Gold nanocages: Photothermal related applications

Gold Nanocages (AuNCs) are made up with a ultra-thin and porous walls of Au or Au-based alloys. It is the class of hollow, porous gold nanoparticles ranging in the size from 10 to over 150 nm. Generally, AuNCs are created by reacting silver nanoparticles with chloroauric acid (HAuCl₄) in boiling water. When AuNCs are irradiated with light, it exhibit a high efficiency for light-to-heat conversion and a large absorption cross section, making them effective photothermal transducers. Tuned AuNCs significantly absorb visible and near-infrared regions to optimize their interaction with the light at different wavelengths. AuNCs exhibits better photo-stability compared to the conventional organic dyes. These perspectives are of AuNCs synthesis and their use in applications involving photothermal conversion.

Figure 1. Structure of NanoRod, NanoShell, NanoCage and AuNCs.

Recently, Qiu et al., have reviewed the synthetic methods of AuNCs and their photothermal related applications, was reported in Chemical Sciences. In the synthesis part of view, they have mentioned that the specific attention of the strategies need to be developed for tuning their size, shape, composition, thickness and porosity of the walls. AuNCs are used in various photothermal related applications such as water evaporation, phase transition, controlled release, and photothermal therapy. It is also widely used as light sensors, imaging contrast agents, photothermal transducers, and drug carriers, which are related to optics, plasmonics, and nanomedicine applications [1].

Two main general methods used for the synthesis of AuNCs

(i) A template-engaged galvanic replacement reaction: This is one of the most commonly used methods for the fabrication of AuNCs are based upon the galvanic replacement reaction between Ag nanocubes and HAuCl₄ in an aqueous medium [2].

(ii) Seed-mediated growth, followed by selective etching: In this new approach, a strong reducing agent like ascorbic acid was introduced to reduce HAuCl₄ while suppressing the galvanic replacement reaction between HAuCl₄ and Ag [3]. The gold over layers were deposited on top of Ag₂O patches would be peeled off, exposing the Ag at the corner sites during this process. Finally, the Ag core could be completely etched away with an aqueous H₂O₂ solution, generating an AuNC with a uniform wall thickness and well defined pores at the corner sites.

Photothermal behavior of AuNCs: One of the most fascinating properties of AuNCs is Localized Surface Plasmon Resonance (LSPR), which refers to the scattering and absorption of incident light at a resonant frequency due to the collective oscillation of conduction electrons at the surface of the nanostructure [4]. LSPR peak position of AuNCs can be readily tuned by the composition, thickness, porosity of the walls and controlling the extent of the galvanic replacement reaction. At same time, the LSPR peak of AuNCs can be easily tuned into the transparent window (700–900 nm) of soft tissues, enabling an array of biomedical applications ranging from photothermal therapy to temperature-controlled drug release.

Applications of AuNCs

Drug delivery: The pores are very important to present in the cage formation, the pores in the walls of AuNCs offer easy and a quick a venue for the release and loading of various types of payloads. These kinds of pores can be designed in advance with the “open or close” gating capability by integration with stimuli-responsive materials to enable on-demand release for the applications in drug delivery [5].

Light detection: AuNCs can be used as a photo-sensitive electrical switch. These kind of devices can also be adapted for the detection of NIR light and light detection through integration with poly(vinylidenefluoride), due to its capability to convert thermal energy to electricity.

Water evaporation: One of the great applications, find in this field are seawater desalination and waste water purification through photothermal evaporation of water [6].

Phase transition: The photothermal effect associated with AuNCs has also been used to trigger the phase-transition of thermo-responsive materials. The phase transferred micropatterned ferroelectric film holds promise for application in NIR sensing and imaging [7].

Controlled release: The AuNC-triggered phase-transition of thermo-responsive materials can also be utilized to manage the release of therapeutic agents used for disease treatment. Thermo-responsive materials can be applied as a coating around or a filler inside AuNCs to serve as an “on or off” gate to regulate the release of payloads under NIR irradiation [8].

Release and decomposition: The photothermal effect associated with AuNCs can also be employed to decompose organic compounds for the production of reactive species such as radicals. For example, 2,2’-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (AIPH) was encapsulated in AuNCs for the generation of reactive radicals and thereby eradication of cancer cells [9]. More significantly, the generation of reactive species with AuNCs through thermal decomposition of AIPH was oxygen independent, making this system suitable for cancer treatment under both normoxic and hypoxic conditions.

Photothermal therapy and potential image-guided treatment: AuNCs have received considerable attention indirectly eradicating cancer cells through localized hypothermia. Tuning their LSPR peaks into the transparent window of soft tissues in the NIR region makes this technique particularly viable for the treatment of malignance occurring deeply below the skin. Generally, AuNCs have bio-inertness, good biocompatibility and photo-stability, making them particularly attractive for the photothermal destruction of solid tumors. Encapsulation of therapeutic drugs into AuNCs will further allow for the combination therapy of photothermal treatment and temperature-controlled drug release. This type of nano sized material with multi-functional properties includes the imaging, drug delivery, controlled release, and photothermal therapy will be of great interest in nanomedicine.

Our SNB team have emphasize this research article to enrich our viewer's knowledge on the main synthetic approach of AuNCs and their photothermal behavior as well as related diverse applications of AuNCs. Perhaps, they will be able to find a way to move through this unique class of nanomaterials to be success for the next level.

References

J. Qiu, et al., Chem. Sci. (2020) DOI: 10.1039/d0sc05146b.
S. E. Skrabalak, et al., Nat. Protoc., 2, 2182 (2007).
X. Sun, et al., ACS Nano, 10, 8019 (2016).
E. Hutter and J. H. Fendler, Adv. Mater., 16, 1685 (2004).
S. Mura, et al., Nat. Mater., 12, 991 (2013).
T. Wu, et al., Mater. Today Energy, 12, 129 (2019).
J. Li, et al., Angew. Chem., Int. Ed., 55, 13828 (2016).
G. D. Moon, et al., J. Am. Chem. Soc., 133, 4762 (2011).
S. Shen, et al., Angew. Chem., Int. Ed., 56, 8801 (2017).

--- Dr. A. S. Ganeshraja

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