Electrical Conductivity of Single Peptide Nucleic Acid (PNA) Duplexes
Group Members:  Emil Wierzbinski
Collaborators: Prof. Catalina Achim (Carnegie Mellon University), Dr. Marcela Madrid (Pittsburgh Supercomputing Center) Prof. Eric Borguet (Temple University), Prof. David Beratan (Duke University)
Hybrid inorganic-nucleic acid structures have great potential in a broad range of fields including molecular electronics, biology, and nanotechnology. We are measuring the conductivity of single peptide nucleic acid (PNA), in partial metal-containing PNA molecules. PNA is a synthetic analogue of DNA, and it has been shown that PNA may act as a scaffold for metals ions.  We are examining whether the incorporation of metal ions in PNA duplexes can produce one-dimensional metal-ion arrays that have novel electronic properties, such as molecular wires.
    To measure the electrical conductivity of single molecules of double stranded Peptide Nucleic Acid (ds-PNA), we have followed two experimental procedures in which Scanning Probe Microscope is used to create molecular junction.
    The first procedure was developed by Lindsay and coworkers (Cui et al. Science, 2001,294, 571) and uses an Atomic Force Microscope (AFM) platform with a metalized AFM tip.
In this procedure molecules of interest are immobilized on gold substrates and diluted within an insulating matrix. In our studies, the matrix contains ss-PNA molecules. Studied ds-PNA molecules are covalently bonded on one end to a gold substrate and on the other end to a gold nanoparticle. The electrical circuit is closed by touching the nanoparticle with a gold-coated AFM tip.
Cartoon of the AFM system used inmeasurements of single molecule conductivity.
Cartoon of the AFM system used inmeasurements of single molecule conductivity.
Cartoon of the AFM system used inmeasurements of single molecule conductivity.
In the second procedure a Scanning Tunneling Microscope (STM) is used to form a molecular junction (Tao and coworkers Science 2003 (301) 1221).
Cartoon of the STM system used in measurements of single molecule conductivity.
Sample current-distance characteristics showing current plateaus. Characteristics taken on the electrode pre-modified with ds-PNA.
In this method, a gold tip is alternately brought into contact with a gold surface that is pre-modified with the molecule to be studied and then withdrawn. The tunneling current-distance dependence is recorded while a constant bias is applied between the tip and the substrate. When one or more molecules is present in the junction, current plateaus are observed on the current-distance profile instead of a simple exponential decay. The procedure is repeated several thousand times to probe various geometries of the molecules in the junction.
Both experimental methods allow measurements of electrical conductivity on a single molecule level with high number of repetitions. Statistical analysis of the results shows that various molecule conformations and contacts conditions may be probed