Colloidal gold is made by dissolving a gold salt in water and then adding a reducing agent that converts the dissolved gold ions back into tiny metallic gold particles, typically 15 to 50 nanometers in diameter. The most widely used method, called the Turkevich method, requires only two main chemicals: a gold salt solution and sodium citrate. The citrate serves double duty, both triggering the formation of gold particles and coating them to prevent clumping. The result is a vivid wine-red liquid that stays suspended indefinitely when prepared correctly.
What You Need Before Starting
The core ingredients are chloroauric acid (the gold salt, often sold as gold(III) chloride) and trisodium citrate. A typical preparation uses 50 mL of a 0.25 mM chloroauric acid solution and a 1% by weight trisodium citrate solution. Both are dissolved in ultrapure deionized water. Even trace contaminants can ruin the synthesis, so water quality matters enormously.
You also need a hotplate with a magnetic stirrer, a stir bar, an Erlenmeyer flask or round-bottom flask, and a way to measure and rapidly inject the citrate solution (a syringe or graduated pipette works). A thermometer is helpful but not essential since you’re aiming for a rolling boil at 100°C.
Cleaning Glassware Properly
Residual metal particles on glass surfaces will interfere with the reaction and produce inconsistent results. The standard cleaning protocol uses aqua regia, a mixture of three parts hydrochloric acid to one part nitric acid by volume. This solution dissolves any metallic residue on the glass. Aqua regia is extremely corrosive and must be prepared fresh each time, as it loses effectiveness quickly. Work in a fume hood with chemical-resistant gloves and safety goggles. After the aqua regia wash, rinse all glassware thoroughly with deionized water.
The Turkevich Method Step by Step
Start by heating 50 mL of the chloroauric acid solution in a clean flask on a hotplate with vigorous magnetic stirring. Bring the solution to a full boil. While waiting, prepare the citrate solution separately.
Once the gold solution is boiling, rapidly inject the citrate solution into the flask. The molar ratio of citrate to gold salt is the single most important variable controlling particle size. A ratio of about 2.8 produces particles around 15 nm in diameter, while lowering the ratio to 1.5 yields particles closer to 50 nm. For a general-purpose colloidal gold in the 15 to 30 nm range, a ratio between 2.0 and 2.8 works well.
After injection, keep the solution boiling with constant stirring. Within 2 to 5 minutes, depending on your citrate-to-gold ratio, the solution will change color. It shifts from pale yellow to colorless, then to dark blue or purple, and finally settles into a deep wine-red. That red color, caused by the way nanoscale gold particles interact with light, is your signal that the reaction is working. Once the color stabilizes, let the solution cool naturally to room temperature. Don’t quench it in ice or cold water.
How Particle Size Affects Color
The striking colors of colloidal gold come from a phenomenon where gold nanoparticles absorb and scatter specific wavelengths of visible light. The exact color depends on particle size. Particles around 16 nm produce a solution with an absorption peak near 520 nm and appear bright red. At 25 nm, the peak shifts slightly to 524 nm. At 40 nm, it moves to 528 nm. All of these appear wine-red when the particles are evenly dispersed.
Color shifts toward purple or blue indicate that particles are aggregating, meaning they’re clumping together. Smaller particles (16 nm) shift from red to blue when they aggregate, while larger particles (40 nm) shift from red to purple. If your freshly made solution looks purple or blue rather than red, something went wrong: the citrate ratio was off, the glassware was contaminated, or the injection wasn’t fast enough.
Why Sodium Citrate Works Best for Beginners
Sodium citrate is a mild reducing agent, meaning it converts gold ions to metallic gold relatively slowly. This slower pace allows particles to grow in a controlled way, producing well-defined spheres in the 15 to 50 nm range with a narrow size distribution. The citrate molecules also coat the surface of each particle, giving it a negative electrical charge that causes particles to repel each other and stay suspended.
Stronger reducing agents like sodium borohydride work at room temperature without any heating, which sounds convenient. But the reaction happens so fast that it generates a huge number of tiny particle seeds simultaneously, resulting in particles too small to display the characteristic red color. Sodium borohydride synthesis tends to produce inconsistent results and requires more advanced techniques to control particle size. For a straightforward synthesis yielding stable, well-sized colloidal gold, citrate reduction is the standard approach.
Keeping Your Colloidal Gold Stable
A well-made citrate-capped colloidal gold solution can remain stable for months. The key metric scientists use to gauge stability is the electrical charge on the particle surface, measured as zeta potential. Values in the range of negative 30 to negative 40 millivolts generally indicate particles that resist clumping, though this isn’t an absolute guarantee.
For long-term storage, keep the solution at room temperature in a clean glass container. Some protocols add a tiny amount of preservative (0.05% sodium azide) to prevent microbial growth. Avoid freezing, which can destabilize the particles. Store the solution away from direct light, and don’t let it sit in metal containers or come into contact with salts or other ionic solutions, which can neutralize the surface charge and trigger aggregation.
If you need particles that survive harsher conditions, you can replace or supplement the citrate coating with polymeric stabilizers. Polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), and chitosan are all commonly used. These molecules wrap around the gold core and provide a physical barrier against clumping, in addition to the electrical repulsion from the citrate layer. For basic colloidal gold, though, the citrate coating alone is usually sufficient.
Safety Considerations
Chloroauric acid is corrosive and can cause chemical burns on contact with skin. It may also cause skin sensitization with repeated exposure. Always handle it wearing chemical-resistant gloves and safety goggles, and work in a well-ventilated area or fume hood. If you’re working with powdered gold salt, a respirator rated for fine particles is recommended to avoid inhaling dust.
Aqua regia, used for cleaning, is far more dangerous than the gold salt itself. It produces toxic fumes, reacts violently with many materials, and causes severe burns. Never store it in a sealed container, as it generates gas pressure. Prepare only the amount you need, use it immediately, and neutralize it carefully before disposal.
The finished colloidal gold solution is far less hazardous than its precursors. Gold nanoparticles in the 15 to 50 nm range are widely used in medical diagnostics and research precisely because of their relative biocompatibility. Still, treat the solution as a laboratory chemical and store it away from food, children, and pets.
Troubleshooting Common Problems
If your solution turns purple or gray instead of red, the most likely cause is contaminated glassware. Even trace amounts of other metals can seed unwanted particle growth. Go back and clean everything with fresh aqua regia. A second common cause is adding the citrate too slowly. The injection needs to be rapid so that all the reducing agent enters the boiling gold solution at once, creating uniform conditions for particle formation.
If the solution stays yellowish and never changes color, the citrate may have degraded (it should be freshly prepared) or the temperature wasn’t high enough. Make sure the gold solution is at a full, rolling boil before you add the citrate. If the solution turns red but becomes purple or cloudy within hours, the particles are aggregating, usually because of ionic contamination in the water or the storage container. Switch to higher-purity deionized water and use only glass containers that have been properly cleaned.