Surface Plasmon Resonance
Kretschmann config · evanescent coupling · biosensing
Analyte refractive index n_a
n_a = 1.330
Gold film thickness (nm)
50 nm
Angle of incidence θ (°)
θ = 55.0°
Physics:
Kretschmann geometry (1971): prism (n_p ≈ 1.5) / thin gold film (50 nm) / analyte. TIR generates evanescent wave that excites SPP at gold/analyte interface when k_x = k_SPP. Resonance condition: n_p·sinθ = Re(k_SPP)/k₀ = √(ε_m·ε_a/(ε_m+ε_a)); sharp reflectivity dip. Sensitivity ~10⁻⁶ RIU (refractive index units); detects 1 pg/mm² protein binding. Shift Δθ_res ≈ 100°/RIU × Δn. Commercial Biacore system (1990) measures drug-target kinetics in real-time. Field enhancement ~100× at resonance amplifies weak signals (SERS).