Darrell Velegol

understanding biophysicochemical interactions at the nano-bio interface

Rapid growth in nanotechnology is increasing the likelihood of engineered nanomaterials coming into contact with humans and the environment. Nanoparticles interacting with proteins, membranes, cells, DNA and organelles establish a series of nanoparticle/biological interfaces that depend on colloidal forces as well as dynamic biophysicochemical interactions. These interactions lead to the formation of protein coronas, particle wrapping, intracellular uptake and biocatalytic processes that could have biocompatible or bioadverse outcomes. For their part, the biomolecules may induce phase transformations, free energy releases, restructuring and dissolution at the nanomaterial surface. Probing these various interfaces allows the development of predictive relationships between structure and activity that are determined by nanomaterial properties such as size, shape, surface chemistry, roughness and surface coatings. This knowledge is important from the perspective of safe use of nanomaterials.

Cell Traction Forces on Soft Biomaterials. I. Microrheology of Type I Collagen Gels

A laser-trap microrheometry technique was used to determine the local shear moduli of Type I collagen gels. Embedded 2.1 microm polystyrene latex particles were displaced 10-100 nm using a near-infrared laser trap with a trap constant of 0.0001 N/m. The trap was oscillated transversely +/- 200 nm using a refractive glass plate mounted on a galvanometric scanner. The displacement of the microspheres was in phase with the movement of the laser trap at frequencies less than 1 rad/s, indicating that at least locally, the gels behaved as elastic media. The local shear modulus was measured at various positions throughout the gel, and, for gels at 2.3 mg/mL and 37 degrees C, values ranged from G = 3 to 80 Pa. The average shear modulus G = 55 Pa, which compares well with measurements from parallel plate rheometry.

Understanding the efficiency of autonomous nano- and microscale motors.

We analyze the power conversion efficiency of different classes of autonomous nano- and micromotors. For bimetallic catalytic motors that operate by a self-electrophoretic mechanism, there are four stages of energy loss, and together they result in a power conversion efficiency on the order of 10(-9). The results of finite element modeling agree well with experimental measurements of the efficiency of catalytic Pt-Au nanorod motors. Modifications of the composition and shape of bimetallic catalytic motors were predicted computationally and found experimentally to lead to higher efficiency. The efficiencies of bubble-propelled catalytic micromotors, magnetically driven flagellar motors, Janus micromotors driven by self-generated thermal gradients, and ultrasonically driven metallic micromotors are also analyzed and discussed.

Boundaries can steer active Janus spheres

The advent of autonomous self-propulsion has instigated research towards making colloidal machines that can deliver mechanical work in the form of transport, and other functions such as sensing and cleaning. While much progress has been made in the last 10 years on various mechanisms to generate self-propulsion, the ability to steer self-propelled colloidal devices has so far been much more limited. A critical barrier in increasing the impact of such motors is in directing their motion against the Brownian rotation, which randomizes particle orientations. In this context, here we report directed motion of a specific class of catalytic motors when moving in close proximity to solid surfaces. This is achieved through active quenching of their Brownian rotation by constraining it in a rotational well, caused not by equilibrium, but by hydrodynamic effects. We demonstrate how combining these geometric constraints can be utilized to steer these active colloids along arbitrary trajectories.

Oriented Adhesion of Escherichia coli to Polystyrene Particles

ABSTRACT The adhesion of nonflagellated Escherichia coli strain K-12 to polystyrene (PS) latex spheres or glass capillaries has been observed by using several techniques. Attention was focused on the orientation of the rod-shaped bacteria as they adhered to the surfaces in 100 mM phosphate-buffered saline. Data show that PS particles adhered to the ends of the bacteria more than 90% of the time. Moreover, the PS particles adhered to one end only, never to both. Similarly, for experiments with bacteria adhering to glass, the bacteria adhered on their ends. In order to determine whether the end of a bacterium had a different charge density from that of the middle, rotational electrophoresis experiments were used. These experiments indicated no measurable charge nonuniformity. In order to examine how strongly adhered the bacteria were to the PS particles, differential electrophoresis was used. Almost always, bacteria were found to be irreversibly adhered to the PS spheres. The cause of the oriented adhesion is not likely due to surface lipopolysaccharides (LPS), since the three strains of K-12 that were used, each having a different length of LPS, showed similar behavior. The results are discussed in terms of bacterial cell polarity. The data indicate that nanodomains on the bacterial ends are important for adhesion and that the time scale for irreversible adhesion is short.

van der Waals forces between nanoclusters: Importance of many-body effects

van der Waals interactions between nanoclusters have been calculated with a self-consistent, coupled dipole method. The method accounts for all many-body (MB) effects. Comparison is made between the exact potential energy, V, and the values obtained with two alternative methods: the sum of two-body interactions and the sum of two-body and three-body interactions. For all cases considered, the three-body term alone does not accurately represent the MB contributions to V. MB contributions are especially large for shape-anisotropic clusters.

Self-generated diffusioosmotic flows from calcium carbonate micropumps.

Calcium carbonate particles, ubiquitous in nature and found extensively in geological formations, behave as micropumps in an unsaturated aqueous solution. The mechanism causing this pumping is diffusioosmosis, which drives flows along charged surfaces. Our calcium carbonate microparticles, roughly ∼10 μm in size, self-generate ionic gradients as they dissolve in water to produce Ca(2+), HCO(3)(-), and OH(-) ions that migrate into the bulk. Because of the different diffusion coefficients of these ions, spontaneous electric fields of roughly 1-10 V/cm arise in order to maintain electroneutrality in the solution. This electric field drives the diffusiophoresis of charged tracers (both positive and negative) as well as diffusioosmotic flows along charged substrates. Here we show experimentally how the directionality and speed of the tracers can be engineered by manipulating the tracer zeta potential, the salt gradients, and the substrate zeta potential. Furthermore, because the salt gradients are self-generated, here by the dissolution of solid calcium carbonate microparticles another manipulated variable is the placement of these particles. Importantly, we find that the zeta potentials on surfaces vary with both time and location because of the adsorption or desorption of Ca(2+) ions; this change affects the flows significantly.

Nanoscale van der Waals interactions

Despite the fact that van der Waals (VDW) interactions are often considered to be weak, they dominate the behaviour of all neutral physical systems at separations of order 0.5 nm or larger. For simple geometries – geometric half spaces, spheres, cylinders, or points – VDW interactions are often calculated using a form of Lifshitz theory, which is based on continuum descriptions. But for nanoscale systems, it is often the case that the geometries involve corners, sharp edges, discrete atom placement or small sizes, so that bulk continuum models do not apply. In these cases it is common to compute the VDW interactions using two-body calculations, for instance from Lennard-Jones parameters, the Derjaguin or Hamaker approximation, or pairwise additivity. In this review, we show that none of these estimates predicts VDW interactions accurately; rather, one must use a ‘nanoscale Lifshitz theory’, which we call the ‘coupled dipole method’ (CDM). The CDM accounts for all many-body interactions in the nonretarded limit. The method uses an exact evaluation of the eigenmodes of the coupled dipole oscillators, which represent the charge fluctuations of the system. A key quantity determining the relative importance of many-body contributions is the dimensionless ratio (ν = α/a 3) of the polarisability to the cube of the interparticle spacing. We assess the accuracy of two-body and three-body calculations against many-body predictions, and then briefly discuss the role of retardation. Several important research questions remain, and these are summarised.

Antimicrobial sand via adsorption of cationic Moringa oleifera protein.

Moringa oleifera (Moringa) seeds contain a natural cationic protein (MOCP) that can be used as an antimicrobial flocculant for water clarification. Currently, the main barrier to using Moringa seeds for producing potable water is that the seeds release other water-soluble proteins and organic matter, which increase the concentration of dissolved organic matter (DOM) in the water. The presence of this DOM supports the regrowth of pathogens in treated water, preventing its storage and later use. A new strategy has been established for retaining the MOCP protein and its ability to clarify and disinfect water while removing the excess organic matter. The MOCP is first adsorbed and immobilized onto sand granules, followed by a rinsing step wherein the excess organic matter is removed, thereby preventing later growth of bacteria in the purified water. Our hypotheses are that the protein remains adsorbed onto the sand after the functionalization treatment, and that the ability of the antimicrobial functionalized sand (f-sand) to clarify turbidity and kill bacteria, as MOCP does in bulk solution, is maintained. The data support these hypotheses, indicating that the f-sand removes silica microspheres and pathogens from water, renders adhered Escherichia coli bacteria nonviable, and reduces turbidity of a kaolin suspension. The antimicrobial properties of f-sand were assessed using fluorescent (live-dead) staining of bacteria on the surface of the f-sand. The DOM that can contribute to bacterial regrowth was shown to be significantly reduced in solution, by measuring biochemical oxygen demand (BOD). Overall, these results open the possibility that immobilization of the MOCP protein onto sand can provide a simple, locally sustainable process for producing storable drinking water.

In-solution assembly of colloidal water

Colloidal trimer assemblies were fabricated by the self-assembly of anisotropically-functionalized particles. These trimer assemblies consisted of one central 4.0 µm diameter melamine formaldehyde particle aggregated with two 4.0 µm diameter polystyrene particles to form a structure that looks like “colloidal water”. Anisotropy was patterned onto the melamine formaldehyde particle using the particle lithography technique, and in the present case the “bond angle” for the three particles was roughly 90 degrees. This fabrication of colloidal trimers demonstrates the ability to (1) use particles as patterning masks, (2) pattern two patches onto one base particle in a controlled manner, (3) use this patterned functionalization to control the bottom-up assembly of a more complex assembly with multiple particles and controlled bond angles, and (4) fabricate multiple materials into the same assembly.