Modifying a known gelator scaffold for nitrite detection

Article abstract of DOI:10.1039/c4cc02504k

The process of selecting and modifying a known gelator scaffold to develop a new nitrite-based sensor is described. Five new azo-sulfonate gelators were discovered and characterized. The most promising scaffold exhibits a stable diazonium intermediate, proceeds in a high yield, and gels nitrite-spiked tap, river, and pond water. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02504k

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Modifying a known gelator scaffold for nitrite
detection†
Cite this:DOI: 10.1039/c4cc02504k
Danielle M. Zurcher, Yash J. Adhia, Julian Dıaz Romero and Anne J. McNeil*
´
´
Received 4th April 2014,
Accepted 27th May 2014
DOI: 10.1039/c4cc02504k
www.rsc.org/chemcomm
The process of selecting and modifying a known gelator scaffold pre-treatment is often required. In contrast, sensors based on
to develop a new nitrite-based sensor is described. Five new molecular gelation are portable and can operate in complex
azo-sulfonate gelators were discovered and characterized. The media (e.g., opaque samples).3,5,13 To develop a molecular
most promising scaffold exhibits a stable diazonium intermediate, gel-based sensor for nitrite, the first step involved identifying
proceeds in a high yield, and gels nitrite-spiked tap, river, and
pond water.
a
chemical transformation that can be mediated by
nitrite. Herein, the Griess reaction was chosen because of
its high selectivity for nitrite, short reaction time, and high
Small molecule-based gels are being explored for a variety of yield.14 In the original Griess reaction, sulfanilic acid reacts
different applications.1 Identifying an appropriate gelator for a with nitrous acid to form a diazonium ion, which is then
specific application can be quite simple if the final gel state is reacted with a-naphthylamine to generate a red-violet azo dye
all that matters. For example, different growth factors have (eqn (1)).15
been added to known peptide-based gelators and used as
scaffolds in tissue engineering.2 On the other hand, identifying
an appropriate gelator can be quite challenging if the solution-to-
gel phase transition is important to the application (e.g., sensing).
(1)
In this case, one needs not only a gelator, but also a non-gelling,
structurally related precursor. As a consequence, in our previous
work, we found it easier to design a new gelator than to modify
a known gelator for sensing applications.3,4 Nevertheless, we
were motivated to explore the alternative approach5 given the
The next step was to identify a gelator that can be formed via
inherent challenges in designing new gelators,6 and the fact the Griess reaction. A search of the small molecule gel literature
that there are over 1000 reported small molecule gelators.7 Herein revealed a surprisingly large number of azo-containing organo-
we demonstrate that modifying a known gelator for a new gelators16–27 and a few hydrogelators.28–31 In most cases, an
application can be successful. Specifically, we will highlight azobenzene moiety was added to a known gelator scaffold to
how a known gelator scaffold was first targeted and then generate a light-responsive transition (via the trans-to-cis azo-
modified to develop a gel-based sensor for detecting nitrite in benzene isomerization). Because the non-azo-functionalized
aqueous solutions.
precursor is also a gelator, most of these scaffolds are unsuitable
Nitrite (NO2ꢀ) contamination of water is a global concern for sensing. We focused our attention solely on hydrogelators
because of its detrimental effects on human health.8,9 Nitrite with scaffolds wherein the azo-functional group was part
sensors based on electrochemical10 and colorimetric methods,11 of the core structure (rather than an appendage). Only one
among others,12 have been developed, however, sample promising scaffold fit these parameters: azo-sulfonates.31
These azo-compounds can be accessed via the Griess reaction
Department of Chemistry and Macromolecular Science and Engineering Program,
University of Michigan, 930 North University Avenue, Ann Arbor, Michigan,
48109-1055, USA. E-mail: ajmcneil@umich.edu
between an aniline derivative and sodium 6-hydroxy-
naphthalene-2-sulfonate, which are both non-gelling in aqu-
eous solutions (eqn (2)). Azosulfonates 3c and 3f were reported
to gel aqueous buffers, albeit at high concentrations.31 Thus,
we began our studies by modifying this scaffold to develop a
gelation-based nitrite sensor for environmental samples.
† Electronic supplementary information (ESI) available: Synthetic procedures, spectro-
scopic data, gel screening data, rheological data, microscopy data, thermogravimetric
analysis data, UV-vis spectroscopic data, and in situ gelation procedures. See DOI:
10.1039/c4cc02504k
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(2)
Fig. 1 (A) Oscillating stress sweep for a gel of 3e (32 mM in borax buffer,
G0 (ꢂ) and G00 (3)). (B) Scanning electron microscope image of a gel of 3e
(37 mM in EtOH/borax buffer (9/1, v/v)).
We focused our efforts on meta-substituted derivatives
because Hamada and co-workers previously reported that most
para-substituted derivatives were nongelators under aqueous
conditions.31 In total, seven azosulfonates (3a–3g) were synthe-
sized and screened for gelation (Table 1 and ESI†).32 All seven
compounds formed gels in either borax buffer (65 mM, pH = 13)
or EtOH/borax buffer (9/1 v/v).33 Because the ultimate goal is to
use aqueous samples from the environment, we focused on
lowering the critical gel concentration (cgc) in neat borax buffer.
The unsubstituted derivative (3g) was too soluble, so a single
methyl- (3a), chloro- (3b) or trifluoromethyl- (3c) substituent
was introduced to increase the hydrophobicity.34 The most
hydrophobic compound (3c) within this series gave the lowest
cgc in borax buffer. Introducing a second, identical substituent
(3d–f) had a large effect on cgc when R = Cl and a surprisingly
small effect on cgc when R = CF3. Overall, the dichloro-
substituted derivative (3e) exhibited the lowest cgc, although
3b, 3c and 3f were only slightly higher.
Fig. 2 (A) Plot of the normalized absorbance versus wavelength for the
reaction of 1e (4.8 ꢃ 10ꢀ4 mmol) with NaNO2 (4.8 ꢃ 10ꢀ4 mmol) at room
temperature in 4 M aq. H2SO4 (0 min (black solid), 2 min (red dashed)
10 min (green dash dot), 30 min (blue dot), 60 min (purple dash dot)).
(B) 1H NMR spectra (in d6-DMSO) acquired before (bottom) and 15 min after
(top) adding NaNO2 (0.03 mmol) to 1e (0.03 mmol) in 4 M aq. H2SO4 at rt.
The mechanical strength and morphology of gels of 3a–g
The Griess reaction is typically carried out at lower temperatures
were characterized using rheology and scanning electron (e.g., 0 1C), presumably due to concerns about diazonium ion
microscopy, respectively.35 Rheological testing of all gelators stability.14 Nevertheless, a practical and portable sensor should
at 1.5 times the cgc revealed the expected 10-fold (or larger) operate at ambient temperatures. Thus, UV-vis spectroscopy was
storage modulus (G0) relative to the loss modulus (G00) in used to monitor the rate of diazonium ion formation (2a–g) and
both the frequency and oscillating stress sweep experiments, decomposition at room temperature. Gratifyingly, diazonium ions
confirming its gel-like nature (cf., Fig. 1A and ESI†).36 Optical 2b (Cl), 2c (CF3), 2e (Cl/Cl) and 2f (CF3/CF3) were stable at ambient
1
and scanning electron microscopy was performed on each gel temperatures for at least an hour (cf., Fig. 2A and ESI†). H NMR
near or above its cgc to determine the morphology.37 Consistent spectroscopy was used to confirm that a single, stable species
with most molecular gels, anisotropic fibers of varying widths was formed during the reaction (Fig. 2B). In contrast, the CH3-
were observed (cf., Fig. 1B and ESI†).
substituted derivatives (2a, 2d) showed presumed loss of nitrogen
after 2 min and the unsubstituted derivative (2g) decomposed after
30 min. Based on these results, our further studies focused solely
on those gelators formed through stable diazonium intermediates
(i.e., 3b, 3c, 3e and 3f).
One concern that emerged from the syntheses of 3a–g was the
moderate-to-low isolated yields (e.g., 17% yield for 3e, see ESI†).
The sensitivity of a gel-based sensor depends on both the critical
gel concentration and the yield of the reaction used to generate
the gelator. For example, although 3e exhibited the lowest overall
cgc, its low reaction yield could ultimately make it less suitable
in the sensor platform. Because the low synthetic yields were
Table 1 Critical gel concentrations (cgc) in different conditionsa
cgc (mM)
Azosulfonate
EtOH/borax bufferb (9/1, v/v)
Borax bufferb
3a
3b
3c
3d
3e
3f
23.5 ꢁ 0.4
43 ꢁ 3
Precipitate
29.4 ꢁ 0.9c
24.2 ꢁ 0.8
Precipitate
21.3 ꢁ 0.5
27 ꢁ 1
Precipitate
35.5 ꢁ 0.2
16.7 ꢁ 0.6
Precipitate
30.0 ꢁ 0.5
3g
Soluble
presumably due to the extensive purification required to remove
1
a
The error reported as a standard deviation is based on 3 samples. the excess salts, we used H NMR spectroscopy to quantify the
Each compound was screened for gelation at concentrations r2 wt%.
in situ reaction yield for 3b, 3c, 3e, and 3f using an internal
standard (cf., Fig. 3 and ESI†). Gratifyingly, all four reactions
proceeded with yields exceeding 85% (ESI†).
The term ‘‘precipitate’’ was used when any amount of precipitate was
b
c
observed. Borax buffer (65 mM, pH = 13). The solvent consisted of
borax buffer (65 mM)/H2SO4 (4 M)/H2O (7.6/2/0.4, v/v/v).
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screening conditions (pH = 13) to the reaction conditions
(pH = 9) might play a role given the acidic hydrazine proton
(pKa B 11).38 Indeed, a similar cgc was observed for isolated 3e when
the pH matched those of the reaction conditions (9.5 ꢁ 0.3 mM, see
ESI†). Overall, the detection limit39 in these studies was 500 ppm,
which is above the EPA minimum set for safe drinking water
(1 ppm).9 We have previously demonstrated that using a smaller
vial leads to a lower cgc, which can be attributed to an increase in the
surface area between the container and the gel.5 Herein, the detec-
tion limit dropped to 90 ppm using a 1.5 mL vial (instead of 4 mL,
see ESI†).40 Further reducing this detection limit will require either
identifying a better gelator or a reaction that is catalytic in nitrite,
wherein each nitrite produces more than one gelator molecule.
In conclusion, these studies reveal that known gelators can be
successfully modified and then utilized in targeted applications.
Herein we demonstrated that by altering the substituents on
the precursor aniline ring, we were able to develop a nitrite
sensor that is operable under ambient temperatures in aqueous,
environmental samples. The impact of these studies is expected
to be large considering that over a thousand small molecules
have been reported to form gels7 and each of these compounds
represent potential starting points for developing other gelation-
based applications.
Fig. 3 1H NMR spectra of the crude reaction mixture (without workup) for
the reaction of 1e (0.0078 mmol) with NaNO2 (0.0071 mmol) and sodium
6-hydroxynaphthalene-2-sulfonate (0.0078 mmol) in deuterated borax
buffer (top), and the corresponding starting materials and products under
the same solvent conditions.
Gelator 3e was ultimately selected for the sensor platform on
the basis of its low cgc, stable diazonium ion intermediate, and
high reaction conversion. We tested the nitrite sensor in
different water sources by spiking each sample with NaNO2
because the natural [NO2ꢀ] in non-polluted water is low. Vials
containing 1e (suspended in 4 M H2SO4) were treated with the
spiked water samples for 10 min, followed by adding sodium
6-hydroxynaphthalene-2-sulfonate (in borax buffer). The resulting
samples were heated to dissolve all solids and then allowed to
cool to room temperature. The bright red/orange color is indica-
tive of azosulfonate formation (Fig. 4).
As evident in Fig. 4, the gel-based nitrite sensor proved to be
quite robust as it gelled tap water, river and pond water, as well
as water drawn from a muddy pond. The non-spiked water samples
serve as a negative control. Surprisingly, the cgc determined
under these reaction conditions was significantly lower (9.3 mM)
than that observed with the isolated (and purified) compound
(21.3 mM). We suspected that the change in pH from our
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excess sodium nitrite (ESI†).
37 Gels of 3c were transparent within the limits of our optical micro-
scope. In addition, the scanning electron microscope images only
revealed salt crystals from the buffer solution.
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21 Sugars: R. Rajaganesh, A. Gopal, T. M. Das and A. Ajayaghosh, Org. 39 The detection limit is based on the total volume of the gel sample
Lett., 2012, 14, 748–751. and not the volume of nitrite-contaminated water.
22 Dicarboxylates: P. Sahoo and P. Dastidar, Cryst. Growth Des., 2012, 40 This gel-based sensor was also able to detect sodium nitrite in a
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certified standard solution from SPEX CertiPrep Group (ESI†).
Chem. Commun.
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