Uwe Beyer

Compensation of Temperature and Concanavalin A Concentratration Effects for Glucose Determination by the Viscometric Affinity Assay

A viscometer suitable for rapid measurements in small volumes of highly viscous liquids is described. Using this device the viscometric affinity assay for glucose was studied under variable conditions in order to obtain basic information for the design of a viscometric glucose sensor. The viscosity of the dextran/Concanavalin A (ConA) solution is sensitive to glucose in a broad range of the shear stress. However, for measuring the glucose concentration with this sensitive liquid the strong dependence of the absolute viscosity on temperature and ConA concentration has to be taken into account. For the purpose of calibration a parameter more suitable than the absolute viscosity is the relative fluidity (Fr) that is defined by the actual measured viscosity at a given glucose concentration, the reference viscosity at a standard glucose concentration, and a constant linearization coefficient. Fr shows a linear dependence on the glucose concentration in the therapeutically interesting range up to 30 mM and is not significantly changed by moderate variations of the ConA concentration or temperature.

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.

Affinitätssensor auf der Grundlage eines Membranosmometers (Affinity Sensor based on a Membrane Osmometer)

Abstract Beim osmotischen Affinitätssensor wird die Konzentration von niedermolekularen Analytmolekülen gemessen, die einen polymeren Affinitätsliganden aus einem Komplex mit einem Affinitätsrezeptor verdrängen können. Der Ligandenaustausch findet in einem Mikro-Membran-Osmometer statt und führt zu einer Konzentrationsänderung der Polymere in der Osmometerzelle. Somit bewirkt der Ligandenaustausch eine Änderung der hydrostatischen Druckdifferenz gegenüber der Außenlösung. Da der Austausch von Dextranmolekülen an dem Lektin Concanavalin A durch Glucose relativ schnell und reversibel erfolgt, kann die Glucosekonzentration mit einem osmotischen Affinitätssensor kontinuierlich gemessen werden.