Plasmon-induced optical control over dithionite-mediated chemical redox reactions

Advances in fundamentals and application of plasmon-assisted CO2 photoreduction

Artificial photosynthesis of hydrocarbons from carbon dioxide (CO2) has the potential to provide renewable fuels at the scale needed to meet global decarbonization targets. However, CO2 is a notoriously inert molecule and converting it to energy dense hydrocarbons is a complex, multistep process, which can proceed through several intermediates. Recently, the ability of plasmonic nanoparticles to steer the reaction down specific pathways and enhance both reaction rate and selectivity has garnered significant attention due to its potential for sustainable energy production and environmental mitigation. The plasmonic excitation of strong and confined optical near-fields, energetic hot carriers and localized heating can be harnessed to control or enhance chemical reaction pathways. However, despite many seminal contributions, the anticipated transformative impact of plasmonics in selective CO2 photocatalysis has yet to materialize in practical applications. This is due to the lack of a complete theoretical framework on the plasmonic action mechanisms, as well as the challenge of finding efficient materials with high scalability potential. In this review, we aim to provide a comprehensive and critical discussion on recent advancements in plasmon-enhanced CO2 photoreduction, highlighting emerging trends and challenges in this field. We delve into the fundamental principles of plasmonics, discussing the seminal works that led to ongoing debates on the reaction mechanism, and we introduce the most recent ab initio advances, which could help disentangle these effects. We then synthesize experimental advances and in situ measurements on plasmon CO2 photoreduction before concluding with our perspective and outlook on the field of plasmon-enhanced photocatalysis.

Tracking interfacial single-molecule pH and binding dynamics via vibrational spectroscopy

Understanding single-molecule chemical dynamics of surface ligands is of critical importance to reveal their individual pathways and, hence, roles in catalysis, which ensemble measurements cannot see. Here, we use a cascaded nano-optics approach that provides sufficient enhancement to enable direct tracking of chemical trajectories of single surface-bound molecules via vibrational spectroscopy. Atomic protrusions are laser-induced within plasmonic nanojunctions to concentrate light to atomic length scales, optically isolating individual molecules. By stabilizing these atomic sites, we unveil single-molecule deprotonation and binding dynamics under ambient conditions. High-speed field-enhanced spectroscopy allows us to monitor chemical switching of a single carboxylic group between three discrete states. Combining this with theoretical calculation identifies reversible proton transfer dynamics (yielding effective single-molecule pH) and switching between molecule-metal coordination states, where the exact chemical pathway depends on the intitial protonation state. These findings open new domains to explore interfacial single-molecule mechanisms and optical manipulation of their reaction pathways.

Thermal effects - an alternative mechanism for plasmon-assisted photocatalysis

Recent experiments claimed that the catalysis of reaction rates in numerous bond-dissociation reactions occurs via the decrease of activation barriers driven by non-equilibrium ("hot") electrons in illuminated plasmonic metal nanoparticles. Thus, these experiments identify plasmon-assisted photocatalysis as a promising path for enhancing the efficiency of various chemical reactions. Here, we argue that what appears to be photocatalysis is much more likely thermo-catalysis, driven by the well-known plasmon-enhanced ability of illuminated metallic nanoparticles to serve as heat sources. Specifically, we point to some of the most important papers in the field, and show that a simple theory of illumination-induced heating can explain the extracted experimental data to remarkable agreement, with minimal to no fit parameters. We further show that any small temperature difference between the photocatalysis experiment and a control experiment performed under external heating is effectively amplified by the exponential sensitivity of the reaction, and is very likely to be interpreted incorrectly as "hot" electron effects.

Classification of analytics, sensorics, and bioanalytics with polyelectrolyte multilayer capsules

Polyelectrolyte multilayer (PEM) capsules, constructed by LbL (layer-by-layer)-adsorbing polymers on sacrificial templates, have become important carriers due to multifunctionality of materials adsorbed on their surface or encapsulated into their interior. They have been also been used broadly used as analytical tools. Chronologically and traditionally, chemical analytics has been developed first, which has long been synonymous with all analytics. But it is not the only development. To the best of our knowledge, a summary of all advances including their classification is not available to date. Here, we classify analytics, sensorics, and biosensorics functionalities implemented with polyelectrolyte multilayer capsules and coated particles according to the respective stimuli and application areas. In this classification, three distinct categories are identified: (I) chemical analytics (pH; K⁺, Na⁺, and Pb²⁺ ion; oxygen; and hydrogen peroxide sensors and chemical sensing with surface-enhanced Raman scattering (SERS)); (II) physical sensorics (temperature, mechanical properties and forces, and osmotic pressure); and (III) biosensorics and bioanalytics (fluorescence, glucose, urea, and protease biosensing and theranostics). In addition to this classification, we discuss also principles of detection using the above-mentioned stimuli. These application areas are expected to grow further, but the classification provided here should help (a) to realize the wealth of already available analytical and bioanalytical tools developed with capsules using inputs of chemical, physical, and biological stimuli and (b) to position future developments in their respective fields according to employed stimuli and application areas. Graphical abstract.

Present and Future of Surface-Enhanced Raman Scattering

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.