Various viologens have been used to control the doping of single-walled carbon nanotubes (SWCNTs) via direct redox reactions. A new method of extracting neutral viologen (V-0) was introduced using a biphase of toluene and viologen-dissolved water. A reductant of sodium borohydride transferred positively charged viologen (V2+) into V-0, where the reduced V-0 was separated into toluene with high separation yield. This separated VI solution was dropped on carbon nanotube transistors to investigate the doping effect of CNTs. With a viologen concentration of 3 mM, all the p-type CNT transistors were converted to n-type with improved on/off ratios. This was achieved by donating electrons spontaneously to CNTs from neutral V-0, leaving energetically stable V2+ on the nanotube surface again. The doped CNTs were stable in water due to the presence of hydrophobic V-0 at the outermost CNT transistors, which may act as a protecting layer to prevent further oxidation from water.