A Continuous Stirred Tank Reactor for the Systematic Control of Gold Nanoparticle Size

As nanoparticle research progresses a challenge still remains in gaining systematic control over size and reproducibility.  A new approach, termed the “extended LaMer mechanism” has been demonstrated on the magnetite (Fe₃O₄) nanoparticle system. The extended LaMer mechanism involves creating steady state growth of nanoparticles achieved by constantly adding precursor to the reaction solution.  With constant addition of precursor a nucleation event occurs leading to an initial drop in precursor concentration. However, instead of a constant decrease in concentration, the precursor rate of consumption and addition eventually become equal leading to the steady state condition.  By achieving this steady state growth, size control not only becomes reproducible, but also predictable. Here the extended LaMer mechanism is expanded to a gold nanoparticle system using the classic citrate reduction of of tetrachloroauric acid (HAuCl₄).  Aqueous solutions of HAuCl₄ and trisodium citrate are injected into a heated reaction flask and monitored using visible spectroscopy through a fiber optic probe to provide real-time spectra of the growing nanoparticles.  Growth is monitored by the shift in the absorption corresponding to the surface plasmon resonance of the gold nanoparticles.   The plasmon extinction coefficient increases approximately proportionally to the cube of particle radius.  To keep absorption values within a convenient range throughout the reaction, a continuous stirred tank reactor is implemented ensuring continuous dilution of nanoparticles.  In addition, particles are characterized ex situ using TEM and SAXS to confirm size and size dispersity.  This approach allows convenient control of gold nanoparticle size over orders of magnitude of size, through a convenient and scalable approach. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.