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 step of introducing said sample fluid into a chamber equipped with a porous substrate, one or more probe molecules being applied to said porous substrate and said probes being able to specifically bind to said one or more target molecules. The method further comprises the step of moving said substrate and said chamber relatively to each other in order to force said sample fluid through the pores of said porous substrate and to capture the one or more target molecules with the one or more probe molecules. Also disclosed is a sensor for the analysis of a sample fluid.