Direct electrochemistry and electrocatalysis of horseradish peroxidase immobilized in sol–gel-derived tin oxide/gelatin composite films

Horseradish peroxidase (HRP) was immobilized into a new type of sol–gel-derived nano-sized tin oxide/gelatin composite film (SnO₂ composite film) using a sol–gel film/enzyme/sol–gel film “sandwich” configuration. Direct electrochemistry and electrocatalysis of HRP incorporated into the composite films were investigated. HRP/SnO₂ composite film exhibited a pair of stable and quasi-reversible cyclic voltammetric peaks for the HRP Fe(III)/HRP Fe(II) redox couple with a formal potential of about −0.25 V (vs. SCE) in a pH 6.0 phosphate buffer solution. The electron transfer between the enzyme and the underlying electrode was greatly enhanced in the microenvironment with nano-SnO₂ particles and nanoporous structures. Morphologies and microstructures of the composite films and HRP/composite films were characterized with TEM, AFM. Electrochemical impedance spectroscopy (EIS) was also used to feature the HRP incorporated into composite films. FTIR and UV–Vis spectroscopy demonstrated that HRP in the composite film could retain its native secondary structure. With the advantages of organic–inorganic hybrid materials, the HRP/SnO₂ composite film modified electrode displayed good stability and electrocatalytic activity to the reduction of H₂O₂, The apparent Michaelis-Menten constant was estimated to be 0.345 mM, indicating a high affinity of HRP entrapped into the composite film toward H₂O₂.