Probing Biopolymer Films with Scanning Force Methods
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gelatin films. Such information is of great importance to improve the performance of photographic emulsions, in which gelatin comprises the binding matrix.9 In thin gelatin films on mica we observe two film components with distinctly different frictional, morphological and adsorptive character. A high-force scanning procedure remarkably transforms the high-friction, primary component into the low-friction component. We discuss our findings based on understandings of protein folding in gelatin films. EXPERIMENTAL DETAILS Freshly-cleaved muscovite mica (Union Mica Corp.) substrates were exposed to 10-3 wt% aqueous gelatin solution for 3 hours and dried slowly following a water rinse. The Nanoscope Ill (Digital Instruments) SFM was used to characterize the resulting adsorbed gelatin films. Topographic and frictional force images were simultaneously collected at constant vertical cantilever deflection using triangular microfabricated 100 gim cantilevers (spring constant = 0.58 N/m) with pyramidal Si 3 N4 tips. The 1231J scanner with lateral/vertical scanning ranges of 160/4.7 gtm was used. Friction-actuated cantilever torsion was enabled by choosing a fast-scan direction perpendicular to the primary cantilever axis. The applied load was varied by changing the vertical cantilever deflection maintained during scanning.
253 Mat. Res. Soc. Symp. Proc. Vol. 355 0 1995 Materials Research Society
Figure 1. Topography/frictional force images (left/right) of typical low-friction islands and surrounding high-friction first layer. Brighter regions correspond to higher elevation or frictional force. Image size is 3000x3000 nm. Repeated high-force scanning (total load -100 nN) in the narrow horizontal strip near the bottom of the imaged region resulted in island removal to leave the underlying first-layer surface. RESULTS AND DISCUSSION Topography/friction SFM images tens of microns in dimension of as-prepared gelatin films reveal scattered low-friction islands surrounded by a dominant high-friction surface. 4 We zoomed in on two representative islands to collect the 3000x3000 nm topography/friction (left/right) image in Figure 1 at a contact force of -25 nN. Higher elevation or frictional force is rendered brighter. The lowest surface regions, which we will call the first layer, appear "granular" as imaged, the smallest resolved "grains" being several tens of nm in lateral dimension. A relatively large frictional force is exerted on the SFM tip in these regions. The lowfriction islands contain sub-regions of characteristically different thickness. The dominant, thinner island portions have mean elevation 1.5 nm higher than the first layer and contain circular pores 10-100 nm in diameter. Friction identical to that of the first layer is imaged at the bases of the pores. The thicker island regions have variable elevation 6-10 nm above the first layer and exhibit friction identical to the thinner island regions. On some samples small, low-friction 4 domains (
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