Polymer mediated peptide immobilization onto amino-containing N-isopropylacrylamide-styrene core-shell particles

Monodisperse cationic core-shell latex particles have been prepared using a shot polymerization process, with N-(3-aminopropyl)-methacrylamide-hydrochloride (APMH) as the functional monomer. The final latexes were characterized with respect to final polymerization conversion, water soluble polymer formation, particle size and size distribution, surface charge density and electrokinetic properties. Then the covalent grafting of maleic anhydride-alt-methyl vinyl ether (MAMVE) copolymer onto aminated latex particles was investigated. The most efficient conditions to obtain derivatised particles with no alteration of the colloidal stability were to control both polymer amount/latex particles concentration ratio and the mixing method of the two species. The charge inversion of the hydrolysed MAMVE functionalized particles was demonstrated by measuring the electrophoretic mobility as a function of pH. Finally, the covalent binding approach was implemented with peptide-MAMVE conjugates, confirming the great potential of this promising methodology for the preparation of reactive latex particles bearing peptides.     

Study of pH-responsive microgels containing methacrylic acid: effects of particle composition and added calcium

pH-responsive microgel dispersions contain cross-linked polymer particles that swell when the pH approaches the pKa of the ionic monomer incorporated within the particles. In recent work from our group, it was demonstrated that the mechanical properties of degenerated intervertebral discs (IVDs) could be restored to normal values by injection of pH-responsive microgel dispersions (Saunders, J. M.; Tong, T.; LeMaitre, C.; Freemont, A. J.; Saunders, B. R. Soft Matter 2007, 3, 486). These dispersions change from a fluid to a gel with increasing pH. The present work investigates the pH-dependent properties of dispersions of microgel particles containing MAA (methacrylic acid) and also the effects of added Ca2+. Two microgels are discussed: microgel A is poly(EA/MAA/AM) (EA and AM are ethyl acrylate and allyl methacrylate), and microgel B is poly(EA/MAA/BDDA) (butanediol diacrylate). The pH-dependent particle properties investigated include hydrodynamic diameters and electrophoretic mobilities. The critical coagulation concentrations (CCC) of dilute dispersions and the elastic modulus (G') of concentrated, gelled microgel dispersions were also investigated. In the absence of added Ca2+, the particle swelling and G' were smallest and largest, respectively, for microgel A. The changes in hydrodynamic diameter and mobility with pH were explained in terms of a core-shell swelling mechanism. Added Ca2+ was found to significantly decrease the CCCs, extents of particle swelling, and magnitude of the electrophoretic mobility. This was attributed to the ionic cross-linking of neighboring RCOO- groups by Ca2+. It is suggested that the formation of ionic cross-links is inefficient within the microgel particles because of the presence of covalent cross-links that oppose the large-scale conformational rearrangement of neighboring RCOO- groups. The effect of Ca2+ on the properties of the gelled dispersions is important from the viewpoint of potential application in vivo. Rheological studies of the gelled microgel dispersions showed that added Ca2+ did not have a specific influence on G'. The differences observed in the presence of Ca2+ were attributed to ionic strength effects (screening). The key parameter that controls G' of the gelled microgel dispersions is pH. The results from this work suggest that the elasticity of the gels would be slightly reduced in vivo as a consequence of the high ionic strength present.     

Electrokinetic Characterization of Poly(N-isopropylacrylamide) Microgel Particles: Effect of Electrolyte Concentration and Temperature

A fundamental study about the characterization of a poly(N-isopropylacrylamide), poly(NIPAM), microgel, cross-linked with N,N'-methylene bisacrylamide (BA), has been carried out. The anionic charge of this system was increased by copolymerization with 2-acrylamido-2methylpropanesulphonic (AMPS) acid. The electrokinetic behavior of these aMPS/NIPAM microgel particles has been investigated. First, the surface charge density was determined by conductimetric titration, and the particle size was obtained by transmission electron microscopy under several conditions. Electrophoretic mobility measurements were carried out as a function of electrolyte concentration and temperature. The hydrodynamic diameter of the particles was also measured by photon correlation spectroscopy as a function of electrolyte concentration and temperature. These size data have been taken into account in order to explain the mobility behavior by using Ohshima's theory for soft particles, with very good agreement between experiments and theoretical predictions. Copyright 2001 Academic Press.     

Cationic amino-containing N-isopropyl- acrylamide-styrene copolymer particles: 2-surface and colloidal characteristics

 In a previous paper [1], the synthesis of various polystyrene– poly[NIPAM] core–shell latexes bearing cationic amidino and/or amino charges has been described. Several colloidal properties of these cationic latexes have been charac-terized such as: particle size, surface charge density, electrophoretic mobility and finally colloidal stability. Due to the poly[NIPAM]-rich layer in the shell, it was found that temperature played a significant role on all these properties, a LCST around 33 °C being exhibited. In addition, ionic strength was also found to affect the colloidal behavior of these latexes, the largest effect being observed with latexes having both amidino and amino surface charges. The critical coagulation concentra-tions (CCC) of the various latexes above and below the LCST were determined, highlighting the contribution of electrostatic and steric repulsive forces to the stability of these particles. Received: 20 January 1998 Accepted: 8 June 1998     

The effect of electrolyte on the colloidal properties of poly(N-isopropylacrylamide-co-dimethylaminoethylmethacrylate) microgel latexes

Monodisperse cationic thermosensitive latex microgels prepared by radical-initiated precipitation copolymerization of N-isopropylacrylamide (NIPAM), methylenebisacrylamide, and dimethylaminoethylmethacrylate (DMAEMA) have been reported (Zha LS, Hu JH, Wang CC, Fu SK, Elaissari A, Zhang Y 2002 Colloid Polym Sci 280:1) and we suggested (Zhang Y, Zha LS, Fu SK J Appl Polym Sci) that the polyelectrolyte chains are rich in their expanded shell layers. The effect of a range of electrolytes on several colloidal properties of these cationic latexes (such as particle size, zeta potential and colloidal stability) has been investigated. The ability of the anions to induce the particle deswelling and flocculation is related to their position in the Hofmeister series. Owing to the DMAEMA-rich layer on the latex particles, the ionic-strength dependence of the particle hydrodynamic size and the zeta potential become more profound with increasing amount of DMAEMA incorporated into the microgel. It is suggested that the effect of electrolytes on the colloidal properties of the copolymer microgel latexes is attributed to the dehydration of the poly(NIPAM) segment and the screening of the electrostatic interaction between the charged DMAEMA units induced by electrolytes.     

The effect of surface modification on the stability characteristics of poly(N-isopropylacrylamide) latices under Brownian and flow conditions

The stability of thermoreversible microgel particles of poly(N-isopropylacrylamide) having carboxylate surface charge groups has been studied in the presence of electrolyte and non-adsorbing polymer. Methylation of the surface charge groups leads to a decrease in the electrophoretic mobility of the particles and also the interparticle electrostatic repulsive potential, resulting in the material becoming more susceptible to flocculation. The Hamaker constant of the microgel particles increases with the decrease in the hydrodynamic size of the particles following heating. This brings about an increase in the van der Waals attractive energy which results in the particles aggregating in the presence of sufficient electrolyte. Under conditions of flow through membranes, where shearing forces are operative, the flocculation observed following the heating of the dispersions results in the blockage of pores.     

Coil-to-globule-type transition of poly(N-isopropylacrylamide) adsorbed on colloidal silica particles

 The temperature dependence of the dimensions of poly(N-isopropylacrylamide) (PNIPAM) adsorbed on two different colloidal silica particles was studied with dynamic light scattering. The hydrodynamic diameter was measured when the temperature was varied stepwise from 10 to 60 °C. PNIPAM molecules free in solution undergo a conformational transition at the θ temperature. We have found that PNIPAM adsorbed onto silica particles also undergoes a transition below the θ temperature. When a small amount of polymer was adsorbed the coil-to-globule transition at the θ temperature did not occur. Potentiometric titrations showed that the surface charge of the silica particles was not affected by the polymer adsorption. Sodium dodecyl sulfate (SDS) (100–1200 mg/l) was added to improve the stability. The particles with a higher zeta potential required a smaller addition of SDS to prevent coagulation compared to the particles with a smaller surface potential. For low additions of SDS the transition curves of adsorbed PNIPAM were unaffected. For larger additions of SDS the collapse of PNIPAM was shifted to higher temperatures. When as much as 1200 mg/l SDS was added, two regions with weak transitions were observed before the collapse. It was also observed that the presence of SDS results in a smaller adsorption of PNIPAM onto the particles. The addition of SDS strongly increased the magnitude of the electrophoretic mobility of the polymer–particle unit. From the electrophoretic measurements an electrokinetic layer thickness was calculated and it was found to be smaller than the corresponding hydrodynamic layer thickness, as obtained by dynamic light scattering. Received: 14 December 1999/In revised form: 22 February 2000/Accepted: 6 March 2000     

Photo-responsive properties of poly(NIPAM-co-AAc) microgel particles with absorbed, hydrophobically modified organic salts

The absorption of two hydrophobically modified organic salts (HMOSs), containing azobenzene units, into poly(N-isopropylacrylamide-co-acrylic acid) microgel particles has been studied at pH 8 and 20 °C. These dispersions were then irradiated with UV light (wavelength 365 nm) for 10 min to observe the effect on the microgel particle properties, such as the adsorbed amount of the HMOS, the particle size, and the electrophoretic mobility. We show that irradiation of these dispersions with UV light can lead to induced, partial desorption of the HMOS molecules, with concomitant changes in the size and electrophoretic mobility of the microgel particles. This is due to a conformational switch (trans-form to cis-form) in the HMOS molecules, which reduces the strength of the hydrophobic interaction between the HMOS molecules and the isopropyl moieties within the microgel network. Moreover, the original absorbed amounts, size, and electrophoretic mobility values can be largely restored after storage in the dark for extended periods.     

Absorption of cetylpyridinium chloride into poly(N-isopropylacrylamide)-based microgel particles, in dispersion and as surface-deposited monolayers

The addition of cetylpyridinium chloride (CPC) to aqueous dispersions of poly(N-isopropylacrylamide) [poly(NIPAM)] and poly(N-isopropylacrylamide-co-acrylic acid) [poly(NIPAM-co-AAc)] microgel particles leads to absorption of the CPC into the particles and to corresponding changes in their hydrodynamic diameter. With the latter set of particles there is a strong pH dependence. The dependence of both hydrodynamic diameter and electrophoretic mobility of the microgel particles on the added CPC concentration show a strong correlation with CPC uptake, as obtained from direct CPC absorption measurements. Various mechanisms for CPC absorption into the microgel particles are postulated, including electrostatic, polar, and hydrophobic interactions. A comparison has also been made between the effect of added CPC on the hydrodynamic diameter of free microgel particles in dispersion, determined by dynamic light scattering, and the thickness of adsorbed monolayers of the same microgel particles deposited on cationically modified, oxidized silicon surfaces, as determined from ellipsometry measurements. The trends observed in both cases are broadly similar. This work opens the way for development of microgel layers for controlled uptake and release applications.     

Characterization of cross-linked poly(N-isopropylmethacrylamide) microgel latexes

 This article reports the colloidal and physicochemical characterization of crosslinked poly (N-isopropylmethacrylamide) (poly[NIPMAM]) latexes. The latex particles were first characterized by determining the lowest critical solubility temperature by measuring the optical density variation as a function of temperature. In addition, the electrophoretic mobility behaviors of all latexes were examined as a function of pH and temperature. Finally, the colloidal stability was investigated by determining the critical coagulation concentration and the critical flocculation temperature, and the corresponding results were discussed by taking into account both the effect of salinity and temperature on the solvency of poly[NIPMAM]. Received: 25 March 1999 Accepted: 5 May 1999     

Symmetric and asymmetric adsorption of pH-responsive gold nanoparticles onto microgel particles and dispersion characterisation

The interaction between carboxylic acid-stabilised gold nanoparticles (AuNP) and pH-responsive microgels is shown. The microgel particles are a copolymer of N-[3-(dimethylamino)propyl]methacrylamide (DMAPMA) and N-isopropylacrylamide (NIPAM). The microgel properties are presented by their hydrodynamic diameter and electrophoretic mobility in response to pH. These microgel particles are pH-responsive under neutral conditions decreasing in diameter beyond pH 7. The dispersion characteristics of AuNP adsorbed onto the microgel network are shown with respect to adsorbed amount and the pH-responsive properties of the AuNP. This data is presented between pH 3 and 6 where the microgel properties remain constant. Asymmetric adsorption of AuNP onto poly(DMAPMA-co-NIPAM) microgels is achieved by adsorption of nanoparticles, from the aqueous phase, onto microgel-stabilised oil-in-water emulsions. These asymmetrically modified microgels display very different dispersion behaviour, in response to pH, due to their dipolar nature.     

Polarizability and complex conductivity of dilute suspensions of spherical colloidal particles with uncharged (neutral) polymer coatings

The polarizability of polymer-coated colloidal particles, as measured via dielectric relaxation spectroscopy, reflects on the degree to which convection, diffusion, and electromigration deform the equilibrium double layer. With a polymer coating, convection and electro-osmosis are resisted by hydrodynamic drag on the polymer segments. The electro-osmotic flow near the underlying bare surface is therefore diminished. Characteristics of the particles and the adsorbed polymer can, in principle, be inferred by measuring the frequency-dependent polarizability. In this work, "exact" numerical solutions of the electrokinetic equations are used to examine how adsorbed polymer changes the particle polarizability and, hence, the conductivity and dielectric constant increments of dilute suspensions. For neutral polymer coatings, the conductivity and dielectric constant increments are found to be very similar to those of the underlying bare particles, so the response depends mostly on the underlying bare particles. These observations suggest that dielectric spectroscopy is best used to determine the underlying surface charge, with characteristics of the coating inferred from the electrophoretic or dynamic mobility, together with the hydrodynamic radius obtained from sedimentation or dynamic light scattering. Addressed briefly are the effects of added counterions and nonspecific adsorption. The electrokinetic model explored in this work can be used to guide experiments (frequency and ionic strength, for example) to either minimize or maximize the sensitivity of the complex conductivity to the coating thickness or permeability.     

Interaction of cetyl-trimethylammonium bromide with swollen and collapsed poly(N-isopropylacrylamide) nanogel particles

The interaction of cetyl-trimethylammonium bromide (CTAB) with swollen and collapsed poly(N-isopropylacrylamide) (pNIPAM) monodisperse nanogel particles was investigated by electrophoretic mobility, dynamic light scattering, and potentiometric surfactant activity measurements. The surfactant binds to the nanogel particles as monomers in the whole CTAB concentration range and binds in the form of surfactant aggregates as well above a critical concentration (cac) in both the swollen and collapsed state of the pNIPAM. The swollen particle system is a thermodynamically stable solution. The collapsed nanogel particle system is an electrically stabilized colloid dispersion, which coagulates when the particles are near the electrically neutral state. An analytically undetectably small amount of surfactant binding (5 × 10(-7) mol/g of pNIPAM) leads to a dramatic effect on the stability of the pNIPAM nanogel system. The electrokinetic potential versus surfactant concentration functions unexpectedly strongly depend upon the temperature around the lower critical solution temperature (LCST) of the polymer, which was interpreted by the change of the polymer segment density in the surface layer of the collapsing nanogel particles.     

The effect of electrosteric interactions on the effective charge of thermoresponsive ionic microgels: Theory and experiments

In this work, the influence of counterion valence and salt concentration on the effective charge of two types of thermoresponsive ionic microgel particles has been studied. The effective charge of the microgel at different swelling states has been experimentally determined from electrophoretic mobility measurements by solving the electrokinetic equations of the solvent for a single polyelectrolyte brush in the presence of an electric field, taking into account the friction of the solvent inside the polymer network. The experimental results have been compared to those obtained by means of the Ornstein‐Zernike integral formalism within the HNC relation. Results show that microgel bare charge is screened by the combined effect of counterion condensation and permeation inside the microgel particle. In addition to the electrostatic interaction, the steric exclusion exerted by the polymer plays an important role on the local ionic concentrations, especially for shrunken configurations. This steric term is responsible for the strong increase of the microgel effective charge experimentally observed when particles shrink for temperatures above the lower critical solution temperature. We also observe that, in the internal region of the microgel, charge electroneutrality is fulfilled, so the effective charge mainly arises from the region close to the microgel surface. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2038–2049.     

The Effect of Mechanical Treatment on Colloidal and Chemical Properties and Reactivity of Powdered Cellulose

The effect of mechanical activation on colloidal and chemical properties of aqueous dispersions of powdered cellulose (particle size, electrokinetic parameters, surface electrical properties, adsorption, etc.) and its solubility in a methylmorpholine N-oxide monohydrate–dimethyl sulfoxide mixture is studied. A rise in the number of mechanical treatment cycles increases the surface charge density, adsorbability, and solubility of cellulose, but decreases particle size and electrophoretic mobility, as well as the viscosity of cellulose solutions due to a reduction in the degree of cellulose polymerization, development of microporosity, and a rise in the internal surface area of polymer particles as a result of their disintegration.     

Colloid Vibration Potential in a Concentrated Suspension of Spherical Colloidal Particles

A relation between the dynamic electrophoretic mobility of spherical colloidal particles in a concentrated suspension and the colloid vibration potential (CVP) generated in the suspension by a sound wave is obtained from the analogy with the corresponding Onsager relation between electrophoretic mobility and sedimentation potential in concentrated suspensions previously derived on the basis of Kuwabara's cell model. The obtained expression for CVP is applicable to the case where the particle zeta potential is low, the particle relative permittivity is very small, and the overlapping of the electrical double layers of adjacent particles is negligible. It is found that CVP shows much stronger dependence on the particle volume fraction φ than predicted from the φ dependence of the dynamic electrophoretic mobility. It is also suggested that the same relation holds between the electrokinetic sonic amplitude of a concentrated suspension of spherical colloidal particles and the dynamic electrophoretic mobility. Copyright 1999 Academic Press.     

Single step hybrid coating process to enhance the electrosteric stabilization of inorganic particles

We report on a single-step coating process and the resulting colloidal stability of silica-coated spindle-type hematite nanoparticles (NPs) decorated with a layer of poly(acrylic acid) (PAA) polyelectrolyte chains that are partially incorporated into the silica shell. The stability of PAA coated NPs as a function of pH and salt concentration in water was compared to bare hematite particles and simple silica-coated hematite NPs, studying their electrophoretic mobility and the hydrodynamic radius by dynamic light scattering. Particles coated with this method were found to be more stable upon the addition of salt at pH 7, and their aggregation at the pH of the isoelectric point is reversible. The hybrid coating appears to increase the colloidal stability in aqueous media due to the combination of the decrease of the isoelectric point and the electrosteric stabilization. This coating method is not limited to hematite particles but can easily be adapted to any silica-coatable particle.     

Electrokinetics of soft polymeric interphases with layered distribution of anionic and cationic charges

Soft surface coatings attract increasing attention due to the versatile options they provide in numerous applications e.g. in the flourishing nanomedicine and nanobiotechnology areas. Optimisation of the performance of such ion- and solvent-permeable polyelectrolytic materials requires a detailed understanding of their electrostatic properties. This task is rendered difficult by the inherent non-uniform distribution of their structural charges. In this article, we review recent advances made in the measurement and theory of the electrokinetics (electrophoresis/streaming current) of soft surface coatings that carry spatially-separated cationic and anionic charges. Examples of such charge-stratified systems are polyelectrolyte-coated particles, polyelectrolyte multilayers, particles with zwitterionic interfacial functionality, microbial cells or hard–soft composite interfaces. It is shown here that the electrokinetic features of such colloidal systems are remarkably different from those of their counterparts with homogeneously distributed cationic and anionic charges. In particular, the interplay between electrostatic and hydrodynamic flow fields developed under electrokinetic conditions in the bulk and interfacial compartments of charge-stratified colloids/films are shown to induce a reversal of their electrokinetic response (electrophoretic mobility/streaming current) that depends on the concentration of monovalent electrolyte in solution. The prerequisites for occurrence of such spectacular behaviour are theoretically identified in terms of the Debye length, the spatial length scales defining charge layering, and the typical length for flow penetration within the colloids/films. Electrophoresis and streaming current results recently reported for poly(amidoamine) carboxylated nanodendrimers, natural rubber colloids and poly(ethyleneimine)-supported lipid bilayers are further discussed to illustrate the generic electrokinetic properties of soft interfaces defined by a given stratification of their anionic and cationic structural charges.     

Hydrophilic stimuli‐responsive particles for biomedical applications

Hydrophilic and stimuli‐responsive submicronic latex particles based on polyalkyl(meth)acrylamide can be prepared owing to simple radical‐initiated polymerizations in heterogeneous media using a water‐soluble initiator and a crosslinker (methylenebisacrylamide). The paper aims at reviewing the synthesis and properties of functionalized polystyrene‐polyN‐isoprpylacrylamide core‐shell particles or polyN‐isopropylmethacrylamide microgel particles. Particle size of analysis showed that a short nucleation period afforded the synthesis of highly monodispersed latexes. The dramatic change of the colloidal properties (particle size, electrophoretic mobility) was found to reflect the thermal sensitivity of such particles. The hydrophilic nature of the particles below the volume phase transition temperature was found to drastically reduce the physical adsorption of proteins. Some examples of biomedical applications of these stimuli‐responsive particles are briefly reported.     

Polymer adsorption onto a micro-sphere from optical tweezers electrophoresis

We explore the design and operation of an optical-tweezers electrophoresis apparatus to resolve polymer adsorption dynamics onto a single micro-sphere in a micro-fluidic environment. Our model system represents a broader class of micro-fluidic electrophoresis experiments for biosensing and fundamental colloid and surface science diagnostics. We track the adsorption of 100 kDa poly(ethylene oxide) homopolymer onto a colloidal silica sphere that is optically trapped in a crossed parallel-plate micro-channel. The adsorption dynamics are probed on the ～1 μm particle length scale with ～1 s temporal resolution. Because the particle electrophoretic mobility and channel electro-osmotic flow are exquisitely sensitive to the polymer layer hydrodynamic thickness, particle dynamics can be complicated by polymer adsorption onto the micro-channel walls. Nevertheless, using experiments and a theoretical model of electro-osmotic flow in channels with non-uniform wall ζ-potentials, we show that such influences can be mitigated by adopting a symmetrical flow configuration. The equilibrium hydrodynamic layer thickness of 100 kDa poly(ethylene oxide) on colloidal silica is ～10 nm at polymer concentrations ?10 ppm (weight percent), with the dynamics reflecting polymer solution concentration, flow rate, and polydispersity.