Polyethylenimine(PEI) magnetic particles are superparamagnetic beads covalently functionalized with PEI. PEI is a kind of branched polymer with a high-density amine group. The ratio of primary amine to secondary amine to tertiary amine is 1:2:1. In each PEI molecule, one nitrogen atom in every two carbon atoms is protonated. Due to the different pKa values of primary, secondary, and tertiary amino groups, PEI could capture protons under different pH conditions, which is called the "proton sponge" mechanism.
