Layer-by-layer assembly of ultrathin films of hemoglobin and clay nanoparticles with electrochemical and catalytic activity

Stable films of clay and hemoglobin (Hb) were assembled layer-by-layer on various solid substrates by alternate adsorption of negatively charged clay platelets from their aqueous dispersions and positively charged Hb from pH 4.5 buffers. Cyclic voltammetry (CV), quartz crystal microbalance and UV-vis spectroscopy were used to monitor film growth. CV of {clay/Hb}(n) films on pyrolytic graphite electrodes showed a pair of well-defined, nearly reversible peaks at about -0.27 V versus saturated calomel electrode (-0.02 V vs normal hydrogen electrode) at pH 5.5, characteristic of the HbFe(III)/Fe-II redox couples. Although the amount of Hb adsorbed in each bilayer was essentially the same, the fraction of electroactive Hb decreased with the number of clay/Hb bilayers (n). Electroactivity of Hb extended to six clay/Hb bilayers. The Soret absorption band of Hb in {clay/Hb}(6) films showed that Hb retained its secondary structure similar to its native state in the medium pH range. A Bragg peak in X-ray diffraction for {clay/Hb}(20) films suggested a partly ordered, layered structure of the films, but without full Hb intercalation. {Clay/Hb}(6) film electrodes catalyzed the electrochemical reduction of trichloroacetic acid, oxygen, and hydrogen peroxide.