All the chemicals used in this work were of analytical grade unless otherwise stated. Horseradish peroxidase (E.C. 1.11.1.7, 169 Units mg1 powder, Sigma) aniline (99%), hydrochloric acid (37%), N,N-dimethylformamide (DMF), disodium hydrogen
phosphate (dehydrated), and sodium dihydrogen phosphate (dehydrated) were all obtained from Sigma-Aldrich (South Africa). Cadmium and copper stock solutions (1000 ppm) were obtained from KIMIX Chemicals & Lab Supplies; and lead
stock solution (1000 ppm) was obtained from Saarchem-Holpro Analytic (PTY) Ltd. Working solutions of hydrogen peroxide were prepared from 30% v=v stock