Microcantilevers functionalized with DNA incorporating a Hind III restriction endonuclease site were digested with Hind III to produce DNA with a single-stranded end on the cantilever surface. Ligase was then used to link a second DNA molecule with a compatible end to the DNA on the cantilever. Nanomechanical motion of the cantilever was monitored throughout the digestion and ligation. Fluorescently labeled DNA was used to confirm that ligation and digestion occurred. The DNA was attached to the silicon side because Hind III and DNA ligase both require dithiothreitol to retain their activity. We therefore avoided the possibility that thiolated DNA on the gold side of the cantilever would be displaced by thiol-containing compounds in solution. Our results show that any natural DNA containing a restriction endonuclease site could be digested and attached to a cantilever functionalized with a compatible DNA. Our results also show that the ligated DNA can be removed, regenerating the cantilever for future use.
In his classic paper on thermal grain boundary grooving Mullins [W.W. Mullins, J. Appl. Physics 28, 333 (1957)] assumes that the dihedral angle at the groove root remains constant and predicts that the groove width and depth grow αt0.25. Here, we derive models describing groove growth while the dihedral angle changes. In our grooving experiments with tungsten at 1350 °C in which the dihedral angle decreased, the growth exponent for the groove depth reached values as high as 0.44 while the growth exponent for the width decreased slightly from Mullins' value of 0.25. Hence groove width data alone are not sufficient for verifying Mullins' growth law unless the dihedral angle is constant. The observed changes in the dihedral angle are used as an input for numerical simulations. With the simulations we are able to extract the surface diffusion constants. Atomic force microscope observations of groove widths and depths in tungsten are in excellent agreement with the simulations.
Grain-boundary grooving has been studied on polished surfaces of polycrystalline tungsten annealed at 1350 8 C. Atomic force microscopy images were taken in the same area for each groove after di erent annealing times. Secondary oscillations next to the main groove maxima (predicted for grooving by surface di usion) were observed, to our knowledge for the ® rst time. The agreement between experimental and calculated groove pro® les (using the surface di usion model of Mullins (1957, J. appl. Phys., 28, 333)) improved when grain-boundary¯uxes were introduced.The theory of grain boundary grooving by surface di usion was rigorously developed by Mullins (1957). Mullins' expressions for the width w and depth d of the grooves can be written as follows:
…3 †In these equations, t is the time of annealing, m the slope of the surface at the groove root,¯D s the product of the thickness of the layer in which surface di usion occurs and the surface self-di usion coe cient, respectively, O the atomic volume of the di using species, ® s the free energy of the solid± gas interface, k the Boltzmann constant and T the absolute temperature.
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