The present invention relates to a method for obtaining intracellular molecules having specific affinity for a polyelectrolyte particle comprising a functionalized magnetic nanoparticle by transferring said polyelectrolyte particle into a eukaryotic cell comprising the intracellular molecule, wherein a conjugate is formed between the polyelectrolyte particle and the intracellular molecule. By recovering the conjugate from the eukaryotic cell and isolating the intracellular molecule from the conjugate, the intracellular molecule can be obtained and subsequently analyzed. The present invention further relates to the use of such a polyelectrolyte particle for identifying and/or isolating a subset of eukaryotic cells comprising the polyelectrolyte particle from a plurality of eukaryotic cells, and for isolating a complex formed between the polyelectrolyte particle and a polyanion from polyelectrolyte particles not in complex with the polyanion. Furthermore, the present invention relates to the use of an intracellular molecule for modifying the expression of a nucleic acid transfected into a eukaryotic cell by means of such a polyelectrolyte particle.
