Aqueous dispersions of single-wall and multiwall carbon nanotubes with designed amphiphilic polycations

Poor solubility of single-walled and multiwalled carbon nanotubes (NTs) in water and organic solvents presents a considerable challenge for their purification and applications. Macromolecules can be convenient solubilizing agents for NTs and a structural element of composite materials for them. Several block copolymers with different chemical functionalities of the side groups were tested for the preparation of aqueous NT dispersions. Poly(N-cetyl-4-vinylpyridinium bromide-co-N-ethyl-4-vinylpyridinium bromide-co-4-vinylpyridine) was found to form exceptionally stable NT dispersions. It is suggested that the efficiency of macromolecular dispersion agents for NT solubilization correlates with the topological and electronic similarity of polymer-NT and NT-NT interactions in the nanotube bundles. Raman spectroscopy and atomic force and transmission electron microcopies data indicate that the polycations are wrapped around NTs forming a uniform coating 1.0-1.5 nm thick. The ability to wind around the NT originates in the hydrophobic attraction of the polymer backbone to the graphene surface and topological matching. Tetraalkylammonium functional groups in the side chains of the macromolecule create a cloud of positive charge around NTs, which makes them hydrophilic. The prepared dispersions could facilitate the processing of the nanotubes into composites with high nanotube loading for electronic materials and sensing. Positive charge on their surface is particularly important for biological and biomedical applications because it strengthens interactions with negatively charged cell membranes. A high degree of spontaneous bundle separation afforded by the polymer coating can also be beneficial for NT sorting.     

The influence of discrete surface charges on the force between charged surfaces

The force between two parallel charged flat surfaces, with discrete surface charges, has been calculated with Monte Carlo simulations for different values of the electrostatic coupling. For low electrostatic coupling (small counterion valence, small surface charge, high dielectric constant, and high temperature) the total force is dominated by the entropic contribution and can be described by mean field theory, independent of the character of the surface charges. For moderate electrostatic coupling, counterion correlation effects lead to a smaller repulsion than predicted by mean field theory. This correlation effect is strengthened by discrete surface charges and the repulsive force is further reduced. For large electrostatic coupling the total force for smeared out surface charges is known to be attractive due to counterion correlations. If discrete surface charges are considered the attractive force is weakened and can even be turned into a repulsive force. This is due to the counterions being strongly correlated to the discrete surface charges forming effective, oppositely directed, dipoles on the two walls.     

Experimental study of electrostatically stabilized colloidal particles: colloidal stability and charge reversal

We consider the interaction of colloidal spheres in the presence of mono-, di-, and trivalent ions. The colloids are stabilized by electrostatic repulsion due to surface charges. The repulsive part of the interaction potential Ψ(d) is deduced from precise measurements of the rate of slow coagulation. These "microsurface potential measurements" allow us to determine a weak repulsion in which Ψ(d) is of the order of a few k(B)T. These data are compared to ζ potential measured under similar conditions. At higher concentrations both di- and trivalent counterions accumulate at the very proximity of the particle surface leading to charge reversal. The salt concentration c(cr) at which charge reversal occurs is found to be always above the critical coagulation concentration c(ccc). The analysis of Ψ(d) and of the ζ potential demonstrates, however, that adsorption of multivalent counterions starts far below c(cr). Hence, colloid stability in the presence of di- and trivalent ions cannot be described in terms of a DLVO ansatz assuming a surface charge that is constant with regard to the ionic strength.     

Ionic electrets: electrostatic charging of surfaces by transferring mobile ions upon contact

This paper describes the fabrication and characterization of ionic electrets-materials that bear a long-lived electrostatic charge because of an imbalance between the number of cationic and anionic charges in the material. Crosslinked polystyrene microspheres that contain covalently bound ions and mobile counterions transfer some of their mobile ions in air, in the absence of bulk liquid, to another material upon contact. According to the ion-transfer model of contact electrification, this selective transfer of mobile ions yields microspheres that have a net electrostatic charge. A tool that operates on the principle of electrostatic induction measures the charge on individual microspheres (50-450 microm in diameter). Microspheres with a variety of covalently bound ionic functional groups (tetraalkylammonium, alkyltriphenylphosphonium, alkylsulfonate, and arylsulfonate) acquire charges consistent with this ion-transfer mechanism. The charge on a microsphere is proportional to its surface area (ca. 1 elementary charge per 2000 nm2) and close to the theoretical limit imposed by the dielectric breakdown of air. The charge density in an atmosphere of SF6 is more than twice that in an atmosphere of N2. These observations suggest that the charge density of these ionic electret microspheres is limited by the dielectric breakdown of the surrounding gas. Functionalizing the surfaces of glass or silicon with covalently bound ions and mobile counterions generates ionic electrets from these inorganic substrates. Soft lithography can pattern charge on a planar silicon surface (with oxide) and on the surface of 250-mum glass microspheres.     

Interaction of similarly charged surfaces mediated by nanoparticles

We study the interaction between two like charged surfaces embedded in a solution of oppositely charged multivalent rod-like counterions.The counterions consist of two rigidly bonded point charges,each of valency Z.The strength of the electrostatic coupling increases with increasing surface charge density or valency of the charges.The system is analyzed by employing a self-consistent field theory,which treats the short and long range interactions of the counterions within different approximations.We find that in the weak coupling limit,the interactions are only repulsive.In the intermediate coupling regime,the multivalent rod-like counterions can mediate attractive interactions between the surfaces. For sufficiently long rods,bridging contributes to the attractive interaction.In the strong coupling limit,the charge correlations can contribute to the attractive interactions at short separations between the charged surfaces.Two minima can then appear in the force curve between surfaces.     

Strong attraction among the fully hydrophilic {Mo72Fe30} macroanions

We report the study on the unique driving forces of the self-assembly of fully hydrophilic, soluble {Mo72Fe30} macroanions into single-layer, vesicle-like "blackberry" structures in water and mixed solvents. The hydrophobic interaction that is responsible for the vesicle formation of amphiphilic surfactants does not contribute to the current blackberry formation because of the absence of hydrophobic moiety. The hydrogen bond, van der Waals force, and chemical interaction only play minor roles. Laser light scattering and conductance measurements on a series of {Mo72Fe30}/ethanol/H2O solutions show that a certain amount of negative charges are necessary for the self-assembly, clearly indicating the existence of long-range attraction between macroanions, presumably due to the small counterions in between. The experimental results suggest that the charges on macroanions play a dual effect: short-range electrostatic repulsion and long-range "like-charge attraction", which is the major source of attractive force between hydrophilic macroanions, while van der Waals force, hydrogen bonds, and temporary inter-{Mo72Fe30} Fe-O-Fe chemical linking may also have minor contributions.     

Chain stiffness, salt valency, and concentration influences on titration curves of polyelectrolytes: Monte Carlo simulations

Monte Carlo simulations have been used to study two different models of a weak linear polyelectrolyte surrounded by explicit counterions and salt particles: (i) a rigid rod and (ii) a flexible chain. We focused on the influence of the pH, chain stiffness, salt concentration, and valency on the polyelectrolyte titration process and conformational properties. It is shown that chain acid-base properties and conformational properties are strongly modified when multivalent salt concentration variation ranges below the charge equivalence. Increasing chain stiffness allows to minimize intramolecular electrostatic monomer interactions hence improving the deprotonation process. The presence of di and trivalent salt cations clearly promotes the chain degree of ionization but has only a limited effect at very low salt concentration ranges. Moreover, folded structures of fully charged chains are only observed when multivalent salt at a concentration equal or above charge equivalence is considered. Long-range electrostatic potential is found to influence the distribution of charges along and around the polyelectrolyte backbones hence resulting in a higher degree of ionization and a lower attraction of counterions and salt particles at the chain extremities.     

Surface Patterns of a Tetrahedral Polyelectrolyte Brush Induced by Grafting Density and Charge Fraction

A tetrahedral polyelectrolyte brush in the presence of trivalent counterions is researched under the condition of good solution by mea ns of molecular dynamics simulati ons.Grafting density and charge fraction are varied to gen erate a series of surface patter ns.Lateral microphase separation happens and various interesting pinned patches appear at appropriate charge fraction and grafting density.Through a careful analysis on the brush thickness,the pair correlation functions,the distributions of net charge,and the four states of trivalent counterions in the brush,we find that the ordered surface patterns and special properties are induced by the pure electrostatic correlation effect of trivalent ions even in the good solvent.Furthermore,the dependences of electrostatic correlation on the charge fraction of tethered chains are evaluated for fixed grafting den sity.Also,our results can serve as a guide for precise control over the stimuli-responsive materials rational and self-assembly of nanoparticles.     

Cobalt–iron decorated tellurium nanotubes for high energy density supercapacitor

We report the synthesis of cobalt-iron (Co–Fe) decorated tellurium nanotubes (Te NTs) using semiconductive Te NTs as a sacrificial template, following a wet chemical method. The interplay of Co and Fe precursor concentrations incorporated with Te NT, residual hydrazine hydrate, and the negative surface charge of Te NT plays a significant role in obtaining various bimetallic telluride structures. The one-dimensional (1-D) structure of Co–Fe decorated Te NTs with Te NTs in the backbone provides superior conductivity and exhibits high electrochemical performance with battery type electrode behavior. A negative surface charge value of ?18.9 mV for Te NTs is obtained due to the presence of an anionic surfactant as sodium dodecyl sulfate (SDS) forms a bilayer on Te NTs. To tune the energy density performance, the Co–Fe decorated Te NTs electrode is combined with the electric double-layer capacitors (EDLC) type electrode activated carbon (AC). The asymmetric assembly shows an excellent specific capacitance of 179.2 F/g (48.7 mAh/g) at a current density of 0.9 A/g in 4 M KOH electrolyte. More importantly, it exhibits a maximum energy density of 62.1 Wh/kg at a power density of 1,138.2 W/kg under a potential window of 1.58 V. This potential finding shows the significant applicability of Te NTs as a template for the synthesis of bimetallic tellurides with unique morphologies. The synergistic effect from multiple metals and anisotropic morphology is beneficial for energy storage applications.     

Controlling electrostatic co-assembly using ion-containing copolymers: from surfactants to nanoparticles

In this review, we address the issue of the electrostatic complexation between charged-neutral diblock copolymers and oppositely charged nanocolloids. We show that nanocolloids such as surfactant micelles and iron oxide magnetic nanoparticles share similar properties when mixed with charged-neutral diblocks. Above a critical charge ratio, core-shell hierarchical structures form spontaneously under direct mixing conditions. The core-shell structures are identified by a combination of small-angle scattering techniques and transmission electron microscopy. The formation of multi-level objects is driven by the electrostatic attraction between opposite charges and by the release of the condensed counterions. Alternative mixing processes inspired from molecular biology are also described. The protocols applied here consist in screening the electrostatic interactions of the mixed dispersions, and then removing the salt progressively as an example by dialysis. With these techniques, the oppositely charged species are intimately mixed before they can interact, and their association is monitored by the desalting kinetics. As a result, sphere- and wire-like aggregates with remarkable superparamagnetic and stability properties are obtained. These findings are discussed in the light of a new paradigm which deals with the possibility to use inorganic nanoparticles as building blocks for the design and fabrication of supracolloidal assemblies with enhanced functionalities.     

Effect of Discrete Macroion Charge Distributions on Electric Double Layer of a Spherical Macroion

The effect of replacing the conventional uniform macroion surface charge density with discrete macroion charge distributions on the structure of electric double layer (EDL) of a spherical macroion has been investigated by Monte Carlo (MC) simulations. Two discrete models have been investigated in addition to the central macroion charge: point charges localized on the macroion surface and finite-sized charges protruding into the solution. Both models have been studied with fixed and mobile macroion charges. The radial functions of local densities and electrostatic potential in EDL, are calculated and compared to the results obtained for the central macroion charge distribution. It is concluded that the model of charge distribution significantly affects the EDL structure close to the macroion, while the effect is much weaker at larger distances. With point charges localized on the macroion surface, counterions become stronger accumulated to the macroion, as a result the absolute values of surface potential ?₀ and zeta ξ potential are decreased. With protruding charges, the excluded volume effect dominates over the increased correlation ability; hence the counterions are less accumulated near the macroions and the absolute values of ?₀ and ξ potentials are increased.     

New results from the contact theorem for the charge profile for symmetric electrolytes

In this paper the contact value of the charge profile at a charged interface is presented as the sum of the normal component of the Maxwell electrostatic tensor and a new electrostatic property defined by the integral from the product of the gradient of the electrical potential and the singlet distribution function of coions (ions with sign of the charge equal to that of the interface). On physical arguments, it is conjectured that this new property is a monotonic function of the electrical charge at the wall and is limited by the bulk electrolyte pressure for large electrical charges at the wall. Using the contact theorems for the density and the charge profiles, the exact expressions for the contact values of the profiles of coions and counterions are derived and some related general properties are discussed.     

生物样品中神经递质的定量测定及其应用研究进展

神经递质（NTs）是神经传递的内源性化学信使，在大脑功能中发挥重要作用。中枢神经系统中神经递质浓度的变化与许多精神和生理疾病有关。神经递质的测定已成为疾病诊断和监测以及治疗干预的重要手段，有效的神经递质体内监测对于疾病诊疗乃至新药研发都至关重要。该文就近年来神经递质的检测方法，包括仪器检测法、电化学检测法以及一些新型检测方法等进行综述，并总结了目前神经递质检测在一些疾病研究中的应用进展。     

Effect of discrete macroion charge distributions in solutions of like-charged macroions

The effect of replacing the conventional uniform macroion surface charge density with discrete macroion charge distributions on structural properties of aqueous solutions of like-charged macroions has been investigated by Monte Carlo simulations. Two discrete charge distributions have been considered: point charges localized on the macroion surface and finite-sized charges protruding into the solution. Both discrete charge distributions have been examined with fixed and mobile macroion charges. Different boundary conditions have been applied to examine various properties. With point charges localized on the macroion surface, counterions become stronger accumulated to the macroion and the effect increases with counterion valence. As a consequence, with mono- and divalent counterions the potential of mean force between two macroions becomes less repulsive and with trivalent counterions more attractive. With protruding charges, the excluded volume effect dominates over the increased correlation ability; hence the counterions are less accumulated near the macroions and the potential of mean force between two macroions becomes more repulsive/less attractive.     

The Effect of Surface Charge on Adsorption of a Cationic Polyelectrolyte

The regularities of adsorption of a cationic polyelectrolyte, poly(diallyldimethylammonium chloride), on the surface of fused quartz are studied at different values of solution pH by capillary electrokinetics. It is shown that the polyelectrolyte adsorption on a negatively charged surface depends on the value of the surface charge and increases with its growth. At a low charge value (pH 3.8), the polyelectrolyte adsorption increases the quartz surface charge. The driving forces of the adsorption are both electrostatic interaction and forces of nonelectrostatic nature, probably hydrophobic interactions and a change in entropy due to the displacement of counterions from a double layer. The adsorption of poly(diallyldimethylammonium chloride) on quartz from alkaline and neutral solutions is irreversible, which indicates the key role of the electrostatic interaction. At low values of the surface charge, the nonelectrostatic interactions play the main role, thereby resulting in polyelectrolyte desorption.     

Counterion condensation on charged spheres, cylinders, and planes

We use the framework of counterion condensation theory, in which deviations from linear electrostatics are ascribed to charge renormalization caused by collapse of counterions from the ion atmosphere, to explore the possibility of condensation on charged spheres, cylinders, and planes immersed in dilute solutions of simple salt. In the limit of zero concentration of salt, we obtain Zimm-Le Bret behavior: a sphere condenses none of its counterions regardless of surface charge density, a cylinder with charge density above a threshold value condenses a fraction of its counterions, and a plane of any charge density condenses all of its counterions. The response in dilute but nonzero salt concentrations is different. Spheres, cylinders, and planes all exhibit critical surface charge densities separating a regime of counterion condensation from states with no condensed counterions. The critical charge densities depend on salt concentration, except for the case of a thin cylinder, which exhibits the invariant criticality familiar from polyelectrolyte theory.     

Investigation of intermolecular interactions in solution polymer‐organic dye by method electronic spectroscopy

It has been shown that specific electrostatic interactions between charges of ion dyes and functional groups of polymer play main role in the cause of polar coulored matrices. These interaction prevent by formation tight ion pairs of dyes and their associates. Therefore, such matrices have practically the same electronic spectra as liquid solution of dyes. Electrostatic interactions between dye counterions dominate in low polarity polymer matrices. Process of formation tight ions pair is became easier. Such pairs cause quenching of the fluorescence and distortion of electronic spectra by comparison with liquids.     

Electronic properties of 3,3'-dimethyl-5,5'-bis(1,2,4-triazine): towards design of supramolecular arrangements of N-heterocyclic Cu(I) complexes

A new efficient and safe synthesis of 3,3'-dimethyl-5,5'-bis-(1,2,4-triazine) is presented. The electron-density distribution and electrostatic properties (charge, electrostatic potential) of this molecule were analyzed. These properties were derived from a high-resolution single-crystal X-ray diffraction experiment at 100 K and compared to the results obtained from ab initio DFT quantum-mechanical calculations. Comparisons of its electrostatic potential features and integrated atomic charges (quantum theory of atoms in molecules, QTAIM) have been made with those of related molecules such as bipyrimidine ligands. Two methods were used to derive integrated charges: one is based on the conventional analytical procedure and the second uses a steepest-ascent numerical algorithm. Excellent agreement was obtained between these two methods. Charges and electrostatic potential were used as predictive indices of metal chelation and discussed in the light of complexation abilities of the title compound and related molecules. The crystal structure of a Cu(I) complex of 3,3'-dimethyl-5,5'-bis(1,2,4-triazine) is reported here. In the solid state, this complex forms a three-dimensional multibranch network with open channels in which counterions and solvent molecules are located. This architecture involves both cis and trans isomers of the title compound.