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.
Exciting progress has been made in the area of solar fuel generation by CO2 reduction. New photocatalytic materials containing well-defined surface catalytic sites have emerged in recent years, including heterogenized...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.