Single-molecule experiments provide fascinating possibilities for studying systems in which molecular individuality matters (see Science 283, special issue on “Frontiers in chemistry: Single molecules” ) . Compared to ensemble measurements, single-molecule experiments offer several important advantages. First, by conducting many sequential measurements, they allow the distribution of molecular properties to be determined, affording the investigation of inhomogeneous systems. Second, single-molecule trajectories provide information that is hidden in ensemble-averaged results. Finally, they permit observation of rarely populated transients that are difficult or impossible to capture using conventional methods. We have develped a novel microcapacitor motion-detector, capable of measuring electronically the motion of single particles, in real time, at submicron resolution, in liquid environments and at ambient temperatures.
Our motion detector, is based on the change in capacitance, DC , induced by the passage of a single particle between the plates of a non-parallel plate capacitor such as the micromachined one shown in the figure above (see appl. phys. lett. 84 (21): 4277-4279 may 24 2004) . When a particle enters the capacitor, the capacitance changes by an amount which depends on the position of the particles along the axis of symmetry of the device. A trace of the capacitance as a function of time can be translated into the position of the particle as a function of time.
Various applications can be envisaged for the device. For example, it may be used to study fluid dynamics in confined media, using suspended particles as probes. For biological systems, macromolecules such as proteins can be tagged with metal or dielectric nanosphere to allow motion to be measured as they perform their tasks. Using labels of different dielectric constants, more than one population of molecules can be followed at the same time.
A unique feature of this scheme is that it allows for resolutions beyond the limits of fabrication encountered in the manufacturing process of the device. The ultimate resolution of the device is limited only by the ability to measure very small changes in capacitance. Simulations show that using a sphere of 50 nm and a VWC whose width is on the order of 100 nm will allow a resolution of ~1 nm. This should enable to obtain real-time dynamic information for a host of situations in liquid environments at ambient temperatures and at the single-molecule level.
This work is carried out in collaboration with Dan Shahar of the Condensed matter physics department at the Weizmann Institute and Joseph Shappir from the school of applied sciences at the Hebrew university in Jerusalem.
