Figure 4

XPS data for the C 1s region of the control samples in surface contamination experiments. (a) Au substrate that was incubated in a blank phosphate buffer for 24 h. Surface contaminants were completely removed by ion beam sputtering and the spot was re-analyzed (b). The sample stage was then exposed to the atmosphere for 1 s (c) or 3 min (d), in addition to the time spent purging the loading dock and pumping down for analysis. The spot was sputter-cleaned in UHV prior to each exposure in (c) and (d).