Effect of Ionic and Nonionic Carriers in Electrical Field-Flow Fractionation

Fabrication of ionic liquid-mesoporous silica/platinum electrode with high hydroelectric stability for electric-field-assisted particle separation

Surface chemistry of electrodes plays a critical role in the fields of electrochemistry and electric-field-assisted separation. In this study, making ingenious use of the ordered mesoporous structure of silica materials and the electrochemical stability of ionic liquids (ILs) when integrated with polyvinylpyrrolidone (PVP), the PVP-modified IL-mesoporous silica/platinum wire (Pt/PVP@meso-SiO₂@IL) was fabricated to increase hydroelectric stability and avoid the problem of electrode polarization. The effect of different amounts of mesoporous silica material used to modify the surface of the Pt electrode was systematically investigated. As a result, we successfully obtained a highly ordered mesoporous Pt/PVP@meso-SiO₂ material with smooth surface. Because pentyl triethylamine bis(trifluoromethylsulfonyl) imide exhibits a wide electrochemical window between -3 to 3 V, this IL was chosen to modify mesopores under vacuum. Even after repeatedly applying electric field on Pt/PVP@meso-SiO₂@IL 100 times, this working electrode remained stable and showed high hydroelectric stability. After verifying the feasibility of this method, it was successfully applied in the electric-field-assisted separation of 2.0 and 3.0 μm polystyrene particles without any impediment from electrode polarization problems. This work provides a brand-new insight for resolving the problem of electrode polarization by developing a versatile tool for the electroseparation of micro-objects.

Insight into the formation and biological effects of natural organic matter corona on silver nanoparticles in water environment using biased cyclical electrical field-flow fractionation

Natural organic matter (NOM) readily interacts with nanoparticles, leading to the formation of NOM corona structures on their surface. NOM corona formation is closely related to the surface coatings and bioavailability of nanoparticles. However, the mechanism underlying NOM corona formation on silver nanoparticles (AgNPs) remains largely unknown due to the lack of effective analytical methods for identifying the changes in the AgNP surface. Herein, the separation ability of biased cyclical electrical field-flow fractionation (BCyElFFF) for same-sized polyvinyl pyrrolidone-coated and poly(ethylene glycol)-coated silver nanoparticles (AgNPs) with different electrophoretic mobilities was evaluated under various electrical conditions. Then, the mechanism behind the NOM corona formation on these AgNP surfaces was elucidated based on the changes in the elution time and off-line characterization of the collected fractions during their elution time in a BCyElFFF run. Finally, the survival rates of E. coli exposed to polyvinyl pyrrolidone-coated and poly(ethylene glycol)-coated AgNPs with or without NOM collected during repeated BCyElFFF runs were observed to increase with increasing NOM concentration, clearly demonstrating the negative effect of NOM corona structures on the bioavailability of AgNPs. These findings highlight the powerful separation and isolation ability of BCyElFFF in studying the transformation and fate of nanoparticles in aqueous environments.

Label-Free Isolation of Exosomes Using Microfluidic Technologies

Exosomes are cell-derived structures packaged with lipids, proteins, and nucleic acids. They exist in diverse bodily fluids and are involved in physiological and pathological processes. Although their potential for clinical application as diagnostic and therapeutic tools has been revealed, a huge bottleneck impeding the development of applications in the rapidly burgeoning field of exosome research is an inability to efficiently isolate pure exosomes from other unwanted components present in bodily fluids. To date, several approaches have been proposed and investigated for exosome separation, with the leading candidate being microfluidic technology due to its relative simplicity, cost-effectiveness, precise and fast processing at the microscale, and amenability to automation. Notably, avoiding the need for exosome labeling represents a significant advance in terms of process simplicity, time, and cost as well as protecting the biological activities of exosomes. Despite the exciting progress in microfluidic strategies for exosome isolation and the countless benefits of label-free approaches for clinical applications, current microfluidic platforms for isolation of exosomes are still facing a series of problems and challenges that prevent their use for clinical sample processing. This review focuses on the recent microfluidic platforms developed for label-free isolation of exosomes including those based on sieving, deterministic lateral displacement, field flow, and pinched flow fractionation as well as viscoelastic, acoustic, inertial, electrical, and centrifugal forces. Further, we discuss advantages and disadvantages of these strategies with highlights of current challenges and outlook of label-free microfluidics toward the clinical utility of exosomes.

Field-flow fractionation for nanoparticle characterization

This review presents field-flow fractionation: The elements of theory enable the link between the retention and the characteristics of the nanometer-sized analytes to be highlighted. In particular, the nature of force and its way of being applied are discussed. Four types of forces which determine four types of techniques were considered: hydrodynamic, sedimentation, thermal, and electrical; this is to show the importance of the choice of technique in relation to the characterization objectives. Then the separation performance is presented and compared with other separation techniques: field-flow fractionation has the greatest intrinsic separation capability. The characterization strategies are presented and discussed; on the one hand with respect to the characteristics needed for the description of nanoparticles; on the other hand in connection with the choice of the nature of the force, and also of the detectors used, online or offline. The discussion is based on a selection of published study examples. Finally, current needs and challenges are addressed, and as response, trends and possible characterization solutions.

Use of electrical field-flow fractionation for gold nanoparticles after improving separation efficiency by carrier liquid optimization

Electrical field-flow fractionation (ElFFF) is a useful separation technique for nanoparticles, however, it has been limited by polarization/electrical double layer formation which reduces an effective field for separation. With an appropriate direct current (DC) applied voltage, sodium carbonate, FL-70, Triton X-100 and acetonitrile were explored as additive substances for preparation of carrier liquid used in normal ElFFF to enhance an amplitude of effective field by their ionic redox-active species, ionic and nonionic surfactant and wide electrochemical potential window nonionic organic solvent properties, respectively. Effective field was indirectly measured in each carrier liquid by investigating retention behavior of polystyrene latex nanoparticles and gold nanoparticles. Effective field improvement was observed in all carrier liquid types (except FL-70) by which the highest effective field existed in 16 μM sodium carbonate at 1.70 V and 0.01% (V/V) Triton X-100 and 50% (V/V) acetonitrile at 1.90 V as compared to deionized water at 1.90 V. In addition, those carrier liquids were applied for separation of 5 nm and 15 nm gold nanoparticles mixture by which Triton X-100 exhibited the best separation resolution (R_s = 1.11).

Characterization and differential retention of Q beta bacteriophage virus-like particles using cyclical electrical field-flow fractionation and asymmetrical flow field-flow fractionation

Virus-like particles (VLPs) are widely used in medicine, but can be difficult to characterize and isolate from aggregates. In this research, primarily cyclical electrical field-flow fractionation (CyElFFF) coupled with multi-angle light scattering (MALS), and dynamic light scattering (DLS) detectors, was used for the first time to perform size and electrical characterization of three different types of Q beta bacteriophage virus-like particles (VLPs): a blank Q beta bacteriophage which is denoted as VLP and two conjugated ones with different peptides. The CyElFFF results were verified with transmission electron microscopy (TEM). Asymmetrical flow field-flow fractionation (AF4) coupled with MALS was also applied using conditions similar to those used in the CyElFFF experiments, and the results of the two techniques were compared to each other. Using these techniques, the size and electrophoretic characteristics of the fractionated VLPs in CyElFFF were obtained. The results indicate that CyElFFF can be used to obtain a clear distribution of electrophoretic mobilities for each type of VLP. Accordingly, CyElFFF was able to differentially retain and isolate VLPs with high surface electric charge/electrophoretic mobility from the ones with low electric charge/electrophoretic mobility. Regarding the size characterization, the size distribution of the eluted VLPs was obtained using both techniques. CyElFFF was able to identify subpopulations that did not appear in the AF4 results by generating a shoulder peak, whereas AF4 produced a single peak. Different size characteristics of the VLPs appearing in the shoulder peak and the main peak indicate that CyElFFF was able to isolate aggregated VLPs from the monomers partially. Graphical abstract.