We perform simultaneous interferometric and fluorescent detection of molecular protein layers on a BioCD. The 488 nm excitation wavelength of fluorescein also provides the interferometric detection channel that operates in a common-path in-line configuration in the condition of phase quadrature set by a thermal oxide on silicon. The simultaneous acquisition of both channels enables a direct correlation between bound mass and fluorescent surface density, which we compare in forward- and reverse-phase immunoassays. Scaling mass sensitivities for immunoassays measured in the interferometric and fluorescent channels are 15 pg/mm and 1.5 pg/mm, respectively, when applied to gel-printed periodic antibody patterns detected in the frequency domain from the spinning disc. These sensitivities are limited by the inhomogeneities of the print. While fluorescence is subject to bleaching, the interferometry signal is robust under long-term laser illumination.
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