Generalizations for the Potential of Mean Force between Two Isolated Colloidal Particles from Monte Carlo Simulations

Unveiling the Role of Electrostatic Forces on Attraction between Opposing Polyelectrolyte Brushes

Electrostatic interaction and molecular excluded-volume effects are responsible for a plethora of nonintuitive phenomena in soft-matter systems, including local charge inversion and attraction between similar charges. In the current work, we study the surface forces and swelling behavior of opposing polyelectrolyte brushes using a classical density functional theory that accounts for electrostatic and excluded-volume correlations. We observe that the detachment pressure between similarly charged brushes is sensitive to salt concentration in both the osmotic and salted regimes and can be negative in the presence of multivalent counterions. A comparison of the theoretical results with the mean-field predictions unravels the role of correlation effects in determining the surface forces and brush structure. For systems containing multivalent counterions, the detachment pressure attains negative values at an intermediate brush-brush separation, and the attractive region in the pressure vs distance plot is magnified in terms of both the depth and width of attraction with increasing counterion valency. However, the interbrush attraction vanishes when the size-induced correlations are switched off. We also investigated the role of counterion size and polymer chain length on the detachment pressure. It is found that smaller counterions are more effective in neutralizing the polymer charge than bigger counterions, leading to a reduced interbrush repulsion and, in some cases, attraction between like-charged brushes at intermediate distances. Meanwhile, varying the chain length of the grafted polymers only shifts the location of the attraction basin, with little influence on the interaction strength. The theoretical predictions show qualitative agreement with experimental observations and offer valuable insights into the interaction between similarly charged polymer brushes in the presence of multivalent ions.

Multivalent Ion-Mediated Attraction between Like-Charged Colloidal Particles: Nonmonotonic Dependence on the Particle Charge

Ion-mediated effective interactions are important for the structure and stability of charged particles such as colloids and nucleic acids. It has been known that the intrinsic electrostatic repulsion between like-charged particles can be modulated into effective attraction by multivalent ions. In this work, we examined the dependence of multivalent ion-mediated attraction between like-charged colloidal particles on the particle charge in a wide range by extensive Monte Carlo simulations. Our calculations show that for both divalent and trivalent salts, the effective attraction between like-charged colloidal particles becomes stronger with the increase of the particle charge, whereas it gradually becomes weakened when the particle charge exceeds a "critical" value. Correspondingly, as the particle charge is increased, the driving force for such effective attraction transits from an attractive electrostatic force to an attractive depletion force, and the attraction weakening by high particle charges is attributed to the transition of electrostatic force from attraction to repulsion. Our analyses suggest that the attractive depletion force and the repulsive electrostatic force at high particle charges result from the Coulomb depletion which suppresses the counterion condensation in the limited region between two like-charged colloidal particles. Moreover, our extensive calculations indicate that the "critical" particle charge decreases apparently for larger ions and smaller colloidal particles due to stronger Coulomb depletion and decreases slightly at higher salt concentrations due to the slightly enhanced Coulomb depletion in the intervening space between colloidal particles. Encouragingly, we derived an analytical formula for the "critical" particle charge based on the Lindemann melting law.

Rejection of Commonly Used Electrolytes in Asymmetric Flow Field Flow Fractionation: Effects of Membrane Molecular Weight Cutoff Size, Fluid Dynamics, and Valence of Electrolytes

Asymmetric flow field flow fractionation (AF4) is an efficient size-based separation technique for the characterization of submicron size particulates. In AF4, membranes having various molecular weight cutoff sizes are used as a barrier to retain particles while allowing the carrier fluid containing electrolytes to permeate. Here, we have hypothesized that electrolyte rejection by the barrier membrane leads to the accumulation of electrolytes in the channel during operation. Electrolyte accumulation can cause various adverse effects that can lead to membrane fouling. An instrument setup containing a conductivity detector was assembled, and the rejection of commonly used carrier electrolytes such as trisodium citrate, ethylenediaminetetraacetic acid, sodium chloride, and ammonium carbonate was evaluated by varying the concentration, cross-flow rate, focusing flow rate, membrane material type, and cutoff sizes. The results showed that electrolyte rejection increased with a decrease in the electrolyte concentration and the molecular weight cutoff size (pore size) or with an increase in the charge state of the anion in the carrier electrolytes. We proposed an electrostatic repulsion-based rejection mechanism and verified it with the measurement of the rejection rate while varying the electrolyte concentration in the running media.

The nature of protein interactions governing globular protein-polymer block copolymer self-assembly

The effects of protein surface potential on the self-assembly of protein-polymer block copolymers are investigated in globular proteins with controlled shape through two approaches: comparison of self-assembly of mCherry-poly(N-isopropylacrylamide) (PNIPAM) bioconjugates with structurally homologous enhanced green fluorescent protein (EGFP)-PNIPAM bioconjugates, and mutants of mCherry with altered electrostatic patchiness. Despite large changes in amino acid sequence, the temperature-concentration phase diagrams of EGFP-PNIPAM and mCherry-PNIPAM conjugates have similar phase transition concentrations. Both materials form identical phases at two different coil fractions below the PNIPAM thermal transition temperature and in the bulk. However, at temperatures above the thermoresponsive transition, mCherry conjugates form hexagonal phases at high concentrations while EGFP conjugates form a disordered micellar phase. At lower concentration, mCherry shows a two-phase region while EGFP forms homogeneous disordered micellar structures, reflecting the effect of changes in micellar stability. Conjugates of four mCherry variants with changes to their electrostatic surface patchiness also showed minimal change in phase behavior, suggesting that surface patchiness has only a small effect on the self-assembly process. Measurements of protein/polymer miscibility, second virial coefficients, and zeta potential show that these coarse-grained interactions are similar between mCherry and EGFP, indicating that coarse-grained interactions largely capture the relevant physics for soluble, monomeric globular protein-polymer conjugate self-assembly.

Nanomaterial-assisted signal enhancement of hybridization for DNA biosensors: a review

Detection of DNA sequences has received broad attention due to its potential applications in a variety of fields. As sensitivity of DNA biosensors is determined by signal variation of hybridization events, the signal enhancement is of great significance for improving the sensitivity in DNA detection, which still remains a great challenge. Nanomaterials, which possess some unique chemical and physical properties caused by nanoscale effects, provide a new opportunity for developing novel nanomaterial-based signal-enhancers for DNA biosensors. In this review, recent progress concerning this field, including some newly-developed signal enhancement approaches using quantum-dots, carbon nanotubes and their composites reported by our group and other researchers are comprehensively summarized. Reports on signal enhancement of DNA biosensors by non-nanomaterials, such as enzymes and polymer reagents, are also reviewed for comparison. Furthermore, the prospects for developing DNA biosensors using nanomaterials as signal-enhancers in future are also indicated.

Enhancement of target-DNA hybridization efficiency by pre-hybridization on sequence-orientated micro-arrayed probes

The enhancement of hybridization efficiency of deoxyribonucleic acid (DNA) targets using oligonucleotide pre-hybridization is studied on two sequence-inversed micro-arrayed probes. The sequences for pre-hybridizing both oligo and target DNA are designed to be fully complementary with their shared DNA probe in a coaxial stacking configuration; i.e. they hybridize immediately alongside each other along the continuous complement probe strand. The pre-hybridizing oligo and target DNA are differentiated by being labeled with two distinct fluorescent dyes, and the cooperative effect on hybridization efficiency is investigated through the comparison of the stacking and individual hybridization configurations based on the detection signals of the labeling dyes. The results show that the pre-hybridization of a DNA oligo enhances the subsequent hybridization efficiency of the target-DNA coupling onto the same probe. The efficiency is enhanced if the hybridization position occurs at a site close to the substrate surface.

Hydration and distribution of ions at the mica-water interface

Molecular-scale structures of mica surfaces in electrolyte solutions reveal how ion and interfacial hydration control cation adsorption. Key differences are obtained for Rb+and Sr2+ using resonant anomalous x-ray reflectivity: Rb+ adsorbs in a partially hydrated state and incompletely compensates the surface charge, but Sr2+ adsorbs in both fully and partially hydrated states while achieving full charge compensation. These differences are driven by balancing the energy cost of disrupting ion and interface hydration with the electrostatic attraction between the cation and charged surface.