A theoretical surface plasmon resonance sensor built from a copper film coated with tin disulfide and molybdenum ditelluride has been optimized in simulation for distinguishing the refractive index signatures of brain tumor tissues, with its designers reporting a maximum sensitivity of 142.15 degrees per refractive index unit and a figure of merit of 31.27 per refractive index unit across a sensing range of 1.33 to 1.50. The work, published in Discover Nano, models a layered structure of an SF5 prism, a copper film, a tin disulfide interlayer, and a monolayer of molybdenum ditelluride facing the sensing medium, and evaluates how well the arrangement responds to the experimentally reported refractive indices of cerebrospinal fluid, gray matter, oligodendroglioma, low-grade glioma, lymphoma, and metastatic brain tissue.
The study is explicitly a numerical exercise rather than a laboratory demonstration. The authors used the Fresnel equations and the transfer matrix method, implemented in MATLAB, to compute reflectance curves for the multilayer stack at a wavelength of 633 nanometers. They stress that the sensor is assessed as a bulk refractive index sensing platform rather than a biomarker-specific biosensor: the sensing medium is modeled as a homogeneous layer whose refractive index is taken from published values for various brain tissues. The goal is to gauge the intrinsic capacity of the structure to register minute refractive index changes before any biomolecular interaction model is added, and the authors are careful to note that experimental validation has not yet been carried out and remains a limitation of the investigation.
The physics underlying the design follows the Kretschmann configuration familiar from decades of plasmonic sensing. A p-polarized light beam strikes the high-refractive-index prism at a controlled angle, and at the condition of total internal reflection an evanescent wave penetrates the thin metal layer. When the incident angle matches the resonance condition, the evanescent field couples to the free electrons of the metal, exciting a surface plasmon wave and producing a sharp dip in reflected intensity. Because the resonance angle shifts when the refractive index of the sample adjacent to the metal changes, tracking that shift provides a label-free route to detecting variations in the sample. Copper was selected over the more conventional gold and silver for its lower cost, strong plasmonic response in the visible spectrum, high electrical conductivity, and compatibility with semiconductor manufacturing, though the authors acknowledge that copper’s susceptibility to oxidation is a central practical concern.
That vulnerability is where the two-dimensional materials come in. The tin disulfide layer, placed between the copper and the sensing medium, is described as chemically stable and acts as a barrier that inhibits oxidation of the copper surface while strengthening light-matter interaction and improving electromagnetic field confinement. The outer molybdenum ditelluride monolayer, with its narrow bandgap, high optical absorption coefficient, and large specific surface area, serves as the sensing interface, enhancing the interaction between the evanescent field and the analyte and offering abundant adsorption sites for future bio-functionalization with antibodies, aptamers, or other recognition elements. In the simulated structure, the combined effect of the three layers produced improved plasmonic coupling and a larger resonance angle shift for a given refractive index change than a conventional copper-based configuration.
The optimization sweep varied the copper thickness from 25 to 60 nanometers and the tin disulfide thickness from 0 to 8 nanometers in one-nanometer computational increments, with the molybdenum ditelluride layer held at a single monolayer. Sensitivity rose steadily as the tin disulfide layer thickened: the modeled sensitivity ranged from 88.75 to 96.24 degrees per refractive index unit with no tin disulfide, up to 121.32 to 142.15 degrees per refractive index unit at 8 nanometers, with the peak of 142.15 degrees per refractive index unit occurring at a copper thickness of 33 nanometers before the value fell back to 128.98 degrees per refractive index unit. The authors caution that layers thicker than roughly 5 to 8 nanometers may begin to suppress the evanescent field and reduce plasmon coupling efficiency, and that the one-nanometer interval reflects computational resolution rather than a fabrication requirement, with about 8 nanometers representing a target design value subject to manufacturing tolerances.
Resonance quality proved equally sensitive to geometry. A minimum reflectance as low as 3.9 times ten to the minus seventh was obtained at a copper thickness of 33 nanometers, and the deepest reflectance dips generally occurred at copper thicknesses of roughly 35 to 50 nanometers depending on the sample refractive index. At the optimized 8-nanometer tin disulfide layer, the resonance dip appeared at 62.40 degrees for a sensing medium of refractive index 1.33 and at 86.56 degrees for 1.50, and the resonance angle shift between those two media grew from 16.12 degrees with no tin disulfide to 24.16 degrees with the full 8-nanometer layer. Electric field analysis showed the evanescent field decaying exponentially into the sensing medium after peaking near the metal-sensing layer interface, with penetration depths of 147.1 nanometers at refractive index 1.33 and 138.7 nanometers at 1.50, and peak field intensities of 1.1 times ten to the fifth and 6.7 times ten to the fourth volts per meter respectively, indicating deeper field penetration in lower refractive index environments.
When the sensing medium was switched to the refractive indices reported for actual brain tissues, the model distinguished them through their resonance angles. Cerebrospinal fluid, with a refractive index of 1.3333, produced the lowest and nearly flat sensitivity across all copper thicknesses, while gray matter at 1.3951 and oligodendroglioma at 1.3531 showed lower sensitivity curves. Low-grade glioma at 1.4320 and lymphoma at 1.4591 exhibited moderate sensitivity, and metastatic brain tissue gave the strongest response, with the best overall performance at copper thicknesses of 40 to 50 nanometers. At a copper thickness of 41 nanometers, resonance angles of 62.19, 62.50, 64.43, and 68.79 degrees were computed for the sensing medium of refractive index 1.33, cerebrospinal fluid, oligodendroglioma, and gray matter respectively. For the higher-index lesions, resonance angles of 73.93, 78.46, and 83.25 degrees were obtained for low-grade glioma, lymphoma, and metastasis at copper thicknesses of 40, 39, and 35 nanometers, and sensitivity climbed to 136.47 degrees per refractive index unit at 35 nanometers, with a figure of merit of 15.01 per refractive index unit.
The authors also lay out a plausible fabrication route, while acknowledging it remains untested. The SF5 prism would be cleaned with hydrofluoric acid and ultrasonic baths of acetone, isopropanol, and deionized water, dried with nitrogen, then coated with copper by physical vapor deposition. Tin disulfide and molybdenum ditelluride layers would be grown by chemical vapor deposition, with the molybdenum ditelluride surface serving as the active sensing face where biomolecule-containing samples would be introduced through a microinjection pump. They identify real obstacles: depositing uniform, ultra-thin two-dimensional layers with consistent thickness and crystallinity demands precise control of temperature, precursor flow, and substrate compatibility; adhesion between copper and the 2D materials may be unstable; copper oxidizes in air without protective capping; and humidity, temperature extremes, corrosive environments, mechanical stress, and surface contamination could all degrade performance. Environmental compensation strategies such as temperature-controlled sample chambers, vibration isolation, stable laser sources, and differential measurement against a reference channel are proposed to improve repeatability.
The comparison table in the paper places the proposed sensor above previously published SPR designs, citing the highest sensitivity of 142.15 degrees per refractive index unit and a figure of merit of 31.27 per refractive index unit for long-range refractive index sensing. The authors conclude that the optimized structure of 35 nanometers of copper, 8 nanometers of tin disulfide, and a monolayer of molybdenum ditelluride delivered the best brain-tissue performance, and they argue that low minimum reflectance and sharp resonance dips support potential use in the early diagnosis of neurological disorders. But they are equally explicit about what stands between the simulation and the clinic: experimental fabrication, stability studies, bio-functionalization techniques, and testing in authentic biomedical environments are all still required, and practical clinical diagnosis would typically demand selective biomarker detection through surface functionalization rather than the bulk refractive index contrast modeled here. Until that experimental stage confirms the simulated sensitivity, resonance sharpness, stability, and reproducibility, the copper-tin disulfide-molybdenum ditelluride stack remains a promising design on paper rather than a working diagnostic device.
Subject of Research: Theoretical optimization of a copper and tin disulfide based surface plasmon resonance sensor for detecting brain tumor tissues through refractive index changes
Article Title: Optimization of copper and tin disulfide based surface plasmon resonance for the detection of brain tumor tissues
Article References: Kumar, R., Brahma, B., Garia, L., kumar, M., & Bhoi, A. K. (2026). Optimization of copper and tin disulfide based surface plasmon resonance for the detection of brain tumor tissues. Discover Nano, 21(1), Article 550. https://doi.org/10.1186/s11671-026-04899-7
Image Credits: AI Generated
DOI: 10.1186/s11671-026-04899-7
Keywords: surface plasmon resonance, brain tumor detection, copper plasmonics, tin disulfide, molybdenum ditelluride, refractive index sensing, 2D materials, biosensor simulation, label-free detection, Kretschmann configuration, evanescent field, Discover Nano
News Source: Nathaniel Bowman. (October 30, 2026). Copper and Tin Disulfide SPR Sensor Simulated to Detect Brain Tumor Tissues. Scienmag.
