Metal-Catalyzed Chemical Reaction of Single Molecules Directly Probed by Vibrational Spectroscopy

Abstract

The study of heterogeneous catalytic reactions remains a major challenge because it involves a complex network of reaction steps with various intermediates. If the vibrational spectra of individual molecules could be monitored in real time, one could characterize the structures of the intermediates and the time scales of reaction steps without ensemble averaging. Surface-enhanced Raman scattering (SERS) spectroscopy does provide vibrational spectra with single-molecule sensitivity, but typical single-molecule SERS signals exhibit spatial heterogeneities and temporal fluctuations, making them difficult to be used in single-molecule kinetics studies. Here we show that SERS can monitor the single-molecule catalytic reactions in real time. The surface-immobilized reactants placed at the junctions of well-defined nanoparticle-thin film structures produce time-resolved SERS spectra with discrete, step-transitions of photoproducts. We interpret that such SERS-steps correspond to the reaction events of individual molecules occurring at the SERS hotspot. The analyses of the yield, dynamics, and the magnitude of such SERS steps, along with the associated spectral characteristics, fully support our claim. In addition, a model that is based on plasmonic field enhancement and surface photochemistry reproduces the key features of experimental observation. Overall, the result demonstrates that it is possible, under well-controlled conditions, to differentiate the chemical and physical processes contributing to the single-molecule SERS signals, and thus shows the use of single-molecule SERS as a tool for studying the metal-catalyzed organic reactions.

Mechanistic Studies of Plasmon Chemistry on Metal Catalysts

“…[40,41,42,47] More complicated situations have emerged from the reactions initiated from pNTP, pATP, and DMAB molecules.W here pNTP or pATPm olecules are concerned, it is reported that the LSP forms an azo bond to generate DMAB molecules; [23][24][25][26][27][28][29][30] however,other observations include reduction from pNTP to pATP, and dissociation of pNTP and the azo bond in DMAB as ac onsequence of the LSP. [40,41,42,47] More complicated situations have emerged from the reactions initiated from pNTP, pATP, and DMAB molecules.W here pNTP or pATPm olecules are concerned, it is reported that the LSP forms an azo bond to generate DMAB molecules; [23][24][25][26][27][28][29][30] however,other observations include reduction from pNTP to pATP, and dissociation of pNTP and the azo bond in DMAB as ac onsequence of the LSP.…”

“…[23-30, 73, 74] Most studies proposed that indirect hot-electron transfer (Figures 2b,e) is ac rucial step in the azo bondformation reaction. [24][25][26][27][28][29][30] In contrast, Jiang et al suggested the direct intramolecular excitation mechanism for the azo bond-formation reaction from an ATPd erivative, S-4-((4-aminophenyl)ethynyl)thiophenol, based on aS ERS study combined with DFT calculations. [23] Ther eaction process is also ac ontroversial issue.W ang and Zhang reported that the photoactivation of O 2 molecules is necessary for plasmon-induced azo bond formation from pATP.…”

“…[24] CortØse tal. [35] To obtain further mechanistic understanding,t imeresolved measurements in Raman signals from as ingle molecule were achieved at the nanogap between aA gN P and Au thin film by SERS, [25] and at the nanogap between aAg-coated AFM tip and Au nanoplate by TERS. [56] The dissociation of the azo bond was induced by the combination of the hot electrons and tunneling electrons,w hich was demonstrated using STM-TERS.…”

“…[16,17] Plasmon chemistry has started to attract the attention of researchers after the discoveries of two important chemical reactions.Insurface-enhanced Raman spectroscopy (SERS), the chemical transformation from p-aminothiophenol (pATP) or p-nitrothiophenol (pNTP) to 4,4'-dimercaptoazobenzene (DMAB) was identified from the observations of spectral changes during the SERS measurements. [23][24][25][26][27][28][29][30] In metal catalysts,a ni mportant finding is that the LSP enhances catalytic oxidation reactions; this was demonstrated by epoxidation on Ag and Cu nanoparticles (NPs). [22] Reactions leading to the formation of azo species by the LPS have been investigated to clarify the reaction mechanisms operating.…”

Thermally Stable TiO2‐ and SiO2‐Shell‐Isolated Au Nanoparticles for In Situ Plasmon‐Enhanced Raman Spectroscopy of Hydrogenation Catalysts

“…[18] Additionally, owing to the highly localized surface sensitivity,i tc an be used to observe possible intermediate structures during reactions. [19,20] To improvet he applicability as au niversal surface characterization technique, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was recently developed. [21] By coating gold or silver cores with SiO 2 ,t he plasmonic core is passivated so that virtually any substrate can be characterized with this technique.…”