12.4 Deposition with the Help of Tips
465
In the above nano-ITIES, the metal deposition took place on a substrate. If the
metal is deposited onto the ITIES, the nanopipette can be transformed into a nanoelectrode if the nanoparticles being formed can agglomerate and form a solid block at
the end of the nanopipette. Two examples are presented for such a process. In the first
case, the water/nitrobenzene interface was used without applying a complexing agent
that facilitates Ag
+ transfer between the two phases [163]. Another important prerequisite of the electrode preparation was that the outer wall of the glass nanopipette
had to be covered with a hydrophobic layer in order to suppress the formation of a
water droplet extending outwards from the pipette’s internal nanochannel. After the
electrode preparation, a contact from the interior of the pipette was formed. Since
there was no way to contact the solid ending of the nanopipette mechanically from
the interior, a silver wire was immersed into the internal solution. When this unit
was used as an electrode, it worked as it is expected for nanoelectrodes – however,
in fact, two electrochemical cells were connected here in serial. The internal cell
in the nanopipette was a symmetrical one in terms of the electrochemical reaction
(Ag oxidation or Ag
+ reduction), and this was a low-impedance cell (with electrodes of high reversibility and small polarization resistance). The electrode response
was determined by the reaction taking place at the external surface of the Ag-filled
nanotip, which made it ideal for using as nanoelectrode.
A variation of the above-described technique is when the bottom liquid phase
(outside the nanopipette) is a liquid metal. In the pioneering work of this technique
[164], eutectic Ga–In (E-GaIn) was used, which dissolves metals much less than
mercury. When the nanopipette filled up with a solution of the metal ion to be
reduced was brought into contact with E-GaIn, the reduction of the noble metal ions
first takes place spontaneously due to the large difference in the standard potentials
of the liquid forming metals (i.e., that of the Ga
3+ /Ga and In
3+ /In redox pairs) and
those to be deposited (i.e., Ag
+ /Ag or Au
3+ /Au). Further electrodeposition of the
nanoelectrode-forming metal took place on the membrane formed spontaneously by
cementation. When the metal filling was finished, pyrrole was polymerized between
the metal tip and an internal wire for providing backside contact. Ion milling of the
outside surface of the tip proved to improve the functionality of the nanoelectrodes.
12.4.4 Miscellaneous Electrochemical Preparation Methods
of Nanoelectrodes
One of the oldest electrochemical nanoelectrode preparation technique is the etching
of STM tips made mostly from tungsten [165–173] and occasionally from other
metals (Re [174], Ag [175, 176], Au [177], PtIr [178, 179], Ni [180] and other
magnetic metals [181]). The fine details of the tip manufacturing process and the
post-treatment modes show a great variety. The typical procedure applies anodic
etching in fairly concentrated (2–3 M) MeOH solution (Me: Na, K), but chlorideand cyanide-based solutions are also applied in some cases. The process starts with
465
In the above nano-ITIES, the metal deposition took place on a substrate. If the
metal is deposited onto the ITIES, the nanopipette can be transformed into a nanoelectrode if the nanoparticles being formed can agglomerate and form a solid block at
the end of the nanopipette. Two examples are presented for such a process. In the first
case, the water/nitrobenzene interface was used without applying a complexing agent
that facilitates Ag
+ transfer between the two phases [163]. Another important prerequisite of the electrode preparation was that the outer wall of the glass nanopipette
had to be covered with a hydrophobic layer in order to suppress the formation of a
water droplet extending outwards from the pipette’s internal nanochannel. After the
electrode preparation, a contact from the interior of the pipette was formed. Since
there was no way to contact the solid ending of the nanopipette mechanically from
the interior, a silver wire was immersed into the internal solution. When this unit
was used as an electrode, it worked as it is expected for nanoelectrodes – however,
in fact, two electrochemical cells were connected here in serial. The internal cell
in the nanopipette was a symmetrical one in terms of the electrochemical reaction
(Ag oxidation or Ag
+ reduction), and this was a low-impedance cell (with electrodes of high reversibility and small polarization resistance). The electrode response
was determined by the reaction taking place at the external surface of the Ag-filled
nanotip, which made it ideal for using as nanoelectrode.
A variation of the above-described technique is when the bottom liquid phase
(outside the nanopipette) is a liquid metal. In the pioneering work of this technique
[164], eutectic Ga–In (E-GaIn) was used, which dissolves metals much less than
mercury. When the nanopipette filled up with a solution of the metal ion to be
reduced was brought into contact with E-GaIn, the reduction of the noble metal ions
first takes place spontaneously due to the large difference in the standard potentials
of the liquid forming metals (i.e., that of the Ga
3+ /Ga and In
3+ /In redox pairs) and
those to be deposited (i.e., Ag
+ /Ag or Au
3+ /Au). Further electrodeposition of the
nanoelectrode-forming metal took place on the membrane formed spontaneously by
cementation. When the metal filling was finished, pyrrole was polymerized between
the metal tip and an internal wire for providing backside contact. Ion milling of the
outside surface of the tip proved to improve the functionality of the nanoelectrodes.
12.4.4 Miscellaneous Electrochemical Preparation Methods
of Nanoelectrodes
One of the oldest electrochemical nanoelectrode preparation technique is the etching
of STM tips made mostly from tungsten [165–173] and occasionally from other
metals (Re [174], Ag [175, 176], Au [177], PtIr [178, 179], Ni [180] and other
magnetic metals [181]). The fine details of the tip manufacturing process and the
post-treatment modes show a great variety. The typical procedure applies anodic
etching in fairly concentrated (2–3 M) MeOH solution (Me: Na, K), but chlorideand cyanide-based solutions are also applied in some cases. The process starts with