Low-ppm-level colorimetric acid detection using gold nanoparticles with electro-steric stabilization

Electro-sterically stabilized gold suspensions were employed in a colorimetric system for the detection of strong acid in water. Using oleyamine and oleic acid as steric stabilizer in 1,2-dichlorobenzene, hydrophobic gold nanoparticles were first synthesized by a reduction reaction of gold salts and were then transferred into water with a cationic surfactant. When the hydrochloric acid solution higher than critical concentration was injected, particles were quickly aggregated and precipitated, creating a clear solution from the colored suspension. The particles were stable against chemical etching by corrosive ion such as chloride. Critical concentration was dependent of the size and concentration of the particles. The minimum concentration of dramatic color change was at 5 ppm level of hydrochloric acid, in which the largest colloidal gold nanoparticles (54 nm) were used. Furthermore, because of their steric repulsive soft layer on particles, particles could be reused for further detection experiments after regeneration by the simple pH-neutralization and washing process.