Metal nanoparticles have promising potential for use in medical applications, such as imaging or targeted drug delivery. Current methods of production of nanoparticles are expensive and make use of harsh reagents. Because of these limitations, nanoparticles are not yet widely available, limiting the use in medical applications. To commercialize nanoparticle production, it is necessary to develop a nanoparticle synthesis process that is cheap, safe and environmentally friendly. Earlier research has shown that biosynthesis of nanoparticles is possible in many different organisms using certain reducing peptide sequences. Biosynthesis of metallic nanoparticles is cheaper, safer and occurs under physiological conditions, making it an ideal process for the up scaling of nanoparticle production. However, shape and size of the nanoparticle are hard to control and yields are low when biosynthesis is used, limiting the use of biosynthesis. To overcome the problem of shape and size control, it is proposed to use viral capsids as protein cages to restrict the size of synthesized nanoparticles to the inside of the capsid. To implement this, metal ion reducing peptides were fused to the monomers of self-assembling capsids in a way that they were displayed on the inside of the capsid. It was shown that with the adjusted monomers, the capsids still self-assemble. Nanoparticle formation was attempted, but results were inconclusive.