Gold Nanocages: From Synthesis to Theranostic Applications

Abstract

CONSPECTUS Gold nanostructures have garnered considerable attention in recent years for their potential to enhance both the diagnosis and treatment of cancer through their advantageous chemical and physical properties. The key feature of Au nanostructures for enabling this diverse array of biomedical applications is their attractive optical properties, i.e. the scattering and absorption of light at resonant wavelengths due to the excitation of plasmon oscillations. This phenomenon is commonly known as localized surface plasmon resonance (LSPR) and is the source of the ruby red color of conventional Au colloids. The resonant wavelength is highly dependent on the size, shape, and geometry of the nanostructures, providing a set of knobs to maneuver the optical properties as needed. For in vivo applications, especially when optical excitation or transduction is involved, the LSPR peaks of the Au nanostructures have to be tuned to the transparent window of soft tissues in the near-infrared (NIR) region (from 700–900 nm) in order to maximize the penetration depth. One class of nanostructures with tunable LSPR peaks in the NIR region is Au nanocages. These versatile nanostructures are characterized by hollow interiors, ultrathin and porous walls, and can be prepared in relatively large quantities using a remarkably simple procedure based on the galvanic replacement between Ag nanocubes and aqueous chloroauric acid. The LSPR peaks of Au nanocages can be readily and precisely tuned to any wavelength in the NIR region by controlling their size and/or wall thickness. Other significant features of Au nanocages that make them particularly intriguing materials for biomedical applications include their compact sizes, large absorption cross sections (almost five orders of magnitude greater than those of conventional organic dyes), bio-inertness, as well as a robust and straightforward procedure for surface modification based on the Au-thiolate chemistry. In this article, we present some of the most recent advances in the use of Au nanocages for a broad range of theranostic applications, including their use: i) as tracers for tracking by multi-photon luminescence; ii) as contrast agents for photoacoustic (PA) and mutimodal (PA/fluorescence) imaging; iii) as photothermal agents for the selective destruction of cancerous or diseased tissue; and iv) as drug delivery vehicles for controlled and localized release in response to external stimuli such as NIR radiation or high-intensity focused ultrasound (HIFU).

Near-infrared–actuated devices for remotely controlled drug delivery

“…NIR-sensitive biomaterials frequently incorporate gold nanomaterials [e.g., shells (20), rods (21), or cages (22)] which readily absorb light within the NIR window and then convert a portion of that light to heat. Modulation of the irradiance allows continuous control of heat production and consequently of drug release from thermosensitive devices.…”

Supramolecular nanoscale assemblies for cancer diagnosis and therapy

“…Gold NPs (AuNPs) are functional inorganic NPs. Different shapes (spheres, rods, tubes, wires, ribbons, cubic, hexagonal, triangular) and sizes can be achieved in a controlled way [155][156][157][158]. Usually spherical AuNPs are used for DDS applications ( Figure 7).…”

“…Colloidal AuNPs can be made for dispersion in different solvents: Hydrophilic AuNPs for water-based solvent and hydrophobic ones in organic solvent [141]. AuNPs have achieved great attention in research due to their inherent unique optical and chemical properties such as high electron density and strong optical absorption [141,163]. Gold colloids present a characteristic strong surface plasmon resonance band at around 520 nm and absorb throughout the visible and near infrared region rendering them active for biological purposes [9,164,165].…”

Facile synthesis of hollow urchin-like gold nanoparticles and their catalytic activity

“…As for the generation of NPs with hollow interiors, the sacrificial template approach is the most adaptable one [18][19][20][21]. Generally, AgNPs have always been taken as sacrificial templates and their reaction with HAuCl 4 or other metal salts leads to the formation of metal NPs with voids [17,[22][23][24][25][26][27][28], in which the morphology of the AgNP seeds directly controls that of the resulting metal shells. These hollow-structured materials have been explored for biomedical and catalytic applications.…”