The invention concerns a method for treating a fluid. According to the invention, the fluid is contacted with a substrate at the surface of which there is a network of organic molecules, hereafter referred to as “network molecules”. There is a central core and at least one lateral arm, the molecules being adsorbed at the surface of the substrate. The invention also concerns a two-dimensional molecular sieve formed by the substrate at the surface of which a network of network molecules is adsorbed. The invention further concerns a module for treating a fluid including a circulating device for the fluid to be treated, the device containing one or more two-dimensional molecule sieves.