A reversible fluorescent chemosensor for the rapid sensing of CN- in water: Utilization of the intramolecular charge transfer blocking

Article abstract of DOI:10.1039/c5nj02413g

A reversible and water-soluble 2,4-dimethyl-7-amino-1,8-naphthyridine (ZR) was synthesized to fluorescently sense CN^- in water. Interestingly, the deprotonation reaction between cyanide and the primary amine would block the intramolecular charge transfer of the electron donor and acceptor, thus, affecting the ICT efficiency and optical properties of the sensing system.

Full text of DOI:10.1039/c5nj02413g

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A reversible fluorescent chemosensor for the
ꢀ
rapid sensing of CN in water: utilization of the
Cite this:DOI: 10.1039/c5nj02413g
intramolecular charge transfer blocking†
Tai-Bao Wei,* Yuan-Rong Zhu, Hui Li, Guo-Tao Yan, Qi Lin, Hong Yao and
You-Ming Zhang*
Received (in Montpellier, France)
9th September 2015,
Accepted 6th January 2016
A reversible and water-soluble 2,4-dimethyl-7-amino-1,8-naphthyridine (ZR) was synthesized to fluorescently
ꢀ
sense CN in water. Interestingly, the deprotonation reaction between cyanide and the primary amine would
DOI: 10.1039/c5nj02413g
block the intramolecular charge transfer of the electron donor and acceptor, thus, affecting the ICT
efficiency and optical properties of the sensing system.
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1
. Introduction
Cyanide, which was first isolated by the Swedish chemist Scheel
1
in 1782, is well known as one of the most toxic materials and
as being extremely harmful to the environment and human
2
health. When cyanide enters the body by oral, inhalation or
dermal exposure, it exerts its acute effects by complexing with
ferric iron atoms in metalloenzymes, resulting in histotoxic
anoxia through inhibition of cytochrome c oxidase. The maxi-
mum permissive level of cyanide in drinking water is therefore
Scheme 1 General strategies to develop ICT-based chemosensors:
0
0
0
(
a) D–p–A to D–p –A; (b) D –p–A to D–p–A; and (c) D–p–A to D–p–A.
set at as low as 1.9 mM by the World Health Organization
3
(
WHO). About 0.5–3.5 mg of cyanide per kilogram of body ICT efficiency of the sensing system, accompanied by the corre-
4
weight is fatal to humans. Despite its extreme toxicity, cyanide sponding fluorescent changes. In the specific case (Scheme 1b),
is used in many industries such as gold mining, electroplating, copper ions could coordinate with the electron donor moiety of
metallurgy and so on, which produce nearly 140 000 tons of the sensor molecule and the original strong electronic donor
cyanide per year worldwide. This makes the selective detection was converted into a weak one with the largely reduced donating
of CN in water absolutely crucial.
5
6
ꢀ
7
ability to participate in the ICT process. However, the added
Generally, for a typical ICT fluorescence probe, the tuning of cyanide could preferentially snatch copper ions in the above
nꢀ
the normal D/p/A structure of fluorophores would lead to complex to form stable [Cu(CN)
different optical nature and concomitant fluorescent or/and
x
]
species.
Different from these two approaches, we found that sensors
8
color changes. As shown in Scheme 1a, the nucleophilic relying on the special deprotonation of cyanide were promising
addition reaction between cyanide and the dicyano-vinyl group owing to their high sensitivity. The deprotonation reaction
would break the conjugation of the electron donor and acceptor, between cyanide and the primary amine would block the intra-
thus, affecting the ICT efficiency and optical properties of the molecular charge transfer of the electron donor and acceptor,
0
sensing system. This transformation from D–p–A to D–p –A thus, affecting the ICT efficiency and optical properties of the
could lead to changes in the whole electronic properties and sensing system (Scheme 1c). If it was the case, this approach
should be a new one for the development of new ICT-based
chemosensors by tuning the ‘‘D–p–A’’ structure.
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of
Education of China, Key Laboratory of Polymer Materials of Gansu Province,
College of Chemistry and Chemical Engineering, Northwest Normal University,
Lanzhou, Gansu, 730070, P. R. China. E-mail: weitaibao@126.com,
zhangnwnu@126.com; Tel: +86 9317973120
In recent years, many research groups have made efforts to
9
make ICT fluorescence sensors. However, most cyanide-ion
sensors are complex and insoluble in water and could be used
only in organic solvents or at least in a mixture of organic
solvents and water. Their application is restricted to two-phase
†
Electronic supplementary information (ESI) available: Complete experimental
procedures and some of the spectroscopic studies. CCDC 1023107. For ESI and
1
0
crystallographic data in CIF or other electronic format see DOI: 10.1039/c5nj02413g (water/organic) detection. Such semi-aqueous detection media
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are not very practical. Other disadvantages include the require- 2.3. General procedure for fluorescence spectra experiments
ments of high temperature, interference with other ions in the
All fluorescence spectra were recorded on a Shimadzu RF-5301
medium, and lack of high sensitivity. Thus developments of
fluorescence spectrometer after the addition of tetrabutyl-
new ICT fluorescence probes that work in water are absolutely
ammonium salts and in water, while keeping the ligand concen-
ꢀ
necessary for detecting CN in water.
ꢀ5
tration constant (2.0 ꢁ 10 M). The excitation wavelength was
ꢀ
In view of the need for CN detection exclusively in water
3
70 nm. Solutions of anions were prepared from the NaCN and
1
1
and as a part of our research devoted to ion recognition,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
4
tetrabutylammonium salts of F , Cl , Br , I , AcO , H PO
HSO4 , ClO4 and SCN . Metal ions were prepared from the
perchlorate salts of Cr .
,
2
herein, we report a simple and water-soluble fluorescent sensor
ꢀ
ꢀ
ꢀ
ꢀ
ZR which can rapidly sense CN with specific selectivity
3+
and high sensitivity in water. This molecule combines a
naphthyridine as signaling subunits, while the presence of 2.4. General procedure for H NMR experiments
1
two nitrogen-atoms and amino groups, which can easily form
multiple hydrogen bonds in water, enhance the water solubility
to the sensor. In addition, the active hydrogen of primary amine,
which could be easily removed, confers the recognition capacity
toward cyanide ions by the way of fluorescence quenching
in water.
1
For H NMR titrations, two stock solutions were prepared in
DMSO-d , one containing the sensor ZR and the second con-
6
taining an appropriate concentration of the anions. Aliquots of
the two solutions were mixed directly in NMR tubes.
3
. Results and discussion
3
.1. Crystal structure of ZR
2
. Experimental section
The synthesis of ZR is shown in Scheme 2. Sensor ZR has been
2.1. Materials and physical methods
1
13
characterized by H NMR (Fig. S1, ESI†), C NMR (Fig. S2,
ESI†), IR spectroscopies and ESI mass spectrometry. The struc-
ture of the sensor was further confirmed by single-crystal X-ray
diffraction (Fig. 1). The single crystal (CCDC: 1023107) was obtained
by slowly vaporizing the water solution of ZR.
Fresh double distilled water was used throughout the experi-
ment. All reagents and solvents were commercially available of
analytical grade and were used without further purification.
1
H NMR spectra were recorded on a Mercury-400BB spectro-
1
3
meter at 400 MHz and C NMR spectra were recorded on a
Mercury-400BB spectrometer at 150 MHz. Chemical shifts are
reported in ppm down field from tetramethylsilane (TMS, d scale
with solvent resonances as internal standards). Melting points
were measured on an X-4 digital melting-point apparatus. The
infrared spectra were performed on a Digilab FTS-3000 FT-IR
spectrophotometer.
3.2. Fluorescent spectroscopic studies of ZR
ꢀ
In order to investigate the CN recognition abilities of the sensor
ZR in water, we carried out a series of host–guest recognition
experiments. The recognition profiles of the chemosensor ZR
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
toward various anions, including F , Cl , Br , I , AcO , H PO
,
2
4
ꢀ
ꢀ
ꢀ
ꢀ
HSO4 , ClO4 , SCN and CN , were primarily investigated using
fluorescence spectroscopy in water. As shown in Fig. 2, sensor ZR
emitted very strongly (lem = 406 nm; lex = 370 nm). Upon
2.2. Synthesis of the sensor ZR
A mixture of 2,4-pentanedione (0.50 mL, 4.58 mmol) and 2,6-
diaminopyridine (0.50 g, 4.58 mmol) in phosphoric acid (2.50 mL)
was heated at reflux for 2 hours (Scheme 2). After cooling to
room temperature, the resulting mixture was neutralized; the
solid formed was collected by vacuum filtration, and washed with
cool water (3 ꢁ 5 mL) and dried in vacuo. The crude product was
recrystallized from ethanol by slow evaporation to give a pale
ꢀ
addition of 50 equivalents of CN , the fluorescence intensity of
sensor ZR (20 mM) decreased rapidly. The apparent fluorescence
emission color change from blue to colorless could be distin-
guished by the naked eye on a UV lamp.
To validate the selectivity of sensor ZR, the same tests were
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
carried out using F , Cl , Br , I , AcO , H PO , HSO4 , ClO4
2
4
ꢀ
and SCN ions. None of these ions induced any significant changes
1
yellow solid. Yield: 0.33 g (41%); m.p: 226–227 1C; H NMR
in the fluorescence spectrum of the sensor (Fig. S3, ESI†). These
(
(
(
1
3
400 MHz, CDCl ) d 7.96 (d, J = 8.8 Hz, 1H), 6.87 (s, 1H), 6.67
3
ꢀ
data explicitly showed the selectivity of compound ZR toward CN .
1
3
d, J = 8.8 Hz, 1H), 5.00 (s, 2H), 2.58 (s, 3H), 2.50 (s, 3H); C NMR
150 MHz, CDCl ) d 161.96, 159.68, 156.94, 145.42, 135.08,
3
ꢀ
+
1
20.35, 115.80, 111.97, 25.85, 18.34; IR (KBr, cm ) v: 3373,
311 (–NH
2
), 1641 (CQN); ESI-MS m/z: (M + H) calcd for
C H N
10 11 3
174.0987; found 174.1061; anal. calcd for C10H N :
11 3
C, 69.34; H, 6.40; N, 24.26; found C 69.27; H, 6.58; N, 24.15.
Scheme 2 Synthesis of the sensor molecule ZR.
Fig. 1 Single-crystal X-ray structure of sensor ZR.
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As shown in Fig. S8 (ESI†), the pH-emission plot shows a
slight enhancement in the emission intensity of ZR in the pH
range 7.0–1.0. However, a significant fluorescence decrease was
observed in the pH range of 7.0–13.0.
ꢀ
3+
3
.3. Optical behavior of ZR + CN with Cr
3
+
The reversibility of the sensor was tested by addition of Cr to
the cyanide-sensor complex. Upon addition of 2.5 equiv. of
3
+
3+
ꢀ
nꢀ
x
]
Cr , the added Cr could snatch CN to form stable [Cr(CN)
species, the optical fluorescence intensity returned to the levels
3
+
ꢀ
observed for the free ZR. Addition of Cr to the ZR + CN
complex shows that the process of titrating ZR with CN is
reversible with little fluorescent efficiency loss (Fig. S9, ESI†).
ꢀ
Fig. 2 Fluorescence spectra of ZR (20 mM) upon excitation at 370 nm in
ꢀ
water before and after addition of CN (50 equiv.). Inset: Photographs
ꢀ
showing the change in the fluorescence of ZR after addition of CN
ꢀ
3.4. Time-dependent fluorescence change of ZR with CN
(50 equiv.) in water.
In addition, many fluorescent sensors are plagued by long response
times. The effect of the reaction time on the interaction process
Achieving high selectivity for the analyte of interest over
a complex background of potentially competing species is a
ꢀ
of CN to ZR was studied as shown in Fig. S10 (ESI†). Following
ꢀ
ꢀ7
the addition of 12.5 equiv. of CN (5.0 ꢁ 10 mol) to ZR (20 mM,
challenging task in sensor development. Fig. S4 (ESI†) illus-
2
mL), the fluorescence intensity of ZR decreased rapidly, reaching
ꢀ
trates the fluorescence response of compound ZR to CN in the
a stable value within 40 seconds. The rapid, stable interaction
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
presence of other anions (F , Cl , Br , I , AcO , H
2
PO
4
,
ꢀ
of CN by ZR and the resulting quick response profile are
ꢀ
ꢀ
ꢀ
HSO4 , ClO4 and SCN ) in water. A background of all selected
anions does not interfere with CN coordination to ZR. In the
presence of these anions, the CN still produced similar emis-
ꢀ
important features for robust, real time detection of CN by a
ꢀ
portable device in field. In contrast, many previously reported
ꢀ
ꢀ
fluorescent sensors showed responses to CN in the time range
sion changes. These results show that the selectivity of sensor ZR
of tens of minutes, generally due to slower sensor reaction
ꢀ
toward CN was not influenced in the presence of other anions
13
processes.
and suggest that it could be used as a fluorescent chemosensor
ꢀ
ꢀ
for CN in water.
To further evaluate the CN -responsive nature of ZR, fluores-
cence titration with CN in varying concentrations was conducted
3.5. Mechanism studies for CN recognition
ꢀ
1
The H NMR spectra of probe before and after addition of
ꢀ
hydrochloric acid and NaCN have been recorded. Upon addition
of hydrochloric acid, the signal of the amine proton at 6.55 ppm
became broad and the signal of the hydrogen atoms in the
naphthyridine rings showed a significant downfield shift, which
(
Fig. 3 and Fig. S5, ESI†). The fluorescence intensity at 406 nm
ꢀ
diminishes remarkably upon addition of CN until 8.5 equiv., and
ꢀ
increasing the CN concentration further, the emission intensity
tends to be the same. The fluorescence quenching of ZR may be
attributable to the deprotonation of primary amine, which can
affect the ICT efficiency. The binding constant Ka was determined
2
corresponds to the protonation of –NH (Fig. S11, ESI†). As shown
ꢀ
in Fig. 4, after adding 1.0 equiv of CN , the –NH
2
proton peak at
6.55 ppm disappeared immediately. At the same time, the signal of
3
ꢀ1
2
to be 8.26 ꢁ 10 M (R = 0.9989) (Fig. S6, ESI†). Furthermore, the
the hydrogen atoms in the naphthyridine rings showed a signifi-
detection limit of the fluorescent spectrum changes calculated on
cant upfield shift, which suggested that the –NH group perhaps
2
12
ꢀ8
ꢀ
the basis of 3s/m is 1.34 ꢁ 10 M for CN , which is far lower
underwent a deprotonation process. The deprotonation of
ꢀ
1
than the WHO guideline of 1.9 mmol L cyanide (Fig. S7, ESI†).
the –NH
2
group suppressed the intramolecular charge transfer
and led to an increase
between naphthyridine rings and –NH
2
in the electronic density of the naphthyridine rings.
1
ꢀ
Fig. 3 Fluorescence spectra of ZR (20 mM) in the presence of different
concentrations of CN (0–8.5 equiv.) in water.
Fig. 4 Partial H NMR spectra of ZR and the presence of 1.0 equiv. of CN
in DMSO-d .
6
ꢀ
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ꢀ
Scheme 3 The proposed mechanism of ZR for CN ions.
In the IR spectra of ZR, the stretching vibration absorption
ꢀ
1
peaks of –NH appeared at 3373 and 3311 cm , respectively.
2
ꢀ
However, when ZR coordinated with CN , the stretching vibra-
tion absorption peaks of –NH
disappeared. At the same time, Fig. 5 The reversible and reproducible fluorimetric switch controlled by
2
3+
ꢀ
alternate addition of CN and Cr into the water solution of ZR (20 mM).
the stretching vibration absorption peaks of –NH appeared
ꢀ1
at 3303 cm , which indicated the deprotonation of –NH
2
occurred, as shown in Fig. S12 (ESI†).
The ESI-MS spectral analyses were also used to explore the
mechanism of the sensor. The products ZR and ZR with CN
reproducible switch is of great interest for molecular-level
information processing. In the field of information technology,
the switching process must be based on a reversible chemical
process to perform any useful calculation. However, most of
newly reported logic systems are actually based on irreversible
chemical processes. The present logic device has great advan-
tage over earlier reported relative systems at least in terms of
the reversible and reproducible characteristics.
ꢀ
were subjected to mass spectral analyses. The ion peak was
detected at m/z 174.1061 (Fig. S13, ESI†), which is corres-
+
ponding to [ZR+ H] . The ion peak at m/z 196.1230 demon-
+
+
strated the presence of [ZR–H + Na ] (Fig. S14, ESI†). Therefore,
ꢀ
+
the presence of NaCN leads to the formation of R–NH –Na ,
and the –NH of ZR was converted to –NH (Scheme 3), producing
2
the MS signals at m/z 196.1230 showing the formation of a 1 : 1
bonding mode between ZR and CN ions.
3
.7. System as a keypad lock
ꢀ
ꢀ
ꢀ
The reversibility and selectivity of ZR/ZR + CN toward CN /
Cr ions prompted us to consider the present system as a
sequence dependent molecular keypad lock using the ZR, CN
and Cr as three different chemical inputs. These chemical
3
+
3.6. The reversible fluorimetric switch with ‘‘IMP’’ logic
ꢀ
functions
3+
As discussed above, changes in emission intensity at 406 nm inputs are designated as ‘W’, ‘G’ and ‘M’, respectively. Among
from ON to OFF were observed in the fluorescent spectra of ZR the six possible input combinations, WGM, WMG, GWM,
ꢀ
concomitant CN binding in water. More interestingly, we GMW, MGW and MWG, the combination WGM gave maximum
found that the cyanide-induced fluorimetric process is totally fluorogenic output, whereas minimum output signaling was
3
+
reversed with the addition of Cr (Fig. S9, ESI†). The fluores- displayed by WMG (Fig. 6). The keypad contains various keys,
cence intensity changes were clearly reflected in the fluores- like that in the electronic keypad (containing A to Z) and follows
3
+
cence spectra, upon addition of 2.5 equiv. of Cr , which results the correct password order (WGM).
in reappearance of the fluorescence band at 406 nm. The
reversible ‘‘ON–OFF–ON’’ cycle could be repeated several times
ꢀ
3+
by alternating the addition of CN and Cr .
The reversible and reproducible fluorimetric switching pro-
cess may be represented by a molecular ‘‘IMPLICATION’’ logic
ꢀ
3+
gate, employing CN (input 1) and Cr (input 2) as the inputs
and the fluorescence intensity at 406 nm as the output. In this
way, a complementary IMP logic function can be realized based
on the ZR, as shown in Fig. S15 (ESI†).
Thus, the fluorescence intensity changes of ZR in water
(
optical output) are controlled by the input of two spices:
ꢀ
3+
CN ‘‘switches’’ OFF the optical output, while Cr ‘‘switches’’
ꢀ
3+
ON the optical output. By alternately adding CN and Cr into
the receptor solution, a reversible fluorimetric switch could be
created in a reproducible manner (Fig. 5). The actual results
from an experiment (fluorescent intensity of ZR at 406 nm as a
function of different input combinations) are shown in Fig. S16
Fig. 6 Output for ZR, corresponding to six possible input combinations at
4
4
06 nm. The inset shows a molecular keypad lock generating emission at
06 nm when a correct password, namely, WGM, is entered. The keys W,
ꢀ
3+
(
ESI†). To the best of our knowledge, this kind of reversible and G and M hold the relevant inputs ZR, CN and Cr , respectively.
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ꢀ
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.8. Test strips
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ꢀ
4
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ꢀ
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ꢀ
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ꢀ
4
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ꢀ
In conclusion, in biological and environmental systems, CN
sensor interactions commonly occur in aqueous solution, there-
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that work in the aqueous phase. The simple 2,4-dimethyl-7-
amino-1,8-naphthyridine was demonstrated to fluorescently
6
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Acknowledgements
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This work was supported by the National Natural Science
Foundation of China (No. 21064006, 21262032 and 21161018), the
Program for Changjiang Scholars and Innovative Research Team in
University of Ministry of Education of China (No. IRT1177), the
Natural Science Foundation of Gansu Province (No. 1010RJZA018),
the Youth Foundation of Gansu Province (No. 2011GS04735) and
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