by PhilipJ on 8 August 2005
While André has been telling us about the research he’s doing in Deutchland, I’ve been busy at home building an optical tweezers apparatus, a different kind of single-molecule technique that has been used quite extensively the past few years to study everything from DNA and motor proteins to colloidal systems.
In simplest terms, a laser beam gets focused by a high numerical aperture objective lens to a very small point, and refractive particles are pulled to this intense spot. A more detailed (but still easy to follow) description can be found at the Optical Tweezer Introduction page that the Block lab has up.
How strong the trapped particles are held in the tweezers depends on a couple of things, such as Laser intensity and the ratio of the refractive indices of the trapped particles and the surrounding medium (often water). To a good approximation, the force that an optical tweezer system applies to a trapped particle is identical to the force applied by springs. Hooke’s law for springs says that the force exterted by a spring on an attached mass is proportional to the displacement from equilibrium, or F = kx, where k is called the “spring constant”, a measure of how stiff the spring is. This same treatment holds for an optical trap, where “k” can now be considered the optical trap stiffness, or how much force one can apply per unit of displacement from the centre of the trap.
Well, I’m happy to say that our trap seems to be quite stiff! Our setup is very similar to one previously described , and it appears as though our trap stiffness is k = ~147 pN/μm. For comparison, that is about 1.3 times the value quoted in ! In more colloquial terms, if a trapped particle is displaced from the centre of the trap by 500 nanometers, it will feel a force of approximately 75 picoNewtons directed back towards the trap’s centre. These may sound like absolutely tiny forces and displacements, and they are, but it just so happens that this is the range of displacements and forces which are relevant to biological activity at the subcellular level!
For the (physi)curious, the data looks like this, with k being the inverse of the slope:
The forces were applied by flowing water back and forth inside our trapping chamber, with a force proportional to the velocity (Stokes drag) of the flow being applied to a trapped 2.1μm polystyrene sphere.
Just what I’ll be doing with this instrument for the rest of my masters will have to wait for another post!
 Gijs J. L. Wuite et al, An Integrated Laser Trap/Flow Control Video Microscope for the Study of Single Biomolecules, Biophysical Journal 79 1155 (2000).