Colorimetric Gold-Nanoparticle Spoilage Sensor

This simulator models the physics behind a real point-of-care food-safety test: a colloid of citrate-capped gold nanoparticles that visibly changes color when it meets the biogenic amines released by spoiling fish or meat. Each nanoparticle diffuses by Brownian motion in a 3D cell; as biogenic-amine concentration and background ionic strength rise, the electrostatic barrier that normally keeps particles apart collapses (DLVO theory), collisions start sticking permanently, and growing aggregates couple their plasmon resonances to redshift the sensor's absorption peak from the red end of the spectrum toward blue. Three real controls — amine concentration, ionic strength, and nanoparticle diameter — drive live readouts of the estimated SPR peak wavelength, its redshift, the weight-average aggregation number, and a rendered solution-color swatch with a fresh/spoiled verdict.