Changes In Charge Density Research Articles

Model of Water Sorption and Swelling in Polymer Electrolyte Membranes: Diagnostic Applications.

This work expands on the poroelectroelastic model of water sorption in polymer electrolyte membranes. It links membrane water sorption and the evolution of the pore size distribution with the microscopic charge density distribution at pore surfaces and the microscopic shear modulus of polymeric pore walls. We evaluate different deformation modes of polymeric pore walls to derive stress-strain relationships that determine the law of swelling. Thereafter, the model is applied to different sets of water sorption data for membranes that were submitted to either hygrothermal aging or chemical degradation. The model-based analysis relates the macroscopic state of swelling to the evolution of the statistical distribution of the pore radius as well as of the microscopic fluid and elastic pressures. Changes in the surface charge density at pore surfaces and elastic pressure of pore walls, induced by degradation, can be rationalized. These insights are useful in view of understanding degradation mechanisms and their structural effects.

Investigation of point and extended defects in electron irradiated silicon—Dependence on the particle energy

This work is focusing on generation, time evolution, and impact on the electrical performance of silicon diodes impaired by radiation induced active defects. n-type silicon diodes had been irradiated with electrons ranging from 1.5 MeV to 27 MeV. It is shown that the formation of small clusters starts already after irradiation with high fluence of 1.5 MeV electrons. An increase of the introduction rates of both point defects and small clusters with increasing energy is seen, showing saturation for electron energies above ∼15 MeV. The changes in the leakage current at low irradiation fluence-values proved to be determined by the change in the configuration of the tri-vacancy (V3). Similar to V3, other cluster related defects are showing bistability indicating that they might be associated with larger vacancy clusters. The change of the space charge density with irradiation and with annealing time after irradiation is fully described by accounting for the radiation induced trapping centers. High resolution electron microscopy investigations correlated with the annealing experiments revealed changes in the spatial structure of the defects. Furthermore, it is shown that while the generation of point defects is well described by the classical Non Ionizing Energy Loss (NIEL), the formation of small defect clusters is better described by the “effective NIEL” using results from molecular dynamics simulations.

Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)5 in solution.

Transition-metal complexes have long attracted interest for fundamental chemical reactivity studies and possible use in solar energy conversion. Electronic excitation, ligand loss from the metal centre, or a combination of both, creates changes in charge and spin density at the metal site that need to be controlled to optimize complexes for photocatalytic hydrogen production and selective carbon-hydrogen bond activation. An understanding at the molecular level of how transition-metal complexes catalyse reactions, and in particular of the role of the short-lived and reactive intermediate states involved, will be critical for such optimization. However, suitable methods for detailed characterization of electronic excited states have been lacking. Here we show, with the use of X-ray laser-based femtosecond-resolution spectroscopy and advanced quantum chemical theory to probe the reaction dynamics of the benchmark transition-metal complex Fe(CO)5 in solution, that the photo-induced removal of CO generates the 16-electron Fe(CO)4 species, a homogeneous catalyst with an electron deficiency at the Fe centre, in a hitherto unreported excited singlet state that either converts to the triplet ground state or combines with a CO or solvent molecule to regenerate a penta-coordinated Fe species on a sub-picosecond timescale. This finding, which resolves the debate about the relative importance of different spin channels in the photochemistry of Fe(CO)5 (refs 4, 16 - 20), was made possible by the ability of femtosecond X-ray spectroscopy to probe frontier-orbital interactions with atom specificity. We expect the method to be broadly applicable in the chemical sciences, and to complement approaches that probe structural dynamics in ultrafast processes.

Electrochemical Tuning of the Dielectric Function of Au Nanoparticles

The tunable dielectric response of Au nanoparticles under electrochemical bias can be interpreted in terms of changes in the surface charge density, surface damping, and the near-surface volume fraction of the nanoparticles that experience a modified dielectric function, as well as changes in the index of refraction of the surrounding electrolyte medium. Using experimental bias-dependent extinction measurements, we derive a potential-dependent dielectric function for Au nanoparticles that accounts for changes in the physical properties contributing to the optical extinction.

Depth profiling charge accumulation from a ferroelectric into a doped Mott insulator.

The electric field control of functional properties is a crucial goal in oxide-based electronics. Nonvolatile switching between different resistivity or magnetic states in an oxide channel can be achieved through charge accumulation or depletion from an adjacent ferroelectric. However, the way in which charge distributes near the interface between the ferroelectric and the oxide remains poorly known, which limits our understanding of such switching effects. Here, we use a first-of-a-kind combination of scanning transmission electron microscopy with electron energy loss spectroscopy, near-total-reflection hard X-ray photoemission spectroscopy, and ab initio theory to address this issue. We achieve a direct, quantitative, atomic-scale characterization of the polarization-induced charge density changes at the interface between the ferroelectric BiFeO3 and the doped Mott insulator Ca(1-x)Ce(x)MnO3, thus providing insight on how interface-engineering can enhance these switching effects.

Multiplexed, label-free detection of biomarkers using aptamers and Tunable Resistive Pulse Sensing (AptaTRPS)

Diagnostics that are capable of detecting multiple biomarkers are improving the accuracy and efficiency of bioassays. In previous work we have demonstrated the potential of an aptamer-based sensor (aptasensor) utilising Tunable Resistive Pulse Sensing (TRPS). Here, we have advanced the technique identifying key experimental designs for potential POC assays. The assay utilised superparamagnetic beads, and using TRPS monitored their translocations through a pore. If the surfaces of the beads are modified with an aptamer, the frequency of beads (translocations/min) through the pore can be related to the concentration of specific proteins in the solution. Herein, we have demonstrated the successful use of TRPS to observe the binding of two proteins, to their specific aptamers simultaneously. We describe a series of experiments illustrating key factors which we believe are integral to bead-based assays and demonstrate a general method for a multiplexed assay. In summary, we have explored the effects of beads size, concentration, potential bias, pH and aptamer affinity to enhance the sensitivity and practically of a TRPS aptasensor. The method utilises the fact the binding of the aptamer to the protein results in a change in charge density on the bead surface, the isoelectric point of the protein then dominates the mobility of the beads, creating a multiplexed assay termed AptaTRPS. By alteration of the applied potential to the instrument it is possible to produce a positive signal in a simple multiplexed assay.

Characterization of human bladder cell membrane during cancer transformation.

Phenomena associated with changes in cell membranes are thought to play an important role in the cancer transformation. We hypothesized that the electrical charge of tumor cells can indirectly represent membrane-based changes that have occurred during cell transformation and may indicate tumor cell status. Here, we describe work showing that phospholipids, proteins content, and electric charge, are all altered in the cell membranes of pT2 stage/grade G3 bladder cancer. Qualitative and quantitative phospholipid composition and the presence of integral membrane proteins were identified using high-performance liquid chromatography. Protein composition was determined using selective hydrolysis of isolated bladder cell membrane proteins and peptide resolution. The surface charge density of human bladder cell membranes was determined using electrophoresis. Our results show that cancer transformation is associated with increased phospholipid levels and a decreased level of integral proteins. Moreover, the process of cancer transformation significantly enhanced changes in the surface charge density of the human bladder cell membrane. In conclusion, this study demonstrates that cell membrane structure and function are modified in bladder cancer cells and that further work in this area is warranted.

Influence of Electric Field in the Adsorption of Atomic Hydrogen on Graphene

The influence of external electric field (EF) in the adsorption of atomic hydrogen on graphene (H/G) was studied by means of electronic structure calculations based on spin-polarized density functional theory with generalized gradient approximation (GGA). The changes in atomic hydrogen physisorption-chemisorption on graphene owed to EF (which ranged between −1.25 V/Å and 0.75 V/Å) were determined. Analysis of the electronic charge density for an H/G system explained the EF influences on the adsorption properties (analyzing changes in electronic charge density for H/G system). A decrease of more than 100% in the chemisorption barrier for an EF of −1.25 V/Å was found. The changes in the electronic charge density confirm the possibility of manipulating the physical-chemical adsorption of hydrogen on graphene by applying electric fields.

Carbon nitride nanosheet-supported porphyrin: a new biomimetic catalyst for highly efficient bioanalysis.

A highly efficient biomimetic catalyst was fabricated based on ultrathin carbon nitride nanosheets (C3N4)-supported cobalt(II) proto-porphyrin IX (CoPPIX). The periodical pyridinic nitrogen units in C3N4 backbone could serve as electron donors for great affinity with Co(2+) in PPIX, which resembled the local electronic structure as vitamin B12 and heme cofactor of hemoglobin. UV-vis kinetics and electrochemistry revealed its competitive (electro)catalysis with conventional peroxidase, while X-ray photoelectron spectroscopy and theoretical calculations suggest that the rehybridization of Co 3d with N orbitals from the backside can result in significant changes in enthalpy and charge density, which greatly promoted the activity of CoPPIX. The prepared nanocatalyst was further conjugated with streptavidin via multiple amines on the edge plane of C3N4 for facile tagging. Using biotinylated molecular beacon as the capture probe, a sensitive electrochemiluminescence-based DNA assay was developed via the electroreduction of H2O2 as the coreactant after the hairpin unfolded by the target, exhibiting linearity from 1.0 fM to 0.1 nM and a detection limit of 0.37 fM. Our results demonstrate a new paradigm to rationally design inexpensive and durable biomimics for electrochemiluminescence quenching strategy, showing great promise in bioanalytical applications.

Binding modes and interaction mechanism between different base pairs and methylene blue trihydrate: a quantum mechanics study.

Different quantum mechanic methods have been evaluated for the calculation of binding modes and interactions between intercalators with different DNA base pairs by comparing with the results of MP2, which is very expensive, indicating that WB97XD method under 6-311+G* basis set is able to efficiently reproduce MP2 results. We discovered that the methylene blue trihydrate intercalated into the DNA base pairs, and DNA intercalation increased the distance between DNA base pairs, depending on the types of DNA bases. According to the binding energy results, it was found that the intercalation of methylene blue trihydrate into AA-TT base pair was more favorable in the orientation of nitrogen than other directions and intercalation, and the electric charge was transferred from methylene blue trihydrate to the AA-TT base pair. The analysis of change in the charge density shows that changes often take place in the heavy atom in the middle of the system which the charge density changes most remarkable.

First-principles calculation on oxygen ion migration in alkaline-earth doped La2GeO5

By using first-principles calculations based on the density functional theory, we investigated the doping effects of alkaline-earth metals (Ba, Sr and Ca) in monoclinic lanthanum germanate La2GeO5 on its oxygen ion conduction. Although the lattice parameters of the doped systems changed due to the ionic radii mismatch, the crystal structures remained monoclinic. The contribution of each atomic orbital to electronic densities of states was evaluated from the partial densities of states and partial charge densities. It was confirmed that the materials behaved as ionic crystals comprising of cations of La and dopants and anions of oxygen and covalently formed GeO4. The doping effect on the activation barrier for oxygen hopping to the most stable oxygen vacancy site was investigated by the climbing-image nudged elastic band method. By tracing the charge density change during the hopping, it was confirmed that the oxygen motion is governed by covalent interactions. The obtained activation barriers showed excellent quantitative agreements with an experiment for the Ca- and Sr-doped systems in low temperatures as well as the qualitative trend, including the Ba-doped system.

Comparison of the influence of polyaspartic acid and polylysine functional groups on the adsorption at the Cr2O3—Aqueous polymer solution interface

Polyamino acids are a group of synthesized polymers obtained by polymerization of a given kind of amino acid monomer. Because of high biodegradability of this class of polymers, they can be used as flocculation or stabilization agents in the environmental aspects. Therefore determination of their influence on the stability of the aqueous suspension of metal oxides is important. An influence of different functional groups of polyamino acids, their molecular weight and concentration on the adsorption at the chromium (III) oxide (Cr2O3)—aqueous solution interface was determined. Experiments were carried out for four values of solution pH varying from 3 to 10 (3, 4, 7.6 and 10, respectively). Two polymers were used: anionic polyaspartic acid (ASP) of 6800 and 27,000 as well as polylysine (LYS) of 4900 and 33,000 molecular weights. Changes of surface charge density of colloidal Cr2O3 in the presence and in the absence of macromolecular substances were determined using potentiometric titration. In these studies the influence of the concentration and molecular weight of the ionic polymers on the pHpzc value was determined. Additionally, due to the lack of appropriate literature data, potentiometric titration of the selected polymers was performed to determine pKa values.

High Precision, Electrochemical Detection of Reversible Binding of Recombinant Proteins on Wide Bandgap GaN Electrodes Functionalized with Biomembrane Models

We report a novel hybrid charge sensor realized by the deposition of phospholipid monolayers on highly doped n‐GaN electrodes. To detect the binding of recombinant proteins with histidine‐tags, lipid vesicles containing chelator lipids were deposited on GaN electrodes pre‐coated with octadecyltrimethoxysilane monolayers. Owing to its optical transparency, GaN allows the confirmation of the fluidity of supported membranes by fluorescence recovery after photo‐bleaching (FRAP). The electrolyte‐(organic) insulator‐semiconductor (EIS) setup enables one to transduce variations in the surface charge density ΔQ into a change in the interface capacitance ΔC p and, thus, the flat‐band potential ΔU FB. The obtained results demonstrate that the membrane‐based charge sensor can reach a high sensitivity to detect reversible changes in the surface charge density on the membranes by the formation of chelator complexes, docking of eGFP with histidine tags, and cancellation by EDTA. The achievable resolution of ΔQ ≥ 0.1 μC/cm2 is better than that obtained for membrane‐functionalized p‐GaAs, 0.9 μC/cm2, and for ITO coated with a polymer supported lipid monolayer, 2.2 μC/cm2. Moreover, we examined the potential application of optically active InGaN/GaN quantum dot structures, for the detection of changes in the surface potential from the photoluminescence signals measured at room temperature.

Ternary, Tunable Polyelectrolyte Complex Fluids Driven by Complex Coacervation

Complex coacervation was achieved by combining poly(allylamine) (PAH) or branched poly(ethylenimine) (PEI) with poly(acrylic acid) (PAA) and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA). We systematically investigated the effects of stoichiometry, salt concentration, and pH. Ternary coacervates formed over a broader range of stoichiometries compared to the base PAA/PDMAEMA system. An enhanced resistance to salt, that is, resistance to dissolution of the complex with added salt, was observed for ternary coacervates. PEI-containing systems showed a considerable difference in salt resistance at pH 6–8 due to the dramatic change in charge density. This change was interpreted in the context of a theoretical treatment that relies on the Voorn–Overbeek model for free energy. Coacervate stability and viscoelastic behavior were affected by stoichiometry, salt, and pH. Ternary coacervates maintain the characteristics and tunability of typical binary coacervates, but the choice of the third component is important, as it significantly affects the response and material properties.

Deswelling and Electrolyte Dissipation in Free Diffusion of Charged PAMAM Dendrimers.

The diffusion coefficient of charged PAMAM dendrimers was measured by fluorescence correlation spectroscopy in aqueous solution at submicromolar concentrations. The solution pH was varied for conditions ranging from a fully charged to neutral charge dendrimer to infer about electrostatic swelling in the dilute regime. The diffusion coefficient of generation G4 increases by as much as 20% between high and low charge conditions due to the combined effects of polyelectrolyte deswelling and loss of electrolyte dissipation. By taking into account the electrolyte dissipation in the friction factor, we have found that the observed deswelling corresponds to a change of hydrodynamic radius between 7-13% for generation G4 and about 12% for generation G7. Simulations of molecular dynamics of dendrimer G4 show that counterion uptake by the dendrimer structure upon full protonation induces a 16% increase of its radius of gyration. The change in dendrimer size is slightly larger than that previously reported from neutron scattering techniques, thereby suggesting that electrostatic swelling is more pronounced at dilute dendrimer concentration and low ionic strength. It is confirmed that even higher generations, which have more congested molecular structures, can experience some degree of conformational change in response to a change of the dendrimer charge density.

Comparison of pH-sensitive degradability of maleic acid amide derivatives

We synthesized five maleic acid amide derivatives (maleic, citraconic, cis-aconitic, 2-(2′-carboxyethyl) maleic, 1-methyl-2-(2′-carboxyethyl) maleic acid amide), and compared their degradability for the future development of pH-sensitive biomaterials with tailored kinetics of the release of drugs, the change of charge density, and the degradation of scaffolds. The degradation kinetics was highly dependent upon the substituents on the cis-double bond. Among the maleic acid amide derivatives, 2-(2′-carboxyethyl) maleic acid amide with one carboxyethyl and one hydrogen substituent showed appropriate degradability at weakly acidic pH, and the additional carboxyl group can be used as a pH-sensitive linker.

Preparation, Characterization of Styrene-Functional Hyperbranched Viologen, and Its Application in Optical Writing and Electrical Erasing Device

Styrene-functional hyperbranched viologen (SHBV2+) was successfully synthesized and characterized in detail by FTIR, 1H NMR, and XPS spectra, respectively. Subsequently, it was cured by visible light to obtain cross-linked hyperbranched viologen film. It is noteworthy that cross-linked SHBV2+ film possessed better photochromic and electrochromic performances. After UV illumination for 60 s, the cross-linked SHBV2+ film can swiftly change its color from pale yellow to deep bluish violet, whereas the absorbance of SHBV2+ film at 610 nm did not change significantly and was still retained about 0.423 after 20 coloring/bleaching cycles. The single-type SHBV2+ EC device can undergo reversible color change in response to the alternate external voltages of −2.0 V and 2.0 V, respectively, while the changes in optical transmission and charge density of EC device at 550 nm did not deteriorate significantly and still remained about 48.2% and 13.85 mC·cm−2 after 10 coloring/bleaching cycles. Furthermore, single-type SHBV2+ PEC device possessed better optical writing and electrical erasing performances. After 10 repeated writing/erasing cycles, the contrast did not decrease significantly and still remained about 48.0%, while the electrical erasing time did not deteriorate significantly and remained about 22.0 s. This device can realize rapid storage, modulation, and erasing of optical information.

Imaging the electro-kinetic response of biological tissues with phase-resolved optical coherence tomography

AbstractIn this study, the electro-kinetic phenomena (EKP) induced in biological tissue by external electric field, while not directly visible in optical coherence tomography (OCT) images, were detected by analyzing their textural speckle features. During application of a low-frequency electric field to the tissue, speckle patterns changed their brightness and shape depending on the local tissue EKP. Since intensities of OCT image speckle patterns were analyzed and discussed in our previous publications, this work is mainly focused on OCT signal phase analysis. The algorithm for extracting local spatial phase variations from unwrapped phases is introduced. The detection of electrically induced optical changes manifest in OCT phase images shows promise for monitoring the fixed charge density changes within tissues through their electro-kinetic responses. This approach may help in the identification and characterization of morphology and function of healthy and pathologic tissues.

Changes in the Physico-Chemical Properties of Human Kidney Cell Membranes during the Cancer Transformation

Kidney tissue is particularly susceptible to reactive oxygen species attack which leads to development of cancer. During oxidative stress, membrane lipids and proteins are major targets of re-active oxygen species (ROS). This work is focused on changes of phospholipids, proteins content and electric charge that occur in cell membranes of kidney cancer of pT3 stage, grade G3 and with metastasis. Qualitative and quantitative phospholipid composition and the presence of integral membrane proteins were determined by high-performance liquid chromatography. Electrophoresis was used to determine the surface charge density of the human kidney cell membrane. It was shown that the process of cancer transformation was accompanied by an increase phospholipid levels and altered the level of integral proteins as determined by decrease phenylalanine, tyrosine, cysteine and arginine. Moreover, the process of cancer transformation significantly enhanced changes in the surface charge density of the human kidney cell membrane. Cell membrane structure and function are modified by neoplasm lesion.

Photocontrolled Adsorption of Polyelectrolyte Molecules on a Silicon Substrate

We report on a change in the properties of monomolecular films of polyelectrolyte molecules, induced by illuminating the silicon substrate on which they adsorb. It was found that under illumination the thickness of the adsorbed layer decreases by at least 27% and at the same time the roughness is significantly reduced in comparison to a layer adsorbed without irradiation. Furthermore, the homogeneity of the film topography and the surface potential is shown to be improved by illumination. The effect is explained by a change in surface charge density under irradiation of n- and p-type silicon wafers. The altered charge density in turn induces conformational changes of the adsorbing polyelectrolyte molecules. Their photocontrolled adsorption opens new possibilities for selective manipulation of adsorbed films. This possibility is of potential importance for many applications such as the production of well-defined coatings in biosensors or microelectronics.