Electroactive Self-assembled Monolayer Research Articles

Lipids Lay Thyselves Down: Redox-Induced Lipid Vesicle Fusion Onto Ferrocene-Terminated Self-Assembled Monolayers

Due to the attractive electrochemical properties of ferrocene (fast electron transfer, low oxidation potential, and two stable redox states), ferrocenylalkanethiolates on gold are the most studied and employed electroactive self-assembled monolayer (SAM) system. Oxidation of the SAM-confined ferrocenes to ferroceniums is accompanied by the formation of ion-pairs with counter-ions in solution [1,2]. Ferrocene-terminated SAMs exhibit a reversible electrochemistry in the presence of anionic surfactants such as n-alkyl sulfates and sulfonates, while the less chaotropic n-alkyl phosphates and carboxylates lead to the irreversible oxidation of the SAM-confined ferrocene (Fig. A) [3]. Cyclic voltammetry indicates that these latter surfactants form strong ion pairs with the oxidized ferroceniums; an oxidation peak is observed for the first anodic potential sweep but no reduction peak is detected during the reverse sweep. No redox peaks are observed in subsequent sweeps. The surface plasmon resonance signal does not return back to its original value during reduction, indicating an irreversible adsorption of material to the surface, which in turn suggests the formation of a stable and insoluble ferrocenium-surfactant anion complex or deposit that blocks the redox process. In this work, atomic force microscopy is used to investigate the association of sodium n-dodecanoate to the SAM surface. This electrochemical approach appears to be an effective strategy for triggering the formation of solid-supported lipid membranes by the fusion of phospholipid vesicles onto the ferrocenylalkanethiolate SAM. In fact, phospholipids are double-tailed amphiphiles and these can associate with the oxidized ferrocene (Fig. B). A potential-step hold at 0,64 V for only five minutes induces the immediate adsorption of phosphatidylserine (DMPS, POPS and DOPS) at the SAM-gold/electrolyte interface with a full surface coverage. This technique is a versatile tool that offers many benefits such as, a faster supported lipid bilayers formation mimicking the biological membranes, a larger variety of lipid composition, structure and chemical nature, and a possibility of using metal surfaces considering their potential applications in electrochemical techniques, spectroscopic techniques and surface plasmon resonance. In this work, AFM-based nano-shaving, force spectroscopy, and surface plasmon resonance are combined to determine the thickness of the obtained lipid membrane.

Modification of gold surface by electrosynthesized mono aza crown ether substituted catechol-terminated alkane dithiol and its application as a new electrochemical sensor for trace detection of cadmium ions.

Among the toxic metals, cadmium is a very dangerous pollutant because it can extremely damage organs in humans and animals. This toxic metal is introduced into water from different industries such as metal plating, batteries, and alloys. Cadmium bioaccumulates in vital organs and unlike organic pollutants does not show any biological degradation. In this study, an electroactive self-assembled monolayer (SAM) was developed by covalent attachment of a novel mono aza-crown ether substituted catechol-terminated hexane dithiol onto the gold surface. The electrochemical behavior of the fabricated SAM electrode was investigated using voltammetry techniques and electrochemical impedance spectroscopy (EIS). The obtained results from voltammetric experiments revealed that the crown ether moiety of SAM forms a selective complex with cadmium ion. Under optimal conditions, Cd2+ could be detected in the range of 15 μM to 65 μM with a detection limit of 4.5 μM. Selectivity measurements reveal that the sensor is specific for Cd2+ even in the presence of high concentrations of other metal ions. The proposed sensor was applied to the determination of cadmium ion in water samples with high sensitivity and good selectivity.

The self-assembly of redox active peptides: Synthesis and electrochemical capacitive behavior.

The present work reports on the synthesis of a redox-tagged peptide with self-assembling capability aiming applications in electrochemically active capacitive surfaces (associated with the presence of the redox centers) generally useful in electroanalytical applications. Peptide containing ferrocene (fc) molecular (redox) group (Ac-Cys-Ile-Ile-Lys(fc)-Ile-Ile-COOH) was thus synthesized by solid phase peptide synthesis (SPPS). To obtain the electrochemically active capacitive interface, the side chain of the cysteine was covalently bound to the gold electrode (sulfur group) and the side chain of Lys was used to attach the ferrocene in the peptide chain. After obtaining the purified redox-tagged peptide, the self-assembly and redox capability was characterized by cyclic voltammetry (CV) and electrochemical impedance-based capacitance spectroscopy techniques. The obtained results confirmed that the redox-tagged peptide was successfully attached by forming an electroactive self-assembled monolayer onto gold electrode. The design of redox active self-assembly ferrocene-tagged peptide is predictably useful in the development of biosensor devices precisely to detect, in a label-free platform, those biomarkers of clinical relevance. © 2016 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 357-367, 2016.

Sensitive label-free electron chemical capacitive signal transduction for D-dimer electroanalysis

D-dimer, an important biomarker in emergency medicine, was determined by applying a label-free electrochemical capacitive approach with an antibody-based selective biosensor consisting of an electroactive self-assembled monolayer mounted on a commercially available gold electrode. Electrochemical measurements were performed by an impedance-derived approach (based on observing the complex capacitance diagrams from where electrochemical capacitance can be obtained as transducer signal) from which it was possible to obtain low limits of detection of 50fmolL−1 in buffer (PBS) and 7pmolL−1 in undiluted human serum (performed without any kind of sample pre-treatment), with good coefficient of determination (r2≥0.99) in a clinically relevant D-dimer concentration range (from 1 to 1,000ngmL−1). These electro analytical/chemistry results pave the way to the creation of low-cost and reliable point-of-care electrochemical approach for the quantitative bedside determination of D-dimer in clinical samples, thus speeding up the diagnosis of potentially lethal, nonetheless treatable, clinical conditions like pulmonary embolism.

In Situ Synthesis of a Novel Quinone Imine Self‐Assembled Monolayer and Consideration of Its Reactivity with L‐Arginine

AbstractIn situ functionalization of a 4‐aminothiophenol (4ATP) self‐assembled monolayer (SAM) on a Au electrode (4ATP/Au SAM) by the Michael addition reaction is considered. Under optimized conditions, the nucleophilic attack of the amino group of 4ATP/Au SAM to give an electrogenerated ortho‐quinone produced a novel electroactive SAM (ESAM). The ESAM could be oxidized to quinone‐imine SAM (QI SAM) for the covalent immobilization of L‐arginine monolayers. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Fourier transform infrared (FTIR) spectroscopy are employed to characterize these systems.. The apparent heterogeneous rate constant (ksapp) for ESAM/Au and the rate constant (k′) of the pseudo‐first order Michael addition reaction of L‐arginine and ESAM/Au are calculated.

Direct Anchoring of Cytochrome c onto Bare Gold Electrode for Sensing Oxidative Stress in Aquatic Cells

A novel third generation biosensor was developed based on one-shot adsorption of chemically-modified cytochrome c (cyt c) onto bare gold electrodes. The introduction of short-chain thiol derivatives (mercaptopropionic acid, MPA) on the lysine residues of cyt c enabled the very fast formation (<5 min) of an electroactive biological self-assembled monolayer (SAM) exhibiting a quasi-reversible electrochemical behavior and a fast direct electron transfer (ET). The heterogeneous ET rate constant was estimated to be ks = 1600 s-1, confirming that short anchors facilitate the ET via an efficient orientation of the heme pocket. In comparison, no ET was observed in the case of native and long-anchor (mercaptoundecanoic acid, MUA) modified cyt c directly adsorbed on gold. However, in both cases the ET was efficiently restored upon in-bulk generation of gold nanoparticles which acted as electron shuttles. This observation emphasizes that the lack of electroactivity might be caused by either an inappropriate orientation of the protein (native cyt c) or a critical distance (MUA-cyt c). Finally, the sensitivity of the bare gold electrode directly modified with MPA-cyt c to hydrogen peroxide (H2O2) was evaluated by amperometry and the so-made amperometric biosensor was able to perform real-time and non-invasive detection of endogeneous H2O2 released by unicellular aquatic microorganisms, Chlamydomonas reinhardtii, upon cadmium exposure.

Label Free DNA Sensors Using PNA Probe Sequence and Electroactive Self-Assembled Monolayer Application to PCR Fragments of Mycobacterium Tuberculosis

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A catechol-terminated self-assembled monolayer at the surface of a gold electrode and its application for the electrocatalytic determination of dopamine

An electroactive self-assembled monolayer was fabricated by covalent attachment of protocatechuic acid at a gold surface and its electrochemical behavior was investigated using cyclic voltammetry. The modification involves a three-step method: (i) preparation of a cysteamine self-assembled monolayer, (ii) activation of the carboxylic groups of protocatechuic acid in solution and (iii) modification of cysteamine self-assembled monolayers by activated protocatechuic acid to functionalize the self-assembled monolayer by catechol-terminated groups. This resulting thin film modified electrode was tested successfully for the electrocatalytic determination of dopamine in aqueous solution.

Spatio‐Temporal Control of Cell Coculture Interactions on Surfaces

Coculture control: We report a combined photochemical and electroactive self-assembled monolayer (SAM)-based substrate strategy to generate a coculture platform with spatial and temporal control of cell-cell interactions. These dynamic substrates can present a variety of ligands on the surface for biospecific interactions between the ligands and cell surface receptors. Photopatterning enables the ligands to be immobilized in patterns and even gradients.

Electrochemical Surface Plasmon Resonance Investigation of Dodecyl Sulfate Adsorption to Electroactive Self-Assembled Monolayers via Ion-Pairing Interactions

The redox-induced assembly of amphiphilic molecules and macromolecules at electrode surfaces is a potentially attractive means of electrochemically modulating the organization of materials and nanostructures on solid substrates via ion-pairing interactions or charge-transfer complexation. In this regard, we have investigated the potential-induced adsorption and aggregation of dodecyl sulfate, a common anionic surfactant, at a ferrocenylundecanethiolate (FcC11SAu) self-assembled monolayer (SAM)/aqueous solution interface by electrochemical surface plasmon resonance (ESPR) spectroscopy. The surfactant anions adsorb onto the electroactive SAM by specific ion-pairing interactions with the oxidized ferricinium species. The ferricinium charge density (QFc+) obtained by cyclic voltammetry and surface coverage measured by SPR indicate that the dodecyl sulfate forms an interdigitated monolayer, where half of the surfactant molecules have their sulfate headgroups paired to the surface and half have their headgroups exposed to the aqueous solution. The surface coverage of dodecyl sulfate was found to depend on both the ferricinium surface concentration and the surfactant aggregation state in solution. A maximum coverage of dodecyl sulfate on the ferricinium surface is obtained below the critical micelle concentration (cmc), in contrast to dodecyl sulfate adsorption to SAM surfaces of static positive charge. This marked difference in adsorption behavior is attributed to the dynamic generation of ferricinium by potential cycling and the specific nature of the ion-pairing interactions versus pure electrostatic ones. The results presented point to a new way of organizing molecules via electrical stimulus.

Self-assembled monolayers of a hydroquinone-terminated alkanethiol onto gold surface. Interfacial electrochemistry and Michael-addition reaction with glutathione

Abstract An electroactive self-assembled monolayer (SAM) was fabricated by covalent attachment of a novel hydroquinone-terminated dodecanethiol onto the gold surface and its electrochemical behavior was investigated using cyclic voltammetry and electrochemical impedance spectroscopy. The capability of the designed SAM in immobilization of organic molecules onto the gold surface was studied utilizing the Michael-addition as a model reaction. The results obtained from cyclic voltammetry, electrochemical impedance and grazing incidence Fourier transform infrared (GI-FTIR) spectroscopy revealed that, upon applying an anodic potential to the Au-SAM electrode system in the presence of glutathione, the electrochemically generated p-quinone participated in a Michael-addition reaction with glutathione and the corresponding Michael adduct was formed at the solid–liquid interface. The kinetic parameters were then derived for this interfacial Michael-addition reaction.

Establishing the Molecular Basis for Molecular Electronics

AbstractFor Abstract see ChemInform Abstract in Full Text.

Discreteness of Charge and Ion Association Effects on Electroactive Self-Assembled Monolayers

The voltammetric response of an electrode covered with an electroactive self-assembled monolayer is modeled including discreteness of charge effects and interfacial ion association. Discreteness of charge potentials are estimated according to the hexagonal array model of Macdonald and Barlow, and results are compared with those obtained in previous work with the cutoff disk model. As a consequence of the slower variation of the discreteness of charge potential on the applied electrode potential, voltammetric waves are predicted to be asymmetrical and wider than those computed from the cutoff disk model. For relatively high redox coverage and/or small values of the integral capacities of the inner and outer part of the monolayer, a negative differential capacity is predicted. This is a consequence of the additional stabilization provided by the discreteness of charge effect, which allows the charge density at the redox plane to increase faster than the charge density on the electrode surface when the monolayer is being oxidized. Comparison with experimental results shows that inclusion of the discreteness of charge effects results in a variation of the absolute value of the interfacial parameters, while their qualitative trends are the same as those obtained on the basis of an average potential model. Therefore, only the physical significance of the fitting parameters may help to discriminate among the different models.

Thionine self-assembly on polyoriented gold and sulphur-modified gold electrodes

Abstract A new type of self-assembled monolayer (SAM) on gold has been reported. Electroactive SAMs of thionine formed on polyoriented gold and sulphur-modified polyoriented gold from aqueous thionine solutions of pH 7.9 have been characterized by voltammetry. In thionine SAMs formed on gold modified by a monolayer of sulphur adatoms the attachment takes place through S-S bonding between the S adatom and S heteroatom of thionine with edge-on orientation. A fast 2c − reduction of thionine molecules in the SAM, at potentials 150 mV more negative than the solution couple, leads to a complete detachment of the reduced molecules leaving the sulphur adlayer intact. In the thionine SAM formed on a flame-treated surface of polyoriented gold, the surface concentration of 4.59 × 10 −10 mol cm −2 corresponds to the most efficient packing of thionine molecules in a monolayer (end-on orientation). On 2c reduction of thionine molecules, the initial thionine SAM transforms irreversibly to a new surface state with one-half of the initial surface concentration.

Ionic interactions in electroactive self-assembled monolayers of ferrocene species

Abstract : The electrochemical and interfacial behavior of two types of electroactive self-assembled monolayer systems is investigated at gold electrodes. The first type is a ferrocene-based surfactant (a redox surfactant) derived from (dimethylamino)methylferrocene via quaternization of the amino group with various n-alkylbromides. These have a long alkyl chain with 16 or 18 carbons in the chain pendent from the cationic ammonium group. We refer to these as C16 and C18. The second type is a ferrocene-based dimeric species with a disulfide functional group capable of providing a permanent anchor to the Au electrode, thus endowing monolayers of this species with exceptional stability towards desorption. The electrochemical quartz crystal microbalance (EQM) is used to monitor the mass changes which occur at the electrode surface during the redox processes of these two species.