SAM Formation Research Articles

Investigation of the modification of gold electrodes by electrochemical molecularly imprinted polymers as a selective layer for the trace level electroanalysis of PAH

Electrochemical molecularly imprinted polymers (e-MIPs) were grafted for the first time as a thin layer to the surface of a gold electrode to perform trace level electroanalysis of benzo(a)pyrene (BaP). This was achieved by controlled/living radical photopolymerization of a redox tracer monomer (ferrocenylmethyl methacrylate, FcMMA) with ethylene glycol dimethacrylate in the presence of benzo(a)pyrene as the template molecule. For that purpose, a novel photoiniferter-derived SAM was first deposited on the gold surface. The SAM formation was monitored by cyclic voltammetry and electrochemical impedance spectroscopy. Then, the “grafting from” of the e-MIP was achieved upon photoirradiation during a controlled time. Differential pulse voltammetry was used to quantify BaP in aqueous solution by following the modification of the signal of FcMMA. A limit of detection of 0.19 nM in water and a linear range of 0.66 nM to 4.30 nM, were determined, thus validating the enhancement of sensitivity induced by the close contact between the e-MIP and the electrode, and the improved transfer electron.

The Puzzling Processes at Electrode | Ionic Liquid Interface

J.M. Lehn stated in his Nobel prize lecture in 1988 that supramolecular chemistry is the chemistry of the intermolecular bond, covering the structure and functions of entities formed by the association of two or more chemical species [1]. The adsorption of organic molecules is the net result of the interactions between the molecules and the two phases, the metal and the electrolyte, and the interactions between the latter two.To this day, the universal understanding that the adsorption of organic molecules results in the formation of an ordered monolayer is based on the phenomenon observed in mainly aqueous solutions. However, the conception might not be straightforwardly transferrable to the organic additive + ionic liquid | electrode interface. For example, the existence of an interfacial multilayer structure of pure IL ions in contrast with an aqueous electrolyte has been previously shown in both experimental and computational studies. Furthermore, it has been shown that in ILs, if the ions form rigid layers on the electrode, it is necessary to apply an overpotential for interfacial restructuring. Therefore, studying the adsorption of organic additives in ionic liquids (IL) should contribute to a better understanding of metal | IL interfaces.In the given presentation we overview the characteristics of solid-liquid interface characteristics of various additives from ionic liquid media at different electrodes.Cyclic voltammetry, electrochemical impedance spectroscopy and in situ scanning tunnelling microscopy measurements were conducted to characterize the electrochemical behavior of the self-assembled layers of 4,4’-bipyridine (4,4’-BP) and 2,2’-bipyridine (2,2’-BP) at the Sb(111) | x-BP+1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) interface [2-3]. The specific adsorption of iodide ions was studied at Bi(111), Cd(0001) and pyrolytic graphite electrodes [4-6]. All the experiments were carried out in a three-electrode electrochemical cell in a glovebox.The halide ions are surface active ions with specific adsorption behavior [4-6]. The strong specific adsorption of halide ions at single crystal electrodes, as well as other electrodes, is a complicated process due to the partial charge transfer between adsorbed ions and the solid electrode surface. The properties of the adsorption layer depend on the electrode potential and the concentration of the surface-active ions in an electrolyte. The theory that the adsorption of organic molecules results in SAM formation is mainly based on the results observed in aqueous solutions. However, these findings may not be as straightforwardly linkable to the organic additive + ionic liquid | electrode interface. The analysis of cyclic voltammetry and impedance results revealed that 2,2′-BP and 4,4′-BP indeed adsorb at the Sb(111) interface, forming a thin dielectric layer at the electrode surface, confirmed by in situ STM measurements, resulting in the differential capacitance values nearly two times lower compared to EMImBF4. Acknowledgments:This work was supported by the Estonian Research Council grant PSG249, and by the EU through the European Regional Development Fund under project TK141 (2014-2020.4.01.15-0011).

In-situ analysis of self-assembled monolayer induced polymorphism in controlled crystallization of carbamazepine using quartz crystal microbalance technique

Compared with traditional control methods in the drug crystallization process, a heterogeneous functional monolayer is an effective approach that could induce needed crystal forms under easier control conditions. In this study, a QCM sensor modified by functional groups was used to control the CBZ polymorphism, analyze the process of SAM formation and demonstrate the crystallization process of template-induced heterogeneous nucleation. The results showed that the heterogeneous functional monolayer reduced the induction time of the crystallization and controlled the polymorphism of pharmaceutical crystals. At the same time, molecular modeling was also applied to study the crystallization growth mechanisms of CBZ on the SAM. This work provides a reliable and promising new idea for the controlled crystallization of more drug systems and the online analysis of the nucleation process, and also provides a precise new means for the exploration of the crystallization mechanism.

Overexpression of the LcCUC2-like gene in Arabidopsis thaliana alters the cotyledon morphology and increases rosette leaf number.

BackgroundThe unique ‘mandarin jacket’ leaf shape is the most famous trait of Liriodendron chinense and this characteristic gives L. chinense aesthetic and landscaping value. However, the underlying regulatory mechanism of genes involved in the leaf development of L. chinense has remained unclear.MethodsBased on transcriptome data of leaves at different developmental stages from L. chinense, we identified differentially expression genes (DEGs) functioning in leaf development. A candidate gene named LcCUC2-like (LcCUC2L) had high similarity in sequence with Arabidopsis thaliana CUC2, and used for further research. We isolated the full-length LcCUC2L gene and its promoter from L. chinense. Subsequently, we analyzed the function of the LcCUC2L gene and its promoter activity via transformation into A. thaliana.ResultsIn this study, we found that the LcCUC2L and AtCUC2 are homologous in sequence but not homologous in function. Unlike the role of AtCUC2 in leaf serration and SAM formation, the LcCUC2L mainly regulates cotyledon development and rosette leaf number. Histochemical β-glucuronidase (GUS) staining revealed that LcCUC2L was expressed in the cotyledons of A. thaliana seedlings, indicating that the LcCUC2L may play a role in cotyledon development. Ectopic expression of LcCUC2L resulted in long, narrow cotyledons without petioles, abnormal lamina epidermis cells and defective vascular tissue in cotyledons, and these results were consistent with the LcCUC2L expression pattern. Further analysis showed that overexpression of LcCUC2L also induced numerous rosette leaves. Also, LcCUC2L and other related genes showed a severe response in L. chinense by introducing exogenous auxin stimulation, partly revealed that LcCUC2L affects the leaf development by regulating the auxin content.ConclusionsThese results suggest that LcCUC2L may play a critical role in leaf development and morphogenesis in L. chinense, and our findings provide insight into the molecular mechanisms of leaf development in L. chinense.

Critical role of additive-induced molecular interaction on the operational stability of perovskite light-emitting diodes

Summary Despite rapid improvements in efficiency and brightness of perovskite light-emitting diodes (PeLEDs), the poor operational stability remains a critical challenge hindering their practical applications. Here, we demonstrate greatly improved operational stability of high-efficiency PeLEDs, enabled by incorporating dicarboxylic acids into the precursor for perovskite depositions. We reveal that the dicarboxylic acids efficiently eliminate reactive organic ingredients in perovskite emissive layers through an in situ amidation process, which is catalyzed by the alkaline zinc oxide substrate. The formed stable amides prohibit detrimental reactions between the perovskites and the charge injection layer underneath, stabilizing the perovskites and the interfacial contacts and ensuring the excellent operational stability of the resulting PeLEDs. Through rationally optimizing the amidation reaction in the perovskite emissive layers, we achieve efficient PeLEDs with a peak external quantum efficiency of 18.6% and a long half-life time of 682 h at 20 mA cm−2, presenting an important breakthrough in PeLEDs.

Allosteric inhibition of MTHFR prevents futile SAM cycling and maintains nucleotide pools in one-carbon metabolism

Methylenetetrahydrofolate reductase (MTHFR) links the folate cycle to the methionine cycle in one-carbon metabolism. The enzyme is known to be allosterically inhibited by SAM for decades, but the importance of this regulatory control to one-carbon metabolism has never been adequately understood. To shed light on this issue, we exchanged selected amino acid residues in a highly conserved stretch within the regulatory region of yeast MTHFR to create a series of feedback-insensitive, deregulated mutants. These were exploited to investigate the impact of defective allosteric regulation on one-carbon metabolism. We observed a strong growth defect in the presence of methionine. Biochemical and metabolite analysis revealed that both the folate and methionine cycles were affected in these mutants, as was the transsulfuration pathway, leading also to a disruption in redox homeostasis. The major consequences, however, appeared to be in the depletion of nucleotides. 13C isotope labeling and metabolic studies revealed that the deregulated MTHFR cells undergo continuous transmethylation of homocysteine by methyltetrahydrofolate (CH3THF) to form methionine. This reaction also drives SAM formation and further depletes ATP reserves. SAM was then cycled back to methionine, leading to futile cycles of SAM synthesis and recycling and explaining the necessity for MTHFR to be regulated by SAM. The study has yielded valuable new insights into the regulation of one-carbon metabolism, and the mutants appear as powerful new tools to further dissect out the intersection of one-carbon metabolism with various pathways both in yeasts and in humans.

Growth and organization of (3-Trimethoxysilylpropyl) diethylenetriamine within reactive amino-terminated self-assembled monolayer on silica

Alkane chains are the most commonly used molecules for monolayer fabrication. Long chains are used for their strong van der Waals interactions inducing good layer organization. Amine function-terminated alkyl chains are of great interest and are widely used for further surface functionalization. Since it is mandatory that such layers be organized to provide amine moieties at the surface, the present study deals with exploring amine-terminated SAM formation as an alternative to the usual aminopropylalkylsilane SAM. Additionally, using a long NH2-terminated alkyl chain allows the formation of hydrogen bonding thanks to the two NH moieties born along the chain. Furthermore, such hydrogen bonding makes possible to shorten the molecule length while preserving a well-organized monolayer. For this purpose we performed a complete study of the grafting of (3-Trimethoxy-silylpropyl) diethylenetriamine (DETAS) on native silicon oxide using various solvents, relative humidity and temperature values. Grafting kinetics was monitored by ellipsometry and goniometry, and SAM structure and organization using AFM and ATR-FTIR spectroscopy. Hydrogen bonding was evidenced within the SAM growth process and in the final complete SAM. We believe such study enables a better control of good quality DETAS SAM in order to improve their efficiency in further surface functionalization applications.

Construction of Dopamine-Releasing Gold Surfaces Mimicking Presynaptic Membrane by On-Chip Electrochemistry.

We report a strategy to construct a dopamine-releasing gold surface mimicking a presynaptic membrane on a microfluidic chip to simulate in vivo neural signaling. We constructed dopamine self-assembled monolayers (DA SAMs) by electrochemical deprotection of methyl group-protected DA SAMs on a gold surface. Electrochemically controllable release of DA SAMs can be realized by applying nonhydrolytic negative potential on the gold surface. Our method in constructing DA SAMs avoids the polymerization and protonation of DA molecules which may lead to the failure of the DA SAM formation. By combining microfluidics, we realized spatial and temporal controllable release of DA by electrochemistry from the gold surface. Furthermore, by culturing neurons on the patterned DA SAMs, the interface between the DA SAMs and the neurons could serve as a presynaptic membrane, and the spatiotemporal release of DA could modulate the neuron activity with high precision. Our study holds great promise in the fields of neurobiology research and drug screening.

Label-free electrochemical immunosensor for quick detection of anti-hantavirus antibody

This report describes the preparation and evaluation of an immunosensor based on chemical modification of gold surface with hantavirus (strain Araucaria) antigen/protein and the antibody detection by electrochemical analysis. The viral nucleoprotein was immobilized onto gold by SAM formation with 3-mercaptopropionic acid (MPA) followed by activation of the surface using N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride and N-Hydroxysuccinimide (EDC/NHS). Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and quartz crystal microbalance (QCM) measurements were used for evaluation of each step involved in the construction of the proposed immunosensor. Several experimental conditions were studied and after optimization of immobilization parameters, the device showed good stability within 21days and selectivity for a qualitative detection of specific antibody (IgG) directed against hantavirus Araucaria. The proposed sensor had shown a linear response for concentration varying between 0.400 and 300μgmL−1 allowing its application in spiked serum samples for detection of specific antibody of hantavirus Araucaria based on a cutoff value. The immunosensor showed a sensitive response to the spiked samples and can be employed as an alternative for the qualitative diagnosis of hantavirus infections.

Characterizing self-assembled molecular layers on weakly interacting substrates: the role of van der Waals and the chemical interactions

The properties of self-assembled molecular layers (SAMs) on weakly interacting substrates depend on a delicate balance between intermolecular and molecule–substrate interactions. In this paper, combining STM experiments and first principle calculations, we study 1,3,5-triazine layers grown on both graphite and G/Pt(111). We have carried out ab initio DFT calculations trying standard and hybrids functionals as well as different approaches for the van der Waals (vdW) interactions to fully characterize the intermolecular—H-bonds and vdW– and molecule–substrate–vdW attraction and Pauli repulsion—interactions. Our results confirm that, regarding the SAM formation, the molecule–substrate interaction is strong enough to fix a relative molecule–substrate orientation even though the intermolecular interaction, which fixes the triangular lattice symmetry of the SAM, is larger. This game between these two interactions explains the appearance of very large moiré patterns between SAM and substrate. However, our simulations, even testing several approaches for the vdW and XC interactions, do not reproduce neither the value of diffusion barriers nor differences between substrates found in the experiments.

Polymer brush on surface with tunable hydrophilicity using SAM formation of zwitterionic 4-vinylpyridine-based polymer

A zwitterionic vinylpyridine-based polymeric SAM was assembled on different surfaces to obtain tunable hydrophilicity.

Surface protection of copper in 3% NaCl solution by using 1-(n-butyl)imidazole self-assembled monolayer

This paper describes the formation of self-assembled monolayer of 1-(n-butyl)imidazole (BI) on electrode surface by self-assembly technique. Electrochemical impedance spectroscopy was conducted to establish the optimum concentration and assembling time of the BI SAM for the SAM formation. It was found that the BI monolayer was formed with the concentration of 1.0 mM of BI at 24 h assembling time and the maximum inhibition efficiency that could be attained is 93.2%. The resulting monolayer was characterized by Fourier transform infrared spectroscopy (FT-IR) and energy dispersive X-ray (EDX) analysis. The surface analysis confirmed that the BI molecule was self-assembled on the copper surface. The corrosion resistance ability of the SAM coated copper in 3% NaCl solution was investigated by potentiodynamic polarization studies, cyclic voltammetry, scanning electron microscopy. The result showed that the presence of BI monolayer on copper electrode resulted in better corrosion resistance against copper corrosion in NaCl medium.

On the decisive role of the sulfur-based anchoring group in the electro-assisted formation of self-assembled monolayers on gold

A reproducible electro-assisted method is proposed to follow the influence of different sulfur-based anchoring groups on the adsorption kinetics and thermodynamics of SAM formation on gold electrodes. This has been made possible thanks to the preparation of several molecules containing various anchoring groups (thiol, thioester, acyclic and cyclic disulfides, thiosulfonate) while preserving the same spacer redox head group. In this paper we focus on the question of a substantial difference between SAMs derived from either alkylthiols or disulfides using cyclic voltammetry. The comparison of equilibrium surface coverage, affinity constant toward gold surface and stability suggest a decisive role of the nature of the anchoring group in the formation of the monolayers. Moreover, a significant difference of the desorption behavior of SAMs on gold, pre-formed from either thiol or disulfide, has clearly been demonstrated.

Molecular Mechanisms of Solvent-Controlled Assembly of Phosphonate Monolayers on Oxide Surfaces

The explicit role of a series of solvents in octadecylphosphonic acid (ODPA) association and monolayer formation on the (0001) surface of α-aluminum oxide is explored from molecular dynamics simulations. For this purpose, molecule-by-molecule attachment and subsequent relaxation is studied in hexane, THF, and 2-propanol, respectively. From a purely structural viewpoint, the simulations show that the packing and ordering of the resulting SAMs closely resembles that of monolayers initially grown in vacuo, followed by immersion into the solvent afterward. In terms of the formation kinetics, we however find significant dissimilarities which result from solvent structuring at the interface and considerable hindering of surfactant association to the template surface at later stages of SAM growth. This leads to drastic deviation from diffusion-controlled kinetics and calls for a time-dependent picture of SAM formation mechanisms.

A novel label free immunosensor based on single-use ITO-PET electrodes for detection MAGE1 protein

This study is aimed to design a label free biosensor based on transparent ITO-PET (Indium tin oxide- polyethylene terephthalate) electrode to determine MAGE1 protein. Firstly, indium tin oxide (ITO) electrodes were modified with NH4OH/H2O2/H2O to obtain the OH groups on the surface. Later, the surface of ITO electrodes were treated with 3-glycidoxypropyltriethoxysilane (3-GOPE). After SAM formation, anti- MAGE1 protein was covalently immobilized on modified ITO electrodes. The parameters such as SAMs concentration, the concentration and incubation time of anti-MAGE1 solution were optimized. For determining the immobilization steps and optimization of the biosensor, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were utilized. To determine analytical characterization of designed biosensor; linear range, repeatability, reproducibility, regeneration processes were studied. In optimum conditions, the linear range of the biosensor was determined as 0.01pg–1.28pgmL−1. Square wave voltammetry technique was applied to the biosensor. These steps were realized in K3 [Fe (CN)6]/K4 [Fe(CN)6] redox probe. Moreover, Single frequency technique was used to understand the interaction between antiMAGE-1 and MAGE-1 protein. This step was realized in pH7 phosphate buffer. And also, shelf life of designed biosensor was investigated. SEM (Scanning Electron Microscope) technique was used to determine the morphology of the surface in every step. The other important parameter for this study was Kramer's Kronig transform. It is used to determine whether the impedance spectra of the designed biosensor are affected from the deviation obtained because of external factors. Lastly, the designed biosensor was applied to real human serum and the results were compared with ELISA (Enzyme-Linked ImmunoSorbent Assay) test.

Binding groups for highly ordered SAM formation: carboxylic versus thiol.

The structure of analogous SAMs chemisorbed on an Ag substrate via a -SH or -COOH binding group is reported. Our observations indicate that COOH binding groups can be considered as a promising alternative for the current standard based on the sulfur binding groups, particularly when 2D ordering, SAM fabrication time and stability under ambient conditions are of great importance.

Nanostructuring of Au(111) during the Adsorption of an Aromatic Isocyanide from Solution.

We present a combined vibrational and morphological characterization of the self-assembly of 1,4-phenylene-diisocyanide (PDI) on Au(111) from methanol solution. Vibrational sum frequency generation (vSFG) and scanning tunneling microscopy (STM) have been applied to determine the adsorption geometry of the PDI-Au adatom complexes as well as the morphological transformations of the Au(111) substrate upon SAM formation from solutions with PDI concentrations in the μM to mM range. At low concentration/coverage, PDI adsorbs in flat adsorption geometry, with both isocyanide groups attached to Au adatoms on the Au(111) surface. Transformation to a standing-up phase is observed with increasing concentration/coverage. In contrast to findings for PDI adsorbed in ultrahigh vacuum, PDI does not form a long-range-ordered monolayer phase when adsorbed from solution. In addition, the Au(111) surface is subjected to structural modifications. Au vacancy islands and ad-islands, which are typical substrate defects formed during the self-assembly of aromatic thiols on Au(111), are also created during PDI adsorption from solution. At low PDI concentration, the Au vacancy islands and ad-islands are found at specific sites mediated by the herringbone reconstruction of the Au(111) surface, giving rise to long-range-ordered structures. These structures do not form during UHV adsorption of PDI on Au(111) nor has a similar ordering effect been observed for any related thiol-SAM system investigated so far.

DPPTE Thiolipid Self-Assembled Monolayer: A Critical Assay.

Supported lipid membranes represent an elegant way to design a fluid interface able to mimic the physicochemical properties of biological membranes, with potential biotechnological applications. In this work, a diacyl phospholipid, the 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (DPPTE), functionalized with a thiol group, was immobilized on a gold surface. In this molecule, the thiol group, responsible for the Au-S bond (45 kJ/mol) is located on the phospholipid polar head, letting the hydrophobic chain protrude from the film. This system is widely used in the literature but is no less challenging, since its characterization is not complete, as several discordant data have been obtained. In this work, the film was characterized by cyclic voltammetry blocking experiments, to verify the SAM formation, and by reductive desorption measurements, to estimate the molecular density of DPPTE on the gold surface. This value has been compared to that obtained by quartz crystal microbalance measurements. Ellipsometry and impedance spectroscopy measurements have been performed to obtain information about the monolayer thickness and capacitance. The film morphology was investigated by atomic force microscopy. Finally, Monte Carlo simulations were carried out, in order to gain molecular information about the morphologies of the DPPTE SAM and compare them to the experimental results. We demonstrate that DPPTE molecules, incubated 18 h below the phase transition temperature (T = 41.1 ± 0.4 °C) in ethanol solution, are able to form a self-assembled monolayer on the gold surface, with domain structures of different order, which have never been reported before. Our results make possible rationalization of the scattered results so far obtained on this system, giving a new insight into the formation of phospholipids SAMs on a gold surface.

Surface Modification of TiO2 Photoanodes with Fluorinated Self-Assembled Monolayers for Highly Efficient Dye-Sensitized Solar Cells

Dye aggregation and electron recombination in TiO2 photoanodes are the two major phenomena lowering the energy conversion efficiency of dye-sensitized solar cells (DSCs). Herein, we introduce a novel surface modification strategy of TiO2 photoanodes by the fluorinated self-assembled monolayer (F-SAM) formation with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFTS), blocking the vacant sites of the TiO2 surface after dye adsorption. The F-SAM helps to efficiently lower the surface tension, resulting in efficient repelling ions, e.g., I3(-), in the electrolyte to decrease the electron recombination rate, and the role of F-SAM is characterized in detail by impedance spectroscopy using a diffusion-recombination model. In addition, the dye aggregates on the TiO2 surface are relaxed by the F-SAM with large conformational perturbation (i.e., helix structure) seemingly because of steric hindrance developed during the SAM formation. Such multifunctional effects suppress the electron recombination as well as the intermolecular interactions of dye aggregates without the loss of adsorbed dyes, enhancing both the photocurrent density (11.9 → 13.5 mA cm(-2)) and open-circuit voltage (0.67 → 0.72 V). Moreover, the combined surface modification with the F-SAM and the classical coadsorbent further improves the photovoltaic performance in DSCs.

Elimination Voltammetry as a New Method for Studying the SAM Formation

Elimination Voltammetry as a New Method for Studying the SAM Formation