Unmasking the Electrochemical Stability of N-Heterocyclic Carbene Monolayers on Gold.

N-heterocyclic carbene (NHC) monolayers are transforming electrocatalysis and biosensor design via their increased performance and stability. Despite their increasing use in electrochemical systems, the integrity of the NHC monolayer during voltage perturbations remains largely unknown. Herein, we deploy surface-enhanced Raman spectroscopy (SERS) to measure the stability of two model NHCs on gold in ambient conditions as a function of applied potential and under continuous voltammetric interrogation. Our results illustrate that NHC monolayers exhibit electrochemical stability over a wide voltage window (-1 V to 0.5 V vs Ag|AgCl), but they are found to degrade at strongly reducing (< -1 V) or oxidizing (>0.5 V) potentials. We also address NHC monolayer stability under continuous voltammetric interrogation between 0.2 V and -0.5 V, a commonly used voltage window for sensing, showing they are stable for up to 43 hours. However, we additionally find that modifications of the backbone NHC structure can lead to significantly shorter operational lifetimes. While these results highlight the potential of NHC architectures for electrode functionalization, they also reveal potential pitfalls that have not been fully appreciated in electrochemical applications of NHCs.

Giving Gold Wings: Ultrabright and Fragmentation Free Mass Spectrometry Reporters for Barcoding, Bioconjugation Monitoring, and Data Storage.

The widespread application of laser desorption/ionization mass spectrometry (LDI-MS) highlights the need for a bright and multiplexable labeling platform. While ligand-capped Au nanoparticles (AuNPs) have emerged as a promising LDI-MS contrast agent, the predominant thiol ligands suffer from low ion yields and extensive fragmentation. In this work, we develop a N-heterocyclic carbene (NHC) ligand platform that enhances AuNP LDI-MS performance. NHC scaffolds are tuned to generate barcoded AuNPs which, when benchmarked against thiol-AuNPs, are bright mass tags and form unfragmented ions in high yield. To illustrate the transformative potential of NHC ligands, the mass tags were employed in three orthogonal applications: monitoring a bioconjugation reaction, performing multiplexed imaging, and storing and reading encoded information. These results demonstrate that NHC-nanoparticle systems are an ideal platform for LDI-MS and greatly broaden the scope of nanoparticle contrast agents.

Probing N-Heterocyclic Carbene Surfaces with Laser Desorption Ionization Mass Spectrometry.

The proliferation of N-heterocyclic carbene (NHC) self-assembled monolayers (SAMs) on gold surfaces stems from their exceptional stability compared to conventional thiol-SAMs. The prospect of biological applications for NHC-SAMs on gold shows the need for biocompatible techniques (e.g., large biomolecule detection and high throughput) that assesses SAM molecular composition. Herein, we demonstrate that laser desorption ionization mass spectrometry (LDI-MS) is a powerful and facile probe of NHC surface chemistry. LDI-MS of prototypical imidazole-NHC- and benzimidazole-NHC-functionalized AuNPs yields exclusively [NHC2Au]+ ions and not larger gold clusters. Employing benzimidazole-NHC isotopologues, we explore how monolayers pack on a single AuNP and the lability of the NHCs once ligated. Quantitative analysis of the homoleptic and heteroleptic [NHC2Au]+ ions is performed by comparing to a binomial model representative of a randomized monolayer. Lastly, the reduction of nitro-NHC-AuNPs to amine-NHC-AuNPs is tracked via LDI-MS signals, illustrating the ability of LDI-MS to probe postsynthetic modifications of the anchored NHCs, which is critical for current and future applications of NHC surfaces.

From Light to Dark: Dancing with Electrons in Colloidal 2D MoS2 Nanosheets.

Extending the lifetime of photogenerated electrons in semiconductor systems is an important criterion for the conversion of light into storable energy. We have now succeeded in storing electrons in a photoirradiated colloidal molybdenum disulfide (MoS2) suspension, showcasing its unique reversible photoresponsive behavior. The dampened A and B excitonic peaks indicate the accumulation of photogenerated electrons and the minimization of interactions between MoS2 interlayers. The stored electrons were quantitatively extracted by titrating with a ferrocenium ion in the dark, giving ca. 0.2 electrons per MoS2 formula unit. The emergence of the photoinduced A1g* Raman mode and the decrease in zeta potential after irradiation suggest intercalation of counterions to maintain overall charge balance upon electron storage. These results provide insights into the mechanism of photogenerated electron storage in 2D materials and pave the way for the potential application of colloidal 2D materials in electron storage.

Stability of N-Heterocyclic Carbene Monolayers under Continuous Voltammetric Interrogation.

N-Heterocyclic carbenes (NHCs) are promising monolayer-forming ligands that can overcome limitations of thiol-based monolayers in terms of stability, surface functionality, and reactivity across a variety of transition-metal surfaces. Recent publications have reported the ability of NHCs to support biomolecular receptors on gold substrates for sensing applications and improved tolerance to prolonged biofluid exposure relative to thiols. However, important questions remain regarding the stability of these monolayers when subjected to voltage perturbations, which is needed for applications with electrochemical platforms. Here, we investigate the ability of two NHCs, 1,3-diisopropylbenzimidazole and 5-(ethoxycarbonyl)-1,3-diisopropylbenzimidazole, to form monolayers via self-assembly from methanolic solutions of their trifluoromethanesulfonate salts. We compare the electrochemical behavior of the resulting monolayers relative to that of benchmark mercaptohexanol monolayers in phosphate-buffered saline. Within the -0.15 to 0.25 V vs Ag|AgCl voltage window, NHC monolayers are stable on gold surfaces, wherein they electrochemically perform like thiol-based monolayers and undergo similar reorganization kinetics, displaying long-term stability under incubation in buffered media and under continuous voltammetric interrogation. At negative voltages, NHC monolayers cathodically desorb from the electrode surface at lower bias (-0.1 V) than thiol-based monolayers (-0.5 V). At voltages more positive than 0.25 V, NHC monolayers anodically desorb from electrode surfaces at similar voltages to thiol-based monolayers. These results highlight new limitations to NHC monolayer stability imposed by electrochemical interrogation of the underlying gold electrodes. Our results serve as a framework for future optimization of NHC monolayers on gold for electrochemical applications, as well as structure-functionality studies of NHCs on gold.

Plasmonic Gold Trimers and Dimers with Air-Filled Nanogaps.

The subwavelength confinement of light energy in the nanogaps formed between adjacent plasmonic nanostructures provides the foundational basis for nanophotonic applications. Within this realm, air-filled nanogaps are of central importance because they present a cavity where application-specific nanoscale objects can reside. When forming such configurations on substrate surfaces, there is an inherent difficulty in that the most technologically relevant nanogap widths require closely spaced nanostructures separated by distances that are inaccessible through standard electron-beam lithography techniques. Herein, we demonstrate an assembly route for the fabrication of aligned plasmonic gold trimers with air-filled vertical nanogaps having widths that are defined with spatial controls that exceed those of lithographic processes. The devised procedure uses a sacrificial oxide layer to define the nanogap, a glancing angle deposition to impose a directionality on trimer formation, and a sacrificial antimony layer whose sublimation regulates the gold assembly process. By further implementing a benchtop nanoimprint lithography process and a glancing angle ion milling procedure as additional controls over the assembly, it is possible to deterministically position trimers in periodic arrays and extend the assembly process to dimer formation. The optical response of the structures, which is characterized using polarization-dependent spectroscopy, surface-enhanced Raman scattering, and refractive index sensitivity measurements, shows properties that are consistent with simulation. This work, hence, forwards the wafer-based processing techniques needed to form air-filled nanogaps and place plasmonic energy at site-specific locations.

1,3,2-Diazaborole-derived carbene complexes of boron.

Reaction of 2-bromo-1,3,2-diazaborole (1) with excess BBr3 induces 1,2-hydrogen migration, giving 1,3,2-diazaborole-derived carbene complexes of boron bromide (2). Compound 2 exists in a dynamic solution equilibrium with 1. The 1H NMR study shows that the equilibrium lies to the right side of the dissociation reaction of 2. Parallel reaction of 1 with excess BI3 gives the corresponding 1,3,2-diazaborole-derived carbene boron iodide complex (3). Notably, in contrast to 2, the dissociation reaction of 3 largely lies to the left side, favouring the formation of 3. The dynamic solution equilibrium behaviours of 2 and 3 are probed by both experimental and theoretical methods.