A simple, rapid and label-free capacitive based immuno-biosensor was developed for the detection of the hepatitis B surface antigen (HBsAg). Two planar gold electrodes were used as capacitor plates. The surfaces of the electrodes were covered by a mixture of HS(CH₂)₁₁(OCH₂CH₂)₃OH and HS(CH₂)₁₁(OCH₂CH₂)₃COOH (20 : 1 weight ratio) as an insulating molecule layer. Then, the primary anti-HBsAg was immobilized on the insulating layer. After the addition of HBsAg, the distance between the plates and also the average surface area of the plates changed. The alteration of these factors led to a change in capacitance which could be used as an indicator of the antibody–antigen interaction. Also, in order to improve the sensitivity of the method, gold nanoparticles were attached to a secondary antibody. Due to the relatively large size of the particles, the thickness of the dielectric layer and thus the capacitance changed remarkably. Therefore, the detection limit was improved to about 10 ng ml⁻¹.