A silicon nanowire-based immunosensor has been developed to determine the presence of 8-hydroxy-2'-deoxyguanosine (8-OHdG). This research demonstrates a process for the detection of 8-OHdG, which is an oxidative stress biomarker, in which elevated concentrations in urine and saliva can act as an indicator of the presence of acute leukaemia, colorectal, breast, lung and prostate cancers. The detection of elevated levels of this biomarker is critical in determining the presence of various mutagenesis and can be used to diagnose early stage cancer before initial symptoms appear. The surface functionalization process has involved the attachment of diazonium salt to the silicon nanowire surface with further stage which involves the reduction of PhNO₂ to PhNH₂. The antibody targeted against 8-OHdG was subsequently bound to the amine NH₂ modified grafted layer of a 370 nm width nanowire. The immunosensor was evaluated through the use of current/voltage electrical probe monitoring in which a 2 μg/ml concentration of 8-OHdG was detected. A further comparison with phosphate buffered saline solution added to the sensor was observed to be negligible. The antibody-functionalised SiNW sensor has been used to detect binding of the 8-OHdG biomarker to the SiNW surface within sixty-seconds of exposure, making this a rapid cancer biomarker test. [DOI: 10.1380/ejssnt.2014.349]