Later Flow Tests and gold colloids

Chemistry

Nanotechnology

Published

July 6, 2025

Lateral Flow Tests

One of the applications of colloidal gold is in lateral flow tests, like pregnancy and COVID tests.

These are the rapid diagnostic tests that give results in 10-15 minutes - the home COVID tests and pregnancy tests you’re familiar with!

┌─────────────────────────────────┐
│    1     │ 3 │ 4 │      5       │
│  Sample  │ T │ C │   Result     │
│   Pad    │   │   │   Window     │
└─────────────────────────────────┘
    2           Test  Control
 Conjugate      Line  Line
   Pad

Those red/pink lines are made of gold nanoparticles.

Usually 20-40 nm gold particles, which appear red. Concentrated gold = visible line

Why Gold?

Intense red color (easy to see)
Stable for years
Easy to attach antibodies/proteins
No power/reader needed

Elements of the Lateral Flow Test

The test consists of several elements:

🦠 = COVID virus
🔴-Y = Gold nanoparticle with anti-COVID antibody
Y' = Different anti-COVID antibody (fixed on test line)
λ = Anti-species antibody (fixed on control line)

Below I explain how it works.

What’s Pre-Deposited in Each Zone?

┌─────────────────────────────────────────────────┐
│   1    │     2        │  3 │ 4 │      5         │
│ Sample │ Conjugate    │  T │ C │   Absorbent    │
│  Pad   │    Pad       │    │   │     Pad        │
│        │   🔴-Y       │ Y' │ λ │                │
└─────────────────────────────────────────────────┘
          Gold-Ab     ←Flow direction←
          stored here

Region	Name	Symbol	Contents	Purpose
1	Sample Pad	-	Nothing pre-deposited	Sample enters here
2	Conjugate Pad	🔴-Y	Gold nanoparticles with anti-COVID antibodies attached	Dried but mobile; ready to flow when liquid added
3	Test Line (T)	Y’	Different anti-COVID antibodies FIXED to membrane	Immobilized; captures virus-gold complexes
4	Control Line (C)	λ	Anti-species antibodies FIXED to membrane	Captures excess gold-antibody conjugates to validate test
5	Absorbent Pad	-	Absorbent material	Pulls liquid through system and stores excess sample

The Movement and Chemistry:

POSITIVE TEST (COVID present):

Step 1: Sample added
COVID virus enters → 🦠🦠🦠

Step 2: In conjugate pad
🦠 + 🔴-Y → 🦠-🔴-Y
Virus binds to gold-antibody

Step 3: Complex flows to Test line
🦠-🔴-Y meets Y' (fixed antibody)
        ↓
    Y'-🦠-🔴-Y
Sandwich forms! Gold trapped at T line

Step 4: Excess gold flows to Control line
Free 🔴-Y meets λ
        ↓
    λ-Y-🔴
Gold antibody recognized and trapped

NEGATIVE TEST (No COVID):

Step 1: Sample added (no virus)

Step 2: In conjugate pad
🔴-Y flows freely (nothing to bind)

Step 3: At Test line
🔴-Y + Y' → No binding! 
Gold-antibody flows past

Step 4: At Control line
🔴-Y + λ → λ-Y-🔴
Gold trapped here only

Why Two Different Antibodies at T?

The "Sandwich Assay":

Gold antibody recognizes "top" of virus
        🔴-Y
         |
        🦠  ← COVID virus
         |
        Y'  ← Test line antibody recognizes "bottom"
    ━━━━━━━━━
    Membrane

This creates specificity - both antibodies must bind for signal!

Timing:

0-2 min: Sample dissolves dried gold conjugates
2-5 min: Complexes flow to test line
5-10 min: Reach control line
15 min: Read result (don’t read after 30 min!)

Result Interpretation

Negative:  │   │ C │  (Only control line)
           │   │━━━│

Positive:  │ T │ C │  (Both lines)
           │━━━│━━━│

Invalid:   │ T │   │  (No control line)
           │━━━│   │  (or no lines)

What You See:

No gold accumulated = No line (colorless)
Gold accumulated = Red line ($10^10$ particles)

Billions of molecular recognition events (antibody-antigen binding) are converted into a simple visual signal through gold nanoparticle accumulation.

--- title: "Later Flow Tests and gold colloids" date: 2025-07-06 categories: ["Chemistry", "Nanotechnology"] draft: false --- ## Lateral Flow Tests One of the applications of [colloidal gold](nano-particle-colloid.qmd) is in lateral flow tests, like pregnancy and COVID tests. These are the rapid diagnostic tests that give results in 10-15 minutes - the home COVID tests and pregnancy tests you're familiar with! ``` ┌─────────────────────────────────┐ │ 1 │ 3 │ 4 │ 5 │ │ Sample │ T │ C │ Result │ │ Pad │ │ │ Window │ └─────────────────────────────────┘ 2 Test Control Conjugate Line Line Pad ``` Those red/pink lines are made of gold nanoparticles. Usually 20-40 nm gold particles, which appear red. Concentrated gold = visible line Why Gold? - Intense red color (easy to see) - Stable for years - Easy to attach antibodies/proteins - No power/reader needed ## Elements of the Lateral Flow Test The test consists of several elements: ``` 🦠 = COVID virus 🔴-Y = Gold nanoparticle with anti-COVID antibody Y' = Different anti-COVID antibody (fixed on test line) λ = Anti-species antibody (fixed on control line) ``` Below I explain how it works. ## What's Pre-Deposited in Each Zone? ``` ┌─────────────────────────────────────────────────┐ │ 1 │ 2 │ 3 │ 4 │ 5 │ │ Sample │ Conjugate │ T │ C │ Absorbent │ │ Pad │ Pad │ │ │ Pad │ │ │ 🔴-Y │ Y' │ λ │ │ └─────────────────────────────────────────────────┘ Gold-Ab ←Flow direction← stored here ``` | Region | Name | Symbol | Contents | Purpose | |--------|------|--------|----------|----------| | 1 | Sample Pad | - | Nothing pre-deposited | Sample enters here | | 2 | Conjugate Pad | 🔴-Y | Gold nanoparticles with anti-COVID antibodies attached | Dried but mobile; ready to flow when liquid added | | 3 | Test Line (T) | Y' | Different anti-COVID antibodies FIXED to membrane | Immobilized; captures virus-gold complexes | | 4 | Control Line (C) | λ | Anti-species antibodies FIXED to membrane | Captures excess gold-antibody conjugates to validate test | | 5 | Absorbent Pad | - | Absorbent material | Pulls liquid through system and stores excess sample | ## The Movement and Chemistry: #### POSITIVE TEST (COVID present): ``` Step 1: Sample added COVID virus enters → 🦠🦠🦠 Step 2: In conjugate pad 🦠 + 🔴-Y → 🦠-🔴-Y Virus binds to gold-antibody Step 3: Complex flows to Test line 🦠-🔴-Y meets Y' (fixed antibody) ↓ Y'-🦠-🔴-Y Sandwich forms! Gold trapped at T line Step 4: Excess gold flows to Control line Free 🔴-Y meets λ ↓ λ-Y-🔴 Gold antibody recognized and trapped ``` #### NEGATIVE TEST (No COVID): ``` Step 1: Sample added (no virus) Step 2: In conjugate pad 🔴-Y flows freely (nothing to bind) Step 3: At Test line 🔴-Y + Y' → No binding! Gold-antibody flows past Step 4: At Control line 🔴-Y + λ → λ-Y-🔴 Gold trapped here only ``` ### Why Two Different Antibodies at T? ``` The "Sandwich Assay": Gold antibody recognizes "top" of virus 🔴-Y | 🦠 ← COVID virus | Y' ← Test line antibody recognizes "bottom" ━━━━━━━━━ Membrane ``` This creates specificity - both antibodies must bind for signal! ### Timing: - 0-2 min: Sample dissolves dried gold conjugates - 2-5 min: Complexes flow to test line - 5-10 min: Reach control line - 15 min: Read result (don't read after 30 min!) ## Result Interpretation ``` Negative: │ │ C │ (Only control line) │ │━━━│ Positive: │ T │ C │ (Both lines) │━━━│━━━│ Invalid: │ T │ │ (No control line) │━━━│ │ (or no lines) ``` ### What You See: No gold accumulated = No line (colorless) Gold accumulated = Red line ($10^10$ particles) Billions of molecular recognition events (antibody-antigen binding) are converted into a simple visual signal through gold nanoparticle accumulation.