Multi-walled carbon nanotubes (MWNTs) were ultrasonicated in concentrated H2SO4/HNO3 for 4 h, and then used to construct a novel immunosensor. After functionalization, the length of the carbon tubes clearly decreased and the arrangement on the electrode surface tended to be more uniform, as confirmed by transmission electron microscopy and scanning electron microscopy measurements. Infrared spectroscopy revealed that carboxyl groups were introduced on the surface of the MWNTs after ultrasonication in mixed acid. Moreover, the functionalized MWNTs exhibited a larger electrochemical response area compared with MWNTs alone. As a result, functionalized MWNTs were successfully used to immobilize the antibodies to CD8+ T-cells. The interaction of antigen and antibody on the electrode interface made a barrier for electrons and inhibited electron transfer, resulting in decreased current signals of the probe K3[Fe(CN)6]. Using this strategy, a convenient, low-cost and rapid electrochemical immunosensor was developed for the counting of CD8+ T-cells.
