Single-Molecule Mechanics of DNA - DNA Nanotechnology: From Structure to Function - page 86

increasing external magnetic field strength, before magnetic saturation occurred.
That is, by controlling the electric current of the coils, the force applied on the
bead can be adjusted easily. After calibration, the applied force on the bead can be
calculated precisely. The advantage of the magnetic tweezers is the high precision
at the relatively low force range compared to other technologies.
The position of the bead in the sample can be observed by a reverse microscope.
These analog signals can be converted into digital signals via CCD and computer.
With the digital positions, the bead can be controlled by the computer in real time.
By controlling the vertical position of the electromagnet, the bead can move along
the Z direction. When the electromagnet rotates around the Z-axis, the bead and the
connected molecule will also rotate. Stretch and rotation are two basic operation
modes of the magnetic tweezers in research.

6.1.2 Optical Tweezers

Photon is a special matter, which exhibits both wave and particle properties. Energy
and momentum can be exchanged when photon interacts with other masses. Forces
can be generated when momentum is exchanged. For example, a pressure of
0.5 dyn/m 2 can be generated on the surface of the Earth for a sunlight of vertical
incidence. However, this force is too weak to be utilized in practice. To enhance the
mechanical effect of light, the intensity of the light must be enhanced greatly. For
common light sources, the emission is random in direction, leading to a relatively
low intensity. The mechanical effect of light becomes significant only after the
invention of laser.
It should be noted that the thermal effect of laser is very strong. If the thermal
is not transferred in time, the object in the focus of laser will evaporate in a wink.
In 1970, Ashkin found that if a transparent particle is suspended in water, the heat
can be transferred to water quickly, and the temperature increase of the particle is
not severe, which avoided the damage by laser heat. He found that the particle at
the edge of the laser beam will be attracted in the axis of the beam, and then the
particle will be pushed forward along the direction of laser by the radiation pressure
of light [ 4 ].
In 1986, Ashkin et al. found that only one strong beam of laser is needed to form
a stable potential well, which can capture a particle (refractive index larger than
that of water) in the focus. The position of the particle will be fixed only when the
center of the particle and the focus of laser superpose together (see Fig. 6.2 [ 5 ]).
This important finding signified the emergence of optical tweezers.
When captured, the particle can be moved by adjusting the focus of the laser.
If a target molecule is tethered on the particle, the molecule can be manipulated
individually (see Fig. 6.3 ). In theory, optical tweezers can generate a high enough
force if a strong enough laser is applied. However, the thermal effect will be a big
problem for a very strong laser. In practice, the power of laser is often limited to
10 W, and the corresponding force of optical tweezers is limited up to 200 pN.
Similarly, optical tweezers can also work in the rotate mode.