Synthesis of pH-responsive shell cross-linked micelles and their use as nanoreactors for the preparation of gold nanoparticles

Poly [(ethylene oxide)-block-glycerol monomethacrylate-block-2-(diethylamino)ethyl methacrylatel (PEO-GMA-DEA) and poly[(ethylene oxide)-block-2-hydroxyethyl methacrylate-block-2-(diethylamino)ethyl methacrylate] (PEO-HEMA-DEA) triblock copolymers were synthesized directly, without recourse to protecting group chemistry, via atom transfer radical polymerization by successive polymerization of GMA (or HEMA) and DEA monomers using a PEO-based macroinitiator. These triblock copolymers dissolved molecularly in aqueous solution at low pH; on addition of NaOH, micellization occurred above pH 7-8 to form three-layer "onionlike"micelles comprising DEA cores, GMA (or HEMA) inner shells, and PEO outer coronas. Selective cross-linking of the GMA (or HEMA) inner shell was successfully achieved by adding divinyl sulfone [DVS] to the alkaline micellar solution at room temperature. Unexpectedly, the PEO-HEMA-DEA triblock proved to be much less reactive toward DVS than the two PEO-GMA-DEA triblocks, and an excess of DVS was required to prepare shell cross-linked (SCL) micelles using the former triblock. The resulting SCL micelles exhibited reversible swelling behavior on varying the solution pH. At low pH, the DEA cores became protonated and hence hydrophilic. The effect of varying the block composition and the [DVS]/[GMA] molar ratio on the structural stability and pH-dependent (de)swelling of the SCL micelles was studied. Longer DEA blocks and lower [DVS]/[GMA] molar ratios led to increased swellability, as expected. Finally, these SCL micelles can serve as nanoreactors for the synthesis of gold nanoparticles. The basic DEA residues in the cores of the SCL micelles were first protonated using HAuCl4, and then the electrostatically bound AuCl4- anions were reduced to nanoparticles of elemental gold using NaBH4 at neutral pH. The gold-loaded SCL micelles exhibited excellent long-term colloid stability.