Functionality-oriented molecular gels: Synthesis and properties of nitrobenzoxadiazole (NBD)-containing low-molecular mass gelators

Article abstract of DOI:10.1039/c4sm01869a

Two nitrobenzoxadiazole (NBD)-containing cholesteryl (Chol) derivatives were prepared by introducing d/l-phenylalanine into the linkers between the NBD and Chol units. The compounds were denoted as NLC and NDC, respectively. The gelation behaviors of them were tested in 34 liquids. It was found that the chirality of the linkers shows a great effect on the gelation ability and the gel properties of the two compounds. SEM studies demonstrated that the gelator in the gel of NDC/DMSO aggregated into uniform fibrous structures. FTIR, ¹H NMR and UV-Vis spectroscopy measurements revealed that intermolecular hydrogen bonding and π-π stacking are two main driving forces to promote the gel formation. Interestingly, the NDC/DMSO gel exhibits rapid and reproducible gel-sol phase transition and fluorescence quenching upon introduction of ammonia. Furthermore, both the gel and the fluorescence emission could be fully recovered upon evaporation of the ammonia gas introduced. Spectroscopy and model system studies revealed the association of ammonia with the nitro group of the NBD unit of the compound, which is recognized as the main reason for the chemical responses of the gel system. On the basis of the discovery, an ammonia sensing film had been fabricated and made into a device. Furthermore, a device-based and conceptual ammonia leaking monitoring instrument was developed. A preliminary test demonstrated that the performance of the system is exceptionally good, a typical and persuasive example to show the important real-life applications of molecular gels. This journal is

Full text of DOI:10.1039/c4sm01869a

Soft Matter
View Article Online
View Journal | View Issue
PAPER
Functionality-oriented molecular gels: synthesis
and properties of nitrobenzoxadiazole (NBD)-
containing low-molecular mass gelators†
Cite this: Soft Matter, 2014, 10, 9159
*
Hang Yu, Yanchao Lu, Xiangli Chen, Kaiqiang Liu and Yu Fang
¨
Two nitrobenzoxadiazole (NBD)-containing cholesteryl (Chol) derivatives were prepared by introducing D/
L-phenylalanine into the linkers between the NBD and Chol units. The compounds were denoted as NLC
and NDC, respectively. The gelation behaviors of them were tested in 34 liquids. It was found that the
chirality of the linkers shows a great effect on the gelation ability and the gel properties of the two
compounds. SEM studies demonstrated that the gelator in the gel of NDC/DMSO aggregated into
uniform fibrous structures. FTIR, 1H NMR and UV-Vis spectroscopy measurements revealed that
intermolecular hydrogen bonding and p–p stacking are two main driving forces to promote the gel
formation. Interestingly, the NDC/DMSO gel exhibits rapid and reproducible gel–sol phase transition and
fluorescence quenching upon introduction of ammonia. Furthermore, both the gel and the fluorescence
emission could be fully recovered upon evaporation of the ammonia gas introduced. Spectroscopy and
model system studies revealed the association of ammonia with the nitro group of the NBD unit of the
compound, which is recognized as the main reason for the chemical responses of the gel system. On
the basis of the discovery, an ammonia sensing film had been fabricated and made into a device.
Furthermore, a device-based and conceptual “ammonia leaking” monitoring instrument was developed.
A preliminary test demonstrated that the performance of the system is exceptionally good, a typical and
persuasive example to show the important real-life applications of molecular gels.
Received 22nd August 2014
Accepted 12th September 2014
DOI: 10.1039/c4sm01869a
www.rsc.org/softmatter
Stimuli-responsive gels have been seen as a new kind of
responsive material, because they combine the elastic behavior
1. Introduction
of solids with the micro-viscous properties of uids. Compared
with chemical gels, of which the gel networks are maintained by
cross-linked covalent bonds, physical gels are built by non-
covalent interactions such as hydrogen bonding, p–p stacking,
van der Waals interaction, host–guest interaction, electrostatic
interaction, etc.14–16 It is the difference in the bonding nature of
the gel networks that makes physical gels rather than chemical
gels to be chosen as responsive systems in smart materials
research.
The stimuli for responsive gels can be divided into physical
stimuli and chemical stimuli. The temperature, mechanical
stress, light illumination, sonication, magnetic eld are the
main physical triggers that gels can respond to.17–19 In contrast,
chemical stimuli are mainly pH variation due to addition of
acids or bases, oxidation/reduction due to the introduction of
oxidants or reductants, and chemical reaction via addition of
other chemicals.19–21 In the past few decades, several LMMG-
based gel systems have been reported for their responses to
chemical stimuli. Huang and co-workers reported some self-
healing molecular gels based on a host–guest interaction.22 Due
to the pH-controllable properties of the host–guest recognition
motif, the gels they obtained can act as degradable materials
triggered by pH stimuli. Chen and co-workers obtained a
Gels are everywhere due to its wide range of applications in
cosmetics, sensor technology, conservation of arts, drug
delivery, biomedical applications, etc.1–5 For realizing and/or
enhancing gelation, saccharide, ureido, steroidal structures, etc.
have been employed as important building blocks in the prep-
aration of low-molecular mass compound-based gelators
(LMMGs).6–8 However, these structural elements are usually
optically inactive, which may limit some specic applications of
the relevant gels.9 Furthermore, considering the fact that
molecular gel formation is a self-assembling process of LMMGs,
and in other words the molecules of gelators in gel-networks are
relatively in a low entropy state or high degree of the ordered
state, it may be reasonable to anticipate that a combination of
the weak interaction-based organization and uorescence
properties of LMMGs in molecular gels may bring opportunities
for preparing novel stimuli-responsive, including sensing,
systems.10–13
Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education),
School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an,
710062, P. R. China. E-mail: yfang@snnu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4sm01869a
This journal is © The Royal Society of Chemistry 2014
Soft Matter, 2014, 10, 9159–9166 | 9159

View Article Online
Soft Matter
Paper
cationic sensitive gel, and developed metal ion sensors by using with THF/petroleum ether (1 : 6, v/v) as an eluent to give a
the gel.23 Weiss and colleagues prepared a class of molecular desired product as orange powders in 70% yield. The proce-
gels, and demonstrated that the gels are responsive to CO2.24 dures used for the preparation of NDC are similar to that for
The gels, however, could be rebuilt by displacing the gas NLC. Satisfactory results were also obtained.
with N2.
For NLC: yield 70%. 1H NMR (CDCl3/Me4Si, 400 MHz): 8.39–
Unlike physical stimuli, most of the responses triggered by 8.41 (d, 1H, H–Ar), 7.29–7.31 (d, 2H, H–Ar), 7.17–7.19 (3H, d, H–
chemical stimuli are hard to be reversed. Furthermore, chem- Ar), 6.57–6.55 (d, H, NH), 6.04–6.02 (d, 1H, H–Ar), 5.38–5.39 (d,
ical stimuli may cause contamination to the gel systems under H, alkenyl), 4.66–4.72 (m, 2H, C–H), 3.23–3.38 (m, 2H, Ph–CH2),
study due to residues remained aer the chemical interactions. 0.68–2.33 (m, 44H, cholesteryl protons) (Fig. S1†). Elemental
Therefore, exploring gel systems that respond to chemical analysis, calcd for C42H56N4O5: C, 72.38; H, 8.10; N, 8.04. Found:
stimuli, and the response which is reversible without extra C, 72.20; H, 8.15; N, 7.98%.
chemical addition are signicant challenges in the area of
supramolecular gel research. It is believed that this kind of gel 8.41 (d, 1H, H–Ar), 7.29–7.30 (d, 2H, H–Ar), 7.16–7.18 (3H, d, H–
will nd important real-life applications.25
Ar), 6.54–6.55 (d, H, NH), 6.04–6.02 (d, 1H, H–Ar), 5.38–5.39 (d,
For NDC: yield 70%. 1H NMR (CDCl3/Me4Si, 400 MHz): 8.39–
To prepare LMMG-based molecular gels with diverse func- H, alkenyl), 4.67–4.75 (m, 2H, C–H), 3.22–3.38 (m, 2H, Ph–CH2),
tionalities, cholesterol (Chol) and nitrobenzoxadiazole (NBD) 0.68–2.32 (m, 44H, cholesteryl protons) (Fig. S2†). Elemental
were specially chosen as main building blocks. This is because analysis, calcd. for C42H56N4O5: C, 72.38; H, 8.10; N, 8.04.
Chol has been proven as an efficient structural unit to enhance Found: C, 72.44; H, 8.04; N, 7.89%.
the self-assembly of its derivatives in the solution state no
matter whether the liquid is hydrophilic or hydrophobic, and
NBD is a widely used uorophore for imaging and sensing in 2.2 Gelation test
life science due to its sensitivity to environmental changes and
A known weight (0.025 g) of a tested compound and a measured
volume (1 mL) of selected pure liquid were placed into a sealed
test tube and the system was sonicated at room temperature for
30 min; then the test tube was inversed to observe if a gel had
high uorescence quantum yields.26,27 The combination of the
two residues has been proven to be an efficient strategy to
develop new LMMGs with superior properties.28 For example,
we, for the rst time, took NBD as a main building block for the
been formed. Gels obtained aer sonication at room tempera-
preparation of new LMMGs, and introduced it into some ALS
ture were denoted as “G*”. If the result was negative, the test
tube was heated until the solid was dissolved completely, and
type Chol derivatives. In this way, several excellent gel systems
with amazing mechanical strength and self-healing properties
then the system was cooled to room temperature in air. Finally,
were developed. Inspired by the discovery, two kinds of NBD
the test tube was inversed to observe if the solution inside could
appended cholesteryl derivatives with L(D)-phenylalaninate as a
still ow. A positive test is obtained if the ow test is negative. A
linker (NLC, NDC) were designed and synthesized. The gelation
gel was denoted as “G” (gel), a mixture of the gel and solution
behaviors of them in 34 liquids were tested to explore the effect
was referred to as “PG” (partial gel), and a system in which only
of spatial conguration on the gelation. Based upon the test, the
solution remained was denoted as “S” (solution). For those, of
system of NDC/DMSO was chosen as an example gel. The
which heating results in dissolution, but cooling was accom-
panied by precipitation, the systems were referred to as “P”
(precipitation). Systems in which the gelators could not be
dissolved even at the boiling point of the liquid were dened as
structure, the formation mechanism, the rheological proper-
ties, and the uorescence behavior of the gel system were
studied systematically. It was found to be surprising that the
uorescence emission of the gel is super-sensitive to the pres-
ence of ammonia, and the response is fast and fully reversible.
Accordingly, an “ammonia leaking” monitor was created. This
paper reports the details.
“I” (insoluble).
2.3 Gel–sol phase transition temperatures (Tgel
)
The falling drop method was employed to measure Tgel. The gels
were placed in the inverted tubes, and then they will fall under
the inuence of gravity when heated to a certain temperature in
an oil bath.30
2. Experimental section
2.1 Preparation of NLC and NDC
The methods used for the preparation of NLC and NDC are
schematically shown in Scheme 1. Specically, cholesteryl L-
phenylalaninate with a primary amine group in its end was
synthesized according to a previous report.29 The compound
2.4 Scanning electron microscopy (SEM) studies
(1.07 g, 2 mmol) was further reacted with NBD-Cl (0.40 g, 2 SEM images of the xerogel were taken on a TM3000 Tabletop
mmol), which had been dissolved in 20 mL of THF, under microscope (Hitachi Limited). The accelerating voltage was 15
stirring in the presence of NaHCO3 (0.20 g, 2.4 mmol) at room kV and the emission current was 10.0 mA. The xerogel for the
temperature. The reaction was monitored by TLC and con- measurement was prepared by freezing the gel formed in the
ducted for another 24 h. Aer the reaction, the mixture was concerned liquid at a measured concentration in liquid
ltered and the ltrate was evaporated to dryness. The residue nitrogen, and evaporated by a vacuum pump for 24 h. Before the
as obtained was puried by silica gel column chromatography examination, the sample was coated with a thin layer of gold.
9160 | Soft Matter, 2014, 10, 9159–9166
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Soft Matter
Scheme 1 Schematic representation of the synthesis of two nitrobenzoxadiazole (NBD)-containing cholesteryl (Chol) derivatives, of which L/D-
phenylalanine was used as a linker (NLC and NDC).
2.5 1H NMR measurements
enough to destroy the gel was applied to the gel for 2 min; (2)
modulus recovery in a time sweep: the high shear force was
The sample containing a gelator under test and the deuterated
removed and a very small monitoring shear stress of 10 Pa was
reagent was prepared in an NMR tube, which was detected by
exerted on the destroyed gel for 2 min. The storage modulus G0
using a Fourier Digital NMR spectrometer (AVANCE 400 MHz)
at a given temperature between 298 K and 338 K or in a given
and the loss modulus G00 of the system were monitored with the
change of time.31
concentration range from 0.1 to 0.25% (w/v). The temperature-/
concentration-dependent 1H NMR spectra were recorded in
2.8 UV-Vis and circular dichroism (CD) measurements
DMSO-d6 to investigate the gelation mechanism. For charac-
terization studies, the 1H NMR spectra of the compounds were
obtained in CDCl3.
UV-Vis and CD spectra were obtained using a chirascan circular
dichroism spectrometer. In the measurement, the hot solution
was poured into a quartz cell (0.1 mm) and cooled to room
temperature to form a stable gel (1%, w/v). The temperature
dependent UV-Vis and CD spectra were recorded, and the
concentration-dependent UV-Vis and CD spectra were also
2.6 FTIR measurements
All FTIR measurements were performed on a Bruker VERTEX 70
V infrared spectrometer in an attenuated total reectance (ATR)
mode. KBr pellets were obtained by mixing a small amount of
the dried gel samples and anhydrous KBr powder. And the
solution sample was prepared by dropping a dilute solution of
the sample on a hot KBr disc, and then evaporating the solvent
immediately.
ꢀ
recorded at 25 C.
2.9 Hansen solubility parameter studies
Hansen solubility parameters (HSPs) are available for each liquid
and quantify liquid's interaction ability via dispersion, dipole–
dipole, and hydrogen bonding. With the Nanomaterials Labora-
tory Hansen Solubility Parameter Data Fitting Soware, 3-D plots
showing regions of solubility (S), gelation (G) and insolubility (I)
were constructed for the system of NDC in different liquids at a
given temperature and concentration, which may allow us to
predict if a compound gels a given liquid or not.32,33
2.7 Rheological measurements
Rheological measurements were carried out with a stress-
controlled rheometer (TA instrument, AR-G2) equipped with a
steel-coated parallel-plate geometry (20 mm diameter). The gap
between two plates was 1 mm. A liquid trapping device was used to
minimize the evaporation. The measurements were conducted
aer the sample was placed on the plate for 30 min at 25 ꢀC.
Firstly, a stress sweep measurement at xed frequency was
conducted, which provides the information about the
mechanical strength of the gel sample. Secondly, the storage
modulus, G0, and the loss modulus, G00, were monitored as
functions of frequency from 0.1 to 628.0 rad sꢁ1 at a constant
stress of 10 Pa well within the linear visco-elastic region.
The thixotropic study was conducted to examine the alter-
ation in the rheological properties of the gel under the appli-
2.10 Fluorescence measurements
Fluorescence measurements were performed on a time-corre-
lated single photon counting Edinburgh FLS 920 uorescence
spectrometer at room temperature. The lm was adhered to the
wall of a sealed quartz cell.
3. Results and discussion
3.1 Gelation behaviors
cation and release of shear. This process includes two steps: (1) The gelation behaviors of NLC and NDC in 34 liquids were
deformation: a constant oscillatory shear stress (5000 Pa) that is tested at a standard concentration of 2.5% (w/v), and the results
This journal is © The Royal Society of Chemistry 2014
Soft Matter, 2014, 10, 9159–9166 | 9161

View Article Online
Soft Matter
Paper
are shown in Table S1.† With reference to the table, it is seen yield stress increased from 248.6 Pa to 8381.0 Pa, a result as
that there is a huge difference in the gelation behavior of them. expected and of value for practical uses, such as crystal growth
Specically, NDC gels seven of the liquids tested, whereas NLC and device fabrication.43,44
gels none of them, indicating that the chirality of the phenyl-
The frequency sweep is important for examining the ability
alanine residue, which connects the Chol unit and the NBD of a material to tolerate external forces.45 Accordingly, the gel of
residue, shows a great effect upon the gelling performances of NDC/DMSO (2.5%, w/v) was employed to conduct the test at a
the compounds since the compositions and basic structures of shear stress of 10 Pa, which is well within the linear region of
them are the same. The data of critical gelation concentrations the gel sample, and the result is shown in Fig. 2b. Reference to
(CGCs) and sol–gel phase transition temperatures (Tgel) of the the gure reveals that with the angle frequency increasing from
gels are also listed in the table.
0.1 rad sꢁ1 to 555.0 rad sꢁ1, the values of the storage modulus
With further reference to Table S1,† it is found that NDC gels (G0) are always higher than those of the loss modulus (G00),
DMSO, kerosene, acetonitrile and several alcohols, but only the associated with the loss of the energy. Furthermore, both G0 and
NDC/DMSO gel exhibits good stability and high mechanical G00 were kept relatively stable within the whole frequency region
strength. Furthermore, its CGC is only 0.3% (w/v), suggesting swept, suggesting that there had been no phase transition
great efficiency of NDC in gelling of the liquid. As expected, the during the sweep process and the gel possesses good tolerance
incorporation of NBD endows the gels with bright uorescence to external forces.
under light irradiation, which may provide a possibility for
them to nd practical uses.
To examine the thixotropic properties of the gel, further
rheological measurements were conducted alternatively under a
shear stress (5000 Pa) signicantly greater than the corre-
sponding yield stress and no shear stress. The results are shown
in Fig. 2c. With reference to the gure, it is seen that the gel
recovers immediately aer removing the shear stress. This sol–
gel phase transition process can be repeated at least ve times,
indicating that the gel is thixotropically reversible. This prop-
erty might be valuable for some specic applications such as
injection molding, drug delivery, etc.46,47
3.2 Scanning electron microscopy (SEM) studies
The micro-structures of the gel networks are important both for
understanding the self-assembly processes and for nding their
practical applications. Accordingly, the morphologies of the
xerogels or aggregates from the NDC/DMSO systems with different
compositions were examined by SEM. Some typical images are
shown in Fig. 1. Reference to the images reveals that with
increasing concentration of NDC, the aggregates change from
loose bers to highly ordered ber arrays, suggesting that the
structures of the aggregates can be modulated by simple variation
of the gelator concentration, which may provide convenience for
device making, a necessity for functionality studies.34–37
3.4 1H NMR and FTIR spectroscopy studies
Temperature-/concentration-dependent 1H NMR measure-
ments were conducted in order to obtain further information
about the formation mechanism of the gel networks due to their
effectiveness in conrming hydrogen-bond formation.38 The
results are shown in Fig. 3. With reference to Fig. 3a, it is seen
that the signal of the N–H proton appeared at 9.28 ppm at 328 K,
but it shied to 8.98 ppm at 368 K, an indication of existence of
hydrogen bonds in the gel networks. However, it is not clear
whether the hydrogen bonds formed inter-molecularly or intra-
molecularly. To reveal the origin of the hydrogen bonding,
concentration-dependent 1H NMR spectroscopy measurements
were also conducted, and the results are shown in Fig. 3b. It is
seen that the signal of the N–H proton is too weak to be seen
when the concentration of the gelator is set at 0.1% (w/v). The
signal appears at 9.06 ppm when the concentration is increased
to 0.18% (w/v). The signal shied to 9.03 ppm when its
concentration is increased to 0.5% (w/v), a result in support of
the inter-molecular interaction. In addition, the signals of NBD
protons at 8.46 ppm and 6.45 ppm slightly shied down-eld to
8.48 ppm and 6.47 ppm, respectively, with increasing gelator
concentration from 0.10% to 0.5% (w/v), a result also found in
temperature-dependent 1H NMR studies, indicating that in
addition to the presence of inter-molecular hydrogen-bonding,
p–p stacking also plays an important role in the formation of
the NDC/DMSO gel.
3.3 Rheological studies
The rheological properties of a gel are extremely important for
its real-life uses.38–42 Therefore, the properties of the gels
prepared were studied by taking NDC/DMSO as an example gel
system. The results from the stress sweep of different concen-
trations of the gelator are shown in Fig. 2a, of which the sweep
was conducted at a constant frequency of 1.0 Hz and at 25 ꢀC. It
is revealed that with increasing gelator concentration from
1.5% to 5.0% (w/v), the value of G0, associated with the energy
storage, increased from 1.39 ꢂ 104 Pa to 8.26 ꢂ 105 Pa and the
FTIR spectroscopy can also provide protable information
on the formation of hydrogen bonds during the gelation
process. Accordingly, the FTIR spectrum of NDC/DMSO in the
Fig. 1 SEM images of the NDC/DMSO gel system with different
concentrations of NBD: (a) 0.05%, (b) 0.50%, (c) 1.00%, (d) 2.50%, and
(e) 5.00% (w/v).
9162 | Soft Matter, 2014, 10, 9159–9166
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Soft Matter
Fig. 2 (a) Evolution of G0 as a function of the applied shear stress at different concentrations of NDC in DMSO; (b) evolution of G0 and G00 as
functions of angle frequency; (c) evolution of G0 as a function of time. The sample under test is the NDC/DMSO gel.
to distinguish different aggregation modes such as H- or J-
aggregation. Accordingly, concentration- and temperature-
dependent absorption spectra of the DMSO solution or gel of
NDC were recorded. The results are shown in Fig. S3.† With
reference to Fig. S3a,† it is clearly seen that the position of the
maximum absorption of NBD shows an obvious red-shi along
with increasing gelator concentration from 0.18% (sol state) to
0.50% (gel state), an indication of p–p stacking and J-type
aggregation. To verify the tentative conclusion, temperature-
Fig. 3 (a) Temperature-dependent 1H NMR spectra of NDC in DMSO-
d6, and (b) concentration-dependent 1H NMR spectra of NDC in
DMSO-d6.
dependent absorption spectroscopy studies were also con-
ducted, and the results are shown in Fig. S3b.† It is obvious that
the maximum absorption of NBD exhibits a signicant red-shi
with decreasing temperature from 338 K (sol state) to 298 K (gel
state), a result in support of the conclusion obtained from the
concentration-dependent measurements, and conrms again
that the structure of NBD possesses a strong tendency to form
aggregates in solution.28
It is known that circular dichroism (CD) can be used to study
not only molecular chirality but also the chirality of a supra-
molecular aggregate.48 Considering the importance of chirality
to the gelation behavior of NDC and NLC, temperature- and
concentration-dependent CD spectroscopy studies of the gel
systems were conducted. The results are shown in Fig. 5. It is
obvious that the sample is CD-active in the gel state at 298 K, but
the CD signal decreased gradually and eventually disappeared
at the sol state with increasing temperature from 298 K to 338 K,
indicating that the CD signal observed originated from the
chirality of the gel networks, the aggregates of the gelator,
rather than from the inherent chirality of the molecules of the
gelator (cf. Fig. 5a).49 This result is further supported by the
gel state and that of NDC/THF in the solution state were
recorded and the results are shown in Fig. 4. With reference to
the traces shown in the gure, it is seen that the stretching
vibration of the N–H bond and that of the C]O bond of NDC in
THF appeared at 3364 cmꢁ1 and 1741 cmꢁ1, respectively. Upon
gelation, however, the bands shied to 3189 and 1663 cmꢁ1
,
respectively, suggesting the existence of hydrogen bonds within
the gel, another driving force to promote the gel formation.
3.5 UV-Vis and CD spectroscopic studies
A UV-Vis spectroscopy study is a useful technique to elucidate
the interaction among gelator molecules, and even can be used
Fig. 5 Temperature-dependent CD spectra of the NDC/DMSO system
at a concentration of 1.0% (w/v) (a), and concentration-dependent CD
spectra of the system recorded at room temperature (b).
Fig. 4 FTIR spectra of NDC in its THF solution (a), and in its DMSO gel
(1.0%, w/v) (b).
This journal is © The Royal Society of Chemistry 2014
Soft Matter, 2014, 10, 9159–9166 | 9163

View Article Online
Soft Matter
Paper
results from concentration-dependent CD spectroscopy studies plots in 2-D is sometimes misleading that is points that appear
because the sample is only CD-active at concentrations greater to be inside a given sphere may actually be outside it). However,
than its CGC, a minimum requirement for gelation (cf. Fig. 5b). some of the liquids points locating within the sphere/shells do
not agree with the experimental results, which are believed to be
an indication of difficulty of using group contribution methods
in the calculation of the HSPs of a complex molecule. Moreover,
the parameters as discussed do not cover all interactions
between the components within a gel system. In other words,
there are some other factors which also affect gelation, for
example, p–p stacking. However, valuable information can be
still acquired from the analysis. For the systems under study, it
is found that among the three parameters only dH exhibits a
good linear relationship with the Tgel of NDC in the liquids
under study (cf. Fig. S4†), indicating the importance of
hydrogen-bonding formation for the gelation of the systems.
3.6 Hansen solubility parameters studies
Hansen solubility parameter (HSP) studies can help us to
understand the reasons why a gel forms and predict whether the
gelator under study gels a given liquid or not. For this reason,
the Hansen space of the systems under study was constructed
by using the Hansen Solubility Data Fitting Soware, the so-
called “concentric sphere shell t method”.32 The results are
shown in Fig. 6.
According to the denition of the solubility parameter by
Hildebrand, the overall energy density (d) can be separated into
three components, which are dispersive (dd), polar (dp) and
hydrogen-bonding (dH), respectively (eqn (1)):
3.7 XRD studies
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
2
d ¼ dd þ dp þ dh
(1)
In order to elucidate the detailed packing mode of the gelator in
the gel networks at a molecular level, XRD measurement of the
xerogel of NDC/DMSO was performed. The result is shown in
Fig. 7. Reference to the trace shown in the gure reveals that it is
characterized by six reection peaks, corresponding to spacings
(d) of 3.32, 4.99, 6.71, 8.02, 9.21 and 10.16 nm, respectively,
which follow the ratio of 1 : (1/O2) : (1/2) : (1/O5) : (1/3) : (1/O8),
suggesting that NDC aggregated in a tetragonal way50 and the
length of a basic structural unit is 3.32 nm, which approxi-
mately equals to the length of a dimer of NDC (cf. Fig. 7a).
Considering the interactions between the gelator molecules
revealed by FTIR, 1H NMR and XRD studies, a model repre-
senting the assembly of NDC in DMSO was proposed (cf.
Fig. 7b).
The HSP parameters for the liquids shown in Table S1† were
obtained from the literature.31 To be simple, the systems of the
liquids with 2.5% (w/v) of NDC were classied into three cate-
gories, which are soluble (S), gel (G) and insoluble (I), respec-
tively. Partial gels, precipitates, and suspensions were treated as
“insoluble”. The distances (R) between the gelator (dp, dH and
dd) and those of a liquid (dlp, dhl and ddl ) in Hansen space were
calculated using eqn (2).
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
À
Á
À
Á
À
Á
2
2
2
R ¼ 4 dd ꢁ dld þ dp ꢁ ddl þ dh ꢁ dhl
(2)
In the Hansen spaces, the solution sphere (blue) are nearest
to the center, the gel shells (red) are next, and the insoluble
shells (green) are outmost. Generally speaking, the liquids t
within the proper sphere/shells (as an aside, viewing the 3-D
3.8 Sol–gel phase transition
One of the most promising properties that molecular gels can
offer is a stimulated response leading to reversible sol–gel phase
transition.20 During the primary test of the uorescence
behavior of NDC gels, it was found by chance that the phase
transition of the NDC/DMSO gel to the sol can be simply
induced by bubbling ammonia. Interestingly, the reversed
process is realized aer evaporation of the gas (cf. Fig. 8). The
process can be repeated many times. Interestingly, the reversed
process is realized aer evaporation of the gas (cf. Fig. 8). The
process can be repeated many times.
Further reference to the gure reveals that the ammonia-
triggered sol–gel phase transition was accompanied with uo-
rescence quenching, and the quenching was reversed upon
evaporation of the ammonia introduced. It is to be noted that
uorescence quenching was not found in temperature-induced
phase transition.
To explore the mechanism of uorescence quenching
induced by ammonia vapor, 1H NMR measurement was con-
ducted. Firstly, the DMSO-d6 solution of NDC at a concentration
of 1.0% (w/v) was prepared in a NMR tube. Secondly, ammonia
vapor was injected into the tube, which was accompanied by
gel–sol phase transition and uorescence quenching. The 1H
NMR spectra recorded before and aer ammonia gas injection
Fig. 6 Solubility data for NDC in the liquids represented in Hansen
space with spheres/shells, where blue stands for the soluble space, red
stands for the gel space, and green stands for the insoluble one. The
dots marked in different colors are from the results shown in Table S1.†
Again, the colors have same meaning with those described for the
space or spheres.
9164 | Soft Matter, 2014, 10, 9159–9166
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Soft Matter
Fig. 7 (a) XRD profile of NDC/DMSO (1.0%, w/v) and the primary assembly structure of a packing unit (inset). (b) Possible packing mode of the
fibrous aggregates of NDC/DMSO gel.
revealed, it is reasonable to believe that it is the interaction of
ammonia and the nitro group of NBD that causes uorescence
quenching and gel–sol phase transition.
The discovery is of great importance for conducting func-
tionality-oriented supra-molecular gel research. For example, an
ammonia sensing lm had been fabricated by in situ growth of
the NDC nanostructures via coating the NDC/DMSO gel on a
glass plate surface. To examine the sensing performance of the
lm, a conceptual device for monitoring “ammonia leaking”
had been developed (cf. Fig. S5†). As shown in a short video (cf.
video S1†), the system works very well, and what is even happier,
the sensing can be repeated many times. A detailed study is in
Fig. 8 Reversible gel–sol phase transition of the NDC/DMSO gel via
progress.
injection of ammonia and evaporation of the gas (a), and the
accompanied fluorescence quenching and fluorescence recovery (b).
4. Conclusions
are shown in Fig. 9. Compared with the results from the
temperature- and concentration-dependent H NMR measure-
1
Two NBD-containing Chol derivatives with L-/D-phenylalanine in
the linkers were designed and synthesized. Gelation tests
demonstrated that the L-phenylalanine containing compound,
NLC, does not gel any of the liquids tested, but the one with D-
phenylalanine in the linker, NDC, gels 7 of the 34 liquids tested.
Interestingly, the gel networks of the NDC/DMSO gel are char-
acterized by highly ordered brous array structures, and show
remarkable thixotropic properties. More interestingly, it was
found that the introduction of ammonia gas could result in gel–
sol phase transition and uorescence quenching of the gel
system. The non-uorescent solution as resulted, however,
could be reverted immediately to a uorescent gel upon evap-
oration of the ammonia gas. On the basis of the discovery, an
ammonia sensing lm had been fabricated, and the lm-based
“ammonia leaking” monitoring device was developed. The
preliminary test demonstrated that performance of the system
is exceptionally good, a typical and persuasive example to show
the important real-life applications of molecular gels.
ments (cf. Fig. 3), of which the changes of the signals triggered
by the phase transition are slight, the changes of the proton
signals of NBD unit in this case are obvious, shiing from 8.50
and 6.39 ppm to 7.69 ppm and 5.88 ppm, respectively. At the
same time, some other changes were also observed. For
example, the signal of N–H disappears. Considering the results
Acknowledgements
We thank the Natural Science Foundation of China (91027017,
21273141, and 21206089) and the Program for Changjiang
Scholars and Innovative Research Team in University (IRT1070).
Fig. 9 (a) 1H NMR spectrum of NDC in DMSO-d6, and (b) 1H NMR
spectrum of NDC in DMSO-d6 with addition of ammonia.
This journal is © The Royal Society of Chemistry 2014
Soft Matter, 2014, 10, 9159–9166 | 9165

View Article Online
Soft Matter
Paper
27 G. D. Zhou, H. L. Wang, Y. Ma and X. Q. Chen, Tetrahedron,
2013, 69, 867–870.
Notes and references
28 Z. Y. Xu, J. X. Peng, N. Yan, H. Yu, S. S. Zhang, K. Q. Liu and
Y. Fang, So Matter, 2013, 9, 1091–1099.
1 A. Wynne, M. Whiteeld, A. J. Dixon and S. Anderson, J.
Dermatol. Treat., 2002, 13, 61–66.
29 Y. G. Li, K. Q. Liu, J. Liu, J. X. Peng, X. L. Feng and Y. Fang,
Langmuir, 2006, 22, 7016–7020.
30 A. Takahashi, M. Sakai and T. Kato, Polym. J., 1980, 12, 335–
341.
31 X. Y. Hou, D. Gao, J. L. Yan, Y. Ma, K. Q. Liu and Y. Fang,
Langmuir, 2011, 27, 12156.
32 K. K. Diehn, H. Oh, R. Hashemipour, R. G. Weiss and
S. R. Raghavan, So Matter, 2014, 10, 2632–2640.
33 N. Yan, Z. Y. Xu, K. K. Diehn, S. R. Raghavan, Y. Fang and
R. G. Weiss, J. Am. Chem. Soc., 2013, 135, 8989–8999.
34 G. Bao and S. Suresh, Nat. Mater., 2003, 2, 715–725.
35 M. M. Stevens and J. H. George, Science, 2005, 310, 1135–
1138.
36 J. C. Tiller, Angew. Chem., Int. Ed., 2003, 42, 3072–3075.
37 S. Mizrahi, J. Gun, Z. G. Kipervaser and O. Lev, Anal. Chem.,
2004, 76, 5399–5404.
2 I. Tokarev and S. Minko, So Matter, 2009, 5, 511–524.
3 P. Baglioni, L. G. Dei, E. Carretti and R. Giorgi, Langmuir,
2009, 25, 8373–8374.
4 L. kumar, R. P. Singh, S. G. Singh and D. kumar, Int. J. Pharm.
Sci. Rev. Res., 2011, 9, 83–91.
5 M. Jaiswal, S. Lale, N. G. Ramesh and V. Koul, React. Funct.
Polym., 2013, 73, 1493–1499.
6 N. Yan, G. He, H. L. Zhang, L. P. Ding and Y. Fang, Langmuir,
2010, 26, 5909–5917.
7 K. Q. Liu and J. W. Steed, So Matter, 2013, 9, 11699–11705.
8 M. Xue, D. Gao, K. Q. Liu, J. X. Peng and Y. Fang, Tetrahedron,
2009, 65, 3369–3377.
9 B. K. An, D. S. Lee, J. S. Lee, Y. S. Park, H. S. Song and
S. Y. Park, J. Am. Chem. Soc., 2004, 126, 10232–10233.
10 K. K. Kartha, S. S. Babu, S. Srinivasan and A. Ajayaghosh, J.
Am. Chem. Soc., 2012, 134, 4834–4841.
38 J. Brinksma, B. L. Feringa, R. M. Kellogg, R. Vreeker and
J. v. Esch, Langmuir, 2000, 16, 9249–9255.
11 C. M. Yu, M. Xue, K. Q. Liu, G. Wang and Y. Fang, Langmuir,
2014, 30, 1257–1265.
39 P. Kirilov, F. Gauffre, S. F. Messant, E. Perez and I. R. Lattes,
J. Phys. Chem. B, 2009, 113, 11101–11108.
12 V. Bhalla, A. Gupta, M. Kumar, D. S. S. Rao and S. K. Prasad,
ACS Appl. Mater. Interfaces, 2013, 5, 672–679.
13 J. W. Chung, B. K. An and S. Y. Park, Chem. Mater., 2008, 20,
6750–6755.
14 X. Z. Yan, F. Wang, B. Zheng and F. H. Huang, Chem. Soc.
Rev., 2012, 41, 6042.
15 L. X. Jiang, Y. Yan and J. B. Huang, So Matter, 2011, 7,
10417–10423.
16 M. Piepenbrock, G. O. Lloyd, N. Clarke and J. W. Steed,
Chem. Rev., 2010, 110, 1960.
´
40 M. Lescanne, P. Grondin, A. d'Aleo, F. Fages, J. L. Pozzo,
O. M. Monval, P. Reinheimer and A. Colin, Langmuir, 2004,
20, 3032–3041.
41 P. D. Sawant and X. Y. Liu, Chem. Mater., 2002, 14, 3793–
3798.
42 A. Dawn, T. Shiraki, H. Ichikawa, A. Takada, Y. Takahashi,
Y. Tsuchiya, L. T. N. Lien and S. Shinkai, J. Am. Chem. Soc.,
2012, 134, 2161–2171.
43 K. C. Mevada, V. D. Patel and K. R. Patel, Arch. Phys. Res.,
2012, 3, 258–263.
44 S. Diring, F. Camerel, B. Donnio, T. Dintzer, S. Toffanin,
R. Capelli, M. Muccini and R. Ziessel, J. Am. Chem. Soc.,
2009, 131, 18177–18185.
45 X. Q. Cai, K. Q. Liu, J. L. Yan, H. L. Zhang, X. Y. Hou, Z. Liu
and Y. Fang, So Matter, 2012, 8, 3756–3761.
46 A. Rudert and R. Schwarze, J. Non-Newtonian Fluid Mech.,
2009, 161, 60–68.
47 P. K. Vemula, J. Li and G. John, J. Am. Chem. Soc., 2006, 128,
8932–8938.
17 Y. Hisamatsu, S. Banerjee, M. B. Avinash, T. Govindaraju and
C. Schmuck, Angew. Chem., Int. Ed., 2013, 52, 12550–12554.
18 X. D. Yu, L. M. Chen, M. M. Zhang and T. Yi, Chem. Soc. Rev.,
2014, 43, 5346–5371.
19 C. Wang, Q. Chen, F. Sun, D. Q. Zhang, G. X. Zhang,
Y. Y. Huang, R. Zhao and D. B. Zhu, J. Am. Chem. Soc.,
2010, 132, 3092–3096.
20 Y. Hisamatsu, S. Banerjee, M. B. Avinash, T. Govindaraju and
C. Schmuck, Angew. Chem., Int. Ed., 2013, 52, 12550–12554.
21 S. Samai and K. Biradha, Chem. Mater., 2012, 24, 1165–1173.
22 M. M. Zhang, D. H. Xu, X. Z. Yan, J. Z. Chen, S. Y. Dong,
B. Zheng and F. H. Huang, Angew. Chem., Int. Ed., 2012,
51, 7011–7015.
48 N. Harada, S. L. Chen and K. Nakanishi, J. Am. Chem. Soc.,
1975, 17, 5343–5352.
49 P. F. Duan, H. Cao, L. Zhang and M. H. Liu, So Matter, 2014,
10, 5428–5448.
23 H. Shao, C. F. Wang, J. Zhang and S. Chen, Macromolecules,
2014, 47, 1875–1881.
50 J. J. van Gorp, Helices by Hydrogen Bonding: Folding and
Stacking of Chiral Supramolecular Scaffolds, Technische
Universiteit Eindhoven, 2004.
24 M. George and R. G. Weiss, J. Am. Chem. Soc., 2001, 123,
10393–10394.
25 R. G. Weiss, J. Am. Chem. Soc., 2014, 136, 7519–7530.
26 S. R. Liu and S. P. Wu, J. Fluoresc., 2011, 21, 1599–1605.
9166 | Soft Matter, 2014, 10, 9159–9166
This journal is © The Royal Society of Chemistry 2014