U.S. patent application number 13/833438 was filed with the patent office on 2014-09-18 for electrochemical detection of beta-lactoglobulin.
This patent application is currently assigned to Wisconsin Alumni Research Foundation. The applicant listed for this patent is WISCONSIN ALUMNI RESEARCH FOUNDATION. Invention is credited to Sundaram Gunasekaran, Jiang Yang.
Application Number | 20140262832 13/833438 |
Document ID | / |
Family ID | 51493314 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140262832 |
Kind Code |
A1 |
Gunasekaran; Sundaram ; et
al. |
September 18, 2014 |
ELECTROCHEMICAL DETECTION OF BETA-LACTOGLOBULIN
Abstract
A method to detect beta-lactoglobulin (BLG) is described. The
method includes the steps of adding a known concentration of
hydrogen peroxide to a sample known to, or suspected of containing
BLG; and electrolyzing the sample using a working electrode at a
fixed potential sufficient to electrolyze BLG, and measuring a
current signal within the sample. A diminution of the current
signal in the sample as compared to a corresponding current signal
from a standard solution containing a known concentration of
hydrogen peroxide and no BLG indicates that the sample contains
BLG.
Inventors: |
Gunasekaran; Sundaram;
(Madison, WI) ; Yang; Jiang; (Madison,
WI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
WISCONSIN ALUMNI RESEARCH FOUNDATION |
Madison |
WI |
US |
|
|
Assignee: |
Wisconsin Alumni Research
Foundation
Madison
WI
|
Family ID: |
51493314 |
Appl. No.: |
13/833438 |
Filed: |
March 15, 2013 |
Current U.S.
Class: |
205/780.5 |
Current CPC
Class: |
G01N 27/28 20130101;
G01N 27/30 20130101; G01N 27/26 20130101 |
Class at
Publication: |
205/780.5 |
International
Class: |
G01N 27/26 20060101
G01N027/26 |
Claims
1. A method to detect beta-lactoglobulin (BLG), the method
comprising: (a) adding a known concentration of hydrogen peroxide
to a sample known to, or suspected of containing BLG; and (b)
electrolyzing the sample of step (a) using a working electrode at a
fixed potential sufficient to electrolyze BLG, and measuring a
current signal within the sample, wherein a diminution of the
current signal in the sample as compared to a corresponding current
signal from a standard solution containing the known concentration
of hydrogen peroxide and no BLG indicates that the sample contains
BLG.
2. The method of claim 1, wherein step (b) comprises electrolyzing
the sample using a working electrode comprising a transition metal
or an oxide of a transition metal.
3. The method of claim 1, wherein step (b) comprises electrolyzing
the sample using a working electrode comprising a transition
metal.
4. The method of claim 1, wherein step (b) comprises electrolyzing
the sample using a working electrode comprising an oxide of a
transition metal.
5. The method of claim 1, wherein step (b) comprises electrolyzing
the sample using a working electrode comprising an element selected
from the group consisting of ruthenium, rhodium, palladium,
platinum, silver, osmium, iridium, gold, mercury, rhenium,
titanium, niobium, tantalum, or any combination thereof.
6. The method of claim 1, wherein step (b) comprises electrolyzing
the sample using a working electrode comprising a metal oxide
selected from the group consisting of Fe.sub.3O.sub.4, FeO, and/or
Fe.sub.2O.sub.3.
7. The method of claim 1, wherein step (b) comprises electrolyzing
the sample at a potential of from about 0.0 V to about 2.0 V.
8. The method of claim 1, wherein step (b) comprises electrolyzing
the sample at a potential of from about 0.2 V to about 1.0 V.
9. The method of claim 1, wherein step (a) comprises adding
hydrogen peroxide to the sample in an amount sufficient to make the
sample about 0.1 M to about 10 mM H.sub.2O.sub.2.
10. The method of claim 1, wherein step (a) comprises adding
hydrogen peroxide to the sample in an amount sufficient to make the
sample about 0.1 to about 5 mM H.sub.2O.sub.2.
11. The method of claim 1, wherein step (a) comprises adding
hydrogen peroxide to the sample in an amount sufficient to make the
sample about 0.1 to about 1 mM H.sub.2O.sub.2.
12. A method to measure concentration of beta-lactoglobulin (BLG),
the method comprising: (a) adding a known concentration of hydrogen
peroxide to a sample known to, or suspected of containing BLG; (b)
electrolyzing the sample of step (a) using a working electrode at a
fixed potential sufficient to electrolyze BLG, and measuring a
current signal within the sample, wherein a diminution of the
current signal in the sample as compared to a corresponding current
signal from a standard solution containing the known concentration
of hydrogen peroxide and no BLG indicates that the sample contains
BLG; and (c) determining the concentration of BLG in the sample by
comparing the diminution of the current signal in step (b) to a
standard curve of current signals compiled using solutions of known
BLG concentration.
13. The method of claim 12, wherein step (b) comprises
electrolyzing the sample using a working electrode comprising a
transition metal or an oxide of a transition metal.
14. The method of claim 12, wherein step (b) comprises
electrolyzing the sample using a working electrode comprising a
transition metal.
15. The method of claim 12, wherein step (b) comprises
electrolyzing the sample using a working electrode comprising an
oxide of a transition metal.
16. The method of claim 12, wherein step (b) comprises
electrolyzing the sample using a working electrode comprising an
element selected from the group consisting of ruthenium, rhodium,
palladium, platinum, silver, osmium, iridium, gold, mercury,
rhenium, titanium, niobium, tantalum, or any combination
thereof.
17. The method of claim 12, wherein step (b) comprises
electrolyzing the sample using a working electrode comprising a
metal oxide selected from the group consisting of Fe.sub.3O.sub.4,
FeO, and/or Fe.sub.2O.sub.3.
18. The method of claim 12, wherein step (b) comprises
electrolyzing the sample at a potential of from about 0.0 V to
about 2.0 V.
19. The method of claim 12, wherein step (b) comprises
electrolyzing the sample at a potential of from about 0.2 V to
about 1.0 V.
20. The method of claim 12, wherein step (a) comprises adding
hydrogen peroxide to the sample in an amount sufficient to make the
sample about 0.1 M to about 10 mM H.sub.2O.sub.2.
21. The method of claim 12, wherein step (a) comprises adding
hydrogen peroxide to the sample in an amount sufficient to make the
sample about 0.1 to about 5 mM H.sub.2O.sub.2.
22. The method of claim 12, wherein step (a) comprises adding
hydrogen peroxide to the sample in an amount sufficient to make the
sample about 0.1 to about 1 mM H.sub.2O.sub.2.
Description
BACKGROUND
[0001] Allergy to cow's milk is a dominant food allergy in babies
and young children. The allergic reaction to cow's milk is most
prevalent in early childhood, with figures generally reported
between about 2 and 6%, and gradually decreases into adulthood to
an incidence of approximately 0.1-0.5%. (Cow's milk allergy ranks
among the most pervasive of human food allergies, alongside
allergies to egg, soy, wheat, peanuts, tree nuts, fish and
shellfish in terms of prevalence.) The long-term prognosis for the
majority of affected infants is good. Roughly 80 to 90% of infants
exhibit allergy to cow's mile naturally acquiring tolerance to
cow's milk by the age of 5 years. However, there remains a strong
trend in infants who recover from an allergy to cow's mile to
develop atopic symptoms later in life, such as asthma, hay fever,
or dermatitis to inhalant allergens later in life. This phenomenon
has been dubbed the so-called "atopic career" or "atopic march" and
infant allergy to cow's milk appears to be an early indicator of
atopy. See, for example, Crittenden, R. G. and Bennett, L. E.
(December 2005) "Cow's Milk Allergy: A Complex Disorder," J Am Coll
Nutr 24(6):5825-5915.
[0002] Numerous milk proteins (beta-lactoglobulin among them) have
been implicated in allergic responses to cow's milk and most of
these allergen proteins have been shown to contain multiple
allergenic epitopes. There is also considerable heterogeneity among
allergic individuals for the particular proteins and epitopes to
which they react. Further complicating a complete understanding of
the allergy, the allergic reactions to cow's milk are driven by
more than one immunological mechanism. Both the incidence and
dominant allergic mechanisms change with age; IgE-mediated
reactions are common in infancy, non-IgE-mediated reactions
dominate in adults. Interestingly, the prevalence of self-diagnosed
allergy to cow's milk is substantially higher than the incidence
reported in blinded and controlled challenge trials, suggesting
that a proportion of the population is unnecessarily avoiding dairy
products (likely due to a confusion between milk allergies and
lactose intolerance, an entirely different malady).
[0003] Beta-lactoglobulin ("BLG") is the major whey protein of cow
and sheep's milk. In fresh, raw cow's milk, it is present in a
concentration of roughly 3 g/L. BLG is also present in many other
mammalian species. However, humans are a notable exception; human
milk does not contain BLG. Thus, BLG is one of the principal
proteins in cow's milk responsible for the allergic response in
humans. (The caseins are the other dominant class of protein
allergens found in cow's milk.) BLG is the most potent of the
allergens found in cow's milk and is responsible for approximately
9% of all diagnosed food allergies. Because BLG is a known allergen
to humans, many countries require that food destined for human
consumption be properly labeled to indicate that it contains BLG.
For example, in Europe, Annex Ma of Directive 2000/13/EC requires
manufacturers to prove the presence or absence of
.beta.-lactoglobulin to ensure their labelling satisfies the
requirements of the directive. Conventionally, food testing
laboratories use enzyme linked immunosorbent assays (ELISA) to
identify and to quantify BLG concentrations in food products.
[0004] Notably, BLG is a whey protein. Whey protein is a mixture of
globular proteins isolated from whey, the liquid material created
as a by-product of cheese production. Whey protein is commonly
marketed and ingested as a dietary supplement, and various health
claims have been attributed to it in the alternative medicine and
body-building communities. The protein in cow's milk is roughly 20%
whey protein and 80% casein. The whey protein fraction of cow's
milk is typically about 65% BLG, 25% alpha-lactalbumin, 8% serum
albumin, and the remainder minor immunoglobulins. Thus, a human who
is allergic to milk due to the presence of BLG will also be
allergic to foods containing any appreciable amount of whey
protein. Whey proteins can be denatured by heat, but even
heat-denatured whey can still cause allergies humans.
[0005] Whey protein is typically sold in three major forms: whey
protein concentrate (WPC), whey protein isolate (WPI), and whey
protein hydrolysate (WPH). These products differ by their level of
purity and other processing parameters. WPC contains a small, but
significant, level of fat, cholesterol, and lactose. WPC's are
typically from about 29% to about 89% protein by weight. WPI is
further processed to remove the fat and lactose. WPI is typically
more than 90% protein by weight. WPH is a whey protein product in
which the proteins have been predigested and partially hydrolyzed.
Highly-hydrolyzed WPH may be less allergenic than other forms of
whey proteins.
[0006] As noted above, food testing laboratories conventionally use
an ELISA to test for and quantify BLG concentrations in food
products. While ELISA's are very sensitive and accurate, they are
also expensive and require specialized equipment to assemble and
read. ELISAs also require enzymes, careful incubation times and
temperatures, and wet-chemical processing to develop. Thus, ELISA's
are not an ideal format for a fast and cheap method to detect and
quantify BLG in foods. Insofar as a significant minority of humans
are allergic to BLG, and not all jurisdictions require that food be
labeled to indicate whether it contains BLG, there remains a
long-felt and unmet need for a quick and easy method to analyze an
unknown sample, especially an unknown sample destined for human
consumption, to determine whether it contains BLG.
SUMMARY OF THE INVENTION
[0007] The present inventors have developed a novel electrochemical
detection technique based on the current signal reduction of
H.sub.2O.sub.2 to detect BLG. The technique is based upon the
detection of H.sub.2O.sub.2 by electrochemical sensing using a
three-electrode system. Any type of working electrode configured to
detect H.sub.2O.sub.2 may be used in the detection method. During
initial testing of an electrode in a dilute solution of
H.sub.2O.sub.2, it was discovered that during the H.sub.2O.sub.2
sensing, there was an increase in the current signal under
detection potential. It was then discovered that upon adding BLG to
the test solution, the current increase quickly dropped (in matter
of seconds) by a detectable amount that was proportional to the
amount of BLG added to the solution. (That is, BLG concentration
and current signal are inversely proportional; the larger the
amount of BLG, the smaller the current.) The serendipitous
observation was then used as a basis to quantify the change in
current to act as an indirect method for detecting the presence of
BLG in samples quickly and easily.
[0008] While not being limited to any specific underlying mechanism
or phenomenon, it is thought that H.sub.2O.sub.2 generates hydroxyl
radicals (.OH, .OOH) under oxidation-reduction potential which in
turn react with BLG. This reaction is thought to generate a
detectable opposing current, thus causing the reduction in current
signal. Because the current is carried by hydroxyl radicals derived
from H.sub.2O.sub.2, the amount of BLG in any test sample can be
determined by first generating a standard curve of the current
generated from control solutions containing various, but fixed
amounts of H.sub.2O.sub.2 and serial dilutions of BLG. (That is,
the standard curve may be generated from a series of control
solutions that provide current data for solutions containing a
fixed concentration of H.sub.2O.sub.2 and a serially-diluted amount
of BLG.) From these control solutions, a series of standard curves
is generated. The current from a test solution containing an
unknown concentration of BLG is then measured after adding known
amount of H.sub.2O.sub.2 to the test solution. The amount of
H.sub.2O.sub.2 in the unknown test solution is then determined by
comparison to the standard curve.
[0009] There are several advantages to the subject method. It is
simple, inexpensive, and does not require the use of antibodies,
enzymes, or labels. The method is both sensitive and rapid; a
reading can be completed in five seconds or less. Because the
method is electrochemistry-based, it is portable. It can be
formatted for multi-sample detection. Suitable electrodes can be
screen-printed very cheaply, to the point that they could be
formatted and packaged for one-time, disposable use. The method
requires only a dilute concentration of H.sub.2O.sub.2 (e.g., 0.1
mM) thus keeping the cost of consumables at a bare minimum.
Additionally, the low detection potential used (about -0.4 V, and
0.0 V with certain electrodes) is easily achieved in a very small
device. Thus, the method can be implemented using a handheld device
and is safe to practice, even at home.
[0010] The method comprises adding a known concentration of
hydrogen peroxide to a sample known to, or suspected of containing
BLG. The sample is then electrolyzed using a working electrode at a
fixed potential sufficient to electrolyze BLG, and measuring a
current signal within the sample. A diminution of the current
signal in the sample as compared to a corresponding current signal
from a standard solution containing the known concentration of
hydrogen peroxide and no BLG indicates that the sample contains
BLG.
[0011] The working electrode used to electrolyze the sample may
comprise a transition metal or an oxide of a transition metal, or
an element selected from the group consisting of ruthenium,
rhodium, palladium, platinum, silver, osmium, iridium, gold,
mercury, rhenium, titanium, niobium, tantalum, or any combination
thereof. The working electrode may, for example, comprise
Fe.sub.3O.sub.4, FeO, and/or Fe.sub.2O.sub.3.
[0012] The sample may be electrolyzed at a voltage suitable to
electrolytically degrade BLG, typically from about 0.0 V to about
2.0 V or from about 0.1 V to about 2.0 V, or from about 0.2 V to
about 1.0 V.
[0013] H.sub.2O.sub.2 is added to the sample typically in an amount
sufficient to make the sample about 0.1 M to about 10 mM
H.sub.2O.sub.2. or about 0.1 to about 5 mM H.sub.2O.sub.2, or about
0.1 to about 1 mM H.sub.2O.sub.2.
[0014] The method can also be used to measure the concentration of
BLG in a sample by comparing the diminution of the current signal
in the sample being tested to a standard curve of current signals
compiled using solutions of known BLG concentration.
[0015] Numerical ranges as used herein are intended to include
every number and subset of numbers contained within that range,
whether specifically disclosed or not. Further, these numerical
ranges should be construed as providing support for a claim
directed to any number or subset of numbers in that range. For
example, a disclosure of from 1 to 10 should be construed as
supporting a range of from 2 to 8, from 3 to 7, from 1 to 9, from
3.6 to 4.6, from 3.5 to 9.9, and so forth.
[0016] All references to singular characteristics or limitations of
the present method shall include the corresponding plural
characteristic or limitation, and vice-versa, unless otherwise
specified or clearly implied to the contrary by the context in
which the reference is made.
[0017] All combinations of method or process steps as used herein
can be performed in any order, unless otherwise specified or
clearly implied to the contrary by the context in which the
referenced combination is made.
[0018] The methods, devices, and kits disclosed herein can
comprise, consist of, or consist essentially of the essential
elements and limitations described herein, as well as any
additional or optional ingredients, components, or limitations
described herein or otherwise useful in electrochemistry.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic diagram of a three-electrode device
for performing voltammetry.
[0020] FIG. 2 depicts superimposed UV-visible absorption spectra of
BLG from a standard solution (circles) and BLG isolated from a
cow's milk (triangles).
[0021] FIG. 3 depicts superimposed UV-visible absorption spectrum
of water as compared to the corresponding UV-visible absorption
spectrum of a dilute solution (1.0 wt %) of BLG in water. There is
a very pronounced absorption peak for BLG at .about.280 nm.
[0022] FIG. 4 depicts superimposed UV-visible absorption spectra of
0.5 mM H.sub.2O.sub.2 (aq) at various time points ranging from 10
min to 2 hr. As shown in the figure, the spectra are literally
superimposed, indicating that the H.sub.2O.sub.2 solution is quite
stable over the two-hour span when the spectra were taken.
[0023] FIG. 5 depicts superimposed UV-visible absorption spectra of
a dilute solution (1.0 wt %) of BLG in water over the course of
electrolysis at -0.4V for time period ranging from 10 min to 2
hours. As evidenced by the diminishing absorption peak at -280 nm,
this figure demonstrates that BLG is electrolyzed over time.
[0024] FIG. 6 depicts superimposed UV-visible absorption spectra of
0.5 mM H.sub.2O.sub.2 (aq) versus water after a two-hour quiescent
period and after two hours of electrolysis at -0.4V. As evidenced
by the diminishing absorption peak at -280 nm, the H.sub.2O.sub.2
is electrolyzed.
[0025] FIG. 7 is a voltammogram depicting the behavior of 1 U
catalase against 0.1 mM H.sub.2O.sub.2 using a Fe.sub.3O.sub.4
working electrode. The gentle current drop due to catalase is
easily distinguished from the sharp current drop due to the
presence of BLG.
[0026] FIG. 8 is a voltammogram generated using a Fe.sub.3O.sub.4
working electrode held constant at -0.4V, in 50 mM
phosphate-buffered saline (PBS), pH 5.9. Hydrogen peroxide was
added at T=30 sec to bring the solution to 0.1 mM H.sub.2O.sub.2,
which caused an immediate jump in current. Bovine serum albumin was
added at 60 sec (no effect on current). BLG was added at 100 sec,
which caused an abrupt drop in current. An aliquot of casein was
added at 160 second (no effect on current).
[0027] FIG. 9 is a control voltammogram for purposes of comparison
to FIG. 8. The voltammogram was generated using a Fe.sub.3O.sub.4
working electrode held constant at -0.4V, against a 0.1 mM
H.sub.2O.sub.2 in 50 mM PBS, pH 5.9. As shown in the figure, the
current reading held steady for 1200 sec.
[0028] FIG. 10 is a control voltammogram showing the current rise
for consecutive additions of aliquots of 0.5 mM H.sub.2O.sub.2. The
voltammogram was generated using a platinum working electrode held
constant at -0.2V, in 11.9 mM PBS, pH 7.4.
[0029] FIG. 11 is a voltammogram depicting the detection of BLG via
a corresponding current drop in the voltammogram. The voltammogram
was generated using a platinum working electrode held constant at
-0.4V, in 11.9 mM PBS, pH 7.4. Hydrogen peroxide was added to 0.5
mM at T=50 seconds, causing a near-instantaneous rise in current. A
1:5 (w/w with H.sub.2O.sub.2) aliquot of BLG was added at T=110
seconds, which resulted in a detectable current drop.
[0030] FIG. 12 is a voltammogram depicting the detection of BLG via
a corresponding current drop in the voltammogram. The voltammogram
was generated using a platinum working electrode held constant at
-0.2V, in 11.9 mM PBS, pH 7.4. Hydrogen peroxide was added to 1.0
mM at T=50 seconds, causing a near-instantaneous rise in current. A
2:5 (w/w with H.sub.2O.sub.2) aliquot of BLG was added at T=110
seconds, which resulted in a detectable current drop.
DETAILED DESCRIPTION
[0031] The present method uses constant-voltage voltammetry to
measure the current needed to reduce H.sub.2O.sub.2 in the presence
of BLG in a test solution, compares the resulting current values
found in the test solution to previously prepared standard curves
for the same current observed in solutions of known concentrations
of H.sub.2O.sub.2 and BLG, and determines the concentration of BLG
in the test solution by comparing the current value from the test
solution to the standard curve. Adding a known amount of
H.sub.2O.sub.2 to a sample to be tested for the presence or
concentration of BLG will yield a robust and reproducible current
increase if the sample does not contain BLG. If the sample does
contain BLG, the current rise due to the added H.sub.2O.sub.2 will
be attenuated in an amount that is proportional to the
concentration of the BLG in the sample tested. In this manner, a
sample can be tested for the presence of BLG by taking a baseline
measurement of the current generated in the test sample when a
fixed potential is applied to the solution. A known amount of
H.sub.2O.sub.2 is then added to the sample (or an aliquot of the
sample), the change in current is measured, and the result compared
to a standard curve (generated previously as noted above) to
determine the presence of BLG in the sample, the concentration of
BLG in the sample, or both the presence and the concentration of
BLG in the sample.
[0032] Voltammetry is the study of current as a function of applied
potential. In the present approach, the half cell reactivity of BLG
with hydroxyl ions generated by the reduction of H.sub.2O.sub.2 is
measured at a constant applied voltage. Unlike cyclic voltammetry,
or other forms of voltammetry, where the applied potential is
varied arbitrarily (either step-wise or continuously) and the
current is measured as the dependent variable, in the present
method the applied potential is held constant at a voltage at or
above the potential required to reduce H.sub.2O.sub.2. In most
milieus, the applied potential used in the present method will
range from about 0.0 V to about 2.0 V, or from about 0.1 V to about
2.0 V, and more typically from about 0.2 V to about 1.0 V. However,
applied potentials above and below these stated ranges are within
the scope of the claimed method.
[0033] To perform the present method requires at least two
electrodes, but for practical purposes it is best conducted with a
three-electrode circuit as depicted in FIG. 1. The minimalist
two-electrode system comprises a working electrode, which makes
contact with the analyte, and which apples the desired potential in
a controlled way and facilitates the transfer of charge to and from
the analyte--in this case an H.sub.2O.sub.2.-BLG complex. A second
electrode acts as the other half of the cell. This second electrode
must have a known potential with which to gauge the potential of
the working electrode. The second electrode must also balance the
charge added or removed by the working electrode. While a
two-electrode device is a viable device configuration for carrying
out the present method, it is not preferred because it has a number
of shortcomings. Most significantly, it is difficult for an
electrode to maintain a constant potential while passing current to
counter redox events at the working electrode. Nevertheless,
carrying out the method using a two-electrode device is within the
scope of the present disclosure.
[0034] It is preferred that the role of supplying electrons versus
providing a referencing potential be divided between two separate
electrodes, as shown in FIG. 1. Referring to FIG. 1, depicted is
the solution 30 to be tested for BLG. The three-electrode
configuration uses a working electrode 10, an auxiliary or counter
electrode 12, and a reference electrode 14. The reference electrode
14 is a half cell with a known reduction potential. Its only role
is to act as reference in measuring and controlling the working
electrode's potential. At no point does the reference electrode 14
pass any current. A power source 24 is used to apply a current to
the working electrode 10 and reference electrode 14 via circuit 18.
Potentiometer 20 is used to measure and control the amount of
voltage applied to the reference electrode 14 and working electrode
14. The auxiliary electrode passes 12 all the current needed to
balance the current observed at the working electrode. The current
passes through circuit 16 and is measured by ammeter 22.
[0035] There are many voltammetric devices which have more than
three electrodes, and which can also be used in the present method.
Their design principles, however, are fundamentally the same as the
three-electrode system illustrated schematically in FIG. 1 and will
not be described in any detail. For example, the rotating ring-disk
electrode has two distinct and separate working electrodes, a disk
and a ring, which can be used to scan or hold potentials
independently of each other. Both of these electrodes are balanced
by a single reference and auxiliary combination for an overall
four-electrode design. As noted above, at least two electrodes are
required to measure the current; three electrodes are preferred.
Devices using more than three electrodes may be used, but they do
not necessarily yield more accurate or precise results.
[0036] The auxiliary electrode 12 can be fabricated from any
electrically conductive material, the only proviso being that the
material chosen must not react with the bulk of the analyte
solution. Suitable auxiliary electrodes are available from a host
of commercial suppliers. See those listed below for the reference
electrodes.
[0037] Likewise, any reference electrode 14 may also be used, with
the same proviso--it must not be reactive with the bulk of the
analyte solution. A large number of reference electrodes are known
in the art and may be used in the present method. Suitable
reference electrodes include the standard hydrogen electrode,
normal hydrogen electrode, reversible hydrogen electrode, saturated
calomel electrode, copper-copper(II) sulfate electrode, silver
chloride electrode, pH-electrode, palladium-hydrogen electrode,
dynamic hydrogen electrode, etc. The foregoing list is exemplary,
not exhaustive. These and other reference electrodes are well known
in the art and will not be discussed in any detail. They can be
purchased from a large number commercial suppliers. For example,
Gamry Instruments (Warminster, Pa.) sells saturated calomel
reference electrodes (Part No. 930-03), silver-silver chloride
reference electrodes (Part No. 930-15), and mercury/mercurous
sulfate reference electrodes (Part No. 930-29), among others. Other
commercial suppliers include Castle Electrodes (Berkshire, UK).
[0038] The working electrode may also be made from any material, so
long as the material chosen is capable of driving the
H.sub.2O.sub.2 redox reaction. For example, electrodes comprising
platinum, sulfonated tetrafluoroethylene-coated platinum, or carbon
fibers can be used. See Roberts, J. G.; Hamilton, K. L, and
Sombers, L. A. (2011) Analyst, 136:3550-3556. Electrodes comprising
other noble metals, such as ruthenium, rhodium, palladium, silver,
osmium, iridium, and gold, may also be used, along with electrodes
comprising mercury, rhenium, titanium, niobium, tantalum, or any
combination of the foregoing may be used. Base metals and base
metal oxides may also be used, such as iron oxide (Fe.sub.3O.sub.4,
FeO, and/or Fe.sub.2O.sub.3). See also the electrode described in
U.S. Patent Publ. 2012/0261273, published Oct. 18, 2012. See also
the electrodes described in U.S. Pat. No. 5,518,591, issued May 21,
1996, and U.S. Pat. No. 5,320,725, issued Jun. 14, 1994. All of the
references cited in this paragraph are incorporated herein by
reference.
[0039] Note also that electrochemical devices for sensing
H.sub.2O.sub.2 using no applied potential (i.e., 0 V) are known.
These devices can be used in the present method for detecting BLG.
See, for example, Jeong et al. (2009) Bull. Korean Chem. Soc.
30(12):2979. This paper describes detecting H.sub.2O.sub.2 using a
glassy carbon electrode that was surface modified with a coating of
single-walled carbon nanotubes and nanowires of
polytetrakis(o-aminophenyl)porphyrin. The nanotubes and nanowires
were adhered to the surface of the glassy carbon electrode using
"Nafion".RTM.-brand resin as a binder. ("Nafion" is a registered
trademark of E.I. DuPont de Nemours & Co., Wilmington, Del.).
The resulting electrode had enhanced sensitivity for H.sub.2O.sub.2
determination at an applied potential of 0.0 V by the amperometric
method. See also Tan et al. (2009) Electroanalysis 21(13)1514-1521,
which describes an amperometric H.sub.2O.sub.2 biosensor based on
glassy carbon electrode surface-modified with
Fe.sub.3O.sub.4/chitosan, and with horseradish peroxidase
immobilized to the modified electrode surface.
[0040] The redox reaction of pure H.sub.2O.sub.2 is a classic
disproportionation reaction:
2H.sub.2O.sub.2.fwdarw.2H.sub.2O+O.sub.2. One half of the
H.sub.2O.sub.2 is oxidized to yield O.sub.2; the other half is
reduced to yield water. Each half reaction requires two (2)
electrons to complete: H.sub.2O.sub.2.fwdarw.2O.sub.2+2 H.sup.++2e-
(oxidation); H.sub.2O.sub.2+2H.sup.++2e-.fwdarw.2H.sub.2O
(reduction). As noted above, while not being limited to any
specific mode of action, it is believed that the presence of BLG
interferes with the H.sub.2O.sub.2 redox reaction by forming
short-lived complexes with hydroxyl intermediates formed during the
course of the redox reaction. This leads to a detectable drop in
current which is proportional to the amount of BLG present in the
sample.
Examples
[0041] The following examples are included solely to provide a more
complete description of the method described and claimed herein.
The examples are not intended to limit the scope of the claims in
any fashion.
[0042] A first step in proving the concept of the present method
was to determine whether BLG could be electrolyzed in the presence
of H.sub.2O.sub.2 and whether the course of the electrolytic
degradation of BLG could be followed via UV-visible spectroscopy.
Thus, as an initial step, the UV-visible spectrum of a commercially
obtained BLG standard (Sigma-Aldrich, St. Louis, Mo.) was compared
to the corresponding spectrum of a BLG isolated via chromatography
from a sample of milk. The results are depicted in FIG. 2, which
depicts the two superimposed UV-visible absorption spectra. The
spectrum of the BLG from the commercially obtained standard is
shown in circles; the spectrum from the BLG isolated from a cow's
milk is shown in triangles. As is readily apparent from FIG. 2, the
two spectra very closely matched, with a marked absorption peak at
.about.280 nm.
[0043] The spectrum of the test BLG isolated from cow's milk was
then run against water to see if the solvent would have an impact
on the absorption maximum (i.e., either shirt the peak absorption
wavelength or change the absorption intensity.) The results are
depicted in FIG. 3, which shows the superimposed UV-visible
absorption spectrum of water as compared to the corresponding
UV-visible absorption spectrum of a dilute solution (1.0 wt %) of
BLG in water. Again, there is a very pronounced absorption peak for
BLG at .about.280 nm, which is not altered by the water. Compare
the BLG curve in FIG. 3 to the spectra in FIG. 2. They are
essentially identical.
[0044] The next preparatory step was to evaluate the corresponding
UV-visible spectrum of H.sub.2O.sub.2 to determine if its spectrum
remained stable over the likely time period of method disclosed
herein. The results are shown in FIG. 4. FIG. 4 depicts
superimposed UV-visible absorption spectra of 0.5 mM H.sub.2O.sub.2
(aq) at various time points: 10 min, 20 min, 30 min, 1 hr, and 2
hr. A key is given in FIG. 4, but each individual spectrum at each
time point was identical. As shown in FIG. 4, the spectra from all
time points tested are literally superimposed. The superimposed
spectra in FIG. 4 indicate that the H.sub.2O.sub.2 solution is
quite stable over the two-hour span during which the spectra were
gathered.
[0045] The next step was then to determine whether BLG could be
electrolyzed. Here, dilute aqueous solutions of BLG were
electrolyzed at various fixed potentials and the progress of the
electrolysis was tracked by UV-visible spectroscopy. Diminution of
the BLG absorption peak at .about.280 nm indicates that the BLG is
being degraded due to the applied voltage. Results for one of the
electrolysis experiments are depicted in FIG. 5. FIG. 5 depicts
superimposed UV-visible absorption spectra of a dilute solution
(1.0 wt %) of BLG in water over the course of electrolysis at -0.4V
for time period ranging from 10 min to 2 hours, using a platinum
working electrode. As evidenced by the diminishing absorption peak
at .about.280 nm, this figure demonstrates that BLG is electrolyzed
over time. Note that the rate of the electrolysis does depend upon
the nature of the working electrode (e.g., the composition of the
electrode, its size and physical structure, the effective surface
area of the electrode, etc.). Thus, when constructing a standard
curve using solutions having known concentrations of BLG,
H.sub.2O.sub.2 and known combinations of the two, a standard curve
must be compiled for each different type of working electrode.
Because the nature of the electrolytic reaction is dependent upon
the nature of the working electrode, a standard curve must be
compiled for each new type of working electrode used in the
method.
[0046] The next step was to evaluate the rate of H.sub.2O.sub.2
degradation via electrolysis to see how it would behave at the
voltages typically used for degrading BLG. Thus, various solutions
of H.sub.2O.sub.2 were electrolyzed at fixed voltages and the
progress of the electrolysis was tracked using UV-visible
spectroscopy. FIG. 6 depicts the results from one such experiment.
FIG. 6 depicts superimposed UV-visible absorption spectra of 0.5 mM
H.sub.2O.sub.2 (aq) versus water after a two-hour quiescent period
and after two hours of electrolysis at -0.4V. As evidenced by the
diminished absorption peak at .about.280 nm after two hours of
electrolysis, the H.sub.2O.sub.2 is electrolyzed essentially
completely.
[0047] Lastly, it needed to be determined whether the drop current
signal attributed to the interaction between H.sub.2O.sub.2 and BLG
would be confounded by the presence of catalase in test samples.
Catalase is an ubiquitous enzyme found in nearly all living
organisms exposed to oxygen. It catalyzes the decomposition of
hydrogen peroxide to water and oxygen. It was unknown whether the
reaction rate of catalase under the electrolytic environment used
in the present method would compete with BLG to degrade
H.sub.2O.sub.2 faster than the H.sub.2O.sub.2 would react with the
BLG and thereby generate the detectable drop in current the forms
the basis of the present method. Catalase has one of the highest
turnover numbers of all known enzymes, thus there was a concern
that catalase would interfere with the electrolysis and the
generation of a current signal proportional to the amount of BLG
present in the sample. This turned out not to be the case, as
evidenced by FIG. 7. FIG. 7 is a voltammogram depicting the
behavior of 1 U catalase against 0.1 mM H.sub.2O.sub.2 using a
Fe.sub.3O.sub.4 working electrode. Note the very gentle current
drop due to catalase being added to the electrolysis reaction at
T=150 sec in FIG. 7. Almost 100 seconds later (T=250 sec), the
current signal has only just returned to the level it was prior to
the addition of 0.1 mM H.sub.2O.sub.2 (at T=50). This gentle drop
in current due to catalase is easily distinguished from the sharp
current drop due to the presence of BLG.
[0048] FIG. 8 demonstrates the basic operation of the method to
detect BLG. Here, a solution is subjected to electrolysis at a
fixed applied potential and then spiked with a known amount of
H2O2. This causes a nearly instantaneous rise in the current
signal. The present inventors discovered that the current signal is
reproducibly attenuated by the subsequent addition of BLG. The
attenuation of the current signal is proportional to the amount of
BLG in the sample being analyzed. Thus, this current signal
attentuation can thus be used to determine both whether BLG is
present in the sample (a simple binary, yes or no result) and/or
the concentration of BLG in the sample. FIG. 8 illustrates the
underlying phenomenon. FIG. 8 is a voltammogram generated using a
Fe.sub.3O.sub.4 working electrode held constant at -0.4V, in 50 mM
phosphate-buffered saline (PBS), pH 5.9. Hydrogen peroxide was
added at T=30 sec to bring the solution to 0.1 mM H.sub.2O.sub.2.
As shown in FIG. 8, this caused a very sharp jump in the current
signal. Bovine serum albumin (BSA) was added at 60 sec to see if
this would have any impact on the current signal. The reaction is
indifferent to added BSA; no change in the current signal was
detected. BLG was added at 100 sec. As shown in FIG. 8, this caused
an abrupt drop in current which was found to be proportional to the
concentration of the added BLG. The reaction was also shown to be
indifferent to added casein. An aliquot of casein was added at 160
second and had no effect on the current signal. For comparison to
FIG. 8, FIG. 9 is a negative control voltammogram of H.sub.2O.sub.2
without any added BLG. The voltammogram depicted in FIG. 9 was
generated using a Fe.sub.3O.sub.4 working electrode held constant
at -0.4V, against a 0.1 mM H.sub.2O.sub.2 in 50 mM PBS, pH 5.9. As
shown in the figure, the current reading held steady for 1200 sec
(20 min). Similarly, FIG. 10 is a negative control voltammogram
showing the current rise for consecutive additions of aliquots of
0.5 mM H.sub.2O.sub.2. As is clearly shown in the figure, each
equal aliquot of H.sub.2O.sub.2 gave a correspondingly identical
bump in the current signal. This signal is likewise attenuated in a
dose-dependent fashion when BLG is added to the solution (data not
shown). The voltammogram depicted in FIG. 10 was generated using a
platinum working electrode held constant at -0.2V, in 11.9 mM PBS,
pH 7.4.
[0049] FIGS. 11 and 12 depict the concentration-dependent current
signal drop when solutions containing BLG are subjected to
electrolysis upon adding different, but known quantities of
H.sub.2O.sub.2 to the test solution. In FIG. 11, the ratio of
H.sub.2O.sub.2 to BLG was 1:5 (w/w). In FIG. 12, the ratio of
H.sub.2O.sub.2 to BLG was 1:5 (w/w). As shown in the two figures,
the drop in the current signal is easily detected in both
scenarios. The voltammogram in FIG. 11 shows the detection of BLG
via a corresponding current drop in the voltammogram. The
voltammogram was generated using a platinum working electrode held
constant at -0.4V, in 11.9 mM PBS, pH 7.4. Hydrogen peroxide was
added to 0.5 mM at T=50 seconds, causing a near-instantaneous rise
in current. A 1:5 (w/w with H.sub.2O.sub.2) aliquot of BLG was
added at T=110 seconds, which resulted in a detectable current
drop. The voltammogram shown in FIG. 12 is similar to the one in
FIG. 11, but was generated using a platinum working electrode held
constant at -0.2V, in 11.9 mM PBS, pH 7.4. Hydrogen peroxide was
added to 1.0 mM at T=50 seconds, causing a near-instantaneous rise
in current. A 2:5 (w/w with H.sub.2O.sub.2) aliquot of BLG was
added at T=110 seconds, which resulted in a detectable current
drop.
* * * * *