The reactivity of organic radicals produced by 1,1'-azobis(cyanocyclohexane) (ACN) is tuned to selectively suppress electrical conduction of small diameter single-walled carbon nanotubes, and the modified tubes are able to produce high-mobility (approximately 10 cm2 V(-1) s(-1)) and solution-processable thin-film transistors with full semiconductor device yield, promising applications in high-performance printable electronics.