A label-free immunosensor for the detection of HbA1c was developed based on gold nanoparticle (AuNP)-aryl diazonium salt modified glassy carbon (GC) electrode where transduction is achieved using electrochemical impedance spectroscopy (EIS). GC electrodes were first modified with 4-aminophenyl (Ph-NH 2) layers to which AuNPs were attached. Thereafter an oligo(ethylene glycol) (OEG-COOH) species were covalently attached to the remaining free amine groups on the Ph-NH 2 surface. The AuNP surfaces were further modified with Ph-NH 2 followed by attachment of a glycosylated pentapeptide (GPP), an analogon to HbA1c. Exposure of this interface to anti-HbA1c IgG resulted in a change in charge transfer resistance (R ct) due to the anti-HbA1c IgG selectively complexing to the surface bound GPP. To detect the amount of HbA1c, a competitive inhibition assay was employed where the surface bound GPP and HbA1c in solution compete for the anti-HbA1c IgG antibodies. The higher the concentration of HbA1c, the less antibody binds to the sensing interface and the lower the change of R ct. The response of the immunosensor is linear with the HbA1c% of total haemoglobin in the range of 0%-23.3%. This competitive inhibition assay can be used for the detection of HbA1c in human blood. The performance of the immunosensor for detection of HbA1c in human blood is comparable to the clinical laboratory method.