Oscillations, Waves, and Interactions - GWDG

1.3 Optical trapping
DPI60plus – a future with biophysics 437
Optical trapping exploits the transfer of momentum due to scattering or refraction
of photons by refractile objects. The forces on a small particle of higher index of
refraction than its surroundings (for example a latex or glass bead in water) can be
made to trap the particle near the focus of the laser beam. Several aspects make
such an optical trap (or “tweezers”) particularly interesting for the study of single
biomolecules: 1.) the force-range accessible with optical traps is – dependent on laser
power – about 0–250 pN which well matches the forces generated by individual motor
proteins and thus fills the gap between load-free conditions in fluorescence experiments
and the minimal forces resolvable by AFM (> 50 pN); 2.) the Brownian motion
of the bead in the trapping potential is well measurable and can thus be used to report
binding and unbinding of individual molecules to their substrate. Binding means
additional spatial confinement or an increase in total system stiffness which results
in a decrease of displacement variance. Such measurements can be performed with a
time resolution of 1 ms which is sufficiently high for the study of many conformational
changes in proteins, for example motor proteins. 3.) The spatial resolution in optical
trapping set-ups using interferometric detection is equally well suited for conformational
changes of many biomolecules, namely in the nanometre range. Acusto-optical
deflectors make it possible to rapidly steer the trap, either to create a time-dependent
force on molecules or to switch between multiple quasi-simultaneous trap positions.
1.4 Microrheology
Currently, optical traps are, on the one hand, used in the lab to measure the forces
and the steps molecular motors produce when they move along cytoskeletal filaments.
Optical trapping and fast and accurate position detection are, on the other hand, also
used for “microrheology”, i. e. to probe the dynamic viscoelastic properties of soft
systems such as colloidal suspensions or polymer networks on mesoscopic scales. Soft
materials are important in technology. Examples are plastics, synthetic polymers,
polymer solutions, colloids and gels. Most biomaterials also classify as soft materials,
such as cytoskeletal protein polymers, polysaccharides, lipid membranes or DNA
solutions. Many of the varied and intriguing properties of soft materials stem from
their complex structures and dynamics with multiple characteristic length and time
scales. One of the characteristic and frequently studied material properties of such
systems is their shear modulus. In contrast to ordinary solids, the shear modulus
of polymeric materials can exhibit significant time or frequency dependence in the
range of milliseconds to seconds or even minutes. In fact, such materials are typically
viscoelastic, exhibiting both a viscous and an elastic response.
Rheology, which is the experimental and theoretical study of viscoelasticity in such
systems, is of both fundamental and immense practical significance. Bulk viscoelasticity
is usually measured with mechanical rheometers that probe macroscopic milliliter
samples at frequencies up to tens of Hertz. Recently, a number of techniques have
been developed to probe the material properties of systems ranging from polymer
solutions to the interior of living cells on microscopic scales. These techniques have
come to be called microrheology, as they can be used to locally measure viscoelastic