Plasmon-Enhanced Autocatalytic N-Demethylation
2018
Tesema, Tefera E. | Annesley, Christopher | Habteyes, Terefe G.
Increasing experimental results indicate that optically excited plasmonic metal nanoparticles can drive photochemical reactions at photon flux comparable to that of solar radiation. However, experimental evidence that provides insight into the mechanism of the reactions on plasmonic surfaces has been limited. Here, using plasmon-enhanced N-demethylation (PEND) of methylene blue (MB) as model reaction, we report mechanistic analysis of photochemical reactions on plasmonic gold nanoparticles under different adsorption and atmospheric conditions using surface-enhanced Raman scattering as operando spectroscopy to monitor the reaction as a function of exposure time to the light source. We found that in air and oxygen atmospheres and in the presence of co-adsorbed water molecules, MB undergoes photochemical N-demethylation to yield thionine (complete N-demethylation product) and other partial N-demethylation products that have distinct vibrational signatures. The product signals are negligible when the MB-particle system is illuminated in nitrogen atmosphere. Consistent with the well-studied mechanism in solution, the PEND reaction appears to be initiated by singlet oxygen generated via energy transfer from the excited state of MB to oxygen molecule, and therefore the reaction may tentatively be described as an autocatalytic photochemical process. The results of this study provide an important insight that electronic excitations of adsorbates pumped by the localized surface plasmon field can lead to selective reaction pathways.
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