Impact of Kapitza resistance on the stability and efficiency of photoacoustic conversion from gold nanorods

Effect of interfacial thermal resistance on the near-field photoacoustic signal from gold nanorod in water: a numerical study

The near-field photoacoustics of gold nanoparticle in water has received much attention for its biomedical applications and is strongly affected by the gold-water interfacial thermal resistance. However, the effect of interfacial thermal resistance on near-field photoacoustics has been very little studied. Here we present the numerical simulations of near-field photoacoustic signal generation from a single gold nanorod in water by considering different thermal resistances at the gold-water interface. It is shown that, different from the reported reduction of far-field photoacoustic signals by interfacial thermal resistance, enhancement of near-field photoacoustic signals is obtained with the typical gold-water interfacial thermal resistance. Further analysis reveals that the higher rate of net heat transfer to the surrounding water at the typical interfacial thermal resistance, which corresponds to faster thermal expansion of the surrounding water, accounts for such enhancement of near-field photoacoustic signal and that the enhancement of near-field photoacoustic amplitude is mainly present at a distance within thermal diffusion length.

Photoacoustic Enhancement of Ferricyanide-Treated Silver Chalcogenide-Coated Gold Nanorods

Plasmonic gold nanorods (AuNRs) are often employed as photoacoustic (PA) contrast agents due to their ease of synthesis, functionalization, and biocompatibility. These materials can produce activatable signals in response to a change in optical absorbance intensity or absorbance wavelength. Here, we report a surprising finding: Ag₂S/Se-coated AuNRs have a ~40-fold PA enhancement upon addition of an oxidant but with no change in absorption spectra. We then study the mechanism underlying this enhancement. Electron micrographs and absorption spectra show good colloidal stability and retention of the core-shell structure after potassium hexacyanoferrate(III) (HCF) addition, ruling out aggregation and morphology-induced PA enhancement. X-ray diffraction data showed no changes, ruling out crystallographic phase changes upon HCF addition, thus leading to induced PA enhancement. Attenuated total reflectance-Fourier transform infrared spectroscopy and zeta potential analysis suggest that PA enhancement is driven by the irreversible displacement of hexadecyltrimethylammonium bromide with HCF. This is further confirmed using elemental mapping with energy-dispersive X-ray analysis. PA characterization after HCF addition showed a four-fold increase in the Grüneisen parameter (Γ), thus resulting in PA enhancement. The PA enhancement is not seen in uncoated AuNRs or spherical particles. Two possible mechanisms for PA enhancement are proposed: first, the photo-induced redox heating at the Ag₂S/Se shell-HCF interface, resulting in an increase in temperature-dependent Γ, and second, an enhanced electrostriction response due to HCF adsorption on a layered plasmonic nanoparticle surface, resulting in a high thermal expansion coefficient (β) that is directly proportional to Γ.

Electron–phonon effects on the photoacoustic response of gold core–silica shell nanoparticles: From the linear regime to nanocavitation

Coating gold nanostructures with a silica shell has been long considered for biomedical applications, including photoacoustic imaging. Recent experimental and modeling investigations reported contradicting results concerning the effect of coating on the photoacoustic response of gold nanostructures. Enhanced photoacoustic response is generally attributed to facilitated heat transfer at the gold/silica/water system. Here, we examine the photoacoustic response of gold core–silica shell nanoparticles immersed in water using a combination of the two temperature model and hydrodynamic phase field simulations. Here, of particular interest is the role of the interfacial coupling between the gold electrons and silica shell phonons. We demonstrate that as compared to uncoated nanoparticles, photoacoustic response is enhanced for very thin silica shells (5 nm) and short laser pulses, but for thicker coatings, the photoacoustic performance are generally deteriorated. We extend the study to the regime of nanocavitation and show that the generation of nanobubbles may also play a role in the enhanced acoustic response of core–shell nanoparticles. Our modeling effort may serve as guides for the optimization of the photoacoustic response of heterogeneous metal–dielectric nanoparticles.

Dependence of the Nonlinear Photoacoustic Response of Gold Nanoparticles on the Heat-Transfer Process

Photoacoustic (PA) imaging using the nonlinear PA response of gold nanoparticles (GNPs) can effectively attenuate the interference from background noise caused by biomolecules (e.g., hemoglobin), thus offering a highly potential noninvasive biomedical imaging method. However, the mechanism of the nonlinear PA response of GNPs based on the thermal expansion mechanism, especially the effect of heat-transfer ability, still lacks quantitative investigation. Therefore, this work investigated the effect of heat-transfer ability on the nonlinear PA response of GNPs using the critical energy and fluence concept, taking into account the Au@SiO2 core-shell nanoparticles (weakened heat transfer) and gold nanochains (enhanced heat transfer). The results showed that the stronger the heat transferability, the smaller the critical energy, indicating that the nonlinear PA response of different nanoparticles cannot be contrasted directly through the critical energy. Moreover, the critical fluence can directly contrast the proportion of nonlinear components in the PA response of different GNPs as governed by the combined effect of heat transferability and photothermal conversion ability.

DNA-Templated ultrasmall bismuth sulfide nanoparticles for photoacoustic imaging of myocardial infarction

Photoacoustic imaging (PAI) has shown great clinical potential in diagnosing various diseases due to its noninvasive, cost-effective, and real-time imaging properties but is limited by the lack of contrast agents with high sensitivity for deep tissue imaging. Here, DNA-templated ultrasmall bismuth sulfide (Bi₂S₃) nanoparticles (NPs) were reported as a photoacoustic (PA) probe for imaging myocardial infarction. We present a simple synthesis strategy of ultrasmall NPs via self-assembly of single-stranded DNA (ssDNA)/metal ion complexes. The in vivo imaging results showed a dramatically enhanced PA signal in the region of myocardial infarction after intravenous injection of DNA-Bi₂S₃ NPs in the myocardial ischaemia/reperfusion (I/R) mouse model. Further near infrared fluorescence imaging indicated that Bi₂S₃ NPs mainly accumulated in the infarcted area, leading to enhancement of PA signals. Moreover, such hybrid NPs possess a well-defined nanostructure, superior photobleaching resistance, excellent water dispersibility and negligible acute toxicity. These results not only demonstrate that ultrasmall DNA-Bi₂S₃ NPs are a potent PA probe for imaging the infarcted region but also provide a new avenue for preparing ultrasmall-sized PA probes by using ssDNA as a template.

Photostability of Contrast Agents for Photoacoustics: The Case of Gold Nanorods

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.