Figure 8. RGD Surface Density Displayed by Adsorbed HFN and Mixed GRGD/OEG₃ SAMs.

XPS was used to estimate the RGD surface density displayed by HFN adsorbed to bare Au or by mixed GRGD/OEG₃ alkanethiol SAMs. (A) Bare Au surfaces were exposed to HFN solution for 30 mins at 37 °C with concentrations from 1.5–50.0 μg ml⁻¹, the surfaces rinsed, and the N_1s spectra acquired with XPS at each concentration (see Supplemental Fig. S1 and Table S1). The area under the N_1s peak was measured for each condition to quantify the amount of protein present. The HFN surface density was estimated by setting the saturation concentration to a previously published fibronectin packing density. The HFN surface density on the remaining surfaces was ratiometrically back calculated using the measured areas under the N_1s peaks. The RGD surface density was determined by multiplying the HFN surface density by the number of RGD motifs, two, displayed on each HFN molecule. (B) A similar method was used to estimate the RGD surface density displayed by mixed SAMs of GRGD- and OEG₃-terminated alkanethiols. Bare Au surfaces were exposed to solutions of mixed alkanethiols at a working concentration of 2 mM for 1 hr. The percent of GRGD-terminated alkanethiol in solution was varied from 0.5–10.0% (for example, the 10% GRGD solution was comprised of equal volumes of 0.2 mM GRGD- and 1.8 mM OEG₃-terminated alkanethiols). The area under the N_1s peak (see Supplemental Fig. S2 and Table S2) for each GRGD concentration was measured and the packing density of the 1% GRGD surface set to a previously published value. The RGD surface density for the remaining surfaces was ratiometrically back calculated using the N_1s peak areas.