radiation-induced reduction of gold ions [36, 37] and (3) sonochemical reduction of
gold ions [38], to name just a few. The interested reader is directed to a comprehensive review by Handley that lists at least one dozen protocols for the syntheses
of gold hydrosols with particle sizes in the range 10–640 A ˚ .
The synthesis of gold nanoparticles in non-polar organic media is a considerably
newer area of research whose origin may be traced to the seminal work of Brust
and co-workers [39]. In this report, the authors have demonstrated the phase
transfer of chloroaurate ions into toluene using a phase transfer molecule such as
tetra-alkylammonium bromide. Thereafter, the gold ions were reduced using sodium borohydride to yield gold nanoparticles of excellent monodispersity capped
with alkanethiol molecules. Analogous to the formation of self-assembled monolayers (SAMs) of alkanethiols on gold thin films [40], Brust and co-workers used
thiolate chemistry to cap the gold nanoparticles with alkanethiols present in the
organic phase during phase transfer and reduction of the gold ions, thus rendering them hydrophobic and soluble in the organic phase [39]. Such surfactantstabilized gold nanoparticles behave like new compounds and can be easily separated out of solution in the form of a powder and re-dissolved in different organic
solvents without significant variation in the particle size distribution. The Brust
report has been rapidly followed by publications on the self-assembly on gold and
silver nanoparticle surfaces of alkanethiol [41, 42], aromatic thiol [43, 44], alkylamine [45, 46], dialkyl disulfide [47] and thiolated cyclodextrin [48, 49] molecules.
Very recently, we have shown in my group that the Brust protocol for the synthesis of hydrophobic metal nanoparticles may be considerably simplified by using a multifunctional molecule, 4-hexadecylaniline [50]. This molecule plays the
role of a phase-transfer molecule, reducing and capping agent and results in the
one-step synthesis of hydrophobic gold [50] and platinum nanoparticles [51] in a
variety of organic solvents. The surface properties of the gold colloids may be
tailored by chemisorption of terminally functionalized thiol molecules resulting
in a number of interesting applications. One of the exciting areas of research using functionalized colloidal gold particles is the study of the reactivity of monolayer protected colloidal particles (or MPCs as they are termed) [52, 53]. Using a
simple place-exchange reaction strategy, Murray and co-workers have demonstrated that alkanethiol derivatized colloidal gold particles in an organic solvent
can be poly-o-functionalized [54]. This led to the possibility of using the polyheterofunctionalized colloidal particles as ‘‘nanofactories’’ where the metal core
scaffolds support complex organic ligand structures that may include polymeric
and hyperbranched domains [54]. Polar terminal functional groups such as carboxylic acid, ammonium ions, sulfonic acid etc. in the monolayers chemisorbed
onto the gold particle surface may be used to increase the solubility of the particles
in polar solvents such as water, thus enabling the synthesis of water-soluble gold
nanoparticles that, like their hydrophobic counterparts, may be dried in the form
of a powder and re-dispersed in water and other polar solvents without significant
degradation in the particle size and monodispersity [55–57].
The two main approaches for the synthesis of gold nanoparticles, viz. synthesis
in water and in non-polar organic solvents, have certain pros and cons that are
enumerated below.
3 Moving Nanoparticles Around: Phase-Transfer Processes in Nanomaterials Synthesis
32
gold ions [38], to name just a few. The interested reader is directed to a comprehensive review by Handley that lists at least one dozen protocols for the syntheses
of gold hydrosols with particle sizes in the range 10–640 A ˚ .
The synthesis of gold nanoparticles in non-polar organic media is a considerably
newer area of research whose origin may be traced to the seminal work of Brust
and co-workers [39]. In this report, the authors have demonstrated the phase
transfer of chloroaurate ions into toluene using a phase transfer molecule such as
tetra-alkylammonium bromide. Thereafter, the gold ions were reduced using sodium borohydride to yield gold nanoparticles of excellent monodispersity capped
with alkanethiol molecules. Analogous to the formation of self-assembled monolayers (SAMs) of alkanethiols on gold thin films [40], Brust and co-workers used
thiolate chemistry to cap the gold nanoparticles with alkanethiols present in the
organic phase during phase transfer and reduction of the gold ions, thus rendering them hydrophobic and soluble in the organic phase [39]. Such surfactantstabilized gold nanoparticles behave like new compounds and can be easily separated out of solution in the form of a powder and re-dissolved in different organic
solvents without significant variation in the particle size distribution. The Brust
report has been rapidly followed by publications on the self-assembly on gold and
silver nanoparticle surfaces of alkanethiol [41, 42], aromatic thiol [43, 44], alkylamine [45, 46], dialkyl disulfide [47] and thiolated cyclodextrin [48, 49] molecules.
Very recently, we have shown in my group that the Brust protocol for the synthesis of hydrophobic metal nanoparticles may be considerably simplified by using a multifunctional molecule, 4-hexadecylaniline [50]. This molecule plays the
role of a phase-transfer molecule, reducing and capping agent and results in the
one-step synthesis of hydrophobic gold [50] and platinum nanoparticles [51] in a
variety of organic solvents. The surface properties of the gold colloids may be
tailored by chemisorption of terminally functionalized thiol molecules resulting
in a number of interesting applications. One of the exciting areas of research using functionalized colloidal gold particles is the study of the reactivity of monolayer protected colloidal particles (or MPCs as they are termed) [52, 53]. Using a
simple place-exchange reaction strategy, Murray and co-workers have demonstrated that alkanethiol derivatized colloidal gold particles in an organic solvent
can be poly-o-functionalized [54]. This led to the possibility of using the polyheterofunctionalized colloidal particles as ‘‘nanofactories’’ where the metal core
scaffolds support complex organic ligand structures that may include polymeric
and hyperbranched domains [54]. Polar terminal functional groups such as carboxylic acid, ammonium ions, sulfonic acid etc. in the monolayers chemisorbed
onto the gold particle surface may be used to increase the solubility of the particles
in polar solvents such as water, thus enabling the synthesis of water-soluble gold
nanoparticles that, like their hydrophobic counterparts, may be dried in the form
of a powder and re-dispersed in water and other polar solvents without significant
degradation in the particle size and monodispersity [55–57].
The two main approaches for the synthesis of gold nanoparticles, viz. synthesis
in water and in non-polar organic solvents, have certain pros and cons that are
enumerated below.
3 Moving Nanoparticles Around: Phase-Transfer Processes in Nanomaterials Synthesis
32