Nanocages

Nanocages

Nanocages are hollow porous gold nanoparticles ranging in size from 10 to over 150 nm. They are created by reacting silver nanoparticles with chloroauric acid (HAuCl) in boiling water. While gold nanoparticles absorb light in the visible spectrum of light (at about 550 nm), gold nanocages absorb light in the near-infrared, where biological tissues absorb the least light. Becausethey are also biocompatible, gold nanocages are promising as a contrast agent for optical coherence tomography, which uses light scattering in a way analogous to ultrasound to produce in-vivo images of tissue with resolution approaching a few micrometers. [3] They can also be used as carriers of drugs and thus may have potential applications in drug delivery and/or controlled drug release. The hollowinteriors can host small objects such as magnetic nanoparticles to construct multifunctional hybrid nanostructures. A contrast agent is required if this technique will be able to image cancers at an early, more treatable stage. Gold nanocages also absorb light and heat up, killing surrounding cancer cells.

This Ag nanostructures could serve as a template for galvanic replacement with HAuCl4to make complementary hollow Au nanostructures with controllable void size, wall thickness, and wall porosity.[1] With Ag nanocubes as a template, Au nanoboxes (with non-porous walls) or nanocages (with porous walls) could be routinely produced on relatively large scales. Because the dimension and wall thickness of the resultant Au nanocages are well controlled by the molar ratio of Ag to HAuCl4,their SPR peaks can be conveniently and precisely shifted to cover a spectral region from 400 to 1200 nm.

Transformation of Ag Nanocubes into Au Nanocages

Because Ag is more reactive than Au, HAuCl4 can be reduced by Ag nanostructures to generate Au atoms:

3Ag(s) + AuCl4- (aq) ->Au(s) + 3Ag+(aq) + 4Cl-(aq)

During this replacement reaction, Au atoms epitaxially nucleate, grow into smallislands, and eventually evolve into a thin shell around each Ag template. The product is a hollow structure with an inner void complementary to that of the Ag template. Based on the stoichiometry (and confirmed by transmission electron microscopy (TEM) studies), the wall thickness is approximately one-eighteenth of the dimension of the Ag template. By performing the reaction in aqueous mediumunder reflux, the Ag+ and Cl- species can be kept in the ionic state without forming AgCl precipitate, which may otherwise interfere with the galvanic replacement reaction.

The results from galvanic replacement reaction between Ag nanocubes 110 nm in edge length and HAuCl4 system indicate that the replacement proceeds through three major steps: i) initiation of galvanic replacement by pitting at aspecific site on the surface of a Ag nanocube; ii) the formation of a pinhole-free nanobox consisting of thin, uniform walls through a combination of galvanic replacement and alloying; and iii) the generation of pores in the wall through a dealloying process, in which Ag atoms are selectively oxidized. As galvanic replacement, alloying, and dealloying proceed, the Ag nanocube template is graduallydissolved, leading first to the formation of a Au nanobox, and then to a series of nanocages with increasing porosities. [2] The dimensions, wall thickness, pore sizes, and pore densities of the resultant Au nanocages can all be controlled by varying the molar ratio between Ag nanocubes and HAuCl4 in a fashion similar to acid-base titration.
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|Figure 1. A) TEM images detailing all morphological and |Figure 2. UV-vis-NIR extinction spectra taken from an aqueous solution |
|structural changes involved in the galvanic replacement |of Ag nanocubes (30 nm in edge length) after it has been titrated with |
|reaction between Ag nano- cubes 30 nm in edge...