Three-dimensional super-resolved live cell imaging through polarized multi-angle TIRF

Near-Infrared Polarimetric Image Sensors Based on Ordered Sulfur-Passivation GaSb Nanowire Arrays

The near-infrared polarimetric image sensor has a wide range of applications in the military and civilian fields, thus developing into a research hotspot in recent years. Because of their distinguishing 1D structure features, the ordered GaSb nanowire (NW) arrays possess potential applications for near-infrared polarization photodetection. In this work, single-crystalline GaSb NWs are synthesized through a sulfur-catalyzed chemical vapor deposition process. A sulfur-passivation thin layer is formed on the NW surface, which prevents the GaSb NW core from being oxidized. The photodetector based on sulfur-passivation GaSb (S-GaSb) NWs has a lower dark current and higher responsivity than that built with pure GaSb NWs. The photodetector exhibits a large responsivity of 9.39 × 10² A/W and an ultrahigh detectivity of 1.10 × 10¹¹ Jones for 1.55 μm incident light. Furthermore, the dichroic ratio of the device is measured to reach 2.65 for polarized 1.55 μm light. Through a COMSOL simulation, it is elucidated that the origin of the polarized photoresponse is the attenuation of a light electric field inside the NW when the angle of incident polarization light rotates. Moreover, a flexible polarimetric image sensor with 5 × 5 pixels is successfully constructed on the ordered S-GaSb NW arrays, and it exhibits a good imaging ability for incident near-infrared polarization light. These good photoresponse properties and polarized imaging abilities can empower ordered S-GaSb NW arrays with technological potentials in next-generation large-scale near-infrared polarimetric imaging sensors.

Polarimetric Image Sensor and Fermi Level Shifting Induced Multichannel Transition Based on 2D PdPS

2D materials have been attracting high interest in recent years due to their low structural symmetry, excellent photoresponse, and high air stability. However, most 2D materials can only respond to specific light, which limits the development of wide-spectrum photodetectors. Proper bandgap and the regulation of Fermi level are the foundations for realizing electronic multichannel transition, which is an effective method to achieve a wide spectral response. Herein, a noble 2D material, palladium phosphide sulfide (PdPS), is designed and synthesized. The bandgap of PdPS is around 2.1 eV and the formation of S vacancies, interstitial Pd and P atoms promote the Fermi level very close to the conduction band. Therefore, the PdPS-based photodetector shows impressive wide spectral response from solar-blind ultraviolet to near-infrared based on the multichannel transition. It also exhibits superior optoelectrical properties with photoresponsivity (R) of 1 × 10³ A W^-1 and detectivity (D*) of 4 × 10¹¹ Jones at 532 nm. Moreover, PdPS exhibits good performance of polarization detection with dichroic ratio of ≈3.7 at 808 nm. Significantly, it achieves polarimetric imaging and hidden-target detection in complex environments through active detection.

Label-free microscopy of mitotic chromosomes using the polarization orthogonality breaking technique

We report how a recently developed polarization imaging technique, implementing micro-wave photonics and referred to as orthogonality-breaking (OB) imaging, can be adapted on a classical confocal fluorescence microscope, and is able to provide informative polarization images from a single scan of the cell sample. For instance, the comparison of the images of various cell lines at different cell-cycle stages obtained by OB polarization microscopy and fluorescence confocal images shows that an endogenous polarimetric contrast arizes with this instrument on compacted chromosomes during cell division.

Narrow-band polymer dots with pronounced fluorescence fluctuations for dual-color super-resolution imaging

Super-resolution optical fluctuation imaging (SOFI) produces fast, background-free, super-resolved images by analyzing the temporal fluorescence fluctuations of independent emitters. With sufficient brightness and fluctuations, a higher order of image processing affords a higher resolution and in principle the resolution enhancement is unbounded. However, it is practically challenging to find suitable probes for high-order SOFI. Herein, we report two types of BODIPY-based polymer dots (Pdots) with narrow-band emissions, pronounced fluctuations, and prominent photostability, thus enabling high-order, dual-color SOFI nanoscopy. Single-particle and subcellular SOFI analysis reveals the superior performance of the BODIPY Pdots as compared to conventional streptavidin-conjugated Alexa dyes. In contrast with wide-field images, the spatial resolution (∼57 nm) was enhanced by ∼6.0-fold in 8^th-order single-particle SOFI nanoscopy. A spatial resolution (61 nm) was obtained for single microtubules labeled by the BODIPY Pdots, while the majority of the subcellular structures were lost for those labeled by streptavidin-Alexa dyes in 8^th-order SOFI. This work indicates the unprecedented performance of Pdot probes for multi-color subcellular SOFI applications.

Variable incidence angle linear dichroism (VALiD): a technique for unique 3D orientation measurement of fluorescent ensembles

A fundamental challenge with fluorophore orientation measurement is degeneracy, which is the inability to distinguish between multiple unique fluorophore orientations. Techniques exist for the non-degenerate measurement of the orientations of single, static fluorophores. However, such techniques are unsuitable for densely labeled and/or dynamic samples common to biological research. Accordingly, a rapid, widefield microscopy technique that can measure orientation parameters for ensembles of fluorophores in a non-degenerate manner is desirable. We propose that exciting samples with polarized light and multiple incidence angles could enable such a technique. We use Monte Carlo simulations to validate this approach for specific axially symmetric ensembles of fluorophores and obtain optimal experimental parameters for its future implementation.

Presegmentation Procedure Generates Smooth-Sided Microfluidic Devices: Unlocking Multiangle Imaging for Everyone?

We present a simple procedure to create smooth-sided, transparent polymer-based microfluidic devices by presegmentation with hydrophobized glass slides. We study the hypothesis that the smooth side planes permit rapid multiangle imaging of microfluidic systems in contrast to the turbid side planes that result from cutting the polymer. We compare the compatibility of the entire approach with the conventional widefield microscopy, confocal and 2-photon microscopy, as well as three-dimensional (3D) rendering and discuss limitations and potential applications.

Super-resolution imaging of fluorescent dipoles via polarized structured illumination microscopy

Fluorescence polarization microscopy images both the intensity and orientation of fluorescent dipoles and plays a vital role in studying molecular structures and dynamics of bio-complexes. However, current techniques remain difficult to resolve the dipole assemblies on subcellular structures and their dynamics in living cells at super-resolution level. Here we report polarized structured illumination microscopy (pSIM), which achieves super-resolution imaging of dipoles by interpreting the dipoles in spatio-angular hyperspace. We demonstrate the application of pSIM on a series of biological filamentous systems, such as cytoskeleton networks and λ-DNA, and report the dynamics of short actin sliding across a myosin-coated surface. Further, pSIM reveals the side-by-side organization of the actin ring structures in the membrane-associated periodic skeleton of hippocampal neurons and images the dipole dynamics of green fluorescent protein-labeled microtubules in live U2OS cells. pSIM applies directly to a large variety of commercial and home-built SIM systems with various imaging modality.

Polarization microscopy has been combined with single-molecule localization, but it’s often limited in either speed or resolution. Here the authors present polarized Structured Illumination Microscopy (pSIM), a method that uses polarized laser excitation to measure dye orientation during fast super-resolution live cell imaging.

Tuning fluorophore excitation in a total-internal-reflection-fluorescence microscopy

In a total-internal-reflection-fluorescence-microscopy method, there is anisotropy in the polarized evanescent wave. Since the evanescent wave is used as an excitation field, the mentioned anisotropy is a disadvantage in using the total-internal-reflection-fluorescence-microscopy technique. Therefore, by theoretical and analytical approaches, and based on the Fresnel coefficients, the effect of three dielectrics media on the anisotropy of the evanescent wave is investigated. Following that, a proper combination of the cover glass, oil immersion, and prism for both living and non-living samples is suggested that not only enhances the intensity of the evanescent wave, but also and importantly, decreases the essential anisotropy of the evanescent wave.

High-speed device synchronization in optical microscopy with an open-source hardware control platform

Azimuthal beam scanning eliminates the uneven excitation field arising from laser interference in through-objective total internal reflection fluorescence (TIRF) microscopy. The same principle can be applied to scanning angle interference microscopy (SAIM), where precision control of the scanned laser beam presents unique technical challenges for the builders of custom azimuthal scanning microscopes. Accurate synchronization between the instrument computer, beam scanning system and excitation source is required to collect high quality data and minimize sample damage in SAIM acquisitions. Drawing inspiration from open-source prototyping systems, like the Arduino microcontroller boards, we developed a new instrument control platform to be affordable, easily programmed, and broadly useful, but with integrated, precision analog circuitry and optimized firmware routines tailored to advanced microscopy. We show how the integration of waveform generation, multiplexed analog outputs, and native hardware triggers into a single central hub provides a versatile platform for performing fast circle-scanning acquisitions, including azimuthal scanning SAIM and multiangle TIRF. We also demonstrate how the low communication latency of our hardware platform can reduce image intensity and reconstruction artifacts arising from synchronization errors produced by software control. Our complete platform, including hardware design, firmware, API, and software, is available online for community-based development and collaboration.

Fluorescence Anisotropy Reloaded-Emerging Polarization Microscopy Methods for Assessing Chromophores' Organization and Excitation Energy Transfer in Single Molecules, Particles, Films, and Beyond

Fluorescence polarization is widely used to assess the orientation/rotation of molecules, and the excitation energy transfer between closely located chromophores. Emerging since the 1990s, single molecule fluorescence spectroscopy and imaging stimulate the application of light polarization for studying molecular organization and energy transfer beyond ensemble averaging. Here, traditional fluorescence polarization and linear dichroism methods used for bulk samples are compared with techniques specially developed for, or inspired by, single molecule fluorescence spectroscopy. Techniques for assessing energy transfer in anisotropic samples, where the traditional fluorescence anisotropy framework is not readily applicable, are discussed in depth. It is shown that the concept of a polarization portrait and the single funnel approximation can lay the foundation for alternative energy transfer metrics. Examples ranging from fundamental studies of photoactive materials (conjugated polymers, light-harvesting aggregates, and perovskite semiconductors) to Förster resonant energy transfer (FRET)-based biomedical imaging are presented. Furthermore, novel uses of light polarization for super-resolution optical imaging are mentioned as well as strategies for avoiding artifacts in polarization microscopy.

One-step deconvolution for multi-angle TIRF microscopy with enhanced resolution

Total internal reflection fluorescence microscopy (TIRF microscopy) uses a rapid decay of evanescent waves to excite fluorophores within several hundred nanometers (nm) beneath the plasma membrane, which can effectively suppress excitation of fluorescence signals in the deep layers. From image stacks obtained with a plurality of different incident angles, a three-dimensional spatial structure of the observed sample can be reconstructed by a Multi-Angle-TIRF (MA-TIRF) algorithm that provides an axial resolution of ~50 nm. Taking into account the point spread function (PSF) of the TIRF microscopes, we further increase its lateral resolution by introducing a fast deconvolution algorithm into the reconstruction of MA-TIRF data (DMA-TIRF), which is approached in just one step of minimizing the reconstruction function. We also introduce a TV regularization term in the deconvolution algorithm to suppress artifacts induced by the excessive noise. Therefore, based on the hardware of existing MA-TIRF microscopes, the proposed DMA-TIRF algorithm has achieved lateral and axial resolutions of ~200 and ~50 nm, respectively.

Nanometric axial resolution of fibronectin assembly units achieved with an efficient reconstruction approach for multi-angle-TIRF microscopy

High resolution imaging of molecules at the cell-substrate interface is required for understanding key biological processes. Here we propose a complete pipeline for multi-angle total internal reflection fluorescence microscopy (MA-TIRF) going from instrument design and calibration procedures to numerical reconstruction. Our custom setup is endowed with a homogeneous field illumination and precise excitation beam angle. Given a set of MA-TIRF acquisitions, we deploy an efficient joint deconvolution/reconstruction algorithm based on a variational formulation of the inverse problem. This algorithm offers the possibility of using various regularizations and can run on graphics processing unit (GPU) for rapid reconstruction. Moreover, it can be easily used with other MA-TIRF devices and we provide it as an open-source software. This ensemble has enabled us to visualize and measure with unprecedented nanometric resolution, the depth of molecular components of the fibronectin assembly machinery at the basal surface of endothelial cells.

Inverse problem based on the fast alternating direction method of multipliers algorithm in multiangle total internal reflection fluorescence microscopy

Multiangle total internal reflection fluorescence microscopy (TIRFM) has become one of the most important techniques for achieving axial superresolution. The key process in this technique is solving the inverse problem. This paper applies an improved alternating direction method of multipliers algorithm to solve the inverse problem and validates the accuracy of the algorithm by reconstructing simulated microtubule structures in multiangle TIRFM images. The reconstruction times for different algorithms and the convergence speeds of the improved and original algorithms are compared. Experimental results show that the improved algorithm can achieve an axial resolution of 40 nm, reduce the influence of the penalty parameter on convergence, and improve the convergence speed of the iterative process while ensuring image reconstruction quality. Based on the algorithm, a three-dimensional image with the depth information of microtubules and mitochondria is reconstructed.

Multi-color live-cell super-resolution volume imaging with multi-angle interference microscopy

Imaging and tracking of near-surface three-dimensional volumetric nanoscale dynamic processes of live cells remains a challenging problem. In this paper, we propose a multi-color live-cell near-surface-volume super-resolution microscopy method that combines total internal reflection fluorescence structured illumination microscopy with multi-angle evanescent light illumination. We demonstrate that our approach of multi-angle interference microscopy is perfectly adapted to studying subcellular dynamics of mitochondria and microtubule architectures during cell migration.

3D super-resolution imaging of dynamic processes in live cells is still challenging, especially in a large field of view. Here the authors combine SIM with multi-angle evanescent light illumination and achieve improved lateral and axial resolution, with stack acquisition time in the range of 1–2 s.

Artifact-free, penetration-adjustable elliptical-mirror-based TIRF microscopy

Evanescent field distribution in the focal region of the elliptical-mirror-based total-internal-reflection fluorescence (e-TIRF) microscopy is analyzed based on vectorial diffraction theory. The simulation demonstrates that the intensity of an evanescent field generated by elliptical mirror decreases exponentially with the penetration depth, and the polarization characteristic of the evanescent wave in various directions is given. We build up an e-TIRF microscope utilizing a focused hollow-cone illumination with all-direction and large range of incidence. The experiment shows the artifact effect can be well suppressed by using the azimuthal-direction illumination method. In addition, the penetration depth of the evanescent field can be controlled by adjusting the sizes of the aperture and obstruction with a large range.

Widefield and total internal reflection fluorescent structured illumination microscopy with scanning galvo mirrors

We present an alternative approach to realize structured illumination microscopy (SIM), which is capable for live cell imaging. The prototype utilizes two sets of scanning galvo mirrors, a polarization converter and a piezo-platform to generate a fast shifted, s-polarization interfered and periodic variable illumination patterns. By changing the angle of the scanning galvanometer, we can change the position of the spots at the pupil plane of the objective lens arbitrarily, making it easy to switch between widefield and total internal reflection fluorescent-SIM mode and adapting the penetration depth in the sample. Also, a twofold resolution improvement is achieved in our experiments. The prototype offers more flexibility of pattern period and illumination orientation changing than previous systems.