Pressing Solids Directly Into Sheets of Plasmonic Nanojunctions Enables Solvent-Free Surface-Enhanced Raman Spectroscopy

Often chemical analysis of solid materials begins with dissolving the sample in a solvent but this is undesirable, particularly if the physical form is important. In principle surfaceenhanced Raman spectroscopy (SERS) should allow detection of solid analytes and offers attomolar sensitivity combined with molecular specificity. SERS requires the target molecules to sit in plasmonic hot-spots, which are normally nanojunctions, just a few nm across. This means that solid samples normally need to be dissolved in a solvent so they can diffuse into the enhancing region. Here, we show that SERS spectra of picograms of solid analytes can be directly obtained by pressing them into a flexible SERS substrate with a dense field of exposed nanojunction hot-spots anchored on its surface. We demonstrate that this can be a powerful tool for straightforward and non-destructive forensic analysis of layered materials (crossing ink lines), solid explosives and illicit drugs as well as for studying previously intractable samples such as pharmaceutical co-crystals, whose important solid state structure is lost when they are dissolved.