Interferometric detection of microRNAs using a capillary optofluidic sensor

We demonstrate biomolecular detection based on the interference of optical modes in a microfiber-capillary optofluidic sensor. The optofluidic sensor is fabricated by aligning a microfiber in lateral contact with the capillary to form a modal interferometer. A biomolecular binding event at the capillary interior induces spectral shift of the interference spectrum as a result of the evanescent-field interaction with the optical modes. The sensitivity of the optofluidic sensor shows significant dependence on the dispersion factor d(Δβ)/dλ of the optical modes. With the pre-immobilization of DNA probes, the biosensor is capable of detecting single-stranded microRNA-let7a (molecular weight: 6.5 k). A log-linear response from 2 nM to 20 μM and a minimum detectable concentration of 212 pM (1.43 ng/mL) have been achieved. The sensor has advantages such as high resistance to environmental perturbations, improved portability, easy fabrication and handling, and intrinsic connection to fiber-optic measurement, and thus is promising for biomarker detection in preclinical applications.