High performance label free localized surface plasmon resonance (LSPR) sensors require resonances that are both sensitive to refractive-index changes and sufficiently narrow for reliable spectral tracking. In this work, a diamond-core/silver-shell nanorod is investigated as a compact refractive-index sensor using three-dimensional finite-element simulations. The nanorod length \(L=30\text {--}70~\textrm{nm}\) , core radius \(r=4\text {--}12~\textrm{nm}\) , and silver-shell thickness \(t=4\text {--}12~\textrm{nm}\) are varied for surrounding refractive indices of \(n_\textrm{env}=1.33\text {--}1.45\) , with wavelength-dependent optical constants assigned to both diamond and silver. All three geometry sweeps demonstrate nearly linear red shifts of the longitudinal plasmon resonance with increasing \(n_\textrm{env}\) . Increasing the nanorod length raises the bulk sensitivity from \(175\) to \(400~\mathrm {nm/RIU}\) , whereas the best length-dependent average figure of merit (FOM) occurs at \(L=60~\textrm{nm}\) because further elongation broadens the resonance. Reducing the core radius improves environmental coupling, giving \(400~\mathrm {nm/RIU}\) for \(r=4\text {--}6~\textrm{nm}\) . In the shell-thickness sweep, \(t=4~\textrm{nm}\) provides the best sensitivity–linewidth balance, with \(375~\mathrm {nm/RIU}\) and an average FOM of \(21.47\) . For the representative high-response geometry \(L=70~\textrm{nm}\) , \(r=4~\textrm{nm}\) , and \(t=4~\textrm{nm}\) , the normalized field map at \(n_\textrm{env}=1.33\) and \(\lambda =498~\textrm{nm}\) shows end-cap hotspots with \(|E|/|E_0|\approx 18.4\) . In this work, absorption cross-section is measured in sqaure meter. Within the smooth-interface classical finite-element model used here, long, slender diamond cores with thin silver coatings give the strongest refractive-index response in the investigated parameter range. Experimental realization, however, will require careful control of silver-shell continuity, roughness, oxidation, and size-dependent damping.
