UV–Vis and gold colloids

Chemistry

Nanotechnology

Published

July 8, 2025

In Nano Particle Synthesis with Machine learning, I read a paper about analysis of nano particles with UV-Vis spectroscopy.

That papers deals with collids, but what is a colloid?

Nano particle colloid

A colloid or colloidal suspension, is a mixture where tiny particles (1 - 1000 nm) are dispersed in another substance, and stay suspended for months or longer.

Gold particles are often suspended in water or another liquid. This often looks like colored transparent liquid.

The color may be red (5-20 nm), purple (50 nm), or blue (100 nm), depending on the size and shape of the particles.

The color comes from surface plasmon resonance (specifically LSPR - localized surface plasmon resonance). See SPR spectra for more details on how this appears in UV-Vis measurements.

Colloid vs Solution vs Suspension:

Property	True Solution	Colloid	Suspension
Particle size	<1 nm (molecules)	1-1000 nm	>1000 nm
Example	Salt water	Gold nanoparticles	Sand in water
Settles?	Never	Very slowly/never	Yes, quickly
Filters through paper?	Yes	Yes	No
Can see particles?	No	No (need electron microscope)	Yes (microscope)
Light scattering	No	Yes (Tyndall effect)	Yes (cloudy)

Gold particles are small enough to stay suspended in a liquid (colloid), but larger than molecules (true solution) and smaller than what would settle out quickly (suspension).

Making a Gold Colloid:

1. HAuCl₄ (gold salt) in water - yellow solution
2. Add reducing agent (citrate)
3. Gold atoms form and cluster
4. Citrate caps particles at ~20 nm
5. Result: Ruby red colloid!

These gold colloids have many applications, including lateral flow tests (like COVID and pregnancy tests).

Aggregated particles

After nono particles have formed, they can aggregate into larger clusters, or aggregates. This changes their optical properties significantly.

Here are stability indicators that tell you where the formation process is at:

Stable: dispersed, clear, consistent color
Aggregating: Color change (red→purple→blue)
Aggregated: Precipitate (visialbe particlates that settle to the bottom) forms

Aggregation changes the surface plasmon resonance (SPR) peak, shifting it to longer wavelengths (red shift). Aggrigation is unwanted. It ruins sensor and drug delivery applications.

Individual particle:     Aggregated particles:
    •                       •••
SPR at 520 nm              •••  SPR shifts to 600-700 nm
(red)                      •••  (blue/purple)

                           Plus new coupling modes

Colloid concentrations

Typically 0.1 - 1 mg/mL of gold nano particles are used in gold colloids.

This gives ~10¹¹-10¹³ particles/mL, depending on particle size.

Particle Size	Mass per Particle	Particles/mL at 0.1 mg/mL	Particles/mL at 1 mg/mL
5 nm	1.3 × 10⁻¹⁵ mg	7.9 × 10¹³	7.9 × 10¹⁴
10 nm	1.0 × 10⁻¹⁴ mg	9.9 × 10¹²	9.9 × 10¹³
20 nm	8.1 × 10⁻¹⁴ mg	1.2 × 10¹²	1.2 × 10¹³
50 nm	1.3 × 10⁻¹² mg	7.9 × 10¹⁰	7.9 × 10¹¹

Separation of colloids

For 10¹² particles/mL (20 nm particles at 0.1 mg/mL):

Volume per particle = 1 mL / 10¹² = 10⁻¹² mL = 10⁻⁹ cm³
Cube root gives average spacing: ∛(10⁻⁹) = 10⁻³ cm = 10 μm
This is 500× the particle diameter - plenty of separation!

This confirms that hat particle-particle interactions are negligible in typical UV/Vis measurements.

--- title: "UV–Vis and gold colloids" date: 2025-07-08 categories: ["Chemistry", "Nanotechnology"] draft: false --- In [Nano Particle Synthesis with Machine learning](index.qmd), I read a paper about analysis of nano particles with UV-Vis spectroscopy. That papers deals with collids, but what is a colloid? ## Nano particle colloid A colloid or colloidal suspension, is a mixture where tiny particles (1 - 1000 nm) are dispersed in another substance, and stay suspended for months or longer. Gold particles are often suspended in water or another liquid. This often looks like colored transparent liquid. The color may be red (5-20 nm), purple (50 nm), or blue (100 nm), depending on the size and shape of the particles. The color comes from surface plasmon resonance (specifically [LSPR](../2025-07-07/lspr.qmd) - localized surface plasmon resonance). See [SPR spectra](spr-spectra.qmd) for more details on how this appears in UV-Vis measurements. ### Colloid vs Solution vs Suspension: | Property | True Solution | Colloid | Suspension | |----------|--------------|---------|------------| | Particle size | <1 nm (molecules) | 1-1000 nm | >1000 nm | | Example | Salt water | Gold nanoparticles | Sand in water | | Settles? | Never | Very slowly/never | Yes, quickly | | Filters through paper? | Yes | Yes | No | | Can see particles? | No | No (need electron microscope) | Yes (microscope) | | Light scattering | No | Yes (Tyndall effect) | Yes (cloudy) | Gold particles are small enough to stay suspended in a liquid (colloid), but larger than molecules (true solution) and smaller than what would settle out quickly (suspension). ## Making a Gold Colloid: ``` 1. HAuCl₄ (gold salt) in water - yellow solution 2. Add reducing agent (citrate) 3. Gold atoms form and cluster 4. Citrate caps particles at ~20 nm 5. Result: Ruby red colloid! ``` These gold colloids have many applications, including [lateral flow tests](lateral-flow-test.qmd) (like COVID and pregnancy tests). ## Aggregated particles After nono particles have formed, they can aggregate into larger clusters, or aggregates. This changes their optical properties significantly. Here are stability indicators that tell you where the formation process is at: - Stable: dispersed, clear, consistent color - Aggregating: Color change (red→purple→blue) - Aggregated: Precipitate (visialbe particlates that settle to the bottom) forms Aggregation changes the surface plasmon resonance (SPR) peak, shifting it to longer wavelengths (red shift). Aggrigation is unwanted. It ruins sensor and drug delivery applications. ``` Individual particle: Aggregated particles: • ••• SPR at 520 nm ••• SPR shifts to 600-700 nm (red) ••• (blue/purple) Plus new coupling modes ``` ## Colloid concentrations Typically 0.1 - 1 mg/mL of gold nano particles are used in gold colloids. This gives ~10¹¹-10¹³ particles/mL, depending on particle size. | Particle Size | Mass per Particle | Particles/mL at 0.1 mg/mL | Particles/mL at 1 mg/mL | |--------------|-------------------|---------------------------|-------------------------| | 5 nm | 1.3 × 10⁻¹⁵ mg | 7.9 × 10¹³ | 7.9 × 10¹⁴ | | 10 nm | 1.0 × 10⁻¹⁴ mg | 9.9 × 10¹² | 9.9 × 10¹³ | | 20 nm | 8.1 × 10⁻¹⁴ mg | 1.2 × 10¹² | 1.2 × 10¹³ | | 50 nm | 1.3 × 10⁻¹² mg | 7.9 × 10¹⁰ | 7.9 × 10¹¹ | ## Separation of colloids For 10¹² particles/mL (20 nm particles at 0.1 mg/mL): - Volume per particle = 1 mL / 10¹² = 10⁻¹² mL = 10⁻⁹ cm³ - Cube root gives average spacing: ∛(10⁻⁹) = 10⁻³ cm = **10 μm** - This is **500× the particle diameter** - plenty of separation! This confirms that hat particle-particle interactions are negligible in typical UV/Vis measurements.