Ole Hammerich

Kinetics and mechanisms of reactions of organic cation radicals in solution

Publisher Summary The dimerization of cation radicals of simple benzene derivatives is usually accompanied by the formation of the corresponding biphenyl derivative. This, by virtue of the extended conjugation is more easily oxidized, and frequently only intractable tars are found as final products. If the reaction medium is carefully selected, so that the biphenyl cation radicals are stable, useful synthetic procedures can be developed. Two mechanisms are probable for the dimerization of cation radicals of anisoles and related compounds. They are termed “radical–radical dimerization” (RRD) and “radical–substrate coupling” (RSC). The oxidation of aromatic hydrocarbons and alkylaromatic compounds in media of low nucleophilicity results in the formation of dimers. The cation radicals of triphenylamines form dimers in high yield when generated in acetonitrile. The products are tetraphenylbenzidines.

The reversible oxidation of aromatic cation radicals to dications. Solvents of low nucleophilicity

Abstract Reversible behavior for both electron transfers for oxidation of aromatic compounds to cation radicals and dications was observed in several common electrolytic solvents. Nitriles, nitro compounds and dichloromethane can all be rendered essentially nucleophile free for voltammetric purposes simply by conducting the voltammetric measurements over neutral alumina shortly after the mixture has been stirred. Solvents containing trifluoroacetic acid and the corresponding acid anhydride are not only useful for voltammetry but can also be used to prepare stable solutions of cation radicals and dications. Equilibrium constants for the disproportionation of cation radicals to dications were calculated from the reversible electrode potentials and the effect of changes in the solvent system on the equilibrium constants is discussed.

Electrochemical activation of screen-printed carbon strips

Screen-printed electrodes, fabricated by thick-film technology, represent an attractive avenue for routine electrochemical sensing. However, the composite character of strips fabricated from commercial carbon strips results in slow rates of heterogeneous electron transfer. The aim of this study was to establish a rapid electrochemical procedure for in situ activation of screen-printed electrodes. Short pre-anodization periods were shown to increase the electrochemical activity for a wide range of irreversible and quasi-reversible redox processes. Activation parameters influencing the enhanced reversibility at strips fabricated from two common carbon inks were explored. The effect of the ink-curing temperature on the redox activity was also examined. Cyclic and differential-pulse voltammetry, X-ray photoelectron spectroscopy and scanning electron microscopy were used for monitoring changes in the electrochemical reversibility, surface area and morphology and the introduction of oxygen surface functionalities. The sensing utility of activated carbon strip electrodes is demonstrated for several analytes, and future prospects are discussed. The simple, yet effective, electrochemical pretreatment is compatible with on-site application of disposable electrodes.

Remarkably selective metallized-carbon amperometric biosensors

Abstract The use of metallized carbon transducers results in remarkably selective amperometric biosensors. Such focus on the transducer eliminates major electroactive interferences in the first place, and hence circumvents the need for anti-interference layers. The remarkable selectivity of metal-dispersed carbons is attributed to their strong, preferential, electrocatalytic action towards the detection of the enzymatically-liberated hydrogen peroxide and reduced nicotinamide cofactor (NADH). Such activity allows tuning of the operating potential to the region (+ 0.1 to −0.2 V) where unwanted reactions do not occur. This article reviews the development of metallized-carbon biosensors, with particular emphasis on metallized carbon-paste enzyme electrodes, miniaturized devices based on electrochemical deposition of the metal/enzyme layer and metal-dispersed sensor strips.

A critical evaluation of a glucose biosensor made by codeposition of palladium and glucose oxidase on glassy carbon

Abstract The preparation of a glucose biosensor by the electrochemical codeposition of palladium and glucose oxidase (GOx) on a glassy carbon electrode is described. The Pd/GOx electrode system allowed a low working potential of + 0.3 V vs. Ag | AgCl which is ca. 0.3 V lower than that reported for equivalent Pt/GOx systems. The sensor required no special pretreatment to suppress interference from galactose. It is demonstrated that the pH of the plating solution has a critical effect on the successful codeposition of palladium and GOx. In addition, the results of a systematic study demonstrated that the deposition time as well as the concentration of the palladium salt and the enzyme level strongly affect the sensitivity of the resulting electrode. During a test period of 50 days the electrode lost approximately 50% of its sensitivity within the first 20 days, after which the response was essentially constant. The morphology of the electrode surface was characterized by scanning electron microscopy.

Analysis of the factors determining the sensitivity of a miniaturized glucose biosensor made by codeposition of palladium and glucose oxidase onto an 8 μm carbon fiber

Abstract The preparation of miniaturized glucose biosensors by the electrochemical codeposition of palladium and glucose oxidase (GOx) onto carbon fiber microelectrodes (8 μm diameter) is described. The sensitivity of the resulting sensors was dependent on the amount of palladium salt in the deposition solution, with the optimal performance found for 2.0 mg K 2 PdCl 6 per 5 ml. This is approximately one order of magnitude lower than found earlier (H. Sakslund et al. J. Electroanal. Chem., 374 (1994) 71) during the preparation of a similar electrode system using a glassy carbon disk (3 mm diameter). This difference in the optimal palladium salt concentration probably reflects that mass transport to the small cylindrical carbon fiber is more effective than to the large planar glassy carbon electrode. The dependence on the amount of GOx and the deposition time was similar for the two electrode systems, with the optimal response found when the amount of GOx was 26 mg or higher (per 5 ml) and the deposition time approximately 10 min. A long-term stability test demonstrated that the sensitivity decreased to 50% of its original value after 25 days and to 30% after 50 days. The morphology of the electrode surfaces was characterized by scanning electron microscopy.

Development and evaluation of glucose microsensors based on electrochemical codeposition of ruthenium and glucose oxidase onto carbon fiber microelectrodes

Abstract Miniaturized glucose biosensors have been prepared by electrochemical codeposition of ruthenium and glucose oxidase onto carbon fiber microelectrodes (11 μm diameter). The ruthenium sites offer an efficient and preferential catalytic action toward the electrochemical reduction of the enzymatically liberated hydrogen peroxide. Highly selective biosensing of glucose is thus accomplished at an optimal potential range around 0.0 V, where contributions from easily oxidizable substances, such as ascorbic or uric acids or acetaminophen, are eliminated. A wide linear range, up to 3 × 10 −2 M, is achieved. Electrodeposition parameters controlling the microstructure and biosensor performance are elucidated.

Mixed valence radical cations and intermolecular complexes derived from indenofluorene-extended tetrathiafulvalenes

Engineering of mixed-valence (MV) radical cations and intermolecular complexes based on π-extended tetrathiafulvalenes (TTFs) is central for the development of organic conductors. On another front, redox-controlled dimerization of radical cations has recently been recognized as an important tool in supramolecular chemistry. Here we show that π-extended TTFs based on the indenofluorene core, prepared by Horner–Wadsworth–Emmons reactions, undergo reversible and stepwise one-electron oxidations and that the detectable, intermediate radical cation forms remarkably strong intermolecular MV ([neutral·cation]) and π-dimer ([cation·cation]) complexes with near-infrared radical cation absorptions. The radical cation itself seems to be a so-called Class III MV species in the Robin–Day classification. The formation of MV dimers was corroborated by ESR spectroelectrochemical studies, revealing two slightly different ESR signals upon oxidation, one assigned to the MV dimer and the other to the cation monomer. Crystals of the radical cation with different anions (PF6−, BF4−, and TaF6−) were grown by electrocrystallization. Conductance studies revealed that the salts behave as semiconductors with the hexafluorotantalate salt exhibiting the highest conductance. Using a custom-built ESR spectrometer with sub-femtomole sensitivity, the magnetic properties of one crystal were investigated. While the spin-to-spin interaction between radical cations was negligible, a high cooperativity coupling to the microwave field was observed – as a result of an exceptionally narrow spin line width and high spin density. This could have great potential for applications in quantum computation where crystalline spin ensembles are exploited for their long coherence times.

Stereoselectivity and mechanism in the electrohydrodimerisation of esters of cinnamic acid

Rate constants (kobs) and reaction orders have been determined for the cathodic reduction in DMF solution of 11 cinnamic acid esters including some derived from chiral alcohols and a dicinnamate derived from trans-cyclohexane-1,2-diol. The cinnamic acid esters typically reduce with high stereoselectivity to all-trans 3,4-diphenylcyclopentanone-2-carboxylates. The enhancement of rates of reaction by addition of water was studied for selected substrates and low energies of activation were found. Changes in the alkoxy or aryloxy groups also caused significant changes in rate and log kobs correlated linearly with E° values. The results from kinetic experiments were complemented by product studies of reactions aimed at probing reversibility of key reaction steps. The combined evidence is interpreted as unambiguous support for radical anion–radical anion coupling as the key step with complexation with water, prior to coupling, being crucial.The relative stereochemistry at C-3 and C-4 is fixed, irreversibly, at the coupling stage and there is strong evidence to suggest that templating in the complex between two radical anions and water determines the stereochemical outcome.

Effects of pH, temperature and reaction products on the performance of an immobilized creatininasecreatinasesarcosine oxidase enzyme system for creatinine determination

Abstract The effects of pH, temperature (t) and reaction products on the performance of enzyme reactors containing immobilized creatininase (CA), creatinase (CI) and sarcosine oxidase (SO) for the determination of creatinine were studied by flow-injection analysis with amperometric detection of the resulting hydrogen peroxide. The optimum performance of the coupled enzyme system was found at pH 7.7 and 25°C. Some of the CI and SO activity was lost irreversibly at t ⩾ 30°C. In contrast, the activity of CA increased with t up to at least 40°C. The effects of the reaction products on the enzyme activities were examined. Glycine caused the CA activity to increase and the SO activity to decrease, whereas the CI activity was unaffected by this compound. Sarcosine caused a decrease in the CI activity. The activities of all three enzymes were insensitive towards the presence of formaldehyde and urea and so was the activity of SO in the presence of creatine and hydrogen peroxide. The fraction, α, of the injected creatinine (or creatine) equilibrated by the CA reactor is introduced as a quantitative measure of the CA activity, and was between 10 and 72% depending on the enzyme loading. The unused immobilized enzymes were found to maintain their activity for at least 6 months. When in heavy daily use, CA and SO lost ca. 25% of the activity over a period of 20–30 days, whereas the activity of CI was found to be essentially unchanged.