Binary Self-assembled Monolayers Research Articles

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.

Molecular-scale topographic design of multicomponent self-assembled monolayers for environmentally adaptable, lyophobic coating

HypothesisThe water wettability of a hydrophilic surface is dictated by the hydration of the region less than 1 nm below the surface plane, yet it hitherto remains a topic rarely touched upon how a subtle change in surface structural features at molecular level affects the surface wettability and its response to the environmental nature. ExperimentsBinary self-assembled monolayers (SAMs) consisting of plain and functional thiols were constructed, where the surface fraction of the functional thiols was varied from 0.1 to 0.3, 0.6, 0.8 and 1.0 and the difference in CH2 unit number between two types of thiols – defined as height difference between surface polar (OH, NH2, COOH, or H2PO3) and non-polar (CH3) groups – from 0 to 1, 3, and 5. The surface wettability of as-prepared binary SAMs and their surface energy were assessed in both air/water/solid and oil/water/solid triphasic systems. FindingsWith the relative height of surface H2PO3 groups being deliberately set as 3 in particular, as-prepared binary SAMs gain unique environmentally adaptable surface lyophobicity, namely, they can be both oleophobic in water and hydrophobic in oil. Thanks to this new environmentally adaptable surface lyophobicity, such binary SAM coating enables copper meshes to realize selective and efficient oil/water separation without need of pre-wetting. Both the polar and dispersion interaction components of surface energy of the binary SAM coatings are found crucial to effective oil/water separation of the coated copper meshes; the former defines critical intrusion pressure and the latter must be larger than 22 nN/m.

Gold-Supported Lipid Membranes Formed by Redox-Triggered Vesicle Fusion on Binary Self-Assembled Monolayers: Ion-Pairing Association and Surface Hydrophilicity.

The assembly of biomimetic, planar supported lipid bilayers (SLBs) by the popular vesicle fusion method, which relies on the spontaneous adsorption and rupture of small unilamellar vesicles from aqueous solution on a solid surface, typically works with a limited range of support materials and lipid systems. We previously reported a conceptual advance in the formation of SLBs from vesicles in the gel or fluid phase using the interfacial ion-pairing association of charged phospholipid headgroups with electrochemically generated cationic ferroceniums bound to a self-assembled monolayer (SAM) chemisorbed to gold. This redox-driven approach lays down a single bilayer membrane on the SAM-modified gold surface at room temperature within minutes and is compatible with both anionic and zwitterionic phospholipids. The present work explores the effects of the surface ferrocene concentration and hydrophobicity/hydrophilicity on the formation of continuous SLBs of dialkyl phosphatidylserine, dialkyl phosphatidylglycerol, and dialkyl phosphatidylcholine using binary SAMs of ferrocenylundecanethiolate (FcC11S) and dodecanethiolate (CH3C11S) or hydroxylundecanethiolate (HOC11S) comprising different surface mole fractions of ferrocene (χFcsurf). An increase in the surface hydrophilicity and surface free energy of the FcC11S/HOC11S SAM mitigates the decrease in the attractive ion-pairing interactions resulting from a reduced χFcsurf. SLBs of ≳80% area coverage form on the FcC11S/HOC11S SAM for all the phospholipid types down to χFcsurf of at least 0.2, composition yielding a water contact angle (θW) of 44 ± 4°. By contrast, a greater number of ion-pairing interactions is required on the hydrophobic FcC11S/CH3C11S surface to drive the vesicle fusion process; bilayers or bilayer patches form at χFcsurf ≳ 0.6 (θW = 97 ± 3°). These findings will aid in tailoring the surface chemistry of redox-active modified surfaces to widen the conditions that yield supported lipid membranes.

Supramolecular Cucurbit[7]uril@Ferrocene Complexation on Surface: From Nanostructure Differentiation to Guest Quantitation

ConspectusCucurbit[7]uril (CB[7]), an important member of the macrocyclic cucurbit[n]uril host family, has attracted much attention due to its ability to form ultrastable inclusion complexes with aromatic or “ring”-structured compounds. In particular, for the CB[7]@ferrocene (Fc) host–guest complex, its exceptionally high binding affinity, ideal redox activity, and extreme stability against biological media have promoted its potential to substitute traditional natural binding pairs (antigen/antibody and biotin/avidin complexes as examples) in many biochemical applications, such as purification, labeling, and biomolecular assembly. In recent years, the immobilization of CB[7]@Fc host–guest complex on electrode surface via various strategies has expanded its use for fabricating electroactive biofunctional devices, such as electrochemical biosensors and switches, where the redox response of Fc/Fc+ can be used as both characterization and sensing signal. These applications require in-depth understanding of the interfacial CB[7]@Fc host–guest binding behavior, which is different from that in a homogeneous solution phase; such studies, in turn, will facilitate the design and development of more efficient interfacial host–guest binding systems.With the advantages of easy preparation, high stability, and reversible redox response, ferrocenyl alkanethiolate self-assembled monolayers (SAMs) on gold have been widely employed as an electroanalytical platform for studying electron transfer behavior and molecular interactions (i.e., ion pairing) and probing microenvironmental changes. In order to achieve an “ideal” redox response for these applications (e.g., a single pair of redox peaks), mixed Fc-alkanethiolate SAMs with low Fc coverage are usually prepared, where the influences from different structural domains and nonuniform distribution of Fc terminal groups in the SAM are minimized.In the past decade, our team has been exploring the renewed applications of Fc-alkanethiolate SAMs, while performing characterizations of their structural properties by electrochemistry and other surface techniques. In particular, we employed redox-active Fc-alkanethiolate SAMs to quantitatively study the interfacial CB[7]@Fc host–guest binding and explored a range of applications for nanostructure differentiation, long-range electron transfer regulation, and quantitation of non-redox-active guests. Besides, we have extended our electrochemical study of interfacial CB[7]@Fc host–guest binding with CB[7]-terminated alkanethiolate SAMs, where the electrochemical quantitation of the CB[7]@Fc host–guest complex formed on an electrode surface becomes straightforward.In this Account, we first provide a brief literature review of the CB[7]@Fc host–guest chemistry, including fundamental studies in solution as well as the challenges for its application on surfaces. Subsequently, we describe our studies of interfacial CB[7]@Fc host–guest complexation on binary SAMs on gold, including fundamental voltammetric evaluation of binding thermodynamics and kinetics, as well as its applications. Lastly, we introduce our recent electrochemical study on interfacial CB[7]@Fc complexation by tethering CB[7] on alkanethiolate SAMs (via click chemistry). These research accomplishments shed light on the profound ability of forming interfacial CB[7]@Fc host–guest binding systems for both fundamental studies and practical applications; the knowledge obtained herein paves the way to further understanding of surface-immobilized supramolecular host–guest systems.

Reversing Molecular Dilution Effect by Varying the Bottom Electrode

AbstractThis paper describes charge tunneling across mixed self‐assembled monolayers (SAMs) and investigates the effect of bottom electrode on molecular dilution and tunneling function in large‐area molecular junctions. Binary mixed SAMs are formed with organic molecular diode (2,2′‐bipyridyl‐terminated n‐undecanethiol (SC11BIPY)) and nonrectifying n‐alkanethiolate diluent (SCn; n = 8, 10, 16, 18) via coadsorption on different electrodes, namely Au and Ag. By taking advantage of self‐referencing feature of rectification, the tunneling–molecular dilution relationship is reliably examined. Notably, rectification ratio of mixed SAMs on the Ag electrode decreases with increasing the diluent's length, whereas that of analogous SAMs on the Au electrode oppositely increases. Simulation of rectification using a parallel circuit model and surface analysis with contact angle goniometry evidence that such an opposite trend is dominated by the supramolecular structural order, which arises from the difference in interthiolate distance between Au and Ag. This work demonstrates that the molecular dilution effect on tunneling can be reversed by varying the bottom electrode.

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.

Large cooperative effects in tunneling rates across van der Waals coupled binary self-assembled monolayers

This paper describes large positive cooperative effects of two orders of magnitude in the tunneling rates across molecular junctions of mixed self-assembled monolayers (SAMs) of rectifying (ferrocenyl undecanethiol HS(CH2)11Fc) and non-rectifying molecules with different terminal groups (11-undecanethiol and its derivatives, denoted as HS(CH2)11X, where X = -H, -NH2 or -NO2). By gradually diluting the surface fraction of HS(CH2)11Fc in the mixed SAM, it is found that the large positive cooperative effect is only important in the coherent tunneling regime but not in the incoherent tunneling regime. Density functional theory (DFT) shows that the measured cooperative effects in the tunneling rates in these binary systems are caused by Fc---X van der Waals interactions which increase in the order of -H< -NH2< -NO2. These strong cooperative effects dramatically alter the operation of a molecular diode, further highlighting the importance of taking cooperative effects into account, in this case driven by van der Waals interactions, in the rational design of electronic devices working at tunneling regime.

Nanoscale Chemical Imaging of Coadsorbed Thiolate Self-Assembled Monolayers on Au(111) by Tip-Enhanced Raman Spectroscopy.

Self-assembled monolayers (SAMs) of thiolates on metal surfaces are of key importance for engineering surfaces with tunable properties. However, it remains challenging to understand binary thiolate SAMs on metals at the nanoscale under ambient conditions. Here, we employ tip-enhanced Raman spectroscopy (TERS) and density functional theory (DFT) calculations to investigate the local information of binary SAMs on Au(111) coadsorbed from an equimolar mixture of p-cyanobenzenethiol (pCTP) and p-aminothiophenol (pATP), including chemical composition, coadsorption behavior, phase segregation, plasmon-induced photocatalysis, and solvation effects. We found that upon competitive adsorption of pCTP and pATP on Au(111) from a methanolic solution, the coadsorption initially occurs randomly and homogeneously; eventually, pATP is replaced by pCTP through the gradual growth of pCTP nanodomains. TERS imaging also allows for visualization of the plasmon-induced coupling of pATP to p,p'-dimercaptoazobenzene (DMAB) and the solvation-induced phase segregation of the binary SAMs into nanodomains, with a spatial resolution of ∼9 nm under ambient conditions. According to DFT calculations, these aromatic thiolates differing only in their functional groups, -CN versus -NH2, show different adsorption energies on Au(111) in vacuum and methanol, and thus, the solvation effect on the adsorption energy of these thiolates in methanol can determine the dispersion state and replacement order of the binary thiolates on Au(111).

Visualization of Thermal Transport Properties of Self-Assembled Monolayers on Au(111) by Contact and Noncontact Scanning Thermal Microscopy.

Thermal transport properties of patterned binary self-assembled monolayers (SAMs) on Au(111) were examined using scanning thermal microscopy (SThM) with both contact and noncontact methods. We fabricated two-dimensional (2D) patterns with two separate domains of n-hexadecanethiol/benzenethiol, benzenethiol/n-butanethiol, or n-hexadecanethiol/n-butanethiol. In the experimental setup, the efficiency of thermal transport from a SThM tip to the SAM surface can be evaluated in terms of the temperature change at the SThM tip. In the contact regime, where a SThM tip physically contacts the SAM surface, direct thermal transport through the SAM and radiation-based thermal transport through the space where SAMs exist may contribute to a drop in temperature at the tip. In the noncontact regime, thermal transport relies on radiation-based heat dissipation from the heated tip to the SAMs. 2D mapping of the spatial temperature distribution on SAMs reflects the difference in thermal transport properties of the two SAM domains. We found that the contact method is effective for visualizing the temperature contrast, which reflects the thermal transport properties of the constituent molecules when the domains of the SAMs have a similar height, while the noncontact method allows visualization of the temperature distribution, which is related to the height of each domain of the SAMs, rather than the chemical structures of the constituent molecules. Combination of contact and noncontact SThM enables 2D imaging of thermal transport properties and topographic imaging simultaneously and represents a new technique for investigating the thermal properties of materials surfaces, which is essential for nanoscale thermal management.

Adsorption Kinetics of Glycated Hemoglobin on Aptamer Microarrays with Antifouling Surface Modification.

Aptamers are oligonucleotides that bind with high affinity to target molecules of interest. One such target is glycated hemoglobin (gHb), a biomarker for assessing glycemic control and diabetes diagnosis. By the coupling of aptamers with surface plasmon resonance (SPR) sensing surfaces, a fast, reliable and inexpensive assay for gHb can be developed. In this study, we tested the affinity of SPR-sensing surfaces, composed of aptamers and antifouling self-assembled monolayers (SAMs), to hemoglobin (Hb) and gHb. First, we developed a gHb-targeted aptamer (GHA) through a modified Systematic Evolution of Ligands by EXponential (SELEX) enrichment process and tested its affinity to gHb using the Nano-Affi protocol. GHA was used to produce three distinct SAM-SPR-sensing surfaces: (Type-1) a SAM of GHA directly attached to a sensor surface; (Type-2) GHA attached to a SAM of 11-mercaptoundecanoic acid (11MUA) on a sensor surface; (Type-3) GHA attached to a binary SAM of 11MUA and 3,6-dioxa-8-mercaptooctan-1-ol (DMOL) on a sensor surface. Type-2 and Type-3 surfaces were characterized by cyclic voltammetry and electrochemical impedance spectroscopy to confirm that GHA bound to the underlying SAMs. The adsorption kinetics for Hb and gHb interacting with each SPR sensing surface were used to quantify their respective affinities. The Type-1 surface without antifouling modification had a dissociation constant ratio (KD,Hb/KD,gHb) of 9.7, as compared to 809.3 for the Type-3 surface, demonstrating a higher association of GHA to gHb for sensor surfaces with antifouling modifications than those without. The enhanced selectivity of GHA to gHb can likely be attributed to the inclusion of DMOL in the SAM-modified surface, which reduced interference from nonspecific adsorption of proteins. Results suggest that pairing aptamers with antifouling SAMs can significantly improve their target affinity, potentially allowing for the development of novel, low cost, and fast assays.

Pronounced Solvent Effect on the Composition of Binary Self-Assembled Monolayers with Embedded Dipole Moments

The formation and properties of binary thiolate self-assembled monolayers (SAMs), comprised of precursors featuring a short heteroaromatic backbone, consisting of a nonpolar phenyl ring and a polar...

Homogeneous Dispersion of Aromatic Thiolates in the Binary Self-Assembled Monolayer on Au(111) via Displacement Revealed by Tip-Enhanced Raman Spectroscopy

Self-assembled monolayers (SAMs) of aromatic thiols on metal surfaces are of interest as covering functional layers. Unlike alkanethiols, multicomponent SAMs of aromatic thiols have not been compre...

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.

Enhanced efficiency and stability of organic light-emitting diodes via binary self-assembled monolayers of aromatic and aliphatic compounds on indium tin oxide

Abstract Regulating hole injection via self-assembled monolayers (SAMs) modification on indium tin oxide (ITO) is significantly favourable for achieving high efficiency and stability of organic light-emitting diodes (OLEDs). In this work, ITO was modified by pentafluorobenzylphosphonic acid (F5BnPA) and heneicosafluorododecylphosphonic acid (HF21DPA) in a binary SAMs way. By altering mole ratios of these two materials, ITO work function can be achieved from 5.20 to 5.82 eV and the roles of F5BnPA and HF21DPA in affecting hole injection and device performances were also explored. The results demonstrate a synergistic effect that HF21DPA improves ITO work function efficiently for its fluorinated alkyl chain and F5BnPA enhances the charge transfer directly due to its aromatic group gathering HOMO level. When α-naphthylphenylbiphenyldiamine (NPB) and 4,4′,4″-tris(carbazol-9-yl) triphenylamine (TCTA) were used as hole transport layers (HTLs) in OLEDs with a structure of ITO/SAM/HTL/tris(8-hydroxyquinolino) aluminum (III) (Alq3)/lithium fluoride (LiF)/Al, brightness up to 36839 and 58189 cd/m2 and current efficiency up to 5.75 and 7.35 cd/A were obtained, respectively. Furthermore, the influence of binary SAMs on the stability of NPB-based OLEDs was studied. The maximum half-lifetime at the initial brightness of 2200 cd/m2 reaches 368.0 h, which is 75.1 times that of the poly(3, 4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-containing device. The high durability is attributed to higher hydrophobicity and lower hole injection barrier caused by SAMs. This is the first report on the synergistic effect of aromatic and aliphatic compounds in binary SAMs on regulating hole injection, showing a new approach of obtaining high efficiency and stability of OLEDs.

Biomolecular charges influence the response of surface plasmon resonance biosensors through electronic and ionic mechanisms

Surface plasmon resonance (SPR) biosensors have become an important label-free optical biomolecular sensing technology and a “gold standard” for retrieving information on the kinetics of biomolecular interactions. Even though biomolecules typically contain an abundance of easily ionizable chemical groups, there is a gap in understanding of whether (and how) the electrostatic charge of a biomolecular system influences the SPR biosensor response. In this work we show that negative static charge present in a biomolecular layer on the surface of an SPR sensor results in significant SPR spectral shifts, and we identify two major mechanisms responsible for such shifts: 1) the formation of an electrical double layer (ionic mechanism), and 2) changes in the electron density at the surface of a metal (electronic mechanism). We show that under low ionic strength conditions, the electronic mechanism is dominant and the SPR wavelength shift is linearly proportional to the surface concentration of biomolecular charges. At high ionic strength conditions, both electric and ionic mechanisms contribute to the SPR wavelength shift. Using the electronic mechanism, we estimated the pKa of surface-bound carboxylic groups and the relative concentration of the carboxyl-terminated alkanethiols in a binary self-assembled monolayer of alkanethiols. The reported sensitivity of SPR to surface charge is especially important in the context of biomolecular sensing. Moreover, it provides an avenue for the application of SPR sensors for fast, label-free determination of the net charge of a biomolecular coating, which is of interest in material science, surface chemistry, electrochemistry, and other fields.

Characterization of binary self-assembled monolayers formed from the sequential deposition of 2-naphthalenethiol and octanethiol

The binary monolayer system of 2-naphthalenethiol (2NT) and octanethiol (OT) was prepared on the surface of Au(111) and structurally characterized by scanning tunneling microscopy (STM) in ambient conditions. X-ray photoelectron spectroscopy (XPS) was conducted for elemental analysis of the adsorbates present in each monolayer. Both the sequence of deposition and the concentration of the 2NT solution used were varied. The mixed monolayer systems formed various surface structures, including the formation of novel packing of 2NT which are observed when the lower concentration of 2NT is adsorbed onto a pre-existing OT monolayer.

Study of binary self-assembled monolayers of a novel anchoring thiol (11-mercaptoundecyl-N′,N″,N‴-trimethylammonium) in the electron transfer with glucose oxidase enzyme

In this study, we explore the fabrication and properties of a sensing electrochemical platform that is based on a novel 11-mercaptoundecyl-N′,N″,N‴-trimethylammonium hexacyanoferrate (MAH) and 6-(ferrocenyl)hexanethiol (FT)/glucose oxidase (GOx) on gold substrate. The hexacyanoferrate and ferrocene moieties that were immobilized on the gold surface were examined as electron transfer mediators with the oxidase glucose. The sensing platform, based on 11-mercaptoundecyl-N′,N″,N‴-trimethylammonium hexacyanoferrate (MAH) and 6-(ferrocenyl)hexanethiol (FT)/GOx modified gold electrode, shows the potential to work with two distinct mechanisms for electrochemical response. The hexacyanoferrate anion that was anchored at positive edge charge of the 11-mercaptoundecyl-N′,N″,N‴-trimethylammonium on the gold surface provided an efficient electron transfer. It was demonstrated that in the design of the GOx/MAH, the FT/Au electrode is facilitating the electron transfer from GOx toward the gold substrate. We believe that this study adds a new facet to nanobioelectronics with applications for biofuels and sensorial platforms.

Kinetic Implications of the Presence of Intermolecular Interactions in the Response of Binary Self-Assembled Electroactive Monolayers.

Quantitative analysis of the influence of intermolecular interactions on the response of electroactive binary monolayers, incorporating the presence of electroinactive coadsorbed species and Marcus–Hush kinetic formalism, has been deduced as an extension of previous models. An expression for the current–potential relationship in terms of two global phenomenological interaction parameters and the apparent charge-transfer rate constant and formal potential has been obtained and applied to multipotential chronoamperometry for the analysis of intermolecular interaction effects on the kinetics of the charge transfer. Experimental verification of the theoretical framework has been performed with binary ferrocenylundecanethiol/decanethiol monolayers on gold and platinum substrates for different coverage degrees of the redox probe. The increase of the ferrocene coverage gives rise to a slowing down of the charge-transfer process through a decrease of the apparent rate constant above a 90% for gold and platinum electrodes when the surface coverage increases from 1–2 to 15–30%. Significant differences in the magnitude of intermolecular interactions and the symmetry degree of the charge-transfer process are observed between gold and platinum electrodes. Moreover, for high surface coverages of ferrocene, two different domains appear, from which it is possible to carry out a kinetic discrimination of the two responses. Values of the interaction and kinetic parameters have been obtained for the different monolayers under study, and the limit of applicability of this treatment is discussed.

A review of molecular phase separation in binary self-assembled monolayers of thiols on gold surfaces(a)(a)Contribution to the Focus Issue Self-assemblies of Inorganic and Organic Nanomaterials edited by Marie-Pule Pileni.

Binary self-assembled monolayers (SAMs) on gold surfaces have been known to undergo molecular phase separation to various degrees and have been subject to both experimental and theoretical studies. On gold nanoparticles in particular, binary SAMs ligand shells display intriguing morphologies. Consequently, unexpected behaviors of the nanoparticles with respect to their biological, chemical, and interfacial properties have been observed. It is critical that the phase separation of binary SAMs be understood at both molecular and macroscopic level to create, and then manipulate, the useful properties of the functionalized surfaces. We look into the current understanding of molecular phase separation of binary SAMs on gold surfaces, represented by Au(111) flat surfaces and Au nanoparticles, from both theoretical and experimental aspects. We point out shortcomings and describe several research strategies that will address them in the future.