Colorimetric Gold-Nanoparticle Spoilage Sensor

Sample Conditions

Biogenic amine (histamine) 0 ppm

Ionic strength (NaCl) 10 mM

AuNP diameter 15 nm

Live Readout

Bulk solution color
#c23b3b

SPR peak λ

520 nm

Redshift Δλ

0 nm

Weight-avg cluster n_w

1.0

Bonded fraction

FRESH — peak unshifted

How it works

Citrate-capped gold nanoparticles (AuNPs) carry a negative surface charge that keeps them electrostatically separated in solution, so their localized surface plasmon resonance (LSPR) sits near λ₀ ≈ 520 nm for a ~15 nm particle — the classic wine-red colloid. Two things collapse that repulsion barrier and let particles stick together:

Debye screening length:  κ⁻¹ ∝ 1 / √I
DLVO stability:          faster aggregation as κ⁻¹ shrinks
Sticking probability:    p ≈ clamp( a·[amine] + b·I , 0, 1 )

Biogenic amines released by spoiling protein (histamine, putrescine, cadaverine) both raise ionic strength and directly cross-link adjacent AuNPs, while dissolved salt independently screens the double layer (DLVO theory). Once two particles touch, they bond permanently, building fractal clusters.

Plasmon coupling (redshift with aggregate size):
λ_peak = λ₀ + Δλ_max · (1 − e^(−β·(n_w − 1)))
n_w = Σ nᵢ² / Σ nᵢ   (weight-average aggregation number, as in DLS)

Neighboring nanoparticle dipoles couple across the shrinking interparticle gap, red-shifting and broadening the plasmon band — the same "plasmon ruler" effect used in real LSPR nanosensors. Macroscopically the sol visibly shifts from red toward purple/blue, which is exactly the readout used in point-of-care colorimetric assays for fish and meat freshness.

Biogenic amine slider — simulated spoilage-marker concentration; raises the per-collision sticking probability.
Ionic strength slider — background salt; independently screens the double layer per DLVO theory.
AuNP diameter slider — sets the baseline monomer plasmon wavelength λ₀ and, via the Stokes–Einstein relation D = k_BT/(6πηr), the particles' diffusion speed.
Reset Sample — redisperses the colloid back to isolated monomers.

Free AuNP monomer Bonded aggregate

⚠ Couldn't load the 3D engineThree.js failed to load from the CDN. Check your connection and reload.