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Tip Insulation with Self-Assembled Monolayers

IP.com Disclosure Number: IPCOM000111891D
Original Publication Date: 1994-Apr-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 2 page(s) / 88K

Publishing Venue

IBM

Related People

Delamarche, E: AUTHOR [+4]

Abstract

Biological molecules like Proteins and DNA need the presence of a water layer to retain their native shape and their biological acitvity. The retracting water film during the process of air drying strongly interacts with the molecules and tears them apart so that topographic studies on such surfaes by STM or AFM or any other local probe method cannot reveal any useful information. Therefore, the true topography of biological surfaces can only be studied woth local probe methods when it is still covered by a film of water or similar solvent which also contains the necessary buffers to maintain pH and ionic strength. When the STM tip is immersed into that solution, ionic currents reduce the sensitivity of the STM at low currents.

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Tip Insulation with Self-Assembled Monolayers

      Biological molecules like Proteins and DNA need the presence of
a water layer to retain their native shape and their biological
acitvity.  The retracting water film during the process of air drying
strongly interacts with the molecules and tears them apart so that
topographic studies on such surfaes by STM or AFM or any other local
probe method cannot reveal any useful information.  Therefore, the
true topography of biological surfaces can only be studied woth local
probe methods when it is still covered by a film of water or similar
solvent which also contains the necessary buffers to maintain pH and
ionic strength.  When the STM tip is immersed into that solution,
ionic currents reduce the sensitivity of the STM at low currents.
Since the current cannot be enlarged without destroying the
biological molecules due to tip-surface interactions, it becomes
almost impossible to study such objects.  Ionic currents can be
compensated by using reference and compensation electrodes in
electrochemical cells [1].  This compensation depends, however, on
the electrochemical potentials of the ions, the surface, the tip and
all other material present in the cell.  Full compensaion is very
difficult especially for biological buffers and biological samples.

      Another approach to reduce ionic currents is by insulating the
major part of the tip with apiezon wax, teflon, and similar
substances [2].  Such coatings typically have thicknesses of more
than 1 micormeter, which require that the coating had to be stopped
several micrometers before the end of the tip leaving several square
microns uncoated, i.e., accessible for ionic currents.  Teflon
coatings are very thick resulting in blunt tips which are only
possible for use in AFM instruments.

      According to the new method, a freshly etched STM tip is coated
with a hydrophobic self assembled monolayer.  Self assembled
monolayers are formed from organic molecules with a linear alkyl
tail, which provides ordering among the molecules and is responsible
for an almost defect free insultation, and a reactive group like
sulfur, silane or carboxylic acid at one end.  A gold tip, for
example, can be passivated with a self assembled monolayer of thiol
or disulfide.  The gold tip is immersed in a millimolar solution of
octadecanethiol (HS-C sub 18 H sub 37) in ethanol for 24 h.  The tip
is th...