Nothing too small to detect

Nanoscale analysis previously unobtainable is now possible, thanks to an unusual approach to atomic force microscopy. The new detection system has successful measured the displacement of very small cantilevers and, most importantly, could be used when studying samples in water.

At the sub-micron scale, the measurement of the position of a scanning probe in relation to the specimen surface is a critical operation, upon which the success of sample analysis can hinge. Current techniques effectively impose limits on the geometrical dimensions of the probes. This, in turn, has an impact on their force sensitivity and the speed of their response.

The new technique promises to reduce these drawbacks and enhance the utility of scanning probe microscopy. Instead of using lasers to measure the deflection of the micro-cantilever force sensors on the microscope, which is the most common technique, it creates a field of evanescent electromagnetic waves just above the sample that is being investigated.

When the tip of a vertically-mounted force sensor enters the field, it causes part of the field to propagate. The scattered electromagnetic radiation is collected and reveals how much the tip of the cantilever has been deflected.

The method has been demonstrated by scientists from Bristol University and Hanover Medical School, who used ion beam milling to cut down commercially-available probes to make even smaller cantilevers. They claim they are sensitive enough to study the dynamic behaviour of biomolecular systems and to enable high-speed measurement from soft samples for the first time.

Max Glaskin