chloroform, benzene and toluene without any indication of sintering of the particles (Figure 3.7B) [73]. In the UV–vis spectra of hydrophobized CdS in the different solvents, the onset of absorption occurs at 475 nm and corresponds to a
particle size of ca. 6 nm. This size is in good agreement with that determined from
TEM studies of CdS nanoparticles transferred into petroleum ether (Figure 3.8). A
fairly regular close-packed assembly of the CdS quantum dots can be observed in
the TEM micrograph. A number of triangular CdS nanoparticles can also be seen
in the figure (identified by arrows).
3.4.2
Transfer of Organically Soluble Gold Nanoparticles to Water
While many different procedures have been developed for the transfer of aqueous gold nanoparticles into organic solvents, the number of reports on the phase
transfer of gold nanoparticles in the reverse direction is much smaller. The key step
is to replace the hydrophobic groups bound to the gold nanoparticle surface with
polar functional groups, thus rendering the nanoparticles water-soluble. Rotello
and co-workers used a place exchange mechanism to functionalize alkanethiolcapped organically soluble gold nanoparticles with carboxylic acid groups [74].
The process is illustrated in Figure 3.9 which depicts replacement of octanethiol molecules bound to the surface of 2 nm diameter gold nanoparticles by 11thioundecanoic acid. Under the experimental conditions adopted by Rotello et al.,
Fig. 3.8. TEM picture of CdS quantum dots capped with
octadecanethiol molecules and phase transferred into
petroleum ether. The arrows in the figure identify some
triangular shaped CdS nanoparticles.
3.4 Moving Gold Nanoparticles Around 43