A device for detecting chemical species in liquids having one or more sensor elements is described. Each sensor element includes a test electrode coated with a perfluorinated ion-exchange polymer film incorporating a redox mediator; an immobilized enzyme layer and, over this, a semipermeable membrane. The invention discloses the method whereby the test electrode is normally kept at a zero potential and then poised at a positive potential for a few milliseconds during which time the test signal is evaluated. The invention formulates a method for calibrating each measurement sample and reference solution wells. These are symetrically arranged with respect to the sensor element so that the center of the sample solution well and the center of the reference solution well are equidistant to the sensor element. Thus the diffusion of chemical species from the sample and reference solutions occurs equally and can be correlated. The invention additionally discloses a method for selectively detecting a number of species in a single sample drop through the provision of multi-sensor elements. The technique proposed in the invention is particularly suitable for the determination of glucose and cholesterol in biological fluids.