Differential near-infrared imaging of heterocysts using single-walled carbon nanotubes

The internalization of near-infrared (NIR) optical nanoprobes in photosynthetic microbes can be exploited for applications ranging from energy conversion to biomolecule delivery. However, the intrinsic, species-dependent properties of microbial cell walls, including their surface charge density, composition, thickness, and elasticity, can severely impact nanoprobe uptake and affect the cellular response. An examination of the interaction of the optical nanoprobe in various species and its impact on cell viability is, therefore, imperative for the development of new imaging technologies. Herein, we extend the technology recently developed for internalizing fluorescent single-walled carbon nanotubes (SWCNTs) in prokaryotes, specifically unicellular Synechocystis sp. PCC 6803, to a filamentous cyanobacterial strain, Nostoc punctiforme. Using a combination of NIR fluorescence, scanning electron microscopy (SEM), and Raman spectroscopy, we investigate uptake in vegetative cells as well as differentiated heterocysts. We demonstrate a strong dependence of long-term cell integrity, activity, and viability on SWCNT surface functionalization. We further show differential uptake of SWCNTs across a single filament, with positively charged functionalized SWCNTs preferentially localizing within the heterocysts of the filament. This cell dependency of the nanoparticle internalization motivates the use of SWCNTs as a NIR stain for monitoring cell differentiation.