The twin cantilever approach to the single molecule detection

author: Marco Lazzarino, Laboratorio Nazionale TASC
published: Feb. 12, 2008,   recorded: October 2007,   views: 3567


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Detection and manipulation of single molecules by means of mechanical systems has recently received constantly growing attention both for fundamental and applied reasons. In particular, great interest has been aroused by sensors based on the principle of frequency shift detection of mechanical resonators, the application of which to medicine and biology has lead to very sensitive diagnostic methods. However, in the effort to push the sensitivity to the single molecule level, extreme conditions of frequency in the GHz range, size with oscillating structures sized below 1m in all directions, vacuum better than 10-10 mbar and temperatures as low as a few Kelvin, have been exploited. Here we present an alternative strategy capable single molecule sensitivity, that uses twin cantilever structures of several micron in size, operated at room temperature and under ordinary vacuum conditions. Two aspects are treated in detail. The formation of a tunable gap with controllable size in the nanometer range has been obtained by a two step process: first a fracture along a Si 100 crystallographic plane has been induced on a suitable designed silicon suspended structure ensuring the formation of two atomically flat edges. The controlled opening of the gap is then induced by bending mechanically the wafer in analogy with what is normally done in mechanical break junction. The detection of one or more molecules placed across the gap can be then obtained either by measuring the perturbation to the mechanical eigenfrequency of the system or by detecting the mechanical cross talk induced by the molecular link between a short driver cantilever and a longer follower cantilever that face each other. In presence of molecules bridging the gap, if the driver is moved electrostatically at the eigenfrequency of the follower, the latter experience large oscillations than can be easily detected by optical or piezoresistive methods, also for weak binding forces. We fabricated an asymmetric twin cantilever system with a link formed by multi walled carbon nanotubes across the gap and we demonstrated that, working at room temperature and under normal vacuum conditions, in a driver-follower scheme, it is possible to detect the presence of a few molecules. Moreover we investigated the evolution of the mechanical properties of the system as the molecules bridging the gap change in number or strength. We believe that with a proper control of the bonding chemistry, the sensitivity of our system could be improved to reach the single molecule level.

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