Plasmonic Nanosensor: LSPR Refractive-Index Shift

Nanoparticle

Radius 30 nm

Sensing Medium

Refractive index n (analyte binding) 1.333

Live Readout

λ resonance

— nm

Shift Δλ

0.0 nm

Sensitivity S

— nm/RIU

FWHM

— nm

How it works

A metal nanoparticle much smaller than the wavelength of light supports a localized surface plasmon resonance (LSPR): the incident field drives the free conduction electrons to slosh collectively against the fixed positive lattice, like a mass on a spring. In the quasi-static (Fröhlich) approximation, this dipole resonance occurs where

Re[ε_metal(ω)] = −2 · n_env²

Drude model:  ε_metal(ω) = 1 − ωp² / (ω² + iγω)
   ⇒  ω_res ≈ ωp / √(1 + 2 n_env²)
   ⇒  λ_res = 2πc / ω_res

Because ω_res depends on the squared refractive index of the surrounding medium, anything that changes the local n — a protein, DNA strand, or virus binding to the particle surface — shifts the resonance wavelength. Real LSPR biosensors track exactly this: nanoparticle arrays functionalized with capture antibodies, illuminated with white light, and read out as a peak-position shift Δλ proportional to bound mass. This is a real, deployed label-free biosensing principle (surface plasmon resonance / LSPR assays), distinct from fluorescent or electrochemical sensors.

Metal (Au / Ag) — sets the bulk plasma frequency ωp and damping γ (silver has a sharper, bluer resonance than gold).
Radius — larger particles add a small radiative-damping redshift and broaden the peak (retardation beyond the pure quasi-static limit).
Refractive index n — the sensing variable; dragging it (or clicking "Bind analyte layer") redshifts λ_res exactly as a real binding event would.
Sensitivity S = Δλ/Δn — the figure of merit reported for every real LSPR sensor design, in nm per refractive-index unit (RIU).

Conduction electron cloud Incident light field (E)

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