Precipitation-redispersion of cerium oxide nanoparticles with poly(acrylic acid): toward stable dispersions

We exploit a precipitation-redispersion mechanism for complexation of short chain polyelectrolytes with cerium oxide nanoparticles to extend their stability ranges. As synthesized, cerium oxide sols at pH 1.4 consist of monodisperse cationic nanocrystalline particles having a hydrodynamic diameter of 10 nm and a molecular weight of 400 000 g mol(-1). We show that short chain uncharged poly(acrylic acid) at low pH when added to a cerium oxide sols leads to macroscopic precipitation. As the pH is increased, the solution spontaneously redisperses into a clear solution of single particles with an anionic poly(acrylic acid) corona. The structure and dynamics of cerium oxide nanosols and their hybrid polymer-inorganic complexes in solution are investigated by static and dynamic light scattering, X-ray scattering, and chemical analysis. Quantitative analysis of the redispersed sol gives rise to an estimate of 40-50 polymer chains per particle for stable suspension. This amount represents 20% of the mass of the polymer-nanoparticle complexes. This complexation adds utility to the otherwise unstable cerium oxide dispersions by extending the range of stability of the sols in terms of pH, ionic strength, and concentration.     

Size, charge, and interactions with giant lipid vesicles of quantum dots coated with an amphiphilic macromolecule

Semiconductor colloidal quantum dots (QDs) are promising fluorescent probes for biology. Initially synthesized in organic solvents, they can be dispersed in aqueous solution by noncovalent coating with amphiphilic macromolecules, which renders the particles hydrophilic and modifies their interactions with other biological compounds. Here, after coating QDs with an alkyl-modified polyacrilic acid, we investigated their colloidal properties in aqueous buffers by electrophoresis, electron microscopy, light scattering, and rate zonal centrifugation. Despite polymer dispersity and variation of the size of the inorganic nanoparticles, the polymer-dot complexes appeared relatively well-defined in terms of hydrodynamic radius and surface charge. Our data show that these complexes contain isolated QD surrounded by a polymer layer with thickness 8-10 nm. We then analyzed their interaction with giant unilamellar vesicles, either neutral or cationic, by optical microscopy. At neutral pH, we found the absence of binding of the coated particles to lipid membrane, irrespective of their lipid composition. An abrupt surface aggregation of the nanoparticles on the lipid membrane was observed in a narrow pH range (6.0-6.2), indicative of critical binding triggered by polymer properties. The overall features of QDs coated with amphiphilic polymers open the route to using these nanoparticles in vivo as optically stable probes with switchable properties.     

Nanoparticles prepared by self-assembly of Chitosan and poly-γ-glutamic acid

The present paper describes the preparation and characterization of novel biodegradable nanoparticles based on self-assembly of poly-gamma-glutamic acid (γ-PGA) and chitosan (CH). The nanosystems were stable in aqueous media at low pH conditions. Solubility of the systems was determined by turbidity measurements. Surface charge and mobility were measured electrophoretically. The particle size and the size distribution of the polyelectrolyte complexes were identified by dynamic light scattering and transmission electron microscopy (TEM). It was found that the size and size distribution of the nanosystems depends on the concentrations of γ-PGA and CH solutions and their ratio as well as on the pH of the mixture and the order of addition. The diameter of individual particles was in the range of 20–285 nm measured by TEM, and the average hydrodynamic diameters were between 150 and 330 nm. These biodegradable, self-assembling stable nanocomplexes might be useful for several biomedical applications.     

Templating CdSe tetrapods at the air/water interface with POPC lipids

ZnS nanoparticles were precipitated in aqueous dispersions of cationic surfactant cetyltrimethylammonium bromide (CTAB). The sphere radii of ZnS nanoparticles calculated by using band-gap energies steeply decreased from 4.5 nm to 2.2 nm within CTAB concentrations of 0.4-1.5 mmol L(-1). Above the concentration of 1.5 mmol L(-1), the radii were stabilized at R=2.0 nm and increased up to R=2.5 nm after 24 h. The hydrodynamic diameters of CTAB-ZnS structures observed by the dynamic light scattering (DLS) method ranged from 130 nm to 23 nm depending on CTAB concentrations of 0.5-1.5 mmol L(-1). The complex structures were observed by transmission electron microscopy (TEM). At the higher CTAB concentrations, ZnS nanoparticles were surrounded by CTA(+) bilayers forming positively charged micelles with the diameter of 10nm. The positive zeta-potentials of the micelles and their agglomerates were from 16 mV to 33 mV. Wurtzite and sphalerite nanoparticles with R=2.0 nm and 2.5 nm covered by CTA(+) were modeled with and without water. Calculated sublimation energies confirmed that a bilayer arrangement of CTA(+) on the ZnS nanoparticles was preferred to a monolayer.     

Maghemite nanoparticles protectively coated with poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)

Superparamagnetic iron oxide particles (SPIO) of maghemite were prepared in aqueous solution and subsequently stabilized with polymers in two layer-by-layer deposition steps. The first layer around the maghemite core is formed by poly(ethylene imine) (PEI), and the second one is formed by poly(ethylene oxide)-block-poly(glutamic acid) (PEO-PGA). The hydrodynamic diameter of the particles increases stepwise from D(h) = 25 nm (parent) via 35 nm (PEI) to 46 nm (PEI plus PEO-PGA) due to stabilization. This is accompanied by a switching of their zeta-potentials from moderately positive (+28 mV) to highly positive (+50 mV) and finally slightly negative (-3 mV). By contrast, the polydispersity indexes of the particles remain constant (ca. 0.15). M?ssbauer spectroscopy revealed that the iron oxide, which forms the core of the particles, is only present as Fe(III) in the form of superparamagnetic maghemite nanocrystals. The magnetic domains and the maghemite crystallites were found to be identical with a size of 12.0 +/- 0.5 nm. The coated maghemite nanoparticles were tested to be stable in water and in physiological salt solution for longer than 6 months. In contrast to novel methods for magnetic nanoparticle production, where organic solvents are necessary, the procedure proposed here can dispense with organic solvents. Magnetic resonance imaging (MRI) experiments on living rats indicate that the nanoparticles are useful as an MRI contrast agent.     

Formation of particles of metal hydroxides in water solution of polymer

The stages of the formation of metal hydroxide particles in water medium were described. The first stage is the formation of complexes between the stabilizer and metal ion or of metal polyions containing a few metal ions. Dependence between the number of metal ions in polyion and its charge on the pH is described. The second stage is the formation of the metal hydroxide particles by the aggregation of polyions or their adsorption on/in the stabilizer. The distribution of the polyion number in the particles in polyion aggregation is described by the equation p(k) = k(ks-1)(k-1)/ksk and in polyion adsorption on/in the stabilizer particles is more narrow and is described by the equation p(k) = exp(-ks)/ks(-k)/k!, where k and ks are the number and the average number of polyions in the particle.     

Effect of poly(ethylene oxide)-silane graft molecular weight on the colloidal properties of iron oxide nanoparticles for biomedical applications

The size, charge, and stability of colloidal suspensions of magnetic nanoparticles with narrow size distribution and grafted with poly(ethylene glycol)-silane of different molecular weights were studied in water, biological buffers, and cell culture media. X-ray photoelectron spectroscopy provided information on the chemical nature of the nanoparticle surface, indicating the particle surfaces consisted of a mixture of amine groups and grafted polymer. The results indicate that the exposure of the amine groups on the surface decreased as the molecular weight of the polymer increased. The hydrodynamic diameters correlated with PEG graft molecular weight and were in agreement with a distributed density model for the thickness of a polymer shell end-grafted to a particle core. This indicates that the particles obtained consist of single iron oxide cores coated with a polymer brush. Particle surface charge and hydrodynamic diameter were measured as a function of pH, ionic strength, and in biological buffers and cell culture media. DLVO theory was used to analyze the particle stability considering electrostatic, magnetic, steric, and van der Waals interactions. Experimental results and colloidal stability theory indicated that stability changes from electrostatically mediated for a graft molecular weight of 750 g/mol to sterically mediated at molecular weights of 1000 g/mol and above. These results indicate that a graft molecular weight above 1000 g/mol is needed to produce particles that are stable in a wide range of pH and ionic strength, and in cell culture media.     

Dissolution of ZnO nanoparticles at circumneutral pH: a study of size effects in the presence and absence of citric acid

Understanding size-dependent processes, including dissolution, of engineered nanoparticles is essential in addressing the potential environmental and health impacts of these materials as well as their long-term stability. In this study, experimental measurements of size-dependent dissolution of well-characterized zinc oxide (ZnO) nanoparticles with particle diameters in the range of 4 to 130 nm have been measured at circumneutral pH (pH 7.5) and compared. Dissolution was found to be enhanced with smaller ZnO nanoparticles compared to larger-sized particles, even though the nanoparticles were present in solution as aggregates with hydrodynamic diameters on the order of 1-3 μm in size. The presence of citric acid significantly enhanced the extent of ZnO dissolution for all sizes, and the greatest enhancement was observed for the 4 nm particles. Although these results are found to be in qualitative agreement with theoretical predictions, a linearized form of the Kelvin equation to calculate a surface free energy yielded quantities inconsistent with expected values from the literature. Reasons for this inconsistency are discussed and include potential deviations of solubility behavior from classical thermodynamics as a result of a lack of detailed knowledge of surface structure and surface properties, including the presence of different surface crystal facets, and the aggregation state.     

Nonpolymeric Coatings of Iron Oxide Colloids for Biological Use as Magnetic Resonance Imaging Contrast Agents

Iron oxide nanoparticles are used in vivo as contrast agents in magnetic resonance imaging. Their widely used polymer coatings are directly involved in their biocompatibility and avoid magnetic aggregation. As these polymer brushes also limit their tissular diffusion due to important hydrodynamic sizes, this work looks to obtain particles coated with thin layers of organic biocompatible molecules. Coating molecules were chosen depending on their fixation site on iron cores; carboxylates, sulfonates, phosphates, and phosphonates, and, among them, analogs of the phosphorylcholine. Two coating procedures (dialysis and exchange resins purification) were evaluated for hydrodynamic size, total iron concentration, electrophoretic mobility, and colloidal stability. Furthermore, a complementary test on stainless steel plates evaluated the contamination by competition of phosphonates as a rough estimation of the biocompatibility of the particles. Coating with bisphosphonates, the more interesting fixation moiety, leads to small (less than 15 nm) and stable objects in a wide range of pH including the neutrality. From stability data, the coating density was evaluated at around 1.6 molecules per nm(2). Including a quaternary ammonium salt to the coating molecule lowers their electrophoretic mobility. Moreover, this type of coating protects steel plates against contamination without significant desorption. All these properties allow further developments of these nanoparticles for biomedical applications. Copyright 2001 Academic Press.     

Binding of methyl orange and its homologs by polyion complexes consisting of a piperidinium cationic polymer and various polyanions in aqueous solution

A study was made of the formation of polyion complexes between a piperidinium cationic polymer and polyanions and of the binding of azo-dye anions (methyl, ethyl, propyl, and butyl orange) by these complexes. Sodium poly(acrylate), poly(styrenesulfonate), dextran sulfate, and carboxy-methylcellulose were used as polyanions. The resultant polyion complexes (insoluble in aqueous solutions) were compared for their ability to bind the small organic molecules in aqueous solutions, for example, of urea and an inorganic electrolyte (KCI), and exhibited a strong binding affinity toward these small anions. Polyion complexes that consisted of sodium poly(acrylate), dextran sulfate, and carboxymethylcellulose as polyanions cooperated in the binding, whereas the polyion complex of sodium poly(styrenesulfonate) did not. It was suggested that small organic anions interact with the polyion complexes primarily through electrostatic and hydrophobic forces.     

Hydrogel-templated growth of large gold nanoparticles: synthesis of thermally responsive hydrogel-nanoparticle composites

In this paper, we describe a unique strategy for preparing discrete composite nanoparticles consisting of a large gold core (60-150 nm in diameter) surrounded by a thermally responsive nontoxic hydrogel polymer derived from the polymerization of N-isopropylacrylamide (NIPAM) or a mixture of NIPAM and acrylic acid. We synthesize these composite nanoparticles at room temperature by inducing the growth of gold nanoparticles in the presence of preformed spherical hydrogel particles. This new method allows precise control of the size of the encapsulated gold cores (tunable between 60 and 150 nm) and affords composite nanoparticles possessing diameters ranging from as small as 200 nm to as large as 550 nm. Variable-temperature studies show that the hydrodynamic diameter of these composite nanoparticles shrinks dramatically when the temperature is increased above the lower critical solution temperature (LCST); correspondingly, when the temperature is lowered below the LCST, the hydrodynamic diameter expands to its original size. These composite nanoparticles are being targeted for use as optically modulated drug-delivery vehicles that undergo volume changes upon exposure to light absorbed by the gold nanoparticle core.     

Formation and properties of positively charged colloids based on polyelectrolyte complexes of biopolymers

Formation of colloids based on polyelectrolyte complexes (PECs) was mainly studied with synthetic polyelectrolytes. In this study, we describe the elaboration of positively charged PEC particles at a submicrometer level obtained by the complexation between two charged polysaccharides, chitosan as polycation and dextran sulfate (DS) as polyanion. The complexes were elaborated by dropwise addition of default amounts of DS to excess chitosan. Quasi-elastic light scattering was used to investigate in detail the influence of the characteristics of components (chain length, degree of acetylation) and parameters linked to the reaction of complexation (molar mixing ratio, ionic strength, concentration in polymer) on the sizes and polydispersity of colloids. Chain length of chitosan is the major parameter affecting the dimensions of the complexes, high molar mass chitosans leading to the largest particles. Variations of hydrodynamic diameters of PECs with the molar mass of chitosan are consistent with a mechanism of particle formation through the segregation of the neutral and then hydrophobic blocks of the polyelectrolyte complexed segments. Resulting particles display probably a structure constituted by a neutral core surrounded by a chitosan shell ensuring the colloidal stabilization. Such a structure was evidenced by measurements of electrophoretic mobilities revealing that the positive charge of particles was decreasing with pH, in relation with the neutralization of excess glucosamine hydrochloride moieties.     

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.     

Copper-Based Hybrid Polyion Complexes for Fenton-Like Reactions

An innovative strategy allowing the development of a new generation of easy-to-prepare and easy-to-use nano-sized catalysts with high tenability is presented. This strategy is based on the formation of hybrid polyion complexes (HPICs) from the complexation of copper with a block copolymer consisting of an ionizable complexing block and a neutral stabilizer block. These complexes have a well-defined structure and size with a hydrodynamic diameter around 29 nm. They are stable in aqueous solution over a pH range from 4 to 8 and are not sensitive to NaCl salt addition or dilution effects. As a proof-of-concept the degradation of naphthol blue black in water through the use of the Fenton or photo-Fenton reaction is studied. Their performances are comparable to a classical homogeneous reaction, whereas HPICs are easily recyclable by simple dialysis.     

Preparation and characterization of nanoparticles formed by chitosan-caseinate interactions

Intermacromolecular complexation between chitosan and sodium caseinate in aqueous solutions was studied as a function of pH (3–6.5), using absorbance measurements (at 600 nm), dynamic light scattering (DLS), and transmission electron microscopy (TEM). The chitosan–caseinate complexes formed were stable and soluble in the pH range 4.8–6.0. In this pH range, the biopolymers had opposite charges. At higher concentrations of chitosan (0.15 wt%), the soluble complexes associated to form larger particles. DLS data showed that, between pH 4.8 and 6.0, the particles formed by the complexation of chitosan and caseinate had sizes between 250 and 350 nm and these nanoparticles were visualized using negative staining TEM. Above pH 6.0, the nanoparticles associated to form larger particles, causing phase separation. Addition of NaCl increased the particle size. The pH dependence of the zeta potential of the mixture solutions was appreciably different from that of the pure protein and pure chitosan solutions.     

Metal Nanoparticle/Polymer Hybrid Particles: The Catalytic Activity of Metal Nanoparticles Formed on the Surface of Polymer Particles by UV-Irradiation

Summary: Polymer particles decorated with metal nanoparticles were prepared by UV-irradiation of polystyrene latex particles incorporating polymethylphenylsilane (PS/PMPS) and P[S-co-NIPAM]/PMPS particles (NIPAM: N-isopropyl acrylamide) in the presence of metal salts. The metal nanoparticle/polymer hybrid particles were used as a catalyst for the reduction of 4-nitrophenol with NaBH₄. The Pd- and Ag-P(S-co-NIPAM)/PMPS hybrid particles had larger metal nanoparticles and the lower catalytic activity than those of Pd- and Ag-PS/PMPS, respectively. The surface functional group of the polymer particles affected the formation of the metal nanoparticles and their catalytic activity.     

Metal Nanoparticle/Polymer Hybrid Particles: The Catalytic Activity of Metal Nanoparticles Formed on the Surface of Polymer Particles by UV-Irradiation

Summary: Polymer particles decorated with metal nanoparticles were prepared by UV-irradiation of polystyrene particles incorporating polymethylphenylsilane (PS/PMPS) and P[S-co-NIPAM]/PMPS particles (NIPAM: N-isopropyl acrylamide) in the presence of metal salts. The metal nanoparticle/polymer hybrid particles were used as a catalyst for the reduction of 4-nitrophenol with NaBH₄. The Pd- and Ag-P(S-co-NIPAM)/PMPS hybrid particles had larger metal nanoparticles and the lower catalytic activity than those of Pd- and Ag-PS/PMPS, respectively. The surface functional group of the polymer particles affected the formation of the metal nanoparticles and their catalytic activity.     

Controlled clustering of superparamagnetic nanoparticles using block copolymers: design of new contrast agents for magnetic resonance imaging

When polyelectrolyte-neutral block copolymers are mixed in aqueous solutions with oppositely charged species, stable complexes are found to form spontaneously. The mechanism is based on electrostatics and on the compensation between the opposite charges. Electrostatic complexes exhibit a core-shell microstructure. In the core, the polyelectrolyte blocks and the oppositely charged species are tightly bound and form a dense coacervate microphase. The shell is made of the neutral chains and surrounds the core. In this paper, we report on the structural and magnetic properties of such complexes made from 6.3 nm diameter superparamagnetic nanoparticles (maghemite gamma-Fe(2)O(3)) and cationic-neutral copolymers. The copolymers investigated are poly(trimethylammonium ethylacrylate methyl sulfate)-b-poly(acrylamide), with molecular weights 5000-b-30000 g mol(-)(1) and 110000-b-30000 g mol(-)(1). The mixed copolymer-nanoparticle aggregates were characterized by a combination of light scattering and cryo-transmission electron microscopy. Their hydrodynamic diameters were found in the range 70-150 nm, and their aggregation numbers (number of nanoparticles per aggregate) from tens to hundreds. In addition, Magnetic Resonance Spin-Echo measurements show that the complexes have a better contrast in Magnetic Resonance Imaging than single nanoparticles and that these complexes could be used for biomedical applications.     

Characterization of ring‐opening polymerization of genipin and pH‐dependent cross‐linking reactions between chitosan and genipin

In this study, a novel chitosan‐based polymeric network was synthesized by crosslinking with a naturally occurring crosslinking agent—genipin. The results showed that the crosslinking reactions were pH‐dependent. Under basic conditions, genipin underwent a ring‐opening polymerization prior to crosslinking with chitosan. The crosslink bridges consisted of polymerized genipin macromers or oligomers (7 ～ 88 monomer units). This ring‐opening polymerization of genipin was initiated by extracting proton from the hydroxyl groups at C‐1 of deoxyloganin aglycone, followed by opening the dihydropyran ring to conduct an aldol condensation. At neutral and acidic conditions, genipin reacted with primary amino groups on chitosan to form heterocyclic amines. The heterocyclic amines were further associated to form crosslinked networks with short chains of dimmer, trimer, and tetramer bridges. An accompanied reaction of nucleophilic substitution of the ester group on genipin by the primary amine group on chitosan would occur in the presence of an acid catalysis. The extent in which chitosan gels crosslinked with genipin was significantly dependent on the crosslinking pH values: 39.9 ± 3.8% at pH 5.0, 96.0 ± 1.9% at pH 7.4, 45.4 ± 1.8% at pH 9.0, and 1.4 ± 1.0% at pH 13.6 (n = 5, p < 0.05). Owing to the different crosslinking extents and different chain lengths of crosslink bridges, the genipin‐crosslinked chitosan gels showed significant difference in their swelling capability and their resistance against enzymatic hydrolysis, depending on the pH conditions for crosslinking. These results indicated a direct relationship between the mode of crosslinking reaction, and the swelling and enzymatic hydrolysis properties of the genipin‐crosslinked chitosan gels. The ring‐opening polymerization of genipin and the pH‐dependent crosslinking reactions may provide a novel way for the preparation and exploitation of chitosan‐based gels for biomedical applications. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1985–2000, 2005     

Synthesis and characterization of iron oxide/polymer composite nanoparticles with pendent functional groups

We describe in this paper an approach to synthesize superparamagnetic iron oxide nanoparticles in the presence of polymerized lactic acid. The resulted particles consisted of clusters of iron oxide monocrystals, embedded inside the polymer chains. The composite particles synthesized in situ were highly dispersible in aqueous solution with good stability. X-ray diffraction and magnetometer data all confirmed the crystalline structure and super-paramagnetic property of the particles. They exhibited narrow size distribution with hydrodynamic diameters close to 80 nm. In addition, the particles were shown to have abundant surface carboxyl groups, which can be used to conjugate various biomolecules. Such a preparation would be especially useful for developing target specific MRI contrast agents or drug delivery vehicles.