Gold nanocage
Gold nanocages are gold nanoparticles of size 20–500 nm with a hollow cubic structure and porous walls. They can be synthesized by reacting silver nanoparticles with chloroauric acid in boiling water. Gold nanocages have been suggested for use in drug delivery, photothermal therapy, and as contrast agents.
Origin and development
Gold nanocages were first created in 2002 by a group at Washington University in St. Louis, led by Younan Xia. He developed the idea for the synthesis when he was teaching galvanic replacement in a general chemistry course at the same time as a method for silver nanocube creation via polyol reduction was being developed.Since the invention of AuNCs, research has focused on the development of alternative synthesis strategies for more precise tuning of nanocage structure and properties. Another significant area of nanocage development has been the investigation of their potential applications in photoacoustic tomography, photothermal cancer therapy, and controlled drug delivery.
Synthetic procedure
Galvanic reaction overview
The fundamental reaction in the preparation of gold nanocages is a galvanic replacement reaction between chloroauric acid and “sacrificial templates” made of Ag nanostructures:3Ag + HAuCl4 → Au + 3AgCl + HCl
Silver nanotemplates can be synthesized via a polyol reduction in which ethylene glycol is oxidized by atmospheric oxygen to form glycolaldehyde. Glycolaldehyde can then be used to reduce Ag+ into elemental Ag.
2Ag+ + HOCH2CHO + H2O → 2Ag + HOCH2COOH + 2H+
AuNCs can be formed from different silver nanostructures, including nanocubes with sharp or truncated corners, single-crystal octahedrons with truncated corners, and polycrystalline quasi-spheres.
Preparation via seed-mediated growth and etching
Seed-mediated growth followed by selective etching has been proposed as a more precise alternative to the traditional template-based galvanic reaction: in the traditional synthesis, the simultaneous reduction of AuCl4 and oxidation of Ag in the galvanic reaction can lead to difficulties in controlling nanocage structure. In this alternative synthesis, a strong reductant like NaOH is added to the reaction mixture, reducing Au3+ ions faster than the galvanic replacement reaction. Patches of Ag2O are formed at the corners of the Ag nanocubes; these patches can then be selectively etched using a weak acid that also dissolves the center of the cube, producing a gold nanocage. While offering control of cage wall thickness down to one atomic layer, seed-mediated growth and etching necessitates further reaction steps and more precise reaction conditions when compared to the traditional synthetic method.Imaging AuNC synthesis
The synthesis and development of gold nanocages at various stages can be visualized using common electron microscopy techniques like scanning electron microscopy and transmission electron microscopy. X-ray ptychography and scanning wide-angle nanoprobe diffraction have also been used to image the galvanic reaction process, allowing for differentiation between different compounds in the developing nanocages and visualization of their crystalline structure.Properties
General properties
The synthesis of AuNCs produces structures that can range in size from 20–500 nm, with wall thicknesses that can be tuned in the range of 2–10 nm. Being bio-inert and nonreactive, AuNCs have been studied for in vivo biomedical applications. Their hollow centers increase surface area and functionality, allowing them to hold payloads for drug delivery.Localized surface plasmon resonance (LSPR)
Much of the optical properties of gold nanocages derive from the phenomenon of LSPR. This LSPR effect leads to the observation that suspensions of gold nanocages can appear to be various colors.LSPR is not unique to gold nanocages, and instead is a property of various classes of metal nanostructures. However, conventional gold nanoparticles exhibit LSPR peaks that are restricted to the visible light region of the electromagnetic spectrum. The hollow nature of AuNCs lead to increased surface area, resulting in a significantly higher absorption cross-section than traditional nanoparticles. This allows for LSPR that can be tunable to 600–1200 nm, in the near-infrared region.1,7 This “tunability” can be achieved by modifying the size of AuNCs and the thickness of their walls, effectively altering the ratio of wall thickness to the overall size of the nanocage.