Single Biomolecule Interaction Measurements Using a COIFM with an Optical Fiber Probe

An optical-fiber tip has previously been fabricated using a chemical etching method and implemented in a cantilever-based optical interfacial force microscope (COIFM) system to study interfacial interactions in aqueous environments. The optical fiber-tip has been successfully applied to measure electrical double layer forces as a function of distances between two surfaces in KCl solutions. A new tip fabrication method using a laser-pulling technique allows for more rapid optical fiber tip fabrication with a smaller tip-diameter than previous chemical etching, less than the order of 100 nm. The newly fabricated tips have been applied to measurements of biomolecule interactions between biotin and avidin molecules at the single molecule level in aqueous environments. The biotin molecules were attached to the optical-fiber tip whereas the avidin molecules were attached to an oxidized silicon surface. The COIFM force-distance curves exhibit abrupt binding events as the distance between the biotin and avidin became smaller. This is the first observation of biological interactions between two biomolecules during approach in aqueous environments. The result successfully demonstrates the capability of the new optical-fiber tip fabrication technique as well as the unique and advantageous abilities of the COIFM system over the conventional AFM system.