Abstract
Plasmonic particles as gold nanorods have emerged as powerful contrast agents for critical applications as the photoacoustic imaging and photothermal ablation of cancer. However, their unique efficiency of photothermal conversion may turn into a practical disadvantage, and expose them to the risk of overheating and irreversible photodamage. Here, we outline the main ideas behind the technology of photoacoustic imaging and the use of relevant contrast agents, with a main focus on gold nanorods. We delve into the processes of premelting and reshaping of gold nanorods under illumination with optical pulses of a typical duration in the order of few ns, and we present different approaches to mitigate this issue. We undertake a retrospective classification of such approaches according to their underlying, often implicit, principles as: constraining the initial shape; or speeding up their thermal coupling to the environment by lowering their interfacial thermal resistance; or redistributing the input energy among more particles. We discuss advantages, disadvantages and contexts of practical interest where one solution may be more appropriate than the other.
Highlights
Plasmonic particles are emerging as a versatile solution for a broad variety of unmet needs at the crossroads of biomedical optics, sensing and imaging, owing to their unique efficiency to enhance, absorb and scatter light [1,2,3]
We present an overview of different approaches to enhance the photostability of gold nanorods for applications in photoacoustic imaging (PAI), either by enhancing their thermal stability by constraining the initial shape and inhibiting the onset of pre-melting through the addition of rigid or soft shells, such as self-assembled monolayers; or by speeding up their rate of dissipative cooling towards the environment by enhancing the thermal coupling at the gold/aqueous interface through a rational design of the surface-to-volume ratio of the metal core; or by redistributing the input energy among more particles through customization of the optical source or a subtle modulation of their plasmonic lineshapes
While we are aware of the pitfalls hidden in a strict compartmentalization, our work intends to bring some order in a literature that is recent but already fragmentary, and to orient the experimentalist who wishes to consider the use of gold nanorods in PAI
Summary
Plasmonic particles are emerging as a versatile solution for a broad variety of unmet needs at the crossroads of biomedical optics, sensing and imaging, owing to their unique efficiency to enhance, absorb and scatter light [1,2,3]. In some cases, the great efficiency of photothermal conversion of gold nanorods may turn into an issue and a practical limitation, as the input energy may trigger their overheating and reshaping, and so cause their permanent deformation, before the pathway of thermal relaxation to the environment prevails In this mini review, we present an overview of different approaches to enhance the photostability of gold nanorods for applications in PAI, either by enhancing their thermal stability by constraining the initial shape and inhibiting the onset of pre-melting through the addition of rigid or soft shells, such as self-assembled monolayers; or by speeding up their rate of dissipative cooling towards the environment by enhancing the thermal coupling at the gold/aqueous interface through a rational design of the surface-to-volume ratio of the metal core; or by redistributing the input energy among more particles through customization of the optical source or a subtle modulation of their plasmonic lineshapes. Nanomaterials 2021, 11, 116 be more pertinent than the other as well as synergistic combinations that collectively target a readier penetration of plasmonic particles into the biomedical practice [6]
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