This thesis aims to synthesise gold-coated magnetic nanoparticles (Au@MNPs). These nanoparticles provide unique properties such as magnetism, localized surface plasmon resonance (LSPR), conductivity, and ease of modification. These properties make Au@MNPs ideal for optical and electrochemical bio-sensing. The main challenges synthetic chemists faced when synthesising these nanoparticles are partial coating, aggregation, low yield, and reduction of saturation magnetisation after coating MNPs with gold. Our strategy to address these issues was to develop two unique types of Au@MNPs and develop a novel method for gold coating of the MNPs. Firstly, the properties of the state-of-the-art commercially available Au@MNPs were investigated. This was performed to increase our understanding of the challenges facing the scientific community regarding these nanoparticles. Then, a novel particle ‘’gold-coated conglomerates of superparamagnetic nanoparticles’’ was synthesized. To make practical biosensors, having MNPs that respond to magnets quickly and are stable against aggregation is crucial. Although using big MNPs as cores guarantees fast magnetic response, they are irreversibly magnetized and easily aggregate once placed next to a magnet. Superparamagnetic nanoparticles are stable against magnetic aggregation, but their saturation magnetization is small. To make an Au@MNPs with a fast magnetic response and good stability against aggregation, a number of superparamagnetic nanoparticles were encapsulated inside each silica shell and then coated with gold. The nanoparticles were characterized and their effectiveness for bio-sensing was demonstrated. Next, gold-coated zero-valent iron core-iron oxide shell nanoparticles were synthesized. Iron has the highest magnetization saturation amongst magnetic elements. The challenge was to avoid the rapid oxidation of iron when exposed to air or water. Thus, a synthetic method was developed to keep parts of the zero-valent iron-core intact while coating with gold. The final gold-coated zero-valent iron core-iron oxide shell showed superparamagnetic behaviour and high magnetization saturation. Finally, a new route for the large scale synthesis of Au@MNPs was developed. The method was designed based on fluidic systems. The gold nanoshell manufacturing systems were consisted of a two and three syringe set-up, delivering streams of reagents to a Y-shaped piece. This system was designed to be fast, cost-effective, and easy to handle.