Induced Surface Charge Research Articles

Monolithic polar conjugated microporous polymers: Optimisation of adsorption capacity and permeability trade-off based on process simulation of flue gas separation

The challenge in developing porous materials that significantly reduce energy consumption in industrial gas separation lies in striking a balance between adsorption capacity and permeability. Herein, the precise anchoring of polar oxygen adsorption sites in a robust porous conjugated backbone is achieved through the directed self-assembly of building blocks, resulting in the formation of oxygen-enriched conjugated microporous polymers (O-CMPs). O-CMPs serve as attractive porous hosts for the synergistic separation of CO2 and PM in exhaust gases emitted from point sources. As analyzed by surface electrostatic potential, the introduction of oxygen-doped π-conjugated systems induced surface charge redistribution, enhancing the CO2 quadrupole-dipole effect within a widely distributed microporous network. The O-CMPs exhibited a high CO2 adsorption capacity of 125.84 mg/g at 273 K and 1.0 bar. The monolithic O-CMPs incorporate permanently integrated hierarchical pores with a high micropore ratio. This transport channel can enhance the transfer rate of gas flow while selectively intercepting CO2 and PM. The rigid conjugated molecular chains and hollow nanotube microstructure effectively prevent material densification, yielding a minimal gas permeation resistance of only 5 Pa. Additionally, O-CMPs also demonstrate outstanding stability and PM separation performance under humid and acid/alkaline condition. The visual process simulations serve to further validate that aforementioned design strategy can optimize the trade-off between adsorption capacity and permeability.

Effect of the Surface Charge-Dependent Boundary Slip on the Electrophoresis of a Hydrophobic Polarizable Rigid Colloid.

The electrophoresis of a hydrophobic charged rigid colloid is studied by considering the lateral movement of the adsorbed surface charge. The slip velocity condition at the hydrophobic surface is modified to take into account the impact of the frictional and electric forces created by the adsorbed laterally mobile surface charge. Though the dependency of the surface charge on the slip velocity in the context of electrophoresis has been addressed before, the effect of the laterally mobile adsorbed surface charge on the electrophoresis of hydrophobic colloids has not been studied. The dielectric colloid is considered to polarize and create an induced immobile surface charge when subjected to an imposed electric field. The impact of the mobile surface charge along with the immobile induced surface charge on electrophoresis of a hydrophobic colloid is elucidated by numerically solving the governing electrokinetic equations in their full form. We have also developed a simplified model under a weak applied field consideration, which can be further reduced to a closed-form analytic expression for the mobility under the Debye-Hückel approximation. This analytic model for mobility is in excellent agreement with the exact numerical solution for an entire range of the Debye length when the ζ-potential is in the order of the thermal potential. One of the notable features of this closed-form mobility expression is that it accounts for the mobile adsorbed surface charge on the hydrodynamic slip condition and the dielectric polarization of the particle. We find that the mobility of the surface charge decreases the electrophoretic mobility of the hydrophobic dielectric colloid. However, the mobile surface charge enhances the mobility of a conducting hydrophobic colloid.

Trifurcated Splitting of Water Droplets on Engineered Lithium Niobate Surfaces.

Controlled splitting of liquid droplets is a key function in many microfluidic applications. In recent years, various methodologies have been used to accomplish this task. Here, we present an optofluidic technique based on an engineered surface formed by coating a z-cut iron-doped lithium niobate crystal with a lubricant-infused layer, which provides a very slippery surface. Illuminating the crystal with a light spot induces surface charges of opposite signs on the two crystal faces because of the photovoltaic effect. If the light spot is sufficiently intense, millimetric water droplets placed near the illuminated spot split into two charged fragments, one fragment being trapped by the bright spot and the other moving away from it. The latter fragment does not move randomly but rather follows one of three well-defined trajectories separated by 120°, which reflect the anisotropic crystalline structure of Fe:LiNbO3. Numerical simulations explain the behavior of water droplets in the framework of the forces induced by the interplay of pyroelectric, piezoelectric, and photovoltaic effects, which originate simultaneously inside the illuminated crystal. Such a synergetic effect can provide a valuable feature in applications that require splitting and coalescence of droplets, such as chemical microreactors and biological encapsulation and screening.

Dense Neural Network for Calculating Solvation Free Energies from Electronegativity-Equalization Atomic Charges.

I propose a dense Neural Network for evaluation of solvation free energies ΔG°solv for molecules and ions in water and nonaqueous solvents, Easy Solvation Energy with Electronegativity Equalization charges and Dense Neural Network (ESE-EE-DNN). As input features, it uses the Conductor-like Screening Model (COSMO) electrostatic energy, atomic cavity surface areas, total cavity volume, and induced surface charges. For the COSMO calculation, electronegativity-equalization atomic charges are employed. ESE-EE-DNN exhibits fairly high accuracy, similar or even superior to that of mainstream density functional theory-based methods. For neutral solutes in water, polar protic, polar aprotic, and nonpolar solvents, ESE-EE-DNN yields a root-mean-square error (RMSE) of 1.25, 1.36, 0.70, and 0.71 kcal/mol, respectively. ESE-EE-DNN is particularly advantageous for ionic solutes, with an RMSE of 2.82 and 1.42 kcal/mol for aqueous and nonaqueous ion solutions, correspondingly. ESE-EE-DNN is very efficient due to a fast evaluation of the electronegativity-equalization charges.

Charge-conversional click polyprodrug nanomedicine for targeted and synergistic cancer therapy.

Polyprodrug nanomedicines hold great potential for combating tumors. However, the functionalization of polyprodrug nanomedicines to improve therapeutic efficacy is restricted by conventional polymerization methods. Herein, we fabricated a charge-conversional click polyprodrug nanomedicine system by metal-free azide-alkyne cycloaddition click polymerization (AACCP) for targeted and synergistic cancer therapy. Specifically, Pt(IV) prodrug-backboned diazide monomer, DMC prodrug-pendent diazide monomer, dialkyne-terminated PEG monomer and azide-modified folate were click polymerized to obtain the target polyprodrug (P1). P1 could self-assemble into nano-micelles (1-NM), where PEG was the hydrophilic shell with folate on the surface, Pt(IV) and DMC prodrugs as the hydrophobic core. Taking advantage of PEGylation and folate-mediated tumor cell targeting, 1-NM achieved prolonged blood circulation time and high tumor accumulation efficiency. Tumor acidic microenvironment-responsive cleavage and cascade activation of pendant DMC prodrug induced surface charge conversion of 1-NM from negative to positive, which promoted tumor penetration and cellular internalization of the remaining 1-NM. After internalization into tumor cells, the reduction-responsive activation of Pt(IV) prodrug to Pt(II) further showed synergetic effect with DMC for enhanced apoptosis. This first designed charge-conversional click polyprodrug nanomedicine exhibited targeted and synergistic efficacy to suppress tumor proliferation in living mice bearing human ovarian tumor model.

A simplified model for the impact of dielectric polarization of a charged droplet on its diffusiophoresis

This study aims to quantify the impact of the dielectric permittivity of a droplet on its diffusiophoresis in different types of electrolytes. The dielectric droplet polarizes by the diffusion field along with the local electric field created by the interactions of the double layer with the imposed ionic concentration gradient, which generates an induced surface charge density anti-symmetrically distributed on the droplet surface. This induced surface charge influences both electrophoresis and chemiphoresis parts. Based on a low imposed concentration gradient, a simplified model is derived through a first-order perturbation technique. Dielectric polarization of the droplet attenuates the spinning force at the interface. This creates the mobility of a droplet of higher dielectric permittivity in the presence of a stronger diffusion field significantly higher than that of a perfectly dielectric droplet, and its value depends on the polarity of the droplet surface charge. In the absence of the diffusion field, the mobility of a conducting droplet remains a positive immaterial of the polarity of its surface charge density. We find that the impact of the dielectric polarization becomes significant as the surface charge density increases and attenuates with the increase in droplet viscosity. For a dielectric droplet at a thinner Debye length, a step-jump in mobility occurs at a higher value of the surface charge density. Such a type of step-jump in mobility does not appear for the conducting droplet due to the absence of the Maxwell stress at the interface.

Tumor acidity-induced surface charge modulation in covalent nanonetworks for activated cellular uptake: targeted delivery of anticancer drugs and selective cancer cell death.

A β-thioester and tertiary amine based covalently cross-linked nanoassembly coined as a nanonetwork (NN) endowed with dual pH responsive features (tumor acidity induced surface charge modulation and endosomal pH triggered controlled degradation) has been designed and synthesized for stable sequestration and sustained release of drug molecules in response to endosomal pH. An amphiphile integrated with tertiary amine and acrylate (ATA) functionalities was synthesized to fabricate the nanonetwork. This amphiphile showed entropically driven self-assembly and micellar nanostructures (nanoassemblies), which can sequester hydrophobic drug molecules at neutral pH. To further stabilize the nanoassemblies and the sequestered drug molecules even below its critical aggregation concentration (CAC), the micellar core was cross-linked via the thiol-acrylate Michael addition click reaction to generate multiple copies of acid labile β-thioester functionalities in the core, which undergo slow hydrolysis at endosomal pH (∼5.0), thus enabling sustained release of the anti-cancer drug doxorubicin at endosomal pH. The nanonetworks showed a significant decrease in drug leakage compared to the nanoassemblies (NAs), which was also justified by a low leakage coefficient calculated from the fluorescence resonance energy transfer experiment. The NN also exhibited dilution insensitivity and high serum stability, whereas the NA disassembled upon dilution and during serum treatment. The biological evaluation revealed tumor extracellular matrix pH (∼6.4-6.8) induced surface charge modulation and cancer cell (HeLa) selective activated cellular uptake of the doxorubicin loaded nanonetwork (NN-DOX). In contrast, the benign nature of NN-DOX towards normal cells (H9c2) suggests excellent cell specificity. Thus, we believe that the ease of synthesis, nanonetwork fabrication reproducibility, robust stability, smart nature of tumor microenvironment sensitive surface charge modulation, boosted tumoral-cell uptake, and triggered drug release will make this system a potential nanomedicine for chemotherapeutic treatments.

Photoswitchable polyurethane based nanoaggregates for on-command release of noncovalent guest molecules

The self-assembly of light responsive amphiphilic polymers has been of great interest in spatially and temporally controlled drug delivery applications. In this article, we report the design, synthesis and characterization of amphiphilic main chain azobenzene polyurethane as well as its self-assembly in aqueous milieu. This polymer self-assembles into micellar nanostructure (investigated by dynamic light scattering and scanning electron microscopy imaging) and shows hydrophobic guest sequestration with high encapsulation stability (probed by FRET experiment). The photoswitching (trans-to-cis) of these nanoaggregates has been investigated by irradiation with UV light (λ = 365 nm), which results significant change in the hydrophobic environment and molecular arrangement in the micellar core. This leads to encapsulated guest release in a controlled manner as probed by UV-vis spectroscopy. Furthermore, we demonstrated tumor-relevant pH (∼6.5-6.8) induced surface charge modulation (neutral to positive) by zeta potential measurements. The light responsive guest release and pH-specific charge modulation, we believe, will have significant impact in development of delivery systems for targeted and controlled delivery of drug molecules.

Theory of asymmetric and piezotronically modified double Schottky barriers

We present a theoretical model for double Schottky barriers at zinc oxide grain boundaries that accounts for piezotronically modified barrier heights resulting in generally asymmetric current–voltage (I–V) characteristics with respect to the applied electrical field direction. The model is based on charge distributions in the vicinity of the barrier and its related electrical potential distributions and can be considered as a generalization of the famous model of Blatter and Greuter. The natural asymmetry of current with respect to forward and reverse bias can be explained by different grain orientations and donor densities. The previously experimentally found change of I–V curves due to the application of mechanical loads can be reproduced via the piezotronic effect, leading to changes in the barrier potential profile due to piezoelectrically induced surface charges. Also, the I–V characteristics of degraded grain boundaries can be interpreted in terms of asymmetric changes in the donor densities. In addition, a second approach is presented that is able to explain experimental data of asymmetric I–V curves of wide grain boundaries with different surface terminations (O and Zn-polar).

Entrapment of Airborne Particles via Simulated Highway Noise-Induced Piezoelectricity in PMMA and EPDM

The US highway system features a huge flux of energy transportation in terms of weight, speed, volume, flow density, and noise levels, with accompanying environmental effects. The adverse effects of high-volume traffic cause health concerns for nearby residential areas. Both chronic and acute exposure to PM 2.5 have detrimental effects on respiratory and cardiovascular health, and motor vehicles contribute 25–35% of direct PM 2.5 emissions. In addition to traffic-related pollutants, residing near major roadways is also associated with exposure to increased noise, and both affect the health and quality of life of residents. While regulatory and policy actions may reduce some exposures, engineering means may offer novel and significant methods to address these critical health and environmental issues. The goal of this study was to harvest highway-noise energy to induce surface charge via a piezoelectric material to entrap airborne particles, including PM 2.5. In this study, we experimentally investigated the piezoelectric effect of a polymethyl methacrylate (PMMA) sheet and ethylene propylene diene monomer (EPDM) rubber foam on the entrapment of copper (II)-2,4 pentanedione powder (Cu II powder). Appreciable voltages were induced on the surfaces of the PMMA via mechanical vibrations, leading to the effective entrapment of the Cu II powder. The EPDM rubber foam was found to attract a large amount of Cu II powder under simulated highway noise in a wide range, of 30–70 dB, and at frequencies of 700–1300 Hz, generated by using a loudspeaker. The amount of Cu II powder entrapped on the EPDM rubber-foam surfaces was found to scale with the SPL, but was independent of frequency. The experimental findings from this research provide a valuable base for the design of a robust piezoelectric system that is self-powered by harvesting the wasted sound energy from highway noise and reduces the amount of airborne particles over highways for effective environmental control.

On the Critical Conditions for Realization of Instability of Azimuthal Modes of Conducting Liquid Jets Carrying Uniform and Nonuniform Induced Surface Charges and Their Electrodispersion

On the Critical Conditions for Realization of Instability of Azimuthal Modes of Conducting Liquid Jets Carrying Uniform and Nonuniform Induced Surface Charges and Their Electrodispersion

Deep Learning in Physics: A Study of Dielectric Quasi-Cubic Particles in a Uniform Electric Field

Solving physics problems for which we know the equations, boundary conditions and symmetries can be done by deep learning. Here, we calculate the induced field inside and outside a dielectric quasi-cube placed in a uniform electric field, wherein the dielectric mismatch at edges and corners of the cube makes accurate calculations challenging. The electric potential is expressed as an ansatz incorporating neural networks with known leading order forms and symmetries, then Laplace's equation with boundary conditions at the dielectric interface is solved by minimizing a loss function inside a large solution domain. We study how the electric potential inside and outside the particle evolves through a sequence of shapes from a sphere to a cube. The neural network being differentiable, it is straightforward to calculate the electric field over the whole domain, the induced surface charge distribution and the polarizability. The neural network being retentive, one can efficiently follow how the field changes upon particle's shape or dielectric constant by iterating from previously converged solution. The present work's objective is two-fold, first to show how can a priori knowledge be incorporated into neural networks to achieve efficient learning and second to apply the method and study how the induced field and polarizability change when a dielectric particle progressively changes its shape from a sphere to a cube.

Interesting molecular adsorption strategy induced surface charge redistribution of commercial TiO2: Boosting 9 times photocatalytic hydrogen production performance

A higher conduction band position and more photogenerated electrons are distinctly important for the high-efficiency photocatalytic hydrogen evolution performance. However, raising conduction band position will enlarge the band gap and reduce the production of photogenerated electrons. Herein, a super simple and efficient ammonia molecular adsorption strategy was proposed and designed to promote efficient hydrogen production by a simple ammonia molecules adsorption on commercial TiO2 surface. The experimental and theoretical calculation results indicated that aniline can be successfully adsorbed on TiO2 surface by a straightforward mechanical grinding method, and the photocatalytic hydrogen production performance of TiO2 can be improved by more than 9 times after the adsorption of aniline. This is because the adsorbed aniline can provide electrons to the TiO2 surface and lead to a charge redistribution and accumulation on the surface of TiO2. Excessive electrons make the conduction band of TiO2 bend upward, thus improving the photogenerated electron reduction ability and the photocatalytic performance of TiO2. In addition, the adsorption of aniline enables TiO2 to show light absorption in the visible region, generating more photogenerated electrons to participate in the photocatalytic water splitting reaction. In this work, the molecular adsorption strategy is not only simple to operate, but also can improve the photoreduction ability and surface charge quantity of materials. It provides a simple and efficient new method for photocatalyst modification in addition to the traditional modification methods such as doping compounds.

Electron energy-loss spectroscopy investigation of multipolar behavior in Al nanostructure as a function of surface coating with PVP and oxidation

The inherent, self-limiting oxidation characteristics of Aluminum (Al) nanospheres and nanocubes of similar dimensions (100 nm edge length) are illustrated utilizing the MATLAB based boundary element method (MNPBEM17) optical and electron energy loss spectroscopy (EELS) simulations. Such model systems conveniently establish not only the distinct dipolar and multi-polar plasmonic resonance behaviors with reference to their respective passivated ligand PVP as well as their self-limiting surface conformal/non-conformal oxide layers, demonstrating the vital fact that Al nanostructures could be a suitable substitution of conventional noble metals, especially for UV-plasmonic applications. In the case of Al nanocubes, both oxidation processes were found to be strongly site-selective (edges, corners and faces), whereas a differential variation is observed in Al nanospheres from the center to the surface, effectively corroborated through the simultaneous correlation of optical spectra with the spatially and energetically resolved EELS maps. Moreover, the robust contribution from dark plasmon resonances was also confirmed in the EELS data along with the induced surface charge distribution calculations, further substantiating their strong dependence on the surrounding dielectric environment concerning their oxide passivation as well. Thus, our present results demonstrate a better understanding of the spatial coherence of the plasmonic behavior with reference to the passivated ligand and (non)-conformal oxidation processes of Al nanostructures, in general, thereby providing a fundamental understanding of the plasmon physics necessary to improve the design and optimization of new nanostructures for nanophotonic, nanoantenna and sensing applications.

Sensitive detection of SARS-CoV-2 spike protein using vertically-oriented silicon nanowire array-based biosensor

The COVID-19 pandemic has caused tremendous damage to the world. In order to quickly and accurately diagnose the virus and contain the spread, there is a need for rapid, sensitive, accurate, and cost-effective SARS-CoV-2 biosensors. In this paper, we report on a novel biosensor based on angiotensin converting enzyme 2 (ACE-2)-conjugated vertically-oriented silicon nanowire (vSiNW) arrays that can detect the SARS-CoV-2 spike protein with high sensitivity and selectivity relative to negative controls. First, we demonstrate the efficacy of using ACE-2 receptor to detect the SARS-CoV-2 spike protein via a capture assay test, which confirms high specificity of ACE-2 against the mock protein, and high affinity between the spike and ACE-2. We then report on results for ACE-2-conjugated vSiNW arrays where the biosensor device architecture is based on a p-n junction transducer. We confirm via analytical modeling that the transduction mechanism of the biosensor involves induced surface charge depletion of the vSiNWs due to negative electrostatic surface potential induced by the spike protein after binding with ACE-2. This vSiNW surface charge modulation is measured via current-voltage characteristics of the functionalized biosensor. Calibrated concentration dependent electrical response of the vSiNW sensor confirms the limit-of-detection for virus spike concentration of 100 ng/ml (or 575 pM). The vSiNW sensor also exhibits highly specific response to the spike protein with respect to negative controls, offering a promising point-of-care detection method for SARS-CoV-2.

Exact polarization energy for clusters of contacting dielectrics.

The induced surface charges appear to diverge when dielectric particles form close contacts. Resolving this singularity numerically is prohibitively expensive because high spatial resolution is needed. We show that the strength of this singularity is logarithmic in both inter-particle separation and dielectric permittivity. A regularization scheme is proposed to isolate this singularity, and to calculate the exact cohesive energy for clusters of contacting dielectric particles. The results indicate that polarization energy stabilizes clusters of open configurations when permittivity is high, in agreement with the behavior of conducting particles, but stabilizes the compact configurations when permittivity is low.

A polarizable coarse-grained model for metal, metal oxide and composite metal/metal oxide nanoparticles: development and implementation.

We present a polarizable coarse-grained model for metal, metal oxide, and composite metal/metal oxide nanoparticles with well-defined crystalline surfaces. The developed model uses a low-resolution polarizable "surface beads" representation of the nanoparticle's geometry and pairwise cross nanoparticle potential consisting of van der Waals and electrostatic interaction terms. The electrostatic interaction term of the cross nanoparticle potential incorporates a crucial physical aspect of electrostatic interaction into the metal and metal oxide systems, such as induced surface charges, making it possible to explore the nanoparticles' behavior in complex environments as well as investigate the interplay between electrostatic and van der Waals interactions in nanoparticle systems. The iterative stability, computational scaling, and performance of the presented model was tested on selected systems of gold, titanium dioxide, and composite gold/titanium dioxide nanoparticle systems. The model exhibits robust iterative stability and is able to converge the charge equilibration equation for fluctuating induced charges and dipoles within 10-60 "tug-tow" iterations in challenging situations, like crowded nanoparticle systems or nanoparticle systems in extreme external electric fields. The computation scaling of the presented model is semi-linear with respect to the number of nanoparticles in the system. It slightly varies depending on the size distribution of nanoparticles in a specific nanoparticle system. The computation cost of the model is significantly lower than that of conventional atomistic polarizable force field models and enables the treatment of large nanoparticle systems that are beyond the reach of currently existing atomistic force field models.

Gamma Ray Induced Surface Charge Redistribution and Change of Surface Morphology in Monolayer Ws2

Gamma Ray Induced Surface Charge Redistribution and Change of Surface Morphology in Monolayer Ws2

How pH induced surface charge modification explains the effect of petrophysical and hydrological factors on recovery trends of water drive gas reservoirs

It is generally known that water drive oil reservoirs are more efficient than water drive gas reservoirs. In the petroleum literature, several studies have documented recovery trends based on petrophysical factors other than the wettability factor.Consistent with the high concentration of carbon dioxide in natural gas systems, the pH induced wetting transition reported in petroleum recovery research associated with low salinity water flooding is a potential factor that can impact gas recovery from natural gas systems. In this paper, we have reviewed extensively the fundamental aspects of pH induced wetting transition associated with surface charge regulation in different lithologies. In so doing, we have developed theoretical models that aid explanation of gas recovery trends from different lithologies associated with natural gas systems under aquifer drive. Most importantly, putting our models in the context of fundamental theories associated with pH induced surface charge modification, we have successfully correlated the literature based wetting transition trends of gas recovery to petrophysical properties of absolute permeability and rock type as well as hydrological effect associated with aquifer type.

Ciprofloxacin conjugated gold nanorods with pH induced surface charge transformable activities to combat drug resistant bacteria and their biofilms

The ever-growing threat of drug-resistant pathogens and their biofilms based persistent, chronic infections has created an urgent call for new strategies to deal with multidrug resistant bacteria (MDR). Near-infrared (NIR) laser-induced photothermal treatment (PTT) of gold nanorods (AuNRs) disinfects microbes by local heating with low possibility to develop resistant. However, PTT disinfection strategy of AuNRs alone shows less efficiency in killing multidrug resistant strains (i.e. Methicillin-resistant Staphylococcus aureus, MRSA) and their matured biofilms. Herein, a novel synergistic chemo-photothermal integrated antimicrobial platform (P(Cip-b-CB)-AuNRs) was fabricated which show enhanced killing efficiency against MRSA in both planktonic and biofilm phenotypes. Polymethacrylate copolymers with pendant ciprofloxacin (Cip) and the carboxyl betaine groups (P(Cip-b-CB)) were synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerization. P(Cip-b-CB) was decorated onto AuNRs via gold−thiol bond which resulted in AuNRs with acidic-induced surface charge-switchable activities and lipase triggered Cip release properties (P(Cip-b-CB)-AuNRs). The lower pH value and overexpress of lipase are characteristics for microenvironment of microbial infections and their biofilms, which ensure the targeting on, penetration into and on-demand release of Cip from the nanocomposites in bacterial infection sites and their biofilms. The bacterial cell membrane was disrupt by photothermal therapy which could improve its permeability and sensitivity to antibiotics, meanwhile lipase-triggered release of Cip ensures a high concentration of antibiotics at the site of bacterial infection. Besides their NIR induced PTT disinfection activities, the increased local temperature generated by NIR light irradiation accelerated Cip release which further enhanced the antibacterial efficiency, leading to synergistic antibacterial activities of chemo-photothermal therapy. Taken together, the designed synergistic chemo-photothermal integrated antimicrobial platform is a promising antibacterial agent for fighting MDR bacterial infections and their biofilms.