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.