Polyarginine has been successfully bound onto iron oxide nanoparticles via carbodiimide activation as a highly positively charged magnetic nano-adsorbent for protein separation. They were nearly superparamagnetic with a mean diameter of 10.3 +/- 2.36 nm, and the binding process did not change the spinel structure of iron oxide. From the analyses of FTIR spectra and zeta potential, the binding of polyarginine on the surface of iron oxide was confirmed and the resultant polyarginine-coated magnetic nanoparticles (PA-MNPs) were positively charged even up to pH 11. By thermogravimetric analysis, the typical product contained about 7.1 wt% of polyarginine. From the adsorption of the proteins with different pl values, the resultant PA-MNPs were found to be quite efficient for the fast and effective adsorption of acid proteins. For the typical acid protein, bovine serum albumin (BSA), the adsorption equilibrium was achieved within few minutes and obeyed the Langmuir isotherm equation. At pH 7 and 25 C. the maximum adsorption capacity and equilibrium constant were 67.6 mg/g and 0.0623 L/mg, respectively. Moreover, by SDS-polyacrylamide gel electrophoresis, the capability of PA-MNPs for the separation of BSA-lysozyme mixture and egg white was further confirmed. Accordingly, the PA-MNPs were useful for the fast and effective magnetic recovery of acid proteins. (C) 2012 Elsevier Ltd. All rights reserved.