Geometry-induced injection dispersion in single-cell protein electrophoresis

Electromigration of Charged Analytes Through Immiscible Fluids in Multiphasic Electrophoresis

Multiphasic buffer systems have been of greatest interest in electrophoresis and liquid-liquid electrotransfer; this study extends that foundation by exploring the interplay of the geometric and viscous properties of an interleaving oil layer on the electrotransfer of a charged analyte from an aqueous solution into a hydrogel. We utilized finite element analysis to examine two complementary configurations: one being electrotransfer of a charged analyte (protein) in an aqueous phase into a surrounding hydrogel layer and another being electrotransfer of the protein from that originating aqueous phase - through an interleaving oil layer of predetermined viscosity and thickness - and into a surrounding hydrogel layer. Results indicate that the presence of an oil layer leads to increased skew of the injected peak. To explain this difference in injection dispersion, we utilize Probstein's framework and compare the Péclet (Pe) number with the ratio between length scales characteristic to the axial and radial dispersion, respectively. The formulation assigns electrotransfer conditions into six different dispersion regimes. We show that the presence or absence of an interleaving oil layer moves the observed peak dispersion into distinct electrotransfer regimes; the presence of an oil layer augments the electrophoretic mobility mismatch between the different phases, resulting in a five-fold increase in Pe and a six-fold increase in the ratio between the axial to radial dispersion characteristic lengths. We further show that oil viscosity significantly influences resultant injection dispersion. A decrease in oil-layer viscosity from 0.08 Pa·s to 0.02 Pa·s results in a >100% decrease in injection dispersion. Our theoretical predictions were experimentally validated by comparing the electrotransfer regimes of three different mineral oil samples. We show that lowering the oil viscosity to 0.0039 Pa·s results in an injection regime similar to that of the absence of an oil layer. Additionally, we measure the migration distance and show that average electromigration velocity over the transit duration is inversely proportional to the viscosity of an interleaving oil layer. Understanding of the impact of electrotransfer of charged species across multiple immiscible fluid layers on peak dispersion informs the design of multiphasic electrophoresis systems.

Segmentation-based analysis of single-cell immunoblots

From genomics to transcriptomics to proteomics, microfluidic tools underpin recent advances in single-cell biology. Detection of specific proteoforms-with single-cell resolution-presents challenges in detection specificity and sensitivity. Miniaturization of protein immunoblots to single-cell resolution mitigates these challenges. For example, in microfluidic western blotting, protein targets are separated by electrophoresis and subsequently detected using fluorescently labeled antibody probes. To quantify the expression level of each protein target, the fluorescent protein bands are fit to Gaussians; yet, this method is difficult to use with noisy, low-abundance, or low-SNR protein bands, and with significant band skew or dispersion. In this study, we investigate segmentation-based approaches to robustly quantify protein bands from single-cell protein immunoblots. As compared to a Gaussian fitting pipeline, the segmentation pipeline detects >1.5× more protein bands for downstream quantification as well as more of the low-abundance protein bands (i.e., with SNR ∼3). Utilizing deep learning-based segmentation approaches increases the recovery of low-SNR protein bands by an additional 50%. However, we find that segmentation-based approaches are less robust at quantifying poorly resolved protein bands (separation resolution, Rs < 0.6). With burgeoning needs for more single-cell protein analysis tools, we see microfluidic separations as benefitting substantially from segmentation-based analysis approaches.

Recent Advances and New Perspectives in Capillary Electrophoresis-Mass Spectrometry for Single Cell "Omics"

Accurate clinical therapeutics rely on understanding the metabolic responses of individual cells. However, the high level of heterogeneity between cells means that simply sampling from large populations of cells is not necessarily a reliable approximation of an individual cell's response. As a result, there have been numerous developments in the field of single-cell analysis to address this lack of knowledge. Many of these developments have focused on the coupling of capillary electrophoresis (CE), a separation technique with low sample consumption and high resolving power, and mass spectrometry (MS), a sensitive detection method for interrogating all ions in a sample in a single analysis. In recent years, there have been many notable advancements at each step of the single-cell CE-MS analysis workflow, including sampling, manipulation, separation, and MS analysis. In each of these areas, the combined improvements in analytical instrumentation and achievements of numerous researchers have served to drive the field forward to new frontiers. Consequently, notable biological discoveries have been made possible by the implementation of these methods. Although there is still room in the field for numerous further advances, researchers have effectively minimized various limitations in detection of analytes, and it is expected that there will be many more developments in the near future.