Danilo C. Pozzo

Pickering Emulsions Stabilized by Nanoparticle Surfactants

Amphiphilic gold nanoparticles are demonstrated to effectively stabilize emulsions of hexadecane in water. Nanoparticle surfactants are synthesized using a simple and scalable one-pot method that involves the sequential functionalization of particle surfaces with thiol-terminated polyethylene glycol (PEG) chains and short alkane-thiol molecules. The resulting nanoparticles are shown to be highly effective emulsifying agents due to their strong adsorption at oil-water and air-water interfaces. The original nonfunctionalized gold nanoparticles are unable to effectively stabilize oil-water emulsions due to their small size and low adsorption energy. Small-angle X-ray scattering and electron microscopy are used to demonstrate the formation of nanoparticle-stabilized colloidosomes that are stable against coalescence and show significant shifts in plasmon resonance enhancing the near-infrared optical absorption.

Nonlinear contrast enhancement in photoacoustic molecular imaging with gold nanosphere encapsulated nanoemulsions

A composite contrast agent, a nanoemulsion bead with assembled gold nanospheres at the interface, is proposed to improve the specific contrast of photoacoustic molecular imaging. A phase transition in the beads core is induced by absorption of a nanosecond laser pulse with a fairly low laser fluence (∼3.5 mJ/cm2), creating a transient microbubble through dramatically enhanced thermal expansion. This generates nonlinear photoacoustic signals with more than 10 times larger amplitude compared to that of a linear agent with the same optical absorption. By applying a differential scheme similar to ultrasound pulse inversion, more than 40 dB contrast enhancement is demonstrated with suppression of background signals.

Fibrin Clot Structure and Mechanics Associated with Specific Oxidation of Methionine Residues in Fibrinogen

Using a combination of structural and mechanical characterization, we examine the effect of fibrinogen oxidation on the formation of fibrin clots. We find that treatment with hypochlorous acid preferentially oxidizes specific methionine residues on the α, β, and γ chains of fibrinogen. Oxidation is associated with the formation of a dense network of thin fibers after activation by thrombin. Additionally, both the linear and nonlinear mechanical properties of oxidized fibrin gels are found to be altered with oxidation. Finally, the structural modifications induced by oxidation are associated with delayed fibrin lysis via plasminogen and tissue plasminogen activator. Based on these results, we speculate that methionine oxidation of specific residues may be related to hindered lateral aggregation of protofibrils in fibrin gels.

Scalable synthesis of self-assembling nanoparticle clusters based on controlled steric interactions

A simple one-pot method is presented for the synthesis of amphiphilic gold nanoparticles that can spontaneously self-assemble into stable clusters with controllable geometry. The technique is based on the control of colloidal interactions through sequential functionalization of the surfaces with long polyethylene glycol (PEG) chains followed by short alkane–thiol molecules. This process renders the gold nanoparticles amphiphilic and initiates the self-assembly of stable clusters in the dispersion. It is shown that the dominant structures and sizes of the clusters (i.e. singlets, doublets, triplets and others) are directly and reproducibly controlled by the concentration of PEG chains that are linked to the particle surface. Changes in the structure of the clusters are systematically characterized by small angle X-ray scattering (SAXS), dynamic light scattering (DLS) and transmission electron microscopy (TEM). Systematic shifts in the plasmon resonance peak of the gold nanoparticle clusters demonstrate that this new method for self-assembly can be effectively used to tune optical properties with great precision. With this new approach, we show that it is possible to fabricate large numbers of amphiphilic nanoparticles and clusters with controllable structure by manipulating the repulsive steric forces that are imparted by surface bound polymers and the attractive interactions of the hydrophobic alkane–thiols. The major advantage of this technique lies in its simplicity, versatility and potential for scalable production of self-assembled ‘colloidal molecules’.

Structure of high density fibrin networks probed with neutron scattering and rheology

The structural and mechanical properties of coarse fibrin networks formed in D2O solutions are investigated over a wide range of concentrations (1–40 mg mL−1). For the first time, this range includes concentrations that are relevant to those found in in vivo blood clots. Small angle neutron scattering (SANS) and ultra small angle neutron scattering (USANS) are used to seamlessly characterize the structure over length scales ranging from 1 nm to several micrometres. Using invariant and Guinier analyses the internal volume fraction of protein within the fiber and their bulk average radius are determined directly. The network properties of fibrin clots are also characterized using a model for fractal structures. The network scale features are shown to be highly dependent on the initial fibrinogen concentration while the average fiber radius is only weakly dependent on this parameter. These results demonstrate the usefulness of combining SANS and USANS as characterization tools for complex biopolymer systems such as fibrin. The linear viscoelastic modulus of fibrin gels is related to the concentration by a power law equation that is valid over the entire range. In contrast, the non-linear rheology of dense networks is altered from the monotonic strain hardening response that is found at lower concentrations. This demonstrates the need for thorough characterization of fibrin at concentrations relevant to those of thrombi formed in vivo.

SANS and SAXS analysis of charged nanoparticle adsorption at oil-water interfaces.

A systematic study of the adsorption of charged nanoparticles at dispersed oil-in-water emulsion interfaces is presented. The interaction potentials for negatively charged hexadecane droplets with anionic polystyrene latex particles or cationic gold particles are calculated using DLVO theory. Calculations demonstrate that increased ionic strength decreases the decay length of the electrostatic repulsion leading to enhanced particle adsorption. For the case of anionic PS latex particles, the energy barrier for particle adsorption is also reduced when the surface charge is neutralized through changes in pH. Complementary small-angle scattering experiments show that the highest particle adsorption for PS latex occurs at moderate ionic strength and low pH. For cationic gold particles, simple DLVO calculations also explain scattering results showing that the highest particle adsorption occurs at neutral pH due to the electrostatic attraction between oppositely charged surfaces. This work demonstrates that surface charges of particles and oil droplets are critical parameters to consider when engineering particle-stabilized emulsions.

Structure and property development of poly(3-hexylthiophene) organogels probed with combined rheology, conductivity and small angle neutron scattering

The structural, mechanical and electrical properties of poly(3-hexylthiophene) (P3HT) organogels have been probed during the sol–gel transition through combined rheology, AC dielectric spectroscopy and small angle neutron scattering (SANS). SANS shows that structural features of P3HT gels, which are crucial for the optimization of organic photovoltaic devices, evolve throughout the gelation process. In situ structure–property analyses also demonstrate that there are very different mechanisms for the formation and dissolution of fibers and networks prepared from these polymeric semiconductors. It is determined that P3HT gels form conductive pathways that are maintained even after up to 50% of the fibers re-dissolve upon heating. P3HT organogels formed in different aromatic solvents also show differences of more than two orders of magnitude in conductivity despite having similar nanoscale fiber structures. These results demonstrate the importance of controlling the self-assembled morphology of fiber networks for maintaining optimal electronic properties. This work also highlights the potential for using organogels as flexible platforms for designing efficient organic photovoltaic devices.

Macroscopic alignment of nanoparticle arrays in soft crystals of cubic and cylindrical polymer micelles

Abstract.We describe a method to organize nanometer-sized hydrophilic particles into ordered arrays by templating them in the soft, micelle-crystal phases (spherical and cylindrical) of a thermoreversible block copolymer. Small-angle neutron scattering (SANS) with contrast variation is used to show that the dispersed particles (in this case, proteins or silica) form structured arrays by being constrained in the interstitial cavities between the polymer micelles in the ordered micelle crystal. Simple shear is used to macroscopically align both phases of the nanocomposites (micelles and particles) into macro-domains. The temperature-induced order-order transition between templates of spherical and cylindrical micelles is demonstrated as a reversible technique to modify the structure of the templated nanoparticle arrays.

Rheology and phase behavior of copolymer-templated nanocomposite materials

The mechanical properties of three-dimensionally organized nanocomposite materials composed of nanometer-sized inorganic particles dispersed in micelle cubic crystals is studied rheologically. The influence of parameters including temperature, relative concentrations, and the relative size of particles and the micelles that make up the cubic crystal is explored using oscillatory shear and creep measurements. These parameters have a significant influence on the mechanical properties of the final nanocomposite materials. Additionally, we find that the properties of the samples depend strongly on the temperature profile that is used to form the micelle crystal. Results indicate the importance of particle-template stoichiometry and relative particle size (to the micelle size) on the mechanical properties of the nanocomposites.

Small angle scattering model for Pickering emulsions and raspberry particles.

Pickering emulsions, raspberry particles and other colloidal particle complexes are often characterized using small angle scattering techniques. The present work derives an analytical scattering model that accounts for the self-correlation of a spherical core and surface adsorbed particles as well as the particle-particle and core-particle correlation terms characteristic of Pickering emulsions and raspberry particles. It is shown that contrast matching of the scattering length density is not essential to obtain meaningful information as long as the scattering contrasts of all phases are precisely known. The derived equations are useful for analyzing data and planning experiments for Small Angle Neutron Scattering (SANS) and Small Angle X-ray Scattering (SAXS) involving these colloidal systems.