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Post‐Stress Thickening of Dextran/Concanavalin A Solutions Used as Sensitive Fluids in a Viscosimetric Affinity Assay for Glucose

Glucose‐dependent low‐stress and high‐stress rheology of a sensitive fluid (SF) containing high‐molecular‐weight dextran (30 g/L) and Concanavalin A (ConA, 10 g/L) was studied to define conditions for reproducible glucose determination by a viscosimetric sensor. The viscosity of the investigated SF was reproducibly glucose‐dependent only at sufficient shear stresses (>0.5 Pa) . Rheometric parameters measured at low stresses (<0.1 Pa) were strongly dependent on the shear history. Low‐stress viscosity measured after laminar flow with a high rate (90 s, 100 s−1, 4.5 Pa) increased by more than 2 orders of magnitude with slow kinetics. Solutions thickened after laminar flow acquired gel‐like properties (storage modulus nearly equal to loss modulus in a broad range of frequencies) . The highly viscous and gel‐like state developed after flow was stable up to a yield stress of ca. 0.05 Pa. The ConA‐mediated contribution to high‐stress viscosity was almost completely suppressed by the presence of glucose (100 mM), whereas thickening after flow was rather insensitive to the competing sugar. Interaction of diffusion‐controlled ligand exchange at the ConA binding sites with convective displacement of dextran molecules has been considered a main reason for the observed thickening effect. It is proposed that the shear field has an orientating effect on lectin distribution in the dextran matrix, whereby far‐reaching lamellar affinity clusters are developed. In the subsequently unloaded fluid, residues of these structures can reorientate and be cross‐linked to a weak metastable gel by redistributing ConA.