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Posted: Nov 28, 2011
DNA nanomaterials: Making contact
(Nanowerk News) Metal-containing DNA structures combine the exceptional functionalities and sequence-specific self-association of DNA with the versatile electrical, magnetic and catalytic properties of metals, making them ideal for the development of biocompatible applications such as molecular electronic circuits and asymmetric catalysis. Yet while numerous synthetic metallo-DNA systems have been developed, determining the electrical conductance of these architectures remains challenging.
Schematic representation of a device consisting of a single metallo-DNA strand.
At the heart of the researchers' system is a nanoscale gap formed by carbon nanotubes bearing carboxylic acid end groups that attach to single molecules. When an individual metallo-DNA strand is laid across the gap, the extremities of the strand attach to the nanotubes through covalent amide bounds.
In addition to natural DNA bases, each metallo-DNA strand included artificial hydroxypyridone bases that pair up to produce stable complexes with metal ions. Introducing one or three of these units in the middle of the strands gave DNA structures that incorporated up to three copper ions. Initial measurements showed that increasing the number of ions enhanced the conductance.
The use of chemically resistant amide bonds allowed the researchers to investigate the coordination chemistry of the metallo-DNA structures under different conditions. Interestingly, sequentially alternating treatments with the polyamino carboxylic acid EDTA and copper ions efficiently switched the electrical properties of the device on and off. Exposing the devices to EDTA effectively removed the ions, causing the conductance to decrease. Conversely, a dramatic increase in conductance occurred upon addition of copper ions. This switching function opens the door to a promising new field of interfacing molecular devices with biological systems.
Guo explains that, in the absence of copper, the hydroxypyridone units act like a mismatched base pair, disrupting the stacking of the bases and charge transport across the double-stranded structure. In contrast, the introduction of metal ions generates a rigid framework that facilitates DNA charge transport. "We are currently developing new approaches to realize single-molecule biosensing and DNA mutations," says Guo. Such DNA-based sensors could lead to the highly accurate molecular diagnosis of various infectious diseases and cancers.