Probing morphological and compositional variations of anodized carbon electrodes with tapping-mode scanning force microscopy.

This paper demonstrates the first application of tapping-mode scanning force microscopy (TM SFM) in the compositional mapping of modified glassy carbon (GC) electrodes. Using TM SFM, we have been able to track both compositional and topographical changes of polished GC induced by electrochemical pretreatment (ECP). Photoresist-based microfabrication techniques were employed to produce surfaces consisting of segregated modified and unmodified regions for direct comparison in the same image. Our results show that ECP of GC via anodization in basic solutions for short times (∼10 s) initially removes the ubiquitous layer of polishing debris via an etching process. Longer anodization in basic electrolyte results in significant etching of the GC surface. ECP in acidic solutions yields little topographic change compared to basic electrolytes. Electrochemical results obtained for three redox systems studied on both modified and unmodified GC electrodes correlate with the TM SFM images collected.

Hydroxylated naphthoquinones as substrates for Escherichia coli anaerobic reductases.

We have used two hydroxylated naphthoquinol menaquinol analogues, reduced plumbagin (PBH2, 5-hydroxy-2-methyl-1,4-naphthoquinol) and reduced lapachol [LPCH2, 2-hydroxy-3-(3-methyl-2-butenyl)-1, 4-naphthoquinol], as substrates for Escherichia coli anaerobic reductases. These compounds have optical, solubility and redox properties that make them suitable for use in studies of the enzymology of menaquinol oxidation. Oxidized plumbagin and oxidized lapachol have well resolved absorbances at 419 nm (epsilon=3.95 mM-1. cm-1) and 481 nm (epsilon=2.66 mM-1.cm-1) respectively (in Mops/KOH buffer, pH 7.0). PBH2 is a good substrate for nitrate reductase A (Km=282+/-28 microM, kcat=120+/-6 s-1) and fumarate reductase (Km=155+/-24 microM, kcat=30+/-2 s-1), but not for DMSO reductase. LPCH2 is a good substrate for nitrate reductase A (Km=57+/-35 microM, kcat=68+/-13 s-1), fumarate reductase (Km=85+/-27 microM, kcat=74+/-6 s-1) and DMSO reductase (Km=238+/-30 microM, kcat=191+/-21 s-1). The sensitivity of enzymic LPCH2 and PBH2 oxidation to 2-n-heptyl-4-hydroxyquinoline N-oxide inhibition is consistent with their oxidation occurring at sites of physiological quinol binding.