Correlation of Photophysical Properties with the Photoacoustic Emission for a Selection of Established Chromophores

Abstract

An archetypal study is presented to correlate both the optical and the photoacoustic (PA) properties for a diverse selection of dyes whose structural properties range across organic and inorganic, symmetric and asymmetric, and neutral and cationic systems. Three distinct classes of molecular PA contrast agents have been identified and classified according to their optical-PA response as either (i) linear absorbers linear PA emitter, (ii) saturable absorbers weak PA emitter, or (iii) reverse saturable absorbers nonlinear PA emitter. The molecular characteristics instrumental in determining the nature of the dyes optical absorption properties, i.e., ground state molar extinction coefficient (εg), excited state molar extinction coefficient (εe), and excited state lifetime (τ), are discussed to aid in the interpretation of a molecule’s optical vs PA response. An excellent linear PA emitter is established in crystal violet, which exhibits the strongest possible PA signal under low laser fluence conditions in both PA Z-scan and tomography experiments. Ultimately, however, nonlinear reverse saturable absorber (RSA) materials are anticipated to be the most promising dye category for generation of an enhanced nonlinear PA response. Effective RSA behavior is expected for materials showing a high ratio of their excited state vs ground state absorption (εe/εg) while also possessing a long-lived excited state lifetime (τ) permitting sequential two-photon absorption. ZnTPP, C60, and methylene blue each show a nonlinear PA response which correlates well with their RSA optical behavior. Relative to the linear PA emission profile of crystal violet, a 3.8-fold enhancement is observed for the PA emission of ZnTPP at the highest laser fluence of 366 mJ cm–2. Similarly, C60 and methylene blue exhibit nonlinear enhancements of 2.15-fold and 1.38-fold, respectively. Finally, to investigate the practical pros and cons with respect to application of these dyes in PA imaging applications, a concentration dependence of their PA emission is presented at both low and high laser fluences, in addition to a complementary photoacoustic tomography study.

Photophysical and Photoacoustic Properties of π‐Extended Curcumin Dyes. Effects of the Terminal Dimethylamino Electron‐donor and the Bridging Aryl Ring

“…This is consistent with the weak PA emission observed for the same dyes across a wider range of laser fluences. As anticipated, the reference CV dye maintains a linear optical absorption at all laser fluences consistent with its steady PA emission, which can be used as a baseline to identify nonlinear PA emission in other dyes . For example, the natural curcumin derivative 1 and DMA‐2 exhibit a modest reverse saturable absorption response comparable to the reference cyanine dye Cy5, all of which exhibit an enhanced nonlinear PA emission relative to the CV linear reference.…”

“…For nonlinear absorption processes, which typically requires a high laser fluence and of course a resonant excited state absorption, a PA signal enhancement is generated by a quadratic dependence on Ι In a prior study we demonstrated that the naphthyl substituted curcumin 4 exhibits the most impressive reverse saturable absorption and nonlinear PA emission of any curcumin dye studied to date . In this study, the optical and photoacoustic responses for compounds 1 , 3 – 5 , DMA‐2 – DMA‐5 and reference dyes crystal violet (CV), Cy3 and Cy5 have been evaluated using the photoacoustic z‐scan (PAZ) method using 532 nm laser irradiation in acetonitrile. For in vivo PA imaging applications it is preferred to employ water‐soluble chromophores with a strong absorption in the biological window (680–980 nm).…”

“…Indeed, interest in the development of synthetic curcumins (16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33) inspired by the natural pigment has been increasing with applications in molecular devices (22), nonlinear optics (20,24), photovoltaics (29), light emitting diodes (32) and photocatalysis (33). However, curcumin derivatives may also serve the medical field from a diagnostic perspective as a molecular contrast agent for fluorescence imaging (25) and even photoacoustics (PA) imaging (30,34,35). With this in mind, the current study aims to modify the feruloyl group of the natural curcumin chromophore with pphenyl, naphtha-4-yl, thien-2-yl and hybrid 4-naphtha-thien-2-yl substituents.…”

Photoacoustic imaging of cells in a three-dimensional microenvironment

“…Except for cells containing melanin, PA contrast is usually undetectable in biomaterials and the cell-laden 3D culture, and so an exogenous contrast agent needs to be introduced for contrast enhancement. Exogenous contrast agents for use in PA molecular imaging have to possess certain photophysical and biological properties, such as efficient optical-to-PA conversion, long-lived excited-state lifetime, biocompatibility, distinct optical absorption spectra (where the endogenous contrast agents have a lower absorption), and the capability to pass through cellular and fibrillar barriers for successful labeling [29,36]. Furthermore, both endogenous and exogenous contrast agents are usually with optical absorption spectra in the nearinfrared (NIR) window (600-1100 nm) so as to ensure their deeper penetration and hence the required imaging depth.…”

Tetramethylbenzidine: An Acoustogenic Photoacoustic Probe for Reactive Oxygen Species Detection

“…The use of fluorescent probes for photoacoustic imaging seems far from efficient because fluorescent dyes will waste a fraction of their excited-state energy by emitting a photon. Hatamimoslehabadi et al have compared the photophysical properties with the photoacoustic response for a selection of established chromophores, revealing the importance of a large non-radiative/radiative decay rate constant ratio for a good photoacoustic performance [ 33 ]. For example, fluorescein is a highly emissive dye with poor photoacoustic properties, while phenolphthalein, a fluorescein derivative, is non-fluorescent yet shows good photoacoustic properties [ 34 ].…”