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Metal Iodate-Based Energetic Composites and Their Combustion and
Biocidal Performance
H. Wang,
†
G. Jian,
†
W. Zhou,
†
J. B. DeLisio,
†
V. T. Lee,
‡,§
and M. R. Zachariah*
,†
†
Department of Chemical and Biomolecular Engineering and Department of Chemistry and Biochemistry, University of Maryland,
College Park, Maryland 20742, United States
‡
Department of Cell Biology and Molecular Genetics, University of Maryland, College Park 20740, Maryland, United States
*
SSupporting Information
ABSTRACT: The biological agents that can be weaponized, such as Bacillus anthracis,
pose a considerable potential public threat. Bacterial spores, in particular, are highly
stress resistant and cannot be completely neutralized by common bactericides. This
paper reports on synthesis of metal iodate-based aluminized electrospray-assembled
nanocomposites which neutralize spores through a combined thermal and chemical
mechanism. Here metal iodates (Bi(IO3)3, Cu(IO3)2, and Fe(IO3)3) act as a strong
oxidizer to nanoaluminum to yield a very exothermic and violent reaction, and
simultaneously generate iodine as a long-lived bactericide. These microparticle-
assembled nanocomposites when characterized in terms of reaction times and temporal
pressure release show significantly improved reactivity. Furthermore, sporicidal
performance superior to conventional metal-oxide-based thermites clearly shows the
advantages of combining both a thermal and biocidal mechanism in spore
neutralization.
KEYWORDS: energetic materials, metal iodate, nanothermite, biocidal, electrospray
1. INTRODUCTION
Thermites is a class of energetic materials that can undergo fast
redox reaction between a fuel (e.g., Al) and an oxidizer (CuO,
Fe2O3,Bi
2O3, etc.), which ,once initiated, release large amounts
of thermal energy.
+→ ++ΔH
A
l3
2MO 1
2Al O 3
2M
23
(1)
Decreasing the reactant length scales from the micron to the
nanoscale greatly increases the interfacial contact and reduces
the diffusion distance between fuel and oxidizer, resulting in as
much as ∼1000×higher reactivity.
1−6
The thermochemical
properties of the materials used result in energy densities, for
the most common mixtures, that are a factor of 2 or more
higher on a volumetric basis than conventional organic-based
energetic materials (e.g., TNT or RDX).
7−22
The potential threats from biological-based weapons, such as
those employing Bacillus anthracis, pose a significant challenge
to global security. Of particular concern are spores of virulent
bacteria that are highly stress resistant and cannot be
completely killed by high-pressure processing (HPP), heat, or
toxic chemicals such as iodophor.
23−25
Conventional energetic
materials produce a thermal event over a relatively short time,
which may not be sufficient for total inactivation. Thus, a
strategy has developed in which, in addition to the thermal
event, a remnant biocidal agent delivered simultaneously would
have a much longer exposure time resulting in a more effective
inactivation. As such, both silver- or halogen-containing
thermites, even difluoroiodate compound-based explosives,
have been considered as biological agent defeating ingre-
dients.
26−29
The two most common methods of incorporating
biocidal agents into energetic systems are to either directly add
silver or halogen into the metallized energetic materials or to
employ silver- or iodine-containing oxidizers into the thermite
formulation.
30−32
The latter option has the potential for Ag and
I to act as part of the energy release landscape rather than as a
passive species. Some extremely strong oxidizers of the latter
kind, such as NaIO4/KIO4, do not release iodine, while others
like I2O5and AgIO3have storage issues because of their
hygroscopicity and light sensitivity.
31,32
The ideal oxidizer
should (1) be easy to handle and (2) decompose to allow the
oxygen to react with the fuel and release molecular iodine.
+→+++Δ
HA
l1
2M(IO ) 1
2Al O 1
2M1
4I
323 2 (2)
In this paper, three metal iodate nanoparticles, Bi(IO3)3,
Cu(IO3)2, and Fe(IO3)3, were synthesized and subsequently
assembled with aluminum nanoparticles using an electrospray
technique to create microsized nanothermites. The confined
burning pressure of the metal iodate-based thermite was found
to be ∼5×higher than conventional Al/CuO nanothermite.
The rapid reaction mechanism and the iodine release from the
metal-iodate-based thermite was investigated using rapid
heating mass spectrometry. The effectiveness of synthesized
Received: May 26, 2015
Accepted: July 10, 2015
Research Article
www.acsami.org
© XXXX American Chemical Society ADOI: 10.1021/acsami.5b04589
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
iodate-based thermites as a biocidal agent was tested, and
results show superior performance in the inactivation of spores.
2. EXPERIMENTAL SECTION
Chemicals. Copper oxide (∼50 nm), bismuth oxide (90−210 nm),
and iron oxide (∼50 nm) were purchased from Sigma-Aldrich, and
aluminum (ALEX, ∼50 nm) was purchased from Argonide Corp. Al
nanoparticles contain ∼70% active aluminum (by weight), which is
confirmed by thermogravity analysis.
33
Collodion solution (4−8%
nitrocellulose, i.e., NC, in ethanol/diethyl ether, by weight) was
purchased from Fluka Corp., and diethyl ether (99.8%)/ethanol
(99.8%) mixture (volume ratio: 1:3) was employed to dissolve the
collodion. The copper iodate, bismuth iodate, and iron iodate
nanoparticles were synthesized by the following procedures. The
formation energies of these oxidizers were calculated on the basis of
the data at https://materialsproject.org/. The formation energies of
Bi(IO3)3, Cu(IO3)2, and Fe(IO3)3are calculated as −1282, −725, and
−1293 kJ/mol, respectively.
Synthesis of the Copper Iodate Nanoparticles. Copper iodate
nanoparticles were synthesized by milling copper(II) nitrate trihydrate
(Sigma-Aldrich) and potassium iodate (Sigma-Aldrich) mixture (mass
ratio: 1:3). After the milling process, the sample was washed with 30
mL of deionized water, and centrifuged for 30 min at 13 500 rpm
(Hermle Z300). The whole process was repeated 4 times to enable full
removal of any impurities. The sample was then dried at 100 °C
overnight. The yield of copper iodate nanoparticles using this milling
process was ∼75%.
Preparation of the Bismuth Iodate and Iron Iodate
Nanoparticles. Bismuth iodate (Bi(IO3)3) and iron iodate (Fe-
(IO3)3) nanoparticles were synthesized by a precipitation method.
Bi(IO3)3nanoparticles were synthesized by the following process: 485
mg bismuth nitrate pentahydrate (Bi(NO3)3·5H2O) was dissolved in 8
mL nitric acid solution (2 M), and 528 mg iodic acid (HIO3) was
dissolved in 8 mL deionized (DI) water. These were then mixed by
dropwise addition of Bi(NO3)3solution. The yield of bismuth iodate
nanoparticles using this milling process was ∼90%. Fe(IO3)3
nanoparticles were synthesized by mixing the Fe(NO3)3solution
and HIO3solution (1:3, by mole) at concentrations of 25 and 66 mg/
mL, respectively. The obtained brown solution was kept at 100 °C
overnight, and the subsequent yellow-green precipitate was collected
and further washed and dried for characterization, with a yield of
∼75%. Both of the Bi(IO3)3and Fe(IO3)3powders were handled with
the same washing, centrifuging, drying, and breaking process as the
Cu(IO3)2.
Preparation of the Physically Mixed Metal Iodate and Metal-
Oxide-Based Composites. The traditional approach to create a
nanocomposite thermite is by physical mixing. For example, to make
the bismuth-iodate-based thermite, 160 mg Bi(IO3)3powder was
dispersed in 1.8 mL of ethanol and sonicated for 60 min. Then, 51 mg
of aluminum nanoparticles (and 0.20 mL collodion for the case of
added NC, from Fluka Corp.) is added and sonicated for another 60
min. The suspension was stirred for 24 h, and then allowed to air-dry
in a hood. The dry powder was then gently broken to a fine powder. A
similar procedure was employed for the other iodates, but the mass
was adjusted to maintain a stoichiometry.
Electrospray Procedure. The electrospray assembly process is
essentially as previously described in ref 22. For example to make the
bismuth-iodate-based thermite 160 mg Bi(IO3)3powder was dispersed
in 1.8 mL of ethanol and sonicated for 60 min. To this was added 51
mg of aluminum nanoparticles (and 0.20 mL collodion for the case
with NC), and the mixture was sonicated for another 60 min. Then,
after stirring for 24 h, the suspension was ready to be electrosprayed.
The electrospray system consisted of a syringe pump to feed the
precursor at a constant speed of 4.5 mL/h, through a stainless steel
0.43 mm inner diameter needle. The distance between the needle and
substrate was 10 cm, with an applied voltage of 19 kV.
SEM/EDS, TEM, and TGA/DSC. Scanning electron microscope
(SEM) characterization was conducted with a Hitachi SU-70
instrument coupled to an energy dispersive spectrometer (EDX).
Transmission electron microscope (TEM) analysis employed a JEOL
2100F field-emission instrument. Thermogravimetry/differential scan-
ning calorimetry (TG/DSC) results were obtained with a TA
Instruments Q600 at a rate of 10 °C/min up to a maximum
temperature of 1000 °C in a nitrogen atmosphere.
Combustion Cell and Burning Products. The details of
combustion cell experiment can be found in refs 5and 34. A confined
combustion cell with a constant volume (∼13 cm3) was used to
measure the pressure and burn time of the samples. In this study, 25.0
mg of the loose thermite sample was placed inside a cell and was
Figure 1. (a) Schematic of electrospray approach. (b−d) Low- and high-magnification SEM images of (b) Al/Bi(IO3)3composites, (c) Al/Cu(IO3)2
composites, and (d) Al/Fe(IO3)3composites. Note: all the above composites contain 5% NC (by weight).
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.5b04589
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B
ignited by a nichrome coil on top of the loose powder. An attached
piezoelectric pressure sensor (PCB) together with an in-line charge
amplifier and signal conditioner were used to record the pressure
history. The optical emission history was simultaneously collected by a
lens tube assembly, containing a planoconvex lens ( f= 50 mm) and a
photodetector to collect the broadband emission. The burn time was
defined as the peak width at half height of the optical emission.
T-Jump Ignition and Time-Resolved Mass Spectrometry
Measurement. The details of the time-of-flight mass spec system
used can be found in refs 5and 33−35. In a typical experiment, a fine
Pt wire (76 μm in diameter, 10 mm in length) was coated with a thin
layer of sample (3−5 mm in length). An applied voltage of ∼24 V was
put across the wire to rapidly heat to ∼1800 K at a heating rate of 5 ×
105Ks
−1. The wire temperature can be calculated by determining the
resistance changing with time. Time-resolved mass spectra were used
for characterization of the species produced during the rapid heating. A
Vision Research Phantom v12.0 digital camera was employed to
capture the burning of the thermite in air. The resolution used was 256
×256 pixels, and the frame rate used was 67 065 fps (14.9 μs per
frame).
Spore Inactivation. The above combustion cell was employed as a
container to evaluate the spore inactivation effectiveness of the
samples. First, metal-iodate-based thermite was placed in the cell of the
internal chamber in the combustion cell (Figure 2a). Aluminum foil
was adopted as the substrate for Bacillus thuringiensis spores, a known
surrogate for Bacillus anthracis.A10μL portion of spore solution (∼7
×105/μL, ∼1.4 μg) was dripped onto the foil surface followed by
spreading and drying. Aluminum foils deposited with or without
spores were placed close to the wall of the chamber, ∼15 mm from
thermite sample. After ignition, the combustion cell was kept closed
for 30 min, following which the foils were extracted and sent for
bacterial spore counting assay. Before the test, explosion-product-
loaded control foils were deposited with the same amount of spores.
Each foil was then immersed in 1 mL of fresh Lysogeny Broth (LB)
medium and incubated at 37 °C for 3 h. Our preliminary test reveals
that this time period is necessary for the initially immobile living
spores to detach from the foil and germinate in the media, enabling us
to measure the spore counts quantitatively later. Since the exact range
of spore viability was unknown, a serial dilution was used. A 100 μL
portion of the medium was extracted and diluted subsequently, with
each dilution of a factor of 10. Then 10 μL of these diluted media with
different bacteria concentrations were spread in parallel lines on an
agar plate. Agar plates were kept overnight at room temperature prior
to numeration of the colony forming units (CFUs) in each line. The
final counts of spores in each sample were obtained statistically in
terms of these serial CFUs. Each experiment was repeated in triplicate.
3. RESULTS AND DISCUSSION
3.1. Metal-Iodate-Based Thermite Synthesis and
Assembly. In this work, micron-scale metal-iodate-based
energetic composites were produced by electrospraying a
precursor solution containing nanoscale components, compris-
ing fuel (nano-Aluminum), the biocidal oxidizer (metal
iodates), and a small quantity of energetic binder nitrocellulose.
As shown in Figure 1a, because of the combined action of
electrostatic forces and surface tension, the precursor
suspension shatters into nearly monodiserse microsized
droplets. Subsequent rapid evaporation of the solvent drives
gelling process within the droplet until the aggregate particles
jam, creating a porous structure. More details of the
electrospray assembly technique can be found in our prior
work.
22,36
The metal iodate nanoparticles used in this study include
bismuth iodate (∼90 nm), iron iodate (∼70 nm), and copper
iodate (∼65 nm), which were prepared by either precipitation
or milling (see Experimental Section and Supporting
Information). For comparison purposes, physically mixed
metal-iodate-based nanothermites were also prepared, and the
SEM images can be found in Figure S9 in the Supporting
Information.
Figure 1b−d shows typical SEM images of the three metal-
iodate-based thermites. The obtained Al/Bi(IO3)3(Figure 1b)
composites have a size distribution of 3−5μm, have a porous
structure, and contain a well-dispersed mixture of the fuel and
oxidizer nanoparticles. Al/Cu(IO3)2composites have similar
structure features and a relatively narrow size of 2−4μm
(Figure 1c), respectively, while the Al/Fe(IO3)3composites
Figure 2. (a) Schematic of combustion cell. (b) Pressure and optical emission profiles of the Al/Bi(IO3)3nanothermite composites. (c) Pressure
profiles of the three metal-iodate-based thermites. (d) Burning times of metal-iodate-based nanothermite composites. Note: The pressure traces of
Cu(IO3)2- and Fe(IO3)3-based thermites have been offset to the right, for better readability.
ACS Applied Materials & Interfaces Research Article
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C
(Figure 1d) were found to have a more open structure, with a
larger size ranging from 5 to 7 μm.
3.2. Reactivity of Metal-Iodate-Based Nanothermite.
The reactivity of nanothermite was assessed in a constant
volume combustion cell (13 cm3), from which the pressure
history and optical emission during combustion can be
obtained simultaneously. Figure 2a shows the schematic of
the combustion cell, which is composed of a pressure sensor
and an optical detector. Pressurization rates obtained from the
pressure history data, which mimic the burning rate, are used to
evaluate the reactivity. Burning times were also obtained by
measuring the peak width at half-maximum (fwhm) of the
optical emission trace.
5,37
Supporting Information Table S1
summarizes the combustion cell results for all the nano-
thermites in this study.
Figure 2b shows the pressure and optical-emission trace for
Al/Bi(IO3)3composites. The peak pressure and burning time
are ∼4.7 MPa and ∼170 μs, respectively. The burning time is
considerably longer than the corresponding pressure rise time,
implying that decomposition of iodates occurs prior to the
primary reaction of the thermite, suggesting that the thermite
burning rate is limited by the aluminum fuel release, rather than
oxidizer availability. The temporal pressure of the three metal-
iodate-based nanothermites are shown in Figure 2c. All three
pressure traces show a rapid rise, of ∼1μs, achieving a peak
pressure of 4−5 MPa with the Cu > Bi > Fe.
For comparison, the reactivity of metal iodates thermite
prepared by conventional physical mixing method was also
evaluated. The pressurization rate results of metal-iodate-based
thermites produced by two approaches were listed in Table 1.
The results show that the pressurization rate of electrosprayed
Al/Bi(IO3)3composites is approximately 2 orders of magnitude
higher than the physical mixed thermites, while the Al/
Cu(IO3)2and Al/Fe(IO3)3systems are more than 4 orders of
magnitude higher, relative to the physically mixed thermites.
The increase in reactivity of the electrosprayed sample can be
attributed to the unique porous inner structure combined with
energetic gas generator nitrocellulose which we have previously
shown minimizes sintering among nanoparticles.
22
Similar to what we have observed for Al/CuO composites,
22
the physically mixed Bi(IO3)3,Cu(IO
3)2,andFe(IO
3)3
nanothermites have a much longer burning time than that of
the corresponding electrosprayed nanothermites, as Figure 2d
shows.
The reactivity difference between metal-iodate-based and
metal oxide-based thermites was compared. The peak pressure
and pressurization rate of the two thermites were simulta-
neously obtained, as listed in Table 2. The reactivity of the
metal-iodate-based thermite is much higher than the reactivity
of the corresponding metal-oxide-based thermite even though
the size of metal iodate nanoparticles is larger. Specifically, the
so-called weak thermite, Al/Fe2O3, has a pressure and
pressurization rate of only 0.06 MPa and 0.02 GPa/s,
respectively. While the corresponding iron-based iodate, Al/
Fe(IO3)3, achieves 1.9 MPa and 590 GPa/s, as shown in Figure
3a. As discussed in our previous work,
38
Al/Fe2O3nano-
thermite combustion pressure and optical emission signals
appear concurrently (Figure 3b), and it has been suggested that
oxidizer decomposition is the rate limited step. In contrast, Al/
Fe(IO3)3features a very rapid pressure increase followed by a
prolonged optical emission profile, indicating the burning is
rate limited by the aluminum fuel. These results show the
advantages of using strong oxidizers, with rapid oxygen release
kinetics in nanothermite formulations.
The adiabatic flame temperatures (AFT) of the three metal-
iodate- and metal-oxide-based thermites have been calculated
and are listed in Table 2. Since there are no experimental data
for thermochemical properties of these metal iodates, the
theoretical estimate results were adopted.
39
These results are
then employed in the Cheetah code, assuming the sample is in
the maximum density and the volume of the burning product is
constant.
40,41
Generally, the flame temperatures of the three
metal-iodate-based thermite reactions are roughly the same,
∼4050 K, which is >1000 K higher than that of the
corresponding metal-oxide-based thermites. The high reaction
temperature can largely improve the potential biocidal
performance for its additional thermal activities which can
synergistically function with the released iodine.
To investigate the ignition mechanism, the nanothermite
samples were coated on a Pt wire with a diameter of 76 μm and
joule heated in air at a heating rate of 5 ×105Ks
−1,as
schematically shown in Figure 4a. The Al/Bi(IO3)3composites
reveal a violent reaction, with an ignition temperature of 590
°C. The ignition temperatures of Cu(IO3)2-based and Fe-
(IO3)3-based nanothermites is 560 and 550 °C, respectively,
Table 1. Peak Pressure, Pressurization Rate, and Burning
Time of Metal-Iodate-Based Nanothermites Made by
Electrospraying (ES) and Physical Mixing (PM)
a
thermite pressure
(MPa) pressurization
(GPa/s) burning
time (μs)
ES Al/Bi(IO3)34.5 3816 235
PM Al/Bi(IO3)30.73 53 298
ES Al/Cu(IO3)24.9 3966 238
PM Al/Cu(IO3)20.14 0.07 2162
ES Al/Fe(IO3)34.0 3186 161
PM Al/Fe(IO3)30.17 0.10 3667
a
All the composites contain 5% NC (by weight). 25.0 mg sample in
each test.
Table 2. Pressure Cell Results of Physically Mixed Nanothermites
a
oxidizer pressure (MPa) pressurization (GPa/s) burning time (μs) AFT (K) stoichiometric ratio
Bi2O3,90−210 nm 1.0 54 240 2333 1.3
Bi(IO3)3,∼90 nm 2.3 770 150 4062 1.0
CuO, ∼50 nm 1.1 100 220 3054 1.0
Cu(IO3)2,∼200 nm 1.8 225 170 4061 1.0
Fe2O3,∼50 nm 0.06 0.02 3330 3027 1.0
Fe(IO3)3,∼70 nm 1.9 590 170 4043 1.0
a
Sample mass: 25.0 mg. All the composites contain no NC. The AFT of Al/Bi2O3thermite is only 3031 K. (Because this was a fuel lean mixture,
stoichiometric ratio = 1, to optimize pressurization rate.)
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which is much lower than that of corresponding CuO-based
(770 °C) and Fe2O3-based nanothermites (1140 °C). The low
ignition temperature is indicative that the metal iodate salts
release oxygen at a relatively low temperature.
5
To further
confirm this, the wire within the extraction region of a mass
spectrometer, i.e., T-jump/time-of-flight mass spectrum (T-
Jump/TOFMS), was employed to investigate the oxygen
release properties of the three metal iodates.
37
Reactivity of a thermite can be evaluated by the explosion
characterization parameters such as peak pressure, pressuriza-
tion rate, and burn time in a confined container (Figure 2a).
Especially, the pressurization rate is directly proportional to the
burn rate of the thermite. In the previous work, it is found that
the reactivity is closely related to its oxygen release behavior at
high heating rates.
42
Figure 4b shows the temporal profile for
oxygen from metal iodates in T-Jump/TOFMS, from which the
oxygen release temperature can be obtained, and the oxygen
release behavior of different oxidizers at high heating rates can
be compared. It is found that the onset temperature of oxygen
release for Bi(IO3)3, Cu(IO3)2, and Fe(IO3)3nanoparticles is
490, 450, and 475 °C, respectively, and thus insensitive to
structure. Interestingly, it is worth noting that while Bi(IO3)3
releases oxygen at 490 °C, and Bi2O3releases gas phase oxygen
at ∼1350 °C, they both show a similar ignition temperature of
∼590 °C.
43
The results imply that Bi(IO3)3and Bi2O3have
different initiation mechanisms. For Bi(IO3)3, it is obvious that
oxygen release from oxidizer contributes to the ignition of
nanothermite, while its condensed phase appears to be the
initiation mechanism for the Al/Bi2O3nanothermite.
As Figure 4b shows, all of the oxygen release traces show a
rapid rise. The oxygen release from Cu(IO3)2initiates at 450
°C, and reaches it maximum in 0.20 ms, significantly
outperforming the commercial CuO nanoparticles, whose rise
time is 1.0 ms,
33
revealing its fast oxygen release kinetics at high
heating rates. Oxygen release profiles of Fe(IO3)3and Bi(IO3)3
nanoparticles are very similar to that of Cu(IO3)3, implying, not
surprisingly, that all metal iodate nanoparticles release oxygen
in a similar manner. It is worth noting that the decomposition
of iodates occurs prior to the thermite ignition, suggesting that
gas phase oxygen release is critical to ignition.
The metal-iodate-based nanothermites were also rapidly
heated in the T-Jump/TOFMS to determine temporal
Figure 3. Pressure and optical emission profiles of (a) Al/Fe(IO3)3and (b) Al/Fe2O3nanothermite reactions.
Figure 4. (a) T-Jump wire ignition method, and burning snapshots of Al/Bi(IO3)3composites. (b) Temporal profile of oxygen release upon heating
the three different metal iodates. (c) T-Jump TOFMS results for the Bi(IO3)3-based, Cu(IO3)2-based, and Fe(IO3)3-based thermite made by
electrospray (5% NC, by weight). Note: The heating pulse time was 3.0 ms.
ACS Applied Materials & Interfaces Research Article
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speciation. As shown in Figure 4c, iodine and oxygen species
are detected in all three metal-iodate-based thermites reactions.
The intensities of oxygen and iodine species, in all of the
samples, reach their maximum values between 500 and 600 °C,
revealing the major weight loss event happening during that
period, and is consistent with TGA results (Supporting
Information).
3.3. Sporicidal Performance. The ability of metal-iodate-
based thermites to produce high heat, pressure, and iodine
makes them ideal biocidal energetic materials with potential
synergistic killing mechanisms. To test the sporicidal perform-
ance of metal iodate-based thermites, a spore inactivation
experiment was conducted in the combustion cell (V=13cm
3,
see Figure 2a). Aluminum foil coupons deposited with or
without ∼7×106CFU Bacillus thuringiensis spores were placed
within the combustion chamber around the wire coil that
ignites the thermite reaction. The control experiment of Al/
Bi(IO3)3thermite reaction without spores shows that iodine
was released after reaction and uniformly dispersed within the
layer of postcombustion products (Figure 5a,b). By loading
different quantities of Al/Bi(IO3)3thermite in the combustion
cell, the survival ratios of Bacillus thurigiensis spores on coupons
were measured after the thermite reactions (Figure 5c). Spores
exposed to 5.0 mg and 25.0 mg of thermite reaction products
had a 2 log or a >4 log reduction in their viability (i.e., the
ability to form bacterial colonies), respectively. Alternatively,
similar tests by employing the reaction of 5.0 mg Al/Bi2O3
thermites show that the viability of spores decreased by 28%.
The major difference of these two thermite systems is that
iodate-based composites can generate free iodine through a
thermite reaction, which has been widely confirmed as a
biocidal agent. Therefore, except for the pressure and thermal
stresses which were also generated from the combustion of Al/
Bi2O3thermites, the combustion of Al/Bi(IO3)3thermites
exposes spores to the additional biocidal stress of iodine. We
estimated that the released iodine efficiently contributed to 27%
loss of spore viability (28% −1%), while the other effects from
pressure, heat, and other reaction products such as Al2O3and
Bi contributed to the majority of the loss as confirmed by the
result after the Al/Bi2O3reaction (Figure 5c). Figure 5c shows
that the postreaction products of Al2O3and Bi induced viability
reduction of spores by 30%, implying that the other heat and
pressure stresses contributed to a 42% loss of spore viability
(72% −30%). Given that other reactions of metal iodate
thermites (Al/Bi(IO3)3and Al/Cu(IO3)2) also led to uniform
iodine release (Supporting Information Figures S13 and S14)
and demonstrate higher sporicidal capabilities compared with
the corresponding oxide-based thermite reactions (Supporting
Information Figure S15), it is proposed that the metal iodate
thermite system can be recognized as an effective biocidal
agent.
The SEM images of spores (Figure 5d) further demonstrate
that fine particles were deposited on the spore surfaces after the
5.0 mg Al/Bi(IO3)3thermite reaction. There is some precedent
for cell death in the absence of morphological damage.
43
However, interestingly, there is no gross morphological change
in the spores after the thermite reaction, suggesting that the
thermite reaction has not compromised the spore structure,
although the viability has been reduced by 2 orders of
magnitude (Figure 5c). Previously, we have shown that, during
rapid heating, spores maintained most of their viability until the
spore coat was melted.
44
Herein, it is further found that, by
applying iodine, the spores were killed before changing
morphologies, indicating the superiority in efficacy of the
Figure 5. (a, b) SEM and energy dispersive X-ray spectroscopy (EDS) results of Al/Bi(IO3)3thermite reaction products. (c) CFU counts from the
spores treated by 5.0 mg Al/Bi(IO3)3postcombustion products, 5.0 mg Al/Bi2O3thermite reaction, and 5.0 mg and 25.0 mg Al/Bi(IO3)3thermite
reactions. (d) SEM images of spores before (top) and after (bottom) undergoing the Al/Bi(IO3)3thermite reaction (5.0 mg). Note: all the
composites are produced by electrospraying with 5% NC (by weight). The labeled numbers in part c are survival ratios of spores.
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.5b04589
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
F
Bi(IO3)3-based thermite complex in killing spores compared
with simple rapid heating.
4. CONCLUSION
This work reports the formation of metal-iodate-based
thermites which have superior performance as sporicidal
agents. Three different metal iodate nanoparticles were
synthesized and incorporated into energetic composites with
nanoaluminum by electrospray or physical mixing, forming
highly reactive nanothermites. We characterized the reactivity
using combustion cell and fast-heating wire experiments. The
pressure and pressurization rate of metal-iodate-based thermites
are several times higher than the corresponding oxide-based
thermites, and the reactivity could be further promoted by
using the electrospray technique to assemble the metal-iodate-
based thermites. The thermal decomposition properties of
metal iodates, as well as the reaction mechanisms of the related
thermites, were separately investigated by TGA/DSC, and
TOF-MS. The results show that the metal iodates decompose
into their corresponding metal oxide, oxygen, and iodine before
the aluminothermic reaction takes place. The sporicidal
performance of the metal-iodate-based thermites was also
assessed, and the results showed that they outperform the
corresponding metal-oxide-based thermites. The working
mechanism was proposed as a synergistic effect of heat/
pressure and iodine production from the highly reactive metal-
iodate-based thermite reaction. This combination has a much
higher sporicidal rate than the individual effect of either heat/
pressure (from the metal-oxide-based thermite reaction) or
iodine (from the burning residue of metal-iodate-based
thermite reaction).
■ASSOCIATED CONTENT
*
SSupporting Information
Characterization of the three different metal iodates nano-
particles was shown, including SEM and TEM images, as well
as thermal decomposition results. The SEM images of
electrosprayed metal-oxide-based thermites and physically
mixed metal-iodate-based thermites. The combustion cell
results of all three kinds of metal-iodate-based thermites and
corresponding metal-oxide-based thermites. The burning snap-
shots of Al/Cu(IO3)2and Al/Fe(IO3)3composites on the wire.
The SEM images and EDS results of the postcombustion
products from Al/Cu(IO3)2and Al/Fe(IO3)3composite
thermite reaction. The Supporting Information is available
free of charge on the ACS Publications website at DOI:
10.1021/acsami.5b04589.
■AUTHOR INFORMATION
Corresponding Author
*Phone: 301-405-4311. E-mail: mrz@umd.edu.
Present Address
§
Department of Safety Engineering, Nanjing University of
Science and Technology, Nanjing, Jiangsu 210094, China.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
This work was supported by the Defense Threat Reduction
Agency and the Army Research Office. We acknowledge the
support of the Maryland Nanocenter and its NispLab. The
NispLab is supported in part by the NSF as a MRSEC Shared
Experimental Facility. H.-Y.W. is grateful for the financial
support from China Scholarship Council.
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