A device for identifying analytes in a biological sample, including a substrate having a surface lying substantially in a first plane, a plurality of targets, each having a wall lying substantially in a second plane offset from the first plane, and a receptor coating applied to one of the surface and the target walls for binding analytes present in the biological sample when the biological sample is applied to the substrate. A laser beam is sequentially directed onto each of the plurality of target, the laser being positioned relative to the substrate such that when the beam is directed onto a target, a first half of the beam is reflected back to the laser from the wall of the target and a second half of the beam is reflected back to the laser from the surface of the substrate adjacent the target. The laser combines the first and second reflected halves to produce a diffraction signal that has a first value when an analyte is not bound to the receptor coating associated with a target and a second value when an analyte is bound to the receptor coating associated with the target, thereby indicating the presence of the analyte.