Biological cells are complex objects that have the potential to act as templates for the subsequent construction of nanoscale structures. We demonstrate the ability to controllably and reversibly manipulate individual, live bacterial cells across micron-sized electrical gaps, and to detect bridging directly through changes in the electrical response. Our model system, Bacillus mycoides, is a rod-shaped bacterium approximately 800 nm wide and 5 microm long, similar in size and shape to many inorganic nanowires.

We combine the use of dielectrophoretic positioning with electrical impedance measurements to detect and discriminate between individual bacterial spores on the basis of their electrical response. Using lithographically defined microelectrodes, we use dielectrophoresis to manipulate individual bacterial spores between the electrodes. The introduction of a single spore between the microelectrodes produces a significant change in electrical response that is species-dependent. When positioned between two electrodes and an AC voltage was applied, single spores caused current increases averaging 6.8 ((2.4) pA for Bacillus mycoides to 1.18 ((0.37) pA for Bacillus licheniformis. Using a mixture of spores of two different species, we demonstrate the ability to distinguish the species of individual spores in real time. This work demonstrates the feasibility of using impedance measurements for real-time detection and discrimination between different types of spores.

Dielectrophoretic manipulation of nanoscale materials is typically performed in nonionic, highly insulating solvents. However, biomolecular recognition processes, such as DNA hybridization and protein binding, typically operate in highly conducting, aqueous saline solutions. Here, we report investigations of the manipulation and real-time detection of individual nanowires bridging microelectrode gaps in saline solutions. Measurements of the electrode impedance versus frequency show a crossover in behavior at a critical frequency that is dependent on the ionic strength. We demonstrate that by operating above this critical frequency, it is possible to use dielectrophoresis to manipulate nanowires across electrode gaps in saline solutions. By using electrical ground planes and nulling schemes to reduce the background currents, we further demonstrate the ability to electrically detect bridging and unbridging events of individual nanowires in saline solutions. The ability to both manipulate and detect bridging events with electrical signals provides a pathway toward automated assembly of nanoscale devices that incorporate biomolecular recognition elements.

Although dielectrophoresis has been used previously to manipulate a variety of nanoscale materials, manipulation in ionic solutions is more difficult due to the high dielectric constant of water and the formation of electrical double-layers. Here, we report experiments aimed at the manipulation of nanowires in aqueous media and real-time detection of nanowire bridging events. Real-time video images demonstrate the ability to manipulate individual nanowires in aqueous media by capturing them along the edges of electrodes, and using a slow fluid flow to transport them until they bridge across micron-sized electrode gaps. By using special cancellation schemes, we demonstrate that it is possible to eliminate the effects of background currents through the electrolyte, and to electrically detect the bridging of electrodes by individual nanowires and nanowire bundles. These results have been obtained using gold nanowires with diameters ranging from ∼50 to 250 nm, ∼50 nm diameter silicon nanowires, and ∼70 nm diameter carbon nanofibres.