Electrospun Nanofibers from a Tricyanofuran-Based Molecular Switch for Colorimetric Recognition of Ammonia Gas

Article abstract of DOI:10.1002/chem.201504448

A chromophore based on tricyanofuran (TCF) with a hydrazone (H) recognition moiety was developed. Its molecular-switching performance is reversible and has differential sensitivity towards aqueous ammonia at comparable concentrations. Nanofibers were fabr

Full text of DOI:10.1002/chem.201504448

DOI: 10.1002/chem.201504448
Full Paper
&
Sensors
Electrospun Nanofibers from a Tricyanofuran-Based Molecular
Switch for Colorimetric Recognition of Ammonia Gas
Tawfik A. Khattab,[b] Sherif Abdelmoez,[b] and Thomas M. Klapçtke*[a]
Abstract: A chromophore based on tricyanofuran (TCF) with
a hydrazone (H) recognition moiety was developed. Its mo-
lecular-switching performance is reversible and has differen-
tial sensitivity towards aqueous ammonia at comparable
concentrations. Nanofibers were fabricated from the TCF–H
chromophore by electrospinning. The film fabricated from
these nanofibers functions as a solid-state optical chemosen-
sor for probing ammonia vapor. Recognition of ammonia
vapor occurs by proton transfer from the hydrazone frag-
ment of the chromophore to the ammonia nitrogen atom
and is facilitated by the strongly electron withdrawing TCF
fragment. The TCF–H chromophore was added to a solution
of poly(acrylic acid), which was electrospun to obtain a nano-
fibrous sensor device. The morphology of the nanofibrous
sensor was determined by SEM, which showed that nanofib-
ers with a diameter range of 200–450 nm formed a nonwo-
ven mat. The resultant nanofibrous sensor showed very
good sensitivity in ammonia-vapor detection. Furthermore,
very good reversibility and short response time were also
observed.
Introduction
also flammable at concentrations of approximately 15–28 vol%
in air.[14–17] Hence, various sensor materials have been devel-
oped for the detection of ammonia, such as tellurium thin
films,[18] nanoporous anodized alumina,[19] CuBr thin films,[20]
acrylic-acid-doped polyaniline,[21] solid-state CuS films,[22] and
Langmuir–Blodgett polypyrrole films.[23] However, the sensors
prepared with the above sensing materials have a limit for am-
monia detection below the ppm level, that is, they are not sen-
sitive enough. Furthermore, these sensing materials are often
slow, irreversible, or necessitate high operating tempera-
tures.[15,16,24,25]
Alkaline gases are defined by their ability to increase the pH
value of an aqueous phase above 7. Ammonia is the techno-
logically most important example of such gases.[1,2] The detec-
tion of ammonia is of special interest because of both the
comparatively high toxicity of ammonia and its huge technical
turnover as an intermediate in the production of artificial fertil-
izers, polymers, textiles, and explosives. Pure ammonia even
finds application as a refrigerating agent.[3–10] Being able to es-
tablish the concentration of ammonia both in gases and in the
aqueous phase is of interest in many fields, such as the food
industry, medical diagnosis, electronic device processing, ecol-
ogy, and quality control.[6–13] A sensitive and fast method for
the determination of ammonia is required because of its detri-
mental effects to human health at low concentrations. Expo-
sure to ammonia gas at levels higher than 1 ppm may cause
nose, eye, and throat irritation. Above 25 ppm, intense burning
of the skin and eyes and permanent damage to the lungs can
occur, and exposure to 300 ppm is absolutely life-threatening.
Furthermore, the maximum value allowed in the work place
for a total of 8 h exposure is only 25 ppm, but the olfactory
limit for the detection of ammonia gas is 55 ppm. Ammonia is
The development of new approaches toward highly sensi-
tive detection techniques, particularly at the ppb level, remains
a major challenge in the field of chemical sensing. The detec-
tion of ammonia by means of a sensitive surface is attractive
because of the possible accumulation by adsorption. As a con-
sequence, the sensitivity of such a sensor is enhanced by in-
creasing the surface area of the material of the detector.
Modern electrospinning would be an attractive method for the
generation of materials even on an industrial scale with well-
defined large inner surfaces where fast response and gas ex-
change can be expected. Moreover, nanofibers obtained by
such a process may form a mechanically stable tissue with ex-
pected long running life. Electrospinning is a process by which
high static voltages are used to produce an interconnected
membranelike web of small fibers with diameters in the range
of 10–1000 nm. This technique can be used with a variety of
polymers along with an active ingredient to produce nanoscale
fibrous membranes.[24,26,27] Electrospun nanofibrous mem-
branes can have approximately 1–2 orders of magnitude more
surface area than is found in continuous thin films, which
makes them excellent candidates for providing much higher
sensitivity and shorter response time in sensing applications.
[a] Prof. Dr. T. M. Klapçtke
Department of Chemistry Energetic Materials Research
Ludwig Maximilian University
Butenandtstrasse 5–13
81377 München (Germany)
Fax: (+49)89-2180-77492
E-mail: tmk@cup.uni-muenchen.de
[b] Dr. T. A. Khattab, Dr. S. Abdelmoez
Dyeing, Printing and Auxiliaries Department
Textile Research Division National Research Centre Dokki
Cairo 12311 (Egypt)
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The large surface area-to-volume ratio of the nanofibers and
their porous nature result in fast diffusion through the mesh
and adsorption onto the surface of the nanofibers.[28]
Herein, we report on a simple method for the preparation of
a reversible sensor that is composed of hydrazone (H) donor
and tricyanofuran (TCF) acceptor moieties (Figure 1) as optical
Figure 1. TCF–H molecular switch 1; R=C9H19
Scheme 1. Synthesis of the hydrazone–tricyanofuran donor–acceptor chro-
mophore; R=C9H19
molecular-switching probe. In a push–pull system, the depro-
tonated hydrazone group can function as a bridge between
donor and acceptor fragments, or itself function as a donor
fragment in conjugation with a strongly electron withdrawing
group.[29] The electrospinning process can produce tunable
nanofibers as a solid-state sensing device, which can be used
like a test paper. SEM is used to study the properties of the re-
sulting nanoscale structures. Due to the increased surface-to-
volume ratio, the miniaturized solid-state film that is formed
exhibits high sensitivity on exposure to ammonia gas. The
characteristics and analytical performance of the sensor, includ-
ing its reversibility and detection limits, are decribed. In addi-
tion, the response of the TCF–H chromophore to aqueous am-
monia in homogenous solution was investigated.
ecule are considerably more acidic those on a carbon atom
that is adjacent to alkyl units. Here, the electron-withdrawing
cyano groups of the TCF fragment help to stabilize the carban-
ion that is produced by removal of a proton from the activated
methyl group. The hydrazone moiety of the TCF chromophore
acts as a signal switcher, which is designed to switch on when
the hydrogen atom on the NH group is abstracted to form
a hydrazone anion that shows extended conjugation. In the
TCF–H anion, the TCF fragment acts as an electron acceptor,
and the hydrazone anion moiety as an electron donor. The
presence of the NH group in the hydrazone form 1 inhibits in-
ternal charge transfer between the TCF moiety and the lone
pair of electrons on the nitrogen atom of the NH group. The
synthetic route is simple and involves azo coupling starting
from tricyanofuran and 2-nonyloxy-4-nitrobenzenamine diazo-
nium chloride. The azo coupling process is a base-mediated re-
action between the diazonium salt and the TCF, which bears
an active methyl group, to afford the corresponding unstable
azo form, which transforms directly into the more stable hydra-
Results and Discussion
Synthesis and characterization of TCF–H chromophore 1
Stimuli-responsive materials are important because they can
potentially afford smart materials that can be applied in a varie-
ty of applications from nanotechnology to pharmacology. Mo-
lecular switches can undergo molecular-structure changes in
response to external physical or chemical stimuli such as light,
pH, and chemical agents. Hydrazone-based molecular switches
are characterized by their modularity, stability, and simple
preparation. Hence, hydrazones can be employed for various
medicinal and supramolecular purposes.[29] The hydrazone
structural unit is related to the structure of ketones and alde-
hydes by replacing the oxygen atom with a =NÀNHÀ group.
Hydrazones can be used for molecular switching since they
contain an imine group that can undergo stimuli-responsive
cis/trans isomerization and/or also have the ability to lose
a proton (Brønsted acid).[29] The simple synthetic approach
used for the preparation of TCF–H sensor 1 is shown in
Scheme 1. The TCF intermediate was prepared in a 68% yield
according to an early literature method.[30] The starting materi-
al 2-amino-5-nitrophenol is commercially available and was
treated with K2CO3 and 1-iodononane in dimethylformamide at
reflux to afford 2-nonyloxy-4-nitrobenzenamine in a relatively
high yield of 83%.[31] The hydrogen atoms located on a carbon
atom adjacent to a highly electron withdrawing group in a mol-
1
zone form.[32] The structure of 1 was characterized by H and
13C NMR spectroscopy, elemental analysis (C, H, N), and FTIR
spectroscopy.
Photophysical properties of chromophore 1 in solution
The optical properties of chromophore 1 in polar and nonpolar
organic solvents are shown in Table 1 and Figure 2. The color
of 1 in various solvents ranges from yellow to purple. General-
ly, the main absorption peak shows a bathochromic shift (posi-
tive solvatochromism) with increasing solvent polarity (ca.
38 nm shift on going from chloroform to DMSO) and diverse
intensities. These features indicate a strongly allowed p–p*
transition with charge-transfer character.
The 4-nitrophenylhydrazone fragment can function as
a donor fragment that, in conjugation with the strongly elec-
tron withdrawing group, results in the formation of a stable
push–pull system. The strongly electron withdrawing nitro
group facilitates the generation of a hydrazone anion. Hence,
distinct solvatochromic activities were observed in polar protic
and aprotic solvents. In protic solvents, a discernible blueshift
was observed due to the increased acidity of the alcohol sol-
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mophore with ammonia in organic solutions was investigated.
The UV/Vis absorption spectra of 1 were monitored on the ad-
dition of increasing concentrations of ammonia by using
a methanolic solution of aqueous ammonia (Figure 3). The
Table 1. Variation of the lmax values of sensor 1 in various solvents.
Solvent
ET(30)[33]
[kcalmolÀ1
lmax [nm]
]
Abs.
Em.
methanol
ethanol
n-propanol
acetonitrile
DMSO
dichloromethane
chloroform
THF
1,4-dioxane
toluene
55.4
51.9
50.7
45.6
45.1
40.7
39.1
37.4
36.0
33.9
484
487
488
477
496
472
469
476
478
478
531
534
536
539
557
538
537
535
532
531
Figure 3. UV/Vis absorption (top), and fluorescence emission (bottom) spec-
tra of TCF–H (cꢀ1.6”10À7) in acetonitrile on addition of increasing concen-
trations of a methanolic solution of aqueous ammonia.
gradual addition of aqueous ammonia results in an increase in
the pH value from 6.12 to 6.43, 6.67, 6.82, and 7.04 with color
changes from yellow to orange, red, violet, and purple, respec-
tively. On reaction with ammonia, the hydrazone NH group is
reversibly converted to a hydrazone anion by deprotonation
(Scheme 2). This acid–base reaction is accompanied by an in-
crease in the electron-donating effect of the hydrazone anion
group, which results in a change in the electron delocalization
within the chromophore molecule, and therefore the absorp-
tion maxima corresponding to the deprotonated molecule is
shifted to longer wavelengths.
Figure 2. UV/Vis absorption (top) and fluorescence emission (bottom) spec-
tra of sensor 1 in various solvents.
vent. In aprotic solvents, the lmax values for a given dye are
generally larger than those in in alcohol solvents with little
consistency in the variation of the transition energy with the
polarity of the medium.
The stepwise addition of a methanolic solution of aqueous
ammonia to a solution of 1 and subsequent recording of the
corresponding absorption maxima lmax in acetonitrile led to
a series of spectra and an isosbestic point at 502 nm. Due to
formation of the hydrazone anion, the intensity of the absorp-
tion at shorter wavelength decreased, while simultaneously an
Optical properties of TCF–H as ammonia sensor
The presence of ammonia in solution affects the pH of the
medium. Therefore, the interaction of the TCF–H sensory chro-
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Scheme 2. Proposed mechanism for the colorimetric and fluorometric sensing of both gaseous and solution phases of ammonia.
absorption at longer wavelength appeared and increased in in-
tensity. The resulting absorption spectra indicated a significant
bathochromic shift between the neutral TCF–H and hydrazone
anion derivative with increasing number of strongly electron-
withdrawing nitro groups in the sensor dye. The fluorescence
wavelength lmax maxima of the sensor dye were recorded with
stepwise addition of a methanolic solution of aqueous ammo-
nia in a variety of solvents. In contrast to the pristine (proton-
ated) TCF–H derivatives, the corresponding hydrazone anions
did not show any fluorescence in the selected solvents. When
the amount of methanolic solution of aqueous ammonia
added to the dye solution is increased, the fluorescence inten-
sity decreases in proportion, which indicates formation of the
corresponding hydrazone anion.
Figure 4. Changes in the UV/Vis absorption spectra of TCF–H in acetonitrile
solution resulting from the addition of aqueous ammonia (c=0–750 nm). At
477 nm, Y1 =À0.0004X+0.2728, R2 =0.981); at 571 nm,
Y2 =0.0003XÀ0.0113, R2 =0.9663.
We were able to construct a calibration curve for ammonia
solutions in the concentration range 0–750 nm by measuring
their UV/Vis absorption spectra. The TCF–H sensor was success-
fully applied to estimate the (unknown) concentration of am-
monia in a solution. For example, the changes in the UV/Vis
absorption spectra of TCF–H in acetonitrile solution on adding
varying amounts of aqueous ammonia solution are shown in
Figure 4.
The reversibility of the sensing effect towards ammonia in
aqueous phase was monitored by warming the chromophore
solution in the cuvette in situ from room temperature to 558C
and maintaining it for a fixed time (5.0 min) to allow ammonia
to evaporate. The UV/Vis electronic absorption and fluores-
cence emission spectra were recorded again after cooling to
room temperature. The same amount of ammonia solution
(c=1.1”10À6 molLÀ1) was then added again to monitor the re-
occurrence of the sensing. The changes in the ratio of the ab-
sorbances at 496 and 572 nm of TCF–H in DMSO solution (c=
2.3”10À5 molLÀ1) were recorded at ambient temperature. The
absorbances of about 0.332 and 0.319 switched back and forth
between 496 and 572 nm, respectively, on adding aqueous
ammonia solution followed by heating. As is shown in
Figure 5, at least for this many cycles, this process clearly ex-
hibits high reversibility towards ammonia sensing without fati-
gue.
Figure 5. Changes in the ratio of the absorbance at 496 and 572 nm of TCF–
H in DMSO solution (c=2.3”10À5 molLÀ1) at ambient temperature.
Morphology of poly(acrylic acid)–TCF–H nanofibrous sensor
Solid-state sensor materials have some advantages, such as
portability, operational simplicity, and reusability, which make
rapid online detection possible at low cost.[15,16] Thus it is very
attractive, but also challenging, to develop solid-state optical
sensor materials for direct detection of a trace target sub-
stance in both solution and vapor phases in real time. In terms
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of practicability, a solid-state film sensor often has more favora-
ble properties than a colorimetric probe dissolved in solution.
The solid film probe may ultimately be employed in basic labo-
ratory assays through portable devices and in household use
as commercial indicators. Therefore, the quest for ecofriendly
solid-film sensors with simple and smart detection is of great
significance.[15–18]
nanofibers exhibit higher sensitivity with respect to the detec-
tion of ammonia gas due to the higher surface-to-volume ratio
and larger interspaced network, which enables the diffusion of
guest ammonia through the matrix.
The solid-state film samples were exposed to ammonia gas
and showed an instant color change from orange to blue,
which was monitored with the naked eye. On removal of the
film sensor from the ammonia gas, the sample quickly recov-
ered its original orange color as the ammonia concentration
decreased. The sensor could be used repeatedly without any
apparent degradation. To test the film sensor, an aqueous am-
monia solution (2 mL) was placed in a 4 mL glass vial. After
generating saturated ammonia vapor, the nanostructure-based
film was placed near the top of the vial and showed an instant
and reversible color change without fatigue (see video ab-
stract).
The electrospinning process used to produce the poly(acrylic
acid) (PAA)–TCF–H nanofibers was developed by carrying out
a series of systematic studies on the effects of flow rate, poly-
mer and dye concentration, applied voltage, and needle tip-to-
receiver distance on the spinnability and morphology of the
polymer fibers. The concentration of TCF–H influences the sen-
sitivity of the final fibrous sensor device. The best concentra-
tion of TCF–H for the preparation of cross-linked PAA–TCF–H
nanofibrous sensor is 1.0 wt%. Increasing the amount of TCF–
H to greater than 1.0 wt% leads to decreased sensitivity of the
final fibrous sensor device. On the other hand, no color
change was detected on decreasing the amount of TCF–H to
less than 1.0 wt%. Electrospinning is an efficient, relatively
simple, and low-cost way to produce polymer and composite
fibers with diameters ranging from several micrometers to
a few nanometers by applying a high voltage to a polymer so-
lution or melt ejected from a microsyringe pump. Porous
three-dimensional membranes assembled from electrospun
fibers generally show large specific surface area, high porosity,
and good interconnectivity. These features of electrospun fi-
brous membranes result in their potential application in ultra-
sensitive and highly miniaturized sensors. An SEM image of
a pure PAA–TCF–H nanofibrous sensor is shown in Figure 6.
The nanofibers with a diameter of 200–450 nm (average diam-
eter of the nanofibers is ca. 325 nm) formed a nonwoven mat
with a porous three-dimensional structure, and the addition of
TCF–H did not affect the morphology of the nanofibrous
sensor. The resulting PAA–TCF–H nanofibrous sensor shows
a porous 3D structure with random fiber orientation and great
potential as a sensor for applications. The thinner solid-state
Conclusion
We have developed a PAA–TCF–H nanofibrous sensor for am-
monia sensing by electrospinning. The TCF–H sensor can
detect ammonia at the ppb level in solution phase. The ammo-
nia sensing is reversible and the nanofibrous sensor can be
reused several times. Compared to small-molecule chemosen-
sors, this nanofibrous sensor may represent a simple and expe-
dient technique for practical detection in both solution and
gaseous phases. The nanostructure aggregates of TCF–H ex-
hibit solid-state optical sensing of ammonia vapor. Exposure to
ammonia vapor resulted in a change of the observed color to-
wards purple. The nanofibrous sensor shows very good sensi-
tivity. In addition, this reusable nanofibrous solid-state sensor
can be easily employed to enable practical sensing of aqueous
ammonia, just like using a test paper.
Experimental Section
Materials and methods
Melting points were determined with an electrothermal melting
point apparatus. IR spectra were recorded with a Bruker Vectra-33
IR spectrometer with a diamond ATR probe. Elemental analyses (C,
H, N) were performed with PerkinElmer 2400 analyzer (PerkinElmer,
Norwalk, CT, USA) at the Microanalytical Center, Cairo University.
UV/Vis absorption spectra were measured with an HP-8453 spec-
trophotometer. Fluorescence spectra were measured with
a VARIAN CARY ECLIPSE spectrophotofluorimeter. NMR spectra
were recorded with
a Bruker AVANCE 400 spectrometer at
400 MHz; chemical shifts are given in parts per million (ppm) rela-
tive to the internal standard TMS at 295 K. The morphology of the
electrospun sample was examined with a Quanta 250 FEG scan-
ning electron microscope operating at 30 kV. The SEM samples
were coated with a layer of gold in vacuum by using an Edwards
S150A sputter coater. Fiber diameters were measured by using the
Image J software of the scanning electron microscope. For fluores-
cence emission and UV/Vis absorption measurements on solid
fibers, the nanofibrous sensor was subjected to ammonia vapor
evolved from aqueous ammonia solution for 10 min, washed with
methanol and water, and dried with nitrogen, after which fluores-
cence emission and UV/Vis absorption measurements were con-
Figure 6. SEM image of electrospun fibers of the PAA–TCF–H composite,
which can be used for solid-state reversible optical sensing of ammonia in
the gas phase.
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ducted. For investigating the reversibility (reusability) of the nanofi-
brous sensor, after exposure to ammonia vapor evolved from
aqueous ammonia solution for 10 min, the nanofibrous sensor was
washed with methanol and water and dried under nitrogen. Fluo-
rescence emission and UV/Vis absorption measurements were then
conducted. The photographs of the nanofibrous sensor before and
after sensing of ammonia vapors evolved from aqueous ammonia
solution were taken by a Canon Power Shot A710 IS digital
camera. Solvents used in this study were obtained from Fluka and
Aldrich. All reactions were monitored by TLC on Merck aluminum
plates precoated with silica gel PF254, 20–20, 0.25 mm, and detect-
ed by visualization of the plate under a UV lamp (254 or 365 nm).
Compounds were purified by recrystallization or flash column chro-
matography on Scharlau silica gel, packed by the slurry method.
switched back and forth between 496 and 572 nm by adding
aqueous ammonia solution (c=1.1”10À6 mollÀ1) and recording
the absorption spectra. This was followed by raising the tempera-
ture of the cuvette inside the spectrophotofluorimeter from room
temperature to 558C and maintaining it for a fixed time (5.0 min)
to allow the ammonia to completely evaporate. The UV/Vis absorp-
tion spectra were recorded again after cooling to room tempera-
ture. The same amount of ammonia solution was added again to
monitor whether sensing occurred again.
Synthesis of 2-(dicyanomethylene)-2,5-dihydro-4-[methyl-(2-
nonyloxy-4-nitro-phenylazo)]-5,5-trimethylfuran-3-carboni-
trile (1)[32]
A
mixture of 4-nitro-2-nonyloxy-4-nitrobenzenamine (1.1 g,
4.0 mmol), hydrochloric acid (3.0 mL), and water (2.0 mL) in
a 25 mL Erlenmeyer flask was stirred and cooled in an ice bath at
0–58C. The aniline diazonium salt was prepared by slow addition
of a solution of sodium nitrite (345 mg, 5.0 mmol) in water (3.0 mL)
at 0–58C. The resulting mixture was stirred for a further 20 min at
0–58C. In a separate round-bottom flask, tricyanofuran (800 mg,
4.0 mmol) was dissolved in acetonitrile (7.0 mL), the solution was
cooled to 0–58C, and acetic acid (2.5 mL) and sodium acetate
(3.0 g) were added. This mixture was vigorously stirred in the ice
bath at 0–58C while the cold solution of diazonium salt was added
dropwise. After stirring the resulting mixture for a further 2 h, the
crude solid product was filtered off with a Buchner funnel, washed
with distilled water (3”15 mL), crystallized from toluene/ethanol,
and air-dried to afford a red solid (1.41 g, 73% yield); m.p. 198–
Electrospinning apparatus
A typical electrospinning device has three main components:
power supply, polymer-solution delivery system, and collector.
Briefly, a programmable syringe pump was used to deliver polymer
solutions at controlled flow rates. The polymer solution was given
an electric potential that exceeds its forces of surface tension
through the high voltage (0–30 kV) power supply. Consequently,
a thin polymer fiber is ejected from the syringe needle (18 gauge,
1.27 mm) towards the collector, which results in ultrathin fibers
being deposited on an aluminum foil of dimensions 10”10 cm.
The aluminum foil was placed in the center of a piece of wood
(15”15 cm), which was attached to a copper cylinder connected
to the grounded cable. The polymer solution was filled into a plas-
tic syringe with a stainless steel blunt needle tip connected to the
dc voltage source. The height of the needle tip was adjusted to
face and align with the center of the vertically oriented aluminum
foil. The distance between the needle tip and collector (air-gap dis-
tance) was varied from 5 to 25 cm. All experiments were carried
out in ventilated fume cupboard made of wood at room tempera-
ture.
1
2008C; H NMR (400 MHz, CDCl3): d=9.68 (s, 1H, NH), 8.02 (s, 1H,
=CH), 7.98 (dd, 1H, J=8.8 Hz, 2.0 Hz), 7.83 (s, 1H, J=2.4 Hz), 7.43
(d, 1H, J=9.2 Hz), 4.22 (t, 2H, J=6.8 Hz), 1.97 (m, 2H), 1.91 (s, 6H),
1.53 (m, 2H), 1.36 (m, 10H), 0.91 ppm (t, 3H, J=4.0 Hz); 13C NMR
(400 MHz, CDCl3): d=175.26, 170.65, 145.85, 143.79, 135.75, 128.82,
118.04, 112.95, 111.52, 111.12, 109.66, 107.33, 100.26, 98.60, 70.00,
57.92, 31.91, 29.65, 29.57, 29.35, 26.77, 25.93, 22.69, 14.13 ppm; IR
(neat): n˜ =3274 (NH), 2231 (CN), 1592 (C=N), 1532, 1332 cmÀ1
(NO2); elemental analysis calcd for C26H30N6O4 (490.23): C 63.66, H
6.16, N 17.13; found: C 63.57, H 6.12, N 17.03.
Preparation of PAA–TCF–H electrospun nanofibrous sensor
Cross linked PAA–TCF–H nanofibrous sensors were prepared as fol-
lows: Firstly, an ethanolic solution with 1 wt% TCF-H, 4 wt% PAA,
and 16 wt% ethylene glycol was prepared by stirring at room tem-
perature for 12 h until a homogeneous solution formed. Secondly,
1 drop of 1m H2SO4 per milliliter of solution was added. The solu-
tion was then used for the electrospinning experiment. Typical pa-
rameters for electrospinning were as follows: the applied electric
voltage was 30 kV, the solution feed rate was 0.8 mLhÀ1, and the
distance between the spinneret and the collector (a grounded
metallic drum) was 20 cm. The grounded rotating metallic drum
with diameter of 10 cm and length of 30 cm was used to collect
the deposited nanofibers at a rotating speed of 180 rpm. The re-
sulting nanofibrous sensor was dried in a vacuum oven at 408C
until constant mass was achieved, and was then kept in a closed
drybox before use.
Acknowledgements
We gratefully acknowledge National Research Center, Cairo for
the support of this work.
Keywords: ammonia
·
colorimetry
·
electrospinning
·
nanostructures · sensors
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Received: November 5, 2015
Published online on February 11, 2016
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim