The influence of surface proximity on photoswitching activity of stilbene-functionalized N-heterocyclic carbene monolayers

N-Heterocyclic Carbene Monolayers on Metal-Oxide Films: Correlations between Adsorption Mode and Surface Functionality

N-Heterocyclic carbene (NHC) ligands have been self-assembled on various metal and semimetal surfaces, creating a covalent bond with surface metal atoms that led to high thermal and chemical stability of the self-assembled monolayer. This study explores the self-assembly of NHCs on metal-oxide films (CuOx, FeOx, and TiOx) and reveals that the properties of these metal-oxide substrates play a pivotal role in dictating the adsorption behavior of NHCs, influencing the decomposition route of the monolayer and its impact on work function values. While the attachment of NHCs onto CuOx is via coordination with surface oxygen atoms, NHCs interact with TiOx through coordination with surface metal atoms and with FeOx via coordination with both metal and oxygen surface atoms. These distinct binding modes arise due to variances in the electronic properties of the metal atoms within the investigated metal-oxide films. Contact angle and ultraviolet photoelectron spectroscopy measurements have shown a significantly higher impact of F-NHC adsorption on CuOx than on TiOx and FeOx , correlated to a preferred, averaged upright orientation of F-NHC on CuOx.

Controllable Assembly of Cu2+ and Chlorin E6 for H2 S-Activatable Recognition of Bacterial Infection and Enhanced Antibacterial Therapy

Antibacterial photodynamic therapy (APDT) has emerged as one of the intriguing strategies to combat bacterial resistance. However, the antibacterial efficacy of APDT is found to be severely impacted by the hydrogen sulfide (H2 S)-overproduced bacterial infection microenvironment. Herein, a multifunctional APDT platform is developed by assembling Cu2+ and chlorin e6 (Ce6), which exhibits unique H2 S-activatable fluorescence (FL) and antibacterial features. Noteworthily, the assembly conditions are crucial for achievement of Cu-Ce6 nanoassemblies (NAs) with the on-demand responsive properties. The quenched FL and photosensitization of Cu-Ce6 NAs can be selectively activated by the overexpressed H2 S in infected area, enabling specific recognition of bacterial infection and localized antibacterial therapy with minimized side effects. Significantly, amplified oxidative stress is achieved owning to the effective consumption of H2 S by Cu2+ in the NAs, leading to an enhanced APDT. The antibacterial mechanisms including broad-spectrum APDT activity of released Ce6, inherent sterilization effects of produced copper polysulfides and the accompanying disturbance of bacterial sulphide metabolism are further identified. This study may pave a new avenue for the rational design of intelligent APDT platform using minimalist biological building units and thus facilitating the clinical translation of nano-antibacterial agents.

N-heterocyclic carbene adsorption states on Pt(111) and Ru(0001)

N-heterocyclic carbene ligands (NHCs) are increasingly used to tune the properties of metal surfaces. The generally greater chemical and thermal robustness of NHCs on gold, as compared to thiolate surface ligands, underscores their potential for a range of applications. While much is now known about the adsorption geometry, overlayer structure, dynamics, and stability of NHCs on coinage elements, especially gold and copper, much less is known about their interaction with the surfaces of Pt-group metals, despite the importance of such metals in catalysis and electrochemistry. In this study, reflection absorption infrared spectroscopy (RAIRS) is used to probe the structure of benzimidazolylidene NHC ligands on Pt(111) and Ru(0001). The experiments exploit the intense absorption peaks of a CF3 substituent on the phenyl ring of the NHC backbone to provide unprecedented insight into adsorption geometry and chemical stability. The results also permit comparison with literature data for NHC ligands on Au(111) and to DFT predictions for NHCs on Pt(111) and Ru(0001), thereby greatly extending the known surface chemistry of NHCs and providing much needed molecular information for the design of metal-organic hybrid materials involving strongly reactive metals.

Selective Deposition of N-Heterocyclic Carbene Monolayers on Designated Au Microelectrodes within an Electrode Array

The incorporation of organic self-assembled monolayers (SAMs) in microelectronic devices requires precise spatial control over the self-assembly process. In this work, selective deposition of N-heterocyclic carbenes (NHCs) on specific electrodes within a two-microelectrode array is achieved by using pulsed electrodeposition. Spectroscopic analysis of the NHC-coated electrode arrays reveals that each electrode is selectively coated with a designated NHC. The impact of NHC monolayers on the electrodes' work function is quantified using Kelvin probe force microscopy. These measurements demonstrate that the work function values of each electrode can be independently tuned by the adsorption of a specific NHC. The presented deposition method enables to selectively coat designated microelectrodes in an electrode array with chosen NHC monolayers for tuning their chemical and electronic functionality.

Self-Assembled Monolayers of N-Heterocyclic Olefins on Au(111)

Self-assembled monolayers (SAMs) of N-heterocyclic olefins (NHOs) have been prepared on Au(111) and their thermal stability, adsorption geometry, and molecular order were characterized by X-ray photoelectron spectroscopy, polarized X-ray absorption spectroscopy, scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. The strong σ-bond character of NHO anchoring to Au induced high geometrical flexibility that enabled a flat-lying adsorption geometry via coordination to a gold adatom. The flat-lying adsorption geometry was utilized to further increase the surface interaction of the NHO monolayer by backbone functionalization with methyl groups that induced high thermal stability and a large impact on work-function values, which outperformed that of N-heterocyclic carbenes. STM measurements, supported by DFT modeling, identified that the NHOs were self-assembled in dimers, trimers, and tetramers constructed of two, three, and four complexes of NHO-Au-adatom. This self-assembly pattern was correlated to strong NHO-Au interactions and steric hindrance between adsorbates, demonstrating the crucial influence of the carbon-metal σ-bond on monolayer properties.

Biocatalysis versus Molecular Recognition in Sialoside-Selective Neuraminidase Biosensing

Sialic acid recognition and hydrolysis are essential parts of cellular function and pathogen infectivity. Neuraminidases are enzymes that detach sialic acid from sialosides, and their inhibition is a prime target for viral infection treatment. The connectivity and type of sialic acid influence the recognition and hydrolysis activity of the many different neuraminidases. The common strategies to evaluate neuraminidase activity, recognition, and inhibition rely on extensive labeling and require a large amount of sialylated glycans. The above limitations make the effort of finding viral inhibitors extremely difficult. We used synthetic sialylated glycans and developed a label-free electrochemical method to show that sialoside structural features lead to selective neuraminidase biosensing. We compared Neu5Ac to Neu5Gc sialosides to evaluate the organism-dependent neuraminidase selectivity-sensitivity relationship. We demonstrated that the type of surface and the glycan monolayer density direct the response to either binding or enzymatic activity. We proved that while the hydrophobic glassy carbon surface increases the interaction with the enzyme hydrophobic interface, the negatively charged interface of the lipoic acid monolayer on gold repels the protein and enables biocatalysis. We showed that the sialoside monolayers can serve as tools to evaluate the inhibition of neuraminidases both by biocatalysis and molecular recognition.

N-Heterocyclic Carbene Based Nanolayer for Copper Film Oxidation Mitigation

The wide use of copper is limited by its rapid oxidation. Main oxidation mitigation approaches involve alloying or surface passivation technologies. However, surface alloying often modifies the physical properties of copper, while surface passivation is characterized by limited thermal and chemical stability. Herein, we demonstrate an electrochemical approach for surface‐anchoring of an N‐heterocyclic carbene (NHC) nanolayer on a copper electrode by electro‐deposition of alkyne‐functionalized imidazolium cations. Water reduction reaction generated a high concentration of hydroxide ions that induced deprotonation of imidazolium cations and self‐assembly of NHCs on the copper electrode. In addition, alkyne group deprotonation enabled on‐surface polymerization by coupling surface‐anchored and solvated NHCs, which resulted in a 2 nm thick NHC‐nanolayer. Copper film coated with a NHC‐nanolayer demonstrated high oxidation resistance at elevated temperatures and under alkaline conditions.