Gold nanoparticles have been extensively studied as probes in biological applications due to their unique optical properties from the localized surface plasmon resonance in UV-vis region and biocompatibility and flexibility in chemical modification. One emerging area of interest is imparting magnetic property to the probe gold nanoparticle. The advantage of such nanoprobes is that the sensitivity of the biological assays can be greatly increased by preconcentrating the target analytes using the magnetic properties. This presentation describes our approach to creating magnetic nanocomposite with golden surfaces with controlled sizes and high monodispersity towards the potential application in biochip based on immunoassays. The key element of our approaches exploits the fine-tunable interfacial reactivity of highly-monodispersed gold nanocomposites. Examples of applications of the magnetic gold composites as biological nanoprobes will be discussed.