Whispering Gallery Mode (WGM) Sensing

WGM resonators are high-Q resonators that trap and store light with high efficiency. Their resonant wavelength depends on the local refractive index which can be used in label-free sensing applications.

Whispering Gallery Mode Sensing using Fluorescence Imaging

The term “whispering gallery” was first introduced in 1910 by Lord Rayleigh to explain the travel of sound through the dome shaped celling of St. Paul’s Cathedral in London. Today the term is also used to describe the confinement of light inside axial symmetric dielectrics. These optical resonators provide an ideal platform for sensitive, robust and label-free biosensors due to their ability to detect changes in the effective refractive index of the surrounding medium. A primary objective of our research is to couple low background fluorescence imaging with microresonators to create a scalable biosensor with low detection limits. We aim to develop a multiplexed bioassay for early detection of ovarian cancer.

Ovarian cancer is one of the most prevalent gynecologic cancers and unfortunately the most fatal as it remains the fifth leading cause of cancer related deaths in North America. Of the females diagnosed with ovarian cancer, over 60% will die from it.1 This staggering statistic is in part due to the difficulty of early diagnosis; when diagnosed early, ovarian cancer is almost entirely treatable. Existing ovarian cancer diagnostic methods include pelvic ultrasounds and monitoring of the antigen CA-125. Current analytical techniques lack sensitivity and specificity preventing effective diagnosis of early stage ovarian cancer. New bioassays with improved detection limits and multiplexed detection capabilities would allow for accurate screening of ovarian cancer and aid in early diagnosis.

Whispering gallery mode imaging allows for a unique bioassay platform that provides label-free, sensitive detection capabilities. The confinement of light in a microsphere results from total internal reflection (TIR) occurring at the air-dielectric interface. This confinement is achieved when a specific wavelength is coupled into the microresonator. The proper coupling wavelength needed to achieve resonance is described by the following equation:

λ=2prneff/m

Where λ is the wavelength of light, r is the resonators radius, neff is the effective refractive index of the medium, and m is an integer. These optical resonances are known as whispering gallery modes (WGMs).

We have developed an imaging platform by fluorescently labeling microresonators in order to measure their WGM via fluorescence intensity. By using WGM imaging, a multiplexed bioassay can be designed to detect multiple ovarian cancer biomarkers. To perform the assay, sensing elements for a specific biomarker, such as antigen CA-125, are attached to the resonators surface. When binding of the analyte occurs, the effective refractive index of the microresonator changes resulting in a shift of the coupling wavelength. This provides a detectable sensor signal to quantitatively measure the analyte present in the biological sample. To complete the assay, a Dove prism is used to evanescently couple light into a field of microresonators. Fluorescence from the microresonators are collected and imaged using a an upright fluorescence microscope and CCD camera.

WGM Imaging Microscope

Light is evanescently coupled into a field of micro resonators using a Dove prism. As the wavelength from the tunable diode laser is scanned, the fluorescence from the labeled WGM resonators is collected and imaged on a CCD camera. When WGM resonators come into resonance, a stripe of fluorescence is observed.

Fluorescence Imaging of WGM Resonators

Representative fluorescence image of 38 μm diameter microsphere labeled with Alexa 633. As the excitation wavelength is scanned, a distinctive ring of enhanced fluorescence is observed when a WGM resonance is encountered. The fluorescence signal also paints the mode structure present in the resonator.

WGM Resonator

Fluorescence images and COMSOL simulations of a resonator when the wavelength is off and on resonance.

Calibration Plots

Three markers of ovarian cancer are detected in a multiplexed WGM imaging experiment.

Selected References

Huckabay, H. A.; Dunn, R. C., Whispering gallery mode imaging for the multiplexed detection of biomarkers. Sensor Actuat B-Chem 2011, 160 (1), 1262-1267.

Huckabay, H. A.; Wildgen, S. M.; Dunn, R. C., Label-free detection of ovarian cancer biomarkers using whispering gallery mode imaging. Biosens Bioelectron 2013, 45, 223-229.

Kim, D. C.; Armendariz, K. P.; Dunn, R. C., Integration of microsphere resonators with bioassay fluidics for whispering gallery mode imaging. Analyst 2013, 138 (11), 3189-3195.

Wildgen, S. M.; Dunn, R. C., Whispering Gallery Mode Resonators for Rapid Label-Free Biosensing in Small Volume Droplets. Biosensors-Basel 2015, 5 (1), 118-130.

Kim, D. C.; Dunn, R. C., Integrating Whispering Gallery Mode Refractive Index Sensing with Capillary Electrophoresis Separations Using Phase Sensitive Detection. Anal Chem 2016, 88 (2), 1426-1433.

Sanders, B. J.; Kim, D. C.; Dunn, R. C., Recent advances in microscale western blotting. Anal Methods-Uk 2016, 8 (39), 7002-7013.