2012 BFY Abstract Detail Page
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||W17 - Optical Trapping in Biophysics
||An optical trap, or laser tweezers, is a device used to manipulate objects between about 20 nm and several microns in diameter and to measure piconewton-sized forces on these objects. This scale of operation makes them a useful tool in biophysics to study mechanical properties of cells, organelles within cells, and single molecules involved in movement and force production. An optical trap is a good learning tool in the physics instructional lab because of the insight it gives into mechanical properties of biological structures, but also because of the physical principles of operation and particularly of the techniques used for calibrating position and force. In Berkeley's physics advanced lab course, students spend about 8 afternoons performing several types of calibrations and performing two biophysics experiments. Building and alignment of a trap could be a challenging semester-long project for a small class, though alignment of the Class 3b IR laser requires safety training and close supervision.
An optical trap is essentially a microscope incorporating a trapping laser and position-detection system. Traps can be made either by adding a laser beam path to a conventional microscope or by building the microscope and beam path from standard optical components. We took the latter approach, which makes the optics easier for students to see (though we shield the collimated IR laser beam with lens tubes for safety), and makes it easy to modify. For instance, last summer our students added a second laser to support fluorescence microscopy needed for a single-molecule experiment. Our trap is patterned after one developed for teaching and research at MIT. A somewhat more expensive version is available in kit form from Thorlabs. Our student write-up and information on experiment development is available on our course wiki at http://advancedlab.org. I also recommend the excellent write-up by Sean Robinson on his MIT Physics Junior Lab web site.
In this workshop, we will do the following activities:
-- Practice trapping 1 micron Silica beads.
-- Observe the effect of laser power on the motion of the bead.
-- Record the position of the bead through the quadrant photodiode (QPD).
-- Use the power spectrum of the QPD data and our understanding of Brownian motion to calibrate sensitivity and stiffness of the trap.
-- Examine student-collected data on (1) E. coli flagellar swimming, (2) internal transport of vesicles in live onion cells, and (3) in vitro stall forces measured from single kinesin motor molecules.