Surface Electric Potential Research Articles

Strain and Temperature Dependence of Defect Formation at AlGaN/GaN High-Electron-Mobility Transistors on a Nanometer Scale

We use depth-resolved cathodoluminescence spectroscopy (DRCLS), Kelvin probe force microscopy (KPFM), and surface photovoltage spectroscopy (SPS) on a nanometer scale to map the temperature, strain, and defects inside GaN high-electron-mobility transistors. DRCLS maps temperature at localized depths, particularly within the 2-D electron gas region during device operation. KPFM maps surface electric potential across the device, revealing lower potential patches that decrease rapidly with increasing off-state stress. CL spectra acquired at these patches exhibit defect emissions that increase with both on- and off-state stresses and that increase with decreasing surface potential. SPS also reveals features of deep level gap states generated after device operation that reduce near-band-edge emission and increase surface band bending. Our nanoscale measurements are consistent with defect generation by inverse piezoelectric field-induced stress at the gate edge on the drain side at high voltage.

Numerical Simulations of Electromigration and Stressmigration Driven Void Evolution in Solder Interconnects

Understanding the effect of high current density on void formation and growth and relating the size of the void to the resulting electrical/mechanical failure is a critical need at the present time to ensure reliable functioning of flip-chip packages. In general, toward this end, the modeling and simulation of geometrical evolution of current induced voids have been relatively few. Simulations considering the coupled effects of mass transport through mechanisms of surface and bulk diffusion under the influence of electrical, thermal, and stress fields in solder joints leading to eventual electromigration failure do not appear to be common. In this study, we develop a phase field model for the evolution of voids under electrical, thermal, and stress fields in a flip-chip solder interconnect. We derive the equations of motion for the void accounting for energetic contributions from the active factors of surface energy, stress, and electric potential, considering both surface diffusion and transfer of the material through the bulk of the material. We describe the implementation of this model using a finite element code written in the PYTHON language, coupled with a commercial finite element solver from which we obtain the electrical, thermal, and stress fields driving the void motion. We demonstrate the implemented methodology through simulations of void evolution in flip-chip solder joints under the effects of mechanical/electrical fields and surface/bulk diffusion.

Applying the Kelvin probe to biological tissues: Theoretical and computational analyses

The Kelvin probe measures surface electrical potential without making physical contact with the specimen. It relies on capacitive coupling between an oscillating metal tip that is normal to a specimen's surface. Kelvin probes have been increasingly used to study surface and electrical properties of metals and semiconductors and are capable of detecting material surface potentials with submillivolt resolution at a micrometer spatial scale. Its capability for measuring electrical potential without being confounded by electrode-specimen contact makes extending its use towards biological materials particularly appealing. However, the theoretical basis for applying the Kelvin probe to dielectric or partially conductive materials such as biological tissue has not been evaluated and remains unclear. This study develops the theoretical basis underlying Kelvin probe measurements in five theoretical materials: highly conductive, conductive dielectric with rapid charge relaxation, conductive dielectric with slow charge relaxation, perfect dielectric, and tissue with a bulk serial resistance. These theoretically derived equations are then computationally analyzed using parameters from both theoretical specimens and actual biomaterials-including wet skin, dry skin, cerebrospinal fluid, and tendon. Based on these analyses, a Kelvin probe performs in two distinct ways depending on the charge relaxation rates of the sample: The specimen is treated either as a perfect dielectric or as highly conductive material. Because of their rapid relaxation rate and increased permittivity biomaterials behave similarly to highly conductive materials, such as metal, when evaluated by the Kelvin probe. These results indicate that the Kelvin probe can be readily applied to studying the surface potential of biological tissue.

Optical properties of a polydispersion of small charged cosmic dust particles

Recent studies of extended red emissions suggest the presence of very small dust particles in the Universe. The sizes of these particles can be some nanometers, thus placing the theoretical problem of their light scattering within the Rayleigh regime. Assuming the wavelength varies over visible or near infrared spectral bands, the efficiency factor for extinction Qext can be written as a power expansion based on size parameter x. Rayleigh theory dictates that the first term dominates; thus, Qext is either proportional to x for absorbing particles, or x4 if the particles are composed of non-absorbing materials. In the latter case, Qext is equivalent to the scattering efficiency Qsca. However, the optical response of charged particles can differ from that of uncharged particles, especially if the surface electric potential is sufficiently large or if resonance conditions are fulfilled. In this study we compare the optical properties of a polydispersion of electrically charged and electrically neutral particles. The resulting optical thickness of such particle ensembles is derived in analytical form and the numerical computations are made for two particle distributions, specifically the power law and modified gamma function. One of the important findings is that the contribution of the smallest nanometer-sized particles to the total optical thickness may be underestimated if the effects of the electric charges are ignored. The distinct optical behavior of these particles also can have an impact on the emission features, since their different optical absorption implies a different temperature of the particle. Finally, the scattering phase function is weakly dependent on electric charges of the scattering particles.

Comparison of Molecular Dynamics with Classical Density Functional and Poisson-Boltzmann Theories of the Electric Double Layer in Nanochannels.

Comparisons are made among Molecular Dynamics (MD), Classical Density Functional Theory (c-DFT), and Poisson–Boltzmann (PB) modeling of the electric double layer (EDL) for the nonprimitive three component model (3CM) in which the two ion species and solvent molecules are all of finite size. Unlike previous comparisons between c-DFT and Monte Carlo (MC), the present 3CM incorporates Lennard-Jones interactions rather than hard-sphere and hard-wall repulsions. c-DFT and MD results are compared over normalized surface charges ranging from 0.2 to 1.75 and bulk ion concentrations from 10 mM to 1 M. Agreement between the two, assessed by electric surface potential and ion density profiles, is found to be quite good. Wall potentials predicted by PB begin to depart significantly from c-DFT and MD for charge densities exceeding 0.3. Successive layers are observed to charge in a sequential manner such that the solvent becomes fully excluded from each layer before the onset of the next layer. Ultimately, this layer filling phenomenon results in fluid structures, Debye lengths, and electric surface potentials vastly different from the classical PB predictions.

Potential Steps at C60–TiOPc–Ag(111) Interfaces: Ultrahigh-Vacuum–Noncontact Scanning Probe Metrology

Nanoscale structure-electric potential relations in films of the organic molecular semiconductors C(60) and titanyl phthalocyanine (TiOPc) on Ag(111) have been measured under UHV conditions. Noncontact force methods were utilized to image domain structures and boundaries with molecular resolution, while simultaneously quantifying the local surface electric potential. Sensitivity and spatial resolution for the local potential measurement were first established on Ag(111) through direct observation of the electrical dipole and potential step, φ(step) = 10 ± 3 mV, of monatomic crystallographic steps. A local surface potential increase of 27 ± 11 mV occurs upon crossing the boundary between the neat Ag(111) surface and C(60) islands. Potential steps in binary C(60)-TiOPc films, nanophase-separated into crystalline C(60) and TiOPc domains, were then mapped quantitatively. The 207 ± 66 mV potential step across the C(60)-to-TiOPc domain boundary exhibits a 3.6 nm width that reflects the spatial resolution for electric potential across a material interface. The absence of potential asymmetry across this lateral interface sets the upper bound for the C(60)-TiOPc interface dipole moment as 0.012 e nm.

XPS Investigation of a CdS-Based Photoresistor under Working Conditions: Operando–XPS

A noncontact chemical and electrical measurement X-ray photoelectron spectroscopy (XPS) technique is performed to investigate a CdS-based photoresistor during its operation. The main objective of the technique is to trace chemical- and location-specified surface potential variations as shifts of the XPS Cd 3d(5/2) peak position without and under photoillumination with four different lasers. The system is also modeled to extract electrical information. By analyzing the measured potential variations with this model, location-dependent resistance values are represented (i) two dimensionally for line scans and (ii) three dimensionally for areal measurements. In both cases, one of the dimensions is the binding energy. The main advantage of the technique is its ability to assess an element-specific surface electrical potential of a device under operation based on the energy deviation of core level peaks in surface domains. Detection of the variations in electrical potentials and especially their responses to the energy of the illuminating source in operando, is also shown to be capable of detecting, locating, and identifying the chemical nature of structural and other types of defects.

X-ray photoelectron spectroscopy of fast-frozen hematite colloids in aqueous solutions. 4. Coexistence of alkali metal (Na+, K+, Rb+, Cs+) and chloride ions

Colloidal suspensions of hematite in contact with aqueous solutions of 50mM alkali metal chloride electrolytes (NaCl, KCl, RbCl, CsCl) were investigated by cryogenic X-ray photoelectron spectroscopy (XPS) and electrophoretic mobility. Suspension pH values were varied from 2 to 11 in order to evaluate effects of positively- and negatively-charged hematite surfaces. XPS revealed coexisting cations and chloride ions both below and above the point of zero charge. Concentration profiles of adsorbed cations point to a Hofmeister series in the order of Na+>K+>Rb+≈Cs+. Binding energies of photoelectrons emitted from electrolyte ions increased with pH at roughly 0.04eV per pH unit. This shift was attributed to variations in the surface electric potential of hematite. This effect, compounded by rises in aliphatic carbon signals with pH, called for referencing of all spectra to the 530.0eV oxide component of the hematite O1s spectrum. This departure from the traditional use of the external C 1s 285.0eV peak is hereby proposed for cryogenic XPS studies of interfacial reactions involving hematite.

Jasmonic acid accumulation and systemic photosynthetic and electrical changes in locally burned wild type tomato, ABA-deficient sitiens mutants and sitiens pre-treated by ABA

Burning the terminal leaflet of younger tomato (Lycopersicon esculentum Mill.) leaf caused local and systemic changes in the surface electrical potential (SEP) and gas exchange (GE) parameters. The local and systemic accumulation of endogenous abscisic acid (ABA) and jasmonic acid (JA) was measured 85min after burning. The experiments were conducted with wild type (WT) plants, ABA-deficient mutant sitiens (SIT) and ABA pre-treated SIT plants (SITA). First changes in SEP were detected within 1.5min after burning and were followed by a decrease in GE parameters within 3-6min in WT, SIT and SITA plants. GE and SEP time courses of SIT were different and wave amplitudes of SEP of SIT were lower compared to WT and SITA. ABA content in WT and SITA control plants was similar and substantially higher compared to SIT, JA content was similar among WT, SIT and SITA. While changes in the ABA content in systemic leaves have not been recorded after burning, the systemic JA content was substantially increased in WT and more in SIT and SITA. The results suggest that ABA content governs the systemic reaction of GE and the SEP shape upon local burning. ABA, JA and SEP participate in triggering the GE reaction. The ABA shortage in the SIT in the reaction to burning is partly compensated by an enhanced JA accumulation. This JA compensation is maintained even in SIT endogenously supplied with ABA. A correlation between the systemic JA content and changes in GE parameters or SEP was not found.

Langmuir Monolayer Characteristics of Erucylphosphocholine - A Novel Anti-Tumor Drug

Erucylphosphocholine, an alkylphosphocholine anticancer drug, was employed for Langmuir monolayer characterization and liquid crystalline studies. Differential scanning calorimetry measurements together with texture observation with polarizing microscope revealed the presence of nematic phase. Film forming properties of erucylphosphocholine at the air/water interface were thoroughly investigated by means of surface pressure–area (π–A) and electric surface potential–area (∆V –A) isotherms. The influence of such factors as subphase temperature, ionic strength, speed of compression, number of molecules spread at the surface on the characteristics of the π–A isotherms was investigated. Erucylphosphocholine was found to form very stable Langmuir monolayers, which are almost not influenced by experimental conditions. The liquid character of its monolayers was confirmed with both compressibility modulus values and homogeneous Brewster angle microscopy images.

Recombinant Cannabinoid Type 2 Receptor in Liposome Model Activates G Protein in Response to Anionic Lipid Constituents

Human cannabinoid type 2 (CB(2)) receptor expressed in Escherichia coli was purified and successfully reconstituted in the functional form into lipid bilayers composed of POPC, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS), and cholesteryl hemisuccinate (CHS). Reconstitution was performed by detergent removal from the protein/lipid/detergent mixed micelles either on an adsorbent column, or by rapid dilution to below the critical micelle concentration of detergent followed by removal of detergent monomers on a concentrator. Proteoliposomes prepared at a protein/phospholipid/CHS molar ratio of 1/620-650/210-220 are free of detergent as shown by (1)H NMR, have a homogeneous protein/lipid ratio shown by isopycnic gradient ultracentrifugation, and are small in size with a mean diameter of 150-200 nm as measured by dynamic light scattering. Functional integrity of the reconstituted receptor was confirmed by quantitative binding of (2)H-labeled agonist CP-55,940-d(6) measured by (2)H magic angle spinning NMR, as well as by activation of G protein. The efficiency of G protein activation by agonist-bound CB(2) receptor was affected by negative electric surface potentials of proteoliposomes controlled by the content of anionic CHS or POPS. The activation was highest at an anionic lipid content of about 50 mol %. There was no correlation between the efficiency of G protein activation and an increase of hydrocarbon chain order induced by CHS or cholesterol. The results suggest the importance of anionic lipids in regulating signal transduction by CB(2) receptor and other class A GPCR. The successful reconstitution of milligram quantities of pure, functional CB(2) receptor enables a wide variety of structural studies.

Theory of SAW Gas Sensor based on Bi-layer Conductivity Changes

The elemental theory of surface acoustic wave (SAW) gas sensor principles is reported within this work. The sensor mechanism is generally based on changes in conductivity of a bi-layer thin sensor structures. In turn, the interaction between electrical surface potential is associated with SAW propagating on piezoelectric and mobile electric charges in bi-layer sensor structures of different electrical conductivities and thicknesses. Both films are very thin in comparison to SAW wavelength and Debye screening length. This allows on one-dimensional treatment of the problem by perturbation theory.

Surface engineering of gold nanoparticles for in vitro siRNA delivery

Cellular uptake, endosomal/lysosomal escape, and the effective dissociation from the carrier are a series of hurdles for specific genes to be delivered both in vitro and in vivo. To construct siRNA delivery systems, poly(allylamine hydrochloride) (PAH) and siRNA were alternately assembled on the surface of 11.8 ± 0.9 nm Au nanoparticles (GNP), stabilized by denatured bovine serum albumin, by the ionic layer-by-layer (LbL) self-assembly method. By manipulating the outmost PAH layer, GNP-PAH vectors with different surface electric potentials were prepared. Then, the surface potential-dependent cytotoxicity of the resultant GNP-PAH particles was evaluated via sulforhodamine B (SRB) assay, while the surface potential-dependent cellular uptake efficiency was quantitatively analyzed by using the flow cytometry method based on carboxyfluorescein (FAM)-labeled siRNA. It was revealed that the GNP-PAH particles with surface potential of +25 mV exhibited the optimal cellular uptake efficiency and cytotoxicity for human breast cancer MCF-7 cells. Following these results, two more positively charged polyelectrolytes with different protonating abilities in comparison with PAH, i.e., polyethylenimine (PEI), and poly(diallyl dimethyl ammonium chloride) (PDDA), were chosen to fabricate similarly structured vectors. Confocal fluorescence microscopy studies indicated that siRNA delivered by GNP-PAH and GNP-PEI systems was better released than that delivered by the GNP-PDDA system. Further flow cytometric assays based on immunofluorescence staining of the epidermal growth factor receptor (EGFR) revealed that EGFR siRNA delivered by GNP-PAH and GNP-PEI exhibited similar down-regulation effects on EGFR expression in MCF-7 cells. The following dual fluorescence flow cytometry assays by co-staining phosphatidylserine and DNA suggested the EGFR siRNA delivered by GNP-PAH exhibited an improved silencing effect in comparison with that delivered by the commercial transfection reagent Lipofectamine 2000.

Change in Zeta Potential with Physicochemical Treatment of Surface of Anatase-form Titania Particles

64 INTRODUCTION Adsorption between the surface of a material and proteins or peptides is primarily influenced by hydrophobic, electrostatic, and van der Waals interactions. The hydrophobicity of a material surface can be determined by measuring contact angle, surface roughness, and surface free energy. It is difficult, however, to determine hydrophilicity by measuring the surface electric potential. Chemical reaction occurs through contact between molecules. Being able to determine the electric charge of a material surface based on intermolecular interactions would be beneficial in evaluating intermolecular adsorption such as between a material Change in Zeta Potential with Physicochemical Treatment of Surface of Anatase-form Titania Particles

Diffusion of Nanoparticles at an Air/Water Interface Is Not Invariant under a Reversal of the Particle Charge

The diffusion of charged nanoparticles at an aqueous/air interface is not invariant under a charge reversal of the particles. Negatively charged particles slow down with the ionic strength of the aqueous phase, while positively charged particles speed up. The diffusion constant of the particles reflects their immersion into the aqueous/air interface. We argue that the opposing behavior of oppositely charged particles is proof that the immersion depth of the particles scales with the contrast in electric surface potential of the particle to the electric surface potential of the air/water interface, not with the particle’s charge density. We therefore propose to incorporate the potential drop across the air/water interface into theories of electrodipping.

Determination of Anisotropic Surface Characteristics of Different Phyllosilicates by Direct Force Measurements

To fundamentally understand the electrokinetic behavior of clay minerals, it is necessary to study the anisotropic surface charge properties of clay surfaces. In this study, two 2:1 layer natural minerals, talc and muscovite, were chosen as representatives of magnesium and aluminum phyllosilicate minerals, respectively. The molecularly smooth basal planes of both platy minerals were obtained by cleavage along the basal planes, while suitable edge surfaces were prepared by an ultramicrotome cutting technique. Silicon nitride atomic force microscopy tip was used as a probe to study the interaction forces between the tip and clay basal/edge surfaces in aqueous solutions of various pH values. The measured interaction force profiles between the tip and clay basal/edge surfaces were fitted with the classical DLVO (Derjaguin-Landau-Verwey-Overbeek) theory, which allows direct determination of electrical surface potential of talc and muscovite surfaces. The surface potential of muscovite basal planes was found to be significantly more negative than the basal plane of talc, both being pH insensitive. In contrast, the surface potential of edge surfaces was highly pH-dependent, exhibiting a point of zero charge (PZC) at pH 7.5 and 8.1 for edges of muscovite and talc, respectively. The observed differences in surface potential of basal planes and edge surfaces for both talc and muscovite are closely related to their crystal structure and ionization characteristics. The protonation reactivity and the contribution of each surface group to the surface charging behavior are modeled using their protonation constants.

Electromagnetic scattering by a polydispersion of small charged cosmic dust particles

Some recent studies on extended red emissions suggest the presence of very small dust particles in the Universe. The sizes of these particles vary from 1 nm to some tens of nanometers, thus situating them deeply in the Rayleigh region if computations are made for visible or near infrared. The optical response of such particles can be a function of the surface charge. In this study we analyse the effect of surface electric potential on the total optical thickness and scattering phase function of the cosmic dust particles. The results are compared with those obtained for electrically neutral dust.

Soil particle surface electrochemical property effects on abundance of ammonia-oxidizing bacteria and ammonia-oxidizing archaea, NH 4+ activity, and net nitrification in an acid soil

Soil surface electrochemical properties may have a strong influence on nitrifying microorganisms, H + and NH 4 + activities, and therefore on the nitrification process. A gradient of surface electrochemical parameters was obtained by amendment of a subtropical acid pine soil (Oxisol) with 0% (control), 3%, 5%, 8%, 10% and 12% pure Ca-Montmorillonite by weight. The H + and NH 4 + activities, the abundance of the ammonia-oxidizing bacterial (AOB) and archaeal (AOA) amoA gene copies, and time-dependent kinetics of net nitrification were investigated. Soil particle surface specific area ranged from 53 to 103 m 2 g −1 and increased with increasing montmorillonite application rate. Similar to specific area, surface charge quantity, surface charge density, electric field strength and surface potential increased after montmorillonite amendment. The H + and NH 4 + activities decreased linearly after montmorillonite addition. AOB amoA gene copy number was 1.82 × 10 5 copies g −1 for unamended soil, and the highest AOB amoA gene copy numbers were found for the 10% montmorillonite amendment (3.11 × 10 7 g −1 soil), which was more than 150 times higher than unamended soil. AOA amoA gene copy numbers were 9.19 × 10 3 copies g −1 dry unamended soil, and the highest AOA amoA gene copy numbers were found in the 8% montmorillonite amendment (1.22 × 10 5 g −1 soil). Although pH significantly decreased during the first three weeks of incubation, no significant difference was observed between the unamended control and different rates of montmorillonite addition treatments during the whole incubation. The largest net nitrification (103 mg N kg −1) was observed in the 10% montmorillonite amendment and the lowest in unamended soil (62 mg N kg −1). While montmorillonite did not change the kinetic patterns of net nitrification, the highest nitrification potential (275 mg N kg −1) for the 10% montmorillonite treatment was more than 3 times higher than unamended soil from simulation of time-dependent kinetics. Nitrification was significantly stimulated after montmorillonite amendment in acid soil mainly due to an increase in the quantity and activity of AOB and AOA. We concluded that soil particle surface parameters can significantly influence nitrification, especially in acid soils.

Insights into bacterial contact angles: Difficulties in defining hydrophobicity and surface Gibbs energy

One of the principal techniques for evaluating the surface hydrophobicity of biological samples is contact angle. This method, applied readily to flat-smooth-inert surfaces, gives rise to an important debate when implemented with microbial lawns. After its initial description, in 1984, several authors have carried out modifications of the technique but the results obtained have not been previously judged. This work focuses on the particularities of contact angle measurements on bacterial lawns and enhances the idea of the impossibility of using water contact angle as a universal measurement of bacterial hydrophobicity. Contact angles can only be used as relative indicators of hydrophobicity, in a similar way to tests based on microbial adhesion to solvents. The strong dependence of contact angles on dried bacterial lawns with measuring time and environmental conditions (e.g. pH of the media) preclude the estimation of their absolute values, and so, of the cells surface Gibbs energy. Yet, for a given measuring time, it is found that the hydrophobicity and the apparent bacterial surface Gibbs energy components are qualitatively related to the bacterial surface electrical potential. In particular, a hydrophobic increase is always accompanied by an increase of the cells Lifshitz-Van der Waals component and a decrease of their acid-base component and absolute zeta potential. Therefore, the present study shows that the physico-chemical surface properties that characterize bacteria are not independent, and one of them can be qualitatively described in terms of the others when measuring contact angles at a fixed time after the drying of the microbial beds.

Computational modeling of the electric potential in biological membrane. A comparison between healthy and cancerous neurons

We have studied the behavior of the electric potential profile across the membrane of the ganglion neuron and the neuroblastoma cell. We considered the physicochemical conditions during the resting and action potential (AP) states of the neuronal cells, and analyzed the influence of the fixed charges of the membrane on the surface electric potential of these cells, based on values of electric parameters obtained from experimental results. The ganglion neuron portrays a healthy neuron and the neuroblastoma cell, which is a tumorous cell, represents a pathologic neuron. We solved the nonlinear Poisson-Boltzmann equation, by considering the volumetric charge densities due to charges dissolved in an electrolytic solution, and also charges fixed on both glycocalyx and cytoplasmic proteins. Data obtained from experimental results [1,2] have been used to solve the first order ordinary differential Poisson-Boltzmann equations [3,4], which have been obtained for the regions of the membrane model we have adopted. Therefore, we have examined the influence of the electric parameters during resting and AP states, analyzing the differences between the healthy ganglion neuron and a neuroblastoma cell. We implemented an algorithm for finding roots of functions with a program in C language, to calculate the surface potentials between the glycocalyx and the bilayer, and between the bilayer and the cytoplasm, regions of the membrane. The surface potential between the electrolytic region and the glycocalyx has been calculated from electrophoretic experimental data. We have calculated the potential profiles using the RungeKutta method, also implemented in C language. We analyzed the electric potential profile across the membranes. Comparing both neuronal types, we verified that the gradual potential fall from the electrolytic region to the surface of glycocalyx is higher for the ganglion neuron than for the neuroblastoma cell, and both curve shapes are similar. Through the glycocalyx we could see that the fall continues for the ganglion neuron, but this fall is negligible for the cancerous cell. The intracellular potential, during the resting state, increases exponentially from the bilayer surface to the bulk cytoplasmic region. During the resting state, the net value of the protein charge in the cytoplasm is predominantly negative [4]. However, in our neuron model the contribution of these charges to the inner potential profile was smaller than the effect of the fixed charges in the inner surface of the bilayer, due to the curvature of the potential in this region, whereas the calculated potential on the surface between the bilayer and the cytoplasm was smaller than the potential in the bulk region. It is known that the neuroblastoma cells, like all the other cancerous cells, multiply themselves quickly. Alterations of the dynamics of the cellular multiplication cause changes in the synthesis, structure and promotes degradation of the membrane components [5], which results in deformations in structure and composition of the plasma membrane. Conclusions Our results may also contribute to the understanding of the neuroblastoma resistance to certain chemotherapeutic treatments. The small change of the surface potential as a response to changes in the culture (pH, for instance) and in the fixed electric charges, due to alterations in the composition and structure of the