The dramatic increase of articles published the last years in the area of nanotechnology reflects the increasing efforts to create new artificial structures like nanodots(1), nanotubes(2) and nanowires(3). This continued incentive to produce functional structures on an atomic scale places severe demands on current technologies. To be able to force a specific design or pattern on surfaces at the nanometer scale, lithographic, electron beam or ion beam processes have been used. On a 10 - 100 nanometer scale, ordered surface heterogeneity can be produced based on the ability of a scanning probe microscope, to mechanically (4) or electronically ( 5 -7 ) "write" on a surface. Alternatively, short-wavelength lithography sources such as an electron (8) or ion (9,10) beams can be employed to write small features on a surface. For example, it has been demonstrated that by using a finely focused ion beam (FFIB) ordered patterns of pits each with a 30 nm diameter could be created(10) Rather than forcing a specific design or pattern, self organisation of molecules is another complementary approach to reach the nanoscopic level of molecule assemblies both free in solution and on surfaces (11-12).
Many complex biological structures are fixed to surfaces by which well organised and ordered arrays of molecules can be obtained to get a high efficiency for different purposes. Good examples of that are biocatalytical enzyme systems, signal systems based on receptor/ligand inteactions at neurotransmittors, respiratory chain or in photosynthesis. On a micron scale it has been shown that by using photolithography, well-ordered surface heterogenity can be obtained resulting in spatial control of the subsequent process of protein adsorption (13,14,15).
Our group has recently published work on site selective adsorption to ion defects at the nanometer scale studied by SPM in a liquid cell which show a more than 2000 preferential adsorption of beta galactosidase to these artefacts compared to human serum albumin (16). The adsorption is in this case selective but the sites for adsorption are randomly distributed over the surface. To add spatial control over surface tailoring, a new generation of finely-focused keV-atomic-ion beams (FIB) can be employed which can sputter-write a line of width as small as 8 nm. Well-ordered arrays of pits have been prepared on gallium arsenide and silicon wafers using a finely-focused ion beam (FFIB). The defect pits in gallium arsenide, examined with tapping mode scanning force microscopy (TM-SFM), had a rim diameter of 60 nm and were spaced 185 nm apart.
TM-SFM images showed that human serum albumin (HSA) adsorption was highly specific to the inner side of the rims of the pits in gallium arsenide, while there was no specific adsorption to the rims of pits in silica.
This study demonstrates that a controlled spatial distribution of adsorbed proteins can be achieved on a nanometer scale and that the choice of material is of importance (17) It is not sufficient however to attain structures at the nanoscale without understanding the forces and underlying mechanisms for ther creation.
To be able to construct even more complex structures from the positioning of single molecules to arrays of different kinds of molecules, the tools need to be refined, the techniques for arraying single molecules to be developed and the mechanisms and conditions for their creation to be determined. Ion defects were created on different kinds of surfaces.
The defects and the surrounding surface were investigated by TEM followed by an evaluation of the degree of preferential adsorption of proteins. Deposition of different metals at site selective positions by use of FFIB make these spots potential sites for site selective covalent immobilisation of macromolecules. With these techniques the possibility exist to get a correctly oriented molecule at a site selective position to be used for building of more or less complex artificial biological structures.
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