44226-G10
Growth of Nanopillar Arrays for Directed Bio-Molecule Assembly
We explore in this project how glancing angle deposited (GLAD) nanostructures can be designed and controllably assembled into meso-scale architectures using biological connector molecules that are attached at predetermined locations on the nanostructure surfaces. The processing steps involve (i) the growth of various sets of multi-stack nanorods and nanosprings by GLAD, followed by (ii) selective attachment of biological molecules to specific locations on these nanostructures, and (iii) the directed assembly of the nanostructures into complex meso-scale architectures. The procedure may be ultimately extended to the controlled assembly of nanorods and nanosprings to form interconnected two-dimensional and three-dimensional architectures like, for example, nanoladders and nanohoneycombs, with potential applications in petroleum sensing and processing.
In the following, we present initial data demonstrating biomolecule-directed assembly of GLAD nanostructures into more complex architectures. For this purpose, two sets of Au capped Si nanorods were grown by GLAD. The long rods, 665+/-10 nm tall and 185+/-10 nm wide, are ~ four times longer than the short rods, 150+/-15 nm tall and 50+/-10 nm wide. Biotin was attached to the Au portion of the short nanorods by first immersing the Si wafer containing the rods into a 5 ml solution of 2 mM 2-aminoethanethiol dissolved in 100% ethanol for 4 hours. The substrate (and rods) were then rinsed with 5 ml of ethanol, immersed in 5 ml 10 mM EZ-Link Sulfo-NHS-LC-Biotin in 0.1 M sodium phosphate buffered saline (0.15 M NaCl, pH 7.2) for 30 min at room temperature, and washed with MilliQ water to remove any unbound linker. Streptavidin was attached to the Au portions of the long rods by first immersing the substrate containing the rods in 5 ml of 2 mM mercapto-undecanoic acid dissolved in 100% ethanol for 4 hours followed by a rinse in 5 ml solution of ethanol, an immersion in 5 ml of aqueous solution containing 0.05 M N-Hydroxysuccinimide (NHS) and 0.2 M 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) for 7 min, and washing with 5 ml of 50 mM MES buffer (pH 6.2). The two silicon supports were then sonicated (separately) for 30 min in MilliQ water, resulting in the formation of aqueous suspensions of functionalized nanorods. The two suspensions, containing short and long nanorods, were mixed together and were allowed to settle for 30 minutes A 5 mL drop of the resulting mixture was dispersed and dried on top of a plasma-cleaned Cu grid with a carbon membrane (Ted Pella, Inc.) for imaging by TEM. In addition, three control samples containing a mixture of (i) biotin-functionalized short nanorods and non-functionalized long nanorods, (ii) streptavidin-functionalized long nanorods and non-functionalized short nanorods and (iii) non-functionalized long and short nanorods, were prepared to demonstrate that biotin-streptavidin linkage is necessary for nanorod assembly.
A representative bright-field TEM micrograph of a Si-Au nanorod assembly shows 3 long nanorods, with a measured length of 674+/-18 nm. The rods are capped with Au, which appears dark due to strong scattering of electrons from crystalline (high Z) Au grains in comparison to the brighter amorphous (low Z) Si rods. The Si portion of the nanorods exhibits strong porosity, associated with atomic shadowing caused by the intentionally high deposition angle in combination with low adatom mobility due to a low homologous growth temperature of Ts/Tm(Si) = 300K/1687K = 0.18. The short nanorods appear as bundles of 1-3 parallel rods. Such bundles form during the nanorod growth, when neighboring rods touch and develop a sufficiently strong bond so that they do not break apart during the subsequent sonication. The short rods have a measured length of 165±10 nm, in reasonable agreement with the length of 150+/-15 nm, measured by SEM. All short rods arrange themselves so that their Au-portions are in close proximity (£ 5 nm) to the Au-portion of the large rods. This is an indication that the biotin at the short rods connects to the streptavidin at the long rods, during the mixing of the two suspensions. TEM analysis of the control samples did not show any assemblies emphasizing the fact that the assembly is due to biotin-streptavidin linkage and not due to any hydrophobic interactions, as observed in the case of Au-polypyrrole amphiphiles. In conclusion, we have shown that Si-Au nanorods can be controllably assembled (end-to-end) using biological connector molecules, in particular biotin and streptavidin, that are selectively attached to the Au portion at the end of the nanorods. This new hybrid physical-vapor-deposition / wet‑chemistry approach will be useful for assembling complex hierarchical nanoarchitectures including nano-honeycombs, nanoladders, and 3D nanorod networks, comprised of controlled materials combinations.
