A biosensor usually contains a layer of biological recognition elements covalently attached to the device. For example, fibre-optic nanoprobes can be covalently bound with antibodies that can selectively target certain biological molecules. The new sensor, made by Marcus Willander and Safa Al-Hilli of Göteborg University, is radically different because the polar and nonpolar surfaces of the hexagonal single-crystal ZnO nanorod act as direct detectors. This means that no separate bioreceptors are needed.

Willander and co-workers made their device by assembling many ZnO nanorods together on silver-coated capillary glass. The nanorods have a diameter of 80–100 nm and are 700 to 900 nm long.

Highly sensitive

The sensor's tip, which contains hundreds of individual ZnO nanorods, is about 1.4 microns across. This small size means that it can penetrate a cell membrane and measure the pH inside the cell in real time. Thanks to the large number of nanorods, the device is highly sensitive and works by monitoring the very small change in electrochemical potential caused by the binding of biomolecular species on the surface of the probe.

The new sensor can pick up signals from individual organelles within a cell – something that is impossible to do with existing glass probes whose electrical resistance increase as the width of the probe shrinks, leading to a noisy signal. "Another important property of the device is that it creates a seal with the cell membrane, so we do not have large holes in the membrane," Willander told nanotechweb.org. "The probe goes inside the cell and we can access all the different parts."

Some of the sensor's other unique advantages: it has a high surface-to-volume ratio, is non-toxic and produces strong, stable and reversible signals with respect to pH changes. It is also stable to a variety of chemicals.

"Using these nano-intracellular sensors, it has become possible to probe individual chemical species in specific locations throughout a single living cell," stated Willander. "We need these types of sensor to better understand the structural and functional relationship between the organelle and the cell as a whole. The intracellular pH value of the cell can also give us enough information to distinguish between healthy and affected cells."

Early days

The team stresses that its research is still at an early stage and much work needs to be done before a real-life ZnO nanorod sensor sees the light of day. The first thing will be to further reduce the size of the tip sensor, says Willander. It might also be possible to change the surface properties of the ZnO nanorods. "This could allow us to attach selective membranes that act as ion sensitive sensors within specific parts of the cell," he explained. "We will also try to increase the pH-sensitivity by using ZnO nanotubes and look at the effect of nonpolar surfaces on the pH measurements."

The results were reported in J. Appl. Phys..