Detection of biomolecular microarrays without fluorescent-labeling agents

We developed an optical oblique-incidence reflectivity difference (OI-RD) scanning microscope for imaging microarrays of label-free protein and DNA. Such a microscope complements currently widely used fluorescence-based optical microscopes by offering the capability to detect biochemical activities of DNA and protein molecules without the influence of fluorescent-labeling molecules. The specific activity and function of protein molecules are particularly subject to binding of small or large foreign molecules either directly through conformational change in the protein molecule itself or indirectly through properties of the attached molecules. We show that an OI-RD microscope can be used to: 1) detect binding reactions on microarrays without labeling, 2) quantitatively measure the optical properties of microarray spots, and 3) detect microarrays submerged in solution (which will enable OI-RD to monitor reaction kinetics on microarrays in reactive solutions). Furthermore, using both OI-RD and fluorescence images of an immunoglobulin-G (IgG) protein microarray, we observed that labeled and unlabeled IgG molecules deposited on the microarray substrate exhibit different wetting behaviors, and a mixture of the two tends to segregate into labeled and unlabeled regions. This illustrates potentially undesirable effects of fluorescent-labeling agents on protein properties that are of interest.