Well-oriented bio-conjugates on gold electrode surfaces will indirectly influence the molecular recognition of antigens to surface bound antibodies thus improving the detection performance of electrochemical immunosensors. This paper describes the modification of self-assembled monolayers (SAMs) on gold electrode surface with 11-mercaptoundecanoic acid (11-MUA). Activation of carboxylic acid terminal was performed by reaction of a mixture of water soluble carbodiimide and N-hydrosuccinimide (NHS) on the electrode surfaces. Characterisation of the SAM formation on the gold electrode was performed using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and contact angle measurements. An amperometric immunosensor was developed for the screening of polycyclic aromatic hydrocarbons (PAHs) in water. The system consists of gold as the working electrode, platinum as the counter electrode and a Ag/AgCl reference electrode. This three electrode system is integrated on a single chip. The measurement employs the enzyme-linked immunosorbent assay (ELISA) principle. Benzo[a]pyrene (BaP) was detected using an immunological reaction by measuring the alkaline phosphatase (AP) enzymatic reaction towards the substrate para-amino phenyl phosphate (pAPP). A competitive assay was performed within the electrode using AP as the labelled-enzyme. A lower limit of detection (5.6 ng ml(-1)) of BaP was achieved after the activation of the mixture of carbodiimide and succinimide with the alkanethiol SAM on the gold electrode in comparison to that obtained for the unmodified electrode (14.2 ng ml(-1)). The developed surface functionalised sensor demonstrated acceptable reproducibility and good stability, with a wide linear response to BaP (4-140 ng ml(-1)).