Area-scaling of interferometric and fluorescent detection of protein on antibody microarrays

The sensitivity limits for an in-line interferometric (IL)/fluorescent (FL) dual-channel BioCD are established as a function of spatial averaging. Forward-phase and sandwich assays at 10 ng/ml were performed on large-scale antibody microarrays (6800-spot) and detected using in-line interferometry and fluorescence channels. The interferometric channel has an extrapolated label-free limit-of-detection (LOD) of 250 pg/ml in a forward-phase assay for which the fluorescent channel is inapplicable. In the sandwich assay, the extrapolated limit-of-detection is 70 pg/ml for the interferometric channel, and for the fluorescent channel it is 30 pg/ml. Intrinsic scale-free sensitivities for the detection channels are defined assuming spatially uncorrelated fluctuations with sensitivities of S′ = 13 pg/mm for interferometric detection and 7 pg/mm for sandwich fluorescent detection. The intrinsic sensitivities become weakly scale dependent in the presence of fractal spatial correlations among the antibody spots.