Development of a novel colorimetric thermometer based on poly(N-vinylcaprolactam) with push-π-pull tricyanofuran hydrazone anion dye

Article abstract of DOI:10.1039/d1nj00221j

Thermochromic poly(N-vinylcaprolactam-co-tricyanofuran hydrazone) [poly(VC-co-TCFH)] gel labeled with a halochromic chromophore was developed using traditional free radical polymerization. A novel vinyl-containing tricyanofuran hydrazone (TCFH) halochromi

Full text of DOI:10.1039/d1nj00221j

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Development of a novel colorimetric
thermometer based on poly(N-vinylcaprolactam)
with push–p–pull tricyanofuran hydrazone
anion dye
Cite this: New J. Chem., 2021,
45, 5382
Meram S. Abdelrahman,*a Tawfik A. Khattab *a and Samir Kamelb
Thermochromic poly(N-vinylcaprolactam-co-tricyanofuran hydrazone) [poly(VC-co-TCFH)] gel labeled
with a halochromic chromophore was developed using traditional free radical polymerization. A novel
vinyl-containing tricyanofuran hydrazone (TCFH) halochromic chromophore bearing a hydrazone sensor
group was generated via an azo-coupling procedure of a tricyanofyran (TCF) heterocycle with the
diazonium chloride salt of 2-hydroxy-4-nitroaniline comprising an allyl group. The molecular structures
of the TCFH chromophore and related intermediates were proved by different techniques, including
FT-IR, mass spectroscopy (MS), 1H/13C NMR and elemental analyses (C, H, N). This chromophore
showed a pronounced solvatochromic activity in various organic solvents of different polarities. pH sensing
activity was also monitored at different pH values. Both solvatochromism and pH-responsiveness can be
ascribed to the delocalization of charge on TCFH. Poly(VC-co-TCFH) acted as a thermochromic sensor
device generating immediate colorimetric changes from yellow to orange, red, wine, violet, and purple with
increasing temperature of aqueous poly(VC-co-TCFH) solution. Such colorimetric changes are
proportionally interrelated with the temperature increase from 33 to 47 1C due to a phase change in an
aqueous solution of poly(VC-co-TCFH) copolymer. The poly(VC-co-TCFH) copolymer composed of
N-vinylcaprolactam (VC) and push–p–pull hydrazone type tricyanofuran chromophore monomers functions
as a temperature-driven chromogenic thermometer. The morphology of the gel nanostructure was
monitored by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Mechanistic
studies accounting for solvatochromism and halochromism of TCFH as well as thermochromism of
poly(VC-co-TCFH) are proposed.
Received 14th January 2021,
Accepted 12th February 2021
DOI: 10.1039/d1nj00221j
rsc.li/njc
influenced by the type and location of substituents.5–7
Tricyanofuran-based dyes with donor–p–acceptor molecular
1. Introduction
The development of colorimetric sensors as environment moni- systems have attracted more interest owing to their non-
toring early warning tools is highly interesting owing to linear optical characteristics. They have demonstrated a high
their simple and fast naked-eye detection without the necessity sensitivity to environmental changes, such as pH of the
for trained personnel, or any expensive or sophisticated medium and solvent polarity.8,9 They have been synthesized
instruments.1–3 Therefore, colorimetric sensors have recently as electro- and photoluminescent compounds in different
attracted more interest in the monitoring of various hazardous fields, such as fluorescent and colorimetric sensors, electro-
materials.4 The simple synthesis of arylhydrazone dyes is an optics, single molecule fluorophores, photo-refraction, logic
effective process due to their versatile chromogenic properties. memories, dye-based lasers and organic light-emitting
Hydrazone dyes are of high significance owing to their physico- devices.10,11 Tricyanofuran hydrazone dyes have significant
chemical properties and potential applications, such as in characteristics, including photo- and thermal stability and
sensors, nonlinear optics, medical purposes. This can be comparatively high yield, as well as their capability to provide
ascribed to their unique molecular structure which is highly a wide range of colors depending on the substituents located on
the arylhydrazone unit. They have been applied for use as
solvatochromic, biochromic, metallochromic, vapochromic,
thermochromic and halochromic sensors. Their solvato-
chromic activity is valuable as potential polarity sensors for
molecular electronics, solvents, in the fabrication of solutions
a Dyeing, Printing and Auxiliaries Department, National Research Centre,
Cairo 12622, Egypt. E-mail: ms.abdel-rahman@nrc.sci.eg, ta.khattab@nrc.sci.eg
b Chemical Industries Research Division, National Research Centre, Cairo 12622,
Egypt
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that absorb light at a certain frequency, and to investigate the characterized by its simple and fast naked-eye monitoring, as well
conformations and binding of proteins.12–15 Tricyanofuran as high sensitivity without the necessity for any complicated electric
hydrazone dyes have been applied as environmental chemo- equipment or electronic parts. The tricyanofuran-based arylhydra-
sensors for bacterial strains as well as for pH and temperature zone was easily synthesized via an azo-coupling process of tricya-
changes.16,20 The cytotoxicity of a tricyanofuran hydrazone nofuran with the diazonium chloride of 2-hydroxy-4-nitroaniline
derivative was evaluated recently at high concentrations to comprising an allyl group. The prepared chromophore exhibited
demonstrate a very low cytotoxicity to human skin.16,20 The solvatochromic and halochromic properties, which were explored
tricyanofuran hydrazones’ performance is attributed to their by UV-Vis absorbance spectra. Mechanisms of solvatochromism
push–p–pull intramolecular charge transfer character. Upon and halochromism of TCFH as well as thermochromism of
deprotonation, the hydrazone recognition moiety functions as poly(VC-co-TCFH) were proposed based on structural differences.
an electron p-bridge connecting electron-push and electron-
pull units or it may function itself as an electron-push unit
2. Experimental
2.1. Materials and reagents
when existing in a conjugated structure with strong electron-
withdrawing substituent(s).16–21 The 4-nitro-substituted phe-
nylhydrazone has an acidic NH group with the ability to
Materials utilized in the current study were obtained from
produce a hydrazone nitrogen anion, after proton abstraction,
commercial sources and were applied with no additional
associated with improved electron donation ability. Conjuga-
purification. The tricyanofuran intermediate was synthesized
tion of the generated hydrazone-anion with a highly electron-
using a previous reported method.6,9 Solvents (spectroscopic
pull moiety should result in potentially attractive vapochromic,
grade) were obtained from Aldrich. Experimental results were
metallochromic, solvatochromic, thermochromic, solvato-
collected under ambient conditions if not stated otherwise.
fluorochromic, halochromic and biochromic dyes.16–21
Temperature is the most commonly employed physical
stimulus in environmentally thermoresponsive polymers. The
2.2. Apparatus and methods
Melting points and thermal properties were measured using a
development of a colorimetric-based thermochromic sensor
TA-2920 differential scan calorimeter (DSC). Elemental analysis
device or chromogenic thermometer has been a desired pro-
perty to monitor temperature changes.22,23 One of the distinc-
(C, H, N) was investigated by a PerkinElmer2400 (Norwalk,
United States). 1H/13
C NMR spectra were studied on a
tive characteristics of thermochromic polymers is the existence
of a critical solution temperature demonstrating a phase
change from the soluble to the insoluble status. Poly
BRUKER-AVANCE at 400 MHz under ambient conditions; sig-
nal shifting was presented in ppm units comparative to tetra-
methylsilane. Fourier-transform infrared (FT-IR) spectra were
determined using a Bruker Vectra 33. Mass spectra were
reported by Shimadzu GC–MS-QP 1000EX at 70 eV. UV-Vis
absorption spectra were determined on a HP-8453 spectro-
photometer. Emission spectra were studied by Varian Cary
Eclipse. The morphological properties were studied by scan-
ning electron microscopy (SEM) on a Hitachi-S2600N at 20 kV.
The gel fabricated from poly(VC-co-TCFH) in aqueous solution
was poured onto carbon tape placed on an aluminum stub, and
then left to air-dry under ambient conditions. The air-dried gel
was annealed for 12 h at 45 1C, and then coated with gold
(10 nm). Transmission electron microscopy (TEM) was utilized
to examine the gel fibrous morphology using JEM-1200EX at 80
kV. The gel fabricated from poly(VC-co-TCFH) in aqueous
solution was poured onto a copper grid (200 mesh), and then
left for 12 h to air-dry under ambient conditions.
(N-substituted-acrylamide) represents
a group of thermo-
chromic polymers with a lower critical solution temperature.
Numerous acrylamide-based polymer systems, such as
poly(acrylamide) and poly(N-isopropylacrylamide), are respon-
sive to the surrounding environmental changes.24–27 In pre-
viously reported research, we described the thermochromic
activity of a novel donor-p-conjugate-electron-acceptor mole-
cular tricyanofuran hydrazone chromophore. Tricyanofuran
hydrazone dyes have been intensively developed for various
purposes, such as sensors and antimicrobial agents as well as
thermochromic, halochromic, biochromic and vapochromic
sensing applications.16 Poly(N-vinylcaprolactam) gels have
recently attracted more interest owing to their unique proper-
ties, including water solubility, and being nonionic, thermally
sensitive, and biocompatible.28–32
In this context, we report on the fabrication of a simple-
structured thermochromic polymer-based colorimetric thermo-
meter based on a poly(N-vinylcaprolactam) as a thermoresponsive
polymer and a tricyanofuran hydrazone as a halochromic
2.3. Synthetic methods
2.3.1. Synthesis of tricyanofuran. In a 500 mL round bot-
chromophore. We synthesized and examined the photo- tom flask positioned in a water bath, Na metal (600 mg,
physical, solavtochromic and halochromic properties of a novel 16 mmol) was added into absolute ethanol (20 mL) under
tricyanofuran hydrazone chromophore containing an allyl moiety. ambient conditions. Both dicyanomethane (malononitrile;
The synthesized tricyanofuran hydrazone dye was applied in a co- 24 g, 362 mmol) and 3-hydroxy-3-methylbutan-2-one (18.0 g,
polymerization process with N-vinylcaprolactam. The polarizability 176 mmol) were then added into the as-prepared CH3ONa
of the tricyanofuran hydrazone molecular system was improved by solution with stirring for 1 h. An additional amount of absolute
the insertion of a strong electron withdrawing nitro group onto ethyl alcohol (60 mL) was added. The admixture was subjected
the phenylhydrazone moiety. This colorimetric thermometer is to refluxing for 1 h and then subjected to cooling in a fridge,
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and the generated precipitate was separated by vacuum filtra- (40 mL) and re-crystallization from chloroform/ethyl alcohol
tion, washed using cold ethanol (20 mL), and finally subjected to give an orange-red powder (yield 86%). Melting point (DSC)
to air-drying to provide an off-white powder (17.05 g). The at 221 1C; 1HNMR (400 MHz, DMSO-d6): 12.14 (s, 1H), 8.52
produced filtrate was concentrated under vacuum to afford (s, 1H), 7.96 (dd, 1H), 7.88 (d, 1H), 7.54 (d, 1H), 6.11 (m, 1H),
extra off-white powder (2.01 g); total yield was 57%; mp 201 1C– 5.49 (dd, 1H), 5.36 (d, 1H), 4.91 (d, 2H), 1.82 (s, 6H); 13C NMR
203 1C. 1HNMR (400 MHz, CDCl3): 2.35 (s, 3H), 1.65 (s, 6H).
2.3.2. Synthesis of 2-allyloxy-4-nitroaniline. 3-Iodo-1- 111, 110, 107, 100, 98, 70, 57, 31, 28, 26, 23; IR (neat, n cmÀ1
(400 MHz, DMSO-d6): 177, 172, 147, 144, 137, 129, 119, 112,
)
propene (12 mmol) and K2CO3 (2 g) were added to the com- 3321 (NH), 2936 (CH aromatic), 2845 (CH aliphatic), 2219 (CN).
mercially available 2-amino-5-nitrophenol (12 mmol) dissolved
in DMF (10 mL) followed by reflux for 4 h. The admixture was
then left to cool to ambient conditions followed by adding an
2.4. Preparation of poly(VC-co-TCFH) co-polymer
aqueous solution of Na2CO3. The generated precipitate was A free radical polymerization reaction was carried out between
then isolated from the aqueous phase by extraction using the commercially available N-vinylcaprolactam and the as-
CH2CL2. The extract was concentrated under vacuum utilizing prepared allyloxy-substituted tricyanofuran hydrazone chromo-
a rotary evaporating system to provide a yellow solid (yield phore in the presence of an initiator to afford the corresponding
83%). Melting point at 61–63 1C; 1HNMR (400 MHz, CDCl3): poly(VC-co-TCFH) co-polymer.
7.83 (dd, 1H), 7.67 (d, 1H), 6.67 (d, 1H), 6.09 (m, 1H), 5.45 (dd,
2H), 4.65 (d, 2H), 4.52 (s, 2H, NH2).
2.5. pH management for sensor testing
2.3.3. Synthesis of tricyanofuran hydrazone. In a 100 ml
A methanolic solution of perfluoroacetic acid (CF3COOH; 1 M)
was added to a tricyanofuran hydrazone solution in acetone or
dimethylsulfoxide to decrease the pH value, while the corres-
ponding hydrazone-anion isomer was produced by adding a
methanol solution of tetrabutylammonium hydroxide
((C4H9)4NOH; 1 M) to the tricyanofuran hydrazone solution in
acetone or dimethylsulfoxide. The pH values of the different
solutions were determined with a Beckman-Coulter pH-I340
meter. The images of the vials containing the tricyanofuran
hydrazone probe at different pH values were taken using a
Canon Power Shot A-710-IS digital camera.
Erlenmeyer flask, a mixture of the as-prepared 2-allyloxy-4-
nitroaniline (10 mmol), distilled water (5 ml) and hydrochloric
acid (5 ml) were stirred in a crushed ice-bath at 0–5 1C, followed
by slowly adding sodium nitrite (10 mmol) dissolved in distilled
water. The admixture was subjected to stirring for an extra 30
minutes at 0–5 1C. The produced diazonium chloride cold
solution was poured slowly into an admixture of sodium acetate
(3 g), acetic acid (4 mL) and the as-prepared tricyanofuran
intermediate (10 mmol) dissolved in acetonitrile at 0–5 1C in a
separate 100 ml Erlenmeyer flask. The crude precipitate was
subjected to filtration under vacuum, washing with water
Scheme 1 Preparation of allyloxy-containing TCFH.
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Scheme 2 Knoevenagel reaction to produce the TCF heterocycle.
2.6. Thermochromic measurements
with an additional equivalent of the reactive methyl and/or
methylene-containing adducts followed by ring-closing results
in strongly electron-deficient heterocyclic moieties as shown in
Scheme 2.
The thermochromic behavior of the poly(VC-co-TCFH) co-
polymer in aqueous solution at neutral pH value was monitored
by raising the temperature from 25 to 47 1C while recording the
ultraviolet-visible absorbance spectra. The reversibility of the
poly(VC-co-TCFH) co-polymer in aqueous solution at neutral pH
value was examined by changing the temperature back and
forth between 25 and 47 1C by increasing the temperature to
47 1C and then cooling it back to 25 1C for several cycles while
recording the absorption spectra for each cycle at 25 and 47 1C.
The H-atoms on a C-atom next to the electron-withdrawing
substituents in
a certain molecular system are highly
acidic.29,30 The electron-withdrawing substituents assist in
stabilizing the carbanion generated via proton abstraction from
an activate methyl or methylene group by a base. The synthesis
was simply carried out through an azo-coupling process of TCF
heterocycle with the aryl diazonium chloride in the presence of
a base to give the corresponding less stable azo-containing
intermediate which switches straight to the more stable
hydrazone-containing molecular isomer. The hydrazone NH
in the novel tricyanofuran-hydrazone spectroscopic probe was
proved by 1H NMR displaying a signal at 12.78 ppm; and by IR
3. Results and discussion
3.1. Synthesis of allyloxy-containing TCFH
Straightforward synthesis of allyloxy-substituted TCFH is
shown in Scheme 1. A tricyanofuran intermediate was synthe-
sized according to a previous literature approach. It should be
useful to illustrate the correlated pieces of the current proposed
research and elucidate their relationship with the present
objective for our new synthesis of tricyanofuran-hydrazone
(TCFH) probes. The Knoevenagel process has been applied in
the synthesis of electron-withdrawing moieties, such as the
current strongly electron-deficient tricyanofuran heterocycle
that has been applied in various donor–acceptor molecular
systems. In a wide range of chemical reactions, the active
methyl and methylene-containing compounds such as dicya-
nomethane or a-nitriles bearing carbonyl functional groups can
interact with a base such as sodium ethanolate in order to
undergo an azo-coupling reaction with the diazonium chloride
of aromatic amines or under a condensation reaction with
aldehydes or ketones. A subsequent one-pot reaction procedure
1
represented through a band at 3360 cmÀ1. The H NMR spec-
trum of the hydrazone NQCH and hydroxyl (OH) groups
displayed signals at 7.68 and 9.72 ppm, respectively. The IR
spectrum showed peaks at 2222 and 1568 cmÀ1 ascribed to
cyano (CN) and CQN groups, respectively.
3.2. Solvatochromic properties
The UV-Vis absorbance spectra of novel allyloxy-substituted
TCFH in several organic solvents of different polarities are
displayed in Table 1. The maximum absorption wavelength
demonstrated a red-shift (known as negative solvatochromic
performance) upon increasing the polarities of the solvents.
These results showed a highly allowed p–p* transition asso-
ciated with intramolecular charge-transfer character. The
colors of the tricyanofuran-hydrazone in organic solvents were
monitored between yellow, orange and purple.
An intramolecular charge transfer can be induced from the
hydrazone electron donating group to the tricyanofuran elec-
tron withdrawing heterocycle in various organic solvents.
Highly distinctive solvatochromism was monitored in both
polar and non-polar solvents as compared to dyes. In polar
protic/alcoholic solvents, the novel allyloxy-substituted tricya-
nofuran hydrazone displayed a distinct red-shift owing to
the increased acidity of the protic solvents. In polar aprotic
solvents, the maximum absorption wavelengths showed a
hypsochromic shift to shorter wavelengths. A smaller value in
the maximum absorption wavelengths was also observed for
the non-polar solvents. The nitro-containing probe revealed a
higher acidity of NH on the hydrazone moiety. Thus, the highly
Table 1 UV-Vis and emission spectra of novel allyloxy-substituted tricya-
nofuran hydrazone in some selected solvents
lmax (nm)
Solvent
Absorption
Emission (F)
Ethanol
Acetic acid
DMSO
473
478
542
527
549
488
491
481
467
545 (0.61)
540 (0.57)
568 (0.54)
545 (0.65)
571 (0.38)
530 (0.67)
575 (0.64)
560 (0.65)
528 (0.33)
Acetone
Benzonitrile
Dichloromethane
Chloroform
Tetrahydrofuran
1,4-Dioxane
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Scheme 3 The lone-pair of electrons on the hydrazone NH group demonstrates a partial contribution to the overall resonance hybrid; R = –CH2–
CHQCH2.
Fig. 1 Deprotonation of hydrazone NH to create a hydrazone moiety as an electron-donor and tricyanofuran as an electron acceptor; R = –CH2–
CHQCH2.
electron-withdrawing nitro substituent improves the stability of associated with intramolecular charge transfer. Similarly, a
the partially generated hydrazone anion in organic solvents. bathochromic shift of solvatofluorochromism was observed as
The solvatochromic performance can be illustrated by two demonstrated in Table 1. Upon the deprotonation of the
resonating contribution forms. One structure is quinoid, non- hydrazone NH group, the hydrazone fragment works as an
polarized and not completely aromatic, whereas the other electron donating bridge to the strongly electron acceptor
structure is a zwitterion, completely aromatic and polarized. tricyanofuran moiety generating a donor–acceptor system
The differences in the absorbance wavelengths that occur upon (Fig. 1). The allyloxy-substituted 4-nitrophenylhydrazone has
increasing the solvent polarity can be attributed to the variation the capacity to generate an acidic NH with the ability to
in the degree of resonance contribution of those canonical
structures to the overall resonance hybrid. Therefore, the
electron-withdrawing substituent certainly plays an important
role in promoting solvatochromism (Scheme 3).
3.3. pH sensory effects
As discussed above, the as-prepared TCFH molecular probe is
soluble in a variety of organic solvents. Both absorption and
emission spectra were explored in both polar and non-polar
solvents. Different colors were monitored, including yellow,
orange, red, wine, violet and purple for the prepared allyloxy-
substituted tricyanofuran hydrazone probe in various solvents.
Fig. 2 Photograph displaying temperature-driven reversible sol–gel tran-
A bathochromic shift was observed upon increasing solvent
sition with a colorimetric change from yellow to purple for an aqueous
solution of poly(VC-co-TCFH).
polarity owing to the strongly allowed p–p* transition state
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Scheme 4 Synthesis of poly(VC-co-TCFH).
produce anion and proton species. Thus, characteristic solva- depended on the traditional free radical polymerization. UV-
tochromism, solvatofluorochromism, and pH responsiveness Vis spectra were employed as a simple approach to determine
were observed as reversible processes. The pH switching (from the lower critical solution temperature.
437 to 567 nm) was monitored around neutral pH value
UV-Vis absorbance spectra of the poly(VC-co-TCFH) in an
employing an aqueous solution of tetrabutylammonium hydro- aqueous medium (0.01 g mLÀ1) at different temperature values
xide to increase the pH value and a methanolic solution of are displayed in Fig. 3. Upon heating the aqueous poly(VC-co-
trifluoroacetic acid to decrease the pH value.
TCFH) solution, it becomes turbid accompanied by a color
change. The temperature-responsiveness was totally reversible
without fatigue. This type of temperature-stimulated gela-
tion activity arises from the balance among the generated
H-bonding with water molecules and the intramolecular hydro-
phobic forces.25 The temperature-dependent ultraviolet-visible
absorbance spectra of poly(VC-co-TCFH) in the temperature
range of 25–47 1C are displayed in Fig. 3.
3.4. Thermal stability of tricyanofuran-hydrazone
The thermal behavior of the novel allyloxy-substituted tricya-
nofuran hydrazone chromophore was gauged by DSC and TGA.
The allyloxy-substituted tricyanofuran hydrazone chromophore
contains an electron-donating allyloxy substituent as well as an
electron-withdrawing nitro substituent on the phenylhydrazone
group showing a thermal stability up to 221 1C (decomposition
temperature).
Upon abstracting a proton from the NH group on the
hydrazone moiety due to increasing the temperature,
a
negatively charged hydrazone anion is generated to function
as an electron donating (push) moiety in conjugation with the
electron withdrawing (pull) tricyanofuran moiety.16,20 Thus, the
thermo-responsiveness of TCFH occurs through an equilibrium
sequence among two isomers. The conjugation of the nitro-
substituted hydrazone anion isomer p-system is longer than the
conjugation of the hydrazone isomeric form. Thus, the color
changes from yellow to purple. To inspect the reversibility of
3.5. Preparation and morphology of poly(VC-co-TCFH)
thermometer
An aqueous solution of poly(VC-co-TCFH) acted as a thermo-
chromic hydrogel producing a colorimetric change from yellow
to purple with increasing temperature as shown in Fig. 2. The
synthetic approach of the poly(VC-co-TCFH) copolymer is
shown in Scheme 4. A novel allyl-containing tricyanofuran
hydrazone dye was synthesized. The essential strategy utilized
for the preparation of the poly(VC-co-TCFH) copolymer
Fig. 4 Changes in the absorbance ratio of the poly(VC-co-TCFH) com-
posite at 437 and 567 nm upon changing the temperature between 25 and
47 1C.
Fig. 3 UV-Vis absorbance spectra at different temperature values (25–
47 1C).
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back to room temperature. The gelation activity was proved by
the formation of nanofibrous dried xerogel. To gain visual
images of the microstructured poly(VC-co-TCFH) gel, the mor-
phological properties of the internal microstructures of the
hydrogel were determined by transmission electron microscopy
(TEM) and scanning electron microscopy (SEM) as shown in
Fig. 5. The internal microstructure of the poly(VC-co-TCFH)
hydrogel demonstrated fibrous aggregation with a 3D architec-
ture. This indicates that the self-assembly of the poly(VC-co-
TCFH) hydrogels generated a stable hydrogel upon increasing
the temperature above the lower critical solution temperature
value of poly(VC-co-TCFH). From the SEM and TEM images, a
three dimensional entangled nanofibrous network with a fiber
diameter at B180 nm can be observed.
The formed supramolecular architectures obviously dis-
played a three-dimensional network generated by intertwined
fibrous system. Interestingly, the xerogel was found to be
reversibly responsive to alkaline vapors with an instant color
shift from orange to purple. Thus, those nanofibers may
display a high sensitivity in the recognition of alkaline ana-
lytes owing to their high surface area and large porosity, which
Fig. 5 SEM (top) and TEM (bottom) images of the microstructured
poly(VC-co-TCFH) gel.
this poly(VC-co-TCFH) against temperature changes, the tem- facilitates the convenient diffusion of the gaseous molecular
perature of the aqueous poly(VC-co-TCFH) solution at a neutral entities through the bulk. It is expected that the xerogel film
pH value was changed back and forth by increasing the can be applied to monitor toxic industrial gases, such as
temperature from 25 to 47 1C while recording UV-Vis absorp- ammonia. Table 2 displays a comparison between different
tion. The composite was left to cool back to room temperature polymer-based thermometers previously reported in the
at 25 1C. This procedure was repeated several times while literature and the current thermometer.
A colorimetric
recording the absorption spectra for each cycle. The absorbance naked-eye color change with a higher and consistent tempera-
ratio at the maximum wavelengths, 437 and 567 nm, was ture range was monitored for poly(VC-co-TCFH). An aqueous
reported as depicted in Fig. 4. It is apparent that such compo- solution of poly(VC-co-TCFH) acted as a thermochromic sen-
site has a good stability and a high reversibility without fatigue. sor displaying a wide range of colorimetric changes from
The produced poly(VC-co-TCFH) thermometer demon- yellow to orange, red, wine, violet and purple with increasing
strated a sol–gel reversible process upon heating and cooling temperature between 33 and 47 1C.
Table 2 Comparison of different polymer-based thermometers (inc. = temperature increase)
Temp.
range
Thermometer
Type of thermometer
Ref.
33
Poly(NIPAM-co-RD), composed of rhodamine (RD) and N-
isopropylacrylamide (NIPAM) units
Simple fluorescent aqueous thermometer displaying
selective emission improvement at a certain temperature
range
25–35 1C
(inc.
10 1C)
Poly(NIPAM-co-BODIPY) composed of boradiazaindacene
(BODIPY) and N-isopropylacrylamide (NIPAM) units
Aqueous copolymer displays weak fluorescence at o23 1C; 23–35 1C
34
however, extreme fluorescence improvement was mon-
itored up to 35 1C
(inc.
12 1C)
33–47 1C
(inc.
Hydrogel composed of poly(N-vinylcaprolactam-co-
tricyanofuran hydrazone)
Reversible colorimetric change from yellow to purple
Current
study
14 1C)
25–40 1C
Poly(NIPAM-co-HC) consisting of N-isopropylacrylamide
(NIPAM) and 4-(4-dimethylaminostyryl)pyridine (hemi-
cyanine, HC) units
Poly(VCa-co-fluorophore) composed of N-vinylcaprolactam
(VCa) and 2D–p–A pyran-based fluorophore units
Highly sensitive and reversible fluorescent thermometer
35
36
37
38
displaying weak fluorescence at o25 1C, and fluorescence (inc.
improvement at 425 1C
Temperature-driven fluorescent thermometer displaying
emission color change from green to blue
15 1C)
27–45 1C
(inc.
18 1C)
25–55 1C
(inc.
30 1C)
25–
Leuco dye-loaded silica nanocapsules immobilized onto
polyester fibers
Durable and reversible colorimetric change from dark
blue to light blue
Organogel fiber-embedded polymer film composed of
cyano-substituted oligo(p-phenylenevinylene) moity, naph-
thyl unit, and dodecyloxy chains
Stable and reversible sol–gel fluorescence changing from
yellow-green to yellow-orange
120 1C
(inc.
95 1C)
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4. Conclusions
References
A novel vinyl-bearing tricyanofuran hydrazone halochromic
probe was synthesized via a simple azo-coupling reaction of a
tricyanofuran heterocyclic moiety with the diazonium chloride
salt of 2-hydroxy-4-nitroaniline bearing an allyl group. The
produced allyl-containing tricyanofuran hydrazone halochro-
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N-vinylcaprolactam to afford poly(N-vinylcaprolactam-co-tricya-
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supervision, Tawfik A. Khattab and Samir Kamel. All authors have
read and agreed to the published version of the manuscript.
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Acknowledgements
This work was supported by the program of the Science and
Technology Development Fund (STDF), Research Support
Grant (STDF-RSG), and Capacity Building Grants (Grant #
34882).
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