Recognizing that the strong interaction of alkanethiol molecules with gold
nanoparticles may be used to hydrophobize gold nanoparticles at the liquid–liquid
interface, Rao and co-workers have demonstrated the acid-facilitated phase transfer
of aqueous gold and indeed, platinum and silver nanoparticles into organic solvents such as toluene [65, 66]. In a typical experiment, the authors took a mixture
of a gold hydrosol and dodecanethiol in toluene. To this biphasic mixture, concentrated HCl was added under stirring. A swift movement of the gold nanoparticles into the organic phase containing the gold nanoparticles was observed,
indicating capping of the metal nanoparticles with alkanethiol molecules [65, 66].
The alkanethiol-stabilized colloidal gold, silver and platinum particles transferred
to toluene as described above could be self-assembled by solvent evaporation on
different substrates yielding close-packed, hexagonal arrays of the nanoparticles
[65, 66]. A concentrated solution of the different sized thiol stabilized-gold particles
was placed on glass substrates and the particles assembled on the surface. Figure
3.2 shows X-ray diffraction patterns from the different films, the size of the particles is indicated next to the corresponding diffractogram.
The d-spacings obtained from the low angle peaks (indicated by arrows in Figure
3.2) are also listed in the figure. The low angle diffraction peaks arise from the
arrangement of the gold particles in the array formed by solvent evaporation. It
is observed that the separation between the clusters decreases as the size of the
particles is reduced (Figure 3.2), in accordance with nanoparticle packing considerations. However, the d-spacing is smaller than that expected from the core þ
surfactant size considerations indicating some degree of interdigitation of the hydrocarbon chains from neighboring particles in the array [66]. The inset of Figure
3.2 shows the optical absorption spectra recorded from the colloidal gold particles
in toluene for gold particles of different sizes. The surface plasmon resonance
from the colloidal gold particles centered around 525 nm is clearly seen for the 4.2
nm and 2.1 nm sized particles and the intensity of the resonance is higher for the
larger sized particles (Figure 3.2, inset). The resonance could not be detected for
the smallest size particles (1.0 nm diameter) and this was attributed to the possibility of the gold particles in this size range being non-metallic [66].
Transmission electron microscopy images recorded from the self-assembled gold
nanoparticle monolayer obtained by solvent evaporation are shown in Figure 3.3
[66]. It can be seen that the thiol-derivatized gold nanoparticles self-assemble into
nanocrystalline arrays over tens of nanometer length scale. The spacing between
the particles is highly regular and roughly 1 nm between the particles. While the
larger particles of mean diameter 4.2 nm assembled into regular, close-packed
domains, assemblies of smaller particles show a large fraction of voids within the
domains [66].
It is known that alkylamine molecules also bind to gold nanoparticles quite
strongly through a ‘weak’ covalent bond, as described by Leff, Brandt and Heath
[45]. Using a process similar to that adopted for phase transfer of gold nanoparticles with alkanethiols, we have recently demonstrated that octadecylamine
molecules present in the organic phase may be used to accomplish the phase
transfer of aqueous gold nanoparticles into toluene [67]. Vigorous stirring of a bi3 Moving Nanoparticles Around: Phase-Transfer Processes in Nanomaterials Synthesis
36
nanoparticles may be used to hydrophobize gold nanoparticles at the liquid–liquid
interface, Rao and co-workers have demonstrated the acid-facilitated phase transfer
of aqueous gold and indeed, platinum and silver nanoparticles into organic solvents such as toluene [65, 66]. In a typical experiment, the authors took a mixture
of a gold hydrosol and dodecanethiol in toluene. To this biphasic mixture, concentrated HCl was added under stirring. A swift movement of the gold nanoparticles into the organic phase containing the gold nanoparticles was observed,
indicating capping of the metal nanoparticles with alkanethiol molecules [65, 66].
The alkanethiol-stabilized colloidal gold, silver and platinum particles transferred
to toluene as described above could be self-assembled by solvent evaporation on
different substrates yielding close-packed, hexagonal arrays of the nanoparticles
[65, 66]. A concentrated solution of the different sized thiol stabilized-gold particles
was placed on glass substrates and the particles assembled on the surface. Figure
3.2 shows X-ray diffraction patterns from the different films, the size of the particles is indicated next to the corresponding diffractogram.
The d-spacings obtained from the low angle peaks (indicated by arrows in Figure
3.2) are also listed in the figure. The low angle diffraction peaks arise from the
arrangement of the gold particles in the array formed by solvent evaporation. It
is observed that the separation between the clusters decreases as the size of the
particles is reduced (Figure 3.2), in accordance with nanoparticle packing considerations. However, the d-spacing is smaller than that expected from the core þ
surfactant size considerations indicating some degree of interdigitation of the hydrocarbon chains from neighboring particles in the array [66]. The inset of Figure
3.2 shows the optical absorption spectra recorded from the colloidal gold particles
in toluene for gold particles of different sizes. The surface plasmon resonance
from the colloidal gold particles centered around 525 nm is clearly seen for the 4.2
nm and 2.1 nm sized particles and the intensity of the resonance is higher for the
larger sized particles (Figure 3.2, inset). The resonance could not be detected for
the smallest size particles (1.0 nm diameter) and this was attributed to the possibility of the gold particles in this size range being non-metallic [66].
Transmission electron microscopy images recorded from the self-assembled gold
nanoparticle monolayer obtained by solvent evaporation are shown in Figure 3.3
[66]. It can be seen that the thiol-derivatized gold nanoparticles self-assemble into
nanocrystalline arrays over tens of nanometer length scale. The spacing between
the particles is highly regular and roughly 1 nm between the particles. While the
larger particles of mean diameter 4.2 nm assembled into regular, close-packed
domains, assemblies of smaller particles show a large fraction of voids within the
domains [66].
It is known that alkylamine molecules also bind to gold nanoparticles quite
strongly through a ‘weak’ covalent bond, as described by Leff, Brandt and Heath
[45]. Using a process similar to that adopted for phase transfer of gold nanoparticles with alkanethiols, we have recently demonstrated that octadecylamine
molecules present in the organic phase may be used to accomplish the phase
transfer of aqueous gold nanoparticles into toluene [67]. Vigorous stirring of a bi3 Moving Nanoparticles Around: Phase-Transfer Processes in Nanomaterials Synthesis
36