In the interests of developing a simple, low-cost, sensitive optical biosensor, researchers at Sumitomo Electric Industries (Yokohama, Japan) created something that is not much different from a telecommunications-style fused, tapered fiber coupler (including the use of a telecom C-band laser), except that the peak-to-valley pitch of the sinusoidal transmission spectrum was made narrower (35 nm) than that for a standard telecom coupler. Because the evanescent field surrounding the tapered portion of the coupler produces a transmission that varies with the refractive index of the surrounding medium, biosubstances that stick to the sensor are easily detected.
After initial tests with ethanol-water solutions of 0.1, 1, and 10 wt% (showing a refractive-index sensitivity of 4 × 106), the researchers prepared the sensor by silanizing it, then coating it with biotin. The sensor was then immersed in a solution of streptavidin at a concentration of 0.5, 1, or 2 µg/ml; over time, the avidin (a protein found in egg white) contained in the solution binds to biotin. The sensor’s response was proportional to the avidin concentration (although temperature drift was observed). Contact Hidehisa Tazawa at [email protected].