The talk is focused on applying single-wall nanotubes (SWNT) as optical reporters of DNA structure. SWNT photoluminescence (PL) was used to explore photo-physics of pi-pi* transitions in the photoexcited DNA.  The ssDNA wrapped CoMoCat SWNTs, water suspended, commonly produce a strong and stable PL under visible excitation^[1]. Recently we demonstrated that under two-color excitation (visible plus UV) such a system shows rich behavior: normal double-band excitonic photogeneration resulting in UV-enhanced PL emission could be one type of response. The other behavior was found in a certain band of excitation wavelength: reversible PL quenching followed the cw-UV illumination, different from photobleaching or photochemical modification of pristine material. The explanation of this intriguing behavior was found and presented in Ref.^[2].

By combination of optical spectroscopy, rate equation analysis for excited states, and semi-empirical quantum mechanical calculation the source of the PL quenching was identified to be free charge carriers introduced by the photoexcited DNA. It is known that the DNA in the course of biological evolution developed mechanisms of recovery after UV irradiation, as well as relaxation mechanisms for photoexcited states. One of those mechanisms, namely, the DNA autoionization (AI) mechanism has been detected experimentally using SWNT as a nanoscale reporter. Theoretical analysis of the effect showed existence of the AI rates shorter than 20 fs and dominated by a hole charge transfer from the ssDNA to the SWNT. The capability to detect a fundamental relaxation mechanism of UV-excited DNA by using two-color spectroscopy proves potential of a DNA-SWNT complex, as a model optical system, for detecting charge transfer from DNA. Furthermore, the model demonstrated that the AI rate is a function of the conformation of the DNA thus opening further avenues for biosensing of DNA architecture in these complexes.

Significant contributions to this work by Dr. T. Ignatova (UC Irvine), Dr. A. Balaeff (University of Central Florida), Dr. M. Zheng (NIST), Mr. M. Blades (LU), and Mr. P. Stoeckl (University of Rochester) are kindly acknowledged; special thinks to Dr. J. Fagan (NIST) for help with instrumentation. Research was partially supported by National Science Foundation (ECCS-1509786).

References:

[1] Carbon Nanotubes and Related Structures, ed. DM Guldi, N Martin.Weinheim:Wiley-VCH. (2010)

[2] T. Ignatova, A. Balaeff, M. Zheng, M. Blades, P. Stoeckl and SV. Rotkin, “Two-color spectroscopy of UV excited ssDNA complex with a single-wall nanotube probe: Fast nucleobase autoionization mechanism”, Nano Research, (2015).