We found a gap mode plasmon induced photocatlaytic oxidation of p-alkyl thiophenol (p-AlTP) to p-mercaptobenzoic acid (p-MBA). In contrast to p-AlTP, o-methyl TP and m-methyl TP were not oxidized, indicating preferential reactivity of the para position in TP molecules. Nonetheless, the site selectivity is not always valid in this type of photocatalytic reactions, as o-, m-, and p-MBAl molecules were oxidized to corresponding MBA. With respect to the reaction mechanism, we confirmed that the oxidation of p-MeTP is not induced by thermal heating of the samples up to 373 K. Subtle temperature increase (<10 K) during the gap mode-induced oxidation was also corroborated by the observed Stokes and anti-Stokes scattering intensity of p-MeTP. Oxygen molecules accelerated the oxidation of p-MeTP at room temperature, whereas nitrogen atmosphere generated an intermediate species attributable to p-mercaptobenzyl radical. We further investigated the reaction process using density functional theory.
A gap mode plasmon induced photocatalytic oxidation of p-alkyl thiophenol (p-AlTP), encompassing methyl, iso-propyl and tert-butyl groups, to p-mercaptobenzoic acid (p-MBA), for which excitation 532 nm laser (<1 μW/μm2) impinged on a sample of silver (Ag) nanoparticles/p-AlTP/Ag films/BK-7 prism. Apparently, the oxidation of p-AlTP yielded p-MBA without leaving any signs of the intermediated species like p-mercaptobenzyl-aldehyde or -alcohol (p-MBAl) in the air. In contrast to p-AlTP, o-methyl TP and m-methyl TP were not oxidized, indicating preferential reactivity of the para position in TP molecules. Nonetheless, the site selectivity is not always valid in this type of photocatalytic reactions, as o-, m-, and p-MBAl molecules were oxidized to corresponding o-, m- and p-MBA. Similarly, all of o-, m-, and p-MBA molecules were decarboxylated to TP in the air. With respect to the reaction mechanism, we confirmed that the oxidation of p-MeTP is not induced by thermal heating of the samples up to 373 K. Subtle temperature increase (<10 K) during the gap mode-induced oxidation was also corroborated by the observed Stokes and anti-Stokes scattering intensity of p-MeTP. Oxygen molecules accelerated the oxidation of p-MeTP at room temperature, whereas nitrogen atmosphere generated an intermediate species attributable to p-mercaptobenzyl radical.
To establish an efficient way to utilize a gap mode plasmon in flocculates of MNPs, under external and ATR configurations, we controlled interaction between adsorbed species and metal nanostructures. We have successfully formed flocculates of AgNPs using electrostatic interaction between dissociated PMBA (-COO-), protonated PATP (-NH3+) and counter ions (Mn+, X-), as well as van der Waals force between neutral PMBAs (-COOH) and PATP (-NH2) on AgNPs. Detailed adsorbed state of PMBA and PATP as well as trapped counter ions were characterized using enormous SERS enhancement in flocculation-SERS. In a gap mode under an external geometry, most of thiol molecules on Ag films immobilized AgNPs through van der Waals force and electrostatic interaction. They showed similar Raman enhancement of 108-109, in accordance with those predicted by FDTD calculations. Only thiols with tert-methyl group did not immobilize any AgNPs due to steric hindrance. In a gap mode under ATR configuration, additional enhancement was obtained by a coupling of PSP and a gap mode.
Conference Committee Involvement (3)
Enhanced Spectroscopies and Nanoimaging 2022
21 August 2022 | San Diego, California, United States
Enhanced Spectroscopies and Nanoimaging 2021
2 August 2021 | San Diego, California, United States
Enhanced Spectroscopies and Nanoimaging 2020
24 August 2020 | Online Only, California, United States
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