Investigating adsorption of synthetic nanoparticles and biological species using surface-grafted molecular and macromolecular gradient assemblies

We utilize novel surface-grafted molecular and macromolecular gradient assemblies to investigate: (1) dispersion of nanoparticles in organic matrices tethered to a substrate, and (2) adsorption of proteins and adhesion of cells to synthetic polymeric surfaces. First, we demonstrate control over the two-dimensional assemblies of nanoparticles bound to a flat substrate by utilizing a concentration gradient template formed via vapor transport of organosilane molecules. Number density of particles is shown to be directly proportional to the surface concentration of organosilane species comprising the monolayer.; Subsequently, we create three-dimensional assemblies of nanoparticles by dispersing particles in surface-anchored polymers. For comprehensive exploration of this new class of nanocomposite materials, we employ novel architectures of surface-grafted polymers that offer either (1) unidirectional variation of polymer molecular weight (linear gradient) or (2) bidirectional, simultaneous variation of molecular weight and grafting density (orthogonal gradient). The number of particles in the polymer brush/particle hybrid increases with increasing polymer molecular weight due to an increase in the number of particle attachment sites. While particles larger than thickness of the brush predominantly reside near the brush-air interface, smaller nanoparticles penetrate deeper into the brush, thus forming a three-dimensional structure. Upon increasing grafting density of the chains, larger particles show a continuous increase in particle loading. In contrast, smaller particles exhibit a maximum in particle concentration at some intermediate value of grafting density. We rationalize the latter behavior in terms of competition between enthalpic gain upon particle attachment to the polymer chains and entropic penalty induced by the insertion of particles in the dense brush.; Finally, we harness gradients of protein repelling polymer to tailor the amount of adsorbed fibronectin (FN) protein, which in turn regulates adhesion of osteoblast precursor cells. FN adsorption is shown to decrease with increasing surface coverage of polymer, which can be achieved by increasing molecular weight and/or grafting density of tethered polymer. Cells are well attached and spread in a polygonal fashion on parts of the gradient with high FN coverage (least polymer coverage) whereas the cells are poorly anchored and elongated in areas of the gradient that are fully decorated by grafted polymer.