Plasmonic nanoparticles allow the sensing of single nanoscale objects (e.g., biomolecules) by transducing changes in the local refractive index into spectral plasmon resonance shifts. Due to the rapid spatial decay of the nanoparticle optical near field into the surrounding medium plasmonic particles imply a high localization of the sensor effect, thus opening the possibility of location-specific sensor concepts.In this thesis an experimental three dimensional characterization of the refractive index sensitivity of plasmonic gold particles is performed. A locally resolved refractive index sensitivity profile of nanodisks and nanorods is obtained by controlled apposition of nm-small dielectric dots to selected particle sites in a two-step electron beam lithography process. Measured sensitivity profiles obtained from tracking the dot-induced spectral shift of the plasmon modes are in excellent agreement with numerical simulation. A locally resolved study of the sensitivity decay normal to the particle surface is accomplished by deposition of nm-thin dielectric films to selected particle sites.Based on the particle sensitivity profiles, sensitive and spatially uniform plasmonic sensors are tailored by masking selected nanoparticle sites. By capping nanodisks with a dielectric layer and masking nanorods with polymer stripes, analyte access is restricted to the sensitive disk rim and rod tips, respectively. A surprisingly high effectivity of masked particles is shown as they give nearly the same optical response as bare particles although having only a fraction of accessible surface.The sensitivity of plasmonic particles upon deposition and binding of organic thin-films is studied utilizing thrombin binding aptamers. Comparing the optical response of bare and top masked nanodisks provides valuable insights into molecular binding behaviour and shows how inhomogeneous sensor surfaces can affect binding dynamics.