A first
approach toward understanding the targeted design of molecular
photoacoustic contrast agents (MPACs) is presented. Optical and photoacoustic
Z-scan spectroscopy was used to identify how nonlinear (excited-state)
absorption contributes to enhancing the photoacoustic emission of
the curcuminBF2 and bis-styryl (MeOPh)2BODIPY dyes relative to Cy3.
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.
The synthesis and characterization of a series of donor-π-acceptor-π-donor (D-A-D) curcuminoid molecules is presented herein that incorporates π-extended aryl and electron-donating amino terminal functionalization. Computational evaluation shows these molecules possess quadrupolar character with the lowest energy transitions displaying high molar extinction coefficients with broad tunability through manipulation of terminal donating groups. Consistent with their quadrupolar nature, these molecules show varying degrees of solvatochromic behavior in both their absorption and emission spectra, which has been analyzed by Lippert-Mataga and Kamlet-Taft analysis. Photophysical and photoacoustic (PA) properties of these molecules have been investigated by the optical photoacoustic z-scan (OPAZ) method. Selected curcuminoid molecules display nonlinear behavior at a high laser fluence through excited state absorption that translates to the production of an enhanced photoacoustic emission. A relative comparison of "molar PA emission" is also presented with the crystal violet linear optical absorbing/linear PA emitting system being utilized as a standard reference material for OPAZ experiments. Furthermore, PA tomography experiments are presented to illustrate the enhanced PA contrast obtainable via an excited state absorption.
The nonlinear optical properties of a series of pyrrolic compounds consisting of BODIPY and aza-BODIPY systems are investigated using 532 nm nanosecond laser and the Z-scan technique. Results show that 3,5-distyryl extension of BODIPY to the red shifted MeO2BODIPY dye has a dramatic impact on its nonlinear absorption properties changing it from a saturable absorber to an efficient reverse saturable absorbing material with a nonlinear absorption coefficient of 4.64 × 10−10 m/W. When plotted on a concentration scale per mole of dye in solution MeO2BODIPY far outperforms the recognized zinc(II) phthalocyanine dye and is comparable to that of zinc(II) tetraphenylporphyrin.
The synthesis and characterization of a bis(2-dimethylaminothien-5-yl)curcumin boron difluoride chromophore is presented. Photophysical, electrochemical and computational investigations establish the properties of its absorption in the Vis-NIR spectral range relative to established curcumin dyes. Application of this thienyl curcumin dye as a photoacoustic contrast agent is investigated against the dicarbocyanine Cy5 dye in the 675-735 nm excitation range.
Excitation of dye-loaded perfluorocarbon nanoparticles (nanobombs) can generate highly localized axially propagating longitudinal shear waves (LSW) that can be used to quantify tissue mechanical properties without transversal scanning of the imaging beam. In this study, we used repetitive excitations of dodecafluoropentane (C5) and tetradecafluorohexane (C6) nanobombs by a nanosecond-pulsed laser to produce multiple LSWs from a single spot in a phantom. A 1.5 MHz Fourier-domain mode-locked laser in combination with a phase correction algorithm was used to perform elastography. Multiple nanobomb activations were also monitored by detecting photoacoustic signals. Our results demonstrate that C6 nanobombs can be used for repetitive generation of LSW from a single spot for the purpose of material elasticity assessment. This study opens new avenues for continuous quantification of tissue mechanical properties using single delivery of the nanoparticles.
The photophysical and electrochemical properties for a series of BODIPY dyes with incremental 3‐ and 3,5‐vinyl conjugation, as well as incremental electron‐donating groups (anisole < triphenylamine < ferrocenyl), are presented. Insight into the influence of each vinyl‐conjugated electron‐donating group on both vis‐NIR absorption and fluorescence emission properties is provided. These trends are further corroborated by density functional theory computational analysis. Two of this series containing the 3,5‐bis(vinyltriphenylamine) and 3,5‐bis(vinylferrocenyl) substituents exhibit significant absorption cross sections in the biological transparency window justifying further investigation of their photoacoustic emission properties via both optical photoacoustic z‐scan and photoacoustic tomography experiments. Both the 3,5‐bis(vinyltriphenylamine) and 3,5‐bis(vinylferrocenyl) substituted BODIPY dyes exhibit quantitative photoacoustic quantum yields. Relative to the commercially available methylene blue and indocyanine green molecular photoacoustic contrast agents, the 3,5‐bis(vinyltriphenylamine)‐derived BODIPY exhibits the greatest photoacoustic emission and contrast upon excited‐state absorption at 685 nm excitation at a low power laser fluence (<20 mJ cm‐2).
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