Optical detection of anions using N-(4-(4-nitrophenylazo)phenyl)-N′- propyl thiourea bound silica film

Article abstract of DOI:10.1039/c2nj40714k

The anion-sensing properties of N-(4-nitrophenylazo)-N′-propyl thiourea chromoionophores have been investigated as free agents in bulk solution and following immobilization on a sol-gel silica film. In addition, the effect of a variety of organic spacers on the immobilized chromoionophores was examined. The binding constants of free chromoionophores with anions were greater than those of the corresponding immobilized chromoionophores in acetonitrile solution and their binding constants revealed a trend were CH ₃CO₂^- > H₂PO₄ ^- > Cl^-. However, the use of an organic spacer with an immobilized chromoionophore can change the binding constants of anions. An octadecyl group spacer, for example, led to an increase in the difference between the binding constants of the acetate ion and other anions and also enhanced selectivity for the acetate ion.

Full text of DOI:10.1039/c2nj40714k

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Optical Detection of Anions using N-(4-(4-nitrophenylazo)phenyl)-N'-propyl thiourea
Bound Silica Film.
Ryo Kato,[a] Akinori Kawai,[b] and Toshiaki Hattori[b,c]
a Cooperative Research Facility Center, Toyohashi University of Technology,
Toyohashi, 441-8580, Japan
b Department of Materials Science, Toyohashi University of Technology,
Toyohashi, 441-8580, Japan
c Department of Electrical & Electronic Information Engineering, Toyohashi University of
Technology,
Toyohashi, 441-8580, Japan
Abstract
The anion sensing properties of N-(4-nitrophenylazo)-N’-propyl thiourea chromoionophores have
been investigated as free agents in bulk solution and following immobilization on a sol-gel silica
film. In addition, the effect of a variety of organic spacers on the immobilized chromoionophores
was examined. The binding constants of free chromoionophores with anions were greater than those
of the corresponding immobilized chromoionophores in acetonitrile solution and their binding
-
-
constants revealed a trend were CH3CO2 > H2PO4 > Cl-. However, the use of an organic spacer with
an immobilized chromoionophore can change the binding constants of anions. An octadecyl group
spacer, for example, led to an increase in the difference between the binding constants of the acetate
ion and other anions and also enhanced selectivity for the acetate ion.

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Key Words
Chromophore, Silica film, Thiourea

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Introduction
The development of a sensing system for biologically and/or environmentally important anions has
gained significant interest in recent years. 1 Anion sensing reagent, which can recognize target anions
and sense us by optical or electrochemical signals, is a useful method for sensing these anions. Until
now, many reagents which have much good affinity and selectivity have been developed. 2
Moreover, in recent years, the research on the elucidation of the anion recognition behavior at the
interface has also attracted attention. Specific reactions which cannot occur in bulk solution can
feasibly occur at the interface, because the physicochemical properties of the interface are very
different from those of the bulk solution. Nucleic acid bases and other biologically important
molecules can be recognized by using multiple hydrogen bonds at the air/water interface.3 Inorganic
anions have been recognized by the thiourea and thiourea-isothiouronium type receptors at the
oil/water interface.4 It has been reported that the anions dissolved in aquerous media can be
recognized through hydrogen bonding by self-assembled mono-layers on gold5 or by LB film on
HOPG.6 The reports mentioned above indicate that hydrogen bonding can be useful in the
recognition of hydrophilic anions at the interface in water. Anion sensing properties at the
silica/water interface have also been studied. Sessler and co-workers immobilized calix[4]pyrroles
onto silica gel and separated anions by HPLC.7 Furthermore, some groups have immobilized anion
recognition chromophores onto the surface of silica gel or mesoporous silica and measured fluoride,8

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hydrophobic carboxylates9 and phosphate anions.10
Herein the fabrication of a new anion sensing silica film on a glass substrate is reported. Anion
sensing silica film was fabricated by sol-gel method using sol solution containing tetraethoxysilane
and a chromogenic anion receptor. Previously, we reported a selectively for the acetate ion in
acetonitrile-water solution using a sensor molecule combining 4-nitrophenyl and thiourea
functionality.11 The substrate that we used as the foundation, which carried out the coat of the anion
sensing silica film in this research, was glass which does not penetrate ultraviolet light. Therefore,
N-(4-(4-nitrophenylazo)phenyl)-N’-propyl thiourea 1, which has an absorption band in a visible
wavelength region, was used as the chromogenic anion receptor of the anion sensing silica film. This
receptor associates with anions by hydrogen bonding with observable changes to the absorption
spectra. The absorption spectra of this film therefore changes with the concentration of anions. To
immobilize 1 on silica films, N-(4-(4-nitrophenylazo)phenyl)-N'-(3-(triethoxysylil)propyl) thiourea 2
was added into sol solution. The structure of 1 and 2 are depicted in scheme 1. The receptor 2 was
created immediately and almost quantitatively by mixing 3-aminopropyl triethoxysilane and
4-nitro-4'-isothiocyanate azobenzene (ITC-AB). This substrate can be fabricated with ease from
commercially inexpensive reagents, easily analyzed and used repeatedly. Three other silica films
with the receptor bound were also produced containing sequentially hydrocarbons (alkyl or phenyl
group), as shown in figure 1. By mixing short chain hydrocarbons sequentially (figures 1b and 1c), a

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space is produced between the anion receptors. The introduction of a long alkyl chain lowered the
polarity near the thiourea (figure 1d). Therefore, compared with the film of only a receptor molecule
(figure 1a), a change in the selectivity and response was expected. Although these anion sensing
films were too thin to measure the transmittance absorption spectrum, the anion sensing ability of the
film was investigated instead by multiple internal reflectance absorption spectra with an optical
waveguide system. Anion chromoionophore-bound silica films were fabricated on glass substrates
and their anion-sensing abilities were evaluated.
Experimental
Reagents and apparatus
NMR spectra were obtained on a Bruker Avance III-400 instrument using the residual solvent signal
of DMSO or CHCl3 as a reference. Transmittance UV-Vis spectra were recorded at room
temperature with a JASCO V-550-DS spectrophotometer (Japan Spectroscopic Co. Ltd., Tokyo,
Japan) using a quartz cell of 1.0 cm path length. All of the reagents used for the synthesis were
special or first grade and used as received. Note that the stabilizer (ethanol, contains about 0.3% -
1.0%) in CHCl3 was not removed. All of the guest anions were commercially available as
tetrabutylammonium salts. HPLC-grade 1,2-dichloroethane (1,2-DCE) and ethanol were purchased
from Wako Pure Chemical Industries (Tokyo, Japan). Spectrometric-grade acetonitrile (MeCN) was
purchased from Nacalai Tesque, Inc. (Kyoto, Japan). 3-aminopropyl triethoxysilane (APTES),

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dimethyldiethoxysilane
(DMDES),
diphenyldiethoxysilane
(DPDES)
and
octadecyldimethylmethoxysilane (ODDMMS) were purchased from Tokyo Kasei Co. Ltd., (Kyoto,
Japan).
Synthesis
Synthesis of 4-nitro-4’-isothicyanatoazobenzene (ITC-AB)
4-nitro-4’-isothiocyanatoazobenzene was synthesized according to an improved version of a
literature method.12 Thiophosgene (0.17 g, 1.48 mmol), triethylamine (0.18 g, 1.78 mmol) and
4-(4-nitrophenylazo)aniline (0.35 g, 1.44 mmol) were dissolved in tetrahydrofuran (THF) and the
solution was refluxed for 3 h. After cooling to room temperature, THF of the reaction mixture was
reduced to about 1/3 by evaporation and the residue was dissolved in CHCl3. This CHCl3 solution
was washed twice with water and then with brine and dried (Na2SO4). The solvent was removed by
evaporation and the residue purified by silica gel column chromatography (hexane/CHCl3 1:1).
Orange solid, 0.135 g. (48%). 1H-NMR (400 MHz, DMSO-d6,):
δ
8.42 (d, 2H, J = 9.0 Hz), 8.09 (d,
2H, J = 9.0 Hz), 8.03 (d, 2H, J = 8.7 Hz), 7.68 (d, 2H, J = 8.7 Hz), 13C-NMR(100 MHz, DMSO-d6,):
δ
154.4, 149.6, 148.1, 135.3, 133.2, 126.7, 124.4, 124.0, 123.0. IR (KBr): 2101 cm-1 (-N=C=S).
ESI-HRMS m/z: [M+H]+ Calcd for C13H8O2N4S 284.0373; Found: 284.0376.

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Synthesis of N-(4-(4-nitrophenylazo)phenyl)-N’-propyl thiourea (1)
4-nitro-4’-isothicyanatoazobenzene (0.142 g, 0.50 mmol) was dissolved in CHCl3. Propylamine (1.0
mL, 12 mmol) was added to the solution and the mixture was stirred at room temperature for 24 h.
The solvent was removed by evaporation and the residue was purified by silica gel column
chromatography (CHCl3). Red solid, 0.166 g (96%).
1H-NMR (400 MHz, DMSO-d6,):
9.91 (s, 1H), 8.42 (d, 2H, J = 10.0 Hz), 8.15 (s, 1H), 8.04 (d, 2H,
δ
J = 10.0 Hz), 7.93 (d, 2H, J = 9.0 Hz), 7.80 (d, 2H, J = 9.0 Hz), 3.44 (s, 2H), 1.58 (q, 2H, J = 7.0 Hz),
0.91 (t, 3H, J = 7.4 Hz). 13C-NMR(100 MHz, DMSO-d6,):
δ
179.5, 154.9, 147.5, 146.9, 143.7, 124.4,
123.4, 122.5, 120.8, 45.0, 20.9, 10.7. ESI-HRMS m/z: [M-H]- Calcd for C16H17O2N5S 342.1030;
Found 342.1033.
Anion Sensing Substrate Preparation
The substrate was the slide glass plate (Matsunami, 98 × 26 × 1.0 mm) with the refractive index of
1.524 at 1.06 ꢀm. The substrate was cut into 26 × 10 mm sections and etched with 0.1 M NaOH
solution for 30 second and then washed with 0.1 M HNO3 solution and water.
Anion sensing silica film was made on this substrate by sol-gel method. ITC-AB (12 mg, 0.042
mmol), APTES (0.01 ml, 0.043 mmol) and either DMDES, DPDES or ODDMMS (0.042 mmol in
either case) were agitated in a 2:1 mixture of ethanol/1,2-DCE at room temperature for 24 h. The

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contents of sol solutions are summarized in table 1. The solution was casted on the substrate with a
spin coater (4000 rpm, 30 second) and heat-treated at 150 °C for 2 h in air on a hot plate.
Total-Internal-Reflection Absorption Spectrum Measurement
The total-internal-reflection absorption spectrum of anion sensing silica film was measured over the
wavelength range of 360–800 nm using a bespoke optical waveguide spectrophotometer which has
previously been described.13 The 10 ꢀl of sample solution (anion-contained acetonitrile) was
dropped onto the film coated glass substrate and a cover glass (Matsunami, 18 × 18 × 0.1 mm) was
placed over the sample solution to prevent evaporation of acetonitrile. All measurements were
repeated three times.
Result and Discussion
The anion recognition behavior of non-immobilized anion receptor 1 in acetonitrile solution was
examined by UV-Vis and NMR spectroscopy. Figure 2 shows the changes in the absorption spectra
of 1 on addition of acetate in acetonitrile. Receptor 1 has an absorption peak at 389 nm assigned to a
charge transfer between the nitrogen atom of the thiourea moiety and nitrophenylazo group. When
acetate was added, the absorption peak was shifted to 425 nm with the isosbestic point at 401 nm.
Such a bathochromic shift was also observed for dihydrogenphosphate and chloride anions. Solution

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color changed from yellow to orange. From the binding constants, the anion selectivity of receptor 1
was acetate > dihydrogenphosphate > chloride, following the anion basicity and the binding
constants (K11) themselves were 1.4 × 105 M-1, 5.3 × 104 M-1 and 2.1 × 103 M-1 respectively. The
binding ability of 1 was found to be superior to other structurally simpler amide,14 urea and
thiourea15 anion recognition molecules. This greater affinity is derived from the enhanced acidity of
the thiourea which was affected by the electron withdrawing effect of the neighboring nitro group.
However, the binding ability of 1 was inferior to well-structured receptors,16 thiourea combined with
nitro phenyl group11 and positively charged receptors such as isothioureas17 and guanidines.18 The
electron withdrawing effect was buffered by the azobenzene group and the acidity of the thiourea
didn’t increase as much. Both spectral and solution color changes were in agreement with those
reported in the literature.19 The process of the hydrogen-bond formation was investigated by
1
1H-NMR titration. Figure 3 shows the changes in the H-NMR spectra of 1 in the absence and the
presence of acetate in CD3CN. Two NH proton peaks were appeared at 8.54 ppm and 7.05 ppm.
Upon addition of acetate anion, significant downfield shifts (up to 3.1 ppm) of the peaks were
observed, indicating that the 1 and acetate form the complex via hydrogen bonding.
The absorption spectra of anion receptor 2 based on silica film on a glass substrate was measured by
the optical waveguide system. Figure 4 shows the changes in the absorption spectra of receptor
molecule only film (substrate AR) on addition of acetate. The absorption peak observed at 389 nm

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was shifted to 412 nm with an isosbestic point at 400 nm. The absorption peak position in the
absence of acetate and the isosbestic point were approximately consisted with the bulk behavior.
However, the absorption spectra were shifted in the 10-4 M – 10-2 M region for acetate, whereas the
absorption spectra of 1 were shifted in the 10-6 M – 10-5 M region in the acetonitrile solution. Similar
reductions in anion response ability were also observed for dihydrogenphosphate and chloride anions.
The results showed that immobilization on substrate led to a reduction in the anion response ability.
The possibility that an anion sensing reagent arranged densely with enough space for an anion to
enter is not securable, as a result of producing a film only using an anion sensing reagent, is worthy
of mention. Furthermore, intermolecular hydrogen bonds may have formed between the adjoining
thiocarbonyl and NH moiety of a thiourea group, as a result of having arranged the anion
recognizing reagent densely (figure 1a).
To improve the anion response ability and selectivity, alkyl chains (CH3 or C18H37) or phenyl groups
were introduced to the anion sensing silica film. The introduction of the methyl group (substrate
AR-Me, Fig. 1b) created some space near anion receptor 2, allowing anion and solvent molecule to
reach deep inside the film. A Phenyl group can provide cover close to the thiourea moiety of 2
(substrate AR-Ph, Fig. 1c), whereas the long alkyl chain of the octadecyl group covers all of 2
(substrate AR-Od, Fig. 1d). It was expected that these differences would affect the anion response
and selectivity.

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All substrates observed the bathochromic shift with anion containing acetonitrile solution. Figure 5
shows the absorption changes of AR, AR-Od and anion receptor 1 at various concentrations of
acetate in acetonitrile solution. Responses for anions were improved in three substrates (AR-Me,
AR-Ph and AR-Od) compared with the substrate AR. However, the response from a substrate did not
reach to the level of strength observed in the acetonitrile. Nonlinear fitting was performed on the
assumption that anion receptor 2 reacts with anions with a 1:1 stoichiometry. The stability constants
are summarized in table 2. The trend in anion selectivity for substrates AR, AR-Me and AR-Ph was
acetate ≈ dihydrogenphosphate > chloride. These results differed from the selectivity observed in
acetonitrile. In contrast, the anion selectivity of substrate AR-Od showed the same tendency as the
result in an acetonitrile. The acetate ion has a methyl group with a high affinity for alkyl groups and
was able to approach the thiourea group between the octadecyl groups with greater ease than the
other anions.
To investigate the anion recognizing behavior mentioned above, the measurement was repeated at
least 20 times in all the substrates, but neither the unfolding of a spectrum nor the decrease of the
absorbance was observed. Therefore, we concluded that the endurance of the anion sensing silica
film fabricated by the present study is high.
Finally, we attempted to use these substrates for anion sensing in water and acetonitrile-water mixed
solution. However, no response was observed in any of the substrate-anion combinations. Other

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groups have achieved anion recognition in water at self-assembled monolayers on a gold electrode5
and with LB film on the HOPG6 using the hydrogen bond-forming receptor. This result showed that
as for the film made from the sol-gel process, a hydrogen bond does not function in the water/silica
film interface.
Conclusion
An anion sensing silica film was fabricated on a glass substrate. The anion sensing silica film
consisted of a N-(4-(4-nitrophenylazo)phenyl)-N’-propyl thiourea which was used as the
chromoionophore for anions and an alkyl or phenyl spacer group set in a silica matrix. The anion
sensing abilities of the films were evaluated by changes in the total-internal-reflection absorption
spectra at various concentrations of anion. As a result, the response from the anion sensing film
decreased to a 1/100 of its value when compared with the response from the anion receptor in bulk
solution. Response was improved by the addition of each functional group spacer, including methyl,
phenyl and octadecyl and the selectivity was also changed by adding these alkyl groups. These
results show that response and selectivity for anions can be controlled by varying the type and the
quantity of a spacer. With this in mind, we are currently trying to improve the response of such films
by investigating the type and the quantity of the spacer molecules involved.

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References
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The anion sensing properties of N-(4-nitrophenylazo)-N'-propyl thiourea chromoionophores have been
investigated as free agents in bulk solution and following immobilization on a sol-gel silica film.
41x21mm (300 x 300 DPI)

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Scheme 1. Synthetic route to chromogenic anion receptors 1 and 2. Reagents and conditions: (i): CSCl2,
(C2H5)3N, THF, reflux (ii): n-C3H7NH2, THF, r.t. (iii): NH2CH2CH2CH2Si(OC2H5)3, EtOH-1,2-DCE, r.t.

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Figure 1. Surface structures and binding images of anion sensing film (a) AR, (b) AR-Me, (c) AR-Ph and (d)
AR-Od.

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Changes in the absorption spectra of 1 on addition of the acetate anion. [1] = 1.0×10-5 M in MeCN. Counter
cation : tetrabutylammonium cation.

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Figure 3. 1H-NMR spectra of 1 in the absence and the presence of 1 equiv. of acetate anion. [1] = 1.0 mM
in CD3CN.

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Figure 4. Changes in the absorption spectra of substrate AR at various concentrations of
the acetate anion in acetonitrile.

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The anion sensing properties of N-(4-nitrophenylazo)-N'-propyl thiourea chromoionophores have been
investigated as free agents in bulk solution and following immobilization on a sol-gel silica film.
41x21mm (300 x 300 DPI)

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Scheme 1. Synthetic route to chromogenic anion receptors 1 and 2. Reagents and conditions: (i): CSCl2,
(C2H5)3N, THF, reflux (ii): n-C3H7NH2, THF, r.t. (iii): NH2CH2CH2CH2Si(OC2H5)3, EtOH-1,2-DCE, r.t.

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Figure 1. Surface structures and binding images of anion sensing film (a) AR, (b) AR-Me, (c) AR-Ph and (d)
AR-Od.

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Changes in the absorption spectra of 1 on addition of the acetate anion. [1] = 1.0×10-5 M in MeCN. Counter
cation : tetrabutylammonium cation.

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Figure 3. 1H-NMR spectra of 1 in the absence and the presence of 1 equiv. of acetate anion. [1] = 1.0 mM
in CD3CN.

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Figure 5. Absorbance at 425 nm of MeCN solution (open squares), at 409 nm of film AR-Od (solid squares)
and at 409 nm of film AR (open circles) for various concentrations of
acetate (0 – 2.0 × 10-2 M) in acetonitrile.

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Table 1. Contents of the silane coupling reagents (APTES, DMDES, DPDES and ODDMS) and isothiocyanate
azobenzene (ITC-AB) of the sol solutions of the anion sensing silica film.

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Table 2. 1:1 Binding Constants for acetate, phosphate and chloride in MeCN (solution) with AR, AR-Me, AR-
Ph and AR-Od (film).