Whispering gallery mode imaging for the multiplexed detection of biomarkers

An approach is presented for combining sensitive fluorescence imaging with whispering gallery mode (WGM) resonators to develop a label-free, multiplexed approach for biosensing. The evanescent field generated by a total internal reflection microscope is used to couple light into a field of microsphere resonators dispersed on a glass coverslip. The microspheres are labeled with the fluorescent dye Alexa 633 which acts as a reporter of the WGM resonance for each microsphere. As the excitation wavelength is scanned, a distinct ring of increased fluorescence intensity is observed when the resonance condition is satisfied. For sensing applications, shifts in the WGM resonance in response to changes in the effective refractive index can be used to track analyte binding. Calibration plots of WGM shifts with refractive index indicate the dye functionalized microspheres have a sensitivity of ∼50 nm/RIU. To illustrate the biosensing capabilities, two markers of ovarian cancer were detected. Antibodies specific for CA-125 and TNF-α were linked to 38 μm and 53 μm spheres, respectively, and used to detect their biomarkers in solution using fluorescence imaging of the WGM resonance shifts. Both assays resulted in linear response curves and the detection limits for the important ovarian cancer marker CA-125 is more than threefold better than commercial ELISA kits. These results illustrate the feasibility of combining WGM resonators with fluorescence imaging for large scale multiplexing of biomarker detection.