Interferometric-type optical biosensor based on exposed core microstructured optical fiber

Abstract

This work presents a novel biosensor using the multimode interference effect in an exposed core microstructured optical fiber (ECF). In this work biotin molecules are immobilized onto the ECF core surface to serve as the capturing probe for streptavidin, the target molecules. Since each distinct guided mode in the ECF interacts with the surrounding medium differently, the interference between any two specific modes will experience a fringe shift (or phase change) upon a change in the refractive index (RI) of the surrounding medium, or a localized RI change on the surface of the ECF core as a result of a biological binding event. In our experiment, the interferometric sensing platform was realized by splicing a section of ECF with lead-in and lead-out single mode fibers (SMFs). An interference pattern is obtained in the transmission spectrum as the result of multiple excited modes (excited and re-collected at the lead-in and lead-out splicing points) propagating in the ECF with different propagation constants. The interference pattern is non-uniform, indicating that there are more than two modes involved. Fast Fourier transform (FFT) is used to separate individual interference patterns that contribute to this complex spectrum and monitor their phase changes upon RI variation of the surrounding medium. In this way multiple RI sensitivities can be realized because each spatial frequency possesses a distinct sensitivity with respect to the surrounding RI. The operation of this device was validated by measuring the phase changes that occur when the sensing platform was subjected to solutions of different RIs or functionalized with different molecules. A biosensor was demonstrated based on this novel platform using biotin as the capturing probe to specifically detect streptavidin with low non-specific adsorption. The proposed platform is reliable, cost-effective, and offers a potential label-free biosensing alternative to the widely used surface plasmon resonance (SPR) technique.