As is happening in so many other fields, nanotechnology looks set to transform biological and chemical sensors. Reducing the scale of detection down to nanometres should allow scientists to analyse minute samples with unprecedented sensitivity. Already, scientists have shown that carbon nanotubes can be used to detect a wide range of chemical compounds and biological particles, such as DNA, proteins, viruses and bacteria.

But detecting objects at these small scales presents its own challenges, not least in getting microscopic biological particles to travel to the detecting part of the sensor. At low concentrations, it can take biological compounds hours to diffuse even a small distance through a sensor to the detector.

Now, a team of US chemical engineers from the University of Notre Dame, Indiana, led by Hsueh-Chia Chang, has come up with a potential solution. Rather than transport the biological particles to the detector, they have used single-walled carbon nanotubes (CNTs) and dielectrophoresis (DEP) to construct the detector around the particles.

DEP is a novel form of electrophoresis that has proved highly effective at gently transporting and capturing biological particles. It involves generating a nonuniform electrical field in a solution of particles, usually by applying an alternating (A/C) current. This displaces the electrons in the particles, producing a dipole moment (in which the particles' positive and negative charges are separated). This means that the particles will now move under the influence of the electric current, with their direction of travel dependent on the frequency of the field and the conductivity of the particle and the solution.

The one drawback with DEP is that it tends to work best with low frequency electric currents, which can only generate weak electric fields that don't extend very far. This means that DEP can only be used to transport biological particles fairly short distances.

Chang and his colleagues decided to see whether they could overcome this problem by using CNTs to bring DEP to the particles. CNTs are good conductors and readily form strong dipole moments in an electric field. The researchers' idea was that the CNTs would act as nanoscale electrodes and spread the electric field throughout the particle solution.

They first tested the plausibility of this idea on a solution of fluorescent nanoparticles. Without CNTs, these particles wouldn't move too far away from electrodes placed 50µm apart. Adding CNTs to the solution caused the particles to spread throughout the entire gap between the electrodes.

Investigating in more detail, Chang and his colleagues discovered that the particles would naturally attach themselves to the CNTs. When they switched on the current, the resultant DEP caused the CNTs to self-assemble into numerous wires that stretched between the two electrodes, in a process that only took a few minutes. As the particles were attached to the CNTs, they also became incorporated in the wires and spread throughout the gap.

Chang and his colleagues then found that the same thing happened with solutions of the bacteria Escherichia coli. Even more interestingly, they found that there was a definite difference in conductivity between wires constructed solely of CNTs and those containing CNTs and E. coli cells. This happened even with low concentrations of bacteria (around 10,000 cells per millilitre).

Chang is now trying to develop a practical biosensor based on this detection technique. "We expect dielectrophoresis to become a common platform for chip-scale bacteria detection," he told separationsNOW.com. "We are currently working on integrating dielectrophoresis with various sensors and on a continuous chromatographic bacteria separation technique using dielectrophoresis." He is also looking at detecting specific species of bacteria, by attaching antibodies to the CNTs.

Related links:

• Electrophoresis, 2006, 27, 1376: "Bacteria capture, concentration and detection by alternating current dielectrophoresis and self-assembly of dispersed single-wall carbon nanotubes"

Article by Jon Evans