Formation of Gold Nanorods by a Stochastic “Popcorn” Mechanism

Abstract

Gold nanorods have significant technological potential and are of broad interest to the nanotechnology community. The discovery of the seeded, wet-chemical synthetic process to produce them may be regarded as a landmark in the control of metal nanoparticle shape. However, the mechanism by which the initial spherical gold seeds acquire anisotropy is a critical, yet poorly understood, factor. Here we examine the very early stages of rod growth using a combination of techniques including cryogenic transmission electron microscopy, optical spectroscopy, and computational modeling. Reconciliation of the available experimental observations can only be achieved by invoking a stochastic, "popcorn"-like mechanism of growth, in which individual seeds lie quiescent for some time before suddenly and rapidly growing into rods. This is quite different from the steady, concurrent growth of nanorods that has been previously generally assumed. Furthermore we propose that the shape is controlled by the ratio of surface energy of rod sides to rod ends, with values of this quantity in the range of 0.3-0.8 indicated for typical growth solutions.

Gold Nanorods as Nanodevices for Bioimaging, Photothermal Therapeutics, and Drug Delivery

“…Optical properties of gold nanorods have been reported by Edgar et al 38) Gold nanorods showed two surface plasmon bands in visible and NIR regions 29) (Fig. 3).…”

“…33,34) Other "replacement" reactions may include thiol groups to gold nanorods. [35][36][37][38][39] Moreover, gold nanorods can be modified with a silica shell following attachment of desired molecules.…”

“…Ellipsoidal approximation of the gold nanorods could predict the peak positions on the aspect ratios (longitude/transverse) of the gold nanorods. 38,39) The absorption band in the visible region corresponds to the transverse mode of SP oscillation.…”

Intertwining Roles of Silver Ions, Surfactants, and Reducing Agents in Gold Nanorod Overgrowth: Pathway Switch between Silver Underpotential Deposition and Gold–Silver Codeposition

“…6,[17][18][19] While the aspect ratios of Au NRs can be fine-tuned by simply varying the concentration of Ag + ions in the growth solution, the exact roles that Ag + ions play in the symmetrybreaking of the isotropic seeds and the subsequent anisotropic NR growth still remain elusive and controversial. 36,[47][48][49][50] In striking contrast to the excellent control over NR aspect ratios, limited success has been achieved so far in the precise facet control of NRs using the seed-mediated growth method.…”

Anisotropic Gold-based Nanoparticles: Preparation, Properties, and Applications

“…However, improvements in the quality of the chemical seeding methods since 2004 have eroded the advantage of the photochemical method. 3943 Recently, detailed mechanisms of nanorod formation were discussed, 44,45 revealing that chemical seeding methods have the potential to form more uniform, reproducible, and shape-controlled gold nanorods.…”