Anodic Methods for Covalent Attachment of Ethynylferrocenes to Electrode Surfaces: Comparison of Ethynyl Activation Processes

A facile approach to create sensitive and selective Cu(ii) sensors on carbon fiber microelectrodes

A facile platform derived from deposition of ethynyl linkers on carbon fiber microelectrodes has been developed for sensitive and selective sensing of Cu(ii). This study is the first to demonstrate the successful anodic deposition of ethynyl linkers, specifically 1,4-diethynylbenzene, onto carbon fiber microelectrodes. Multi-scan deposition of DEB on these microelectrodes resulted in an increased sensitivity and selectivity towards Cu(ii) that persists amidst other divalent interferents and displays sustained performance over four days while stored at room temperature. This method can be extended to other ethynyl terminal moieties, thereby creating a versatile chemical platform that will enable improved sensitivity and selectivity for a new frontier of biomarker measurement.

Fast and Versatile Functionalization of Glassy Carbon

A rapid procedure for the functionalization of glassy carbon surfaces (GCSs) is disclosed. A three-step sequence of bromomethylation, azide displacement, and copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) allows ethynylated molecules to be attached covalently to the carbon surface through a methylene functional group. Redox-active ethynyl ferrocene and [Ru^II(Cl)(DMSO)(ethynyl-TPA)]¹⁺ (DMSO = dimethylsulfoxide; TPA = tris(2-pyridylmethyl)amine) are attached with high coverages as assessed by cyclic voltammetry, and the elemental composition of the surface is confirmed by X-ray photoelectron spectroscopy. In less than 1 h, surface coverages of 1 × 10¹⁴ molecules/cm² are possible that exhibit good durability in both acidic and basic media. Attached [Ru^II(Cl)(DMSO)(ethynyl-TPA)]¹⁺ catalytically oxidizes alcohols, yet the currents and potentials are less impressive compared to an attachment without the intervening methylene group. The advantages of this covalent attachment procedure for GCSs are its short reaction times, mild reaction conditions, and the use of standard laboratory reagents and glassware, allowing for many types of ethynylated molecules to be attached rapidly to the surface.

Microelectrode arrays, electrosynthesis, and the optimization of signaling on an inert, stable surface

Microelectrode arrays are powerful tools for monitoring binding interactions between small molecules and biological targets. In most cases, molecules to be studied using such devices are attached directly to the electrodes in the array. Strategies are in place for calibrating signaling studies utilizing the modified electrodes so that they can be quantified relative to a positive control. In this way, the relative binding constants for multiple ligands for a receptor can potentially be determined in the same experiment. However, there are applications of microelectrode arrays that require stable, tunable, and chemically inert surfaces on the electrodes. The use of those surfaces dictate the use of indirect detection methods that are dependent on the nature of the stable surface used and the amount of the binding partner that is placed on the surface. If one wants to do a quantitative study of binding events that involve molecules on such a stable surface, then once again a method for calibrating the signal from a positive control is needed. Fortunately, the electrodes in an array are excellent handles for conducting synthetic reactions on the surface of an array, and those reactions can be used to tune the surface above the electrodes and calibrate the signal from a positive control. Here, we describe how available Cu-based electrosynthetic reactions can be used to calibrate electrochemical signals on a polymer-coated electrode array and delineate the factors to be considered when choosing a polymer surface for such a study.

Ethynylphenyl-Derivatized Free Base Porphyrins: Anodic Oxidation Processes and Covalent Grafting onto Glassy Carbon Electrodes

In six of seven cases, direct anodic oxidation of the ethynyl group of an ethynylphenyl-derivatized free-base porphyrin gave modified glassy carbon electrodes in which the porphyrin was strongly surface-bound, most likely in a perpendicular geometry through covalent attachment of the ethynyl group to a surface carbon atom. The porphyrins each contained an ethynylphenyl group in one meso position and varied in the groups present in the other three meso positions. Electrografted 5,10,15,20-tetrakis(ethynylphenyl)porphyrin, H₂1, which has ethynyl moieties in all four meso positions, has well-defined surface voltammetry and grows to multilayer levels upon repeated cyclic voltammetry (CV) deposition scans. Multilayering was not observed to the same degree for monoethynylphenyl-substituted porphyrins and became progressively less for porphyrins having groups in the 15-meso position that were more protective against ethynyl radical attack. Clean molecular monolayer-level coverage was observed for 5-ethynylphenyl-10,20-bis(3-methoxyphenyl)-15-hexylporphyrin, H₂5. Owing to the fact that the ethynyl oxidation potential (1.1 to 1.5 V vs ferrocene) is more positive than that of the second macrocycle oxidation, the longevities and follow-up reactions of the porphyrin dications were also studied by CV, chemical oxidation, and optical spectroscopy in homogeneous solution. The primary follow-up products of the doubly oxidized porphyrins, whether surface-bound or in solution, were pyrrole-protonated species that were easily reduced back to the neutral porphyrin.

Effect of Large Electrolyte Anions on the Sequential Oxidations of Bis(fulvalene)diiron Attached to Glassy Carbon by an Ethynyl Linkage

Two ethynyl-derivatized isomers of bis(fulvalene)diiron (BFD, 1,1'-biferrocenylene) were prepared and covalently attached to glassy carbon electrodes through their ethynyl group by three different electrode modification methods. Cyclic voltammetry and square wave (SW) voltammetry were used to characterize surface coverages of 1.4-5.5 × 10^-10 mol cm^-2, the higher of these corresponding to roughly a monolayer, based on computation of an idealized close-packing structure for ethynylbis(fulvalene)diiron (E-BFD) on a solid surface. In a dichloromethane solution containing a smaller electrolyte anion such as [PF₆]^- or [ClO₄]^-, the E-BFD-modified electrodes exhibited two quasi-Nernstian one-electron oxidations. In contrast, the current for the second oxidation process, [E-BFD]^+/2+, was diminished in electrolytes containing one of the large fluoroaryl borate anions, [B(C₆F₅)₄]^- or [B(C₆H₃(CF₃)₂)₄]^-. The effect was enhanced for electrodes having higher surface coverages being probed at shorter voltammetric time scales. SW voltammetry showed that the diminished currents for [E-BFD]^+/2+ in large-anion electrolytes are not caused by slow electron transfer. Rather, they are attributed to mixed diffusivity of the counter-anions at the electrode/solution interface, as [E-BFD]⁺ and the anion form the optimum (lowest-energy) configuration of a 1:1 ion pair. The interior transport of the anion required to reach this configuration may be sterically encumbered, accounting for the diminished charge transfer observed with electrolytes containing large anions.

Surface Functionalization of Metals by Alkyl Chains through a Radical Crossover Reaction

Alkyl chains are covalently attached onto metal surfaces by indirect reduction of the bromoalkyl derivative (RBr). This indirect reaction involves the formation (by spontaneous or electrochemical reduction of the 2,6-dimethylbenzenediazonium salt) of a sterically hindered aryl radical that abstracts a Br atom from RBr but does not react with the surface. This crossover reaction furnishes an alkyl radical that reacts with the surface. Starting from 6-bromohexanoic acid, carboxylic functionalized gold surfaces are prepared. "Layer-by-layer" assemblies are built from these surfaces and present some ionic selectivity.