What Is Colloidal Gold?
Direct answer: Colloidal gold is a suspension of gold nanoparticles — typically around 40 nm — that serves as the visible reporter in lateral-flow rapid tests. Antibodies are adsorbed onto the gold surface; when the labeled complex accumulates at a capture line, the concentrated particles appear as a red line readable by eye.
What colloidal gold is
Colloidal gold consists of nanometre-scale metallic gold particles suspended in liquid. Their small size and surface chemistry make them an ideal label: they are dense enough to produce an intense color when concentrated, yet stable enough to conjugate to proteins.
Why gold
- Visible — gold nanoparticles appear red at test concentrations, readable without an instrument.
- Stable — gold is chemically inert and stable at room temperature.
- Conjugatable — antibodies adsorb onto the gold surface by electrostatic and hydrophobic interaction.
- High signal density — a small captured amount produces a clearly visible line.
How it produces the result
- Antibodies are conjugated to gold nanoparticles (see colloidal gold conjugation).
- The gold-labeled antibody binds the target analyte in the sample.
- The labeled complex is captured at the test line by immobilized antibodies.
- Accumulated gold forms a visible red line, whose intensity reflects the amount of captured label.
Particle size and performance
Particle size influences signal and flow. Larger particles give stronger color but may flow less freely and bind less efficiently; smaller particles flow better but give weaker signal. The optimal size (~20–40 nm) balances signal intensity, conjugate stability and membrane flow — a formulation choice, not a fixed universal value.
Advantages
- Instrument-free visual detection
- Room-temperature-stable label
- Low-cost, scalable manufacturing
Limitations
- Visual readout is qualitative (faint lines are subjective)
- Conjugation quality and particle aggregation affect reproducibility
Frequently Asked Questions
Why is the line red? Gold nanoparticles appear red when concentrated at the capture line.
What size are colloidal gold particles? Typically around 20–40 nm for lateral-flow tests, chosen to balance signal and flow.
Is colloidal gold the only label? No — colored latex and fluorescent particles are also used; fluorescent labels enable reader-based quantification.
Related knowledge
References
- Posthuma-Trumpie GA, Korf J, van Amerongen A. Lateral flow (immuno)assay: its strengths, weaknesses, opportunities and threats. Anal Bioanal Chem. 2009;393(2):569-582.
- Bahadır EB, Sezgintürk MK. Lateral flow assays: principles, designs and labels. TrAC Trends Anal Chem. 2016;82:286-306.
- 侧向层析标记物研究进展 (Research advance of lateral flow assay labels). Journal of Biomedical Engineering. PMC9935128.
By Progene Bio Regulatory Affairs Team · Medical Device QA · Medical disclaimer · Last reviewed: 2026-09-12