Porous membrane sensor element

A sensor element (101) for detecting an analyte in a fluid sample by optical probing comprises a translucent film (110) having a front side (111) defining a sensor surface for contacting the fluid sample and a back side (112) facing away from the front side (111). The sensor element (101) has a reflective layer (114) on the front side (111) of the translucent film (110). An optical input port (120) is connected to the back side (112) of the translucent film (110) for feeding probe light to the translucent film (110) through the back side (112). An optical output port (130) is connected to the back side (120) of the translucent film (110) for collecting an optical response from the translucent film (110) through the back side (112). The sensor element further comprises an optical contracting element (140) having a screen element (141) arranged between the input port (120) and the output port (130) so as to optically separate the output port (130) from the input port (140). The shrink element (140) further comprises a reflective element (142) arranged at the back side (112) of the translucent film (110). The reflective element (142) faces the reflective layer (114) at a vertical distance D from the reflective layer (114) to define a detection zone (116). Light propagating from the input port (120) to the output port (130) is confined to pass through the detection region (116), wherein at least the detection region (116) comprises a hole (118) extending from a corresponding opening at the sensor surface into the translucent film (110). Another aspect relates to a sensor assembly (101) comprising a sensor element (101) integrated with a sample chamber (102).