DOI: 10.1002/adma.200501712 Surface-Functionalized Polymer Nanoparticles for Selective Sequestering of Heavy Metals** By Craig A. Bell, Suzanne V. Smith, Michael R. Whittaker, Andrew K. Whittaker, Lawrence R. Gahan, and Michael J. Monteiro* In this communication, we demonstrate the synthesis of sur- face-functionalized polymer nanoparticles for the selective se- questering 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-copoly- mer formation, and particle morphology, but also aid in the selective binding to Hg II over Co II at low concentrations (be- low parts per million, ppm). Mercury, lead, and cadmium are considered harmful to hu- mans, 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 selec- tive 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 dis- persed in water with controlled molecular weight and particle morphology. [1–4] 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 pro- duction 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 concentra- tions, metal radioisotopes have been employed. The radioiso- topes employed in this study were Hg-197/Hg-203 and Co-57, the choice dictated in part by their different ionic radii (Hg II : 102 pm; Co II : 74 pm; Co III : 61 pm), their propensity for coor- dination to these types of macrobicyclic ligand, [8–11] their hard (Co II /Co III ) and soft (Hg II ), [12] labile (Hg II , Co II ) and inert (Co III ) characteristics. The ab-initio emulsion polymerization of styrene in the presence of the xanthate was carried out as reported pre- viously. [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 la- tex 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 polymer- ization 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 calibra- tion curve. The increase in M n and decrease in PDI support block formation, [13,14] in which the decrease in PDI is through the random coupling process. The increase in M n of 869 (∼ 4 AAEMA units per polymer chain) is consistent with the full consumption of 2 wt.-% of AAEMA in the polymeriza- tion, and this suggests that AAEMA does form blocks but does not account for its reactivity in the RAFT process. Based on the average diameter (73 nm), the number of AAEMA units per particle was calculated to be close to 10 000, with ap- proximately 2500 SC(OEt)S groups per particle (i.e., four times an excess of AAEMA units to RAFT end groups). The functional surface of the polystyrene particles, coated COMMUNICATIONS 582 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2006, 18, 582–586 – [*] Prof. M. J. Monteiro, C. A. Bell, Dr. M. R. Whittaker, Prof. A. K. Whittaker Australian Institute of Bioengineering and Nanotechnology The University of Queensland Brisbane QLD 4072 (Australia) E-mail: m.monteiro@uq.edu.au Prof. M. J. Monteiro, C. A. Bell, Dr. L. R. Gahan School of Molecular and Microbial Sciences The University of Queensland Brisbane QLD 4072 (Australia) Dr. S. V. Smith Institute of Materials and Engineering Science Australian Nuclear Science and Technology Organization (ANSTO) Private Mail Bag No. 1, Menai, NSW 2234 (Australia) Prof. A. K. Whittaker Centre for Magnetic Resonance, University of Queensland Brisbane QLD 4072 (Australia) [**] The authors thank the Australian Institute of Nuclear Science and Engineering (AIN Award: AINGRA04054P) for supporting the pro- ject and the Australian Research Council (ARC DP0453105). Sup- porting Information is available online from Wiley InterScience or from the author.