This paper describes the first detailed analytical characterization of the surface properties of a new class of biomolecular interfaces based on derivatized poly(L-lysine)-grafted-poly(ethylene glycol) (PLL-g-PEG) copolymers. Such copolymers spontaneously adsorb to negatively charged surfaces under physiological pH and efficiently repel nonspecific protein adsorption while providing PEG-tethered functional/active sites for specific biomolecular recognition. As a model system, we synthesized biotin-derivatized (PLL-g-PEG) copolymers, PLL-g-[(PEG)1-x(PEG-biotin)x], where x varies from 0 to 1. The copolymers were adsorbed on titanium dioxide substrates. Surface characteristics and biorecognition properties were investigated using X-ray photoelectron spectroscopy and radiometry. We show that the monolayer formed is ∼20-25 Å in thickness. It is organized with its PLL backbone located within the first 10 Å on the substrate and with the PEG side chains located above the PLL. The resulting biotin surface concentration depends linearly on the biotin concentration in the bulk copolymer. This aspect implies that the surface concentration of functional groups can be adjusted by adapting their concentration within the bulk copolymer. The PLLg-PEG(-biotin) monolayers are efficient in repelling nonspecific protein adsorption but can specifically bind streptavidin (SA). Within the biotin range considered, the SA surface concentration increases linearly with the biotin surface concentration of the monolayer.

In x-ray photoelectron spectroscopy (XPS) the Auger parameter is often used to study the electronic properties of elements, particularly in insulator materials, because this parameter is assumed to be independent of charging effects. In this paper we report on subtle differences in sample structure and experimental conditions for which the sample potential may not remain constant during the measurements for some spectrometers or experimental arrangements; for such conditions the Auger parameter is not independent of charging. We compare a series of measurements with insulating plate substrates of Al 2 O 3 on which different amounts of SnO 2 and Au were deposited. X-ray photoelectron spectra were collected for different conditions of the sample that was placed either grounded or left floating on a metallic sample holder during measurement. It is found that the Auger parameter is independent of the experimental conditions for Au but substantial differences were found for deposited SnO 2 . Surprisingly, measurement artifacts due to charging appeared in the Auger parameter for Sn when the sample holder was grounded but not when it was left floating. In the grounded samples differences up to 0.6 eV in the Auger parameter for Sn were found with respect to the actual value of this parameter measured with substrates where charging effects were not significant. Because no differences in peak broadening have been observed under different measurement conditions, it has been assumed that the shift was not caused by a conventional differential charging phenomenon. Considering the different response of the substrate and the deposited layer on stabilizing the charge when the sample is grounded, we have worked out a possible explanation to account for the observed artifacts. Instrumental specifications should be optimized very carefully, especially if (as here) relatively high charging shifts point to a non-optimum self-biasing of the surface potential at the insulating samples.