Core--shell nanocluster films of hemoglobin and clay nanoparticle: direct electrochemistry and electrocatalysis

A novel core-shell protein nanocluster film, designated as clay-(Hb/PSS)(n), was fabricated on pyrolytic graphite (PG) electrodes. Positively charged hemoglobin (Hb) at pH 5.5 and negatively charged poly(styrenesulfonate) (PSS) were first assembled layer by layer on surface of clay nanoparticles from their solutions mainly by electrostatic attraction, forming a core-shell nanocluster structure in which clay nanoparticles were the "cores" and (Hb/PSS)(n) multilayers were the "shells". The aqueous dispersion of clay-(Hb/PSS)(n) nanoclusters was then cast on surface of PG electrodes, forming clay-(Hb/PSS)(n) nanocluster films after evaporation of solvent. Hb in clay-(Hb/PSS)(n) films exhibited a pair of well-defined and reversible cyclic voltammetric (CV) peaks at about -0.36 V vs. SCE in pH 7.0 buffers, characteristic of Hb heme Fe(III)/Fe(II) redox couples. Compared with other Hb-containing clay films, clay-(Hb/PSS)(n) films displayed smaller CV peak separation (DeltaE(p)), indicating the better electrochemical reversibility of Hb in these nanocluster films. The partially ordered structure of the films was characterized by X-ray diffraction (XRD) experiments. UV-VIS and reflection absorption infrared (RAIR) spectroscopy suggests that Hb retains its near-native structure in clay-(Hb/PSS)(n) films. Oxygen, hydrogen peroxide, and nitrite were catalytically reduced at clay-(Hb/PSS)(n) film electrodes, showing the potential applicability of the films as the new type of biosensors or bioreactors based on protein direct electrochemistry. The electrochemical and electrocatalytic activity of the films could be tailored by controlling the number of bilayers of the (Hb/PSS)(n) shells on the surface of clay nanoparticle cores.