Magnetic particles in motion: magneto-motive imaging and sensing

Superparamagnetic nanoparticles have become an important tool in biomedicine. Their biocompatibility, controllable small size, and magnetic properties allow manipulation with an external magnetic field for a variety of diagnostic and therapeutic applications.
Recently, the magnetically-induced motion of superparamagnetic nanoparticles has been investigated as a new source of imaging contrast.
In magneto-motive imaging, an external, time-varying magnetic field is applied to move a magnetically labeled subject, such as labeled cells or tissue.
Several major imaging modalities such as ultrasound, photoacoustic imaging, optical coherence tomography, and laser speckle tracking can utilize magneto-motive contrast to monitoring biological events at smaller scales with enhanced contrast and sensitivity.
In this review article, an overview of magneto-motive imaging techniques is presented, including synthesis of superparamagnetic nanoparticles, fundamental principles of magneto-motive force and its utility to excite labeled tissue within a viscoelastic medium, current capabilities of magneto-motive imaging modalities, and a discussion of the challenges and future outlook in the magneto-motive imaging domain.

Direct Spectroscopic Quantification of the Absorption and Scattering Properties for Single Aerosol Particles.

Understanding the optical properties of micrometer-scale light-absorbing aerosol particles is of paramount importance in addressing key challenges in atmospheric and physical chemistry.
For example, the absorption of solar radiation by atmospheric aerosols represents one of the largest uncertainties in climate models. Moreover, reaction acceleration within the unique environments of aerosol droplets cannot be replicated in bulk solutions.
The causes of these reaction rate enhancements remain controversial, but ultrasensitive spectroscopic measurements of evolving aerosol optical properties should provide new insights.
We demonstrate a new approach using cavity ring-down spectroscopy that allows the first direct spectroscopic quantification of the continuously evolving absorption and scattering cross sections for single, levitated, micrometer-scale particles as their size and chromophore concentration change.
For two-component droplets composed of nigrosin and 1,2,6-hexanetriol, the unprecedented sensitivity of our measurements reveals the evolving real and imaginary components of the refractive index caused by changes in concentration as 1,2,6-hexanetriol slowly evaporates.

Gold-viral particle identification by deep learning in wide-field photon scattering parametric images.

The ability to identify virus particles is important for research and clinical applications. Because of the optical diffraction limit, conventional optical microscopes are generally not suitable for virus particle detection, and higher resolution instruments such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) are required.
In this paper, we propose a new method for identifying virus particles based on polarization parametric indirect microscopic imaging (PIMI) and deep learning techniques.
By introducing an abrupt change of refractivity at the virus particle using antibody-conjugated gold nanoparticles (AuNPs), the strength of the photon scattering signal can be magnified.
After acquiring the PIMI images, a deep learning method was applied to identify discriminating features and classify the virus particles, using electron microscopy (EM) images as the ground truth.
Experimental results confirm that gold-virus particles can be identified in PIMI images with a high level of confidence.

Holographic 3D particle reconstruction using a one-stage network.

Volumetric reconstruction of a three-dimensional (3D) particle field with high resolution and low latency is an ambitious and valuable task. As a compact and high-throughput imaging system, digital holography (DH) encodes the 3D information of a particle volume into a two-dimensional (2D) interference pattern.
In this work, we propose a one-stage network (OSNet) for 3D particle volumetric reconstruction. Specifically, by a single feed-forward process, OSNet can retrieve the 3D coordinates of the particles directly from the holograms without high-fidelity image reconstruction at each depth slice.
Evaluation results from both synthetic and experimental data confirm the feasibility and robustness of our method under different particle concentrations and noise levels in terms of detection rate and position accuracy, with improved processing speed.
The additional applications of 3D particle tracking are also investigated, facilitating the analysis of the dynamic displacements and motions for micro-objects or cells. It can be further extended to various types of computational imaging problems sharing similar traits.

A statistical approach to boost soluble expression of E. coli-derived virus-like particles in shake-flask cultivation.

Hepatitis B core virus-like particles (HBc-VLP) have been widely used as carrier platforms to present an epitope of interest. Escherichia coli expression system is cost effective and produces high yields of recombinant protein. However major drawbacks include difficulties in obtaining soluble expression and tendency to form inclusion bodies.
To boost solubility of proteins during expression of E. coli-derived HBc-VLPs carrying EBNA1 epitope, a statistical approach involving fractional factorial design (FFD) and response surface methodology (RSM) was used.
For the first time, this approach was applied to quantitatively determine the impact of key parameters in shake-flask cultivation.
Expression conditions including post-induction temperature and shaker-speed, and cell density at induction were optimized. Based on native agarose gel electrophoresis, optimized soluble protein cellular yield was 210.5mgg-1 dry cell mass and volumetric yield was 272mgL-1 of culture media.
Findings highlight: 1) the significant interaction between post-induction temperature and shaker-speed on production, and; 2) sufficient oxygen level is required during induction.
It is concluded that this statistical approach can be practically applied to optimize expression of HBc-VLP in shake-flask cultivation, and to determine key parameters for large-scale productions.

Spirally rotating particles with structured beams generated by phase-shifted zone plates.

The emerging field of structured beams has led to optical manipulation with tremendous progress.
Beyond various methods for structured beams, we use phase-shifted zone plates known as beam-shaping diffractive optical elements to generate beams whose phase exclusively or both phase and intensity are twisted along a curve.
These beams can trap and guide particles on open curved trajectories for continuous motion, not necessarily requiring a closed symmetric intensity distribution.
We show the feasibility and versatility of the proposed method as a promising technique in optical manipulation in which the trajectory of the spiral rotation and the rate of rotation of trapped particles can be controlled.

Backscatter multiple wavelength digital holography for color micro-particle imaging.

This work applies digital holography to image stationary micro-particles in color. The approach involves a Michelson interferometer to mix reference light with the weak intensity light backscattered from a distribution of particles.
To enable color images, three wavelengths are used, 430, 532, and 633 nm, as primary light sources. Three separate backscattered holograms are recorded simultaneously, one for each wavelength, which are resolved without spectral cross talk using a three-CMOS prism sensor.
Fresnel diffraction theory is used to render monochrome images from each hologram. The images are then combined via additive color mixing with red, green, and blue as the primary colors.
The result is a color image similar in appearance to that obtained with a conventional microscope in white-light epi-illumination mode.

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A variety of colored polyethylene micro-spheres and nonspherical dust particles demonstrate the feasibility of the approach and illustrate the effect of simple speckle-noise suppression and white balance methods.
Finally, a chromaticity analysis is applied that is capable of differentiating particles of different colors in a quantitative and objective manner.

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