The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

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Present and Future of Surface-Enhanced Raman Scattering

In the last two decades, plasmon resonance in gold nanoparticles (Au NPs) has been the subject of intense research efforts. Plasmon physics is intriguing and its precise modelling proved to be challenging. In fact, plasmons are highly responsive to a multitude of factors, either intrinsic to the Au NPs or from the environment, and recently the need emerged for the correction of standard electromagnetic approaches with quantum effects. Applications related to plasmon absorption and scattering in Au NPs are impressively numerous, ranging from sensing to photothermal effects to cell imaging. Also, plasmon-enhanced phenomena are highly interesting for multiple purposes, including, for instance, Raman spectroscopy of nearby analytes, catalysis, or sunlight energy conversion. In addition, plasmon excitation is involved in a series of advanced physical processes such as non-linear optics, optical trapping, magneto-plasmonics, and optical activity. Here, we provide the general overview of the field and the background for appropriate modelling of the physical phenomena. Then, we report on the current state of the art and most recent applications of plasmon resonance in Au NPs.

Surface plasmon resonance in gold nanoparticles: a review.

Core–shell nanoparticles are at the leading edge of the hot research topics and offer a wide range of applications in optics, biomedicine, environmental science, materials, catalysis, energy, and so forth, due to their excellent properties such as versatility, tunability, and stability. They have attracted enormous interest attributed to their dramatically tunable physicochemical features. Plasmonic core–shell nanomaterials are extensively used in surface-enhanced vibrational spectroscopies, in particular, surface-enhanced Raman spectroscopy (SERS), due to the unique localized surface plasmon resonance (LSPR) property. This review provides a comprehensive overview of core–shell nanoparticles in the context of fundamental and application aspects of SERS and discusses numerous classes of core–shell nanoparticles with their unique strategies and functions. Further, herein we also introduce the concept of shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) in detail because it overcomes the long...

Core–Shell Nanoparticle-Enhanced Raman Spectroscopy

Single-molecule (SM) surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) have emerged as analytical techniques for characterizing molecular systems in nanoscale environments. SERS and TERS use plasmonically enhanced Raman scattering to characterize the chemical information on single molecules. Additionally, TERS can image single molecules with subnanometer spatial resolution. In this review, we cover the development and history of SERS and TERS, including the concept of SERS hot spots and the plasmonic nanostructures necessary for SM detection, the past and current methodologies for verifying SMSERS, and investigations into understanding the signal heterogeneities observed with SMSERS. Moving on to TERS, we cover tip fabrication and the physical origins of the subnanometer spatial resolution. Then, we highlight recent advances of SMSERS and TERS in fields such as electrochemistry, catalysis, and SM electronics, which all benefit from the vibrational characterization of si...

Single-Molecule Chemistry with Surface- and Tip-Enhanced Raman Spectroscopy

The journal of physical chemistry letters

Surface- and tip-enhanced Raman spectroscopy (SERS and TERS) techniques exhibit highly localized chemical sensitivity, making them ideal for studying chemical reactions, including processes at catalytic surfaces. Catalyst structures, adsorbates, and reaction intermediates can be observed in low quantities at hot spots where electromagnetic fields are the strongest, providing ample opportunities to elucidate reaction mechanisms. Moreover, under ideal measurement conditions, it can even be used to trigger chemical reactions. However, factors such as substrate instability and insufficient signal enhancement still limit the applicability of SERS and TERS in the field of catalysis. By the use of sophisticated colloidal synthesis methods and advanced techniques, such as shell-isolated nanoparticle-enhanced Raman spectroscopy, these challenges could be overcome.

Surface- and Tip-Enhanced Raman Spectroscopy in Catalysis

Electrocatalytic reduction of carbon dioxide (CO2) into value-added products (e.g., ethylene) is a promising approach for greenhouse gas mitigation, but many details of electrocatalytic CO2 reduction reactions (CO2RR) remain elusive. Raman spectroscopy is suitable for in situ characterization of CO2RR mechanisms, but the low signal intensity and resulting poor time resolution (often up to minutes) hampers the application of conventional Raman spectroscopy for the study of the dynamic CO2 reduction reaction, which requires sub-second time resolution. By using Time-Resolved Surface Enhanced Raman Spectroscopy (TR-SERS) we were able to successfully monitor CO2RR over Cu surfaces with sub-second time resolution. Anodic treatment at 1.55 V vs. the reversible hydrogen electrode (RHE) and subsequent surface oxide reduction (below -0.4 V vs. RHE) induced roughening of the Cu electrode surface, which resulted in hot-spots for TR-SERS, enhanced time resolution (down to ~ 0.7 s) and improved CO2RR efficiency (i.e., four-fold increase in ethylene faradaic efficiency). With TR-SERS, the initial formation of hot-spots for SERS and CO2RR was followed (<7 s), after which a stable copper surface surrounded by increased local alkalinity was formed. Our measurements revealed that a highly dynamic CO intermediate, with a characteristic vibration below 2060 cm-1, is related to C-C coupling and ethylene production (-0.9 V vs. RHE), whereas lower cathodic bias (-0.7 V vs. RHE) resulted in gaseous CO production from isolated and static CO surface species with a distinct vibration at 2092 cm-1. Our results provide valuable time-resolved insights into the dynamic nature of the electrode surface and adsorbed intermediates during CO2 electrochemical reduction on copper and showcase the potential of TR-SERS in copper-based electrocatalysis to follow reaction dynamics.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.202104114

Sub-second Time-resolved Surface Enhanced Raman Spectroscopy Reveals Dynamic CO Intermediates during Electrochemical CO2 Reduction on Copper.

The development of improved catalysts requires insights into the relationship between catalytic activity and catalyst structure, including the underlying reaction mechanism. Here, we demonstrate a unique set of catalyst extrudate sensors that allow for the simultaneous detection of local temperature by luminescence thermometry, and of surface species by shell-isolated nanoparticle-enhanced Raman spectroscopy. This sensing approach was applied to the characterization of direct conversion of syngas into hydrocarbons and C2+ oxygenates over supported Rh and RhFe catalysts. Luminescence thermometry demonstrated a mismatch between the set temperature and the local catalyst temperature, with variations up to 40 °C. Furthermore, by investigating the surface species on varying extrudate and catalyst compositions, we identified tilted carbonyl species on the Rh/SiO2 interface that are probable precursors for the hydrogen-assisted CO dissociation. The implementation of extrudate catalyst sensors as a characterization tool provides a unique approach towards the further understanding of the relevant parameters in catalysis. Multi-modal approaches to simultaneously characterize different aspects of a reaction in situ are not readily accessible. Here, catalyst extrudates equipped with both luminescence thermometry and Raman spectroscopy sensors are introduced, providing an in-depth picture for the conversion of syngas on a supported rhodium catalyst.

Operando monitoring of temperature and active species at the single catalyst particle level

An era of circularity requires robust and flexible catalysts and reactors. We need profound knowledge of catalytic surface reactions on the local scale (i.e., angstrom–nanometer), whereas the react...

Operando Nanoscale Sensors in Catalysis: All Eyes on Catalyst Particles.

Raman spectroscopy is known as a powerful technique for solid catalyst characterization as it provides vibrational fingerprints of (metal) oxides, reactants, and products. It can even become a strong surface-sensitive technique by implementing shell-isolated surface-enhanced Raman spectroscopy (SHINERS). Au@TiO2 and Au@SiO2 shell-isolated nanoparticles (SHINs) of various sizes were therefore prepared for the purpose of studying heterogeneous catalysis and the effect of metal oxide coating. Both SiO2 - and TiO2 -SHINs are effective SHINERS substrates and thermally stable up to 400 °C. Nano-sized Ru and Rh hydrogenation catalysts were assembled over the SHINs by wet impregnation of aqueous RuCl3 and RhCl3 . The substrates were implemented to study CO adsorption and hydrogenation under in situ conditions at various temperatures to illustrate the differences between catalysts and shell materials with SHINERS. This work demonstrates the potential of SHINS for in situ characterization studies in a wide range of catalytic reactions.

https://chemistry-europe.onlinelibrary.wiley.com/doi/pdf/10.1002/chem.201704370

Thomas Hartman

Papers

Surface- and Tip-Enhanced Raman Spectroscopy in Catalysis

Sub-second Time-resolved Surface Enhanced Raman Spectroscopy Reveals Dynamic CO Intermediates during Electrochemical CO2 Reduction on Copper.

Operando monitoring of temperature and active species at the single catalyst particle level

Operando Nanoscale Sensors in Catalysis: All Eyes on Catalyst Particles.

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