A dynamic view of self-assembled monolayers

Probing surface properties of organic molecular layers by scanning tunneling microscopy

In view of the relevance of organic thin layers in many fields, the fundamentals, growth mechanisms, and dynamics of thin organic layers, in particular thiol-based self-assembled monolayers (SAMs) on Au(111) are systematically elaborated. From both theoretical and practical perspectives, dynamical and structural features of the SAMs are of great intrigue. Scanning tunneling microscopy (STM) is a remarkably powerful technique employed in the characterization of SAMs. Numerous research examples of investigation about the structural and dynamical properties of SAMs using STM, sometimes combined with other techniques, are listed in the review. Advanced options to enhance the time resolution of STM are discussed. Additionally, we elaborate on the extremely diverse dynamics of various SAMs, such as phase transitions and structural changes at the molecular level. In brief, the current review is expected to supply a better understanding and novel insights regarding the dynamical events happening in organic SAMs and how to characterize these processes.

Nanoparticle Targeting with Antibodies in the Central Nervous System

Treatments for disease in the central nervous system (CNS) are limited because of difficulties in agent penetration through the blood-brain barrier, achieving optimal dosing, and mitigating off-target effects. The prospect of precision medicine in CNS treatment suggests an opportunity for therapeutic nanotechnology, which offers tunability and adaptability to address specific diseases as well as targetability when combined with antibodies (Abs). Here, we review the strategies to attach Abs to nanoparticles (NPs), including conventional approaches of chemisorption and physisorption as well as attempts to combine irreversible Ab immobilization with controlled orientation. We also summarize trends that have been observed through studies of systemically delivered Ab-NP conjugates in animals. Finally, we discuss the future outlook for Ab-NPs to deliver therapeutics into the CNS.

Controlled Hysteresis of Conductance in Molecular Tunneling Junctions

The problem this paper addresses is the origin of the hysteretic behavior in two-terminal molecular junctions made from an EGaIn electrode and self-assembled monolayers of alkanethiolates terminated in chelates (transition metal dichlorides complexed with 2,2'-bipyridine; BIPY-MCl₂). The hysteresis of conductance displayed by these BIPY-MCl₂ junctions changes in magnitude depending on the identity of the metal ion (M) and the window of the applied voltage across the junction. The hysteretic behavior of conductance in these junctions appears only in an incoherent (Fowler-Nordheim) tunneling regime. When the complexed metal ion is Mn(II), Fe(II), Co(II), or Ni(II), both incoherent tunneling and hysteresis are observed for a voltage range between +1.0 V and -1.0 V. When the metal ion is Cr(II) or Cu(II), however, only resonant (one-step) tunneling is observed, and the junctions exhibit no hysteresis and do not enter the incoherent tunneling regime. Using this correlation, the conductance characteristics of BIPY-MCl₂ junctions can be controlled. This voltage-induced change of conductance demonstrates a simple, fast, and reversible way (i.e., by changing the applied voltage) to modulate conductance in molecular tunneling junctions.

Electrochemical and X-ray Photoelectron Spectroscopy Surface Characterization of Interchain-Driven Self-Assembled Monolayer (SAM) Reorganization

Herein, we report a combined strategy encompassing electrochemical and x-ray photoelectron spectroscopy (XPS) experiments to investigate self-assembled monolayer (SAM) conformational reorganization onto an electrode surface due to the application of an electrical field. In particular, 3-mercaptopriopionic acid SAM (3MPA SAM) modified gold electrodes are activated with a 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysulfosuccinimide (NHSS) (EDC-NHSS) mixture by shortening the activation time, from 2 h to 15/20 min, labelled as Protocol-A, -B and -C, respectively. This step, later followed by a deactivation process with ethanolamine (EA), plays a key role in the reaction yields (formation of N-(2-hydroxyethyl)-3-mercaptopropanamide, NMPA) but also in the conformational rearrangement observed during the application of the electrical field. This study aims at explaining the high performance (i.e., single-molecule detection at a large electrode interface) of bioelectronic devices, where the 3MPA-based SAM structure is pivotal in achieving extremely high sensing performance levels due to its interchain interaction. Cyclic voltammetry (CV) experiments performed in K4Fe(CN)6:K3Fe(CN)6 for 3MPA SAMs that are activated/deactivated show similar trends of anodic peak current (IA) over time, mainly related to the presence of interchain hydrogen bonds, driving the conformational rearrangements (tightening of SAMs structure) while applying an electrical field. In addition, XPS analysis allows correlation of the deactivation yield with electrochemical data (conformational rearrangements), identifying the best protocol in terms of high reaction yield, mainly related to the shorter reaction time, and not triggering any side reactions. Finally, Protocol-C’s SAM surface coverage, determined by CV in H2SO4 and differential pulse voltammetry (DPV) in NaOH, was 1.29 * 1013 molecules cm−2, being similar to the bioreceptor surface coverage in single-molecule detection at a large electrode interface.

Translational Diffusion Dynamics in Divalent Metal-Phosphonate Monolayers

Self-assembled monolayers are attractive for surface modification due to their ease of synthesis and the range of chemical functionality that can be applied. Metal-phosphonate monolayer properties can be controlled through the metal ions that can be used in their formation. The organization and fluid properties of these monolayers can be understood in the context of their thermodynamic properties and the association and dissociation kinetics that proceed at the metal-phosphonate complex. In this work, four different M(II)-phosphonate monolayers were synthesized and the diffusional behavior of free and tethered chromophores was evaluated using fluorescence recovery after photobleaching measurements. The ω-terminal group identity of the metal-phosphonate monolayer was varied to determine its effect on monolayer dynamics.

Machine learning for condensed matter physics

Condensed matter physics (CMP) seeks to understand the microscopic interactions of matter at the quantum and atomistic levels, and describes how these interactions result in both mesoscopic and macroscopic properties. CMP overlaps with many other important branches of science, such as chemistry, materials science, statistical physics, and high-performance computing. With the advancements in modern machine learning (ML) technology, a keen interest in applying these algorithms to further CMP research has created a compelling new area of research at the intersection of both fields. In this review, we aim to explore the main areas within CMP, which have successfully applied ML techniques to further research, such as the description and use of ML schemes for potential energy surfaces, the characterization of topological phases of matter in lattice systems, the prediction of phase transitions in off-lattice and atomistic simulations, the interpretation of ML theories with physics-inspired frameworks and the enhancement of simulation methods with ML algorithms. We also discuss in detail the main challenges and drawbacks of using ML methods on CMP problems, as well as some perspectives for future developments.

Tuning the magnetism of gold nanoparticles by changing the thiol coating

Room-temperature ferromagnetic behaviour has been reported in nanoscale materials expected to be diamagnetic, including gold. However, it is yet unclear which factors (size, shape, surface coating) predominantly influence the magnitude of the magnetic response. In this work, we study the magnetic and electronic properties of similarly-sized gold nanoparticles (Au NPs) coated with four different n-alkanethiols, as well as hydroxyl- and carboxyl-functionalized alkanethiols using superconducting quantum interference device (SQUID) magnetometry and ultraviolet photoelectron spectroscopy (UPS). We find room-temperature behaviour (hysteresis in magnetization vs. field strength loops) in all samples, as well as large effective magnetic anisotropy. Importantly, we find the nanoparticles coated with polar chain end-groups (-OH and -COOH) show markedly higher magnetization; this increased magnetization correlates with a higher work function. This work establishes chemical handles to enhance magnetism in nanoscale gold particles.

Dynamic behaviors of interfacial water on the self-assembly monolayer (SAM) heterogeneous surface

Dynamic behaviors of water molecules near the surface with mixed hydrophobic and hydrophilic areas are studied by molecular dynamics simulation. More specifically, the diffusion coefficient and hydrogen bond lifetime of interfacial water on the self-assembly monolayer composed of hydrophobic and hydrophilic groups and their dependence on the mixing ratio are studied. The diffusion dramatically slows down, and the hydrogen bond lifetime considerably increases when a few hydrophilic groups are added to the hydrophobic surface. When the percentage of hydrophilic groups increases to 25%, the behavior of interfacial water is similar to the case of the pure hydrophilic surface. The sensitivity to the hydrophilic group can be attributed to the fact that the grafted hydrophilic groups can not only retard the directly bound water molecules but also affect indirectly bound water by stabilizing hydrogen bonds among interfacial water molecules.

Organic Field-Effect Transistor Platform for Label-Free, Single-Molecule Detection of Genomic Biomarkers

The increasing interest in technologies capable of tracking a biomarker down to the physical limit points toward new opportunities in early diagnostics of progressive diseases. Indeed, single-molecule detection technologies are foreseen to enable clinicians to associate the tiniest increase in a biomarker with the progression of a disease, particularly at its early stage. Bioelectronic organic transistors represent an extremely powerful tool to achieve label-free and single-molecule detection of clinically relevant biomarkers. These electronic devices are millimetric in size and in the future could be mass-produced at low cost. The core of the single molecule with a large transistor (SiMoT) platform, based on an electrolyte-gated field-effect transistor, is a gold gate electrode biofunctionalized with a self-assembled monolayer, a densely packed layer of recognition elements. So far, only the SiMoT detection of proteins, using the corresponding antibodies as recognition elements, has been reported. In this study, the SiMoT sensing response toward genomic biomarkers is proposed. Herein, the gate is functionalized with a genomic biomarker for multiple sclerosis (miR-182). This is relevant, not only because a limit of detection of a single molecule is achieved but also because it proves that the SiMoT label-free, single-molecule detection principle is the only one of its kind that can detect, by means of the same platform, both protein and genomic markers.

Nuclear Spin Incoherent Neutron Scattering from Quantum Well Resonators

We report the detection and quantification of nuclear spin incoherent scattering from hydrogen occupying interstitial sites in a thin film of vanadium. The neutron wave field is enhanced in a quantum resonator with magnetically switchable boundaries. Our results provide a pathway for the study of dynamics at surfaces and in ultrathin films using inelastic and/or quasielastic neutron scattering methods.

Direct Measurement of Water Permeation in Submerged Alkyl Thiol Self-Assembled Monolayers on Gold Surfaces Revealed by Neutron Reflectometry

Self-assembled monolayers (SAMs) of alkyl thiols are frequently used to chemically functionalize gold surfaces for applications throughout materials chemistry, electrochemistry, and biotechnology. Despite this, a detailed understanding of the structure of the SAM-water interface generated from both formation and use of the SAM in an aqueous environment is elusive, and analytical measurements of the structure and chemistry of the SAM-water interface are an ongoing experimental challenge. To address this, we used neutron reflectometry (NR) to measure water association with both hydrophobic and hydrophilic SAMs under both wet and dry conditions. SAMs used for this study were made from hydrophobic decanethiol mixed with hydrophilic 11-azido-1-undecanethiol with compositions of 0-100% of the azide-terminated thiol. All SAMs were formed by conventional solution incubation of a Au substrate immersed in ethanol. Each SAM was characterized by grazing incidence angle reflection-absorption Fourier transfer infrared spectroscopy, contact angle goniometry, and electrochemical methods to confirm it was a completely formed monolayer with evidence of extensive crystalline-like domains. NR measured significant absorption of water into each SAM, ranging from 1.6 to 5.7 water molecules per alkyl thiol, when SAMs were immersed in water. Water infiltration was independent of SAM composition and terminal group hydrophilicity. These results demonstrate that water accesses defects, fluid regions, and heterogeneous domains inherent to even well-formed SAMs.

Solution NMR Analysis of Ligand Environment in Quaternary Ammonium-Terminated Self-Assembled Monolayers on Gold Nanoparticles: The Effect of Surface Curvature and Ligand Structure

We report a solution NMR-based analysis of (16-mercaptohexadecyl)trimethylammonium bromide (MTAB) self-assembled monolayers on colloidal gold nanospheres (AuNSs) with diameters from 1.2 to 25 nm and gold nanorods (AuNRs) with aspect ratios from 1.4 to 3.9. The chemical shift analysis of the proton signals from the solvent-exposed headgroups of bound ligands suggests that the headgroups are saturated on the ligand shell as the sizes of the nanoparticles increase beyond ∼10 nm. Quantitative NMR shows that the ligand density of MTAB-AuNSs is size-dependent. Ligand density ranges from ∼3 molecules per nm² for 25 nm particles to up to 5-6 molecules per nm² in ∼10 nm and smaller particles for in situ measurements of bound ligands; after I₂/I^- treatment to etch away the gold cores, ligand density ranges from ∼2 molecules per nm² for 25 nm particles to up to 4-5 molecules per nm² in ∼10 nm and smaller particles. T₂ relaxation analysis shows greater hydrocarbon chain ordering and less headgroup motion as the diameter of the particles increases from 1.2 nm to ∼13 nm. Molecular dynamics simulations of 4, 6, and 8 nm (11-mercaptoundecyl)trimethylammonium bromide-capped AuNSs confirm greater hydrophobic chain packing order and saturation of charged headgroups within the same spherical ligand shell at larger nanoparticle sizes and higher ligand densities. Combining the NMR studies and MD simulations, we suggest that the headgroup packing limits the ligand density, rather than the sulfur packing on the nanoparticle surface, for ∼10 nm and larger particles. For MTAB-AuNRs, no chemical shift data nor ligand density data suggest that two populations of ligands that might correspond to side-ligands and end-ligands exist; yet T₂ relaxation dynamics data suggest that headgroup mobility depends on aspect ratio and absolute nanoparticle dimensions.

Vibrational Properties of Thiolate-Protected Gold Nanoclusters

Over recent years, the field of thiolate-protected gold nanoclusters has made remarkable progress. The successful determination of the structure of some of these clusters by X-ray crystallography was a milestone in this field. X-ray crystallography is arguably the most important technique in the field up to now, and it enabled the study of structure evolution as a function of cluster size. It also shed light on the structure of the Au-S interface. Recently, it has been realized that thiolate-protected gold clusters are very dynamic systems. Metal atoms and ligands can exchange easily between clusters. Furthermore, the adsorbed ligands bear conformational dynamics. Such dynamic effects call for experimental methods that can cope with it. Future efforts in this field will be directed toward applications of thiolate-protected clusters, and many of them will rely on dissolved clusters. Therefore, structure determination in solution is an important issue, though it is very challenging. The structure of the metal core and the Au-S interface is not expected to change in solution with respect to the crystal. However, the structure of the adsorbed ligand itself is sensitive to the environment and may be different in the solid state and in solution, as has been shown in fact in the past. It is this (dynamic) structure of the ligand that determines the interaction between the cluster and its environment, which is crucial, for example, for sensing applications. Vibrational spectroscopy is a promising technique to characterize thiolate-protected clusters in different environments. A vibrational spectrum is sensitive to structure (conformation) although this information is often "hidden" in the spectrum, requiring detailed analysis and support from theory to be deciphered. Compared to other techniques like UV-vis spectroscopy and mass spectrometry, vibrational spectroscopy was not extensively used in the field of thiolate-protected clusters, but we believe that the technique will be very valuable for the future developments in the field. We have used vibrational spectroscopy to investigate thiolate-protected gold clusters for mainly two lines of research. In the first, we studied in detail the low energy region of the vibrational spectrum, in particular the Au-S vibrational modes, in order to understand the structure sensitivity. It emerges that the Au-S vibrational spectrum is indeed sensitive to the structure of the interface but also to other factors, especially the organic part of the thiol, in a complex way. The ability to directly correlate structure, from X-ray crystallography, and vibrational spectra for thiolate-protected clusters, should lead to a database that will help in the future the structure determination of the Au-S interface by vibrational spectroscopy for systems where direct structure determination is not possible, for example, for flat surfaces. A second line of research focused on the determination of the structure of the adsorbed ligands for dissolved clusters. Such information is mostly extracted by the comparison of theoretical and calculated spectra for different conformers. In this respect, vibrational circular dichroism (VCD) is particularly powerful as it strongly depends on the conformation, more than conventional infrared spectroscopy. VCD can be applied to chiral nonracemic compounds, and it is a sensitive probe for chirality. Using this method, it was possible to demonstrate that a cluster can transfer its chirality to achiral thiolate ligands. In this Account, we summarize the possibilities and challenges of vibrational spectroscopy in the field of thiolate-protected clusters.

Peptide Modified ZnO Nanoparticles as Gas Sensors Array for Volatile Organic Compounds (VOCs)

In this work a peptide based gas sensor array based of ZnO nanoparticles (ZnONPs) has been realized. Four different pentapeptides molecularly modeled for alcohols and esters having cysteine as a common spacer have been immobilized onto ZnONPs. ZnONPs have been morphologically and spectroscopically characterized. Modified nanoparticles have been then deposited onto quartz crystal microbalances (QCMs) and used as gas sensors with nitrogen as carrier gas. Analysis of the pure compounds modeled demonstrated a nice fitting of modeling with real data. The peptide based ZnONPs had very low sensitivity to water, compared to previously studied AuNPs peptide based gas sensors allowing the use of the array on samples with high water content. Real samples of fruit juices have been assayed; stability of the signal, good repeatability, and discrimination ability of the array was achieved.

Self-assembled monolayers in organic electronics

Self-assembly is possibly the most effective and versatile strategy for surface functionalization. Self-assembled monolayers (SAMs) can be formed on (semi-)conductor and dielectric surfaces, and have been used in a variety of technological applications. This work aims to review the strategy behind the design and use of self-assembled monolayers in organic electronics, discuss the mechanism of interaction of SAMs in a microscopic device, and highlight the applications emerging from the integration of SAMs in an organic device. The possibility of performing surface chemistry tailoring with SAMs constitutes a versatile approach towards the tuning of the electronic and morphological properties of the interfaces relevant to the response of an organic electronic device. Functionalisation with SAMs is important not only for imparting stability to the device or enhancing its performance, as sought at the early stages of development of this field. SAM-functionalised organic devices give rise to completely new types of behavior that open unprecedented applications, such as ultra-sensitive label-free biosensors and SAM/organic transistors that can be used as robust experimental gauges for studying charge tunneling across SAMs.

Tunable Mechanical Properties of Nanoparticle Monolayer Membranes via Ligand Phase Control and Defect Distribution

In this study, the effects of ligand phase, morphology, and temperature on the elastic modulus of free-standing alkanethiol-capped gold nanoparticle membranes are reported. Langmuir films of 2.5 nm gold nanoparticles capped with tetradecanethiol were prepared at temperatures above and below the phase transition temperature (T _m) of the ligand shell and transferred to holey carbon grids (containing 1.2 μm holes) to form free-standing membranes. Force-indentation measurements are used to measure the elastic modulus of the membranes using an atomic force microscope in the temperature range 10-40 °C. These films are compared with membranes of dodecanethiol-capped gold nanoparticles, which do not undergo a ligand order-disorder transition in the temperature range investigated. The ligand phase effect is observed in the tetradecanethiol-capped gold nanoparticle films, where an abrupt change in the elastic modulus is seen near T _m. The temperature (relative to T _m) during the fabrication of the films is determined to play an important role in tuning the mechanical strength of these films in this temperature range by both changing the nature of the interparticle interactions and by affecting microscale film morphology.

Influence of Order within Nonpolar Monolayers on Hydrophobic Interactions

We report an experimental investigation of the influence of molecular-level order (crystallinity) within nonpolar monolayers on hydrophobic interactions. The measurements were performed using gold film-supported monolayers formed from alkanethiols (CH₃(CH₂)_nSH, with n ranging from 3 to 17), which we confirmed by using polarization-modulation infrared reflection-adsorption spectroscopy to exhibit chain-length-dependent order (methylene peak moves from 2926 to 2919 cm^-1, corresponding to a transition from liquid- to quasi-crystalline-like order) in the absence of substantial changes in chain density (constant methyl peak intensity). By using monolayer-covered surfaces immersed in either aqueous triethanolamine (TEA, 10 mM, pH 7.0) or pure methanol, we quantified hydrophobic and van der Waals contributions to adhesive interactions between identical pairs of surfaces (measured using an atomic force microscope) as a function of the length and order of the aliphatic chains within the monolayers. In particular, we measured pull-off forces arising from hydrophobic adhesion to increase in a nonlinear manner with chain length (abrupt increase between n = 5 and 6 from 2.1 ± 0.3 to 14.1 ± 0.7 nN) and to correlate closely with a transition from a liquid-like to crystalline-like monolayer phase. In contrast, adhesion in methanol increased gradually with chain length from 0.3 ± 0.1 to 3.2 ± 0.3 nN for n = 3 to 7 and then did not change further with an increase in chain length. These results lead to the hypothesis that order within nonpolar monolayers influences hydrophobic interactions. Additional support for this hypothesis was obtained from measurements reported in this paper using long-chain alkanethiols (ordered) and alkenethiols (disordered). The results are placed into the context of recent spectroscopic studies of hydrogen bonding of water at nonpolar monolayers. Overall, our study provides new insight into factors that influence hydrophobic interactions at nonpolar monolayers.

Description of the Adsorption and Exciton Delocalizing Properties of p-Substituted Thiophenols on CdSe Quantum Dots

This work describes the quantitative characterization of the interfacial chemical and electronic structure of CdSe quantum dots (QDs) coated in one of five p-substituted thiophenolates (X-TP, X = NH2, CH3O, CH3, Cl, or NO2), and the dependence of this structure on the p-substituent X. (1)H NMR spectra of mixtures of CdSe QDs and X-TPs yield the number of X-TPs bound to the surface of each QD. The binding data, in combination with the shift in the energy of the first excitonic peak of the QDs as a function of the surface coverage of X-TP and Raman and NMR analysis of the mixtures, indicate that X-TP binds to CdSe QDs in at least three modes, two modes that are responsible for exciton delocalization and a third mode that does not affect the excitonic energy. The first two modes involve displacement of OPA from the QD core, whereas the third mode forms cadmium-thiophenolate complexes that are not electronically coupled to the QD core. Fits to the data using the dual-mode binding model also yield the values of Δr1, the average radius of exciton delocalization due to binding of the X-TP in modes 1 and 2. A 3D parametrized particle-in-a-sphere model enables the conversion of the measured value of Δr1 for each X-TP to the height of the potential barrier that the ligand presents for tunneling of excitonic hole into the interfacial region. The height of this barrier increases from 0.3 to 0.9 eV as the substituent, X, becomes more electron-withdrawing.

Dielectric relaxation properties of carboxylic acid-terminated n-alkyl monolayers tethered to Si(1 1 1): dynamics of dipoles and gauche defects

Molecular-level insights into the organization and dynamics of n-alkyl monolayers covalently bonded to Si(1 1 1) were gained from admittance measurements of dipolar relaxation in rectifying Hg|| HOOC-C10H(25-n) Si junctions performed as a function of applied voltage and temperature. A collective behavior of dipole dynamics is inferred from the non-Debye asymmetric relaxation peak shape and strong coupling of the dipole relaxation path with some bending vibrations of the n-alkyl OML (multi-excitation entropy model). A variety of relaxation mechanisms is observed in the frequency range (0.1 Hz-10 MHz) with different dependence of relaxation frequency and dipolar strength on measurement temperature and applied voltage. Their microscopic origin is discussed by comparing the activation energy of relaxation frequency with previous molecular mechanics calculations of saddle point energy barriers for structural defects such as gauche conformations or chain kinks in n-alkanes assemblies. Gauche conformations organized in pairs (kinks) have vanishing relaxation strength below an order-disorder transition temperature T(D) =  175 K and their probability strongly increases with applied reverse voltage, above T(D). The presence of hydrogen bonds between terminal carboxylic acid functionalities is inferred from a comparison with a similar junction bearing a low density of carboxylic acid end groups. This temperature-dependent hydrogen-bond network provides some additional stiffness against external electrostatic stress, as deduced from the rather weak sensitivity of relaxation frequencies to applied bias voltage.

Approaching the size limit of organometallic layers: synthesis and characterization of highly ordered silver–thiolate lamellae with ultra-short chain lengths

Synthesis of single crystal silver alkanethiolate (any chain length) lamellae with highly ordered chain conformations, interlayer interfaces and intralayer lattices.

Observation of uniform ligand environments and 31P–197Au coupling in phosphine-terminated Au nanoparticles

Phosphine-terminated gold nanoparticles exhibit unexpectedly uniform ligand environments that enable the identification of ³¹P–¹⁹⁷Au coupling for the first time in these systems.

Temperature-Dependent Permeability of the Ligand Shell of PbS Quantum Dots Probed by Electron Transfer to Benzoquinone

This paper describes an increase in the yield of collisionally gated photoinduced electron transfer (electron transfer events per collision) from oleate-capped PbS quantum dots (QDs) to benzoquinone (BQ) with increasing temperature (from 0 to 50 °C), due to increased permeability of the oleate adlayer of the QDs to BQ. The same changes in intermolecular structure of the adlayer that increase its permeability to BQ also increase its permeability to the solvent, toluene, resulting in a decrease in viscous drag and an apparent increase in the diffusion coefficient of the QDs, as measured by diffusion-ordered spectroscopy (DOSY) NMR. Comparison of NMR and transient absorption spectra of QDs capped with flexible oleate with those capped with rigid methylthiolate provides evidence that the temperature dependence of the permeability of the oleate ligand shell is due to formation of transient gaps in the adlayer through conformational fluctuations of the ligands.

Entropy effects in the collective dynamic behavior of alkyl monolayers tethered to Si(111)

Dynamic properties of n-alkyl monolayers covalently bonded to Si(111) were studied by broadband admittance spectroscopy as a function of the temperature and the applied voltage using rectifying Hg/C12H25/n-type Si junctions. Partial substitution of methyl end groups by polar (carboxylic acid) moieties was used to enhance the chain end relaxation response. Two thermally activated dissipation mechanisms (B1 and B2, with f B1 < f B2) are evidenced for all reverse bias values. The strong decrease of both relaxation frequencies with increasing reverse dc bias reveals increasing motional constraints, attributed to electrostatic pressure applied to the densely-packed nanometer-thick monolayer. Spectral decomposition of the frequency response shows a power-law dependence of their activation energies on |V DC|. A large reverse bias reversibly increases the B2 response attributed to the distribution of gauche defects, in contrast with the constant strength of the acid dipole loss (B1). A trans-gauche isomerization energy of 50 meV is derived from the temperature dependence of the B2 dipolar strength. For both dissipation mechanisms, the observed linear correlation between activation energy and logarithm of pre-exponential factor is consistent with a multi-excitation entropy model, in which the molecular reorientation path is strongly coupled with a large number of low energy excitations (here the n-alkyl bending vibrational mode) collected from the thermal bath. This collective dynamic behavior of alkyl chains tethered to Si is also confirmed by the asymmetric relaxation peak shape related to many-body interactions in complex systems.

Preparation and characterization of Au nanoparticles capped with mercaptocarboranyl clusters

The preparation of 3-4 nm and 10 nm gold nanoparticles capped with neutral carborane-based mercaptocarboranes, via two different preparative routes, is reported. The resulting boron-enriched nanomaterials exhibit complete dispersibility in water, opening the way for the use of these monolayer protected clusters (MPCs) in medical applications, such as boron neutron capture therapy (BNCT). These newly prepared MPCs have been characterized by FTIR, (1)H and (11)B NMR spectroscopy, UV-visible, centrifugal particle sizing (CPS), and, in some cases, inductively coupled plasma atomic emission spectrometry (ICP-AES). Water dispersibility exhibited by these MPCs allowed the study of the cellular uptake by HeLa cells.

Electron transfer through 3D monolayers on Au25 clusters

The monolayer protecting small gold nanoparticles (monolayer-protected clusters, MPCs) is generally represented as the 3D equivalent of 2D self-assembled monolayers (SAMs) on extended gold surfaces. However, despite the growing relevance of MPCs in important applied areas, such as catalysis and nanomedicine, our knowledge of the structure of 3D SAMs in solution is still extremely limited. We prepared a large series of monodisperse Au25(SCnH2n+1)18 clusters (n=2, 4, 6, 8, 10, 12, 14, 16, 18) and studied how electrons tunnel through these monolayers. Electron transfer results, nicely supported by 1H NMR spectroscopy, IR absorption spectroscopy, and molecular dynamics results, show that there is a critical ligand length marking the transition between short ligands, which form a quite fluid monolayer structure, and longer alkyl chains, which self-organize into bundles. At variance with the truly protecting 2D SAMs, efficient electronic communication of the Au25 core with the outer environment is thus possible even for long alkyl chains. These conclusions provide a different picture of how an ultrasmall gold core talks with the environment through/with its protecting but not-so-shielding monolayer.

Free energy change for insertion of charged, monolayer-protected nanoparticles into lipid bilayers

Charged, monolayer-protected gold nanoparticles (AuNPs) with core diameters smaller than 10 nm have recently emerged as a prominent class of nanomaterial for use in targeted drug delivery and biosensing. In particular, recent experimental studies showed that AuNPs protected by a binary mixture of purely hydrophobic and anionic, end-functionalized alkanethiol ligands were able to spontaneously penetrate through cell membranes via a non-endocytic, non-disruptive mechanism. The critical step in the penetration process is a fusion step during which the AuNPs insert into the hydrophobic core of the bilayer. This fusion step is driven by hydrophobic forces as inserted AuNPs minimize their exposed hydrophobic surface area and thereby lower their free energy compared to particles in the bulk. Here, we explore the effect of the large parameter space of composition, size, ligand length, morphology, and hydrophobicity strength on the change in the free energy upon insertion. Using a newly developed implicit bilayer, implicit solvent simulation model, our work shows that there is a size cutoff for insertion that has a strong dependence on surface composition and ligand chemistry. Our results agree well with previous experimental findings for a particular value of the hydrophobicity strength. This work provides physical insight that may be used to both understand the insertion of AuNPs into bilayers and guide the design of monolayers to either encourage or inhibit insertion.

Effects on the self-assembly of n-alkane/gold nanoparticle mixtures spread at the air-water interface

Nanoparticle films formed at the air-water interface readily form rigid films, where the nanoparticles irreversibly associate into floating "islands", often riddled with voids and defects, upon solvent evaporation. Improving the nanoparticle mobility in these films is key to achieving control over the nanoparticle packing parameters, which is attractive for a variety of applications. In this study, a variety of n-alkanes were mixed with tetradecanethiol-capped 2 nm gold nanoparticles and studied as Langmuir films at 18 and 32 °C. Pressure-area isotherms at 18 °C reveal a mixed liquid-expanded phase of nanoparticles and alkane at the air-water interface, but only for n-alkanes that are equal to or exceed the nanoparticle capping ligand in carbon chain length. Transmission electron microscopy images of the corresponding films suggest that the nanoparticles are mixed with a continuous hydrocarbon phase at 0 mN/m and that the hydrocarbon is squeezed out of the nanoparticle film during compression.

Orientational ordering of passivating ligands on CdS nanorods in solution generates strong rod-rod interactions

We present the first nearly atomistic molecular dynamics study of nanorod-nanorod association in explicit solvent, showing that inter-rod forces can be dominated by microscopic factors absent in common continuum descriptions. Specifically, we find that alkane ligands on faceted CdS nanorods in n-hexane undergo a temperature-dependent order-disorder transition akin to that of self-assembled monolayers on macroscopic substrates. This collective ligand alignment organizes nearby solvent molecules, strongly influencing the statistics of rod-rod separation. The strong temperature dependence of this mechanism could be exploited in the laboratory to manipulate and optimize the assembly of ordered structures.

Organic-sulfur-zinc hybrid nanoparticle for optical applications synthesized via polycondensation of trithiol and Zn(OAc)2

Organic-sulfur-zinc hybrid materials were prepared via polycondensation of Zn(OAc)2 and trithiols bearing various alkyl groups. A soluble nanoparticle could be obtained by the polycondensation using a trithiol bearing octadecyl moieties. The good dispersing ability as nano-scaled particles was confirmed by dynamic light scattering and atomic force microscopy analyses. This hybrid nanoparticle was miscible with poly(methyl methacrylate) and served as a refractive additive to increase refractive indexes. The calculated refractive index value for the nanoparticle was 1.58.

Improving the binding characteristics of tripodal compounds on single layer graphene

Graphene is an atomically thin, transparent, and conductive electrode material of interest for sensors and energy conversion and storage devices, among others. Fully realizing its potential will require robust and general methods to anchor active functionality onto its pristine basal plane. Such strategies should not utilize covalent bond formation, which disrupts the graphene's π-electron system, from which most of its desirable properties arise. We recently introduced a tripodal binding motif, which forms robust monolayers on graphene capable of immobilizing active proteins and preventing their denaturation. Here we describe structure-property relationships for a series of tripod binding groups with "feet" of different sizes. Each derivative adsorbs strongly (ΔGads ≈ -39 kJ mol(-1)) to graphene's basal plane, yet the resulting monolayers exhibit kinetic stabilities that vary over 2 orders of magnitude and molecular densities that vary by a factor of 2. This study identifies phenanthrene as a superior anchor relative to pyrene on the basis of its increased monolayer density and similar kinetic stability. We also demonstrate that varying the length of the methylene linkers between the feet and tripodal core does not affect binding substantially. These results represent the first demonstration of structure-property relationships in the assembly of molecular adsorbates on graphene and provide a paradigm for designing effective graphene binding motifs.

Asymmetric organic/metal(oxide) hybrid nanoparticles: synthesis and applications

Asymmetric particles (APs) with broken centrosymmetry are of great interest, due to the asymmetric surface properties and diverse functionalities. In particular, organic/metal(oxide) APs naturally combine the significantly different and complementary properties of organic and inorganic species, leading to their unique applications in various fields. In this review article, we highlighted recent advances in the synthesis and applications of organic/metal(oxide) APs. This type of APs is grounded on chemical or physical interactions between metal(oxide) NPs and organic small molecular or polymeric ligands. The synthetic methodologies were summarized in three categories, including the selective surface modifications, phase separation of mixed ligands on the surface of metal(oxide) NPs, and direct synthesis of APs. We further discussed the unique applications of organic/metal(oxide) APs in self-assembly, sensors, catalysis, and biomedicine, as a result of the distinctions between asymmetrically distributed organic and inorganic components. Finally, challenges and future directions are discussed in an outlook section.

Hydrolytic stability of mesoporous zirconium titanate frameworks containing coordinating organic functionalities

The hydrolytic stability of lanthanide and actinide selective mono- and polyphosphonate-functionalized mesoporous zirconium titanium oxide adsorbents has been investigated in nitric acid solutions. Hydrolytic degradation of the surfaces, as measured through the fractional loss of phosphorus and elements of the oxide framework, increased by more than an order of magnitude as the nitric acid concentration was increased from 0 to 2 mol/L. The unfunctionalized parent oxide suffered considerable dissolution in 2 mol/L acid over a period of 72 h. Under identical conditions, the fractional Zr and Ti release was reduced to 1 × 10(-2) for monophosphonate functionalized hybrids and reached as low as 1 × 10(-6) for trisphosphonate functionalized variants. The bisphosphonates showed intermediate values. The leaching of P, Zr and Ti was found to be incongruent with the Zr leaching to a lesser extent implying enhanced stability of the Zr-O-P bond. Quantitative analysis of the dissolution kinetics indicated a parabolic dissolution model with a rate constant in the range of 0.5-1.5 mg g(-1) min(-1/2) for the elemental leaching of P, Ti, and Zr. The leaching of Zr from the mesoporous matrix was relatively more complex than for the other elements with evidence of a leaching mechanism involving two processes. ToF-SIMS and DRIFT analysis demonstrated that after leaching in 2 M HNO3 for 24 h, a significant proportion of grafted ligands remained on the surface. The oxide functionalized with amino trismethylenephosphonic acid, which had previously shown excellent (153)Gd(3+) selectivity, was demonstrated to have outstanding stability, with low fractional elemental losses and preservation of mesoporous texture even after leaching for 24 h in 2 M HNO3. This suggests this particular hybrid to be worthy of additional study.

Nitro-substituted arene sulfenyl chlorides as precursors to the formation of aromatic SAMs

The formation of aromatic SAMs on Au(111) using three nitro-substituted arene sulfenyl chlorides (4-nitrophenyl sulfenyl chloride (1), 2-nitrophenyl sulfenyl chloride (2), and 2,4-dinitrophenyl sulfenyl chloride (3)) is studied. The formation of SAMs and their quality are investigated as a function of the position of the nitro substituent(s) on the aromatic ring. The modified surfaces are characterized by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), polarization modulation infrared reflection absorption spectroscopy (PMIRRAS), and cyclic voltammetry (CV). The results show that all three compounds are deposited on Au within very short times. The corresponding coverages are determined using CV. However, only compound 1 forms stable, long-range, well-ordered SAMs. The 4-nitrophenyl thiolate is adsorbed nearly vertically on the Au surface. Compounds 2 and 3 both form lower-quality SAMs where the adsorbed nitro-phenyl thiolates are more tilted. These SAMs are less stable than the ones obtained with the 4-nitrosubsituted precursor and decompose with time, leaving only sulfur on the gold surface.

Effect of Alkyl Chain Length on Carboxylic Acid SAMs on Ti-6Al-4V

The formation of methyl-terminated carboxylic acid self-assembled monolayers (SAMs) with even numbers of carbons, from eighteen to thirty, was investigated on the oxide surface of Ti-6Al-4V and component metal oxides. Modified surfaces were characterized using diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) and contact angle analysis. Infrared spectroscopy indicated that using aerosol spray deposition techniques, stable, all-trans SAMs of octacosanoic (28 carbons) and triacontanoic (30 carbons) acids were formed on the alloy. Films were similarly formed on titanium and aluminum oxide. The surface of vanadium oxide exhibited limited reactivity. MALDI-TOF MS confirmed that formed films were monolayers, without multilayers or aggregates present. Water contact angles are indicative of the presence of hydrophobic methyl groups at the interface. This stable carboxylic acid SAM formation could be a useful alternative to phosphonic acid SAMs for corrosion and other applications.

Lanthanide-based NMR: a tool to investigate component distribution in mixed-monolayer-protected nanoparticles

Gd(3+) ions, once bound to the monolayer of organic molecules coating the surface of gold nanoparticles, produce a paramagnetic relaxation enhancement (PRE) that broadens and eventually cancels the signals of the nuclear spins located nearby (within 1.6 nm distance). In the case of nanoparticles coated with mixed monolayers, the signals arising from the different coating molecules experience different PRE, depending on their distance from the binding site. As a consequence, observation of the signal broadening patterns provides direct information on the monolayer organization.

Preparation of water-soluble maleimide-functionalized 3 nm gold nanoparticles: a new bioconjugation template

We present an efficient methodology to prepare maleimide-tethered, water-soluble gold nanoparticles (maleimide-AuNPs). The maleimide-AuNPs were prepared in the protected form and are readily recovered via a retro-Diels-Alder reaction. The maleimide-AuNPs were fully characterized by (1)H NMR, TGA, TEM, and XPS and were determined to have a gold core with an average size of 3.2 ± 0.8 nm; each core contains about 1000 gold atoms and is surrounded by 30 maleimide-terminated ligands and 60 thiolated PEG ligands. The maleimide-AuNPs efficiently react with rhodamine 123 and cysteine and are a promising template for biological applications.

Effect of alkanethiol chain length on gold nanoparticle monolayers at the air-water interface

In this study, we report the effects of the alkyl chain length on alkanethiol-capped gold nanoparticle Langmuir films. Gold nanoparticles (2-3 nm) capped with C(n)H(2n+1)SH (n = 5-12, 14-16, 18) were prepared via a two-phase synthesis. The films were sampled by Langmuir-Schaefer horizontal transfer at various points in the pressure-area isotherm and monitored with transmission electron microscopy. Changes in surface pressure, temperature, and alkyl chain length did not lead to observable differences in the mesoscale film morphology. Pressure-area isotherms at 22 °C, however, revealed that the work of compression and the collapse pressure are directly dependent on alkyl chain lengths of 14 carbons or greater. Variable temperature isotherms suggest that the work of compression is strongly affected by the phase state (i.e., crystalline vs liquid-like) of the gold-thiolate self-assembled monolayer (SAM) capping the nanoparticles.

Rational size control of gold nanoparticles employing an organometallic precursor [Au-C≡C- t-Bu]4 and tunable thiolate-functionalized ionic liquids in organic medium

Gold nanoparticles (Au NPs) stabilized with six different thiolate-functionalized ionic liquids (TFILs) were synthesized in an organic solvent. The size and optical properties of the TFIL-stabilized Au NPs can be rationally controlled by altering the N-alkyl chain length and (or) the counteranion of the TFIL-stabilizing ligand. Au NPs were prepared from the reduction of a gold precursor (HAuCl4 or [Au-C≡C-t-Bu]4) employing NaBH4 in the presence of the different disulfide precursors. A model, based on Israelachvili theory, is proposed to account for the dependence of NP size on the N-alkyl chain length and the counteranion of the surfactantlike TFIL stabilizers.

Mercaptocarborane-capped gold nanoparticles: electron pools and ion traps with switchable hydrophilicity

A simple single-phase method for the preparation of ca. 2 nm gold nanoparticles capped with mercaptocarborane ligands is introduced. The resultant monolayer protected clusters (MPCs) exhibit redox-dependent solubility and readily phase transfer between water and nonpolar solvents depending on the electronic and ionic charge stored in the metal core and in the ligand shell, respectively. The particles and their properties have been characterized by high angle annular dark field imaging in a scanning transmission electron microscope, elemental analysis, centrifugal particle sizing, UV-vis and FTIR spectroscopy, and thermogravimetric analysis and by (1)H, (11)B, and (7)Li NMR spectroscopy. Cellular uptake of the MPCs by HeLa cells has been studied by TEM, and the subsequent generation of reactive oxygen species inside the cells has been evaluated by confocal fluorescence microscopy. These MPCs qualitatively showed significant toxicity and the ability to penetrate into most cell compartments with a strong tendency of finally residing inside membranes. Applications in catalysis, electrocatalysis, and biomedicine are envisaged.