of phenol to form o-quinone that is electrochemically active. Gold nanoparticles
allow a large degree of freedom in the orientation of the enzyme molecules. Additionally, the catalytic sites are easily accessible to substrate molecules. Casella and
co-workers have electrochemically deposited gold nanoparticles on GCE surfaces
and used these for sensing carbohydrates in flow injection and liquid chromatography modes [162]. Copper electrodeposited in a similar way leads to a sensor for
glucose [163]. A very low detection limit of 0.8 pmol of glucose is obtained with a
dynamic range extending over four orders of magnitude of the analyte.
Three-dimensional superstructures consisting of gold nanoparticles and a hemecontaining undecapeptide, microperoxidase-11 (MP-11) have been assembled on
ITO electrodes [164]. The base monolayer consists of an aminosilane that is crosslinked with MP-11 using 1-(3-dimethylaminopropyl)-3-methylcarbodiimide (EDC).
When this assembly is treated with citrate-stabilized gold (13 nm diameter) colloid,
the amino groups associated with the lysine and valine residues of the enzyme
concentrate the metallic particles. This is further treated with cystamine to yield
amino groups on the surface of the anchored gold particles. The procedure of enzyme modification is continued to obtain multilayers as shown in Scheme 20.3.
Electrocatalytic reduction of H 2 O 2 is demonstrated using this assembly. Dong and
co-workers [165] used the concept of immobilizing gold nanoparticles into a silicate network assembled on an electrode surface through specific AuaS interactions. This assembly is used to anchor horseradish peroxidase enzyme to analyze
hydrogen peroxide. The biosensor has resulted in a fast amperometric response of
2.5 s for the detection of H 2 O 2 with a detection limit of 2 mM. The sensor is also
reported to exhibit high sensitivity, good reproducibility and long-term stability.
The co-deposition of aminosilicate-stabilized gold particles and an enzyme, glucose
oxidase on an ITO electrode surface is reported to result in a biosensor for glucose
[166]. Since an all-aqueous medium is used in this process, the denaturation
of the enzyme is largely hindered. Typical chronoamperometric traces (Figure
20.13) show the response of the sensor for various glucose additions. Chen and
co-workers suggested yet another method for the amperometric determination
of H 2 O 2 using HRP immobilized gold nanoparticles of different size via a long
cysteamine/glutaraldhyde/cysteamine bridge [167]. This assembly, however, does
not show an efficient direct electron transfer and hence catechol is used as an
electron transfer mediator. Subsequently, the same authors directly immobilized
HRP to uniformly small Au colloids via a short cystamine chain on a gold electrode
[168]. This reduced the chain length, eliminating the need for an electron transfer
mediator. D’Souza and Sampath have prepared bimetallic nanocomposites of
PtaPd in a single step using organically modified silicates as a matrix as well as a
stabilizer. XPS and CO adsorption studies on PdaPt bimetals suggest a Pd core
and Pt shell type of structure. Thin films of PdaPt bimetal deposited on a GCE are
found to be very good for the amperometric determination of ascorbic acid. The
response of the sensor is fast, of the order of a few tens of seconds [169a]. The
concept of immobilization of gold nanoparticles on a cystamine-modified gold
mesh has been used to determine and separate dopamine (DA) from ascorbic acid
20.6 Nanosensors 671
allow a large degree of freedom in the orientation of the enzyme molecules. Additionally, the catalytic sites are easily accessible to substrate molecules. Casella and
co-workers have electrochemically deposited gold nanoparticles on GCE surfaces
and used these for sensing carbohydrates in flow injection and liquid chromatography modes [162]. Copper electrodeposited in a similar way leads to a sensor for
glucose [163]. A very low detection limit of 0.8 pmol of glucose is obtained with a
dynamic range extending over four orders of magnitude of the analyte.
Three-dimensional superstructures consisting of gold nanoparticles and a hemecontaining undecapeptide, microperoxidase-11 (MP-11) have been assembled on
ITO electrodes [164]. The base monolayer consists of an aminosilane that is crosslinked with MP-11 using 1-(3-dimethylaminopropyl)-3-methylcarbodiimide (EDC).
When this assembly is treated with citrate-stabilized gold (13 nm diameter) colloid,
the amino groups associated with the lysine and valine residues of the enzyme
concentrate the metallic particles. This is further treated with cystamine to yield
amino groups on the surface of the anchored gold particles. The procedure of enzyme modification is continued to obtain multilayers as shown in Scheme 20.3.
Electrocatalytic reduction of H 2 O 2 is demonstrated using this assembly. Dong and
co-workers [165] used the concept of immobilizing gold nanoparticles into a silicate network assembled on an electrode surface through specific AuaS interactions. This assembly is used to anchor horseradish peroxidase enzyme to analyze
hydrogen peroxide. The biosensor has resulted in a fast amperometric response of
2.5 s for the detection of H 2 O 2 with a detection limit of 2 mM. The sensor is also
reported to exhibit high sensitivity, good reproducibility and long-term stability.
The co-deposition of aminosilicate-stabilized gold particles and an enzyme, glucose
oxidase on an ITO electrode surface is reported to result in a biosensor for glucose
[166]. Since an all-aqueous medium is used in this process, the denaturation
of the enzyme is largely hindered. Typical chronoamperometric traces (Figure
20.13) show the response of the sensor for various glucose additions. Chen and
co-workers suggested yet another method for the amperometric determination
of H 2 O 2 using HRP immobilized gold nanoparticles of different size via a long
cysteamine/glutaraldhyde/cysteamine bridge [167]. This assembly, however, does
not show an efficient direct electron transfer and hence catechol is used as an
electron transfer mediator. Subsequently, the same authors directly immobilized
HRP to uniformly small Au colloids via a short cystamine chain on a gold electrode
[168]. This reduced the chain length, eliminating the need for an electron transfer
mediator. D’Souza and Sampath have prepared bimetallic nanocomposites of
PtaPd in a single step using organically modified silicates as a matrix as well as a
stabilizer. XPS and CO adsorption studies on PdaPt bimetals suggest a Pd core
and Pt shell type of structure. Thin films of PdaPt bimetal deposited on a GCE are
found to be very good for the amperometric determination of ascorbic acid. The
response of the sensor is fast, of the order of a few tens of seconds [169a]. The
concept of immobilization of gold nanoparticles on a cystamine-modified gold
mesh has been used to determine and separate dopamine (DA) from ascorbic acid
20.6 Nanosensors 671