Ling Qi

Redispersible Hybrid Nanopowders: Cerium Oxide Nanoparticle Complexes with Phosphonated-PEG Oligomers

Rare earth cerium oxide (ceria) nanoparticles are stabilized using end-functional phosphonated-PEG oligomers. The complexation process and structure of the resulting hybrid core-shell singlet nanocolloids are described, characterized, and modeled using light and neutron scattering data. The adsorption mechanism is nonstoichiometric, yielding the number of adsorbed chains per particle N(ads) = 270 at saturation. Adsorption isotherms show a high affinity of the phosphonate head for the ceria surface (adsorption energy DeltaG(ads) approximately -16kT) suggesting an electrostatic driving force for the complexation. The ease, efficiency, and integrity of the complexation is highlighted by the formation of nanometric sized cerium oxide particles covered with a well anchored PEG layer, maintaining the characteristics of the original sol. This solvating brushlike layer is sufficient to solubilize the particles and greatly expand the stability range of the original sol (<pH 3) up to pH = 9. We underscore two key attributes of the tailored sol: (i) strong UV absorption capability after functionalization and (ii) ability to redisperse after freeze-drying as powder in aqueous or organic solvents in varying concentrations as singlet nanocolloids. This robust platform enables translation of intrinsic properties of mineral oxide nanoparticles to critical end use.

Influence of the Formulation Process in Electrostatic Assembly of Nanoparticles and Macromolecules in Aqueous Solution: The Mixing Pathway

Complex Assemblies of Soft Matter Laboratory (COMPASS), CNRS UMI3254, Rhodia Center for Research and Technology in Bristol, 350 Georges Patterson BouleVard, Bristol, PennsylVania 19007, Matière et Systèmes Complexes (MSC), UMR 7057 CNRS, UniVersité Denis Diderot Paris-VII, Bâtiment Condorcet, 10 rue Alice Domon et Léonie Duquet, 75205 Paris, France, Institut Laue LangeVin, 6 rue Jules Horowitz, F-38042 Grenoble Cedex 9, France, and Centre de Recherche Paul Pascal (CRPP), CNRS, UniVersité Bordeaux 1, 33600 Pessac, France

Microfluidics with on-line dynamic light scattering for size measurements

We present a detailed investigation on the feasibility of on-line dynamic light scattering measurements of colloidal sizes in a pressure-driven microfluidic flow. We review some theoretical arguments showing that such experiments are difficult to perform due to the Poiseuille flow that induces interferences of different Doppler shifts. Such a theoretical approach is however very useful to figure out the range of parameters where on-line size measurements are possible. We then build a dynamic light scattering setup around a microfluidic chip that enables us to estimate the size of Brownian scatterers flowing in PDMS-based microchannels, thus validating experimentally the theoretical estimations. We finally present a microfluidic chip that can mix two reactants in approximately 200 ms, and allows size measurements using dynamic light scattering at about 300 ms after complete mixing. Two applications are presented: the continuous monitoring of the viscosity of a two-fluid mixture, and the electrostatic co-assembly of oppositely charged nanoparticles and block copolymers.

Universal scattering behavior of coassembled nanoparticle-polymer clusters.

Water-soluble clusters made from 7-nm inorganic nanoparticles have been investigated by small-angle neutron scattering. The internal structure factor of the clusters was derived and exhibited a universal behavior as evidenced by a correlation hole at intermediate wave vectors. Reverse Monte Carlo calculations were performed to adjust the data and provided an accurate description of the clusters in terms of interparticle distance and volume fraction. Additional parameters influencing the microstructure were also investigated, including the nature and thickness of the nanoparticle adlayer.

Organic versus hybrid coacervate complexes: co-assembly and adsorption properties

We report the co-assembly and adsorption properties of coacervate complexes made from polyelectrolyte-neutral block copolymers and oppositely charged nanocolloids. The nanocolloids put under scrutiny were ionic surfactant micelles and highly charged 7 nm cerium oxide (CeO2) nanoparticles. Static and dynamic light scattering was used to investigate the microstructure and stability of the organic and hybrid complexes. For five different systems of nanocolloids and polymers, we first demonstrated that the electrostatic complexation resulted in the formation of stable core-shell aggregates in the 100 nm range. The microstructure of the CeO2-based complexes was resolved using cryogenic transmission electronic microscopy (Cryo-TEM), and it revealed that the cores were clusters made from densely packed nanoparticles, presumably through complexation of the polyelectrolyte blocks by the surface charges. The cluster stability was monitored by systematic light scattering measurements. In the concentration range of interest, c = 10-4-1 wt.%, the surfactant-based complexes were shown to exhibit a critical association concentration (cac) whereas the nanoparticle-polymer hybrids did not. The adsorption properties of the same complexes were investigated above the cac by stagnation point adsorption reflectometry. The adsorbed amount was measured as a function of time for polymers and complexes using anionically charged silica and hydrophobic poly(styrene) substrates. It was found that all complexes adsorbed readily on both types of substrates up to a level of 1-2 mg m-2 at stationary state. Upon rinsing however, the adsorbed layer was removed for the surfactant-based systems, but not for the cerium oxide clusters. As for the solution properties, these finding were interpreted in terms of a critical association concentrations, which are very different for organic and hybrid complexes. Combining the efficient adsorption and strong stability of the CeO2-based core-shell hybrids on various substrates, it is finally suggested that these systems could be used appropriately for coating and anti-biofouling applications.