2. High concentrations of the gold nanoparticles in solution may be easily prepared.
3. Functionalized gold nanoparticles may be stored as a powder without sintering
and irreversible aggregation of the particles.
4. The nanoparticles spontaneously assemble into close-packed, hexagonal monolayers upon solvent evaporation [39, 41, 42, 45, 46]. The collective properties of
the nanoparticle assembly may be controlled by varying the interparticle separation via capping with different chain length alkanethiols.
3.3.2
Disadvantages
1. The procedure is a multi-step one involving, independently, phase transfer of
the gold ions followed by their reduction and capping.
2. While close-packed monolayers of the gold nanoparticles may be deposited by
solvent evaporation, there is little control over the process of assembly. Furthermore, superlattices of the gold nanoparticles cannot be readily deposited, in
contrast with the layer-by-layer assembly that is possible for electrostatically
stabilized gold nanoparticles in water.
3. Formation of bioconjugates with the gold nanoparticles is not possible in an
organic environment and consequently, biological application of gold nanoparticles becomes difficult.
It is clear that both methods for the synthesis of gold nanoparticles have characteristic advantages. Depending on the particular application of the nanoparticles,
the ideal condition would be to somehow marry the two methods and thus maximize their advantages. This may conveniently be done by effecting a phase transfer
of gold nanoparticles synthesized in one medium (water/organic solvent) to the
second medium (organic solvent/water). In addition to maximizing the benefits
accruing from a combination of the two syntheses methods, the ability to move
nanoparticles across liquid interfaces into environments of specific physicochemical properties to probe, for example, variation in the optical properties of the
nanoparticle solution [64] is an attractive feature of phase-transfer protocols. In the
remaining part of this chapter, I discuss some of the methods developed to carry
out the phase transfer of gold nanoparticles in both directions. The experimental
methods are quite general and may be extended to other chemical compositions
such as metal sulfide nanoparticles. Examples will also be given wherever possible.
3.4
Moving Gold Nanoparticles Around
3.4.1
Phase Transfer of Aqueous Gold Nanoparticles to Non-Polar Organic Solvents
The movement of aqueous gold nanoparticles into non-polar organic solvents requires hydrophobization of the nanoparticles. The many techniques developed to
3 Moving Nanoparticles Around: Phase-Transfer Processes in Nanomaterials Synthesis
34
3. Functionalized gold nanoparticles may be stored as a powder without sintering
and irreversible aggregation of the particles.
4. The nanoparticles spontaneously assemble into close-packed, hexagonal monolayers upon solvent evaporation [39, 41, 42, 45, 46]. The collective properties of
the nanoparticle assembly may be controlled by varying the interparticle separation via capping with different chain length alkanethiols.
3.3.2
Disadvantages
1. The procedure is a multi-step one involving, independently, phase transfer of
the gold ions followed by their reduction and capping.
2. While close-packed monolayers of the gold nanoparticles may be deposited by
solvent evaporation, there is little control over the process of assembly. Furthermore, superlattices of the gold nanoparticles cannot be readily deposited, in
contrast with the layer-by-layer assembly that is possible for electrostatically
stabilized gold nanoparticles in water.
3. Formation of bioconjugates with the gold nanoparticles is not possible in an
organic environment and consequently, biological application of gold nanoparticles becomes difficult.
It is clear that both methods for the synthesis of gold nanoparticles have characteristic advantages. Depending on the particular application of the nanoparticles,
the ideal condition would be to somehow marry the two methods and thus maximize their advantages. This may conveniently be done by effecting a phase transfer
of gold nanoparticles synthesized in one medium (water/organic solvent) to the
second medium (organic solvent/water). In addition to maximizing the benefits
accruing from a combination of the two syntheses methods, the ability to move
nanoparticles across liquid interfaces into environments of specific physicochemical properties to probe, for example, variation in the optical properties of the
nanoparticle solution [64] is an attractive feature of phase-transfer protocols. In the
remaining part of this chapter, I discuss some of the methods developed to carry
out the phase transfer of gold nanoparticles in both directions. The experimental
methods are quite general and may be extended to other chemical compositions
such as metal sulfide nanoparticles. Examples will also be given wherever possible.
3.4
Moving Gold Nanoparticles Around
3.4.1
Phase Transfer of Aqueous Gold Nanoparticles to Non-Polar Organic Solvents
The movement of aqueous gold nanoparticles into non-polar organic solvents requires hydrophobization of the nanoparticles. The many techniques developed to
3 Moving Nanoparticles Around: Phase-Transfer Processes in Nanomaterials Synthesis
34