A device for detection of an organic analyte in a liquid sample, characterized by the device comprising: an assay substrate configured with an assay substrate surface comprising a first component and a second component; the first component comprising a sensor molecule labeled with a first fluorescent marker immobilized to the assay substrate surface with a first linker, the first linker being a bi-polar linker comprising a first binding group for specific binding of the first fluorescent marker and a second binding group for specific binding of the assay substrate surface, the sensor molecule having a specific binding site for the organic analyte, the sensor molecule labeled with the first fluorescent marker in a position that has no effect on the organic analyte binding the specific binding site; the second component comprising a chemical analogue of the organic analyte, the chemical analogue labeled with a second fluorescent marker, the chemical analogue linked to the first fluorescence marker with a second linker having a length exceeding Förster radius, and the chemical analogue reversibly binding the specific binding site of the sensor molecule of the first component; the assay substrate defining an assay substrate compartment for applying the liquid sample to the assay substrate surface; a light source configured to emit a specific spectrum to excite fluorescence of the first fluorescent marker; an opto-electronic detector configured to detect fluorescence of the second fluorescent marker and generate a signal corresponding to fluorescence intensity; a controller configured to record the signal from the opto-electronic detector and determine presence of the organic analyte based on a decrease of the detected fluorescence. an assay substrate configured with an assay substrate surface comprising a first component and a second component; the first component comprising a sensor molecule labeled with a first fluorescent marker immobilized to the assay substrate surface with a first linker, the first linker being a bi-polar linker comprising a first binding group for specific binding of the first fluorescent marker and a second binding group for specific binding of the assay substrate surface, the sensor molecule having a specific binding site for the organic analyte, the sensor molecule labeled with the first fluorescent marker in a position that has no effect on the organic analyte binding the specific binding site; the second component comprising a chemical analogue of the organic analyte, the chemical analogue labeled with a second fluorescent marker, the chemical analogue linked to the first fluorescence marker with a second linker having a length exceeding Förster radius, and the chemical analogue reversibly binding the specific binding site of the sensor molecule of the first component; the assay substrate defining an assay substrate compartment for applying the liquid sample to the assay substrate surface; a light source configured to emit a specific spectrum to excite fluorescence of the first fluorescent marker; an opto-electronic detector configured to detect fluorescence of the second fluorescent marker and generate a signal corresponding to fluorescence intensity; a controller configured to record the signal from the opto-electronic detector and determine presence of the organic analyte based on a decrease of the detected fluorescence. <EOS>