The authors propose a direct electrochemistry-based reagentless, separation-free, immunosensor for quantification of cancer antigen 125 (CA125) based on direct ELISA technique. HRP-conjugated monoclonal anti-CA125 antibody, helps in direct electron transfer from electroctalytic center of enzyme to electrode on immunocomplex formation via covalent linkage between immobilized tagged antibodies, amino acid coated colloidal gold nanoparticles and electrode. Optimum temperature, pH and incubation time for efficient electron transfer and hence amperometric response of the immunosensor was 30 „aC, pH 7.0 and 15 min respectively. The electrocatalytic current response was proportional to CA125 concentration with a dynamic range of 1.0 to 9 kIU/mL, lower limit of detection as 1 IU/mL and linear range 1.0¡V1500 IU/mL with sensitivity of 29.7 nA/IU/cm2. The fabricated immunosensor showed good stability and could be used for onsite clinical diagnosis of CA125 since it requires no processing steps. Keywords: Immunosensor, biomarker, ovarian carcinoma, CA125, direct electrochemistry
