The Determination of Halides and Pseudohalides by the Vanadium Haloperoxidase Based Biosensor

Abstract

A biosensor containing vanadium haloperoxidase (VHP) immobilized on a graphite electrode responded to the halides and hydrogen peroxide at electrode potentials from -0.1 to 0.25 V vs SCE. An apparent Michaelis-Menten constant (Km) of the biosensor was 12–14 μM for hydrogen peroxide at 0.1–0.2 mM of KCl and 0.1 mM of sodium vanadate. Km decreased up to 2.3 μM in a vanadate free solution. At the same conditions and at a fixed concentration (0.1 mM) of hydrogen peroxide Km for chloride changed from 0.6 to 2.5 mM. Km for bromide was 9–16 μM and for iodide it was 5.5 μM. The biosensor response was largest at pH 5.1–5.3 and at pH 6.1 in presence of chloride and iodide, respectively. KNO3 in a competitive manner inhibited the biosensor response; Ki was 0.6 and 0.27 mM, for bromide and chloride. Ki for thiocyanate and for the same halides was 0.25 and 0.08 μM. 10.1 μM of cyanide inhibited 50 % of the biosensor response at 5 mM of chloride. Sodium azide irreversibly inhibited the biosensor response at 1–2 nM and in the presence of 10 μM of bromide or 5 mM of chloride.