Short multi-walled carbon nanotubes (MWCNTs) 8–15 nm in diameter (Nanostructured & Amorphous Materials, Inc.) were acid oxidized and purified as previously described 35 (link). Preparations of 1% Pluronic (Sigma), 1% 1,2-Distearoyl-sn-Glycero-3-Phosphoethanolamine conjugated to Polyethylene Glycol (DSPE-PEG) or 2% DSPE-PEG (Nanocs, Inc.) were made by dissolving the surfactants at a concentration of 1% or 2% weight to volume in deionized water. Nanoparticle dispersions were prepared by hydrating 10 mg of unmodified or acid oxidized MWCNTs with 10 mL of degassed Milli-Q (type I) water (with or without surfactants) in a 20 mL glass vial, followed by 30 minutes of bath sonication (Branson 2510). Nanoparticle suspensions were rendered isotonic by the addition of one part in 10 of 10× phosphate-buffered saline (PBS) (Invitrogen, Carlsbad, CA, USA) prior to dilution in cell culture media.