The optical properties of metal nanoparticles make them useful in the fundamental study of light-matter interactions at the nanoscale. These nanoparticles can behave as antennas for optical electromagnetic radiation and can be used to investigate nanoscopic processes by measuring the optical spectral response of individual nanoantennas. In this thesis, original research is presented that expands our knowledge and understanding of how plasmonic nanoantennas respond to electrochemical potential control. In addition to minor spectral shifts from charge density tuning, nanoparticle plasmons respond very strongly to potential-controlled chemical reactions at the nanoparticle surface. With this knowledge, nanostructures were engineered to produce desired optical characteristics that are switchable via electrochemistry. The optical responses can then be used to either sense electrochemical processes occurring at the nanoscale or to create desired optical phenomena. Two engineered systems were studied to this end. First, single nanospheres and pairs of closely spaced nanospheres called dimers were placed onto two working electrode substrates and the effects of the substrate on sensitivity to potential-controlled electroadsorption of sulfate anions was investigated. The most sensitive nanoantenna geometry was then used to develop and demonstrate a single-nanoantenna analog to bulk electrochemistry’s cyclic voltammogram (CV). Using this technique, the single-nanoantenna CV analog was used to detect the potential controlled adsorption and desorption of sulfate, acetate, and perchlorate anions. A simpler intensity-based alternate method was also tested to increase the accessibility and applicability of the single-nanoantenna technique. In another application, a core/shell strategy was employed in which the shell was electrochemically switchable between a semiconductor and a pure metal. This method was employed for both Au nanospheres and dimers. In the case of a dimer fused by an Ag bridge, the bridge was found to be electrochemically switchable between electrically conductive and nonconductive, allowing a change in the plasmonic coupling mechanism using only electrochemical potential control. The scattering spectra of these dimers experienced large changes during this switching process and dynamic measurements allowed the observation of the first in situ switchable and tunable charge transfer plasmon resonance mode. In sum, the work presented in this thesis demonstrates the value of single-nanoantenna spectroelectrochemistry both for fundamental research and application.