This thesis focuses on how self-assembled optical nanostructures can play a crucial role in nanochemistry. It starts with an introduction to the fundamental plasmonic properties of metal nano-architectures, followed by various self-assembly strategies I explored to form controlled plasmonic aggregates. I then discuss in detail how these structures can be combined with optics to control and probe chemical and physical processes at the nanoscale. Chain aggregates of metal nanoparticles are used to optically control thermally-activated redox processes. These aggregates are further dressed by core-shell quantum dots through direct self-assembly and the resultant hybrid aggregates are used to probe the chemical kinetics using surface-enhanced Raman spectroscopy during heterogenous photo-catalytical reaction induced by quantum dots. Single quantum dots are further sandwiched in individual plasmonic cavities. Manipulation of excitonic nonlinear excitation and relaxation pathways of quantum dots is realised through the coupling between two-photon excited excitons and a plasmon mode provided by the plasmonic cavity. Single atomic features within these plasmonic nanocavities are shown to further enhance the field-confinement. Such extremely confined light is then utilised to monitor single-molecule surface coordination dynamics in real-time. From bulk solution down to single-molecule, a range of systems are demonstrated to illustrate the powerful interplay between plasmonics and nanochemistry.