Role of Valence Band States and Plasmonic Enhancement in Electron-Transfer-Induced Transformation of Nitrothiophenol

Plasmon-driven substitution of 4-mercaptophenylboronic acid to 4-nitrothiophenol monitored by surface-enhanced Raman spectroscopy

Plasmon-driven reactions on plasmonic nanoparticles (NPs) occur under significantly different conditions from those of classical organic synthesis and provide a promising pathway for enhancing the efficiency of various chemical processes. However, these reactions can also have undesirable effects, such as 4-mercaptophenylboronic acid (MPBA) deboronation. MPBA chemisorbs well to Ag NPs through its thiol group and can subsequently bind to diols, enabling the detection of various biological structures by surface-enhanced Raman scattering (SERS), but not upon its deboronation. To avoid this reaction, we investigated the experimental conditions of MPBA deboronation on Ag NPs by SERS. Our results showed that the level of deboronation strongly depends on both the morphology of the system and the excitation laser wavelength and power. In addition, we detected not only the expected products, namely thiophenol and biphenyl-4,4-dithiol, but also 4-nitrothiophenol (NTP). The crucial reagent for NTP formation was an oxidation product of hydroxylamine hydrochloride, the reduction agent used in Ag NP synthesis. Ultimately, this reaction was replicated by adding NaNO2 to the system, and its progress was monitored as a function of the laser power, thereby identifying a new reaction of plasmon-driven -B(OH)2 substitution for -NO2.

Condensed Matter Systems Exposed to Radiation: Multiscale Theory, Simulations, and Experiment

This roadmap reviews the new, highly interdisciplinary research field studying the behavior of condensed matter systems exposed to radiation. The Review highlights several recent advances in the field and provides a roadmap for the development of the field over the next decade. Condensed matter systems exposed to radiation can be inorganic, organic, or biological, finite or infinite, composed of different molecular species or materials, exist in different phases, and operate under different thermodynamic conditions. Many of the key phenomena related to the behavior of irradiated systems are very similar and can be understood based on the same fundamental theoretical principles and computational approaches. The multiscale nature of such phenomena requires the quantitative description of the radiation-induced effects occurring at different spatial and temporal scales, ranging from the atomic to the macroscopic, and the interlinks between such descriptions. The multiscale nature of the effects and the similarity of their manifestation in systems of different origins necessarily bring together different disciplines, such as physics, chemistry, biology, materials science, nanoscience, and biomedical research, demonstrating the numerous interlinks and commonalities between them. This research field is highly relevant to many novel and emerging technologies and medical applications.

Surface-enhanced Raman scattering spectroscopy monitoring and degradation of organic pollutants using a novel nanowire

A multifunctional Ag/AlOOH nanowires (ANW) composite substrate was constructed, which not only accomplishes highly sensitive detection of organic dye molecules, but also has excellent performance in the degradation of pollutants. The ANW in the Ag/ANW substrate possesses a high aspect ratio, which extends the distribution area of Ag and enables a large number of hot spots on the active substrate. Additionally, due to the abundant OH groups on the ANW, there is an increased number of anchor sites for adsorbed metal ions in the Ag/ANW compound, thus contributing to the enhancement and degradation of molecules. Moreover, the constructed multifunctional Ag/ANW nanocomplexes also show great promise for practical applications, providing a reference for the detection and degradation of contaminants.

Ligand-mediated electron transport channels enhance photocatalytic activity of plasmonic nanoparticles

Photoexcitation of noble metal nanoparticles creates surface plasmons which further decay to form energetic charge carriers. These charge carriers can initiate and/or accelerate various chemical processes at nanoparticle surfaces, although the efficiency of such processes remains low as a large fraction of these carriers recombine before they can reach the reaction sites. Thus efficient utilization of these charge carriers requires designing nanostructures that promote the separation of charges and their transport toward the reaction sites. Here we demonstrate that covalently bound surface-coating ligands with suitable orbital alignment can provide electron transport channels boosting hot electron extraction from a gold nanostructure leading to a huge enhancement in the rate of hydrogen evolution reaction (HER) under NIR excitation. A (p)Br-Ph-SH substituted gold nanoprism (AuTP) substrate produced ∼4500 fold more hydrogen compared to a pristine AuTP substrate under 808 nm excitation. Further experimental and theoretical studies on a series of substituted benzene-thiol bound AuTP substrates showed that the extent of the ligand-mediated HER enhancement depends not only on the polarity of the ligand but on the interfacial orbitals interactions.

Molecular Cocatalyst of p-Mercaptophenylboronic Acid Boosts the Plasmon-Mediated Reduction of p-Nitrothiophenol

Localized surface plasmon resonance (LSPR) has been demonstrated to be highly effective in the initialization or acceleration of chemical reactions because of its unique optical properties. However, because of the ultrashort lifetime (fs to ps) of plasmon-generated hot carriers, the potential of LSPR in photochemical reactions has not been fully exploited. Herein, we demonstrate an acceleration of the plasmon-mediated reduction of p-nitrothiophenol (PNTP) molecules on the surface of silver nanoparticles (AgNPs) with in situ Raman spectroscopy. p-Mercaptophenylboronic acid (PMPBA) molecules coadsorbed on AgNP surfaces act as a molecular cocatalyst in the plasmon-mediated reaction, resulting in a boosting of the PNTP reduction. This boosting is attributed to the improved transfer and separation of the plasmon-generated hot carriers at the interface of the AgNPs and coadsorbed PMPBA molecules. Our finding provides a highly simple, cost-effective, and highly effective strategy to promote plasmonic photochemistry by introducing a molecular cocatalyst, and this strategy can be extended to promote various plasmon-mediated reactions.

Plasmonic reactivity of halogen thiophenols on gold nanoparticles studied by SERS and XPS

Localized surface plasmon resonances on noble metal nanoparticles (NPs) can efficiently drive reactions of adsorbed ligand molecules and provide versatile opportunities in chemical synthesis. The driving forces of these reactions are typically elevated temperatures, hot charge carriers or enhanced electric fields. In the present work the dehalogenation of halogenated thiophenols on the surface of AuNPs has been studied by surface enhanced Raman scattering (SERS) as a function of the photon energy to track the kinetics and identify reaction products. Reaction rates are found to be surprisingly similar for the different halothiophenols studied here, although the bond dissociation energies of the C-X bonds differ significantly. Complementary information about the electronic properties at the AuNP surface, namely work-function and valence band states, have been determined by X-ray photoelectron spectroscopy (XPS) of isolated AuNPs in the gas-phase. In this way, it is revealed how the electronic properties are altered by the adsorption of the ligand molecules, and we conclude that the reaction rates are mainly determined by the plasmonic properties of the AuNPs. SERS spectra reveal differences in the reaction product formation for the different halogen species and on this basis the possible reaction mechanisms are discussed to approach an understanding of opportunities and limitations in the design of catalytical systems with plasmonic NPs.

Microscopic Understanding of Reaction Rates Observed in Plasmon Chemistry of Nanoparticle-Ligand Systems

Surface-enhanced Raman scattering (SERS) is an effective and widely used technique to study chemical reactions induced or catalyzed by plasmonic substrates, since the experimental setup allows us to trigger and track the reaction simultaneously and identify the products. However, on substrates with plasmonic hotspots, the total signal mainly originates from these nanoscopic volumes with high reactivity and the information about the overall consumption remains obscure in SERS measurements. This has important implications; for example, the apparent reaction order in SERS measurements does not correlate with the real reaction order, whereas the apparent reaction rates are proportional to the real reaction rates as demonstrated by finite-difference time-domain (FDTD) simulations. We determined the electric field enhancement distribution of a gold nanoparticle (AuNP) monolayer and calculated the SERS intensities in light-driven reactions in an adsorbed self-assembled molecular monolayer on the AuNP surface. Accordingly, even if a high conversion is observed in SERS due to the high reactivity in the hotspots, most of the adsorbed molecules on the AuNP surface remain unreacted. The theoretical findings are compared with the hot-electron-induced dehalogenation of 4-bromothiophenol, indicating a time dependency of the hot-carrier concentration in plasmon-mediated reactions. To fit the kinetics of plasmon-mediated reactions in plasmonic hotspots, fractal-like kinetics are well suited to account for the inhomogeneity of reactive sites on the substrates, whereas also modified standard kinetics model allows equally well fits. The outcomes of this study are on the one hand essential to derive a mechanistic understanding of reactions on plasmonic substrates by SERS measurements and on the other hand to drive plasmonic reactions with high local precision and facilitate the engineering of chemistry on a nanoscale.

The electronic structure of the metal-organic interface of isolated ligand coated gold nanoparticles

Light induced electron transfer reactions of molecules on the surface of noble metal nanoparticles (NPs) depend significantly on the electronic properties of the metal-organic interface. Hybridized metal-molecule states and dipoles at the interface alter the work function and facilitate or hinder electron transfer between the NPs and ligand. X-ray photoelectron spectroscopy (XPS) measurements of isolated AuNPs coated with thiolated ligands in a vacuum have been performed as a function of photon energy, and the depth dependent information of the metal-organic interface has been obtained. The role of surface dipoles in the XPS measurements of isolated ligand coated NPs is discussed and the binding energy of the Au 4f states is shifted by around 0.8 eV in the outer atomic layers of 4-nitrothiophenol coated AuNPs, facilitating electron transport towards the molecules. Moreover, the influence of the interface dipole depends significantly on the adsorbed ligand molecules. The present study paves the way towards the engineering of the electronic properties of the nanoparticle surface, which is of utmost importance for the application of plasmonic nanoparticles in the fields of heterogeneous catalysis and solar energy conversion.

Spatial Separation of Plasmonic Hot-Electron Generation and a Hydrodehalogenation Reaction Center Using a DNA Wire

Using hot charge carriers far from a plasmonic nanoparticle surface is very attractive for many applications in catalysis and nanomedicine and will lead to a better understanding of plasmon-induced processes, such as hot-charge-carrier- or heat-driven chemical reactions. Herein we show that DNA is able to transfer hot electrons generated by a silver nanoparticle over several nanometers to drive a chemical reaction in a molecule nonadsorbed on the surface. For this we use 8-bromo-adenosine introduced in different positions within a double-stranded DNA oligonucleotide. The DNA is also used to assemble the nanoparticles into nanoparticles ensembles enabling the use of surface-enhanced Raman scattering to track the decomposition reaction. To prove the DNA-mediated transfer, the probe molecule was insulated from the source of charge carriers, which hindered the reaction. The results indicate that DNA can be used to study the transfer of hot electrons and the mechanisms of advanced plasmonic catalysts.

Electron attachment to microhydrated 4-nitro- and 4-bromo-thiophenol

We investigate the effect of microhydration on electron attachment to thiophenols with halogen (Br) and nitro (NO₂) functional groups in the para position. We focus on the formation of anions upon the attachment of low-energy electrons with energies below 8 eV to heterogeneous clusters of the thiophenols with water. For nitro-thiophenol (NTP), the primary reaction channel observed is the associative electron attachment, irrespective of the microhydration. On the other hand, bromothiophenol (BTP) fragments significantly upon the electron attachment, producing Br^- and (BTP-H)^- anions. Microhydration suppresses fragmentation of both molecules, however in bromothiophenol, the Br^- channel remains intense and Br(H₂O)_n^- hydrated fragment clusters are observed. The results are supported by the reaction energetics obtained from ab initio calculations. Different dissociation dynamics of NTP and BTP can be related to different products of their plasmon induced reactions on Au nanoparticles. Computational modeling of the simplified BTP(H₂O) system indicates that the electron attachment products reflect the structure of neutral precursor clusters - the anion dissociation dynamics is controlled by the hydration site.

Kinetics and Mechanism of Plasmon-Driven Dehalogenation Reaction of Brominated Purine Nucleobases on Ag and Au

Plasmon-driven photocatalysis is an emerging and promising application of noble metal nanoparticles (NPs). An understanding of the fundamental aspects of plasmon interaction with molecules and factors controlling their reaction rate in a heterogeneous system is of high importance. Therefore, the dehalogenation kinetics of 8-bromoguanine (BrGua) and 8-bromoadenine (BrAde) on aggregated surfaces of silver (Ag) and gold (Au) NPs have been studied to understand the reaction kinetics and the underlying reaction mechanism prevalent in heterogeneous reaction systems induced by plasmons monitored by surface enhanced Raman scattering (SERS). We conclude that the time-average constant concentration of hot electrons and the time scale of dissociation of transient negative ions (TNI) are crucial in defining the reaction rate law based on a proposed kinetic model. An overall higher reaction rate of dehalogenation is observed on Ag compared with Au, which is explained by the favorable hot-hole scavenging by the reaction product and the byproduct. We therefore arrive at the conclusion that insufficient hole deactivation could retard the reaction rate significantly, marking itself as rate-determining step for the overall reaction. The wavelength dependency of the reaction rate normalized to absorbed optical power indicates the nonthermal nature of the plasmon-driven reaction. The study therefore lays a general approach toward understanding the kinetics and reaction mechanism of a plasmon-driven reaction in a heterogeneous system, and furthermore, it leads to a better understanding of the reactivity of brominated purine derivatives on Ag and Au, which could in the future be exploited, for example, in plasmon-assisted cancer therapy.

Decoding the kinetic limitations of plasmon catalysis: the case of 4-nitrothiophenol dimerization

Plasmon-mediated chemistry presents an intriguing new approach to photocatalysis. However, the reaction enhancement mechanism is not well understood. In particular, the relative importance of plasmon-generated hot charges and photoheating is strongly debated. In this article, we evaluate the influence of microscopic photoheating on the kinetics of a model plasmon-catalyzed reaction: the light-induced 4-nitrothiophenol (4NTP) to 4,4'-dimercaptoazobenzene (DMAB) dimerization. Direct measurement of the reaction temperature by nanoparticle Raman-thermometry demonstrated that the thermal effect plays a dominant role in the kinetic limitations of this multistep reaction. At the same time, no reaction is possible by dark heating to the same temperature. This shows that plasmon nanoparticles have the unique ability to enhance several steps of complex tandem reactions simultaneously. These results provide insight into the role of hot electron and thermal effects in plasmonic catalysis of complex organic reactions, which is highly important for the ongoing development of plasmon based photosynthesis.

Kinetics of molecular decomposition under irradiation of gold nanoparticles with nanosecond laser pulses-A 5-Bromouracil case study

Laser illuminated gold nanoparticles (AuNPs) efficiently absorb light and heat up the surrounding medium, leading to versatile applications ranging from plasmonic catalysis to cancer photothermal therapy. Therefore, an in-depth understanding of the thermal, optical, and electron induced reaction pathways is required. Here, the electrophilic DNA nucleobase analog 5-Bromouracil (BrU) has been used as a model compound to study its decomposition in the vicinity of AuNPs illuminated with intense ns laser pulses under various conditions. The plasmonic response of the AuNPs and the concentration of BrU and resulting photoproducts have been tracked by ultraviolet and visible (UV-Vis) spectroscopy as a function of the irradiation time. A kinetic model has been developed to determine the reaction rates of two parallel fragmentation pathways of BrU, and their dependency on laser fluence and adsorption on the AuNP have been evaluated. In addition, the size and the electric field enhancement of the decomposed AuNPs have been determined by atomic force microscopy and finite domain time difference calculations, respectively. A minor influence of the direct photoreaction and a strong effect of the heating of the AuNPs have been revealed. However, due to the size reduction of the irradiated AuNPs, a trade-off between laser fluence and plasmonic response of the AuNPs has been observed. Hence, the decomposition of the AuNPs might be limiting the achievable temperatures under irradiation with several laser pulses. These findings need to be considered for an efficient design of catalytic plasmonic systems.

Kinetic and Mechanistic Investigation of the Photocatalyzed Surface Reduction of 4-Nitrothiophenol Observed on a Silver Plasmonic Film via Surface-Enhanced Raman Scattering

Hot electrons generated by photoinduced plasmon decay from a plasmonic metal surface can reduce 4-nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP). Compared to the reduction with a reducing agent such as sodium borohydride, surface-enhanced Raman scattering (SERS) measurements were performed here to elucidate the complex molecular mechanism of the reduction in the presence of halide ions and hydrogen ions. The SERS measurements were performed using a simply prepared silver plasmonic film (AgPF), which enables monitoring of the reaction under different conditions at a solid-liquid surface and eliminates the need for the use of a reducing agent. As the concentration of H⁺ and Cl^- could be controlled, the observation of the reaction under a systematic set of conditions was possible. Based on the kinetic traces of the intermediates, a reaction mechanism for the 4-NTP to 4-ATP reduction is suggested. Rate constants for the individual reactions are presented that fit the measured kinetic traces, and the role of hydrogen in each reaction step is characterized. This work provides clarification on the molecular transformation directly using protons as the hydrogen source and demonstrates an effective method of applying a simple and low-cost silver surface catalyst for SERS studies. Moreover, the monitoring of Cl^--concentration-dependent spectra provides insight into the hot-electron conversion process during the photoreduction and strongly supports the formation of AgCl for the activation of H⁺.