This research work reports the plasmonic-based biosensing using attenuated total reflection (ATR). A photon beam falls on the metallic film through an angle higher than critical angle of glass. SPPs are produced along the metal/analyte interface. An optimum gold (Au) film has been used to sense the refractive index of analyte lying next to it while utilizing the sensitivity power of electric field of the surface plasmon polaritons (SPPs). Such Nanoplasmonic devices have been designed and optimized utilizing finite element analysis (FEA) in licensed version of COMSOL 5.4, RF module. Reflection spectra have been extracted for far-field analysis, whereas shift in the position of SPP dip has been exploited to compute sensitivity of the device. By controlling the film thickness and angle of light incidence, the sensitivity of the device is enhanced. It is worth mentioning that the spectral range was brought in visible region where most of the practical devices work very well. The sensitivity of 18200 nm/RIU has been achieved for optimum angle and gold film thickness. Hence such devices have various sensing applications in medical diagnostics.