Direct detection of RDX vapor using a conjugated polymer network

Article abstract of DOI:10.1021/ja402668e

1,3,5-Trinitroperhydro-1,3,5-triazine (RDX) is a principal component of plastic explosives used in acts of terrorism and within improvised explosive devices, among others. Approaches to detect RDX compatible with remote, stand-off sampling that do not require preconcentration strategies, such as the swabs commonly employed in airports, will benefit military and civilian security. Such detection remains a significant challenge because RDX is 10³ less volatile than 1,3,5-trinitrotoluene (TNT), corresponding to a parts-per-trillion vapor pressure under ambient conditions. Therefore, while fluorescence quenching of conjugated polymers is sufficiently sensitive to detect TNT vapors, RDX vapor detection is undemonstrated. Here we report a cross-linked phenylene vinylene polymer network whose fluorescence is quenched by trace amounts of RDX introduced from solution or the vapor phase. Fluorescence quenching is reduced, but remains significant, when partially degraded RDX is employed, suggesting that the polymer responds to RDX itself. The polymer network also responds to TNT and PETN similarly introduced from solution or the vapor phase. Pure solvents, volatile amines, and the outgassed vapors from lipstick or sunscreen do not quench polymer fluorescence. The established success of TNT sensors based on fluorescence quenching makes this a material of interest for real-world explosive sensors and will motivate further interest in cross-linked polymers and framework materials for sensing applications.

Full text of DOI:10.1021/ja402668e

Article
pubs.acs.org/JACS
Direct Detection of RDX Vapor Using a Conjugated Polymer Network
Deepti Gopalakrishnan and William R. Dichtel*
Baker Laboratory, Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853-1301, United States
S
* Supporting Information
ABSTRACT: 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX) is a
principal component of plastic explosives used in acts of
terrorism and within improvised explosive devices, among
others. Approaches to detect RDX compatible with remote,
“stand-off” sampling that do not require preconcentration
strategies, such as the swabs commonly employed in airports,
will benefit military and civilian security. Such detection remains
a significant challenge because RDX is 103 less volatile than
1,3,5-trinitrotoluene (TNT), corresponding to a parts-per-
trillion vapor pressure under ambient conditions. Therefore,
while fluorescence quenching of conjugated polymers is sufficiently sensitive to detect TNT vapors, RDX vapor detection is
undemonstrated. Here we report a cross-linked phenylene vinylene polymer network whose fluorescence is quenched by trace
amounts of RDX introduced from solution or the vapor phase. Fluorescence quenching is reduced, but remains significant, when
partially degraded RDX is employed, suggesting that the polymer responds to RDX itself. The polymer network also responds to
TNT and PETN similarly introduced from solution or the vapor phase. Pure solvents, volatile amines, and the outgassed vapors
from lipstick or sunscreen do not quench polymer fluorescence. The established success of TNT sensors based on fluorescence
quenching makes this a material of interest for real-world explosive sensors and will motivate further interest in cross-linked
polymers and framework materials for sensing applications.
high sensitivity, the simplicity of photoemission measurements
facilitates the design of stand-off detection devices. Many
conjugated polymers fluoresce intensely upon exposure to
visible light. Following light absorption, excited states, or
excitons, diffuse along and among polymer chains, where they
may encounter the analyte of interest. The analyte non-
radiatively deactivates the mobile excitons, interrupting the
fluorescence of the material. Since excitons sample many
polymer chains during their lifetimes, a small amount of the
analyte can quench much of the fluorescence of a thin polymer
film; this amplification strategy provides the high sensitivity
necessary for vapor-phase detection.14 The most sensitive
conjugated polymers sense TNT at parts per quadrillion
levels,15,16 and these materials have been commercialized for
military and security use. Recently developed material classes
with high surface area to interact with analytes, including
metal−organic frameworks (MOFs),17,18 conjugated micro-
porous polymers,19 and self-assembled conjugated macro-
cycles,20 have shown promise for detecting energetic
compounds. However, most of these materials respond to
TNT or more volatile nitroaromatics; none has demonstrated a
response to RDX. Lei and co-workers observed unexpectedly
high fluorescence quenching sensitivity from electrospun
pyrene/polystyrene nanofibers for many nitro-containing
explosives,21 although their sensitivity to RDX vapor was
slow and demonstrated qualitatively. Here we describe a
1. INTRODUCTION
Low-volatility explosives, such as 1,3,5-trinitroperhydro-1,3,5-
triazine (RDX), 1,3,5-trinitrotoluene (TNT), and pentaery-
thritol tetranitrate (PETN), pose major security risks and safety
hazards throughout the world. RDX in particular is one of the
most common components of plastic explosives. The detection
of trace amounts of these compounds, ideally from the vapor
phase, is essential for identifying explosive devices and
individuals who have handled them recently. Vapor detection
of 1,3-dinitrotoluene (DNT) and TNT has been studied
extensively for the past two decades, resulting in several
technologically relevant solutions.1,2 However, detecting RDX
vapor remains an important and difficult challenge, as it is three
orders of magnitude less volatile than TNT and is a weaker
electron acceptor than nitroaromatic explosives.3
Analytical techniques that have been applied for trace RDX
sensing include colorimetric immunoassays,4,5 surface enhanced
Raman spectroscopy,6,7 ion mobility spectroscopy,8 and
changes in the surface plasmon resonance of Au nano-
particles.9,10 Many of these techniques require specialized
equipment and/or require preconcentration strategies.11
Andrew and Swager recently reported that dihydroacridine
derivatives show a “turn-on” fluorescence response to nitration
induced by the photodegradation of RDX and PETN.12,13 This
strategy demonstrated 100 pg sensitivity to RDX, similar to that
of current swab-based transportation security systems.
Fluorescence quenching of conjugated polymers by appro-
priate analytes is a highly sensitive phenomenon that enables
the direct detection of explosive vapors.1,14 In addition to its
Received: March 15, 2013
© XXXX American Chemical Society
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Figure 1. Schematic of RDX vapor detection by the TPV conjugated polymer network.
Figure 2. (A) Synthesis of the TPV network under ADMET reaction conditions. (B) Structure of soluble model compound 2.
conjugated polymer network prepared using olefin metathesis.
Its films exhibit fluorescence quenching when exposed to RDX,
TNT, and PETN delivered from either solution or vapor phase
(Figure 1). This quenching response is not observed for solvent
vapors, various amines, or volatile compounds outgassed from
common household items. These results are relevant for
practical RDX sensors, and we expect them to inspire further
investigations of polymer networks and framework materials for
sensing applications.
also observed in the TPV FT-IR spectrum, which correspond to
unreacted styrene moieties that remain within the polymer
network. These peaks are prominent in the FT-IR spectrum of
monomer 1. TPV powders were also characterized by solid-
state 13C cross-polarization magic angle spinning (CP-MAS)
NMR, which was also consistent with the expected stilbene-
linked structure. The spectrum indicates two broad aromatic
resonances centered at 141 and 137 ppm, as well as a peak
corresponding to the vinylic carbons at 128 ppm (Figure S3).
Despite the known reversibility of olefin metathesis, which
might give rise to a crystalline network, all samples of TPV
prepared under these conditions are amorphous and non-
porous, as indicated by their low N2 uptake (Figures S13 and
S14). They may be heated to 600 °C before decomposing to
volatile products, as determined by thermogravimetric analysis
(Figure S12).
Although characterizing the powder form of 1 provides
important confirmation of the network structure, these powders
strongly scatter light and are not ideal for detecting explosives
via fluorescence quenching. Linear conjugated polymers are
readily processed into films using solution-based methods, such
as spin-coating or drop-casting.22 In contrast, cross-linked
conjugated networks are inherently insoluble. We prepared thin
films of TPV instead by including a fused SiO2 substrate in the
reaction vessel during its synthesis. The resulting films were
washed extensively with CH2Cl2 to remove soluble byproducts
and then activated by solvent evaporation under ambient
conditions. The films show similar FT-IR spectra (Figure S9) as
2. RESULTS AND DISCUSSION
2.1. Polymer Synthesis and Characterization. Con-
jugated networks were synthesized through an acyclic diene
metathesis (ADMET) polymerization of a trifunctional styrene
derivative 1 (Figure 2). Olefin metathesis was chosen for its
high functional group tolerance, mild reaction conditions,
benign stoichiometric byproducts (ethylene), and synthetically
convenient monomers. 1 was polymerized under solvothermal
conditions in CH2Cl2 at 45 °C in the presence of 6 mol % of
the Grubbs second-generation olefin metathesis catalyst, which
provided tris(phenylene)vinylene (TPV) as an insoluble
amorphous powder. FT-IR spectra of the TPV powders are
consistent with the formation of a network linked by stilbene
moieties and feature CC stretches at 1593 cm−1 and vinyl
C−H out-of-plane bends at 963 cm−1 (Figure 3A). We
prepared tris(1,3,5-stilbenyl)benzene 2 as a model of the TPV
network, which exhibits similar vibrations at 1594 and 959
cm−1, respectively. Very weak peaks at 1627 and 987 cm−1 are
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Figure 3. (A) Infrared spectra of 1, 2, and TPV powder. (B) Normalized UV/vis spectra (each in CH2Cl2) of 4-phenylstyrene (yellow), 1 (blue),
4,4′-biphenylstilbene (black), 2 (green), and TPV film grown on fused SiO2 (red).
the TPV powders and are also amorphous, as determined by
grazing incidence X-ray diffraction at the Cornell High Energy
Synchrotron Source (CHESS). X-ray photoelectron spectros-
copy indicated a narrow carbon signal centered at 284 eV,
corresponding to the sp2-hybridized carbons within the TPV
network. Strong Si and O peaks were also observed that
originate from the fused SiO2 substrate. Peaks corresponding to
Ru were not observed, and only trace N was detected,
indicating low amounts of catalyst decomposition products in
the films (Figures S63−S65).
The electronic absorption and fluorescence of the films show
subtle but reproducible differences as a function of the reaction
time, which also proved to strongly impact their RDX response
(see below). UV/vis spectra of the TPV films yield local
maxima at 287 and 358 nm. The 358 nm peak increases in
intensity relative to that of 287 nm as a function of reaction
time. Films grown for 1 h showed an I358/I287 ratio of 0.87,
which increased to ∼1.2 at 24 and 48 h reaction times. The 287
nm peak of films grown for 72 h typically appeared only as a
shoulder of the 358 nm peak (Table S1). We attribute these
spectral changes to increased conversion of styrene to stilbene
moieties in the films. This conclusion was further supported by
the solution absorption spectra of relevant model compounds.
The λmax of 4-phenylstyrene (279 nm) and monomer 1 (287
nm) correspond to the 287 nm absorption observed in the
TPV film. The spectra of model compound 2 (332 nm) and
4,4′-biphenylstilbene (342 nm) are indicative of extended
conjugation (Figure 3B). These peaks correspond well to the
358 nm absorption of the TPV film, which is red-shifted further
due to its greater conjugation and/or aggregated thin-film
structure. In summary, UV/vis spectroscopy provides a
qualitative indication of the extent of the olefin metathesis
reaction in the TPV films and suggests increased conversion of
the metathesis reaction over reaction times up to 72 h.
tetrazocine (HMX) (see Figure 5B). HMX is a cyclic
tetranitramine explosive related to RDX and has an even
lower vapor pressure. A RDX stock solution (1 mg/mL
dissolved in 1:1 v/v CH3CN:MeOH) was prepared and diluted
further to deliver picogram quantities of RDX to the films. After
the film’s initial fluorescence was recorded, the RDX solution
was introduced and the solvent evaporated under high vacuum.
The film’s fluorescence was again recorded. This procedure was
repeated to introduce cumulative RDX dosages between 0 and
18 765 pg and reproduced in triplicate for films grown for 24,
48, and 72 h. Each TPV film showed reduced emission intensity
when exposed to even 1 pg of RDX, and this response saturated
at cumulative doses of ∼1800 pg. The films exhibited increased
quenching response as a function of their reaction time. The 72
h films showed 51 15% quenching when exposed to 25 pg of
RDX and saturated at 71 9% at larger doses. In contrast, the
48 h films showed 24 6% when exposed to 25 pg of RDX and
saturated at 53 10%, while the 24 h films showed only 9
4% when exposed to 25 pg of RDX and saturated at 14 9%
(Figure 4A−D). Although each film responded to low
picogram quantities of RDX, this response is nonlinear at this
dosage. Nonlinear, reproducible quenching responses were also
observed for 72 h films exposed to lower dosages of 0−2900 fg
(Figure S24) and 0−1700 ag of RDX (Figure S25). Upon still
further reduction of the RDX dosage, 72 h films exhibited an
approximately linear quenching response over the 1−30 ag
range (Figure 4E), a promising level of sensitivity for detecting
RDX from the vapor phase. We also performed an important
control experiment, in which the fluorescence responses of
TPV films grown for 72 h were measured after sequential
introduction and evaporation of the same 1:1 v/v
CH3CN:MeOH mixture used to prepare the dilute RDX
solutions. We noted that residual amounts of these solvents
induce an increase in TPV fluorescence intensity if they are not
evaporated completely (Figure 4F). Therefore, quenching
observed in response to the RDX-containing solution cannot
be attributed to residual solvent or the presence of trace
impurities found in these solvents. Similar responses were
observed for 72 h films exposed to picogram amounts of TNT
and PETN under the same conditions (Figure S37). We
hypothesize that the increased response of the 72 h films
compared to those grown for shorter times derives from longer
exciton diffusion lengths that arise from the increased degree of
polymerization of the network.
2.2. Fluorescence Response to Explosives Introduced
from Solution. We first evaluated the fluorescence response of
TPV films polymerized for 24, 48, and 72 h to RDX introduced
from solution. RDX was extracted from commercial samples of
the compound adsorbed onto sand that are used to train bomb-
detecting canines. RDX was crystallized twice, stored at low
temperature, and protected from light prior to performing
quenching experiments (see SI for experimental details and
safety considerations). 1H NMR of the twice-crystallized
material indicated the expected resonance for RDX, along
with a trace amount of octahydro-1,3,5,7-tetranitro-1,3,5,7-
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Figure 4. (A) Quenching of fluorescence of TPV film grown for 24 h. (B) Quenching of fluorescence of TPV film grown for 48 h. (C) Quenching of
fluorescence of TPV film grown for 72 h. (D) Relative quenching % of TPV films on exposure to RDX in solution with varying time of reaction.
Each data point indicates the average response of three different films. The error bars show one standard deviation from the average. (E)
Fluorescence quenching % of TPV films grown for 72 h in response to attogram (ag) quantities of RDX. (F) Response of 72 h TPV films to 1:1 v/v
CH3CN:MeOH solution in the absence of RDX.
Given RDX’s poor photostability, which was utilized in the
detection strategy of Andrew and Swager,12,13 we evaluated the
response of the 72 h TPV film to partially degraded samples of
RDX to determine whether the polymer responds to RDX itself
or its degradation products. RDX photolysis produces a
complex mixture of products: hexahydro-1-nitroso-3,5-dinitro-
1,3,5-triazine (MNX), hexahydro-1,3,5-trinitroso-1,3,5-triazine
(TNX), 1,3-dinitro-1,2,3,4-tetrahydro-1,3,5-triazine (MUX),
1,3,5-triazine (TUX), and perhaps others, any of which might
quench the fluorescence of the TPV network (Figure 5A).23 To
assess this possibility, we photolyzed a solution of recrystallized
RDX (10 mg/mL in 1:1 v/v CD3CN:CD3OD) for 88 h using a
2.3. Fluorescence Response to Explosive Vapors.
Military and civilian security applications will greatly benefit
from directly detecting RDX vapor at equilibrium or
subequilibrium vapor pressures without the need for
preconcentration strategies. We evaluated each of the TPV
films in this context by storing the film in the headspace of a
closed glass vessel containing a sample of crystallized RDX at
ambient temperature and pressure. Prior to the quenching
experiments, the vessel containing the RDX was placed under
high vacuum for 6 h at 25 °C while protected from light
exposure to minimize the presence of other volatile
compounds. The fluorescence of 24, 48, and 72 h films was
recorded in triplicate as a function of exposure time. As before,
the 72 h film showed both the fastest and largest response, with
22 11% quenching within 30 s, which saturated at 50 11%
at longer exposure times. The 48 and 24 h films showed
qualitatively similar quenching behavior, but with a reduced
response (Figure 6A). We also evaluated the response of the
films directly to the canine training aids, which contain RDX,
presumably alongside its degradation products. Similar, albeit
reduced responses were observed (Figure S36), which are
consistent with our solution RDX degradation study. The 72 h
TPV films also responded to vapors of canine training samples
of TNT and PETN vapors (Figure S40). We also subjected
films grown for 72 h to vapors of compounds or household
items typically encountered in airport security environments:
EtOH, lipstick, and sunscreen (Figure 6B). None of these
compound mixtures quenched the fluorescence of the TPV
films, which instead showed increased fluorescence intensity in
1
hand-held UV lamp. H NMR analysis of the solution clearly
indicated significant degradation, along with ∼42% residual
RDX (Figure 5B). This solution was diluted with 1:1 v/v
CH3CN:CH3OH to a 10 μg/mL concentration based on the
initial concentration of crystallized RDX. A 72 h film exposed
to 25 pg of RDX from this solution showed 26% quenching and
saturated at 46% (Figure 5C). The reduced saturated
quenching response, despite the photolyzed solution containing
significant amounts of RDX, indicates that the photo-
degradation products do not quench the fluorescence of the
TPV network. A similar reduction in quenching efficiency was
observed when commercial RDX standard solutions, which had
been stored for prolonged periods, were employed. The films
showed 27
13% quenching on exposure to 25 pg and
saturated at 53 14% (Figure 5C). These observations suggest
that TPV films respond to RDX itself and not its degradation
products.
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1
Figure 5. (A) Schematic of several possible RDX photolysis products (see refs 12, 13, and 23). (B) Partial H NMR spectra (300 MHz 1:1
CD3CN:CD3OD, 298 K) of crystallized RDX before (blue) and after irradiation (red) with UV light. The photolyzed spectrum shows clear evidence
of RDX degradation. (C) Fluorescence quenching curves of 72 h TPV films exposed to solutions of crystallized (blue) and photolyzed (red) and a
RDX commercial standard (green). Each data point indicates the average response of three different films. The error bars show one standard
deviation from the average.
Figure 6. (A) Response of TPV films to RDX vapor exposure. (B) Response of 72 h TPV films to outgassed vapors from EtOH, and household
items, lipstick and sunscreen. Each data point indicates the average response of three different films. The error bars show one standard deviation
from the average.
each case. This response is similar to that observed when
residual volatile solvents remain in the films. Electron-rich
aromatics have also been shown to increase the fluorescence
intensity of conjugated microporous polymers capable of
sensing DNT.19,24 Vapors of several volatile amines, including
NEt3, pyridine, aniline, and N-methylpiperidine, also induce
increases in emission intensity (Figure S56), also indicating that
quenching occurs only in the presence of the electron deficient
explosives. The reduced potential for false positives and high
sensitivity to RDX make TPV polymers a potential material for
commercial sensing technology. These findings represent a
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(7) Farrell, M. E.; Holthoff, E. L.; Pellegrino, P. M. Proc. SPIE 2012,
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3. CONCLUSIONS
We synthesized a conjugated polymer network whose
fluorescence is quenched by explosive vapors. Analysis of the
insoluble powder form of the polymer confirmed that the olefin
metathesis reactions employed in its synthesis provided the
anticipated chemical linkages. Thin films of these polymers
were grown on fused SiO2 substrates, and UV/vis absorption
spectroscopy of these films suggested increased conversion of
their reactive groups over 72 h. The fluorescence of these films
decreased upon exposure to RDX, introduced either from
solution or from the vapor phase, and depended strongly on the
film’s growth time. Degradation studies also suggest that the
films respond to RDX itself and retain most of their sensitivity
to partially degraded samples. These promising findings warrant
future inquiry into conjugated polymer networks, as their
performance might be improved by achieving long-range order,
high surface area, and improved control over film thickness.
While it remains challenging to characterize and control the
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and characterization data for all new
compounds and materials and fluorescence response of TPV
films to explosives. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
wdichtel@cornell.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the NSF CAREER award
(CHE-1056657). We also made use of the Cornell Center for
Materials Research Shared Facilities, which are supported
through the NSF MRSEC program (DMR-1120296). CHESS
is supported by the NSF and NIH/NIGMS via NSF award
DMR-0936384. The authors thank J. W. Colson for acquiring
grazing incidence X-ray diffraction at CHESS. D.G. acknowl-
edges the award of a Faculty for the Future fellowship from the
Schlumberger Foundation.
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