SHEAR FORCE IMAGING OF DNA IN A NEAR-FIELD SCANNING OPTICAL MICROSCOPE (NSOM)

A near-field scanning optical module has been constructed as an access ory for a Nanoscope IIIa commercial scanning probe microscope. Distanc e feedback and topographic registration are accomplished with an uncoa ted optical fibre scanning tip by implementation of the shear force te chnique. The tip is driven by a piezoelectric actuator at a resonance frequency of 8-80 kHz. A laser diode beam is scattered by the tip and detected by a split photodiode, with lock-in detection of the differen ce signal, The amplitude (r) and phase (theta) responses were characte rized as a function of the calibrated tip-sample separation. Using an r cos theta feedback signal, imaging of pUC18 relaxed circular plasmid DNA spread on mica precoated with cetylpyridinium chloride was achiev ed. The apparent width (28 +/- 5 nm) was approximately four times that achieved by scanning force measurements with the same instrument; the apparent height of the DNA (0.6 +/- 0.3 nm) was similar with the two techniques. These results demonstrate the applicability of the shear f orce signal for imaging biological macromolecules according to topogra phy and in conjunction with the optical signals of a near-field scanni ng optical microscope (NSOM).