Controlled Patterning of Carbon Nanotube Energy Levels by Covalent DNA Functionalization

Single-wall carbon nanotubes (SWCNTs) are a special one-dimensional class of carbon materials. They consist of sheets of graphene, rolled up to form hollow tubes with walls one atom thick. Their unusual electronic and optical properties have attracted intense attention from basic and applied researchers. At present, sparse chemical functionalization at random positions on the sidewalls of semiconducting SWCNTs has been shown to introduce local band gap changes that can trap mobile excitons and cause spectrally shifted luminescence. Existing literature reports show that such chemically altered SWCNTs hold promise as single-photon emitters for quantum cryptography. To date, however, it has not been possible to control the spatial pattern of covalent functionalization or to smoothly adjust the band gap modulation.

To address this goal, a group of researchers from the Rice University: Yu Zheng, Dr. Sergei Bachilo and Professor Bruce Weisman have developed a facile method for using chemical reactions with single-stranded DNA (ssDNA) to accomplish such modulation. The report a simple but effective method to achieve tailored sidewall functionalization of SWCNTs that can modulate electronic energy levels along the nanotube axis with customized spatial patterns and depths. Their work is currently published in the research journal ACS Nano.

In brief, the research team started by dispersing raw SWCNTs into a water solution of ssDNA to obtain a stable suspension. Then the suspended SWCNTs were exposed to singlet (excited state) oxygen, which was formed by irradiating a common dye with visible light. In a room temperature reaction, the nanotubes selectively formed covalent bonds to the guanine nucleobases. Further, they established that the resulting covalent hybrids displayed near-infrared fluorescence that was red-shifted, broadened, and intensified compared to pristine SWCNTs.

In summary, Rice University scientists successfully established that the exposure of ssDNA-coated SWCNTs to singlet oxygen yielded covalent functionalization of guanine nucleobases to the nanotube sidewall. In addition, the degree of spectral shift and broadening was directly related to the spatial density of guanines in the ssDNA oligo and could therefore be selected by choice of ssDNA structure. In a statement to Advances in Engineering, Professor Bruce Weisman, the corresponding author, highlighted that their new approach may find applications in biosensing to help in medical diagnostics and to detect bacteria, viruses, and other targets of interest. Moreover, the method may lead to applications in optoelectronics and single-photon emission for quantum information processing and may further motivate the development of other methods for spatial control of nanotube electronic structure.