Small angle neutron scattering studies and kinetic analysis of laponite–enzyme hydrogels in view of biosensors construction

Clay hydrogels were designed as an immobilization matrix for electroactive enzymes at the surface of electrodes. Such materials are prepared by drying an aqueous colloidal suspension of laponite clay and enzyme on the surface of the electrodes. The properties of such materials have been investigated with regards to their structure and permeability, which determine the entrapment efficiency and the accessibility of the analyses to the enzymatic sites. The structure of such materials containing similar amounts of globular protein (bovine serum albumin (BSA)) and laponite have been investigated both in the dry and swollen states by means of small angle neutron scattering (SANS). The macroscopic orientation parallel to the faces of the dry films and the short-range ordering in the direction perpendicular to the faces was observed. This structure corresponds to that of the clay template swollen by the proteins. These structural features resist against moderate swelling by a concentrated electrolytic solution. The macroscopic orientation and the short-range order progressively disappear upon swelling with aqueous solutions of decreasing ionic strength. Since the structure turns from a quasi-impermeable stack of clay sheets to an open isotropic structure with enhanced accessibility below 100 mM of phosphate buffer, the sensitivity of an enzymatic clay-modified electrode (CME) working with the polyphenol oxidase (PPO) increases when the ionic strength of the swelling medium decreases. However, the gain of sensitivity remains moderate although the structural changes are huge. The rate determining mechanism seems to be a diffusion of the reacting species through defects (microchannels) inside the film as was previously proposed based on electrochemical and kinetics studies on the electrodes.