Investigating Adsorption of Synthetic Nanoparticles and Biological Species using Surface-grafted Molecular and Macromolecular Gradient Assemblies

Abstract

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. Application of gradient surfaces facilitates unambiguous identification and analysis of the key parameters governing these complex, multivariate phenomena.

First, we demonstrate the formation of and control over the two-dimensional assemblies of nanoparticles bound to a flat substrate via self-assembled monolayer adhesive coating. A molecular gradient template formed by vapor transport of organosilane is used to tune the number of surface-bound particles. Number density of particles is shown to be directly proportional to the surface concentration of organosilane species comprising the monolayer coating. The gradient geometry is further utilized to elucidate the inverse relationship between surface concentration and degree of ionization of organosilane species required to achieve a given particle number density.

We also form a new class of nanocomposite materials by dispersing nanoparticles in surface-anchored polymer assemblies. In order to systematically probe the influence of various polymer properties on the structure of resulting nanocomposite, we employ novel architectures of surface-grafted polymers that offer either 1) unidirectional variation of polymer molecular weight (so-called linear gradient) or 2) bidirectional, simultaneous variation of molecular weight and grafting density (called as orthogonal gradient). The number of particles in the polymer brush/particle hybrid is found to increase with increasing polymer molecular weight due to an increase in the number of sites, to which particles can bind. For a given grafting density of polymer brush, larger particles predominantly reside near the brush-air interface. In contrast, smaller nanoparticles penetrate deeper into the polymer brush, thus forming a three-dimensional structure. Upon increasing grafting density of the chains, the number of attached particles exhibits different trends depending on the particle size. For larger particles, a continuous increase in particle loading is detected as a function of increasing grafting density. In contrast, polymer brushes containing 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. We also demonstrate that polymer brushes that respond to changes in environmental conditions (temperature in particular) can be harnessed to further tune nanoparticles loading in polymer brush-particle composites. Our experimental results concur very well with theories describing organization of nanoparticles in polymer brushes.

Finally, we apply gradients in molecular weight and grafting density of protein-repelling polymer to tailor protein adsorption and cell adhesion to surfaces. Protein 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. Polymer gradient substrates are utilized to tailor the amount of adsorbed fibronectin (FN), which in turn regulates adhesion of osteoblast precursor cells. 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.