A gold nanoparticle's localized surface plasmon resonance (LSPR) sits wherever the metal's free-electron oscillation matches the incoming light — and that wavelength depends on the refractive index of whatever sits right at the particle's surface, within a few nanometres. Coat the particle with antibody receptors, and each antigen that docks onto a receptor locally raises that refractive index by a tiny amount, nudging the resonance to a longer wavelength (a redshift). No dye, no label — the shift itself is the signal.
dθ/dt ≈ k_on·[C]·(1-θ) (θ = fraction of receptor sites bound)
λ_peak(θ) = λ0 + Δλmax·θ
- Target concentration — sets how often antigen molecules reach the surface and attempt to bind; higher concentration fills the receptor sites faster.
- Antibody affinity (kon) — sets how readily a molecule that reaches a receptor actually docks.
- Sensor nanoparticle (left, functionalized) — antibody receptors capture matching antigen; each binding event nudges the resonance red.
- Control nanoparticle (right, bare surface) — antigens bounce off an unfunctionalized surface; no binding occurs, so its resonance never shifts, even at the same solution concentration.
Real-world relevance: this label-free readout — track the peak wavelength, not a fluorescent tag — is how LSPR biosensor chips detect antibody-antigen binding, DNA hybridization and other molecular recognition events in real time.