Abstract
Photoacoustic responses induced by laser-excited photothermal bubbles (PTBs) in colloidal gold solutions are relevant to the theranostics quality in biomedical applications. Confined to the complexity of nonstationary, multiscale events, and multiphysical parameters of PTBs, systematic studies of the photoacoustic effects remain obscure. Photoacoustic effects mediated by PTB dynamics and a physical mechanism are studied based on a proof-of-principle multimodal platform integrating side-scattering imaging, time-resolved optical response, and acoustic detection. Results show excitation energy, nanoparticle (NP) size, and NP concentration have strong influence on photoacoustic responses. Under the characteristic enhancement regime, the photoacoustic signal amplitude increases linearly with excitation energy and increases quadratically with the NP diameter. As for the effects of the NP concentration (characterized by absorption coefficient), a higher photoacoustic signal amplitude is generally induced by a dense NP distribution. However, with an increase in the NP size, the shielding effect of NP swarm prevents the increase of photoacoustic responses. This study presents experimental evidence of some key physical phenomena governing the PTB-induced photoacoustic signal generation in gold NP suspensions, which may help enrich theranostic approaches in clinical applications by rationalizing operation parameters.
Highlights
The method based on forwarding deflection signal (FDS) is sensitive to bubble formation, the oscillation behavior cannot be observed when the excitation energy is close to the bubble threshold for small-sized NPs, because the FDS with short oscillation lifetime is usually undetectable or quenched somewhere by the presence of other NPs.[41]
We developed a proof-of-principle multimodal platform to systematically study the photoacoustic signal induced by the laserexcited photothermal bubbles (PTBs) in colloidal gold NP suspensions
The results show that compared with the bubble generation threshold, a conservative estimation of the critical level is ∼40 folds higher, below which the photoacoustic signal exhibits a linear dependence on the PTB size
Summary
Plasmonic nanoparticles (NPs) have received extensive attention as exogenous cellular agents because of their good biocompatibility, relative safety in vivo, ease of bioconjugation with biomarkers, and superior optical properties.[1,2,3,4] These factors benefit the potential applications of NP-based photoacoustic detection, including biomedical imaging,[5,6] in vivo image-guided theranostics,[7,8] and in vitro biosensing.[9,10] In conventional photoacoustic imaging, noninvasive and biologically safe modality is indispensable, where photoacoustic signals induced by thermoelastic mechanism might satisfy this demand exactly.[11]. Compared with other vapor bubbles (e.g., induced by endogenous absorbers through bulk heating28), the PTB induced by plasmonic NPs is uniquely provided with nanoscale effect due to a high level of localized thermal field. This effect could thermally insulate the outer environment from the high-temperature NP, reduce the risk and range of thermal damage to the minimum.[28] we confined our work to the plasmonic NP (the Au NP) in background solvent of water. To probe into the mechanism of multiscale responses of photoacoustic signals to excitation energy, a pump laser was modulated to yield an upper limit high enough to induce an optical breakdown
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