Reactive photonic film for label-free and selective sensing of cyanide

Article abstract of DOI:10.1002/smll.201101934

Three-dimensional ordered inverse-opal films bearing a reactive trifluoroacetyl group are successfully constructed. Through the specific reaction between cyanide and trifluoroacetyl, the photonic films can selectively detect sub-micromolar levels of cyani

Full text of DOI:10.1002/smll.201101934

full papers
Sensors
Reactive Photonic Film for Label-Free and Selective
Sensing of Cyanide
Xuesong Li, Lihua Peng, Jiecheng Cui, Weina Li, Changxu Lin, Dan Xu, Tian Tian,
Guanxin Zhang, Deqing Zhang,* and Guangtao Li*
T
hree-dimensional ordered inverse-opal films bearing a reactive trifluoroacetyl
group are successfully constructed. Through the specific reaction between cyanide
and trifluoroacetyl, the photonic films can selectively detect sub-micromolar levels of
cyanide by distinct structural color change. Labeled molecules are not necessary for
the sensing mechanism.
affinity between the cyanide anion and copper cation, mas-
1
. Introduction
sive efforts have been made to explore metal-organic cyanide
As one of the most toxic chemicals in physiological systems
and environmental domains, cyanide-containing salts have
attracted considerable attention from researchers all over
the world. Cyanide-containing salts originate not only from
industrial processes, such as electroplating and metallurgy,
but also from micro-organisms and foods that contain cya-
[4]
chemosensors. Most of these chemosensors, however, require
complicated processing in their syntheses, and are detrimental
to the physical health of human beings. Various approaches
based on the excellent nucleophilicity of cyanide have also
[
1]
[5]
been designed. Tian et al. developed chemosensors based
on fused indoline and benzooxazine, whose C–O bond can be
[2]
nogenic glycosides, including cassava and bamboo shoots.
cleaved when cyanide appears in a MeCN/H O (19/1) solution,
2
To prevent human beings from being exposed to cyanide,
the World Health Organization (WHO) has suggested that
cyanide concentration in drinking water should not be more
than 0.07 mg/L. Consequently, there is a growing concern in
the development of chemosensors for cyanide detection. It is
of great significance to develop an effective cyanide chemo-
sensor with high sensitivity and selectivity.
and the cleavage of the C–O bond leads to a change in the
[5b]
color and absorption properties of the solution.
Shiraishi
et al. reported a chemosensor containing a spiropyran deriva-
tive which could perform an addition reaction with cyanide
in MeCN/H O solution. In this reaction, the sensor operates
2
using a color change or UV–vis absorption variation when
[5c]
cyanide is present. All of these sensors mentioned above,
In recent years, an increasing number of researchers
focused on this subject and have completed a considerable
however, can only work when the related sensing elements
are dissolved in solvent(s), and at the same time show lower
[3]
amount of remarkable research. Based on the powerful
[4]
sensitivity than metal-organic chemosensors. The specifical
reaction between cyanide and the trifluoroacetyl group has
[
6]
X. Li, J. Cui, W. Li, C. Lin, D. Xu, T. Tian, Prof. G. Li
Key Lab of Organic Optoelectronic
and Molecular Engineering
Department of Chemistry
Tsinghua University
been advantageously employed to detect cyanide. Owing
to the strong nucleophilicity, cyanide has the ability to attack
the electron-deficient carbonyl of the trifluoroacetyl group,
leading to a negatively charged fragment which can influence
the optical properties of the whole molecule, such as fluo-
rescence, color, and UV–vis absorption. These chemosensors
however can only work with special “labeled” molecules, the
modification of whose properties (e.g., fluorescence, color,
absorption) is the essence of sensing cyanide. In many cases,
it is highly desirable to develop more convenient and “label-
free” cyanide chemosensors, which can be easily operated like
pH test paper. In this work, based on the reactive inverse-
opal photonic structure, we report a new kind of cyanide
Beijing, 100084, P. R. China
E-mail: lgt@mail.tsinghua.edu.cn
Dr. L. Peng, Dr. G. Zhang, Prof. D. Zhang
Organic Solids Laboratory
Institute of Chemistry
Chinese Academy of Sciences
Beijing, 100190, P. R. China
E-mail: dqzhang@iccas.ac.cn
DOI: 10.1002/smll.201101934
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Photonic Film for Label-Free and Selective Sensing of Cyanide
chemosensor that can selectively detect sub-micromolar levels
of cyanide without the use of labels; the presence of cyanide is
indicated by a distinct structural color change.
An inverse opal is a 3D ordered macroporous structure.
This periodical pore structure displays unique optical proper-
ties and gives rise to bright colors through the diffraction of
light. Its optical properties are determined by the Bragg equa-
tion and are extremely sensitive to the change of its refrac-
[7,8]
tive index and lattice parameter.
Based on this property,
various functional materials and devices have been devel-
oped, and they show a great potential for various applications
including chemosensors. In this respect, our group has made
a series of chemosensors which have the ability to detect a
Figure 1. Scanning electron microscopy (SEM) images of the used
SiO2 colloidal crystal template (a) and the resulting inverse-opal film
(b); and the diffraction curve of the photonic film (c). The scale bar in
these images stands for 1 μm.
[9]
[10]
variety of molecules or to sense certain special ions.
Along this research direction, by making use of the specific
addition reaction between cyanide and the trifluoroacetyl
group, we designed a reactive inverse-opal film, in which a
novel pH-paper-like cyanide chemosensor was developed.
The negatively charged fragment generated via the reac-
tion between cyanide and trifluoroacetyl is the key point for
the function of the fabricated sensor films. Upon exposure
to analyte (cyanide) solution, the inverse-opal film changes
from a neutral (hydrophobic) state to a charged (hydrophilic)
state, leading to considerable osmotic pressure, and thus, the
swelling (optical change) of the inverse-opal structure. Based
on this response, the constructed chemosensor can easily
sense cyanide with high sensitivity and selectivity (Scheme 1).
A chemosensor with this kind of sensing scheme possesses
the following advantages: firstly, this film sensor can serve as
a pH-paper-like detector, and its use is more convenient; sec-
ondly, sensing events of the sensor can be efficiently converted
to a readable optical signal without the need of molecular
reporters (label-free) and sophisticated technical equipments,
usually being easily visible to the naked eye, when analyte
exists at high concentration in sample.
(
Scheme 1). A three-step approach was involved: fabrica-
[11]
tion of silica (SiO ) opaline template;
infiltration of the
2
colloidal crystal template with the mixture of functional
monomer (details of the synthesis in the Experimental Sec-
tion), cross-linker and initiator (2,2′-azoisobutyronitrile,
AIBN) followed by thermal-induced polymerization; and
selective dissolution of the SiO template to afford the 3D
ordered macroporous structure (inverse opal). Uniform SiO2
particles with a diameter of about 330 nm used in this work
were used to form close-packed face-centered cubic (fcc)
photonic crystal films with thicknesses of about 2 to 9 μm
2
(Figure 1a). For improving the film formability of the inverse
opal, methyl methacrylate was utilized in our polymerization
mixture. SiO2 templates were selectively etched by dilute
HF, affording the inverse opal film with the ordered struc-
ture replicated from the SiO crystal template (Figure 1b).
2
This kind of polymer film possesses fascinating optical prop-
erties (Figure 1c) due to its periodic structure. When the
inverse opal film was embedded into the cyanide solution, it
is interesting to find that the color of polymer film changes
from green to red when the concentration of cyanide is
1
mm (Figure 2a), and the diffraction peak gradually shifts to
longer wavelength with increasing concentration of cyanide
(
2
. Results and Discussion
Figure 2b). Even at the concentration of 0.1 μm (10− m),
7
Colloidal crystal templates were used for preparation of
D ordered inverse-opal films bearing trifluoroacetyl group
which is lower than the requirement of the WHO for drinking
3
[2]
water (1.9 μm), the prepared photonic film still exhibits
detection capability (Figure 2b), and the shift of diffrac-
tion peak is about 8 nm from 542 to 550 nm. As mentioned
above, the specific reaction between cyanide and trifluoro-
acetyl should be responsible for the observed red-shifts in the
spectra of the photonic film. As cyanide attacks the carbonyl
of trifluoroacetyl pendant groups of the photonic film with
the formation of a nagatively charged state, the lattice spacing
of photonic films increases, leading to the red-shift of the dif-
fraction peak. As a control experiment, the same photonic
film was embedded in water containing fluorate at different
concentrations (Figure 2c). Only small irregular fluctuations
of the diffraction peak were observed. The contact angles of
the fabricated photonic films that reacted at different concen-
−
4
−3
trations of cyanide (0, 10 , 10 m) were analyzed to explain
the swelling kinetics (Figure 3). As shown in Figure 3, the ini-
tial film, without the treatment of cyanide addition reaction,
Scheme 1. Schematic illustration of the fabrication of reactive photonic
film for label-free and selective sensing of cyanide through the specific has a large contact angle compared with other films treated
reaction between cyanide and trifluoroacetyl group.
with cyanide solution, and the contact angles gradually
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X. Li et al.
Figure 3. Contact angles of the photonic films after the treatment with
−
4
−3
cyanide solution of a) 0, b) 10 , and c) 10 m.
work, it is found that the observed color changes for each
cyanide concentration correspond to the equilibrium state of
every swollen photonic film. In our case, the resulting inter-
connected porous structure is especially beneficial in sensor
Figure 2. Optical images (a) and the responses of the photonic film
upon exposure to different concentrations of cyanide (b) and fluorate
(
c); the selectivity (d) of the photonic film towards cyanide, acetate, applications that require high specific surface area, more
fluorate, and dihydrogen phosphate (DHP).
interaction sites, efficient mass transport, and easier accessi-
bility to the active sites. The reaction of the prepared reactive
decrease with the increase of cyanide concentration. This film with cyanide is very fast and takes only several seconds.
result indicates that with the increase of cyanide concentra-
The response of the photonic film was examined upon
tion, more hydrophobic trifluoroacetyl groups in the sensor contact with a sequentially more concentrated cyanide
film convert into negatively charged cyanohydrin units and solution. It is found that, with the increase of cyanide con-
thus the film becomes more hydrophilic and swellable. In our centration, the diffraction peak of the photonic film shifts
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Photonic Film for Label-Free and Selective Sensing of Cyanide
Figure 5. 1H-NMR spectra of the samples before and after the addition
of different quantity of cyanide in the d -DMSO solution of the
6
trifluoroacetyl-functionalized monomer.
of our prepared films with these species was not observed.
Additionally, it should be noted that, due to hydrophobic
character of the fabricated photonic films, several drops of
organic solvent (MeCN, ∼10% v/v) were added to aqueous
solution (pH = 7) of cyanide to facilitate the reaction
between the trifluoroacetyl group and cyanide ion within the
polymer film.
The addition reaction between cyanide and trifluoro-
1
acetyl group has been confirmed by H NMR experiments.
Figure 5 shows the 1H-NMR spectra of the synthesized
Figure 4. a) Diffraction spectra showing shifts of the diffraction peak of functional monomer in the presence of different quantities
the prepared photonic films upon contact with different concentrations of cyanide. The proton signals of the aromatic moiety were
of cyanide and b) the dependence of the diffraction peak position on
concentrations of cyanide.
obviously shifted up-field. As 1.0 equivalent of cyanide was
introduced into the d6-dimethylsulfoxide (d -DMSO)
6
monomer solution, the signals at 7.55 and 7.30 ppm were
shifted to ca. 7.40 and 7.20 ppm, respectively. When more
continuously to longer wavelength range (Figure 4). However, than 1.0 equivalent of cyanide was dissolved in d -DMSO
6
after a threshold concentration, the magnitude of peak shifts solution, the signals remain unchanged, revealing the
decrease (Figure 4). A similar phenomenon was observed in 1:1 reaction stoichiometry between cyanide and the functional
[12]
the literature.
strength in the external solution caused by the a higher con- previously reported work.
centration of cyanide and thus the decrease of the inner
The regeneration of the described sensor was exam-
Donnan potential of the film, which is the essence of swelling ined. It is found that upon exposure of the fabricated
in photonic films and the shift of their diffraction peak.
photonic film to acetonitrile solution (1 m) of trifluoro-
The reason is due to the increase of ionic monomer. This result is consistent with the observation in the
[
13]
In order to study the sensing selectivity of the fabricated acetic acid (TFA) under shaking, the chemisorbed cya-
photonic film, several important anions including acetate, nide can be completely removed from the sensor film, and
fluorate, and dihydrogen phosphate (DHP) were chosen to the color change of the photonic film is reversible. In our
test its selectivity in comparison with cyanide. After being work, the photonic sensor film was repeatedly used three
soaked in different anion solutions, photonic films were times (Figure 6), showing an excellent regenerative capa-
[
14]
characterized using UV–vis spectrometry. Figure 2d displays bility. In the literature,
it is reported that cyanide can
the selectivity of the photonic film towards the different attack the carbonyl group activated by the strong electron-
anions under the same characterization conditions. Clearly, withdrawing effect of the trifluoromethyl group, and thus
a significant shift of the diffraction peak was detected only cyanohydrin formation takes place. When TFA is added
in the presence of cyanide, indicating the excellent cyanide to the reaction medium, the chemisorbed cyanide can be
selectivity of the prepared photonic film. The reason why the extracted and cyanohydrin is converted into a carbonyl
selectivity occurs might be that cyanide is more nucleophilic group. The same mechanism should occur in our case
than other anions, such as acetate, fluorate, and DHP. It has described above.
been reported that trifluoroacetyl suffers nucleophilic attack
In this work, we found that the sensing performance of
with amines and alcohols. In our case, however, the reaction the prepared reactive photonic films is strongly dependent on
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X. Li et al.
Figure 7. a,b) SEM images of the photonic films with different
thicknesses, and c) the corresponding magnitudes of their diffraction
peak shifts.
Figure 6. a) Diffraction spectra showing the reversible change of the
diffraction peak of the prepared photonic film treated alternatively by
KCN and TFA solutions, and b) the diffraction peak of photonic sensor
film as it was repeatedly used three times, showing an excellent
regenerative capability.
3
. Conclusion
In summary, based on the combination of the unique optical
properties of photonic crystals and the specific reaction
between cyanide and trifluoroacetyl, a novel label-free cya-
nide chemosensor has been successfully fabricated. This
solid sensor is characterized with high sensing capability and
selectivity for cyanide under ambient conditions with a swift
detecting velocity. Moreover, the detection can induce a dis-
tinct structural colour change, which is easily observable by
the naked eye. We expect this work may create a new per-
spective in the design of chemosensors for cyanide detection.
the preparation and the analytical solution. The shift extent
of the photonic film can be tuned by adjusting the elastic
modulus of the photonic structure or the property of ana-
lytical solution. It can, in general, be operated by altering the
ratio of functional monomer to cross-linker in polymeriza-
tion precursor, or the proportion of H O in H O/MeCN. It is
2
2
well known that the more the amount of cross-linker is used,
the stiffer the as-prepared photonic film becomes, and vice
[
9c]
versa. In our case, the photonic films have been prepared
by using different ratios of cross-linker to monomer (from 3/1
to 1/1, mol/mol). As expected, the sensitivity of the prepared
photonic films is consistent with the supposition mentioned
above. Additionally, it should be noted that the observed dif-
fraction shift or colour change is dependent on the thickness
4. Experimental Section
Chemicals: All of the chemicals and solvents used in the exper-
of the photonic film. It is found that the photonic film with a iments were of reagent quality, without further purification unless
thick thickness shows a larger magnitude of diffraction peak mentioned. Anhydrous ethanol, ammonia, acrylic acid, tetra-
shift than the thin one (Figure 7). The reason is that the sub- ethoxysilane (TEOS), and dichloromethane were all from Beijing
strate on which the photonic film is deposited has different Chemicals. Methyl methacrylate, ethylene glycol dimethylacrylate
extent of confinement on the photonic films with different (EGDMA) and AIBN were purchased from Acros Organics. Trifluoro-
[15]
thickness.
In our work, in order to obtain comparative acetate acid, 4-aminophenethyl ethanol, 2,2,2-trifluoroacetic anhy-
results, the photonic film with a given thickness (9 μm) was dride and triethylamine were obtained from Alfa Aesar. DCC, DMAP,
used through our work.
potassium cyanide, and other affiliated chemicals were supplied
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Photonic Film for Label-Free and Selective Sensing of Cyanide
on silica gel (eluent: dichloromethane) to offer [4-(trifluoroacetyl)
amino]phenylethanol in the next step.[16] The mixture of [4-(tri-
fluoroacetyl)amino]phenylethanol (10 mmol, 2.37 g), acrylic acid
(15 mmol, 1.08 g), DCC (15 mmol, 3.09 g), and DMAP (15 mmol,
2
.02 g) in anhydrous dichloroethane (150 mL) was stirred at room
temperature for over 24 h. After urea was filtered off, the resulting
organic layer was washed with 4% hydrochloric acid, saturated
aqueous sodium bicarbonate, and brine, and it was dried using
anhydrous magnesium sulfate, following evaporation in vacuum.
The crude product was purified by flash chromatography on silica
gel (eluent: dichloromethane: ethyl acetate = 6:1) to afford a vis-
1
cous liquid. H NMR (300 MHz, CDCl ): 9.46 (s, 1H), 8.21 (s, 1H),
3
7
2
.93 (d, 2H), 7.29 (q, 2H), 5.93 (d, 1H), 5.45 (d, 1H), 3.52 (m, 2H),
.50 (m, 2H).
Preparation of SiO2 Colloid Crystal Template: The mono-
disperse silica particles were synthesized following the Stöber
method.[17] Approximately 10 mL NH ·H O and 120 mL anhydrous
3
2
ethanol were poured into a flask that was treated by dichromate
lotion beforehand, and equipped by a magnetic beater. After being
mixed uniformly, 4 mL TEOS was slowly added dropwise, and then
the mixture was stirred at room temperature for ca. 5 h. Monodis-
perse SiO2 was obtained after centrifugation and dispersion in
anhydrous ethanol for several times to remove the residues. The
size of the SiO2 spheres could be controlled by the quantity of
NH ·H O. Afterward, the obtained SiO spheres, which dispersed
3
2
2
in anhydrous ethanol with a weight concentration of about 1–4%,
were allotted into 10 mL cleaning vials, into which the glass slide
treated by H SO /H O was inserted vertically or slantways. As the
2
4
2 2
ethanol evaporated, the colloidal dispersion slowly traced down
the surface of the glass slides, coated by SiO2 spheres, which
Scheme 2. Synthesis of the used functional monomer. DCC is N, could assemble into an fcc crystal arrangement driven by capillary
N′-dicyclohexyl carbodiimide and DMAP is 4-dimethylaminopyridine.
force.
Preparation of Reactive Inverse-Opal Films using the SiO Col-
2
loid Crystal Template: The reactive inverse opal films were prepared
by local suppliers. The flasks were treated by dichromate before via a template technique. An assortment of precursors preparing
use. All glass slides were sheared out to be 50 mm × 20 mm, fol- with mixing predetermined proportions of functional monomer
lowing H SO /H O treatment and rinsing with deionized water and (0.287 g, 1 mmol), MMA (0.100 g, 1 mmol), EGDMA (~0.198–
2
4
2 2
anhydrous ethanol, and the surface of poly(methyl methacrylate) 0.595 g, ~1–3 mmol), and AIBN (1%) were infiltrated into a slot of
PMMA) slides whose dimensions were similar to the glass slides sandwich, which was made of a glass slide cover with SiO colloid
(
2
above, was treated by plasma in order to make it hydrophilic.
crystal and two PMMA slides, until the sandwich was transparent,
Characterization: The 1H NMR of functional monomer was and then polymerized at 80 °C for 24 h. After etching SiO with 1%
2
characterized by JEOL ECA 300 NMR spectrometer (Japan). The HF, the photonic film was gotten and placed into deionized water
ultrasonic cleaner and the centrifuge used to treat with the silica for use.
nanospheres employed KQ-200KDE ultrasonic cleaner (Kunshan,
Characterizations of the Photonic Film in Response to Cyanide:
China) and Anke TGL-16C (Shanghai, China) respectively. The All the curves of the characterizations of photonic films were
UV–vis curves of photonic crystals and reactive photonic films tested by Olympus BX51M. The photonic films containing 1 mmol
were obtained from Lambda 35 spectrophotometer (Perkin Elmer, functional monomer, 1 mmol MMA, and ˜1–3 mmol EGDMA were
USA) and The SEM images were taken by LEO-1503 field emission analyzed using 10− m KCN dissolved in H O with a small amount
3
2
scanning electron microscope (Germany).
of CH CN. To determine the influence of the components of film,
3
Synthesis of Functional Monomer–2-(methacryloyloxy)ethyl photonic films with various reagent ratios (functional monomer,
4
4
-(2,2,2-trifluoroacetamido) Benzoate Scheme 2: A mixture of MMA, and EGDMA) were prepared, and characterized in the pres-
-aminophenethyl ethanol (30 mmol, 4.12 g) and triethylamine ence of 10− m KCN dissolved in H O (with a small amount of
3
2
(
35 mmol, 3.54 g) in anhydrous dichloromethane was slowly CH CN). Based on these considerations (the mixture of H O with
3 2
added to trifluroacetic anhydride (35 mmol, 7.35 g) at 0 °C, and a small amount of CH CN as solvent and using the relatively better
3
then stirred at room temperature to react for 1 h. The solution was reagent ratios for the photonic film), we investigated various prop-
washed with saturated aqueous sodium bicarbonate and brine. ertie of the photonic films, such as sensitivity, selectivity, and
After the mixture was filtered, the resulting organic layer was dried reversibility.
over anhydrous magnesium sulfate, filtered, and concentrated in
Sensitivity of Photonic Film: After acquisition of the photonic
vacuum. The crude residue was purified by flash chromatography films with equivalent quantities of functional monomer, MMA and
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X. Li et al.
EGDMA (1:1:1, mol/mol/mol) and after obtaining their UV–vis
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spectra, the solvent (water with a small amount of CH CN) was
3
dropped onto the film, and the corresponding UV–vis spectra was
obtained. Cyanide solutions containing various concentrations
of KCN was then dropped onto the film, and their UV–vis spectra
were recorded. The red-shift caused by cyanide is the result of the
red-shift of the cyanide solution minus the Einstein shift of the sol-
vent. The corresponding optical images of the photonic films were
obtained by immersing them into vials full of cyanide solutions.
Selectivity of Photonic Film: The diffractive peaks of the photonic
films were recorded before adding the solvent and after dripping the
solvent onto the films; the corresponding curves were also recorded.
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−
−
−
Solution containing various anions (H PO , F , and Ac ) were then
2
4
added to the film, and the diffractive peaks were recorded. The red-
shifts caused by the various anions are the results of the red-shifts
of the anion solutions minus the Einstein shift of the solvent.
Reversibility Cycle of the Photonic Film Responding to Cyanide:
After acquisition of the UV–vis curves of the photonic film before
and after dropping solvent onto them, the cyanide solution con-
taining 10− m KCN was added to the film, and the corresponding
diffractive spectrum was recorded. This film was then soaked in the
trifluoroacetyl acid solution and vibrated. After rinsed with deion-
ized water 3 times, this photonic film was dried, the solvent was
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[
[
Acknowledgements
2
We gratefully acknowledge the financial support from the NSFC (No
20533050, 50873051 and 50673048), MOST (2007AA03Z07) and
the transregional project (TRR61).
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Received: September 15, 2011
Revised: December 13, 2011
Published online: January 27, 2012
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