The detection of HBsAg was conducted by incubating the SWCNT-Ab chemiresistive biosensor with the 100 µL sample solution for 30 min to ensure maximum adsorption of HBsAg. Then PBS wash was used to remove excessive proteins. A resistance change was measured before and after the incubation and expressed as the signal (S), as described in Equation 1: S = (Raft -Rbef)/ Rbef (1)