A method for determining the concentration of an
analyte in a sample liquid, in which a) by means of a
radiation source (3, 10, 17, 25, 31), excitation
radiation is directed onto a carrier unit (5, 13, 20, 26, 33) which is in contact with the sample liquid and which has immobilized molecules of a sensor dye that is sensitive to the
analyte, b) a portion of
luminescence radiation of the sensor dye, this radiation is induced by the excitation radiation, is applied to a radiation
detector (4, 14, 21, 28, 34,35) in order to generate an output
signal therefrom, and c) an evaluation routine ascertains the
analyte concentration from the
detector output
signal, wherein a dependence of a property of the
luminescence radiation on the concentration of the analyte in the sample liquid is used, characterized in that d) dependence of the examined property of the
luminescence radiation on an
exchange interaction between the individual molecules of the sensor dye is used to ascertain analyte concentration, wherein the
exchange interaction takes place when the molecules of the sensor dye interact with the particles of the analyte. An
optode designed to carry out the method is also proposed.