Whispering Gallery Mode Microresonator Sensor Integrated in Exposed Core Fiber for Label-Free DNA Hybridization Detection

A novel optical fiber sensor using exposed-core fiber (ECF) to excite whispering gallery modes (WGMs) within a high refractive index barium titanate glass microsphere is proposed for achieving label-free real-time specific DNA detection. The microsphere is encapsulated into the exposed channel of the ECF allowing for physical stability while excited WGMs via the evanescent field. The structure enables the sensing area to be exposed to a large amount of aqueous solution and better contact with the sample to be analyzed. Using a transmission measurement, a high Q-factor in the order of 10 ⁴ is obtained in an aqueous environment. To the best of our knowledge, this is the best Q-factor level obtained by the in fiber WGM sensor in an aqueous environment. Specific sequences of probe DNA are then bound to the surface of the microsphere so that specific detection of complementary DNA can be realized by tracking the wavelength shift the WGM resonance. Experimental results show a red shift of 5.6 pm for complementary DNA hybridization detection and less than 1 pm fluctuation for non-complementary DNA detection. The portable integrated sensor with miniature size has great potential for aqueous environmental sensing applications in the fields of diseases diagnostics, chemical analysis, and environmental protection.