Recent research on stimulated emission depletion microscopy for reducing photobleaching

Coherent Interference Fringes of Two-Photon Photoluminescence in Individual Au Nanoparticles: The Critical Role of the Intermediate State

The interaction between light and metal nanoparticles enables investigations of microscopic phenomena on nanometer length and ultrashort timescales, benefiting from strong confinement and enhancement of the optical field. However, the ultrafast dynamics of these nanoparticles are primarily investigated by multiphoton photoluminescence on picoseconds or photoemission on femtoseconds independently. Here, we presented two-photon photoluminescence (TPPL) measurements on individual Au nanobipyramids (AuNP) to reveal their ultrafast dynamics by double-pulse excitation on a global timescale ranging from subfemtosecond to tens of picoseconds. Two orders of magnitude photoluminescence enhancement, namely, coherent interference fringes, has been demonstrated. Power-dependent measurements uncovered the transform of the nonlinearity from 1 to 2 when the interpulse delay varied from tens of femtoseconds to tens of picoseconds. We proved that the real intermediate state plays a critical role in the observed phenomena, supported by numerical simulations with a three-state model. Our results provide insight into the role of intermediate states in the ultrafast dynamics of noble metal nanoparticles. The presence of the intermediate states in AuNP and the coherent control of state populations offer interesting perspectives for imaging, sensing, nanophotonics, and in particular, for preparing macroscopic superposition states at room temperature and low-power superresolution stimulated emission depletion microscopy.

Exploring the interstitial system in the brain: the last mile of drug delivery

Brain interstitial system (ISS) is a nanoscale network of continuously connected tubes and sheets surrounding each neural cell in the central nervous system. ISS usually accounts for ∼20% of the brain volume, far more than the cerebral blood vessels, which account for 3%. The neuronal function, signaling pathways, and drug delivery are all closely related to the microenvironment provided by ISS. The objective of this paper is to give the readers a clear outline of detection, anatomy, function, and applications of ISS. This review describes the techniques propelling the exploration for ISS in chronological order, physiological function and pathological dysfunction of ISS, and strategies for drug delivery based on ISS. Biophysical features are the focus of ISS research, in which the diffusion characteristics have dominated. The various techniques that explore ISS take advantage of this feature. ISS provides an essential microenvironment for the health of cells and brain homeostasis, which plays an important functional role in brain health and disease. Direct intracranial administration allows the diffusion of drugs directly through ISS to successfully bypass the blood-brain barrier that prevents most drugs from reaching the brain. With the deepening of understanding of the brain ISS, the new research model that takes into account brain cells, cerebral vessels, and ISS will provide a new perspective and direction for understanding, utilizing, and protecting the brain.

Cumulative effects of photobleaching in volumetric STED imaging-artefacts and possible benefits

In stimulated emission depletion (STED) imaging, the excitation and depletion laser beams extend well beyond the focal plane in the imaged sample. We investigated how photobleaching resulting from this irradiation can affect STED images, by acquiring 3D images of fluorescent polystyrene beads using a 2D STED microscope, and applying different Z pixel sizes, scanning speeds, resulting in different laser light doses. While higher STED beam irradiances can increase the spatial resolution, they can also significantly increase photobleaching and thereby reduce signal-to-background levels. In 2D STED imaging, based on a single scan within the focal plane, scan parameters can often be selected to avoid photobleaching effects. Upon 3D optical sectioning experiments however, using the same scan parameters, additional cumulative effects of photobleaching may appear, due to the extension of the excitation and depletion laser beams beyond the focal planes being scanned. Apart from a reduction in signal-to-background levels, such photobleaching can lead to an apparent shift of the axial localization of the objects in the images, but also to an increased resolution in the axial dimension. These findings, confirmed by simulations based on a simplified model for photobleaching, suggests some caution in volumetric STED imaging experiments, but also a possibility for enhanced axial resolution in such experiments.

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.

Ultrashort laser based two-photon phase-resolved fluorescence lifetime measurement method

This paper presents a two-photon phase-resolved fluorescence-lifetime measurement method based on the use of an ultrashort pulse laser. The proposed method also involves the use of a lock-in amplifier to control the phase difference between the reference and fluorescence signals, thereby facilitating the use of an alternative method for determining fluorescence lifetimes. Verification of the fluorescence lifetimes as measured in this study was performed using rhodamine B and a cellular thermoprobe as samples. In this study, we assume that the fluorescence decay was monoexponential in all cases. Rhodamine B was observed to exhibit an average fluorescence lifetime of 2.15 ns, whereas a temperature sensitivity of 1.39 ns C^-1 over a temperature range of 33.79-37.2 °C was demonstrated for the cellular thermoprobe. These results validate the feasibility of the proposed method for accurate measurement of fluorescence lifetimes using a simple laser configuration.

Organic nanoparticles with ultrahigh stimulated emission depletion efficiency for low-power STED nanoscopy

Stimulated emission depletion (STED) nanoscopy is a powerful sub-diffraction imaging tool to probe subcellular structures and organelles. Conventional organic dyes require high STED power (PSTED) to obtain sub-diffraction resolution, leading to serious photo-bleaching. Herein, this study demonstrates highly emissive silica-coated core-shell organic nanoparticles (CSONPs) as a new type of photostable probe with ultrahigh stimulated emission depletion efficiency for low-power super-resolution STED nanoscopy. The CSONPs offer (i) efficient red emission with high solid-state fluorescence quantum yields around 0.6, (ii) large Stokes shift of 150 nm and (iii) high photostability owing to silica shell protection. The stimulated emission depletion efficiency (η) of CSONPs was extremely high up to η = 99% (the highest value reported so far) with a saturation intensity as low as Isat = 0.18 MW cm-2. Moreover, this research demonstrates the super-resolution imaging of living HeLa cells stained using CSONPs with a lateral spatial resolution of 63 nm at an extremely low depletion power of ISTED = 0.89 MW cm-2 and a long-term stability >600 s at η = 80% without obvious fatigue. The excellent and comprehensive performances of the CSONPs are promising for super-resolution imaging in biological applications.

Fight against background noise in stimulated emission depletion nanoscopy

STimulated emission depletion (STED) nanoscopy has been proposed to extend greatly our capability of using light to study a variety of biological problems with nanometer-scale resolution. However, in practice the unwanted background noise degrades the STED image quality and precludes quantitative analysis. Here, we discuss the underlying sources of the background noise in STED images, and review current approaches to alleviate this problem, such as time-gating, anti-Stokes excitation removal, and off-focus incomplete depletion suppression. Progress in correcting uncorrelated background photons in fluorescence correlation spectroscopy combined with STED (STED-FCS) will also be discussed.

Full volume super-resolution imaging of thick mitotic spindle using 3D AO STED microscope

Stimulated emission depletion (STED) nanoscopy is one of a suite of modern optical microscopy techniques capable of bypassing the conventional diffraction limit in fluorescent imaging. STED makes use of a spiral phase mask to enable 2D super-resolution imaging whereas to achieve full volumetric 3D super-resolution an additional bottle-beam phase mask must be applied. The resolution achieved in biological samples 10 µm or thicker is limited by aberrations induced mainly by scattering due to refractive index heterogeneity in the sample. These aberrations impact the fidelity of both types of phase mask, and have limited the application of STED to thicker biological systems. Here we apply an automated adaptive optics solution to correct the performance of both STED masks, enhancing robustness and expanding the capabilities of this nanoscopic technique. Corroboration in terms of successful high-quality imaging of the full volume of a 15µm mitotic spindle with resolution of 50nm x 50nm x 150nm achieved in all three dimensions is presented.