Harnessing ultrasensitivity from optical structures to detect tiny changes in the targeted samples is the main goal of scientists in the field of sensor design. In this study, an uncommon rhombus-shape plasmonic structure is proposed for providing blue-shift ultrasensitivity. The physical origin of this optical response relies on multi-faces of gold rhombus and their electromagnetic coupling with their induced images in a high-refractive-index substrate (Si3N4). A characteristic of blue-shift emerges as the Fano resonance in the reflection spectrum. We have experimentally shown that this novel structure has the surface sensitivity to the refractive index difference in the order of 10(-5). These characteristics have been applied for non- and conditioned- cell culture medium with refractive differences in this order.This level of sensitivity is interesting for enhanced fingerprinting of minute quantities of targeted molecules and interfacial ion redistribution. (c) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement