Colorimetric Gold-Nanoparticle Spoilage Sensor (2D)

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). Every collision between two free particles in this 2D field is tested against that per-collision sticking probability; once they stick, they bond permanently and diffuse together as one fractal cluster.

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. The strip chart under the particle field plots that peak wavelength live, so you can watch higher amine concentration visibly speed up the color shift.

Biogenic amine slider — simulated spoilage-marker concentration; raises the per-collision sticking probability, which is why higher settings shift color faster, not just further.
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 and clears the color-vs-time trace.

Free AuNP monomer Bonded aggregate

SPR peak λ vs. time