Glucose Concentration Measurements by Cavity Enhanced Sensing

Our refractive index (RI) cavity sensor consists of a double chamber Fabry-Perot etalon that measures the differential RI between a reference chamber and a measurement chamber. The etalon chambers have wavelength dependent transmission maxima which are linearly dependent on the RI inside them. In our device, an RI difference of Δn = 1.5*10^-6 changes the spectral position of a transmission maximum by 1 pm. We sweep the wavelength of a single-mode VCSEL linearly in time (1kHz, nominal central wavelength is 850 nm) and detect the maximum transmission times of the etalon. Thereby we achieve an RI accuracy of Δn = ±3.5*10^-6 in the Δn = 0…1.75*10^-4 and a 2% accuracy in the Δn = 1.75*10^-4…9.8*10^-4 RI-difference detection range. The accuracy is primarily limited by our reference measurement. The temperature stability of the measurement was determined by varying the temperature between 32 C and 42 C. The standard deviation of a RI measurement under these conditions is typically Δn = ±1.4*10^-6. We are working towards a continuous glucose monitor for subcutaneous long term implantation. The size of the tethered prototype sensor that includes a laser and detectors is suitable for implantation (12mm x 2.5mm x 8mm).

The RI difference between the etalon chambers can be made specific to glucose by competitively and reversibly releasing dextran from immobilized Concanavalin A (Con A). Con A and dextran bound to it, is positioned outside the optical detection path. Dextran that has been released from the receptor by glucose, diffuses into the optical path and changes the RI of the detection chamber. At the same time other factors, for example buffer variations and temperature changes, affect the RI in measurement and detection chamber equally. Therefore, we have created a label-free, glucose specific concentration measurement on the basis of our cavity sensor platform. We will present our current results and calculations about the release of different variations of Con A, response times of the measurement, sensitivity and specificity.