Advances in surface design and biomedical applications of magnetic nanoparticles

Despite significant efforts by scientists in the development of advanced nanotechnology materials for smart diagnosis devices and drug delivery systems, the success of clinical trials remains largely elusive. In order to address this biomedical challenge, magnetic nanoparticles (MNPs) have gained attention as a promising candidate due to their theranostic properties, which allow the simultaneous treatment and diagnosis of a disease. Moreover, MNPs have advantageous characteristics such as a larger surface area, high surface-to-volume ratio, enhanced mobility, mass transference and, more notably, easy manipulation under external magnetic fields. Besides, certain magnetic particle types based on the magnetite (Fe3O4) phase have already been FDA-approved, demonstrating biocompatible and low toxicity. Typically, surface modification and/or functional group conjugation are required to prevent oxidation and particle aggregation. A wide range of inorganic and organic molecules have been utilized to coat the surface of MNPs, including surfactants, antibodies, synthetic and natural polymers, silica, metals, and various other substances. Furthermore, various strategies have been developed for the synthesis and surface functionalization of MNPs to enhance their colloidal stability, biocompatibility, good response to an external magnetic field, etc. Both uncoated MNPs and those coated with inorganic and organic compounds exhibit versatility, making them suitable for a range of applications such as drug delivery systems (DDS), magnetic hyperthermia, fluorescent biological labels, biodetection and magnetic resonance imaging (MRI). Thus, this review provides an update of recently published MNPs works, providing a current discussion regarding their strategies of synthesis and surface modifications, biomedical applications, and perspectives.

Layer-by-Layer Immobilization of DNA Aptamers on Ag-Incorporated Co-Succinate Metal-Organic Framework for Hg(II) Detection

Layer-by-layer (LbL) immobilization of DNA aptamers in the realm of electrochemical detection of heavy metal ions (HMIs) offers an enhancement in specificity, sensitivity, and low detection limits by leveraging the cross-reactivity obtained from multiple interactions between immobilized aptamers and developed material surfaces. In this research, we present a LbL approach for the immobilization of thiol- and amino-modified DNA aptamers on a Ag-incorporated cobalt-succinate metal-organic framework (MOF) (Ag@Co-Succinate) to achieve a cross-reactive effect on the electrochemical behavior of the sensor. The solvothermal method was utilized to synthesize Ag@Co-Succinate, which was also characterized through various techniques to elucidate its structure, morphology, and presence of functional groups, confirming its suitability as a host matrix for immobilizing both aptamers. The Ag@Co-Succinate aptasensor exhibited extraordinary sensitivity and selectivity towards Hg(II) ions in electrochemical detection, attributed to the unique binding properties of the immobilized aptamers. The exceptional limit of detection of 0.3 nM ensures the sensor's suitability for trace-level Hg(II) detection in various environmental and analytical applications. Furthermore, the developed sensor demonstrated outstanding repeatability, highlighting its potential for long-term and reliable monitoring of Hg(II).

A General One-Step Synthesis of Alkanethiyl-Stabilized Gold Nanoparticles with Control over Core Size and Monolayer Functionality

In spite of widespread interest in the unique size-dependent properties and consequent applications of gold nanoparticles (AuNPs), synthetic protocols that reliably allow for independent tuning of surface chemistry and core size, the two critical determinants of AuNP properties, remain limited. Often, core size is inherently affected by the ligand structure in an unpredictable fashion. Functionalized ligands are commonly introduced using postsynthesis exchange procedures, which can be inefficient and operationally delicate. Here, we report a one-step protocol for preparing monolayer-stabilized AuNPs that is compatible with a wide range of ligand functional groups and also allows for the systematic control of core size. In a single-phase reaction using the mild reducing agent tert-butylamine borane, AuNPs that are compatible with solvents spanning a wide range of polarities from toluene to water can be produced without damaging reactive chemical functionalities within the small-molecule surface-stabilizing ligands. We demonstrate that the rate of reduction, which is easily controlled by adjusting the period over which the reducing agent is added, is a simple parameter that can be used irrespective of the ligand structure to adjust the core size of AuNPs without broadening the size distribution. Core sizes in the range of 2-10 nm can thus be generated. The upper size limit appears to be determined by the nature of each specific ligand/solvent pairing. This protocol produces high quality, functionally sophisticated nanoparticles in a single step. By combining the ability to vary size-related nanoparticle properties with the option to incorporate reactive functional groups at the nanoparticle-solvent interface, it is possible to generate chemically reactive colloidal building blocks from which more complex nanoparticle-based devices and materials may subsequently be constructed.

Cationic Polymer Coating Increases the Catalytic Activity of Gold Nanoparticles toward Anionic Substrates

Organic coatings on catalytic metal nanoparticles (NPs) typically hinder their activity due to the blocking of active sites. Therefore, considerable effort is made to remove organic ligands when preparing supported NP catalytic materials. Here, cationic polyelectrolyte coatings are shown to increase the catalytic activity of partially embedded gold nanoislands (Au NIs) toward transfer hydrogenation and oxidation reactions with anionic substrates compared to the activity of identical but uncoated Au NIs. Any potential steric hindrance caused by the coating is countered by a decrease in the activation energy of the reaction by half, resulting in overall enhancement. The direct comparison to identical but uncoated NPs isolates the role of the coating and provides conclusive evidence of enhancement. Our findings show that engineering the microenvironment of heterogeneous catalysts, creating hybrid materials that cooperatively interact with the reactants involved, is a viable and exciting path to improving their performance.

Ligand Chemistry in Antitumor Theranostic Nanoparticles

ConspectusTheranostic nanoparticles' potential in tumor treatment has been widely acknowledged thanks to their capability of integrating multifaceted functionalities into a single nanosystem. Theranostic nanoparticles are typically equipped with an inorganic core with exploitable physical properties for imaging and therapeutic functions, bioinert coatings for improved biocompatibility and immunological stealth, controlled drug-loading-release modules, and the ability to recognize specific cell type for uptake. Integrating multiple functionalities in a single nanosized construct require sophisticated molecular design and precise execution of assembly procedures. Underlying the multifunctionality of theranostic nanoparticles, ligand chemistry plays a decisive role in translating theoretical designs into fully functionalized theranostic nanoparticles. The ligand hierarchy in theranostic nanoparticles is usually threefold. As they serve to passivate the nanoparticle's surface, capping ligands form the first layer directly interfacing with the crystalline lattice of the inorganic core. The size and shape of nanoparticles are largely determined by the molecular property of capping ligands so that they have profound influences on the nanoparticles' surface chemistry and physical properties. Capping ligands are mostly chemically inert, which necessitates the presence of additional ligands for drug loading and tumor targeting. The second layer is commonly utilized for drug loading. Therapeutic drugs can either be covalently conjugated onto the capping layer or noncovalently loaded onto nanoparticles via drug-loading ligands. Drug-loading ligands need to be equally versatile in properties to accommodate the diversity of drugs. Biodegradable moieties are often incorporated into drug-loading ligands to enable smart drug release. With the aid of targeting ligands which usually stand the tallest on the nanoparticle surface to seek and bind to their corresponding receptors on the target, theranostic nanoparticles can preferentially accumulate at the tumor site to attain a higher precision and quantity for drug delivery. In this Account, the properties and utilities of representative capping ligands, drug-loading ligands, and targeting ligands are reviewed. Since these types of ligands are often assembled in close vicinity to each other, it is essential for them to be chemically compatible and able to function in tandem with each other. Relevant conjugation strategies and critical factors with a significant impact on ligands' performance on nanoparticles are discussed. Representative theranostic nanoparticles are presented to showcase how different types of ligands function synergistically from a single nanosystem. Finally, the technological outlook of evolving ligand chemistry on theranostic nanoparticles is provided.

Singlet Oxygen Generation Driven by Sulfide Ligand Exchange on Porphyrin-Gold Nanoparticle Conjugates

Here, we report a switching method of singlet oxygen (¹O₂) generation based on the adsorption/desorption of porphyrins to gold nanoparticles driven by sulfide (thiol or disulfide) compounds. The generation of ¹O₂ by photosensitization is effectively suppressed by the gold nanoparticles and can be restored by a sulfide ligand exchange reaction. The on/off ratio of ¹O₂ quantum yield (Φ_Δ) reached 7.4. By examining various incoming sulfide compounds, it was found that the ligand exchange reaction on the gold nanoparticle surface could be thermodynamically or kinetically controlled. The remaining gold nanoparticles in the system still suppress the generation of ¹O₂, which can be precipitated out simultaneously with porphyrin desorption by the proper polarity choice of the incoming sulfide to restore the ¹O₂ generation.

Single-Electron Charging of Thioctic Acid Monolayer-Protected Gold Clusters

There is great interest in the use of Monolayer-Protected Gold Clusters (AuMPCs) as nanoscale capacitors in aqueous media for nanobiotechnological applications, such as bioelectrocatalysts, biofuel cells, and biosensors. However, AuMPCs exhibiting subattofarad double-layer capacitance at room temperature, and the resolution of single-electron charging, has been mainly obtained in an organic medium with nonfunctional capping ligands. We report here the synthesis of Thioctic Acid Monolayer-Protected Au Clusters (TA-AuMPCs) showing electrochemical single electron quantized capacitance charging in organic and aqueous solutions and when immobilized onto different self-assembled monolayer-modified gold electrodes. The presence of functional carboxylic groups opens a simple strategy for interfacing a nanoparticle assembly to biomolecules for their use as electron donors or acceptors in biological electron transfer reactions.

Mixed-ligand strategy for synthesizing water-soluble chiral gold clusters with phosphine ligands

Water-soluble chiral metal clusters have drawn much attention by virtue of their fascinating physicochemical properties and potential biomedical applications, but currently, phosphine-protected Au clusters with both chirality and water-solubility are still very limited. In this article, we demonstrate a mixed-ligand strategy for the facile synthesis of atomically precise, water-soluble chiral Au clusters protected by phosphine alone. The clusters are obtained by the reduction of aurate ions in the presence of a phosphine mixture consisting of highly hydrophilic monophosphine (i.e., triphenylphosphine trisulfonate; TPPTS) and hydrophobic chiral diphosphine (i.e., S-Segphos or S-BINAP), both of which are commercially available. The clusters are size/composition-separated via gel electrophoresis, and notably, heptanuclear cluster Au₇(S-Segphos)₃(TPPTS)₂ exhibits a large chiroptical activity with the maximum anisotropy factor (g-factor) of 4.7 × 10^-3, one of the largest values in such Au clusters. Quantum chemical calculations for model Au₇ cluster species suggest two important factors to obtain large chiroptical activity: (i) more than two axially-chiral diphosphine ligands, and (ii) the absence of configurational isomer averaging. Consequently, despite the experimental use of a mixture containing both chiral and achiral phosphines, a large chiroptical activity can be created in Au clusters with high water-solubility.

Gold Nanoclusters as Nanoantibiotic Auranofin Analogues

Auranofin, a gold(I)-complex with tetraacetylated thioglucose (Ac4 GlcSH) and triethylphosphine ligands, is an FDA-approved drug used as an anti-inflammatory aid in the treatment of rheumatoid arthritis. In repurposing auranofin for other diseases, it was found that the drug showed significant activity against Gram-positive but was inactive against Gram-negative bacteria. Herein, the design and synthesis of gold nanoclusters (AuNCs) based on the structural motif of auranofin are reported. Phosphine-capped AuNCs are synthesized and glycosylated, yielding auranofin analogues with mixed triphenylphosphine monosulfonate (TPPMS)/Ac4 GlcSH ligand shells. These AuNCs are active against both Gram-negative and Gram-positive bacteria, including multidrug-resistant pathogens. Notably, an auranofin analogue, a mixed-ligand 1.6 nm AuNC 4b, is more active than auranofin against Pseudomonas aeruginosa, while exhibiting lower toxicity against human A549 cells. The enhanced antibacterial activity of these AuNCs is characterized by a greater uptake of Au by the bacteria compared to AuI complexes. Additional factors include increased oxidative stress, moderate inhibition of thioredoxin reductase (TrxR), and DNA damage. Most intriguingly, the uptake of AuNCs are not affected by the bacterial outer membrane (OM) barrier or by binding with the extracellular proteins. This contrasts with AuI complexes like auranofin that are susceptible to protein binding and hindered by the OM barrier.

A Review of State of the Art in Phosphine Ligated Gold Clusters and Application in Catalysis

Atomically precise gold clusters are highly desirable due to their well-defined structure which allows the study of structure-property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal-ligand interaction and type of ligands. This critical feature renders gold-phosphine clusters unique and distinct from other ligand-protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold-phosphorous (Au-P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold-phosphine clusters and to present an in-depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold-phosphine clusters. Advanced characterization techniques, including synchrotron-based spectroscopy, have unraveled substantial effects of Au-P interaction on the composition-, structure-, and size-dependent properties. State-of-the-art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold-phosphine clusters in catalysis is presented.

Tethered Blatter Radical for Molecular Grafting: Synthesis of 6-Hydroxyhexyloxy, Hydroxymethyl, and Bis(hydroxymethyl) Derivatives and Their Functionalization

Synthetic access to 7-CF₃-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl radicals containing 4-(6-hydroxyhexyloxy)phenyl, 4-hydroxymethylphenyl or 3,5-bis(hydroxymethyl)phenyl groups at the C(3) position and their conversion to tosylates and phosphates are described. The tosylates were used to obtain disulfides and an azide with good yields. The Blatter radical containing the azido group underwent a copper(I)-catalyzed azide-alkyne cycloaddition with phenylacetylene under mild conditions, giving the [1,2,3]triazole product in 84% yield. This indicates the suitability of the azido derivative for grafting Blatter radical onto other molecular objects via the CuAAC "click" reaction. The presented derivatives are promising for accessing surfaces and macromolecules spin-labeled with the Blatter radical.

Mixed-Charge Nanocarriers Allow for Selective Targeting of Mitochondria by Otherwise Nonselective Dyes

Targeted delivery of molecular cargos to specific organelles is of paramount importance for developing precise and effective therapeutics and imaging probes. This work describes a disulfide-based delivery method in which mixed-charged nanoparticles traveling through the endolysosomal tract deliver noncovalently bound dye molecules selectively into mitochondria. This system comprises three elements: (1) The nanoparticles deliver their payloads by a kiss-and-go mechanism - that is, they drop off their dye cargos proximate to mitochondria but do not localize therein; (2) the dye molecules are by themselves nonspecific to any cellular structures but become so with the help of mixed-charge nanocarriers; and (3) the dye is engineered in such a way as to remain in mitochondria for a long time, up to days, allowing for observing dynamic remodeling of mitochondrial networks and long-term tracking of mitochondria even in dividing cells. The selectivity of delivery and long-lasting staining derive from the ability to engineer charge-imbalanced, mixed [+/-] on-particle monolayers and from the structural features of the cargo. Regarding the former, the balance of [+] and [-] ligands can be adjusted to limit cytotoxicity and control the number of dye molecules adsorbed onto the particles' surfaces. Regarding the latter, comparative studies with multiple dye derivatives we synthesized rationalize the importance of polar groups, long alkyl chains, and disulfide moieties in the assembly of fluorescent nanoconstructs and long-lasting staining of mitochondria. Overall, this strategy could be useful for delivering hydrophilic and/or anionic small-molecule drugs difficult to target to mitochondria by classical approaches.

Rapidly Adaptive All-covalent Nanoparticle Surface Engineering

Emerging nanotechnologies demand the manipulation of nanoscale components with the same predictability and programmability as is taken for granted in molecular synthetic methodologies. Yet installing appropriately reactive chemical functionality on nanomaterial surfaces has previously entailed compromises in terms of reactivity scope, functionalization density, or both. Here, we introduce an idealized dynamic covalent nanoparticle building block for divergent and adaptive post-synthesis modification of colloidal nanomaterials. Acetal-protected monolayer-stabilized gold nanoparticles are prepared via operationally simple protocols and are stable to long-term storage. Tunable surface densities of reactive aldehyde functionalities are revealed on-demand, leading to a wide range of adaptive surface engineering options from one nanoscale synthon. Analytically tractable with molecular precision, interfacial reaction kinetics and dynamic surface constitutions can be probed in situ at the ensemble level. High functionalization densities combined with rapid equilibration kinetics enable environmentally adaptive surface constitutions and rapid nanoparticle property switching in response to simple chemical effectors.

Probing Multiscale Factors Affecting the Reactivity of Nanoparticle-Bound Molecules

The structures and physicochemical properties of surface-stabilizing molecules play a critical role in defining the properties, interactions, and functionality of hybrid nanomaterials such as monolayer-stabilized nanoparticles. Concurrently, the distinct surface-bound interfacial environment imposes very specific conditions on molecular reactivity and behavior in this setting. Our ability to probe hybrid nanoscale systems experimentally remains limited, yet understanding the consequences of surface confinement on molecular reactivity is crucial for enabling predictive nanoparticle synthon approaches for postsynthesis engineering of nanoparticle surface chemistry and construction of devices and materials from nanoparticle components. Here, we have undertaken an integrated experimental and computational study of the reaction kinetics for nanoparticle-bound hydrazones, which provide a prototypical platform for understanding chemical reactivity in a nanoconfined setting. Systematic variation of just one molecular-scale structural parameter-the distance between reactive site and nanoparticle surface-showed that the surface-bound reactivity is influenced by multiscale effects. Nanoparticle-bound reactions were tracked in situ using ¹⁹F NMR spectroscopy, allowing direct comparison to the reactions of analogous substrates in bulk solution. The surface-confined reactions proceed more slowly than their solution-phase counterparts, and kinetic inhibition becomes more significant for reactive sites positioned closer to the nanoparticle surface. Molecular dynamics simulations allowed us to identify distinct supramolecular architectures and unexpected dynamic features of the surface-bound molecules that underpin the experimentally observed trends in reactivity. This study allows us to draw general conclusions regarding interlinked structural and dynamical features across several length scales that influence interfacial reactivity in monolayer-confined environments.

Kinetics of Intercluster Reactions between Atomically Precise Noble Metal Clusters [Ag25(DMBT)18]- and [Au25(PET)18]- in Room Temperature Solutions

The kinetics of intercluster metal atom exchange reactions between solvated [Ag₂₅(DMBT)₁₈]^- and [Au₂₅(PET)₁₈]^- (DMBT and PET are 2,4-dimethylbenzenethiol and 2-phenylethanethiol, respectively, both C₈H₁₀S) were probed by electrospray ionization mass spectrometry and computer-based modeling. Anion mass spectra and collision induced dissociation (CID) measurements show that both cluster monomers and dimers are involved in the reactions. We have modeled the corresponding kinetics assuming a reaction mechanism in which metal atom exchange occurs through transient dimers. Our kinetic model contains three types of generic reactions: dimerization of monomers, metal atom exchange in the transient dimers, and dissociation of the dimers to monomers. There are correspondingly 377 discrete species connected by in total 1302 reactions (i.e., dimerization, dissociation and atom exchange reactions) leading to the entire series of monomeric and dimeric products [Ag_mAu_25-m]^- (m = 1-24) and [Ag_mAu_50-m]^2- (m = 0-50), respectively. The rate constants of the corresponding reactions were fitted to the experimental data, and good agreement was obtained with exchange rate constants which scale with the probability of finding a silver or gold atom in the respective monomeric subunit of the dimer, i.e., reflecting an entropic driving force for alloying. Allowing the dimerization rate constant to scale with increasing gold composition of the respective reactants improves the agreement further. The rate constants obtained are physically plausible, thus strongly supporting dimer-mediated metal atom exchange in this intercluster reaction system.

Pt-S Bond-Mediated Nanoflares for High-Fidelity Intracellular Applications by Avoiding Thiol Cleavage

The Au-S bond is the classic way to functionalize gold nanoparticles (AuNPs). However, cleavage of the bond by biothiols and other chemicals is a long-standing problem hindering practical applications, especially in cells. Instead of replacing the thiol by a carbene or selenol for stronger adsorption, it is now shown that the Pt-S bond is much more stable, fully avoiding cleavage by biothiols. AuNPs were deposited with a thin layer of platinum, and an AuNP@Pt-S nanoflare was constructed to detect the miRNA-21 microRNA in living cells. This design retained the optical and cellular uptake properties of DNA-functionalized AuNPs, while showing high-fidelity signaling. It discriminated target cancer cells even in a mixed-cell culture system, where the Au-S based nanoflare was less sensitive. Compared to previous methods of changing the ligand chemistry, coating a Pt shell is more accessible, and previously developed methods for AuNPs can be directly adapted.

Photoswitchable Macroscopic Solid Surfaces Based On Azobenzene-Functionalized Polydopamine/Gold Nanoparticle Composite Materials: Formation, Isomerization and Ligand Exchange

The facile preparation of dynamic interfaces is presented based on the combination of photoisomerizable azobenzenes and polydopamine (PDA)/Au nanoparticle composite materials. Azobenzenes with different spacer lengths (C₃ , C₆ ) and surface-binding groups (SH, NH₂ ) were synthesized. The polymer layer on macroscopic quartz surface was prepared by the facile aerobic autopolymerisation of dopamine hydrochloride under basic conditions. The presence of redox-active catechol moieties meant that gold nanoparticles were formed on the polymer surface. The obtained UV-Vis spectroscopic results confirmed that following their successful assembly, the switching of azobenzenes on PDA/Au was not affected by the surface binding group and the spacer length of the azobenzene molecules under the measurement conditions. Furthermore, facilitated by the curved nature of the Au particles, the surface-bound azobenzene layer could be reconstructed by ligand-exchange processes, and the photochemical characterization of the mixed layer was performed.

Structural characterization of Au nano bipyramids: reshaping under thermal annealing, the capping agent effect and surface decoration with Pt

Anisotropic gold nanoparticles offer potential applications due to their functionalities and shape-dependent properties. Reshaping noble metal nanoparticles is an interesting field with optical, surface-enhanced Raman spectroscopy, catalytic applications and potential application as a photothermic therapy. This work comprises a structural study on gold nano bipyramids (Au NBPs) and nanodumbbells, and the evolution of Au NBPs capped with cetyltrimethylammonium bromide and dodecanethiol through an in situ and ex situ heating process in high vacuum. Also, we study the reshaping of Au NBPs by the addition of Pt to study the surface modification and the strain generated on a single particle by geometric phase analysis.

Chirality in Au9 clusters protected by chiral/achiral mixed bidentate phosphine ligands: influence of the metal core and ligand array

Modulation of chiroptical activity in Au₉ clusters by mixed-diphosphine ligation is associated with the difference in the degree of chirality of the cluster core.

Ultrastable Gold Nanoparticles Modified by Bidentate N-Heterocyclic Carbene Ligands

Highly stable gold nanoparticles (Au NPs) functionalized by bidentate N-heterocyclic carbene (NHC) ligands have been synthesized by top-down and bottom-up approaches. A detailed study of the effect of alkylation, denticity, and method of synthesis has led to the production of NHC-stabilized nanoparticles with higher thermal stability than bi- and tridentate thiol-protected Au NPs and than monodentate NHC-stabilized NPs. Importantly, bidentate NHC-protected NPs also displayed unprecedented stability to external thiol, which has been an unsolved problem to date with all nanoparticles. Thus, multidentate NHC ligands are an important, and as yet unrecognized, step forward for the preparation of high stability nanomaterials.

Multiscale simulations of ligand adsorption and exchange on gold nanoparticles

Multiscale simulations of CTAB/PEG-SH ligand adsorption and exchange kinetics on gold nanoparticles.

Continuum tuning of nanoparticle interfacial properties by dynamic covalent exchange

Dynamic covalent modification of the surface-stabilizing monolayer accesses a continuum of nanoparticle properties from a single starting point.

Surface chemical composition is fundamental to determining properties on the nanoscale, making precise control over surface chemistry critical to being able to optimise nanomaterials for virtually any application. Surface-engineering independent of the preparation of the underlying nanomaterial is particularly attractive for efficient, divergent synthetic strategies, and for the potential to create reactive, responsive and smart nanodevices. For monolayer-stabilised nanoparticles, established methods include ligand exchange to replace the ligand shell in its entirety, encapsulation with amphiphilic (macro)molecules, noncovalent interactions with surface-bound biomolecules, or a relatively limited number of covalent bond forming reactions. Yet, each of these approaches has considerable drawbacks. Here we show that dynamic covalent exchange at the periphery of the nanoparticle-stabilizing monolayer allows surface-bound ligand molecular structure to be substantially modified in mild and reversible processes that are independent of the nanoparticle–molecule interface. Simple stoichiometric variation allows the extent of exchange to be controlled, generating a range of kinetically stable mixed-monolayer compositions across an otherwise identical, self-consistent series of nanoparticles. This approach can be used to modulate nanoparticle properties that are defined by the monolayer composition. We demonstrate switching of nanoparticle solvent compatibility between widely differing solvents – spanning hexane to water – and the ability to tune solubility across the entire continuum between these extremes, all from a single nanoparticle starting point. We also demonstrate that fine control over mixed-monolayer composition influences the assembly of discrete, colloidally stable nanoparticle clusters. By carefully assessing monolayer composition in each state, using both in situ and ex situ methods, we are able to correlate the molecular-level details of the nanoparticle-bound monolayer with system-level properties and behaviour. These empirically determined relationships contribute fundamental insights on nanoscale structure–function relationships, which are currently beyond the capabilities of ab initio prediction.

Assembly Behavior of Organically Interlinked Gold Nanoparticle Composite Films: A Quartz Crystal Microbalance Investigation

Thin films based on dodecylamine stabilized gold nanoparticles interlinked with different organic molecules are prepared by automatic layer-by-layer self-assembly in a microfluidic quartz crystal microbalance (QCM) cell, to obtain an in situ insight on the film formation by ligand/linker exchange reactions. The influence of interlinking functional groups and the length of the organic linker molecule on the assembly behavior is investigated. Alkyldithiols with different lengths are compared to alkyldiamines and alkylbisdithiocarbamates with a C8 alkylic molecular backbone. The stepwise layer-by-layer assembly occurs independently of the linker molecule, while the largest frequency changes always correspond to the gold nanoparticle step. During the solvent rinsing and ligand/linker exchange reaction step, the frequency is almost constant with slight increases or decreases dependent on the molar mass of the linker compared to the exchanged ligand. The assembly efficiency is higher for shorter molecules and for molecules with stronger interacting functional groups. The densities of the composite films are calculated from QCM data and independent thickness measurements. They reflect the higher fraction of organic material in the films comprising longer organic linkers. The plasmon resonance band of the gold nanoparticles in the final assemblies is measured with UV/vis spectroscopy. Band positions in films prepared from dithiols and diamines of comparable lengths are very similar, while the spectrum of the bisdithiocarbamate film exhibits a distinct blue-shift. This observation is explained by the longer molecular structure of the linker due to a larger binding group, in conjunction with a delocalization of particle charge on the organic molecule. Obtained results play an essential role in the understanding of thin film layer-by-layer self-assembly processes, and enable the formation of new gold nanoparticle networks with organic diamine and bisdithiocarbamate molecules.

Nanoscale Surface Curvature Effects on Ligand-Nanoparticle Interactions: A Plasmon-Enhanced Spectroscopic Study of Thiolated Ligand Adsorption, Desorption, and Exchange on Gold Nanoparticles

The interfacial adsorption, desorption, and exchange behaviors of thiolated ligands on nanotextured Au nanoparticle surfaces exhibit phenomenal site-to-site variations essentially dictated by the local surface curvatures, resulting in heterogeneous thermodynamic and kinetic profiles remarkably more sophisticated than those associated with the self-assembly of organothiol ligand monolayers on atomically flat Au surfaces. Here we use plasmon-enhanced Raman scattering as a spectroscopic tool combining time-resolving and molecular fingerprinting capabilities to quantitatively correlate the ligand dynamics with detailed molecular structures in real time under a diverse set of ligand adsorption, desorption, and exchange conditions at both equilibrium and nonequilibrium states, which enables us to delineate the effects of nanoscale surface curvature on the binding affinity, cooperativity, structural ordering, and the adsorption/desorption/exchange kinetics of organothiol ligands on colloidal Au nanoparticles. This work provides mechanistic insights on the key thermodynamic, kinetic, and geometric factors underpinning the surface curvature-dependent interfacial ligand behaviors, which serve as a central knowledge framework guiding the site-selective incorporation of desired surface functionalities into individual metallic nanoparticles for specific applications.

Quantitative Measurement of Ligand Exchange with Small-Molecule Ligands on Iron Oxide Nanoparticles via Radioanalytical Techniques

Ligand exchange on the surface of hydrophobic iron oxide nanoparticles is a common method for controlling surface chemistry for a desired application. Furthermore, ligand exchange with small-molecule ligands may be necessary to obtain particles with a specific size or functionality. Understanding to what extent ligand exchange occurs and what factors affect it is important for the optimization of this critical procedure. However, quantifying the amount of exchange may be difficult because of the limitations of commonly used characterization techniques. Therefore, we utilized a radiotracer technique to track the exchange of a radiolabeled ¹⁴C-oleic acid ligand with hydrophilic small-molecule ligands on the surface of iron oxide nanoparticles. Iron oxide nanoparticles functionalized with ¹⁴C-oleic acid were modified with small-molecule ligands with terminal functional groups including catechols, phosphonates, sulfonates, thiols, carboxylic acids, and silanes. These moieties were selected because they represent the most commonly used ligands for this procedure. The effectiveness of these molecules was compared using both procedures widely found in the literature and using a standardized procedure. After ligand exchange, the nanoparticles were analyzed using liquid scintillation counting (LSC) and inductively coupled plasma-mass spectrometry. The labeled and unlabeled particles were further characterized by transmission electron microscopy (TEM) and dynamic light scattering (DLS) to determine the particle size, hydrodynamic diameter, and zeta potential. The unlabeled particles were characterized via attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and vibrating sample magnetometry (VSM) to confirm the presence of the small molecules on the particles and verify the magnetic properties, respectively. Radioanalytical determination of ¹⁴C-oleic acid was used to calculate the total amount of oleic acid remaining on the surface of the particles after ligand exchange. The results revealed that the ligand-exchange reactions performed using widely cited procedures did not go to completion. Residual oleic acid remained on the particles after these reactions and the reactions using a standardized protocol. A comparison of the ligand-exchange procedures indicated that the binding moiety, multidenticity, reaction time, temperature, and presence of a catalyst impacted the extent of exchange. Quantification of the oleic acid remaining after ligand exchange revealed a binding hierarchy in which catechol-derived anchor groups displace the most oleic acid on the surface of the nanoparticles and the thiol group displaces the least amount of oleic acid. Thorough characterization of ligand exchange is required to develop nanoparticles suitable for their intended application.

Dynamic Covalent Nanoparticle Building Blocks

Rational and generalisable methods for engineering surface functionality will be crucial to realising the technological potential of nanomaterials. Nanoparticle-bound dynamic covalent exchange combines the error-correcting and environment-responsive features of equilibrium processes with the stability, structural precision, and vast diversity of covalent chemistry, defining a new and powerful approach for manipulating structure, function and properties at nanomaterial surfaces. Dynamic covalent nanoparticle (DCNP) building blocks thus present a whole host of possibilities for constructing adaptive systems, devices and materials that incorporate both nanoscale and molecular functional components. At the same time, DCNPs have the potential to reveal fundamental insights regarding dynamic and complex chemical systems confined to nanoscale interfaces.

Water-Soluble Phosphine-Protected Au₁₁ Clusters: Synthesis, Electronic Structure, and Chiral Phase Transfer in a Synergistic Fashion

Synthesis of atomically precise, water-soluble phosphine-protected gold clusters is still currently limited probably due to a stability issue. We here present the synthesis, magic-number isolation, and exploration of the electronic structures as well as the asymmetric conversion of triphenylphosphine monosulfonate (TPPS)-protected gold clusters. Electrospray ionization mass spectrometry and elemental analysis result in the primary formation of Au11(TPPS)9Cl undecagold cluster compound. Magnetic circular dichroism (MCD) spectroscopy clarifies that extremely weak transitions are present in the low-energy region unresolved in the UV-vis absorption, which can be due to the Faraday B-terms based on the magnetically allowed transitions in the cluster. Asymmetric conversion without changing the nuclearity is remarkable by the chiral phase transfer in a synergistic fashion, which yields a rather small anisotropy factor (g-factor) of at most (2.5-7.0) × 10(-5). Quantum chemical calculations for model undecagold cluster compounds are then used to evaluate the optical and chiroptical responses induced by the chiral phase transfer. On this basis, we find that the Au core distortion is ignorable, and the chiral ion-pairing causes a slight increase in the CD response of the Au11 cluster.

Magnetic and Electrochemical Properties of a TEMPO-Substituted Disulfide Diradical in Solution, in the Crystal, and on a Surface

A study of the magnetic and electrochemical properties of a TEMPO-substituted disulfide diradical in three different environments was carried out: in solution, in the crystal, and as a self-assembled monolayer (SAM) on an Au(111) substrate, and the relationship between them was explored. In solution, this flexible diradical shows a strong spin-exchange interaction between the two nitroxide functions that depends on the temperature and solvent. Structural, dynamic, and thermodynamic information has been extracted from the EPR spectra of this dinitroxide. The magnetic interactions in the crystal include intra- and intermolecular contributions, which have been studied separately and shown to be antiferromagnetic in both cases. Finally, we demonstrate that both the magnetic and electrochemical properties are preserved upon chemisorption of the diradical on a gold surface. The resulting SAM displayed anisotropic magnetic properties, and angle-resolved EPR spectra of the monocrystal allowed a rough determination of the orientation of the molecules in the SAM.

Influence of Ligand Shell Composition upon Interparticle Interactions in Multifunctional Nanoparticles

The interactions of nanoparticles with biomolecules, surfaces, or other nanostructures are dictated by the nanoparticle's surface chemistry. Thus, far, shortcomings of syntheses of nanoparticles with defined ligand shell architectures have limited our ability to understand how changes in their surface composition influence reactivity and assembly. We report new synthetic approaches to systematically control the number (polyvalency), length, and steric interactions of omega-functionalized (targeting) ligands within an otherwise passivating (diluent) ligand shell. A mesofluidic reactor was used to prepare nanoparticles with the same core diameter for each of the designed ligand architectures. When the targeting ligands are malonamide groups, the nanoparticles assemble via cross-linking in the presence of trivalent lanthanides. We examined the influence of ligand composition on assembly by monitoring the differences in optical properties of the cross-linked and free nanoparticles. Infrared spectroscopy, electron microscopy, and solution small-angle X-ray scattering provided additional insight into the assembly behavior. Lower (less than 33%) malonamide ligand densities (where the binding group extends beyond the periphery of diluent ethylene glycol ligands) produce the strongest optical responses and largest assemblies. Surprisingly, nanoparticles containing a higher surface number of targeting ligand did not produce an optical response or assemble, underscoring the importance of an informed mixed ligand strategy for highest nanoparticle performance.

Reversible Photoisomerization of Spiropyran on the Surfaces of Au25 Nanoclusters

Au25 nanoclusters functionalized with a spiropyran molecular switch are synthesized via a ligand-exchange reaction at low temperature. The resulting nanoclusters are characterized by optical and NMR spectroscopies as well as by mass spectrometry. Spiropyran bound to nanoclusters isomerizes in a reversible fashion when exposed to UV and visible light, and its properties are similar to those of free spiropyran molecules in solution. The reversible photoisomerization entails the modulation of fluorescence as well as the light-controlled self-assembly of nanoclusters.

Charge Transfer at Hybrid Interfaces: Plasmonics of Aromatic Thiol-Capped Gold Nanoparticles

Although gold nanoparticles stabilized by organic thiols are the building blocks in a wide range of applications, the role of the ligands on the plasmon resonance of the metal core has been mostly ignored until now. Herein, a methodology based on the combination of spectroscopic ellipsometry and UV-vis spectroscopy is applied to extract dielectric functions of the different components. It is shown that aromatic thiols allow a significant charge transfer at the hybrid interface with the s and d bands of the gold core that yields "giant" red shifts of the plasmon band, up to 40 nm for spherical particles in the size range of 3-5 nm. These results suggest that hybrid nanoplasmonic devices may be designed through the suitable choice of metal core and organic components for optimized charge exchange.

Reversible Control of Nanoparticle Functionalization and Physicochemical Properties by Dynamic Covalent Exchange

Existing methods for the covalent functionalization of nanoparticles rely on kinetically controlled reactions, and largely lack the sophistication of the preeminent oligonucleotide-based noncovalent strategies. Here we report the application of dynamic covalent chemistry for the reversible modification of nanoparticle (NP) surface functionality, combining the benefits of non-biomolecular covalent chemistry with the favorable features of equilibrium processes. A homogeneous monolayer of nanoparticle-bound hydrazones can undergo quantitative dynamic covalent exchange. The pseudomolecular nature of the NP system allows for the in situ characterization of surface-bound species, and real-time tracking of the exchange reactions. Furthermore, dynamic covalent exchange offers a simple approach for reversibly switching-and subtly tuning-NP properties such as solvophilicity.

Reversible control of nanoparticle functionalization and physicochemical properties by dynamic covalent exchange

Existing methods for the covalent functionalization of nanoparticles rely on kinetically controlled reactions, and largely lack the sophistication of the preeminent oligonucleotide-based noncovalent strategies. Here we report the application of dynamic covalent chemistry for the reversible modification of nanoparticle (NP) surface functionality, combining the benefits of non-biomolecular covalent chemistry with the favorable features of equilibrium processes. A homogeneous monolayer of nanoparticle-bound hydrazones can undergo quantitative dynamic covalent exchange. The pseudomolecular nature of the NP system allows for the in situ characterization of surface-bound species, and real-time tracking of the exchange reactions. Furthermore, dynamic covalent exchange offers a simple approach for reversibly switching—and subtly tuning—NP properties such as solvophilicity.

Gold nanocrystal arrays as a macroscopic platform for molecular junction thermoelectrics

The thermoelectric properties of gold nanocrystal arrays with thiol-terminated ligands are compared to molecular junction experiments.

Comparative STM studies of mixed ligand monolayers on gold nanoparticles in air and in 1-phenyloctane

Scanning tunnelling microscopy (STM) studies have found stripe-like domains on gold nanoparticles (NPs) coated with certain binary mixtures of ligand molecules. The majority of these NPs' properties have been investigated for particles in solvents. Yet, most STM studies are for NPs in a dry state. Images of the same particles in air and liquid have not been obtained yet. In this work, a judicious choice of ligand molecules led to NPs with close-to-ideal STM imaging conditions in air and in 1-phenyloctane (PO). Large datasets under both conditions were acquired and rapidly evaluated through power spectral density (PSD) analysis. The result is a quantitative comparison of stripe-like domains in air and PO on the same NPs. PSD analysis determines a characteristic length-scale for these domains of ~1.0 nm in air and in PO showing persistence of striped domains in these two media. A length scale of ~0.7 nm for homoligand NPs was found.

Structurally similar triphenylphosphine-stabilized undecagolds, Au11(PPh3)7Cl3 and [Au11(PPh3)8Cl2]Cl, exhibit distinct ligand exchange pathways with glutathione

Ligand exchange is frequently used to introduce new functional groups on the surface of inorganic nanoparticles or clusters while preserving the core size. For one of the smallest clusters, triphenylphosphine (TPP)-stabilized undecagold, there are conflicting reports in the literature regarding whether core size is retained or significant growth occurs during exchange with thiol ligands. During an investigation of these differences in reactivity, two distinct forms of undecagold were isolated. The X-ray structures of the two forms, Au₁₁(PPh₃)₇Cl₃ and [Au₁₁(PPh₃)₈Cl₂]Cl, differ only in the number of TPP ligands bound to the core. Syntheses were developed to produce each of the two forms, and their spectroscopic features correlated with the structures. Ligand exchange on [Au₁₁(PPh₃)₈Cl₂]Cl yields only small clusters, whereas exchange on Au₁₁(PPh₃)₇Cl₃ (or mixtures of the two forms) yields the larger Au₂₅ cluster. The distinctive features in the optical spectra of the two forms made it possible to evaluate which of the cluster forms were used in the previously published papers and clarify the origin of the differences in reactivity that had been reported. The results confirm that reactions of clusters and nanoparticles may be influenced by small variations in the arrangement of ligands and suggest that the role of the ligand shell in stabilizing intermediates during ligand exchange may be essential to preventing particle growth or coalescence.

A technique to functionalize and self-assemble macroscopic nanoparticle-ligand monolayer films onto template-free substrates

This protocol describes a self-assembly technique to create macroscopic monolayer films composed of ligand-coated nanoparticles. The simple, robust and scalable technique efficiently functionalizes metallic nanoparticles with thiol-ligands in a miscible water/organic solvent mixture allowing for rapid grafting of thiol groups onto the gold nanoparticle surface. The hydrophobic ligands on the nanoparticles then quickly phase separate the nanoparticles from the aqueous based suspension and confine them to the air-fluid interface. This drives the ligand-capped nanoparticles to form monolayer domains at the air-fluid interface. The use of water-miscible organic solvents is important as it enables the transport of the nanoparticles from the interface onto template-free substrates. The flow is mediated by a surface tension gradient and creates macroscopic, high-density, monolayer nanoparticle-ligand films. This self-assembly technique may be generalized to include the use of particles of different compositions, size, and shape and may lead to an efficient assembly method to produce low-cost, macroscopic, high-density, monolayer nanoparticle films for wide-spread applications.

New frontiers in hybrid materials: noble metal nanoparticles – supramolecular gel systems

Recent developments in hybrid materials composed of noble metal nanoparticles and supramolecular gels are reviewed, with particular emphasis on synthetic procedures, characterization and potential applications.

High thermal stability of block copolymer-capped Au and Cu nanoparticles

Pyridine-containing block copolymers (PS-P4VP) can effectively stabilize metal nanoparticles which survive prolonged heating in solutions at high temperatures.