(Figure 3.12B). A comparison of the two TEM pictures shows that little sintering/
aggregation of the gold nanoparticles had occurred after one month of storage.
Gittins and Caruso have speculated on the mode of binding of the DMAP molecules on the gold nanoparticle surface during the phase-transfer process [76].
Based on a series of control experiments, they have proposed a mechanism for the
phase transfer as illustrated in Figure 3.13.
Addition of an aqueous DMAP solution to the nanoparticle dispersion in toluene
leads to partitioning of the DMAP molecules across the toluene/water boundary
as shown in Figure 3.13. The DMAP molecules replace the tetraalkyl ammonium
salts and form a labile donor–acceptor complex with the gold atoms on the surface
of the nanoparticles through the endocyclic nitrogen atoms. The surface charge
on the gold nanoparticles that stabilizes the particles in water arises due to partial
Fig. 3.11. UV–vis spectra of a diluted solution of gold
nanoparticles in toluene (solid line) and the same sample
transferred into an equal volume of 0.1 M DMAP solution at
pH 10.5 (dashed line). (Reprinted with permission from [76],
8 2001, WILEY-VCH Verlag GmbH).
3 Moving Nanoparticles Around: Phase-Transfer Processes in Nanomaterials Synthesis
46