A hybrid electrochemical-colorimetric sensing platform for detection of explosives

Full text of DOI:10.1021/ja809104h

Published on Web 01/14/2009
A Hybrid Electrochemical-Colorimetric Sensing Platform for Detection of
Explosives
Erica S. Forzani,† Donglai Lu,† Matthew J. Leright,† Alvaro Diaz Aguilar,† Francis Tsow,†
Rodrigo A. Iglesias,† Qian Zhang,‡ Jin Lu,‡ Jinghong Li,*,‡ and Nongjian Tao*,†
Center for Bioelectronics & Biosensors, The Biodesign Institute, Department of Electrical Engineering, Arizona
State UniVersity, Tempe, Arizona 85287, and Department of Chemistry, Tsinghua UniVersity, Beijing 100084, China
Received November 20, 2008; E-mail: jhli@mail.tsinghua.edu.cn; njtao@asu.edu
Detection of trace explosives is a leading priority in security. In
spite of significant advances made recently,1–3 a portable device
that can be reliably deployed in a field environment still faces
formidable challenges. These challenges include low levels of
explosives in most practical scenarios, multiple interferences from
common household and personal care products, variable environ-
mental conditions (e.g., temperature, humidity, and pollutants), and
the need for low-cost and highly portable detection methods. Here,
we present a novel hybrid electrochemical-colorimetric (EC-C)
sensing platform to meet these challenges. The hybrid sensor is
based on consorted operations of electrochemical reactions of trace
explosives, colorimetric detection of the reaction products, and
unique properties of the explosives in an ionic liquid (IL) (Figure
1). Unlike previous electrochemical detection methods, the elec-
trochemical reactions in the present work are mainly used to
generate reaction products (Figure 1A) which are detected with an
optical imaging device (Figure 1B). This approach affords not only
Figure 1. Hybrid electrochemical-colorimetric sensor with a thin layer
increased sensitivity but also selectivity as evident from the
demonstrated null rate of false positives and low detection limits
(Supporting Information, SI).
of ionic liquid as a selective preconcentration medium. (A) Cyclic
voltammograms of blank IL, BMIM-PF6 (black line) and 2 ppm TNT in
IL (red line) at 100 mV/s. Arrows indicate peak currents of TNT. (B) Color
(absorbance) changes during the electrochemical reduction of TNT in
BMIM-PF6. The absorbance change for each color is defined as the
logarithmic ratio of the intensity in a sensing area (working electrode) to
the intensity in a reference area (reference electrode). Insets in part B show
two images taken before (0.0 V) and after (-1.5 V) TNT reduction. The
distinct color change provides a fingerprint for identification and quantifica-
tion of the explosive.
A basic version of the device is constituted of merely a
microcontroller-based potentiostat, a typical webcam, a transparent
substrate of indium tin oxide (ITO) printed electrodes, and a thin
layer of ionic liquid (IL) covering it (Figure 1). Excellent selectivity
is achieved by (1) controlling the electrode potential to induce the
electrochemical reactions of the target analytes, (2) specific
interactions between the reaction products and the supporting
electrolyte (IL), and (3) distinct optical absorption patterns. In
addition, the device hardware can be rather simple and compact,
allowing the benefits of low cost and high portability.
One of the key elements of the hybrid sensing platform is the
use of a thin layer of 1-butyl-3-methylimidazolium hexafluoro-
phosphate (BMIM-PF6),4–6 an IL with extraordinary properties (see
SI for synthesis details). The BMIM-PF6 coating provides (1) a
medium that selectively preconcentrates explosives and quickly
transports the analytes to the electrodes, (2) an electrolyte that is
highly stable under ambient conditions for electrochemical reactions,
and (3) a medium that promotes the formation of colored reduction
products. We applied this hybrid sensing platform to detect
nitroaromatic explosives and explosive signatures, including 2,4,6-
trinitrotoluene (TNT), picric acid (PA), and 2,4-dinitrotoluene
(DNT). This approach should also be applicable to other
nitroexplosives.
Figure 2. (A) Optical properties of the electrochemical reduction products
of TNT in BMIM-PF6, where the differential absorbance is the spectrum
of the reduction products after subtraction of the spectrum before reduction.
Inset: TNT spectrum in BMIM-PF6 before the reduction (background due
to BMIM-PF6 absorption was corrected). (B) Comparison of preconcen-
tration capacities of various analytes in BMIM-PF6. Inset: Absorption/
desorption kinetics of TNT vapor determined with a BMIM-PF6-coated
tuning fork sensor at room temperature (SI).
Figure 1A is a cyclic voltammogram of TNT in BMIM-PF6,
which shows three irreversible reduction peaks as marked by arrows.
We found that reduction processes produce distinctive red colored
products (Figure 1B) with an absorption peak at ∼450 nm (Figure
† Arizona State University.
‡ Tsinghua University.
2A) and a large extinction coefficient of 6060 M-1 cm-1. The
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1390 J. AM. CHEM. SOC. 2009, 131, 1390–1391
10.1021/ja809104h CCC: $40.75
2009 American Chemical Society

C O M M U N I C A T I O N S
reduction products were identified to be azo and azoxy derivatives,
using Fourier transform infrared spectroscopy (FTIR) and liquid
chromatography/mass spectrometry (LC/MS) (SI). This identifica-
tion is also supported by work on nitrosobenzene in acetonitrile.7,8
No visible color changes have been found in the electrochemical
reduction of TNT in aqueous solution, so the IL medium is essential
to provide a stable environment for the reaction products with a
distinct color for TNT. Electrochemical reaction products with
distinctive colors can also be produced in the IL for other
nitroaromatic explosives, such as PA and DNT (SI).
Figure 3. (A) RGB color changes due to reduction products of nitroex-
plosives: TNT, DNT, and PA and realistic concentrations of interferents:
RH ) RH change of 21%, Perf ) mist (Coty, US), MW ) mouth wash
(Listerine), HC ) hydrocarbons, hexane/toluene mixture, 42 ppmV e.a.,
CN: general cleaner (3M). The RGB signal changes were recorded after
2.5 min exposure at a potential of -2.0 V. The 3 blocks at the top represent
the final colors of the 3 explosives after reduction. (B) RGB color changes
on sensing (Sens) area normalized to the RGB color changes on reference
(ref) area vs concentration of TNT in IL.
In addition to providing a desirable medium4–6 to promote the
formation of stable colored reaction products, the IL has an
astonishing preconcentration capability for the explosives. Figure
2B shows the relative preconcentration capacities of BMIM-PF6
toward different analyte vapors determined on a BMIM-PF6-coated
quartz crystal mass loading sensor (SI). The partition coefficients
obtained from these measurements as well as from UV-visible
quantification (SI) are as follows: 5.0 × 105 for TNT, 4.0 × 105
for DNT, 1.2 × 105 for nitrotoluene (NT), 2.9 × 103 for xylenes,
7.1 × 102 for benzene, 3.7 × 102 for water. The coefficient increases
rapidly with the number of nitro groups in the aromatic structure.
This effect may be related to a combination of several factors: (1)
electron-withdrawing power of nitro groups, (2) electron-donating
properties of the IL, due to deprotonated BMIM cations (acidic),4
and (3) hydrophobic anions (PF6-).4 According to the mass sensor
measurements (Figure 2B), PA shows 6 times larger affinity than
TNT, indicating that the hydroxyl group may promote hydrogen
bond interactions with a BMIM cation and lead to the larger
partition coefficient.
The high and selective preconcentration capability of BMIM-
PF6 for the explosives is partially responsible for the high sensitivity
and selectivity of the hybrid sensor. The achieved detection limit
is in the ppbV range (SI). However, based on the noise level
(∼10-4) of the webcam/LED and IL preconcentration factor, the
estimated detection limit is a few tens of pptV. The detection limit
can be further improved by using a more sophisticated optical9 and
more efficient electrochemical detection10 system (SI).
of a reference area as a function of analyte concentration and
observed a quasi linear dependence (Figure 3B), which can be used
for quantitative detection of explosives.
In summary, a highly selective, sensitive, and low-cost hybrid
sensing platform is developed based on extraordinary properties
of explosives in an ionic liquid (BMIM-PF6) and an integrated
electrochemical and colorimetric approach. High selectivity is
achieved due to a selective preconcentration effect of BMIM-PF6,
distinct electrochemical activity, and RGB color patterns of the
reaction products. High sensitivity is possible because of the large
preconcentration factors and large optical extinction coefficient of
reduced products. Using an inexpensive webcam we have achieved
a detection limit of ppbV and demonstrated selective detection of
explosives in the presence of common interferences (perfumes,
mouth wash, cleaners, petroleum products, etc.).
Acknowledgment. The authors thank funding from the NSF,
National Natural Science Foundation of China (20575-032), and
National Basic Research Program of China (2007CB310500) and
Dr. A. Cagan for helpful discussions.
We have also demonstrated highly selective detection of
explosives with various common interferents with realistic con-
centrations such as large humidity changes (∆RH ) 21%),
perfumes, mouth wash vapors, cleaners (all at ∼1% of saturation
vapors), and high vapor pressure petroleum derivatives (84 ppmV).
Figure 3A shows the relative color (Red (R), Green (G), Blue (B))
changes due to different explosives and as well as the interferents.
Large color changes are detected only in the presence of the
explosives, and no color changes are visible within the noise level
for all the interferents, which demonstrate discriminative detection
of nitroexplosives from common interferences. Furthermore, the
data show distinct color patterns for different explosives, allowing
us to identify different nitroexplosives. We note that additional
discrimination can be achieved by (1) controlling the electrochemi-
cal potential because different analytes have different electrochemi-
cal activities (SI) and (2) following the kinetics of the color changes
due to the different reaction mechanisms. We have determined the
total color change on a sensing area normalized by the color change
Supporting Information Available: Synthesis of IL, characteriza-
tion of reduction products, and vapor phase detection. This material is
available free of charge via the Internet at http://pubs.acs.org.
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