Development of dry-state SERS substrates for the noninvasive detection of artistic dyes in textiles

Surface-enhanced Raman spectroscopy (SERS) is a powerful technique for the detection of natural dyes, allowing one to overcome the limitations of conventional Raman spectroscopy, i.e., the weakness of the signals and the intense fluorescence background. In the artistic field, a noninvasive approach is recommended; however, SERS is still an inherently invasive method, requiring the extraction of the analyte or close contact with the object and wet substrate. Inexpensive and easy-to-produce substrates allowing for dry-state measurements were fabricated, exploiting only the electromagnetic interaction between a nanostructured substrate and the dye molecules absorbed onto a textile. To this aim, different substrates were developed starting from two colloids (silver nanospheres and nanostars) immobilized onto a functionalized microscope glass slide, and some parameters connected to their synthesis were optimized to obtain a great enhancement of the signal in dry conditions. Finally, dry-state SERS spectra of four different dyes were successfully obtained from reference dyed wool threads.