U.S. patent application number 10/715489 was filed with the patent office on 2007-07-12 for electrochemical method and sensor for the detection of traces of expolsives.
This patent application is currently assigned to MEDIS TECHNOLOGIES LTD.. Invention is credited to Boris Filanovsky.
Application Number | 20070158212 10/715489 |
Document ID | / |
Family ID | 34619904 |
Filed Date | 2007-07-12 |
United States Patent
Application |
20070158212 |
Kind Code |
A1 |
Filanovsky; Boris |
July 12, 2007 |
ELECTROCHEMICAL METHOD AND SENSOR FOR THE DETECTION OF TRACES OF
EXPOLSIVES
Abstract
A system for highly sensitive electrochemical detection of trace
nitro-aromatic compounds in air, uses a carbon or carbon/gold
working electrode with a surface that is modified to increase the
electron transfer kinetics of nitro-aromatic compounds. Chemical
modifiers of the working electrode surface include amino-aromatic
compounds such as aniline and its derivatives. The detection method
involves dissolving trace nitro-aromatic compounds in an
electrolyte including aprotonic solvents, or dipolar solvents, in
the electrochemical cell including a working electrode, a reference
electrode and an auxiliary electrode. Voltage is varied across the
working electrode and the reference electrode, and an electrical
current is measured between the working electrode and the auxiliary
electrode. The measured electrical peak current is a sensitive
indication of the concentration of the trace compounds. This
invention is appropriate for portable, field-testing of trace
explosive compounds in air.
Inventors: |
Filanovsky; Boris;
(Jerusalem, IL) |
Correspondence
Address: |
DR. MARK FRIEDMAN LTD.;c/o Bill Polkinghorn
Discovery Dispatch
9003 Florin Way
Upper Marlboro
MD
20772
US
|
Assignee: |
MEDIS TECHNOLOGIES LTD.
|
Family ID: |
34619904 |
Appl. No.: |
10/715489 |
Filed: |
November 19, 2003 |
Current U.S.
Class: |
205/780.5 ;
204/400 |
Current CPC
Class: |
G01N 33/0057 20130101;
G01N 27/4045 20130101 |
Class at
Publication: |
205/780.5 ;
204/400 |
International
Class: |
G01F 1/64 20060101
G01F001/64; G01N 27/26 20060101 G01N027/26 |
Claims
1-25. (canceled)
26. A system for electrochemical assay of nitro-aromatic compounds,
comprising: (a) a working electrode having a surface of carbon and
gold, wherein said surface is modified by a monomeric
amino-aromatic compound by treatment thereof with said monomeric
amino-aromatic compound dissolved in an organic polar solvent.
27. The system, according to claim 26 wherein said monomeric
amino-aromatic compound is selected from the group consisting of
alkyl-aniline compounds, halide derivatives of alkyl-aniline
compounds and hydroxyl-aniline compounds.
28. The system, according to claim 26, wherein said monomeric
amino-aromatic compound is selected from the group consisting of
phenylene-diamine, diphenylene-diamine, and
diphenylene-triamine.
29. The system, according to claim 26, wherein said monomeric
amino-aromatic compound is aniline.
30. The system, according to claim 26, wherein said organic polar
solvent is a polar aprotonic solvent.
31. The system, according to claim 26, wherein said organic polar
solvent is dimethylsulfoxide.
32. The system, according to claim 26 wherein said monomeric
amino-aromatic compound is in a range of one to five percent
solution in said organic polar solvent.
33. The system, according to claim 26, wherein said working
electrode includes at least one element selected from the group
consisting of carbon and gold.
34. The system, according to claim 26, wherein said working
electrode includes submicron particles.
35. The system, according to claim 26, wherein said working
electrode includes elemental gold deposited on carbon, wherein the
gold is of average thickness less than one nanometer.
36. The system, according to claim 26, wherein said working
electrode includes carbon paper.
37. The system, according to claim 26, further comprising, (b) an
electrolyte for dissolving the nitro-aromatic compounds; wherein
said electrolyte is a mixed solvent including water and an organic
solvent.
38. The system, according to claim 37, further comprising (c) a
mechanism for inputting air suspected to include the nitro-aromatic
compounds, into said electrolyte in order to dissolve the
nitro-aromatic compounds in said electrolyte.
39. The system, according to claim 37, wherein said organic solvent
is selected from the group consisting of aprotonic solvents, and
organic dipolar solvents.
40. The system, according to claim 37, wherein said organic solvent
is selected from the group consisting of dimethylformamide,
acetonitrile, propylene carbonate.
41. The system, according to claim 37, wherein said organic solvent
is selected from the group consisting of ethanol, propanol,
ethylene-glycol, and propylene-glycol.
42. The system, according to claim 37, wherein said electrolyte has
a pH greater than 8.
43. The system, according to claim 37, wherein said electrolyte has
a pH greater than 7.
44. A system for electrochemical assay of nitro-aromatic compounds,
comprising: (a) a working electrode having a surface modified by a
monomeric amino-aromatic compound by treatment thereof with said
monomeric amino-aromatic compound dissolved in an organic polar
solvent; and (b) an electrolyte for dissolving the nitro-aromatic
compounds; wherein said electrolyte is a mixed solvent including a
water buffer of pH greater than 8, ethanol and acetonitrile.
45. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the detection of trace
amounts of explosive materials such as nitro-aromatic compounds in
air, using an electrochemical measurement technique, and
specifically to improving the sensitivity of the measurement of
ti-ace explosive materials, and decreasing measurement time. More
specifically, the present invention relates to a method of
explosives detection of low cost that is suitable for portable
field-testing.
BACKGROUND OF THE INVENTION
[0002] As a consequence of recent efforts to thwart the recent
upsurge in international terrorism, there is an increased interest
in the detection of explosive materials. These materials include
nitro-aromatic compounds including 2,4,6-trinitrotoluene (TNT),
dinitrotoluene (DNT) and similar derivatives.
[0003] Many detection methods have been used to detect explosive
materials. These methods include gas and HPLC chromatography, x-ray
scattering neutron analysis, nuclear quadrupole resonance, and mass
spectrometry (U.S. Pat. No. 6,571,649). These methods generally
require expensive and sophisticated equipment, (e.g. high vacuum),
equipment that is not portable (e.g. cylinders of compressed
gases), and/or have a complicated sample preparation. These
techniques, are therefore, not appropriate for low cost portable
field-testing for trace explosive materials. A recent review of
some of these methods for explosives detection is "Explosives
detection systems (EDS) for aviation security" (Singh, S., Signal
Processing vol. 83, 2003, p. 31-55).
[0004] Another known method for the detection of trace amounts of
explosive materials utilizes immunochemical sensors. For example,
U.S. Pat. No. 6,573,107 is directed towards the immunochemical
detection of explosive substances in the gas phase using surface
plasmon resonance spectroscopy. Immunochemical detection methods
potentially offer high selectivity and high sensitivity.
[0005] Electrochemical detection refers to the use of electrodes,
immersed in an electrolyte, and connected to an instrument that
varies the voltage applied to the electrodes. The instrument
measures the current flow between the electrodes. Typically, the
electrode potential is varied; and an electric current flows
between the electrodes that is characteristic of the presence of
electrochemical active substances in the electrolyte. The magnitude
of the current is proportional to the concentration of the
electrochemical active substances. It is well known that TNT and
other nitro-aromatic compounds are reduced electrochemically at the
cathode and may be detected by electrochemical detection. Wang et.
al. (Analytica Chimica Acta, vol. 485 (2003) p. 139-144) reported
the monitoring of TNT in natural waters using an electrochemical
technique. They reported a measurement sensitivity of 0.003
.mu.A/ppb of TNT in natural seawater. This sensitivity level was
achieved by Wang et. al. by subtracting the background signal, in
natural seawater not contaminated by TNT, caused by the reduction
of dissolved oxygen. The applicant reported (Reviews Analytical
Chemistry vol. 18 no. 5, 1999, p. 293) the use of carbon/Hg film
electrode materials in an aqueous solvent. This electrode material
was successful to minimize the background by separating the
atmospheric O.sub.2 background current from the TNT current,
however the sensitivity reported was only .about.0.7 .mu.A/.mu.M
(.about.0.003 .mu.A/ppb) and was comparable to the sensitivity
reported by Wang. Despite these positive developments in the prior
art, the sensitivity is still insufficient, and the kinetics of the
TNT reduction reaction are too slow to achieve a practical portable
field test for trace explosive materials. A practical
electrochemical sensor for trace explosive materials should have
high sensitivity, a short measurement time and in addition a way of
cleaning the electrodes rapidly to perform further testing
[0006] There is thus a widely recognized need for an
electrochemical method and sensor for the detection of traces of
explosives, and it would be highly advantageous to have an
electrochemical method and sensor for the detection of traces of
explosives, with high sensitivity, and fast reaction kinetics.
SUMMARY OF THE INVENTION
[0007] According to the present invention, there is provided a
system for electrochemical assay of nitro-aromatic compounds,
including: (a) a working electrode having a surface modified with a
chemical that increases electron transfer kinetics of the
nitro-aromatic compounds.
[0008] Preferably, the chemical that increases the electron
transfer kinetics is an aromatic compound, for example an
amino-aromatic compound, an alkyl-aniline compound, a halide
derivative of an alkyl aniline compound and/or an hydroxyl-aniline
compound. Most preferably, the chemical modifier is
phenylene-diamine, diphenylene-diamine, diphenylene-triamine, or
aniline.
[0009] Preferably, the working electrode contains elemental carbon
or gold; the working electrode includes submicron particles and the
elemental gold is a coating on the electrode surface. Preferably,
the working electrode includes carbon paper.
[0010] Preferably, the system includes in addition, (b) an
electrolyte for dissolving the nitro-aromatic compounds, and the
electrolyte is chosen to minimize background current resulting from
oxygen reduction.
[0011] Preferably, the electrolyte includes an aprotonic solvents,
and/or dipolar solvents; such as dimethylfomamide, acetonitrile,
propylene carbonate and optionally also a solvent such as ethanol,
propanol, ethylene-glycol, and/or propylene-glycol.
[0012] Preferably, the system further includes (c) a mechanism for
inputting air suspected to include the nitro-aromatic compounds,
into the electrolyte in order to dissolve the nitro-aromatic
compounds in the electrolyte.
[0013] According to the present invention, there is provided an
electrochemical method of assaying trace compounds in air,
including the steps of (a) dissolving the trace compounds in an
electrolyte that includes aprotonic solvents, and/or dipolar
solvents; (b) immersing a working electrode in the electrolyte; (c)
applying a varying potential to the working-electrode; and (d)
measuring an electrical current consequent to the varying
potential, thereby providing measurement results indicative of a
concentration of the trace compounds.
[0014] Preferably, after measurement, the electrochemical method
includes (e) regenerating the working electrode by applying a
negative potential to the working electrode.
[0015] Preferably, the dissolving of trace compounds is performed
by bubbling air containing the trace compounds through the
electrolyte.
[0016] Preferably, the electrochemical method includes, prior to
dissolving the trace compounds in the electrolyte, the steps of:
(f) measuring a background electrical current, while applying a
varying potential, thereby obtaining background current results;
and (g) subtracting the background current results from the
measurement results, thereby obtaining calibrated measurement
results. Preferably, the electrolyte used in the electrochemical
method includes dimethylformamide, acetonitrile, and/or propylene
carbonate; optionally also ethanol, propanol, ethylene-glycol,
and/or propylene-glycol and preferably the electrolyte has pH
greater than 7. Preferably, the electrochemical method includes
preconditioning the working electrode thereby increasing electron
transfer kinetics of the trace compounds. The preconditioning
modifies the working electrode with a chemical such as
amino-aromatic compounds, alkyl-aniline compounds, halide
derivatives of alkyl aniline compounds and/or hydroxyl-aniline
compounds. According to the present invention, there is provided an
electrochemical method of assaying nitro-aromatic compounds in air,
including the steps of: (a) dissolving the nitro-aromatic compounds
in an electrolyte that includes an aprotonic solvent, and/or a
dipolar solvent; (b) immersing a working electrode in the
electrolyte; (c) applying a varying potential to the working
electrode; (d) measuring an electrical current consequent to the
varying potential, thereby providing measurement results,
indicative of a concentration of the nitro-aromatic compounds.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The invention is herein described, by way of example only,
with reference to the accompanying drawings, wherein:
[0018] FIG. 1 is a drawing of an electrochemical cell used to sense
trace explosive materials, according to the present invention;
[0019] FIG. 2 is a graph of background current measured in an
electrochemical cell using both a conventional aqueous solvent and
an electrolyte of the present invention;
[0020] FIG. 3 is a graph showing the sensitivity of the TNT
detection, using an electrolyte of the present invention, and a
conventional working electrode.
[0021] FIG. 4 is a graph showing the measurement sensitivity at
pH=4 with a modified electrode, according to the present
invention;
[0022] FIG. 5 is a graph showing the measurement sensitivity with a
modified electrode and with electrolyte of pH=9, according to the
present invention;
[0023] FIG. 6 is a graph showing the measurement sensitivity with
an electrode surface further modified according to the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] The present invention is an electrochemical method and
sensor for the detection of traces of explosives. Specifically, the
present invention can be used for the detection of trace amounts in
air of nitro-aromatic compounds including 2,4,6-trinitrotoluene
(TNT), dinitrotoluene (DNT) and similar derivatives.
[0025] The principles and operation of an electrochemical method
and sensor for the detection of traces of explosives, according to
the present invention, may be better understood with reference to
the drawings and the accompanying description.
[0026] Referring now to the drawings, FIG. 1 illustrates an
electrochemical cell 101 including a working electrode 103, a
reference electrode 105, an auxiliary electrode 106, an electrolyte
109, an air inlet 107 that passes air into the cell through a
perforated tube 108 allowing air to bubble through electrolyte 109,
and an air outlet 115 that lets air out of the cell. Reference
electrode 105 is an Hg/HgCl electrode that includes an element 111
to protect reference electrode 105 from air bubbles. Electrolyte
109 is a solvent or a mixture of solvents including trace materials
dissolved in the solvent(s). These trace materials, including
nitro-aromatic compounds dissolved in the solvent(s), are admitted
to the solvent mixture from the air using air inlet 107 and
dissolved in electrolyte 109 by bubbling the air through
electrolyte 109. Air is output through air outlet 115. A metal
screen 113 is used to prevent electrolyte 109 from escaping through
air outlet 115. Voltage is applied between working electrode 103
and reference electrode 105. A current is measured which flows
between reference electrode 103 and auxiliary electrode 106, as a
result of oxidation-reduction reactions on the electrode surfaces
in electrolyte 109. Working electrode 103 is prepared by a
technique of galvanic Au plating of carbon on an ordinary carbon
paper surface in an aqueous solution of HAuCl, K.sub.4Fe(CN).sub.6,
and Na.sub.2CO.sub.3 at current density 1 ma/cm.sup.2. The carbon
particles are of typical dimension 0.1-1 .mu.m. The carbon paper
with density 0.4-0.8 g/cc, part number P2 or P3 was to supplied by
E-TEK Inc. (Somerset, N.J., USA). The gold layer deposited on the
carbon particles is of approximate thickness, 0.30-0.60 nm. All
reagents used were obtained from Sigma-Aldrich (USA).
[0027] Voltammetric measurements were performed using a CV-50W
potentiostat of Bioanalytical Systems Inc. (West Lafayette, Ind.,
USA). Measurements were performed in the differential pulse (DIP)
mode. A background current was measured using background
electrolyte 109 before trace elements are introduced into
electrolyte 109. Trace elements are then introduced into
electrolyte 109 through air inlet 107 using a standard air pump
(not shown in FIG. 1) with throughput 1500 ml/min for 20 sec. The
current measurement is then performed and the background current is
subtracted from the measured current to yield the measured results.
The potential range is from -150 mV to -500 mV; the scan rate is
from 30 to 50 mV/sec. Electrochemical active substances in the air
including nitro-aromatic compounds (e.g. TNT) are dissolved in
electrolyte 109 and are detected electrochemically.
[0028] FIG. 2 is a graph of DIP voltammetric data measured with
C/Au working electrode 103. For data trace 201, electrolyte 109 is
0.1M KCLO.sub.4 in water with pH 9.0. For data trace 203,
electrolyte 109 is 0.1M KCLO.sub.4 in a mixture of water, ethanol,
and acetonitrile (1:1:1 v/v/v), with pH 9.0. FIG. 2 shows a window
of about 250 mV, between potentials -0.5 and -0.6V where trace 302
has a lower background current. The higher background current of
trace 201 is attributed to dissolved gaseous oxygen that interferes
with the measurement. Consequently, the mixed electrolyte of trace
203 is preferred.
[0029] FIG. 3 is a graph of DIP voltammetric data measured using an
electrochemical cell shown in FIG. 1, according to the present
invention. However, the data of FIG. 3 were measured with a working
electrode C/Au 103 that was immersed in pure dimethylsulfoxide
(DMSO) for 5 minutes at room temperature. Trace 301 is the
background voltammetic data with electrolyte 109 of pH 4 consisting
of water/ethanol/acetonitrile 1:1:1 (v/v). Trace 302 shows the
voltammetric data with 200 ug/l (200 ppb) TNT dissolved in
electrolyte 109. Each subsequent trace 302 to 304 shows
voltammetric data each with an additional 200 ug/l (200 ppb) TNT
dissolved in electrolyte 109. The TNT measurement sensitivity is
0.003 .mu.A/ppb.
[0030] FIG. 4 is a graph of DIP voltammetric data measured in
accordance with the present invention. Working electrode 103 is
preconditioned by immersing working electrode 103 in a 2% solution
of aniline in DMSO for 5 minutes at room temperature. Electrolyte
109 is the same as that of FIG. 3, of pH 4, consisting of
water/ethanol/acetonitrile 1:1:1 (v/v). Trace 401 shows the
background voltammetric data. Each subsequent trace 402 to 407
shows voltammetric data each with an additional 60 .mu.g/l (60 ppb)
TNT dissolved in electrolyte 109. The sensitivity of the
measurement is shown in the graph of inset 410, in which the
abscissa is the TNT concentration (ppb) and the ordinate is the
measured peak current in .mu.A. The measured sensitivity is 0.11
.mu.A/ppb. These data compared with the data of FIG. 3 show that
the surface modification of working electrode 103 with aniline
significantly improves the measurement sensitivity of TNT.
[0031] FIG. 5 is a graph of DIP voltammetric data measured in
accordance with the present invention. The surface of working
electrode 103 is immersed as in the measurement of FIG. 4, in a 2%
solution of aniline in DMSO. Electrolyte 109 is similar to that
used in the measurement of FIG. 4, consisting of
water/ethanol/acetonitrile 1:1:1 (v/v), but with pH 9. The pH was
achieved using Merck buffer capsules, no. CPM90L4 pH=9. Traces 501
to 505 show the calibrated data after subtracting the background
data. Traces 501 to 505 show the calibrated data each with an
additional 10 .mu.g/l (10 ppb) dissolved in electrolyte 109,
starting with 10 .mu.g/l in trace 501. Graph inset 510 shows a
sensitivity of 0.50 .mu.A/ppb. These data compared with the
measurement of FIG. 4 show that the high pH increases the
measurement sensitivity.
[0032] FIG. 6 is a graph of DIP voltammetric data measured in
accordance with the present invention. The surface of working
electrode 103 is preconditioned by immersing working electrode 103
in a 3% solution of aniline in DMSO for 5 minutes at room
temperature. Electrolyte 109 is the same as that of the measurement
of FIG. 5, consisting of water/ethanol/acetonitrile 1:1:1 (v/v),
with pH 9. Trace 601 shows the background data. Traces 602 to 606
show the measured data without subtracting the background data.
Traces 602 to 606 show the measured data, each when an additional
15 ppb of TNT is dissolved in electrolyte 109, starting with 15 ppb
of TNT in trace 602. The measured sensitivity is shown in graph
inset 610. The measured sensitivity is 0.66 .mu.A/ppb, showing that
the stronger aniline treatment significantly improves the
sensitivity.
[0033] The measurement of nitro-aromatic compounds, according to
the present invention requires about 30-40 sec. After the detection
of nitro-aromatic compounds, working electrode 103 is regenerated,
if necessary, using a high negative potential about -1000 to -1300
mV. Regeneration time is about 5 seconds. Therefore, the
electrochemical technique, according to the present invention, is
suitable for rapid and portable field-testing of nitro-aromatic
compounds. The experimental results show that the use of solvents
which are aprotonic, organic dipolar, or have a high dielectric
constant, as part of electrolyte 109, are expected to improve the
detection sensitivity, according to the present invention. These
solvents include acetonitrile, dimethyl-formamide and
propylene-carbonate and mixtures thereof. Other polar solvents such
as ethanol, propanol, ethylene-glycol, and propylene-glycol may be
added as diluents. A preferred diluent has a high boiling point and
is stable against evaporation. The electrode used, according to the
present invention, is manufactured from ordinary carbon paper.
Carbon paper is readily available, of low cost and therefore
suitable for a disposable electrode. An electrode, including a
carbon particle layer on other substrates including cloth or glass,
will function in a similar way.
[0034] The experimental results furthermore show that the chemical
modification of a, carbon or carbon/gold electrode, with compounds
similar to aniline, preconditions the electrode surface to increase
the sensitivity of the measurement, according to the present
invention. These compounds include aromatic compounds containing
amino groups, including derivatives that are mono-alkyl (e.g.
methyl, ethyl, propyl, . . . ), di-alkyl or tri-alkyl. The chemical
modifiers, according to the present invention include
aromatic-anilile compounds such as, phenylene-diamine,
diphenylene-diamine, diphenylene-triamine and similar compounds and
derivatives. This chemical modification is necessary because the
reduction of nitro-aromatic compounds has slow kinetics. The
modification successfully increases the electron transfer rate from
the solution to electrode 103 and therefore increases the
sensitivity of the measurement and lowers the detection limit.
Other modifiers that increase electron transfer, according to the
present invention, include compounds such as J. Meisenheimers
complexes, known to mediate electron transfer, and other
nitro-amine complexes; and alkyl-aniline and its derivatives; and
halide derivatives of alkyl aniline compounds as well as
hydroxyl-aniline compounds.
[0035] While the invention has been described with respect to a
limited number of embodiments, it will be appreciated that many
variations, modifications and other applications of the invention
may be made.
* * * * *