2014 bailey, nano research
TRANSCRIPT
Functionalized, carbon nanotube material for the catalyticdegradation of organophosphate nerve agents
Mark M. Bailey1,† (), John M. Heddleston1, Jeffrey Davis2, Jessica L. Staymates2, and
Angela R. Hight Walker1 ()
1 National Institute of Standards and Technology (NIST), Semiconductor and Dimensional Metrology Division, Gaithersburg, MD, USA2 National Institute of Standards and Technology (NIST), Materials Measurement Science Division, Gaithersburg, MD, USA † Present Address: United States Army Medical Research Institute of Infectious Diseases (USAMRIID), Center for Aerobiological Sciences,
Fort Detrick, MD, USA
Received: 14 February 2013
Revised: 20 November 2013
Accepted: 17 December 2013
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2013
KEYWORDS
single-wall carbon
nanotube
functionalization,
catalytically-active
nanomaterial,
chemical warfare agent
ABSTRACT
Recent world events have emphasized the need to develop innovative, functional
materials that will safely neutralize chemical warfare (CW) agents in situ to
protect military personnel and civilians from dermal exposure. Here, we
demonstrate the efficacy of a novel, proof-of-concept design for a Cu-containing
catalyst, chemically bonded to a single-wall carbon nanotube (SWCNT) structural
support, to effectively degrade an organophosphate simulant. SWCNTs have
high tensile strength and are flexible and light-weight, which make them a
desirable structural component for unique, fabric-like materials. This study aims
to develop a self-decontaminating, carbon nanotube-derived material that can
ultimately be incorporated into a wearable fabric or protective material to minimize
dermal exposure to organophosphate nerve agents and to prevent accidental
exposure during decontamination procedures. Carboxylated SWCNTs were
functionalized with a polymer, which contained Cu-chelating bipyridine groups,
and their catalytic activity against an organophosphate simulant was measured
over time. The catalytically active, functionalized nanomaterial was characterized
using X-ray fluorescence and Raman spectroscopy. Assuming zeroth-order reaction
kinetics, the hydrolysis rate of the organophosphate simulant, as monitored
by UV–vis absorption in the presence of the catalytically active nanomaterial,
was 63 times faster than the uncatalyzed hydrolysis rate for a sample containing
only carboxylated SWCNTs or a control sample containing no added nanotube
materials.
Nano Research 2014, 7(3): 390–398
DOI 10.1007/s12274-014-0405-3
Address correspondence to A.R. Hight Walker, [email protected]; M. Bailey, [email protected]
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391 Nano Res. 2014, 7(3): 390–398
1 Introduction
Recent world events in the Middle East have de-
monstrated the continued reality of chemical warfare
(CW) threats, specifically the threat of organophosphate-
based nerve agents, such as Sarin (GB), Tabun (GA),
and Soman (GD). During the Gulf War, military
personnel were exposed to nerve agents during the
destruction of the Khamisiyah Ammunition Storage
Facility [1]. In 1995, domestic terrorists released the
organophosphate nerve agent Sarin within a crowded
Tokyo subway, tragically killing 13 people and injuring
nearly a thousand others [2]. These incidents emphasize
the need to engineer advanced functional materials
that will safely neutralize organophosphate chemical
agents to protect military service members and civilians
from dermal exposure.
Organophosphate nerve agents act by irreversibly
binding the enzyme acetylcholine esterase (AChE)
via phosphorylation with the active site [3, 4]. This
enzyme is present in the blood and in the peripheral
and central nervous systems, and its function is to
catabolize the neurotransmitter acetylcholine (ACh).
When AChE is inhibited, excess ACh accumulates at
neuron synapses, and the victim presents symptoms
associated with exposure to organophosphate nerve
agents. Exposure to high doses can lead to symptoms
within minutes or hours. These can include miosis,
nausea, vomiting, hyperhidrosis, hypersalivation,
hyperlacrimation, fasciculation, diarrhea, anxiety,
tremor, and other symptoms indicative of cholinergic
overstimulation [4]. Death usually occurs through
severe depression of the central nervous system,
and by asphyxiation caused by sustained diaphragm
paralysis and increased bronchial secretions [5]. Severe
poisoning that does not result in death has been
reported to precipitate long-term physiological and
psychological effects, such as myocardial damage,
post-traumatic stress disorder (PTSD), and diminished
intellectual and motor capabilities [5].
Current physical protective measures deployed
against CW agents utilize gas masks and chemical-
resistant suits, boots, and gloves that protect the user
from exposure [6]. Chemical protective suits will
prevent dermal exposure to the chemical agent, but
if the CW threat is environmentally persistent (i.e.,
does not spontaneously degrade over short periods
of time), the personal protective equipment would
still require decontamination and disposal [7]. This
limitation could lead to accidental exposure, and
necessitates the development of novel materials that
will combine protective measures with in situ CW
agent degradation.
While detection of the agent is critical [8] studies
have also aimed to develop polymeric catalysts [2, 7,
9–10], metal oxide nanoparticle-containing materials
[11–13], organometallics [14], and enzyme-containing
materials [15–17] to accelerate the chemical degradation
of organophosphate nerve agents. Carbon nanotubes
have been used to develop sensors for the detection
of organophosphates [18–20], and carbon nanotube-
based materials have been designed for the degradation
of blistering agents [21]. Other types of non-carbon
nanotubes have also been explored for this purpose
[22]. However, single-wall carbon nanotubes (SWCNTs)
have unique mechanical properties and are very
light-weight, which make them a desirable structural
material for a variety of applications, including
threads and fabric-like materials with high tensile
strength [23–25]. Unlike multiwall carbon nanotubes
(MWCNT), SWCNTs offer a more homogenous sample
with protocols available for detailed, reproducible
characterization [26]. Chemical modification of carbon
nanotube threads and fabrics could lead to mul-
tifunctional materials that merge mechanical strength
with chemical functionality, such as catalytically
active materials that degrade CW agents in situ. This
study aims to develop a self-decontaminating, SWCNT-
derived material that can ultimately be incorporated
into a wearable fabric or protective material to
minimize dermal exposure to organophosphate nerve
agents and to prevent accidental exposure during
decontamination procedures.
2 Experimental
Certain commercial equipment, instruments, or materi-
als are identified in this article to specify adequately
the experimental procedure. Such identification does
not imply recommendation or endorsement by the
National Institute of Standards and Technology, nor
does it imply that the materials or equipment identified
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392 Nano Res. 2014, 7(3): 390–398
are necessarily the best available for the purpose.
Carboxylated SWCNTs were purchased from Carbon
Solutions, Inc. (Riverside, CA). Polyallylamine HCl
was purchased from Sigma-Aldrich (St. Louis, MO).
2-2’-Bipyridine-4-carboxylic acid (BCA) was purchased
from Atlantic Research Chemicals Ltd (Cornwall,
United Kingdom). Spectra/Por cellulose ester dialysis
membrane (500 D to 1,000 D molecular weight cut-off)
was purchased from Spectrum Labs (Greensboro, NC).
Cellulose ester filter discs (0.05 μm pore size) were
purchased from EMD Millipore (Billerica, MA). All
other chemicals were purchased from Sigma-Aldrich
(St. Louis, MO). All materials were used as received
unless otherwise specified.
2.1 Polyallylamine-carboxylated bipyridine copoly-
mer synthesis (Scheme 1)
The copper-chelating copolymer 2-2’-bipyridine-4-
amido polyallylamine (with 70% of the amine groups
functionalized with bipyridine) was synthesized from
polyallylamine (Scheme 1(a)) and BCA (Scheme 1(b)).
First, 2-(N-morpholino)ethanesulfonic acid (MES)
buffer was made by dissolving MES in water (2 wt.%)
and adjusting the pH to 6.5 with aqueous sodium
hydroxide and hydrochloric acid solutions. Next,
250 mg of polyallyamine were dissolved in 1 mL of
MES buffer and stirred using a magnetic stir plate.
Approximately 345 mg of BCA were dissolved by
titrating with drops of 0.1 M NaOH. This solution
was then added to the polyallylamine solution
under stirring. Next, approximately 3 g of 1-ethyl-3-
(3-dimethylaminopropyl)carbodiimide (EDC) (10 molar
excess relative to 2-2’-bipyridine-4-carboxylic acid) were
dissolved in 1 mL of MES buffer, and approximately
3.2 g of N-hydroxysuccinimide (NHS) (1.5 molar
excess relative to BCA) were dissolved in 400 μL of
dimethyl sulfoxide (DMSO). The NHS solution was
added to the polyallylamine solution, after which the
EDC solution was added. Sufficient volume of MES
buffer was then added such that the final reaction
volume was 10 mL. The reaction was allowed to proceed
overnight under stirring and at ambient conditions.
The final product was then dialyzed against deionized
water and lyophilized.
2.2 Synthesis of catalytically active, functional
nanomaterial (Scheme 1)
The copper-chelating copolymer 2-2’-bipyridine-4-
Scheme 1 Reaction scheme of copolymer synthesis, conjugation to carboxylated SWCNTs via amidation, and catalyst formation. For all amidation reactions, EDC/NHS carbodiimide chemistry was used.
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393 Nano Res. 2014, 7(3): 390–398
amino polyallylamine (Scheme 1C) was conjugated to
carboxylated SWCNTs (Scheme 1(d)) using EDC/NHS
carbodiimide chemistry. Typically, 6 mg of SWCNTs
and 60 mg of the copolymer were dissolved in 10 mL
of MES buffer. Next, approximately 14 mg of EDC were
dissolved in 2 mL of MES buffer, and approximately
3 mg of NHS were dissolved in 0.5 mL DMSO. The
NHS solution was first added to the carboxylated
SWCNTs/copolymer solution, then the EDC solution
was added under stirring. The reaction was allowed
to proceed overnight under stirring and at ambient
conditions. The final product (Scheme 1(e)) was then
dialyzed against deionized water and lyophilized.
For the Cu-containing catalyst (Scheme 1(f)),
approximately 50 mg of lyophilized SWCNT–
copolymer was first dissolved in 10 mL of deionized
water and 2 mL of 1 M aqueous CuCl2 and allowed to
stir overnight. Next, 1 mL of 1 M sodium hydroxide
solution was added and the suspension was allowed
to stir for several hours at room temperature. The
suspension was then vortexed and filtered through
a 0.05 μm cellulose ester filter and washed with
deionized water. The filtrate was then allowed to dry
overnight.
2.3 X-ray fluorescence (XRF) and Fourier transform
infra-red spectroscopy (FTIR)
XRF experiments were performed using an EDAX Eagle
III μXRF (Mahwah, NJ) containing a polychromatic
rhodium source filtered by an aluminum window.
For non-functionalized SWCNTs and carboxylated
SWCNTs, suspensions in acetone were drop cast onto
a SiO2/Si substrate and allowed to dry in a chemical
hood. Catalytically active, functional nanomaterial
was separated from the cellulose ester membrane and
placed on the SiO2/Si substrate. Spectral peaks were
assigned using NIST Desktop Spectrum Analyzer II
(DTSA-II) software [27]. The FTIR method is described
in the Electronic Supplementary Material (ESM,
Section 1).
2.4 Raman spectroscopy
Raman data were acquired on a Renishaw InVia
MicroRaman Spectrometer (Hoffman Estates, IL)
equipped with a 632.8 nm He–Ne laser, and a 514.5 nm
Ar+ laser. The measurements were calibrated against
the Si peak at 521 cm–1 prior to each measurement.
For sample preparation, carbon nanotube material
suspensions in acetone were drop cast onto a SiO2/Si
substrate. For the catalytically active, functional nano-
material, dried material was removed from the filter
and analyzed on the SiO2/Si substrate. Samples were
then illuminated using a 50× objective, which probes
approximately 2 m. Spectra were acquired and
analyzed using WiRE 33 software and Origin 8.6 at
multiple spots. A typical measurement used a 10 s
exposure time with three accumulations and a laser
power on the order of 50 μW for both the 633 nm and
514 nm band lines. The D and G band areas were
calculated by first normalizing the peak intensity to
G and then fitting the peaks with either a Gaussian
curve (D bands) or a Lorentzian curve (G bands) and
integrating. Combined standard uncertainties were
calculated using the root-sum-of-squares method.
2.5 Kinetic measurements
All kinetic studies were conducted using the chro-
mogenic organophosphate simulant, 4-nitrophenol
phosphate disodium salt, by measuring the UV–vis
absorbance of the evolved product, p-nitrophenol,
(Scheme 2) at 410 nm ± 10 nm wavelength [28].
A flask containing approximately 50 mg of hetero-
geneous catalyst on cellulose-ester membrane along
with the organophosphate simulant solution was stirred
at room temperature in parallel with an identical
apparatus containing simulant solution without any
material (“control”), or a sample containing only
carboxylated SWCNTs. For the SWCNT sample,
approximately 10 mg of carboxylated SWCNTs were
suspended in water and filtered onto a cellulose-ester
membrane. All reaction vessels were charged with
50 mL of 50 mM disodium phosphate solution in
deionized water and were sampled over time. For the
smaller-scale statistical experiments, 10 mL scintillation
vials containing 7 mL of 50 mM 4-nitrophenol phos-
phate disodium salt solution in deionized water
were charged with either 10 mg of catalytically active,
functional nanomaterial, or 2 mg of carboxylated
SWCNTs on cellulose ester-membrane, or no material
(“control”), with n = 3 for each treatment. For all
experiments, UV–vis spectra were acquired on a
Perkin Elmer Lambda 25 UV–vis spectrophotometer
(Waltham, MA) at several different time points over
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394 Nano Res. 2014, 7(3): 390–398
40 hours, and were analyzed using Origin 8.6. Area
under the curve between 400 nm and 420 nm was
calculated using the trapezoid rule. Background
was subtracted, and absorbance measurements were
normalized to the baseline (time 0) measurement.
Statistical analysis was performed using a Students’
two-tailed t-test.
3 Results and discussion
A bipyridine-containing copolymer (Scheme 1) was
prepared from polyallylamine (Scheme 1(a)) and a
carboxylated bipyridine, 2,2’-bipyridine-4-carboxylaic
acid (Scheme 1(c)) using carbodiimide amidation
chemistry. Carboxylated SWCNTs (Scheme 1(d)) were
functionalized with the bipyridine-containing polymer
(Scheme 1(c)), forming a SWCNT–copolymer com-
posite (Scheme 1(e)). With the addition of copper(II)
chloride and sodium hydroxide to the SWCNT–
copolymer composite, the bipyridine groups form a
copper chelate, which is the catalytically active
site against organophosphates (“catalytically active,
functional nanomaterial” will refer to the SWCNT–
copolymer containing the chelated copper ions) [1].
The presence of carboxyl groups on the SWCNTs,
which ease functionalization, was confirmed using
FTIR (Fig. S1 in the ESM). These data corroborate the
manufacturer’s carboxylation claim, based on their solid
state NMR results (data not shown). The presence of
copper in the catalytically active, functional nano-
material was determined using X-ray fluorescence. In
Fig. 1, the spectra show strong CuK-family peaks in the
catalyst sample (Fig. 1(a)), confirming the presence of
copper. The as-received carboxylated SWCNTs were
also analyzed (Fig. 1(b)), and the spectrum showed
NiK-family peaks, which are most likely residual material
Scheme 2 Evolution of p-nitrophenol.
Figure 1 X-ray fluorescence (XRF) spectra of carboxylated SWCNTs (a), and the catalytically active, functional nanomaterial (b).
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395 Nano Res. 2014, 7(3): 390–398
from the manufacturing process. The SiK-family peak is
from the substrate. The spectrum also showed small
AsK-family peaks, which are from a dopant in the Si
substrate. The catalytically active, functional nano-
material sample did not show any NiK-family peaks, thus
it is unlikely that the Ni would interfere with the results
of the kinetic study. The slight increase in intensity of
the background around 15 keV is due to Compton
scattering and scattering from the Rh source [29].
Raman spectra of carboxylated SWCNTs, and the
catalytically active, functional nanomaterial are shown
in Fig. 2. The spectra show radial breathing modes
(RBM) for all samples at excitation wavelengths of both
514 nm (Fig. 2(a)) and 633 nm (Fig. 2(b)), confirming the
presence of SWCNTs. The RBMs for the catalytically
active, functional nanomaterial at 514 nm excitation
are difficult to resolve, due to background photo-
luminescence of the polymer. RBM peaks are observed
with the carboxylated SWCNTs and the catalytically
active, functional nanomaterial at this excitation
wavelength. An increase in the intensity of the D peak
(reported as a decrease in the G/D ratio) is a measure
of defects, and increases at both excitation energies.
This is due to the introduction of defects in the carbon
nanotube lattice during carboxylation and further
functionalization, and corroborates the carboxylation
and functionalization of the SWCNTs. Homogeneity of
samples was accessed at three points in each sample,
which were found to be in good agreement, thus only
spectra from single measurements are reported. G/D
ratios from single measurements, with combined
standard uncertainties, are listed in Table 1.
Kinetic experiments were performed to evaluate the
catalytic activity of the catalytically active, functional
nanomaterial, based on Scheme 1. Figure 3 shows the
activity of the catalytically active, functional nano-
material against the organophosphate simulant,
4-nitrophenol phosphate disodium salt, in water.
Table 1 G/D ratios +/ combined standard uncertainty (UC)
Sample Excitation laser (nm) G/D ± UC
633 10.08 ± 0.232Carboxylated SWCNTs (as received) 514 27.20 ± 2.33
633 3.639 ± 0.157Catalytically active, functional nanomaterial 514 3.856 ± 0.130
The absorbance of the evolved hydrolysis product,
p-nitrophenol, was measured by calculating the area
under the curve from 400 nm to 420 nm, as described
previously [2]. Absorbance spectra from 400 nm to
550 nm are included in Fig. S2 (in the ESM). For
the uncatalyzed reaction (“control”), the absorbance
change of the organophosphate simulant without the
functional nanomaterial was measured over time. The
absorbance change of carboxylated SWCNTs without
the polymer/Cu functionalization, was also measured
over time. Assuming zeroth-order (linear) reaction
kinetics from 0 to 1,500 min, the relative reaction
rate of the catalyzed reaction is about 63 times faster
than the rates of both the control reaction and the
SWCNTs only reaction, both of which showed no
significant change in absorbance (Fig. 3(a)). In the
Figure 2 Raman spectra of samples using excitation at (a) 514 nm and (b) 633 nm for the as-received carboxylated SWCNTs, and the catalytically active, functional nanomaterial. G/D ratios are listed in Table 1.
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396 Nano Res. 2014, 7(3): 390–398
second kinetic study, three reactors for each treatment
(catalyzed, control, and SWCNTs only) were sampled
at three different time points. At 16 hours, 24 hours, and
40 hours, the catalyzed samples showed a statistically
significant increase (p<0.01) in absorbance verses
the control sample and the SWCNT-only sample.
Additionally, the absorbance of the catalyzed sample
at 40 hours was significantly higher (p<0.05) than
the absorbance of the catalyzed sample at 16 hours,
showing an increase in absorbance over time. In a third
kinetic study, two reactors of control and catalytically
active nanomaterial were compared for their ability
to degrade the organophosphate substrate over an
extended observational period (Fig. S3, in the ESM).
After 18 days and 3 separate rounds of experiments,
the catalytic material maintained its function without
any discernible loss in catalysis of the substrate.
The proposed reaction mechanism is described in
Scheme S1 (in the ESM).
4 Conclusions
This study demonstrates the efficacy of a novel,
proof-of-concept design for a Cu-containing catalyst,
chemically bonded to a SWCNT support that shows
catalytic activity statistically better than carboxylated
SWCNTs and an uncatalyzed hydrolysis reaction
(“control”). The carbon nanotube matrix provides
structural support for the catalytic polymer, potentially
making it suitable for a variety of applications. We
also demonstrate that this novel material maintains
its catalytic function after repeated use for up to
several weeks of constant catalysis. Future work will
examine the possibility of functionalizing carbon
nanotube yarn with the copper-containing polymer
and weaving it into fabric for in situ chemical defense
applications.
Acknowledgements
The authors gratefully acknowledge funding from the
United States National Research Council Post-Doctoral
Research Associateship Program.
Electronic Supplementary Material: Supplementary
material, including FTIR data, proposed hydrolysis
reaction mechanism, and UV–vis absorption data,
is available in the online version of this article at
http://dx.doi.org/10.1007/s12274-014-0405-3.
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Figure 3 Kinetic measurements of the CW agent simulant hydrolysis. (a) The relative absorbance change of the hydrolysis product overtime in the presence of the catalytically active, functional nanomaterial (catalyzed), with carboxylated SWCNTs, and without anymaterial (control). (b) A comparison of the relative absorbance change at three different time points with n = 3 for each treatment.Uncertainty bars indicate standard deviation, and an * indicates a statistically significant difference with p<0.01 and ** indicates p<0.05.
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