Nanoalloys and nanocomposites are widely studied classes of nanomaterials within the context of biological systems. These are of immense interest because of the possibility of tuning the optical, magnetic, electronic and chemical properties through particle composition and internal architecture. In principle these properties can therefore be tuned for application in biological detection, designed for example for detection of DNA sequences, bacteria, viruses, antibodies, antigens, and cancer cells. This article presents an overview of methods currently used for nanoalloy and nanocomposite synthesis and characterisation, focusing on Au-Ag and FexOy@Au structures as primary components in detection platforms for plasmonic and magnetically enabled plasmonic bio-sensing.