Surface plasmon resonance sensor for milk adulteration detection using a CaF2 prism, Ag film, and black phosphorus layer

We propose a novel surface plasmon resonance (SPR) sensor comprising a calcium fluoride (CaF₂) prism, silver (Ag) film, and black phosphorus (BP) layer for assessing milk quality and detecting adulteration. The sensor operates by measuring refractive index (RI) changes in adulterated milk samples placed on the BP layer. The CaF₂ prism, configured in the Kretschmann setup, facilitates the generation of surface plasmons in the Ag film. The resonance characteristics are calculated using the transfer matrix method (TMM). Refractive indices for CaF₂ and Ag are derived using the Sellmeier equation and Lorentz-Drude model, respectively, at a wavelength of 662 nm. Our proposed design, using the CaF₂ prism, achieves a sensitivity of 574°/RIU for RI variations between 1.331 and 1.336 ($\Delta n_s=0.005$), representing a significant improvement of 98.62% compared to the 289°/RIU sensitivity obtained with a BK7 prism under similar conditions. This enhancement demonstrates the superior performance of CaF₂ over BK7 in SPR-based sensing. To optimize performance, the thicknesses of the Ag and BP layers are systematically adjusted. The results are benchmarked against recent metal-membrane-based SPR sensors, demonstrating superior performance. This innovative design holds significant potential for precise milk quality assessment.