Method for grafting, in a covalent manner, onto the surface of a solid substrate, a coloured indicator having at least one aromatic primary amine function, comprising the following steps: a) preparing, from said coloured indicator having at least one aromatic primary amine function, the corresponding diazonium salt; b) placing the solid substrate in contact with a solution containing the diazonium salt prepared in step (a) and subjecting said solution to non-electrochemical conditions for an optimal grafting time whereby organic chains in which the units are derivatives of said diazonium salt are grafted, in a covalent manner, onto the surface of said solid substrate said optimal grafting time having been determined, before said step (b), for a given coloured indicator by a method involving: α) carrying out a plurality of experiments, each experiment being carried out for a period of time T, regularly subdivided into n sub-periods, n being a whole number greater than or equal to 1 that is different for each experiment, each sub-period corresponding to a step of free-radical chemical grafting of the coloured indicator onto the surface of a solid substrate and each grafting step being followed by at least one washing step, β) after the period of time T, determining the quantity of coloured indicator grafted for each experiment, normalising the quantity determined for each experiment with respect to the quantity determined for the experiment in which n is equal to 1, and determining the experiment E for which the normalised quantity is maximum whereby the optimal period of time is T/n e with n e corresponding to the value of n during the experiment E; c) washing the solid substrate thus grafted; d) repeating steps (b) and (c) or possibly steps (a) to (c) at least once. a) preparing, from said coloured indicator having at least one aromatic primary amine function, the corresponding diazonium salt; b) placing the solid substrate in contact with a solution containing the diazonium salt prepared in step (a) and subjecting said solution to non-electrochemical conditions for an optimal grafting time whereby organic chains in which the units are derivatives of said diazonium salt are grafted, in a covalent manner, onto the surface of said solid substrate said optimal grafting time having been determined, before said step (b), for a given coloured indicator by a method involving: α) carrying out a plurality of experiments, each experiment being carried out for a period of time T, regularly subdivided into n sub-periods, n being a whole number greater than or equal to 1 that is different for each experiment, each sub-period corresponding to a step of free-radical chemical grafting of the coloured indicator onto the surface of a solid substrate and each grafting step being followed by at least one washing step, β) after the period of time T, determining the quantity of coloured indicator grafted for each experiment, normalising the quantity determined for each experiment with respect to the quantity determined for the experiment in which n is equal to 1, and determining the experiment E for which the normalised quantity is maximum whereby the optimal period of time is T/n e with n e corresponding to the value of n during the experiment E; α) carrying out a plurality of experiments, each experiment being carried out for a period of time T, regularly subdivided into n sub-periods, n being a whole number greater than or equal to 1 that is different for each experiment, each sub-period corresponding to a step of free-radical chemical grafting of the coloured indicator onto the surface of a solid substrate and each grafting step being followed by at least one washing step, β) after the period of time T, determining the quantity of coloured indicator grafted for each experiment, normalising the quantity determined for each experiment with respect to the quantity determined for the experiment in which n is equal to 1, and determining the experiment E for which the normalised quantity is maximum whereby the optimal period of time is T/n e with n e corresponding to the value of n during the experiment E; c) washing the solid substrate thus grafted; d) repeating steps (b) and (c) or possibly steps (a) to (c) at least once.