Label-free in-situ real-time DNA hybridization kinetics detection employing microfiber-assisted Mach-Zehnder interferometer

A label-free DNA biosensor based on microfiber-assisted Mach-Zehnder interferometer (MAMZI) for in-situ real-time DNA hybridization kinetics detection has been proposed and experimentally demonstrated. A microfiber of hundreds of microns in length is fabricated by tapering a segment of standard single-mode fiber (SMF) to construct the U-shaped microcavity between the lead-in and lead-out SMFs. Thanks to the mode field mismatching between the SMF and microfiber, the incident guided mode light would separate into two beams that respectively propagate in the air microcavity and the microfiber. Consequently, interference between different light modes would occur at the joint between the microfiber and the lead-out SMF. Experimental results indicate that owing to the participation of opening cavity modes in the modal interference process, the interferometric spectrum of our proposed microcavity sensor is highly sensitive to the variation of environmental refractive index (RI), especially for the RI range around 1.34 which is useful for most biological applications. The microfiber functionalization is achieved by stepwise modifying the microfiber with monolayer Poly-l-lysine (PLL) and single-stranded DNA (ssDNA) probes to produce the sensitive surface that could uniquely attach specific target ssDNAs. The fiber surface functionalization as well as DNA hybridization processes have been experimentally investigated for different target ssDNA solutions in real time. The interferometric transmission spectrum shows large wavelength shift for different biological phases, and a detection limit conservatively down to 0.0001pmol/μL has been acquired by employing the U-shaped microcavity of 176.88μm in length. Our proposed DNA biosensor possesses several advantages such as compact size, ease of fabrication, and strong response for DNA hybridization, which make it a promising candidate for potential applications in such rapidly expanding areas as medical diagnosis, cancer screenings, medicine examination and environmental engineering, etc.