A single molecule can be covalently attached to an individual carbon nanotube by using a novel electrochemical method, according to a new study. The technique provides a way to functionalize nanotube sidewalls controllably and doubles as an interrogation method for probing chemical reactions as they occur.
In the developing field of molecular electronics, researchers use various methods to manipulate an individual molecule and pin it in the clutches of a set of electrodes. The idea is to construct single-molecule circuits and probe their electronic properties with the goal of understanding how to assemble and exploit microscopic and densely packed molecule-based electronic devices. But making minuscule electrodes, coaxing the molecules (one per circuit) to line up in exactly the right spot, and verifying the molecules' presence in the circuits are challenging tasks.
Now, researchers at the University of California, Irvine, report a simple procedure that does not require tricky molecular manipulations or advanced fabrication methods. Starting with easily prepared circuits that each contain an individual single-walled carbon nanotube, the group treats the nanotubes with an acidic electrolyte solution in an electrochemical cell while monitoring the circuit conductance.
By applying an electrochemical potential, the researchers switch on an electrooxidation reaction that induces large jumps in conductance that they attribute to individual oxidation events. The potential and hence the chemical reaction can then be switched off with microsecond resolution, thereby stopping the reaction after just one chemical event (Science 2007, 315, 77).
The method was developed by Philip G. Collins, an assistant professor of physics; graduate students Brett R. Goldsmith and John G. Coroneus; and their colleagues. An enabling feature of the new method, according to Collins, is the strong dependence of electrical conductivity on the formation of defects (bonding changes) in the initially pristine and defect-free nanotubes. For nanotubes exposed to nitric acid or sulfuric acid solutions, for example, electrooxidation disrupts the carbon sp2 hybridization, causing large changes in conductance as nitrate or sulfate groups form C-O bonds and become attached to the nanotube wall.
The team demonstrated that after modifying the nanotubes with the small molecules, those individual points of chemical functionality can be used as sites for subsequent reactions, which are readily monitored by further changes in circuit conductance. On the basis of electronic measurements and microscopy methods, they showed that single nickel clusters and goldlabeled streptavidin molecules can be attached to the nanotube walls. The group is looking into ways of using the method to probe antibody-antigen interactions and other types of biomolecular reaction dynamics.
Robert C. Haddon, a professor of chemical engineering at UC Riverside, notes that the Irvine investigation stands out from previous nanotube-electronics studies, which were based on indirect spectroscopy methods. Haddon says the new study "directly and elegantly" demonstrates the reported results by measuring the electrical conductance of individual carbon nanotubes that are controllably functionalized by in situ solution-phase electrochemistry.