(599r) Noncovalent Ternary Dispersions of Single Wall Carbon Nanotubes for Controlled Cellular Delivery
Single wall carbon nanotubes (SWCNTs) are hydrophobic and require stabilization in aqueous solution for cellular applications using a biocompatible dispersing agent such as polyethylene glycol copolymers, lipids, or proteins. Covalent modification of SWCNTs to add the dispersing agent has traditionally been used to impart targeted cellular delivery, increased molecular loading, and controllable release profiles. However, covalent functionalization introduces defects into SWCNTs which destroy inherent SWCNT properties and limits their application potential. Here, we exploit the noncovalent interaction between bovine serum albumin (BSA) and SWCNTs to disperse SWCNTs for cellular delivery. Serum albumin is a natural surfactant-like protein known to aid in the transport of materials through interactions with hydrophobic subdomains. We further produce a noncovalent ternary complex of SWCNTs coated in BSA wherein small hydrophobic molecules have been loaded into the hydrophobic pockets of BSA. We observe changes in quantum yield for hydrophobic fluorescein molecules trapped within these so-called pockets of BSA which can be recovered upon protease mediated degradation or thermal denaturation of BSA in vitro. Alone, model hydrophobic molecules cannot efficiently disperse SWCNTs, but when complexed with BSA create high yield, stable dispersions of individual SWCNTs with ~20-70 molecules per SWCNT. Using fluorescence imaging, we observe efficient delivery of complexed molecules to the cytoplasm of macrophages and epithelial cells. We also loaded the BSA with drug molecules including the chemotherapeutic daunomycin, which accumulated in the nucleus and reduced cell proliferation. To further investigate intracellular release from the SWCNT complex we use a combination of fluorescence lifetime imaging microscopy (FLIM) and NIR fluorescence imaging. FLIM of the loaded molecules reveals a bimodal distribution of fluorescence lifetimes corresponding to partial release from SWCNTs and quenching from molecules retained on SWCNTs. NIR fluorescence imaging of SWCNTs confirms individual SWCNTs within cells and shows regions of spatially separated SWCNTs and released fluorescent molecules. We suggest a noncovalent coupling approach can be useful for labeling SWCNTs without covalent disruption of the carbon backbone and may be generalized to include other molecular loading combinations.