Exemplary embodiments provide microfluidic devices and methods for their use. The microfluidic device can include an array of M×N reaction sites formed by intersecting a first and second plurality of fluid channels of a flow layer. The flow layer can have a matrix design and/or a blind channel design to analyze a large number of samples under a limited number of conditions. The microfluidic device can also include a control layer including a valve system for regulating solution flow through fluid channels. In addition, by aligning the control layer with the fluid channels, the detection of the microfluidic devices, e.g., optical signal collection, can be improved by piping lights to/from the reaction sites. In an exemplary embodiment, guard channels can be included in the microfluidic device for thermal cycling and/or reducing evaporation from the reaction sites.