pH-dependent size distribution of gold nanoparticles investigated by in situ pH-monitoring throughout the synthetic reaction

•With in situ pH-monitoring, the relation between pH and Au nanoparticle size distribution was investigated.•Au nanoparticles were formed through two distinctively different stages: a pH-decreasing early stage and a pH-increasing later stage.•The narrowest size distribution was obtained when the starting pH was about 7.5.•Size distribution could be improved by pH adjustment at the pH turning point.