SPR biosensors based on low-dimensional carbon materials for trace urine glucose

Surface plasmon resonance (SPR) biosensors are attractive for non-invasive screening of urine glucose, but their performance depends sensitively on the choice of low-dimensional carbon overlayer and metal stack. Here we numerically compare graphene, semiconducting single-walled carbon nanotubes SWCNT (s-SWCNT), graphene oxide (GO), or reduced graphene oxide (rGO) integrated in a standardized CaF₂/Cu/Si₃N₄/(low-D)/urine stack. Using a transfer-matrix model at 633 nm, with a realistic analyte refractive index change (Δn ≈ 4.6 × 10 −4 RIU) corresponding to 0.015–0.225 g dL −1 urine glucose, we extract angular sensitivity, linewidth, quality factor, figure of merit, a combined sensitivity factor and theoretical limits of detection. Within an effective-medium description of the carbon films, rGO delivers the highest sensitivity and lowest detection limits but broader resonance dips; GO yields the narrowest dips and the highest precision; graphene and s-SWCNTs offer intermediate responses with stable linewidths and minimal angle drift. We also identify practical thickness windows for each overlayer on Cu that balance sensitivity against spectral sharpness. Compared with prior SPR studies that focus on a single carbon coating or Au-based stacks, this work provides a unified framework and quantitative design rules to select Cu-based low-dimensional overlayers for resolving small, bulk urine-glucose-induced refractive-index changes above the clinical baseline and to guide subsequent experimental implementations.