By controlling the presence of twin defects in silver nanoparticle seeds, we can selectively grow them into pentagonal nanowires, nanocubes with controllable corner truncation, or right bipyramids. The key to removal of twinned particles is oxidative etching. Without oxygen in the reaction, five-fold twinned decahedrons preferentially form due to their lower surface energy, and these seeds grow anisotropically into pentagonal nanowires. With the addition of chloride and oxygen, twinned particles are etched away, leaving only single crystal seeds that grow to form nanocubes. If bromide is added in place of chloride, an intermediate level of etching results in formation of seeds with a single twin that grow to form right bipyramids. The symmetry of plasmon resonance within each of these nanostructures is distinct, and thus each has a unique spectral signature that may find application in multiple-analyte surface plasmon resonance sensors. We can further tune the spectral properties of silver nanostructures through a galvanic replacement reaction that converts them into hollow gold nanostructures. Gold nanocages obtained from silver nanocubes have extremely high extinction coefficients in the near-IR (800-1000nm, transparent window for soft tissues). For this reason, we functionalized gold nanocages with tumor antibodies to enable contrast-enhanced imaging of cancer tissue and photodynamic therapy of tumor cells.