Assembly of Nanoparticles using Surface-Grafted Orthogonal Polymer Gradients

Polymer-coated surfaces are routinely used in industrially important applications including prevention of biofouling (in fabrication of protein-resistant surfaces), improving stability of colloidal dispersions, surface lubrication, or enhancing wettability and adhesion. Polymer chains chemically bound to the surface represent a special class of polymer coatings that has gained considerable attention in the past few years in nanoand biotechnology applications. This is primarily due to the ease of growing polymer chains on a variety of functional surfaces and the ability of grafted polymer chains to withstand harsh operational conditions of solvent and temperature. The chemical grafting approach also allows for manipulation of the length (or equivalently molecular weight, MW) and grafting density (s, number of chains per unit area of grafted surface) of polymer chains attached to the surface. MW ands determine the conformation of polymer chains, which in turn, governs the effectiveness of polymer coating for a given application. For example, it has been predicted that adsorption of small proteins (and the subsequent biofouling) on a surface can be minimized by using polymer brushes having high grafting density (high s) whereas thicker brushes, i.e., large molecular weight, MW) are preferred to achieve similar repellency for larger proteins. Polymer brushes are widely used to prevent coagulation of colloidal particles. This steric stabilization is best realized by using long grafted chains with relatively high surface density. Adhesion promotion between a polymer melt and brush-coated surface is found to depend critically on s. Short chains with very high s are suitable for enhancing the lubrication of a polymer coated surface with a sliding surface. Thus, it is desirable to be able to systematically manipulate the conformation by controlling MW and s of the chains. Our group has recently developed combinatorial methods for preparing surface-tethered polymers with smoothly Summary: We report on preparing poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) gradient substrate, wherein molecular weight (MW) and grafting density (s) of the surface-anchored PDMAEMA chains vary continuously in two orthogonal directions. Such a specimen is used to control the assembly of charged gold nanoparticles. Increasing MW and s of the grafted PDMAEMA cause an enhanced binding of the nanoparticles to PDMAEMA, thus leading to an orthogonal number density gradient of surface-bound gold nanoparticles.