Surface enhanced Raman scattering (SERS) and FTIR characterization of the sepia melanin pigment used in works of art

Surface enhanced Raman scattering (SERS) and FTIR were used for the vibrational characterization of the sepia melanin pigment extracted from the cuttlefish or Sepia officinalis, a material used in works of art from at least the late 18th century. The heterogeneous polymeric nature of the pigment, together with its fluorescent background, makes its identification by normal Raman spectroscopy difficult. In the present study, SERS and FTIR spectra of sepia were obtained. SERS proved to be suitable to characterize this complex material in micro-samples because of its sensitivity, ability to quench fluorescence, and minimal preparation required. For the SERS measurements, different substrates were tested in contact with the sepia pigment. It was found that a sodium borohydride reduced silver colloid gave the best performance, particularly when applied as a drop on top of a solid sepia sample, and subsequently dried. A larger enhancement was observed in the SERS spectrum of the sepia sample extracted in the laboratory, when compared to the more pure commercial sample, consistently with the higher metal binding capacity of less pure sepia melanin reported by other authors. The FTIR and SERS frequencies observed were found to be consistent with those reported in the literature for closely related compounds, such as indole, pyrrole and substituted pyrroles, and were assigned by comparison with them and with other published data for functional groups in organic chemistry.