I have investigated gap plasmon mode of an eccentric coaxial waveguide structure using effective index method. The results found good agreement with fully-vectorial numerical calculation. In the eccentric structure, a strong field localization has been noticed at and around the smallest gap. Analysis showed the increase of effective index of lowest-order waveguide mode to 3.7 in the structure considered with a 2 nm minimum gap for a wavelength of 4 micrometer. In the visible regime, the effective index increases to over 10 for the same structure. Nanohole arrays, both flowover and flow-through formats, have been fabricated using focused ion beam (FIB). A 2-color LED-based nanohole sensor has been presented. The objective of the sensing platform was to register mutually opposite intensity change of transmitted light when the dielectric medium of metal-dielectric interface of the nanohole sensor undergoes a change. A number of tests with microfluidics setup demonstrated the proof-of-concept.