Electrostatic sensing and electrochemistry with single carbon nanotubes

Electrostatic sensing and electrochemistry with single carbon nanotubes

Heller, I.

Dekker, C. (promotor)
Lemay, S.G. (promotor)

Applied Sciences

2009-01-16

This thesis describes the experimental study of devices based on single carbon nanotubes in the context of (bio)sensing in aqueous solutions. Carbon nanotubes are cylindrical molecules of sp2- carbon, about one nanometer in diameter and typically several micrometers long, which have semiconducting or metallic electronic properties. Nanotube devices can interact both electrostatically and electrochemically with the solution and the (bio)molecules dissolved in it. We study these interactions electronically with the aim to learn how carbon nanotube devices interact with their environment and how they can be used as the active elements in highly sensitive nanoscale (bio)sensors. First, we study the electrochemical interaction of redox molecules with carbon nanotube devices. An applied potential difference over the interface between a carbon nanotube and the solution can drive the electrochemical transfer of electrons from dissolved redox molecules to the nanotube and vice versa. We demonstrate that individual carbon nanotubes, both metallic and semiconducting, can be used as nanoelectrodes for electrochemistry. Due to the small diameter of nanotubes, the relative influx of electrochemically active molecules is so high that the kinetics of charge transfer become rate limiting. We provide a theoretical description of electrochemical charge transfer at nanotube and graphene electrodes. We find that, although the distinct electronic structure of nanotubes does play a role in the charge transfer process, metallic and semiconducting nanotubes cannot readily be distinguished. Even when a semiconducting nanotube is switched OFF, charge transfer can still take place at high rates. Next we explore carbon nanotubes employed as liquid-gated field-effect transistors. Although the literature contains an increasing amount of studies that use nanotubes for sensing purposes, a thorough fundamental understanding of how exactly these transistors interact with their environment is lacking. We elucidate and demonstrate several physical mechanisms that allow nanotubes to act as nanoscale electrostatic sensors. We show that the sensor response can be affected by an artifact related to the reference electrode. By eliminating this artifact we can study the effect of biomolecule adsorption near nanotube sensors unambiguously. Then we describe a method to identify the different mechanisms that can lead to a sensor response. We find that the origin of sensor response to biomolecule adsorption is a combination of a change in surface potential, and alterations to the tunnel barrier at the nanotube-metal contact. Contact effects make sensing unreliable, but these can be suppressed by covering up the contact regions. Finally, we show that carbon nanotube and graphene transistors are sensitive to changes in the ionic strength, the pH, and even the type of ions of the electrolyte. Changes in these electrolyte properties lead to a sensor response by changing the surface charge and the spatial distribution of ions, and thus the surface potential. We proceed by studying the signal-to-noise ratio for biosensing with liquid-gated carbon nanotube transistors. We show that the low-frequency noise is consistent with the fluctuation of nearby charges that gate the nanotube through a field-effect. The power of the noise is inversely proportional to the length of the nanotube. Surprisingly, the signal-to-noise ratio is highest in the sub-threshold regime. The decrease of the signal-to-noise ratio in ON state is related to additional noise sources and depends on device architecture. In specific cases the back gate can enhance the signal-to-noise ratio. Finally, we report our exploratory studies of carbon nanotube sensors as probes to study living cells. Although our results are suggestive that we can successfully detect cellular activity, the transistor stability and electrochemical sensitivity need to be improved. We show that the electrochemical sensitivity can be improved by coating nanotubes with catalytic nanoparticles. In conclusion, we have studied carbon nanotube devices in aqueous solution. The work presented in this thesis elucidates a number of different physical mechanisms, both electrochemical and electrostatic, through which carbon nanotube devices can interact with their environment. In addition, many of the concepts developed and studied here may be extended to other nanoscale sensors, such as nanowires and graphene. This knowledge can be used to further exploit the unique properties of carbon nanotubes, and pursue the ultimate goals of single-molecule detection and single-cell probing.

carbon nanotube
graphene
nanowire
nanotechnology
sensor
biosensing
field-effect-transistor
cell

http://resolver.tudelft.nl/uuid:201c982a-296e-488b-b0c6-a07c5fe149b7

978-90-8593-047-1

Institutional Repository

doctoral thesis

(c) 2008 Heller, I.