Introduction In the past ten years, numerous reviews have been devoted to the application of magnetic beads [1±9] in the biomedical ®eld. These composite particles can be used for therapeutic or analytical purposes. In the ®rst case, magnetic particle carriers permit either the guiding and release of a drug in a speci®c site of the body [6, 10] or the extraction of tumour cells from the organism and their curing in vitro [8, 9]. In both cases, toxicity and the secondary eects of a strong chemical in vivo treatment are avoided. Analytical applications often require a separation step. Magnetic particles were found suitable as a solid phases in immunoas- says, since after the capture step the target molecules can easily be separated upon applying a magnetic ®eld [7, 11]. Magnetic particles can be prepared according to three main dierent strategies. 1. First, inorganic and polymer materials are synthesised separately. Then, polymer chains can be grafted or adsorbed onto an inorganic core, generally composed of iron oxide [12, 13]. Otherwise, when mixed with magnetic material [14, 15], polymer chains can eventually be cross-linked [16, 17] to form large and polydisperse polymer beads containing magnetic material. 2. The second procedure consists of preparing iron oxides inside the polymer particles and was pioneered by Ugelstad and coworkers [18, 19] for preparing supermi- Colloid Polym Sci 277:846±855 (1999) Ó Springer-Verlag 1999 ORIGINAL CONTRIBUTION Received: 28 December 1998 Accepted in revised form: 15 April 1999 F. Sauzedde á A. ElaõÈssari (&) á C. Pichot Ecole Normale Superieure de Lyon Unite mixte CNRS-bioMeÂrieux ENS, 46 alleÂe d'Italie F-69364 Lyon, France e-mail: hamid.elaissari@ens-bma.cnrs.fr Tel.: +33-4-7272-8364 Fax: +33-4-7272-8533 Abstract With a view to preparing monosized hydrophilic functional magnetic latex particles based on a two-step strategy using anionic iron oxide and cationic polymer latexes, the adsorption step was systemati- cally investigated for a better control of the subsequent encapsulation step. The iron oxide nanoparticles were ®rst obtained according to the classical precipitation method of ferric and ferrous chloride salt using a concentrated sodium hydroxide solution, whereas the polystyrene (PS), P(S/N-isopropylacrylamide (NIPAM)) core±shell and PNIPAM latexes were produced via emulsion and precipitation polymerizations, respectively. The polymer and inor- ganic colloids were then character- ised. The adsorption of iron oxide nanoparticles onto the three types of polymer latexes via electrostatic in- teraction was studied as a function of iron oxide particle concentration, charge density and the cross-linking density of the hydrophilic layer. The maximum amounts of magnetic nanoparticles adsorbed onto the various latexes were found to increase in the following order: PS < P(S/NIPAM) < P(NIPAM). This signi®cant dierence is discus- sed by taking into account the charge distribution in the hydrogel layer and diusion phenomena inside the cross-linked hydrophilic shell. Key words Cationic latexes á Hydrophilic particles á Iron oxide á Adsorption á Electrostatic interactions F. Sauzedde A. ElaõÈssari C. Pichot Hydrophilic magnetic polymer latexes. 1. Adsorption of magnetic iron oxide nanoparticles onto various cationic latexes