Negative Surface Charge Density Research Articles

Enhanced effect of aggregated gold nanoparticles on luminol chemiluminescence system and its analytical application

Some organic compounds containing groups of OH, NH2, or SH, which could induce the aggregation of gold nanoparticles (AuNPs), were observed to enhance effectively the luminol–H2O2–2.6nm AuNPs CL system. It was found that the aggregation of AuNPs was an important effect factor for the catalytic activity of AuNPs on luminol CL system. The aggregated AuNPs could effectively enhance luminol CL signal compared with the dispersed one. The enhanced effect was closely related to the sizes of AuNPs. Among the studied AuNPs with seven sizes, 2.6nm AuNPs had the greatest enhancement effect on luminol CL system after its aggregation. The CL enhancement mechanism was investigated, and the marked enhancement of aggregated 2.6nm AuNPs for luminol CL system was supposed to originate from the decrease of AuNPs’ surface negative charge density compared to its dispersed state. For the luminol–H2O2–2.6nm AuNPs CL system in the presence of organic compounds containing groups of OH, NH2, or SH, more than one factor played the role in influencing the CL intensity. It was found that the enhanced effect of aggregated 2.6nm AuNPs induced by such organic compounds was much more significant than the inhibition effect of reducing groups of OH, NH2, or SH, which made it applicable for the determination of this kind of compounds.

Effect of clay nanoparticles on the structure and performance of polyethersulfone ultrafiltration membranes

In this work, the effect of clay nanoparticles as a casting solution additive on the morphology and performance of polyethersulfone ultrafiltration membranes was evaluated. Ultrapure water permeability was used to initially survey membrane performance and the membranes cast with 1% clay+1% sodium hexametaphosphate (SHMP) and 2% clay had the highest permeabilities. The pure water permeability had no correlation to membrane thickness, porosity, contact angle, or negative surface charge density. Scanning electron microscopy (SEM) images of the membrane surface and cross-section were used to determine the membrane surface porosity, surface pore size, and internal structure, and all of these membrane features were affected by the clay addition and concentration. The increased permeability of the 2% clay membrane is attributed to its greater surface pore size and porosity. The 1% clay+1% SHMP, 2% clay, and control membrane were subject to further cross-flow permeability, retention, and fouling evaluations using ultrapure water, sodium alginate solution, and natural surface water. The membranes cast with clay additions had a greater potential for fouling as compared to the control. A simple detergent cleaning procedure indicated the membrane fouling was completely reversible. Membrane structure as a predictor for drinking water treatment performance is discussed.

Effect of Fe/Al Hydroxides on Desorption of K+ and NH+4 from Two Soils and Kaolinite

Potassium (K) and nitrogen (N) are essential nutrients for plants. Adsorption and desorption in soils affect K+ and NH+4 availabilities to plants and can be affected by the interaction between the electrical double layers on oppositely charged particles because the interaction can decrease the surface charge density of the particles by neutralization of positive and negative charges. We studied the effect of iron (Fe)/aluminum (Al) hydroxides on desorption of K+ and NH+4 from soils and kaolinite and proposed desorption mechanisms based on the overlapping of diffuse layers between negatively charged soils and mineral particles and the positively charged Fe/Al hydroxide particles. Our results indicated that the overlapping of diffuse layers of electrical double layers between positively charged Fe/Al hydroxides, as amorphous Al(OH)3 or Fe(OH)3, and negatively charged surfaces from an Ultisol, an Alfisol, and a kaolinite standard caused the effective negative surface charge density on the soils and kaolinite to become less negative. Thus the adsorption affinity of these negatively charged surfaces for K+ and NH+4 declined as a result of the incorporation of the Fe/Al hydroxides. Consequently, the release of exchangeable K+ and NH+4 from the surfaces of the soils and kaolinite increased with the amount of the Fe/Al hydroxides added. The greater the positive charge on the surfaces of Fe/Al hydroxides, the stronger was the interactive effect between the hydroxides and soils or kaolinite, and thus the more release of K+ and NH+4. A decrease in pH led to increased positive surface charge on the Fe/Al hydroxides and enhanced interactive effects between the hydroxides and soils/kaolinite. As a result, more K+ and NH+4 were desorbed from the soils and kaolinite. This study suggests that the interaction between oppositely charged particles of variable charge soils can enhance the mobility of K+ and NH+4 in the soils and thus increase their leaching lsos.

Structural characteristics of mixed oxides MOx/SiO2 affecting photocatalytic decomposition of methylene blue

A series of photocatalysts based on silica (nanoparticulate) supported titania, ceria, and ceria/zirconia were synthesized and characterized by a variety of techniques including surface area measurements, X-ray diffraction, Fourier transform infrared spectroscopy, zeta potential, surface charge density, and photocatalytic behavior toward methylene blue decomposition. Thermal treatment at 600°C increases the anatase content of the titania based catalysts detected by XRD. Changes in the infrared spectra before and after thermal treatment indicate that at low temperature there are more SiOTi bonds than at high temperature. As these bonds break upon heating the SiO2 and TiO2 separate, allowing the TiO2 anatase phase to form. This results in an increased catalytic activity for the thermally treated samples. Nearly all titania based samples exhibit a negative surface charge density at pH 7 (initial pH of photocatalytic studies) which aids adsorption of methylene blue. The crystallinity of ceria and ceria/zirconia based catalysts are in some cases limited, and in others non-existent. Even though the energy band gap (Eg) can be lower for these catalysts than for the titania based catalysts, their photocatalytic properties are inferior.

Fibronectin adsorption and cell response on electroactive poly(vinylidene fluoride) films

Due to the large potential of electroactive materials in novel tissue engineering strategies, the aim of this work is to determine if the crystalline phase and/or the surface electrical charge of electroactive poly(vinylidene fluoride), PVDF, have influence on the biological response in monolayer cell culture. Non-polar α-PVDF and electroactive β-PVDF were prepared. The β-PVDF films were poled by corona discharge to show negative or positive electrical surface charge density. It has been concluded that hydrophilicity of the PVDF substrates depends significantly on crystalline phase and polarity. Furthermore, by means of atomic force microscopy and an enzyme-linked immunosorbent assay test, it has been shown that positive or negative poling strongly influences the behavior of β-PVDF supports with respect to fibronectin (FN) adsorption, varying the exhibition of adhesion ligands of adsorbed FN. Culture of MC3T3-E1 pre-osteoeblasts proved that cell proliferation depends on surface polarity as well. These results open the viability of cell culture stimulation by mechanical deformation of a piezoelectric substrate that results in varying electrical charge densities on the substrate surface.

Separation and purification of fullerenols for improved biocompatibility

Isoelectric focusing was used to separate the different components of fullerenols based on their different electronegativities induced by the surface chemical functionalities and structures. Purified fractions of fullerenols were analyzed and characterized by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The results showed that four fractions of fullerenols could be separated by isoelectric focusing, and all fractions had a general formula of C 60(OH) m (O) n where the values of m and n might vary with their isoelectric points. Results from the capillary electrophoresis studies showed that the negative surface charge densities were different among the fractions of fullerenols, which indicated that the net surface negative charge was determined by the degree of C O functionalization. In vitro cytotoxicity assays were also performed to determine the effect of purified fullerenols on cytotoxicity and DNA stability in Raw 264.7 cells. Although the fullerenols had low cytotoxicity, the composition of fullerenols with the highest negative charge density (−1.913 ± 0.008 × 10 −4 C) and smaller size (3.92 ± 0.71 nm) was found to induce DNA damage. Therefore, crude fullerenols must be separated and purified in order to obtain fullerenols with low degree of C O functionalization and good biocompatibility.

The thermal stability of the external invertase isoforms from Saccharomyces cerevisiae correlates with the surface charge density

Understanding the effect of surface charge on the stability of proteins is one prerequisite for “tailoring” proteins with increased thermal stability. Here, we investigated the origin of the altered thermal stability observed between the four recently isolated isoforms (EINV1–EINV4) of external invertase. External invertase from yeast Saccharomyces cerevisiae, a homodimeric glycoprotein, represents a widely used model for studying the influence of the glyco component on protein stability. The stability of the four isoforms of invertase decreases from EINV1 to EINV4, which is accompanied by an increase in negative surface charge density. Mass spectrometry analysis revealed that the isoforms share identical protein parts indicating that the differences in stability are the result of post-translational modifications. 31P NMR analysis revealed that the isoforms contain negatively charged phosphate groups in diester and monoester forms attached to the glycan part. The total amount of phosphate bound to the polymannan component varies between the different isoforms. These results, together with the analysis of the amount of polymannan components, show that negative surface charge density does not entirely depend on the amount of phosphate but rather on its distribution. This suggests that charged groups bound to the glyco-component of a protein can influence the stability of glycoproteins.

Characterization of surface charge and mechanical properties of chitosan/alginate based biomaterials

This study aims to examine mechanical properties and surface charge characteristics of chitosan/alginate-based films for biomedical applications. By varying the concentrations of chitosan and alginate, we have developed films with varying surface charge densities and mechanical characteristics. The surface charge densities of these films were determined by applying an analytical model on force curves derived from an atomic force microscope (AFM). The average surface charge densities of films containing 60% chitosan and 80% chitosan were found to be − 0.46 mC/m 2 and − 0.32 mC/m 2, respectively. The surface charge density of 90% chitosan containing films was found to be neutral. The elastic moduli and the water content were found to be decreasing with increasing chitosan concentration. The films with 60%, 80% and 90% chitosan gained 93.5 ± 6.6%, 217.1 ± 22.1% and 396.8 ± 67.5% of their initial weight, respectively. Their elastic moduli were found to be 2.6 ± 0.14 MPa, 1.9 ± 0.27 MPa and 0.93 ± 0.12 MPa, respectively. The trend observed in the mechanical response of these films has been attributed to the combined effect of the concentration of polyelectrolyte complexes (PEC) and the amount of water absorbed. The Fourier transform infrared spectroscopy experiments indicate the presence of higher alginate on the surface of the films compared to the bulk in all films. The presence of higher alginate on surface is consistent with negative surface charge densities of these films, determined from AFM experiments.

On charge transfer in the adsorbed molecules-graphene monolayer-SiC substrate system

A step-by-step consideration of charge transfer in the molecule-single-layer graphene-SiC substrate system is presented. At the first step, a simple model of the density of states of a single graphene monolayer adsorbed on silicon carbide (the graphene-SiC system) is suggested, which allows the calculation of the corresponding occupation numbers of graphene atoms. It is shown that the graphene monolayer accumulates a negative charge. At the second step, the graphene-SiC system is considered as a substrate that adsorbs molecules with a high electron affinity. The charge of these molecules as a function of their surface concentration is calculated. It is shown that, in the case of a monolayer coating, the negative surface charge density of molecules in the molecule-graphene monolayer-SiC substrate system is considerably higher than the surface charge density transferring from the SiC substrate to the graphene layer. This suggests that it is possible to neutralize the excess charge in the graphene layer via adsorption of proper particles on the layer.

Вплив аміксину на електрофоретичну рухомість Т-лімфоцитів миші

The amixine-induced early changes in the electrophoretic mobility (EPM) of murine splenic T lymphocytes were studied in vitro by the microelectrophoresis technique. It has been found that T lymphocytes treated with amixine have a greater EPM within the first hours of amixine addition than control cells. This change in EPM depends on the concentration of amixine in the medium and the duration ofamixine exposure. It was concluded that the amixine-treated cells have a greater net negative surface charge density than control cells. This effect may play an important role in the cell-cell interaction during the immune response.

Isolated Silicon Dangling Bonds on a Water-Saturated n+-Doped Si(001)-2 × 1 Surface: An XPS and STM Study

Using Si 2p core-level X-ray photoelectron spectroscopy we have measured the upward band bending at the surface of n+-doped water-saturated Si(001)-2 × 1 and inferred the macroscopic negative surface charge density of the surface. These macroscopic results are in excellent accord with the microscopic view provided by dual-bias scanning tunneling microscopy showing that the isolated silicon dangling bonds (∼1.2 × 10−2 defects per Si atom) bear indeed a negative charge. Noting the structural analogy between isolated dangling bonds on water-saturated Si(001) and H-terminated Si(001), in the final, prospective section of the paper, we raise the question of the possible role that these defects could play in radical chain reactions with π-bonded molecules, in relationship with the hydride/hydroxyl patterns that are resolved in the scanning tunneling images.

Different synthesis methods allow to tune the permeability and permselectivity of dopamine–melanin films to electrochemical probes

Different methods to prepare melanin films by dopamine oxidation are compared with respect to their permeability to electrochemical probes being either neutral or carrying a positive or a negative charge. To examine charge-dependence of the permeability differently charged probes of similar size (hexaamineruthenium, ferrocenemethanol and hexacyanoferrate) are employed. We deliberately not investigated the permeability of metal cations through the melanin films owing to their possible complexation by the catechol groups of melanin. The films prepared by solution oxydation methods (O 2 or Cu 2+ as oxidants) are impermeable to hexacyanoferrate as soon as their thickness reaches 5–10 nm but remain permeable to hexaamineruthenium and ferrocenemethanol up to thickness reaching 25–40 nm before becoming also impermeable. The melanin films prepared by electrochemical deposition display a marked difference; they remain permeable to hexacyanoferrate up to 35 nm in thickness and to hexaamineruthenium and ferrocenemethanol up to the maximal thickness that can be reached by this deposition method. When the maximal film thickness of 45 nm is reached the film becomes also impermeable to these two redox probes. These results are explained on the basis of structural differences of the melanin films at the molecular level. The permselectivity of the melanin coatings is related to their negative surface charge density at the working pH of 7.5.

The Effect of Specific Adsorption of Cations and Their Size on the Charge-Compensation Mechanism in Carbon Micropores: The Role of Anion Desorption

Combined application of cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) technique reveals a complicated interplay between the adsorption of ammonium and lower molecular weight tetraalkyl ammonium cations and desorption of Cl(-) anions inside carbon micropores at low surface charge densities, which results in failure of their permselectivity. Higher negative surface charge densities induce complete exclusion (desorption) of the Cl(-) co-ions, which imparts purely permselective behavior on the carbon micropores. The second fundamental effect discovered herein relates to the dominant role of anion desorption (as compared to cation adsorption), that is, overwhelming failure of permselectivity extends to high negative charge densities of the electrode in the presence of bulky tetraalkyl ammonium cations, which tend to be confined in the micropores of the carbon. The results obtained are important for advancement of high power density carbon-based supercapacitors, nanofiltration technologies with porous carbon membranes, and studies of ionic transport across biological membranes.

Polyacrylate adsorbents for the selective adsorption of cholesterol-rich lipoproteins from plasma or blood.

Polyacrylate (PAA) adsorbents selectively bind low density lipoproteins (LDL) from human plasma and blood, whereas very low density lipoproteins (VLDL) are only minimally adsorbed. The adsorption of cholesterol-rich lipoproteins to PAA adsorbents is related to the molecular weight (mw) of the polyanion ligand. Ca++ and Mg++ inhibit the binding of LDL to PAA adsorbents. The chemical composition of the organic hardgels of the adsorbents does not have an influence on adsorption. The selective adsorption of LDL to PAA adsorbents can be explained to result from their low negative surface charge density and the specific colloid-chemical properties of the surface-bound PAA, which do not prevent LDL from binding to charge-like domains of the ligand. By contrast, VLDL and high density lipoproteins (HDL) are repelled from the adsorbents due to their higher negative surface charge density.

Adhesion of osteoblasts to a nanorough titanium implant surface

This work considers the adhesion of cells to a nanorough titanium implant surface with sharp edges. The basic assumption was that the attraction between the negatively charged titanium surface and a negatively charged osteoblast is mediated by charged proteins with a distinctive quadrupolar internal charge distribution. Similarly, cation-mediated attraction between fibronectin molecules and the titanium surface is expected to be more efficient for a high surface charge density, resulting in facilitated integrin mediated osteoblast adhesion. We suggest that osteoblasts are most strongly bound along the sharp convex edges or spikes of nanorough titanium surfaces where the magnitude of the negative surface charge density is the highest. It is therefore plausible that nanorough regions of titanium surfaces with sharp edges and spikes promote the adhesion of osteoblasts.

Nonequilibrium carrier distribution in semiconductor photodetectors: Surface leakage channel under illumination

The nonequilibrium carrier distribution in an InGaAs/InP avalanche photodiode under light illumination is obtained by cross-sectional scanning capacitance microscopy combined with numerical simulation. The sheet density of negative surface charge is determined to be 1.85×1010 cm−2 on the native-oxidized InGaAs (110) face. This surface charge is found responsible for the accumulation of minority holes, which leads to an inversion layer at the sidewall surface of device in the absorption region under illumination exceeding 0.1 mW/cm2. The inversion depth increases up to 200 nm along with the enhancement of excitation intensity. This work suggests that a surface leakage channel may form in semiconductor photodetectors through detection light excitation.

Proton transfer to charged platinum electrodes. A molecular dynamics trajectorystudy

A recently developed empirical valence bond (EVB) model for proton transfer onPt(111) electrodes (Wilhelm et al 2008 J. Phys. Chem. C 112 10814) has beenapplied in molecular dynamics (MD) simulations of a water film in contactwith a charged Pt surface. A total of seven negative surface charge densitiesσ between − 7.5 and − 18.9 µC cm − 2 were investigated.For each value of σ, between 30 and 84 initial conditions of a solvated proton within a water slab weresampled, and the trajectories were integrated until discharge of a proton occurred on thecharged surfaces. We have calculated the mean rates for discharge and for adsorptionof solvated protons within the adsorbed water layer in contact with the metalelectrode as a function of surface charge density. For the less negative values ofσ we observe a Tafel-like exponential increase of discharge rate with decreasingσ. At the more negative values this exponential increase levels off and the discharge processis apparently transport limited. Mechanistically, the Tafel regime corresponds to a stepwiseproton transfer: first, a proton is transferred from the bulk into the contact water layer,which is followed by transfer of a proton to the charged surface and concomitant discharge.At the more negative surface charge densities the proton transfer into the contact waterlayer and the transfer of another proton to the surface and its discharge occur almostsimultaneously.

Screening platelet–surface interactions using negative surface charge gradients

Negative surface charge density gradients were prepared on fused silica slides using selective oxidation of a 3-mercaptopropyltrimethoxysilane (MTS) monolayer converting surface thiol groups (–SH) into negatively charged sulfonate (−SO 3 −) groups. The sulfonate-to-thiol gradient samples were characterized by water contact angle and electron spectroscopy for chemical analysis (ESCA). Gradients were pre-adsorbed with proteins from three different solutions: platelet free plasma (PFP), fibrinogen, or albumin in phosphate buffered saline (PBS). Washed platelets were perfused over gradient samples in a parallel plate flow chamber and platelet adhesion was measured across the gradients using differential interference contrast (DIC) microscopy. Gradients pre-adsorbed with PFP showed adhesion contrast inversely related to the negative surface charge density. The magnitude of the adhesion contrast along the gradient was also dependent on PFP concentration. Gradients pre-adsorbed with fibrinogen showed an adhesion maximum in the center of the gradient region. Albumin coating of the gradients resulted in low overall platelet adhesion with increased adhesion in regions of high negative charge density. The effect of gradient orientation with respect to the flow was also investigated. Gradients pre-adsorbed with 10% PFP showed different adhesion contrast when the platelets were perfused in opposite directions. This suggests that platelet adhesion is, in addition to responding to the local surface properties, also dependent on the upstream conditions.

Modeling of Bottle-Brush Polymer Adsorption onto Mica and Silica Surfaces: Effect of Side-Chain Length

Adsorption of a series of charged bottle-brush polymers with side chains of different length on solid surfaces is modeled using a lattice mean-field theory. The bottle-brush polymers are modeled Lis being composed of two types of main-chain segments: charged segments and uncharged segments with ill attached side chain. The composition variable X denotes the percentage of charged main-chain segments and ranges from X = 0 (uncharged bottle-brush polymer) to X = 100 (linear polyelectrolyte). Two types of surfaces are considered: mica-like and silica-like. The mica-like surface possesses a constant negative surface charge density and no nonelectrostatic affinity for either main-chain or side-chain segments, whereas the silica-like Surface has a constant negative surface potential and a positive affinity for the side chains of the bottle-brush polymers. With the mica-like Surface. ill low X the surface excess becomes smaller and at X >= 25 it becomes larger with increasing side-chain length. Hence, the value of X at which the surface excess displays a maximum increases with the side-chain length. However, with the silica-like Surface the surface excess increases with increasing side-chain length at all X < 100, and the maximum of the surface excess appears at X approximate to 10 independent of the side-chain length.

Charge Inversion and Ion−Ion Correlation Effects at the Mercury/Aqueous MgSO4 Interface: Toward the Solution of a Long-Standing Issue

Charge inversion is the phenomenon in which an electric double layer contains more counterions than needed to compensate the surface charge. For colloidal particles this has the consequence that the apparent surface charge, as inferred from electrophoresis or interaction studies, has a sign opposite to that of the actual surface charge, obtainable by titration. This phenomenon has been known for over a century. According to the traditional interpretation, the inversion is caused by (chemical) specific adsorption of ions. However, beginning in the early 1980s it has been predicted by a large number of workers that charge inversion should occur as a consequence of many-body ion−ion correlations. For surfaces of sufficiently high surface charge density in the presence of divalent or multivalent counterions, charge inversion is expected to be ubiquitous even in the absence of specific adsorption. Testing this prediction has proved difficult because chemical specific adsorption is a very common phenomenon and can outweigh the effects of ion correlations. So far, no experimental systems have been thoroughly investigated where strong specific adsorption could be unambiguously ruled out under conditions where charge inversion due to ion−ion correlations is predicted. Here, we solve this problem by studying the mercury/aqueous MgSO4 interface. This system has the advantage that highly accurate double layer data are available for a variety of conditions, including some where chemical specific adsorption is known to be absent (or at least very weak). From precise data for this system [Harrison, J. A.; Randles, J. E. B.; Schiffrin, D. J. J. Electroanal. Chem. 1970, 25, 197] one can establish the ionic components of charge and surface charge density. To extract quantitative theoretical predictions about the consequences of ion−ion correlations, we use the highly accurate anisotropic hypernetted chain (AHNC) method, where ion−ion correlations in the double layer are taken into account in a fully self-consistent fashion. It is found that for moderate to large negative surface charge densities and not too high concentration, the variation in the ionic components of charge with the surface charge density can to a large extent be quantitatively explained by enrichment of ions close to the surface due to ion−ion correlations. That chemical specific adsorption of Mg2+ is negligible is supported by considering the properties of the double layer close to the electrocapillary maximum. In view of the large body of evidence indicating that the counterions tend to specifically adsorb on the mercury surface for positive polarization but not for negative, the agreement between theory and experiment for negative surface charge constitutes strong evidence for ion−ion correlations as the origin of charge inversion.