Determination Of Surface Charge Research Articles

A Hybrid Molecular Dynamics/Multiconformer Continuum Electrostatics (MD/MCCE) Approach for the Determination of Surface Charge of Nanomaterials

The surface charge of nanomaterials determines their stability in solution and interaction with other molecules and surfaces, yet experimental determination of surface charge of complex nanomaterials is not straightforward. We propose a hybrid approach that iteratively integrates explicit solvent molecular dynamics simulations and a multiconformer continuum electrostatic model (MCCE) to efficiently sample the configurational and titration spaces of surface ligands of nanomaterials. Test calculations of model systems indicate that the iterative approach converges rapidly even for systems that contain hundreds of titratable sites, making the approach complementary to more elaborate methods such as explicit solvent-based constant-pH molecular dynamics. The hybrid method is applied to analyze the pKa distribution of alkylamines attached to a carbon-based nanoparticle as a function of ligand density, nanoparticle surface curvature, and ligand heterogeneity. The results indicate that functionalization strategie...

2-Hydroxyoleic acid-inserted liposomes as a multifunctional carrier of anticancer drugs

Studies have shown that insertion of oleic acid into lipid bilayers can modulate the membrane properties of liposomes so as to improve their function as drug carriers. Considering that 2-hydroxyoleic acid (2OHOA), a potential antitumor agent currently undergoing clinical trials, is a derivative of oleic acid, we explored the possibility of developing 2OHOA-inserted liposomes as a multifunctional carrier of antitumor drugs in the present study. The insertion of 2OHOA into lipid bilayers was confirmed by surface charge determination and differential scanning calorimetry. 2OHOA insertion greatly decreased the order of dimyristoylphosphatidylcholine packing, produced a nanosized (<100 nm) dispersion, and improved the colloidal stability of liposomes during storage. Moreover, 2OHOA-inserted liposome forms exhibited greater growth inhibitory activity against cancer cells compared with free 2OHOA, and the growth-inhibitory activity of liposomal 2OHOA was selective for tumor cells. 2OHOA insertion greatly increased the liposome-incorporated concentration of hydrophobic model drugs, including mitoxantrone, paclitaxel, and all-trans retinoic acid (ATRA). The in vitro anticancer activity of ATRA-incorporated/2OHOA-inserted liposomes was significantly higher than that of ATRA-incorporated conventional liposomes. In a B16-F10 melanoma syngeneic mouse model, the tumor growth rate was significantly delayed in mice treated with ATRA-incorporated/2OHOA-inserted liposomes compared with that in the control group. Immunohistochemical analyses revealed that the enhanced antitumor activity of ATRA-incorporated/2OHOA-inserted liposomes was due, at least in part, to increased induction of apoptosis. Collectively, our findings indicate that 2OHOA-inserted liposomes exhibit multiple advantages as antitumor drug carriers, including the ability to simultaneously deliver two anticancer drugs – 2OHOA and incorporated drug – to the tumor tissue.

Laser Doppler Electrophoresis and electro-osmotic flow mapping: A novel methodology for the determination of membrane surface zeta potential

A novel technique employing an Uzigirs dip cell arrangement is used in conjunction with Laser Doppler Electrophoresis for the determination of the surface zeta potential for a UF, NF, and RO membrane. To the authors best knowledge this is the first study employing Laser Doppler Electrophoresis and Electro-osmotic Flow Mapping for membrane surface charge determination. High correlation of the regression fit (R2>0.95) for a carboxylated polystyrene latex particle electrophoretic mobility was achieved at low electrolyte concentrations (1mM and 10mM NaCl), but the reliability and accuracy of the extrapolated zeta potential values were problematic at higher concentration due to high measurement uncertainty (>10% in some cases). Changes in the applied electric field increased the phase resolution of 50mM NaCl electrolyte solutions between 0.5–2.0V. However, the effects of Joule heating at higher voltages compromised 50mM NaCl sample integrity. When compared with the established Tangential Streaming Potential method, Laser Doppler Electrophoresis measurements provided similar zeta potential values and trends indicating that this new methodology can indeed be employed for membrane characterization purposes; however, further research needs to be conducted in order to optimize this new technique and set appropriate operating limits.

Stimulation of bone regeneration with pigment epithelium-derived factor microparticles: evidence in silico, in vitro and in vivo.

The occurrence of bone defects can be due to a variety of factors not limited to bone fractures and tumours. Most diseased bone is removed and the patient fitted with prosthetics, prior to use of certain factors such as bone morphogenetic proteins (BMPs) to aid healing. Recently, the protein pigment epithelium-derived factor (PEDF) and the polysaccharide chitosan have been found to have promising effects on the regeneration of bone, with the major advantage of these agents being their safety to date. A study was performed to determine whether the combination of both chitosan and PEDF would enhance greater bone regeneration effects. Post-formulation, in silico tests (particle sizing and surface charge determination) were followed by several cell-based assays (microparticle cellular uptake, cytotoxicity, mitochondrial abundance, bone mineral formation, colony formation in matrigel, and colony formation in collagen I matrix), and finally in vivo testing where microparticles were injected periosteally in the hindlimb. Collectively, these findings support the idea that PEDF microencapsulated within chitosan promotes bone regeneration, and has potential for bone trauma management. Future studies will examine the ability of this promising bone regeneration microparticle to heal bone in disease states such as fracture and tumour-mediated osteolysis.

Injectable Liposomal Formulations: Systematic Analysis for Regulatory Purposes

Background: Among the drug delivery systems, liposomes stand out for their biocompatible and biodegradable characteristics and high versatility. There are many marketed liposomal drug products, most of them injectables. From a regulatory perspective, some documents were published by FDA and EMA, but a systematic analysis of the factors influencing efficacy, toxicity and stability of injectable liposomal drug products has not yet been carried out in order to identify regulatory relevant items. Method: We use cause and effect diagrams (Ishikawa) to provide knowledge for regulatory action and to support the development of regulation or guidelines containing specific requirements for injectable liposomal drug products, especially with regard to their quality control and stability. Result: It was possible to identify the critical quality attributes to the efficacy, safety and stability of injectable liposomal drug products, and some of the tests to be considered in their characterization, quality control and stability. Some important aspects of the main analytical methods used for particle size and electric surface charge determination were described. An analysis of some specific topics related to Good Manufacturing Practices of injectable liposomal drug products is presented. Conclusion: The documents published by FDA and EMA do not include all the items that need to be assessed for injectable liposomal drug products quality, and do not address analytical methods for quality attributes evaluation. It is urgent to establish regulatory requirements covering these products. Keywords: Analytical methods, characterization, efficacy, liposomes, regulation, stability, toxicity.

Anomalous Transit Time and Pulse Amplitude of Highly Charged Particles in Resistive Pulsing

With resistive pulse technology, three types of particles with similar size but different surface charge densities have been investigated in track-etched polyethylene terephthalate (PET) single micropores. We have shown that pores with axially undulating diameter constitute a sensitive tool for spherical particles’ detection and sizing. Shape of resistive pulses carries information on the shape and charge of passing particles, as well as shape and charge of pores. Passage of single particles causes a decrease of the recorded current whose amplitude is correlated with the particle volume. However, our experiments have revealed, the blockage current of the strongly charged particles can be significantly larger than those from the other two types of particles and this observation cannot be described by the classical resistive-pulse model. This finding is also in contrast to earlier reports showing that pulse amplitude created by a charged molecule/particle is smaller compared to an uncharged particle of the same volume. Our results are explained by the distortion of the electrical double layer adjacent to the charged particle, which create a region with depleted ionic concentrations. We have also found that the particle passage time results from an interplay of three effects: (i) ion condensation, (ii) formation of an asymmetric electrical double layer around the particle, and (iii) electroosmotic flow induced by the charges on the pore walls and the particle surfaces. Our results are important for applying resistive-pulse technique to determine surface charge density and zeta potential of the particles. This research has implications for separation between particles with different surface charge properties, particle detection and sizing, and for the fundamental understanding of fluid flow at the meso-scale.

Anomalous Mobility of Highly Charged Particles in Pores

Single micropores in resistive-pulse technique were used to understand a complex dependence of particle mobility on its surface charge density. We show that the mobility of highly charged carboxylated particles decreases with the increase of the solution pH due to an interplay of three effects: (i) ion condensation, (ii) formation of an asymmetric electrical double layer around the particle, and (iii) electroosmotic flow induced by the charges on the pore walls and the particle surfaces. The results are important for applying resistive-pulse technique to determine surface charge density and zeta potential of the particles. The experiments also indicate the presence of condensed ions, which contribute to the measured current if a sufficiently high electric field is applied across the pore.

Biobased polymers and cationic microfibrillated cellulose as retention and drainage aids in papermaking: Comparison between softwood and bagasse pulps

Water soluble cationic chitosan (C-Ch) and surface-modified microfibrillated cellulose (C-MFC) were prepared and characterized. C-MFC was prepared by reaction of MFC (microfibrillated cellulose) with β-chloroethyldiethylamine followed by quaternization using methyl iodide while C-Ch was prepared by reaction of a commercial chitosan (Ch) with 2,3-epoxypropyltrimethyl ammonium chloride. C-Ch and C-MFC were characterized by elemental analysis (nitrogen content), Fourier transform infrared spectroscopy (FTIR) and surface charge determination. The prepared C-Ch and C-MFC were used with bentonite in order to improve drainage and filler retention in paper handsheets made from softwood and bagasse pulps. Dosage rates were 0.05–0.2% (w/w, weight of additive by weight of fibres) and 0.05–1% (w/w) for C-Ch and C-MFC, respectively. When used alone, these polymers did not induce any noticeable effect on drainage but they improved the retention of ground calcium carbonate (GCC). The effect of C-MFC/bentonite and C-Ch/bentonite systems on filler retention was more pronounced in case of bagasse pulp than for softwood pulp. The use of C-Ch improved filler retention more than C-MFC did. Comparisons with a commercially available cationic polyacrylamide (CPAM) showed that CPAM was the most efficient additive regarding both drainage and GCC retention, followed by Ch, C-Ch, and finally C-MFC.

The determination of surface charge on nitrogen-containing reduced graphene oxides and its application to adsorb molybdate

To speculate the performance of chemically reduced graphene oxide from different reductants, detailed investigation of chemical structures needs diverse characterizations. Surface charge, mainly resulting from surface functional groups, may provide a facile method to comprehensively predict performance on certain levels. Surface charge of hydrazine-reduced graphene oxide (HrGO) and p-phenylenediamine (p-PDA)-reduced graphene oxide (ArGO) was investigated by cyclic voltammetry. The charge generation mechanism was further analysed by X-ray photoelectron spectroscopy. Electrical impedance spectroscopy was applied to investigate the effect of surface charge on the adsorptive properties. Furthermore, the experiments on MoO42− adsorbed by ArGO were carried out at different pH values. Results indicate that the surface charge on ArGO was more positive than that on HrGO, which is mainly due to protonation of amino group. MoO42− adsorption experiments indicate that relative positive charge made ArGO more efficient than HrGO to adsorb MoO42−.

Concentration dependence of nanochannel impedance and the determination of surface charge.

In this paper, we demonstrate the variation of nanochannel impedance with bulk (reservoir) electrolyte concentration. The impedance of a nanochannel is shown to correspond to a characteristic deformed semicircular arc. The degree of deformation decreases with increasing concentration, and at a sufficiently low concentration the complex impedance saturates, becoming essentially independent of the reservoir concentration. This behavior is indicative of a surface-conduction dominant regime. Here we demonstrate that this effect extends beyond dc conductance and affects the ac response of the system as well, including both phase relationship and magnitude. The nanochannel resistance, obtained from low-voltage ac measurements, is then used to extract the nanochannel surface charge density. This is found to increase in magnitude with increasing electrolyte concentration.

Novel insights into the development of barrier discharges by advanced volume and surface diagnostics

The comprehensive characterization of microdischarges (MDs) requires complementary diagnostics of volume and surface processes at the same discharge configuration under identical conditions. This contribution summarizes the results from optical, spectroscopic and electric investigations as well as the determination of surface charges and metastable nitrogen molecules in filamentary and diffuse barrier discharges. The feasibility of such an approach is demonstrated on selected examples.Fast optical and spectroscopic methods are reviewed for the example of a pulsed driven single filament dielectric barrier MD. It is demonstrated that the methods of streak recording and cross-correlation spectroscopy can complement one another for a comprehensive study of the MD development. Using these techniques it is shown that the so-called prephase is present also in sub-microsecond pulsed barrier discharges. The excitation starts directly with the voltage increase. In the case of diffuse barrier discharges in nitrogen, the combination of spectroscopic and electrical characterization, surface charge measurement by the Pockels effect, and the determination of nitrogen metastables N2(A) by laser-induced fluorescence provides detailed knowledge about the time-integrated surface charge which correlates with the discharge current for each half cycle, whereas the temporal maximum of the metastables of the order of few 1013 cm−3 is delayed in relation to the current maximum. The spatial (axial) maximum of the metastable density is located near the anode like the emission maximum from N2 second positive system at λ = 337 nm. Furthermore, the lifetime of surface charges beyond a typical discharge period has been investigated.

Titrimetric methods for the determination of surface and total charge of functionalized nanofibrillated/microfibrillated cellulose (NFC/MFC)

Total and surface charge of three different carboxymethylated nanofibrillated/microfibrillated cellulose (NFC/MFC) samples were investigated by using titrimetric methods (conductometric and polyelectrolyte (PE) titrations). Conductometric titration was found to be suitable method for the NFC total charge measurements when the back titration with HCl was applied. Surface charge measurements of NFC/MFC were conducted by using both indirect and direct PE titrations. The direct PE titration was found to be a more suitable method for the surface charge determination of NFC/MFC whereas the indirect PE titration produced too high surface charge values. This is presumably due to kinetically locked polyelectrolyte conformations on the NFC/MFC surfaces or entrapment of residual polymer after adsorption onto the NFC/MFC gel network. Finally, NFC was propargyl-functionalized and the changes in surface and total charge were successfully monitored and compared to those of propargyl-functionalized pulp. A good correlation between the titrimetric methods and elemental analysis was observed.

Photo-induced switchable TiO2 thin films for biological applications

Titanium and its alloys are key materials for biomedical applications due to their unique combination of high biocompatibility and mechanical strength. The interface between implant and tissue can be improved by titanium oxide coatings. Titanium oxide thin films prepared by electron beam deposition with well-defined structure, phase composition, morphology and local surface chemistry have been used in a study of the correlation between physicochemical surface characteristics and their biocompatibility, which have been shown dependent on preparation method and deposition parameters. In this study, three independent measurement methods were used for the determination of surface groups and charge: X-ray photoelectron spectroscopy, electrokinetic and contact angle measurements. The behavior of the thin titanium oxide films exposed to fibroblast L929 cells was studied. The aim of this study was to functionalize the oxide surfaces by controlling their properties through either defects introduced using high-temperature annealing, UV-light and/or modifier atoms as well as by molecules adsorbed at the surface, in particular, the formation of hydroxide groups. It was found that UV-light drives a switchable biocompatible titanium oxide surface, changing the surface charge polarity which increases or decreases the wettability of the films and the growth of the cells on the film’s surface correspondingly.

Surface charge and interfacial potential of titanium dioxide nanoparticles: Experimental and theoretical investigations

Size dependent surface charging and interfacial potential of titanium dioxide (TiO2) nanoparticles are investigated by experimental and theoretical methods. Commercially available TiO2 (P25) nanoparticles were used for surface charge determinations by potentiometric titrations. Anatase particles, 10 and 22nm in diameter, were synthesized by controlled hydrolysis of TiCl4, and electrophoretic mobilities were determined at a fixed pH but at increasing salt concentrations. Corrected Debye–Hückel theory of surface complexation (CDH-SC) was modified to model the size dependent surface charging behavior of TiO2 nanoparticles. Experimentally determined surface charge densities of rutile and P25 nanoparticles in different electrolytes were accurately modeled by the CDH-SC theory. Stern layer capacitances calculated by the CDH-SC theory were in good agreement with the values found by the classical surface complexation approach, and the interaction of protons with OH groups is found to be less exothermic than for iron oxide surfaces. Moreover, the CDH-SC theory predicts that the surface charge density of TiO2 nanoparticles of diameter <10nm is considerably higher than for larger particles, and pH at the point of zero charge (pHPZC) shifts to higher pH values as the particle size decreases. The importance of including the particle size in calculating the zeta potentials from mobilities is demonstrated. Smoluchowski theory showed that 10nm particles had lower zeta potential than 22nm particles, whereas a reverse trend was seen when zeta potentials were calculated by Ohshima’s theory in which particle size is included. Electrokinetic charge densities calculated from zeta potentials were found to be only one third of the true surface charge densities.

Gene identification and comparative molecular modeling of a Trypanosoma rangeli major surface protease

Trypanosoma rangeli is a hemoflagellate parasite which is able to infect humans. Distinct from Trypanosoma cruzi, the causative agent of Chagas disease, T. rangeli is non-pathogenic to the vertebrate host. The manner by which the T. rangeli interacts with the host is still unknown, but it certainly depends on the surface molecules. Major surface proteins (MSP) are GPI-anchored, zinc-dependent metalloproteases present in the surface of all trypanosomatids studied so far, which are implicated as virulence factors in pathogenic trypanosomatids, such as Leishmania spp and T. cruzi. The aims of this work were to generate the complete sequence of a T. rangeli MSP (TrMSP) gene and to determine the 3D-structure of the predicted protein by homology modeling. The plasmid bearing a complete copy of a TrMSP gene was completely sequenced and the predicted protein was modeled using Modeller software. Results indicate that TrMSP open reading frame (ORF) codes for a predicted 588 amino acid protein and shows all elements required for its posttranslational processing. Multiple sequence alignment of TrMSP with other trypanosomatids' MSPs showed an extensive conservation of the N-terminal and central regions and a more divergent C-terminal region. Leishmania major MSP (LmMSP), which had its crystal structure previously determined, has an overall 35% identity with TrMSP. This identity allowed the comparative molecular modeling of TrMSP, which demonstrated a high degree of structural conservation between MSPs from other trypanosomatids (TrypMSPs). All modeled MSPs have a conserved folding pattern, apart from structural divergences in the C-domain and discrete differences of charge and topology in the catalytic cleft, and present the same geometry of the canonical HEXXH zinc-binding motif. The determination of surface charges of the molecules revealed that TrMSP is a predominantly positive protein, whereas LmMSP and Trypanosoma cruzi MSP (TcMSP) are negative proteins, suggesting that substrates recognized by TcMSP and LmMSP could not interact with TrMSP. Moreover, the comparison between TrMSP and TcMSP protein sequences has revealed 45 non-neutral amino acid substitutions, which can be further assessed through protein engineering. The characteristics of TrMSP could explain, at least in part, the lack of pathogenicity of T. rangeli to humans and point to the necessity of identifying the biological targets of this enzyme.

Novel permittivity test for determination of yeast surface charge and flocculation abilities

Yeast flocculation has been found to be important in many biotechnological processes. It has been suggested that flocculation is promoted by decreasing electrostatic repulsion between cells. In this study, we used an unconventional rapid technique—permittivity test—for determination of the flocculation properties and surface charge values of three industrial yeast strains with well-known flocculation characteristics: Saccharomyces cerevisiae NCYC 1017 (brewery, ale), S. pastorianus NCYC 680 (brewery, lager), and Debaryomyces occidentalis LOCK 0251 (unconventional amylolytic yeast). The measurements of permittivity were compared with the results from two classical methods for determination of surface charge: Alcian blue retention and Sephadex DEAE attachment. The permittivity values for particular strains correlated directly with the results of Alcian blue retention (r = 0.9). The results also confirmed a strong negative relationship between the capacitance of yeast suspensions and their flocculation abilities. The highest permittivity was noted for the ale strain NCYC 1017, with weak flocculation abilities, and the lowest for the flocculating lager yeast NCYC 680. This paper is the first to describe the possibility of using a rapid permittivity test to evaluate the surface charge of yeast cells and their flocculation abilities. This method is of practical value in various biotechnological industries where flocculation is applied as a major method of cell separation.

Surface charge characterization of metal oxides by potentiometric acid–base titration, revisited theory and experiment

The increased interest in architecting nanocomposites and in clarifying their impact on health and environment necessitates a standardized method of surface characterization, because the behaviour of nanoparticles is governed largely by their surface free energy and interfacial chemistry. Potentiometric acid–base titration has been used for decades to determine surface charge density of oxides, tremendously useful in understanding and modelling oxide/electrolyte interfaces. The method has solid theoretical and experimental foundation. Yet, published values of the point of zero charge (PZC) of chemically identical oxides scatter widely: the deviation of seven units of pH is the rule, rather than an exception. The PZC must reflect the inherent charging behaviour, linked exclusively to the protonation, and determined by the chemistry of the oxides. Assignment of PZC (just like assignment of equilibrium constant of any chemical reaction) requires strict conditions and consistent protocol. We present here such a protocol, and some examples of application. The protocol leads through the basic steps: pH buffer calibration, proton concentration calibration in the background electrolytes, and titrations of oxides. Built-in tests help to eliminate artificial side effects simultaneously from the measurement of pH and from the oxide/electrolyte interfacial equilibria. Although these may seem self-evident, literature survey proves the need to go back to basics in order to obtain precise, reliable and comparable pH-dependent charging of oxides. It is most general that the oxides contain some unrecognized surface impurities and have a strongly pH-dependent dissolution. These distort the results of titrations; their effect increases with decreasing particle size and can become dominant for nanoparticles. The protocol presented here contains the necessary feedback loops to easily recognize and exclude these side effects.

Synthesis and sorption performance of highly specific imprinted particles for the direct recovery of carminic acid

Carminic acid (CA) is a colorant of natural origin, which is demanded by the food, cosmetic, and pharmaceutical industries. In this work, a selective finite bath process was developed based on the utilization of molecularly imprinted polymeric particles. Such adsorbent was synthesized in a (porous) particle shaped form employing methacrylic acid (MAA) and 4-vinylpyridine (4Vpy) as monomers and ethylene glycol dimethacrylate (EDGMA) as a cross-linker. The imprinted particles were characterized by surface area, surface charge and pore size determination. The adsorption behavior of CA on such a material followed a Langmuir–Freundlich isotherm. Maximum capacity at equilibrium reached 0.64mmol/g (316mg/g) and maximum available binding sites 1.8mmol/g (917mg/g) were observed for an association coefficient value of 1.5mM−1. Further, the imprinting factor; showing the strength of interaction of CA to the polymer, was calculated as 13 while the selectivity factor depicted a value of 15. The data presented indicates that the imprinted adsorbent could be conveniently utilized for the recovery of CA, from cochineal extract, in the finite bath mode of operation.

Detailed characterization of calcium silicate precipitation tube (CaSPT) as a multi-cation adsorbent in aqueous medium

Calcium silicate precipitation tube (CaSPT) produced through ‘silica garden’ route has been physico-chemically characterized as a multi-cation adsorbent in aqueous medium. Characterizations include phase identification, morphology, surface area, pore volume, iso-electric point and surface charge determination, thermogravimetric analysis, functional group identification and initial assessment as a multi-cation adsorbent. These tubes are hierarchically built from smaller interconnected tubes of about 20nm diameter. Exterior and interior walls of the ‘as grown’ tubes, analyzed through SEM-EDAX, show different chemical composition. One of the prominent phases in ‘as grown’ tubes was identified as γ-Ca2SiO4. CaSPT has moderately high surface area. Iso-electric point (IEP) at pH 3.41 indicates the surface is suitable for adsorbing cationic species. Initial adsorption studies corroborate that CaSPT is a promising adsorbent for Cu(II), Cd(II), Zn(II), Pb(II) and Cr(III). CaSPT as an adsorbent is superior to conventional non tubular calcium silicate.

Potentiometric Titrations as a Tool for Surface Charge Determination

This article summarizes methods for determining proton surface charge at mineral/water inter- faces. It covers conventional experimental procedures and discusses problems with the techniques. Also it involves recommendations for obtaining reasonable and results. The term comparable re- sults refers to comparison between results for the same solid as obtained in different laboratories. The most important parameters for the surface titrations are discussed. We also propose a reference titration procedure that would allow direct, unbiased comparisons of experimental data. The article finally includes a check-list for researchers and reviewers which should allow limiting the amount of titration data that are not useful for future uses. (doi: 10.5562/cca2062)