Size-Controlled Synthesis of Sub-10-nanometer Citrate-Stabilized Gold Nanoparticles and Related Optical Properties.

Abstract

Highly-monodisperse biocompatible and functionalizable sub-10 nm citrate-stabilized gold nanoparticles (Au NPs) have been synthesized following a kinetically controlled seeded-growth strategy. The use of traces of tannic acid together with an excess of sodium citrate during nucleation is fundamental in the formation of a high number (7•10 13 NP/mL) of small ~3.5 nm seeds with a very narrow distribution. A homogeneous nanometric growth of these seeds is then achieved by adjusting the reaction parameters: pH, temperature, sodium citrate concentration and gold precursor to seeds ratio. We use this method to produce Au NPs with a precise control over their sizes between 3.5 and 10 nm and a versatile surface chemistry allowing studying the size-dependent optical properties in this transition size regime lying between clusters and nanoparticles. Interestingly, an inflexion point is observed for NPs smaller than 8 nm in which the sensitivity of the Localized Surface Plasmon Resonance (LSPR) peak position as a function of NPs size and surface modifications dramatically increased. These studies are relevant in the design of the final selectivity, activity and compatibility of Au NPs, especially in those (bio)applications where size is a critical parameter (e.g. biodistribution, multiplex labeling and protein interaction).

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

“…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.…”

“…As a stabilizing agent, citrate has been shown to be capable of stabilizing gold nanoparticles of ca. 3.5 nm (Piella et al, 2016). Thus, in order to synthesize sub-5 nm gold nanoparticles reduced by citrate, the key point is the enhancement of the nucleation rate, as the final particle size can be determined by the amount of nuclei formed.…”

Modeling the Optical Responses of Noble Metal Nanoparticles Subjected to Physicochemical Transformations in Physiological Environments: Aggregation, Dissolution and Oxidation

“…Interestingly, in this case, the maximum absorbance value constantly increases as NP size increases, which can be explained by the higher amount of metal atoms present in the solutions with larger NPs. These results are more representative of the growth mechanism of NPs by monomer addition used in many synthetic strategies [45][46][47]. Thus, when the growth process takes place via the incorporation of monomer from the solution (the total number of NPs in solution remains unaltered), a constant increasing of maximum extinction should be observed by UV-vis characterization ( Figure 3).…”

Hybrid nanomaterial: biocolloidals

“…On the other hand, this encapsulation is also applicable to NPs that are not biocompatible. SiO 2 layers can be polymerized on the surface of those toxic particles to form a shell to generate biocompatible hybrid NPs (Pokhrel et al, 2014;Pla-Roca et al, 2015;Del Mercato et al, 2016;Filip et al, 2016;Li et al, 2016;Piella et al, 2016). Fabrication techniques, such as hybrids, are actually quite hard to achieve by applying silane chemistry, but many SiO 2 NP biohybrids are still synthesized and employed due to their utility (Polavarapu et al, 2014).…”