We report the direct observation of surface-enhanced Raman scattering (SERS) effect using metal-organic frameworks (MOFs) as substrates. Without the aid of any metal colloids or enhancing agents, the SERS signals of methyl orange (MO) adsorbed in MOFs were observed and even remained active if the organic linkers in MOFs were completely removed by high temperature and O2 plasma treatments. It implies that the SERS active site is at the metal oxide clusters. The ultraviolet-visible spectra of MO, MOFs, and MO-MOF complexes show that absorption peaks are far from laser excitation line. Thus, conventional resonance enhancement effect should be ruled out, and charge-transfer mechanism is the most likely scenario responsible for the observed SERS effect. Density functional theory (DFT) was used to interpret the chemical enhancement mechanism and the adsorption orientation-dependent SERS spectra in our observation. The preferred adsorption orientations calculated by DFT method are consistent with the observed SERS results.
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