the magnetic colloidal particles comprise a core and an envelope in which the core is magnetic and is coated with at least one polymer comprising functional groups x chosen from amine , hydroxyl , thiol , aldehyde , ester , anhydride , acid chloride , carbonate , carbamate , isocyanate and isothiocyanate groups , or mixtures thereof , at least one fraction of which has reacted with other functional groups of the envelope , and the envelope comprises a polymer bearing ionizable functional groups , z and z ′, which may be identical or different , chosen from amine , carboxylic acid , ester , anhydride , aldehyde , thiol , disulfide , α - halocarbonyl , sulfonic acid , maleimide , isocyanate and isothiocyanate groups , which have partially reacted with the functional groups x of the core . these magnetic colloidal particles can be used to isolate biological material .