A Hot Dip Before Swimming

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Science  28 Sep 2007:
Vol. 317, Issue 5846, pp. 1834
DOI: 10.1126/science.317.5846.1834c

Most solution routes for nanoparticle synthesis proceed in nonpolar solvents and achieve size selectivity in part by capping the surfaces with hydrophobic groups. However, after this preparation, many applications require dispersing the nanoparticles in aqueous solvents. Ligand exchange reactions can be used to introduce capping agents that bear hydrophilic groups on their ends, but these reactions, which often run near room temperature, tend to be incomplete, and can lead to aggregation if ligand desorption dominates and exposes the underlying surfaces. Zhang et al. have developed a robust method for exchanging hydrophobic capping groups with short-chain polyelectrolytes such as poly(acrylic acid). The reactions run in polar solvent such as diethylene glycol with a high boiling point (in this case, ~245°C). The multivalent polyelectrolytes help displace the hydrophobic ligands while minimizing surface exposure. The properties of several nanoparticles—magnetism for iron oxides, photocatalysis for titanium dioxide, and photoluminescence for cadmium selenide—were maintained or even improved after such processing. — PDS

Nano Lett. 7, 10.1021/nl071928t (2007).

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