Allan Hjarbæk Holm

Nucleophilic and electrophilic displacements on covalently modified carbon: introducing 4,4′-bipyridinium on grafted glassy carbon electrodes

4,4′-Bipyridinium (i.e., viologen) was immobilized on 4-(chloromethyl)phenyl grafted glassy carbon electrodes by a nucleophilic substitution reaction involving 1-ethyl- or 1-benzyl-4-(4′-pyridyl)pyridinium. Reaction times of about 5 days were required for these surface-constrained processes to go to completion in aqueous solution at room temperature. The applicability of the described procedure was demonstrated by performing the equivalent modification in 2 steps by reacting first with 4,4′-bipyridine, followed by quaternization of the available nitrogen to obtain the viologen functionality, that is, the surface acts as a nucleophile in a substitution reaction. However, the quaternization step was found to be possible for introducing the benzyl group but not the ethyl group. The covalently modified electrodes were reasonably stable to repeated electrochemical sweeping in acetonitrile with a 25% decrease in the observed electroactivity after 100 sweeps at a sweep rate of 2 V s−1. The coverage was determined from the electrochemical response of the viologen moiety to be approximately 3 × 10−10 mol cm−2. In addition to cyclic voltammetry, the presence of viologen was demonstrated by means of X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. Scanning images (500 × 500 µm2) obtained by the latter technique indicated that the molecules were distributed uniformly over the entire surface. Scanning tunnelling microscopy was used to follow the individual steps of the modification procedure on highly ordered pyrolytic graphite.

Improved Adhesion Between PMMA and Stainless Steel Modified with PMMA Brushes

In this work, various lengths and densities of poly(methyl methacrylate) (PMMA) brushes were synthesized on stainless steel (SS) surfaces via surface initiated atom transfer radical polymerization. Subsequently, the joints between the bulk PMMA and the PMMA brushed stainless steel were obtained by injection molding, and for these the degree of adhesion was assessed by tensile testing. Several conditions are required to facilitate the mixing between the brushes and the bulk polymer and to reduce the residual stress at the interface: preheating of the SS samples before the injection molding; a long packing time; and a mold temperature above the glass transition temperature (Tg) of PMMA during the injection molding. This treatment leads to a cohesive failure in the bulk PMMA. It was observed that the stress concentrated at the rim, due to contraction of bulk PMMA during cooling, results in a weak adhesion at the rim of the joint. A combination of high density and long brush length of PMMA film provides better adhesion. The large number of PMMA brush chains apparently promotes good penetration into the bulk PMMA chains and ultimately results in high adhesion strength.

Versatile Transformations of Alkylamine-Derivatized Glassy Carbon Electrodes using Aryl Isocyanates

The reaction between a nucleophilic 4-(2-aminoethyl)phenyl-tethered glassy carbon surface and various para-substituted aryl isocyanates [ONC-PhX; X = NO(2), COPh, Cl, H, and NMe(2)] has been studied in toluene. It is demonstrated that the nucleophilic addition reaction is relatively fast occurring within two hours while providing an efficient and versatile route for derivatizing alkylamine-functionalized surfaces. An often overlooked issue in surface reactions is the possibility for competing physisorption processes. In such cases, the solution-based reactants become adsorbed to the surface or embedded in the grafted layer rather than chemically bonded to the surface. It is shown that for two of the aryl isocyanates (X = NO(2) and COPh) this physical adhesion can be so strong that even prolonged ultrasonic treatment cannot remove the adsorbant. However, a single potential excursion is capable of desorbing most of the physisorbed layers. The isocyanate-based method is also compared with the well-known approaches involving diazonium salts for assembling similar chemical systems directly. It is concluded that the method can be used advantageously not only in cases, where such approaches should fall short, but also if the goal is to achieve better control of the positioning of, e.g., redox active molecules in a well-defined layer with the ultimate goal of obtaining distinct electrochemical responses.