Quantitative Fluorescence Excitation Spectra for Synthetic Eumelanin

TL;DR

This study provides the first quantitative excitation spectra for synthetic eumelanin, correcting for probe beam attenuation and emission reabsorption, revealing new insights into eumelanin's photophysical properties.

Contribution

It introduces a correction method for excitation spectra of eumelanin, enabling accurate measurement and revealing a consistent excitation peak at 365 nm.

Findings

Probe beam attenuation and emission reabsorption significantly affect spectra.

Corrected spectra resemble absorption profiles.

Identified a consistent excitation peak at 365 nm.

Abstract

Previously reported excitation spectra for eumelanin are sparse and inconsistent. Moreover, these studies have failed to account for probe beam attenuation and emission reabsorption within the samples, making them qualitative at best. We report for the first time quantitative excitation spectra for synthetic eumelanin, acquired for a range of solution concentrations and emission wavelengths. Our data indicate that probe beam attenuation and emission reabsorption significantly affect the spectra even in low-concentration eumelanin solutions and that previously published data do not reflect the true excitation profile. We apply a correction procedure (previously applied to emission spectra) to account for these effects. Application of this procedure reconstructs the expected relationship of signal intensity with concentration, and the normalised spectra show a similarity in form to the absorption profiles. These spectra reveal valuable information regarding the photophysics and photochemistry of eumelanin. Most notably, an excitation peak at 365 nm (3.40 eV), whose position is independent of emission wavelength, is possibly attributable to a DHICA component singly linked to a polymeric structure.