Highly sensitive sensing of volatile organic compound ethylamine

Article abstract of DOI:10.1016/j.dyepig.2014.04.030

Two new hemicyanine dyes with dihydroxybenzene moiety as a donor group have been investigated in terms of interacting with volatile organic compounds (VOCs) EtNH_2. The sensing behavior of the hemicyanines toward EtNH₂ was studied by UV-vis absorption spectroscopy. Different sensing mechanism of the dyes to EtNH₂ was confirmed by ¹NMR studies, together with theoretical calculations based on DFT and PPP-MO methods. We have also investigated the fast EtNH₂ gas sensing of these dyes loaded poly(acrylonitrile) nanofiber. Reversible response and recovery were achieved using alternating gas exposure. This system shows a fast EtNH₂ gas sensing within 5 s.

Full text of DOI:10.1016/j.dyepig.2014.04.030

Dyes and Pigments 108 (2014) 93e97
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Highly sensitive sensing of volatile organic compound ethylamine
*
Seong-Min Ji, Seon-Yeong Gwon, Sung-Hoon Kim
Department of Textile System Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new hemicyanine dyes with dihydroxybenzene moiety as a donor group have been investigated in
terms of interacting with volatile organic compounds (VOCs) EtNH2. The sensing behavior of the hem-
icyanines toward EtNH2 was studied by UVevis absorption spectroscopy. Different sensing mechanism of
the dyes to EtNH2 was confirmed by 1NMR studies, together with theoretical calculations based on DFT
and PPP-MO methods. We have also investigated the fast EtNH2 gas sensing of these dyes loaded
poly(acrylonitrile) nanofiber. Reversible response and recovery were achieved using alternating gas
exposure. This system shows a fast EtNH2 gas sensing within 5 s.
Received 28 March 2014
Received in revised form
21 April 2014
Accepted 22 April 2014
Available online 30 April 2014
Keywords:
Volatile organic compounds (VOCs)
Ethylamine
Ó 2014 Elsevier Ltd. All rights reserved.
Sensing
Hemicyanine dye
HOMOeLUMO
Nanofiber
1. Introduction
ethylamines have especially been postulated to be involved in at-
mospheric nucleation, i.e. the natural process whereby new parti-
Recently there is an increased awareness of the importance to
detect specific volatile organic compounds (VOCs) especially for
indoor air quality control but also in food processing industry [1].
Determination of VOCs discharged from industries, vehicles,
homes, and living organisms is of particular concern, because they
can cause serious impacts on the environment and public health. A
wide range of amines are pollutants in industrial and
manufacturing areas because they are extensively used in the
preparation of fertilizer, herbicides, pharmaceuticals, surfactants,
rubber latex, biological buffer substances and colorants. Turner
et al. investigated the ability of a hemithioacetal-based polymer to
react with primary amines and to form a fluorescent isoindole
complex [2]. They are toxic both in the gas phase and dissolved in
liquid. Because of their extensive use in industries, it is necessary to
develop new and effective sensors for amines. Amine compounds
have been found at trace levels in the atmosphere. Many analytical
methods have been developed for the separation and determina-
tion of amines such as isotachophoresis [3], ion chromatography
[4], thin-layer chromatography, gas chromatography [5,6], high-
performance liquid chromatography [7], and capillary electropho-
resis [8]. Recently, various optical-sensing methods are also widely
used for the detection of amines [9e11]. Mono-, di- and tri-
cles are generated [12]. Among various types of amines, EtNH2 is a
colorless, flammable liquid and an ammonia-like odor. It has an air
odor threshold concentration of 0.95 ppm of air. Inhalation expo-
sure to EtNH2 may cause eye irritation, tearing, conjunctivitis, and
corneal edema. Qiu et al. worked with a supramolecular metal-
organic framework (MOF) constructed by two-dimensional infin-
ite coordination polymers, [Zn(1,4-benzenedicarboxyate)(H2O)]m,
and they evaluated the fluorescence response of the MOF nano-
sheets in the presence of EtNH2 solution [13]. We have previously
reported the colorimetric signaling of mono-, di-, and tri-ethyl-
amine based on intermolecular charge-transfer interaction [14] and
have also described the ultrafast EtNH2 gas sensing of 2-chloro-3,5-
dinitrobenzotrifluoride loaded poly(acrylonitrile) nanofiber based
on an intermolecular charge-transfer complexation [15]. This sys-
tem showed a fast EtNH2 gas sensing with 0.4 s. We report herein
the synthesis and highly sensitive EtNH2 sensing properties of
hemicyanine dyes.
2. Experimental
2.1. Materials and methods
2-Methylbenzothiazole,
2.3.3-trimethylindolenine,
2,5-
dihydroxybenzaldehyde, were purchased from SigmaeAldrich.
The rest of chemicals were commercially available with high grade
* Corresponding author.
E-mail address: shokim@knu.ac.kr (S.-H. Kim).
and
were
used
without
further
purification.
2,3-
http://dx.doi.org/10.1016/j.dyepig.2014.04.030
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

94
S.-M. Ji et al. / Dyes and Pigments 108 (2014) 93e97
Dimethylbenzothiazolium
iodide
2
and
1,2,3,3-
2,3-dimethylbenzothiazolium
iodide
2
and
1,2,3,3-
tetramethylindolenium iodide 3 were obtained by synthesis ac-
cording to the literature methods [16].
tetramethylindolenium iodide 3 were synthesized by the reaction
of methyl iodide with 2-methylbenzothiazole and 2,3,3-
trimethylindolenine, respectively.
The analytical characterizations were conducted by 1H NMR, MS
and elemental analysis to the obtained dyes. For the interpretation
of the ICT process of dye 1 and dye 2, the quantum chemical DMol3
approach was used. All the theoretical calculations were performed
by DMol3 program in the Materials Studio 4.4 package [18,19]
which is the quantum mechanical code using density functional
theory. PerdeweBurkeeErnzerhof (PBE) function of generalized
gradient approximation (GGA) level [20] with double numeric
polarization basis set was used to calculate the energy level of the
frontier molecular orbital.
Fig. 1 displays the calculated and optimized electron density
distribution and HOMO/LUMO energy levels of dye 1 and 2. Com-
parison of the electron distribution in the frontier MOs reveals that
the HOMOeLUMO excitation moves the electron distribution from
the dihydroxybenzene moiety to the acceptor, which showed an
electron density migration of ICT character of these dyes. Upon
addition of EtNH2 to the solution of dye 1, the absorption band at
359 and 442 nm progressively decreased in intensity and a new
peak at 589 nm appeared; an isobestic point at 497 nm also
developed. The appearance of this isobestic point suggests that at
least one stable dye 1-EtNH2 species is present in solution
(Fig. 2(a)). Analysis of the adduct formation, studied by the changes
in absorption spectra on addition of EtNH2, indicated a 1:2 dye to
EtNH2 stoichiometry (Fig. 2(a) inset).
2.2. Instruments
Melting points were determined using an Electrothermal IA900
apparatus and were uncorrected. Elemental analyses were recor-
ded on a Carlo Elba Model 1106 analyzer. Mass spectra were
recorded on a JMS-700 high resolution mass spectrometer using an
FAB ion source. 1H NMR spectra were recorded on Varian Unity
Inova 400 MHz FT-NMR spectrophotometer with TMS as internal
standard. The UVeVis absorption spectra were measured on an
Agilent 8453 spectrophotometer. Calculation of the HOMO and
LUMO energy levels and electron densities were carried with
PiSystems XTE ver. 6.2 package and Material Studio 4.3 program.
2.3. Synthesis of dye 1 and 2
A mixture of 2,5-dihydroxybenzaldehyde 1 (0.3 g, 2.17 mmol),
2,3-dimethylbenzothiazolium iodide 2 (0.6 g, 2.12 mmol), and ab-
solute ethanol (20 mL) was refluxed for 4 h under a nitrogen at-
mosphere. After cooling, the crude product was filtered and
recrystallized from ethanol. Yield: 10%.
1H NMR (400 MHz, DMSO-d6):
d
4.30 (s, 3H, NþeCH3), 6.86 (d,
J ¼ 8.8 Hz, 1H), 6.93 (d, J ¼ 8.8 Hz, 1H), 7.37 (d, J ¼ 2.84, 1H), 7.91e
7.76 (m, 3H), 8.25e8.19 (m, 2H), 8.38 (d, J ¼ 7.44 Hz, 1H), 9.18 (s, 1H,
eOH), 10.24 (s, 1H, eOH). EA: anal. calcd. C16H14INO2S: C 46.73, H
3.43, N 3.41, S 7.80, Found C 46.39, H 3.42, N 3.70, S 7.77%.
[M]þꢀI ¼ 284.3. mp: 281e282 ꢁC.
This finding can be supported by Fig. 1(a). In Fig. 1, the resulting
energy gap between HOMO and LUMO with EtNH2 addition was
Dye 2 was obtained using similar procedure. Yield 16%. 1H NMR
decreased from
D
E ¼ 1.526 eV to E ¼ 1.349 eV that of the dye 1-
D
(400 MHz, DMSO-d6):
d
1.74 (s, 6H, e(CH3)2), 4.08 (S, 3H, eNþe
EtNH2 is smaller than that of dye 1. This means that the dye 1-
EtNH2 is expected to exhibit the bathochromic absorption
compared with that for dye 1. The dihydroxybenzene part of dye 1
has two OH fragments that can be form intermolecular proton
transfer complexes with the EtNH2. The large changes in the ab-
sorption spectrum by the addition of EtNH2 implied that the
electron-donor Oꢀ promote the ICT process. The bathochromic
changes in absorption spectra associated with the formation of dye
CH3) 6.87 (d, J ¼ 8.84 Hz, 1H), 6.99 (d, J ¼ 8.84 Hz, 1H), 7.47 (s, 1H),
7.57e7.65 (m, 3H), 7.86e7.89 (m, 2H), 8.48 (d, J ¼ 16.36 Hz,1H), 9.23
(S, 1H, eOH), 10.42 (S, 1H, eOH). EA: anal. calcd. C19H20INO2: C
54.17,
H 4.79, N 3.32, Found C 53.94, H 4.83, N 3.29%.
[M]þꢀI ¼ 294.3. mp: 266e268 ꢁC.
3. Results and discussion
1-EtNH2 species is due to the delocalization of
p-electrons on the
entire molecule. To confirm the nature of the intermolecular proton
transfer between dye 1 and EtNH2, the 1H NMR spectrum of dye 1
was recorded in the absence and presence of EtNH2 (Fig. 3).
As shown in Fig. 3, original OH protons of dye 1 appear at 9.18
and 10.24 ppm; however, EtNH2 addition leads to an upfield shift.
With addition of EtNH2, the olefinic and aromatic protons showed
an upfield shift due to the OHeN bond formation which increased
the electron density of the olefinic and phenyl ring. These results
indicate that an intermolecular proton transfer between dye 1 and
EtNH2 is formed. The olefinic Hb proton of the dye 1-EtNH2 adduct
shows a large upfield shift, indicating that the interaction of Hb
with C]O produces an intermolecular hydrogen bonding.
Hemicyanine dyes are widely applied in different areas of
technology due to their diversified properties. Because of their
excellent spectroscopic properties of large molecular extinction
coefficient (ε ¼ 104 Mꢀ1 cmꢀ1) and good fluorescence quantum
yield, they are commonly used as laser dyes and fluorescence
probes. We have developed new hemicyanine dye 1 and 2 in which
dihydroxybenzene moiety was connected through a conjugated
system with the acceptor moiety. Aromotic aldehydes react with
compounds possessing active methylene groups giving the hemi-
cyanine dyes [17]. The synthesis of dye 1 and 2 was performed by
condensation reaction as shown in Scheme 1, the key intermediates
Fig. 2(b) shows the changes in the absorption spectra of the
originally yellow colored solution of 5 ꢂ 10ꢀ5 M dye 2 in 1:1
DMSO:water imparted by the addition of 0e1.5 equivalents of
EtNH2, from which it is apparent that the absorbance of the dye 2 at
373 and 462 nm gradually decreased with increasing concentration
of EtNH2 and the color of the solution changed from yellow to
colorless. The detection limits of dye 1 and 2 for EtNH2 were found
to be 1 ꢂ 10ꢀ4 M and 7.5 ꢂ 10ꢀ5 M, respectively. The absorption
spectral change of dye 2 upon EtNH2 addition is due to the nucle-
ophilic addition between EtNH2 and Ca]Cb (in Scheme 1) of dye 2.
To explain this finding, it is approached to consider EtNH2 addition
behavior toward dye 2 molecule, namely nucleophilic addition ef-
OH
S
N
S
N
HO
I
CHO
I
OH
dye 1
2
+
OH
HO
β
1
α
N
N
HO
I
I
3
dye 2
Scheme 1. Synthesis of dye 1 and dye 2.
fect. The nucleophile of EtNH2 can react to the a position, where a

S.-M. Ji et al. / Dyes and Pigments 108 (2014) 93e97
95
Fig. 1. Electron distribution of the HOMO and LUMO energy levels of (a) dye 1 and (b) dye 2.
Fig. 2. The changes in the absorption spectra of (a) dye 1 and (b) dye 2 upon titration
Fig. 3. 1H NMR spectral changes of dye 1 in DMSO-d6 upon addition of (a) 0, (b)
3 equiv. of EtNH2.
with EtNH2 in DMSO/H2O (1:1, v/v); [dye] ¼ 5 ꢂ 10ꢀ5 mol Lꢀ1
.

96
S.-M. Ji et al. / Dyes and Pigments 108 (2014) 93e97
Fig. 4.
p-Electron density values of LUMO of dye 2.
position of dye 2 molecule having lower electron density in LUMO
can be attacked by nucleophile of EtNH2 (Fig. 1(b)).
p-Electron density of LUMO of dye 2 was also calculated using
ParisereParrePople (PPP) MO method. Fig. 4 represents the
p-
electron density values of LUMO of dye 2. It clearly shows that
a
carbon position of dye 2 is the lowest value of 0.034 and where the
resulting interaction with EtNH2 can be proposed.
This is confirmed by 1H NMR spectra of dye 2 obtained without
and with EtNH2. When the dye 2 was treated with amount of
1 equiv EtNH2, all its aromatic protons were shifted to the upfield
region (Fig. 5). An olefinic proton Ha at 6.99 ppm of dye 2 was
upfield shifted to 5.71 ppm, while the other proton is slightly up-
field shifted. This is because the olefinic Ha proton of the adduct is
magnetically shielded and shifts upfield as compared to the original
dye 2. These 1H NMR studies indicate that the EtNH2 was added to
the olefinic bridge region via a simple addition reaction of the
EtNH2 to dye 2. The observation is in good agreement with the
afore-mentioned mechanism.
Electrospinning is a well-known and convenient technique for
generating ultrathin nanofiber materials, with diameters in the
range of nanometers to several micrometers [21,22]. The electro-
spinning technique allows for rapid and cost-effective fabrication of
fibrous polymer membranes with great length and high specific
surface area. In the process of electrospinning, the polymer solution
for electrospinning is injected from a small nozzle under the in-
fluence of an electric field and electrospun fibers are collected on
the grounded collector screen. Thus, the EtNH2 gas sensing per-
formance of the dye 2 loaded poly(acrylonitrile) (PAN) nanofiber
was next investigated. A typical procedure used for the fabrication
of electrospun nanofiber is schematically presented in Fig. 6(a). A
viscous DMF solution containing PAN and dye 2 is placed in a sy-
ringe. A high voltage (20 kV) is then applied to the syringe needle.
Fig. 6. (a) Schematic representation of the preparation and EtNH2 gas sensing of dye 2
loaded PAN nanofiber. (b) Still images of fast and reproducible responses of PAN
nanofiber mat upon exposure of EtNH2 gas as a function of time.
The nanofibers are collected on the surface of
a grounded
aluminum foil. Fig. 6(b) shows a span of 2.5 s still images in which
PAN nanofiber containing dye 2 have been exposed to EtNH2 gas for
5 s. These still images were selected from video clip (video camera-
VIXIA HFM 31 Canon, Japan). Upon exposing PAN nanofiber mat to
EtNH2 gas, a dark red color was promptly appeared and color in-
tensity increased, depending on the exposing time. After removal of
the EtNH2 gas stimuli, the original pattern is reversibly recovered
within 5 s. We can judge from Fig. 6 that the fast color changing
response exhibits an excellent response and recovery characteristic.
4. Conclusions
We have designed and synthesized new hemicyanine dye che-
mosensors containing dihydroxybenzene moiety as the donor.
Hemicyanine dye 1 and dye 2 exhibited marked changes in UVevis
absorption peak upon the addition of EtNH2. These two chemo-
sensors provide a different sensing mechanism. However, other
amines did not affect any color change in absorption properties. We
Fig. 5. 1H NMR spectral changes of dye 2 in DMSO-d6 upon addition of (a) 0, (b)
1 equiv. of EtNH2.

S.-M. Ji et al. / Dyes and Pigments 108 (2014) 93e97
97
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This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea Government (MSIP)
(2008-0062617). This research was supported by a grant from the
Fundamental R&D Program for Core Technology of Materials
(10037193) funded by the Ministry of Trade, Industry & Energy,
Republic of Korea.
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