Two-component organogel for visually detecting nitrite anion

Article abstract of DOI:10.1039/c3tc32158d

Low molecular weight two-component gels were obtained from naphthalene diimide undecanoic acid mixed with several aliphatic and aromatic diamines. The gelation properties, structural characteristics and fluorescence of these two-component gels were studied via scanning electron microscopy, X-ray diffraction and spectroscopy. Specifically, the gel containing diaminoanthraquinone (DAQ) was sensitive to nitrite anions. The absorbance and emission intensity of the gel in the visible range obviously decreased upon addition of an aqueous solution of sodium nitrite. The faded gel collapsed and changed to a sediment after detection and the organic acid could be recycled. Te NO₂^- detection with the gel provides a convenient method of nitrite analysis that is visible to the "naked eye".

Full text of DOI:10.1039/c3tc32158d

Journal of
Materials Chemistry C
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PAPER
Two-component organogel for visually detecting
nitrite anion†
Cite this: J. Mater. Chem. C, 2014, 2,
1854
a
a
b
a
a
*
Qian Xia, Yueyuan Mao, Junchen Wu, Tianmin Shu and Tao Yi
Low molecular weight two-component gels were obtained from naphthalene diimide undecanoic acid
mixed with several aliphatic and aromatic diamines. The gelation properties, structural characteristics and
fluorescence of these two-component gels were studied via scanning electron microscopy, X-ray
diffraction and spectroscopy. Specifically, the gel containing diaminoanthraquinone (DAQ) was sensitive
to nitrite anions. The absorbance and emission intensity of the gel in the visible range obviously
decreased upon addition of an aqueous solution of sodium nitrite. The faded gel collapsed and changed
Received 1st November 2013
Accepted 5th December 2013
DOI: 10.1039/c3tc32158d
ꢀ
to a sediment after detection and the organic acid could be recycled. Te NO2 detection with the gel
www.rsc.org/MaterialsC
provides a convenient method of nitrite analysis that is visible to the “naked eye”.
sensing nitrite using a new kind of two-component low
molecular weight (LMW) organogel containing DAQ, a probe
for visually sensing nitrite in neutral aqueous solutions.
Introduction
Nitrite (NO2ꢀ) is known to be able to form carcinogenic
nitrosamines in the human stomach, from the reaction of
nitrite with secondary amines under acidic conditions.1 In
addition, nitrate can oxidize Fe(II) in oxyhaemoglobin to
Fe(III) and the oxyhaemoglobin turns into methaemoglobin.
This process is associated with infantile meth-
ahemoglobinemia.2 However; sodium nitrite is frequently
used as a colour xative and preservative in meat and sh. If
the nitrite was over added, it would be harmful to human's
health. Therefore, detecting the nitrite content of food and
drinking water is really important and vital. Although
numerous techniques have been developed for nitrite deter-
mination such as colorimetric procedures,3 spectroscopic
determination aer enzymatic reduction,4 and electro-
chemical techniques,5 most of them involve time-consuming
and tedious procedures. In traditional spectroscopic
methods, aromatic ortho-diamines are commonly used as
reactive probes to detect nitrite, which are converted into
Over the past decades, LMW organogels have attracted
considerable interest because of their diverse applications as
smart so materials.8 The solvent immobilized can be either
organic or aqueous. The formation of organogels is facilitated
by highly directional self-assembly through non-covalent
interactions such as p–p stacking, hydrogen bonding, metal–
ion coordination, dipole–dipole interactions9 into various 3D
aggregates such as rods, long bers, strands, tubules, or glob-
ules.10 Those gels have been applied as functional so materials
for sensors,11 electrophoretic and electrically conductive
matrices,12 templates for cell growth or the growth of sol–gel
structures,13 and in many other industrial elds such as
cosmetics, oils, and foods.14 However, most gelators have
complicated structures, such as aminophenol, long hydrophilic
substituent chains, carbohydrates, dendritic substituents, cho-
lesteryl groups and so on.15 The rationalized design and
synthesis of a specically functionalized gelator is still a chal-
lenge. So we hope to nd an easier way to make functional gels
without complex molecule structures and intricate synthesis
processes.
Basit et al. reported a new kind of hydrogel expediently made
from fatty acids and amines.16 Here, by introducing a uores-
cent naphthalimide group,17 we designed and synthesized two
novel fatty acids with emission characteristics (compounds 1
and 2, Scheme 1). When the fatty acids are mixed with various
amines, complexes could be formed that subsequently self-
assembled into supramolecular networks in organic solvents.
Interestingly, the red gel formed with 1 and diaminoan-
thraquinone (DAQ) has a colorimetric and uorescent response
to neutral solutions of sodium nitrite. The organic acid in the
gel is recyclable without producing solid wastes.
ꢀ
benzotriazoles in the presence of NO2 .6 However, the reac-
tion could only take place in acidic conditions. Recently,
Luis's group proved that lms including acrylic acid and 1,2-
diaminoanthraquinone (DAQ) could detect nitrite.7 Herein,
for consideration of environmental protection and limiting
the production of solid wastes, we propose a new way for
aDepartment of Chemistry and Collaborative Innovation Center of Chemistry for
Energy Materials, Fudan University, Shanghai 200433, China. E-mail: yitao@fudan.
edu.cn; Fax: +86 21 5566 4621
bKey Laboratory for Advanced Materials and Institute of Fine Chemicals, East China
University of Science and Technology, Shanghai 200237, China
† Electronic supplementary information (ESI) available: Additional absorption
and emission spectra, IR spectra and LC-mass spectra. See DOI:
10.1039/c3tc32158d
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solvent was removed by evaporation. The water phase was
acidied to pH 2–3 with HCl. The slurry was ltered and the
solid was washed with ether to give 1 as a white powder. Yield
1
ꢁ
83%, m.p. 110–112 C. H NMR (400 MHz, DMSO): d 11.90 (s,
1H), 8.69 (d, J ¼ 8.4 Hz, 1H), 8.57 (d, J ¼ 7.3 Hz, 1H), 8.41 (d, J ¼
8.3 Hz, 1H), 7.95–7.86 (m, 1H), 7.54 (t, J ¼ 7.9 Hz, 2H), 7.39–7.25
(m, 3H), 6.98 (d, J ¼ 8.3 Hz, 1H), 4.08–3.95 (m, 2H), 2.16 (t, J ¼
7.3 Hz, 2H), 1.61 (s, 2H), 1.45 (s, 2H), 1.37–1.13 (m, 12H); 13C
NMR (100 MHz, CDCl3): d 177.87, 159.86, 154.91, 132.86,
131.94, 130.42, 129.73, 128.56, 126.53, 125.57, 123.99, 122.72,
120.77, 116.71, 110.70, 77.34, 77.02, 76.70, 40.45, 33.74, 29.36,
29.27, 29.11, 28.98, 28.13, 27.09, 24.68. MS (EI) calculated for
C
C
29H32NO5: 474.2 [M + H]+, found: 474.4; anal. calcd for
29H31NO5: C, 73.49; H, 6.55; N, 2.96. Found: C, 73.63; H, 6.55;
N, 2.91%.
Synthesis of compound 2a. To a 50 mL anhydrous MeOH
solution of (R)-2-amino-3-phenylpropanoic acid (4.50 g, 27.23
mmol), drops of thionyl chloride (98%) were added slowly
during stirring. The mixture was reuxed for 8 h. Then the
solvent was removed under reduced pressure. The pale powder
was washed with ethyl acetate. This dried powder (2.35 g, 10.9
mmol) was then mixed with 1 (4.3 g, 9.08 mmol), DCC (3.75 g,
18.16 mmol), HOBt (1.23 g, 9.08 mmol) and Et3N (9.18 g, 9.08
mmol) in dry CH2Cl2 (150 mL). The mixture was stirred and
reuxed for 12 h at room temperature in nitrogen atmosphere.
Then the solvent was removed under reduced pressure and the
residue was subjected to column chromatography on silica gel
(chloroform/petroleum ether ¼ 4 : 1 (v/v)) to give 2a as a white
powder. Yield 67%, m.p. 81–83 ꢁC. 1H NMR (400 MHz, CDCl3): d
8.65 (dd, J ¼ 17.6, 7.6 Hz, 2H), 8.43 (d, J ¼ 8.2 Hz, 1H), 7.76 (t, J ¼
7.7 Hz, 1H), 7.47 (t, J ¼ 7.5 Hz, 2H), 7.34–6.94 (m, 8H), 6.88 (d, J
¼ 8.2 Hz, 1H), 5.93 (d, J ¼ 7.3 Hz, 1H), 4.82 (dd, J ¼ 12.9, 7.0 Hz,
1H), 4.23–4.09 (m, 1H), 3.89 (q, J ¼ 7.0 Hz, 1H), 3.66 (s, 3H), 3.29
(dd, J ¼ 14.5, 5.2 Hz, 1H), 3.06 (dd, J ¼ 14.2, 5.6 Hz, 1H), 2.27 (t, J
¼ 7.4 Hz, 2H), 1.63 (m, 4H), 1.32 (m, 12H). 13C NMR (100 MHz,
CDCl3): d 174.45, 165.32, 164.45, 163.83, 159.91, 154.89, 134.14,
132.84, 131.93, 130.49, 129.73, 128.60, 127.73, 126.56, 125.90,
125.66, 123.97, 122.72, 120.87, 116.68, 110.64, 77.44, 77.12,
76.80, 53.12, 51.54, 40.48, 35.02, 34.20, 29.54, 29.47, 29.43,
29.30, 29.21, 28.22, 27.21, 25.04. MS (EI): m/z calculated for
Scheme 1 Chemical structures and synthesis routes to 1 and 2.
Experimental section
Materials and general methods
All reagents were used as received from the supplier without
further purication unless stated otherwise. 11-Amino-
undecanoic acid (99.5%), 4-bromo-1,8-naphthalic anhydride
($95%), hexane-1,6-diamine (CP) and anhydrous ethane-1,2-
diamine (99%) were obtained from Sinopharm Chemical
Reagent Co., Ltd. (Shanghai). Other chemicals and solvents
1
C
39H43N2O6 [M + H]+: 635.3, found: 635.7.
were provided from Shanghai No. 1 Chemical Reagent Co. H
NMR and 13C NMR spectra were recorded on a Mercuryplus, at
400 and 100 MHz, respectively. Proton chemical shis were
reported in parts per million downeld from tetramethylsilane.
ESI-MS data were recorded on a Waters Quattro Micro API LC-
MS-MS spectrometer (Waters, USA). Elemental analysis was
carried out on a VARIOEL3 apparatus (ELEMENTAR). Melting
points were recorded on a hot-plate melting point apparatus
XT4-100A without further correction.
Synthesis of compound 2: LiOH (5.8 g, 241.66 mmol) in 50
mL water was added to a THF solution (50 mL) of 2b (5.0 g, 8.06
mmol). The mixture was stirred for 3 days and organic solvent
was removed by evaporation. The water phase was acidied to
pH 2–3 with HCl. The slurry was ltered and the solid was
washed with ether to give 2 as a white powder. Yield 77%, m.p.
1
ꢁ
86–88 C. H NMR (400 MHz, CDCl3): d 8.69 (ddd, J ¼ 8.4, 7.8,
1.1 Hz, 1H), 8.46 (d, J ¼ 8.3 Hz, 1H), 7.78 (dd, J ¼ 8.3, 7.4 Hz,
1H), 7.57–7.41 (m, 1H), 7.38–7.09 (m, 9H), 6.91 (d, J ¼ 8.3 Hz,
1H), 5.94 (d, J ¼ 7.5 Hz, 1H), 4.87 (dd, J ¼ 13.3, 6.0 Hz, 1H), 4.23–
4.07 (m, 1H), 3.73 (q, J ¼ 7.0 Hz, 1H), 3.42 (s, 1H), 3.25 (dd, J ¼
Synthesis
The synthesis procedure for 1 and 2 is shown in Scheme 1. 1a 14.1, 5.6 Hz, 1H), 3.14 (dd, J ¼ 14.0, 6.2 Hz, 1H), 2.16 (t, J ¼ 7.3
was synthesized according to our previous report.17
Hz, 1H), 1.92 (d, J ¼ 12.7 Hz, 1H), 1.71 (d, J ¼ 9.3 Hz, 2H), 1.64–
Synthesis of compound 1. LiOH (6.6 g, 276.87 mmol) in 1.46 (m, 2H), 1.46–1.01 (m, 12H). 13C NMR (100 MHz, CDCl3) d
40 mL water was added to a THF solution (40 mL) of 1a (4.5 g, 173.57, 165.10, 164.52, 163.93, 159.97, 154.86, 132.95, 132.00,
9.23 mmol). The mixture was stirred for 3 days and organic 130.43, 129.71, 129.42, 128.55, 127.06, 126.53, 125.62, 123.97,
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122.63, 120.79, 116.57, 110.67, 77.34, 77.22, 77.02, 76.71, 53.17,
49.64, 40.53, 36.49, 33.58, 29.20, 29.08, 28.04, 27.06, 25.52,
25.49, 24.79. MS (EI): m/z calculated for C38H41N2O6 [M + H]+:
621.3, found: 621.2; anal. calcd for C38H40N2O6: C, 73.53; H,
6.50; N, 4.51. Found: C, 73.63; H, 6.50; N, 4.43%.
Results and discussion
Gel formation and characterization
Naphthalimide undecanoic acids 1 and 2 are used as the
fundamental gel component. These two compounds have no
capability to form gels by themselves. Aer mixing with various
kinds of diamines (Scheme 2, details of preparation are given in
the Experimental section), the two-component complexes can
gel in a variety of organic solvents. Gelation behaviour of the
complexes was checked in 14 organic solvents with the molar
ratio of 1/amine or 2/amine of 2 : 1 (Table 1). It was observed
that the complexes of 1 and amines were more capable of
gelation than those of 2. 1 could form gel in alcohols, cyclo-
hexane, acetonitrile and toluene with both aliphatic and
aromatic amines. 2 could only gelate long chain alcohols in the
presence of aliphatic amines. It was also found that the
complexes with aliphatic amines, such as EDA and HDA, can
gelate more kinds of solvents, especially in alcohol. In contrast,
the complexes with aromatic amines such as NBDA and DAQ
make compounds more soluble in most of the tested solvents.
1+DAQ has better gelation behavior in acetonitrile and ethanol,
Gelation test of organic uids
When 1 or 2 mixed with diamines (shown in Scheme 2), dis-
solved in some organic solvent, the complexes show gelation
behavior. For example, when 0.1 g (0.21 mmol) 1 and 7.35 mL
(0.11 mmol) 1,2-diaminoethane were dissolved in chloroform,
the two-component gelator 1+EDA was obtained aer removing
chloroform. Generally, the molar proportion of compound 1 or
2 with diamine is 2 : 1. The gelator (5 mg) and the solvent (200
mL) were put in a hermetic test tube (1.5 mL) and heated (>75 ꢁC)
until the solid was dissolved. Then the sample vial was cooled to
25 ꢁC (prepared sample). Qualitatively, gelation was considered
successful if no sample owed upon inverting the container at
room temperature. The critical gelation concentrations (CGC)
were tested using dilution method aer the gels were formed
(Table 1). For example, to a 200 mL gel sample in a test tube,
additional 50 mL solvent was added, followed by a heating–
cooling process to see whether it could form a gel again. This
process was repeated until the gel began ow, and this
concentration was recorded as the CGC.
with critical gelation concentrations of 12.5 and 7.7 mg mLꢀ1
respectively.
,
The viscoelastic behaviour of gels of 1 with both aliphatic
(1+HDA) and aromatic diamines (1+DQA) (1/amine ¼ 2 : 1, 25
mg mLꢀ1 in ethanol) was studied by dynamic oscillatory
measurements. The linear viscoelastic region (LVR), as a func-
tion of increasing amplitude of deformation due to shearing,
was determined with strain amplitude in the range 0.005–
1000% at an angular frequency of 6.28 rad sꢀ1 (Fig. 1a). The two
gels show similar viscoelastic behaviour. The strength (storage
modulus, G0) of the gel is around 150 Pa with a small strain of
0.01%, which indicates that these gels were relatively weak
compared to other gels in the eld.10 With the small strain
amplitudes, the storage modulus (G0) is larger than the loss
modulus (G00) in both gels, and G0 and G00 remain nearly
constant (LVR) up to approximately 0.3% and 2% strain,
respectively. At larger strain amplitudes, the gels showed a
catastrophic disruption accompanied by a step decrease in the
values of both moduli, and the reversal of the viscoelastic signal
(G0 < G00) is 15% in 1+DAQ and 20% in 1+HDA. The imple-
mentation of a frequency sweep between 0.3 and 100 rad sꢀ1 at a
strain of 0.2% shows G0 > G00 in both gels, which conrms that
the gels have a predominantly elastic character (Fig. 1b). The
elasticity of the gels was further evident from the facts that the
G0 and G00 values are minimally sensitive to frequency (u), which
indicated that the gel system formed a stable network
structure.18
Techniques
UV-visible spectra were recorded on a UV-vis 2550 spectrometer
(Shimadzu). Fluorescent spectra were obtained using an Edin-
burgh Instruments FLS 900 with an Xe lamp as the excitation
source. Fourier transform infrared (FT-IR) spectra were recor-
ded using an IRPRESTIGE-21 spectroscope (Shimadzu) with KBr
pellets. SEM images of the xerogels were obtained using a SSX-
550 (Shimadzu) with an accelerating voltage of 15 kV. Samples
were prepared by diluting the gel and dropping it on the surface
of a mica ake, which was then freeze dried and coated with Au.
Powder X-ray diffraction patterns were generated by using a
Philips PW3830 sealed-tube X-ray generator (Cu target, l ¼
0.1542 nm) with a power of 40 kV and 50 mA. Samples were
prepared as freeze dried gels. Freezing dry was performed in a
FDU-1200 Freeze Dryer (Tokyo Rikakikai, Japan) with the trap
ꢁ
temperature of ꢀ45 C.
The network morphologies of the freeze-dried gels derived
from gelators of compound 1 and 2 mixed with various amines
in organic solvent were investigated by scanning electron
microscopy (SEM) (Fig. 2). From the images, it can be seen that
the architectures from gels of 1 with a variety of amines are
similar in appearance. Those gels show ribbon structures with
the width from 2–5 mm (Fig. 2a–d). As for compound 2 with
aliphatic amines, the morphologies show networks interweaved
Scheme 2 Structures and abbreviations of different amines checked
for gelation.
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Table 1 Gelation ability of 1 and 2 with several aminesa
EDA
HDA
NBDA
DAQ
Solvent
1
2
1
2
1
2
1
2
n-Octanol
n-Pentanol
Methanol
Ethanol
1,4-Dioxane
DMSO
Chloroform
Acetone
Cyclohexane
Acetonitrile
Ethyl acetate
Toluene
G (10)
G (10)
G (9)
G (9)
PG
G (12.5)
S
P
P
G (14.3)
S
S
P
G (5)
G (5)
G (5)
G (5)
PG
S
S
P
G (7.7)
G (5)
PG
PG
S
G (12.5)
G (25)
S
S
S
S
P
S
S
S
I
P
S
S
S
S
S
S
S
S
I
P
P
S
S
S
I
I
I
P
I
S
P
G (10)
G (7.7)
S
S
I
S
G (10)
G (10)
S
I
S
S
G (10)
G (7.7)
S
S
S
S
G (12.5)
G (12.5)
S
S
S
S
P
S
G (10)
G (12)
G (10)
PG
G (7.1)
S
S
I
I
I
P
I
G (10)
G (25)
G (12.5)
S
P
P
P
S
S
G (12.5)
S
S
1-Methyl-2-pyrrolidone
Tetrahydro-furan
S
PG
S
a
G ¼ gel; PG ¼ partial gel; P ¼ precipitation; S ¼ solution; I ¼ insoluble. Molar ratio of acid : amine ¼ 2 : 1, concentration ¼ 25 mg mLꢀ1. The
critical gelation concentration is in parentheses (mg mLꢀ1). Gels were made by a heating–cooling process from 75 ꢁC to 25 ꢁC.
stretching vibration of C]O of 1 at around 1710 cmꢀ1 attrib-
uted to carboxylic acid drastically decreased and shied to 1700
cmꢀ1 when interacted with EDA and DAQ in the gel formation
(also see Fig. S2 in the ESI†). The stretching vibration of C–O of
1 at 1090 and 1020 cmꢀ1 was shied to 1080 and 1030 cmꢀ1
,
which also suggested the strong interaction between carboxylic
acid and amines. The disappearance of the strong vibration at
1600 cmꢀ1 of EDA (belonging to N–H bonds) and the appear-
ance of the peak at 1608 cmꢀ1 in 1+EDA are attributable to the
complex formation between –NH2 of diamine and –COOH of 1.
Moreover, the peaks of the stretching vibration of N–H at 3360
and 3280 cmꢀ1 almost vanished in 1+EDA gel, and the presence
of multiple peaks in the range 3000–3100 cmꢀ1 strongly sug-
gested the formation of ammonium ions. All these data were
indicative of carboxylate salt formation between 1 and amines.
X-ray diffractions (XRD) can provide structural information
for the self-assembly in the gel state. Here XRD patterns were
recorded for powder of 1, DAQ and xerogel of the complex from
ethanol (Fig. 4A). The molar ratio of 1 and diamines in the
complex gelator is 2 : 1, just like all the above. The X-ray
Fig. 1 Dynamic oscillatory data for gel 1+DAQ and 1+HDA (25 mg
mLꢀ1 in ethanol) at 20 ꢁC. (a) Strain sweep of the gels at a frequency of
6.28 rad sꢀ1. (b) Frequency sweep of the gel at a strain of 0.2%.
Fig.
2 SEM images of self-assembled complexes: (a) 1+EDA in
ethanol, (b) 1+HDA in ethanol, (c) 1+NBDA in ethanol, (d) 1+DAQ in
ethanol, (e) 2+EDA in octanol, (f) 2+HDA in octanol; scale bars for a, b,
c, d, e and f are 20, 20, 20, 50, 5 and 10 mm, respectively.
by ne exible bers with diameters smaller than 100 nm
(Fig. 2e and f and Fig. S1 in the ESI†).
To investigate the interactions between 1 and amines, IR
analysis of 1 and the complexes of 1+EDA and 1+DAQ was
Fig. 3 IR spectra of powder of 1, EDA and xerogel of 1+EDA formed in
performed. As shown in Fig. 3, the characteristic peak of the toluene (25 mg mLꢀ1).
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Fig. 5 Emission and absorption spectra of the sols and gels of (a and c)
1+EDA in toluene and (b and d) 1+DAQ in acetonitrile. Insets are
pictures of gels in 365 nm UV light (a and b) or in natural light (c and d).
Molar ratio of acid : amine ¼ 2 : 1, the concentration for sols are
5 ꢂ 10ꢀ4 M and for gels are 25 mg mLꢀ1
.
Fig. 4 (A) XRD spectra of 1 (a), DAQ (b), and the xerogel of 1+DAQ (c).
Numbers marked on the XRD peaks stand for values of distance (A˚). (B)
Proposed interaction mode of 1 and DAQ (left) and the supposed
stacking method (right).
and b and S5 in ESI.† It is clear that all emission spectra have a
red shi from sol to gel state. In the case of 1+EDA in toluene,
the emission band of the naphthalimide group was modulated
from 420 nm in sol to 478 nm in the gel, while for 1+DAQ in
acetonitrile, this peak shied from 421 to 434 nm. The weak
emission band at 625 nm of 1+DAQ belongs to DAQ in the gel
state, by comparison with pure DAQ solution (Fig. S4 in ESI†). A
red shi of the absorption band from sol to gel state could also
be observed as the absorption peak for the naphthalimide
group shied from 359 to 401 nm in 1+EDA and from 359 to 364
nm in 1+DAQ, respectively (Fig. 5c and d). This indicates that
the p–p interaction between naphthalimide groups has a great
contribution to the gel formation in complex gels.
diffraction prole of the xerogel of the complex is quite different
from those of the individual components 1 or DAQ, indicating
the formation of the complex. The X-ray diffraction prole of the
xerogel of 1 with aromatic amines showed a series of peaks at
˚
˚
24.35, 12.30, 8.43 A for 1+DAQ, and 25.14, 12.58, 8.42 A for
1+NBDA (Fig. S3 in ESI†), with a ratio of 1 : 1/2 : 1/3, charac-
teristic of lamellar packing of the complexes. The distance of
˚
24.35 A for 1+DAQ is in agreement with the combined length of
˚
˚
the molecules 1 (16 A) and DAQ (9 A). The dimensions of about
ꢀ
Response to NO2
˚
4.1 A in the complexes may suggest p–p interactions. The
ꢀ
existence of p–p stacking in 1+DAQ was also supported by
spectral study discussed in the following part. The proposed
aggregation mode of 1+DAQ is shown in Fig. 4B, in which the
complexes of 1+DAQ are formed by acid–base interaction and
then the complexes further aggregate into a layered structure.
The X-ray diffraction in the gels of 1 with aliphatic amines, such
as EDA and HDA, was much weaker and broader than that with
aromatic amines, indicating the less ordered structure in those
gels (Fig. S3†).
The spectroscopic study also supports the formation of p–p
stacking between gelators. The maximum absorption and
emission of compound 1 in CH3CN (1 ꢂ 10ꢀ5 M) is located at
360 nm and 429 nm, respectively (Fig. S4 in ESI†). The uo-
rescent quantum yield (4) of 1 was 0.1366 using quinine sulfate
as a reference. DAQ solution has an absorption band at 500 nm
in the visible range and an emission band at 625 nm (4 ¼
0.0061). To investigate the uorescence characteristics in the
aggregated state, emission spectra of gels (25 mg mLꢀ1) and
related sols (5 ꢂ 10ꢀ4 M) were studied and are shown in Fig. 5a
Gel 1+DAQ was found to be sensitive to NO2 as we imagined
(Fig. 6). The original colour of the two-component gel was red as
shown in Fig. 6a due to the strong absorption of DAQ at 517 nm.
With the addition of an aqueous solution of NaNO2, the gel
collapsed to a suspension and the colour of the gel gradually
faded. Quantitatively, when 100 equiv. of NaNO2 aqueous
solution was added to the top surface of the gel of 1+DAQ
(25 mg mLꢀ1 in acetonitrile, molar ratio of 1 : DAQ ¼ 2 : 1), in
two hours which equals the average level of using DAQ to sense
nitrite anions in the literature,19 the colour of the broken gel was
completely faded with an obvious decrease of the absorption at
517 nm (Fig. 6b and d). What's more, the uorescence emission
of 1 at 434 nm quenched to 40% of its original intensity and
uorescence emission of the DAQ at 639 nm fell to nearly zero
because of the existence of colourless triazole formed from the
ꢀ
reaction of DAQ and NO2 (Fig. 6c and e). Results of LC-MS
demonstrate this reaction. Aer reaction, the peak for DAQ (m/z
¼ 239) in mass spectra was totally changed into benzotriazole
(m/z ¼ 249), but the peak for compound 1 (m/z ¼ 474) was
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Journal of Materials Chemistry C
Fig. 7 (a) UV-visible absorption spectra of the liquid part of gel 1+DAQ
treated with pure water and different equiv. of NaNO2 aqueous solu-
tion (cell: 1 mm); (b) the absorbance change at 520 nm with the
addition of different molar ratios of NaNO2. (c) The absorbance at 520
nm with the addition of different kinds of anions (100 equiv. of DAQ).
Samples all diluted in THF.
Fig. 6 Photo of gel 1+DAQ ( 25 mg mLꢀ1 in acetonitrile) (a), and the gel
with added NaNO2 solution (left) and pure water (right) in natural light
(b) and with 365 nm light irradiation (c). (d) Absorption and (e) emission
spectra of gel 1+DAQ before and after treatmeꢀnt with NO2ꢀ. (f) The
scheme of the reaction between DAQ and NO2
.
absorbance at 359 nm belonging to 1 from the liquid part and
the precipitation part of 1+DAQ+NaNO2 solution in THF shows
that only 9% of 1 remains in the liquid part, which means the
majority of 1 could be reused.
High-level selectivity is of paramount importance for an
excellent chemosensor. In the present work, the common
environmental salts such as NaF, NaHCO3, Na2SO3 and Na3PO4
were checked with 1+DAQ by means of absorption spectroscopy.
A gel of 1+DAQ (20 mM) has little variation of absorption upon
the addition of excess anions (100 equiv.) such as NaF, NaHCO3,
NaSO3, Na3PO4 (Figꢀ. 7c). This indicates a high selectivity of
1+DAQ towards NO2 compared with other anions.
unchanged (Fig. S6 in ESI†). The mechanism is shown in Fig. 6f.
These facts indicate that the complex 1+DAQ can serve as an
indicator of NO2 that is visible to the naked eye. It should be
pointed out that pure water or other solutions of salt can also
destroy the gel, but the colour and emission characteristics
would not be changed (Fig. 6b and c).
ꢀ
As for the practical application and recycling considerations,
the quantitative change of detecting of gel 1+DAQ for both pure
water and different concentrations of NO2ꢀ (0–200 eq. of DAQ)
were checked. Aer reaction, the mixture consists of two parts:
the liquid part (L) mainly including solvent, triazole product of
DAQ or remaining DAQ, and the precipitation part mainly
including 1 (P), as 1 could hardly dissolve in acetonitrile or
water. The liquid part was taken out by a lter membrane and
diluted with 4 mL THF solutions to make sure that the
proportion stayed unchanged. The precipitation part was also
dissolved in 4 mL of THF to see how much 1 remained that
could be taken for reuse by means of absorption spectroscopy.
As we observed from the absorption and emission spectra of the
liquid part of the broken gel treated with nitrite (1+DAQ+-
NaNO2) (L) in Fig. 7, the gel mixture of 1+DAQ quickly faded
aer addition of 5 equiv. of NaNO2 (40% of absorbance
quenched), and gradually decreased aer further addition of
NaNO2. The reaction was complete within two hours aer 200
equiv. NaNO2 (relative to DAQ) was added (98% of the absor-
bance quenched, Fig. 7a). Fig. 7b shows the absorbance change
of 1+DAQ (liquid part) with the addition of different molar
ratios of NaNO2, from which we can see that the 1+DAQ gel is
very sensitive to NO2ꢀ. Besides, because 1 hardly dissolved in
the gelling solvent CH3CN, it was easily recovered and reused.
As shown in Fig. S7 in the ESI,† the comparison of the
Compared with the previous reported NO2ꢀ probes, the main
advantages of this kind of two-component low molecular weight
orgaꢀnogel are as follows: (1) it affords an easy way for sensing
NO2 with obvious colour and emission change through self-
assembly of an organic acid and DAQ; (2) additional HCl is not
needed in the sensing process, as reported previously,6 because
1 provides necessary protons for the reaction between DAQ and
NO2ꢀ; (3) the two-component gel is decomposable and the part
of the organic acid 1 could be recovered and used as a circu-
lating material, showing that it is environmentally benign.
Conclusions
In conclusion, we made a kind of two-component low molecular
weight organogel from naphthalene diimide undecanoic acid
mixed with several aliphatic and aromatic diamines. The spec-
troscopic and morphological properties of the gels could be
tuned by changing the associated amines. Besides, this conve-
niently prepared two-component gel containing diaminoan-
thraquinone can be used as a new kind of probe for sensing
ꢀ
NO2 in neutral environments. What's outstanding is that,
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besides the colorimetric change, this gel state probe has a
uorescence emission response towards NO2ꢀ, which expands
the eld for more practical applications.20 Aer sensing, the
and B. Escuder, Chem. Soc. Rev., 2013, 42(17), 7086;
J. W. Steed, Chem. Soc. Rev., 2010, 39, 3686–3699; X. Yang,
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organic acid could be recycled. This two-component gel is thus 10 L. A. Estroff, A. A. Lazarides, R. C. Mucic, C. A. Mirkin,
an ideal platform for the design of supramolecular visual
sensing systems with tuneable optical properties.
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Acknowledgements
This work was supported by the National Basic Research
Program of China (2013CB733700), the China National Funds
for Distinguished Young Scientists (21125104), National
Natural Science Foundation of China (91022021), Program for
Innovative Research Team in University (IRT1117), Program of
Shanghai Subject Chief Scientist (12XD1405900), and Shanghai
Leading Academic Discipline Project (B108).
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