Two-Dimensional, Shell-Cross-linked Nanoparticle Arrays

Abstract

Ordered arrays of shell-cross-linked (SCK) nanoparticles are formed at substrate surfaces through the precise manipulation of charged groups in the SCK shell. The shell cross-linking chemistry adjusts the particle surface charge and structural rigidity to control the interparticle spacing and particle shape when assembled on a substrate. With the establishment of synthetic methodologies for preparing well-defined nanostructured materials, 1 the development of techniques to manipulate and assemble nanoscale components into a new generation of functional, addressable superstructured materials is the new frontier challenging the advancement of this field. 2 Nanomaterial assembly into two-dimensional (2-D) arrays represents a first step toward the construction of designed superstructured materials. The technological importance of 2-D nanoscale assembly has been widely recognized as evidenced by the development of 2-D colloidal arrays 3 for use as coatings, 4 chemical sensors, 5 and photonic crystals. 6 Until recently, emulsion polymerized latex particles typically have been employed in 2-D colloidal array assembly 7 because they exhibit narrow particle size distributions and strong interparticle interactions. Polymer micelles are a class of technologically important supramolecular materials of well-defined nanoscale dimension that possess an amphiphilic, core-shell morphology. Möller and co-workers 8 used block copolymers of polystyrene and poly(2-vinylpyridine) to form inverse micelles that ordered noble metals into nanoparticle arrays on mica surfaces. The general applicability of polymer micelles is, however, limited by their dynamic structure. In this communication, we report the formation of ordered arrays resulting from robust SCKs. The inherent chemical control of SCK shell composition needed to manipulate nano-array formation is demonstrated.Preparation of the SCKs employed a previously described procedure 9 (Scheme 1), which produced SCKs containing acrylamide-based cross-links. The stoichiometry of the amine-to-acid functionalities controlled the extent of cross-linking and the proportion of remaining carboxylates.SCKs derived from a PAA 90 -b-PMA 240 block copolymer having 40% conversion of the acrylic acid groups were nearly monodisperse in size and had a number-average hydrodynamic diameter (D n ) of 48 ( 2 nm in water (Nanopure 18 MΩ/cm), as measured by dynamic light scattering (DLS). Atomic force microscopy (AFM) characterization of the SCKs deposited onto a mica substrate from this aqueous dispersion yielded images of particles with a number-average height of 6.2 ( 2.2 nm and diameter of 51.2 ( 5.8 nm, without compensation for AFM tip effects. Transmission electron microscopy (TEM) imaging, obtained for a sample deposited onto a carbon-coated copper grid, gave a number-average diameter of 30.3 ( 2.6 nm. The structural model for the SCK supported by DLS, AFM, and TEM data is a swollen nanoparticle (due to the hydrogel-like shell layer) of maximum diameter in aqueous solution, and a flattened nanopartic...

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