Direct electrochemistry of hemoglobin at silver electrode modified by lipoic acid monolayer

The direct electrochemistry of hemoglobin (Hb) was studied by cyclic voltammetry(CV) and flow injection analysis(FIA) on a silver electrode modified by a self-assembled monolayer of lipoic acid(LA). Lipoic acid molecules can strongly adsorb onto the Ag electrode surface through the cleavage of the S-H bond and the formation of the Ag-S bond. The observed adsorption coverage of LA demonstrates that the LA molecules spontaneously form a self-assembled monolayer. Experimental data show that LA can promote the redox process of Hb at the modified electrode surface. This chemically modified electrode (CME) exhibits good stability in the CV and FlA. Linear sweep voltammetric measurement of Hb at the CME reveals a linear relationship between the oxidative peak current and the concentration of Hb in the range of 5.0x10^-7-1.5x10^-5 mol/L. The relative standard derivation (RSD) for six replicate measurements of 5.0x10^-6 mol/L Hb in FIA is 2.8%. The detection limit is 2.0x10^-7 mol/L. The reaction mechanism involves the hydrogen bond/salt bridge formation between the carboxylate of LA and the protonated lysine residues of Hb that can enhance the electron transfer reaction. It can be used to detect Hb in real examples.