A Cyanine Dye Encapsulated Porous Fibrous Mat for Naked-Eye Ammonia Sensing

Article abstract of DOI:10.1002/asia.201600818

Electrospun ultrathin fiber-based sensors are desirable because of their practicality and sensitivity. Ammonia-detection systems are in high demand in different areas, including the industrial and agricultural fields. However, current technologies rely on large and complex instruments that restrict their actual utilization. Herein, we report a flexible naked-eye ammonia sensor, the polylactic acid–cyanine (PLA-Cy) fibrous mat, which was fabricated by blending a carboxyl-functionalized cyanine dye (D1) into electospun PLA porous fibers. The sensing mat was shown to undergo a naked-eye-detectable color change from white to blue upon exposure to ammonia vapor. The mat showed high selectivity to ammonia gas with a detection limit of 3.3 ppm. Aggregated D1 was first encapsulated by PLA and was then ionized by NH₃. These mechanisms were examined by photophysical studies and scanning electron microscopy. The aggregation–deaggregation process of D1 in the PLA-Cy fibrous mat led to the color change. This work provides a facile method for the naked-eye detection of ammonia and a novel strategy for the use of organic dyes in ammonia sensing.

Full text of DOI:10.1002/asia.201600818

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Accepted Article
Title: A Cyanine Dye Encapsulated Porous Fibrous Mat for Naked Eye Ammonia
Sensing
Authors: Kai Pan; Chengdong Ji; Lijing Ma; Meizhen Yin; Wantai Yang
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DOI: 10.1002/ ((please add manuscript number))
Article type: Full Paper
A Cyanine Dye Encapsulated Porous Fibrous Mat for Naked Eye Ammonia
Sensing
Chendong Ji, Lijing Ma, Meizhen Yin*, Wantai Yang, Kai Pan*
Mr. C. Ji, Mr. L. Ma, Prof. M. Yin, Prof. W. Yang, Prof. K. Pan,
State Key Laboratory of Chemical Resource Engineering, Key Laboratory of Carbon
Fiber and Functional Polymers of Ministry of Education, Beijing Laboratory of
Biomedical Materials, Beijing University of Chemical Technology, Beijing, 100029,
China
Email: pankai@mail.buct.edu.cn; yinmz@mail.buct.edu.cn
Keywords: Electrospinning, Fibrous Mat, Cyanine Dye, Naked Eye Sensor,
Ammonia Detection.
Abstract: Electrospun ultrathin fiber based sensor is desirable because of its
practicality and sensitivity. Ammonia detection systems are in high demand in
different areas such as industrial and agricultural fields. However, current
technologies rely on large and complex instruments that restrict their actual utilization.
Herein, we report a flexible naked eye ammonia sensor, the PLA-Cy fibrous mat,
which was fabricated by blending a carboxyl-functionalized cyanine dye (D1) into
electospun polylactic acid (PLA) porous fibers. The sensing mat undergoes a naked
eye detectable color change from white to blue upon exposure to ammonia vapor. The
mat showed high selectivity to ammonia gas with the detection limit of 3.3 ppm.
Aggregated D1 was first encapsulated by PLA and then ionized by NH . These
3
mechanisms were examined by photophysical studies and scanning electron
microscopy (SEM). The aggregation-deaggregation process of D1 in the PLA-Cy
1

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fibrous mat leads to the color change. This work provides a facile method for naked
eye ammonia detection and a novel strategy for using organic dyes in ammonia
sensing.
1
. Introduction
Due to the potential applications of ultrathin fibers, such as nanofibers, a large
number of advanced techniques have been developed to prepare ultrathin fibers with
[
1]
different structures and functions in recent years.
Among these methods,
electrospinning was considered as a simple and versatile method for fabricating
[
2]
ultrathin fibers from a variety of polymers. The electrospun ultrathin fibers feature
[
3]
nanoscale diameters, large surface area to volume ratios, and long lengths. By
controlling the processing conditions, many different fiber morphologies can be
[
4]
[5]
[6]
[7]
obtained, such as core-shell, hollow, nanoweb, and porous structures. At the
same time, much work have been performed to control the array structure of fibers by
[
8]
designing or adjusting the collector. In addition, multiple extra features can be
incorporated into ultrathin fibers to broaden their application. Up to now, electrospun
[9]
ultrathin fibers have found use in a wide range of applications, including filtration,
[
10]
[11]
[12]
[13]
[14]
adsorption, catalysis, textiles, energy, sensors and more.
Generally, the sensitivity of a sensor that detects analytes by interacting with
[15]
molecules on the surface will increase with increasing surface area per unit mass.
Therefore, the large surface area to volume ratio and good interconnectivity make
electrospun ultrathin fibers highly attractive for sensor applications. In order to
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fabricate ultrathin fiber sensors, the research can be classified into two main
directions. One is to introduce sensing elements onto the surface of electrospun
[
16]
ultrathin fibers. The sensors made by this method have higher sensitivity but are
easily affected by the external environment. The other strategy is to fabricate
nanofiber sensors directly by adding sensing elements into the spinning solution, but
[
17]
the sensitivity needs to be improved further.
However, increasing demands for
ultrasensitive sensors for a variety of analytes has led to an intense interest in ultrathin
fibers. Developing a new approach toward ultrasensitive sensors remains a major
challenge in the field. It is worth noting that a porous nanostructured fiber can be
[
7]
generated directly by controlling phase separation during electrospinning. The
enhanced surface to volume ratio of the porous nanofibers is expected to improve the
sensitivity of these sensors.
Ammonia (NH ) is widely employed in industrial and agricultural systems such as
3
[
18]
refrigeration, stock farming, fertilizer production, and food processing. In addition
to its broad applications, ammonia is also toxic, irritating, and corrosive to human
[
19]
skin, eyes, and respiratory system. Therefore, efficient methods for its detection is
in high demand. Conventional detection methods for ammonia are based on
[
20]
[21]
chromatography-mass spectrometry (GC-MS),
electrochemical systems,
and
[
22]
spectrophotometers.
In this process, large stationary instruments and complex
components are required, which limit their use in resource limited fields. Recently,
fluorescent sensors for ammonia and other amines have shown great promise with
[
23]
their low cost, easy use, and high sensitivity. Nevertheless, most organic dyes are
3

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colored, and the slight color variations are not easily perceived by visual inspection,
especially under low concentrations of ammonia. The “turn on” fluorescence of these
sensors also needs ultraviolet (UV) light excitation, which is inconvenient for some
practical applications. Moreover, the preparation of the organic sensors requires many
complex synthetic steps, and these compounds have limited stability. Substrate
material fabricated by electrospinning provides stable support for dyes and enhanced
[24]
sensitivity, which are essential for industrial and agricultural applications.
Therefore, the development ammonia sensing material that is compatible with naked
eye detection, practical, sensitive, and easy to be prepared would be of great
significance.
Herein, we prepared a cyanine dye (D1) encapsulated porous polylactic acid
(PLA) fibrous mat (PLA-Cy) by electrospinning. The PLA-Cy fibrous mat undergoes
a naked eye detectable color change when exposed to ammonia. With its porous
surface, the mat performed as a highly sensitive and selective ammonia sensor.
Interestingly, the color of dye in the PLA matrix didn’t emerge until exposure to
ammonia. This color changing mechanism was investigated by optical spectroscopy
and scanning electron microscope (SEM). Aggregated D1 covered by PLA fiber was
white. When treated with ammonia vapor, D1 ionized gradually and deaggregated,
which led to the emergence of its intrinsic blue color. This work describes a flexible
sensor for naked eye ammonia detection.
2
2
. Results and Discussion
.1 Preparation of PLA-Cy mat
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D1 with four carboxylic acid groups (Scheme 1) was synthesized according to our
[
25]
previous report.
The carboxylic acid groups afforded strong polarity, stable
[
23c]
intermolecular hydrogen bonds,
and potential reaction sites for alkaline reagents.
The functionalized dye was encapsulated into PLA, and then PLA-Cy mat was
fabricated via electrospinning (Figure 1a). By controlling the humidity, the porous
PLA-Cy ultrathin fibers were obtained as shown in Figure 1b and c. The mat is white
and soft in appearance. Its microstructure features a regular pore structure distributed
on the surface of the PLA-Cy fibers. The average pore size was in the order of 50 nm
in width and 200 nm in length with the long axis oriented along the fiber axis. The
results suggested that the rapid phase separation to form pores and the elongation
[7a]
process were carried out at the same time during the electrospinning process. Thus,
the surface area was greater compared to fibers with a smooth surface, which
increased the sensitivity of sensors.
In order to understand the chemical composition of the PLA-Cy fibrous mat,
X-ray photoelectron spectroscopy (XPS) measurements were performed. The N1s
peak at 398.39 eV appeared (Figure S4), indicating the blending of the cyanine dye.
To investigate the different surface effects of the fibers, porous and non-porous PLA
fibers were further fabricated as controls. With nanoscale pores, the contact angle of
the PLA fibrous mat increased from 117° to 130°, indicating the enhancement of
[
26]
hydrophobicity. As shown in Figure S5a, nitrogen adsorption-desorption isotherms
of the porous PLA fibrous mat exhibited a hysteresis loop when the relative pressure
[27]
P/P was higher than 0.8, implying the presence of mesopores and macropores. The
0
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pore size distribution also confirmed that the pore size was bewteen 100 nm to 200
nm (Figure S5b). Notably, the Brunner-Emmet-Teller (BET) surface area showed an
2
obvious increase from 3.67 to 7.69 m /g as the pores exist (Table S1). Thus, with an
enhanced surface to volume ratio, the porous PLA fibrous mat may work as a
potential material for gas sensing with improved sensitivity.
2
.2 PLA-Cy fibrous mat for ammonia vapor detection
First, the ammonia response of the PLA-Cy fibrous mat was investigated by the
headspace method. As shown in Figure 2, the mat exhibited an obvious color change
from white to blue after exposure to ammonia vapor. This color change could be
observed by naked eye directly. Also, the optical spectrum change of the PLA-Cy
fibrous mat is consistent with the color difference, and both the absorption and
fluorescence intensities of the mat increased rapidly. The maximum absorption and
fluorescence peaks of the PLA-Cy fibrous mat (absorption peak at 647 nm and
fluorescence peak at 670 nm (Figure S6 and S7)) appeared due to ionization process
of the cyanine dye in the mat matrix when treated with ammonia vapor. The
responding process was time dependent. PLA-Cy fibrous mat responded within 1 min
under high concentrations (ca. 2500 ppm, 25 °C, 71.5% RH) of ammonia. The color
of the mat turned light blue and was stable after 3 min of contact with ammonia vapor.
The absorption peak maxima increased gradually and reached the saturation point in 5
to 10 min, indicating that the dye was fully saturated. Interestingly, the absorption
maximum displayed a blue shift of 48 nm from 695 nm to 647 nm, and this was
[
28]
probably caused by the disaggregation of the cyanine dye.
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The ammonia response behavior of the PLA-Cy fibrous mat under low
concentrations of ammonia (below 150 ppm, 25 °C, 70 % RH) was also investigated.
The absorption signal exhibited an increase at concentrations as low as 13 ppm when
the mat was treated with dilute ammonia vapor for 3 min. With increasing ammonia
concentration, the absorption of the mat increased and a blue shift also appeared
(Figure 3), which was in accordance with observations made with high ammonia
concentrations. Optical images showed the color changes of PLA-Cy fibrous mat
from white to light blue below 130 ppm ammonia (Figure S8).The calculated
detection limit of the PLA-Cy fibrous mat is 3.3 ppm according to 3δ/S, where δ is
the standard deviation of the blank signal for six repeat measurements and S is the
slope of the calibration curve (Figure S9). Under 50 ppm of ammonia gas, the
[
18]
maximum allowed indoor concentration , the PLA-Cy fibrous mat displayed an
easily recognizable color change and thus could serve as a naked eye available sensor
for industrial ammonia detection.
As shown in Figure 4, the PLA-Cy fibrous mat exhibited outstanding selectivity
to ammonia vapor as expected. The mat also had a weak response to other volatile
organic amines, such as ethylenediamine, 2-aminoethanol, ethylamine and
triethylamine, whereas it did not respond to an acidic gas, hydrochloric acid, or
neutral organic vapors, such as ethanol or acetone. Compared with volatile organic
amines, ammonia is more volatile and easily to hydrate, which may contribute to a
high detection selectivity. This selectivity of the PLA-Cy fibrous mat to ammonia
made it a valuable candidate for detecting ammonia in complex mixtures.
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2
.3 Response mechanism of PLA-Cy fibrous mat
The alkaline vapor response of the PLA-Cy fibrous mat was attributed to the
carboxylic acids on the cyanine molecule. To study the response mechanism, an
unsubstituted cyanine dye D2 was synthesized and subsequently introduced to the
PLA mat (PLA-CyH) prepared by electrospinning. Surprisingly, at the same molar
concentration of dye as compared with the PLA-Cy fibrous mat, the PLA-CyH
fibrous mat was sky blue. When the PLA-CyH fibrous mat was treated with ammonia
vapor, no significant change was observed in both the apparent color and optical
properties over the course of 10 min (Figure S10, S11 and S12). Therefore, the
carboxylic groups of D1 must have played a key role in the chromogenic process. The
reaction of carboxylic groups with ammonia contributed to the “light-up” of the dye.
In addition, the intermolecular hydrogen bonds between the carboxylic acid groups
led to the formation of cyanine aggregates, which might influence the homogenization
process of the dye during electrospinning.
First, the response mechanism was investigated with cyanine dissolved in DMF.
As shown in Figure 5, the absorbance of D1 increased gradually upon addition of an
aqueous ammonia solution (the initial sudden absorption decline was caused by
addition of water, Figure S13). The absorption increased as D1 ionized upon reaction
4
-
+
with NH (Figure 6). Formation of ammonium salts (D1 ·4NH ) disrupted
3
4
intermolecular hydrogen bonds resulting in disaggregating of D1. In addition,
4
-
+
D1 ·4NH exhibited better solubility in highly polar solvents such as DMF and
4
water. An insignificant blue shift of the absorption maximum further suggested the
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dispersion of D1 molecules. By contrast, the absorption spectrum of D2 in DMF
showed no response upon adding ammonia, which indicated that D2 and NH did not
3
interact in the same way (Figure S14). Ionization of D1 with alkali was further studied.
Upon the addition of NaOH, similar absorption changes were observed, supporting
the proposed mechanism for NH detection (Scheme S2 and Figure S15). As a result,
3
D1 is responsive to different alkaline compounds including but not limited to organic
amines. However, the PLA-Cy fibrous mat responded best to ammonia, which is
highly volatile and hydratable.
Based on the ionization of the dye, we further investigated the color changing
mechanism of the PLA-Cy fibrous mat. Figure 6 illustrates the possible response
[
29]
process. Aggregated D1 was first encapsulated in PLA.
The material was dried
during electrospinning, forming dye encapsulated PLA fibers which was white. When
the mat was treated with ammonia vapor, the ionized dye partially deaggregated and
dissolved, leading to the color change from white to blue. In contrast, D2 dispersed
homogenously in both the solution and the fiber (Figure S16), and no response to
ammonia was observed. Therefore, the PLA-CyH fibrous mat was blue both before
and after treatment with ammonia vapor.
To furthe support this hypothesis, the micromorphology of D1 in solid state was
determined by SEM. Aggregated particles with sizes up to 300 nm formed in the
-5
DMF solution of D1 (1×10 M) due to hydrogen bonding between carboxylic acids
[
23c]
and π-π stacking between the D1 backbones (Figure 7a).
PLA without the addition
of dyes would have formed a film-like morphology instead of the nanocapsule
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morphology (Figure 7b). When the DMF solution of D1 was added dropwise to the
solution of PLA in dichloromethane, the nanocapsule structure was formed (Figure
c). D1 aggregated heavily in dichloromethane and was encapsulated within the
7
hydrophobic cavity of the PLA coil. Absorption spectrum also comfirmed the
aggregate state of D1 in PLA mitrax. As shown in Figure S17, the absorption peak of
D1 in solid state was broad and red-shifted (similar to the case in Figure 2), indicating
D1 was highly aggregated in PLA matrix. While in DMF solution, D1 showed a sharp
peak at 650 nm, which indicated good dispersity of the dye. In addition, D1 with PLA
showed a similar absorption curve compared with D1 in solution, indicating there
were no interaction between D1 and PLA. Upon aggregation-deaggregation of the dye,
the PLA-Cy fibrous mat underwent an obvious color change from white to blue. In
general, the carboxylic acid groups play
a
crucial role in the
aggregation-deaggregation process. By taking advantage of the carboxylic acid
mediated aggregation, we were ultimately able to devise a feasible and effective
material for ammonia sensing.
Humidity is known as a unfavorable factor which effects on the sensitivty of
[
30]
ammonia sensors (especially electrochemical sensors).
According to the ionizing
and deaggregation mechanism of D1, the PLA-Cy mat is likely to perform a more
obvious color change under higher RH level. Then we tested the responding property
of the mat in dry ammonia (0% RH) and 70% RH level (Figure S18). The mat showed
no response when exposed in dry ammonia, which indicated water molecules were
1
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required during the ionizing and deaggregation process of D1. Thus, PLA-Cy fibrous
mat would exhibit better sensing property under moisture circumstances.
Based on a carboxylic acid functionalized cyanine, different approaches were
[
23c]
employed for ammonia sensing in this and previous works.
D1 could be embedded
into different materials. In addition to the D1 encapsulated mat, D1 in solution and in
solid state could also be used for ammonia sensing. The characteristics of each
material are listed below for comparison (Table S2). Each method has its unique
characteristics, and the PLA-Cy fibrous mat stands out in terms of its preparation and
application. With enhanced sensitivity and practicality, the PLA-Cy fibrous mat could
serve as a naked eye sensor for industrial and agricultural ammonia detection.
3
. Conclusion
In summary, we reported a feasible way to prepare a cyanine dye (D1 or D2)
encapsulated PLA mat by electrospinning. The D1 encapsulated PLA-Cy fibrous mat
performed as a naked eye ammonia sensor with high sensitivity and selectivity. The
color changed upon exposure to ammonia due to an aggregation-deaggregation
process mediated by the four carboxylic acids in D1. Aggregated D1 covered by PLA
fibers was white colored. When treated with ammonia vapor, D1 ionized gradually
and become deaggregated, leading to the emergence of its intrinsic blue color. Using
D2 as contrast, the ionization of D1 in the presence of ammonia was verified by
optical analysis. SEM revealed the formation of nanocapsule complexed of D1 and
PLA. With porous and hydrophobic surfaces, the PLA-Cy fibrous mat exhibited
enhanced sensitivity and specific gas responses with good practicality. This work
1
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provides a facile method for the preparation of a naked eye ammonia sensor and a
new strategy for organic dye sensing based on an aggregation-deaggregation process.
4
. Experimental Section
Materials and Methods: 4-Hydrazinobenzoic acid hydrochlorid (97%),
Hydrazinobenzoic acid hydrochlorid (97%), 4-bromophenylhydrazine hydrochloride
(97%), 3-methyl-2-butanone (98%), 4-bromomethylbenzoic acid (97%), and
3
,4-dihydroxycyclobut-3-ene-1,2-dione (98%) were purchased from Alfa Aesar.
Polylactide (PLA) was provided by Jinan Daigang Biomaterials Co., Ltd, the
weight-average molecule weight (Mw) was 100,000. Dimethylformamide (DMF),
dichloromethane (DCM), sodium hydroxide (NaOH), calcium oxide (CaO), chlorine
hydride (HCl), ethylenediamine, 2-aminoethanol, ethylamine, triethylamine ethanol
and acetone were supplied by Beijing Chemical Co., Ltd. (Beijing, China). All
reagents were of analytical grade and directly used as received without further
purification.
1
13
1
H and C NMR spectra were recorded on a Bruker 400 (400 MHz H; 100 MHz
1
3
C) spectrometer at room temperature. Mass spectra (MS) were measured with a
XEVO-G2QTOF (ESI, Waters, USA). UV-visible spectra were obtained on a
spectrometer (UV 2600, Shimadzu, Japan). Fluorescence spectroscopic studies were
performed on a fluorescence spectrophotometer (Horiba Jobin Yvon FluoroMax-4
NIR, NJ, USA). All the optical measurements of PLA mats were performed in
reflection mode. Morphologies fibrous mat and D1 aggregations were observed with a
1
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Hitachi S-4700 scanning electron microscope (SEM). Relative humidity (RH) levels
were recorded by humidity meter (DT-625, CEM, China).
Synthesis of cyanine dyes: D1 and D2 were synthsized by three steps processes
[
25]
according to the previous literatures.
information.
The details were given in the supporting
Fabrication of porous PLA-Cy (H) fibrous mats: Spinning solution was prepared by
dissolving the polylactide in dichloromethane and dimethylformamide at room
temperature with continuous stirring until the polylactide solution became transparent
and homogenous. Then the dye was added to spinning solution with continuous
stirring. The ratio of dichloromethane/dimethylformamide (v/v) is 9/1. The
polylactide concentration was fixed at 6%, and the dye concentration was 0.03%. The
desired PLA solution was loaded into syringe with a settled vertically spinneret (D =
0
.7 mm). The PLA fibers were collected on the aluminium foil at a high voltage
ranging from 15 to 20 kV at relative air humidity of 40-50%. The solution feed rate
was 0.5 mL/h, and the distance between the collector and the spinneret was 18 cm.
o
The resulting membranes were dried in vacuum oven at 50 C till constant weight.
Fabrication of non-porous PLA fibrous mats:
Spinning solution was prepared by dissolving the polylactide in dichloromethane and
dimethylformamide at room temperature with continuous stirring until the polylactide
solution
became
transparent
and
homogenous.
The
ratio
of
dichloromethane/dimethylformamide (v/v) is 9/1. The polylactide concentration was
fixed at 6%. The desired PLA solution was loaded into syringe with a settled vertically
1
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10.1002/asia.201600818
spinneret (D = 0.7 mm). The non-porous PLA fibers were collected on the aluminium
foil at a high voltage ranging from 15 to 20 kV at relative air humidity of 20%. The
solution feed rate was 0.5 mL/h, and the distance between the collector and the
o
spinneret was 18 cm. The resulting membranes were dried in vacuum oven at 50 C
till constant weight.
Ammonia sensing experiments: The sensing system was conducted using a headspace
method as shown in Figure S19. The container was airtight with mat samples sticked
inside. Ammonia vapor concentration was controlled by gas-liquid equilibrium of
[
31]
ammonia solution in the container according to Henry’s law.
All the sensing
experiments were conducted in 25 °C. The RH level of each measurement was kept
same (around 70%). This method was also used to evaluate the selectivity of PLA-Cy
mat towards various volatile solutions including ethylenediamine, 2-aminoethanol,
ethylamine and trimethylamine, hydrochloric acid, ethanol and acetone. Dry ammonia
(0% RH) was obtained by adding stronger ammonia water into NaOH followed by
drying with CaO.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the
author.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of
China (21574009 and 51521062), the Beijing Natural Science Foundation (2142026),
Beijing collaborative innovative research center for cardiovascular diseases, and the
Innovation and Promotion Project of Beijing University of Chemical Technology.
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Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
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Scheme 1. The structures of squarylium cyanine D1 and D2.
Figure 1. (a) Schematic of the electrospinning process. (b) Photograph of the
PLA-Cy fibrous mat. (c) SEM images of the electrospun porous PLA-Cy fibers. The
inset in (c) is the amplified image of selected fiber.
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Figure 2. Time-dependent UV-vis absorption spectra and apparent color in natural
light of PLA-Cy fibrous mat under NH3.
Figure 3. Absorption spectra of PLA-Cy fibrous mat under low concentrations of
ammonia vapor.
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Figure 4. Selective response of PLA-Cy fibrous mat to ammonia vapor.
Figure 5. UV-vis absorption spectra of D1 in DMF solution with ammonia addition.
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Figure 6. Illustration for color changing mechanism of the PLA-Cy fibrous mat.
Figure 7. SEM images of (a) D1 only, (b) PLA only and (c) PLA encapsulated D1
complex.
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The table of contents entry
Polylactic acid porous fibrous mat with cyanine dye blended is used for ammonia
sensing. When exposed to ammonia vapor, the aggregated dye deaggregated due to
ionization of the four carboxylic acid groups. Correspondingly, the fibous mat
undergoes a color change from white to blue as a naked eye ammonia sensor.
Keyword
Naked eye ammonia sensor
Chendong Ji, Lijing Ma, Meizhen Yin*, Wantai Yang, Kai Pan*
Title
A Cyanine Dye Encapsulated Porous Fibrous Mat for Naked Eye Ammonia Sensing
ToC figure
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