A fluorescence affinity hollow fiber sensor for continuous transdermal glucose monitoring.
A novel concept of a fluorescence affinity hollow fiber sensor for transdermal glucose monitoring is demonstrated. The glucose-sensing principle is based on the competitive reversible binding of a mobile fluorophore-labeled Concanavalin A (Con A) to immobile pendant glucose moites inside of intensely colored Sephadex beads. The highly porous beads (molecular weight cutoff of 200 kDa) were colored with two red dyes, Safranin O and Pararosanilin, selected to block the excitation and spectrum of the fluorophore Alexa488. The sensor consists of the dyed beads and Alexa488-Con A confined inside a sealed, small segment of a hollow fiber dialysis membrane (diameter 0.5 mm, length 0.5 cm, molecular weight cutoff 10 kDa). In the absence of glucose, the majority of Alexa488-Con A resides inside the colored beads bound to fixed glucose. Thus, excitation light at 490 nm impinging on the sensor is strongly absorbed by the dyes, resulting in a drastically reduced fluorescence emission at 520 nm from the Alexa488-Con A residing within the beads. However, when the hollow fiber sensor is exposed to glucose, glucose diffuses through the membrane into the sensor chamber and competitively displaces Alexa 488-Con A molecules from the glucose residues of the Sephadex beads. Thus, Alexa 488-Con A appears in the void space outside of the beads and is fully exposed to the excitation light, and a strong increase in fluorescence emission at 520 nm is measured. At a medium to high loading degree of Sephadex with Alexa488-Con A (10 mg mL(-1) bead suspension), the absolute fluorescence increase due to 20 mM glucose was very large. It exceeded the response of other sensor devices based on FRET by a factor of 50 (Meadows and Schultz Anal. Chim. Acta 1993, 280, 21-30; Russell et al. Anal. Chem. 1999, 71, 3126-3132). The new sensor featured a glucose detection range extending from 0.15 to 100 mM, exhibiting the strongest dynamic signal change from 0.2 to 30 mM. It showed a reasonably fast response time (4-5 min). The combination of all the beneficial sensor features makes this sensor extremely attractive for future in vivo implantation studies for glucose monitoring in subdermal tissue.