Turkevich in New Robes: Key Questions Answered for the Most Common Gold Nanoparticle Synthesis

Abstract

This contribution provides a comprehensive mechanistic picture of the gold nanoparticle synthesis by citrate reduction of HAuCl4, known as Turkevich method, by addressing five key questions. The synthesis leads to monodisperse final particles as a result of a seed-mediated growth mechanism. In the initial phase of the synthesis, seed particles are formed onto which the residual gold is distributed during the course of reaction. It is shown that this mechanism is a fortunate coincidence created by a favorable interplay of several chemical and physicochemical processes which initiate but also terminate the formation of seed particles and prevent the formation of further particles at later stages of reaction. Since no further particles are formed after seed particle formation, the number of seeds defines the final total particle number and therefore the final size. The gained understanding allows illustrating the influence of reaction conditions on the growth process and thus the final size distribution.

Continuous flow synthesis of ultrasmall gold nanoparticles in a microreactor using trisodium citrate and their SERS performance

“…Even though the model agreed with several literature results, it does not yield satisfactory predictions in other cases, because it does not take account of the acid-base properties of the reactants (Agunloye et al, 2017). Wuithschick et al (2015) demonstrated experimentally that the Au(III) species are the critical element to control particle size. In their study, the formation of monodispersed gold nanoparticles in the Turkevich method was investigated.…”

“…As the gold precursor has a higher concentration, which is associated with lower pH and the presence of more [AuCl 4 ] À at the early stages, the final particle size obtained by the inverse Turkevich method could be as low as 7.6 nm without any additional strong reducing agents or ligands. It has been established that fast nucleation is the key in order to achieve monodispersed and small gold nanoparticles (Wuithschick et al, 2015). However, since citrate is a moderately weak reducing agent, the nucleation rate using citrate reduction is not as fast as with some strong reducing agents such as NaBH 4 .…”

“…However, since citrate is a moderately weak reducing agent, the nucleation rate using citrate reduction is not as fast as with some strong reducing agents such as NaBH 4 . That is why the typical minimum nanoparticle diameter is in the range of 7 nm (Wuithschick et al, 2015). However, the low toxicity (Murphy et al, 2008) and the ease with which such citratecapped gold nanoparticles can be functionalised, keeps researchers motivated to improve this method.…”

“…This is also the reason why the current study has focused on a citrate-only system. In order to decrease the overall size and particle size distribution of the citrate-capped gold nanoparticles, several approaches have been implemented, such as using a stronger reducing agent (Brown et al, 1996;Piella et al, 2016;Singh et al, 2009;Slot and Geuze, 1985), tuning citrate concentration (Kimling et al, 2006), reaction temperature (Wuithschick et al, 2015), solution pH , reactant pre-mixing at room temperature, adding Ag + ions (Xia et al, 2009), employing UV irradiation (du Toit et al, 2017 and ultrasound (Su et al, 2003). In batch systems, it seems that only with a stronger reducing agent can the size of citrate-capped gold nanoparticles be made smaller than 5 nm.…”

Light-Induced Polarization-Directed Growth of Optically Printed Gold Nanoparticles

“…For instance, tight control over the well-known Turkevich 8 reaction introduced more than fifty years ago remained elusive until very recently 9,10 . Moreover, in the case of metallic nanoparticles, the presence of light can dramatically change the outcome of a synthesis.…”