Single-molecule analysis techniques promise new insights into the natural processes occurring within our bodies. Tiny cantilevers have been investigated to measure the mass of individual biomolecules, but using such delicate ‘scales’ in water remains quite challenging. Shunichi Sawano, Takayuki Arie and Seiji Akita of Osaka Prefecture University and the Japan Science and Technology Agency1 have now used the vibrations of carbon nanotube (CNT) resonators in water for single-molecule measurements.

The resonators are devices that oscillate at specific frequencies — according to Arie, the outstanding mechanical and chemical characteristics of CNTs provide unique advantages for molecular detection. “Unlike a conventional resonator, the tip of a nanotube can be easily chemically functionalized for mass measurements of single molecules,” he says.

Weighing individual biomolecules in water with nanometer-sized resonators has previously failed to yield precise results because the viscosity of the fluid interferes with the ability of the resonators to vibrate. This effect, known as viscous damping, results from drag forces that prevent objects from moving through fluids and restricts their oscillations.

Fig. 1: Scanning electron microscopy image of a carbon nanotube cantilever attached to a silicon substrate.© 2010 S. Sawano

Arie and his co-workers turned to CNTs to minimize the viscous damping effects. First, the team manufactured an array of CNT cantilevers at the edge of a silicon chip (Fig. 1) and placed it in a glass microscope chamber filled with water. Next, they used an actuator to vibrate the cantilevers, and subsequently detected the resulting oscillations with laser light. The difference in scattered light intensity between vibrating and non-vibrating nanotubes provided the resonant frequencies, which could then be related directly to the load attached to the cantilever tip.

The researchers found that several CNTs displayed two modes of oscillations in vacuum, but only one, higher-frequency mode in water. The persistence of these higher-frequency modes suggests that viscosity has little effect on the oscillations. Moreover, the quality factor, which reflects the oscillation energy loss, decreased due to the viscous resistance of the liquid. “Higher modes of oscillation possess higher quality factors, leading to a higher sensitivity of the resonator,” says Arie.

The team is now planning to study the oscillations of nanotube resonators at higher frequencies in water and investigate their use as analytical tools for biological reactions in living cells.