Twist and shake: Making microcantilevers move in new ways

Up and down

A flexible diving board, or springboard, usually just vibrates up and down when a swimmer dives off the far end. But a springboard can also potentially move in other ways.

If the swimmer stands at one far corner of the springboard, then that corner will move down while the opposite far corner moves up; this is known as torsional motion. If the swimmer stands at the far end and shimmies from side to side, then the end of the springboard will also move from side to side; this is known lateral motion. It now turns out that tiny microcantilevers may be able to move in similar ways, potentially providing a novel and more sensitive way to detect analytes.

Microcantilevers are thin microscopic strips of metal, anchored at just one end, leaving the opposite end free to move, just like a springboard. Unlike a springboard, however, this free end tends to vibrate up and down continuously at a characteristic frequency, known as the resonant frequency. Any particles or compounds that land on the end of the microcantilever reduce this resonant frequency, with larger particles or more compounds causing a larger reduction. It’s the same as twanging a ruler on the end of a table: the ruler will vibrate slower if it has an eraser on the end of it.

Monitoring changes in the resonant frequency of a microcantilever thus provides a way to detect analytes. Line up an array of cantilevers, each coated with a substance designed to bind with a specific analyte, and you have a way to detect multiple analytes. Such microcantilever arrays have already been used to detect multiple analytes after a chromatographic separation (see Microcantilevers bend to accommodate GC).

 

Torsional and lateral

Up to now, the free ends of microcantilevers have been designed just to vibrate up and down. But Raj Mutharasan at Drexel University in Philadelphia, US, wondered whether the ends could vibrate in other ways, such as engaging in torsional or lateral motion, and whether analytes landing on the end of the microcantilever would also alter the frequency of this kind of vibration.

To induce these other forms of motion, Mutharasan and his colleagues experimented with changing how the cantilevers were anchored. Rather than merely anchoring the rear end, they also anchored sections of the side and top of the cantilevers. To make the whole process a bit easier, they conducted these initial experiments on millimeter-size cantilevers, around 5mm long and 1mm wide, made from lead zirconate titanate coated in gold.

To induce lateral motion, Mutharasan tried anchoring a small rear section of one side of the cantilever, giving one side of the cantilever more freedom of horizontal movement than the other. To induce torsional motion, Mutharasan tried anchoring the top rear section of the cantilever at an angle, such that the top rear section was anchored at one side but not the other, giving one side of the cantilever more freedom of vertical movement than the other.

 

Three ways to move

By monitoring how well these microcantilevers conducted electricity, which is influenced by their resonant frequency, Mutharasan showed that both of them did indeed vibrate when placed in an aqueous solution. Furthermore, the cantilever anchored at the back and side did seem to display lateral motion and the cantilever anchored at the back and top did seem to display torsional motion.

Next, Mutharasan added the gold-binding thiol 6-mercapto-1-hexanol to the solution and found that this reduced the resonant frequency of the cantilevers, presumably as a result of the thiol binding to the far end of the cantilevers. Using a mathematical model to extrapolate these findings down to the level of microcantilevers, Mutharasan found that lateral and torsional motion may offer a more sensitive detection mechanism than normal up and down motion. The model predicted that a microcantilever engaging in lateral motion should be almost twice as sensitive as a conventional microcantilever.

Building on this work, Mutharasan is now looking to detect analytes using a combination of all three forms of motion. ‘We can design sensors that exhibit bending, torsional and lateral modes simultaneously, but at different distinct frequencies,’ he told separationsNOW. ‘Such a design can offer us three simultaneous measurements for the same target analyte.’