Joseph Plewa

Processing carbon nanotubes with holographic optical tweezers

We report the first demonstration that carbon nanotubes can be trapped and manipulated by optical tweezers. This observation is surprising because individual nanotubes are substantially smaller than the wavelength of light, and thus should not be amenable to optical trapping. Even so, nanotube bundles, and perhaps even individual nanotubes, can be transported at high speeds, deposited onto substrates, untangled, and selectively ablated, all with visible light. The use of holographic optical tweezers, capable of creating hundreds of independent traps simultaneously, suggests opportunities for highly parallel nanotube processing with light.

Conserved linking in single- and double-stranded polymers

We demonstrate a variant of the bond fluctuation lattice Monte Carlo model in which moves through cis conformations are forbidden. Ring polymers in this model have a conserved quantity that amounts to a topological linking number. Increased linking number reduces the radius of gyration mildly. A linking number of order 0.2 per bond leads to an 8% reduction of the radius for 128-bond chains. This percentage appears to rise with increasing chain length, contrary to expectation. For ring chains evolving without the conservation of linking number, we demonstrate a substantial anticorrelation between the twist and writhe variables whose sum yields the linking number. We raise the possibility that our observed anticorrelations may have counterparts in the most important practical polymer that conserves linking number, DNA.

High-sensitivity measurement of free-protein concentration using optical tweezers

We have applied optical tweezers to measure the free-protein concentration of a microscopic sample in the nM range by measuring the optical tweezer laser power at which protein coated beads are ruptured from an antibody coated coverslip surface. We used silica beads that were covalently coated with a target protein and a glass coverslip coated with antibodies specific against the target protein, causing the coated beads to stick to the surface. The unknown unlabelled target protein concentration was added, which then competed with the bead-bound target protein for antibody binding sites on the coverslip surface. In this way the number of bead-surface bonds were modulated by the free protein concentration in solution affecting the threshold laser power necessary to rupture the bead from the surface. An optical tweezer was used to probe the number of bead-surface bonds by measuring the threshold power required to pull the bead away from the surface. We positioned an optical tweezer (1064 nm) slightly above the bead and linearly ramped the laser power until the bead ruptured from the surface. The power at which this occurred was used to determine the free protein concentration. Our measured calibration curve of threshold power versus free protein concentration was fitted to a single binding site equilibrium model which yielded an estimate for the equilibrium dissociation coefficient that is comparable to literature values.