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Chemical Approaches
to the Detection of ExplosivesAnthony S. Breitbach
Under the supervision ofHelen E. Blackwell, PhD
February 12, 2009
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Outline
2
Explosives
Explosive Detection
Colorimetric Tests
Amplifying Fluorescent Polymers
Enzymatic-Facilitated Chemical Sensors
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Explosives are Classified by Performance
Singh, S.J. Hazard. Mater. 2007, 144, (1-2), 15-28.3
ExplosivesEnergetic material that can undergo a very rapid
combustion releasing large amounts of energy .
Low
Subsonic Combustion
High
Supersonic Combustion
Pyrotechnics Propellants Primary Secondary
Initiators Main charge
Pb(N3)2
TNT
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Why are Compounds Explosive?
4
Exothermic combustion
Release large amounts of energy and gas
Internal oxidant
N-oxides (NO, NO2, NO3) as source of O
Nitromethane (NM)cH [kJ/mol] -709.2
Combustion velocity [km/s] 6.49
MethanecH [kJ/mol] -890.8
Combustion velocity [km/s] 1.4-1.64
Akhavan, J. Kirk-Othmer Encycl. Chem Technol., 5th ed.; Wiley: New York, 2005; Vol. 10, pp 719-744.
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Nitro Groups Easily Incorporated By Nitration
5
N-nitration
Aryl-nitration
O-nitration
Nitramines
Nitroaromatics
Nitrate Esters
Nitration via electrophilic substitution with nitronium ion
Olah, G. A. Nitration Methods and Mechanisms; VCH Publishers: New York, 1989.
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The Usual Suspects
6
Nitro
Aliphatics
Acid Salts
Nitro
AromaticsNitrate Esters
Peroxides
Nitramines
2,4,6-trinitroluene (TNT)
DV 7.07km/s
Nitromethane (NM)
DV 6.49 km/s
Ammonium Nitrate
Triacetone triperoxide (TATP)
DV 5.2 km/s
Hexamethylene triperoxide diamine (HMTD)
DV 4.5 km/s
Nitroglycerine(NG)
DV 8.08 km/s
Trinitro-triazacylohextane (RDX)
DV 8.63 km/s
Keshavarz, M. H.J. Hazard. Mater. 2007, 141, (3), 536-539.
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Detection Needed for Safety
7
Airports reduced number
of scheduled flights by
30% to allow increase
time for security checks
120 million
unexploded landmines
in 62 countries
unaccounted for
Kill or injure 30,000
people each year
TNT classed as toxic at >2ng/mL
Forensics
Homeland Security Global de-mining
Environmental Detection and Remediation
Singh, S.J. Hazard. Mater. 2007, 144, (1-2), 15-28.
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Canines Are Unreliable and Costly
8
PROS: Display high selectivity and sensitivity (up to 500ppt)
CONS: Unreliable, high cost and maintenance
High demand for new approaches to explosive detection
Habib, M. K. Biosens. Bioelectron. 2007, 23, (1), 1-18.
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Chemosensors Lead the Way
9
Dilemma types of radiation that yield
specific info about molecules and bonds
cannot penetrate objects
Trace
Detectors
Explosives
Detectors
Bulk
Detectors
Mass
SensorsBiosensorsChemosensors
Singh, S.J. Hazard. Mater. 2007, 144, (1-2), 15-28. Yinon, J.Anal. Chem. 2003, 75, (5), 98A-105A.
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Difficulties for Detection
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Low vapor pressure
Vapor pressure depression
Mixtures
Binders
Packaging
Thermal vaporization causes degradation
Lack characteristic absorption bands
Increased usage of non-conventional explosives
Explosive
Vapor Pressure
[ppt (v/v)]
Chloroform 500,000,000
EGDN 100,000,000
NG 580,000
DNT 56,000
TNT 9,500
PETN 18
RDX 6
Increasing Difficulty for Detection
Kolla, P.Angew. Chem. Int. Ed. 1997, 36, (8), 800-811.
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Indirect Colorimetric Detection
12
Indirect detection using color reactions
Commercial kits available
PROS: simple, fast (30s), portable, disposable, versatile (>30 explosives), LOD 25-500ng
CONS: selectivity (false positives), poor sensitivity (requires large sample size)
Field Forensics, Inc.: ELITE Explosive Detector. http://www.fieldforensics.com/ (accessed Feb 2009)
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Nitroaromatics detected by reacting with nucleophiles to form colored JM complex.
Jackson-Meisenheimer (JM) Complex
13
-OH -OH
2,4,6-trinitrotoluene (TNT) 2,6-dinitrotoluene (DNT)
C3 JM Complex C1 JM ComplexTNT
Chen, H.; et. al.J. Am. Soc. Mass. Spectrom. 2004, 15, (7) 998-1004.
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Nitrite Assayed with Griess Test
14
Nitrite reduced to nitrosonium ion by treatment with reductant
Nitrate esters and nitramines undergo alkaline hydrolysis to release nitrite
RDX
Jenkins, T. F.; et. al. Talanta 1992, 39, (4), 419-428.
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Nitosonium Reacts to Colored Azo
15
RDX
CON: Interference with
other nitrites is an issue
Sulfanilic Acid Diazonium ion
1-naphthylamine
Colored Azo
Jenkins, T. F.; et. al. Talanta 1992, 39, (4), 419-428.
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Multiple Explosives Detection
16
Color reactions can be built into arrays and coupled with pattern detection
to build a nano-nose which mimics the olfactory sensing of dogs.
Color Reagents
Optical Fiber
Pattern Response
to Explosives
Yinon, J.Anal. Chem. 2003, 75, (5), 98A-105A.
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Fluorescent Quenching for Detection
17
Exploits electron accepting capability of nitro-containing explosives
HOMO
LUMO
HOMO
LUMO
LUMO
ERed: NB (-1.15V) < DNT (-0.9V) < TNT (-0.7V)Increased Reduction Potential =
Increased Quenching
Toal, S. J.; et. al.J. Mater. Chem 2006, 16, (28), 2871-2883.
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Amplified Fluorescent Quenching
18
Analyte
Analyte
Excited Fluorophores Quenched Fluorophore
Excited Fluorescent Polymer Amplified Quenched Polymer
Yinon, J.Anal. Chem. 2003, 75, (5), 98A-105A.
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Conjugated Fluorescent Polymers
19
Polyacetylenes Poly(p-phenylene-
vinylenes)Poly(p-phenylene-
ethynylenes)
Polymeric
Porphyrins
Polysilanes Polymetalloles
Toal, S. J.; et. al.J. Mater. Chem 2006, 16, (28), 2871-2883.
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Polymetalloles Have Unique Electronics
20
Metallacyclopenta-2,4-diene moiety
Highly fluorescent
Highly quenchable
Tunable electronics
2,5-diphenylsilol from ab initio calculations at the HF/6-31G* level
HOMO LUMO
*
*
Toal, S. J.; et. al.J. Mater. Chem 2006, 16, (28), 2871-2883. Sohn, H.; et. al.JACS 2003, 125, (13), 3821-3830.
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+TNT
29Si NMR Polymetallole
Downfield
Shift
Polymetalloles Form Selective Helix
21
Crystal structures and
models show polymetallolesform selective helices
29Si NMR studies indicate
weak Lewis Acid-Lewis Base
interaction
Sohn, H.; et. al.JACS 2003, 125, (13), 3821-3830. Sanchez, J. C.; et. al. Chem. Mater. 2007, 19, (26), 6459-6470.ppm
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Dehydrocoupling of Dihydrometallole
22
Poly(tetraphenylsilol)
[PSi]
Toal, S. J.; et. al.J. Mater. Chem 2006, 16, (28), 2871-2883. Sohn, H.; et. al.JACS 2003, 125, (13), 3821-3830.
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Quenching Efficiency Depends on Explosive
23
Quenching
Efficiency [M-1]
TNT DNT NB
4340 2420 1200
Quenching efficiency directly related to reduction potential:
TNT (-0.7V) > DNT (-0.9V) > NB (-1.15V)
Relative
Fluorescence
Quenching
Concentration of Explosive [M]
Poly(tetraphenylsilol)
[PSi]
TNT
DNT NB
Sohn, H.; et. al.JACS 2003, 125, (13), 3821-3830.
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PROS: excellent sensitivity and selectivity, cheap, easily synthesized,
readily fielded for on-site use, and nontoxic
CONS: limited to nitroaromatics, interference from UV absorbers
PSi Detects Nitroaromatics
24
LOD [ng]DNT 50
TNT 30
Poly(tetraphenylsilol)
[PSi]
Toal, S. J.; et. al.J. Forensic Sci. 2007, 52, (1), 79-83.
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PSiV More Delocalized and Stable
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Metallole-vinylene polymers increased delocalization across the backbone and
is Si-C more thermally stable vs Si-Si bridge (> by 150 kJ/mol)
Si-vinylene easily incorporated with hydrosilylation.
Poly(tetraphenylsilol-vinylene)
[PSiV]
Sanchez, J. C.; et. al. Chem. Mater. 2007, 19, (26), 6459-6470.
60-70% Yield
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HOMO and LUMO Energies Calculated for Explosives and
Trimers PSiV and PSFV at the B3LYP/631G* Level of Theory
LUMO
HOMO
SOMO
4.6eV3.5eV
PSFV, Better Orbital Energy Matching
26
UV
Larger Band Gap = Higher Energy = Better Reduction Driving Force=
= Greater Sensitivity?
Poly(silafluorene-
vinylene) [PSFV]
Sohn, H.; et. al.JACS 2003, 125, (13), 3821-3830. Sanchez, J. C.; et. al. Chem. Mater. 2007, 19, (26), 6459-6470.
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PSiV and PSFV Have Increased Quenching
27
Quenching Efficiency [M-1]in Toluene
Polymer % Yield MW (GPC) n TNT DNT RDX
PSi 88 1500 4 4300 2400 x
PSiV 73 4000 10 10500 4300 x
PSFV 66 4300 21 10200 5500 x
Poly(tetraphenylsilol)
[PSi]
Poly(tetraphenylsilol-vinylene)
[PSiV]
Poly(silafluorene-vinylene)
[PSFV]
Greater degree of polymerization
Greater quenching efficiency
Nitramine, RDX, not detected
Sanchez, J. C.; et. al. Chem. Mater. 2007, 19, (26), 6459-6470.
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PSiV and PSFV More Accessible
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Vinylene and silafluorene functionalities create more open helices than PSi
allowing greater access to Si centers.PSi PSiV PSFV
Sanchez, J. C.; et. al. Chem. Mater. 2007, 19, (26), 6459-6470.
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PSFV More Ordered in Solid-State
29
PSFV red shifts in solid-state
Framework more ordered
Interchain - stacking
RelativeFluorescence
Intensity
Re
lativeFluorescence
Intensity
Solution
Solid
Solution
Solid
[PSiV] [PSFV]
Sanchez, J. C.; et. al. Chem. Mater. 2007, 19, (26), 6459-6470.
No Change
RED Shift
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PSFV Detects RDX in Solid-State
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Expl. PSiV PSFVPA 2 2
TNT 2 0.3
DNT 3 2
RDX X 2
HMX X 3PETN X 2
TNG X 3
PSFV detected nitramines and nitrate esters in solid-state
PSFV detected TNT from real-life contaminated samples
PSFV more susceptible to interferents (benzophenone 3 ng/cm2)
Solid-State LOD [ng cm-2]
Nitroaromatics
Nitramines
Nitrate Esters
Sanchez, J. C.; et. al. Chem. Mater. 2007, 19, (26), 6459-6470.
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Fido, Worlds Most Sensitive
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LOD in femto-gram
range (ppq)
Detection of vapor and
particulates
Used in Iraq
ICX Technologies: Amplifying Fluorescent Polymers Fido Detector. http://www.icxt.com (accessed Feb 2009)
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Peroxides a Challenge for Detection
32
Easily Synthesized
As powerful as TNTMore sensitive
Difficult to detect
Increased usage by terrorists
Schulte-Ladbeck, R.; et. al.Anal. Bioanal. Chem. 2006, 386, (3), 559-565.Dubnikova, F.; et. al.JACS 2005, 127, (4), 1146-1159.
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TATP Detected Indirectly with POD
33
TATP
2,2-azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS) Colored Radical Cation
H+
Schulte-Ladbeck, R.; et. al.Anal. Bioanal. Chem. 2006, 386, (3), 559-565.
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ABTS Oxidizes to Colored Cation
34Kadnikova, E. N.; et al.J. Mol. Catal. B: Enzym. 2002, 18, (1-3), 39-48.
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ACRO P.E.T.
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Acid catalyzed decomposition
Horse-radish peroxidase
ABTS color detection
PRO: high selectivity, fast, cheap, disposable
CON: moderate sensitivity (mg), interference
TATP
Itzhaky, H.; Keinan, E. Method and Kit For the Detection of Explosives. U.S. Patent 6,767,717, 2004.ACRP Security Technologies: ACRO-P.E.T. www.acrosec.com/index.asp (accessed Feb 2009)
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Summary
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Chemical approaches offer a more
reliable solution to explosive detection
Future Directions
More selective and sensitive color
reagents and fluorescent polymers
Incorporation of bio-selective elements
Arrays for Pattern Detection
Steinfeld, J. I.; et. al.Annu. Rev. Phys. Chem. 1998, 49, (1), 203-232. 48.
Yeah, but it doesnt
smell like a bomb
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Acknowledgements
37
Special Thanks:
Prof. Helen Blackwell
Blackwell group
Practice talk attendeesDrew Palmer
Joey Stringer
Margie Mattmann
Reto Frei
Christie McInnis
Aaron Crapster
Teresa Beary
JP Gerdt
Melissa Mennig
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