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Posted: Jan 15, 2008
Biosensing mechanism with carbon nanotubes explained
(Nanowerk Spotlight) Transistors are the key elements of many types of electronic (bio)sensors. Since the discovery that individual carbon nanotubes (CNTs) can be used as nanoscale transistors, researchers have recognized their outstanding potential for electronic detection of biomolecules in solution, possibly down to single-molecule sensitivity.
To detect biologically derived electronic signals, CNTs are often - but not always - functionalized with (conductive) linkers such as proteins and peptides to interface with soluble biologically relevant targets (linkers need not be conductive as long as they are capable of localizing the target molecule in close vicinity of the tube).
Although CNT transistors have been used as biosensors for some years now, the ultimate single-molecule sensitivity, which is theoretically possible, has not been reached yet. One of the reasons that hampers the full exploitation of these promising nanosensors is that the sensing mechanism is still not well understood. Although a variety of different sensing mechanisms has been suggested previously, various studies contradict one another, and the sensing mechanism remained under debate.
Researchers in The Netherlands - through modeling and specific control experiments - now have succeeded in identifying the sensing mechanism. They found that the majority of their experiments can be explained by a combination of electrostatic gating and Schottky barrier effects. Because these two mechanisms have different gate-potential dependence, the choice of gate potential can strongly affect the outcome of real-time biosensing experiments.
Atomic force microscopy data (3D topography pictures) of a CNT transistor. (Image: Molecular Biophysics group, Kavli Institute of NanoScience, TU Delft)
"We address an issue that is crucial to understanding and further exploiting these new nanosensors, i.e. the physical mechanism that actually leads to a change in the transistor conductance upon biomolecule adsorption and thus allows carbon nanotubes to act as biosensors" Iddo Heller explains to Nanowerk. "By carefully modeling how the gate response of the transistor changes upon biomolecule adsorption we found that in specific cases (ambipolar devices, where the majority charge carriers can be both electrons and holes) each suggested physical mechanism gives a very characteristic change, which allows the sensing mechanism to be identified unambiguously."
Comparing the modeling to their experiments, the researchers found that sensing is caused by a combination of the 'bulk gating mechanism' – induced by the charge of biomolecules that adsorb along the length of the nanotube – and the 'contact mechanism' – where biomolecules that adsorb close to the contact change the nature of the metal-nanotube contact.
Heller, a PhD student in the Molecular Biophysics group in the Kavli Institute of NanoScience at Delft University of Technology in The Netherlands, is first author of a recent paper in Nano Letters ("Identifying the Mechanism of Biosensing with Carbon Nanotube Transistors") in which the Kavli Institute researchers resolve the issue of the sensing mechanism through both extensive protein-adsorption experiments and modeling.
Artist's impression of proteins on or near a CNT transistor: red near contact, i.e. contact mechanism, and blue along length of tube, i.e. bulk mechanism. (Image: Molecular Biophysics group, Kavli Institute of NanoScience, TU Delft)
Single-walled carbon nanotubes (SWCNTs), with their diameter of about 1 nm, are directly comparable to the size of single biomolecules and to the electrostatic screening length in physiological solutions. SWCNTs arguably are the ultimate biosensor among nanoscale semiconducting materials not only for their size but also because their low charge-carrier density is directly comparable to the surface charge density of proteins. Furthermore, a SWCNT consists solely of surface such that every single carbon atom is in direct contact with the environment, allowing optimal interaction with nearby biomolecules.
"We have found that sensing is dominated by two physical mechanisms, of which the contact mechanism appears to be rather unreliable" says Heller. "We showed that by passivating the contact area, the unreliable mechanism (the Schottky barrier effect) is canceled out, providing a reliable platform for biomolecule sensors based on individual nanotubes."
The Dutch scientists point out that a crucial issue for design and applications of SWCNT sensors concerns the region of the device where protein adsorption causes significant conductance changes. "Although earlier reports, which suggest workfunction modulation to be the dominant sensing mechanism, imply that the sensitive region is limited to the nanoscale contact regions, our results clearly indicate that in addition, strong electrostatic gating reliably occurs along the bulk of the SWCNT channel" says Heller.
A second important issue for protein sensing is whether the conductance changes can be directly related to the charge of the biomolecules.
Through their experiments, the researchers were able to confirm that the electrostatic gating shift can be entirely attributed to the protein charge.
In summary, these results indicate that If the SWCNT contact region is passivated, sensing is dominated by electrostatic gating, which demonstrates that the sensitive part of a nanotube transistor is not limited to the contact region, as previously suggested.
Such a layout provides a reliable platform for biosensing with nanotubes.
The ultimate goal that the Dutch scientist are pursuing is to reach single-molecule sensitivity. When this becomes possible, nanotubes can be a very interesting electronic tool to study for instance enzymatic activity on the single molecule level.