tering and accumulation at the sides of the scanned area is
clearly visible in the region which previously experienced high
loads. Scanning the sample in the contact mode at di†erent
loading forces allowed us to Ðnd an upper limit to non-
disruptive loads at about 1 nN. The evaluation of the protein
height from contact mode images gives reliable results (about
Acknowledgements
We are grateful to Caterina Arcangeli, Salvatore Cannistraro,
Alessandro Desideri and Carlo DellÏErba for helpful dis-
`
cussions. We thank Adriana Dacca for the XPS measure-
ments. This work was partially supported by the University of
Genoa and MURST (Italian Ministry for ScientiÐc Research).
3
nm) under low load (1 nN or less). The images acquired with
higher forces show di†erent surface topologies: molecules are
either squeezed by the scanning tip, resulting in artiÐcially
thin and deformed features, or pushed laterally and aggre-
gated into clusters with vertical dimensions between 10 and 30
nm, well above the molecular size.
At this stage of the work, no conclusive statement about the
detailed mechanism of sample damage can be given. Taking
into account the existence of a chemical bond between the
protein and the thiol chain, it is conceivable that the whole
thiolÈprotein ensemble is displaced by the scanning tip. Thiol
displacement by the AFM tip has been observed in previous
AFM studies, which investigated the reversible displacement
of thiol molecules chemisorbed on gold obtained by increasing
the loading force.39,40
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