01-10-2012, 04:57 PM
Electrochemical Glucose Biosensors
[attachment=34959]
Introduction
Diabetes mellitus is a worldwide public health problem.
This metabolic disorder results from insulin deficiency and
hyperglycemia and is reflected by blood glucose concentrations
higher or lower than the normal range of 80-120 mg/
dL (4.4-6.6 mM). The disease is one of the leading causes
of death and disability in the world. The complications of
battling diabetes are numerous, including higher risks of heart
disease, kidney failure, or blindness. Such complications can
be greatly reduced through stringent personal control of blood
glucose. The diagnosis and management of diabetes mellitus
thus requires a tight monitoring of blood glucose levels.
Accordingly, millions of diabetics test their blood glucose
levels daily, making glucose the most commonly tested
analyte. Indeed, glucose biosensors account for about 85%
of the entire biosensor market.
First-Generation Glucose Biosensors
First-generation glucose biosensors rely on the use of the
natural oxygen cosubstrate and generation and detection of
hydrogen peroxide (eqs 1 and 3). The biocatalytic reaction
involves reduction of the flavin group (FAD) in the enzyme
by reaction with glucose to give the reduced form of the
enzyme (FADH2)
Electroactive Interferences
The amperometric (anodic) measurement of hydrogen
peroxide at common working electrodes requires application
of a relatively high potential at which endogenous reducing
species, such as ascorbic and uric acids and some drugs (e.g.,
acetaminophen), are also electroactive. The current contributions
of these and other oxidizable constituents of biological
fluids can compromise the selectivity and hence the overall
accuracy of measurement. Considerable efforts during the
late 1980s were devoted to minimizing the interference of
coexisting electroactive compounds.
Oxygen Dependence
Since oxidase-based devices rely on the use of oxygen as
the physiological electron acceptor, they are subject to errors
resulting from fluctuations in oxygen tension and the
stoichiometric limitation of oxygen. These errors include
changes in sensor response and a reduced upper limit of
linearity. This limitation (known as the “oxygen deficit”)
reflects the fact that normal oxygen concentrations are about
1 order of magnitude lower than the physiological level of
glucose.
Several avenues have been proposed for addressing this
oxygen limitation. One approach relies on the use of masstransport-
limiting films (such as polyurethane or polycarbonate)
for tailoring the flux of glucose and oxygen, i.e.,
increasing the oxygen/glucose permeability ratio.1,47,48 A twodimensional
cylindrical electrode, designed by Gough’s
group,47,48 has been particularly attractive for addressing the
oxygen deficit by allowing oxygen to diffuse into the enzyme
region of the sensor from both directions while glucose
diffuses only from one direction (of the exposed end). This
was accomplished by using a two-dimensional sensor design
with a cylindrical gel containing GOx and an outside silicone
rubber tube which is impermeable to glucose but highly
permeable to oxygen.
Wired Enzyme Electrodes
Enzyme wiring with a redox polymer offers additional
improvements in the electrical contact between the redox
center of GOx and electrode surfaces (Figure 2). An elegant
nondiffusional route for establishing a communication link
between GOx and electrodes was developed by Heller’s
group.16,55 This was accomplished by ‘wiring’ the enzyme
to the surface with a long flexible hydrophilic polymer
backbone [poly(vinylpyridine) or poly(vinylimidazole)] having
a dense array of covalently linked osmium-complex
electron relays. The redox polymer penetrates and binds the
enzyme (through multiple lysine amines) to form a threedimensional
network that adheres to the surface. Such folding
along the GOx dramatically reduces the distance between
the redox centers of the polymer and the FAD center of the
enzyme. The resulting film conducts electrons and is
permeable to the substrate and product of the enzymatic
reaction. Electrons originating from the redox site of GOx
are thus transferred through the gel’s polymer network to
the electrode. The resulting three-dimensional redox-polymer/
enzyme networks thus offer high current outputs and fast
response and stabilize the mediator to electrode surfaces.
Modification of GOx with Electron Relays
Chemical modification of GOx with electron-relay groups
represents another attractive route for facilitating the electron
transfer between the GOx redox center and the electrode
surface. In 1984 Hill described the covalent attachment of
ferrocene-monocarboxylic acid to the lysine residues of
GOx using isobutyl choloformate,11 while Heller16 used
carbodimide coupling for attaching the same mediator to
GOx. Such covalent attachment of ferrocene groups led to
direct oxidation of the flavin center of GOx at unmodified
electrodes with the bound ferrocenes allowing electron
tunneling in a number of consecutive steps. Bartlett described
the carbodimide-based covalent attachment of TTF to the
peptide backbone of GOx.20 Direct oxidation of the FAD
centers of the enzyme was demonstrated without the need
for soluble species.
Solid-State Glucose Sensing Devices
The unique electrical properties of 1-dimensional nanomaterials,
such as carbon nanotubes, have been shown to be
useful for developing conductivity based nanosensors for
glucose.69 Dekker’s group demonstrated that GOx-coated
semiconducting SWNTs act as sensitive pH sensors and that
the conductance of GOx-coated semiconducting SWNTs
changes upon addition of glucose substrate (Figure 5). A
conductivity-based glucose nanobiosensor based on conducting-
polymer-based nanogap has been developed by Tao and
co-workers.70 Such nanojunction-based sensor was formed
by using polyaniline/glucose oxidase for bridging a pair of
nanoelectrodes separated with a small gap (ca. 20-60 nm).
[attachment=34959]
Introduction
Diabetes mellitus is a worldwide public health problem.
This metabolic disorder results from insulin deficiency and
hyperglycemia and is reflected by blood glucose concentrations
higher or lower than the normal range of 80-120 mg/
dL (4.4-6.6 mM). The disease is one of the leading causes
of death and disability in the world. The complications of
battling diabetes are numerous, including higher risks of heart
disease, kidney failure, or blindness. Such complications can
be greatly reduced through stringent personal control of blood
glucose. The diagnosis and management of diabetes mellitus
thus requires a tight monitoring of blood glucose levels.
Accordingly, millions of diabetics test their blood glucose
levels daily, making glucose the most commonly tested
analyte. Indeed, glucose biosensors account for about 85%
of the entire biosensor market.
First-Generation Glucose Biosensors
First-generation glucose biosensors rely on the use of the
natural oxygen cosubstrate and generation and detection of
hydrogen peroxide (eqs 1 and 3). The biocatalytic reaction
involves reduction of the flavin group (FAD) in the enzyme
by reaction with glucose to give the reduced form of the
enzyme (FADH2)
Electroactive Interferences
The amperometric (anodic) measurement of hydrogen
peroxide at common working electrodes requires application
of a relatively high potential at which endogenous reducing
species, such as ascorbic and uric acids and some drugs (e.g.,
acetaminophen), are also electroactive. The current contributions
of these and other oxidizable constituents of biological
fluids can compromise the selectivity and hence the overall
accuracy of measurement. Considerable efforts during the
late 1980s were devoted to minimizing the interference of
coexisting electroactive compounds.
Oxygen Dependence
Since oxidase-based devices rely on the use of oxygen as
the physiological electron acceptor, they are subject to errors
resulting from fluctuations in oxygen tension and the
stoichiometric limitation of oxygen. These errors include
changes in sensor response and a reduced upper limit of
linearity. This limitation (known as the “oxygen deficit”)
reflects the fact that normal oxygen concentrations are about
1 order of magnitude lower than the physiological level of
glucose.
Several avenues have been proposed for addressing this
oxygen limitation. One approach relies on the use of masstransport-
limiting films (such as polyurethane or polycarbonate)
for tailoring the flux of glucose and oxygen, i.e.,
increasing the oxygen/glucose permeability ratio.1,47,48 A twodimensional
cylindrical electrode, designed by Gough’s
group,47,48 has been particularly attractive for addressing the
oxygen deficit by allowing oxygen to diffuse into the enzyme
region of the sensor from both directions while glucose
diffuses only from one direction (of the exposed end). This
was accomplished by using a two-dimensional sensor design
with a cylindrical gel containing GOx and an outside silicone
rubber tube which is impermeable to glucose but highly
permeable to oxygen.
Wired Enzyme Electrodes
Enzyme wiring with a redox polymer offers additional
improvements in the electrical contact between the redox
center of GOx and electrode surfaces (Figure 2). An elegant
nondiffusional route for establishing a communication link
between GOx and electrodes was developed by Heller’s
group.16,55 This was accomplished by ‘wiring’ the enzyme
to the surface with a long flexible hydrophilic polymer
backbone [poly(vinylpyridine) or poly(vinylimidazole)] having
a dense array of covalently linked osmium-complex
electron relays. The redox polymer penetrates and binds the
enzyme (through multiple lysine amines) to form a threedimensional
network that adheres to the surface. Such folding
along the GOx dramatically reduces the distance between
the redox centers of the polymer and the FAD center of the
enzyme. The resulting film conducts electrons and is
permeable to the substrate and product of the enzymatic
reaction. Electrons originating from the redox site of GOx
are thus transferred through the gel’s polymer network to
the electrode. The resulting three-dimensional redox-polymer/
enzyme networks thus offer high current outputs and fast
response and stabilize the mediator to electrode surfaces.
Modification of GOx with Electron Relays
Chemical modification of GOx with electron-relay groups
represents another attractive route for facilitating the electron
transfer between the GOx redox center and the electrode
surface. In 1984 Hill described the covalent attachment of
ferrocene-monocarboxylic acid to the lysine residues of
GOx using isobutyl choloformate,11 while Heller16 used
carbodimide coupling for attaching the same mediator to
GOx. Such covalent attachment of ferrocene groups led to
direct oxidation of the flavin center of GOx at unmodified
electrodes with the bound ferrocenes allowing electron
tunneling in a number of consecutive steps. Bartlett described
the carbodimide-based covalent attachment of TTF to the
peptide backbone of GOx.20 Direct oxidation of the FAD
centers of the enzyme was demonstrated without the need
for soluble species.
Solid-State Glucose Sensing Devices
The unique electrical properties of 1-dimensional nanomaterials,
such as carbon nanotubes, have been shown to be
useful for developing conductivity based nanosensors for
glucose.69 Dekker’s group demonstrated that GOx-coated
semiconducting SWNTs act as sensitive pH sensors and that
the conductance of GOx-coated semiconducting SWNTs
changes upon addition of glucose substrate (Figure 5). A
conductivity-based glucose nanobiosensor based on conducting-
polymer-based nanogap has been developed by Tao and
co-workers.70 Such nanojunction-based sensor was formed
by using polyaniline/glucose oxidase for bridging a pair of
nanoelectrodes separated with a small gap (ca. 20-60 nm).