| The assembly of nanoparticles in controllable and predictable ways would not only aid practical nanoscale assembly, which requires accurate and scalable assembly of large and complex nanoscale structures, but also would increase their utility for many applications, including electronics, optics, sensing and imaging, medical diagnostics, etc. Well-defined and controlled functionality and directionality of the building blocks are essential to actively control the molecular assembly processes at the nanometer scale. Such controls over the functionality and directionality would enable us to construct sophisticated nanostructures to take advantage of the increasing number of available nanocomponents and ultimately to approximate the complexity and the functionality of current microfabrication. We have developed a serial solid-phase placement approach to synthesize anisotropically or symmetrically functionalized gold nanoparticles (AuNPs), in which the functionality and directionality (e.g., numbers, locations, and orientations) of the functional ligands are controlled. Two types of bi-functionalized (bif-) AuNPs were synthesized at a site-specific manner with increased yield and accuracy: (1) homo-bif-AuNPs with two carboxyl groups at ∼180° angle (para-configuration) and (2) hetero-bif-AuNPs with one carboxyl and one amine functional groups at less than 180°, but greater than 90° angle (meta-configuration). With such control, we successfully demonstrated the assembly of intentionally designed one-dimensional (1D) chains with homo-bif-AuNPs and two-dimensional (2D) rings with hetero-bif-AuNPs, confirming the high functional as well as directional selectivity of the functionalized NPs. This study represents an important step towards accurate, reliable, and scaled-up manufacturing of complex nanoscale structures, potentially making 'bottom-up' nanofabrication of practical use. We have further developed the ligand replacement technology to achieve such active controls in biologically relavant aqueous solutions. Specifically, this was accomplished with passivating monolayers on AuNPs into water-soluble form and we successfully demonstrated the controlled assembly of the anisotropic structures such as dimers and 2D rings in aqueous solution. Furthermore, both control and complexity were further increased by conjugating DNA oligonucleotides. We could achieve functionalizations of up to six DNA in all x, y, and z directions. The oligonucleotide sequences used in this study were non-crosshybridizing, with which unplanned defects and errors from undesigned duplex formation in the assembly are minimized. Finally, our functionalization reaction schemes were evaluated mathematically using computational modeling. The ligand replacement reaction fit to a Langmurian kinetic equation at unsteady state. This indicates that the Langmurian kinetics could be an efficient model for nanoparticle-based chemical ligand reactions. |
