Stability Of Self-assembled Monolayers Research Articles

Electrically Stable Self-Assembled Monolayers Achieved through Repeated Surface Exchange of Molecules

Electrically Stable Self-Assembled Monolayers Achieved through Repeated Surface Exchange of Molecules

Development of a Multiplexed Electrochemical Aptasensor for the Detection of Cyanotoxins.

In this study, we report a multiplexed platform for the simultaneous determination of five marine toxins. The proposed biosensor is based on a disposable electrical printed (DEP) microarray composed of eight individually addressable carbon electrodes. The electrodeposition of gold nanoparticles on the carbon surface offers high conductivity and enlarges the electroactive area. The immobilization of thiolated aptamers on the AuNP-decorated carbon electrodes provides a stable, well-orientated and organized binary self-assembled monolayer for sensitive and accurate detection. A simple electrochemical multiplexed aptasensor based on AuNPs was designed to synchronously detect multiple cyanotoxins, namely, microcystin-LR (MC-LR), Cylindrospermopsin (CYL), anatoxin-α, saxitoxin and okadaic acid (OA). The choice of the five toxins was based on their widespread presence and toxicity to aquatic ecosystems and humans. Taking advantage of the conformational change of the aptamers upon target binding, cyanotoxin detection was achieved by monitoring the resulting electron transfer increase by square-wave voltammetry. Under the optimal conditions, the linear range of the proposed aptasensor was estimated to be from 0.018 nM to 200 nM for all the toxins, except for MC-LR where detection was possible within the range of 0.073 to 150 nM. Excellent sensitivity was achieved with the limits of detection of 0.0033, 0.0045, 0.0034, 0.0053 and 0.0048 nM for MC-LR, CYL, anatoxin-α, saxitoxin and OA, respectively. Selectivity studies were performed to show the absence of cross-reactivity between the five analytes. Finally, the application of the multiplexed aptasensor to tap water samples revealed very good agreement with the calibration curves obtained in buffer. This simple and accurate multiplexed platform could open the window for the simultaneous detection of multiple pollutants in different matrices.

A Guide for the Electrochemical Stability of Thiolate Self-Assembled Monolayers on Gold in Various Electrolytes

Self-assembled monolayers (SAMs) are widely used in surface chemistry to modify surfaces for various applications such as biosensing1, electronics2, and electrocatalysis3. SAMs are attractive due to their high surface coverage and stability. Although studies have been conducted to test the effects of temperature, air exposure, and solvent degradation on SAMs stability 4,5, the electrochemical stability of SAMs has not been as extensively studied. Existing literature on this topic mainly focuses on the reductive desorption limits of thiolate SAMs in alkaline conditions6,7. Recent work by Ramos et al. demonstrated pH-dependent stable potential windows in which SAMs remained intact on electrode surfaces of different metals8. However, there is still a need to verify and expand on this work.To address this need, we aim to create a comprehensive guide on the electrochemical stability of thiolate SAMs on gold electrodes. We will use cyclic voltammograms (CV), electrochemical impedance spectroscopy (EIS), and electrochemical surface plasmon resonance (EC-SPR) to probe and verify the stable electrochemical potential windows in both aqueous and non-aqueous electrolytes. We have conducted studies on three aqueous solutions at three distinct pH, perchloric acid, potassium hydroxide, and sodium chloride. For non-aqueous solvents we tested a wide range of different organic solvents including acetonitrile, dimethylformamide, propylene carbonate, and various alcohols. Our findings demonstrate that the main factor for SAM stability in aqueous and non-aqueous electrolytes is the pH and solubility of the SAM, respectively. Our poster will present our current work and outlook on this topic.(1) Dauphin-Ducharme, P.; Ploense, K. L.; Arroyo-Currás, N.; Kippin, T. E.; Plaxco, K. W. Electrochemical Aptamer-Based Sensors: A Platform Approach to High-Frequency Molecular Monitoring In Situ in the Living Body. In Biomedical Engineering Technologies: Volume 1; Ossandon, M. R., Baker, H., Rasooly, A., Eds.; Methods in Molecular Biology; Springer US: New York, NY, 2022; pp 479–492. https://doi.org/10.1007/978-1-0716-1803-5_25.(2) Hatton, R. A.; Willis, M. R.; Chesters, M. A.; Rutten, F. J. M.; Briggs, D. Enhanced Hole Injection in Organic Light-Emitting Diodes Using a SAM-Derivatised Ultra-Thin Gold Anode Supported on ITO Glass. J. Mater. Chem. 2003, 13 (1), 38–43. https://doi.org/10.1039/B208169P.(3) Shang, H.; Wallentine, S. K.; Hofmann, D. M.; Zhu, Q.; Murphy, C. J.; Baker, L. R. Effect of Surface Ligands on Gold Nanocatalysts for CO2 Reduction. Chem. Sci. 2020, 11 (45), 12298–12306. https://doi.org/10.1039/D0SC05089J.(4) Schlenoff, J. B.; Li, M.; Ly, H. Stability and Self-Exchange in Alkanethiol Monolayers. J. Am. Chem. Soc. 1995, 117 (50), 12528–12536. https://doi.org/10.1021/ja00155a016.(5) Laibinis, P. E.; Whitesides, G. M. Self-Assembled Monolayers of n-Alkanethiolates on Copper Are Barrier Films That Protect the Metal against Oxidation by Air. J. Am. Chem. Soc. 1992, 114 (23), 9022–9028. https://doi.org/10.1021/ja00049a038.(6) Salvarezza, R. C.; Carro, P. The Electrochemical Stability of Thiols on Gold Surfaces. J. Electroanal. Chem. 2018, 819, 234–239. https://doi.org/10.1016/j.jelechem.2017.10.046.(7) Beulen, M. W. J.; Kastenberg, M. I.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Electrochemical Stability of Self-Assembled Monolayers on Gold. Langmuir 1998, 14 (26), 7463–7467. https://doi.org/10.1021/la981031z.(8) Ramos, N.; Medlin, J. W.; Holewinski, A. Electrochemical Stability of Thiolate Self-Assembled Monolayers on Au, Pt, and Cu; preprint; Chemistry, 2022. https://doi.org/10.26434/chemrxiv-2022-dt7dk. Figure 1

Electrochemical Deposition of N-Heterocyclic Carbene on Steels As a Corrosion Protective Layer

Steels play a vital role in every aspect of day-to-day life. They are major components in construction,transportation, medicine, and etc. Despite their larger usability, steels are suffering from inevitablecorrosion. Hence, finding a sustainable solution for corrosion has been a hot topic in the research field forcenturies. One extensively studied strategy is applying a protective layer on top of metals. The recentfindings show that N-heterocyclic carbene (NHC) has the potency to form a carbon-metal bond throughself-assembling1 or electrochemical deposition2 on different metals such as Au, Pt, Pd, and Ag. Herein, weare trying to coat NHC on mild steel (MS) through electrochemical deposition to act as a corrosionprotective layer. Deposition conditions will be optimized by varying the material concentrations, appliedvoltages, time durations, and substrate roughness. The NHC-coated surface is characterized by X-rayPhotoelectron Spectroscopy (XPS), Matrix-Assisted Laser Desorption/Ionization (MALDI), and AtomicForce Microscope-Infrared Spectroscopy (AFM-IR). In addition, the contact angle measurements can beattributed to visualizing the differences in coatings under different conditions. The corrosion studies areutilized to understand the corrosion-resistant properties of the NHC coating. Moreover, the NHC moietycan be modified to perform better to inhibit corrosion and improve the properties of mild steel on theirapplications. Ultimately, this benchtop approach will be utilized and tested on a large scale and industriallevel.(1) Crudden, C. M.; Horton, J. H.; Ebralidze, I. I.; Zenkina, O. V.; McLean, A. B.; Drevniok, B.; She, Z.; Kraatz,H.-B.; Mosey, N. J.; Seki, T. Ultra stable self-assembled monolayers of N-heterocyclic carbenes on gold. Nat. Chem. 2014, 6 (5), 409-414.(2) Amit, E.; Dery, L.; Dery, S.; Kim, S.; Roy, A.; Hu, Q.; Gutkin, V.; Eisenberg, H.; Stein, T.; Mandler, D.Electrochemical deposition of N-heterocyclic carbene monolayers on metal surfaces. Nat. Commun. 2020, 11 (1), 1-10.

Extreme long-lifetime self-assembled monolayer for air-stable molecular junctions.

The molecular electronic devices based on self-assembled monolayer (SAM) on metal surfaces demonstrate novel electronic functions for device minimization yet are unable to realize in practical applications, due to their instability against oxidation of the sulfur-metal bond. This paper describes an alternative to the thiolate anchoring group to form stable SAMs on gold by selenides anchoring group. Because of the formation of strong selenium-gold bonds, these stable SAMs allow us to incorporate them in molecular tunnel junctions to yield extremely stable junctions for over 200 days. A detailed structural characterization supported by spectroscopy and first-principles modeling shows that the oxidation process is much slower with the selenium-gold bond than the sulfur-gold bond, and the selenium-gold bond is strong enough to avoid bond breaking even when it is eventually oxidized. This proof of concept demonstrates that the extraordinarily stable SAMs derived from selenides are useful for long-lived molecular electronic devices and can possibly become important in many air-stable applications involving SAMs.

Thermal Stability and Orthogonal Functionalization of Organophosphonate Self-Assembled Monolayers as Potential Liners for Cu Interconnect.

In this study, we investigated the thermal stabilities of butylphosphonic acid (BPA) and aminopropyltriethoxysilane (APTES) self-assembled monolayers (SAM) on a Si substrate. The thermal desorption and the thermal cleavage of the BPA and APTES SAM film on the Si substrate were studied by X-ray photoelectron spectroscopy (XPS) upon thermal treatment from 50 to 550 °C. XPS analyses show that the onset of the thermal desorption of the APTES monolayer occurs at 250 °C and the APTES SAM completely decomposed at 400 °C. Conversely, BPA SAM on Si shows that the onset of thermal desorption occurs at 350 °C, and the BPA SAM completely desorbed at approximately 500 °C. Our study revealed that the organophosphonate SAM is a more stable SAM in modifying the dielectric sidewalls of a Cu interconnect when compared to organosilane SAM. To overcome the spontaneous reaction of the organophosphonate film on the metal substrate, a simple orthogonal functionalization method using thiolate SAM as a sacrificial layer was also demonstrated in this study.

N-Heterocyclic carbene-based gold etchants.

N-Heterocyclic carbenes (NHCs) are an emerging alternative to thiols for the formation of stable self-assembled monolayers (SAMs) on gold. We examined several different species that have been used to produce NHC-based monolayers on gold, namely 1,3-diisopropyl-5-nitrobenzimidazolium iodide, 1,3-diisopropyl-5-nitrobenzimidazolium hydrogen carbonate, bis(1,3-diisopropyl-5-nitrobenzimidazolium)gold(I) iodide, and 1,3-diisopropyl-5-nitrobenzimidazole-2-ylidene. Contrary to expectation, solutions containing the first two species in tetrahydrofuran and dichloromethane caused visible loss of gold from thin-film-coated glass slides. The use of toluene solutions of all species resulted in no apparent dissolution of gold. We present scanning electron micrographs and elemental imaging analyses by energy dispersive X-ray spectroscopy to examine the effect of solutions of each species on the gold film. This work highlights the risk of unwanted etching during some routes to NHC-based surface functionalization but also the potential for deliberate etching, with the outcome determined by choice of chemically synthesized organic species and solvent.

A Complementary Experimental and Theoretical Approach for Probing the Surface Functionalization of ZnO with Molecular Catalyst Linkers

AbstractThe application of ZnO materials as solid‐state supports for molecular heterogeneous catalysis is contingent on the functionalization of the ZnO surface with stable self‐assembled monolayers (SAMs) of catalyst linker molecules. Herein, experimental and theoretical methods are used to study SAMs of azide‐terminated molecular catalyst linkers with two different anchor groups (silane and thiol) on poly and monocrystalline (0001, ) ZnO surfaces. Angle‐resolved and temperature‐dependent X‐ray photoelectron spectroscopy (XPS) is used to study SAM binding modes, thermal stabilities, and coverages. The binding strengths and atomistic ordering of the SAMs are determined via atom‐probe tomography (APT). Density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations provide insights on the influence of the ZnO surface polarity on the interaction affinity and conformational behavior of the SAMs. The investigations show that SAMs based on 3‐azidopropyltriethoxysilane possess a higher binding strength and thermal stability than the corresponding thiol. SAM surface coverage is strongly influenced by the surface polarity of ZnO, and the highest coverage is observed on the polycrystalline surface. To demonstrate the applicability of linker‐modified polycrystalline ZnO as a catalyst support, a chiral Rh diene complex is immobilized on the azide‐terminal of the SAM and its coverage is evaluated via XPS.

Effect of alkanethiol chain length on the oxidation resistance of self-assembled monolayer passivated Ge(100) surfaces

• The resistance to oxidation is dependent on the length of thiol chain • Oxidation resistance increases with increasing chain length. • Acceptable oxidation resistance down the C4 chain length. • Van der Waals interactions are significant for SAM stability. The use of self-assembled monolayers (SAMs) of heteroalkanes as passivation layers to protect against oxidation has been studied on a variety of materials. However, typically heteroalkanes with C8 and longer have been used, since the contributing van der Waals between the carbon chains is a significant stabilising force and the longer the chain the greater the force. The industry requirement for passivating semiconducting materials is that they remain oxide free for a queue time of 24 h. The remit of this study is to explore whether passivation using shorter chain alkanethiols, which would be beneficial for reducing carbon, will work to prevent re-oxidation over this time frame. A series of 1-alkanethiols, with chain lengths from C2 to C12 are used to create SAMs on Ge(100) and a study of the re-oxidation of the passivated Ge upon exposure to ambient conditions is undertaken in an effort to determine how chain length effects oxidation resistance of the passivated Ge. X-ray photoelectron spectroscopy is used, complimented by water contact angle measurements to show that the longer thiol molecules outperform their shorter-chain counterparts at inhibiting re-oxidation over 168 h of exposure to ambient. Nonetheless, Ge surfaces passivated by the short-chain thiols, down to C4, still display acceptable resistance to re-oxidation. Finally, a detailed summary of density functional theory simulations whereby the most stable SAM structures and coverages are explored.

Copolymerization of zwitterionic sulfobetaine and hydrophobic acrylamide based antifouling electrochemical biosensors for detection of CA125 in clinical serum samples

It is crucial to construct antifouling electrochemical biosensors with long-term stability and high sensitivity for the detection of biomarkers in complex biological environments to prevent the unspecific adsorption of proteins and other biomolecules. Herein, a glassy carbon electrode electropolymerization with polyaniline (PANI/GCE) was further modified with a copolymerization (PSN) of zwitterionic sulfobetaine methacrylate (SBMA) and hydrophobic N-isopropyl acrylamide (NIPAM) by simple hydrophobic interaction to construct an antifouling biosensor. PSN forms a stable self-assembled monolayer on the PANI/GCE surface and exhibits good antifouling performance in serum due to the advantages of hydrophobic anchoring and the antifouling performance of zwitterionic polymers. Ascribe to the large specific surface area and excellent conductivity, the present biosensor can detect CA125 in undiluted samples in the range of 0.01–1000 U mL−1 with a limit of detection of 2.7 mU mL−1 (3σ/k). Both with good biocompatibility and biological stability, PSN modified sensing surface realizes a long-term antifouling for 15 days in a buffer solution. The present work provides a simple strategy for the direct analysis of CA125 in human serum without a complicated construction of biosensors, indicating the potential application in clinical diagnosis.

Crosslinked organosulfur-based self-assembled monolayers: formation and applications

Self-assembled monolayers (SAMs) have found use in diverse applications that range from corrosion prevention to biosensing. However, for all of these applications, stability remains a key challenge for the utilization of SAMs. Over the last decade, intermolecular crosslinking as a method to enhance the thermal and chemical stability of SAMs has attracted increased attention from scientists and engineers. As such, this review introduces a variety of crosslinked SAMs: (1) aromatic thiol-based SAMs; (2) olefinic- and acetylenic-based alkanethiols; (3) other aliphatic alkanethiols; (4) silane-based alkanethiols; (5) boronic acid-based alkanethiols; and (6) crosslinked SAMs realized by hydrogen bonding. By offering insight into the structure-application relationships of the aforementioned SAMs, this review seeks to inspire researchers toward the development of new classes of SAMs with enhanced stabilities and working lifetimes.

An AIE-Active Ultrathin Polymeric Self-Assembled Monolayer Sensor for Trace Volatile Explosive Detection.

This work has prepared polymeric self-assembled monolayer (SAM) sensors for the detection of trace volatile nitroaromatic compound (NAC) explosives by fluorescence quenching. A typical aggregation-induced emission (AIE) luminogen 1,1,2,2-tetraphenylethene (TPE) polymerizes into PTPE to increase the fluorescence intensity in the SAMs, and the phosphoric acid acts as the anchor group to form stable covalent bonds with the Al2 O3 substrate. This design takes advantage of the high sensitivity and good stability of SAMs, and high fluorescence intensity, and "wire effect" of the conjugated polymers. The polymeric SAM sensors are prepared on the Al2 O3 silicon wafer and testing paper. Both of them show good response speed, reversibility, selectivity, and sensitivity. The detection limits down to 0.07, 0.35, and 4.11ppm for TNT, DNB, and NB, respectively, are achieved on the inorganic testing paper. Furthermore, due to the higher fluorescence intensity by interlacing and overlapping of fibers, the detection of the paper can be distinguished by naked eyes even with a low-power handheld UV lamp, which provides an experimental basis for the development of cheap and easy trace NAC explosive sensors.

An estimation on the mechanical stabilities of SAMs by low energy Ar+ cluster ion collision

The stability of the molecular self-assembled monolayers (SAMs) is of vital importance to the performance of the molecular electronics and their integration to the future electronics devices. Here we study the effect of electron irradiation-induced cross-linking on the stability of self-assembled monolayer of aromatic 5,5′-bis(mercaptomethyl)-2,2′-bipyridine [BPD; HS-CH2-(C5H3N)2-CH2-SH] on Au (111) single crystal surface. As a refence, we also study the properties of SAMs of electron saturated 1-dodecanethiol [C12; CH3-(CH2)11-SH] molecules. The stability of the considered SAMs before and after electron-irradiation is studied using low energy Ar+ cluster depth profiling monitored by recording the X-ray photoelectron spectroscopy (XPS) core level spectra and the UV-photoelectron spectroscopy (UPS) in the valance band range. The results indicate a stronger mechanical stability of BPD SAMs than the C12 SAMs. The stability of BPD SAMs enhances further after electron irradiation due to intermolecular cross-linking, whereas the electron irradiation results in deterioration of C12 molecules due to the saturated nature of the molecules. The depth profiling time of the cross-linked BPD SAM is more than 4 and 8 times longer than the profiling time obtained for pristine and BPD and C12 SAMs, respectively. The UPS results are supported by density functional theory calculations, which show qualitative agreement with the experiment and enable us to interpret the features in the XPS spectra during the etching process for structural characterization. The obtained results offer helpful options to estimate the structural stability of SAMs which is a key factor for the fabrication of molecular devices.

Temperature dependence of resistivity increases induced by thiols adsorption in gold nanosheets

By utilizing stable thiol self-assembled monolayers (SAMs) on gold (Au) nanosheets, we investigated the temperature dependence of the amount of resistivity increases by SAM modification. increased with increasing temperature from 5 to 375 K in all the measured nanosheets for various thicknesses (9–20 nm). We consider from the DFT calculation that the temperature dependence originates from unoccupied states of sulfur atoms above the Fermi level of Au. The available unoccupied states increase at higher temperatures, which enhances scattering rates and thus increases the resistivity. Furthermore, we calculated the temperature dependence of using the Bloch–Grüneisen theory and Fuchs–Sondheimer (FS) model. Based on the results, we conclude that for ultra-thin metal films, the temperature-dependent specularity parameter which represents the fraction of electrons scattered specularly should be introduced into the conventional FS model to predict over a wide temperature range.

Porosity Effects on the Ordering and Stability of Self-Assembled Monolayers on Nanoporous Gold

We investigated the stability, ordering, and composition of mixed self-assembled monolayers (SAMs) on nanoporous gold surfaces formed by dealloying silver–gold leaf. We characterized surface featur...

Preparation of methylacridinium iodides self-assembled monolayers and its anti-corrosion properties for mild steel in seawater: Experimental and computational studies

Two kinds of environmentally stable acridinium-based self-assembled monolayer materials, namely 9,10-dimethylacridinium iodide (DMAI) and 9-phenyl-10-methylacridinium iodide (PMAI), were produced on the surface of mild steel by a dip-coating strategy. The resultant self-assembled monolayers have been characterized by ATR-FTIR, XPS, and contact angle measurements. The anti-corrosion performances of the self-assembled monolayers were evaluated by weight-loss tests and electrochemical techniques. The experimental results indicated that the properties of self-assembled monolayers are mixed-type inhibitor, and the ηmax of PMAI and DMAI reached up to 92.27% and 89.47% respectively at 300 mg/L based on polarization studies. The surface morphology of corroded/inhibited mild steel has been studied by atomic force microscope (AFM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX). Quantum chemical calculations provided insightful quantitative information for revealing the relationship between the molecular structures and inhibition performance. Molecular dynamics (MD) simulations were carried out to explore the configurationally adsorption behavior of DMAI and PMAI on Fe (110) surface.

Optimization of Fc-Thiolate SAMs for Electrochemically Mediated Bacterial Biosensors

Alkanethiol Self-Assembled Monolayers (SAMs), typically deposited on Au substrates, have been commonly utilized to immobilize receptor biomolecules in the construction of a variety of biosensors, such as the detection of glucose oxidase by the immobilization of calixarenes thiols 1. The appeal of using SAMs derives from their ease of fabrication and component flexibility, allowing for more than one alkanethiol to be deposited in a single monolayer. This results in the formation of SAMs in which each thiol component adds a different property or role, thus introducing versatility to the construction and operation of resulting sensors 2.In our recent work, alkanethiol SAMs, composed of three different thiol molecules, were constructed for use as an electrochemical Toll-Like Receptor 4 (TLR4)-based sensor that can detect Gram-negative bacteria. TLRs are a family of immunoproteins in the innate immune system, which are able to detect a broad category of pathogens, rather than specific targets, as is the case with the adaptive system (e.g., antibodies). In our published work 3, SAMs consisted of a carboxylic acid terminated thiol that allowed for the conversion to a nitrilotriacetic (NTA) group. This moiety could then chelate a Ni2+ cation, followed by attachment of the TLR4 immunoprotein via its His-tag. The second thiol in the SAM contained a terminal ferrocene (Fc) group, where the Fc group was bound to the C11 chain via an amide group. Here, the Fc group serves as a mediator for charge transfer between ferrocyanide in solution and the Au substrate, resulting in relatively high current signals, thus making the sensor potentially field deployable. The third thiol in these SAMs was a hydroxy-terminated thiol, added as a diluent between the immobilized TLR4 receptors. SAMs were prepared by co-deposition on sputter Au on glass.Electrochemical testing involved cyclic voltammetry (CV) studies in pH 7 buffer solution in order to determine the amount of Fc-thiol present, as well as CV and electrochemical impedance spectroscopy (EIS) studies in ferrocyanide solutions to examine its redox-mediation response and to obtain a measure of the charge transfer rate. In the presence of lysed bacteria, the immobilized TLR4 dimerizes, which blocks electrochemical mediation between ferrocyanide and the surface bound Fc. This results in an increase in RCT which is our biosensor signal. However, a key problem with these sensors is that their RCT increased with electrochemical testing in the absence of lysed bacteria, producing signal drift. Thus, in the present work, the reasons for sensor signal drift and methods to overcome it were explored.It was found in recent work that the drift of the full TLR4 sensor was correlated with the loss in the Fc redox signal, although it was not clear if underlying SAM was at the root of the problem. If so, then it was necessary to determine if the loss in the Fc redox activity was caused by the physical loss of the Fc thiol from the Au surface or deactivation of the Fc surface complex and/or its electrochemistry. To determine which of these options was responsible for signal drift, the relative % v/v fraction of the three thiols was varied, and in later work, based on chemical considerations, the Fc molecule containing an amide group was replaced with a C11 chain bonded directly to the Fc. It was found that this latter strategy led to a very stable SAM, showing negligible drift in the Fc CV charges and in the RCT values in ferrocyanide solutions, with or without the TLR4 receptor attached on the outer surface of the SAM.This presentation will focus on presenting the electrochemistry of the various binary and ternary thiol SAMs investigated in this work and examining the Fc CV peak charges, potential, and kinetics, as well as the double layer capacitance, to better understand how and why the Fc redox response was stabilized. This is important because this correlates directly with the stability of the TLR4/Gram-negative bacteria sensor, as will be demonstrated for a TLR2 biosensor tested against Gram-positive bacteria cell wall ligands.

Alkynyl Linkers as a Design Tool to Gain Control over the Self-Assembly of Meso-Substituted Porphyrins on HOPG.

Self-assembled monolayers (SAMs) fall generally into two broad categories: those that are covalently bound either to the surface or to each other and those that rely on weaker forces such as hydrogen bonding or van der Waals forces. The engineering of the structure of SAMs formed from weaker forces is an exciting and complex field that often utilizes long alkane substituents bound to core moieties. The core provides the unique optical, electronic, or catalytic property desired, while the interdigitation of the alkane chains provides the means for creating well-regulated patterns of cores on the substrate. This design technique sometimes fails because some of the alkane substituents remain extended into solution rather than become interdigitated on the substrate. One contributor to this is steric hindrance between elements of the core and of the alkane chain. It is shown that the use of an alkyne linker between the core and the alkane chain can, in the case of meso-substituted porphyrins, significantly reduce this steric barrier and allow more stable and predictable surface structures to form. In particular, 5,15-bis(1-octynyl)porphyrin and 5,15-bis(1-tetradecynyl)porphyrin are shown to form significantly more stable SAMs than their alkane-linked counterparts. Scanning tunneling microscopy is used to provide detailed surface structures. Temperature and solution concentration dependence of the surface coverage is also reported. Density functional theory (DFT) is used to determine the energetic effects associated with alkane substitution at both the meso and β positions and the beneficial energetic effects of the alkyne linker.

Vapor-phase deposition-based self-assembled monolayer for an electrochemical sensing platform

Vapor-phase deposition was investigated to prepare electrochemically stable self-assembled monolayers (SAMs) since the promising coating technology can improve the surface functionality for electrochemical biosensors. In our experiments, mercaptopropionic acid (MPA)–Au SAMs in vapor-phase deposition were compared with those in the liquid-phase process, and their electrochemical properties were interrogated by measuring cyclic voltammetry and impedance spectroscopy. As a result, Au–MPA SAMs prepared in a vapor-phase process exhibited much higher electrochemical stability than those in a liquid-phase process. Furthermore, parameters in the vapor–phase deposition process were optimized for reproducible impedance signals, and the as-prepared Au–MPA SAMs under the conditions were adopted for detecting target IgE. It would pave the way for the development of a highly reproducible electrochemical bio-sensing platform.

Self-assembled monolayers from symmetrical di-thiols: Preparation, characterization and application for the assembly of electrochemically active films

1,3-dimercaptopropan-2-ol, a symmetrical di-thiol, has been synthesized and applied as a new type of anchor molecule to prepare a self-assembled monolayer (SAM) on the gold surface. The formed monolayers were studied by cyclic voltammetry, impedance spectroscopy, X-ray photoelectron spectroscopy, kinetic capacitance, and contact angle measurements. The SAM structure depends on the adsorption conditions. A short incubation time of the electrode at high concentration of this di-thiol leads to the predominating binding through one thiol group of the adsorbate to the gold surface, while a long incubation at low concentration leads to the predominating binding by both thiol groups. A comparative study of the desorption and replacement of SAMs indicates a strong stability increase when the SAM molecules bond gold surface by two bonds mainly. This monolayer was used to immobilize electrochemically active p-benzoquinone moiety. The surface concentration of p-benzoquinone obtained from cyclic voltammetry is 2.5 ± 0.2 × 10−10 mol·cm−2 which corresponds to the functionalization of 65 ± 5% of SAM molecules. The obtained highly stable SAM with redox-active terminal group can be applied for different tasks of chemical sensing and biosensing. As an example, an application of this system for electrocatalytical oxidation of dihydronicotinamide adenosine dinucleotide (NADH) was tested.