Composition Of Self-assembled Monolayers Research Articles

Electrochemical SEIRAS Analysis of Imidazole-Ring-Functionalized Self-Assembled Monolayers.

An essential amino acid, histidine, has a vital role in the secondary structure and catalytic activity of proteins because of the diverse interactions its side chain imidazole (Im) ring can take part in. Among these interactions, hydrogen donating and accepting bonding are often found to operate at the charged interfaces. However, despite the great biological significance, hydrogen-bond interactions are difficult to investigate at electrochemical interfaces due to the lack of appropriate experimental methods. Here, we present a surface-enhanced infrared absorption spectroscopy (SEIRAS) and density functional theory (DFT) study addressing this issue. To probe the hydrogen-bond interactions of the Im at the electrified organic layer/water interface, we constructed Au-adsorbed self-assembled monolayers (SAMs) that are functionalized with the Im group. As the prerequisite for spectroelectrochemical investigations, we first analyzed the formation of the monolayer and the relationship between the chemical composition of SAM and its structure. Infrared absorption markers that are sensitive to hydrogen-bonding interactions were identified. We found that negative electrode polarization effectively reduced hydrogen-bonding strength at the Im ring at the organic layer–water interface. The possible mechanism governing such a decrease in hydrogen-bonding interaction strength is discussed.

Electron Transfer in Binary Hemin-Modified Alkanethiol Self-Assembled Monolayers on Gold: Hemin's Lateral and Interfacial Interactions.

Orientated coupling of redox enzymes to electrodes by their reconstitution onto redox cofactors, such as hemin conjugated to self-assembled monolayers (SAMs) formed on the electrodes, poses the requirements for a SAM design enabling reconstitution. We show that the kinetics of electron transfer (ET) in binary SAMs of alkanethiols on gold composed of in situ hemin-conjugated 11-amino-1-undecanethiol (AUT) and diluting OH-terminated alkanethiols with 11, 6, and 2 methylene groups (MC11OH, MC6OH, and MC2OH) depends on both the SAM composition and surface density of hemin, Γheme. In AUT/MC11OH SAMs composed of equal linker/diluent lengths, the heterogeneous ET rate constant ks decreased with the Γheme and varied between 70 and 500 s-1. For shorter diluents, the ks of 245-330 s-1 (C6) and 300-340 s-1 (C2) showed a little (if any) Γheme dependence. In AUT/MC11OH SAMs, the increasing Γheme resulted in the steric crowding of hemin species and their neighboring lateral interactions in the plane of hemin localization, affecting the potential distribution at the SAM/electrode interface and inducing local electrostatic effects interfering with hemin oxidation. In AUT/MC6OH and AUT/MC2OH SAMs, hemin discharged at the plane of the closest approach to the gold surface, equal to the diluent length and permeable to electrolyte ions, which lessened those effects. All studied binary SAMs provided steric hindrance for protein reconstitution on the hemin cofactor conjugated to the extended AUT linker. Further use of SAM-modified electrodes with the covalently attached hemin as interfaces for heme proteins' reconstitution should consider SAMs with loosely dispersed redox centers terminating more rigid molecular wires. Such wires place hemin at fixed distances from the electrode surface and thus ensure the interfacial properties required for the effective on-surface reconstitution of proteins and enzymes.

Concept of Embedded Dipoles as a Versatile Tool for Surface Engineering.

ConspectusControlling the physical and chemical properties of surfaces and interfaces is of fundamental relevance in various areas of physical chemistry and a key issue of modern nanotechnology. A highly promising strategy for achieving that control is the use of self-assembled monolayers (SAMs), which are ordered arrays of rodlike molecules bound to the substrate by a suitable anchoring group and carrying a functional tail group at the other end of the molecular backbone. Besides various other applications, SAMs are frequently used in organic electronics for the electrostatic engineering of interfaces by controlling the interfacial level alignment. This is usually achieved by introducing a dipolar tail group at the SAM–semiconductor interface. Such an approach, however, also changes the chemical character of that interface, for example, affecting the growth of subsequent layers. A strategy for avoiding this complication is to embed polar groups into the backbones of the SAM-forming molecules. This allows disentangling electronic interface engineering and the nucleation of further layers, such that both can be optimized independently. This novel concept was successfully demonstrated for both aliphatic and aromatic SAMs on different application-relevant substrates, such as gold, silver, and indium tin oxide. Embedding, for example, ester and pyrimidine groups in different orientations into the backbones of the SAM-forming molecules results in significant work-function changes. These can then be fine-tuned over a wide energy range by growing mixed monolayers consisting of molecules with oppositely oriented polar groups. In such systems, the variation of the work function is accompanied by pronounced shifts of the peaks in X-ray photoelectron spectra, which demonstrates that electrostatically triggered core-level shifts can be as important as the well-established chemical shifts. This illustrates the potential of X-ray photoelectron spectroscopy (XPS) as a tool for probing the local electrostatic energy within monolayers and, in systems like the ones studied here, makes XPS a powerful tool for studying the composition and morphology of binary SAMs. All these experimental observations can be rationalized through simulations, which show that the assemblies of embedded dipolar groups introduce a potential discontinuity within the monolayer, shifting the energy levels above and below the dipoles relative to each other. In molecular and monolayer electronics, embedded-dipole SAMs can be used to control transition voltages and current rectification. In devices based on organic and 2D semiconductors, such as MoS2, they can reduce contact resistances by several orders of magnitude without adversely affecting film growth even on flexible substrates. By varying the orientation of the embedded dipolar moieties, it is also possible to build p- and n-type organic transistors using the same electrode materials (Au). The extensions of the embedded-dipole concept from hybrid interfaces to systems such as metal–organic frameworks is currently underway, which further underlines the high potential of this approach.

Correlating the Influence of Disulfides in Monolayers across Photoelectron Spectroscopy Wettability and Tunneling Charge-Transport.

Despite their ubiquity, self-assembled monolayers (SAMs) of thiols on coinage metals are difficult to study and are still not completely understood, particularly with respect to the nature of thiol–metal bonding. Recent advances in molecular electronics have highlighted this deficiency due to the sensitivity of tunneling charge-transport to the subtle differences in the overall composition of SAMs and the chemistry of their attachment to surfaces. These advances have also challenged assumptions about the spontaneous formation of covalent thiol–metal bonds. This paper describes a series of experiments that correlate changes in the physical properties of SAMs to photoelectron spectroscopy to unambiguously assign binding energies of noncovalent interactions to physisorbed disulfides. These disulfides can be converted to covalent metal–thiolate bonds by exposure to free thiols, leading to the remarkable observation of the total loss and recovery of length-dependent tunneling charge-transport. The identification and assignment of physisorbed disulfides solve a long-standing mystery and reveal new, dynamic properties in SAMs of thiols.

Self-Assembled Monolayers for Anti-Fouling and Highly Selective Electrode Interfaces

The detection, collection and/or conversion of biological-based molecules has applications to sensing, wearables, and biofuels. Flexible, wearable electronics for real-time detection of, for example, Neuropeptide Y in human sweat [1], or flagella protein in biofuels, enables deviation from bulky laboratory benchtop analyses such as ELISA. Challenges include required limits of detection, as biological materials can be present in small amounts (nM to pM concentrations) in complex high ionic strength environments[2], the need for a highly selective sensor interface that avoids non-specific binding, and a quick response to account for degradation of analytes.This work will discuss the implementation of a mixed self-assembled monolayer (SAM) interface with highly selective bio-recognition elements (BREs) on gold disk electrodes, focusing on target application for aqueous sensing in biofuels, with room for expansion to sensing in other biological fluids. The SAM composition was systematically varied to assess functionality towards preventing fouling/non-specific binding and increase the selectivity of the interface for the analyte of interest. Briefly, mixed monolayer combinations included pseudo-zwitter surfaces to prevent fouling, blocking groups similarly employed in the literature[3] to help rectify BREs, and dithiobis(succinimidyl propionate) (DSP) cross-linking chemistry[4] to attach various BREs to the gold electrode. BREs include concanavilin A, streptavidin, nanobodies, and synthetic lectins. Streptavidin and it’s interaction with biotin was chosen as a control, due to their known strong interaction. Electrochemical impedance spectroscopy (EIS) and square wave voltammetry (SWV) were used to estimate changes in the resistance to charge transfer to redox mediators either immobilized or in solution. EIS is one of the most common modern techniques used in biosensing, but as a time-intensive measurement it can be undesirable for accurate sensing in analyte-degrading environments. Therefore, SWV was employed as a substitute which reduces times of measurements by an order of magnitude, and is shown to provide the same information as EIS. To corroborate these electrochemical effects with a definite surface coverage/binding event, ellipsometric experiments of functionalized electrodes are underway.Our SAM interface produces nM level detection for yeast through the employment of gold immobilized charge transfer mediators and concanavalin A with selective binding to the yeast cell wall. Specifically, a decrease in charge transfer resistance (or increase in peak current as measured in SWV) occurs upon binding of yeast cells to concanavalin A. The streptavidin-biotin interaction also manipulated the charge transfer resistance similarly, confirming our method effectiveness. Nanobodies (binding fragment of antibodies) were also shown to have some success in detecting flagella protein with redox mediators in solution.These proof-of-concept results demonstrate feasibility and indicate forward paths towards highly selective sensor electrodes with simultaneous anti-fouling properties that prevents non-specific binding. Ongoing efforts include modifying electronic device interfaces with SAMs for combined signaling and amplification, such as modification of gate electrodes in printable electrochemical transistors.

Impact of Self-Assembled Monolayer Design and Electrochemical Factors on Impedance-Based Biosensing.

Real-time sensing of proteins, especially in wearable devices, remains a substantial challenge due to the need to convert a binding event into a measurable signal that is compatible with the chosen analytical instrumentation. Impedance spectroscopy enables real-time detection via either measuring electrostatic interactions or electron transfer reactions while simultaneously being amenable to miniaturization for integration into wearable form-factors. To create a more robust methodology for optimizing impedance-based sensors, additional fundamental studies exploring components influencing the design and implementation of these sensors are needed. This investigation addresses a sub-set of these issues by combining optical and electrochemical characterization to validate impedance-based sensor performance as a function of (1) biorecognition element density, (2) self-assembled monolayer chain length, (3) self-assembled monolayer charge density, (4) the electrochemical sensing mechanism and (5) the redox reporter selection. Using a pre-existing lysozyme aptamer and lysozyme analyte combination, we demonstrate a number of design criteria to advance the state-of-the-art in protein sensing. For this model system we demonstrated the following: First, denser self-assembled monolayers yielded substantially improved sensing results. Second, self-assembled monolayer composition, including both thickness and charge density, changed the observed peak position and peak current. Third, single frequency measurements, while less informative, can be optimized to replace multi-frequency measurements and in some cases (such as that with zwitterionic self-assembled monolayers) are preferred. Finally, various redox reporters traditionally not used in impedance sensing should be further explored. Collectively, these results can help limit bottlenecks associated with device development, enabling realization of next-generation impedance-based biosensing with customize sensor design for the specific application.

D-shaped plastic optical fibre aptasensor for fast thrombin detection in nanomolar range

The development of optical biosensors for the rapid and costless determination of clinical biomarkers is of paramount importance in medicine. Here we report a fast and low-cost biosensor based on a plasmonic D-shaped plastic optical fibre (POF) sensor derivatized with an aptamer specific for the recognition of thrombin, the target marker of blood homeostasis and coagulation cascade. In particular, we designed a functional interface based on a Self Assembled Monolayer (SAM) composed of short Poly Ethylene Glycol (PEG) chains and biotin-modified PEG thiol in ratio 8:2 mol:mol, these latter serving as baits for the binding of the aptamer through streptavidin-chemistry. The SAM was studied by X-ray Photoelectron Spectroscopy (XPS) analysis, static contact angle (CA), Surface Plasmon Resonance (SPR) in POFs, and fluorescence microscopy on gold surface. The optimized SAM composition enabled the immobilization of about 112 ng/cm2 of aptamer. The thrombin detection exploiting POF-Aptasensor occurred in short times (5–10 minutes), the reached Limit of Detection (LOD) was about 1 nM, and the detection range was 1.6–60 nM, indicating the POF-Aptasensor well addresses the needs for a low-cost, simple to use and to realize, rapid, small size and portable diagnostic platform.

Mixed Organic Ligand Shells: Controlling the Nanoparticle Surface Morphology toward Tuning the Optoelectronic Properties.

Precise control over the ratio of perylene bisimide (PBI) monomers and aggregates, immobilized on alumina nanoparticle (NP) surfaces, is demonstrated. Towards this goal, phosphonic acid functionalized PBI derivatives (PA-PBI) are shown to self-assemble into stoichiometrically mixed monolayers featuring aliphatic, glycolic, or fluorinated phosphonic acid ligands, serving as imbedding matrix (PA-M) to afford core-shell NPs. Different but, nevertheless, defined PBI monomer/aggregate composition is achieved by either the variation in the PA-PBI to PA-M ratios, or the utilization of different PA-Ms. Various steady-state as well as time-resolved spectroscopy techniques are applied to probe the core-shell NPs with respect to changes in their optical properties upon variations in the shell composition. To this end, the ratio between monomer and excimer-like emission assists in deriving information on the self-assembled monolayer composition, local ordering, and corresponding aggregate content. With the help of X-ray reflectivity measurements, accompanied by molecular dynamics simulations, the built-up of the particle shells, in general, and the PBI aggregation behavior, in particular, are explored in depth.

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.

SAM Composition and Electrode Roughness Affect Performance of a DNA Biosensor for Antibiotic Resistance.

Antibiotic resistance is a growing concern in the treatment of infectious disease worldwide. Point-of-care (PoC) assays which rapidly identify antibiotic resistance in a sample will allow for immediate targeted therapy which improves patient outcomes and helps maintain the effectiveness of current antibiotic stockpiles. Electrochemical assays offer many benefits, but translation from a benchtop measurement system to low-cost portable electrodes can be challenging. Using electrochemical and physical techniques, this study examines how different electrode surfaces and bio-recognition elements, i.e. the self-assembled monolayer (SAM), affect the performance of a biosensor measuring the hybridisation of a probe for antibiotic resistance to a target gene sequence in solution. We evaluate several commercially available electrodes which could be suitable for PoC testing with different SAM layers and show that electrode selection also plays an important role in overall biosensor performance.

Self-Assembled Monolayers for Dental Implants.

Implant-based therapy is a mature approach to recover the health conditions of patients affected by edentulism. Thousands of dental implants are placed each year since their introduction in the 80s. However, implantology faces challenges that require more research strategies such as new support therapies for a world population with a continuous increase of life expectancy, to control periodontal status and new bioactive surfaces for implants. The present review is focused on self-assembled monolayers (SAMs) for dental implant materials as a nanoscale-processing approach to modify titanium surfaces. SAMs represent an easy, accurate, and precise approach to modify surface properties. These are stable, well-defined, and well-organized organic structures that allow to control the chemical properties of the interface at the molecular scale. The ability to control the composition and properties of SAMs precisely through synthesis (i.e., the synthetic chemistry of organic compounds with a wide range of functional groups is well established and in general very simple, being commercially available), combined with the simple methods to pattern their functional groups on complex geometry appliances, makes them a good system for fundamental studies regarding the interaction between surfaces, proteins, and cells, as well as to engineering surfaces in order to develop new biomaterials.

Formation Kinetics of Mixed Self-Assembled Monolayers of Alkanethiols on GaAs(100).

We report on the formation kinetics of mixed self-assembled monolayers (SAMs) comprising 16-mercaptohexadecanoic acid (MHDA) and 11-mercapto-1-undecanol (MUDO) thiols on GaAs(100) substrates. These compounds were selected for their potential in constructing highly selective and efficient architectures for biosensing applications. The molecular composition and quality of one-compound and mixed SAMs were determined by the Fourier transform infrared absorption spectroscopy measurements. The formation of enhanced-quality mixed SAMs was investigated as a function of the molecular composition of the thiol mixture and the proportion of ethanol/water solvent used during their arrangement. Furthermore, the formation of mixed SAMs has been carried out by successive immersion of MHDA SAMs in MUDO thiol solutions and MUDO SAMs in MHDA thiol solution through the process involving thiol-thiol substitution. Our results, in addition to confirming that water-ethanol-based solvents improve the packing density of single thiol monolayers, demonstrate the attractive role of water-ethanol solvents in forming superior quality mixed SAMs.

Two Are Better than One: Bidentate Adsorbates Offer Precise Control of Interfacial Composition and Properties

Mixed self-assembled monolayers (SAMs) generated from perfluoro-, n-alkyl-, and oligo(ethylene glycol)-terminated alkanedithiols were utilized to prepare two-dimensional interfaces with precise composition and wettability. Interfacial control was afforded simply by adjusting the mole fraction of the adsorbates present in the development solutions and was modulated by the dual contributions of the tailgroups of the bidentate thiols. In contrast, the composition and wettability of mixed SAMs generated from traditional monodentate thiols bearing analogous tailgroups failed to track systematically with the mole fractions of the adsorbates in the development solutions, reflecting a greater dependence on solvent/adsorbate interactions. All of the SAMs were thoroughly characterized by contact angle goniometry, ellipsometry, X-ray photoelectron spectroscopy (XPS), and polarization modulation infrared reflection–absorption spectroscopy (PM-IRRAS). Furthermore, solution-phase displacement tests performed to evaluat...

(Invited) Effects of Sensitization Conditions on Photovoltaic and Photophysical Properties of Porphyrin-Sensitized Solar Cells

Elucidating the structural details of molecular assemblies formed on surfaces is essential to understand a variety of fundamental surface and interface phenomena as well as develop new types of functional surfaces and interfaces. Examples include molecular electronic devices and adsorption of biological molecules and materials. In the past the vast majority of such studies have been performed for thiol-based self-assembled monolayers (SAMs) which can be formed on gold surfaces by immersion. In contrast, the use of thiol-based SAMs on other metal and semiconducting surfaces has been limited technologically. Meanwhile, carboxylic and phosphonic acids have been frequently employed as an anchoring group for the immobilization of functional molecules on semiconducting surfaces. However, their studies have focused mainly on function rather than structure. As such, adsorption/desorption and exchange behaviour of functional molecules on semiconducting surfaces and their structure-function relationship have not been fully addressed. As a promising alternative to silicon-based solar cells, considerable efforts have been devoted to the development of dye-sensitized solar cells (DSSCs) where a SAM of dye is formed on a highly porous semiconducting electrode. In particular, porphyrin-sensitized solar cells (PSSCs) have drawn much attention in DSSCs because of potential advantage of porphyrins over ruthenium dyes in terms of cost, molecular design, and light-harvesting properties. On the basis of push-pull concept as well as asymmetrical pi-elongation, cell performances of PSSCs have been improved rapidly. In particular, push-pull porphyrins have been developed intensively in recent years with the aim of better matching between the absorption and solar spectra, reaching a power conversion efficiency more than 10%. To achieve further improvement of the cell performances, it is also important to elucidate close relationship between the dye structure, film structure, photophysics and photovoltaic properties, providing a guideline for the rational molecular design of high-performance dyes. During the course of our studies, we accidently encountered “memory effects” of the photovoltaic properties in the porphyrin SAMs on the TiO2 that could be regulated by choice of the sensitization solvents (MeOH vs. t-BuOH/MeCN) with and without the porphyrins. Although there are some examples of memory effects of SAMs induced by external stimuli, “memory effects” of photovoltaic properties by SAMs have not been reported in organic photovoltaics devices. Herein we report the first example of the memory effects in porphyrin SAMs on TiO2 electrodes that can be controlled by sensitization solvents, which have been evaluated by the photovoltaic properties of PSSCs, complemented by measurements of electron transfer kinetics and SAM composition and thickness. CN-labeled ZnP carboxylic acid (CNMP) and the reference porphyrin (MP) were used in this study.

Structural Investigations of Self-Assembled Monolayers for Organic Electronics: Results from X-ray Reflectivity

Self-assembled monolayers (SAMs) have been established as crucial interlayers and electronically active layers in organic electronic devices, such as organic light emitting diodes (OLEDs), organic photovoltaics (OPVs), organic thin film transistors (OTFTs), and nonvolatile memories (NVMs). The use of self-assembling functionalized organic molecules is beneficial due to mainly three advantages compared with common thin film deposition approaches. (1) Molecular self-assembly occurs with surface selectivity, determined by the interaction between the functional anchor group of the organic molecules and the target surface. (2) The film thickness of the resulting layers is perfectly controllable on the angstrom scale, due to the self-terminating film formation to only a single molecular layer. And finally, (3) the wide variability in the chemical structure of such molecules enables different SAM functionalities for devices, ranging from electrical insulation to charge storage to charge transport. The SAM approach can be further expanded by employing several functionalized molecules to create mixed SAMs with consequently mixed properties. The function of SAMs in devices depends not only on the chemical structure of the molecules but also on their final arrangement and orientation on the surface. A reliable and nondestructive in-depth characterization of SAMs on nonconductive oxide surfaces is still challenging because of the very small thickness and the impracticality of methods such as scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). In this Account, we illustrate how X-ray reflectivity (XRR) provides analytical access to major questions of SAM composition, morphology, and even formation by means of investigations of pure and mixed SAMs based on phosphonic acids (PAs) of various chain structures on flat alumina (AlOx) surfaces. XRR is an analytical method that provides access to spatially averaged structural depth profiles over a relatively large area of several square micrometers. The key outcome of XRR, the surface-normal electron density profile of the SAMs, leads to precise information on the SAM thickness with subangstrom resolution and allows for the determination of molecular tilt angles and packing densities. We have systematically increased the chemical complexity of PA molecules and the resulting SAMs, utilizing XRR to provide insight into the SAM structures. In SAMs composed of functionalized molecules or complex chain structures, the distribution of electron rich and electron poor signatures is detected and thus the molecular order within the SAM is determined. In mixed SAMs of two different molecular species, electron density profiles reveal the morphology and how the surface-normal structure changes if one component of the mixed SAM is altered. Furthermore, XRR was applied to investigate in situ the self-assembly of functionalized PA molecules from solution by tracking the monolayer growth over time. Even though the results provided by XRR on in-plane molecular arrangement are limited, it presents excellent information on the molecular scale along the surface normal and in addition allows for drawing conclusions on the intermolecular interactions within the SAM.

Neutralized chimeric avidin binding at a reference biosensor surface.

We describe the development of a reference biosensor surface, based upon a binary mixture of oligo-ethylene glycol thiols, one of which has biotin at the terminus, adsorbed onto gold as self-assembled monolayers (SAMs). These surfaces were analyzed in detail by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) to establish the relationship between the thiol solution composition and the surface composition and structure. We report the use of argon cluster primary ions for the analysis of PEG-thiols, establishing that the different thiols are intimately mixed and that SIMS may be used to measure surface composition of thiol SAMs on gold with a detection limit better than 1% fractional coverage. The adsorption of neutralized chimeric avidin to these surfaces was measured simultaneously using ellipsometry and QCM-D. Comparison of the two measurements demonstrates the expected nonlinearity of the frequency response of the QCM but also reveals a strong variation in the dissipation signal that correlates with the surface density of biotin. These variations are most likely due to the difference in mechanical response of neutralized chimeric avidin bound by just one biotin moiety at low biotin density and two biotin moieties at high density. The transition between the two modes of binding occurs when the average spacing of biotin ligands approaches the diameter of the avidin molecule.

Reconstitution of peroxidase onto hemin-terminated alkanethiol self-assembled monolayers on gold

Reconstitution of enzymes onto surface tethered cofactors may be considered as a powerful tool both for design of biorecognition interfaces with a molecular-level control over spatial distribution and orientation of biosensing elements and for fundamental studies of their structure–function relationships. Here, reconstitution of horseradish peroxidase (HRP) on the heme cofactor covalently attached to the mixed self-assembled monolayers (SAM) on gold electrodes was studied. The efficiency of the holoenzyme formation was assessed by analysis of the hemin electrode reaction and bioelectrocatalytic activity of HRP. The formal potential, E0′, of the electrode-tethered hemin approached −340mV vs. Ag/AgCl, while the heterogeneous electron transfer (ET) rate constant, ks, for its redox transformation ranged between 245 and 340s−1, depending on the SAM composition. After enzyme reconstitution, both the E0′ and ks values for the hemin ET reaction showed a very little variation, while the onset of electrocatalytic reduction of H2O2 shifted by ca. 500mV more positive by this approaching the potentials of compound I of native HRP. Both the onset potential and bioelectrocatalytic currents were lower than observed with native HRP adsorbed on gold (Ferapontova and Gorton, 2001), being consistent with a lower surface population of the holoenzyme and/or less efficient ET pathway.

Bell‐Shaped Superhydrophilic–Superhydrophobic–Superhydrophilic Double Transformation on a pH‐Responsive Smart Surface

Superhydrophobic to neutral water droplets, superhydrophilic to acidic or basic. This double transition of surface wettability in response to a single stimulus - pH - is demonstrated for the first time. The smart surface is composed of a rough gold surface modified with a self-assembled monolayer (SAM) containing three thiols, HS(CH2 )11 CH3 , HS(CH2 )10 COOH, and HS(CH2 )11 NH2 . A ternary diagram is generated that describes wettability as a function of the SAM composition and the pH of the surrounding solution.

Electrochemical characterization of dehaloperoxidase adsorbates on COOH/OH mixed self-assembled monolayers

Electrochemical characterization of Amphitrite ornata dehaloperoxidase, a monomeric hemoglobin, adsorbed on COOH/OH-terminated alkanethiolate mixed self-assembled monolayers (SAMs) is reported. Adsorption was achieved by simple electrode exposure to low ionic strength protein solution at pH 6.0, and cyclic voltammetry (CV) was employed to measure surface concentration, electron transfer rate, and formal potential under anaerobic conditions. Surface concentration values determined by CV were found to be inversely proportional to scan rate, a behavior that is different than that observed for electron transfer proteins such as cytochrome c. We attribute this behavior to a dynamic heterogeneous adsorbate layer that permits some fraction of the molecules to undergo reorientation during a CV scan. A model was proposed that classifies adsorbates among three populations with respect to their most stable orientations, namely, electroactive adsorbates, latent electroinactive adsorbates capable of undergoing reorientation to electroactive states, and electroinactive adsorbates. Electrochemical results and the proposed model are discussed in terms of the tertiary structure and surface properties of dehaloperoxidase and are compared to the docking and electron transfer reactions of the myoglobin/cytochrome b5 system. Examination of the effects of ionic strength and mixed SAM composition provide support for the proposed model.

Photoactive self-assembled monolayers for optically switchable organic thin-film transistors

We investigate the photoconductive and photovoltaic effects in organic thin-film transistors with thin hybrid dielectrics composed of aluminum oxide and self-assembled monolayers (SAMs). By using SAM molecules with an electro-optical functionality tuning of the photoinduced charge transfer at the interface of semiconductor and SAM upon illumination with laser light can be achieved. Control of the threshold voltage by the SAM composition enables the optical operation of the transistors without applying a gate voltage and affects the dynamics of photoinduced charge transfer.