Label-free, needle-type biosensor for continuous glucose monitoring based on competitive binding

With the goal of developing a method for the continuous monitoring of blood glucose, an implantable sensor was developed by placing an optical fiber probe within the internal hollow space of a syringe needle. A glucose binder, concanavalin A (Con A), was immobilized on the probe tip and a protein (e.g., bovine serum albumin) chemically coupled with a sugar ligand (e.g., mannose) was loaded as a solution inside of the needle, which were then closed using a semi-permeable membrane. Upon immersion in the glucose sample, small molecules were able to freely pass through the membrane and compete with the ligand conjugate for Con A binding. This changed the molecular layer thickness on the probe surfaces depending on the glucose concentration, which shifted the wavelength of the guided light along the fiber. Such interference in the wavelength pattern was measured using a commercial sensor system, Octet, without employing a label. Using this analytical approach, two major steps controlling the performance of glucose detection were overcome: permeation of glucose (optimum with 50 nm-porous polycarbonate membrane under the experimental conditioned used) and molecular diffusion of the ligand conjugate within the sensor compartment (19 gauge-needle, offering minimal demensions for the probe). Under optimal conditions, the sensor was able to monitor glucose fluctuations, even in serum medium, with a response time of less than 15 min in a range 10-500 mg/dL. This, however, could be further shortened down to about 5 min in principle by miniaturizing the sensor dimensions.