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We have previously demonstrated a fabrication method of single-walled carbon nanotubes (SWCNTs) based gas sensor by employing dielectrophoresis. Because this method uses the SWCNTs that are synthesized in bulk, it is a cheaper and more flexible method than that of on-site synthesized. This method can quantify the amount of trapped nanotubes on a real time basis by monitoring the electrical impedance of the sensor simultaneously with its fabrication. In this study, semiconducting SWCNTs, which served as transducer in gas detection, were separated from commercially available mixture of semiconducting and metallic ones, and were further enriched by using a spin column and dextran-based gel. The separation and enrichment of the semiconducting SWCNTs were confirmed by measuring their optical and electrical properties. The CNT gas sensor fabricated using separated semiconducting SWCNTs was highly sensitive against nitrogen dioxide gas, - more sensitive by two times than that fabricated using the pristine SWCNTs mixture.