Chemistry Reference
In-Depth Information
Another interesting application of ILs is that related with the synthesis of metal nanoparticles. It has been
demonstrated that imidazolium ionic liquids constitute a suitable working medium for the preparation of
catalytically active transition-metal nanoparticles (NPs). So, platinum, rhodium and iridium NPs were
prepared in 1-butyl-3-methyl-imidazolium with control of size, near-monodispersity, shape and stabilization
[76]. Recently, the first synthesis of anisotropic gold nanocrystals (AuNC) by means of a seeding approach
using the greener ionic liquid, 1-butyl-3-methylimidazolium lauryl sulfate, [BMIM] [C
12
H
25
OSO
3
] has been
reported. Crystal growth was successfully achieved by successive addition of the growth solution containing
HAuCl
4
and ascorbic acid to 2.5-nm Au seeds. These nanocrystals were stabilized by the ionic liquid
that served as solvent and capping agent via a two layer micellar formation by the butyl chain of the organic
cation and the lauryl chain of the anion. [77].
14.3.2 Supercritical fluids
Since the beginning of the use of microelectrodes in electroanalysis, supercritical fluids (SCFs) were
conceived as appropriate media to carry out electrochemical measurements. These fluids are considered
important from the point of view of Green Analytical Chemistry as they can be employed instead of some
organic solvents. Despite this and because of the high electrical resistivity of these media, their use was
unfeasible for electrodes of conventional size. Microelectrodes have a critical surface of size approaching the
diffusion layer. The current magnitude they provide is low and this minimizes the ohmic drop effects, thus
being possible to use them in supercritical fluids such as the carbon dioxide which, moreover, is non-toxic,
relatively inert and easy to be purified. Some separation methods by supercritical fluids chromatography
using microelectrodes to perform electrochemical detection of compounds of environmental significance
were described. For instance, a platinum microelectrode 25
m in diameter allowed the determination of
components in a mixture of phenols and nitroaromatic compounds working with both oxidative and reductive
amperometric detection modes, using carbon dioxide modified with a low concentration (1-5
μ
) of methanol
as the mobile phase. The comparison with the results obtained with an UV detector revealed among other
advantages the significantly less pronounced baseline drift and higher signal-to-noise ratio values (between
20 and 40 versus 4-10 obtained with UV detection) [78].
More recent interesting applications of SCFs have to do with the preparation of materials employed in
electrochemistry and, in particular, with nanomaterials. SFCs have been used to deposit thin films of metal
on a variety of surfaces and to incorporate metal nanoparticles on several organic and inorganic substrates.
Different approaches have been reported to achieve these preparations. Among them, supercritical fluid
deposition, especially using supercritical carbon dioxide, is particularly attractive because no liquid wastes
are generated, no solvent residues remain on the substrate and the mass transfer rates are faster [79]. The
synthesis of carbon nanotubes is another area where the use of SCFs is of high interest. The synthesis of this
nanomaterial in gas phase produces low yields and poor crystallinity, and high temperatures are required. As
an alternative, SCFs have been used to prepare well-crystallized CNTs using carbon dioxide as the carbon
source in a relatively low temperature process and lithium metal as a reducing agent [80]. Compared with
CO
2
gas, scCO
2
enhanced the adsorption of carbon dioxide on the surface of lithium and therefore facilitated
and accelerated the electron transfer process. Large amounts of CNTs were produced with a diameter of 55
nm and more than one micrometer in length, without the use of a catalyst and in relatively mild and cost-
effective reaction conditions. A similar approach was also reported to produce multiwalled carbon nanotubes
(MWNTs) in supercritical toluene [81]. The concept was to take advantage of pressure effects to lower the
high temperatures usually required to synthesize CNTs. In this process, supercritical toluene served both as
the carbon source for nanotube growth, and as the reaction medium, and ferrocene, iron or platinum iron
nanocrystals were used as catalysts.
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