Disclosed is a method of preparing a sensor for the analysis of a sample fluid, said sample fluid containing one or more target molecules. The method comprises the steps of applying a non-activated porous organic polymer membrane with probes in the form of an array of probe locations, said probes being able to specifically bind to said one or more target molecules. Furthermore, the method comprises the steps of blocking areas remaining free of probes of said porous organic polymer membrane with one or more blocking substances and forcing the sample fluid repeatedly in one or two directions through the pores of said porous organic polymer membrane. Also disclosed is a sensor for the analysis of a sample fluid.