Electrochemical study of a metallothionein modified gold disk electrode and its action on Hg2+ cations

A novel protein monolayer modified electrode has been prepared by the self-assembly of metallothionein (MT) at a gold disk electrode. The properties of MT in Tris–HCl buffer and in the monolayer are studied by using cyclic voltammetry and differential pulse voltammetry with a gold disk electrode. In the negative sweep, the voltammogram of MT in buffer shows two small peaks and different electrochemical behaviour from that at a mercury electrode. Cd2+ complexed to the thionein can easily be replaced by Hg2+ ions, and Hg2+ ions can firmly adsorb in the MT monolayer with a saturation coverage of (2.78±0.29)×10−10 mol cm−2. This behaviour has been used to preconcentrate trace Hg2+ for its determination by cathodic stripping differential pulse voltammetry. The cathodic stripping peak current is proportional to Hg2+ concentration in the range of 0.15–3 μM and the detection limit is ca. 0.08 μM (16 ppb) with a 2 min open circuit accumulation step. The relative standard deviation is 7.2% at 0.4 μM Hg2+ concentration (n=4). At higher concentration the adsorption of Hg2+exhibits a response similar to that expected for a Langmuir adsorption isotherm with the stability constant of (4.0±0.2)×105 M−1.