Carbon nanotubes (CNTs) are well known as materials for nanoelectronics and show great potential to be used as the sensing elements in chemical and biological sensors. Recently, CNTs have been shown to be effective nanofluidic channels and the transport of substances through small diameter CNTs is intrinsically fast, selective, and operates at the single molecule level. It has been shown that the transport characteristics of semiconducting single-walled CNT (SWCNT) field effect transistors (FETs) are sensitive to internal water wetting. We report here that the characteristics of semiconducting SWCNT FETs are also sensitive to the concentration, pH and ion type of the ionic solution when the electrolyte is inside the CNT. Such sensitivity is not observed at the outside surface of a semiconducting SWCNT. This opens a new avenue for building new types of CNT sensor devices in which the SWCNT concurrently functions as a nanochannel and an electronic detector.