Water-Soluble N-Heterocyclic Carbene-Protected Gold Nanoparticles: Size-Controlled Synthesis, Stability, and Optical Properties

An organometallic hybrid antibiotic of metronidazole with a Gold(I) N-Heterocyclic Carbene overcomes metronidazole resistance in Clostridioides difficile

Antimicrobial resistance (AMR) has been emerging as a major global health threat and calls for the development of novel drug candidates. Metal complexes have been demonstrating high efficiency as antibacterial agents that differ substantially from the established types of antibiotics in their chemical structures and their mechanism of action. One strategy to exploit this potential is the design of metal-based hybrid organometallics that consist of an established antibiotic and a metal-based warhead that contributes an additional mechanism of action different from that of the parent antibiotic. In this communication, we describe the organometallic hybrid antibiotic 2c, in which the drug metronidazole is connected to a gold(I) N-heterocyclic carbene warhead that inhibits bacterial thioredoxin reductase (TrxR). Metronidazole can be used for the treatment with the obligatory anaerobic pathogen Clostridioides difficile (C. difficile), however, resistance to the drug hampers its clinical success. The gold organometallic conjugate 2c was an efficient inhibitor of TrxR and it was inactive or showed only minor effects against eucaryotic cells and bacteria grown under aerobic conditions. In contrast, a strong antibacterial effect was observed against both metronidazole-sensitive and -resistant strains of C. difficile. This report presents a proof-of-concept that the design of metal-based hybrid antibiotics can be a viable approach to efficiently tackle AMR.

A Resilient Platform for the Discrete Functionalization of Gold Surfaces Based on N-Heterocyclic Carbene Self-Assembled Monolayers

We describe the synthesis and characterization of a versatile platform for gold functionalization, based on self-assembled monolayers (SAMs) of distal-pyridine-functionalized N-heterocyclic carbenes (NHC) derived from bis(NHC) Au(I) complexes. The SAMs are characterized using polarization-modulation infrared reflectance-absorption spectroscopy, surface-enhanced Raman spectroscopy, and X-ray photoelectron spectroscopy. The binding mode is examined computationally using density functional theory, including calculations of vibrational spectra and direct comparisons to the experimental spectroscopic signatures of the monolayers. Our joint computational and experimental analyses provide structural information about the SAM binding geometries under ambient conditions. Additionally, we examine the reactivity of the pyridine-functionalized SAMs toward H2SO4 and W(CO)5(THF) and verify the preservation of the introduced functionality at the interface. Our results demonstrate the versatility of N-heterocyclic carbenes as robust platforms for on-surface acid-base and ligand exchange reactions.

Synthesis of N-heterocyclic carbene gold(I) complexes from the marine betaine 1,3-dimethylimidazolium-4-carboxylate

The marine natural product norzooanemonin (1,3-dimethylimidazolium-4-carboxylate) has been used to prepare a series of carboxyl- or carboxylate-functionalized N-heterocyclic carbene (NHC) gold(I) complexes from [(Me2S)AuCl] in the presence of potassium carbonate. The potential of the resulting mono- and dicarbene complexes to act as cytotoxic or antibacterial drugs was investigated.

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.

Sandwich-Kernelled AgCu Nanoclusters with Golden Ratio Geometry and Promising Photothermal Efficiency

Metal nanoclusters have recently attracted extensive interest from the scientific community. However, unlike carbon-based materials and metal nanocrystals, they rarely exhibit a sheet kernel structure, probably owing to the instability caused by the high exposure of metal atoms (particularly in the relatively less noble Ag or Cu nanoclusters) in such a structure. Herein, we synthesized a novel AgCu nanocluster with a sandwich-like kernel (diameter≈0.9 nm and length≈0.25 nm) by introducing the furfuryl mercaptan ligand (FUR) and the alloying strategy. Interestingly, the kernel consists of a centered silver atom and two planar Ag10 pentacle units with completely mirrored symmetry after a rotation of 36 degrees. The two Ag10 pentacles and some extended structures show an unreported golden ratio geometry, and the two inner five-membered rings and the centered Ag atom form an unanticipated full-metal ferrocene-like structure. The featured kernel structure causes the dominant radial direction transition of excitation electrons, as determined via time-dependent density functional theory calculations, which affords the protruding absorption at 612 nm and contributes to the promising photothermal conversion efficiency of 67.6 % of the as-obtained nanocluster, having important implications for structure-property correlation and the development of nanocluser-based photothermal materials.

N-Heterocyclic Carbene Boranes: Dual Reagents for the Synthesis of Gold Nanoparticles

N-Heterocyclic carbenes (NHCs) have drawn considerable interest in the field of nanomaterials chemistry as highly stabilizing ligands enabling the formation of strong and covalent carbon-metal bonds. Applied to gold nanoparticles synthesis, the most common strategy consists of the reduction of a preformed NHC-AuI complex with a large excess of a reducing agent that makes the particle size difficult to control. In this paper, we report the straightforward synthesis of NHC-coated gold nanoparticles (NHC-AuNPs) by treating a commercially available gold(I) precursor with an easy-to-synthesize NHC-BH3 reagent. The latter acts as both the reducing agent and the source of surface ligands operating under mild conditions. Mechanistic studies including NMR spectroscopy and mass spectrometry demonstrate that the reduction of gold(I) generates NHC-BH2 Cl as a by-product. This strategy gives efficient control over the nucleation and growth of gold particles by varying the NHC-borane/gold(I) ratio, allowing unparalleled particle size variation over the range of 4.9±0.9 to 10.0±2.7 nm. Our strategy also allows an unprecedented precise and controlled seeded growth of gold nanoparticles. In addition, the as-prepared NHC-AuNPs exhibit narrow size distributions without the need for extensive purification or size-selectivity techniques, and are stable over months.

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.

Monomeric and Polymeric Mesoionic N-Heterocyclic Carbene-Tethered Silver Nanoparticles: Synthesis, Stability, and Catalytic Activity

In recent years, N-heterocyclic carbenes (NHCs) have garnered significant attention as promising alternatives to thiols to stabilize metallic nanoparticles and planar surfaces. While most studies thus far have focused on NHC-functionalized gold nanoparticles (AuNPs), as an ideal platform to investigate the role of NHCs in stabilizing such nanoparticles, their ability to protect more unstable coinage metal nanoparticles, such as silver nanoparticles (AgNPs), has been largely overlooked. This is despite the fact that AgNPs possess a much more sensitive optical response that, upon their enhanced stability, can broaden their scope of application in various fields, including nanomedicine and catalysis. In this study, the synthesis and use of monomeric and polymeric mesoionic NHC-Ag(I) complexes as precursors to mono- and multidentate NHC-tethered AgNPs are reported. The polymeric analog was obtained by first synthesizing a polymer, containing 1,2,3-triazole repeat units, employing the copper-catalyzed alkyne-azide cycloaddition click polymerization of monomers containing diazide- and dialkyne functional groups. Subsequent quaternization of the triazole moieties and Ag insertion yielded the target NHC-Ag-containing polymer. Using this polymer as well as its monomeric analog as substrates, AgNPs with either catenated networks of NHCs or monomeric NHCs were fabricated by their reduction using borane-tert-butylamine complex. Our stability studies demonstrate that while monomeric NHCs impart some degree of stability to AgNPs, particularly at elevated temperatures in aqueous as well as organic medium, their polymeric analogs further enhance their stability in acidic environment (pH = 2) and against glutathione (3 mM), as an example of a biologically relevant thiol, in aqueous media. To highlight the application of these NHC-functionalized AgNPs in catalysis, we explore the aqueous phase reduction of methyl orange and 4-nitrophenol.

Coordination-Induced Trigger for Activity: N-Heterocyclic Carbene-Decorated Ceria Catalysts Incorporating Cr and Rh with Activity Induction by Surface Adsorption Site Control

A coordination-induced trigger for catalytic activity is proposed on an N-heterocyclic carbene (NHC)-decorated ceria catalyst incorporating Cr and Rh (ICy-r-Cr_0.19Rh_0.06CeO_z). ICy-r-Cr_0.19Rh_0.06CeO_z was prepared by grafting 1,3-dicyclohexylimidazol-2-ylidene (ICy) onto H₂-reduced Cr_0.19Rh_0.06CeO_z (r-Cr_0.19Rh_0.06CeO_z) surfaces, which went on to exhibit substantial catalytic activity for the 1,4-arylation of cyclohexenone with phenylboronic acid, whereas r-Cr_0.19Rh_0.06CeO_z without ICy was inactive. FT-IR, Rh K-edge XAFS, XPS, and photoluminescence spectroscopy showed that the ICy carbene-coordinated Rh nanoclusters were the key active species. The coordination-induced trigger for catalytic activity on the ICy-bearing Rh nanoclusters could not be attributed to electronic donation from ICy to the Rh nanoclusters. DFT calculations suggested that ICy controlled the adsorption sites of the phenyl group on the Rh nanocluster to promote the C-C bond formation of the phenyl group and cyclohexenone.

Trans-[bis(benzimidazol-2-ylidene)dichlorido]platinum(II) complexes with peculiar modes of action and activity against cisplatin-resistant cancer cells

Three series of cis- and trans-[bis(benzimidazol-2-ylidene)dichlorido]platinum(II) and cis-[(benzimidazol-2-ylidene)(DMSO)dichlorido]platinum(II) complexes were synthesised and screened for cytotoxicity against six human cancer cell lines. Depending on their N-alkyl and 5-alkoxycarbonyl substituents, two-digit nanomolar to single-digit micromolar IC₅₀ values against cancer cell lines intrinsically resistant to or ill-responding to cisplatin were reached by both cis- and trans-configured complexes. The stability of the complexes under aqueous biotest conditions was shown via ¹H and ¹⁹⁵Pt NMR monitoring to be dependent on their configuration and their N-substituents. Localisation studies employing click reactions with 1-alkyne- or cyclopropene-tagged derivatives revealed that the cis-complexes accumulated in the cell nuclei and the trans-complexes in the mitochondria. While the most active cis-complexes showed modes of action akin to those of cisplatin, the most active trans-complexes differed from cisplatin by much lower rates of cellular uptake and ROS production, and by their non-interaction with the cell cycle and the DNA of cancer cells. Thus, we identified structural key elements for the synthesis of optimised trans-configured NHC platinum(II) complexes with high activity also against cisplatin-refractory cancer cells.

Bottom-up Synthesis of Water-Soluble Gold Nanoparticles Stabilized by N-Heterocyclic Carbenes: From Structural Characterization to Applications

N-heterocyclic carbenes (NHCs) have become attractive ligands for functionalizing gold nanoparticle surfaces with applications ranging from catalysis to biomedicine. Despite their great potential, NHC stabilized gold colloids (NHC@AuNPs) are still scarcely explored and further efforts should be conducted to improve their design and functionalization. Here, the 'bottom-up' synthesis of two water-soluble gold nanoparticles (AuNP-1 and AuNP-2) stabilized by hydrophilic mono- and bidentate NHC ligands is reported together with their characterization by various spectroscopic and analytical methods. The NPs showed key differences likely to be due to the selected NHC ligand systems. Transmission electron microscopy (TEM) images showed small quasi-spherical and faceted NHC@AuNPs of similar particle size (ca. 2.3-2.6 nm) and narrow particle size distribution, but the colloids featured different ratios of Au(I)/Au(0) by X-ray photoelectron spectroscopy (XPS). Furthermore, the NHC@AuNPs were supported on titania and fully characterized. The new NPs were studied for their catalytic activity towards the reduction of nitrophenol substrates, the reduction of resazurin and for their photothermal efficiency. Initial results on their application in photothermal therapy (PTT) were obtained in human cancer cells in vitro. The aforementioned reactions represent important model reactions towards wastewater remediation, bioorthogonal transformations and cancer treatment.

Recent applications of nanoparticles in organic transformations

A variation in the size of metal nanoparticles leads to a difference in their properties. As the size of metal nanoparticles decreases, the surface area increases which leads to an increase in the reactivity of metal nanoparticles. Metals like Au, Ag, Pd, and Pt have interesting properties when used in nanometric dimensions. They function efficiently in significant industrial processes as electrocatalysts and photocatalysts in various organic reactions. Recently, the green biosynthesis of nanoparticles has attracted the attention of researchers. With environmental pollution rising over the past few decades, metal nanoparticle catalysts could be the key to subdue the toxic effects. Being versatile, they can be used to degrade pollutants, develop solar cells, convert toxic nitroaromatic compounds, significantly reduce CO₂ emissions per unit of energy, and many more. Owing to their unique properties, nanoparticles have wide applications in biomedicine, for example, gold cages are promising agents for cancer diagnosis and therapy. Transition metal-oxide nanoparticles have been considered one of the best supercapacitor electrodes with high electrochemical performance. In this review, we have summarised fundamental concepts of metal nanoparticles over the last decade's main emphasis from 2010 to 2021. It focuses on the exceptional use of these nanocatalysts in various organic reactions. Additionally, we have also discussed the utility of these reactions and their crucial role in solving the problems of today. Through this article, we hope to provide the necessary framework needed to further advance the applications of metal nanoparticles as catalysts.

Growth of N-Heterocyclic Carbene Assemblies on Cu(100) and Cu(111): From Single Molecules to Magic-Number Islands

N‐Heterocyclic carbenes (NHCs) have superior properties as building blocks of self‐assembled monolayers (SAMs). Understanding the influence of the substrate in the molecular arrangement is a fundamental step before employing these ligands in technological applications. Herein, we study the molecular arrangement of a model NHC on Cu(100) and Cu(111). While mostly disordered phases appear on Cu(100), on Cu(111) well‐defined structures are formed, evolving from magic‐number islands to molecular ribbons with coverage. This work presents the first example of magic‐number islands formed by NHC assemblies on flat surfaces. Diffusion and commensurability are key factors explaining the observed arrangements. These results shed light on the molecule‐substrate interaction and open the possibility of tuning nanopatterned structures based on NHC assemblies.

NHC-Stabilized Au10 Nanoclusters and Their Conversion to Au25 Nanoclusters

Herein, we describe the synthesis of a toroidal Au₁₀ cluster stabilized by N-heterocyclic carbene and halide ligands via reduction of the corresponding NHC-Au-X complexes (X = Cl, Br, I). The significant effect of the halide ligands on the formation, stability, and further conversions of these clusters is presented. While solutions of the chloride derivatives of Au₁₀ show no change even upon heating, the bromide derivative readily undergoes conversion to form a biicosahedral Au₂₅ cluster at room temperature. For the iodide derivative, the formation of a significant amount of Au₂₅ was observed even upon the reduction of NHC-Au-I. The isolated bromide derivative of the Au₂₅ cluster displays a relatively high (ca. 15%) photoluminescence quantum yield, attributed to the high rigidity of the cluster, which is enforced by multiple CH-π interactions within the molecular structure. Density functional theory computations are used to characterize the electronic structure and optical absorption of the Au₁₀ cluster. ¹³C-Labeling is employed to assist with characterization of the products and to observe their conversions by NMR spectroscopy.

N-Heterocyclic Carbene Ligand Stability on Gold Nanoparticles in Biological Media

The ability to functionalize gold nanoparticle surfaces with target ligands is integral to developing effective nanosystems for biomedical applications, ranging from point-of-care diagnostic devices to site-specific cancer therapies. By forming strong covalent bonds with gold, thiol functionalities can easily link molecules of interest to nanoparticle surfaces. Unfortunately, thiols are inherently prone to oxidative degradation in many biologically relevant conditions, which limits their broader use as surface ligands in commercial assays. Recently, N-heterocyclic carbene (NHC) ligands emerged as a promising alternative to thiols since initial reports demonstrated their remarkable stability against ligand displacement and stronger metal-ligand bonds. This work explores the long-term stability of NHC-functionalized gold nanoparticles suspended in five common biological media: phosphate-buffered saline, tris-glycine potassium buffer, tris-glycine potassium magnesium buffer, cell culture media, and human serum. The NHCs on gold nanoparticles were probed with surface-enhanced Raman spectroscopy (SERS) and X-ray photoelectron spectroscopy (XPS). SERS is useful for monitoring the degradation of surface-bound species because the resulting vibrational modes are highly sensitive to changes in ligand adsorption. Our measurements indicate that imidazole-based NHCs remain stable on gold nanoparticles over the 21 days of examination in all tested environments, with no observed change in the molecule's SERS signature, XPS response, or UV-vis plasmon band.

Fundamentals and applications of N-heterocyclic carbene functionalized gold surfaces and nanoparticles

Improved stability and higher degree of synthetic tunability has allowed N-heterocyclic carbenes to supplant thiols as ligands for gold surface functionalization.  This review article summarizes the basic science and applications of NHCs on gold.

Catalytically Active Gold Nanomaterials Stabilized by N-heterocyclic Carbenes

Solid supported or ligand capped gold nanomaterials (AuNMs) emerged as versatile and recyclable heterogeneous catalysts for a broad variety of conversions in the ongoing catalytic 'gold rush'. Existing at the border of homogeneous and heterogeneous catalysis, AuNMs offer the potential to merge high catalytic activity with significant substrate selectivity. Owing to their strong binding towards the surface atoms of AuMNs, NHCs offer tunable activation of surface atoms while maintaining selectivity and stability of the NM even under challenging conditions. This work summarizes well-defined catalytically active NHC capped AuNMs including spherical nanoparticles and atom-precise nanoclusters as well as the important NHC design choices towards activity and (stereo-)selectivity enhancements.

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.

High Stability Au NPs: From Design to Application in Nanomedicine

In recent years, Au-based nanomaterials are widely used in nanomedicine and biosensors due to their excellent physical and chemical properties. However, these applications require Au NPs to have excellent stability in different environments, such as extreme pH, high temperature, high concentration ions, and various biomatrix. To meet the requirement of multiple applications, many synthetic substances and natural products are used to prepare highly stable Au NPs. Because of this, we aim at offering an update comprehensive summary of preparation high stability Au NPs. In addition, we discuss its application in nanomedicine. The contents of this review are based on a balanced combination of our studies and selected research studies done by worldwide academic groups. First, we address some critical methods for preparing highly stable Au NPs using polymers, including heterocyclic substances, polyethylene glycols, amines, and thiol, then pay attention to natural product progress Au NPs. Then, we sum up the stability of various Au NPs in different stored times, ions solution, pH, temperature, and biomatrix. Finally, the application of Au NPs in nanomedicine, such as drug delivery, bioimaging, photothermal therapy (PTT), clinical diagnosis, nanozyme, and radiotherapy (RT), was addressed concentratedly.

N-Heterocyclic Carbene-Stabilized Ultrasmall Gold Nanoclusters in a Metal-Organic Framework for Photocatalytic CO2 Reduction

Ultrafine gold nanoclusters (Au-NCs) are susceptible to migrate and aggregate, even in the porosity of many crystalline solids. N-heterocyclic carbenes (NHCs) are a class of structurally diverse ligands for the stabilization of Au-NCs in homogeneous chemistry, showing catalytic reactivity in CO₂ activation. Herein, for the first time, we demonstrate a heterogeneous nucleation approach to stabilize ultrasmall and highly dispersed gold nanoclusters in an NHC-functionalized porous matrix. The sizes of gold nanoclusters are tunable from 1.3 nm to 1.8 nm based on the interpenetration of the metal-organic framework (MOF) topology. Control experiments using amine or imidazolium-functionalized MOFs afforded the aggregation of Au species. The resultant Au-NC@MOF composite exhibits a steady and excellent activity in photocatalytic CO₂ reduction, superior to control mixtures without NHC-ligand stabilization. Mechanistic studies reveal the synergistic catalytic effect of MOFs and Au-NCs through the MOF-NHC-Au covalent-bonding bridges.

Surface engineering strategies of gold nanomaterials and their applications in biomedicine and detection

Gold nanomaterials have potential applications in biosensors and biomedicine due to their controllable synthesis steps, high biocompatibility, low toxicity and easy surface modification. However, there are still various limitations including low water solubility and stability, which greatly affect their applications. In addition, some synthetic methods are very complicated and costly. Therefore, huge efforts have been made to improve their properties. This review mainly introduces the strategies for surface modification of gold nanomaterials, such as amines, biological small molecules and organic small molecules as well as the biological applications of these functionalized AuNPs. We aim to provide effective ideas for better functionalization of gold nanomaterials in the future.

Multifunctional Gold Nanorod for Therapeutic Applications and Pharmaceutical Delivery Considering Cellular Metabolic Responses, Oxidative Stress and Cellular Longevity

Multifunctional gold nanorods (GNR) have drawn growing interest in biomedical fields because of their excellent biocompatibility, ease of alteration, and special optical properties. The great advantage of using GNR in medicine is their application to Photothermal therapy (PPTT), which is possible thanks to their ability to turn luminous energy into heat to cause cellular hyperthermia. For this purpose, the relevant articles between 1988 and 2020 were searched in databases such as John Wiley, Free paper, Scopus, Science Direct, and Springer to obtain the latest findings on multifunctional gold nanorods for therapeutic applications and pharmaceutical delivery. In this article, we review recent progress in diagnostic and therapeutic applications of multifunctional GNR, highlighting new information about their toxicity to various cellular categories, oxidative stress, cellular longevity, and their metabolic effects, such as the effect on the energy cycles and genetic structures. The methods for the synthesis and functionalization of GNR were surveyed. This review includes new information about GNR toxicity to various cellular categories and their metabolic effects.

Gold(I) and Gold(III) N-Heterocyclic Carbene Complexes as Antibacterial Agents and Inhibitors of Bacterial Thioredoxin Reductase

A series of (NHC)Au(I)Cl monocarbene complexes and their gold(III) analogues (NHC)Au(III)Cl₃ were prepared and investigated as antibacterial agents and inhibitors of bacterial TrxR. The complexes showed stronger antibacterial effects against the Gram-positive MRSA and E. faecium strains than against several Gram-negative bacteria. All complexes were efficient inhibitors of bacterial thioredoxin reductase, indicating that inhibition of this enzyme might be involved in their mechanism of action. The efficacy of gold(I) and gold(III) analogues was comparable in most of the assays. The cytotoxicity of the gold NHC compounds against cancer and human cells was overall weaker than the activity against the Gram-positive bacteria, suggesting that their optimization as antibacterials warrants further investigation.

Self-assembly of N-heterocyclic carbenes on Au(111)

Although the self-assembly of organic ligands on gold has been dominated by sulfur-based ligands for decades, a new ligand class, N-heterocyclic carbenes (NHCs), has appeared as an interesting alternative. However, fundamental questions surrounding self-assembly of this new ligand remain unanswered. Herein, we describe the effect of NHC structure, surface coverage, and substrate temperature on mobility, thermal stability, NHC surface geometry, and self-assembly. Analysis of NHC adsorption and self-assembly by scanning tunneling microscopy and density functional theory have revealed the importance of NHC-surface interactions and attractive NHC-NHC interactions on NHC monolayer structures. A remarkable way these interactions manifest is the need for a threshold NHC surface coverage to produce upright, adatom-mediated adsorption motifs with low surface diffusion. NHC wingtip structure is also critical, with primary substituents leading to the formation of flat-lying NHC2Au complexes, which have high mobility when isolated, but self-assemble into stable ordered lattices at higher surface concentrations. These and other studies of NHC surface chemistry will be crucial for the success of these next-generation monolayers.

Preparation, Functionalization, Modification, and Applications of Nanostructured Gold: A Critical Review

Gold nanoparticles (Au NPs) play a significant role in science and technology because of their unique size, shape, properties and broad range of potential applications. This review focuses on the various approaches employed for the synthesis, modification and functionalization of nanostructured Au. The potential catalytic applications and their enhancement upon modification of Au nanostructures have also been discussed in detail. The present analysis also offers brief summaries of the major Au nanomaterials synthetic procedures, such as hydrothermal, solvothermal, sol-gel, direct oxidation, chemical vapor deposition, sonochemical deposition, electrochemical deposition, microwave and laser pyrolysis. Among the various strategies used for improving the catalytic performance of nanostructured Au, the modification and functionalization of nanostructured Au produced better results. Therefore, various synthesis, modification and functionalization methods employed for better catalytic outcomes of nanostructured Au have been summarized in this review.

A PEGylated N-heterocyclic carbene-gold(i) complex: an efficient catalyst for cyclization reaction in water

A recyclable, PEGylated N-heterocyclic carbene(NHC)–gold(i) catalyst was synthesized, which exhibited high reactivity and regio-selectivity in the cyclization of alkynoic acids and cascade reaction of tryptamine and alkynoic acids in water.

Perylene diimide-tagged N-heterocyclic carbene-stabilized gold nanoparticles: How much ligand desorbs from surface in presence of thiols?

N-Heterocyclic carbenes (NHCs) have recently emerged as viable alternatives to commonly used thiols to stabilize a variety of metal surfaces and nanoparticles. In this context, thanks to their biocompatibility and novel optical properties, NHC-stabilized gold nanoparticles (AuNPs) have been extensively studied. It has been shown that such materials exhibit improved stabilities in acidic and basic solutions, high temperatures, electrolyte solutions, cell culture media, and to some extent to nucleophilic thiols. Despite intense efforts, instability of NHC-functionalized AuNPs to thiols has been an ongoing challenge. In order to circumvent this problem, quantification of NHC desorption from nanoparticle surface by the invading thiols would constitute a necessary first step. To do this, we have first developed water-soluble azide decorated NHC-stabilized "clickable" AuNPs. Optically active perylene diimide (PDI)-tagged AuNP hybrids are then obtained by means of Cu-catalyzed alkyne-azide cycloaddition between these AuNPs and an alkyne-decorated PDI derivative. Investigation of photophysical properties of these AuNP/PDI hybrids revealed that the fluorescence of PDI molecules is effectively quenched by AuNPs in aqueous solution. The extent of NHC desorption from AuNP surface in presence of glutathione (4 mM), as a biologically relevant thiol, is then quantified by means of fluorescence emission restoration of PDI molecules upon detachment from AuNP surfaces. Our results demonstrate that while ∼20% of surface NHCs are displaced by glutathione within the first 24 h of their exposure to the thiol, ligand desorption reaches ∼45% after one week. We believe that these findings will provide more insight on true stability of NHC-stabilized materials.

An ICP-MS-based assay for characterization of gold nanoparticles with potential biomedical use

Most of potential diagnostic and therapeutic nanoparticles fail to reach clinical trials because assessment of their 'drug-like' properties is often overlooked during the discovery stage. This compromises the results of cell culture and animal experiments, making them insufficient to evaluate the lead candidates for testing on patients. In this study, we demonstrate the potential of high-resolution inductively coupled plasma mass spectrometry (ICP-MS) as a nanoparticle qualification tool. Using novel gold nanoparticles stabilized by N-heterocyclic carbenes as test nanoparticles, it was shown that important prerequisites for biomedical applications, such as resistance to the action of human serum milieu or reactivity toward serum biomolecules, can be reliably assessed by recording the signals of gold or sulfur isotopes. Implemented during the screening stage, the method would provide benefits in shortening timelines and reducing cost for selection and initial testing of medicinal nanoparticle candidates.

Synthetically Versatile Nitrogen Acyclic Carbene Stabilized Gold Nanoparticles

N-heterocyclic carbenes (NHCs) have received significant attention as gold nanoparticle stabilizers due to their strong binding affinity towards gold. However, their tunability is limited by the difficulty in obtaining nonsymmetric NHCs. In this regard, N-acyclic carbenes (NACs) are attractive alternatives due to their high synthetic versatility, allowing easy tuning of their steric and electronic properties towards specific applications. This work reports the first series of stable and monodisperse NAC-functionalized gold nanoparticles. These particles with sizes ranging 3.8 to 11.6 nm were characterized using NMR, UV/Vis and TEM. The nanoparticles display good stability at elevated temperatures and for extended periods both dried or dispersed in a medium, as well as in the presence of exogenous thiols. Importantly, these NAC-stabilized gold nanoparticles offer a promising and versatile alternative to NHC-stabilized gold nanoparticles.

Electrochemical deposition of N-heterocyclic carbene monolayers on metal surfaces

N-heterocyclic carbenes (NHCs) have been widely utilized for the formation of self-assembled monolayers (SAMs) on various surfaces. The main methodologies for preparation of NHCs-based SAMs either requires inert atmosphere and strong base for deprotonation of imidazolium precursors or the use of specifically-synthesized precursors such as NHC(H)[HCO₃] salts or NHC–CO₂ adducts. Herein, we demonstrate an electrochemical approach for surface-anchoring of NHCs which overcomes the need for dry environment, addition of exogenous strong base or restricting synthetic steps. In the electrochemical deposition, water reduction reaction is used to generate high concentration of hydroxide ions in proximity to a metal electrode. Imidazolium cations were deprotonated by hydroxide ions, leading to carbenes formation that self-assembled on the electrode’s surface. SAMs of NO₂-functionalized NHCs and dimethyl-benzimidazole were electrochemically deposited on Au films. SAMs of NHCs were also electrochemically deposited on Pt, Pd and Ag films, demonstrating the wide metal scope of this deposition technique.

N-heterocyclic carbenes (NHCs) have been widely used for the formation of monolayers but self-assembly methods come with drawbacks such as need for dry environment or using specifically-synthesized precursors. Here, the authors demonstrate an approach for surface-anchoring of NHCs which overcomes these limitations by using electrochemically-assisted deprotonation.

Simulations of X-ray absorption spectroscopy and energetic conformation of N-heterocyclic carbenes on Au(111)

It has recently been demonstrated that N-heterocyclic carbenes (NHCs) form self-assembled monolayers (SAMs) on metal surfaces. Consequently, it is important to both characterize and understand their binding modes to fully exploit NHCs in functional surface systems. To assist with this effort, we have performed first-principles total energy calculations for NHCs on Au(111) and simulations of X-ray absorption near edge structure (XANES). The NHCs we have considered are N,N-dimethyl-, N,N-diethyl-, N,N-diisopropylbenzimidazolylidene (BNHCX, with X = Me, Et, and iPr, respectively) and the bis-BNHCX-Au complexes derived from these molecules. We present a comprehensive analysis of the energetic stability of both the BNHCX and the complexes on Au(111) and, for the former, examine the role of the wing group in determining the attachment geometry. Further structural characterization is performed by calculating the nitrogen K-edge X-ray absorption spectra. Our simulated XANES results give insight into (i) the relationship between the BNHCX/Au geometry and the N(1s) → π*/σ*, pre-edge/near-edge, absorption intensities, and (ii) the contributions of the molecular deformation and molecule-surface electronic interaction to the XANES spectrum. These simulated spectra work not only as a map to the BNHCX conformation, but also, combined with electronic structure calculations, provide a clear understanding of recent experimental XANES findings on BNHCX/Au.

Ultrasensitive and Label-Free Detection of the Measles Virus Using an N-Heterocyclic Carbene-Based Electrochemical Biosensor

With the current intense need for rapid and accurate detection of viruses due to COVID-19, we report on a platform technology that is well suited for this purpose, using intact measles virus for a demonstration. Cases of infection due to the measles virus are rapidly increasing, yet current diagnostic tools used to monitor for the virus rely on slow (>1 h) technologies. Here, we demonstrate the first biosensor capable of detecting the measles virus in minutes with no preprocessing steps. The key sensing element is an electrode coated with a self-assembled monolayer containing the measles antibody, immobilized through an N-heterocyclic carbene (NHC). The intact virus is detected by changes in resistance, giving a linear response to 10-100 μg/mL of the intact measles virus without the need to label or process the sample. The limit of detection is 6 μg/mL, which is at the lower limit of concentrations that can cause infections in primates. The NHC-based biosensors are shown to be superior to thiol-based systems, producing an approximately 10× larger response and significantly greater stability toward repeated measurements and long-term storage. This NHC-based biosensor thus represents an important development for both the rapid detection of the measles virus and as a platform technology for the detection of other biological targets of interest.

Chemoselective H/D exchange catalyzed by nickel nanoparticles stabilized by N-heterocyclic carbene ligands

With this work, we report the synthesis and full characterization of nickel nanoparticles (NPs) stabilized by N-heterocyclic carbene (NHC) ligands, namely 1,3-bis(cyclohexyl)-1,3-dihydro-2H-imidazol-2-ylidene (ICy) and 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IMes). Although the resulting NPs have the same size, they display different magnetic properties and different reactivities, which result from ligand effects. In the context of H/D exchange on pharmaceutically relevant heterocycles, Ni@NHC shows a high chemoselectivity, avoiding the formation of undesired reduced side-products and enabling a variety of H/D exchange on nitrogen-containing aromatic compounds. Using 2-phenylpyridine as a model substrate, it was observed that deuteration occurred preferably at the α position of the nitrogen atom, which is the most accessible position for the C-H activation. In addition, Ni@IMes NPs are also able to fully deuterate the ortho positions of the phenyl substituents.

Synthesis of glucoside-based imidazolium salts for Pd-catalyzed cross-coupling reaction in water

Sugar-based imidazolium salts (IMSs) represent an outstanding type of material making them eye-catching for a wide variety of applications. Herein, a series of glucoside-based IMSs (Glu-IMSs) combining glucoside and imidazolium head groups with different substituents were synthesized. The catalytic activities of these Glu-IMSs were evaluated by Pd-catalyzed Heck-Mizoroki and Suzuki-Miyaura reactions in water. Among them, the Glu-IMSs contain both -OH and NHCs coordination sites was found to be the most efficient ancillary ligand in comparison with other Glu-IMSs with just single NHCs coordination site. The HR-TEM analysis showed that the palladium nanoparticles stabilized by the Glu-IMSs with an average size of ~4.0 nm was formed in the reaction system, which may be act as an efficient real catalytic species. Under the optimized reaction conditions, a series of novel fluorine-cored organic small molecule functional materials were synthesized with favorable yields.

Polyethylenimine-Assisted Generation of Optical Nanoprobes for Biosensing Applications

Detection of analytes in biological systems is pivotal to explore their physiological roles and provide diagnostic and treatment options for related diseases, which however remains a great challenge. Optical nanoprobes that exhibit absorption or fluorescence signal changes in response to the targets of interest have emerged as a versatile class of biosensors in the field. Polyethylenimine (PEI) with abundant amine groups plays indispensable roles in the construction of optical nanoprobes and mediating the sensing processes. After interaction with analytes, PEI-based optical nanoprobes can be induced to form aggregates, be disassembled or separated into individual units, or undergo structure/component alterations. As such, the optical properties of these nanoprobes have corresponding changes, allowing for sensitive and selective detection of a wide variety of analytes in biological environment. Up to now, detections of reactive oxygen species, pH, metal ions, biothiols, neurotransmitters, therapeutic agents, oxygen levels, enzyme activities, and virus/bacteria have been successfully demonstrated using PEI-based optical nanoprobes. Herein, we summarize the recent developments of PEI-based optical nanoprobes for biosensing applications and highlight the probe designs and sensing mechanisms. The existing challenges and prospects regarding biosensing applications of PEI-based optical nanoprobes are also briefly discussed.

Ultrastable and Highly Catalytically Active N-Heterocyclic-Carbene-Stabilized Gold Nanoparticles in Confined Spaces

Controlling the size and surface functionalization of nanoparticles (NPs) can lead to improved properties and applicability. Herein, we demonstrate the efficiency of the metal-carbene template approach (MCTA) to synthesize highly robust and soluble three-dimensional polyimidazolium cages (PICs) of different sizes, each bearing numerous imidazolium groups, and use these as templates to synthesize and stabilize catalytically active, cavity-hosted, dispersed poly-N-heterocyclic carbene (NHC)-anchored gold NPs. Owing to the stabilization of the NHC ligands and the effective confinement of the cage cavities, the as-prepared poly-NHC-shell-encapsulated AuNPs displayed promising stability towards heat, pH, and chemical regents. Most notably, all the Au@PCCs (PCC=polycarbene cage) exhibited excellent catalytic activities in various chemical reactions, together with high stability and durability.