The stability and functionality of chemically crosslinked microtubules.

A variety of bifunctional crosslinking agents have been explored for stabilizing microtubule shuttles used for the active transport of nanomaterials in artificial environments. Crosslinking agents that target amine residues form intertubulin crosslinks that produce crosslinked microtubules (CLMTs) with structural and functional lifetimes that can be up to four times as long as those achieved with taxol stabilization. Such CLMTs are stable at temperatures down to -10 degrees C, are resistant to depolymerization induced by metal ions such as Ca2+, and yet continue to be adsorbed and transported by self-assembled monolayers containing the motor protein kinesin. However, crosslinkers that target cysteine residues depolymerize the MTs, probably by interfering with the guanosine triphosphate binding site. The impact of crosslink attributes, including terminal group chemistry, chain length, crosslink density, and specific location on the tubulin surface, on microtubule stability and functionality are discussed.

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.

Metalloporphyrin assemblies on pyridine-functionalized titanium dioxide.

Porphyrin adsorption on TiO2 nanoparticles has been achieved for multiple porphyrins, and in mixed porphyrin assemblies, via axial ligation to surface-bound pyridine anchored by either para carboxylic or phosphonic functionalizations. Homogenous assemblies were prepared and characterized, while mixed metalloporphyrin assemblies were demonstrated by controlling the concentration ratios of respective porphyrins in the modifying solution. Evaluation of the assemblies using spectroscopic techniques and electrochemistry confirms high porphyrin retention, while exhibiting their surface bound optical and electrochemical properties. A thorough study is discussed where several metalloporphyrins have been evaluated (Ru(CO)OEP, Ru(CO)TPP, and ZnTPP) for relative comparisons and relationships to pyridyl axial binding strengths. The systematic study evaluates multiple background cases using either H2TPP, TiO2 modification with benzoic acid, or unmodified TiO2 to confirm the high affinity of Ru and Zn porphyrins for surface-anchored pyridyl sites. The simple method of step-by-step coordinative anchoring of porphyrins to TiO2 using small, commercially available molecules is highly adaptable for use in dye-sensitized solar cells (DSSC) where intimate contact between the absorbing dye and the semiconductor is required. DSSC devices with novel mixed porphyrin assemblies were shown to give higher power performance than DSSCs utilizing sensitization with only one type of porphyrin.

Direct control of the magnetic interaction between iron oxide nanoparticles through dendrimer-mediated self-assembly.

Cationic superparamagnetic iron oxide nanoparticles were assembled using a series of anionic polyamidoamine dendrimers. The resulting assemblies featured systematically increasing average interparticle spacing over a 2.4 nm range with increasing dendrimer generation. This increase in spacing modulated the collective magnetic behavior by effective lowering of the dipolar coupling between particles. The results obtained in these studies deviate from the predicted dependence of collective behavior on interparticle spacing, suggesting that a dense assembly of magnetically "free" particles can exist with a surprisingly small space between particles.

Radial control of recognition and redox processes with multivalent nanoparticle hosts.

Mixed Monolayer Protected Gold Clusters (MMPCs) featuring both hydrogen bonding and aromatic stacking molecular recognition functionalities have been used to create multivalent hosts for flavins. Multitopic binding of these hosts to flavin was shown to have a strong radial dependence: when the recognition site was brought closer to the MMPC surface, recognition was enhanced approximately 3-fold due to increased preorganization. The effect of preorganization is reversed upon reduction of flavin, where the MMPC with longer side chains bind the flavin guest approximately 7-fold stronger than the short chain counterpart due to unfavorable dipolar interactions between the electron-rich aromatic stacking units of the host and the anionic flavin guest. This fine-tuning of recognition and redox processes provides both a model for enzymatic systems and a tool for the fabrication of devices.

Controlled interparticle spacing through self-assembly of Au nanoparticles and poly(amidoamine) dendrimers.

Control of particle-particle spacing is a key determinant of optical, electronic, and magnetic properties of nanocomposite materials. We have used poly(amidoamine) (PAMAM) dendrimers to assemble carboxylic acid-functionalized mixed monolayer protected clusters (MMPCs) through acid/base chemistry between the particle and dendrimer. Small angle X-ray scattering was then used to establish average inter-MMPC distances. Five generations of PAMAM dendrimer (0, 1, 2, 4, 6) were investigated, with a monotonic increase in interparticle spacing from 4.1 to 6.1 nm observed with increasing generation.

Recognition-mediated assembly of nanoparticles into micellar structures with diblock copolymers.

Polystyrene-based diblock copolymers, featuring diaminotriazine functionality on one of the blocks were used to assemble complementary uracil-functionalized nanoparticles into micellar aggregates. The size of these self-assembled aggregates was controlled by block length, as determined in solution (using dynamic light scattering), and in thin films (using transmission electron microscopy).

Supramolecular assembly on surfaces: manipulating conductance in noncovalently modified mesoscale structures.

Molecules capable of complementary hydrogen bonding were used to control the noncovalent self-assembly and electronic properties of a chemically well-defined surface mesostructure. In this work, we patterned a footprint region for molecular assembly on a surface and used moieties featuring complementary recognition to tune the current-voltage properties of the patterned region. With the appropriate functionalities on the complementary moieties, we were able to increase and decrease the observed conductance in surface-bound mesoscale structures imaged by scanning tunneling microscopy (STM).