Study of Near-Infra Red (NIR) fluorescence properties of Single-Walled Carbon Nanotubes (SWCNTs) as possible candidates for imaging of some biological objects

Single-walled carbon nanotubes (SWCNTs) are quasi one-dimensional materials with small band-gap that have unique physical and chemical properties for applications in life sciences and medicine, including in vivo sensing and imaging. For the development of biologically applicable fluorescence-based sensors, the important characteristics are the quantum yield, the photostability and the tissue transparency in the sensor emission range. Semiconducting SWCNTs are characterized by a fluorescence emission in the near-infrared (NIR) region, from 820 to 1600 nm, where the absorption by biological tissues is usually minimal. Furthermore SWCNT have demonstrated an essentially infinite photobleaching threshold, with a steady fluorescence emission even under high excitation fluence. The ability shown by the SWCNTs to maintain constant fluorescence emission under continuous illumination ensures reproducibility in fluorescence measurements but also permits the possibility to develop sensors with indefinite lifetimes. Despite SWCNTs are considered the only fluorophores to date that do not photobleach over extended periods of time, a previous research of the group of interdisciplinary physics of the Laboratory for Radiopharmaceutical and Molecular Imaging (LARIM) in the National Laboratories of Legnaro (LNL), demonstrated that the fluorescence emission from sodium cholate water solutions of SWCNTs, excited by a laser with a wavelength of 830 nm, diminishes with the time. The dependence of the intensity emission with respect to the storage time was studied for an overall period of 8 hours, keeping the sample in a dark storage at a temperature of 20 °C. This first observation suggested the importance to perform a more detailed investigation in order to better understand the processes involved over a longer time scale. For these reasons the photoluminescence intensity of a SWCNT solution has been investigated as a function of the storage time and the storage temperature. A long-term constant-temperature storage compartment for solutions with NIR emitting nanoparticles has been designed, constructed, tested and incorporated in the setup for NIR studies. The fluorescence emission of SWCNTs suspended in sodium cholate aqueous solutions has been collected for periods of about 250 hours and studied as a function of the storage time and storage temperature. It was found out that the SWCNTs fluorescence emission yield has a fading storage-time dependent behavior, which confirms the previous research made by the LARIM research team. The fading behavior can be described by a storage-time dependent exponential function summed with a constant term. The half time coefficients for the exponential part were found to vary according to the wavelength region in which the NIR spectra have been subdivided and also with the storage temperature. The results of this experiment suggest the possibility to apply sufficiently good corrections to the observed intensity emission of SWCNTs. These corrections can be very important in the development of SWCNT sensors with long lifetimes. The temperature dependence of the fading behaviour suggests the need to perform further measurements to evaluate and explain the origin of this phenomenon