Charged Beads Research Articles

Conformational transitions and helical structures of a dipolar chain in external electric fields.

The conformational behavior of a single dipolar chain in a uniform electric field is investigated by molecular dynamics simulations. The dipolar chain is modeled as a backbone bead-on-spring chain of equally charged beads, each connected by a rigid spring with an oppositely charged side bead that can freely rotate around the backbone bead. In the strong coupling regime, when the dipolar chain is in the globular state due to a strong electrostatic correlational attraction, the application of an electric field causes the chain swelling and elongation along the field direction. In the weak coupling regime, a qualitatively new regime is found when the swollen dipolar chain shrinks along the field direction adopting flattened conformations due to the field-induced anisotropy of the chain rigidity and the head-to-tail attraction of the dipoles orienting along the field lines. A novel helical conformation is detected for low-polar media and strong electric fields. An increasing rigidity of the backbone chain leads to some stabilization of the helical conformation and the formation of double and triple helices as well as flat spread springs. Fine tuning of the interplay between dipolar and volume interactions by external electric fields induces re-orientation of rod-like dipolar chains in dilute solutions. The obtained results can provide new ways to control dipolar polymer conformations and design materials with responsive properties.

Transport of Colloidal Particles in Microscopic Porous Medium Analogues with Surface Charge Heterogeneity: Experiments and the Fundamental Role of Single-Bead Deposition.

Understanding colloid transport in subsurface environments is challenging because of complex interactions among colloids, groundwater, and porous media over several length scales. Here, we report a versatile method to assemble bead-based microfluidic porous media analogues with chemical heterogeneities of different configurations. We further study the transport of colloidal particles through a family of porous media analogues that are randomly packed with oppositely charged beads with different mixing ratios. We recorded the dynamics of colloidal particle deposition at the level of single grains. From these, the maximum surface coverage (θmax = 0.051) was measured directly. The surface-blocking function and the deposition coefficient (kpore = 3.56 s-1) were obtained. Using these pore-scale parameters, the transport of colloidal particles was modeled using a one-dimensional advection-dispersion-deposition equation under the assumption of irreversible adsorption between oppositely charged beads and colloids, showing very good agreement with experimental breakthrough curves and retention profiles at the scale of the entire porous medium analogue. This work presents a new approach to fabricate chemically heterogeneous porous media in a microfluidic device that enables the direct measurement of pore-scale colloidal deposition. Compared with the conventional curve-fitting method for deposition constant, our approach allows quantitative prediction of colloidal breakthrough and retention via coupling of direct pore-scale measurements and an advection-dispersion-deposition model.

Multiwalled Carbon Nanotubes in Microfluidic Chip for the Separation of X- and Y-Sperm Based on a Photolithography Technique

This study examined the performance of a microfluidic device for sperm separation assuming that X- and Y-sperm have different surface electrical charges. The proposed microfluidic chip was first tested with electrically charged particles, namely TiO2-coated polystyrene (PS) beads, to mimic spermatozoa. Negatively charged TiO2-coated PS beads were fabricated in a simple and efficient manner. These beads were characterised using X-ray diffraction, tungsten scanning electron microscopy with energy-dispersive X-ray spectroscopy mode, and X-ray absorption spectroscopy to determine the reason for the persistence of the negative surface charge. The fabricated TiO2-coated PS beads were partly coated in mixed forms of amorphous Ti4+, which caused a sustained negative charge on the surface after fabrication. The microfluidic device was simulated to obtain an optimal structure for fabrication. Negatively charged TiO2-coated PS beads were tested in designed microfluidic devices. Detailed structures for microfluidic device design with integrated microelectrodes were fabricated using a photolithographic technique, and the finished device was tested with PS beads. Based on PS-bead separation, a microfluidic device with a $150~\mu \text{m}$ microchannel and $100~\mu \text{m}$ distance between electrodes was the best performing prototype at 87.07% with a confidence level of 95%. Commercial positively and negatively charged PS beads were tested in the selected design for additional validation. Finally, sperm separation using the proposed device was conducted; the specially designed microfluidic device with multiwalled carbon nanotubes was fabricated, and the proposed device showed a X- and Y-sperm separation accuracy of 74.62%. [2020-0278]

Modern tools characterize corpses and sequence sacred texts

Mummy postmortem Getting an up close look at the chemical makeup of an ancient artifact generally requires researchers to destroy a bit of it in the process. But Beatrice Demarchi, a molecular archaeologist at the University of Turin, thought a new tactic was in order for a rare specimen known as “the young woman with the pleated dress.” It’s one of the few mummies from Egypt’s Old Kingdom (ca. 2649–2130 BCE) still in existence. Demarchi teamed up with Pier Giorgio Righetti, professor emeritus of chemistry at the Politecnico di Milano, and Gleb Zilberstein, CEO of the sensor company Spectrophon, to extract proteins from the mummy using a noninvasive protein-harvesting technique. An ethylene vinyl acetate (EVA) film studded with charged plastic beads attracts proteins from an object’s surface without significantly altering it in the process (J. Archaeol. Sci. 2020, DOI: 10.1016/j.jas.2020.105145). Demarchi looked at the proteins’ patterns of degradation to suss

Dissipative particle dynamics for systems with polar species: Interactions in dielectric media.

In this work, we develop a method for simulating polar species in the dissipative particle dynamics (DPD) method. The main idea behind the method is to treat each bead as a dumb-bell, i.e., two sub-beads kept at a fixed distance, instead of a point-like particle. The relation between the bead dipole moment and the bulk dielectric permittivity was obtained. The interaction force of single charges in polar liquid showed that the effective dielectric permittivity is somewhat smaller than that obtained for the bulk case at large separation between the charges. In order to understand the reasons behind the observed drop in the dielectric permittivity, we calculate the electric field of an isolated charge in a polar liquid; no permittivity drop is observed for this case. We can assume that the behavior observed for the force is due to the fact that the probing point is always associated with the charged bead, which is a force center, which essentially leads to a non-homogeneous density distribution around it on average; this is not the case when the field is measured. The interaction of a single charge with an interface between two liquids with different permittivities was studied after that; the model is found to correctly reproduce the "mirror image" effects. Finally, we show why it is necessary to treat the polar species in DPD explicitly by investigating the behavior of a charged colloidal particle at a liquid-liquid interface.

Larval exposure to a pyrethroid insecticide and competition for food modulate the melanisation and antibacterial responses of adult Anopheles gambiae

The insecticides we use for agriculture and for vector control often arrive in water bodies, where mosquito larvae may be exposed to them. Not only will they then likely affect the development of the larvae, but their effects may carry over to the adults, potentially affecting their capacity at transmitting infectious diseases. Such an impact may be expected to be more severe when mosquitoes are undernourished. In this study, we investigated whether exposing larvae of the mosquito Anopheles gambiae to a sub-lethal dose of permethrin (a pyrethroid) and forcing them to compete for food would affect the immune response of the adults. We found that a low dose of permethrin increased the degree to which individually reared larvae melanised a negatively charged Sephadex bead and slowed the replication of injected Escherichia coli. However, if mosquitoes had been reared in groups of three (and thus had been forced to compete for food) permethrin had less impact on the efficacy of the immune responses. Our results show how larval stressors can affect the immune response of adults, and that the outcome of exposure to insecticides strongly depends on environmental conditions.

Effect of the Fraction and Size of Polar Groups on the Formation of Compact Conformations of a Polymer Chain with Variable Stiffness in Low-Polar Media

The conformational behavior of a single semiflexible polymer chain with a variable fraction of polar groups (dipoles) is studied by the molecular dynamics method. Dipoles of the chain are simulated as two oppositely charged beads, one of which belongs to the backbone and another bead is a side group. The charged bead of the side group may freely rotate around the backbone, and the size of the side bead and the distance between oppositely charged groups (dipole length) are varied. The main attention is focused on studying the effect of backbone stiffness and an interplay of excluded volume and electrostatic interactions on the conformational behavior of a chain and the structure of multiplets formed due to a strong electrostatic attraction in low-polar media.

Fast Computation of Electrostatic Interactions for a Charged Polymer with Applied Field

Using a hybrid simulation approach that combines a finite difference method with a Brownian dynamics, we investigated the motion of charged polymers. Owing to the fact that polymer-solution systems often contain a large number of particles and the charged polymer chains are in a state of random motion, it is a time-consuming task to calculate the electrostatic interaction of the system. Accordingly, we propose a new strategy to shorten the CPU time by reducing the iteration area. Our simulation results illustrate the effect of preset parameters on CPU time and accuracy, and demonstrate the feasibility of the “local iteration” method. Importantly, we find that the increase in the number of charged beads has no significant influence on the time of global iterations and local iterations. For a number of 80 × 80 × 80 grids, when the relative error is controlled below 1.5%, the computational efficiency is increased by 8.7 times in the case that contains 500 charged beads. In addition, for a number of 100 × 100 × 100 grids with 100 charged beads, the computational efficiency can be increased up to 12 times. Our work provides new insights for the optimization of iterative algorithms in special problems.

Osmolyte-Induced Collapse of a Charged Macromolecule.

In the recent surge of investigations on osmolyte-induced conformational landscape of hydrophobic macromolecules notwithstanding, there is a lack of understanding of how the presence of Coulombic charges in the macromolecule dictates its own conformational preference in aqueous media of osmolyte. Toward this end, in this work, we have computationally simulated the trimethyl amine N-oxide (TMAO)-induced collapse behavior of a charge-neutral polymer by varying the number of oppositely charged monomeric beads of a given charge density. From our free-energy-based analysis, at low charge density, there emerges a nonmonotonic trend in the extent of osmolyte-induced protection of collapsed conformation of the charge-neutral polymer as a function of the number of periodically distributed charged monomers: specifically, we observe that, at low charge density, with incremental introduction of oppositely charged monomers in the charge-neutral polymer, the process of osmolyte-induced polymer collapse first gets free-energetically destabilized relative to that in uncharged polymer. However, with further increase in the number of charged monomers of low charge density, there is a recurrence of osmolyte-induced stabilization of polymer collapse. On the contrary, the nonmonotonic trend in osmolyte-induced polymer collapse across the number of charged monomer beads diminishes with an increase in charge density: the aqueous TMAO solution becomes a denaturant of the polymer collapse at higher charge distribution in charge-neutral polymer with higher charge density. A molecular-level analysis of the polymer-osmolyte interaction reveals that the differential interaction of nitrogen and oxygen atoms of TMAO with the charged polymer beads, together with the competing effect of polymer-TMAO dispersion interaction and electrostatic interaction, holds the key in dictating the trend in the osmolyte-induced protection of the polymer collapse across various charge densities and charge distributions.

Preparation and Cu(II) ion removal capacities of Schiff base-based polystyrene nanocomposites for wastewater treatment

In this paper, new composite nanomaterials by encapsulating Schiff base molecules with hydrophilic spacers into sulfonate groups charged polystyrene beads (PS) have been prepared. The obtained composite adsorbent, Schiff base-based polystyrene nanocomposites (PS-GN1), demonstrated more preferable Cu(II) ion removal capacity in the presence of Ca2+ co-ions in solutions. And the obtained PS-GN1 composites with inner well-dispersed Schiff base molecules demonstrated much faster breakthrough for Cu(II) ion removal than PS beads mainly due to the electrostatic forces and coordinating reaction with Cu(II) ion. The present research work showed it potential to prepare excellent nanocomposites for the removal of toxic metal ions and other charged pollutants from wastewater.

Ultrafast and Ultrasensitive Naked-Eye Detection of Urease-Positive Bacteria with Plasmonic Nanosensors.

Identifying the pathogen responsible for an infection is a requirement in order to personalize antimicrobial treatments. Detecting bacterial enzymes, such as proteases, lipases, and oxidoreductases, is a winning approach for detecting pathogens at the point of care. In this Article, a new method for detecting urease-producing bacteria rapidly and at ultralow concentrations is reported. In this method, longsome bacteriological culture steps are substituted for a 10 min capture procedure with positively charged magnetic beads. The presence of urease-positive bacteria on the particles is then queried with a plasmonic signal generation step that generates blue- or red-colored nanoparticle suspensions upon addition of the enzyme substrate. These colorimetric signals, which can be easily identified by eye, are generated by the NH3-dependent assembly of gold nanoparticles in the presence of bovine serum albumin (BSA). The proposed method can detect Proteus mirabilis with a limit of detection of 101 cells mL-1, with a total assay time of 40 min, even in the presence of a large excess of urease-negative bacteria ( Pseudomonas aeruginosa). Furthermore, it does not require bulky equipment, and it can detect P. mirabilis at clinically relevant concentrations within minutes, making it suitable for detecting urease-positive pathogens at the point of care.

Charge Separating Microfiltration Membrane with pH-Dependent Selectivity.

Membrane filters are designed for selective separation of components from a mixture. While separation by size might be the most common approach, other characteristics like charge can also be used for separation as presented in this study. Here, a polyether sulfone membrane was modified to create a zwitterionic surface. Depending on the pH value of the surrounding solution the membrane surface will be either negatively or positively charged. Thus, the charged state can be easily adjusted even by small changes of the pH value of the solution. Charged polystyrene beads were used as model reagent to investigate the pH dependent selectivity of the membrane. It was found that electrostatic forces are dominating the interactions between polystyrene beads and membrane surface during the filtration. This enables a complete control of the membrane’s selectivity according to the electrostatic interactions. Furthermore, differently charged beads marked with fluorescent dyes were used to investigate the selectivity of mixtures of charged components. These different components were successfully separated according to their charged state proving the selectivity of the invented membrane.

Preparation of Template-Free Robust Yolk-Shell Gelled Particles from Controllably Evolved All-in-Water Emulsions.

A template-free all-aqueous bulk preparation of robust hollow capsules having a gelatin shell from all-in-water double emulsions is reported. The hot (>40 °C) quaternary system water/polyethylene glycol (PEG)/gelatin/alginate is shown to spontaneously form PEG-in-gelatin-in-PEG double water emulsion droplets having a multinuclear core. These droplets are stable upon cooling below the temperature at which gelatin gelled. In contrast, above the melting temperature of gelatin, multinuclear double emulsion droplets controllably evolve into stable mononuclear yolk (aqueous PEG)-shell (gelatin) capsules dispersed in the aqueous PEG continuous phase. It is demonstrated that the gelatin shell can accommodate negatively charged latex beads and be re-enforced by glutaraldehyde or silica. These capsules are also shown to encapsulate payloads, suggesting possible applications in microencapsulation, drug delivery, and synthetic biology.

Adsorption Dynamics and Structure of Polycations on Citrate-Coated Gold Nanoparticles

Despite the widespread application of engineered multilayered polyelectrolyte-coated gold nanoparticles (AuNPs), their interparticle interaction is not fully understood in part because of a lack of molecular scale observation of the layer-by-layer assembly of the polyelectrolyte coating. While top-down coarse-grained models of polyelectrolyte-coated AuNPs have focused on polyelectrolytes of short length, from 10 to a 100 monomers represented as one charged bead per monomer, here we use molecular dynamics and bottom-up coarse-grained approaches to access more typical polymer lengths on the order of 200 monomers. Specifically, we simulate the adsorption dynamics and structure of one or two such long polycations on negatively charged 4 nm citrate-coated AuNPs within implicit or explicit solvents. The first polycation coats approximately half of the AuNP surface regardless of solvent model and leaves a significant part of the anionic citrate layer exposed to absorption by a second polycation. We find that the...

Electrostatic Purification of Mixed-Phase Metal–Organic Framework Nanoparticles

Although macroscopic metal–organic framework (MOF) single crystals have been routinely synthesized, undesired impurity phases are sometimes obtained in MOF nanoparticle (NP) syntheses, where purification remains challenging. Herein we report an electrostatic adsorption strategy to separate mixed phases of MOF NPs on the basis of their metal cluster-dependent surface charge differences. As a proof of concept, two groups of mixed-phase MOF NPs were synthesized and subsequently separated on the basis of their different Coulombic attraction to negatively charged magnetic beads (MBs). Different frameworks form on the basis of the conditions used. In the first group, a combination of three possible iron–terephthalate frameworks were evaluated: MIL-53, MIL-88B, and MIL-101 (MIL = Material Institute Lavoisier). In the second group, two zirconium–terephthalate frameworks were separated: MIL-140A and UiO-66 (UiO = University of Oslo). MIL-53 and MIL-140A are not positively charged and do not adsorb onto the MBs. Th...

Novel Optical Flocculation Approach for Chemical Contaminations in the Water Treatment Using Micro/Nano Polymeric Beads

The optogenetic is a biological technique that involves the use of light to control cells in living tissue, typically neurons that have been genetically modified to express light-sensitive ion channels. In this paper, we exploit the use of the optogenetic to create a novel approach for water treatment and purification using a nano charged dielectric polymeric beads.

Condensation of Counterions Gives Rise to Contraction Transitions in a One-Dimensional Polyelectrolyte Gel.

The equilibrium volume of a polyelectrolyte gel results from a balance between the tendency to swell caused by outbound polymer/counterion diffusion along with Coulomb interactions on the one hand; and, on the other, the elastic resilience of the cross-linked polymer network. Direct Coulomb forces contribute both to non-ideality of the equilibrated Donnan osmotic pressure, but also to stretching of the network. To isolate the effect of polyelectrolyte expansion, we have analyzed a “one-dimensional” version of a gel, a linear chain of charged beads connected by Hooke’s law springs. As in the range of weak Coulomb strengths previously studied, the springs are significantly stretched by the repulsive interactions among the beads even when the Coulomb strength is strong enough to cause condensation of counterions. There is a quasi-abrupt transition from a stretched state to a partially collapsed state in a transition range between weak and strong Coulomb strengths. Fluctuations between stretched and contracted conformations occur within the transition range. As the solvent quality decreases past the transition range, a progressive collapse can result if the condensed counterions strengthen the spring constant.

Graphene as an Imaging Platform of Charged Molecules.

Graphene, a single atom layer of carbon atoms, provides a two-dimensional platform with an extremely high sensitivity to charges due to its unique band structure and high surface-to-volume ratio. Graphene field-effect transistor (G-FET) biosensors have, indeed, demonstrated a detection limit of subnanomolar or even subpicomolar. However, in G-FET, signal is averaged throughout the whole channel, so there remains a need to visualize the spatial distribution of target molecules on a single G-FET, to provide further insight into target molecules and/or biological functions. Here, we made use of graphene as an imaging platform of charged molecules via Raman microscopy. Positively (or negatively) charged microbeads with a diameter of 1 μm were dispersed in a buffer solution and were attached on graphene. We found out that Raman peaks of graphene, where positively (or negatively) charged beads contacted, were up-shifted (or down-shifted) significantly, indicating that the carrier density in the graphene was locally modulated by the charged beads and the charge state of the beads was represented by the peak-shift direction. From the peak shift, the change in the carrier density was calculated to be +1.4 × 1012 cm–2 (or −1.0 × 1012 cm–2). By taking Raman peak-shift images, we visualized distribution of charged molecules on graphene with a spatial resolution below 1 μm. The technique described here overcomes the limitation of spatial resolution of G-FET and provides a new route to graphene-based chemical and biosensors.

Coarse-Grain Molecular Dynamics Simulations To Investigate the Bulk Viscosity and Critical Micelle Concentration of the Ionic Surfactant Sodium Dodecyl Sulfate (SDS) in Aqueous Solution.

The first critical micelle concentration (CMC) of the ionic surfactant sodium dodecyl sulfate (SDS) in diluted aqueous solution has been determined at room temperature from the investigation of the bulk viscosity, at several concentrations of SDS, by means of coarse-grain molecular dynamics simulations. The coarse-grained model molecules at the mesoscale level are adopted. The bulk viscosity of SDS was calculated at several millimolar concentrations of SDS in water using the MARTINI force field by means of NVT shear Mesocite molecular dynamics. The definition of each bead in the MARTINI force field is established, as well as their radius, volume, and mass. The effect of the size of the simulation box on the obtained CMC has been investigated, as well as the effect of the number of SDS molecules, in the simulations, on the formation of aggregates. The CMC, which was obtained from a graph of the calculated viscosities versus concentration, is in good agreement with the reported experimental data and does not depend on the size of the box used in the simulation. The formation of a spherical micelle-like aggregate is observed, where the dodecyl sulfate tails point inward and the heads point outward the aggregation micelle, in accordance with experimental observations. The advantage of using coarse-grain molecular dynamics is the possibility of treating explicitly charged beads, applying a shear flow for viscosity calculation, and processing much larger spatial and temporal scales than atomistic molecular dynamics can. Furthermore, the CMC of SDS obtained with the coarse-grained model is in much better agreement with the experimental value than the value obtained with atomistic simulations.

Selective Cu(II) ion removal from wastewater via surface charged self-assembled polystyrene-Schiff base nanocomposites

The design and preparation of functionalized composite adsorbent materials are of key importance for wide environmental fields. This study demonstrates a new route to fabricate hybrid nanomaterial by encapsulating Schiff base molecules into sulfonate groups charged polystyrene beads (PS). The prepared composite adsorbent, polystyrene-Schiff base ((PS-SB)) nanocomposites, showed more preferable Cu(II) ion removal capacity in the presence of Mg2+ and Ca2+ co-ions in solutions. Comparatively, the obtained (PS-SB) composites with inner well-dispersed Schiff base molecules and high sorption ability demonstrated much faster breakthrough for Cu(II) ion removal than PS beads, mainly due to the electrostatic forces and coordinating reaction of Schiff base groups with Cu(II) ion. In addition, the used composite adsorbent material could be easily regenerated by HCl-NaCl solution for repeated utilization without obvious capacity loss. Present research work showed that charged functional groups in host nanostructures improved the dispersion and connection of anchored organic molecules, enhancing the removal capacity of targeted metal ions. Suitably designed guest molecules in composite structures are crucial in the preparation of excellent nanocomposites for the removal of toxic metal ions and other charged pollutants from wastewater.