Alkanethiol Self-assembled Monolayers Research Articles

Effect of Solution Composition on the Desorption Behavior of Self-Assembled Monolayers of Alkanethiols with Different Terminal Groups

Effect of Solution Composition on the Desorption Behavior of Self-Assembled Monolayers of Alkanethiols with Different Terminal Groups

Investigating the Blocking Effect of Alkanethiol Self-assembled Monolayer on Electrochemical Response of the Split Gq-based DNA-NTs-based Biosensor

Investigating the Blocking Effect of Alkanethiol Self-assembled Monolayer on Electrochemical Response of the Split Gq-based DNA-NTs-based Biosensor

Vibrational spectrum perturbations of alkanethiol self-assembled monolayers with noble gases and chlorinated species

This study aims to improve our understanding of some differences in odd and even chain-length alkanethiol self-assembled monolayers (SAMs) using infrared reflection absorption spectroscopy (IRRAS) and density functional theory (DFT). Xe, Kr, CH2Cl2, CD2Cl2, and CDCl3 were used experimentally to perturb the CH2 and CH3 stretches arising from the tail region of the SAM films and changes in position and intensity were monitored in the infrared. Using DFT methods, noble gases and CH2Cl2 were added to models of the SAM monolayers, and energies and vibrational spectra were calculated. It was observed that perturbing species affected the CH3 symmetric and asymmetric stretches on hexadecanethiol SAMs (an “even” SAM with a 16-carbon backbone) while on pentadecanethiol SAMs (an “odd” SAM with a 15-carbon backbone) both CH2 and CH3 symmetric and asymmetric stretches were affected. Films formed with shorter chain-length species, hexanethiol (even) and pentanethiol (odd), had less consistent results, likely due to more disorder in the alkanethiol chains from weaker van der Waals interactions. Adsorption energies for different perturbing species on the monolayers were higher for the hexanethiol than the pentanethiol. The vibrational spectrum of pentanethiol monolayers with adsorbed species mainly showed shifts to higher frequencies for CH2 and CH3 stretches; for hexanethiol, shifts to lower frequencies for CH2 stretches and higher frequencies for CH3 stretches were observed. Thus, the odd and even alkanethiol SAMs were affected differently by the perturbing species as observed both experimentally and computationally.

Odd-even effects in aryl-substituted alkanethiolate SAMs: nonsymmetrical attachment of aryl unit and its impact on the SAM structure.

Aryl-substituted alkanethiolate (AT) self-assembled monolayers (SAMs) exhibit typically so-called odd-even effects, viz. systematic variations in the film structure, packing density, and molecular inclination depending on the parity of the number of the methylene units in the alkyl linker, n. As an exception to this rule, ATs carrying an anthracen-2-yl group (Ant-n) as tail group were reported to have different behavior due the non-symmetric attachment of the anthracene unit to the AT linker, providing additional degree of freedom for the molecular organization and allowing for partial compensation of the odd-even effects. In this context, the structure of SAMs formed by adsorption of anthracene-substituted ATs (Ant-n; n = 1-6) at room temperature on Au(111) substrate was investigated by high-resolution scanning tunnelling microscopy (STM). Most of these SAMs exhibit a coexistence of two different ordered phases, some of which are common for either n = odd or n = even while other vary over the series, showing a broad variety of different structures. The average packing density of the Ant-n SAMs, derived from the analysis of the STM data, varies by 7.5-10% depending on the parity of n, being, as expected, higher for n = odd. The respective extent of the odd-even effects is noticeably lower than that usually observed for other aryl-substituted monolayers (∼25%), which goes in line with the previous findings and emphasizes the impact of the non-symmetric attachment of the aromatic unit.

Characterization and application of alkanethiolate self-assembled monolayers on Au-coated chips for Ir(IV) and Rh(III) sorption

Au-coated Si alpha-particle detectors have been widely used during the chemical characterization of superheavy elements that have been an area of great interest to scientists for decades. Further experiments are aimed at the functionalization of this type of detector to study the chemical properties of meitnerium. It is expected to be a member of the group 9 elements of the periodic table, and its chemical properties have not been studied before. In this study, self-assembled monolayers (SAMs) of 1-(11-mercaptoundecyl)imidazole (Im-C11-SH) and 12-mercaptododecanoic acid (MDDA) on 1 × 1 cm2 Au-coated Si and glass chips were prepared and characterized using several analytical and optical techniques: atomic force microscopy (AFM), nanoprojectile secondary ion mass spectrometry (NP-SIMS), X-ray photoelectron spectroscopy (XPS), ellipsometry, and instrumental neutron activation analysis (INAA). The coverage of Au-coated Si chips with Im-C11-SH SAMs was (99 ± 6)%. NP-SIMS showed quantitative sorption of Rh(III) from an HCl medium, the coverage of Im-C11-SH SAMs with Rh(III) was (81.3 ± 3.8)%. INAA showed quantitative sorption of Ir(IV) and illustrated that it can be adsorbed from a 5.4 μM initial Ir(IV) solution in 0.55 M HCl on the Im-C11-SH and MDDA SAMs with a surface saturation of (77 ± 12)% and (84 ± 16)%, respectively.

Impact of Surface Functionalization and Deposition Method on Cu-BDC surMOF Formation, Morphology, Crystallinity, and Stability.

For direct integration into device architectures, surface-anchored metal-organic framework (surMOF) thin films are attractive systems for a wide variety of electronic, photonic, sensing, and gas storage applications. This research systematically investigates the effect of deposition method and surface functionalization on the film formation of a copper paddle-wheel-based surMOF. Solution-phase layer-by-layer (LBL) immersion and LBL spray deposition methods are employed to deposit copper benzene-1,4-dicarboxylate (Cu-BDC) on gold substrates functionalized with carboxyl- and hydroxyl-terminated alkanethiol self-assembled monolayers (SAMs). A difference in crystal orientation is observed by atomic force microscopy and X-ray diffractometry based on surface functionalization for films deposited by the LBL immersion method but not for spray-deposited films. Cu-BDC crystallites with a strong preferred orientation perpendicular to the substrate were observed for the films deposited by the LBL immersion method on carboxyl-terminated SAMs. These crystals could be removed upon testing adhesive properties, whereas all other Cu-BDC surMOF film structures demonstrated excellent adhesive properties. Additionally, film stability upon exposure to water or heat was investigated. Ellipsometric data provide insight into film formation elucidating 7 and 14 Å average thicknesses per deposition cycle for films deposited by the immersion method on 11-mercapto-1-undecanol (MUD) and 16-mercaptohexadecanoic acid (MHDA), respectively. In contrast, the films deposited by the spray method are thicker with the same average thickness per deposition cycle (21 Å) for both SAMs. While the spray method takes less time to grow thicker films, it produces similar crystallite structures, regardless of the surface functionalization. This research is fundamental to understanding the impact of deposition method and surface functionalization on surMOF film growth and to provide strategies for the preparation of high-quality surMOFs.

Molecular Mechanism of Selective Al2O3 Atomic Layer Deposition on Self-Assembled Monolayers.

Area-selective atomic layer deposition (AS-ALD) of insulating metallic oxide layers could be a useful nanopatterning technique for making increasingly complex semiconductor circuits. Although the alkanethiol self-assembled monolayer (SAM) has been considered promising as an ALD inhibitor, the low inhibition efficiency of the SAM during ALD processes makes its wide application difficult. We investigated the deposition mechanism of Al2O3 on alkanethiol-SAMs using temperature-dependent vibrational sum-frequency-generation spectroscopy. We found that the thermally induced formation of gauche defects in the SAMs is the main causative factor deteriorating the inhibition efficiency. Here, we demonstrate that a discontinuously temperature-controlled ALD technique involving self-healing and dissipation of thermally induced stress on the structure of SAM substantially enhances the SAM's inhibition efficiency and enables us to achieve 60 ALD cycles (6.6 nm). We anticipate that the present experimental results on the ALD mechanism on the SAM surface and the proposed ALD method will provide clues to improve the efficiency of AS-ALD, a promising nanoscale patterning and manufacturing technique.

Probing charge transfer through antifouling polymer brushes by electrochemical methods: The impact of supporting self-assembled monolayer chain length

Ultrathin surface-tethered polymer brushes represent attractive platforms for a wide range of sensing applications in strategically vital areas such as medicine, forensics, or security. The recent trends in such developments towards “real world conditions” highlighted the role of zwitterionic poly(carboxybetaine) (pCB) brushes which provide excellent antifouling properties combined with bio-functionalization capacity. Highly dense pCB brushes are usually prepared by the “grafting from” polymerization triggered by initiators on self-assembled monolayers (SAMs). Here, multi-methodological experimental studies are pursued to elucidate the impact of the alkanethiolate SAM chain length (C6, C8 and C11) on structural and functional properties of antifouling poly(carboxybetaine methacrylamide) (pCBMAA) brush. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a custom-made 3D printed cell employing [Ru(NH3)6]3+/2+ redox probe were used to investigate penetrability of SAM/pCBMAA bilayers for small molecules and interfacial charge transfer characteristics. The biofouling resistance of pCBMAA brushes was characterized by surface plasmon resonance; ellipsometry and FT-IRRAS spectroscopy were used to determine swelling and relative density of the brushes synthesized from initiator-bearing SAMs with varied carbon chain length. The SAM length was found to have a substantial impact on all studied characteristics; the highest value of charge transfer resistance (Rct) was observed for denser pCBMAA on longer-chain (C11) SAM when compared to shorter (C8/C6) SAMs. The observed high value of Rct for C11 implies a limitation for the analytical performance of electrochemical sensing methods. At the same time, the pCBMAA brushes on C11 SAM exhibited the best bio-fouling resistance among inspected systems. This demonstrates that proper selection of supporting structures for brushes is critical in the design of these assemblies for biosensing applications.

Thermodynamic expression of surface reconstruction impact on surface wettability

In this work we demonstrated noticeable change in the surface wettability in response to environmental changes for self-assembled monolayers (SAM) of functional alkanethiols as a result of the orientation of surface polar groups as well as the permeation of the environmental phase into the SAM surface region with the aid of sum frequency generation spectroscopy and quartz microcrystal balance. Yet the revealed surface reconstruction caused unexpectedly subtle change in SAM surface energy (within ± 8 %) due to checks and balances of the dispersive and polar components of surface energy. By introducing the work of wetting as the difference between the work of adhesion of a fluid on a solid and its surface tension, here we managed to consistently express surface reconstruction impact in SAM surface wettability as an exponential function of the relative change in work of wetting for the wetting fluid. Furthermore, we also succeeded in harnessing the relative change ratio of the work of wetting of water to that of oil to demarcate surface polar groups, which are > 1 for weakly polar groups (e.g., Br and pyrrole), <1 for small, highly polar groups (e.g., OH, NH2 and COOH), and environmentally sensitive for large, highly polar groups (e.g., H2PO3).

Permeability of Alkanethiols Self-Assembled Monolayers on Gold Prepared by Neat Thiols and Ethanolic Thiols

Permeability of Alkanethiols Self-Assembled Monolayers on Gold Prepared by Neat Thiols and Ethanolic Thiols

Lateral dipole moments induced by all-cis-pentafluorocyclohexyl groups cause unanticipated effects in self-assembled monolayers

All-cis-hexafluoro- and all-cis-pentafluoro-cyclohexane (PFCH) derivatives are new kinds of materials, the structures and properties of which are dominated by the highly dipolar Janus-face motif. Here, we report on the effects of integrating the PFCH groups into self-assembled monolayers (SAMs) of alkanethiolates on Au(111). Monolayers with an odd (eleven) and even (twelve) number of methylene groups were characterized in detail by several complementary experimental tools, supported by theoretical calculations. Surprisingly, all the data show a high similarity of both kinds of monolayers, nearly lacking the typically observed odd-even effects. These new monolayers have a packing density about 1/3 lower than that of non-substituted alkanethiolate monolayers, caused by the bulkiness of the PFCH moieties. The orientations of the PFCH groups and the alkyl chains could be determined independently, suggesting a conformation similar to the one found in the solid state structure of an analogous compound. Although in the SAMs the PFCH groups are slightly tilted away from the surface normal with the axial fluorine atoms pointing downwards, most of the dipole moments of the group remain oriented parallel to the surface, which is a unique feature for a SAM system. The consequences are much lower water contact angles compared to other partly fluorinated SAMs as well as rather moderate work function values. The interaction between the terminal PFCH moieties results in an enhanced stability of the PFCH-decorated SAMs toward exchange reaction with potential molecular substituents in spite of the lower packing density of these films.

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.

Guiding Metal Organic Framework Morphology via Monolayer Artificial Defect-Induced Preferential Facet Selection.

Guiding metal organic framework (MOF) morphology, especially without the need for chemical additives, still remains a challenge. For the first time, we report a unique surface guiding approach in controlling the crystal morphology formation of zeolitic imidazole framework-8 (ZIF-8) and HKUST-1 MOFs on disrupted alkanethiol self-assembled monolayer (SAM)-covered Au substrates. Selective molecule removal is applied to generate diverse SAM matrices rich in artificial molecular defects in a monolayer to direct the dynamic crystal growth process. When a 11-mercaptoundecanol alkanethiol monolayer is ruptured, the hydroxyl tail groups of surface residue molecules act as nucleating sites by coordination with precursor metal ions. Meanwhile, the exposed alkane chain backbones stabilize a particular facet of MOF nuclei in the dynamic growth by slowing down their crystal growth rates along a specific direction. The competitive formation between the [110] and [100] planes of ZIF-8 ultimately regulates the crystal shapes from rhombic dodecahedron, truncated rhombic dodecahedron, and truncated cube to cube. Similarly, changeable morphologies of HKUST-1 crystals are also achieved from cube and tetrakaidekahedron to octahedron, originating from the competitive selection between the [100] and [111] planes. In addition to the artificial matrix preferred orientation of initial nucleation, parameters such as temperature also play a crucial role in the resulting crystal morphology. Standing on the additive-free MOF crystal morphology growth control, porous architectures prepared in this approach can act as templates for ligand-free metal (Au, Ag, and Cu) nanocluster synthesis. The nanocluster-embedded MOF structures represent distinct crystal morphology-dependent optical properties, and interestingly, their fluorescence emission can be highly enhanced by facet-induced nanocluster packing alignments. These findings not only provide a unique thought on MOF crystal morphology guidance but also pave a new route for the accompanied property investigation and further application.

One-step impedimetric NT-proBNP aptasensor targeting cardiac insufficiency in artificial saliva

Currently, sensitive and accurate approaches for diagnosis, rapid assessment, and cardiac biomarker monitoring in patients with heart failure are needed. In this context, the advantages of aptamers over traditional antibodies have been employed to fabricate a single-step impedimetric N-terminal pro b-type natriuretic peptide (NT-proBNP)-modified gold microelectrode array. The development of an electrochemical aptasensing platform was based on the coimmobilization of alkanethiol self-assembled monolayers and amine-terminated aptamer that specifically recognized cardiac NT-proBNP protein resulting in charge electron transfer. Electroimpedimetric signals of the sensor were observed to be linear to the NT-proBNP concentrations in the range of 5.0 × 10−3 to 1.0 pg mL−1 (R2 = 0.9624), while achieving a low detection limit of 5.0 × 10−3 pg mL−1. Clinically relevant detection levels for NT-proBNP were achieved in a simple, rapid, and label-free measurement using artificial saliva, which was highlighted to be specific, regenerative, and selective over potential interferers occurring during the processes of cardiac insufficiency, Therefore, the novel NT-proBNP aptasensor is a promising point-of-care tool exhibiting safe, non-invasive, affordable, and non-prescription home use accessible to overcome the limitations associated with conventional ELISA and previous aptasensing.

Gap-directed chemical lift-off lithographic nanoarchitectonics for arbitrary sub-micrometer patterning.

We introduce a unique soft lithographic operation that exploits stamp roof collapse-induced gaps to selectively remove an alkanethiol self-assembled monolayer (SAM) on Au to generate surface patterns that are orders of magnitude smaller than structures on the original elastomer stamp. The smallest achieved feature dimension is 5 nm using a micrometer-scale structured stamp in a chemical lift-off lithography (CLL) process. Molecular patterns retained in the gaps between stamp features and their circumscribed or inscribed circles follow mathematical predictions, and their sizes can be tuned by altering the stamp structure dimensions, including height, pitch, and shape. These generated surface molecular patterns can function as biorecognition arrays or be transferred to the underneath Au layer for metallic structure creation. By combining CLL process with this gap phenomenon, soft material properties that are previously thought as demerits can be used to achieve sub-10 nm features in a straightforward sketch.

Effects of Interfacial Interactions on Electrocatalytic Activity of Cytochrome c Oxidase in Biomimetic Lipid Membranes on Gold Electrodes.

Effects of interfacial interactions on the electrocatalytic activity of protein-tethered bilayer lipid membranes (ptBLMs) containing cytochrome c oxidase (CcO) for the oxygen reduction reaction are studied by using protein film electrochemistry and surface-enhanced infrared absorption (SEIRA) spectroscopy. Mammalian CcO was immobilized on a gold electrode via self-assembled monolayers (SAMs) of mixed alkanethiols. The protein orientation on the electrode is controlled by SAM-CcO interactions and is critical to the cytochrome c (cyt c) binding. The CcO-phospholipid and CcO-cyt c interactions modulate the electrocatalytic activity of CcO, and more densely packed ptBLMs show higher electrocatalytic activity. Our study indicates that spectroscopic and electrochemical studies of ptBLMs can provide insights into the effects of relatively weak protein-protein and protein-lipid interactions on the enzymatic activity of transmembrane enzymes.

AFM Manipulation of EGaIn Microdroplets to Generate Controlled, On-Demand Contacts on Molecular Self-Assembled Monolayers.

Liquid metal droplets, such as eutectic gallium-indium (EGaIn), are important in many research areas, such as soft electronics, catalysis, and energy storage. Droplet contact on solid surfaces is typically achieved without control over the applied force and without optimizing the wetting properties in different environments (e.g., in air or liquid), resulting in poorly defined contact areas. In this work, we demonstrate the direct manipulation of EGaIn microdroplets using an atomic force microscope (AFM) to generate repeated, on-demand making and breaking of contact on self-assembled monolayers (SAMs) of alkanethiols. The nanoscale positional control and feedback loop in an AFM allow us to control the contact force at the nanonewton level and, consequently, tune the droplet contact areas at the micrometer length scale in both air and ethanol. When submerged in ethanol, the droplets are highly nonwetting, resulting in hysteresis-free contact forces and minimal adhesion; as a result, we are able to create reproducible geometric contact areas of 0.8-4.5 μm2 with the alkanethiolate SAMs in ethanol. In contrast, there is a larger hysteresis in the contact forces and larger adhesion for the same EGaIn droplet in air, which reduced the control over the contact area (4-12 μm2). We demonstrate the usefulness of the technique and of the gained insights in EGaIn contact mechanics by making well-defined molecular tunneling junctions based on alkanethiolate SAMs with small geometric contact areas of between 4 and 12 μm2 in air, 1 to 2 orders of magnitude smaller than previously achieved.

Towards Ge-based electronic devices: Increased longevity of alkanethiol-passivated Ge(100) in low humidity environments

Germanium is a critically important material for future complementary metal-oxide-semiconductor devices, however, to maximise its potential it is necessary to develop a robust passivation process that prevents Ge re-oxidation for a queue time of 24 h. Self-assembled monolayers (SAMs) of alkanethiols on Ge have previously been shown to inhibit oxidation; however, re-oxidation eventually occurs when exposed to ambient conditions. Herein, it is shown that humidity plays a key role in the degradation of the SAM, ultimately resulting in re-oxidation. To demonstrate this, thiol-passivated Ge(100) surfaces are exposed to controlled humidity environments with different levels of relative humidity (RH). The rate of re-oxidation of the Ge surfaces are tracked using X-ray photoelectron spectroscopy and water contact angle analysis to discern what role RH plays in the re-oxidation of the Ge and the degradation of the SAM passivation. Atomic force microscopy data is presented to show that humidity-mediated re-oxidation of the Ge has little or no impact on the route mean square roughness of those surfaces. Finally, atomistic modelling of thiol-SAM passivated Ge in the presence of water molecules has been studied using first principles density functional theory in order to simulate experimental conditions and to understand the atomic level processes that determine stability in hydrophilic and hydrophobic configurations.

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.

Observing Real-Time Formation of Self-Assembled Monolayers on Polycrystalline Gold Surfaces with Scanning Electrochemical Cell Microscopy.

Self-assembled monolayers (SAMs) of alkanethiols on gold have become a central focus of controllable surface chemistry because they can be easily formed from the solution phase and characterized using various techniques. Understanding the formation processes occurring at a nanoscale level is crucial to form defect-free SAMs for tailored applications in bio- and nanotechnology. Although many reports study and characterize SAMs after they are formed on gold surfaces, typical methods have not extensively studied the SAM formation process at the nanoscale. This paper focuses on the formation of defect-free SAMs and elucidates the formation mechanism occurring at the nanoscale level during the formation process. Exploiting the strength of scanning electrochemical cell microscopy, we monitored SAM formation via a soluble redox reporter on a polycrystalline gold foil using voltammetric and amperometric techniques. We formed SAMs by varying the concentration of 3-mercapto-1-propanol [HS(CH2)3OH], 6-mercapto-1-hexanol [HS(CH2)6OH], and 9-mercapto-1-nonanol [HS(CH2)9OH] to determine the effects of the thiol chain length, concentration, and location on the substrate (grain boundaries) on monolayer formation. We observed real-time changes in the quasisteady-state current of our redox reporter during the self-assembly process. Importantly, we formed defect-free SAMs at the nanoscale level using different concentrations of HS(CH2)6OH and HS(CH2)9OH and found that SAM formation at the nanoscale is concentration-dependent and varies when at a boundary between two crystal grains.