U.S. patent number 10,948,441 [Application Number 15/764,811] was granted by the patent office on 2021-03-16 for high-resolution in situ electrochemical nmr with interdigitated electrodes.
This patent grant is currently assigned to Georgetown University. The grantee listed for this patent is Georgetown University. Invention is credited to Eric G. Sorte, YuYe J. Tong.
United States Patent |
10,948,441 |
Tong , et al. |
March 16, 2021 |
High-resolution in situ electrochemical NMR with interdigitated
electrodes
Abstract
A system for carrying out electrochemical nuclear magnetic
resonance spectroscopy (EC-NMR) is disclosed, along with methods of
manufacturing the EC-NMR system, and methods of using the EC-NMR
system to monitor electrochemical reactions. The system comprises
interdigitated electrodes arranged in a cylindrically symmetric
manner. The system allows for nuclear magnetic resonance
spectroscopy to be carried out on a sample during electrolysis with
minimal effect to its sensitivity.
Inventors: |
Tong; YuYe J. (Gaithersburg,
MD), Sorte; Eric G. (Albuquerque, NM) |
Applicant: |
Name |
City |
State |
Country |
Type |
Georgetown University |
Washington |
DC |
US |
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Assignee: |
Georgetown University
(Washington, DC)
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Family
ID: |
1000005424401 |
Appl.
No.: |
15/764,811 |
Filed: |
September 30, 2016 |
PCT
Filed: |
September 30, 2016 |
PCT No.: |
PCT/US2016/054959 |
371(c)(1),(2),(4) Date: |
March 29, 2018 |
PCT
Pub. No.: |
WO2017/059337 |
PCT
Pub. Date: |
April 06, 2017 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
|
US 20180284042 A1 |
Oct 4, 2018 |
|
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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62236772 |
Oct 2, 2015 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N
24/088 (20130101); G01R 33/46 (20130101); G01R
33/30 (20130101); C25B 1/00 (20130101) |
Current International
Class: |
G01N
24/08 (20060101); G01R 33/46 (20060101); G01R
33/30 (20060101); C25B 1/00 (20210101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Hawkins; Dominic E
Attorney, Agent or Firm: Klarquist Sparkman, LLP
Government Interests
STATEMENT OF GOVERNMENT INTEREST
This invention was made with government support under grant number
DE-FG02-07ER15895 awarded by the Department of Energy. The
government has certain rights in the invention.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application is the U.S. National Stage of International
Application No. PCT/US2016/054959, filed Sep. 30, 2016, which was
published in English under PCT Article 21(2), which in turn claims
the benefit of the earlier filing date of U.S. provisional
application No. 62/236,772, filed Oct. 2, 2015, which is
incorporated herein by reference in its entirety.
Claims
The invention claimed is:
1. A method of performing nuclear magnetic resonance spectroscopy
during an electrochemical reaction comprising: supplying an
interdigitated electrode acting as a working electrode; supplying
an interdigitated electrode acting as a counter electrode; and
supplying an NMR sample tube, wherein the interdigitated electrodes
are positioned inside the NMR sample tube such that the electrodes
are in an NMR detection region of the NMR sample tube, and wherein
the interdigitated electrodes are cylindrically symmetric; and
performing nuclear magnetic resonance spectroscopy in the NMR
detection region that comprises applying a magnetic field having a
skin depth, and wherein the working electrode and the counter
electrode each have a thickness of 0.1 to 2.5% of the magnetic
field skin depth.
2. The method of performing nuclear magnetic resonance spectroscopy
during an electrochemical reaction of claim 1, wherein the working
and counter electrodes are located on a support.
3. The method of claim 1, wherein the electrochemical reaction is a
sample undergoing electrolysis.
4. The method of claim 1, wherein the interdigitated electrodes
comprise gold or platinum deposited on a support.
5. The method of claim 1, further comprising an electrical
feed-through cap that fits on the NMR sample tube.
6. The method of claim 5, wherein the electrical feed-through cap
comprises: a working electrode metal finger extending through the
cap, a counter electrode metal finger extending through the cap;
wherein, when fitted to the NMR sample tube, the electrical fingers
make sliding contact with the corresponding interdigitated
electrodes; electrical connectors attached to each metal finger,
wherein the electrical connectors allow each metal finger to be
connected to a potentiostat; and a reference electrode positioned
through the cap.
7. A method of manufacturing an electrochemical nuclear magnetic
resonance (EC-NMR) system comprising: (a) cutting a support; (b)
applying a mask to the support; (c) depositing electrodes on the
support; (d) removing the mask to reveal an interdigitated
electrode pattern; and (e) rolling the support with the
interdigitated electrode pattern into a cylinder for inserting into
an NMR tube.
8. The method of claim 7, wherein the electrodes each have a
thickness in a range of 5-100 nanometers.
9. The method of claim 8, wherein the interdigitated electrodes
comprise gold or platinum.
10. The method of claim 7, wherein the interdigitated electrode
pattern comprises gold or platinum.
11. The method of claim 10, wherein the support is a glass,
ceramic, or polymer support, and the thickness of the
interdigitated electrode pattern is in a range of 5-25
nanometers.
12. The method of claim 7, wherein the support is a glass, ceramic,
or polymer support.
13. The method of claim 7, further comprising applying an
electrocatalyst to the interdigitated electrode pattern.
14. The method of claim 7, wherein the thickness of the
interdigitated electrode pattern is in a range of 5-100
nanometers.
15. The method of claim 7, wherein the thickness of the
interdigitated electrode pattern is in a range of 5-50
nanometers.
16. The method of claim 7, wherein the thickness of the
interdigitated electrode pattern is in a range of 5-25
nanometers.
17. The method of claim 7, further comprising cleaning the support
prior to applying the mask to the support.
Description
FIELD OF THE INVENTION
This invention relates to nuclear magnetic resonance (NMR)
spectroscopy and electrochemistry (EC).
BACKGROUND OF THE INVENTION
Nuclear magnetic resonance (NMR) spectroscopy is a powerful
analytic technique for determining the composition of unknown
samples. NMR utilizes the magnetic properties of atomic nuclei in
order to determine the physical and chemical properties of samples.
Despite its extensive presence as a powerful analytic technique it
remains underutilized in organic chemistry when studying
electrochemical reactions. This is due to the presence of
conducting metallic electrodes that are required for maintaining
current and the presence of conducting electrolytes. The presence
of the electrodes and electrolytes in electrochemical reactions is
known to warp the magnetic field. Since a homogenous magnetic field
is necessary for accurate and sensitive NMR spectroscopy, the
inclusion of such electrodes and electrolytes poses a problem.
Since the electrodes and electrolytes have adverse effects on
conventional NMR probes, which compromise their sensitivity, their
usefulness thus far has been minimal.
Additionally, electrolysis is a firmly established fundamental
technique for driving non-spontaneous chemical reactions in organic
chemistry. The ability to use NMR during electrolysis experiments
in electrochemistry and electro-synthesis would allow for a better
understanding of the reactions and their products. Despite
considerable efforts to create a system that would allow NMR
spectroscopy in the field, the community has not adopted the
technique as a general in situ tool. There is considerable need for
a simple, electrolysis-friendly, NMR design that can overcome these
remaining challenges in order to allow this powerful tool to
achieve large-scale use in electrolysis and electrochemical
investigations.
SUMMARY OF THE INVENTION
One aspect of the present invention provides for an electrochemical
NMR (EC-NMR) system that allows for high-resolution in situ NMR on
samples undergoing electrolysis. An aspect of the invention
provides a system for EC-NMR that is robust and compatible with
commercially available NMR instruments. Another aspect of the
invention provides a system for use in NMR that does not require
modification to commercial NMR probes and can be set up quickly and
inexpensively without compromising the high-resolution features of
solution NMR that are critical for molecular spectroscopy.
According to one aspect of the invention, a system for in situ
EC-NMR includes an electrode assembly comprising a plurality of
interdigitated electrodes that comprise conductive metals deposited
on a support. In an embodiment, the electrode assembly is
cylindrically symmetric. The electrodes include at least a working
electrode and a counter electrode. In certain embodiments, an
electro-catalyst is further deposited on the one or more of the
electrodes.
An embodiment of the system includes an EC-NMR system comprising:
an interdigitated electrode acting as a working electrode; an
interdigitated electrode acting as a counter electrode; which can
be inserted into an NMR sample tube. The interdigitated electrodes
can be positioned inside the NMR sample tube such that the
electrodes are in the NMR detection region of the NMR sample tube.
The electrodes may be arranged in a manner such they exhibit
cylindrical symmetry.
Examples of conductive metals for use as electrodes include, but
are not limited to, gold, copper, platinum, palladium, silver,
aluminum, zinc, nickel, brass, iron, steel, lead, and alloys
thereof.
In an embodiment of the system, the thickness of the conductive
metal layer on the electrodes is in a range of about 5-100
nanometers. Alternatively, the range is about 5-50 nanometers. In
another embodiment the range is about 5-25 nanometers.
Examples of materials to make supports include, but are not limited
to, glass, ceramics, and polymers. The support may be flexible or
rigid. For example, the support may be made from materials,
including, but not limited to polytetrafluoroethylene (TFE),
fluorinated ethylene propylene (FEP), polyethylene, polypropylene,
silicon, and silicate glasses. In an embodiment, the support is a
polyimide based support.
In an embodiment, the support has a thickness ranging from about 10
to about 100 microns. In an alternative embodiment, a range of
about 25-50 microns is used.
In certain embodiments, the electrodes comprise gold or platinum
deposited on a support, wherein the symmetry is maintained when the
support is rolled into a cylinder. The system may further comprise
an electrocatalyst, wherein the electrocatalyst is applied to one
or both interdigitated electrodes.
In an embodiment, the electrodes are made from gold and are
deposited on the support using any one of a variety of masking
techniques. Optionally, strips of copper tape may be applied to the
ends of electrodes that are not in the NMR sensitive region of the
electrode systems. The film is then rolled into a cylinder and
placed within an NMR tube. The placement of the assembly is such
that it is within the NMR detection region (e.g. located in a
specific area of an NMR tube where NMR spectroscopy will
occur).
An embodiment of the system further includes, an electrical
feed-through cap that fits on the NMR sample tube wherein the
electrical feed-through cap comprises: a working electrode metal
finger extending through the cap, a counter electrode metal finger
extending through the cap; wherein, when fitted to the NMR sample
tube, the electrical fingers make sliding contact with the
corresponding interdigitated electrodes; electrical connectors
attached to each metal finger, wherein the electrical connectors
allow each metal finger to be connected to a potentiostat; and a
reference electrode positioned through the cap.
The electrodes of the present invention may be used for NMR-EC
experiments inside NMR sample tubes, wherein the electrodes extend
into the detection region of the NMR instrument. The NMR sample
tube may have a diameter ranging from about 3-15 mm. In certain
embodiments the NMR sample tube has a diameter ranging from about
5-10 mm. In certain embodiments, the NMR sample tube has a diameter
of about 5 mm or about 10 mm.
A further object of one aspect of the invention is to provide a
method of manufacturing an EC-NMR system that is capable of driving
electrochemical reactions and or electrolysis while maintaining NMR
sensitivity comprising a thin electrode assembly that is
symmetrical when rolled into a cylindrical shape.
A further object of another aspect of the invention is to provide a
method of carrying out electrolysis and/or an electrochemical
reaction with an EC-NMR system in order to determine the make up of
a sample and or any intermediate species using NMR
spectroscopy.
In a further embodiment the invention includes a method of
performing nuclear magnetic resonance spectroscopy during an
electrochemical reaction comprising: supplying an interdigitated
electrode acting as a working electrode; supplying an
interdigitated electrode acting as a counter electrode; and
supplying an NMR sample tube, wherein the interdigitated electrodes
are positioned inside the NMR sample tube such that the electrodes
are in an NMR detection region of the NMR sample tube; and
performing nuclear magnetic resonance spectroscopy in the NMR
detection region.
In an additional embodiment the invention a method of manufacturing
electrochemical nuclear magnetic resonance (EC-NMR) system
comprising: (a) cutting a support such that it will fit, when
rolled into a cylinder, into an NMR tube; (b) applying a mask to
the support; (c) depositing electrodes on the support; and (d)
removing the mask to reveal an interdigitated electrode
pattern.
The system further includes an electrical feed-through cap that
attaches to and seals the NRM tube. The feed-through cap can be
constructed from a standard NMR tube cap. The cap includes two
metal fingers that are located there through via slits in the cap.
The fingers can be fixedly attached to the cap via epoxy or the
like. The ends of the metal fingers that protrude from the top have
male end connectors attached. Additionally, the cap includes a hole
that a reference electrode is fixed through. In an embodiment the
reference electrode is a silver wire sealed within a glass tube.
Another male end connector is attached to the reference electrode
on the top of the feed-through cap in order to provide electrical
connectivity.
When the feed-through cap and NMR tube are mated, the metal fingers
from the cap make electrical contact with the working and counter
electrode located on the film. In an embodiment the electrical
contact is made via the copper tape located on the firm.
In an alternative embodiment, the system further includes
components that provide a current to the electrodes. According an
aspect of the invention the system includes a potentiostat. The
potentiostat is electrically connected to the working, counter, and
reference electrode via electrical lines that are terminated at
female connectors that correspond to the male connectors located on
the feed-through cap. These electrical lines are further grounded
in order to reduce systemic noise.
A further aspect of the invention comprises a top cover that
removeably attaches to the feed-through cap and a tube mount. The
top cover includes a housing, the electrical cables that terminate
at female connectors with associated inductors, and a support rod.
The housing, also referred to as a delrin cap is machined to fit
into the tube mount tightly.
According to another aspect of the invention, the electrode
assembly is manufactured by (1) cutting a support (e.g. a polyimide
film) such that it will fit within an NMR tube (i.e. for a 5 mm NMR
tube the dimensions are about 1.2 cm.times.17 cm); (2) cleaning the
support by washing it in a 1:1 nitric acid in water and then in
piranha solution, with deionized water rinses in between washes;
(3) applying a mask to the film in the shape in the shape of a
designed IGE pattern; (5) depositing the electrodes on the film;
(6) optionally applying an electro-catalyst to the electrodes; (7)
apply a thin strip of copper tape to each of the leads of the
electrodes.
According to another aspect of the invention, the steps for
carrying out the method of NMR detection include; (1) providing an
electrode assembly within an NMR tube; (2) loading the NMR tube
with a desired sample; (3) applying the feed-through cap to the NRM
tube in order to seal the tube and provide electrical connectivity
for the working, counter, and reference electrode; (4) apply a
potential to the electrodes such that an electrochemical process
occurs within the tube; (5) performing NMR on the sample at set
intervals in order to determine the composition of the sample over
time.
BRIEF DESCRIPTION OF THE DRAWINGS
The following description, given by way of example and not intended
to limit the invention to the disclosed details, is made in
conjunction with the accompanying drawings, in which like
references denote like or similar elements and parts, and in
which:
FIG. 1A is a front view of an interdigitated gold electrode (IGE)
assembly placed within a 5 mm NMR tube with the feed-through cap
installed;
FIG. 1B is a close up of the IGE assembly before placement in an
NMR tube
FIG. 2A is an embodiment of the feed-through cap omitting the
reference electrode and connectors;
FIG. 2B is a an embodiment of the feed-through cap where the female
connectors have been attached;
FIG. 2C is a an embodiment of the feed-through cap where the female
connectors and reference electrode have been attached;
FIG. 3A shows an embodiment where the IGE assembly, feed-through
cap, and top cover are shown prior to the top cover fitting over
the NMR tube support;
FIG. 3B shows an embodiment where the top cover is fitted over the
NMR tube support;
FIG. 4A shows the effect of adding various concentrations of
perchloric acid to the system;
FIG. 4B shows the effect on signal intensity of the IGE electrode
assembly during NMR;
FIG. 4C shows the resistance and quality of the system versus
electrode thickness;
FIG. 5 shows a current density output of feasibility test of the
system using methanol oxidation;
FIG. 6A shows the NMR spectra for the feasibility test using
methanol oxidation;
FIG. 6B shows the integrated intensities of carbon resonances in
the NMR spectra for the feasibility test using methanol
oxidation;
FIG. 7 shows the integrated areas of the time-series of the peak
areas of the methanol and intermediate products during the
oxidation cycle during the feasibility test;
FIG. 8A shows the integrated intensities of carbon resonances in
the NMR spectra for the feasibility test using methanol
oxidation;
FIG. 8B shows the integrated areas of the time-series of the peak
areas of the methanol and intermediate products during the
oxidation cycle during the feasibility test;
FIG. 9A shows an electrochemically reversible FeIII/II or
ferrocenium/ferrocene redox couple's cyclic voltammogram as a
result of NMR spectroscopy according to the invention.
FIG. 9B. shows a graph of the concentration of paramagnetic
ferrocenium over time as compared to the applied voltage.
DETAILED DESCRIPTION
Embodiments of the invention are described below with reference to
the accompanying drawings which depict different embodiments.
However, it is to be understood that application of the invention
encompasses other uses for the invention in applications involving
nuclear magnetic resonance (NMR) spectroscopy. Also, the invention
is not limited to the depicted embodiments and the details thereof,
which are provided for purposes of illustration and not
limitation.
FIG. 1A shows a system design of an embodiment of an
electrochemical nuclear magnetic resonance spectroscopy system
(EC-NMR) including an interdigitated gold electrode (IGE) assembly,
a feed through cap, and NMR tube. Included on the IGE assembly are
interdigitated working electrode F, interdigitated counter
electrode H, and copper tape D. As can be seen in FIG. 1A the
copper tape abuts the ends of the working electrode and counter
electrode. Alternatively the tape may be applied on top of the
leads of the electrodes (so long as the tape and electrodes are in
electrical contact). Additionally, the feed-through cap includes a
reference electrode and metallic fingers B and C. The assembly also
includes 200 .mu.h indicators A as RF chokes. IGE assembly is
rolled into a cylinder and inserted into the NMR tube. The IGE
assembly is placed within the tube such that it is in the NMR
detection region (the NMR coils are designated as "I" in FIG. 1A).
An illustrative electrolyte level F is shown in FIG. 1A. The
detection region is the portion of the tube that is exposed to the
magnetic field of the NMR system.
The feed-through cap attaches to the top of NMR tube in order to
seal the interior such that NMR spectroscopy can be carried out on
a sample therein. It can also be seen from the figure that the
working electrode, counter electrode, and reference electrode are
all electrically connected to male connectors (further described
below) in order to provide and electrical current to the
electrodes. Although the specification will refer to the electrode
assembly as IGE assembly it is to be understood that "gold" is
merely a metal to be used in one embodiment of the system and the
specification and invention are not intended to be limited as such.
As will be evident from the below disclosure other materials can be
used in the creation of the electrodes. The important features of
the selected metal include: inertness, lack of reactivity to the
electrochemical reaction, and conductivity.
IGE assembly is manufactured in order to create a symmetrical
electrode arrangement that consists of a working and counter
electrode. The first step in creating IGE assembly is sizing a
support such that it will fit within NMR tube. In an embodiment of
the system the NMR tube is a 5 mm tube (e.g. 5 mm tube width),
while in an alternative embodiment the tube is a 10 mm tube.
However, a wide range of NMR tubes are suitable for use (3-10 mm
tubes). The support can be fabricated from a 25 or 50 micrometer
thick polyimide film. However, the support is not limited to
polyimide and can be constructed from an array of polymers,
ceramics, and glasses (e.g. Teflon, Kaflon, and other plastics). It
is further possible to have a non-flexible substrate that is
already formed into the desired cylindrical shape prior to use.
Additionally, the thickness of the support is not limited to 25 or
50 microns and is contemplated to be in a range of about 10-100
microns. When using a 5 mm tube, the support is cut, such that its
dimensions are approximately 1.2 cm.times.17 cm. In alternative
embodiments the dimensions range from a width of about 0.5-3 cm and
a length about of 10-40 cm. The support is then cleaned by washing
for 30 minutes in 1:1 nitric acid in water and then for 30 minutes
in piranha solution, with deionized water rinses in between washes.
Alternatively, the cleaning can be for shorter or longer periods of
time (10 minutes to an hour) and the wash can be in a wide array
concentrations (e.g. 1:10 up to 10:1 ratios and beyond). This
cleaning step is critical in order to ensure that subsequent
deposition of the electrodes provides adequate adhesion. Although
polyimide is disclosed as the material for the support above, other
materials have been contemplated as discussed.
After cleaning, a mask is applied to the support in a shape that is
complementary to the desired pattern for the electrodes. The mask
may be created using a variety of techniques including but not
limited to: cutting adhesive tape, creating a mask using a 3d
printer, cutting a mask from a thin Teflon sheet. Once the mask is
placed on the support electrodes are formed on the exposed areas.
In an embodiment, the electrodes are gold. However, other materials
have been contemplated and are suitable including silver,
silver-chloride, platinum, gold-platinum, copper, zinc, brass,
other alloys and other metals. Key characteristics in selecting the
metal are conductivity and inertness. The means for deposition of
the electrodes can be any one of the following; high-vacuum
magnetron plasma deposition (CVC); thermal evaporation; or atomic
layer deposition. It should be noted that this is not an exhaustive
list and other methods are contemplated. The electrodes are
deposited directly to the support to the desired thickness.
Additionally, the shape of the IGE electrodes is an important
aspect of the invention. In order to further reduce the warping
effects of having metal in the NMR detection region the electrodes
must be symmetrical when rolled into the cylindrical shape. It is
further advantageous to shape the working and counter electrode in
an interdigitated fashion. In such a configuration the working
electrode and counter electrode each comprise a plurality of
"digits" that alternate across a longitudinal axis of the assembly.
In an embodiment the electrodes may be patterned in differing
configurations so long as a symmetrical shape is maintained.
The thickness of the electrodes is critical to aspects of the
invention. Having thinner electrodes on the support reduces the
impact the IGE assembly will have on the magnetic field during use.
In an embodiment, the thickness is determined based upon the skin
depth of the magnetic field. Depending on the frequency of the
magnetic field produced, the NRM system will have a specific RF
penetration depth (i.e. a skin depth). The higher the frequency
(e.g. in the gigahertz range) the greater the skin depth. In an
embodiment of the system, the thickness of the electrodes is a
function of the skin depth of the magnetic field. In an aspect of
the invention, the thickness of the working and counter electrodes
is about 0.5-1.5% of the skin depth. In another aspect of the
invention the thickness is about 0.1-2.5% of the skin depth.
In an alternative embodiment, the thickness of the electrodes of
the IGE assembly is about 20-100 nm. In another alternative
embodiment a thickness of 50 nm is used. A 50 nm thickness for the
electrode deposition has found to be optimal through testing when a
frequency of 300 Mhz is used. As FIG. 4C shows the thickness of
electrodes is proportional to the ohmic drop experience from the
top of the NMR tube to the bottom. As more gold was deposited, the
ohmic drop from the top of the tube to the bottom decreased,
leading to better electrochemical performance. However, a thicker
gold coating also lead to lower quality factors and a frequency
shift of the probe resonance such that our narrow-band probe had
trouble tuning to the correct frequency. Both effects cause a
reduction in probe sensitivity. A thickness of 50 nm results in a
final loaded quality factor of 100 while keeping the resistance
under 200 ohms.
If an electrocatalyst is desired (shown as G in FIG. 1A), it is
then applied to either the working or counter electrodes (or both,
depending on application) after electrode deposition but before the
mask is removed to prevent shorting the electrodes and ensure their
electrical isolation. The mask is then removed, revealing the
interdigitated electrode.
After the electrode deposition and mask removal, thin strips of
copper tape 2 cm in length are applied to the top of each of the
leads (i.e. the work and counter electrode ends) for reinforcement.
These copper leads are important in order to ensure that when
assembled, and electrical connections are made, the electrodes are
not damaged or scratched. Although copper is mentioned above other
suitable electrical conductors have been contemplated including but
not limited to: gold, silver, platinum, zinc, brass and other
alloys. Additionally the length of the copper strips is not limited
to 2 cm but can range from about 1-10 cm in length.
FIGS. 2A-2C show the construction of the feed-through cap
previously shown in FIG. 1A. The first step in construction of the
cap, as shown in FIG. 2A, to create small slits within the cap.
These slits allow for gold-plated metal fingers to be inserted into
the cap. The metal fingers can be glued into the cap with a simply
epoxy or other glue for additional stability. The curved portions
of each of the metal fingers are intended to make electrical
contact with the copper tape such that when assembled the working
electrode and counter electrode electrically terminate outside of
the tube.
As FIG. 2B shows, once the metal fingers are secured into the cap,
the ends of the fingers that protrude from the top of the cap are
soldered to the male ends of push connectors. The use for these
push connectors will become evident based upon further disclosure
below. Lastly, as shown in FIG. 2C, a reference electrode is
attached to the cap. An additional hole is drilled in the center of
the cap for the lead to the reference electrode. In an embodiment,
this electrode is a thin chlorinated silver wire sealed in a 1.5 mm
OD glass tube terminated with a glass frit for ion conduction.
However, the wire may be made from other metals and sealed in a
glass tube of varying sizes. Like the metal fingers, the portion of
reference electrode that protrudes from the top is also fit with a
male push connector. It should be noted that the gold plating on
the metal fingers is important to avoid corrosion that can
interrupt electrical contact. Although the disclosure makes
reference to the metal fingers being coated gold other materials
have been contemplated, including but not limited to: gold, silver,
platinum, zinc, brass and other alloys.
Once assembled the feed-through cap, NMR tube, and IGE assembly are
ready for use in NMR spectroscopy. Electrical lines from a
potentiostat are terminated in female components of metal
connectors that are then attached to the NMR sample tube via male
connectors as described above. In an embodiment, the electrical
lines are RG-316 coaxial cables. However any suitable coaxial cable
may be used. For noise control purposes, it is important to ground
the coaxial cables to the magnet of the NMR spectroscopy apparatus
to establish a common electrical ground potential between the
potentiostat and the magnet system. To ground the lines, about 2 cm
of outer plastic insulation from the three electrical lines is
removed and the lines are joined together with a grounding strap.
The grounding strap is attached to the magnet, effectively
establishing a common ground.
FIGS. 3A-3B show an optional component of the system, a top cover
D. The top cover is designed to mate with the assembled system
previously described while also stabilizing the tube when it is
placed in an NMR tube mount. In an embodiment, the top cover is
constructed of 2 inch Delrin and machined to fit a wide-bore 300
MHz magnet (5 cm bore diameter for sample loading). However, other
suitable materials have been contemplated including an array of
polymers and ceramics. The coaxial cables A (i.e. the electrical
lines referenced above) for electrochemistry feed through the top
of the top cover as shown in FIG. 3A, and the attached aluminum rod
B facilitates rapid sample changes without the concerns associated
with pulling directly on the coaxial cables A to retrieve the
sample. It is convenient to mount RF isolation inductors C into the
top cover to further isolate the DC electrolysis electronics from
the high frequency NMR electronics (not shown). The coaxial cables
terminate in female components E of connectors that provide
electrical connection to the male connectors F of the feed-through
cap H. Thus, when the top cover is mounted into tube mount the male
connectors and female connectors join, creating an electrically
path between the electrodes and the potentiostat.
EXAMPLES AND TESTING
The in situ electrochemical nuclear magnetic resonance spectroscopy
system was characterized by evaluating the effects of an
electrolyte and the IGE assembly on the sensitivity and performance
of a commercial NMR probe. To determine the performance of the
design, several standard oxidation/reduction reactions on different
nuclei were performed: .sup.13C NMR during ethanol oxidation,
.sup.13C NMR methanol oxidation, and .sup.1H NMR during ferrocene
oxidation-reduction cycles. NMR was performed on a 300 MHz
(.sup.13C Larmor frequency 74.47 MHz) wide-bore magnet (not shown)
with a hybrid Bruker/Tecmag spectrometer and a standard 5 mm probe.
Electrochemistry was performed with a VoltaLab PGZ100 Radiometer
potentiostat.
Example 1
FIG. 4a shows the effect of adding various concentrations of
perchloric acid supporting electrolyte to the NMR tube with the
gold electrodes present. Clearly the ionic liquid has a dramatic
effect on the probe sensitivity. The experiments were operated at
0.01M HClO.sub.4, thereby sacrificing only approximately 20% of the
available sensitivity. With 0.01M HClO.sub.4 present in the NMR
tube, nutation curves were measured with and without the IGE
assembly to determine the effect of the gold electrodes on the
probe sensitivity. FIG. 4B shows that while there is a small
decrease in signal intensity, the length of the 90-degree pulse (at
constant RF power) is approximately the same with and without the
IGE assembly present. Thus the thin gold electrodes are not
significantly distorting or absorbing the applied RF pulse. After
shimming, the width of the resonance spectra was the same with and
without the IGE present.
Test 1: Methanol Oxidation
As a feasibility test, methanol oxidation was performed in the in
situ electrochemical nuclear magnetic resonance spectroscopy
system. For this experiment 5M .sup.13CH.sub.3OH with 0.01M
HClO.sub.4 in 1 mL D.sub.2O was used. To catalyze the reaction, 0.1
mL of PtRu black was drop-cast onto the IGE assembly, covered with
dilute Nafion solution and air-dried. Before measurements were
performed, the catalyst was activated by alternating the electrode
potential between -0.5 V and 0.8 V (vs. Ag/AgCl) for 5 s
alternatively for 10 cycles. During oxidation, the working
electrode was held at +600 mV with respect to Ag/AgCl chlorinated
silver quasi-reference electrode. While the potential was applied,
NMR was performed at 30 minute intervals to observe the buildup of
oxidation products. FIG. 5 shows the chromoamperimetric data taken
while holding the potential fixed at +0.6 V. The current of
approximately 3 mA decayed by a factor of 2 during the length of
the 15 hour experiment. The CV is shown in the inset of FIG. 5.
FIG. 6A, shows initial (lower trace) and final (upper trace)
.sup.13C NMR spectra for the methanol oxidation. The initial trace
shows only the methanol resonance at 49 ppm, as expected. The final
trace, taken after the 15 hour oxidation period, shows the presence
of the stable intermediate oxidation products formaldehyde (90.3
ppm, 82.5 ppm, and 55.3 ppm), formic acid (166.4 ppm), and the
formate ion (165.2 ppm), as well as the unoxidized methanol. FIG.
6B shows the buildup over time of the formic acid resonance,
demonstrating the in situ data that can be obtained from the
electrochemical nuclear magnetic resonance spectroscopy system.
The integrated intensities of carbon resonances in .sup.13C NMR
like those in FIG. 6b is a reliable method to monitor carbon
content, as .sup.13C peaks often do not overlap and baseline
problems common to .sup.1H NMR spectra are largely absent. The
assumption is that the area under the resonance peaks for a given
species in .sup.13C NMR spectra is proportional to the number of
carbon atoms in that species. FIG. 7A shows the integrated areas of
the time-series of the peak areas of the methanol and intermediate
products during the oxidation cycle. At the end of the oxidation
period, 80.6% of the .sup.13C atoms remained in unoxidized
methanol, 7.1% had formed formaldehyde molecules, and 5.7% had
formed formic acid as shown in FIG. 7B. Therefore 6.6% of the
original carbon content was fully oxidized to CO.sub.2 (and escaped
as gas).
Test 2: Ethanol Oxidation
Ethanol oxidation was also performed in the in situ electrochemical
nuclear magnetic resonance spectroscopy system. For this experiment
2M .sup.13CH.sub.3CH.sub.2OH (98%, Cambridge Isotope) in 0.01M
HClO.sub.4 with 1 mL D.sub.2O was used. The working electrode was
held at +600 mV with respect to the Ag/AgCl quasi-reference. While
the potential was applied, NMR was performed on the liquid
reactants at 30 minute intervals to observe the buildup of
oxidation products. The entire experiment lasted for a period of 20
hours. Activated commercial PtRu black was used as the working
electrode catalyst in this experiment.
FIG. 8A shows the buildup of the stable intermediate of ethanol
oxidation product acetic acid at 177.5 ppm as a function of time.
Acetalydehyde, another stable intermediate, is also present at 207
ppm (not shown), as is the unoxidized ethanol resonance at 58 ppm.
As with methanol oxidation, the ethanol resonance and the
intermediate products' resonances are integrated, and their areas
plotted as a function of time in FIG. 8B.
Ethanol has a much more complicated and less well-understood
oxidation pathway. The two observed stable intermediates, acetic
acid and acetaldehyde, that were observed have been seen in other
investigations of this reaction. As shown in FIG. 8B, the original
ethanol carbon content decreased sharply in the first 4 hours of
oxidation, and thereafter decreased much more slowly.
Correspondingly, the stable intermediates also increased rapidly
during the first 4 hours, and thereafter increased at a reduced
rate. At the end of 20 hours, the total carbon distribution reveals
that of the 29.5% carbon lost from the initial carbon content of
the ethanol, 8.4% ended up in acetic acid molecules and 5.5% in
acetaldehyde. The remaining 15.6% was, presumably, completely
oxidized to CO.sub.2 gas and is thus absent from the integrated
intensities of the solution state products.
Test 3: Ferrocene Redox Reactions
To demonstrate the high-resolution and multi-nuclear capabilities
of electrochemical nuclear magnetic resonance spectroscopy system,
.sup.1H NMR during ferrocene reduction was performed. Ferrocene,
Fe(C.sub.5H.sub.5).sub.2, in acetonitrile is a reversible
electrochemical system often used as a pseudo-reference for
reporting standard reduction potentials in organic phase
voltammetry. It was one of the first organometallics to be
synthesized, and is remarkably stable at room temperature. The
electrochemically reversible Fe.sup.III/II or ferrocenium/ferrocene
redox couple's cyclic voltammogram is shown in FIG. 9A.
The electrochemical nuclear magnetic resonance spectroscopy system
was prepared as described above, with the exception that no
catalyst was drop-cast on the electrodes after the gold deposition.
A solution of 15 mM ferrocene in 0.1M Bu.sub.4NPF.sub.6 in
d-acetonitrile was prepared and deoxygenated by bubbling nitrogen
through the solution for 10 minutes. Thereafter, the tip of the
nitrogen flow tube was raised above the solution while a gentle
flow of nitrogen continued to flow. This is important when applying
a negative potential as dissolved O.sub.2 may be reduced at the
working electrode and interfere with the desired reaction.
Initially, the ferrocene sample contains no paramagnetic
ferrocenium species, and a high-resolution linewidth of .DELTA.v=5
Hz (0.015 ppm) at 4.19 ppm was measured. A cyclic voltagram was
measured and the oxidation and reduction potential peaks were
identified. The sample was then held at +40 mV to oxidize the
ferrocene to ferrocenium while NMR was measured to monitor the
reaction in situ. As the concentration of paramagnetic ferrocenium
increased, the linewidth increased dramatically and shifted
downfield due to fast electron transfer among the aromatic protons
(see FIG. 9B). The potential was then changed to -60 mV
(corresponding to the reduction peak position) and NMR performed
again to monitor the reduction of ferrocenium back to ferrocene.
The reaction proceeds quickly, and the resonance is seen to narrow
and shift back to its original position as the paramagnetic species
is consumed. This reaction was repeated for many cycles by
alternating the applied potential.
It will be appreciated by those skilled in the art that various
modifications and changes can be made to the invention without
departing from the spirit and scope of this invention. Accordingly,
all such modifications and changes fall within the scope of the
appended claims and are intended to be part of this invention.
* * * * *