Study of color change in seed-mediated growth of gold nanoparticles and insights into Turkevich method

Abstract

Gold nanomaterials have received great attention within the scientific community mostly due to their unique optical properties. Many forms of gold nanomaterials have been developed for various applications. The simplest form of gold nanomaterials is spherical in shape and known as gold nanoparticles (AuNPs), whose dispersion is ruby red in color (for widely used AuNPs of diameter ~13 nm). The widely adopted synthesis methods of AuNPs include Turkevich and seed-mediated growth. For the two methods with citrate as reducing agent, a purple/blue intermediate stage has been reported by some literatures. However, no consensus has been reached on the underlying reason for such coloration. There are five main schools of thoughts proposed in the literature: (i) formation of fluffy large aggregates from AuNPs which are spherical in shape; (ii) formation of solid polycrystalline nanowires via aggregation from AuNPs or deposition of gold along certain facet of AuNP seeds; (iii) formation of spherical aggregates from "glue" particles (liquid globules or Au(I)/sodium acetone dicarboxylate (SAD) complex) within which AuNPs nucleate and grow, without AuNPs directly interacting with each other; (iv) kinetic intermediate from closely spaced gold clusters/particles before compacting into AuNPs without any aggregates formation; (v) attachment of gold ions at electric double layer of the AuNPs without any aggregates formation. This study closely examined the seed-mediated growth of 5 nm citrate-capped AuNPs at room temperature. By close monitoring of the different color stages and manipulation of the colors, the purple/blue color appearance and disappearance have been proposed in this thesis to result from the formation and disintegration of chain-like structures which form from AuNPs interconnected by some liquid "glue". Key components within such liquid "glue" include Au(III) complex ions (in the form of [AuCl3(OH)]-) and SAD (oxidation product of sodium citrate), cross-linking probably via hydrogen bonds and ligand replacement of chloride ions within [AuCl3(OH)]- by the carboxylate groups of SAD. The close proximity of the connected AuNPs results in red shift of the localized surface plasmon resonance peak and increase in the near-infrared absorption for the red-to-purple/blue color change. With complete consumption of Au(III) complex ions by reducing agents (e.g., citrate, SAD, and acetone), the "glue" is depleted and such chain-like structures can no longer maintain, redispersing AuNPs for purple/blue-to-red color change. The red-to-purple/blue-to-red color reversal can be viewed as the dynamic process between chain-like structure formation (facilitated by liquid "glue" formation from interaction between Au(III) complex ions and SAD; SAD is gradually produced from citrate oxidation) and chain-like structure disintegration (induced by depletion of liquid "glue" from consumption of Au(III) complex ions). This proposed mechanism is also applicable to illustrate the red-to-purple/blue-to-red color reversal in seed-mediated growth of 13 nm citrate-coated AuNPs as well as the purple/blue-to-red color change in Turkevich method, offering new insights into the underlying physicochemical processes for the two widely adopted AuNP synthesis methods.