Direct measurements of forces between different charged colloidal particles and their prediction by the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO)

A comprehensive review of emerging 3D-printing materials against bacterial biofilm growth on the surface of healthcare settings

A significant burden on the healthcare system, microbial contamination of biomedical surfaces can result in hospital-acquired illnesses. Bacteria, viruses, and fungi may live on surfaces for days or months and spread to patients and medical personnel. This article describes the 3D printing technologies, such as fused deposition modeling, bioprinting, binder jetting/inkjet, poly-jet, electron beam manufacturing, stereolithography, selective laser sintering, and laminated object manufacturing used for manufacturing the healthcare setting's surface to reduce bacterial contamination with exploring anti-biofilm activity against different bacterial species responsible for infections, based on the critical evaluation of published reports. This strategy has immense potential to become an upcoming approach for advancing the coating concept on the material's surface in healthcare settings. Our literature evaluation identifies beneficial 3D printing materials and associated technologies against microorganisms' growth, mainly bacteria involved in implant-based infection, emphasizing the development of anti-biofilm 3D-printed surfaces. Additionally, the authors have identified a few key areas where research and development are critically required to advance 3D-printing technology in healthcare settings.

On the formation and stability mechanisms of diverse lipid-based nanostructures for drug delivery

In the evolving landscape of nanotechnology and pharmaceuticals, lipid nanostructures have emerged as pivotal areas of research due to their unique ability to mimic biological membranes and encapsulate active molecules. These nanostructures offer promising avenues for drug delivery, vaccine development, and diagnostic applications. This comprehensive review explores the complex mechanisms underlying the formation and stability of various lipid nanostructures, including lipid liquid crystalline nanoparticles and solid lipid nanoparticles. Drawing upon a wide array of studies, we integrate current knowledge on the physicochemical properties of lipids that contribute to nanostructure formation, such as lipid composition, charge, and the role of environmental factors such as pH and ionic strength. We further discuss the stabilisation mechanisms that preserve the integrity and functionality of these nanostructures in biological systems, highlighting the influence of surface modification, PEGylation, and the incorporation of stabilising agents. Through a methodical examination of both classical theories and cutting-edge research, our review highlights the critical factors that dictate the self-assembly of lipids into nanostructures, the dynamics of their formation, and the interplay between different stabilising forces. The implications of these insights for the design of lipid-based delivery systems are vast, offering the potential to enhance the bioavailability of therapeutics, target specific tissues or cells, and minimise adverse effects. The integration of lipid nanostructures in pharmaceutical nanotechnology not only stands to revolutionise the delivery of therapeutic agents but also paves the way for innovative applications in targeted therapy, personalised medicine, and vaccine adjuvant development. By bridging the gap between fundamental biophysical studies and applied research, this review contributes to the ongoing discourse on lipid nanostructures, advocating for a multidisciplinary approach to harness their full potential.

Recent Developments in the Methods and Applications of Electrostatic Theory

ConspectusThe review improves our understanding of how electrostatic interactions in the electrolyte, gas phase, and on surfaces can drive the fragmentation and assembly of particles. This is achieved through the overview of our advanced theoretical and computational modeling toolbox suitable for interpretation of experimental observations and discovery of novel, tunable assemblies and architectures. In the past decade, we have produced a significant, fundamental body of work on the development of comprehensive theories based on a rigorous mathematical foundation. These solutions are capable of accurate predictions of electrostatic interactions between dielectric particles of arbitrary size, anisotropy, composition, and charge, interacting in solvents, ionized medium, and on surfaces. We have applied the developed electrostatic approaches to describe physical and chemical phenomena in dusty plasma and planetary environments, in Coulomb fission and electrospray ionization processes, and in soft matter, including a counterintuitive but widespread attraction between like-charged particles.Despite its long history, the search for accurate methods to provide a deeper understanding of electrostatic interactions remains a subject of significant interest, as manifested by a constant stream of theoretical and experimental publications. While major international effort in this area has focused predominantly on the computational modeling of biocatalytic and biochemical performance, we have expanded the boundaries of accuracy, generality, and applicability of underlying theories. Simple solvation models, often used in calculating the electrostatic component of molecular solvation energy and polarization effects of solvent, rarely go beyond the induced dipole approximation because of computational costs. These approximations are generally adequate at larger separation distances; however, as particles approach the touching point, more advanced charged-induced multipolar descriptions of the electrostatic interactions are required to describe accurately a collective behavior of polarizable neutral and charged particles. At short separations, the electrostatic forces involving polarizable dielectric and conducting particles become nonadditive which necessitates further developments of quantitatively accurate many-body approaches. In applications, the electrostatic response of materials is commonly controlled by externally applied electric fields, an additional complex many-body problem that we have addressed most recently, both theoretically and numerically.This review reports on the most significant results and conclusions underpinning these recent advances in electrostatic theory and its applications. We first discuss the limitations of classical approaches to interpreting electrostatic phenomena in electrolytes and complex plasmas, leading to an extended analytical theory suitable for accurate estimation of the electrostatic forces in a dilute solution of a strong electrolyte. We then introduce the concept and numerical realization of many-body electrostatic theory focusing on its performance in selected experimental cases. These experiments underpin, among other applications, electrostatic self-assembly of two-dimensional lattice structures, melting of ionic colloidal crystals in an external electric field, and coalescence of charged clusters.

Structural correlations in highly asymmetric binary charged colloidal mixtures

We explore structural correlations of strongly asymmetric mixtures of binary charged colloids within the primitive model of electrolytes considering large charge and size ratios of 10 and higher. Using computer simulations with explicit microions, we obtain the partial pair correlation functions between the like-charged colloidal macroions. Interestingly the big-small correlation peak amplitude is smaller than that of the big-big and small-small macroion correlation peaks, which is unfamiliar for additive repulsive interactions. Extracting optimal effective microion-averaged pair interactions between the macroions, we find that on top of non-additive Yukawa-like repulsions an additional shifted Gaussian attractive potential between the small macroions is needed to accurately reproduce their correct pair correlations. For small Coulomb couplings, the behavior is reproduced in a coarse-grained theory with microion-averaged effective interactions between the macroions. However, the accuracy of the theory deteriorates with increasing Coulomb coupling. We emphasize the relevance of entropic interactions exerted by the microions on the macroions. Our results are experimentally verifiable in binary mixtures of micron-sized colloids and like-charge nanoparticles.

Comparative study of the hydrophobic interaction effect of pH and ionic strength on aggregation/emulsification of Congo red and amyloid fibrillation of insulin

Amyloid fibrillation is provoked by the conformational rearrangement of its source. In our previous study, we claimed that the conformational rearrangement of hen egg white lysozyme requires intermolecular aggregation/packing induced. Our proposed causality of the aggregation and amyloid formation was demonstrated by the quantitative dependence of amyloid fibrillation on pH difference from its isoelectric point (pI) and on the square root of ionic strength in order to reduce the intermolecular repulsion due to the shielding effect of electrolytes (DLVO effect). When Congo red has dianionic form at the pH higher than its pKa, it forms ribbon-like micelle colloids under lower ionic strength, while it loses electrostatic repulsion and aggregates to be emulsified in the octanolic phase under the higher ionic strength. These behaviors of Congo red were resembling to molecular assembly of surfactants. In contrast, the amyloid formation of insulin was proportional to the square root of ionic strength at the pH lower than its isoelectric point. Therefore, the trigger for conformational rearrangement of amyloid fibrillation is predominantly gripped by hydrophobic hydration and an electrostatic shielding effect. We concluded that the both behaviors of Congo red and insulin were derived from a driving force related to the hydrophobic hydration.

Rigorous treatment of pairwise and many-body electrostatic interactions among dielectric spheres at the Debye-Hückel level

Electrostatic interactions among colloidal particles are often described using the venerable (two-particle) Derjaguin-Landau-Verwey-Overbeek (DLVO) approximation and its various modifications. However, until the recent development of a many-body theory exact at the Debye-Hückel level (Yu in Phys Rev E 102:052404, 2020), it was difficult to assess the errors of such approximations and impossible to assess the role of many-body effects. By applying the exact Debye-Hückel level theory, we quantify the errors inherent to DLVO and the additional errors associated with replacing many-particle interactions by the sum of pairwise interactions (even when the latter are calculated exactly). In particular, we show that: (1) the DLVO approximation does not provide sufficient accuracy at shorter distances, especially when there is an asymmetry in charges and/or sizes of interacting dielectric spheres; (2) the pairwise approximation leads to significant errors at shorter distances and at large and moderate Debye lengths and also gets worse with increasing asymmetry in the size of the spheres or magnitude or placement of the charges. We also demonstrate that asymmetric dielectric screening, i.e., the enhanced repulsion between charged dielectric bodies immersed in media with high dielectric constant, is preserved in the presence of free ions in the medium.

Charged dielectric spheres interacting in electrolytic solution: A linearized Poisson-Boltzmann equation model

We present an analytical theory of electrostatic interactions of two spherical dielectric particles of arbitrary radii and dielectric constants, immersed into a polarizable ionic solvent (assuming that the linearized Poisson-Boltzmann framework holds) and bearing arbitrary charge distributions expanded in multipolar terms. The presented development entails a novel two-center re-expansion analytical theory that expands upon and improves the existing ones, bypassing the conventional expansions in modified Bessel functions. On this basis, we develop a specific matrix formalism that facilitates the construction of asymptotic expansions in ascending order of Debye screening terms of potential coefficients, which are then employed to find exact closed-form expressions for the total electrostatic energy. In particular, this work allows us to explicitly and precisely quantify the k-screened terms of the potential coefficients and mutual interaction energy. Specific cases of monopolar and dipolar distributions are described in particular detail. Comprehensive numerical examples and tests of series convergence and the relative balance of leading and higher-order terms of the mutual interaction energy are presented depending on the inter-particle distance and particles' radii. The results of this work find application in soft matter modeling and, in particular, in computational biophysics and colloid science, where the availability of increasingly larger experimental structures at the atomic-level resolution makes numerical treatment challenging and calls for more efficient expressions and an increased range of validity.

Rapid Characterization and Modeling of Natural and Undefined Charge-Regulated Surfaces in Aqueous Systems

The surfaces of most materials in aqueous systems are charged due to the ionization of surface functional groups. When these surfaces interact, the surface charge, electrostatic potential, and pH will vary as a function of separation distance, and this process is termed the charge-regulation effect. Charge regulation is a controlling factor in the adhesion and transport of colloids and microorganisms in aqueous systems, and its modeling requires representation of the pH-charge response of the surfaces, typically provided as the equilibrium constants (K) and site densities (N) of the dominant surface functional groups. Existing methods for obtaining these parameters demonstrate shortcomings when applied to many natural and man-made materials, such as weathered materials, materials with undefined or complex surface structures, and permeable materials, and for materials that do not provide the requisite high surface area in suspension due to small sample sizes. This hinders inclusion of the charge-regulation effect in colloid and microbial transport studies, and most studies of colloidal and microbial surface interactions use simplifying assumptions; a key example is the routine use of the constant potential assumption in DLVO modeling. Here we present a robust method that overcomes these issues and provides a rapid means to characterize charge-regulated surfaces using zeta potential data, without requiring a priori knowledge of the material composition. Applying a combined charge-regulation and Gouy-Chapman model, K and N values are obtained that accurately represent the electrostatic response of a charge-regulated surface. This method is demonstrated using activated carbon, aluminum oxide, iron (hydr)oxide, feldspar, and silica sand. The resulting K and N values are then used to show the variations in surface charge, electrostatic potential, and pH that can occur as these charge-regulated surfaces interact. This method provides a readily applied experimental approach for characterizing charge-regulated surfaces, with the overall goal to promote the inclusion of charge-regulated interactions into adhesion and transport studies with natural and undefined materials.

Decoupling conservative forces and hydrodynamic interactions between optically trapped spheres

Characterizing the interactions between colloidal particles is important, both from a fundamental perspective as well as due to its technological importance. However, current methods to measure the interaction forces between two colloids have significant limitations. Here we describe a method that exploits the fluctuation spectra of two optically trapped microspheres in order to extract, and decouple, the conservative forces acting between them and their hydrodynamic coupling. We demonstrate the proposed method with two silica microspheres, and find good agreement between our results and previous predictions for the hydrodynamic and electrostatic interactions between the spheres.

Does capillary evaporation limit the accessibility of nonaqueous electrolytes to the ultrasmall pores of carbon electrodes?

Porous carbons have been widely utilized as electrode materials for capacitive energy storage. Whereas the importance of pore size and geometry on the device performance has been well recognized, little guidance is available for identification of carbon materials with ideal porous structures. In this work, we study the phase behavior of ionic fluids in slit pores using the classical density functional theory. Within the framework of the restricted primitive model for nonaqueous electrolytes, we demonstrate that the accessibility of micropores depends not only on the ionic diameters (or desolvation) but also on their wetting behavior intrinsically related to the vapor-liquid or liquid-liquid phase separation of the bulk ionic systems. Narrowing the pore size from several tens of nanometers to subnanometers may lead to a drastic reduction in the capacitance due to capillary evaporation. The wettability of micropores deteriorates as the pore size is reduced but can be noticeably improved by raising the surface electrical potential. The theoretical results provide fresh insights into the properties of confined ionic systems beyond electric double layer models commonly employed for rational design/selection of electrolytes and electrode materials.

Humic acids modify the pulse size distributions in the characterization of plastic microparticles by Tunable Resistive Pulse Sensing

Tunable Resistive Pulse Sensing, TRPS, is an emerging technique used in quantification and measuring the size (particle-by-particle) of viruses, exosomes and engineered colloidal spheres in biological fluids. We study the features of TRPS to enhance size characterization and quantification of submicron-sized microplastics, also called plastic microparticles, MP, in freshwater environments. We report alterations on the detection of the resistive pulses in the TRPS caused by humic acids, HA, during the size measurement of polystyrene microspheres used as MP surrogate. We discuss the alteration of the electric field in the measuring channel of the TRPS apparatus induced by the passage of HA. TRPS is a fast and precise technique for counting and size determination of MP but needs the evaluation of the influence of the organic matter on the current blockades. We show that statistical clustering models of the magnitude distribution of the resistive pulses can help to detect and quantify changes in the pulse size distributions induced by flocculation of humic acids. Conclusions of this study indicate that TRPS can be a valuable tool to improve the knowledge of the MP fate in surface waters, in the vadose zone and groundwater.

Electrostatic interactions between charged dielectric particles in an electrolyte solution: constant potential boundary conditions

The problem of electrostatic interactions between colloidal particles in an electrolyte solution has been solved within the Debye-Hückel approximation using the boundary condition of constant potential. The model has been validated in two independent ways - by considering the limiting cases obtained from DLVO theory and comparison with the available experimental data. The presented methodology provides the final part of a complete theory of pairwise electrostatic interactions between spherical colloidal particles; one that embraces all possible chemical scenarios within the boundary conditions of constant potential and constant charge.

The application of atomic force microscopy in mineral flotation

During the past years, atomic force microscopy (AFM) has matured to an indispensable tool to characterize nanomaterials in colloid and interface science. For imaging, a sharp probe mounted near to the end of a cantilever scans over the sample surface providing a high resolution three-dimensional topographic image. In addition, the AFM tip can be used as a force sensor to detect local properties like adhesion, stiffness, charge etc. After the invention of the colloidal probe technique it has also become a major method to measure surface forces. In this review, we highlight the advances in the application of AFM in the field of mineral flotation, such as mineral morphology imaging, water at mineral surface, reagent adsorption, inter-particle force, and bubble-particle interaction. In the coming years, the complementary characterization of chemical composition such as using infrared spectroscopy and Raman spectroscopy for AFM topography imaging and the synchronous measurement of the force and distance involving deformable bubble as a force sensor will further assist the fundamental understanding of flotation mechanism.

Attractive non-DLVO forces induced by adsorption of monovalent organic ions

Direct force measurements between negatively charged colloidal particles were carried out using an atomic force microscope (AFM) in aqueous solutions containing monovalent organic cations, namely tetraphenylarsonium (Ph₄As⁺), 1-hexyl-3-methylimidazolium (HMIM⁺), and 1-octyl-3-methylimidazolium (OMIM⁺).

Long-ranged and soft interactions between charged colloidal particles induced by multivalent coions

Forces between charged particles in aqueous solutions containing multivalent coions and monovalent counterions are studied by the colloidal probe technique. Here, the multivalent ions have the same charge as the particles, which must be contrasted to the frequently studied case where multivalent ions have the opposite sign as the substrate. In the present case, the forces remain repulsive and are dominated by the interactions of the double layers. The valence of the multivalent coion is found to have a profound influence on the shape of the force curve. While for monovalent coions the force profile is exponential down to separations of a few nanometers, the interaction is much softer and longer-ranged in the presence of multivalent coions. The force profiles in the presence of multivalent coions and in the mixtures of monovalent and multivalent coions can be accurately described by Poisson-Boltzmann theory. These results are accurate for different surfaces and even in the case of highly charged particles. This behavior can be explained by the fact that the force profile follows the near-field limit to much larger distances for multivalent coions than for monovalent ones. This limit corresponds to the conditions with no salt, where the coions are expelled between the two surfaces.

Charge regulation in ionic solutions: thermal fluctuations and Kirkwood-Schumaker interactions

We study the behavior of two macroions with dissociable charge groups, regulated by local variables such as pH and electrostatic potential, immersed in a monovalent salt solution, considering cases where the net charge can either change sign or remain of the same sign depending on these local parameters. The charge regulation in both cases is described by the proper free-energy function for each of the macroions, while the coupling between the charges is evaluated on the approximate Debye-Hückel level. The charge correlation functions and the ensuing charge fluctuation forces are calculated analytically and numerically. Strong attraction between like-charged macroions is found close to the point of zero charge, specifically due to asymmetric, anticorrelated charge fluctuations of the macroion charges. The general theory is then implemented for a system of two proteinlike macroions, generalizing the form and magnitude of the Kirkwood-Schumaker interaction.