Identifying brazilwood’s marker component, urolithin c, in historical textiles by surface-enhanced raman spectroscopy

The fugitive nature of the colorants obtained from sappanwood (Caesalpinia sappan L.) or the South American species commonly known as ‘brazilwoods’ (including other Caesalpinia species and Paubrasilia echinata (Lam.)) makes the identification of brazilwood dyes and pigments in historic artefacts analytically challenging. This difficulty has been somewhat alleviated recently by the recognition and structural elucidation of a relatively stable marker component found in certain brazilwood dyes and pigments—the benzochromenone metabolite urolithin C. This new understanding creates an ideal opportunity to explore the possibilities for urolithin C’s localization and identification in historical artefacts using a variety of analytical approaches. Specifically, in this work, micro-destructive surface-enhanced Raman spectroscopic methods following a one-sample two-step (direct application of the colloid and then subsequent exposure of the same sample to HF before reapplication of the colloid) approach are utilized for the examination of four historical brazilwood dyed textiles with the results confirmed via HPLC-DAD analysis. It is shown that characterization of reference urolithin C is possible, and diagnostic features of this molecule can also be traced in faded historical linen, silk and wool textiles, even in the presence of minor quantities of flavonoid, indigoid and tannin components. The exploitation of the same micro-sample through a series of SERS analyses affords a fuller potential for confirming the characterization of this species.