Lignosulfonate-modified electrodes: electrochemical properties and electrocatalysis of NADH oxidation.

Lignosulfonic acid (LS1) and partially desulfonated lignosulfonic acid (LS2) were oxidatively deposited on a preactivated glassy carbon (GC) electrode, giving rise to redox active films showing three distinct redox couples at midpeak potentials (E degrees ') of 0.22, 0.44, and 0.53 V (vs Ag/AgCl in 0.1 M H(2)SO(4)). The redox activity was assigned to quinone moieties of different degrees of substitution, formed upon the oxidation of electroactive groups in the lignosulfonate structure. The most predominant couple (E degrees ' = 0.44 V) shifted negatively with pH at a rate of 59.5 mV per pH unit. In neutral electrolytes, the LS1- and LS2-modified electrodes behaved as anionic coatings, showing an increase in the charge transfer resistance (R(ct)) for the ferrocyanide/ferricyanide redox couple. The change in R(ct) was highly dependent on the LS sulfonation degree, and in comparison to an unmodified electrode it increased by ca. 490% for LS1-modified electrodes and by only 53% for LS2-modified electrodes. The LS-modified electrodes showed high electrocatalytic activity toward oxidation of reduced nicotinamide adenine dinucleotide (NADH). Electrocatalysis was studied in TRIS-HNO(3) buffers having pH of 5.0, 7.5, and 8.5 in the absence and presence of 20 mM Mg(2+), using the rotating disk electrode technique. Determined kinetic constants revealed that the impact of electrocatalysis depended strongly on the pH, the LS sulfonation degree, and the presence of bivalent metal ions. At fixed pH, the observed oxidation rate constant was lower for LS1-based electrodes than for LS2-based electrodes. On the other hand, the relative enhancement of this constant caused by the presence of Mg(2+) ions was much higher for LS1-based electrodes than for LS2-based electrodes. This phenomenon was explained by the participation of sulfonic groups in the formation of a ternary complex between quinone moiety, metal ions, and NADH. The values of other kinetic constants, including the Michaelis-Menten constant (K(M)), suggested that the formation of such a complex is preferred in alkaline pHs.