Colorimetric detection of nitroaromatics using organic photochromic compounds

Article abstract of DOI:10.1071/CH15337

An organic photochromic compound is explored as a new portable colorimetric sensor for nitroaromatics. This photochromic compound switches from colourless to pink upon exposure to ultraviolet light. In the presence of nitroaromatic explosive derivatives the photoswitching behaviour of the dithienylethene is suppressed, while a potential false positive (toluene) has little effect. The degree of photoswitching inhibition was determined by comparing the integrated visible absorption with the concentration of the analyte to give the pseudo Stern-Volmer constant (KPSV). The KPSVs measured varied from 12900 (p-nitrotoluene) to 236M-1 (toluene), which were directly related to the analyte absorption at the excitation wavelength.

Full text of DOI:10.1071/CH15337

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2015, 68, 1723–1726
Full Paper
http://dx.doi.org/10.1071/CH15337
Colorimetric Detection of Nitroaromatics Using Organic
Photochromic Compounds
A
,
B
,
C
B
George Vamvounis
and Nicholas Sandery
A
College of Science, Technology and Engineering, James Cook University, Townsville,
Qld 4811, Australia.
B
School of Chemistry and Molecular Biosciences, The University of Queensland,
Brisbane, Qld 4072, Australia.
C
Corresponding author. Email: george.vamvounis@jcu.edu.au
An organic photochromic compound is explored as a new portable colorimetric sensor for nitroaromatics. This
photochromic compound switches from colourless to pink upon exposure to ultraviolet light. In the presence of
nitroaromatic explosive derivatives the photoswitching behaviour of the dithienylethene is suppressed, while a potential
false positive (toluene) has little effect. The degree of photoswitching inhibition was determined by comparing the
integrated visible absorption with the concentration of the analyte to give the pseudo Stern–Volmer constant (KPSV). The
ꢀ1
KPSVs measured varied from 12900 (p-nitrotoluene) to 236 M (toluene), which were directly related to the analyte
absorption at the excitation wavelength.
Manuscript received: 9 June 2015.
Manuscript accepted: 2 July 2015.
Published online: 5 August 2015.
Introduction
in biologically relevant applications such as mimicking pyri-
[
16]
[17]
The discovery of inexpensive portable devices for sensing
explosives is needed because of the escalating threat towards
public security. In particular, the detection of high explosives
such as 2,4,6-trinitrotoluene (TNT) is in high demand because of
their widespread availability. Although TNT can be detected
with devices based on X-ray scanning and ion mobility spec-
trometry, they lack the portability required for on-the-spot
sensing. Therefore, devices based on fluorescence quenching of
light-emitting conjugated polymers and dendrimers have
attracted much attention because they can be used in portable
doxal phosphate
and imaging in living cells.
Regulation of the switching behaviour of DAEs has been an
area of great interest for sensing applications. To that end, the
[
18]
[19]
[20]
chemosensing of metals,
fluoride ions,
and cysteine
has been demonstrated. In these instances, the DAE interacted
with the analyte to obtain the desired selectivity. Alternatively,
if the analyte has a unique absorption spectrum, then inhibition
of photoswitching could be used to detect that compound at a
specific wavelength. This would be particularly applicable to
nitroaromatic compounds as they have high molar absorptiv-
ities, meaning a small amount of compound could afford a large
change in the photoswitching property. To that end, the inhibi-
tion of photoswitching of a dithienylethene (DTE) derivative,
Scheme 1, in the presence of various nitroaromatic compounds
is investigated herein.
[
1–3]
nitroaromatic explosive sensing devices.
The portability
and cost of explosive sensors still needs to be improved to fur-
ther the accessibility of this useful technology. Colorimetric
detectors, such as the litmus test for acidity, are powerful sensors
because they are cost-effective, portable, and permit visual
[4]
detection allowing use by non-experts. Therefore, the color-
imetric detection of TNT and their derivatives is of great
Results
[
5–7]
interest.
an organic photochromic compound is explored.
In this manuscript, the colorimetric detection using
Fig. 1 depicts the photoswitching of a DTE as a function of DNB
concentration. It is apparent that as the concentration of the DNB
increases, the extent of photoswitching decreases.
A method to quantify, and therefore directly compare, this
photoswitching inhibition of the various nitroaromatic analytes
was developed, which is analogous to Stern–Volmer fluores-
cence quenching. In doing so, the change in the fluorescence
intensity area is replaced with the absorption intensity area in the
Stern–Volmer equation. This pseudo Stern–Volmer relationship
is represented by Eqn 1:
Organic photochromic compounds are a widely applicable
class of molecules that undergo a reversible colour change upon
irradiation with light.
diarylethenes (DAEs) have shown significant promise in terms
8]
of ease of synthesis, solid state switching, switching speed,
and fatigue-resistance.
switching properties, DAEs have been investigated as active
materials for ophthalmic lenses, carbon dioxide capture,
optical data storage, and logic components where the two
[
8–10]
Of these photochromic molecules,
[
[
11,12]
Because of the promising photo-
[
8]
[13]
[
14]
photoisomers can display a unique electronic
output.
or optical
Furthermore, these photochromes have been used
A0
[15]
¼ 1 þ KPSV ½Anꢁ ð1Þ
www.publish.csiro.au/journals/ajc
A
Journal compilation Ó CSIRO 2015

1
724
G. Vamvounis and N. Sandery
F
F
F
F
F
F
F
F
F
F
F
F
S
UV light
ϩ
Nitroaromatic
S
S
S
Colourless
Pink
Scheme 1. Photoswitch inhibition of a dithienylethene using a nitroaromatic compound.
0
0
0
0
0
0
.6
.5
.4
.3
.2
.1
0
a linear relationship between the decrease in the absorption area
between 425 and 600 nm and the concentration of the DNB (ꢂ) at
various excitation wavelengths. Upon exposure at 280 nm, the
KPSV was 8200 ꢃ 520 Mꢀ . Importantly, the corrected KSV of
fluorescent polymers, dendrimers, and small molecules are
0
0
0
0
0
0
mM DNB
.02 mM DNB
.04 mM DNB
.06 mM DNB
.07 mM DNB
.08 mM DNB
1
generally in the tens or hundreds, and rarely in the thousands
ꢀ1 [1–3]
M
.
The KPSVs were found to be excitation wavelength
ꢀ
1
dependent, where KPSV varied from 4000 to 4900 M when
excited at 300 and 320 nm.
A key characteristic of an explosives sensor is its selectivity
to avoid false positives. To determine the photoswitching
inhibition selectivity of the DTE, the photoswitching
inhibition of p-nitrotoluene (pNT), 2,4-dinitrotoluene (DNT),
1,4-dinitrobenzene (DNB), benzophenone (BP), and toluene
ꢀ
1
2
75
325
375
425
475
525
575
were investigated (Fig. 3a). The K was 12900 ꢃ 745 M for
PSV
pNT, 9650 ꢃ 650 Mꢀ for DNT, 6030 ꢃ 310 M for the non-
1
ꢀ1
Wavelength [nm]
ꢀ
1
explosive BP and only 236 ꢃ 38 M for toluene at 280 nm
excitation. The different response to the analytes was correlated
with the molar absorptivity (e) of the analyte at the excitation
wavelength (280 nm). To that end, Fig. 3b illustrates the
absorption traces of all the analytes in dichloromethane. It is
clear the greater the analyte’s molar absorptivity at 280 nm, the
greater the photoswitching inhibition. For instance, pNT has
Fig. 1. Absorption spectrum of dithienylethene solutions (0.5 mM) as
a function of DNB concentration after exposing each solution to 280 nm
for 200 s.
1
1
1
1
1
1
1
.6
.5
.4
.3
.2
.1
.0
λex ϭ 280 nm
ꢀ1
ꢀ1
the highest molar absorptivity (11700 cm
and gives the greatest photoswitching inhibition (K
M
) at 280 nm
¼
PSV
ꢀ1
9
molar absorptivity (,10 cm
650 M ), while the non-explosive toluene has the lowest
ꢀ1
ꢀ
1
M ) and has the lowest photo-
ꢀ
1
switching inhibition (KPSV ¼ 236 M ). The comparison of the
KPSV of all the analytes with respect to their molar absorptivity
is given in Fig. 3c. It is clear that the analyte absorption at
280 nm is directly correlated with the photoswitching inhibi-
tion, meaning that the photoswitching inhibition is primarily
attributed to competing absorption of the photochrome and the
analyte at the given excitation wavelength. That is, as the
analyte concentration increases, the analyte absorbs more light,
which increases photoswitching inhibition. To further demon-
strate this difference in selectivity, Fig. 3a inset depicts the
response after UV-irradiation in the absence of an analyte
λex ϭ 300 nm
0
0.00005
0.0001
[
DNB] [M]
(
DTE), in the presence of pNT (DTE þ pNT) and in the
Fig. 2. Photoswitching inhibition of the dithienylethene as a function
presence of toluene (DTE þ toluene). This demonstrates that
the photoswitching is inhibited in the presence of pNT while
virtually unaffected in the presence of toluene. Therefore, given
the excitation wavelength dependence (Fig. 2) and the absorp-
tion profile of any analyte, the selective detection at a particular
excitation wavelength can be predicted.
of DNB concentration and excitation wavelength.
where A is the integrated absorption intensity in the absence of
0
analyte, A is the integrated absorption intensity after the analyte
addition, [An] is the concentration of the analyte, and KPSV is the
pseudo Stern–Volmer constant. An additional advantage of this
quantification method is that it enables the responsiveness of
this colorimetric detection to be directly compared with tradi-
tional fluorescence quenching technologies (Stern–Volmer con-
Experimental
Materials
n-Butyl lithium, 2,5-dimethylthiophene, N-bromosuccinimide,
DNB, PNT, DNT, BP, and toluene were obtained from
stant, K ) for explosives detection. In doing so, Fig. 2 illustrates
SV

Colorimetric Detection of Nitroaromatics
1725
(
a) 2.0
CH3
The photochrome solutions were prepared (,0.5 mM) and
that solution was used to prepare the analyte solutions to avoid
changes in the photochrome concentration after the addition of
the analyte. The photostationary state is primarily in the open
state. The pseudo Stern–Volmer constants were calculated by
comparing the integrated absorption area from 425 to 600 nm,
and dividing by the total volume of the solution. The KPSV
coefficients and errors reported were based on an average of five
measurements. Absorption spectra were recorded on a Varian
Cary 5000 UV-visꢀNIR spectrophotometer using freshly dis-
tilled dichloromethane as the solvent. A Horiba Jobin-Yvon
Fluoromax 4 spectrometer was used as a light source for
photoswitching the photochrome solutions with a 5 nm excita-
tion slit width for 200 s at the specified wavelength.
NO2
1.8
1.6
1.4
1.2
1.0
0
0.0003
0.0006
0.0009
0.0012
Analyte concentration [M]
(
b) 25000
Toluene
Benzophenone
Synthesis
2
1
1
0000
5000
0000
Dinitrobenzene
Dinitrotoluence
[21]
3-Bromo-2,5-dimethylthiophene
A 500 mL round bottom flask was charged with 2,5-dimethyl
thiophene (5.1 mL, 44.6 mmol) in glacial acetic acid (220 mL)
and N-bromosuccinimide (7.94 g, 44.6 mmol) was added slowly
portion wise at room temperature with stirring. The solution was
allowed to stir for 3 h at this temperature. The reaction was
quenched with a mixture of ice and water (300 mL) followed by
extraction with dichloromethane (3 ꢄ 100 mL). The combined
organic fractions were washed with 2 M Na CO (5 ꢄ 100 mL),
p-Nitrotoluence
5
000
0
2
30
250
270
290
310
330
350
2
3
Wavelength [nm]
3
water (2 ꢄ 100 cm ), and dried over MgSO . The mixture was
4
filtered and the solvent was removed under reduced pressure.
The crude product was purified by column chromatography over
silica (500 mL) using hexanes as the eluent to afford the title
compound as a colourless oil (6.63 g, 77 %). d (CDCl ) 2.32
(
c) 14
White: ε (cmϪ1 MϪ1
)
Grey: KPSV(MϪ1
)
at 280 nm
1
1
2
0
8
6
4
2
0
H
3
þ
(3H, s), 2.39 (3H, s), 6.55 (1H, m). m/z (GCMS): 192 (M ).
0
3
,3 -(Perfluorocyclopent-1-ene-1,2-diyl)bis(2,5-
[
22]
dimethylthiophene), DTE
A 100 mL round bottom flask was charged with 3-bromo-
,5-dimethylthiophene (3.66 g, 19.4 mmol) in dry THF (30 mL)
2
and cooled to ꢀ788C. n-Butyl lithium (19.4 mmol) was added
dropwise allowed to stir at ꢀ788C for 1 h before perfluorocy-
clopentene (1.19 mL, 9.23 mmol) was added. The solution was
warmed slowly to room temperature under stirring. The reaction
was quenched with 3 M hydrochloric acid (5 mL) and allowed to
stir for 30 min, followed by extraction with diethyl ether
(3 ꢄ 15 mL). The combined organic fractions were dried over
Toluene
BP
DNB
DNT
pNT
Fig. 3. (a) Photoswitching inhibition as a function of p-nitrotoluene (r),
,4-dinitrotoluene (&), 1,4-dinitrobenzene (
), benzophenone (ꢄ), and
toluene (W) at 280 nm excitation. Inset: DTE, DTE with pNT (1.8 mg,
.2 mM), and DTE with toluene (3.0 mg, 13.0 mM) solution depicting
photoswitching inhibition after irradiating with UV light for 30 s. (b) the
2
ꢂ
5
anhydrous MgSO , filtered, and the solvent was removed under
4
ꢀ1
ꢀ1
molar absorptivity (L mol cm ) of the analytes in dichloromethane as a
function of wavelength. (c) Comparison of the analyte absorption at 280 nm
and their pseudo Stern–Volmer constants.
reduced pressure. The crude product was purified by column
chromatography over silica using hexanes as the eluent and
followed by recrystallisation from hexanes to afford the title
compound as a white-pink crystalline solid (0.80 g, 21 %). n
(
max
ꢀ
1
neat)/cm 2960, 2924, 1622, 1497, 1436, 1333, 1262, 1208,
1
1
185, 1145, 1125, 1085, 1043, 980, 888, 822, 731. d (CDCl )
H 3
.81 (3H, s), 2.41 (3H, s), 6.70 (1H, s).
Sigma-Aldrich and perfluorocyclopentene was obtained from
SynQuest Laboratories. All chemicals were used as received.
Characterisation
Conclusion
1
H NMR spectra were recorded on a Bruker Avance 300 MHz
spectrometer. Gas chromatography–mass spectrometry
We have demonstrated a simple approach for the visible
detection of nitroaromatics using a DTE photochrome. It was
found that DTE is highly sensitive and selective towards
nitroaromatics because of the high molar absorptivity of the
nitroaromatics and the good absorption overlap of the analyte
with the DTE. This technology would be useful for the pro-
duction of cost-effective and portable explosive sensors.
(
5
GCMS) chromatograms were recorded on a Hewlett Packard
971A gas chromatograph–mass spectrometer. Fourier trans-
form infrared (FT-IR) spectra were recorded as a powder using a
Nicolet 6700 FT-IR equipped with a smart iTR attenuated total
reflectance crystal.

1
726
G. Vamvounis and N. Sandery
Acknowledgements
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This project was supported by the Australian Research Council through
G.V.’s Australian Research Fellowship (DP 1095404). We thank The
University of Queensland for providing support through the Early
Career Researcher grant and the Strategic Initiative – Centre for Organic
Photonics & Electronics.
[
[
[
[
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