Biomedical Engineering Reference
In-Depth Information
Chapter 3
Imaging and Manipulation of Biomolecules
Abstract
In this chapter, we present the main methods able to image and manip-
ulate biomolecules, with the purpose of getting more information about their
behavior. The main characterization tools related to nanotechnologies, such as the
atomic force microscope and the scanning tunneling microscopy, will be presented
in relation to biomolecule imaging and manipulation. Other applications will be
presented as well.
The imaging of biomolecules and other biological entities is dominated by
optical microscopy, fluorescence microscopy, confocal microscopy, and labeling
with organic fluorophores, which are nanoparticles that emit light for in vivo
analysis or in vitro detection. However, this subject is beyond the aims of this topic.
The reader interested in these subjects is advised to read the excellent review of
Roncali et al.
(
2010
). This chapter deals mainly with imaging methods directly
linked to nanotechnologies
3.1
Bioapplications of Atomic Force Microscopy
The atomic force microscope (AFM) can detect very small forces, up to attonewtons
(1 aN
D
10
18
N). These forces are applied between a cantilever ended with a tip
and a surface that undergoes testing and are sensed via the cantilever deflection.
Several principles can be used to measure the deflection of the cantilever, which has
a very small mass, but by far, the most widespread is optical interferometry, which
enables the sensing of displacements as small as 10
4
A.
The type of force detected by the AFM depends on the tip-sample distance,
which dictates the interaction type between the tip and the sample. For example,
interatomic forces prevail at very small tip-sample separations. These forces
have two components: a short-range Born repulsive force, dominant at distances
smaller than
0:1 nm, and a longer-range van der Waals force, which manifests
up to 10-15 nm. As the tip-sample distance increases further, the exponentially
decaying van der Waals force becomes negligible, and long-range electromagnetic,
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