U.S. patent application number 13/517471 was filed with the patent office on 2012-10-11 for carbon-based electrodes with graphene modification.
This patent application is currently assigned to PROXIM DIAGNOSTICS. Invention is credited to Mikhail Briman, Vikram Joshi.
Application Number | 20120255860 13/517471 |
Document ID | / |
Family ID | 44196120 |
Filed Date | 2012-10-11 |
United States Patent
Application |
20120255860 |
Kind Code |
A1 |
Briman; Mikhail ; et
al. |
October 11, 2012 |
CARBON-BASED ELECTRODES WITH GRAPHENE MODIFICATION
Abstract
Certain embodiments of the present application describe a
carbon-based electrode with graphene platelets. The addition of
graphene platelets is intended to improve properties of the
electrode. These properties include, but are not limited to,
physical, electrical, and biochemical properties of the electrode.
Enhanced reproducibility of these properties can also result from
the addition of the graphene platelets.
Inventors: |
Briman; Mikhail; (Sunnyvale,
CA) ; Joshi; Vikram; (Santa Monica, CA) |
Assignee: |
PROXIM DIAGNOSTICS
Sunnyvale
CA
|
Family ID: |
44196120 |
Appl. No.: |
13/517471 |
Filed: |
December 21, 2010 |
PCT Filed: |
December 21, 2010 |
PCT NO: |
PCT/US10/61503 |
371 Date: |
June 20, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61290130 |
Dec 24, 2009 |
|
|
|
Current U.S.
Class: |
204/403.15 ;
204/400; 977/734 |
Current CPC
Class: |
H01B 1/04 20130101; G01N
27/308 20130101; G01N 27/3275 20130101 |
Class at
Publication: |
204/403.15 ;
204/400; 977/734 |
International
Class: |
G01N 27/30 20060101
G01N027/30; G01N 27/327 20060101 G01N027/327 |
Claims
1. A first plurality of sensor electrodes, each comprising: a
carbon-based layer; and a layer of graphene platelets, wherein the
layer of graphene platelets modifies surface properties of the
carbon-based layer such that the first plurality of sensor
electrodes has a narrower distribution of electrochemical
properties than does a second plurality of sensor electrodes
comprising the carbon-based layer but not the layer of graphene
platelets.
2. The first plurality of sensor electrodes of claim 1, wherein the
first plurality of sensor electrodes is electrochemically activated
such that the first plurality of sensor electrodes has a narrower
distribution of biochemical properties than does a third plurality
of sensor electrodes comprising the carbon-based layer and the
layer of graphene platelets but that has not been electrochemically
activated.
3. The first plurality of sensor electrodes of claim 2, wherein the
first plurality of sensor electrodes is electrochemically activated
through voltammetric cycling until a pre-defined value of
electrochemical capacitance is met.
4. The first plurality of sensor electrodes of claim 1, wherein the
first plurality of sensor electrodes is characterized by at least
one of lower nonspecific binding of biomolecules, increased
signal-to-noise ratio, lower detection limits and higher
electrochemical activity as compared to the second plurality of
sensor electrodes.
Description
[0001] This application claims priority to PCT Application No.
PCT/US2010/061503, filed Dec. 21, 2010 and entitled "CARBON-BASED
ELECTRODES WITH GRAPHENE MODIFICATION," and U.S. Provisional
Application No. 61/290,130, filed Dec. 24, 2009 and entitled
"CARBON-BASED ELECTRODES WITH GRAPHENE MODIFICATION," both of which
are hereby incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to carbon-based
electrodes that are modified by graphene platelets.
BACKGROUND OF THE INVENTION
[0003] In electrochemistry, the performance of the working
electrode in a sensor is of prime importance. In electrochemical
sensor applications, the electrode of choice often consists of a
carbon-based material.
[0004] Graphene platelets are a material that has nano-scale size,
high conductivity, and a chemical resemblance to traditional
graphite. Certain processes to form graphene platelets and
formulate into ink are known in the art.
[0005] Sensors with carbon-based electrodes have then been used to
detect analytes such as glucose and dopamine. However, use of
carbon-based electrodes in sensors is held back by the magnitude
and lack of reproducibility of the electrochemical response. The
electrode's exposed surface materials, roughness and other critical
properties are most responsible for the poor sensor behavior.
SUMMARY OF THE INVENTION
[0006] The present invention describes a material that modifies a
carbon-based electrode. For biological and chemical sensing
applications, the carbon-based electrode is the transducer element
of the electrochemical sensor. As used herein, a "carbon-based"
electrode may include, but is not limited to, one that is a glassy
carbon, pyrolytic carbon film(s) and/or screen printed
electrode(s), and/or any basic electrode composed of graphene,
carbon black, carbon particles, carbon nanotubes, graphite, and/or
other form of carbon that can be electrically conductive.
[0007] Certain embodiments of the present invention may improve the
electrochemical characteristics of the carbon-based electrode. This
improvement may take the form of greater electrochemical activity
and greater overall conductivity of the electrode. Additionally,
the modification may enhance the overall efficiency of the electron
transduction process whereby the electrode is used in an
electrochemical process such as sensing.
[0008] Certain embodiments of the present invention may improve the
surface properties of the carbon-based electrode. Because the
modification can result in a surface morphology that has a feature
size comparable with the size scale of various biological molecules
(for example, IgG antibodies), the coverage or packing density as
well as coverage uniformity of the electrode surface would be
augmented. Using the electrode as part of an electrochemical
sensor, the better packing density and uniformity can translate
ultimately to greater sensitivity in measurement.
[0009] Certain embodiments may provide advantage over alternate
modification methods in regards to the reproducibility of the
carbon-based electrode in its physical, electrical and biochemical
properties. The result of the present invention may be that a group
of electrodes modified with graphene platelets would have a
narrower distribution in their physical, electrical, and
biochemical properties when compared to a group of conventional
carbon-based electrodes (i.e., that are not modified by graphene
platelets).
[0010] Electrodes modified with graphene platelets, according to
certain embodiments of the present invention, may display improved
electrochemical properties (as compared to conventional
carbon-based electrodes) including, but not limited to, lower
nonspecific binding of biomolecules, increased signal-to-noise
ratio, lower detection limits and higher electrochemical
activity.
[0011] Other features and advantages of the invention will be
apparent from the accompanying drawings and from the detailed
description. One or more of the above-disclosed embodiments, in
addition to certain alternatives, are provided in further detail
below with reference to the attached figures. The invention is not
limited to any particular embodiment disclosed; the present
invention may be employed in not only sensor applications, but in
other applications as well.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The invention is better understood from reading the
following detailed description of the preferred embodiments, with
reference to the accompanying figures in which:
[0013] FIG. 1 is a drawing depicting a film, according to certain
embodiments of the present invention, having at least one monolayer
of graphene platelet material with some overlap between individual
platelets.
[0014] FIG. 2 is a schematic of an electrode, according to certain
embodiments of the present invention, wherein the top layer is
comprised of graphene platelets (labeled "graphene layer").
[0015] FIGS. 3A-3E are schematic representations of an electrode
apparatus and device fabrication process according to certain
embodiments of the present invention, wherein top layer
modification by graphene is employed.
[0016] FIG. 4 is graph of the cyclic voltammetry plots for
graphene-modified and bare electrodes under exposure to a
tetramethylbenzidine (TMB) based aqueous buffer.
[0017] FIG. 5 is a table showing the response of graphene modified
electrodes and unmodified electrodes acting as immunological
sensors under exposure to thyroid stimulating hormone in positive
and negative control conditions.
[0018] FIG. 6 is a schematic of an electrode, according to certain
embodiments of the present invention, having a multi-layer
structure wherein at least one layer of the assembly is comprised
of graphene platelets.
[0019] FIG. 7 is a schematic of an electrode, according to certain
embodiments of the present invention, comprised of a composite of
various materials, one of which is graphene nanoplatelets.
[0020] FIG. 8 is a flow chart of a general process, according to
certain embodiments of the present invention, that may be used to
fabricate carbon-based electrodes with graphene modification.
[0021] Features, elements, and aspects of the invention that are
referenced by the same numerals in different figures represent the
same, equivalent, or similar features, elements, or aspects in
accordance with one or more embodiments of the system. Those of
ordinary skill in the art will appreciate that features, elements
and aspects of the invention depicted in the figures in a similar
or identical manner may be similar or identical, even if, for
example, a plurality of such features, elements and aspects are not
individually labeled.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0022] The invention primarily comprises modifications of
carbon-based electrodes in various arrangements with graphene
platelets. A graphene platelet is a nano-scale flake of graphene,
which may have a thickness ranging from a monolayer to many
monolayers. Referring to FIG. 1, the graphene platelets 110 can be
arranged onto a surface 120 to form a layer of material. There may
be overlap between adjacent graphene platelets to maintain an
electrically conducting pathway through the layer of material.
[0023] Such modification of a carbon-based electrode with graphene
platelets may alter the electronic properties, surface properties,
and/or reproducibility of the carbon-based electrode.
[0024] Graphene platelets, for example as described above, can
provide a sensitive transduction layer, e.g., where the
electrochemical activity of the layer is high. The electrical
conductivity of the interconnected graphene platelets may be higher
than that of the carbon-based electrode (without graphene
functionalization), and this increased conductivity can lead to
better electron transduction. For the example of screen printed
sensors, the electrons generated by captured or
electrochemically-labeled biomolecules would be more efficiently
transduced and detected, yielding greater sensitivity, i.e.,
ability to detect lower concentrations of analytes.
[0025] Graphene modification according to embodiments of the
present invention may also smooth the surface of a carbon-based
electrode, e.g., where graphene flakes having nanoscale dimensions
fill gaps (holes, pores, channels, unevenness, etc.) in a
carbon-based electrode comprised of relatively larger structured
forms of carbon (e.g., graphite). This smoothing effect can
significantly improve application (e.g., sensor properties). For
example, smoothing of the electrode surface can make the surface
more amenable to sensor molecule attachment, allowing improved,
more reliable and/or more uniform attachment; and thereby
increasing sensitivity and/or decreasing variability in sensor
applications. In addition to smoothing the surface, graphene
modification can increase the surface area of a carbon-based
electrode (e.g., due to graphene flakes' nanoscale dimensions). In
one embodiment of the present invention, graphene modified
electrodes allow greater attachment of antibodies and/or DNA onto
the electrode surface, and corresponding greater capture of target
molecules.
[0026] In addition to the structural effects of graphene
modification, graphene can be chemically more amenable to sensor
molecule attachment than other materials in the carbon-based
electrode (e.g., due to functionalization at the edges of graphene
flakes). In certain embodiments of the present invention,
antibodies can be attached to the electrode surface via adsorption
or through carbon chemistry to covalently attach them to the
graphene layer. There may further be an advantage due to more
carboxyl groups on the graphene surface for this covalent
attachment of the antibodies.
[0027] Because of the aforementioned electrical effect, structural
effect and smoothing effect of graphene modification according to
embodiments of the present invention, such graphene modification
can narrow the distribution and variability in a carbon-based
electrode's surface, electronic, chemical properties. Hence, the
electrodes modified with graphene according to the present
invention would be more reproducible--important not only for
functionality, but also for manufacturability.
First Embodiment--Top Layer
[0028] For the first embodiment, there exists a base electrode
layer that is composed of some amount of carbon-based material that
sits atop a substrate. On top of the base electrode layer is a top
layer of graphene platelets that may be in combination with a
binder.
[0029] Referring to FIG. 2, the entire structure may be formed atop
a substrate 210. A carbon-based electrode is the base layer 220.
The graphene layer may then be deposited as to be the top layer
230.
[0030] The top layer can thus modify the base electrode layer and
would be at least one monolayer in thickness. The effective
structure setup such that the top layer is would be exposed to the
environment. The top layer may or may not be continuous in all
places.
[0031] An electrode apparatus according to certain embodiments of
the present invention comprised an 8 pad electrode screen-printed
sensor ("8-plex device"). This device was employed to demonstrate
improvement of the electrochemical properties of the electrode and
witness the performance effects of the electrode acting as an
immunological sensor after graphene modification of the
electrode.
[0032] Referring to FIG. 3A, the 8-plex device may be fabricated by
first screen-printing silver ink contact traces 310 on a plastic
flexible substrate.
[0033] Referring to FIG. 3B, silver/silver chloride (Ag/AgCl) inks
may then be screen-printed to form a reference electrode 320.
[0034] Referring to FIG. 3C, carbon inks (e.g., proprietary inks)
may then be screen-printed to form 8 sensors (working electrodes,
sensor electrodes) 330.
[0035] Referring to FIG. 3D, an insulation layer 340 may then be
screen-printed, for example, to have only carbon sensing pads and
reference electrodes be exposed to liquid.
[0036] Referring to FIG. 3E, a plastic top part 350 with 8 openings
may then be glued on top of the plastic to form 8 isolated wells
for liquid application.
[0037] Electrode apparatuses, e.g., 8-plex devices as described
above, according to certain embodiments of the present invention
may be connected to an electronic measuring setup that allows
setting the user-defined voltage V (V) on the reference electrode
and measuring individual pad currents I (A).
[0038] The 8-plex device may be modified by graphene in the
following method. The 8-plex device may be placed on a heated
surface at 50 C. Then a solution of graphene ink that may consist
of graphene platelets in ethanol may be drop-casted onto an
individual sensor pad of an 8-plex device and may be repeated as to
modify other pads. Some pads of the device may be left unmodified.
The method may be used to deposit at least a single monolayer of
graphene or many monolayers by controlling the number of drops
deposited onto the sensor pad (note: the monolayer(s) may or may
not be continuous in all places). In an experimental embodiment of
the present invention, each modified pad had an average of 3
monolayers of graphene, i.e., the equivalent of 1 nm extra
thickness was introduced onto the bare electrode surface.
[0039] Referring to FIG. 4, the 8-plex device was measured by
cyclic voltammetry to compare the effect of top layer graphene
modification versus bare electrodes under exposure to
Tetramethylbenzidine (TMB) based aqueous buffer (substrate). TMB is
an electrochemically active substance that can be oxidized and
reduced on carbon-based working electrodes at potentials ranging
from approximately 0.2 to 0.5 V vs. Ag/AgCl reference electrode.
TMB was selected because of its contribution to amperometric signal
generation when employed in immunological sensors.
[0040] Analysis of the experiment, corresponding to certain
embodiments of the present invention, shows a typical result
comparing electrochemical performance of the graphene modified
electrode and the bare carbon-based electrode in FIG. 4. It was
observed that after graphene modification, the magnitude of
oxidation and reduction currents increased by 26% and 27%
respectively, indicating an improvement in electrochemical activity
of electrode interaction with a TMB-based substrate. This change in
activity would improve performance characteristics of chemical and
biological sensors, in particular immunological sensors, that
employ graphene-modified electrodes.
[0041] The graphene-modification procedure according to certain
embodiments of the present invention, e.g. as described above, may
be employed to improve properties of an immunological
electrochemical biosensor that has carbon-based electrodes. The
sample embodiment below describes a Thyroid Stimulating Hormone
assay that served as a model system.
[0042] Construction of an Immunological Sensor
[0043] Immunological sensors according to certain further
embodiments of the present invention were constructed on two 8-plex
devices. First, 4 pads of an 8-plex device were modified with
graphene, leaving the other 4 pads unmodified. Next, all 8 pads
were covered with 30 .mu.g/ml monoclonal anti-TSH (Thyroid
Stimulating Hormone) capture antibodies in a proprietary coating
buffer and left in a humidity chamber for 2 hours at room
temperature. After this, the devices were rinsed twice with PBST
(phosphate buffered saline+0.05% Tween 20) and covered with a
proprietary protein blocking agent to cover areas on the sensor
surface where no antibodies attached. After 1 hour of blocking at
room temperature, the blocking agent was aspirated and devices were
dried.
[0044] Two different detection analytes were prepared. The first
consisted of protein-based buffer that contained 500 ng/ml anti-TSH
antibodies labeled with HRP (horseradish peroxidase) to serve as
detection antibodies. This analyte was designed to serve as
negative control. The second analyte designed to serve as the test
material (hereafter, positive control) was made exactly in the same
manner as negative control, but in addition the analyte was spiked
with TSH reference material to a concentration of 25 .mu.IU/mL.
[0045] Of the sensor pads, 4 were exposed to the positive control,
while another 4 pads were exposed to the negative control. The
devices were put on a rotational shaker and were allowed to react
with analyte for 30 minutes at room temperature. As is common with
ELISA experiments, the positive control case, had antibodies on the
electrode surface that would capture TSH from one side, and the
detection antibodies would attach to the other side of TSH protein
forming an antibody "sandwich" structure. In contrast, for the
negative control case, the undesired non-specific binding of the
reporter antibodies to the electrode surface would be present and
no "sandwich" would be formed.
[0046] After 30 minutes of incubation, the 8-plex devices were
washed several times with PBST to remove unbound analyte and
detection antibodies. All pads were then exposed to TMB-based
substrate that upon reaction with HRP labels on the reporter
antibodies produces an electrochemically active species that is
detected amperometrically by an electronic measurement of the
device.
[0047] Referring to FIG. 5, the table displays sensor response for
two 8-plex devices, 4 positive and 4 negative control signals on
the graphene modified electrodes, as well as similar data points
for unmodified electrodes. In the case of graphene-modified
electrodes, the average of 4 positive control signals was 1359 nA,
while the average of 4 negative controls was 2.35 nA, providing a
signal-to-noise ratio was 579.4. For unmodified electrodes, the
average positive and negative signals were 1159 nA and 2.56 nA,
respectively. The signal-to-noise ratio here was 453.2. From the
measurements, the graphene modification showed improvement in
signal-to-noise ratio by approximately 28%. Therefore, electrode
modification by graphene would result in greater sensitivity, which
translates to improvement in the limit of detection for a given
immunological sensor.
[0048] Further improvements in the graphene modification process
described above may lead to even greater improvement in
signal-to-noise ratio. And more automated and non-manual
depositions methods for graphene ink may lead to reduced
variability amongst the response of modified electrodes. Those of
ordinary skill in the art will appreciate that the scope of the
present invention is not limited to improvement levels described
above (e.g., data from present experimental embodiments).
Second Embodiment--Multilayer Structure
[0049] For the second embodiment, a multilayer structure could
provide more structural integrity, add functionality, provide
conductivity, enhance electrochemical properties or be used as
adhesion layer(s) for other carbon-based layers. Any number of
layers or composite layers could be used. In particular, multiple
layers of graphene could be incorporated adjacent to other material
layers. Alternatively, the multilayer structure could be
exclusively made up of graphene platelets whereby it is built up
layer by layer. For this structure, there would be at least one
graphene layer with a minimum thickness of a monolayer (note: the
monolayer(s) may or may not be continuous in all places).
[0050] Referring to FIG. 6, carbon layers may sit atop a substrate
610. For the next layers 620, 630, and 640, at least one layer may
comprise graphene platelets and at least one layer may be a
carbon-based electrode. Other layers may comprise other materials
that may include polymers, small molecules, biological molecules,
metals, ceramics, or nanostructured materials in the form of wires,
particles, flakes, etc.
[0051] For example, one may start with a graphene ink as stated
above and other materials that may be assembled into a multi-layer
structure. For this example, the base electrode layer may be screen
printed carbon on top of a plastic substrate. Then layers of other
materials consisting of polymers, metal nanoparticles, carbon
micro/nanostructures, including graphene or the like, may be
constructed on top of each other. The top layer 640 may be composed
of graphene platelets.
Third Embodiment--Composite Electrode
[0052] For the third embodiment, a composite material is used to
form the entirety of the carbon-based electrode. The composite
electrode would primarily start with a recipe for that of a
conventional carbon-based electrode. The composite material would
encompass any ratio of materials that involves some amount of the
graphene nanoplatelets. The other material components may be other
carbon micro/nanomaterials, metals, inorganics, or organic
species.
[0053] Referring to FIG. 7, the composite electrode sits atop a
substrate 710. The electrode 720 comprises various materials, e.g.,
nanoparticles 730, graphene platelets 740, and/or
nanorods/nanotubes 750 in a matrix 760 that binds the assembly
together.
[0054] For example, one can form this composite by establishing an
ink formulation consisting of at least graphite and graphene
platelets, solvent, and a binder. Additionally, other non-carbon
materials may be used such as metal nanoparticles. This ink or
paste would then be deposited, e.g., through a screen printed
process, to form the composite electrode on top of the
substrate.
Fourth Embodiment--Deposition Techniques for Graphene Layer
Formation
[0055] In certain embodiments of the present invention, various
techniques may be used in the formation of the structures described
above. In a typical format, the graphene platelets are part of an
ink. This ink is then deposited through a given method to form a
film with the intent of placement onto a substrate or a material
layer of interest. The various ink deposition methods may include
but are not limited to spray painting, drop casting, spin coating,
vacuum filtration, dip coating, screen printing, and ink-jet
printing. Certain methods may provide droplet size or other
conditions that lead to ultra-thin layers, greater uniformity of
films, and control of surface and thickness parameters.
[0056] Referring to FIG. 8, a general process is stated for
graphene layer formation. It starts with an ink where at least one
component comprises graphene platelets. Then a deposition method is
applied to a surface or substrate. Next, a possible drying phase is
performed to evaporate the solvent. Finally, the assembly is
subjected to post-processing, which may include, but is not limited
to, various chemical, biological, or physical treatments.
[0057] For example, one may use a commercial inkjet printer from
Dimatix to apply the graphene layer in a very controlled fashion.
The typical drop volume from the printer cartridge can be around 1
pL, so the drop size is approximately 1 .mu.m. Such a fine mist
allows accurate control of the surface thickness to the nano-scale.
This control is in contrast to conventional screen printing
whereby, the ink is pushed through a metal mesh to make a several
hundred micron thick ink layer that later is dried. After the
curing process, such a procedure produces typical surface roughness
on the order of several microns as opposed to nanometers when using
inkjet.
Fifth Embodiment--Electrochemical Activation
[0058] For the fifth embodiment, a carbon-based electrode may be
electrochemically activated before, after and/or during graphene
modification. Electrochemical activation may be used to increase
the uniformity and reproducibility of carbon-based electrodes, and
may employ measurement of electrochemical capacitance (e.g.,
because this value may increase as more electrochemically active
surface groups are introduced during a given activation process,
and the activation process may be allowed to continue until a
target degree of activation is met, which may be some predefined
electrode capacitance value).
[0059] For example, a carbon-based electrode modified with graphene
according to embodiments of the present invention may be exposed to
an aqueous solution, and a potential may be applied to the sensor
in the cyclic manner. After application of activation potential
(Vact) versus the potential of the reference electrode for a
certain period (Tact), the potential may be brought back down to
the value (Vcontrol) where the sensor does not normally participate
in faradic reactions with electrolyte, and the controlling
capacitance measurement is performed. After remaining at this
potential for a predefined amount of time (Twait), a small sine
wave may be superimposed around Vcontrol for a time that would be
necessary enough to measure the capacitance (Tmeasure). For a given
measurement, both sensor potential V(t) and current I(t) may be
recorded, and at the end of each cycle the last sine wave may be
analyzed and its amplitude Iampl, offset Iofst, and phase shift
(.phi.) between voltage and current may be calculated. The
capacitance may then be estimated by the following equation:
C = I ampl V ampl 2 .pi. f sin ( .PHI. ) EQ . 1 ##EQU00001##
[0060] where Vampl is the amplitude of the applied potential wave,
and f is the frequency.
[0061] The activation cycle may be ended when the capacitance
becomes greater or equal to some predefined target value
Cfinal.
[0062] In certain sample embodiments of the present invention,
preferred values of the parameters mentioned above may be set but
not limited to Vact=1.5 V, Tact=1 sec, Vcontrol=0.2 V, Twait=2 sec,
Tmeasure=2 sec, f=10 Hz, Vampl=0.015 V.
[0063] The present invention has been described above with
reference to preferred features and embodiments. Those skilled in
the art will recognize, however, that changes and modifications may
be made in these preferred embodiments without departing from the
scope of the present invention. For example, those skilled in the
art will recognize that although exemplary embodiments have been
described above with respect to carbon-based electrodes, the
present invention is also applicable to electrodes comprising other
conductive materials (e.g., gold, platinum, palladium, etc.).
* * * * *