The signal transduction and enzyme activity were investigated in biosensors based on the glucose oxidase (GOx) and carbon nanotubes (CNT) embedded in a bio-adhesive film of chitosan (CHIT). The voltammetric studies showed that, regardless of CHIT matrix, the GOx adsorbed on CNT yielding a pair of surface-confined current peaks at -0.48 V. The anodic peak did not increase in the presence of glucose in an O2-free solution indicating the lack of direct electron transfer (DET) between the enzymatically active GOx and CNT. The voltammetric peaks were due to the redox of enzyme cofactor flavin adenine dinucleotide (FAD), which was not the part of active enzyme. The presented data suggest that DET may not be happening for any type of GOx/CNT-based sensor. The biosensor was sensitive to glucose in air-equilibrated solutions indicating the O2-mediated enzymatic oxidation of glucose. The signal transduction relied on the net drop in a biosensor current that was caused by a decrease in a 4-e- O2 reduction current and an increase in a 2-e- H2O2 reduction current. The enzyme assays showed that CNT nearly doubled the retention of GOx in a biosensor while decreasing the average enzymatic activity of retained enzyme by a factor of 4-5. Such inhibition should be considered when using a protein-assisted solubilization of CNT in water for biomedical applications. The proposed analytical protocols can be also applied to study the effects of nanoparticles on proteins in assessing the health risks associated with the use of nanomaterials.

The relationship between the state of the surface of carbon nanotubes (CNT) and their electrochemical activity was investigated using the enzyme cofactor dihydronicotinamide adenine dinucleotide (NADH) as a redox probe. The boiling of CNT in water, while nondestructive, activated them toward the oxidation of NADH as indicated by a shift in the anodic peak potential of NADH (E NADH ) from 0.4 to 0.0 V. The shift in E NADH was due to the redox mediation of NADH oxidation by traces of quinone species that were formed on the surface of treated CNT. The harsher treatment that comprised of microwaving of CNT in concentrated nitric acid had a similar effect on the E NADH and, additionally, it increased the anodic peak current of NADH. The latter correlated with the formation of defects on the surface of acid-microwaved CNT as indicated by their Raman spectra. The increase in current was discussed considering a role of surface mediators on the buckled graphene sheets of acid-microwaved CNT. The other carbon allotropes including the edge plane pyrolytic graphite, graphite powder, and glassy carbon did not display a comparable activation toward the oxidation of NADH.

The shape-dependent activity of gold nanoparticles (AuNP) was studied by testing them as electrocatalysts for the notoriously slow non-enzymatic oxidation of glucose in neutral solutions. The AuNP of spherical and irregular (including polyhedral) morphologies were synthesized and attached to glassy carbon electrodes with chitosan. Voltammetric and mass spectrometric studies showed that the irregular AuNP were more catalytically active toward the oxidation of glucose to gluconic acid. No obvious differences between both morphologies were found based on their X-ray diffraction patterns and HRTEM images suggesting that the crystallographic orientation alone did not account for their catalytic properties. While both morphologies contain the (111) crystallographic planes that are catalytic toward glucose oxidation, the better activity of irregular AuNP was ascribed to a higher surface density of incipient gold oxide acting as a fast redox mediator for glucose oxidation. Supporting this, the AuNP of both morphologies oxidized glucose after their anodic activation, although not to the same extent. The amperometric (0.30 V) determination of glucose at electrodes made of irregular AuNP yielded a wide linear calibration plot (0.20–110 mM; R2, 0.998), sensitivity of 66 μA M−1 cm−2, limit of detection of 100 μM (S/N, 3), and a response time below 5 s. The advantage of low-cost irregular AuNP over macro gold is that they are catalytic toward glucose oxidation without any need for prior activation.

The conventional gold electrodes were compared with recently published electrodes based on gold nanoparticles and gold nanostructured films as amperometric sensors for glucose in pH 7.40 phosphate buffer solutions. The conventional electrodes provided similar electroanalytical benefits while required much simpler and shorter preparation. It is recommended that the future reports on the development of electrochemical sensors based on metal nanoparticles/nanostructures include also the analytical figures of merit obtained at relevant conventional metal electrodes. The voltammetric studies indicated that, in contrast to phosphate buffers, the Tris buffers were not suitable for activation of gold surface toward the direct oxidation of glucose.Keywords: Direct oxidation of glucose, Amperometric glucose sensors, Gold electrodes, Phosphate buffers, Tris buffers, Voltammetry, Nanostructures DOI: 10.1002/elan.201000006 The direct oxidation of glucose at solid electrodes is important for the development of new electrochemical devices including fuel cells and electrochemical sensors for glucose. In particular, the direct oxidation of glucose at gold electrodes has been extensively investigated in alkaline [1 -10], acidic [11 -13], and neutral solutions [14 -22]. Recently, the elaborate electrodes based on the gold nanoparticles and gold nanostructured films have been introduced for the direct determination of glucose in neutral solutions [23 -26]. The present communication demonstrates that the conventional gold disk electrodes can be as effective as these complex electrodes in the determination of glucose at physiological pH 7.40. In addition, the importance of solution composition for the activation of gold toward the oxidation of glucose is briefly discussed. Figure 1A presents typical cyclic voltammograms that were recorded at conventional gold disk electrodes in the absence (1) and presence (2) of glucose in a solution. They show that the oxidation of glucose takes place in the potential window from À 0.20 to 0.50 V with the current peak at 0.25 V. The oxidation of glucose was observed only after the gold electrodes were activated by cycling their potential between À 0.20 and 0.80 V before the addition of glucose to a solution. Practically no current due to the oxidation of glucose was recorded at the electrodes that did not undergo such activation. Figure 1B shows that the glucose peak current at 0.25 V reached a plateau after the electrode was activated by 50 potential cycles. The necessity for such activation is consistent with the hypothesis that the electrooxidation of glucose requires the presence of the so-called "incipient oxide" on the gold surface [1,5,14,15,17,27,28]. Apparently, the cyclization of electrode potential in a phosphate buffer solution generates the gold-oxygen species on the electrode surface that catalyze the oxidation of glucose. The Inset in Figure 1B presents a typical calibration plot for the amperometric detection of glucose at 0.25 V. The plot yields the sensitivity of 0.72 AE...

The Diels-Alder (D-A) reaction of the twisted hydrocarbon 1,4-diphenylbenz[a]aceanthrylene (4) with dienophiles maleic anhydride, bromomaleic anhydride, and N-phenylmaleimide and with benzyne is reported. The stereochemistry of the products derived from the D-A reaction of 4 is compared with the products derived from reaction of planar benz[a]aceanthrylene (5) with maleic anhydride as a model. The endo regiochemical π-facial selectivity of 4 appears to be the result of the steric effect of a phenyl substituent as the transition state is reached. Compound 5 produces both endo and exo D-A adducts when treated with maleic anhydride. X-ray crystallographic analysis verifies the topology of the bromomaleic anhydride adduct of 4 and the maleic anhydride adduct of 5.