Ultralow detection limit label-free biosensor based on a microbottle resonator for the detection of human chorionic gonadotropin

Due to their high quality factor (Q-factor), strong evanescent field, and small mode volume, whispering gallery mode (WGM) resonators are ideally suited to applications in the biosensing area. In this work, a label-free bioimmunoassay method based on a WGM microbottle resonator (MBR) is proposed and demonstrated experimentally for the detection of human chorionic gonadotropin (hCG). The MBR was prepared using a modified electric arc discharge method, and a tapered optical fiber was used for coupling into and out of the MBR. The measured Q-factor is close to 105. When the MBR is functionalized with anti-β-hCG antibody, a stable layer is formed on the surface of the WGM MBR to capture hCG, with a resultant measurable shift in the resonant spectral dips. The spectral characteristics of the WGM resonance were studied to evaluate the dynamic process of specific interaction between antigen and antibody. The experimental results show that the developed sensor can directly detect hCG in solution with a concentration as low as 5 × 10-5 mIU/mL. The limit of detection (LoD) of the sensor for hCG is estimated as 2.14 × 10-5 mIU/mL, which is among the most sensitive label-free optical biosensors reported so far. In addition, specificity tests were also carried out using pure and mixed solutions of human-IgG, sheep-IgG, rabbit-IgG, pig-IgG, and hCG, and the result shows that the sensor has excellent specificity for HCG. Tests were also undertaken on the recovery of urine samples, with a recovery rate in the range 108.75% and 127.7%, with the urine matrix having no significant effect on the detection of hCG. Beyond use as an hCG sensor, the sensor has the potential to be adapted for a wide range of bio and chemical sensing applications, such as food safety, medical diagnostics, and environmental monitoring.