Undergraduate · 30 min

Why gold sols are red: the plasmon band

Compare UV–Vis absorbance curves of gold nanoparticles from 5 nm to aggregated 100 nm and connect λmax to particle size — the basis of colourimetric nano-assays.

Goal

Estimate λmax for each size; explain the red sol colour from complementary absorption and predict the spectrum of an aggregated sample.

Apparatus and reagents

Virtual bench: citrate-capped gold sols of four nominal diameters and one deliberately aggregated sample; the sim plots absorbance from 400–800 nm.

Procedure

Select the 5 nm dataset and read λmax (~519 nm): the sol absorbs green light, so transmitted light looks red.
Step through 20 nm and 50 nm: measure how λmax red-shifts and the band widens.
Open the aggregated 100 nm sample: besides the main band, find the shoulder near 700 nm and explain it via coupled plasmons of touching particles.
Design the assay: if a target ion crosslinks the sol, which spectral feature would you monitor — λmax shift or the 700 nm shoulder?

What to observe

Bigger particles absorb longer wavelengths and scatter more: the band drifts from ~519 to ~560 nm and broadens.
Aggregation produces a second, weaker feature at long wavelength rather than simply shifting the main peak.
All curves nearly vanish below ~450 nm where interband transitions of gold dominate.

Explanation

In a metal nanoparticle smaller than the wavelength of light, the conduction electrons oscillate collectively — a localized surface plasmon resonance (LSPR). For spheres its frequency is set by the dielectric function of gold and the surrounding medium, and Mie theory predicts the red-shift and broadening with size that you measured. Because colour reports size and inter-particle distance, gold sols power lateral-flow tests (pregnancy tests, COVID antigen strips) where aggregation flips a line from absent to visible.

History of the experiment

Michael Faraday made ruby gold sols in 1857 and guessed the colour came from “the divided state” of the metal. Gustav Mie’s electromagnetic solution (1908) later explained exactly how particle size sets the plasmon band.

Chemists behind it

Michael Faraday