The electrochemical characteristics of anthraquinone-derivatized Au clusters prepared by an electroreduction method are similar to those observed on a planar
surface. However, the double layer capacitance values reveal that it is possible
to control the charging phenomenon stepwise when multiple redox activity is
present.
The incorporation of polar ligands such as nitrothiophenyl moieties (NTP) in
hexanethiolate-stabilized gold MPCs by the place exchange method leads to an
increase in the observed double layer capacitance [65]. The value increases from
0.61 aF for hexane thiolate protected gold clusters (HT-MPCs) to 0.73 aF for nitrophenyl incorporated ones (HT þ NTP-MPCs). The HT þ NTP-MPCs exhibit two
well-defined reversible voltammetric waves corresponding to the reduction of NTP
moiety in aprotic media while planar gold electrodes modified with NTP do not
exhibit these features. Water soluble tiopronin-based MPCs have been prepared
and used for further place exchange with other functionalities such as viologen.
Capacitance values as high as 4.5 aF have been observed for tiopronin MPCs in
aqueous solutions containing NH 4 PF 6 as supporting electrolyte [66]. The carboxylic acid functional groups available on tiopronin MPCs can be titrated as weak
acids that would lead to charge repulsion effects [67]. Polyethylene glycol attached
MPCs have been reported to show ionic conductivities when lithium salts are dissolved in the matrix [68]. The ionic conductivity is found to be higher than the
corresponding polymer–lithium salt composites. The same group has recently reported a novel method to prepare a molecular melt of a few nm sized quantum
dots [69]. The PEG-based quantum dots are also able to dissolve lithium salts. Dry
films of arenethiolate or alkanethiolate protected MPCs show electronic conductivity as governed by electron tunneling through the thiolate ligands. The current–
potential curves under high potential gradients show characteristics of hopping
type conductivity [70]. A tunneling constant (b) of 1.2 per A ˚ is obtained that is
similar to 2-D SAMs on planar electrodes.
20.3.2
Nanoelectrode Ensembles
Martin and co-workers have extensively studied the preparation of templated electrodes using microporous membranes and their electrochemical behavior [71–80].
The process involves the synthesis of a desired material within the pores of a
uniform membrane. This has been shown to be a very general procedure for the
preparation of ensembles of a variety of electrode materials [71, 76, 78, 79, 81]. The
availability of membranes with high monodispersity allows the preparation of uniform nanostructures. Track etched plastic/mica-based filtration membranes and
microporous alumina have been used as templates for a long time [74, 77–79, 82].
‘Nanochannel glass’ is a novel type of membrane introduced recently [83]. This
is formed by heating and drawing glass capillaries until the desired pore size is
obtained. Pore sizes of the order of 17 nm with pore densities as high as 3 Â 10
9
pores cm
À2 have been achieved. Highly ordered arrays with excellent monodispersity can be fabricated using this method. Many metals such as copper, platinum
20.3 Electrochemistry with Metallic Nanoparticles 653
surface. However, the double layer capacitance values reveal that it is possible
to control the charging phenomenon stepwise when multiple redox activity is
present.
The incorporation of polar ligands such as nitrothiophenyl moieties (NTP) in
hexanethiolate-stabilized gold MPCs by the place exchange method leads to an
increase in the observed double layer capacitance [65]. The value increases from
0.61 aF for hexane thiolate protected gold clusters (HT-MPCs) to 0.73 aF for nitrophenyl incorporated ones (HT þ NTP-MPCs). The HT þ NTP-MPCs exhibit two
well-defined reversible voltammetric waves corresponding to the reduction of NTP
moiety in aprotic media while planar gold electrodes modified with NTP do not
exhibit these features. Water soluble tiopronin-based MPCs have been prepared
and used for further place exchange with other functionalities such as viologen.
Capacitance values as high as 4.5 aF have been observed for tiopronin MPCs in
aqueous solutions containing NH 4 PF 6 as supporting electrolyte [66]. The carboxylic acid functional groups available on tiopronin MPCs can be titrated as weak
acids that would lead to charge repulsion effects [67]. Polyethylene glycol attached
MPCs have been reported to show ionic conductivities when lithium salts are dissolved in the matrix [68]. The ionic conductivity is found to be higher than the
corresponding polymer–lithium salt composites. The same group has recently reported a novel method to prepare a molecular melt of a few nm sized quantum
dots [69]. The PEG-based quantum dots are also able to dissolve lithium salts. Dry
films of arenethiolate or alkanethiolate protected MPCs show electronic conductivity as governed by electron tunneling through the thiolate ligands. The current–
potential curves under high potential gradients show characteristics of hopping
type conductivity [70]. A tunneling constant (b) of 1.2 per A ˚ is obtained that is
similar to 2-D SAMs on planar electrodes.
20.3.2
Nanoelectrode Ensembles
Martin and co-workers have extensively studied the preparation of templated electrodes using microporous membranes and their electrochemical behavior [71–80].
The process involves the synthesis of a desired material within the pores of a
uniform membrane. This has been shown to be a very general procedure for the
preparation of ensembles of a variety of electrode materials [71, 76, 78, 79, 81]. The
availability of membranes with high monodispersity allows the preparation of uniform nanostructures. Track etched plastic/mica-based filtration membranes and
microporous alumina have been used as templates for a long time [74, 77–79, 82].
‘Nanochannel glass’ is a novel type of membrane introduced recently [83]. This
is formed by heating and drawing glass capillaries until the desired pore size is
obtained. Pore sizes of the order of 17 nm with pore densities as high as 3 Â 10
9
pores cm
À2 have been achieved. Highly ordered arrays with excellent monodispersity can be fabricated using this method. Many metals such as copper, platinum
20.3 Electrochemistry with Metallic Nanoparticles 653