The invention relates to a method for identifying organic or biochemical substances and for determining their concentration in a fluid medium using a nanogap sensor that comprises at least two electrodes. The invention is characterized in that: a nanogap sensor) with electrodes of different materials is used, a respective probe molecule is bonded to each surface of the two electrodes of the sensor and the free remainder of the probe molecules have at least one bondable group with specificity for bonding to a sought substance or to an analyte molecule in the fluid medium. The analyte molecule has at least two binding sites and passes selectively out of the fluid medium in which it is contained, binds to the free ends of the probe molecules, forming a bridge with the probe molecule, and modifies the resulting impedance between the electrodes. The concentration of the substance in the fluid medium can be determined as a result of the modification.