Synthesising gold nanoparticles with chloroauric acid

Gold nanoparticles form when gold(III) in solution is reduced to gold(0) faster than the atoms can find existing surfaces to grow on. A burst of nucleation consumes most of the gold within seconds, the remaining gold(III) is reduced onto the nuclei that already exist, and a capping agent (citrate, a thiol or a surfactant) stops the particles coalescing. Everything that controls particle size in practice, including the reductant-to-gold ratio, temperature, pH, mixing and the purity of the water, does so by shifting the balance between nucleation and growth.

The gold source is tetrachloroaurate(III), [AuCl₄]⁻, supplied as chloroauric acid (HAuCl₄·3H₂O), sodium tetrachloroaurate or potassium tetrachloroaurate. The three salts give the same ion; they differ in hygroscopicity and in the pH of the resulting solution, which matters for citrate syntheses because citrate is both reductant and buffer. Published methods almost always use the acid, and the quantities below assume it.

• Clean all glassware in aqua regia (3:1 concentrated HCl:HNO₃, in a fume hood), or at least in laboratory detergent followed by dilute nitric acid, then rinse at least five times with ultrapure (18 MΩ·cm) water. Traces of gold from earlier batches seed uncontrolled growth; traces of chloride or surfactant cause aggregation.
• Use a PTFE-coated stirrer bar reserved for gold work. Never use a metal spatula with gold(III) solutions.
• Make a gold stock by dissolving a whole pack rather than weighing the deliquescent solid: 1.00 g of chloroauric acid trihydrate (393.83 g/mol) made up to 100.0 mL gives 25.4 mM. Store it in the dark in glass; it keeps for months. Correct the concentration using the gold assay on the certificate of analysis if exact molarity matters.
• Prepare reductant solutions fresh on the day. Sodium borohydride solutions lose activity within an hour; trisodium citrate solutions keep for a week in the fridge.

1. Place 100 mL of 1.0 mM HAuCl₄ (3.94 mL of the 25.4 mM stock made up to 100 mL) in a 250 mL round-bottomed flask fitted with a reflux condenser and bring it to a rolling boil with vigorous stirring.
2. Add 10 mL of 38.8 mM trisodium citrate (1.14 g of the dihydrate per 100 mL) in one portion while still boiling and stirring hard.
3. Watch the colour: pale yellow to colourless within seconds, then grey-blue, then purple, and finally a clear wine red after one to two minutes. Keep boiling for a further 10–15 minutes, then cool to room temperature with stirring.

The product is citrate-capped particles of about 13–15 nm with a surface plasmon resonance peak near 520 nm and a particle concentration of roughly 10 nM. The citrate-to-gold ratio sets the size: Frens showed that reducing the citrate (0.25 to 1 mL of 1% citrate per 50 mL of 0.01% gold) gives progressively larger particles, up to 100 nm or more, at the cost of broader size distributions. For particles above 40 nm, seeded growth gives better monodispersity than a single Frens step.

The pH of the gold solution influences the result because the hydrolysis products of [AuCl₄]⁻ at higher pH react more slowly. Adding the gold to boiling citrate (the inverse Turkevich method) and adjusting the pH to about 6.5 gives narrower distributions; it is worth trying if the standard method gives you tailing in the size histogram.

The two-phase Brust–Schiffrin synthesis transfers gold(III) into toluene with tetraoctylammonium bromide, then reduces it with borohydride in the presence of an alkanethiol. The thiol monolayer makes the particles isolable as a dry powder that redissolves in organic solvents.

1. Dissolve 0.30 mmol of HAuCl₄ (118 mg of the trihydrate) in 10 mL of water. Dissolve 0.70 mmol of tetraoctylammonium bromide (383 mg) in 27 mL of toluene. Combine and stir vigorously until all the gold has transferred to the organic phase (the water goes colourless, the toluene orange-brown).
2. Add 1-dodecanethiol: 0.30 mmol (72 µL) for particles of about 2 nm, less thiol for larger particles. The organic phase may lighten as gold(I)–thiolate polymer forms.
3. Add 25 mL of a freshly prepared, ice-cold 0.1 M sodium borohydride solution (2.5 mmol) dropwise over about two minutes with very fast stirring. The organic phase darkens to deep brown immediately. Stir for three hours.
4. Separate the toluene layer, evaporate it to about 5 mL, add 200 mL of ethanol and leave at −18 °C overnight to precipitate the particles. Filter, wash with ethanol and acetone to remove excess thiol and phase-transfer salt, and dry.

Gold-to-thiol ratio is the main size control: 1:1 gives about 2 nm, 6:1 gives about 5 nm. Fast reductant addition and vigorous stirring narrow the distribution.

Nanorods are grown from tiny seeds in a surfactant growth solution containing silver nitrate, which directs anisotropic growth. The standard protocol (Nikoobakht and El-Sayed) uses cetyltrimethylammonium bromide (CTAB), which must be kept above its Krafft temperature: hold everything at 27–30 °C.

1. Seeds: mix 5 mL of 0.20 M CTAB with 5 mL of 0.50 mM HAuCl₄, then add 0.60 mL of ice-cold 10 mM NaBH₄ with vigorous stirring for two minutes. The solution turns brownish yellow. Use within two hours.
2. Growth solution: to 5 mL of 0.20 M CTAB add 0.20 mL of 4.0 mM silver nitrate and 5 mL of 1.0 mM HAuCl₄, then 70 µL of 78.8 mM ascorbic acid; the solution turns from orange to colourless as gold(III) is reduced to gold(I).
3. Add 12 µL of seed solution, mix gently and leave undisturbed at 27–30 °C for two to three hours. The colour develops over 10–20 minutes, from pale to blue-green, purple or brown depending on aspect ratio.

The silver nitrate volume (50–250 µL) is the main handle on aspect ratio: more silver, longer rods, with the longitudinal plasmon moving from about 650 nm to beyond 850 nm. Purify by centrifuging twice (8,000 g, 15 minutes) and redispersing in water to remove excess CTAB, which is cytotoxic.

• UV-visible spectroscopy is the first check. Citrate spheres of 13 nm absorb at 518–520 nm; the peak shifts red by roughly 1 nm per nanometre of diameter above 20 nm, and a shoulder or second peak above 600 nm means aggregation. Particle size and concentration can be estimated from the ratio of absorbance at the peak to absorbance at 450 nm (Haiss et al., Anal. Chem. 2007).
• Dynamic light scattering gives the hydrodynamic diameter, which is larger than the core; it is sensitive to the small fraction of aggregates.
• Transmission electron microscopy is the reference method for size and shape; count at least 200 particles.
• Zeta potential around −30 mV or more negative indicates a stable citrate sol.

• Purple or blue instead of red: aggregation, usually from dirty glassware, chloride or other electrolyte in the water, too little citrate, or stirring that stopped during the colour change.
• No colour change: the gold stock has been reduced by light or organic contamination, the citrate solution is old, or the solution was not actually boiling when the citrate went in.
• Black precipitate: reduction too fast or too concentrated. Check that the gold concentration is 1 mM, not 10 mM.
• Broad size distribution: slow or uneven mixing of the reductant. Add it in one shot into a vortex; use a larger stirrer bar.
• Batch-to-batch variation: almost always the gold stock. Use the same stock for a series, make it from a whole pack, and record the lot number and certificate assay.