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Probing nanoscale adhesion and structure at soft interfaces

University of British Columbia

Physical measurement at soft interfaces presents special problems. The compliance of the interface makes positional accuracy of secondary importance to force sensitivity. Only minuscule forces are required to displace a soft surface or to slowly overcome a small energetic barrier in an aqueous environment. The thesis is organized according to three aspects of measurement at soft interfaces and the parts are labelled by Roman numerals I—III. Each of these parts (I—III) contains results conclusions and discussions specific to the particular topic of the segment. I) To test adhesion and probe compliance at soft interfaces, the developement of a general, ultra-sensitive force measurement technique is described. The technique exploits a tunable force transducer comprised of a biomembrane capsule held under tension chemically bonded to a glass microsphere probe. The technique has a high sensitivity (in the range of pico- to nano-Newtons) and a large span of force loading rates. II) To demonstrate nanoscale mechanical testing of interfacial compliance, the ultrasensitive force probe was used to determine the thickness compressibility of the human red blood cell membrane. The membrane was found to be 100-fold softer than an ideal rubber. The minimum thickness of the red cell membrane was 58 ± 4 nm. This thickness implies that there are large proteins associated with the membrane that act to expand the tethered spectrin network. III) To demonstrate nanoscale testing of molecular adhesive strength, the probe has been used to rupture single receptor-ligand bonds (avidin-biotin). Recognizing that thermal activation underlies dissociation of weak bonds, a theory for the force-driven failure of point physical bonds is presented. The predictions from theory are verified by Brownian dynamics simulation. Bond failure is a kinetic process. The experimentally measured strength of the bond depends on the rate which force is applied to the linkage. A universal logarithmic rate dependence regime reveals information about the underlying bonding potential along the force-driven reaction pathway. The form of the strength over a spectrum of loading rates gives the position and height of major energetic barriers along the bond failure pathway. Complex bonding potentials can thus be partially reconstructed and compared to the structures of the bonding species, if available.

Thesis/Dissertation

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2009-06-25

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http://hdl.handle.net/2429/9585

Physics

University of British Columbia

UBCV

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