Separation of polystyrene nanoparticles bearing different carboxyl group densities and functional groups quantification with capillary electrophoresis and asymmetrical flow field flow fractionation

A targeted fluorescent nanosensor for ratiometric pH sensing at the cell surface

The correlation between altered extracellular pH and various pathological conditions, including cancer, inflammation and metabolic disorders, is well known. Bulk pH measurements cannot report the extracellular pH value at the cell surface. However, there is a limited number of suitable tools for measuring the extracellular pH of cells with high spatial resolution, and none of them are commonly used in laboratories around the world. In this study, a versatile ratiometric nanosensor for the measurement of extracellular pH was developed. The nanosensor consists of biocompatible polystyrene nanoparticles loaded with the pH-inert reference dye Nile red and is surface functionalized with a pH-responsive fluorescein dye. Equipped with a targeting moiety, the nanosensor can adhere to cell membranes, allowing direct measurement of extracellular pH at the cell surface. The nanosensor exhibits a sensitive ratiometric pH response within the range of 5.5-9.0, with a calculated pKa of 7.47. This range optimally covers the extracellular pH (pHe) of most healthy cells and cells in which the pHe is abnormal, such as cancer cells. In combination with the nanosensors ability to target cell membranes, its high robustness, reversibility and its biocompatibility, the pHe nanosensor proves to be well suited for in-situ measurement of extracellular pH, even over extended time periods. This pH nanosensor has the potential to advance biomedical research by improving our understanding of cellular microenvironments, where extracellular pH plays an important role.

Capillary Electrophoresis as a Complementary Analytical Tool for the Separation and Detection of Nanoplastic Particles

Capillary electrophoresis (CE) is presented as a technique for the separation of polystyrene nanoparticles (NPs, particle diameters ranging from 30 to 300 nm) through a bare fused silica capillary and ultraviolet detection. The proposed strategy was also assessed for other types of nanoplastics, finding that stronger alkaline conditions, with an ammonium hydroxide buffer (7.5%, pH = 11.9), enabled the separation of poly(methyl methacrylate), polypropylene, and polyethylene NP for the first time by means of CE for particle diameters below 200 nm. Particle behavior has been investigated in terms of its effective electrophoretic mobility, showing an increasing absolute value of effective electrophoretic mobility from the smaller to the larger sizes. On the other hand, the absolute value of surface charge density decreased with increasing size of NPs. It was demonstrated and quantified that the separation mechanism was a combination of linear and nonlinear electrophoretic effects. This work is the first report on the quantification of nonlinear electrophoretic effects on nanoplastic particles in a CE system.

Quantitative separation of polystyrene nanoparticles in environmental matrices with picogram detection limits using capillary electrophoresis

We developed a capillary electrophoresis method to separate polystyrene particles (PSPs) with different sizes or different surface functionalities. Separations were performed in buffer and 100 mg L-1 clay or 100 mg L-1 Suwanee River humic acid. In all solutions, PSPs were baseline or near-baseline resolved in less than 15 minutes.

Development of Two-Dimensional Offline Coupling of Asymmetrical Flow Field-Flow Fractionation and Capillary Electrophoresis for the Separation of a Five-Component Submicrometer Particle Mixture

Submicrometer colloidal particles are widely applied in a variety of industrial products. While precise size and surface charge control is crucial to the stability and functionality of these materials, a tool to determine these properties with sufficient resolution, detection sensitivity, and robustness is still not available. The recently reported offline coupling of asymmetrical flow field-flow fractionation and capillary electrophoresis (AF4 × CE) shows success in improving the separation resolution for nanoparticles; however, challenges remain for sensitive multiple-component submicrometer particle analysis because of wide size and mobility distributions. We here report offline coupling of an AF4 method and a CE method, which utilized the online reversed electrode polarity stacking mode, to successfully characterize a five-component, submicrometer particle mixture. The mixture was successfully separated and detected with an improved inter- and intracomponent resolution. Therefore, our developed platform holds great potential for industrial applications involving multiple-component particle mixtures.

Analyzing the surface of functional nanomaterials-how to quantify the total and derivatizable number of functional groups and ligands

Functional nanomaterials (NM) of different size, shape, chemical composition, and surface chemistry are of increasing relevance for many key technologies of the twenty-first century. This includes polymer and silica or silica-coated nanoparticles (NP) with covalently bound surface groups, semiconductor quantum dots (QD), metal and metal oxide NP, and lanthanide-based NP with coordinatively or electrostatically bound ligands, as well as surface-coated nanostructures like micellar encapsulated NP. The surface chemistry can significantly affect the physicochemical properties of NM, their charge, their processability and performance, as well as their impact on human health and the environment. Thus, analytical methods for the characterization of NM surface chemistry regarding chemical identification, quantification, and accessibility of functional groups (FG) and surface ligands bearing such FG are of increasing importance for quality control of NM synthesis up to nanosafety. Here, we provide an overview of analytical methods for FG analysis and quantification with special emphasis on bioanalytically relevant FG broadly utilized for the covalent attachment of biomolecules like proteins, peptides, and oligonucleotides and address method- and material-related challenges and limitations. Analytical techniques reviewed include electrochemical titration methods, optical assays, nuclear magnetic resonance and vibrational spectroscopy, as well as X-ray based and thermal analysis methods, covering the last 5-10 years. Criteria for method classification and evaluation include the need for a signal-generating label, provision of either the total or derivatizable number of FG, need for expensive instrumentation, and suitability for process and production control during NM synthesis and functionalization.

Fluorescence coupled capillary electrophoresis as a strategy for tetrahedron DNA analysis

A strategy based on fluorescence coupled capillary electrophoresis (CE-FL) was developed for analyzing tetrahedron DNA (TD) and TD-doxorubicin (DOX) conjugate. Capillary gel electrophoresis exhibited desirable performance for separating TD and DNA strands. Under the optimized conditions, satisfactory repeatability concerning run-to-run and interday repeatability was obtained, and relative standard deviation value of resolution (n = 6) was 0.64%. Furthermore, the combination of CE and fluorescence detection provided a sensitive platform for quantifying TD concentration and calculating the damage degree of TD. The electrophoretograms indicated that CE-FL was a suitable TD assay method with high specificity and sensitivity. In addition, the application of CE-FL for TD fluorescence resonance energy transfer (FRET) research was also explored. Two types of DNA strands were utilized to interfere the formation of TD. The impact of partially complementary chain and completely complementary chain on FRET signal was explored, and the influence mechanism was discussed. After applying CE-FL for characterizing TD, we also combine CE and FRET to analyze TD-DOX conjugate. CE presented a favourable technique to monitor DOX loading and releasing processes. These noteworthy results offered a stepping stone for DNA nanomaterials assay by using CE-FL.

Asymmetric flow field-flow fractionation as a multifunctional technique for the characterization of polymeric nanocarriers

The importance of polymeric nanocarriers in the field of drug delivery is ever-increasing, and the accurate characterization of their properties is paramount to understand and predict their behavior. Asymmetric flow field-flow fractionation (AF4) is a fractionation technique that has gained considerable attention for its gentle separation conditions, broad working range, and versatility. AF4 can be hyphenated to a plurality of concentration and size detectors, thus permitting the analysis of the multifunctionality of nanomaterials. Despite this potential, the practical information that can be retrieved by AF4 and its possible applications are still rather unfamiliar to the pharmaceutical scientist. This review was conceived as a primer that clearly states the "do's and don'ts" about AF4 applied to the characterization of polymeric nanocarriers. Aside from size characterization, AF4 can be beneficial during formulation optimization, for drug loading and drug release determination and for the study of interactions among biomaterials. It will focus mainly on the advances made in the last 5 years, as well as indicating the problematics on the consensus, which have not been reached yet. Methodological recommendations for several case studies will be also included.

Contributions of Capillary Electrophoresis in Analytical Nanometrology: A Critical View

An overview on the increasing role of capillary electrophoresis in characterization and direct analysis of nanomaterials is herein presented. The niche of electrophoretic approaches in nanometrology is so relevant that nonmetallic, metal, metal oxide nanoparticles, and quantum dots have been analyzed to be targeted via capillary electrophoresis with conventional detection systems or coupling arrangements aimed at increasing selectivity and sensitivity toward either pristine or conjugated nanoparticles. Moreover, parameters altering intrinsic properties of nanoparticles may be optimized to gather the desired results and identify nanomaterials according to their size, shape, or associations with binding agents. The usefulness and quickness of capillary electrophoresis for quantifying or screening ultrasmall-sized particles enables this technique to set an example for analysis of standards or previously synthesized nanostructures in research or routine laboratories. Abundant evidence of the suitability of electrophoretic approaches for characterization and direct determination of nanomaterials in actual samples has been provided in this review, together with a discussion about hyphenation with state-of-the art detectors and comparison between capillary electrophoresis with other separation approaches. This permits scientific community to be optimistic in the short term.