The invention relates to the use of diazonium salts for the formation of thick layers on at least one surface of a substrate. The diazonium salts used in the invention have the following formula (I): wherein R is a group that can be electrochemically reduced in a reversible manner to a cathodic potential between 0 and −1.5 V, E is a spacer chain consisting of at least one aromatic cyclic group having between 5 and 6 links, optionally containing at least one heteroatom preferably selected from N, S, O or P and optionally substituted by at least one group selected from the C1-C5 alkyls and the halogens, r is 1 or 2, m is a whole number between 1 and 5 inclusive, p is a whole number between 1 and 5, and B is a counter-ion. The invention can be especially applied to the field of gas sensors.