Bard and co-workers have reported on the attainment of equilibrium between
the nanosized particles and an electrode in the presence of a redox mediator [25a].
The study refers to the production of a mediator (methyl viologen radical cation)
that reduces water in the presence of colloidal gold and platinum metal catalyst.
An electrochemical model based on the assumption that the kinetic properties are
controlled by the half-cell reactions is proposed to understand the catalytic properties of the colloidal metals. The same authors have used 15 nm electrodes to detect
single molecules using scanning electrochemical microscopy (SECM) [25b]. A
PtaIr tip of nm size diameter is used along with a ferrocene derivative in a positive
feedback mode of SECM. The response has been found to be stochastic and Faradaic currents of the order of pA are observed.
The possibility of constructing electrochemical interfaces with tunable kinetic
barriers has recently been reported [42]. A planar gold electrode is modified with a
hydrolyzed mercaptosilane sol to yield a surface with high blocking properties. The
barrier properties are demonstrated using a freely diffusing [Fe(CN) 6 ]
3À=4À redox
couple in the solution phase. Different loading of the metallic particles is achieved
by immersing the silane-modified electrode in citrate-stabilized gold sol for varying periods of time. The barrier properties of the resulting surfaces are different,
depending on the extent of metal loading.
Henglein and co-workers [43–46] have developed a radiolytic method to deposit
excess electrons onto colloidal metal particles in aqueous solutions. These excess
electrons can then be used to initiate electrochemical reactions on the surface of
the colloidal particle. This mechanism has been utilized to deposit metals such as
gold, cadmium, lead and indium on silver nanoparticles. The Ag nanoparticles
behave like ‘microelectrodes’ in solution. They become charged via the electron
transfer of free radicals that are generated in situ in the solution by radiolysis. The
nanoparticles can store a large number of excess electrons and can be used to reduce different metals on their surface. A similar concept has been used to polarize
a nanocapacitor consisting of a metal (Au) core with a semiconducting (SnO 2 ) shell
[47]. Since there is a large difference in the Fermi-energy level of the core and the
conduction band energy of the shell, mobile electrons remain trapped within the
core for long periods of time. The charge can be stored for up to 10 h in the Au
core in the absence of oxygen but when O 2 is introduced into the system, there is a
slow discharge of electrons at the gold/SnO 2 interface. Charging and discharging
processes in aqueous solutions have been monitored by observing changes in the
surface plasmon position [47].
The corrosion behavior of supported and unsupported iron nanoparticles has
been followed by Ponder and co-workers [48]. The material referred to as ‘ferragels’
is found to be stable in air and the electrochemical behavior is observed to be similar to that of iron filings. Gold nanoparticle enhanced microchip capillary electrophoresis has been developed by Pumera and co workers [49a]. This has resulted in
achieving a good selectivity of solutes and an increase in the efficiency of separation. The walls of a microchannel are coated with a polymer [poly(diallyl-dimethylammonium chloride)] and subsequently with citrate-stabilized gold nanoparticles.
The separation of aminophenol is demonstrated using this material. Voltammetry
20 Electrochemistry with Nanoparticles
650
the nanosized particles and an electrode in the presence of a redox mediator [25a].
The study refers to the production of a mediator (methyl viologen radical cation)
that reduces water in the presence of colloidal gold and platinum metal catalyst.
An electrochemical model based on the assumption that the kinetic properties are
controlled by the half-cell reactions is proposed to understand the catalytic properties of the colloidal metals. The same authors have used 15 nm electrodes to detect
single molecules using scanning electrochemical microscopy (SECM) [25b]. A
PtaIr tip of nm size diameter is used along with a ferrocene derivative in a positive
feedback mode of SECM. The response has been found to be stochastic and Faradaic currents of the order of pA are observed.
The possibility of constructing electrochemical interfaces with tunable kinetic
barriers has recently been reported [42]. A planar gold electrode is modified with a
hydrolyzed mercaptosilane sol to yield a surface with high blocking properties. The
barrier properties are demonstrated using a freely diffusing [Fe(CN) 6 ]
3À=4À redox
couple in the solution phase. Different loading of the metallic particles is achieved
by immersing the silane-modified electrode in citrate-stabilized gold sol for varying periods of time. The barrier properties of the resulting surfaces are different,
depending on the extent of metal loading.
Henglein and co-workers [43–46] have developed a radiolytic method to deposit
excess electrons onto colloidal metal particles in aqueous solutions. These excess
electrons can then be used to initiate electrochemical reactions on the surface of
the colloidal particle. This mechanism has been utilized to deposit metals such as
gold, cadmium, lead and indium on silver nanoparticles. The Ag nanoparticles
behave like ‘microelectrodes’ in solution. They become charged via the electron
transfer of free radicals that are generated in situ in the solution by radiolysis. The
nanoparticles can store a large number of excess electrons and can be used to reduce different metals on their surface. A similar concept has been used to polarize
a nanocapacitor consisting of a metal (Au) core with a semiconducting (SnO 2 ) shell
[47]. Since there is a large difference in the Fermi-energy level of the core and the
conduction band energy of the shell, mobile electrons remain trapped within the
core for long periods of time. The charge can be stored for up to 10 h in the Au
core in the absence of oxygen but when O 2 is introduced into the system, there is a
slow discharge of electrons at the gold/SnO 2 interface. Charging and discharging
processes in aqueous solutions have been monitored by observing changes in the
surface plasmon position [47].
The corrosion behavior of supported and unsupported iron nanoparticles has
been followed by Ponder and co-workers [48]. The material referred to as ‘ferragels’
is found to be stable in air and the electrochemical behavior is observed to be similar to that of iron filings. Gold nanoparticle enhanced microchip capillary electrophoresis has been developed by Pumera and co workers [49a]. This has resulted in
achieving a good selectivity of solutes and an increase in the efficiency of separation. The walls of a microchannel are coated with a polymer [poly(diallyl-dimethylammonium chloride)] and subsequently with citrate-stabilized gold nanoparticles.
The separation of aminophenol is demonstrated using this material. Voltammetry
20 Electrochemistry with Nanoparticles
650