The invention relates to a solution comprising: a heteropolyacid and a lanthanide salt in a solvent consisting of a water/organic solvent mixture or an organic solvent. Optionally, the solution can also comprise an organic or inorganic buffer with a pH comprised in the interval of 4-6 or a strong base or acid. The object of the present invention also includes a use of said solution and any products of the reaction between the components thereof as a contrast medium for biological samples and a method for preparing biological samples that uses said solution in at least one step, for analysis by either correlative microscopy (CLEM) and electron microscopy (EM).
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