Surface‐Functionalized Polymer Nanoparticles for Selective Sequestering of Heavy Metals

Abstract

In this communication, we demonstrate the synthesis of surface-functionalized polymer nanoparticles for the selective sequestering of heavy metals. This is the first example where the reversible addition-fragmentation chain transfer (RAFT; xanthate, 1) end groups on the surface of the particles can not only be utilized to control the molecular weight, block-copolymer formation, and particle morphology, but also aid in the selective binding to Hg II over Co II at low concentrations (below parts per million, ppm). Mercury, lead, and cadmium are considered harmful to humans, as they inhibit enzymatic processes because of their strong interactions with cystine residues to form secondary macromolecular structures. We have sought a methodology for the preparation of new nanoscale materials for the selective removal of heavy-metal ions from water at below ppm levels. The synthesis was carried out using RAFT emulsion polymerization to create novel polymer nanoparticles dispersed in water with controlled molecular weight and particle morphology. [1][2][3][4] The advantage of the approach is that the use of xanthates (1) (RAFT agents shown to be located at the polymer/water interface) [2,5] as the controlling agent permits the polymer to grow from the interface and enables the production of core/shell-type morphologies and thus controlled surface functionality (Scheme 1).In this work, we synthesized a block-copolymer particle consisting of a polystyrene (PSTY) core and a thin shell of poly((2-acetoacetoxy)ethyl methacrylate) (polyAAEMA). The side chain of polyAAEMA has ketone functionality [2] that can be coupled with metal-binding ligands to produce a surface-functionalized nanoparticle. The choice of ligand is dictated in part by the capacity of the ligand to be selective for a particular type of metal ion. We have chosen to employ a macrobicyclic ligand (NH 2 capten, 2, Scheme 1) containing both secondary amine (r donor) and thioether (p acceptor) metal-coordination sites as well as a suitably positioned amine functionality for attachment to the nanoparticle (Scheme 1). [7] In order to evaluate the extent of complexation of metal ions with the functionalized nanoparticles at such low concentrations, metal radioisotopes have been employed. The radioisotopes employed in this study were The ab-initio emulsion polymerization of styrene in the presence of the xanthate was carried out as reported previously. [2,5] The number-average molecular weight, M n , was equal to 41 917, and a polydispersity index (PDI) of 2.22 was measured by size exclusion chromatography. The resulting latex was dialyzed for three days, and the particle size of 73 nm was measured by dynamic light scattering. These results are consistent with previous findings. [1,3] The polystyrene latex was then used in a second-stage (seeded) emulsion polymerization to make nanoparticles, in which the second block (2 wt.-%) consisted of AAEMA. The M n increased to 42 786, and the PDI decreased to 2.17, based on a polystyrene calibration curve. The increase in M n a...

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