A method for forming a sensor is provided, together with a sensor formed according to the method. Photoresist material is deposited on a surface of the sensor, and is then patterned and etched to form an array of microlens structures. The structures are spaced close together in a predetermined pattern so that when a reflow process is performed, the structures melt and coalesce to form a barrier. The barrier defines a region for constraining or channeling the flow of reagent and analyte samples used in bio-optical sensors.