SERS of molecules that do not adsorb on Ag surfaces: a metal–organic framework-based functionalization strategy

The potential for discriminating between analytes by their unique vibrational signature makes surface-enhanced Raman scattering (SERS) extremely interesting for chemical detection. However, for molecules that weakly adsorb to non-functionalized plasmonic materials, detection by SERS remains a key challenge. Here we present an approach to SERS-based detection where a polycrystalline metal-organic framework (MOF) film is used to recruit a range of structurally similar volatile organic compounds for detection by SERS. MOF films were grown on the surface of Ag "films-over-nanospheres" (FONs), which have previously been shown to enhance Raman signals of surface adsorbates by a factor of 10(7). Upon exposing the MOF-coated FON to benzene, toluene, nitrobenzene, or 2,6-di-tert-butylpyridine, the MOF film traps the vapors at the FON surface, allowing the unique Raman spectrum of each vapor to be recorded. By contrast, these analytes do not adsorb to a bare FON surface and thus cannot be detected by conventional SERS substrates. Pyridine was also tested as a Ag-adsorbing control analyte. Concentration dependence and time resolved measurements provide evidence for the hypothesis that the vapors are reversibly adsorbed on the surfaces of MOF nanocrystals exposed at grain boundaries. This represents a generalized approach for confining aromatic molecules through interactions with the MOF surface, which can be applied for future SERS-based sensors.