Continuous synthesis of PVP stabilized biocompatible gold nanoparticles with a controlled size using a 3D glass capillary microfluidic device

A reliable glass capillary microfluidic method was developed for a continuous production of well-controlled gold nanoparticles (AuNPs) capped with polyvinylpyrrolidone (PVP) of different molecular weights (PVP K15, PVP K30 and PVP K90). A two-phase co-flow glass capillary microfluidic device with an injection orifice diameter ranging between 100 and 240 µm was used to synthesise 100–240 µm was used to synthesise AuNPs via the chemical reduction between tetrachloroaurate trihydrate (HAuCl4·3H2O) and ascorbic acid. AuNPs with an average diameter between 48 and 135 nm were synthesised, as determined by DLS measurements. Decreasing the injection orifice diameter, increasing the flow rate of ascorbic acid stream and its pH resulted in smaller AuNPs. The polydispersity index (PDI) was found to be independent on the injection orifice diameter or the molecular weight of PVP, but increased with the increase of flow rate and the pH of ascorbic acid stream. The stability study over 6-week period confirmed that PVP K30 with an average Mw of 40000 g/mol was the best capping agent to synthesize and stabilise smaller AuNPs. The reactor fouling due to deposition of AuNPs on reactor walls and orifices was mitigated by hydrophobization of reactor/capillary walls with octadecyltrimethoxisilane and the use of ascorbic acid solution of higher pH.