Oxidative desorption of thiols as a route to controlled formation of binary self assembled monolayer surfaces

Spectroelectrochemical determination of thiolate self-assembled monolayer adsorptive stability in aqueous and non-aqueous electrolytes

Self-assembled monolayers (SAM) are ubiquitous in studies of modified electrodes for sensing, electrocatalysis, and environmental and energy applications. However, determining their adsorptive stability is crucial to ensure robust experiments. In this work, the stable potential window (SPW) in which a SAM-covered electrode can function without inducing SAM desorption was determined for aromatic SAMs on gold electrodes in aqueous and non-aqueous solvents. The SPWs were determined by employing cyclic voltammetry, attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), and surface plasmon resonance (SPR). The electrochemical and spectroscopic findings concluded that all the aromatic SAMs used displayed similar trends and SPWs. In aqueous systems, the SPW lies between the reductive desorption and oxidative desorption, with pH being the decisive factor affecting the range of the SPW, with the widest SPW observed at pH 1. In the non-aqueous electrolytes, the desorption of SAMs was observed to be slow and progressive. The polarity of the solvent was the main factor in determining the SPW. The lower the polarity of the solvent, the larger the SPW, with 1-butanol displaying the widest SPW. This work showcases the power of spectroelectrochemical analysis and provides ample future directions for the use of non-polar solvents to increase SAM stability in electrochemical applications.

A new cysteamine-copper chemically modified screen-printed gold electrode for glyphosate determination

A chemically modified screen-printed gold electrode has been prepared by covering the electrode surface with a cysteamine-copper self-assembled monolayer (SAM). The sensor was effective for the voltammetric sensing of glyphosate. The method exploits the interaction of glyphosate with copper ions complexed by cysteamine, which results in a decrease in the intensity of copper redox current. Cyclic voltammetry was employed as a measuring technique. When dealing with voltammograms with numerous peaks changing in shape and size, it is difficult to define which signal is the most significant for the analyte determination; in these cases, a helpful approach is chemometrics. In this work, PLS (Partial Least Square regression) has been applied to build models to correlate the signal with the glyphosate concentration in standard aqueous solutions and tap water samples (matrix-matched calibration). The method's figures of merits were evaluated, obtaining a limit of quantification of about 5 μM. The reliability of the proposed sensor was verified by analyzing tap water spiked with glyphosate; recoveries higher than 90 % were achieved.

Site-selective functionalization of in-plane nanoelectrode-antennas

Stacked organic optoelectronic devices make use of electrode materials with different work functions, leading to efficient large area light emission. In contrast, lateral electrode arrangements offer the possibility to be shaped as resonant optical antennas, radiating light from subwavelength volumes. However, tailoring electronic interface properties of laterally arranged electrodes with nanoscale gaps - to e.g. optimize charge-carrier injection - is rather challenging, yet crucial for further development of highly efficient nanolight sources. Here, we demonstrate site-selective functionalization of laterally arranged micro- and nanoelectrodes by means of different self-assembled monolayers. Upon applying an electric potential across nanoscale gaps, surface-bound molecules are removed selectively from specific electrodes by oxidative desorption. Kelvin-probe force microscopy as well as photoluminescence measurements are employed to verify the success of our approach. Moreover, we obtain asymmetric current-voltage characteristics for metal-organic devices in which just one of the electrodes is coated with 1-octadecanethiol; further demonstrating the potential to tune interface properties of nanoscale objects. Our technique paves the way for laterally arranged optoelectronic devices based on selectively engineered nanoscale interfaces and in principle enables molecular assembly with defined orientation in metallic nano-gaps.

Self-Assembled Monolayer of Monomercaptoundecahydro-closo-dodecaborate on a Polycrystalline Gold Surface

In this work, we present an electrochemical study of the boron cage monomercaptoundecahydro-closo-dodecaborate [B₁₂H₁₁SH]²⁻ in solution and in a self-assembled monolayer over a polycrystalline gold electrode. Cyclic voltammetry of the anion [B₁₂H₁₁SH]²⁻ in solution showed a shift in the peak potentials related to the redox processes of gold hydroxides, which evidences the interaction between the boron cage and the gold surface. For an Au electrode modified with the anion [B₁₂H₁₁SH]²⁻, cyclic voltammetry response of the probe Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ showed a ΔEp value typical for a surface modification. Electrochemical impedance spectroscopy presented R_tc and C_dl values related to the formation of a self-assembled monolayer (SAM). A comparison of electrochemical responses of a modified electrode with thioglycolic acid (TGA) reveals that the boron cage [B₁₂H₁₁SH]²⁻ diminishes the actives sites over the Au surface due to the steric effects. Differential capacitance measurements for bare gold electrode and those modified with [B₁₂H₁₁SH]²⁻ and (TGA), indicate that bulky thiols enhance charge accumulation at the electrode–solution interface. In addition to electrochemical experiments, DFT calculations and surface plasmon resonance measurements (SPR) were carried out to obtain quantum chemical descriptors and to evaluate the molecular length and the dielectric constant of the Boron cage. From SPR experiments, the adsorption kinetics of [B₁₂H₁₁SH]²⁻ were studied. The data fit for a Langmuir kinetic equation, typical for the formation of a monolayer.

Immobilization of molecular catalysts on electrode surfaces using host-guest interactions

Anchoring molecular catalysts on electrode surfaces combines the high selectivity and activity of molecular systems with the practicality of heterogeneous systems. Molecular catalysts, however, are far less stable than traditional heterogeneous electrocatalysts, and therefore a method to easily replace anchored molecular catalysts that have degraded could make such electrosynthetic systems more attractive. Here we applied a non-covalent 'click' chemistry approach to reversibly bind molecular electrocatalysts to electrode surfaces through host-guest complexation with surface-anchored cyclodextrins. The host-guest interaction is remarkably strong and enables the flow of electrons between the electrode and the guest catalyst. Electrosynthesis in both organic and aqueous media was demonstrated on metal oxide electrodes, with stability on the order of hours. The catalytic surfaces can be recycled by controlled release of the guest from the host cavities and the readsorption of fresh guest.

Electrochemical Activation of Self-Assembled Monolayers for the Binding of Effectors

A self-assembled monolayer (SAM) on gold was prepared from a diaminoterephthalate (DAT) derivative as functional molecule and 1-decanthiol as a backfiller. The DAT derivative is N-protected by a tert-butyloxycarbonyl (Boc) group and is anchored to the gold surface via a liponic acid as a stable anchor group. The terminal DAT moiety exhibits interesting effector properties such as fluorescence and electrochemical activity. Irreversible oxidation of the monolayer at 0.4 V (Hg|Hg₂SO₄) in 0.1 M HClO₄ triggers deprotection of the DAT group and subsequent chemical reactions, during which 10% of the DAT groups of the original SAM are transformed to a new surface-bound, quasi-reversible redox couple with a formal potential of 0.0 V (Hg|Hg₂SO₄) and a standard rate constant of 8 s^-1 in 0.1 M HClO₄. Immersion of the mixed SAM in 0.1 M HClO₄ at open circuit potential or oxidation in 0.1 M H₂SO₄ did not produce this surface-bound redox couple. The monolayers were thoroughly characterized by X-ray photoelectron spectroscopy (XPS) and polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) after the different preparation steps indicating only minor changes in the overall composition of the monolayer, in particular, the preservation of the heteroatoms. The new redox couple is likely a diimine, in agreement with its ability to bind nucleophiles such as anilines by conjugate addition that could be followed by multicycle voltammetry and XPS. The DAT effector group is especially interesting because it can also report the binding reaction by changed electrochemical and fluorescence signals.

Microfluidic Surface Titrations of Electroactive Thin Films

We report the use of microfluidic surface titrations (MSTs) for studying electroactive self-assembled monolayers (eSAMs) and other thin films. The technique of MST utilizes a microfluidic generation-collection dual channel electrode (DCE) configuration to quantify the charge associated with electroactive thin films that might or might not be in direct contact with an electrode surface. This technique allows for quantitative measurement of surface coverages, Γ, as low as 30 pmol cm^-2 for electrodeposited Cu thin films. Additionally, we show that it is possible to quantify Γ for ferrocene (Fc)-terminated alkylthiols in mixed-monolayer eSAMs. Interestingly, MSTs sometimes reveal a two-fold higher eSAM concentration compared to direct electrochemical measurements. This finding suggests that in these instances not all the constituent Fc-moieties of the eSAM are in sufficiently close proximity to the surface to be addressable via direct electrochemistry.

Selective in situ potential-assisted SAM formation on multi electrode arrays

The selective modification of individual components in a biosensor array is challenging. To address this challenge, we present a generalizable approach to selectively modify and characterize individual gold surfaces in an array, in an in situ manner. This is achieved by taking advantage of the potential dependent adsorption/desorption of surface-modified organic molecules. Control of the applied potential of the individual sensors in an array where each acts as a working electrode provides differential derivatization of the sensor surfaces. To demonstrate this concept, two different self-assembled monolayer (SAM)-forming electrochemically addressable ω-ferrocenyl alkanethiols (C₁₁) are chemisorbed onto independent but spatially adjacent gold electrodes. The ferrocene alkanethiol does not chemisorb onto the surface when the applied potential is cathodic relative to the adsorption potential and the electrode remains underivatized. However, applying potentials that are modestly positive relative to the adsorption potential leads to extensive coverage within 10 min. The resulting SAM remains in a stable state while held at potentials <200 mV above the adsorption potential. In this state, the chemisorbed SAM does not significantly desorb nor do new ferrocenylalkythiols adsorb. Using three set applied potentials provides for controlled submonolayer alkylthiol marker coverage of each independent gold electrode. These three applied potentials are dependent upon the specifics of the respective adsorbate. Characterization of the ferrocene-modified electrodes via cyclic voltammetry demonstrates that each specific ferrocene marker is exclusively adsorbed to the desired target electrode.

Electrochemical growth of CoNi and Pt-CoNi soft magnetic composites on an alkanethiol monolayer-modified ITO substrate

CoNi and Pt-CoNi magnetic layers on indium-tin oxide (ITO) substrates modified by an alkanethiol self-assembled monolayer (SAM) have been electrochemically obtained as an initial stage to prepare semiconducting layer-SAM-magnetic layer hybrid structures. The best conditions to obtain the maximum compactness of adsorbed layers of dodecanethiol (C12-SH) on ITO substrate have been studied using contact angle, AFM, XPS and electrochemical tests. The electrochemical characterization (electrochemical probe or voltammetric response in blank solutions) is fundamental to ensure the maximum blocking of the substrate. Although the electrodeposition process on the SAM-modified ITO substrate is very slow if the blocking of the surface is significant, non-cracked metallic layers of CoNi, with or without a previously electrodeposited seed-layer of platinum, have been obtained by optimizing the deposition potentials. Initial nucleation is expected to take place at the pinhole defects of the C12-SH SAM, followed by a mushroom-like growth regime through the SAM interface that allows the formation of a continuous metallic layer electrically connected to the ITO surface. Due to the potential of the methodology, the preparation of patterned metallic deposits on ITO substrate using SAMs with different coverage as templates is feasible.

Electrochemical fabrication of surface chemical gradients in thiol self-assembled monolayers with tailored work-functions

The studies on surface chemical gradients are constantly gaining interest both for fundamental studies and for technological implications in materials science, nanofluidics, dewetting, and biological systems. Here we report on a new approach that is very simple and very efficient, to fabricate surface chemical gradients of alkanethiols, which combines electrochemical desorption/partial readsorption, with the withdrawal of the surface from the solution. The gradient is then stabilized by adding a complementary thiol terminated with a hydroxyl group with a chain length comparable to desorbed thiols. This procedure allows us to fabricate a chemical gradient of the wetting properties and the substrate work-function along a few centimeters with a gradient slope higher than 5°/cm. Samples were characterized by cyclic voltammetry during desorption, static contact angle, XPS analysis, and Kelvin probe. Computer simulations based on the Dissipative Particle Dynamics methods were carried out considering a water droplet on a mixed SAM surface. The results help to rationalize the composition of the chemical gradient at different position on the Au surface.