Permanent Surface Charge Research Articles

In-situ measurement of dust charge density in nanodusty plasma

A dust grain immersed in a low-pressure gas discharge obtains a permanent negative surface charge due to the high mobility of electrons compared to that of ions. This charge essentially governs all fundamental processes in dusty and complex plasmas involving dust grains, neutrals, (an)ions and electrons and—consequently—virtually all industrial applications of these types of plasmas are affected and steered by it. In this work, we have measured the surface charge by application of laser-induced electron detachment from nanosized dust grains in concert with microwave cavity resonance spectroscopy and laser light extinction. The main result is that the electron release is governed by photodetachment rather than by thermionic emission, and that recharging of the dust grains occurs on timescales that are well in agreement with the orbital-motion-limited (OML) theory. The total surface charge density residing on the dust grains inside the laser volume follows from the saturation of the photodetachment signal, which was used in combination with dust density values derived from extinction measurements to estimate the mean dust charge. The negative dust charge on the 140 nm (average) diameter dust grains in this work is obtained to be in the range of 273 – 2519 elementary charges, of which the lower bound matches well with analytical predictions using the OML theory.

Influence of UV-irradiation intensity and exposure duration on the hemobiocompatibility enhancement of a novel synthesized phosphobetaine zwitterions polyethersulfone clinical hemodialysis membranes.

To improve the biocompatibility of polyethersulfone (PES) membranes utilized for biomedical hemodialysis (HD) applications, surface grafting with hydrophilic polymers has become a reliable modification strategy. Like most photochemical catalyzed reactions, UV-assisted grafting is distinctly advantageous for inducing permanent surface chemistry, enhancing hydrophilicity, improving morphology, and surface charge of membranes. PES membranes may be hydrophilic and chemically stable; however, they also have low protein-binding capacity and very susceptible to fouling and target analyte binding. In this study, novel zwitterionic polymers (PVP-ZW) have been synthesized by UV-assisted grafting PVP to a phosphobetaine monomer in a reaction involving dimethylamino and dioxaphospholane-2-oxide terminal groups in an NVP monomer solution at varying UV exposure conditions. The highlight of the present study is the investigation of the hemocompatibility of coated PES HD membranes at varying UV exposure conditions with respect to membrane chemistry and morphology and its influence on human serum protein adsorption. A clinical investigation of inflammatory biomarker release from incubated coated membranes within uremic blood samples of HD patients reveals they are weak complement and coagulation activators compared to bare PES membrane. The trend of fibrinogen adsorption on coated PES membranes was observed to increase with reducing UV intensity and exposure duration. Fibrinogen adhesion only increased with roughened membrane surfaces, and this also led to the formation of biological activation pathways hindering biocompatibility. Resistance against fibrinogen absorption on zwitterionic modified PES membrane could be linked with the creation of electrostatically induced neutral zwitterionic PVP-phosphobetaine hydration layer with hydrophilic character. Experimental results are accompanied by spectroscopic and morphological imaging evidence. Zwitterion coated PES membranes (PES-PVP-ZW) fabricated from higher UV intensities through longer exposure durations showed significant presence of surface deformations in the forms of inherent exfoliations due to harsh UV reaction conditions. The zeta potential and surface roughness of coated membranes also played significant role in the fibrinogen adsorption on PES membranes during ultrafiltration.

SIGNIFICANCE OF SOIL DEVELOMENT AND IRON OXIDES CONTENT ON PROPERTIIES OF PERMENANT CHARGE ON CLAY SURFACES IN SOME SELECTED SOILS FROM NORTHERN IRAQ

The study was conducted on nine clay samples collected from three different soils of northern Iraq (Mukhmour, Shikhan and Zakho) which varied in climatic conditions (precipitations and temperatures).Permanent surface charge on clay surfaces was measured through adsorption of lithium chloride experience before and after removing iron oxides. Results showed that the highest values of permanent negative charge on clay surfaces before removing iron oxides was found in Mukhmour soil with average 34.5 C.mol. kg-1. clay, while the lowest values appeared in clay fraction of Zakho soil (18.1 C.mol. kg-1. clay), but in Shikhan soil,the negative charge values was at average of (20.0 C.mol. kg-1 clay). The highest stability of negative surface charge was found at pH between values (3-5). The distribution of negative charges within soil profiles showed an increase in values with depth and in all studied soils. Results also reveals very low positive surface charge compared with negative surface charge. However, the results of the lithium chloride adsorption experiment have indicated that the removal of iron oxides from clay fraction had a clear effect on increasing in the values of negative surface charge in all study soils.The rates of increase associated mainly with the iron oxides content of the clay fractions but the impact of removing iron oxides on the values of positive surface charge were minor and had no clear behavior. From the above study it can be concluded that the variations in the values of surface of clay surface charge clearly reflects the levels of weathering and the degrees of soil development.

Evolution of soil surface charge in a chronosequence of paddy soil derived from Alfisol

Limited information is available on the changes in surface electro–chemical properties of paddy soils during pedogenesis. This study evaluated the category, evolution and origin of soil surface charge along an Alfisol–derived paddy soil chronosequence. Ammonium acetate adsorption, as well as cesium (Cs+) adsorption and sequential desorption methods were applied to reveal the surface charge evolution along a paddy soil chronosequence. Results indicated that organic matter increased by 52.79%, 84.90%, 144.35%, and 126.52% in the surface paddy soils after 3, 8, 30, and 70 years of rice cultivation, respectively. The main phyllosilicates in Alfisol and Alfisol–derived paddy soils were montmorillonite and illite, which increased with paddy cultivation over time. Soil negative charge, as determined by ammonia (NH4+) adsorption, was strongly correlated with data from Cs+ adsorption methods. The negative charge on the surface paddy soil, as estimated by NH4+ adsorption, increased by 16.41%, 33.21%, 56.49%, and 31.68% after 3, 8, 30, and 70 years of rice cultivation, respectively. Further investigations revealed that an increasing variable surface charge is mainly attributed to soil organic matter accumulation. Also, hydroponics cultivation was shown to prompt the destruction and weathering of primary minerals and formation of soil secondary phyllosilicate minerals, which are mainly responsible for increasing permanent surface charge along the paddy soil chronosequence. Thus, the nutrients holding capacities of paddy soils are maintained or even improved, thereby increasing agricultural productivity, when compared with upland soils.

How the particle interaction forces determine soil water infiltration: Specific ion effects

Many studies have shown that K+ has more favourable effects on soil water infiltration than Na+, which is a result of specific ion effects or Hofmeister effects. In this study, we employed four alkali metal cation species (Li+, Na+, K+, and Cs+) to determine how the particle interactions influence the water infiltration in a soil with permanent surface charges. Moreover, the defined Hofmeister energy was employed to characterize the specific ion effects. The DLVO force plus the hydration force between two adjacent soil particles mainly determines water infiltration rate through affecting the soil pore-size distribution, which exhibits strong specific ion effects. The hydration force may originate from the surface hydration of soil particles and the hydration of cations in the double layer. The hydration pressure between adjacent soil particles was complexly influenced by electrolyte concentration, Debye length, cationic distribution in the double layer through the Hofmeister energy of cation.

Enhanced demulsification from aqueous media by using magnetic chitosan-based flocculant

A series of quaternized chitosan (QC)-grafted magnetic nanoparticles (MNPs) were successfully synthesized for demulsification from aqueous environments. Fe3O4 MNPs were synthesized by using a coprecipitation method, followed by surface coating with silica and aminopropyl to form a surface for further grafting of QC molecular chains. The synthetic magnetic flocculants were characterized by various technologies and their demulsification performances were evaluated in detail as a function of dosage, QC grafting ratio (Gq), pH and magnetic field. Results showed that pH did not significantly affect oil–water separation performance and MNPs with high Gq exhibited enhanced separation efficiency. The separation capacity was estimated to be >105 mg of diesel oil/mg of magnetic flocculant. Recycling experiment indicated the magnetic flocculant could be recycled up to at least 7 cycles at various pH levels. The grafted QC layer endowed the hybrid MNPs with permanent positive surface charges, thus allowing them to flocculate negatively charged oil droplets via electrostatic patching. The magnetic field could not only accelerate the separation of resulting flocs, but also remove the MNPs-coated dispersed oil droplets. In conclusion, QC-grafted MNPs provide a potentially new technique for developing environmentally friendly and highly efficient magnetic flocculant for practical demulsification applications.

Spectral induced polarization of Na-montmorillonite dispersions

Montmorillonite (Mt) clays have a high specific surface area and surface charge, which confer them remarkable adsorption properties. Nevertheless, their electrochemical and aggregation behavior are not completely elucidated because of the complexity of their microstructural and interfacial properties. In this work, the conductive and dispersive properties of Na-Mt suspensions of weight fractions 0.5–5.2% were investigated for the first time using the spectral induced polarization method. A four-electrode system was used to reduce errors introduced by electrode polarization and contact resistances. Complex conductivity spectra in the low-frequency range of 0.1Hz to 45kHz were successfully described using a triple layer model of the basal surface of Mt and a complex conductivity model that considers conduction of the diffuse layer and polarization of the Stern layer. Aggregate size distributions were inferred from inverted relaxation time distributions. We found that the negative and permanent surface charge of the basal plane of Na-Mt controls its quadrature (imaginary) conductivity, which is not very sensitive to pH and salinity (NaCl) in the 100Hz to 45kHz frequency range. For lower frequencies, the sudden increase of the quadrature conductivity at the highest salinities was explained by considering coagulation of Na-Mt particles.

Highly Dispersed Fe3+-Substituted Colloidal Silica Nanoparticles for Defect-Free Tungsten Chemical Mechanical Planarization

Silica nanoparticles (NPs) are used as abrasives for tungsten chemical mechanical planarization (CMP) at acidic pH. However, the use of silica NPs at pH near their isoelectric point remains a problem because agglomeration due to low surface charge leads to defects on the tungsten surface during CMP. Herein, we report a simple strategy to increase the surface charge of silica NPs at acidic pH for defect-free tungsten CMP. The isomorphic substitution of Si4+ by Fe3+ ions on the surface of silica NPs by hydrothermal reaction led to a pH-independent permanent negative surface charge, which increased as the concentration of substituted Fe3+ ions increased. At acidic pH, the increased negative surface charge of Fe3+-substituted silica (Fe-silica) NPs resulted in a reduction in the number of agglomerated large particles relative to that of pure silica NPs. As a result, highly negatively-charged Fe-silica NPs showed high performance in the reduction of defect count on the tungsten surface after CMP.

Molecular catalysis at polarized interfaces created by ferroelectric BaTiO3.

The local environment at polarized solid-liquid interfaces provides a unique medium for chemical reactions that could be exploited to control the selectivity of non-faradaic reactions. Polarized interfaces are commonly prepared by applying a voltage to an electrode in an electrolyte solution, but it is challenging to achieve high surface charge densities while suppressing faradaic reactions. Ferroelectric materials have permanent surface charge densities that arise from the dipole moments of ferroelectric domains and can be used to create polarized solid-liquid interfaces without applying a voltage. We studied the effects of ferroelectric oxides on the selectivity of a Rh porphyrin-catalyzed carbene rearrangement. The addition of ferroelectric BaTiO3 nanoparticles to the reaction solution changed the product ratio in the same direction and by a similar magnitude as performing the reaction at an electrode-electrolyte interface polarized by a voltage. The results demonstrate that colloidal suspensions of BaTiO3 nanoparticles act as a dispersible polarized interface that can influence the selectivity of non-faradaic reactions.

Three-dimensional simulation of biological ion channels under mechanical, thermal and fluid forces

In this article we address the three-dimensional modeling and simulation of biological ion channels using a continuum-based approach. Our multi-physics formulation self-consistently combines, to the best of our knowledge for the first time, ion electrodiffusion, channel fluid motion, thermal self-heating and mechanical deformation. The resulting system of nonlinearly coupled partial differential equations in conservation form is discretized using the Galerkin Finite Element Method. The validation of the proposed computational model is carried out with the simulation of two ion nanochannels. The first is a voltage operated channel with K+ and Na+ ions, and the second is the sodium-potassium pump in which also chlorine (Cl−) and bicarbonate (HCO3−) ions are considered. In the first case study, we investigate the coupling between electrochemical and fluid-dynamical effects. Then, we enrich the modeling picture by investigating the influence of a thermal gradient. Finally, we add a mechanical stress responsible for channel deformation and investigate its effect on the functional response of the channel. Results show that fluid and thermal fields have no influence in absence of mechanical deformation whereas ion distributions and channel functional response are significantly modified if mechanical stress is included in the model. These predictions agree with biophysical conjectures on the importance of protein conformation in the modulation of channel electrochemical properties. In the second case study, we exploit our multiphysical mathematical formulation to investigate the effect of permanent surface charge on the function of the sodium-potassium pump. In particular, we consider the biophysical application in which the pump actively participates to the process of aqueous humor production across the transepithelial membrane in the ciliary body of the eye. In this study we are motivated by the fact that several data are available to verify the accuracy of model predictions and that the role of surface charge has not yet been mathematically analyzed in the specific context at hand. Results show that the model is able to predict in a very accurate manner the correct aqueous humor flow direction, the magnitude of aqueous velocity and the experimentally measured transepithelial membrane potential only if the (negative) surface permanent charge is larger than a limiting value, whereas if the charge is below this value, fluid inversion occurs.

Defects in structure as the sources of the surface charges of kaolinite

The sign and magnitude of the surface charge govern the particle interactions and hence determine the physical properties of dispersions. The present work is to evaluate the distortions, isomorphic and broken surface bond as the sources of the permanent or variable surface charges of kaolinite. The theoretical permanent charge density (σcc) was calculated by XRF. The experimental permanent charge density (σp) and variable charge density (σv) were obtained by the method of potentiometric titration. The degree of broken bonds (Db, b) of Al–O–Al and Si–O–Si at the side of kaolinite structure were characterized by the numbers and strength of surface hydrogen in 1H MAS NMR, absorbance of Al–OH or Si–OH in FTIR and edge fractures in FESEM. The surface area was measured by N2 adsorption–desorption. The defects such as basal z corrugation (∆z), tetrahedral rotation angles (θ) and octahedral flattening angles (ψ) were obtained by the XRD rietveld refinement. The negative σcc or σp of three samples KA, KB and KC are 0.012, 0.007 and 0.009C·m−2 or 0.24, 0.19 and 0.22C·m−2, respectively. Db, b of three samples are Db, b-KA>Db, b-KC>Db, b-KB. The σv depend on the degree of the broken bonds at edges of kaolinite and can be affected by pH significantly. The negative variable charge density at alkaline condition decreased with the increase of the degree of broken bonds at the side of kaolinite structures. Surface charges of kaolinite come from the defects such as isomorphic substitutions and broken bonds Al–O–Al and Si–O–Si. The distortions in structure e.g. ∆z, θ and ψ may not directly create the surface charges.

Mapping and screening risk assessment of heavy metals concentrations in soils of the Bahr El-Baker Region, Egypt

Mapping the spatial distribution of soil pollutants is essential for delineating contaminated areas ofBahr El-Baker Soils, Egypt. Geostatistical interpolation and kriging was used to estimate pollutant levels in soils. The X- Ray Fluorescence (XRF) and X- Ray Diffraction (XRD) analyses results showed similarity of the studied soils in the most of chemical and mineralogical (dominant montmorillonite) properties; thus revealing to these exposed soils the same geological and pedological conditions. These soils exhibited that they possess both permanent and variable surface charges due to smectite minerals, amorphous iron oxides, and organic matter which ensures high metal sorption capacity thus, increasing the cation-exchange capacity (CEC) values. The spatial patterns showed the homogeneity in Cd, Pb, Cr, and Ni contents. In contrast, Cu and Zn contents, since their data distributions were characterized by great variation and high skewness (1.27 and 1.22 respectively). The best fit model for Cd and Pb were Circular and Exponential model with Mean Standardized (MS) values for them -0.0255 and 0.0600, respectively which is closest to zero. Root Mean Square Standardized (RMSS) values for Cd and Pb were 1.096 and 0.9789 respectively which are closest to 1. The best fit model for Zn was Pentaspherical model (MS = 0.0002) and Rational Quadratic model for Ni and Cr with -0.0029 and -0.0046 MS, respectively which is closest to zero. The data showed that the concentrations of heavy metals Zn, Cr, Ni, and Pb measured in these soils are still under the threshold values. However, the concentrations of Cd and Cu in some soils exceeded the threshold values for non-polluted soils. Since the whole study area (17818.93 ha) became contaminated by Cd, and 6370.23 ha only became contaminated by Cu. Although, this study is a screening presentation which helps to identify the important areas to sample in the future, the results indicate that these soils are still able to load with over concentrations of these metals and retained them, since they do not reach their saturation capacities except for Cu and Cd in some areas. But under bad management with continuous accumulation of these toxic metals in the environment, region will be exposed to real disaster. Geostatistics provides effective methods to quantify the contaminated soil which support decision-making about redevelopment scenarios or remediation techniques. Key words: Heavy metals, geostatistics, risk assessment, Egypt.

Development and Evaluation of Chitosan-Coated Liposomes for Oral DNA Vaccine: The Improvement of Peyer’s Patch Targeting Using a Polyplex-Loaded Liposomes

The aim of this study was to develop chitosan-coated and polyplex-loaded liposomes (PLLs) containing DNA vaccine for Peyer's patch targeting. Plain liposomes carrying plasmid pRc/CMV-HBs were prepared by the reverse-phase evaporation method. Chitosan coating was carried out by incubation of the liposomal suspensions with chitosan solution. Main lipid components of liposomes were phosphatidylcholine/cholesterol. Sodium deoxycholate and dicetyl phosphate were used as negative charge inducers. The zeta potentials of plain liposomes were strongly affected by the pH of the medium. Coating with chitosan variably increased the surface charges of the liposomes. To increase the zeta potential and stability of the liposome, chitosan was also used as a DNA condensing agent to form a polyplex. The PLLs were coated with chitosan solution. In vivo study of PLLs was carried out in comparison with chitosan-coated liposomes using plasmid encoding green fluorescence protein as a reporter. A single dose of plasmid equal to 100 μg was intragastrically inoculated into BALB/c mice. The expression of green fluorescence protein (GFP) was detected after 24 h using a confocal laser scanning microscope. The signal of GFP was obtained from positively charged chitosan-coated liposomes but found only at the upper part of duodenum. With chitosan-coated PLL540, the signal of GFP was found throughout the intestine. Chitosan-coated PLL demonstrated a higher potential to deliver the DNA to the distal intestine than the chitosan-coated liposomes due to the increase in permanent positive surface charges and the decreased enzymatic degradation.

Chaotic mixing in electro-osmotic flows driven by spatiotemporal surface charge modulation

This paper presents an investigation into chaotic mixing in an electro-osmotic flow through a microchannel. In the mixing system, the continuous throughput flow has the form of a pluglike electro-osmotic flow induced by a permanent surface charge on the wall surface, while electro-osmotic flows contributed by spatiotemporal surface charge variations act as a perturbed flow. The spatiotemporal surface charge variations are achieved using the field-effect control method. The analyses consider two different spatiotemporal surface charge modulation schemes, designated as “MS I” and “MS II,” respectively. It is shown that both modulation schemes prompt the crossing of the flow streamlines at different instances in time and produce a chaotic mixing effect. Utilizing the thin double layer assumption, the study commences by solving the biharmonic equation for the electro-osmotic flow fields analytically. The mixing phenomena induced by the two modulation schemes are then analyzed using the Lagrangian particle tracing method. Finally, the mixing performances of the two schemes are evaluated analytically using the Poincaré section method, the finite-time Lyapunov exponent (FTLE) technique, and a stretching value distribution analysis method, respectively. It is found that the mean FTLE combined with the coefficient of variance of the FTLE distribution provides the most suitable criterion for obtaining quantitative estimates of the mixing performance and therefore provides a feasible means of estimating the amplitude and time-switching period of the perturbed flows which optimize the mixing performance. The validity of the analytical results is confirmed via a comparison with the results obtained from the back-trace imaging method and direct numerical simulations based on a species convection-diffusion equation, respectively. In addition, the direct numerical simulation results show that the dimensionless mixing length and dimensionless mixing time required to achieve a 90% mixing both vary as a logarithmic function of the Péclet number when the mixing system is in a nearly fully chaotic state.

Diffuse sorption modeling: Apparent H/Na, or the same, Al/Na exchange on clays

Clay minerals are specified by permanent negative surface charge. In solutions of sodium salts, the surface of clay is covered by exchangeable sodium ions. In an acidic field (pH < 4–6), sodium ions are displaced from the surface. This apparent H/Na exchange is conditioned by dissolution of alumina, followed by Al/Na exchange. Two kinds of published experimental data were considered in order to follow Al/Na exchange: the first is direct measurement of exchangeable sodium and aluminum in clay, and the second is exchange sorption of trace metal. Because of the equivalency of ionic exchange, trace metal acts as a probe, indicating the sodium content in clay. These experimental data were successfully modeled with use of the Poisson–Boltzmann equation, with the assumption that all exchange cations are located in the diffuse layer.

Acid–base behavior of a colloidal clays fraction extracted from natural quartz sand: Effect of permanent surface charge

The electrical double-layer model (EDLM), generally used in order to describe the behaviour of the solids bearing only amphoteric groups at the surface (i.e. oxides), can be extended to solid based dispersion presenting simultaneously amphoteric sites (pH-dependent charge) and permanent structural charges (pH-independent charge) at the surface (i.e. clays). In this study, the acid–base behaviours of a fraction of colloidal clays extracted from a natural quartz sand have been modelled within the EDLM frameworks. These colloidal clays also exhibit heterogeneous macroscopic surface charge distribution. We have considered the case where the colloidal fraction presents only amphoteric sites at the surface (case 1) and the case where both pH-depending and pH-independent charges are present simultaneously at the surface (case 2). The modelling results show a great difference between the theory and the experimental data for case 1 but a good corroboration for case 2. Therefore, in order to describe correctly the physical-chemistry properties of the colloidal fraction it is necessary to take into account the permanent charges (microscopic surface charge heterogeneity) to explain the increase in the zero protonic charge (ZPC) and in the protonation amount with decreasing ionic strength. In case 1, the double-layer model leads to a constant ZPC and to an increase in the protonation amount as a function of ionic strength, which is in disagreement with the experimental results.

Probing Surface Charge Potentials of Clay Basal Planes and Edges by Direct Force Measurements

The dispersion and gelation of clay suspensions have major impact on a number of industries, such as ceramic and composite materials processing, paper making, cement production, and consumer product formulation. To fundamentally understand controlling mechanisms of clay dispersion and gelation, it is necessary to study anisotropic surface charge properties and colloidal interactions of clay particles. In this study, a colloidal probe technique was employed to study the interaction forces between a silica probe and clay basal plane/edge surfaces. A muscovite mica was used as a representative of 2:1 phyllosilicate clay minerals. The muscovite basal plane was prepared by cleavage, while the edge surface was obtained by a microtome cutting technique. Direct force measurements demonstrated the anisotropic surface charge properties of the basal plane and edge surface. For the basal plane, the long-range forces were monotonically repulsive within pH 6-10 and the measured forces were pH-independent, thereby confirming that clay basal planes have permanent surface charge from isomorphic substitution of lattice elements. The measured interaction forces were fitted well with the classical DLVO theory. The surface potentials of muscovite basal plane derived from the measured force profiles were in good agreement with those reported in the literature. In the case of edge surfaces, the measured forces were monotonically repulsive at pH 10, decreasing with pH, and changed to be attractive at pH 5.6, strongly suggesting that the charge on the clay edge surfaces is pH-dependent. The measured force profiles could not be reasonably fitted with the classical DLVO theory, even with very small surface potential values, unless the surface roughness was considered. The surface element integration (SEI) method was used to calculate the DLVO forces to account for the surface roughness. The surface potentials of the muscovite edges were derived by fitting the measured force profiles with the surface element integrated DLVO model. The point of zero charge of the muscovite edge surface was estimated to be pH 7-8.

Kinetics of Enzyme Attack on Substrates Covalently Attached to Solid Surfaces: Influence of Spacer Chain Length, Immobilized Substrate Surface Concentration and Surface Charge

The use of alpha-chymotrypsin to cleave covalently bound N-acetyl- l-tryptophan (Ac-Trp-OH) from the surfaces of aminopropylated controlled pore glass (CPG) and the polymer PEGA 1,900 was investigated. Oligoglycine spacer chains were used to present the covalently attached Ac-Trp-OH substrate to the aqueous enzyme. In the absence of the oligoglycine spacer chain, the rate of release was relatively slow, especially from the PEGA 1,900. These slow rates reflect the position of the amino group to which Ac-Trp-OH is covalently attached. On the glass there was a clear optimum with a chain of four glycine residues. For PEGA 1,900 there is no real apparent change beyond two glycine residues. The decline in rate beyond these optima are a possible result of changes in oligoglycine structure. Comparing different surface loadings of bound substrate the rate of release of Ac-Trp-OH from CPG with a pore diameter of 1,200 A was optimal when using 83% of the maximum that can be coupled, then fell again at higher loading. The rate of Ac-Trp-OH release from CPG was the same for surface coverages of 0.4 and 1.0. The introduction of permanent surface charges on CPG 1,200 exhibits a distinct influence on enzymatic cleavage with an increase in the rate of biocatalysis at the surface. Optimal presentation of covalently immobilized substrate on different supports by use of appropriate linkers leads to favorable biocatalysis from the support.

Nanoprecipitation-assisted ion current oscillations

Nanoscale pores exhibit transport properties that are not seen in micrometre-scale pores, such as increased ionic concentrations inside the pore relative to the bulk solution, ionic selectivity and ionic rectification. These nanoscale effects are all caused by the presence of permanent surface charges on the walls of the pore. Here we report a new phenomenon in which the addition of small amounts of divalent cations to a buffered monovalent ionic solution results in an oscillating ionic current through a conical nanopore. This behaviour is caused by the transient formation and redissolution of nanoprecipitates, which temporarily block the ionic current through the pore. The frequency and character of ionic current instabilities are regulated by the potential across the membrane and the chemistry of the precipitate. We discuss how oscillating nanopores could be used as model systems for studying nonlinear electrochemical processes and the early stages of crystallization in sub-femtolitre volumes. Such nanopore systems might also form the basis for a stochastic sensor.

Characterisation of ferroelectric-calcium phosphate composites and ceramics

This paper studies the effect of composite composition on the electrical properties of BaTiO 3-hydroxyapatite (HA) composites and examines the high temperature electrical properties of pure HA. At least 80% volume of BaTiO 3 was needed for the composites to exhibit any stress-induced piezoelectric potential. At elevated temperatures, pure HA exhibits an increase in real part of conductivity and permittivity. The increased ionic conductivity at elevated temperatures can be used to manufacture polarised HA bioceramics with permanent surface charges as polarised HA exhibited a depolarisation current at elevated temperatures.