Simultaneously controlled directionality and valency on a water-soluble gold nanoparticle precursor for aqueous-phase...

The anisotropic interaction of gold nanoparticles (AuNP's) into a highly accurate, scalable complex structure not only would aid practical nanoscale assembly but also would increase their utility in many applications, including electronics, optics, and biosensing. Particularly for biological purposes, here we demonstrate an aqueous-phase serial solid-phase monofunctionalization approach to synthesize water-soluble AuNP linkers that contain distinctive single or double diametric functionalities in a site-specific way. Using a mild, rapid, effective single-phase 1:1 ligand replacement reaction between mixed-ligand-protected AuNP precursors and ligand-group-attached silica gels, we successfully synthesized (1) two types of monofunctionalized (monof-) carboxyl- or amino-AuNP's with enhanced yield and accuracy and (2) heterobifunctionalized (bif-) AuNP's with one carboxyl and one amine end group at a discrete angle (∼160°). The controlled coupling chemistry in aqueous solution allowed the covalent bond-directed assembly of intentionally designed 1D dimers with monof-AuNP's and 2D rings with heterobif-AuNP's, confirming the highly functional as well as directional selectivity of the functionalized NPs. This study thus represents an important step toward active control over the design and assembly of bottom-up nanostructures with increased complexity and biocompatibility.