Reversal Of Surface Charge Research Articles

Self-Regulated Assembly and Disassembly of Gold Nanoparticles for Low-Temperature Time Indication.

The color-changing self-assembly and autonomous disassembly of colloidal gold nanoparticles (AuNPs) is reported by simply mixing negatively charged phosphine ligand-capped AuNPs with partially oxidized polyethylene glycol (PEG). The assembly of AuNPs is initiated by PEG adsorption, which disrupts the hydration layer of AuNPs, leading to depletion attraction and reduction of hydration repulsion among the AuNPs. The oxidative species in PEG subsequently oxidize and remove the charged ligands from the AuNP surface, resulting in a decrease and reversal of the negative surface charge. This causes the PEG to adsorb on AuNPs in a tighter and more direct manner, providing strong steric shielding to the AuNPs, thereby triggering the disassembly of the AuNP assemblies. The self-regulated assembly-disassembly process can be tuned widely by controlling chemical conditions of PEG, nanoparticle concentration, and the environmental conditions, suggesting potential applications as colorimetric time-temperature indicators for food and medicine storage conditions. As a proof of concept, it is demonstrated that the lifetime of the color-changing assembly-disassembly process can be extended from tens of minutes to weeks when subjected to a refrigerated environment, with tunability achievable through varying polymer conditions and storage atmospheres.

Poly-L-lysine Coated Oral Nanoemulsion for Combined Delivery of Insulin and C-Peptide

An attempt of co-delivery of insulin and C-peptide enclosed in linseed oil globules has been made employing a protective coating of positively charged poly-L-lysine to manage diabetes-associated complications. Oral water in oil in water (w/o/w) nanoemulsion manufactured by double emulsification method showed good entrapment efficiency of 87.6 ± 7.48% for insulin and 73.4 ± 6.44% for C-peptide. The optimized uncoated nanoemulsion showed a mean globule size of 210.6 ± 9.87 nm with a good PDI of 0.145 ± 0.033 and -21.7 ± 4.5 mV ZP. The poly-L-lysine coating of the nanoemulsion resulted in the reversal of surface charge to positive i.e. 18.3 ± 2.7 mV due to the cationic nature of poly-L-lysine. In vitro drug release showed an initial burst of 15-20% release within 4 h followed by controlled release up to 24 h. The poly-L-lysine coated nanoemulsion showed an 8.28-fold higher uptake than fluorescein isothiocyanate (FITC) solution in HCT116 intestinal cell lines. In vivo studies confirmed that orally administered insulin and C-peptide bearing coated nanoemulsion has the potential to improve glycemic control confirmed by blood glucose level under 200 mg/dL for 12 h compared to that of subcutaneous administration of insulin. The formulation was found stable at 25 °C as well as 4°C for up to 3 months. These findings show a promising approach for delivering oral insulin along with C-peptide for effective glycemic control and management of complications associated with diabetes.

Reversal of Electroosmotic Flow in Charged Nanopores with Multivalent Electrolyte.

We study the reversal of electroosmotic flow in charged cylindrical nanopores containing multivalent electrolyte. Dissipative particle dynamics is used to simulate the hydrodynamics of the electroosmotic flow. The electrostatic interactions are treated using 3D Ewald summation, corrected for a pseudo-one-dimensional geometry of the pore. We observe that, for sufficiently large surface charge density, condensation of multivalent counterions leads to the reversal of the pore's surface charge. This results in the reversal of electroosmotic flow. Our simulations show that the Smoluchowski equation is able to quantitatively account for the electroosmotic flow through the nanopore, if the shear plane is shifted from the position of the Stern contact surface.

Separation and anti-dye-deposition properties of polyamide thin-film composite membrane modified via surface tertiary amination followed by zwitterionic functionalization

This work focused on modification of polyamide based thin-film composite membrane via a novel strategy combining surface tertiary amination and zwitterionic functionalization for improved separation and anti-dye-deposition properties. Surface tertiary amination was conducted through grafting polyethyleneimine (PEI) onto the surface of base membrane at the sites of carboxylic groups activated by 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide. Zwitterionic functionalization was implemented through in situ ring-open reaction between tertiary amine groups of grafted PEI molecules and 1, 4-butanesultone. It was found that grafting of PEI could enhance CaCl2/NaCl rejection ratio from 0.36 to 3.05 through reversal of surface charge at a sacrifice of pure water permeation coefficient from 9.7 to 8.6 l/m2 h bar. The following incorporation of zwitterions under desired condition could resume pure water permeation coefficient to a higher value of 12.6 l/m2 h bar and further enhance CaCl2/NaCl rejection ratio to 3.11. Furthermore, proper surface tertiary amination and zwitterionic functionalization could even significantly improve membrane resistance to deposition by both cationic and anionic dyes. Membrane's water permeation coefficients to aqueous Congo red, Victoria blue B and methylene blue solutions under steady state were enhanced by 72.5, 78.8 and 53.2%, respectively, and relative flux decline ratios were lowered by 80.5, 80.3 and 79.0%.

Inversion of Montmorillonite Ion-Exchange Characteristics

The mechanism of montmorillonite modification with sodium metasilicate for inversing its ion-exchange characteristics has been studied. The blockage of the interlayer space in montmorillonite by amorphous silicon–oxygen clusters, which are formed as a result of sodium metasilicate hydrolysis, hinders the access of ions to the internal negatively charged surface of the mineral and increases the strength of bonding between aluminosilicate layers. As a result, adsorption becomes possible only on the outer surface of particles, namely, on active silanol and aluminol groups. This fact has been confirmed by the high rate of the process, as well as by the appearance of the ability to adsorb anions only after the reversal of montmorillonite surface charge. The value of the adsorption of Cr(VI) oxoanions by modified montmorillonite has appeared to be 0.26–1.05 mg/g depending on the synthesis conditions of the samples.

The recognition of proteasomal receptors by Plasmodium falciparum DSK2

One of the pathways by which proteins are targeted for degradation by the proteasome involve transport by shuttle proteins to proteasomal receptors. The malaria parasite Plasmodium falciparum has recently been found to possess a similar pathway, with the shuttle protein PfDsk2 being the major player. In this study, we have demonstrated how PfDsk2 and its recognition by proteasomal receptors differ from the mammalian system. Our crystal structure of unbound PfDsk2 UBL domain at 1.30 Å revealed an additional 310-helix compared to the human homolog, as well as a few significant differences in its putative binding interface with the proteasome receptors, PfRpn10 and PfRpn13. Moreover, the non-binding face of UBL showed a reversal of surface charge compared to HsDsk2 shuttle protein, instead resembling HOIL-like E3 ligase UBL domain. The affinity of the interaction with the proteasomal receptors remained similar to the human system, and dissociation constants of the same order of magnitude. On the other hand, we have found evidence of a novel interaction between PfRpn13DEUBAD with the PfDsk2UBL suggesting that PfDsk2 may work in cooperation with deubiquitinating enzymes for proofreading ubiquitinated substrates. Our study provides the first molecular look at shuttle proteins in Apicomplexan parasites and hints at how their interaction landscape might be broader than what we may expect.

Mechanistic Understanding of the Interactions of Cationic Conjugated Oligo- and Polyelectrolytes with Wild-type and Ampicillin-resistant Escherichia coli

An in-depth understanding of cell-drug binding modes and action mechanisms can potentially guide the future design of novel drugs and antimicrobial materials and help to combat antibiotic resistance. Light-harvesting π-conjugated molecules have been demonstrated for their antimicrobial effects, but their impact on bacterial outer cell envelope needs to be studied in detail. Here, we synthesized poly(phenylene) based model cationic conjugated oligo- (2QA-CCOE, 4QA-CCOE) and polyelectrolytes (CCPE), and systematically explored their interactions with the outer cell membrane of wild-type and ampicillin (amp)-resistant Gram-negative bacteria, Escherichia coli (E. coli). Incubation of the E. coli cells in CCOE/CCPE solution inhibited the subsequent bacterial growth in LB media. About 99% growth inhibition was achieved if amp-resistant E. coli was treated for ~3–5 min, 1 h and 6 h with 100 μM of CCPE, 4QA-CCOE, and 2QA-CCOE solutions, respectively. Interestingly, these CCPE and CCOEs inhibited the growth of both wild-type and amp-resistant E. coli to a similar extent. A large surface charge reversal of bacteria upon treatment with CCPE suggested the formation of a coating of CCPE on the outer surface of bacteria; while a low reversal of bacterial surface charge suggested intercalation of CCOEs within the lipid bilayer of bacteria.

Surface potential study of ceria/poly(sodium 4-styrenesulfonate) aqueous solution interface

The interfacial properties of the system cerium(IV) oxide/poly(sodium 4-styrenesulfonate) (NaPSS) aqueous solution were studied in a broad pH region by means of inner surface potential, zeta potential and adsorption measurements. Surface potential data were obtained using a single crystal electrode, while zeta potential and adsorption data were obtained using synthetized CeO2 nanoparticles. In order to examine the adsorption of NaPSS on cerium(IV) oxide in detail, surface and electrokinetic potentials of ceria particles were measured in the absence and in the presence of NaPSS.The results of surface potential measurements showed that negatively charged NaPSS molecules exhibit high adsorption affinity towards the CeO2 surface. Titration with NaPSS in the acidic region, where the ceria surface is originally positively charged, suggested that adsorption of negatively charged NaPSS molecules compensates positive charge at the surface and adsorption proceeds until the reversal of surface charge produces a critical value of the potential affecting the state of adsorbed NaPSS molecules. Adsorption of NaPSS on cerium(IV) oxide in the examined pH range was additionally confirmed by zeta potential and adsorption measurements in sodium chloride aqueous solution. From the measured pH dependence of the electrokinetic potential of CeO2 nanoparticles, isoelectric point was determined to be pHiep ≈ 5.4, while the electrokinetic potential in the presence of NaPSS was found to be negative in the whole investigated pH region. Finally, adsorption measurements were carried out which enabled the determination of the maximum surface concentration of the adsorbed polyelectrolyte as Γmax ≈ 1 μmol m−2.

Interaction between Gemini Dodecyl O-Glucosides-Based Multilayer Vesicles and β-Lactoglobulin: The Dominant Role of Surface Charge.

Novel Gemini dodecyl O-glucoside-based primary, secondary, and tertiary vesicles were developed in this work utilizing layer-by-layer deposition of polysaccharides (e.g., sodium carboxymethyl cellulose and chitosan), and their interaction with β-lactoglobulin (BLG) was carefully investigated. The increase of polysaccharide layers on primary vesicles led to a monotonic increase in size and consecutive reversal of surface charge. Polysaccharide deposition significantly retarded the vesicle aggregation and degradation of entrapped catechin laurate during storage. Steady-state fluorescence, isothermal titration calorimetry, and protein precipitation analyses revealed the surface charge dependence of the interactions between vesicles and a model milk protein BLG, which were much stronger when they were charged oppositely than when they presented the same type of surface charge. It was highlighted that the surface charge of vesicles could be tuned by differently charged coatings to accommodate to that of the milk proteins in the food matrix. This work will contribute to the practical application of niosomal vesicles loaded with bioactive compounds to fortify dairy products.

Gas-phase ions produced by freezing water or methanol for analysis using mass spectrometry.

Introducing water or methanol containing a low concentration of volatile or nonvolatile analyte into an inlet tube cooled with dry ice linking atmospheric pressure and the first vacuum stage of a mass spectrometer produces gas-phase ions even of small proteins that can be detected by mass spectrometry. Collision-induced dissociation experiments conducted in the first vacuum region of the mass spectrometer suggest analyte ions being protected by a solvent cage. The charges may be produced by processes similar to those proposed for charge separation under freezing conditions in thunderclouds. By this process, the surface of an ice pellet is charged positive and the interior negative so that removal of surface results in charge separation. A reversal of surface charge is expected for a heated droplet surface, and this is observed by heating rather than cooling the inlet tube. These observations are consistent with charged supercooled droplets or ice particles as intermediates in the production of analyte ions under freezing conditions.

Synthesis, characterization and antimicrobial property of Fe3O4-Cys-HNQ nanocomplex, with l-cysteine molecule as a linker

Lawsone, (2-hydroxy-1,4-naphthoquinone or HNQ), is known for its antimicrobial activity, corrosion inhibition and metal chelation properties. We describe here a simple methodology to functionalize Fe3O4 nanoparticles with lawsone to produce Fe3O4-Cysteine-HNQ nanocomplex, using cysteine as a linker. The characterization of the nanocomplex is done using Fourier Transform Infrared (FTIR) spectroscopy, Thermogravimetric Analysis (TGA), Dynamic Light Scattering size measurement, zeta potential and contact angle measurement. The zeta potential of uncoated nanoparticles and Fe3O4-Cysteine-HNQ nanocomplex is found to be +48 mV and −38 mV respectively. The reversal of surface charge from positive to negative corroborates the presence of lawsone monolayer at particle interface. Further, TGA result shows a weight loss of 32% for these nanocomplexes due to the decomposition of lawsone and cysteine present on the nanoparticles. The FTIR spectrum of the nanocomplex exhibits CC and CO stretching bands of lawsone, with a slight shift in their position, which confirms the interaction of lawsone with the cysteine coated nanoparticles. Fe3O4-Cysteine-HNQ nanocomplexes exhibit interesting properties such as increased wettability (contact angle ∼20°) on stainless steel substrate and antimicrobial activity against Staphylococcus aureus, with a minimum inhibitory concentration of 700 μg mL−1. These results show that lawsone functionalized magnetite nanoparticles can be used as response stimulus antimicrobial agents.

Adsorptive property of Cu 2+–ZnO/cetylpyridinium–montmorillonite complexes for pathogenic bacterium in vitro

Cu 2+–ZnO/cetylpyridinium–montmorillonite (Cu 2+–ZnO/CP–MMT) complexes were prepared using montmorillonite (MMT), Cu 2+, Zn 2+, and cetylpyridinium (CP). The goal was to assess comparatively the adsorption properties of Cu 2+–ZnO/CP–MMT in vitro using pathogenic Escherichia coli. The results showed that Cu 2+–ZnO/CP–MMT adsorbed significantly ( P < 0.05) more E. coli compared with the parent clay. The adsorption process of bacterial cells occurring on the modified MMT surface reached equilibrium after 90 min. The percentages of E. coli adsorbed onto the surfaces of Cu 2+–ZnO/CP–MMT and MMT in adsorption equilibrium were 84.66% and 47.01%, respectively. Adsorption data from the bacteria-clay systems followed the Langmuir and Freundlich isotherms, but not the BET isotherm. Adsorption of E. coli in acidic medium was higher than in alkaline medium. The extent of bacteria adsorption onto the modified MMT increased with decreasing ionic strength, and with increasing temperature. The processes of E. coli adsorption onto the tested adsorbents were endothermic and spontaneous at the experimental temperature. The mechanism of adsorption of bacteria on Cu 2+–ZnO/CP–MMT may involve enhanced hydrophobicity and the reversal of surface charge from negative to positive.

Preparation and Controlled Self-Assembly of Janus Magnetic Nanoparticles

Janus magnetic nanoparticles (~20 nm) were prepared by grafting either polystyrene sodium sulfonate (PSSNa) or polydimethylamino ethylmethacrylate (PDMAEMA) to the exposed surfaces of negatively charged poly(acrylic acid) (PAA)-coated magnetite nanoparticles adsorbed onto positively charged silica beads. Individually dispersed Janus nanoparticles were obtained by repulsion from the beads on reversal of the silica surface charge when the solution pH was increased. Controlled aggregation of the Janus nanoparticles was observed at low pH values, with the formation of stable clusters of approximately 2-4 times the initial size of the particles. Cluster formation was reversed, and individually dispersed nanoparticles recovered, by restoring the pH to high values. At intermediate pH values, PSSNa Janus nanoparticles showed moderate clustering, while PDMAEMA Janus nanoparticles aggregated uncontrollably due to dipolar interactions. The size of the stable clusters could be controlled by increasing the molecular weight of the grafted polymer, or by decreasing the magnetic nanoparticle surface availability for grafting, both of which yielded larger cluster sizes. The addition of small amounts of PAA-coated magnetic nanoparticles to the Janus nanoparticle suspension resulted in a further increase in the final cluster size. Monte Carlo simulation results compared favorably with experimental observations and showed the formation of small, elongated clusters similar in structure to those observed in cryo-TEM images.

Charge Inversion at High Ionic Strength Studied by Streaming Currents

We report charge inversion, the sign reversal of the effective surface charge in the presence of multivalent counterions, for the biologically relevant regimes of divalent ions and mixtures of monovalent and multivalent ions. Using streaming currents, the pressure-driven transport of countercharges in the diffuse layer, we find that charge inversion occurs in rectangular silica nanochannels at high concentrations of divalent ions. Strong monovalent screening is found to cancel charge inversion, restoring the original surface charge polarity. An analytical model based on ion correlations successfully describes our observations.

Probing Microbial Cell Surface Charges by Atomic Force Microscopy

Most microorganisms possess a negative surface charge under physiological conditions due to the presence of anionic carboxyl and phosphate groups. Cell surface charge plays an important role in controlling cell adhesion and aggregation phenomena, as well as antigen-antibody, cell-virus, cell-drug, and cell-ions interactions. We have used atomic force microscopy (AFM) with chemically functionalized probes to investigate the surface charges of yeast cells. Force-distance curves and adhesion maps recorded with probes terminated with ionizable carboxyl groups were strongly influenced by pH: while no adhesion was measured at neutral/alkaline pH, multiple adhesion forces were recorded at pH : less than or equal to 5. Three pieces of evidence indicated that these changes were related to differences in the ionization state of the cell surface functional groups. First, the adhesion force vs pH curve was correlated with microelectrophoresis data, the pH of the largest adhesion force corresponding to the cell isoelectric point, i.e., pH 4. Second, treating the cells with Cu(II) ions caused a reversal of the cell surface charge at neutral pH and promoted the adhesion toward the negatively charged probe. Third, control experiments using nonionizable hydroxyl-terminated probes indicated that the changes in adhesion forces were not simply due to the titration of the probe surface charges. This study shows that AFM with chemically modified probes is a valuable approach in microbiology and biophysics for probing the local electrostatic properties of microbial cell surfaces.

Effect of cationic polymer additives on the adsorption of humic acid onto iron oxide particles

Improving the removal of natural organic matter (e.g. humic acid) during drinking water treatment is important in order to minimize the formation of disinfection by-products (DBPs). Although polymeric flocculants are often used to improve turbidity removal during the coagulation process, little information is available regarding the influence of these polymers on NOM removal. In this study, the adsorption of humic acid onto polymer-coated iron oxide particles was investigated as a way to improve humic acid removal during the coagulation process. Monodisperse iron oxide particles and well-characterized humic acid were used as a model system. The adsorption of humic acid onto bare iron oxide particles decreased as solution pH increased. When iron oxide particles were coated with a cationic polymer, adsorption doubled at high pH (∼9.5) and low salt concentration (0.001 M NaCl). The greater adsorption of humic acid on polymer-coated surfaces at high pH was largely due to changes in the electrostatic interactions between humic acid and the particle surface. Coating the iron oxide particles with cationic polymer resulted in the reversal of the negative particle surface charge, thus providing more favorable conditions for humic acid adsorption. Little change in humic acid adsorption was observed, however, for polymer-coated particles at near neutral pH values (∼6.8) or at high salt concentration. These data suggest that under certain conditions polymers may enhance humic acid adsorption onto iron oxide surfaces, a process which may improve DBP precursor removal during drinking water treatment.

Cooperative Effect of Adsorbed Cations and Iodide on the Interception of Back Electron Transfer in the Dye Sensitization of Nanocrystalline TiO2

Specific adsorption of cations (H+, Li+, ...) on TiO2 nanocrystalline particles is known to control the energetics of the conduction band and therefore the ability for molecular sensitizers to inject electrons into the semiconductor upon irradiation. In photoelectrochemical energy conversion devices employing dye-sensitized titanium dioxide mesoporous electrodes, back electron transfer is generally intercepted by the use of the iodide/triiodide couple as a charge mediator. Kinetics of the oxidation of I- by the oxidized state of cis-RuII(dcbpy)2(NCS)2 sensitizer adsorbed on TiO2 was measured by flash photolysis in propylene carbonate. The rate of this reaction was found to depend on the nature and concentration of added cations such as Mg2+, Li+, Na+, and K+. A brusque acceleration of the process was in particular observed at a critical concentration. Electrophoretic measurements showed that this step in the dye regeneration reaction kinetics corresponds to the reversal of particle surface charge upon adsorption of potential-determining species, which causes I- to efficiently adsorb onto the oxide. These observations strongly suggest that the specific adsorption of cations on TiO2 nanoparticles governs the formation of (I-, I-) ion pairs on the surface, and allows the more energetically favorable and faster mechanism involving oxidation of I- to I2•- radical to take place.

The point of zero charge of monazite and xenotime

The point of zero charge (PZC) for monazite and xenotime reported in the literature has a wide range. Chemical modelling from this study shows that variation in the chemical composition of the monazite and xenotime samples can have detrimental effects on the chemical speciation of the mineral surfaces. It has been found that any slight variation of the chemical composition of the mineral surface would, therefore, cause a reversal of surface charge. This explains the wide variation of PZC of different rare earth mineral samples from different mines.

Reversal of the surface charge of a mineral powder: application to electrophoretic deposition of silica for anticorrosion coatings

Electrophoretic deposition is used to produce a silica layer on zinc-coated steel and chromated zinc-coated steel. This type of coating is commonly employed in car industries to protect steel against corrosion. Because of the presence of zinc and chromated layers on the substrate, the polarization must be cathodic to avoid oxidation. Reversal of the silica surface charge is obtained using different methods, such as cation adsorption, cationic polymer (polyvinylimidazole) adsorption or oxide (cerium oxide) adsorption. The production of deposits using cations is quite difficult because it is necessary to add a high concentration of salt to reverse the silica charge, provoking high conductivity and gas release at the cathode. With polyvinylimidazole, the instability of the polymer is also a restrictive factor for the application of electrophoresis. The most promising results are obtained with ceriacovered silica. In fact the system is stable under the electric field and allows homogeneous coatings to be produced. The polarization resistance measurements indicate the good protective effect compared with the untreated substrate.

Transport of Humic-Coated Iron Oxide Colloids in a Sandy Soil: Influence of Ca2+ and Trace Metals

Understanding colloid release, transport, and deposition in natural heterogeneous porous media is a prerequisite for evaluating the potential role of colloids in subsurface contaminant transport. In this study, we investigate the influence of adsorbed humic acid, solution Ca2+ concentration, and adsorbed trace metals (Cu2+, Pb2+) on the transport and deposition kinetics of colloidal hematite particles (α-Fe2O3; 122 nm diameter) in a sandy soil matrix. A short-pulse chromatographic technique was used to measure colloid deposition rate coefficients and collision efficiencies (α). At pH 5.7, pure hematite was positively charged and deposited rapidly (α ≈ 1) even at low electrolyte concentrations (10-4 M CaCl2). Adsorption of humic acid to the hematite caused reversal of surface charge from positive to negative. As a result, colloid deposition rates were decreased by approximately 2 orders of magnitude (α ≈ 0.01). Deposition rates of humic-coated hematite colloids strongly increased with increasing Ca2+ concentration. A transition from the slow (α < 1) to the fast (α = 1) deposition regime was observed at approximately 10-3 M CaCl2. Substituting Ca2+ with Cu2+ or Pb2+ decreased electrophoretic mobility and colloid mobility, but the effects were small compared with Ca2+ concentration effects. The results of this study demonstrate that adsorbed natural organic matter and solution ionic strength play a key role in controlling colloid mobility in soils and surface near groundwater aquifers.