SERS detection of shikonin dye by metal-coordinated tethering on gold nanofilm

Abstract

We developed a spectroscopic detection method of dye molecules by combining surface-enhanced Raman spectroscopy (SERS) and a metal-coordinated tethering method. In order to mimic the surface of cellulose fiber, a gold nanofilm (AuNF) surface was modified with hydroxyl groups, and then treated following a tethering process to form a metal ion-mediated dye ligand complex (Al3+-shikonin dye complex) on the AuNF. Then, the colloidal gold nanoparticles (AuNPs) were dropped and dried to form SERS hot-spots, where shikonin locates in the interstitial gap between AuNP and AuNF. The SERS enhancement factor (EF) of AuNP-on-AuNF was calculated to be 4.4 × 107 via SERS measurement of 4-mercaptobenzoic acid (4-MBA). The SERS heatmap of shikonin from the ‘dyed AuNP-on-AuNF’ substrate showed that shikonin was uniformly tethered on AuNF surface in the same way as the dyeing of dye molecules on cellulose fibers. Furthermore, the spectral differences were observed between ‘dyed AuNP-on-AuNF’ and salt-induced AuNP aggregates due to the difference in vibrational modes between dyed shikonin and free shikonin. This method can apply to various natural dyes owing to the versatility of the dyeing method with a metal ion. Moreover, SERS libraries obtained using this method can directly compare with dyes tethered on fibers.