Smart Colloidal Materials

Thermoresponsive poly(N-isopropylacrylamide) (pNIPAm) core/shell particles bearing primary amine groups in either core or shell were prepared via two-stage, free radical precipitation polymerization, using 2-aminoethyl methacrylate (AEMA) as a comonomer. The amine groups were then used to initiate ring-opening polymerization of γ-benzyl L-glutamate N-carboxyanhydride (BLG-NCA), producing poly(γ-benzyl L-glutamate) (PBLG) side chains covalently anchored to the particles. Photon Correlation Spectroscopy (PCS) and ¹H NMR were employed to characterize these particles. A shift of phase transition to a lower temperature and an increase in particle des welling volume ratios were observed as a result of grafting hydrophobic PBLG chains to the particles. Further studies by ¹H NMR in different solvents indicate that the PBLG chains grafted from the particle shell phase separate on the pNIPAm networks in aqueous media but remain well solvated in DMSO. Together, these results suggest that both core- and shell-grafted architectures can be synthesized with equal ease, and that the particle structure and colloidal behavior can be manipulated by tuning the relative solubility of the network and graft portions of the particle.

This short article is a condensed review of recent work devoted to thermally sensitive based polymer particles and their potential applications as biomolecules carriers in biomedical diagnostic. Firstly, several aspects related to synthesis of different thermally sensitive colloidal particles are presented. Secondly, the general colloidal properties of such particles are reported and illustrated. Finally, some fine applications of reactive, hydrophilic thermally sensitive particles in biomedical diagnostic are briefly presented.

Microparticles offer various significant advantages as drug delivery systems, including: (i) an effective protection of the encapsulated active agent against (e.g. enzymatic) degradation, (ii) the possibility to accurately control the release rate of the incorporated drug over periods of hours to months, and (iii) an easy administration (compared to alternative parenteral controlled release dosage forms, such as macro-sized implants). Desired, pre-programmed drug release profiles can be provided which match the therapeutic needs of the patient. This article gives an overview on the most important past, current and future strategies using drug-loaded microparticles to improve the efficiency of various medical treatments. Special emphasis is laid on the different types of preparation techniques that are commonly used, the physicochemical properties of the devices and practical examples illustrating the considerable benefits of this type of advanced drug delivery systems. But also the major challenges and obstacles to be overcome during the development and production of these pharmaceutical dosage forms are pointed out.

Molecular exchange through membranes of dispersed vesicles is studied using nuclear magnetic resonance spectroscopy combined with pulsed field gradients. Encapsulated molecules are differentiated from those in the continuous phase by their mean square displacement depending on a variable time interval Δ. Generally, two field gradient pulses are combined with a stimulated pulse echo sequence. In case of small capsules and vesicles with diameters below 1 µm, the Brownian motion of the capsules dominates the lateral motion for the encapsulated fraction and the echo decay curve can be analyzed with a simple analytical approach. In case of larger vesicles and capsules with diameters above 1 µm, the diffusion inside the encapsulated domain becomes the dominating phenomenon and the echo decay has to be fitted with a numerical approach based on a finite element approximation. For very slow exchange processes with average residence times above 10 s, permeation is directly observed in a time resolved measurement on the non-equilibrium state. In all cases, a careful analysis yields data on the release of a given encapsulated ingredient as well as, under variation of the tracer molecule, on the specific permeability of the capsule walls for molecules of various size, polarity or flexibility.

Copolymerization of methylmethacrylate (MMA) and the polymerisable surfactant (surfmer) p-(11(acrylamido)undecanoyloxy) — phenyldimethylsulfonium methylsulfate (AUPDS) by emulsion polymerization resulted p(MMAco-AUPDS) particles with a yield > 95%. The particle diameter depended on the surfmer feed concentration and was to be chosen in the range of 116 nm to 145 nm. The particle surface area per activated ester surfmer residue depended on the surfmer concentration and amounted to 0.7 nm² to 1.24 nm². All areaper — charge values were found to be smaller then the analogous value from cmc measurements of an AUPDS solution in ultrapure water which was found to be 1.42 nm². The protein conjugate streptavidin peroxidase (SAv-POD) was immobilized covalently on the particle surface by incubating particles in PBS buffer containing this enzyme. By addition of the enzyme substrate 3,3′,5,5′ tetramethylbenzidine (TMB) the activity of the particle-bound enzymes was determined at pH 7.2 to (5.3±0.2) mUmg⁻¹. The enzyme activity was raised to (15.5±0.2) mUmg⁻¹ when the particles were incubated in a pH 8.0 buffer. The non-specific binding was determined to 37% when compared to the total binding by incubating particle blank samples with the same amount of SAv-POD.

Swelling properties of doubly temperature sensitive core-shell microgels consisting of two thermosensitive polymers namely poly-N-isopropylacrylamide (PNIPAM) with a lower critical solution temperature (LCST) at ca. 34 °C and poly-N-isopropylmethacrylamide (PNIPMAM) with a LCST of ca. 44 °C have been investigated by small-angle neutron scattering (SANS). Two types of microgels with different core-shell composition were studied: PNIPAM-core — PNIPMAM-shell microgels as well as a microgel with inverse structure, i.e. PNIPMAM-core — PNIPAM-shell. A core-shell form factor has been employed to evaluate the structure and the real space particle structure is expressed by radial density profiles. By this means the influences of composition and temperature on the internal structure have been revealed. At temperatures between the LCSTs the swelling of the PNIPMAM-shell leads to an expansion of the PNIPAM-core. At temperatures below the core LCST, the core cannot swell to its native size (i.e. in the absence of a shell) because the maximum expanded shell network prohibits further swelling. Thus depending on temperature the shell either expands or compresses the core. The inverse PNIPMAM-core — PNIPAM-shell microgel displays qualitatively different behavior. At intermediate temperatures, the segment density of the shell is higher as compared to the core. Since the density ratio of shell and core depends on temperature, such core-shell microgels provide interesting opportunities for encapsulation and controlled release.

We prepared giant unilamellar vesicles (GUVs) enclosing solutions or covalent gels of Poly(Nisopropylacrylamide) (PolyNipam). Concentrated suspensions of GUVs were prepared by applying an alternative field on a lipid film hydrated by a monomer solution containing N-isopropylacrylamide, crosslinker (N,N′-methylene-bis-acrylamide), initiator and sucrose. Vesicle inner medium was polymerised and cross-linked by UV irradiation of the suspension, yielding viscous vesicles enclosing a solution of linear PolyNipam chains (when no bisacrylamide was used) or elastic vesicles filled with a covalent PolyNipam gel. We show that gel-filled vesicles are responsive systems triggered by the temperature: they shrink, reducing by a factor eight their volume below the critical temperature (32 °C in water, lower in glucose solution) and re-swell in a reversible and reproducible way upon decreasing temperature. In both cases, we show that the vesicle lipid membrane interacts with the internal polymer, resulting in an strong resistance of the vesicles to external mechanical stresses (enhanced tension of lysis).

We investigated the potential profile between colloidal probes floating above a polyelectrolyte multilayer (PEM), built up by the layer by layer technique with Total Internal Reflection Microscopy (TIRM). The interaction between a single poly(ethyleneimine) layer and an amino-terminated polystyrene latex sphere can be accurately described by the superposition of electrostatic repulsion and gravity. PEM with more than one layer exhibit laterally inhomogeneous potentials with extremely long ranging repulsive contributions.

We review recent experiments on the interaction of proteins with anionic polyelectrolytes in aqueous solution. Data from literature demonstrate that proteins can form soluble complexes even on the “wrong side” of the isoelectric point, that is, for pH values above the isoelectric point of the proteins under which the polyelectrolytes and the proteins are like-charged. All data published so far demonstrate that this type of adsorption becomes weaker with increasing ionic strength. A much stronger interaction is found if the polyelectrolyte chains are grafted onto solid surfaces to form polyelectrolyte brushes. Here it has been shown that spherical polyelectrolyte brushes consisting of a core of ca. 100 nm diameter and long attached polyelectrolyte chains strongly adsorb proteins at low ionic strength (“polyelectrolyte-mediated protein adsorption”; PMPA). Virtually no adsorption takes place onto the spherical polyelectrolyte brushes at high ionic strength. A critical comparison of data obtained on free polyelectrolytes and on polyelectrolyte brushes shows that both phenomena can be traced back to patches of positive charge on the surface of the proteins. Moreover, the PMPA may directly be related to the Donnan-pressure within the brush layer.

The DLVO theory of colloidal particle interactions has been at the core of colloid science for a long time. Quantitatively, agreement between experiment and theory was illusory except at salt concentrations less than about 10-2 molar. The same problem with theory exists for pH measurements, buffers, electrochemistry, zeta potentials, electrolyte activities, interfacial tension of salt solutions and a host of phenomena that depend on so called specific ion effects, This is so, most dramatically in biology, but also in colloid, polymer and surface science generally. The problems date back to Hofmeister whose work stands in the scheme of things as Mendel’s did to genetics. Where problems occurred we have tended to argue them away, capturing specificity in unquantifiable terms embodied in words like cosmotropes, chaotropes, hydrophilicity, hydrophobicity, soft and hard ions, pi-cation interactions, hydration and hydrophobic forces, water structure. To complicate the puzzle further the role of dissolved atmospheric gas or other sparingly soluble (hydrophobic) solutes is sometimes major, and has been completely ignored in theories or simulations.

Some progress in unravelling these difficulties has been made. It turns out that theories have been seriously flawed. They depend on an ansatz that separates electrostatic forces from the totality of non electrostatic (NES) quantum mechanical electrodynamic fluctuation (Lifshitz or dispersion) forces. These NES forces are ignored, as for the Born self energy of an ion, or its decorations. Or else the electrostatic forces are treated in a non linear theory (e.g. Poisson Boltzman), and the quantum forces via Lifshitz theory as for DLVO. Even for the continuum solvent approximation this violates both the Gibbs adsorption equation, and the gauge condition on the electromagnetic field.

These problems are highly non trivial and occur equally in quantum field theories and biophysical problems that couple electron and photon transfer.

When the faults are repaired, the revised theory does seem to account for ion specificity and a veritable zoo of postulated new forces begin to fall into place quantitatively. An account will be given of the emerging situation, the role of dissolved gas and “hydrophobic” forces. This leads to new insights into the necessary cooperativity that occurs with water in biological and other systems.

For the classical DLVO theory, which deals only with electrostatic forces acting between ions and colloids, all ions in solution with the same charge should result in the same force between colloids. Ion specificity does occur in the opposing attractive Lifshitz forces but only very weakly. Ion size parameters, inner and outer Helmholtz planes are used to fit the specificity but that do not work. At, and above, biological salt concentrations other, non electrostatic (NES) ion specific forces act that are ignored in such modeling. To exemplify the general ideas we use a system that corresponds to pairs of nanoparticles. We show that ion specific double layer forces can be understood once NES forces acting between ions and colloids are included consistently in non-linear theory and in Monte Carlo simulations.

We consider a model to describe starlike polymers featuring a steric repulsion accompanied by a dispersion- or depletion-induced tunable attraction. The range and depth of the latter can be controlled by suitable choices of the solvent, salt concentration and/or depletant size and type, whereas the strength of the steric repulsions is set by the arm number f of the stars. We focused on star polymers with arm number f=32. Depending on the choice of the attraction characteristics and on the temperature, the system exhibits, in addition to the usual ultra-soft repulsion, a relatively short range barrier at longer distances. Our results show a variety of structurally distinct states. In the fluid phase we find evidence for cluster formation which is accompanied by fluid-phase separation. Moreover the system presents unexpected fluid-solid transitions which are completely absent for the purely repulsive case. The dependence of the cluster and solid regions, and the location of the critical point on the potential parameters is quantitatively analyzed.

The Debye-Hückel-Potential in combination with an effective or renormalized charge is a widely and often successfully used concept to describe the interaction in charged colloidal model systems and the resulting suspension properties. In particular the phase behaviour can be described in dependence of the parameters particle number density, salt concentration and effective charge. We performed simultaneous measurements of the phase behaviour, the shear modulus and the low frequency conductivity of deionised aqueous suspensions of highly charged colloidal spheres. From the shear modulus the interaction potential at the nearest neighbour distance in terms of a Debye-Hückel potential can be determined with an effective charge Z*_G as free parameter. Conductivity measures the number of freely moving small ions Z*_σ and thus relates to the ion condensation process in the electric double layer under conditions of finite macroion concentrations. We present the first experimental access of the pair energy of interaction in charged colloidal suspensions which describes both the elastic properties and the fluid crystalline phase behaviour. This means that a consistent description of the suspension properties is obtained, when Z*_G is taken from the elasticity measurement.

A recently derived Difusion equation [Uvarov A, Fritzsche S (2004) J Chem Phys 121(13):6561] is utilized to analyze the restricted rotational motion of macromolecules in solution if they are immobilized on a surface. Both, the bead-bead and bead-surface interactions are taken into account in order to describe the orientational dynamics of non-rigid macromolecules and its relaxation in time after a perturbation has occured. Using several realistic bead-bead and bead-surface potentials, detailed numerical investigations have been carried out for the rotational diffusion coefficient as well as for the conformational phase-space distribution function of the macromolecules. From this phase-space distribution, the orientational correlation function are derived and compared with phenomenological computations from the literature. Such correlation function can be observed in dielectric relaxation and fluorescence depolarization experiments.

We review recent progress concerning an understanding of the rheological properties of foams, both in bulk form and confined in narrow channels, and including the problem of foam sliding along a solid wall. Our calculations contribute not only to the interpretation of rheological data, but also to the coupling of foam drainage and rheology.

A photo-destructible surfactant sodium 4-hexylphenylazosulfonate (C6-PAS, Scheme 1) has been employed in normal AOT-stabilized water-in-heptane microemulsions. Phase studies are consistent with initially homogeneous microemulsions, for which significant changes in stability as a function of UV irradiation time are observed. Photolysis of C6-PAS in these systems results in eventual separation of water to yield a Winsor II system. Proton NMR spectra show that C6-PAS undergoes UV-induced decomposition, to yield a mixture of 4-hexylphenol and the non-surface active hexylbenzene as main product. This photo-triggered breakdown gives rise to changes in adsorption and aggregation properties of C6-PAS, representing a unique route to induce microemulsion destabilization. Small-angle neutron scattering (SANS) was used to follow the resulting UV-induced shrinkage of the water nanodroplets: a maximum volume decrease was found to be in the order of 60–70%. Multi-contrast SANS experiments gave further insight, for example it was demonstrated that the surfactant shell thickness remained constant (∼ 9 Å). This study represents a novel example of light-induced microemulsion destabilization.

We studied the thermal diffusion behavior of C₁₀E₈ (decyl octaethylene glycol ether) in water by means of thermal diffusion forced Rayleigh scattering (TDFRS). We determined the two diffusion coeffi- cients D _T, D and the Soret coefficient S _T in a concentration range from w = 5 wt% to 25 wt% in a temperature range from T = 20 °C to 40 °C. The obtained Soret coefficients S _T were positive for all temperatures and concentrations. Additionally, we also performed dynamic light scattering experiments in the same temperature range in order to compare the measured diffusion constants and characterize the system. Special attention was paid to the tiny amount of inert dye which needs to be added for absorption and thermalization of the light energy. The influence of an organic dye and an organic coloured salt on the experimentally determined transport properties has been studied. The results show that all coefficients are independent of the choice of the dye for this particular surfactant system.

Spectroscopic techniques (UV absorption, fluorescence and circular dichroism) are applied for probing the conformational stability of lysozyme as a model protein after the impact of surfactants. The investigations allow the equilibrium constant, K, and the free energy change, ΔG, of the transition from the folded (native) to the unfolded (denatured) state to be estimated. ΔG at 25 °C in the absence of additives allows quantifying the conformational stability of the protein. Though the results are based on the validity of several assumptions regarding folding/unfolding mechanism, evaluation procedure, and environmental conditions, the thermodynamics of surfactant-induced unfolding may be estimated. Compared to the unfolding induced by the chaotropic denaturant guanidinium chloride, cationic and zwitterionic surfactants are found to yield lower ΔG values. In the case of lysozyme, anionic and nonionic surfactants did not result in transition curves. The interpretation of the transition curves indicated the existence of a two-state behavior. Quantities which do not significantly depend on the unfolding mechanism, such as the midpoints of denaturant concentrations and thermal unfolding curves, c _1/2 and T _m, may also be applied for comparing conformational stabilities of proteins, even in the case of irreversible transitions. The evaluation of the thermal denaturation allows the derivation of enthalpy and entropy changes, ΔH and ΔS.

The title cationic surfactants were synthesized by the scheme in Fig. 1, where RCO₂H refers to decanoic, dodecanoic, tetradecanoic and hexadecanoic acid, respectively. In aqueous solution, the micelle/water interface may be located at the quaternary ammonium ion or at the amide group. The following pieces of evidence indicate that the interface lies at the latter site: theoretically calculated aggregation numbers and those determined by static light scattering; dependence on surfactant concentration, below and above the critical micelle concentration, cmc, of both the IR frequency of amide I band and ¹H NMR chemical shifts of the discrete surfactant protons. Solution conductance and calorimetric titration have been employed to study the aggregation of these surfactants in water at 25 °C. Increasing the length of R resulted in a decrease of the cmc and the degree of counter-ion dissociation, α_mic. Gibbs free energies of micelle formation were calculated and divided into contributions from the methylene groups of the hydrophobic tail, and the terminal methyl plus head-group. The former are similar to those of other surfactants, whereas the latter are more negative, i.e., the transfer of the head-group from bulk water to the micelle is more favorable. This is attributed to direct or water-mediated H-bonding of the micellized surfactant molecules, via the amide group, in agreement with the IR data presented.

Microscopic tracking of silica tracer particles suspended in mixtures of a nematogen (5CB) and small amounts of alkane reveals that during an isotropic-nematic phase transition induced by slow cooling particle transport is dominated by long-range interaction with the isotropic-nematic interface for the most part of phase transition. According to the phase diagram of liquid crystal-alkane mixtures, this long-range interaction was attributed to hydrodynamic (advection) or thermodynamic (local concentration gradients) effects of alkane redistribution from the nematic into the isotropic domains during phase transformation.

Nano-scaled spherical silica particles were directly coated with titania nanoparticles by means of heterocoagulation. Silica was prepared by the Stöber method, titania by a hydrolysis-condensation reaction of tetrapropyl-orthotitanate under acidic conditions. The photocatalytic activity of the coated spheres was investigated by a smog chamber technique, designed for the simulation of tropospheric photodegradation of hydrocarbons adsorbed on aerosol surfaces. In the chamber different solar spectra and irradiation intensities can be simulated and therefore the degradation can be monitored under different conditions. The degradation itself is dominated by OH radicals. Besides the monitoring, the reaction kinetics of particular hydrocarbon degradations was determined in the smog chamber. The steady-state concentration of OH radicals is higher in the presence of titania, and furthermore a heterogeneous photodegradation on the titania surface occurs.

In polycation-modified SDS/decanol systems, dense multilamellar structures, i.e. multilamellar vesicles are formed by self-organization, which were used as an organic template for CdS nanoparticle preparation. Specific amounts of precursors, CdCl₂ and Na₂S, were incorporated into the multilamellar vesicles without losing the multilamellar structure. Structural changes of the lamellar liquid crystalline template induced by the incorporation of the polycation and the inorganic precursors were investigated by differential scanning calorimetry in combination with small angle X-ray scattering. By mixing both precursors within the multilamellar vesicles CdS nanoparticles are formed. After decomposition of the vesicle template, quite different shaped and sized CdS-nanoparticles were observed by transmission electron microscopy. At lower polymer concentration spherical CdS nanoparticles of about 10 nm can be obtained. At higher polymer concentration predominantly rod-like CdS aggregates were produced with an average length of 120 nm and width of 30 nm.

Polishing slurries for the copper chemical mechanical planarization (CMP) process consist of a complex composition of highly stable nanoparticle suspensions in the presence of chemical additives for etching and protecting the surface. The target of such dispersions is to achieve perfectly smooth surfaces of low topography. Key factors are the surface roughness, local geometry and total flatness of the whole wafer. Surface defects like scratches, dishing, etching and erosion e.g. due to strong particle adsorption, aggregates or chemical impact have to be avoided. Particle wafer interactions between silica particles of a silica CMP-dispersion and a copper wafer surface were analyzed by electron cpectroscopy for chemical analysis (ESCA), scanning electon microscopy (SEM) and zetapotential-measurements. The colloidal stability of the silica dispersion over a broad pH-range of 2.3 to 9.8 was analyzed in the presence of Na⁺-, Mg²⁺-, Cu²⁺-, and Al³⁺-ions in terms of the critical coagulation concentration (CCC), change in particle size and pH. Silica particles strongly and irreversibly adsorbed to the predominantly oxidic copper wafer surface at pH 2.3 but not at pH > 4. Over the whole pH-range a high colloidal stability was observed with a maximum at pH 2.3. CCC-values of 100–300 mmol/L versus Cu²⁺ were obtained. Even at high pH of 9.8, the behaviour could not be explained by the Derjaguin- Landauer-Verley-Overbeek (DLVO) theory. In the presence of Cu²⁺-ions a higher colloidal stability compared to divalent Mg²⁺-ions was observed at high initial slurry-pH of 9.8. Due to the acidic reaction of Cu²⁺ in the aqueous environment, the pH was reduced to 2–3, where colloidal silica showed the highest stability. Even at high removal rates in the polishing process of 1000 nm/min, the released Cu²⁺-concentration (40 mmol/L) was lower than the critical coagulation concentration (100 mmol/L).

Porous bead celluloses have wide ranging applications as separation media and carrier systems. Physicochemical characterization of these materials and related suspensions is essential for quality control and technical applications. This includes their surface properties, porosities and mechanical properties.

The paper describes the application of multisample analytical centrifugation for porous bead celluloses.

Bead celluloses of different sizes and differing in the preparation process were chosen and analysed in respect to porosity and particle interactions.

The consolidation, packing behaviour and elasticity of the bead celluloses was analysed in an alternating centrifugal field.

Differences in packing density obtained under controlled conditions were used to calculate the porosities of the samples. Multisample analytical centrifugation provides a powerful alternative for determination of the cumulative porosity, which was elaborated in this study. This approach is generally applicable for other particulate material if strong flocculation between particles can be avoided.

Further, different preparation processes lead to an alteration of the surface properties of the bead celluloses. Bead celluloses revealed a markedly different behaviour than cellulose powder manufactured from native cellulose.