Plasmon-driven chemical conversion is a burgeoning field in heterogeneous catalysis. Surface plasmons, collective oscillations of free electrons at a metal-dielectric interface, decay to generate energetic "hot electrons" that can drive chemical reactions. This phenomenon, known as plasmon chemistry, has been demonstrated with various small molecules and organic compounds. The study expands upon previous work by investigating the reactivity of a secondary amide, a key binding motif in peptides and proteins, within plasmonic nanocavities. The research question focuses on understanding the factors governing the reactivity of secondary amides at plasmonic interfaces, specifically the role of hot electrons versus thermal effects, the influence of photon density and energy, and the identification of reaction products. The importance lies in elucidating the mechanisms of plasmon-induced chemistry and its potential applications in various fields, including biomolecule modification and organic synthesis. The study uses N-methyl-4-sulfanylbenzamide (NMSB) as a model secondary amide molecule to investigate the transformation process, and aims to determine the influence of various factors like the laser power density, photon energy, external temperature, and plasmonic material on the reaction rates. Understanding these factors is critical for controlling and optimizing plasmon-driven reactions. The choice of NMSB is motivated by its relevance as a common binding motif in peptides and proteins, making this study highly relevant to various fields such as biophysics and chemical biology.

Existing literature extensively covers plasmon-induced processes, often explained by models like desorption induced by electronic transitions (DIET) and desorption induced by multiple electronic transitions (DIMET). These models involve the formation of transient negative ions (TNIs) upon electron transfer from the metal to the adsorbate. While DIET and DIMET assume reversible electron transfer, TNIs can also undergo non-ergodic bond dissociation. A similar dissociative electron attachment (DEA) mechanism has been proposed in previous studies. The probability of electron transition-driven transformations is influenced by factors including the availability of low-lying molecular orbitals, excitation wavelength, power density, potential energy landscape, surface temperature, and interfacial chemistry. Linic and coworkers proposed two mechanisms for hot carrier injection: direct (chemical interface damping) and indirect (Landau damping) charge transfer, with the success depending on the accessibility of molecular orbitals. Light absorption can lead to energy transfer via phonon coupling (heating) or electron transfer to an unoccupied molecular orbital (LUMO), or hole transfer between the HOMO and the metal. The HOMO-LUMO gap shifts in metal-molecule hybrid states, and interfacial chemistry can influence the reaction pathway. Electron capture dissociation (ECD) and electron transfer dissociation (ETD) are commonly employed in MS/MS analysis of peptides, involving fragmentation via H atom loss, N-Ca bond dissociation, ammonia release, side-chain group release, and disulfide bond cleavage. This prior research provides the foundation for the current study, which aims to investigate the specific reactivity of a secondary amide under plasmonic excitation and determine the dominant reaction mechanisms.

The study utilized N-methyl-4-sulfanylbenzamide (NMSB) adsorbed on self-aggregated structures of silver (Ag) and gold (Au) nanoparticles. Scanning electron microscopy (SEM) characterized the morphology of the NMSB-coated nanoparticle aggregates. Surface-enhanced Raman scattering (SERS) served as an in-situ spectroscopic tool to monitor the chemical transformation under 633 nm and 785 nm laser excitation. The time evolution of the SERS spectra revealed the changes in the vibrational fingerprint of the molecule during the reaction. Potential reaction products were considered, and their calculated Raman spectra were compared with experimental SERS data to identify the products. A dissociative electron attachment (DEA) study was conducted on NMSB in the gas phase to gain insights into the electronic processes involved in the plasmon-driven transformation. Density functional theory (DFT) calculations supported the spectral assignments and provided structural information. Kinetic analysis was performed to determine the reaction rate constants under various conditions (different laser powers, wavelengths, and temperatures). The dependence of the reaction rate on laser power and temperature provided information about the thermal versus non-thermal nature of the reaction. UV-vis spectroscopy was employed to investigate the plasmon absorbance bands of the nanoparticle aggregates. The computational studies used various DFT functionals (wB97XD, B3LYP) and basis sets (aug-cc-pVDZ, aug-cc-pVTZ) to optimize structures and calculate energies. The DEA experiment used a crossed electron-molecular beam instrument with a hemispherical electron monochromator (HEM) to analyze the resulting ions.

SERS measurements showed a gradual disappearance of the NMSB amide III band at 1320 cm⁻¹, accompanied by the appearance of peaks at 1189 cm⁻¹ (NH₂ rocking of primary amide) and 2230 cm⁻¹ (nitrile stretching), indicating the conversion of NMSB to p-mercaptobenzamide (MBAm) and p-mercaptobenzonitrile (MBN). DFT calculations supported the spectral assignments. The peak shift from 315 cm⁻¹ to 345 cm⁻¹ in the SERS spectra, assigned to NH₂ rocking and C-S-Ag bending vibrations, respectively, suggested a transformation from MBAm to MBN. Gas-phase DEA studies showed S-H bond dissociation at ~1.1 eV, consistent with the plasmon-induced reaction. The lack of significant Ca-N cleavage in the gas phase DEA study, contrasted with the observation in SERS experiments, highlights the influence of the metal surface. Kinetic analysis indicated a near-linear dependence of the reaction rate constant on laser power under 633 nm excitation, suggesting a non-thermal pathway. Little temperature dependence on the reaction rate further supported this conclusion. The reaction rates were higher under 633 nm excitation than under 785 nm excitation, consistent with stronger plasmon absorption at 633 nm. No reaction occurred without nanoparticles, confirming the necessity of plasmon excitation. The overall reaction involved a hot-electron-mediated DEA mechanism, where hot electrons generated under plasmonic excitation transferred energy to NMSB, facilitating bond cleavage via TNI states. The formation of MBN is proposed to proceed via a dehydration reaction of the primary amide, MBAm.

The findings demonstrate that plasmonic excitation drives the chemical transformation of NMSB via a predominantly hot-electron-mediated mechanism, rather than a thermally driven process. The near-linear relationship between reaction rate and laser power, coupled with the weak temperature dependence, strongly supports this conclusion. The discrepancy between the gas-phase DEA results and the condensed-phase SERS observations underscores the importance of the metal-molecule interface in facilitating the reaction. The transformation of NMSB to MBAm and subsequently to MBN offers a novel route for nitrile formation on plasmonic surfaces under visible light irradiation. The study's findings contribute significantly to our understanding of plasmon-driven catalysis and provide insights into the reactivity of secondary amides in plasmonic environments. These insights are crucial for designing and optimizing plasmon-enhanced catalytic systems for various applications.

This study reveals a hot-electron-mediated transformation of a secondary amide (NMSB) into a primary amide (MBAm) and an aromatic nitrile (MBN) on plasmonic Ag and Au nanoparticles. The findings highlight the non-thermal nature of the reaction and emphasize the critical role of hot electrons in driving the bond cleavage. Future research could explore the reactivity of other amides and peptides on plasmonic interfaces, investigate the role of different metal nanoparticles, and explore potential applications in peptide modification and organic synthesis.

The study focuses on a specific model secondary amide (NMSB) and two types of plasmonic nanoparticles (Ag and Au). The generalizability of the findings to other amides, peptides, and nanoparticles requires further investigation. The kinetic analysis relies on the assumption of a pseudo-first-order rate law, which might not be entirely accurate for complex reaction pathways. The error bars in the temperature-dependent experiments limit the definitive conclusions about the effect of temperature on the reaction rate.