Dielectrophoretic assembly of semiconducting carbon nanotubes separated and enriched by spin column chromatography and its application to gas sensing

The present authors have previously demonstrated the electrokinetic fabrication of a single-walled carbon nanotube (SWCNT) gas sensor by employing dielectrophoresis. Because this method employs mass-produced SWCNTs, it can realize cheaper and more flexible SWCNT gas sensor fabrication than that based on the on-site synthesis of SWCNTs. In this study, a new protocol was proposed and tested for the separation and enrichment of semiconducting SWCNTs, aiming to improve the SWCNT gas sensor sensitivity. The protocol employed a spin column filled with size-exclusion dextran-based gel beads as well as two surfactants (sodium dodecyl sulfate and sodium deoxycholate), which had different affinities to metallic and semiconducting SWCNTs. The separation and enrichment of the semiconducting SWCNTs were confirmed by measuring their optical and electrical properties. The CNT gas sensor fabricated using enriched semiconducting SWCNTs was highly sensitive to nitrogen dioxide (NO ₂) gas,-more sensitive by 10 times than that fabricated using the pristine SWCNT mixture.