aminosilane-stabilized gold sol onto planar electrodes [33]. This involves the stabilization of metal nanoparticles in an aminosilane matrix that is simultaneously
hydrolyzed to form a sol. The sol is then electrolysed to give a coating of silanestabilized metallic particles on a conducting substrate.
Musick and co-workers have prepared nanoelectrodes [34], which are derived
from self-assembled monolayers of colloidal gold on glass surfaces. The base
monolayer for colloidal gold deposition is formed by the silanization of glass surfaces with amino- or mercaptosilane groups. Further functionalization is carried
out by two different methods. One involves the stepwise assembly of Au nanoparticles cross-linked with bifunctional groups, which gives good control over the
number of layers of deposited gold. The second procedure involves the electroless
deposition of gold on gold nanoparticles using NH 2 OH and Au
3þ ions as initiators
for the process. Electrochemical potential control has also been shown to assist in
varying the rate of Au monolayer formation and interparticle spacing, thus giving a
good handle on the morphology of the surface [35]. These electrodes can also be
characterized by several different techniques such as optical spectroscopy, AFM
and electrochemistry. Such electrodes exhibit conductivities and electrochemical
properties comparable to macroelectrodes and also have advantages in terms of
ease of preparation and ability to control the geometric shape [34]. Electrodes prepared using a layer-by-layer growth technique show a transition from microelectrode array type of voltammetry to the usual reversible behavior observed at a
bulk, planar gold electrode, depending on the number of metallic layers [34]. The
active electrochemical area of these surfaces is larger than the geometric area due
to the roughness associated with the nanocrystalline phases [34]. Microband electrodes can also be fabricated using this method and they are found to exhibit
almost ideal microelectrode behavior [34].
Another novel way of encapsulating gold particles into an electropolymerized
matrix has been developed by Willner and co-workers [36]. This involves the use of
hydrogels based on polyacrylamide that exhibit reversible swelling and shrinking
properties. The swelling is accomplished in the presence of a citrate-stabilized gold
colloid while the shrinking property is brought about by the use of acetone as
the solvent. Gold particles stabilized by citrate molecules have a high hydrogen
bonding potential and hence the particles get trapped in the acrylamide matrix
while the shrinking mechanism is operative. The method is described in Scheme
20.1. The extent of incorporation of gold particles varies as a function of the number of swelling–shrinking (breathing) cycles. This is confirmed by following the
charge transfer resistance using impedance measurements in the presence of a
ferrocyanide/ferricyanide (Fe(CN) 6
3À=4À ) redox couple. The enhanced conductivity
due to gold particle incorporation is shown in the form of a decreased charge
transfer resistance as a function of the number of breathing cycles. Bard and
co-workers [37] reported the preparation of gold nanoparticles at the tip end of
glass micropipettes and used it for scanning electrochemical microscopy (SECM)
studies. Shao and co-workers [38] developed a method based on a laser-based micropipette puller to prepare nanometer-sized electrodes and used them in sensing
applications. Other techniques to form nanostructures include Langmuir–Blodgett
20 Electrochemistry with Nanoparticles
648
hydrolyzed to form a sol. The sol is then electrolysed to give a coating of silanestabilized metallic particles on a conducting substrate.
Musick and co-workers have prepared nanoelectrodes [34], which are derived
from self-assembled monolayers of colloidal gold on glass surfaces. The base
monolayer for colloidal gold deposition is formed by the silanization of glass surfaces with amino- or mercaptosilane groups. Further functionalization is carried
out by two different methods. One involves the stepwise assembly of Au nanoparticles cross-linked with bifunctional groups, which gives good control over the
number of layers of deposited gold. The second procedure involves the electroless
deposition of gold on gold nanoparticles using NH 2 OH and Au
3þ ions as initiators
for the process. Electrochemical potential control has also been shown to assist in
varying the rate of Au monolayer formation and interparticle spacing, thus giving a
good handle on the morphology of the surface [35]. These electrodes can also be
characterized by several different techniques such as optical spectroscopy, AFM
and electrochemistry. Such electrodes exhibit conductivities and electrochemical
properties comparable to macroelectrodes and also have advantages in terms of
ease of preparation and ability to control the geometric shape [34]. Electrodes prepared using a layer-by-layer growth technique show a transition from microelectrode array type of voltammetry to the usual reversible behavior observed at a
bulk, planar gold electrode, depending on the number of metallic layers [34]. The
active electrochemical area of these surfaces is larger than the geometric area due
to the roughness associated with the nanocrystalline phases [34]. Microband electrodes can also be fabricated using this method and they are found to exhibit
almost ideal microelectrode behavior [34].
Another novel way of encapsulating gold particles into an electropolymerized
matrix has been developed by Willner and co-workers [36]. This involves the use of
hydrogels based on polyacrylamide that exhibit reversible swelling and shrinking
properties. The swelling is accomplished in the presence of a citrate-stabilized gold
colloid while the shrinking property is brought about by the use of acetone as
the solvent. Gold particles stabilized by citrate molecules have a high hydrogen
bonding potential and hence the particles get trapped in the acrylamide matrix
while the shrinking mechanism is operative. The method is described in Scheme
20.1. The extent of incorporation of gold particles varies as a function of the number of swelling–shrinking (breathing) cycles. This is confirmed by following the
charge transfer resistance using impedance measurements in the presence of a
ferrocyanide/ferricyanide (Fe(CN) 6
3À=4À ) redox couple. The enhanced conductivity
due to gold particle incorporation is shown in the form of a decreased charge
transfer resistance as a function of the number of breathing cycles. Bard and
co-workers [37] reported the preparation of gold nanoparticles at the tip end of
glass micropipettes and used it for scanning electrochemical microscopy (SECM)
studies. Shao and co-workers [38] developed a method based on a laser-based micropipette puller to prepare nanometer-sized electrodes and used them in sensing
applications. Other techniques to form nanostructures include Langmuir–Blodgett
20 Electrochemistry with Nanoparticles
648