Gold nanoparticles (AuNPs) are attractive nanomaterials due to their specific physical and chemical properties. The use of synthetic deoxyribonucleic acid (DNA) to functionalise nanoparticles and bring them into programmed 2 or 3D structures has further enhanced their attractiveness as materials. These novel nanomaterials exhibit tuned properties, depending on their structures. The design process of such structures for specific applications can be facilitated by the study of physicochemical properties. In this project, the aim was to synthesise programmed DNA-gold nanoparticle assemblies and to study and compare their physicochemical properties. Gold nanoparticles of various sizes (5 to 50 nm) were synthesised and functionalised with designed oligonucleotides. They were then self-assembled to form dimer assemblies. A new strategy was developed to functionalise large spherical gold nanoparticles with a specific number of oligonucleotides. Physicochemical properties of both single and dimer assembled gold nanoparticles were studied using a range of methods. Extinction and scattering as well as two photon photoluminescence spectroscopies were used to characterise optical properties of gold nanoparticles. The diffusion properties were studied using microfluidics and two photon excited photoluminescence-fluctuation correlation spectroscopy. Finally, the sedimentation process of gold nanoparticles was followed using digital photography. The results obtained showed differences in the properties between single and assembled gold nanoparticles. Finally, a new way of synthesising DNA-AuNP assemblies was developed using a seeded-growth method. DNA-gold nanoparticle dimers of 5 nm were introduced into the growth process and either large spherical or branched DNA-gold nanoparticle dimers were obtained. After purification, the assemblies were studied using extinction and scattering spectroscopies. This new way of synthesising DNA-gold nanoparticle assemblies has exciting potential for the development of a larger library of gold nanoparticle structures.