organic media (1.05 aF in aqueous media vs. 0.75 aF in organic solvents such as
CH 2 Cl 2 ) [113]. The binding of PF 6
À on the surface-immobilized MPCs probably
increases the effective dielectric constant and hence the capacitance. Rectification
is induced only in the presence of specific soft, hydrophobic anions, as mentioned
earlier, and to a small extent in the presence of soft cations such as (tetramethylammonium, TMA)
þ and (tetraethylammonium, TEA)
þ . In effect, both hydrophobic and electrostatic forces play a significant role in ion binding.
Other metallic clusters that have been demonstrated to show the QDL effect are
palladium [116, 117], silver [118] and copper [119]. Palladium MPCs capped with
mixed monolayers of hexanethiolate/dodecanethiolate and ferrocene thiolate ligands are prepared in a manner similar to that employed for gold MPCs. The DPV
studies exhibit a quantized charging effect but the current peaks are not as well
defined as those observed for Au-MPCs. Capacitance values of the order of 0.35 aF
are obtained, indicating smaller core sizes or thicker monolayer dielectrics [116].
Fresh copper nanoparticles show the QDL effect and the aged particles behave like
semiconductors due to the formation of the oxide film [119].
An important observation related to the QDL effects is that the MPCs can be
electrolytically charged to controllable potentials [120]. The pattern and the height
of the current peaks can be used to estimate the number of electrons (or holes)
stored on the cluster. The charged MPCs are stable for several hours in the solution phase and the interesting fact is that these particles retain most of their charge
when isolated in the dry form. Some charge is lost during this process due to several factors, but the remaining charge is reasonably stable. These charged clusters
thus hold great promise as oxidizing and reducing agents. Charging the MPCs at
positive potentials results in increased solubility in polar solvents. Thus, a substituent effect can be incorporated by electrochemical tuning of the clusters. Preliminary experiments have indicated that they can be used to carry out redox
reactions with electron acceptor or donor molecules like ethylferrocene (EtFc) and
7,7,8,8-tetracyanoquinodimethane (TCNQ) respectively. A small amount of these
charged clusters is added to a solution of EtFc or TCNQ and the subsequent electron transfer reactions can be monitored like any regular potentiometric titration
[120]. Differently charged MPC clusters can also undergo electron transfer reactions with each other. These mixed valent solutions follow the Nernstian behavior as given by:
E ¼ E
0
MPC À 0:059 logf½N z =½N zÀ1 g; where E
0
MPC ¼ E PZC þ ½z À ð1=2Þe=C clu :
Thus, the electronic charges associated with double layer charging of MPCs have
numerous redox-like formal potentials that can be taken to a desired charge state
[120].
The dynamics of self-exchange between the MPCs is probed by linking them
through carboxylate–metal ion–carboxylate bridges [121] on an electrode surface.
The diffusion-like electron transport in multilayer films is used to obtain information on the nanoparticle–nanoparticle electron transfer dynamics. Potential step
experiments that give an idea of the rate of electron hopping (electron diffusion)
20.4 Single Electron Events 663
CH 2 Cl 2 ) [113]. The binding of PF 6
À on the surface-immobilized MPCs probably
increases the effective dielectric constant and hence the capacitance. Rectification
is induced only in the presence of specific soft, hydrophobic anions, as mentioned
earlier, and to a small extent in the presence of soft cations such as (tetramethylammonium, TMA)
þ and (tetraethylammonium, TEA)
þ . In effect, both hydrophobic and electrostatic forces play a significant role in ion binding.
Other metallic clusters that have been demonstrated to show the QDL effect are
palladium [116, 117], silver [118] and copper [119]. Palladium MPCs capped with
mixed monolayers of hexanethiolate/dodecanethiolate and ferrocene thiolate ligands are prepared in a manner similar to that employed for gold MPCs. The DPV
studies exhibit a quantized charging effect but the current peaks are not as well
defined as those observed for Au-MPCs. Capacitance values of the order of 0.35 aF
are obtained, indicating smaller core sizes or thicker monolayer dielectrics [116].
Fresh copper nanoparticles show the QDL effect and the aged particles behave like
semiconductors due to the formation of the oxide film [119].
An important observation related to the QDL effects is that the MPCs can be
electrolytically charged to controllable potentials [120]. The pattern and the height
of the current peaks can be used to estimate the number of electrons (or holes)
stored on the cluster. The charged MPCs are stable for several hours in the solution phase and the interesting fact is that these particles retain most of their charge
when isolated in the dry form. Some charge is lost during this process due to several factors, but the remaining charge is reasonably stable. These charged clusters
thus hold great promise as oxidizing and reducing agents. Charging the MPCs at
positive potentials results in increased solubility in polar solvents. Thus, a substituent effect can be incorporated by electrochemical tuning of the clusters. Preliminary experiments have indicated that they can be used to carry out redox
reactions with electron acceptor or donor molecules like ethylferrocene (EtFc) and
7,7,8,8-tetracyanoquinodimethane (TCNQ) respectively. A small amount of these
charged clusters is added to a solution of EtFc or TCNQ and the subsequent electron transfer reactions can be monitored like any regular potentiometric titration
[120]. Differently charged MPC clusters can also undergo electron transfer reactions with each other. These mixed valent solutions follow the Nernstian behavior as given by:
E ¼ E
0
MPC À 0:059 logf½N z =½N zÀ1 g; where E
0
MPC ¼ E PZC þ ½z À ð1=2Þe=C clu :
Thus, the electronic charges associated with double layer charging of MPCs have
numerous redox-like formal potentials that can be taken to a desired charge state
[120].
The dynamics of self-exchange between the MPCs is probed by linking them
through carboxylate–metal ion–carboxylate bridges [121] on an electrode surface.
The diffusion-like electron transport in multilayer films is used to obtain information on the nanoparticle–nanoparticle electron transfer dynamics. Potential step
experiments that give an idea of the rate of electron hopping (electron diffusion)
20.4 Single Electron Events 663