Studying the internalization of SWCNTs in microalgal cells: positive side-effects of the charge transfer

Over the past several years, efforts have been dedicated to study the effects of SWCNT internalization in both animal and plant living cells. The need for a fast and reliable method for tracking carbon nanotubes in biological samples is of increasing interest both for studying toxicity of carbon nanotubes and developing CNT based bio-applications. An example of an emerging technology based on the functional coupling of nanomaterials, such as CNTs, with plant cell structures is the plant nanobionics, whose advances would require the development of new and effective diagnostic tools. Common techniques able to detect SWCNTs in plant cells are high-resolution microscopies as electron transmission microscopy and fluorescence microscopy by using either the characteristic fluorescence of nanotubes in the NIR region [Wong] or CNTs coupled with fluorescence labels. However, these diagnostic tools do not give information on cell metabolic processes and, in the case of the fluorescence markers, they are non-specific probes. Raman microscopy is an exciting alternative to already existing methods for imaging of CNTs, enabling also observation of living cells under physiological conditions. One of the key advantages of Raman spectroscopy is the ability to distinguish cellular macromolecules such as proteins, lipids and DNA, that can be visualized according to their vibrational Raman spectra. In this work, a visible Raman microscopy has been used to study the internalization of SWCNTs in algal cells and the interaction between the nanotubes and photosynthetic machinery into the cell chloroplast. The Raman signal was used to map and compare single algal cells hosting SWCNTs and cells free of nanotubes. One of the major hurdles that still limits the prompt application of Raman microscopy and imaging to photosynthetic organisms is the strong autofluorescence of photosynthetic pigments that hamper the detection of Raman scattering. The overcoming of this problem imposes specific experimental approaches aimed at suppressing of the chlorophyll fluorescence usually present under excitation with visible light. Our approach was successful in demonstrating the occurrence of charge transfer between the chlorophyll molecules and nanotubes. This phenomenon cuts down the fluorescence background, enabling to record Raman signals originating from cellular compounds distributed near to the nanotube, without the use of a complicated experimental set up. The potential of the SWCNTs to be used as a spectroscopic probe in photosynthetic cells and the origin of the SWCNTs/pigments-interaction will be discussed.