Abstract: One-step aqueous dispersion and diameter-selective separation of HiPco single-walled carbon nanotubes（SWCNTs）has been accomplished through noncovalent complexation of the nanotubes with a water-soluble,biocompatible polymer chitosan（CHI）at room temperature."Smaller-diameter" SWCNTs are preferentially dispersed and wrapped by CHI in the aqueous supernatant that results following noncentrifuged precipitation,while "larger-diameter" SWCNTs exist in the precipitate.Such CHI wrapped small-diameter individual SWCNTs have been used for the immobilization of microperoxidase-11（MP-11）for direct electrochemistry and electrocatalytic purposes.We found that SWCNTs facilitate the direct electron transfer between MP-11 and the electrode surface,with a formal potential of MP-11 at about-0.36 V（vs.SCE）in phosphate buffer solution（pH 7.2）.The heterogenous electron transfer rate constant and surface coverage of MP-11 are estimated to be 78 s^-1 and 8.76×10^-10 mol/cm²,respectively.Further study demonstrates that the modified electrode retains its electrocatalytic activity toward the reduction of both O₂ and H₂O₂.The immobilized MP-11 can catalyze O₂ through a four-electron process to water,which could be used as a cathode in a bio fuel cell.Such SWCNT based biosensor exhibits a rapid response time,of less than 4 s,and a good linear detection range for H₂O₂ concentration,from².5 μM to 70 μM with a detection limit of 0.8 μM.The apparent Michaelis-Menten constant（K_m）and the maximum electrode sensitivity（i_max /K_m）are evaluated to be 1.0 mM and 22.4 μA/mM,respectively.