Mixed Self-assembled Monolayers Research Articles

Molecular simulations of mixed self-assembled monolayer coated gold nanoparticles in water.

Molecular dynamics simulations have been employed to study the hydration of a series of nanoparticles, each of which was coated with a mixed self-assembled monolayer (SAM) comprising methyl- and hydroxy-terminated alkane thiol chains. The mixing ratio of those chains are different for each nanoparticle. The simulations focused on the wetting behavior of the SAM-coated gold nanoparticles and the distribution and structure of their interfacial water molecules. The interactions of the mixed SAM-coated gold nanoparticles with water were analyzed by evaluating the radial distribution function, hydrogen bonds, the dipole orientations of the water molecules, and the water residence time in the interfacial region. The wettability of the mixed SAM-coated gold nanoparticles improved as the concentration of terminal hydroxy moieties was increased. The distribution and dynamics of the interfacial water molecules were found to be influenced by the mixing ratio of the terminal moieties of the SAM chains. The results of our simulations suggest that the surface interactions of the mixed SAM-coated gold nanoparticles with the aqueous medium can be modulated by systematically altering the mixing ratio of the terminal methyl and hydroxy moieties. This work may lead to new biological and technological applications and inspire the development of novel biomimetic materials. Graphical Abstract Mixed SAM-coated gold nanoparticles.

Simultaneous enhancement of photocurrent and open circuit voltage in a ZnO based organic solar cell by mixed self-assembled monolayers

In this study, we utilized the carboxylic acid functionalized molecules, namely, benzoic acid (BZA) and C61-dicarboxylic acid (CDCA) as interfacial modifiers to form self-assembled monolayers (SAM) in polymer hybrid solar cell (HSC) consisting of poly (3-hexylthiophene) (P3HT) and zinc oxide (ZnO). We found that the short circuit current density (Jsc) and the open circuit voltage (Voc) of the device could be drastically increased by using CDCA–SAM and BZA–SAM, respectively. On the other hand, the optimized mixture of CDCA–SAM and BZA–SAM showed better improvement in both the Jsc and Voc. The devices modified with mixed SAMs in an optimized ratio showed significant increment in the overall conversion efficiencies compared to unmodified devices consisting of both bi-layer and nanorod (NR) structured HSC. In particular, almost 1.2-fold improvement could be achieved on the NR device with the efficiencies of 0.73%. We attribute the improvement of Jsc and Voc to higher carrier mobility of CDCA–SAM and work function modulation of ZnO by BZA–SAM, respectively.

Mixed Self-Assembled Monolayers of Mercaptoundecanoic Acid and Thiolactic Acid for the Construction of an Enzymatic Biosensor for Hydroquinone Determination

Self-assembled monolayers (SAMs) have attracted much interest due to their potential applications in biosensors, biomolecular electronics and nanotechnology. Due to its homogeneity, ease of preparation and ability to vary both the length of the chains and terminal functional groups, which provide a good versatility for immobilizing biocatalytically active compounds. Mixed SAM (designated as SAMmix), that combines the properties of alkanethiols with different carbon chain lengths or different functional groups, is one of the most attractive methods for promoting different arrangements on the electrode surface. SAMmix of long and short chains are reported to show better electron transfer rates than unicomponent SAMs due to the flexibility of redox species distribution at the SAM interface. Therefore, in this work, a horseradish peroxidase (HRP) biosensor was constructed using methods of physical adsorption and covalent binding for the enzyme immobilization on binary SAMs of 11-mercaptoundecanoic acid (MUA) and thiolactic acid (TLA) on gold surface for biosensor construction. The characterization of the biosensors was evaluated by voltammetry and electrochemical impedance spectroscopy. The immobilization method by covalent binding provided greater stability when compared to the adsorption method. For the preparation of the mixed self-assembled monolayers (SAMmix), the gold electrode was incubated in a solution containing different proportions of MUA and TLA and the best concentration ratio of these molecules was 0.5 and 1.0 mmol L-1, respectively. The results obtained by cyclic voltammetry and electrochemical impedance spectroscopy showed that the electrode surface dramatically changes when varying the concentration of these molecules. The rate of electron transfer was substantially affected because TLA enables an increase in the conductivity due to the formation of SAMmix "islands", whereas MUA although partially block the surface, it gives stability to the monolayer for the enzyme immobilization. The SAMmix obtained using 1 mmol L-1 of MUA had the lowest sensitivity due to the high concentration of the long-chain molecule. The HRP enzyme was efficiently immobilized on the SAMmix, designated as Au-SAMmix-HRP. The HRP enzyme was immobilized by the method of covalent attachment, in which the reaction occurs between the -COOH end groups of the SAM ligands with EDC (N-(3-9 dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride) and NHS (N-hydroxysuccinimide ester), which favors a more stable immobilization of the enzyme. This biosensor was applied for the determination of hydroquinone (HQ). It was observed that the presence of the enzyme on the monolayer increases the peak current compared to SAMmix, however best results were obtained for determination of HQ when a fixed concentration of H2O2 was added in solution. Therefore, the effect of the H2O2 concentration on the biosensor response was investigated. It was observed that as the concentration of H2O2 was increased, there was a linear increase in the biosensor current response from of 30.0 to 300.0 µmmol L-1 and 0.1 to 1.0 mmol L-1, with sensitivities of 10.35 μA/mol L-1 and and 2.38 μA/mol L-1, respectively. Lower sensitivity was observed at high concentrations of the substrate, probably due to the saturation of the active site of the enzyme, reducing its sensitivity. Michaelis–Menten kinetics, KM app of 0.4 mmol L-1 was obtained, indicating that the electrode architecture employed presents advantages for the fabrication of enzymatic biosensor. The resulting Au-SAMmix-HRP exhibited an excellent electrocatalytic activity toward the hydroquinone, which presents a wide linear range from 3.0 to 30.0 mmol L with a linear equation, I= 2209.69 [HQ] - 0.00534 with a correlation coefficient of 0.985. The standard deviation for the calibration curve was estimated at SB = 0.00101. This data was used to calculate the detection limit and quantification limit of the biosensor which were DL = 1.37 µmol L-1 and QL = 4.57 µmol L-1. The repeatability of the biosensor Au-SAMmix-HRP was performed on a series of 10 measurements (n = 10) in a solution containing HQ concentration of 0.3 mmol L-1 ([H2O2]= 0.3 mmol L-1) in PBS buffer by means of cyclic voltammetry, with 0.99% RSD. The biosensor stability was evaluated after 100 consecutive cycles by cyclic voltammetry in these same conditions and only a slight decay of the cathodic current peak was observed, at about 14 %. The long-term stability of the biosensor was also investigated and the biosensor was stable for at least 6 days without change in the response. After this period, the signal decreased 10% over a period of 15 days. Therefore, the biosensor Au-SAMmix-HRP displayed good reproducibility, sensitivity and stability for the determination of hydroquinone.

Intermolecular Interactions in Mixed Self‐Assembled Monolayers of Ferrocene

AbstractElectrochemical characterization of mixed self‐assembled monolayers (SAMs) of 6‐ferrocenyl‐1‐hexanethiol (FcH) and mercaptoundecanoic acid tyrosinamide (MUATyr) on gold is reported. Single‐component SAMs of FcH presented repulsive intermolecular interactions (vGθT=−1.12), while mixed SAMs of FcH/MUATyr (1 : 1) exhibited attractive interactions (vGθT=+0.20), with a homogeneous distribution of both components. Electrochemical kinetic determinations on mixed SAMs of FcH/MUATyr, indicated a secondary electron transfer pathway between the redox centers of both components. Higher amounts of FcH in the mixed SAMs lowered the observed rate of electron transfer of MUATyr. The oxidation of FcH caused an anodic shift of 160 mV in the voltammetric wave of MUATyr.

Stealth surface modification of surface-enhanced Raman scattering substrates for sensitive and accurate detection in protein solutions.

Reliable surface-enhanced Raman scattering (SERS) based biosensing in complex media is impeded by nonspecific protein adsorptions. Because of the near-field effect of SERS, it is challenging to modify SERS-active substrates using conventional nonfouling materials without introducing interference from their SERS signals. Herein, we report a stealth surface modification strategy for sensitive, specific and accurate detection of fructose in protein solutions using SERS by forming a mixed self-assembled monolayer (SAM). The SAM consists of a short zwitterionic thiol, N,N-dimethyl-cysteamine-carboxybetaine (CBT), and a fructose probe 4-mercaptophenylboronic acid (4-MPBA). The specifically designed and synthesized CBT not only resists protein fouling effectively, but also has very weak Raman activity compared to 4-MPBA. Thus, the CBT SAM provides a stealth surface modification to SERS-active substrates. The surface compositions of mixed SAMs were investigated using X-ray photoelectron spectroscopy (XPS) and SERS, and their nonfouling properties were studied with a surface plasmon resonance (SPR) biosensor. The mixed SAM with a surface composition of 94% CBT demonstrated a very low bovine serum albumin (BSA) adsorption (∼3 ng/cm(2)), and moreover, only the 4-MPBA signal appeared in the SERS spectrum. With the use of this surface-modified SERS-active substrate, quantification of fructose over clinically relevant concentrations (0.01-1 mM) was achieved. Partial least-squares regression (PLS) analysis showed that the detection sensitivity and accuracy were maintained for the measurements in 1 mg/mL BSA solutions. This stealth surface modification strategy provides a novel route to introduce nonfouling property to SERS-active substrates for SERS biosensing in complex media.

An impedimetric biosensor to test neat serum for dengue diagnosis

NS1, a non-structural dengue protein, has been used as a biomarker for the detection of viremia in dengue diagnosis. In this work, an impedimetric label-free immunosensor based on an anti-NS1 modified gold electrode was developed. To immobilize anti-NS1, a mixed self-assembled monolayer consisting of 11-mercaptoundecanoic acid (for covalent anti-NS1 attachment) and 6-mercaptohexanol (as a spacer) was prepared. The impedance spectra were recorded in the presence of a redox probe ([Fe(CN)6]3−/4−) to monitor changes in charge-transfer resistance associated with target binding both in PBS and neat serum. A calibration curve was constructed in order to obtain analytical parameters. NS1 was diluted in PBS to final concentrations in the linear range of 0.01–2.00μgmL−1 with a sensitivity of 14.1percentagedecade−1 (R∼0.99) and a limit of detection (LOD) of 3ngmL−1. A linear range 0.01–1.00μgmL−1 of NS1 diluted in neat serum was also obtained (R∼0.98) with a sensitivity of 10.4percentagedecade−1 and a LOD of 30ngmL−1. The results open new insights into the potential use of this biosensor for point-of-care and bedside applications for dengue diagnosis.

Development and evaluation of a SPR-based immunosensor for detection of anti-Trypanosoma cruzi antibodies in human serum

Protozoan Trypanosoma cruzi is the etiological agent of Chagas disease, which needs urgent progress in its immunological diagnosis. Surface plasmon resonance (SPR) is a promising technique for the development of immunosensors with biomedical applications. In this work, a SPR-based immunosensor has been developed by the first time for real time and label free immunoassay for detection of anti-T. cruzi antibodies in serum samples. T. cruzi antigen was successfully immobilized on a SPR sensor chip via activated mixed self-assembled monolayer (SAM) of 3-mercaptopropionic acid (3-MPA) and 11-mercaptoundecanoic acid (11-MUA), by amide coupling. After the sensor construction, a pool of human sera infected with T. cruzi was added to its surface and the antibodies were detected in sera diluted up to 1280 times, indicating excellent sensitivity of the technique for detection of antigen–antibody interaction. The addition of a pool of negative human serum at dilutions lower than 1:160 to the sensor surface was accompanied by a null or very low response. Then, the following operational parameters of the immunoassay were optimized and defined: time of immobilization and antigen concentration at 20min and 30mgmL−1, serum dilution at 1:320, preventing of nonspecific bindings with solution of BSA 1.0% and surface regeneration by injection of SDS 1.0%. The immunoassay, here termed SPRCruzi, showed high capability in the discrimination of positive and negative sera, including those infected with other pathogens usually sources of false positives results in conventional serodiagnosis. Therefore, the proposed immunosensor was successfully developed and the immunoassay allowed a simple, effective, faster and specific detection of anti-T. cruzi antibodies, which represents an encouraging field for the progress of the diagnosis of Chagas disease.

Investigation of the heparin–thrombin interaction by dynamic force spectroscopy

BackgroundThe interaction between heparin and thrombin is a vital step in the blood (anti)coagulation process. Unraveling the molecular basis of the interactions is therefore extremely important in understanding the mechanisms of this complex biological process. MethodsIn this study, we use a combination of an efficient thiolation chemistry of heparin, a self-assembled monolayer-based single molecule platform, and a dynamic force spectroscopy to provide new insights into the heparin–thrombin interaction from an energy viewpoint at the molecular scale. ResultsWell-separated single molecules of heparin covalently attached to mixed self-assembled monolayers are demonstrated, whereby interaction forces with thrombin can be measured via atomic force microscopy-based spectroscopy. Further these interactions are studied at different loading rates and salt concentrations to directly obtain kinetic parameters. ConclusionsAn increase in the loading rate shows a higher interaction force between the heparin and thrombin, which can be directly linked to the kinetic dissociation rate constant (koff). The stability of the heparin/thrombin complex decreased with increasing NaCl concentration such that the off-rate was found to be driven primarily by non-ionic forces. General significanceThese results contribute to understanding the role of specific and nonspecific forces that drive heparin–thrombin interactions under applied force or flow conditions.

Structure of Mixed Self-Assembled Monolayers on Gold Nanoparticles at Three Different Arrangements

In this work, we performed atomistic simulations to study the structural properties of mixed self-assembled monolayers (SAM) of hydrophilic and hydrophobic alkylthiols, with two different chain lengths (C5 and C11), on gold nanoparticles (NPs) at three different arrangements, namely: random, patchy, and Janus domains. In particular, we report the effect of mixing of thiols with unequal carbon chain lengths (C5 and C11) at three different arrangements on the structural properties and hydration of SAMs. Our simulation study reveals that the arrangement of thiols having unequal carbon chains in mixed SAMs is a key parameter in deciding the hydrophilicity of the coated gold NPs. Thus, our findings suggest that the hydration of the SAMs-protected gold NPs is not only dependent on the molecular composition of the thiols, but also on the organization of their mixing. In addition, our results show that the bending of longer thiols, when these are mixed with shorter thiols, depends on the arrangement of thiols as well as the chemical nature of their terminal groups.

A nanostructured genosensor for the early diagnosis of systemic arterial hypertension.

The rapid progress of nanomedicine, especially in areas related to medical imaging and diagnostics, has motivated the development of new nanomaterials that can be combined with biological materials for specific medical applications. One such area of research involves the detection of specific DNA sequences for the early diagnosis of genetic diseases, using nanoparticles-containing genosensors. Typical genosensors devices are based on the use of sensing electrodes - biorecognition platforms - containing immobilized capture DNA probes capable of hybridizing with specific target DNA sequences. In this paper we show that upon an appropriate design of the biorecognition platform, efficient sandwich-type genosensors based upon DNA-AuNPs nanocomplexes can be efficiently applied to the detection of a Systemic Arterial Hypertension (SAH) polymorphism located in intron 16 of the Angiotensin-converter enzyme (ACE) gene. Since SAH is intimately related to heart diseases, especially blood hypertension, its early detection is of great biomedical interest. The biorecognition platforms were assembled using mixed self-assembled monolayers (SAMmix), which provided the immobilization of organized architectures with molecular control. Detection of the DNA target sequence at concentrations down to 1 nM was carried out using electrochemical impedance spectroscopy (EIS). We show that the use of EIS combined with specific nanobiocomplexes represents an efficient method for the unambiguous detection of complementary DNA hybridization for preventative nanomedicine applications.

Tailoring the properties of surface-immobilized azobenzenes by monolayer dilution and surface curvature.

Photoswitching in densely packed azobenzene self-assembled monolayers (SAMs) is strongly affected by steric constraints and excitonic coupling between neighboring chromophores. Therefore, control of the chromophore density is essential for enhancing and manipulating the photoisomerization yield. We systematically compare two methods to achieve this goal: First, we assemble monocomponent azobenzene-alkanethiolate SAMs on gold nanoparticles of varying size. Second, we form mixed SAMs of azobenzene-alkanethiolates and "dummy" alkanethiolates on planar substrates. Both methods lead to a gradual decrease of the chromophore density and enable efficient photoswitching with low-power light sources. X-ray spectroscopy reveals that coadsorption from solution yields mixtures with tunable composition. The orientation of the chromophores with respect to the surface normal changes from a tilted to an upright position with increasing azobenzene density. For both systems, optical spectroscopy reveals a pronounced excitonic shift that increases with the chromophore density. In spite of exciting the optical transition of the monomer, the main spectral change in mixed SAMs occurs in the excitonic band. In addition, the photoisomerization yield decreases only slightly by increasing the azobenzene-alkanethiolate density, and we observed photoswitching even with minor dilutions. Unlike in solution, azobenzene in the planar SAM can be switched back almost completely by optical excitation from the cis to the original trans state within a short time scale. These observations indicate cooperativity in the photoswitching process of mixed SAMs.

Investigation of the deposition and thermal behavior of striped phases of unsymmetric disulfide self-assembled monolayers on Au(111): the case of 11-hydroxyundecyl decyl disulfide.

Self-assembled monolayers (SAMs) of unsymmetric disulfides on Au(111) are used to form mixed SAMs that can be utilized in many applications. Here, we have studied 11-hydroxyundecyl decyl disulfide (CH3-(CH2)9-S-S-(CH2)11-OH, HDD) SAMs produced by supersonic molecular beam deposition and characterized by He diffraction. The film growth was monitored at different temperatures up to a coverage which corresponds to a full lying down phase and the diffraction analysis shows that below 250 K the phase is different from the phase measured above 300 K. During the annealing of the film, two phase transitions were observed, at 250 K and 350 K. The overall data suggest that the former is related to an irreversible phase separation of HDD above 250 K to decanethiolate (-S-(CH2)9-CH3, DTT) and hydroxyundecylthiolate (-S-(CH2)11-OH, MUDT), while the latter to a reversible melting of the film. Above 450 K, the specular intensity shows an increase related to film desorption and different chemisorbed states were observed with energies in the same range as observed for decanethiol (H-S-(CH2)9-CH3, DT) and mercaptoundecanol (H-S-(CH2)11-OH, MUD) SAMs.

New approach for natural products screening by real-time monitoring of hemoglobin hydrolysis using quartz crystal microbalance

The hemoglobin (Hb) released from erythrocytes is a primary nutritive component for many blood-feeding parasites. The aspartic protease cathepsin D is a hemoglobinase that is involved in the Hb degradation process and is considered an interesting target for chemotherapy intervention. However, traditional enzymatic assays for studying Hb degradation utilize spectrophotometric techniques, which do not allow real-time monitoring and can present serious interference problems. Herein, we describe a biosensor using simple approach for the real-time monitoring of Hb hydrolysis as well as an efficient screening method for natural products as enzymatic inhibitors using a quartz crystal microbalance (QCM) technique. Hemoglobin was anchored on the quartz crystal surface using mixed self-assembled monolayers. The addition of the enzyme caused a mass change (frequency shift) due to Hb hydrolysis, which was monitored in real time. From the frequency change patterns of the Hb-functionalized QCM, we evaluated the enzymatic reaction by determining the kinetic parameters of product formation (kcat). The QCM enzymatic assay using immobilized human Hb was shown to be an excellent approach for screening possible inhibitors in complex mixtures, opening up a new avenue for the discovery of novel inhibitors.

A Tailored Self-Assembling Monolayer for Monitoring Biomarkers in Cell Culture Media Using the liSPR System

Surface plasmon resonance (SPR) spectroscopy is a promising chip-based biosensor technology that can quantify various biomarkers without molecular labels or time-consuming sample preparation. The liSPR spectrometer is an inexpensive, convenient, portable, and robust SPR system. A drawback is that the chips’ sensing surfaces are fragile as they consist of a thin layer of gold sputtered directly on a polymer slide, without the Cr or Ti adhesive layers typically used in other SPR chips. Furthermore, interfering substances, particularly non-target proteins, may adsorb to the gold due to its hydrophobicity. This can lead to non-specific sensor signals or even inactivation of the ligands and hence false SPR signals. Thus, to improve the system’s reliability the bare gold surface must be modified. Here, we present a tailored sensor surface for measuring human serum albumin (HSA) levels of human hepatocytes cultivated in vitro in media containing huge numbers of proteins capable of adsorbing to a bare gold surface. The key feature is a mixed self-assembled monolayer (SAM) comprising two types of alkanethiols: one for covalently attaching ligand molecules – here antibodies – to the chip and one for suppressing non-specific adsorption of interfering sample components. In addition to describing the new sensor surface we present data on its levels of immobilized antibody, non-specific adsorption of cell culture components, and HSA binding capacities. Calibration curves (obtained using HSA-spiked media samples) are also presented showing that the rapid, convenient sensor system provides similar sensitivity and reliability to standard ELISA methodology.

Original Covalent Approach for Gold Nanorods Immobilization onto Acid-Terminated-Cysteamine Self-Assembled Monolayers for FT-SPR Biosensor Applications

In this paper, we report an original method to immobilize gold nanorods onto mixed self-assembled monolayers (SAMs) of Mercaptoundecanoic Acid (MUA), Mercaptohexanol (MOH) and cysteamine (CYS) onto planar gold surface. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images revealed a remarkable shape and size narrow distribution on well functionalized gold surfaces, as well as a tendency to form linear assemblies after immobilization. The results highlight the good distribution of gold nanorods with an average length of 32.6 ± 0.9 nm and width of 13 ± 1.8 nm, the simplicity of the immobilization procedure of gold nanorods and the interest of using them as labels to enhance the sensitivity of FT-SPR-based sensors. gold nanostructured surface FT-SPR measurements of biorecognition using gold nanorods immobilized onto gold surfaces were performed at various prostatic antigen specific (PSA) concentrations, from 5 mg/L to 0.5 mg/L reaching a system sensitivity of 37 ± 2 cm-1/ mgL-1.

Mixed Self-Assembled Monolayers of Mercaptoundecanoic Acid and Thiolactic Acid for the Construction of an Enzymatic Biosensor for Hydroquinone Determination

A horseradish peroxidase (HRP) biosensor was constructed using binary self-assembled monolayers (SAM) of 11-mercaptoundecanoic acid (MUA) and thiolactic acid (TLA) on gold surface. The advantages of using mixed SAM for the enzyme immobilization is that the long carbon chain molecules act as a support for the enzyme while the short chain molecules favor the electron transfer process. In order to obtain this modified surface, the gold electrode was incubated in a solution containing different proportions of MUA and TLA and the best concentration ratio of these molecules was 0.5 and 1.0 mmol L−1, respectively. The preparation steps and the biosensor response were monitored by electrochemical techniques. The biosensor proposed was applied to determine hydroquinone in a 0.10 mol L−1 phosphate buffer solution containing H2O2 0.3 mmol L−1. The Au-SAMmix-HRP electrode, in the presence of hydrogen peroxide, catalyzes the oxidation of hydroquinone to the corresponding quinone, which is electrochemically reduced back to hydroquinone at −0.08 V vs Ag/AgCl. The analytical curve was linear for hydroquinone concentrations from 5.0 to 30 μmol L−1 and the detection limit was 1.26 μmol L−1. The lifetime of this biosensor was 15 days. The modified electrode displayed good reproducibility, sensitivity and stability for the determination of hydroquinone.

Mixed self-assembled monolayer gate dielectrics for low-voltage solution-processed polymer field-effect transistors

Highly insulating gate dielectrics based on AlOx and on mixed phosphonic-acid SAMs terminated with methyl/carboxylic acid groups are demonstrated.

Influence of surface structure on single or mixed component self-assembled monolayers via in situ spectroelectrochemical fluorescence imaging of the complete stereographic triangle on a single crystal Au bead electrode.

The use of a single crystal gold bead electrode is demonstrated for characterization of self-assembled monolayers (SAM)s formed on the bead surface expressing a complete set of face centered cubic (fcc) surface structures represented by a stereographic projection. Simultaneous analysis of many crystallographic orientations was accomplished through the use of an in situ fluorescence microscopic imaging technique coupled with electrochemical measurements. SAMs were prepared from different classes of molecules, which were modified with a fluorescent tag enabling characterization of the influence of electrical potential and a direct comparison of the influence of surface structure on SAMs adsorbed onto low index, vicinal and chiral surfaces. The assembly of alkylthiol, Aib peptide and DNA SAMs are studied as a function of the electrical potential of the interface revealing how the organization of these SAMs depend on the surface crystallographic orientation, all in one measurement. This approach allows for a simultaneous determination of SAMs assembled onto an electrode surface onto which the whole fcc stereographic triangle can be mapped, revealing the influence of intermolecular interactions as well as the atomic arrangement of the substrate. Moreover, this method enables study of the influence of the Au surface atom arrangement on SAMs that were created and analyzed, both under identical conditions, something that can be challenging for the typical studies of this kind using individual gold single crystal electrodes. Also demonstrated is the analysis of a SAM containing two components prepared using thiol exchange. The two component SAM shows remarkable differences in the surface coverage, which strongly depends on the surface crystallography enabling estimates of the thiol exchange energetics. In addition, these electrode surfaces enable studies of molecular adsorption onto the symmetry related chiral surfaces since more than one stereographic triangle can be imaged at the same time. The ability to observe a SAM modified surface that contains many complete fcc stereographic triangles will facilitate the study of the single and multicomponent SAMs, identifying interesting surfaces for further analysis.

Tuning the optical emission of MoS2 nanosheets using proximal photoswitchable azobenzene molecules

We report photoluminescence measurements performed on monolayer- and two-layer-MoS2 placed on two types of mixed self-assembled monolayers (mSAMs) of photoswitchable azobenzene molecules. The two mSAMs differ via the electronegative character of the azobenzene derivatives. Thin layers of a transition metal dichalcogenide—MoS2—were mechanically exfoliated on mSAM to allow for direct interaction between the molecules and the MoS2 layers. When the MoS2 nanosheet is in contact with the electropositive azobenzene molecules in trans configuration, an emission side band at lower energies and at low excitation powers suggest n-type doping. The photoisomerization of the molecules from trans to cis configuration lowers the doping, quenching the side band and enhancing the overall PL efficiency by a factor of ∼3. Opposite results were observed with the chlorinated, more electronegative molecules, exhibiting a reversed trend in the PL efficiency between trans and cis, but with an overall larger intensity. The type of doping induced by the two types of mSAMs was determined by Kelvin probe force microscopy technique.

Structure and Electronic and Charge-Transfer Properties of Mercaptobenzoic Acid and Mercaptobenzoic Acid–Undecanethiol Mixed Monolayers on Au(111)

The surface structure, molecular conductance, and charge-transfer properties of pure mercaptobenzoic acid (MBA) and mixed MBA–undecanethiol (UDT) self-assembled monolayers (SAMs) on Au(111) are studied by scanning tunneling microscopy, scanning tunneling spectroscopy, electrochemical techniques, and density functional theory calculations. MBA forms ordered diluted lattices without evidence of Au adatom complexes at terraces (absence of vacancy islands) at low MBA concentrations, whereas disordered lattices with possible formation of these complexes are found at high concentrations (presence of vacancy islands). In addition, some other interesting facts have been established. The stability of the MBA SAMs is improved in mixed MBA–UDT SAMs, revealing aromatic–aliphatic attractive interactions. The mixed SAMs at equivalent amounts of thiols hinder the vacancy island formation typical of the pure UDT SAMs. The MBA SAMs exhibit good molecular conductance with filled states from S and aromatic ring contribution...