Radiative Relaxation of Sepia Eumelanin is Affected by Aggregation¶

Abstract

The steady-state and time-resolved emission properties of aqueous solutions containing different aggregation state distributions of eumelanin are reported. Excitation spectra of the size-selected samples reveal, for the first time, differences in absorption bands due to varying levels of aggregation. These size-dependent absorption properties result in size-dependent emission band shapes and quantum yields. For all size fractions, absorption and emission overlap significantly. The emission yield for small eumelanin aggregates in 5.7 times greater than that for large eumelanin aggregates. Time-resolved population decays reveal that small eumelanin aggregates are responsible for long-lived emission dynamics (lifetimes greater than 1 ns), while large eumelanin aggregates are the source of short emission decay (lifetimes less than 1 ns). Polarized emission decays for the large and small aggregates reveal that energy transfer occurs both within the same and between the separate fundamental building blocks of eumelanin. The observed energy transfer dynamics can be accounted for using Förster theory.

Optical and Paramagnetic Properties of Size-Controlled Ink Particles Isolated fromSepia officinalis

“…It is well known that Sepia ink shows a characteristic absorption from melanin pigments in the ultraviolet region, [4][5][6][7] but the correlation between the structure of Sepia-ink particles on the order of nanometers and their optical properties have not yet been established. Furthermore, the paramagnetic properties of Sepia-ink particles of different sizes have barely been investigated.…”

“…Nofsinger and Simon discovered that the absorption of melanin in the UV-VIS spectrum changes as a function of its aggregation state. 5) Since the particle radius after natural aggregation of synthetic melanin in aqueous solutions is theoretically estimated to be 38 nm, 12) the ideal sizes of the particles in aqueous solution are expected to be in the order of large ink particle size > synthetic melanin particle size > small ink particle size. Hence the spectral shape obtained is dependent on the size of the particles in a solution.…”

Excitonic effects from geometric order and disorder explain broadband optical absorption in eumelanin

“…Besides its pigmentation, eumelanin is also an efficient photoprotective pigment. The broadband absorption spectrum of eumelanin [1][2][3][4] is a result of biological evolution for protecting living organisms from intense sunlight 5,6 . Any gaps in the spectrum would leave living organisms vulnerable to these specific wavelengths of sunlight.…”

Thermal lensing in the ocular media

“…Though some fluorophores and their isomers have been studied, to date, the absorptive properties of the RPE in the NIR are largely unknown (Eldred and Katz 1988;Vitkin, Woosley et al 1994;Nofsinger, Forest et al 1999;Nofsinger and Simon 2001;Fishkin, Sparrow et al 2005). Numerous physiological changes occur in the aging RPE including a reduction in cell density and changes in pigmentation (Strauss 2005).…”

“…Lipofuscin build-up is just one example which can cause variability in optical absorption between species and subjects. In lieu of these facts, 20 characterization of reported chromophores such as eumalanin has revealed a significant decrease in absorption towards longer wavelengths (Vitkin, Woosley et al 1994;Nofsinger, Forest et al 1999;Nofsinger and Simon 2001). Given the variability in pigmentation content and density of the retina between subjects, a trend in absorption of the RPE, neural retina and choroid in the NIR is a suitable analysis.…”