Microstructured optical fibers (MOFs) show plasmonic properties after deposition of metal nanoparticles on the surface of their capillaries. A method of enhancing the functionality of such fibers by immobilizing different nanoparticles in the different capillaries of an MOF is described. Silver and gold nanoparticles show well-separated localized surface plasmon resonances (LSPRs). Measurements confirm calculations, according to which both resonance wavelengths shift with changes in the refractive index of the nanoparticles’ immediate environment. Such modified MOFs can be used in LSPR sensing; they may also be used in multiplex detection of bio-analytes, in particular.