Mechanisms of Colloidal Stabilization of Oxidized Nanocarbons in the Presence of Polymers: Obtaining Highly Stable Colloids in Physiological Media

Abstract

There are successful protocols for dispersing carbon nanotubes and graphene oxide in physiological media by using biocompatible polymers, which enable their use in nanomedicine. However, there is not a clear understanding regarding the mechanisms of the colloidal stabilization manifested (i.e., electric, steric, electrosteric, or through depletion forces). Here we show that the manifestation of a particular mechanism of stabilization for oxidized carbon nanotubes (CNTs) and graphene oxide (GO) in the presence of Pluronic F-127 (PF127) and short- and long-chain polyethylene glycol (PEG 1500 or 35000, respectively) depends on a proper matching between the nanocarbon morphology and the polymer chain length, chemical structure, and concentration. The high aspect ratio one-dimensional morphology of CNTs enables an initial steric and electrosteric stabilization through the nanotube wrapping (i.e., adsorption) by PF127 present in low concentrations (<0.1%). Depletion stabilization for CNTs manifests when PF127 is present in high concentrations (≥1.0%), thus enabling the formation of highly stable CNT colloids even in a 0.9% NaCl saline solution. This depletion stabilization depends little on the CNT structure (i.e., single-walled or multiwalled), surface charge (i.e., ζ potential), oxidation and carboxylation degrees, or nanotube length. On the other hand, large-sized sheets of GO could be colloidally stabilized in 0.9% NaCl only in the presence of PEG 35000 through repulsive depletion forces, whose manifestation occurs with a polymer concentration threshold of 5.0 wt %. Comparatively, in a physiological saline solution, PF127 is able to colloidally stabilize CNTs to a much larger extent than PEG 35000 stabilizes the large GO sheets.