AuNP Synthesis

Overview

Gold nanoparticles (AuNPs) for the total amine screen are synthesized by the citrate reduction method — one of the oldest and most reliable nanoparticle synthesis protocols, well within citizen science capability.

Target: ~25nm diameter AuNPs (optimal for histamine/amine colorimetric sensitivity per published literature)

Materials

Item	Source	Notes
Chloroauric acid (HAuCl₄·3H₂O)	Sigma-Aldrich, Strem	~$20-40 for 1g. Handle carefully — irritant, stains skin/surfaces
Trisodium citrate dihydrate	Lab supplier, grocery (as “citric acid salt”)	CAS 6132-04-3
Distilled water	Grocery	Use throughout — tap water will ruin the synthesis
Clean glassware		Thoroughly cleaned — contamination causes aggregation
Hot plate or stove
Amber storage bottle		Light protection

Procedure

Cleaning Glassware (Critical)

1. Wash all glassware with aqua regia or piranha solution if available, otherwise:
2. Wash with concentrated HCl, then rinse thoroughly with distilled water
3. Final rinse with distilled water × 3
4. Do not use soap — surfactant residue causes aggregation

Synthesis

1. Prepare stock solutions:

   • 1% HAuCl₄: dissolve 0.1g in 10ml distilled water
   • 1% sodium citrate: dissolve 0.1g in 10ml distilled water

2. Bring 100ml distilled water to a rolling boil in a clean Erlenmeyer flask

3. Add 1ml of 1% HAuCl₄ solution while stirring (solution turns pale yellow)

4. Immediately add 2ml of 1% sodium citrate solution while stirring vigorously

5. Continue boiling with stirring for 10-15 minutes

   • Solution progresses: pale yellow → colorless → grey-purple → wine red
   • Wine red = successful ~25nm AuNPs

6. Remove from heat, continue stirring until cool (~30 min)

7. Transfer to amber bottle, store at 4°C

Quality Check

• Color: clear wine red (not purple, not grey, not blue)
• SPR peak: 522-524nm (measure with colorimeter vs water blank)
• Stability: no color change after 24h at 4°C

If solution is purple or blue: aggregation has occurred — discard and repeat with cleaner glassware.

Stability

• Shelf life: ~30 days at 4°C
• Check SPR peak before each use — still at 522nm = good
• Red-shifted or broadened peak = partial aggregation — replace batch
• Do not freeze

Safety

• Chloroauric acid: irritant, oxidizer. Wear gloves. Avoid skin contact. Stains permanently.
• Gold nanoparticles in suspension: treat as potentially hazardous nanomaterial. Do not pour down drain — collect for disposal.
• All reagents: use in ventilated area

Yield and Cost

• 100ml batch provides ~100-200 individual tests (0.5ml per test)
• Cost per batch: ~$0.20-0.40 in gold + negligible citrate
• Cost per test: effectively cents once reagents are on hand

Troubleshooting

Problem	Likely Cause	Fix
Solution stays yellow	Too little citrate or not hot enough	Check temperature, remake citrate solution
Solution turns grey/black	Contamination in glassware	Re-clean all glassware thoroughly
Solution turns blue immediately	HAuCl₄ concentration too high	Dilute stock solution
Aggregation on storage	Temperature too high, or light exposure	Store at 4°C in amber bottle

References

• Turkevich et al. (1951) — original citrate reduction method
• ACS Omega (2024) — 25nm AuNP characterization for histamine sensing
• Scientific Reports (2025) — paper-based AuNP sensor validation
• Histamine Testing Methods