum Dots for Improved Single-Molecule Localization Microscopy

Colloidal semiconductor quantum dots (QDs) have long established their versatility and utility for the visualization of biological interactions. On the single-particle level, QDs have demonstrated superior photophysical properties compared to organic dye molecules or fluorescent proteins, but it remains an open question as to which of these fundamental characteristics are most significant with respect to the performance of QDs for imaging beyond the diffraction limit. Here, we demonstrate significant enhancement in achievable localization precision in QD-labeled neurons compared to neurons labeled with an organic fluorophore. Additionally, we identify key photophysical parameters of QDs responsible for this enhancement and compare these parameters to reported values for commonly used fluorophores for super-resolution imaging.

Orthogonal Emissive Upconversion Nanoparticles: Material Design and Applications

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

Improving the image quality in STED nanoscopy using frequency spectrum modulation

Since stimulated emission depletion (STED) nanoscopy was invented in 1994, this technique has been widely used in the fields of biomedicine and materials science. According to the imaging principle of STED technology, increasing the power of the depletion laser within a certain threshold can improve the resolution. However, it will cause not only severe photo-damage to the samples and photo-bleaching to the fluorophores but also serious background noise, leading to the degeneration of the quality of STED images. Here we propose a new processing method based on frequency spectrum modulation to improve the quality of STED images, abbreviated as FM-STED. We have demonstrated the performance of FM-STED in improving the signal-to-noise ratio and the resolution using fluorescent beads and biological cells as samples.

Desilylation Induced by Metal Fluoride Nanocrystals Enables Cleavage Chemistry In Vivo

Metal fluoride nanocrystals are widely used in biomedical studies owing to their unique physicochemical properties. The release of metal ions and fluorides from nanocrystals is intrinsic due to the solubility equilibrium. It used to be considered as a drawback because it is related to the decomposition and defunction of metal fluoride nanocrystals. Many strategies have been developed to stabilize the nanocrystals, and the equilibrium concentrations of fluoride are often <1 mM. Here we make good use of this minimum amount of fluoride and unveil that metal fluoride nanocrystals could effectively induce desilylation cleavage chemistry, enabling controlled release of fluorophores and drug molecules in test tubes, living cells, and tumor-bearing mice. Biocompatible PEG (polyethylene glycol)-coated CaF₂ nanocrystals have been prepared to assay the efficiency of desilylation-induced controlled release of functional molecules. We apply the strategy to a prodrug activation of monomethyl auristatin E (MMAE), showing a remarkable anticancer effect, while side effects are almost negligible. In conclusion, this desilylation-induced cleavage chemistry avails the drawback on empowering metal fluoride nanocrystals with a new function of perturbing or activating for further biological applications.

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.

Lanthanide-Doped Near-Infrared Nanoparticles for Biophotonics

Light in the near-infrared (NIR) spectral region is increasingly utilized in bioapplications, providing deeper penetration in biological tissues owing to the lower absorption and scattering in comparison with light in the visible range. Lanthanide-doped luminescent nanoparticles with excitation and/or emission in the NIR range have recently attracted tremendous attention as one of the prime candidates for noninvasive biological applications due to their unique optical properties, such as large Stokes shift, spectrally sharp luminescence emissions, long luminescence lifetimes, and excellent photostability. Herein, recent advances of lanthanide-doped nanoparticles with NIR upconversion or downshifting luminescence and their uses in cutting-edge biophotonic applications are presented. A set of efficient strategies for overcoming the fundamental limit of low luminescence brightness of lanthanide-doped nanoparticles is introduced. An in-depth literature review of their state-of-art biophotonics applications is also included, showing their superiority for high-resolution imaging, single-nanoparticle-level detection, and efficacy for tissue-penetrating diagnostics and therapeutics.

Nanoscale optical writing through upconversion resonance energy transfer

Nanoscale optical writing using far-field super-resolution methods provides an unprecedented approach for high-capacity data storage. However, current nanoscale optical writing methods typically rely on photoinitiation and photoinhibition with high beam intensity, high energy consumption, and short device life span. We demonstrate a simple and broadly applicable method based on resonance energy transfer from lanthanide-doped upconversion nanoparticles to graphene oxide for nanoscale optical writing. The transfer of high-energy quanta from upconversion nanoparticles induces a localized chemical reduction in graphene oxide flakes for optical writing, with a lateral feature size of ~50 nm (1/20th of the wavelength) under an inhibition intensity of 11.25 MW cm-2 Upconversion resonance energy transfer may enable next-generation optical data storage with high capacity and low energy consumption, while offering a powerful tool for energy-efficient nanofabrication of flexible electronic devices.

Hybrid Device Architecture Using Plasmonic Nanoparticles, Graphene Quantum Dots, and Titanium Dioxide for UV Photodetectors

In this work, a nanoscale device architecture is demonstrated for a UV photodetector application on sapphire (0001), incorporating the plasmonic hybrid nanoparticles (HNPs), graphene quantum dots (GQDs), and titanium oxide (TiO₂) for the first time. The hybrid GQDs/TiO₂/HNPs photodetector exhibits the photocurrent of 1.58 × 10^-5 A under the 1.64 mW/mm² of 275 nm illumination at 10 V, which is around two order increase from the bare TiO₂ device. The proposed architecture demonstrates a low dark current of ∼1 × 10^-10 A at 10 V and thus the device demonstrates an excellent photo to dark current ratio along with the improved rise and fall time on the order of several hundred millisecond. The enhanced performance of device architecture is attributed to the efficient utilization of localized surface plasmon resonance (LSPR) induced hot carriers as well as scattered photons from the plasmonic HNPs that are fully encapsulated by the photoactive TiO₂ layers. Furthermore, the addition of GQDs on the TiO₂ can offer an additional photon absorption pathway. The proposed hybrid architecture of GQDs/TiO₂/HNPs demonstrates the integration of the photon absorption and carrier transfer properties of plasmonic HNPs, GQDs, and TiO₂ for an enhanced ultraviolet (UV) photoresponse. The photocurrent enhancement mechanisms of the hybrid device architecture are thoroughly investigated based on the finite-difference time domain (FDTD) simulation along with the energy band analysis. This work demonstrates a great potential of the hybrid device architecture for high-performance UV photodetectors.

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

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

Super-resolution Microscopy for Biological Imaging

Studying the ultra-fine structures and functions of the subcellular organelles and exploring the dynamic biological events in depth are the key issues in contemporary biological research. Fluorescence bio-imaging has been used to study cell biology for decades. However, the structures and functions of the subcellular organelles which fall under the diffraction limit are still not explored fully at a nanoscale level. Several super-resolution microscopy (SRM) techniques have been devised over the years which can be utilized to overcome diffraction limit. These techniques have opened a new window in biological research. However, SRM methods are highly vulnerable to the lack of appropriate fluorophores and other sophisticated technical considerations. Therefore, this chapter briefly summarizes the basic principles of various SRM methods which have been frequently utilized in biological imaging. The chapter not only gives an overview of the technical advantages and drawbacks about using different SRM techniques for bio-imaging applications but also briefly articulates the nitty-gritties of selecting a proper fluorescent probe for a specific SRM experiment with biological samples.

Recent advances in luminescent materials for super-resolution imaging via stimulated emission depletion nanoscopy

Recent progress on STED fluorophores for super-resolution imaging and also their characteristics are outlined here, thus providing some guidelines to select proper probes and even develop new materials for super-resolution imaging via STED nanoscopy.

Light-Emitting Diode Excitation for Upconversion Microscopy: A Quantitative Assessment

Lanthanide-based upconversion nanoparticles (UCNPs) generally require high power laser excitation. Here, we report wide-field upconversion microscopy at single-nanoparticle sensitivity using incoherent excitation of a 970 nm light-emitting diode (LED). We show that due to its broad emission spectrum, LED excitation is about 3 times less effective for UCNPs and generates high background compared to laser illumination. To counter this, we use time-gated luminescence detection to eliminate the residual background from the LED source, so that individual UCNPs with high sensitizer (Yb³⁺) doping and inert shell protection become clearly identified under LED excitation at 1.18 W cm^-2, as confirmed by correlated electron microscopy images. Hydrophilic UCNPs are obtained by polysaccharide coating via a facile ligand exchange protocol to demonstrate imaging of cellular uptake using LED excitation. These results suggest a viable approach to bypassing the limitations associated with high-power lasers when applying UCNPs and upconversion microscopy to life science research.

Low threshold lasing emissions from a single upconversion nanocrystal

Cross-relaxation among neighboring emitters normally causes self-quenching and limits the brightness of luminescence. However, in nanomaterials, cross-relaxation could be well-controlled and employed for increasing the luminescence efficiency at specific wavelengths. Here we report that cross-relaxation can modulate both the brightness of single upconversion nanoparticles and the threshold to reach population inversion, and both are critical factors in producing the ultra-low threshold lasing emissions in a micro cavity laser. By homogenously coating a 5-μm cavity with a single layer of nanoparticles, we demonstrate that doping Tm³⁺ ions at 2% can facilitate the electron accumulation at the intermediate state of ³H₄ level and efficiently decrease the lasing threshold by more than one order of magnitude. As a result, we demonstrate up-converted lasing emissions with an ultralow threshold of continuous-wave excitation of ~150 W/cm² achieved at room temperature. A single nanoparticle can lase with a full width at half-maximum as narrow as ~0.45 nm.

Nanorods with multidimensional optical information beyond the diffraction limit

Precise design and fabrication of heterogeneous nanostructures will enable nanoscale devices to integrate multiple desirable functionalities. But due to the diffraction limit (~200 nm), the optical uniformity and diversity within the heterogeneous functional nanostructures are hardly controlled and characterized. Here, we report a set of heterogeneous nanorods; each optically active section has its unique nonlinear response to donut-shaped illumination, so that one can discern each section with super-resolution. To achieve this, we first realize an approach of highly controlled epitaxial growth and produce a range of heterogeneous structures. Each section along the nanorod structure displays tunable upconversion emissions, in four optical dimensions, including color, lifetime, excitation wavelength, and power dependency. Moreover, we demonstrate a 210 nm single nanorod as an extremely small polychromatic light source for the on-demand generation of RGB photonic emissions. This work benchmarks our ability toward the full control of sub-diffraction-limit optical diversities of single heterogeneous nanoparticles.

Development of functional nanostructures can enable a range of applications in imaging and nanoscale science. Here, the authors fabricate and characterize complex heterogeneous nanorods with diverse, tunable sub-wavelength structures.

Enhancing FRET biosensing beyond 10 nm with photon avalanche nanoparticles

Förster Resonance Energy Transfer (FRET) between donor (D) and acceptor (A) molecules is a phenomenon commonly exploited to study or visualize biological interactions at the molecular level. However, commonly used organic D and A molecules often suffer from photobleaching and spectral bleed-through, and their spectral properties hinder quantitative analysis. Lanthanide-doped upconverting nanoparticles (UCNPs) as alternative D species offer significant improvements in terms of photostability, spectral purity and background-free luminescence detection, but they bring new challenges related to multiple donor ions existing in a single large size UCNP and the need for nanoparticle biofunctionalization. Considering the relatively short Förster distance (typically below 5-7 nm), it becomes a non-trivial task to assure sufficiently strong D-A interaction, which translates directly to the sensitivity of such bio-sensors. In this work we propose a solution to these issues, which employs the photon avalanche (PA) phenomenon in lanthanide-doped materials. Using theoretical modelling, we predict that these PA systems would be highly susceptible to the presence of A and that the estimated sensitivity range extends to distances 2 to 4 times longer (i.e. 10-25 nm) than those typically found in conventional FRET systems. This promises high sensitivity, low background and spectral or temporal biosensing, and provides the basis for a radically novel approach to combine luminescence imaging and self-normalized bio-molecular interaction sensing.

Binary Nanoparticle Superlattices for Plasmonically Modulating Upconversion Luminescence

Engineering a facile and controllable approach to modulate the spectral properties of lanthanide-doped upconversion nanoparticles (UCNPs) is always an ongoing challenge. Herein, long-range ordered, distinct two-dimensional (2D) binary nanoparticle superlattices (BNSLs) composed of NaREF₄ :Yb/Er (RE = Y and Gd) UCNPs and plasmonic metallic nanoparticles (Au NPs), including AB, AB₃ , and AB₁₃ lattices, are fabricated via a slow evaporation-driven self-assembly to achieve plasmonic modulation of upconversion luminescence (UCL). Optical measurements reveal that typical red-green UCL from UCNPs can be effectively modulated into reddish output in BNSLs, with a drastically shortened lifetime. Notably, for AB₃ - and AB₁₃ -type BNSLs with more proximal Au NPs around each UCNP, modified UCL with fine-structured spectral lineshape is observed. These differences could be interpreted by the interplay of collective plasmon resonance introduced by 2D periodic Au arrays and spectrally selective energy transfer between UCNPs and Au. Thus, fabricating UCNP-Au BNSLs with desired lattice parameters and NP configurations could be a promising way to tailor the UCL through controlled plasmonic modulation.

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.

Revisiting the Luminescence Decay Kinetics of Energy Transfer Upconversion

Energy transfer upconversion (ETU) can efficiently upconvert near-infrared photons into higher-energy photons. Although a comprehensive understanding of ETU is fundamental to the design of ETU materials, the basic excited-state decay kinetics of ETU remains a complicated problem. Here we unravel the mechanism underlying ETU decay in benchmark β-NaYF₄:Er³⁺ and β-NaYF₄:Ln³⁺/Yb³⁺ (Ln = Er, Ho, Tm) ETU microcrystals by combining rate equation analyses with ETU decay measurements. The results show that all of the excited states of one ETU system decay concordantly, with the ETU decay of the emitting state determined by only its intrinsic decay and the product of the ETU decays of the two intermediate states directly responsible for the emitting-state photon upconversion. This general mechanism may serve as a basic rule for excited-state kinetics in upconversion microparticles and nanoparticles, which could provide detailed insight into ETU processes and guide the design of efficient ETU materials.

Helix Shape Power-Dependent Properties of Single Upconversion Nanoparticles

Nonblinking, nonbleaching, and superbright single upconversion nanoparticles have been recently discovered with nonlinear power-dependent properties and can be switchable under dual-beam excitations, which are ideal for super-resolution microscopy, single-molecule tracking, and digital assays. Here, we report that the brightness of Nd³⁺-Yb³⁺-Er³⁺-doped nanoparticles displays a pair of unusual double helix shapes as the function of power densities of 976 and 808 nm excitations. We systemically analyze the power-dependent emission spectra, lifetimes, and power-intensity double-log slopes of single upconversion nanoparticles, which reveal that the dynamic roles of Nd³⁺ ions in the tridoped nanosystem with underlining electron population pathways are power dependent. That is, at high power 808 nm excitation, Nd³⁺ ions can directly emit upconverted luminescence, with their conventional role of sensitization saturated in the Nd³ → Yb³⁺ → Er³⁺ energy transfer systems. Moreover, we confirm that the universal helix shape phenomena commonly exist in a set of eight batches of core-shell nanoparticles regardless of the doping concentrations of Nd³⁺, Yb³⁺, and Er³⁺ ions in the sensitization shell, migration shell, and active core, though the crossing nodes occur at different excitation power ranges. This study emphasizes the important role of power-dependent properties in both improving the upconversion emission efficiency and the design of nonlinear responsive probes for imaging and sensing.

Combating Concentration Quenching in Upconversion Nanoparticles

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

Recent advances of lanthanide-doped upconversion nanoparticles for biological applications

Near infrared (NIR) excited lanthanide-doped upconversion nanoparticles (UCNPs) are emerging as a new type of fluorescent tag for biological applications, which can emit multi-photon ultraviolet, visible or NIR luminescence for imaging or activation of photosensitive molecules. Here, we present a comprehensive review on recent advances of UCNPs for a manifold of biological applications, including upconversion mechanisms, building bright multicolor upconversion nanocrystals, single nanoparticle and super resolution imaging, in vivo optical and multimodal imaging, photodynamic therapy, light-controlled drug release, biosensing, and toxicities. Our perspectives on the future development of UCNPs are also described.

Super-Resolution Mapping of Single Nanoparticles inside Tumor Spheroids

Cancer spheroids have structural, functional, and physiological similarities to the tumor, and have become a low-cost in vitro model to study the physiological responses of single cells and therapeutic efficacy of drugs. However, the tiny spheroid, made of a cluster of high-density cells, is highly scattering and absorptive, which prevents light microscopy techniques to reach the depth inside spheroids with high resolution. Here, a method is reported for super-resolution mapping of single nanoparticles inside a spheroid. It first takes advantage of the self-healing property of a "nondiffractive" doughnut-shaped Bessel beam from a 980 nm diode laser as the excitation, and further employs the nonlinear response of the 800 nm emission from upconversion nanoparticles, so that both excitation and emission at the near-infrared can experience minimal loss through the spheroid. These strategies lead to the development of a new nanoscopy modality with a resolution of 37 nm, 1/26th of the excitation wavelength. This method enables mapping of single nanoparticles located 55 µm inside a spheroid, with a resolution of 98 nm. It suggests a solution to track single nanoparticles and monitor their release of drugs in 3D multicellar environments.

Strategies for Constructing Upconversion Luminescence Nanoprobes to Improve Signal Contrast

Lanthanide-doped upconversion nanoparticles (UCNPs) can convert two or more lower-energy near-infrared photons to a single photon with higher energy, which makes them particularly suitable for constructing nanoprobes with large imaging depth and minimal interference of autofluorescence and light scattering from biosamples. Furthermore, they feature excellent photostability, sharp and narrow emissions, and large anti-Stokes shift, which confer them the capability of long-period bioimaging and real-time tracking. In recent years, UCNPs-based nanoprobes (UC-nanoprobes) have been attracting increasing interest in biological and medical research. Signal contrast, the ratio of signal intensity after and before the reaction of the probe and target, is the determinant factor of the sensitivity of all reaction-based probes. This progress report presents the methods of constructing UC-nanoprobes, with a focus fixed on recent strategies to improve the signal contrast, which have kept on promoting the bioapplication of this type of probe.

Super-Resolution Visualization of Self-Assembling Helical Fibers Using Aggregation-Induced Emission Luminogens in Stimulated Emission Depletion Nanoscopy

Organic fluorophores for stimulated emission depletion (STED) nanoscopy usually suffer from quenched emission in the aggregate state and inferior photostability, which largely limit their application in real-time, in situ, and long-term imaging at an ultrahigh resolution. Herein, an aggregation-induced emission (AIE) luminogen of DP-TBT with bright emission in solid state (photoluminescence quantum yields = 25%) and excellent photostability was designed to meet the requirements in STED nanoscopy. In addition to its excellent fluorescence properties, DP-TBT could also easily form self-assembling helixes and finally be well-visualized by super-resolution STED nanoscopy. The observations showed that helical fibers of DP-TBT as dashed lines had a much decreased fiber width with also a full width at half-maximum value of only 178 nm, which is ∼6 times higher than solid lines obtained by confocal microscopy (1154 nm). The STED nanoscopic data were also used to reconstruct 3D images of assembled helixes. Finally, by long-term tracking and dynamic monitoring, the formation and growth of helical fibers by DP-TBT in self-assembly processes were successfully obtained. These findings imply that highly emissive AIEgens with good photostability are highly suitable for real-time, in situ, and dynamic imaging at super-resolution using STED nanoscopy.

3D sub-diffraction imaging in a conventional confocal configuration by exploiting super-linear emitters

Sub-diffraction microscopy enables bio-imaging with unprecedented clarity. However, most super-resolution methods require complex, costly purpose-built systems, involve image post-processing and struggle with sub-diffraction imaging in 3D. Here, we realize a conceptually different super-resolution approach which circumvents these limitations and enables 3D sub-diffraction imaging on conventional confocal microscopes. We refer to it as super-linear excitation-emission (SEE) microscopy, as it relies on markers with super-linear dependence of the emission on the excitation power. Super-linear markers proposed here are upconversion nanoparticles of NaYF4, doped with 20% Yb and unconventionally high 8% Tm, which are conveniently excited in the near-infrared biological window. We develop a computational framework calculating the 3D resolution for any viable scanning beam shape and excitation-emission probe profile. Imaging of colominic acid-coated upconversion nanoparticles endocytosed by neuronal cells, at resolutions twice better than the diffraction limit both in lateral and axial directions, illustrates the applicability of SEE microscopy for sub-cellular biology.

Microlens array enhanced upconversion luminescence at low excitation irradiance

The dearth of high upconversion luminescence (UCL) intensity at low excitation irradiance hinders the prevalent application of lanthanide-doped upconversion nanoparticles (UCNPs) in many fields ranging from optical bioimaging to photovoltaics. In this work, we propose to use microlens arrays (MLAs) as spatial light modulators to manipulate the distribution of excitation light fields in order to increase UCL, taking advantage of its nonlinear response to the excitation irradiance. We show that multicolored UCL from NaYF₄:Yb³⁺,Er³⁺@NaYF₄:Yb³⁺,Nd³⁺ and NaYF₄:Yb³⁺,Tm³⁺@NaYF₄:Yb³⁺,Nd³⁺ core/shell UCNPs can be increased by more than one order of magnitude under either 980 or 808 nm excitation, by simply placing a polymeric MLA onto the top of these samples. The observed typical green (525/540 nm) and red (654 nm) UCL bands from Er³⁺ and a blue (450/475 nm) UCL band from Tm³⁺ exhibit distinct enhancement factors due to their different multi-photon processes. Importantly, our ray tracing simulation reveals that the MLA is able to spatially confine the excitation light (980 and 808 nm) by orders of magnitude, thus amplifying UCL by more than 225-fold (the 450 nm UCL band of Tm³⁺) at low excitation irradiance. The proposed MLA method has immediate ramifications for the improved performance of all types of UCNP-based devices, such as UCNP-enhanced dye sensitized solar cells demonstrated here.

Optically Robust and Biocompatible Mechanosensitive Upconverting Nanoparticles

Upconverting nanoparticles (UCNPs) are promising tools for background-free imaging and sensing. However, their usefulness for in vivo applications depends on their biocompatibility, which we define by their optical performance in biological environments and their toxicity in living organisms. For UCNPs with a ratiometric color response to mechanical stress, consistent emission intensity and color are desired for the particles under nonmechanical stimuli. Here, we test the biocompatibility and mechanosensitivity of α-NaYF4:Yb,Er@NaLuF4 nanoparticles. First, we ligand-strip these particles to render them dispersible in aqueous media. Then, we characterize their mechanosensitivity (∼30% in the red-to-green spectral ratio per GPa), which is nearly 3-fold greater than those coated in oleic acid. We next design a suite of ex vivo and in vivo tests to investigate their structural and optical properties under several biorelevant conditions: over time in various buffers types, as a function of pH, and in vivo along the digestive tract of Caenorhabditis elegans worms. Finally, to ensure that the particles do not perturb biological function in C. elegans, we assess the chronic toxicity of nanoparticle ingestion using a reproductive brood assay. In these ways, we determine that mechanosensitive UCNPs are biocompatible, i.e., optically robust and nontoxic, for use as in vivo sensors to study animal digestion.