A sensor (100) for detecting a state of a fluid within a microfluidic plate (120) is disclosed. The sensor (100) comprises at least a first planar electrode (102) and a second planar electrode (104) arranged parallel to one another so as to form an electric capacitor (106). The first planar electrode (102) and the second planar electrode (104) are configured to generate an electric field (114) within the fluid, wherein the sensor (100) is configured to detect the state of the fluid by means of a capacitance of the electric capacitor (106). Further, a system (116) for controlling a preparation of a sample including a fluid is disclosed. Furthermore, a method for detecting a state of a fluid within a microfluidic plate (120) is disclosed.