Light-scattering detection within the difficult size range of protein particle measurement using flow cytometry

Measuring the size of oil droplets in a flow cytometer using Mie resonances: A possible size calibration ladder for 0.5-6 μm

An emulsion of silicone oil droplets in aqueous buffer produces a distinctive series of peaks or resonances in the side scatter histogram in a flow cytometer. As many as 12 peaks are observed in the violet-side scatter channel at 405 nm, with half that number observed in the blue side scatter channel at 488 nm. Using the index of refraction of the oil and buffer, the wavelength of light, and the collection angle and gain of the instrument, we assign the peaks to specific diameters at which Mie resonances occur. With the close match for the index of refraction of silicone oil (n = 1.417 at 405 nm) to biological materials, these resonances could form the basis of a finely spaced size calibration ladder in the range 0.5-6 μm for estimating the size of biological particles in a flow cytometer. Resonances were also observed using mineral oil (n = 1.483 at 405 nm) suggesting that investigating and modeling resonances for emulsion systems may be useful for understanding these systems.

Opportunities in optical and electrical single-cell technologies to study microbial ecosystems

New techniques are revolutionizing single-cell research, allowing us to study microbes at unprecedented scales and in unparalleled depth. This review highlights the state-of-the-art technologies in single-cell analysis in microbial ecology applications, with particular attention to both optical tools, i.e., specialized use of flow cytometry and Raman spectroscopy and emerging electrical techniques. The objectives of this review include showcasing the diversity of single-cell optical approaches for studying microbiological phenomena, highlighting successful applications in understanding microbial systems, discussing emerging techniques, and encouraging the combination of established and novel approaches to address research questions. The review aims to answer key questions such as how single-cell approaches have advanced our understanding of individual and interacting cells, how they have been used to study uncultured microbes, which new analysis tools will become widespread, and how they contribute to our knowledge of ecological interactions.

Global Analysis of Aggregation Profiles of Three Kinds of Immuno-Oncology mAb Drug Products Using Flow Cytometry

Accurately quantifying the protein particles in both subvisible (1-100 μm) and submicron (≤1 μm) ranges remains a prominent challenge in the development and manufacturing of protein drugs. Due to the limitation of the sensitivity, resolution, or quantification level of various measurement systems, some instruments may not provide count information, while others can only count particles in a limited size range. Moreover, the reported concentrations of protein particles commonly have significant discrepancies owing to different methodological dynamic ranges and the detection efficiency of these analytical tools. Therefore, it is extremely difficult to accurately and comparably quantify protein particles within the desired size range at one time. To develop an efficient protein aggregation measurement method that can span the entire range of interest, we established, in this study, a single particle-sizing/counting method based on our highly sensitive lab-built flow cytometry (FCM) system. The performance of this method was assessed, and its capability of identifying and counting microspheres between 0.2 and 25 μm was demonstrated. It was also used to characterize and quantify both subvisible and submicron particles in three kinds of top-selling immuno-oncology antibody drugs and their lab-produced counterparts. These assessment and measurement results suggest that there may be a role for an enhanced FCM system as an efficient investigative tool for characterizing and learning the molecular aggregation behavior, stability, or safety risk of protein products.

High-Throughput Counting and Sizing of Therapeutic Protein Aggregates in the Nanometer Size Range by Nano-Flow Cytometry

Protein aggregation is one of the greatest challenges in biopharmaceuticals as it could decrease therapeutic efficacy, induce immunogenicity, and reduce shelf life of protein drugs. However, there lacks high-throughput methods than can count and size protein aggregates in the nanometer size range, especially for those smaller than 100 nm. Employing a laboratory-built nano-flow cytometer (nFCM) that enables light scattering detection of single silica nanoparticles as small as 24 nm with sizing resolution and accuracy comparable to those of electron microscopy, here, we report a new benchmark to analyze single protein aggregates as small as 40 nm. With an analysis rate of up to 10,000 particles/min, the size distribution and particle concentration of nanometer protein aggregates can be acquired in 2-3 min. Employing heat-induced aggregation of bovine serum albumin (BSA) at high concentrations as the model system, effects of different categories of excipients, including sugars, polyols, salts, and amino acids on the inhibition of protein aggregation were investigated. Strikingly enough, as high as 10¹⁰ to 10¹² particles/mL of protein aggregates were observed in the size range of 40 to 200 nm for therapeutic proteins of human serum albumin injection, reconstituted recombinant human interieukin-2 solution, and human immunoglobulin injection. nFCM opens a new avenue to count and size nanometer protein aggregates, suggesting its future usability in the quality assessment and formulation promotion of therapeutic proteins.

New Advances and Applications in Field-Flow Fractionation

Field-flow fractionation (FFF) is a family of techniques that was created especially for separating and characterizing macromolecules, nanoparticles, and micrometer-sized analytes. It is coming of age as new nanomaterials, polymers, composites, and biohybrids with remarkable properties are introduced and new analytical challenges arise due to synthesis heterogeneities and the motivation to correlate analyte properties with observed performance. Appreciation of the complexity of biological, pharmaceutical, and food systems and the need to monitor multiple components across many size scales have also contributed to FFF's growth. This review highlights recent advances in FFF capabilities, instrumentation, and applications that feature the unique characteristics of different FFF techniques in determining a variety of information, such as averages and distributions in size, composition, shape, architecture, and microstructure and in investigating transformations and function.

Detection and Sizing of Submicron Particles in Biologics With Interferometric Scattering Microscopy

We demonstrate the application of interferometric scattering microscopy (IFS) for characterizing submicron particles in stir-stressed monoclonal antibody. IFS uses a layered silicon sensor and modified optical microscope to rapidly visualize and determine the particle size distribution (PSD) of submicron particles based on their scattering intensity, which is directly proportional to particle mass. Limits for particle size and optimal solution concentration were established for IFS characterization of submicron particles. We critically compare IFS data with dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA) and find IFS is superior to NTA and DLS for determining the realistic shape of the number-based PSD, whereas NTA and DLS provide superior information about absolute particle size. Together, IFS, NTA, and DLS provide complementary information on submicron particles and enable quantitative characterization of the PSD of submicron aggregates. Finally, we explore quantifying particle size with IFS by developing a calibration curve for particle scattering intensity based on correlative scanning electron microscopy imaging. We found that only a subset of isotropic-shaped particles followed the expected proportionality between IFS intensity and particle mass. Overall, this study demonstrates IFS is a simple approach for detecting and quantifying submicron aggregate PSD in protein-based therapeutics.