Poly-L-cysteine (PLCys(n)) (n = 20) was immobilized onto the surface of commercially available magnetic gamma-Fe2O3 nanoparticles, and its use as a selective heavy metal chelator was demonstrated. Magnetic nanoparticles are an ideal support because they have a large surface area and can easily be retrieved from an aqueous solution. PLCys(n) functionalization was confirmed using FTIR and the quantitative Ellman's test. Metal binding capacities for As(III),Cd(II), Cu(II), Ni(II), Pb(II) and Zn(II) were determined at pH 7.0 and compared to adsorption capacities for unfunctionalized gamma-Fe2O3 nanoparticles. The effect of pH on the PLCys(n) functionalized nanoparticles was also investigated. For all of the metals examined, binding capacities (mu mol metal/g support) were more than an order of magnitude higher than those obtained for PLCys(n) on traditional supports. For As(III), Cu(II), Ni(II) and Zn(II), the binding capacities were also higher than the metal adsorption capacities of the unfunctionalized particles. Metal uptake was determined to be rapid (< 2.5 min) and metal recoveries of > 50% were obtained for all of the metals except As(III). PLCysn, which has a general metal selectivity towards soft metals acids, was chosen to demonstrate the proof of concept. Greater metal selectivity may be achievable through the use of combinatorial peptide library screening or by using peptide fragments based on known metal binding proteins. (C) 2008 Elsevier B.V. All rights reserved.