Super-resolution fluorescence microscopy by stepwise optical saturation

Fluorescence saturation imaging microscopy: molecular fingerprinting with a standard confocal microscope

Molecular specificity in fluorescence imaging of cells and tissues can be increased by measuring parameters other than intensity. For instance, fluorescence lifetime imaging became a widespread modality for biomedical optics. Previously, we suggested using the fluorescence saturation effect at pulsed laser excitation to map the absorption cross-section as an additional molecular contrast in two-photon microscopy [Opt. Lett.47(17), 4455 (2022).10.1364/OL.465605]. Here, it is shown that, somewhat counterintuitive, fluorescence saturation can be observed under cw excitation in a standard confocal microscopy setup. Mapping the fluorescence saturation parameter allows obtaining additional information about the fluorophores in the system, as demonstrated by the example of peptide hydrogel, stained cells and unstained thyroid gland. The suggested technique does not require additional equipment and can be implemented on confocal systems as is.

Antimicrobial peptide-conjugated phage-mimicking nanoparticles exhibit potent bactericidal action against Streptococcus pyogenes in murine wound infection models

Streptococcus pyogenes is a causative agent for strep throat, impetigo, and more invasive diseases. The main reason for the treatment failure of streptococcal infections is increased antibiotic resistance. In recent years, infectious diseases caused by pyogenic streptococci resistant to multiple antibiotics have been rising with a significant impact on public health and the veterinary industry. The development of antibiotic resistance and the resulting emergence of multidrug-resistant bacteria have become primary threats to the public health system, commonly leading to nosocomial infections. Many researchers have turned their focus to developing alternative classes of antibacterial agent based on various nanomaterials. We have developed an antibiotic-free nanoparticle system inspired by naturally occurring bacteriophages to fight antibiotic-resistant bacteria. Our phage-mimicking nanoparticles (PhaNPs) display structural mimicry of protein-turret distribution on the head structure of bacteriophages. By mimicking phages, we can take advantage of their evolutionary constant shape and high antibacterial activity while avoiding the immune reactions of the human body experienced by biologically derived phages. We describe the synthesis of hierarchically arranged core-shell nanoparticles, with a silica core conjugated with silver-coated gold nanospheres to which we have chemisorbed the synthetic antimicrobial peptide Syn-71 on the PhaNPs surface, and increased the rapidity of the antibacterial activity of the nanoparticles (PhaNP@Syn71). The antibacterial effect of the PhaNP@Syn71 was tested in vitro and in vivo in mouse wound infection models. In vitro, results showed a dose-dependent complete inhibition of bacterial growth (>99.99%). Cytocompatibility testing on HaCaT human skin keratinocytes showed minimal cytotoxicity of PhaNP@Syn71, being comparable to the vehicle cytotoxicity levels even at higher concentrations, thus proving that our design is biocompatible with human cells. There was a minimum cutoff dosage above which there was no evolution of resistance after prolonged exposure to sub-MIC dosages of PhaNP@Syn71. Application of PhaNP@Syn71 to a mouse wound infection model exhibited high biocompatibility in vivo while showing immediate stabilization of the wound size, and infection free wound healing. Our results suggest the robust utility of antimicrobial peptide-conjugated phage-mimicking nanoparticles as a highly effective antibacterial system that can combat bacterial infections consistently while avoiding the emergence of resistant bacterial strains.

Multiplane differential saturated excitation microscopy using varifocal lenses

Saturated excitation microscopy, which collects nonlinear fluorescence signals generated by saturation, has been proposed to improve three-dimensional spatial resolution. Differential saturated excitation (dSAX) microscopy can further improve the detection efficiency of a nonlinear fluorescence signal. By comparing signals obtained at different saturation levels, high spatial resolution can be achieved in a simple and efficient manner. High-resolution multiplane microscopy is perquisite for volumetric imaging of thick samples. To the best of our knowledge, no reports of multiplane dSAX have been made. Our aim is to obtain multiplane high-resolution optically sectioned images by adapting differential saturated excitation in confocal laser scanning fluorescence microscopy. To perform multiplane dSAX microscopy, a variable focus lens is employed in a telecentric design to achieve focus tunability with constant magnification and contrast throughout the axial scanning range. Multiplane fluorescence imaging of two different types of pollen grains shows improved resolution and contrast. Our system's imaging performance is evaluated using standard targets, and the results are compared with standard confocal microscopy. Using a simple and efficient method, we demonstrate multiplane high-resolution fluorescence imaging. We anticipate that high-spatial resolution combined with high-speed focus tunability with invariant contrast and magnification will be useful in performing 3D imaging of thick biological samples.

Defect extraction method for additive manufactured parts with improved learning-based image super-resolution and the Canny algorithm

Additive manufacturing (AM) is a highly competitive, low-cost, and high-degree-of-manufacturing technology. However, AM still has limitations because of some defects. Thus, defect detection technology is essential for quality enhancement in the AM process. Super-resolution (SR) technology can be utilized to improve defect image quality and enhance defect extraction performance. This study proposes a defect extraction method for additive manufactured parts with improved learning-based image SR and the Canny algorithm (LSRC), which is based on direct mapping methodology. The LSRC method is compared with the bicubic interpolation algorithm and the neighbor embedding (NE) algorithm in SR reconstruction quality and robustness. The experimental results show that the proposed LSRC method achieves satisfactory performance in terms of the averaged information entropy (E), standard deviation (SD), peak signal-to-noise ratio (PSNR), and structural similarity (SSIM), which are 7.259, 45.301, 27.723, and 0.822, respectively. The accordingly average improvement rates of the E, SD, PSNR, and SSIM, are 0.45%, 7.15%, 5.85%, and 6.35% in comparison with the bicubic interpolation algorithm, while the comparison data are 0.97%, 13.40%, 10.55%, and 15.35% in terms of the NE algorithm. This indicates that the LSRC method is significantly better than the comparison algorithm in reconstruction quality and robustness, which is of great significance for the extraction and analysis of key defect information of additive manufactured parts.

Biocompatible, Multi-Mode, Fluorescent, T2 MRI Contrast Magnetoelectric-Silica Nanoparticles (MagSiNs), for On-Demand Doxorubicin Delivery to Metastatic Cancer Cells

There is a need to improve current cancer treatment regimens to reduce systemic toxicity, to positively impact the quality-of-life post-treatment. We hypothesized the negation of off-target toxicity of anthracyclines (e.g., Doxorubicin) by delivering Doxorubicin on magneto-electric silica nanoparticles (Dox-MagSiNs) to cancer cells. Dox-MagSiNs were completely biocompatible with all cell types and are therapeutically inert till the release of Doxorubicin from the MagSiNs at the cancer cells location. The MagSiNs themselves are comprised of biocompatible components with a magnetostrictive cobalt ferrite core (4−6 nm) surrounded by a piezoelectric fused silica shell of 1.5 nm to 2 nm thickness. The MagSiNs possess T2-MRI contrast properties on par with RESOVIST™ due to their cobalt ferrite core. Additionally, the silica shell surrounding the core was volume loaded with green or red fluorophores to fluorescently track the MagSiNs in vitro. This makes the MagSiNs a suitable candidate for trackable, drug nanocarriers. We used metastatic triple-negative breast cancer cells (MDAMB231), ovarian cancer cells (A2780), and prostate cancer cells (PC3) as our model cancer cell lines. Human umbilical vein endothelial cells (HUVEC) were used as control cell lines to represent blood-vessel cells that suffer from the systemic toxicity of Doxorubicin. In the presence of an external magnetic field that is 300× times lower than an MRI field, we successfully nanoporated the cancer cells, then triggered the release of 500 nM of doxorubicin from Dox-MagSiNs to successfully kill >50% PC3, >50% A2780 cells, and killed 125% more MDAMB231 cells than free Dox.HCl. In control HUVECs, the Dox-MagSiNs did not nanoporate into the HUVECS and did not exhibited any cytotoxicity at all when there was no triggered release of Dox.HCl. Currently, the major advantages of our approach are, (i) the MagSiNs are biocompatible in vitro and in vivo; (ii) the label-free nanoporation of Dox-MagSiNs into cancer cells and not the model blood vessel cell line; (iii) the complete cancellation of the cytotoxicity of Doxorubicin in the Dox-MagSiNs form; (iv) the clinical impact of such a nanocarrier will be that it will be possible to increase the current upper limit for cumulative-dosages of anthracyclines through multiple dosing, which in turn will improve the anti-cancer efficacy of anthracyclines.

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.

Saturated-excitation image scanning microscopy

Image scanning microscopy (ISM) overcomes the trade-off between spatial resolution and signal volume in confocal microscopy by rearranging the signal distribution on a two-dimensional detector array to achieve a spatial resolution close to the theoretical limit achievable by infinitesimal pinhole detection without sacrificing the detected signal intensity. In this paper, we improved the spatial resolution of ISM in three dimensions by exploiting saturated excitation (SAX) of fluorescence. We theoretically investigated the imaging properties of ISM, when the fluorescence signals are nonlinearly induced by SAX, and show combined SAX-ISM fluorescence imaging to demonstrate the improvement of the spatial resolution in three dimensions. In addition, we confirmed that the SNR of SAX-ISM imaging of fluorescent beads and biological samples, which is one of the challenges in conventional SAX microscopy, was improved.

Reporting on the future of integrative structural biology ORAU workshop

Integrative and hybrid methods have the potential to bridge long-standing knowledge gaps in structural biology. These methods will have a prominent role in the future of the field as we make advances toward a complete, unified representation of biology that spans the molecular and cellular scales. The Department of Physics and Astronomy at Clemson University hosted The Future of Integrative Structural Biology workshop on April 29, 2017 and partially sponsored by partially sponsored by a program of the Oak Ridge Associated Universities (ORAU). The workshop brought experts from multiple structural biology disciplines together to discuss near-term steps toward the goal of a molecular atlas of the cell. The discussion focused on the types of structural data that should be represented, how this data should be represented, and how the time domain might be incorporated into such an atlas. The consensus was that an explorable, map-like Virtual Cell, containing both spatial and temporal data bridging the atomic and cellular length scales obtained by multiple experimental methods, represents the best path toward a complete atlas of the cell.

Super-resolution fluorescence imaging of carbon nanotubes using a nonlinear excitonic process

Highly efficient exciton-exciton annihilation process unique to one-dimensional systems is utilized for super-resolution imaging of air-suspended carbon nanotubes. Through the comparison of fluorescence signals in linear and sublinear regimes at different excitation powers, we extract the efficiency of the annihilation processes using conventional confocal microscopy. Spatial images of the annihilation rate of the excitons have resolution beyond the diffraction limit. We investigate excitation power dependence of the annihilation processes by experiment and Monte Carlo simulation, and the resolution improvement of the annihilation images can be quantitatively explained by the superlinearity of the annihilation process. We have also developed another method in which the cubic dependence of the annihilation rate on exciton density is utilized to achieve further sharpening of single nanotube images.

Generating intravital super-resolution movies with conventional microscopy reveals actin dynamics that construct pioneer axons

Super-resolution microscopy is broadening our in-depth understanding of cellular structure. However, super-resolution approaches are limited, for numerous reasons, from utilization in longer-term intravital imaging. We devised a combinatorial imaging technique that combines deconvolution with stepwise optical saturation microscopy (DeSOS) to circumvent this issue and image cells in their native physiological environment. Other than a traditional confocal or two-photon microscope, this approach requires no additional hardware. Here, we provide an open-access application to obtain DeSOS images from conventional microscope images obtained at low excitation powers. We show that DeSOS can be used in time-lapse imaging to generate super-resolution movies in zebrafish. DeSOS was also validated in live mice. These movies uncover that actin structures dynamically remodel to produce a single pioneer axon in a ‘top-down’ scaffolding event. Further, we identify an F-actin population – stable base clusters – that orchestrate that scaffolding event. We then identify that activation of Rac1 in pioneer axons destabilizes stable base clusters and disrupts pioneer axon formation. The ease of acquisition and processing with this approach provides a universal technique for biologists to answer questions in living animals.

Summary: Actin dynamics are examined in zebrafish axons using DeSOS, a new super-resolution technique combining deconvolution with stepwise optical saturation microscopy that allows detailed intravital imaging of cells in their native environments.

Generalized stepwise optical saturation enables super-resolution fluorescence lifetime imaging microscopy

We present a novel super-resolution fluorescence lifetime microscopy technique called generalized stepwise optical saturation (GSOS) that generalizes and extends the concept of the recently demonstrated stepwise optical saturation (SOS) super-resolution microscopy [Biomed. Opt. Express9, 1613 (2018)]. The theoretical basis of GSOS is developed based on exploring the dynamics of a two-level fluorophore model and using perturbation theory. We show that although both SOS and GSOS utilize the linear combination of M raw images to increase the imaging resolution by a factor of M , SOS is a special and the simplest case of GSOS. The super-resolution capability is demonstrated with theoretical analysis and numerical simulations for GSOS with sinusoidal and pulse-train modulations. Using GSOS with pulse-train modulation, super-resolution and fluorescence lifetime imaging microscopy (FLIM) images can be obtained simultaneously. The super-resolution FLIM capability is experimentally demonstrated with a cell sample on a custom-built two-photon frequency-domain (FD) FLIM system based on radio frequency analog signal processing. To our knowledge, this is the first implementation of super-resolution imaging in FD-FLIM.