Single-walled carbon nanotubes (SWCNTs) have attracted a great deal of attention during the past two decades because of their unique optoelectronic properties and their potential applications in biomedical theranostics as well as nanoelectronics. It was recently discovered that it is possible to spatially modulate the exciton band gap of carbon nanotubes through a process known as guanine functionalization. In this reaction, the guanine nucleotides of an ssDNA strand wrapping a nanotube become covalently bonded to sidewall carbon atoms upon exposure to singlet oxygen.¹ In the original work, the authors used xanthene derivatives such as rose bengal and erythrosine B as photosensitizers in order to generate singlet oxygen. It was assumed that this process was an independent step that did not involve the nanotube. However, we have found that rose bengal has a strong tendency to form a charge transfer complex with SWCNTs, and these complexes appear to be essential in the functionalization reaction.

We have studied the complexation through a spectroscopic study of SWCNT fluorescence quenching and the appearance of a charge transfer absorption feature near 600 nm. Both are interpreted as results of nanotube p-doping when the dye adsorbs to the sidewall. We also observe that the charge transfer complex formation accelerates when the rose bengal is optically excited, apparently proceeding through the dye’s triplet state. A correlation between extent of guanine functionalization and the concentration of charge transfer complexes is found as ionic strength is changed. We infer that the functionalization reaction is enabled by adsorbed photosensitizers. Interestingly, the extent of SWCNT fluorescence quenching by rose bengal is found to vary significantly with (n,m) structure. This quenching effect therefore provides a simple approach to monitor structure-specific interactions between SWCNTs and coatings, including ssDNA and conventional ionic surfactants.

(1) Zheng, Y.; Bachilo, S. M.; Weisman, R. B. Controlled Patterning of Carbon Nanotube Energy Levels by Covalent DNA Functionalization. ACS Nano 13, 5222 (2019). https://doi.org/10.1021/acsnano.9b03488.