Andrew K. Boal

Interactions between cargo-carrying biomolecular shuttles

Microtubule shuttles propelled by the motor protein kinesin embedded in self-assembled monolayers are being developed for active transport functions in artificial microfluidic systems. As a model system, biotinylated microtubules have been laden with streptavidin-coated particles as cargo. The behaviour of cargo-laden microtubules has been observed using fluorescence microscopy upon activation of kinesin-driven transport processes. Collisions between mobile microtubules and their particulate cargo result in six distinct behaviours: bypass, microtubule bending, particle knock-off, particle transfers between microtubules, co-joining of microtubules to a common particle, and particle-induced severing of microtubules. The distribution of observed events can be described qualitatively on the basis of the mechanics of motor proteins and microtubules, the geometry of the collision events, and the loading rate dependence of the strength of microtubule–particle binding. Implications of the results for the use of motor proteins in active transport and cargo-handling systems for nanomaterials are described.

Investigation of the use of chlorine based advanced oxidation in surface water: Oxidation of natural organic matter and formation of disinfection byproducts

Abstract Disinfection byproduct (DBP) formation during the treatment of raw Colorado River Water (CRW) using aqueous chlorine and ultraviolet (UV) light advanced oxidation processes (AOPs) was investigated. Here, CRW was combined with aqueous chlorine from two distinct sources (electrochemically-generated Mixed Oxidant Solution (MOS) and commercial sodium hypochlorite) and then exposed to ultraviolet C (UV-C) and ultraviolet A (UV-A) light. The impact of the treatment process on the structure of (NOM) in the CRW was examined, as well as the resulting production of various halogenated organic Disinfection By-Products (DBPs). Both AOP conditions tested resulted in destruction of chromophoric components of the NOM, while formation of total amounts of haloacetic acids and trihalomethanes was far below the US EPA regulated maximum contaminant level values for these contaminants, even though the UV and chlorine doses used in these studies were much higher than the typical doses used in an actual treatment process.

Templated Nanocrystal Assembly on Biodynamic Artificial Microtubule Asters

Microtubules (MTs) and the MT-associated proteins (MAPs) are critical cooperative agents involved in complex nanoassembly processes in biological systems. These biological materials and processes serve as important inspiration in developing new strategies for the assembly of synthetic nanomaterials in emerging techologies. Here, we explore a dynamic biofabrication process, modeled after the form and function of natural aster-like MT assemblies such as centrosomes. Specifically, we exploit the cooperative assembly of MTs and MAPs to form artificial microtubule asters and demonstrate that (1) these three-dimensional biomimetic microtubule asters can be controllably, reversibly assembled and (2) they serve as unique, dynamic biotemplates for the organization of secondary nanomaterials. We describe the MAP-mediated assembly and growth of functionalized MTs onto synthetic particles, the dynamic character of the assembled asters, and the application of these structures as templates for three-dimensional nanocrystal organization across multiple length scales. This biomediated nanomaterials assembly strategy illuminates a promising new pathway toward next-generation nanocomposite development.

Microtubule Templated Synthesis of Inorganic Nanomaterials

Protein microtubules (MTs) have been used as templates for the biomimetic synthesis of metal oxide, metal sulfide, and metallic nanomaterials. These materials were coated onto MTs via three distinct synthetic pathways: metal ion hydrolysis which yielded iron oxide or zinc oxide-coated microtubules, metal ion/sulfide co-precipitation which yielded zinc sulfide coated MTs, and metal ion reduction which yielded gold-coated MTs. The growth process of metal oxide coating involves heterogeneous nucleation on the MT surface and produces even, microcrystalline films. Metal sulfide and metal coating initially involves the formation of nanoparticle arrays that decorate the MT surface and can eventually lead to either semi- or fully continuous coatings.

Incorporation of bioactive materials into integrated systems

Sandia is exploring two classes of integrated systems involving bioactive materials: 1) microfluidic systems that can be used to manipulate biomolecules for applications ranging from counter-terrorism to drug delivery systems, and 2) fluidic systems in which active biomolecules such as motor proteins provide specific functions such as active transport. An example of the first class involves the development of a reversible protein trap based on the integration of the thermally-switchable polymer poly(N-isopropylacrylamide)(PNIPAM) into a micro-hotplate device. To exemplify the second class, we describe the technical challenges associated with integrating microtubules and motor proteins into microfluidic systems for: 1) the active transport of nanoparticle cargo, or 2) templated growth of high-aspect ratio nanowires. These examples illustrate the functions of bioactive materials, synthesis and fabrication issues, mechanisms for switching surface chemistry and active transport, and new techniques such as the interfacial force microscope (IFM) that can be used to characterize bioactive surfaces.

Stability of Chemically Crosslinked Microtubules

Microtubules (MT) are dynamic protein-based polymers with numerous applications in materials science including the active transport of nanomaterials and as templates for biomimetic materials synthesis. Some of these applications require that the dynamic nature of the MT be suppressed, and in this report we will discuss the preparation and stability of chemically crosslinked microtubules (CLMTs). MT reaction with gluteraldehyde results in the formation of protein dimers and higher molecular weight oligimers as observed by gel electrophoresis, confirming the formation of covalent inter-protein linkages. While extensive crosslinking was found to destabilize MTs and inactivate them with regards to kinesin binding, moderate amounts of crosslinking lead to CLMTs that had functional lifetimes of at least twice that of uncrosslinked MTs. Further studies demonstrated that CLMTs exhibited a wider thermal stability window and were far more resistant to metal-ion induced depolymerization than uncrosslinked MTs.

Conjugation and transport of quantum dots and nanoparticles by microtubule-associated kinesin motor proteins

In contrast to synthetic materials that passively assemble to form static structures through equilibrium dynamics, Nature has evolved complex, nonequilibrium strategies for the active assembly, reconfiguration and trafficking of nanomaterials. Microtubule-associated kinesin motor proteins play important roles in cell division, intracellular transport, and material assembly/reconfiguration in eukaryotic organisms. The primary goals of our work are to (1) integrate kinesin-microtubule shuttles for transporting synthetic nanoparticles, and (2) utilize the unique properties of these molecular shuttles to design dynamic and adaptive materials at synthetic interfaces. Towards this end, we are currently developing schemes by which kinesin-microtubule complexes are utilized in the transport of inorganic nanoparticles. Microtubule copolymers have been prepared from unlabeled and biotinylated tubulin and conjugated with streptavidin-coated gold nanoparticles and CdSe quantum dots. A variety of composite structures, ranging from centrosome-like assemblies to fully decorated microtubules, have been observed, and appear to be dependent on both the degree of biotinylation as well as the ratio of microtubules to quantum dots. Further, we have observed that these nanoparticle-laden microtubules can be transported by surface bound kinesins with an efficiency comparable to dye-labeled microtubules. To further facilitate microtubule-based nanoparticle transport, we have modified the microtubule polymerization and composition to create unique areas for attachment of nanoparticles and increase the efficacy for nanoparticle transport. Overall, these data demonstrate the ability to efficiently assemble and transport inorganic nanoparticle arrays using biological transport mechanisms, and provide a platform for the further development of dynamic nanomaterials based on these strategies.