Single-stranded DNA and single-walled carbon nanotubes (SWNTs) form ssDNA-SWNT hybrids that have many potential applications ranging from sensing to drug delivery. DNA-SWNT hybrids are prepared by performing sonication in aqueous solution.  However, sonication may damages the DNA molecules on the SWNT surfaces. In this study, we examined whether the biological function of DNA molecules on the SWNT surfaces is maintained by using biomolecular recognition of RecA proteins.  For comparison, we prepared the carboxymethylcellulose-SWNT (CMC-SWNT) sample as a control sample to study the biomolecular recognition ability of RecA proteins for DNA on the SWNT surfaces more clearly.
Analysis of the diameter distribution using an AFM revealed that the ssDNA-SWNT hybrids with RecA proteins had larger diameters than the ssDNA-SWNT sample without RecA proteins. Although we also compared the mixture of RecA proteins and CMC-SWNT hybrids with CMC-SWNT sample in diameter distribution of rodlike hybrids, there was no significant change in diameter distribution. Furthermore, agarose gel electrophoresis complied with the results obtained from the AFM observations. Migration speed of the ssDNA-SWNT hybrids was obviously decreased in the presence of RecA proteins. The difference of mobility suggests that RecA proteins bind to ssDNA on the SWNT surfaces. Contrarily, the band of CMC-SWNT hybrids was remained at the similar position even after adding RecA proteins. The results demonstrate that the binding of RecA proteins with ssDNA molecules on SWNT is based on biomolecular recognition not non-specific physisorption.