Quantification of free polymer removal from polymer-SWNT complexes. (a) Schematic of polymer-SWNT washing via spin filtration. (b) Schematic of polymer-SWNT washing via vacuum frit filtration. (c) Filtrate from the first polymer-SWNT wash step loaded with DNA and run on an agarose gel for all polymers. (d) Filtrate from the sixth polymer-SWNT wash step loaded with DNA and run on an agarose gel for all polymers. From left to right: (1) free plasmid, (2) LPEI-5000, (3) LPEI-800, (4) BPEI-800, (5) low-phi-BPEI, (6) branched polylysine, (7) med-phi-BPEI, (8) BPEI-750k, (9) BPEI-25k. (e) Measurements taken after each wash step for BPEI-25k polymer-SWNTs show a steady increase in the plasmid migration distance, corresponding to a decrease in free polymer, after each wash step.