Solid polymer substrates and coated fibers containing 2,4,6-trinitrobenzene motifs as smart labels for the visual detection of biogenic amine vapors

Article abstract of DOI:10.1002/chem.201501365

Attempts to polymerize trinitrobenzene derivatives (TNB) have been fruitless so far. Accordingly, polymers containing TNB have not been exploited in spite of their envisaged potential applications. Here, we describe two ways for preparing polymers with TNB moieties thus overcoming the previously reported polymerization impairments. We also report on the exploitation of the materials, both obtained as tractable transparent films and coated fibers, as smart labels for the visual detection of amine vapors. More precisely, amines in the atmosphere surrounding the sensory materials diffuse into them reacting with the TNB motifs forming highly colored Meisenheimer complexes, giving rise to development of color and to the naked eye sensing phenomenon. This is the case of highly volatile amines, such as trimethylamine, produced in food spoilage, specifically in the deterioration of fish or meat, for which the color development of the smart labels can be used as a visual test for food freshness.

Full text of DOI:10.1002/chem.201501365

DOI: 10.1002/chem.201501365
Communication
&
Amine Sensors
Solid Polymer Substrates and Coated Fibers Containing 2,4,6-
Trinitrobenzene Motifs as Smart Labels for the Visual Detection of
Biogenic Amine Vapors
Jesffls L. Pablos,* Saffll Vallejos, Asunción MuÇoz, María J. Rojo, Felipe Serna, FØlix C. García,
and JosØ M. García*[a]
occurs in food spoilage because bacteria enzymes decarboxyl-
ize amino acids. Samples of BA are spermine, tryptamine, sper-
midine, histamine, tyramine, b-phenylethylamine, cadaverine,
putrescine, ethylenediamine, and trimethylamine. The latter
originates in the reduction of trimethylamine-N-oxide (TMAO)
by spoilage bacteria, such as Pseudomonas.[4,5] Accordingly, we
undertook the preparation of sensory polymers that give rise
to visual color change upon interaction with BA vapors.
Our recent preparation of colorimetric sensory materials for
nitro-explosives based on polymers containing amino sensing
motifs[6] made us think the other way round, the naked eye
recognition of BAs based on polymers that contain trinitroben-
zene subunits (TNB).[7] The sensing and recognition phenom-
ena come from the formation of Meisenheimer complexes.[8] In
this work we prepared a methacrylamide monomer containing
the trinitrophenyl core N-(2,4,6-trinitrophenyl)methacrylamide
(NPhMA), and carried out without success different attempts
of thermally and photochemically initiated radical polymeri-
zation under conventional conditions and using different co-
monomers. The initial fiasco in the polymerization of the mo-
nomer was in agreement with the known radical scavenging
capabilities of TNB units, which prevent polymerization even in
very low ratio in copolymerization mixtures, as initially de-
scribed by Wiley and Behr in the 1950s.[9,10] The phenomenon
was ascribed by Kadoma et al. to the reaction of the nitro
groups with the growing radicals resulting in chain termina-
tion.[11] The presence of TNB-containing monomers also inhibit-
ed cationic and anionic polymerization.
Abstract: Attempts to polymerize trinitrobenzene deriva-
tives (TNB) have been fruitless so far. Accordingly, poly-
mers containing TNB have not been exploited in spite of
their envisaged potential applications. Here, we describe
two ways for preparing polymers with TNB moieties thus
overcoming the previously reported polymerization im-
pairments. We also report on the exploitation of the mate-
rials, both obtained as tractable transparent films and
coated fibers, as smart labels for the visual detection of
amine vapors. More precisely, amines in the atmosphere
surrounding the sensory materials diffuse into them react-
ing with the TNB motifs forming highly colored Meisen-
heimer complexes, giving rise to development of color
and to the naked eye sensing phenomenon. This is the
case of highly volatile amines, such as trimethylamine,
produced in food spoilage, specifically in the deterioration
of fish or meat, for which the color development of the
smart labels can be used as a visual test for food fresh-
ness.
The development of chemical sensors, or chemosensors, with
optical output is a topic of current interest. Envisaged applica-
tions in food, healthcare, household, and homeland security
are fostering the research on this field. This is especially true
with sensors with chromogenic response, that is, in which the
presence of a target chemical induces a color change, so that
the human eye acts as detector.[1] Furthermore, if the sensor is
a polymer, the material can be processed in different and man-
ageable shapes as smart labels, for instance, as films or tex-
tiles.[2] Among target molecules, biogenic amines (BAs) are of
special interest, specifically if they can be detected in the
vapor phase, because their increase in packaged food, espe-
cially fish and meat, is related with the food freshness.[3] BAs
Therefore, our initial efforts were directed to overcome the
above-mentioned difficulties to prepare sensory materials, fol-
lowed by their study in the naked eye or colorimetric recogni-
tion of amines.
We initially tackled the preparation of the materials by firstly
preparing a polymer membrane with benzene units (M1),
coming from N-phenylmethacrylamide (PhMA), followed by its
nitration to render a material containing the TNB sensory
motifs as sensory membrane M1sen (Section S2 in the Sup-
porting Information). More precisely, the nitration was carried
out with the conventional sulfonitric mixture with the peculiar-
ity that the solid was a membrane swelled in this mixture. This
methodology has the advantage of overriding the polymeri-
zation inhibition capability of TNB-containing monomers. How-
ever, the inherent difficulties associated to polymer modifica-
tions, spurred in the solid state, gave us only partial control on
membrane reproducibility.
[a] Dr. J. L. Pablos, Dr. S. Vallejos, Dr. A. MuÇoz, Prof. Dr. M. J. Rojo, Dr. F. Serna,
Prof. Dr. F. C. García, Prof. Dr. J. M. García
Departamento de Química, Facultad de Ciencias, Universidad de Burgos
Plaza de Misael BaÇuelos s/n, 09001 Burgos (Spain)
E-mail: jlpablos@ubu.es
jmiguel@ubu.es
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501365.
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Communication
For this reason we undertook a polymerization study of mo-
nomer mixtures containing NPhMA increasing the polymeri-
zation temperature beyond reasonable when using AIBN as
thermal initiator, which has a half-life of 6 min at 1018C (and
estimated value of 38 s at 1208C).[12] Thus, we carried out what
we call a flash polymerization at 1208C and succeeded in ob-
taining the sensory membrane M2sen (Scheme 1 and Sec-
The procedure followed to prepare M2sen was also used to
obtain the smart textile (Tsen) based on white cotton fabric.
Thus, the fabric was coated with the monomers by immersion,
and after wringing, the coating was polymerized in oxygen
free atmosphere at high temperature (1208C).
Previously to the preparation and characterization of the
sensory materials, the colorimetric sensory behavior toward
biogenic amines of monomer NPhMA was analyzed visually
and by UV/Vis. Pale yellowish solutions of NPhMA in acetone
turned reddish upon adding the amines (Figure 1). The color
Figure 1. Response of materials to the presence of amines (from left to
right: blank, tyramine, tryptamine, spermine, spermidine, cadaverine, putres-
cine, histamine, trimethylamine, phenylethylamine, b-ethylenediamine):
a) acetone solutions of NPhMA (a1: 1.9”10À4 m; a2: 9.4”10À4 m) upon
adding a set or biogenic amines (concentration of each amine, top:
4.8”10À3 m, bottom: 6.3”10À3 m), b) discs cut from M2sen upon dipping for
35 min in water (pH 10, sodium bicarbonate buffer 0.05m) containing differ-
ent amines (0.05m); c) samples cut as squares from Tsen in atmospheres of
different amines (sealed vials of 8.40 cm3 in which 100 mL of a 0.3m water
solution of different amines were dropped at the bottom of the vial (resi-
dent time=45 min, temperature=258C).
Scheme 1. Monomers and chemical structure of membranes M2T (polymeri-
zation temperatures (T)=60, 80, 100, 120 and 1508C) and sensory mem-
brane M2sen (polymerization temperature=1208C).
tion S2 in the Supporting Information). For testing the ap-
proach to this flash polymerization, we carried out polymeri-
zations at conventional temperatures with the mixture of co-
monomers used for preparing M2sen, excluding NPhMA, that
is, 60–808C, and compared the thermal and mechanical prop-
erties of films prepared at temperatures up to 1508C (films
M2T, Scheme 1). Surprisingly, both mechanical and thermal
properties were fairly similar (Section S2 and Table S1 in the
Supporting Information), as well as the water swelling percent-
age (WSP). The properties showed negligible dependence on
the polymerization temperature, and this result is not in agree-
ment with the expected deterioration of the properties accord-
ing to the steady-state radical polymerization theory. The prep-
aration of different batches of M2sen showed good reproduci-
bility of spectroscopic characteristics and properties. The FTIR
spectra showed the signals corresponding to the nitro groups
(Figure S5 in the Supporting Information). On the other hand,
the nitration process of M1 to render M1sen greatly lowered
the WSP, probably because of side reactions that increased the
crosslinking density, for example, by condensation of hydroxyl
end groups to render ether linkages, which is in agreement
with the increment in the char yield at 8008C observed ther-
mogravimetrically. In parallel, the handability of M1sen was
impaired by a brittleness increase.
intensity and hue were dependent both on the NPhMA and
on the amine type and concentration. On the other hand, the
response, as derived from the UV/Vis spectra (Figure S7 in the
Supporting Information), was insensitive to the type of amine,
in terms of spectral patterns, with differences in absorbance
probably because of variations in the nucleophilicity and
number of amino groups per target molecule, that lead to dif-
ferent concentration of Meisenheimer complexes.
However, we have centered our study in the detection of
amine vapors, because these compounds are generated in the
food spoilage and their visual detection is important for the
consumer as well as for other fields related with food safety
and with health and safety at work. More precisely, we have
analyzed the color development of sensory materials upon en-
tering into contact with atmospheres containing TMA
(Figure 2). This amine is at gas phase at room temperature,
and is usually used in water solution. We have mimicked the
spoilage of packaged food in relation with the gradual incre-
ment of amine concentration in the packaging atmosphere, es-
pecially TMA for its high vapor pressure (Table S2 in the Sup-
porting Information). Thus, the in-lab performance of the
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The proof of concept related with the performance of the
labels were carried out with fresh fish (tuna) and meat (beef)
bought in the local market and tray-packaged in facilities of
the Biotechnology and Food Science Department at the Uni-
versity of Burgos (for conditions and pictures see Supporting
Information, Section S3 and S7). Sensory discs cut from M1sen
and M2sen were placed in the tray without physical contact
with the food, and the color evolution along time was fol-
lowed using a reference disc outside the packaging film. The
study was carried out at constant temperature, both under
and without refrigeration, 3 and 258C, respectively (Figure 4).
Figure 2. UV/Vis spectra along time of M2sen in an empty cuvette
(4.40 cm3) upon adding 100 mL of a water solution of TMA 0.3m at the
bottom of the cuvette (temperature=258C). The spectra were taken every
10 min. A blank film of M2sen was used as reference. Inset: picture of
M2sen took before adding the amine and after 140 min of the addition.
Figure 4. Color development of sensory discs cut from M2sen inside refri-
gerated (38C) and non-refrigerated (258C) tray-packaged tuna and beef. The
evolution along days can be seen in Supporting Information, Figure S12 and
S13.
The spoilage of the food was accompanied by the darkening
of the yellow sensory discs to orange or reddish, demonstrat-
ing that the material may encounter practical uses, for exam-
ple, for visually informing the consumer of the freshness. As it
has been pointed out, M2sen has the advantage of precise
control of the polymer structure, though the content of the
sensory monomer is restricted to a low rate in relation with
the other co-monomers. Alternatively, the reaction carried out
in the parent M1 membrane to render M1sen limits the struc-
ture control, but affords a much higher content of TNB units,
increasing the response and at the same time probably the ap-
plicability from a commercial viewpoint. The performance anal-
ysis of sensory material also require of the assessment of their
stability, which can be evaluated from a mechanical, thermal,
and ambient viewpoint. The mechanical and thermal stability
was evaluated by strain–stress studies (universal testing ma-
chine) and thermogravimetic analysis (TGA), respectively (Sup-
porting Information, Section S2.2., Figure S4 and Table S1). Re-
garding the stability of sensory membranes to ambient condi-
tions (light, oxygen, and humidity), the different batches were
prepared and stored between filter papers without special
care, without differences in the sensing performance. More-
over, a disc cut form the recently prepared M2sen was irradiat-
ed with UV and visible light for 50 h in a UV/Vis spectropho-
tometer by successive scans from 350 to 800 nm for 2 days in
air (1 scan per hour). The collected spectra were virtually iden-
tical showing no-degradation of the TNB sensory motifs with
light, even with UV irradiation (Supporting Information, Fig-
ure S6).
Figure 3. Titration of TMA vapors with discs obtained from M2sen and
smart textile squares cut from Tsen. a) and b) picture of discs of M2sen and
squares of Tsen, respectively, after being in contact with an atmosphere con-
taining TMA. The number of moles of aqueous solution of TMA added to
the chamber (vial of 8.4 cm3) are indicated. c) Titration curve obtained by
treatment of the digital picture data (CP1 is the principal component of
a PCA analysis using the red and green values of the RGB parameters that
define the digital color of each disc). The titration curve corresponding to
the textiles is depicted in the Supporting Information, Figure S10.
smart labels, obtained as discs and squares from M2sen films
and Tsen textiles, respectively, with punchers or scissors, were
studied by adding different amounts of 0.3m TMA, from 1”
10À7 to 1”10À4 mol, to nine sealed vials (8.40 cm3) containing
sensory discs at 258C (Figure 3). The pictures taken of the nine
discs or squares allowed for the preparation of the titration
curve by using the digital color data of each disc or square, as
previously reported,[13] or by registering the UV/Vis spectra of
the discs (the UV/Vis spectra of the discs and the derived titra-
tion curve can be found in the Supporting Information, Fig-
ure S9).
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J. M. Barat, I. Escriche, P. Amorós, Chem. Commun. 2006, 2239–2241; H.
Xiaowei, Z. Xiaobo, Z. Jiewen, S. Jiyong, L. Zhihua, S. Tingting, Int. J.
Food Sci. Technol. 2007, 117, 258–269; G. J. Mohr, Sens. Actuators B
2003, 90, 31–36.
In short, trinitrobencene (TNB) motifs have been chemically
incorporated into the polymer structure of transparent films
and coated textiles to give smart labels that respond with
a color change in presence of amines in its environment. The
color change comes from the formation of Meisenheimer com-
plexes between the TNB units and the amines. As TNB are well
known polymerization inhibitors, two different approaches for
the preparing the polymers were developed, by flash polymeri-
zation at unusual high temperature and by chemical modifica-
tion of a parent polymer. Besides demonstrating ability to
detect amines in the gas phase in the laboratory, these sensory
materials have shown their potential in the visual indication of
the spoilage of samples of tray-packaged fresh food, such as
fish and meat.
[2] S. Rochat, T. M. Swager, Angew. Chem. Int. Ed. 2014, 53, 9792–9796;
G. J. Mohr, N. Tirelli, C. Lohse, U. E. Spichiger-Keller, Adv. Mater. 1998, 10,
1353–1357.
[3] N. A. Rakow, A. Sen, M. C. Jancen, J. B. Ponder, K. S. Suslick, Angew.
Chem. Int. Ed. 2005, 44, 4528–4532; Angew. Chem. 2005, 117, 4604–
4608; J. M. Landete, B. de Las Rivas, A. Marcobal, R. MuÇoz, Int. J. Food
Microbiol. 2007, 117, 258–269.
[4] M. R. Adams, M. O. Moss, Food Microbiology, 3rd Ed., RSC Publishing,
Cambrdige, 2008, p. 143.
[5] J. M. Jay, M. J. Loessner, D. A. Golden, Modern Food Microbiology, 7th
Ed., Springer, New York, 2005, p. 116.
[6] J. L. Pablos, M. Trigo-Lopez, F. Serna, F. C. Garcia, J. M. Garcia, RSC Adv.
2014, 4, 25562–25568; J. L. Pablos, M. Trigo-Lopez, F. Serna, F. C. Garcia,
J. M. Garcia, Chem. Commun. 2014, 50, 2484–2487.
[7] J. A. Vinson, J. F. Evans, H. E. Holets, Microchim. Acta 1983, 81, 301–306.
[8] J. Meisenheimer, Justus Liebigs, Ann. Chem. 1902, 323, 205–246; F. Ter-
rier, Chem. Rev. 1982, 82, 777–152; H. Chen, H. W. Chen, G. Cooks, J.
Am. Soc. Mass Spectrom. 2004, 15, 998–1004; Y. Liu, H. H. Wang, J. E. In-
dacochea, M. L. Wang, Sens. Actuators B 2011, 160, 1149–1158; E. Ercag,
A. Uzer, R. Apak, Talanta 2009, 78, 772–780.
[9] R. H. Wiley, L. C. Behr, J. Am. Chem. Soc. 1950, 72, 1822–1824.
[10] C. F. Bjork, W. A. Gey, J. H. Robson, R. W. Van Dolah, J. Am. Chem. Soc.
1953, 75, 1988.
Acknowledgements
We gratefully acknowledge the financial support provided by
the Spanish Ministerio de Economía y Competitividad-Feder
(MAT2014-54137-R) and by the Consejería de Educación -
Junta de Castilla y León (BU232U13).
[11] Y. Kadoma, T. Toida, K. Takeda, K. Uni, Y. Iwakura, Polym. Sci. Polym.
Chem. Ed. 1975, 13, 707–716.
[12] Polymer Handbook, 4th Ed. (Eds.: J. Brandrup, E. H. Immergut, E. A.
Grulke), Wiley, New York, 1999, II/2–69.
Keywords: amine sensors · biogenic amines · chemosensors ·
packaged food · polymers
[13] H. El Kaoutit, P. Estevez, F. C. Garcia, F. Serna, J. M. Garcia, Anal. Methods
2013, 5, 54–58; S. Vallejos, A. MuÇoz, S. Ibeas, F. Serna, F. C. Garcia,
J. M. Garcia, J. Mater. Chem. A 2013, 1, 15435–15441.
[1] J.-F. Lin, J. Kukkola, T. Spinola, D. Raut, A. Samikannu, J.-P. Mikkola, M.
Mohl, G. Toth, W.-F. Su, T. Laurila, K. Kordas, J. Mater. Chem. A 2015, 3,
4687–4694; B. Lee, R. Scopelliti, K. Severin, Chem. Commun. 2011, 47,
9639–9641; B. García-Acosta, M. Comes, J. L. Bricks, M. A. Kudinova,
V. V. Kurdyukov, A. I. Tolmachev, A. B. Descalzo, M. D. Marcos, R. Martí-
nez. MµÇez, A. Moreno, F. Sancenón, J. Soto, L. A. Villaescusa, K. Rurack,
Received: April 8, 2015
Published online on April 27, 2015
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