Quantified Binding Scale of Competing Ligands at the Surface of Gold Nanoparticles: The Role of Entropy and Intermolecular Forces

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.

Oriented immobilization of antibodies onto sensing platforms - A critical review

The immunosensor has been proven a versatile tool to detect various analytes, such as food contaminants, pathogenic bacteria, antibiotics and biomarkers related to cancer. To fabricate robust and reproducible immunosensors with high sensitivity, the covalent immobilization of immunoglobulins (IgGs) in a site-specific manner contributes to better performance. Instead of the random IgG orientations result from the direct yet non-selective immobilization techniques, this review for the first time introduces the advances of stepwise yet site-selective conjugation strategies to give better biosensing efficiency. Noncovalently adsorbing IgGs is the first but decisive step to interact specifically with the Fc fragment, then following covalent conjugate can fix this uniform and antigens-favorable orientation irreversibly. In this review, we first categorized this stepwise strategy into two parts based on the different noncovalent interactions, namely adhesive layer-mediated interaction onto homofunctional support and layer-free interaction onto heterofunctional support (which displays several different functionalities on its surface that are capable to interact with IgGs). Further, the influence of ligands characteristics (synthesis strategies, spacer requirements and matrices selection) on the heterofunctional support has also been discussed. Finally, conclusions and future perspectives for the real-world application of stepwise covalent conjugation are discussed. This review provides more insights into the fabrication of high-efficiency immunosensor, and special attention has been devoted to the well-orientation of full-length IgGs onto the sensing platform.

Turning over on sticky balls: preparation and catalytic studies of surface-functionalized TiO2 nanoparticles

We have investigated the reactivity of rhodium(iii) complex-functionalized TiO2 nanoparticles and demonstrate a proof-of-principle study of their catalytic activity in an alcohol oxidation carried out under aqueous conditions water in air. TiO2 nanoparticles (NPs) have been treated with (4-([2,2':6',2''-terpyridin]-4'-yl)phenyl)phosphonic acid, 1, to give the functionalized NPs (1)@TiO2. Reaction between (1)@TiO2 NPs and either RhCl3·3H2O or [Rh2(μ-OAc)4(H2O)2] produced the rhodium(iii) complex-functionalized NPs Rh(1)2@TiO2. The functionalized NPs were characterized using thermogravimetric analysis (TGA), matrix-assisted laser desorption ionization (MALDI) mass spectrometry, 1H NMR and FT-IR spectroscopies; the single crystal structures of [Rh(1)2][NO3]3·1.25[H3O][NO3]·2.75H2O and of a phosphonate ester derivative were determined. 1H NMR spectroscopy was used to follow the reaction kinetics and to assess the recyclability of the NP-supported catalyst. The catalytic activity of the Rh(1)2@TiO2 NPs was compared to that of a homogeneous system containing [Rh(1)2]3+, confirming that no catalytic activity was lost upon surface-binding. Rh(1)2@TiO2 NPs were able to withstand reaction temperatures of up to 100 °C for 24 days without degradation.

Metallic-Nanoparticle-Based Sensing: Utilization of Mixed-Ligand Monolayers

The last two decades have seen great advancements in fundamental understanding and applications of metallic nanoparticles stabilized by mixed-ligand monolayers. Identifying and controlling the organization of multiple ligands in the nanoparticle monolayer has been studied, and its effect on particle properties has been examined. Mixed-ligand protected particles have shown advantages over monoligand protected particles in fields such as catalysis, self-assembly, imaging, and drug delivery. In this Review, the use of mixed-ligand monolayer protected nanoparticles for sensing applications will be examined. This is the first time this subject is examined as a whole. Mixed-ligand nanoparticle-based sensors are revealed to be divided into four groups, each of which will be discussed. The first group consists of ligands that work cooperatively to improve the sensors' properties. In the second group, multiple ligands are utilized for sensing multiple analytes. The third group combines ligands used for analyte recognition and signal production. In the final group, a sensitive, but unstable, functional ligand is combined with a stabilizing ligand. The Review will conclude by discussing future challenges and potential research directions for this promising subject.

From mono-PEGylation towards anti-nonspecific protein interaction: comparison of dihydrolipoic acid versus glutathione-capped fluorescent gold nanoclusters using gel electrophoresis

Ultrafine fluorescent gold nanoclusters (AuNCs) have emerged as biocompatible nanoprobes for biomedical imaging in vivo, and the precision surface chemistry of AuNCs is the key for attaining their clinical application. Comparison of two promising candidates for future nanomedicine, i.e. dihydrolipoic acid- versus glutathione-capped AuNCs (AuNC@DHLA vs. AuNC@GSH), was conducted for the first time to clarify their polyethylene glycol-related bioconjugate chemistry (PEGylation) and protein interactions. Gel electrophoresis was performed to separate the number of AuNCs PEGylation, and the molecular weight of the PEG spacer dominated the resolution of the separation in the gel. We have engineered and isolated the mono-PEGylated AuNCs either from the indirect carbodiimide bioconjugate chemistry or the direct Au-S binding. One-pot synthesis showed great efficiency for isolating mono-PEGylated AuNC@GSH from the tailored controlled aggregation of Au(i)-thiolate complexes on in situ generated Au(0) cores. Post-PEGylation of AuNC@GSH was also feasible using monodendate thiol-terminated PEG, but bidendate ligands of AuNC@DHLA exhibited low PEGylated efficiency by Au-S binding. In addition, mono-PEGylated AuNC@GSH significantly enhanced the ability of anti-nonspecific protein adsorption, but mono-PEGylated AuNC@DHLA cannot avoid the nonspecific binding with serum albumin. In addition, specific nano-assembly involving mono-biotinylated AuNCs with streptavidin were also compared using gel electrophoresis. These results provide key insights into the selection, preparation and design of functional AuNCs as nanoprobes for versatile biomedical applications.

Effect of the Ligand Binding Strength on the Morphology of Functionalized Gold Nanoparticles

Functionalized gold nanoparticles are investigated by density functional theory calculations in the context of cancer radiotherapy. Several typical experimental shapes, including nanostars, nanospheres, and nanorods, are modeled by optimizing Au clusters covered by organic monolayers composed of hydrated short-chain polyethylene glycol (PEG) ligands. The PEGylation stabilizes significantly the stellation of decahedral Au₅₄ by deforming significantly its geometry at the spikes. The higher stability of the PEG molecules adsorbed on this stellated nanocluster with respect to the more spherical icosahedral Au₅₅ and truncated octahedral Au₇₉ leads to a larger energy cost to desorb them and thus a weaker propensity for the starred nanoparticle to exchange ligands with the cell membrane, in agreement with experiments. These results open interesting possibilities for advancing our understanding of the cellular uptake of gold nanoparticles.

The SALSAC approach: comparing the reactivity of solvent-dispersed nanoparticles with nanoparticulate surfaces

We demonstrate that the 'surface-as-ligand, surface-as-complex' (SALSAC) approach that we have established for annealed nanoparticulate TiO₂ surfaces can be successfully applied to nanoparticles (NPs) dispersed in solution. Commercial TiO₂ NPs have been activated by initial treatment with aqueous HNO₃ followed by dispersion in water and heating under microwave conditions. We have functionalized the activated NPs with anchoring ligands 1-4; 1-3 contain one or two phosphonic acid anchoring groups and 4 has two carboxylic acid anchors; ligands 1, 2 and 4 contain 6,6'-dimethyl-2,2'-bipyridine (Me₂bpy) metal binding domains and 3 contains a 2,2':6',2''-terpyridine (tpy) unit. Ligand functionalization of the activated NPs has been validated using infrared and ¹H NMR spectroscopies, and thermogravimetric analysis. NPs functionalized with 1, 2 and 4 react with [Cu(MeCN)₄][PF₆] and those with 3 react with FeCl₂·4H₂O; metal binding has been investigated using solid-state absorption spectroscopy and scanning electron microscopy (SEM). Competitive binding of ligands 1-4 to TiO₂ NPs has been investigated and shows preferential binding of phosphonic acid over carboxylic acid anchors. For the phosphonic acids, the binding orders are 3 > 1 > 2 which is rationalized in terms of relative pK _a values (phosphonic acid and [HMe₂bpy]⁺ or [Htpy]⁺) and the number of anchoring groups in the ligands. Ligand exchange between ligand-functionalized NPs and homoleptic metal complexes gives NPs functionalized with heteroleptic copper(i) or iron(ii) complexes.

AuNPs-Based Thermoresponsive Nanoreactor as an Efficient Catalyst for the Reduction of 4-Nitrophenol

A new AuNPs-based thermosensitive nanoreactor (SiO₂@PMBA@Au@PNIPAM) was designed and prepared by stabilizing AuNPs in the layer of poly(N,N'-methylenebisacrylamide) (PMBA) and subsequent wrapping with the temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) layer. The new nanoreactor exhibited high dispersibility and stability in aqueous solution and effectively prevented the aggregation of AuNPs caused by the phase transformation of PNIPAM. The XPS and ATR-FTIR results indicated that AuNPs could be well stabilized by PMBA due to the electron transfer between the N atoms of amide groups in the PMBA and Au atoms of AuNPs. The catalytic activity and thermoresponsive property of the new nanoreactor were invested by the reduction of the environmental pollutant, 4-nitrophenol (4-NP), with NaBH₄ as a reductant. It exhibited a higher catalytic activity at 20 °C and 30 °C (below LCST of PNIPAM), but an inhibited catalytic activity at 40 °C (above LCST of PNIPAM). The PNIPAM layer played a switching role in controlling the catalytic rate by altering the reaction temperature. In addition, this nanoreactor showed an easily recyclable property due to the existence of a silica core and also preserved a rather high catalytic efficiency after 16 times of recycling.

A Versatile AuNP Synthetic Platform for Decoupled Control of Size and Surface Composition

While a plethora of protocols exist for the synthesis of sub-10-nm gold nanoparticles (AuNPs), independent control over the size and surface composition remains restricted. This poses a particular challenge for systematic studies of AuNP structure-function relationships and the optimization of crucial design parameters. To this end, we report on a modular two-step approach based on the synthesis of AuNPs in oleylamine (OAm) followed by subsequent functionalization with thiol ligands and mixtures thereof. The synthesis of OAm-capped AuNPs enables fine-tuning of the core size in the range of 2-7 nm by varying the reaction temperature. The subsequent thiol-for-OAm ligand exchange allows a reliable generation of thiol-capped AuNPs with target surface functionality. The compatibility of this approach with a vast library of thiol ligands provides detailed control of the mixed ligand composition and solubility in a wide range of solvents ranging from water to hexane. This decoupled control over the AuNP core and ligand shell provides a powerful toolbox for the methodical screening of optimal design parameters and facile preparation of AuNPs with target properties.

N-Heterocyclic carbene-stabilized gold nanoparticles with tunable sizes

A simple and straightforward synthesis of N-heterocyclic carbene (NHC)-protected gold nanoparticles is derived from (benz)imidazolium-AuX4 complexes and NaBH4 only. The proposed method allows size tuning, from 3 to 6 nm, by adding (benz)imidazolium bromide. Changing the reducing agent to tBuNH2BH3 shifts the size range to ca. 6-12 nm. A one pot protocol is also reported from AuCl, (benz)imidazolium bromides and NaBH4, thereby providing an even more straightforward way of producing NHC-capped gold nanoparticles. In addition, X-ray photoelectron spectroscopy (XPS) is used to unambiguously evidence, on the nanoparticles, the covalent bond formed between the NHC and the surface gold atoms.

Beyond the Compositional Threshold of Nanoparticle-Based Materials

The design of inorganic nanoparticles relies strongly on the knowledge from solid-state chemistry not only for characterization techniques, but also and primarily for choosing the systems that will yield the desired properties. The range of inorganic solids reported and studied as nanoparticles is however strikingly narrow when compared to the solid-state chemistry portfolio of bulk materials. Efforts to enlarge the collection of inorganic particles are becoming increasingly important for three reasons. First, they can yield materials more performing than current ones for a range of fields including biomedicine, optics, catalysis, and energy. Second, looking outside the box of common compositions is a way to target original properties or to discover genuinely new behaviors. The third reason lies in the path followed to reach these novel nano-objects: exploration and setup of new synthetic approaches. Indeed, willingness to access original nanoparticles faces a synthetic challenge: how to reach nanoparticles of solids that originally belong to the realm of solid-state chemistry and its typical protocols at high temperature? To answer this question, alternative reaction pathways must be sought, which may in turn provide tracks for new, untargeted materials. The corresponding strategies require limiting particle growth by confinement at high temperatures or by decreasing the synthesis temperature. Both approaches, especially the latter, provide a nice playground to discover metastable solids never reported before. The aim of this Account is to raise attention to the topic of the design of new inorganic nanoparticles. To do so, we take the perspective of our own work in the field, by first describing synthetic challenges and how they are addressed by current protocols. We then use our achievements to highlight the possibilities offered by new nanomaterials and to introduce synthetic approaches that are not in the focus of recent literature but hold, in our opinion, great promise. We will span methods of low temperature "chimie douce" aqueous synthesis coupled to microwave heating, sol-gel chemistry and processing coupled to solid state reactions, and then molten salt synthesis. These protocols pave the way to metastable low valence oxyhydroxides, vanadates, perovskite oxides, boron carbon nitrides, and metal borides, all obtained at the nanoscale with structural and morphological features differing from "usual" nanomaterials. These nano-objects show original properties, from sensing, thermoelectricity, charge and spin transports, photoluminescence, and catalysis, which require advanced characterization of surface states. We then identify future trends of synthetic methodologies that will merit further attention in this burgeoning field, by emphasizing the importance of unveiling reaction mechanisms and coupling experiments with modeling.

Tripodal tris-disulfides as capping agents for a controlled mixed functionalization of gold nanoparticles

New tris(SS)-tripodal capping ligands afford AuNP surface composition fine control and SPR sensing for TNT-like explosives.

Domain Formation and Conformational Changes in Gold Nanoparticle Conjugates Studied Using DPD Simulations

A gold nanoparticle (AuNP) conjugate formed with 11-mercaptoundecanoic acid (MUA) and thiolated polyethylene glycol (SH-PEG) is simulated using dissipative particle dynamics (DPD) methods, obtaining an excellent agreement with previous experimental observations. The simulations cover the isolated components (AuNP, MUA, and SH-PEG), as well as pairs of components, and finally the all three components at the same time. In this latter case, changes in the order of addition of MUA and SH-PEG over the AuNP are also considered. The AuNP is formed by independent gold beads and keeps an almost spherical shape throughout the simulation. MUA forms micelles of four to six MUA units when dispersed in water, while SH-PEG stays individually and well solvated. When exposed to AuNP, both molecules show a tendency to form patches on the surface. SH-PEG displays two different conformations (radial and tangential) depending on its relative concentration and the presence of other molecules at the NP surface. When combined at subsaturation concentrations, MUA arrives faster to the AuNP surface than SH-PEG and forms patches while SH-PEG occupies the remaining free surface. In these conditions, the order of addition of the different components partially alters these results. When SH-PEG is added over an already formed MUA/AuNP partial layer, it adopts a radial conformation over the MUA formed patches; on the contrary, if MUA is added over an already formed SH-PEG/AuNP partial layer, much less SH-PEGs adopt a radial conformation and MUA patches are significantly smaller.

Nanophase Segregation of Self-Assembled Monolayers on Gold Nanoparticles

Nanophase segregation of a bicomponent thiol self-assembled monolayer is predicted using atomistic molecular dynamics simulations and experimentally confirmed. The simulations suggest the formation of domains rich in acid-terminated chains, on one hand, and of domains rich in amide-functionalized ethylene glycol oligomers, on the other hand. In particular, within the amide-ethylene glycol oligomers region, a key role is played by the formation of interchain hydrogen bonds. The predicted phase segregation is experimentally confirmed by the synthesis of 35 and 15 nm gold nanoparticles functionalized with several binary mixtures of ligands. An extensive study by transmission electron microscopy and electron tomography, using silica selective heterogeneous nucleation on acid-rich domains to provide electron contrast, supports simulations and highlights patchy nanoparticles with a trend toward Janus nano-objects depending on the nature of the ligands and the particle size. These results validate our computational platform as an effective tool to predict nanophase separation in organic mixtures on a surface and drive further exploration of advanced nanoparticle functionalization.