New biocomposite materials, based on the incorporation of nucleic acid dopants within an electronically conducting polypyrrole network, are described. The growth patterns and ion-exchange properties of these electropolymerized polypyrrole-oligonucleotide (PPy/ODN) films are characterized using an in situ electrochemical quartz crystal microbalance (EQCM). The EQCM and corresponding voltammetric data indicate that nucleic acids can serve as the sole charge-compensating counterions during the film formation. While the incorporation of ODNs is similar to that of small inorganic anions, such large nucleic acid dopants could not be readily expelled from the PPy network. As a result, the electrochemistry is dominated by the movement of the electrolyte cation: PPy(ODNn-)(X)(Na+)(nX)double left right arrow PPyX+(ODn-)(X)+nxNa(+)nxe(-) Various parameters, such as the ODN length or concentration and the potential range, have a marked effect on the properties of the new conducting biomaterials. Very favorable growth patterns are observed for biocomposites containing 20-30-mer long ODNs, while films based on shorter oligonucleotides or chromosomal DNA display inferior properties. The composite films can be prepared using low (similar to 1 x 10(-5) M) concentrations of the nucleic acid dopant, in the absence of additional electrolyte. Such biomaterials open up new opportunities, including genoelectronic devices, composite materials, bioactive interfaces, genetic analysis, or probing of DNA charge transfer.