An official website of the United States government
A facile methodology to prepare N-heterocyclic carbene (NHC)-terminated polymers as surface ligands to functionalize gold nanoparticles (AuNPs) is reported. Our method highlights a mild, aerobic synthesis of NHC-functionalized polymers and a simple ligand exchange approach towards surface modification of AuNPs prepared in aqueous solution. Two methods, including end-group functionalization of halogen-ended polymers from a conventional atom transfer radical polymerization (ATRP) and post-polymerization functionalization of imidazole-containing polymers using imidazole-containing ATRP initiator, have been investigated to prepare imidazolium-ended polymers. Using a one-step, oxygen and moisture tolerant procedure, the polymer–NHC–Cu( i ) species can be synthesized from imidazolium-ended polymers and readily bind to citrate-capped AuNPs likely through transmetalation, yielding robust polymer-stabilized AuNPs. Our synthetic method significantly simplifies the preparation and use of polymer–NHC ligands for surface functionalization of metal NPs. Our methodology is general and potentially applicable to any polymers prepared by ATRP to functionalize metal NPs via NHC–metal coordination; therefore, it will likely broaden the applications of polymer–NHC ligands for metal nanoparticles in the fields of catalysis and nanomedicine.
more »
« less
The Landscape of Gold Nanocrystal Surface Chemistry
ConspectusGold nanoparticles (AuNPs) exhibit unique size- and shape-dependent properties not obtainable at the macroscale. Gold nanorods (AuNRs), with their morphology-dependent optical properties, ability to convert light to heat, and high surface-to-volume ratios, are of great interest for biosensing, medicine, and catalysis. While the gold core provides many fascinating properties, this Account focuses on AuNP soft surface coatings, which govern the interactions of nanoparticles with the local environments. Postmodification of AuNP surface chemistry can greatly alter NP colloidal stability, nano-bio interactions, and functionality. Polyelectrolyte coatings provide controllable surface-coating thickness and charge, which impact the composition of the acquired corona in biological settings. Covalent modification, in which covalently bound ligands replace the original capping layer, is often performed with thiols and disulfides due to their ability to replace native coatings. N-heterocyclic carbenes and looped peptides expand the possible functionalities of the ligand layer.The characterization of surface ligands bound to AuNPs, in terms of ligand density and dynamics, remains a challenge. Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for understanding molecular structures and dynamics. Our recent NMR work on AuNPs demonstrated that NMR data were obtainable for ligands on NPs with diameters up to 25 nm for the first time. This was facilitated by the strong proton NMR signals of the trimethylammonium headgroup, which are present in a distinct regime from other ligand protons’ signals. Ligand density analyses showed that the smallest AuNPs (below 4 nm) had the largest ligand densities, yet spin–spin T2 measurements revealed that these smallest NPs also had the most mobile ligand headgroups. Molecular dynamics simulations were able to reconcile these seemingly contradictory results.While NMR spectroscopy provides ligand information averaged over many NPs, the ligand distribution on individual particles’ surfaces must also be probed to fully understand the surface coating. Taking advantage of improvements in electron energy loss spectroscopy (EELS) detectors employed with scanning transmission electron microscopy (STEM), a single-layer graphene substrate was used to calibrate the carbon K-edge EELS signal, allowing quantitative imaging of the carbon atom densities on AuNRs with sub-nanometer spatial resolution. In collaboration with others, we revealed that the mean value for surfactant-bilayer-coated AuNRs had 10–30% reduced ligand density at the ends of the rods compared to the sides, confirming prior indirect evidence for spatially distinct ligand densities.Recent work has found that surface ligands on nanoparticles can, somewhat surprisingly, enhance the selectivity and efficiency of the electrocatalytic reduction of CO2 by controlling access to the active site, tuning its electronic and chemical environment, or denying entry to impurities that poison the nanoparticle surface to facilitate reduction. Looking to the future, while NMR and EELS are powerful and complementary techniques for investigating surface coatings on AuNPs, the frontier of this field includes the development of methods to probe the surface ligands of individual NPs in a high-throughput manner, to monitor nano-bio interactions within complex matrices, and to study structure–property relationships of AuNPs in biological systems.
more »
« less
- PAR ID:
- 10417893
- Date Published:
- Journal Name:
- Accounts of Chemical Research
- ISSN:
- 0001-4842
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
null (Ed.)Metal nanoparticles (NPs) tethered by synthetic polymers are of broad interest for self-assembly, nanomedicine and catalysis. The binding motifs in polymer ligands usually as the end functional groups of polymers are mostly limited to thiolates. Since the binding motif only represents a tiny fraction of many repeating units in polymers, its importance is often ignored. We herein report the uniqueness of polymeric N-heterocyclic carbene (NHC) ligands in providing oxidative stability and promoting the catalytic activity of noble metal NPs. Two “grafting to” methods were developed for polymer NHCs for pre-synthesized metal NPs in various solvents and with different sizes. Remarkably, imidazolium-terminated polystyrene can modify gold NPs (AuNPs) within 2 min while reaching a similar grafting density to polystyrene-thiol (SH) requiring 6 h modification. We demonstrate that polymer NHCs are extremely stable at high temperature in air. Interestingly, the binding motifs of polymer ligands dominate the catalytic activity of metal NPs. Polymer NHC modified metal NPs showed improved activity regardless of the surface crowdedness. In the case of AuNPs, AuNPs modified with polystyrene NHCs are approximately 5.2 times more active than citrate-capped ones and 22 times more active than those modified with polystyrene thiolates. In view of ligand-controlled catalytic properties of metal NPs, our results illustrate the importance of binding motifs that has been overlooked in the past.more » « less
-
Abstract In recent decades, extensive studies have been devoted to assembling nanoparticles (NPs) into various ordered structures to achieve novel optical properties. However, it still remains a challenging task to assemble NPs into cyclic one‐dimensional (1D) shapes, such as rings and frames. Herein, we report a directed assembly method to precisely assemble NPs into well‐defined, free‐standing frames using polymer single crystals (PSCs) as the template. Preformed poly(ethylene oxide) (PEO) single crystals were used as the template to direct the crystallization of block copolymer (BCP) poly(ethylene oxide)‐b‐poly(4‐vinylpyridine) (PEO‐b‐P4VP), which directs the gold NPs (AuNPs) to form AuNP frames. By controlling the PSC growth, we were able to, for the first time, precisely tune both the size and width of the AuNP frame. These novel AuNP frames topologically resemble NP nanorings and cyclic polymer chains, and show unique surface plasmon resonance (SPR) behaviors.more » « less
-
The exceptional stability of N-heterocyclic carbene (NHC) monolayers on gold surfaces and nanoparticles (AuNPs) is enabling new and diverse applications from catalysis to biomedicine. Our understanding of NHC reactivity at surfaces; however, is quite nascent when compared to the long and rich history of NHC ligands in organometallic chemistry. In this work, well-established transmetalation reactions, previously developed for NHC transfer in homogeneous organometallic systems, are explored to determine how they can be used to create carbene functionalized gold surfaces. Two classes of NHCs, based on imidazole and benzimidazole scaffolds, were tested. The resulting AuNP surfaces were analyzed using X-ray photoelectron spectroscopy (XPS), laser desorption ionization mass spectrometry (LDI-MS), and surface-enhanced Raman spectroscopy (SERS). Reaction of either a Au( i ) or Ag( i ) isopropyl benzimidazole NHC complex with citrate-capped AuNPs yields, in both cases, a chemisorbed NHC that is bound through a Au adatom. Theoretical calculations additionally illustrate that binding through the Au adatom is favored by more than 10 kcal mol −1 , in good agreement with experiments. Surprisingly, reaction of Au( i ), Ag( i ), and Cu( i ) diisopropylphenyl imidazole NHCs do not follow the same pattern. The Cu complex undergoes transmetalation with very little deposition of Cu; whereas, unexpectedly, the Ag complex foregoes transmetalation and instead adducts to the AuNP with retention of the Ag–C bond. Theoretical calculations illustrate that the imidazole ligand affords significant dispersion interactions with the gold surface, which may stabilize binding through the Ag adatom motif, despite its less favorable bonding energies. Taken together these results suggest a unique ability to tune the reactivity by changing the carbene structure and raise critical questions about how established transmetalation reactions in organometallic chemistry can be applied to form NHC functionalized surfaces.more » « less
-
The surface properties of biologically active nanoparticles (NPs) are often dictated by synthetic ligands that are grafted to the NP core to form a protecting monolayer. Ligand selection is thus critical in determining NP surface properties and corresponding interactions at the nano-bio interface, which are relevant to numerous applications including drug delivery and biosensing. However, chemically specific structure–property relationships for rationally selecting ligands to achieve desired biointeractions are largely lacking. In this Focus Article, we review the challenges associated with relating ligand chemical properties to monolayer-protected NP surface properties due to the interplay of ligand–ligand, ligand–solvent, and ligand–biomolecule interactions that are difficult to anticipate. In particular, we highlight unexpected spatially varying properties that emerge even for uniformly functionalized NPs due to the fluctuations of ligands at the nanoscale. We further review the capability of physics-based molecular simulations to reveal these unexpected behaviors, providing powerful computational methods to predict NP properties. Finally, we discuss the opportunity for such simulations to be combined with machine-learning methods to guide the computational design of monolayer-protected NPs prior to synthesis.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.accounts.3c00109
