Poly(l-lysine)-graft-poly(ethylene glycol) Assembled Monolayers on Niobium Oxide Surfaces:  A Quantitative Study of the Influence of Polymer Interfacial Architecture on Resistance to Protein Adsorption by ToF-SIMS and in Situ OWLS

Poly(l-lysine) grafted with poly(ethylene glycol) (PLL-g-PEG), a polycationic copolymer that is positively charged at neutral pH, spontaneously adsorbs from aqueous solution onto negatively charged surfaces, resulting in the formation of stable polymeric monolayers and rendering the surfaces protein-resistant to a degree related to the PEG surface density. A set of PLL-g-PEG polymers with different architectures was synthesized. The grafting ratio, g, of the polymer, defined as the ratio of the number of lysine monomers to the number of PEG side chains, was systematically varied between 2 and 23, and PEG molecular weights of 1, 2, and 5 kDa were used. The polymers were adsorbed onto niobium oxide-coated substrates, leading to highly different but well-controlled PEG surface densities with maximal values of 0.9, 0.5, and 0.3 chains/nm2 for the three PEG molecular weights, respectively. Time-of-flight secondary-ion mass spectrometry (ToF-SIMS) was used in conjunction with the in situ optical waveguide lightmode spectroscopy (OWLS) technique to investigate the interface architecture. While ToF-SIMS provided surface-analytical data on the polymeric adlayer, OWLS allowed the quantitative determination of the adsorbed polymer mass. Extremely good correlations were established between the ToF-SIMS data (obtained in UHV) and the in situ OWLS results. The amount of serum adsorbed, determined quantitatively by OWLS, was found to depend systematically on the surface coverage in terms of the ethylene glycol (EG) density, controlled by both PEG molecular weight and grafting ratio, g. Serum adsorption dropped gradually from 590 ng/cm2 on bare Nb2O5 to <2 ng/cm2 (=detection limit of the OWLS technique) for EG densities ≥ 20 nm-2. The PLL-g-PEG technology shows itself to be an efficient, cost-effective, and robust tool for the immobilization of PEG chains onto metal oxide surfaces. The precise control over PEG surface density across a wide range allows for the production of tailored surfaces with controlled degrees of bio-interactiveness. Such surfaces are expected to have a substantial potential for applications in biomedical and bioanalytical devices.