We optimized the resolution of nanohole arrays in metal films for refractive index sensing by increasing the sensitivity with modifications to the hole-array parameters and by reducing the noise of the sensor system. The nanohole array parameters (including film thickness, periodicity and diameter) were first optimized by finite difference time-domain simulations, and then fabricated and tested, showing good agreement between the two cases (theory and experiment) in terms of optimal parameters. To improve the sensitivity and to reduce the noise, the laser source wavelength was optimized (including the efficiency of the camera for detection) and the intensity was increased to reduce shot noise. A bulk resolution of 6×10^-7 RIU was

demonstrated. Due to the collinear microscope geometry and potential for multiplexing of nanohole arrays, these results are encouraging for future biosensing applications.