A novel and fast responsive turn-on fluorescent probe for the highly selective detection of Cd2+ based on photo-induced electron transfer

Article abstract of DOI:10.1039/c9ra06356k

A novel, highly sensitive and fast responsive turn-on fluorescence probe, 2,2′-((1E,1′E)-((1,10-phenanthroline-2,9-diyl)bis(methanylylidene)) bis(azanylylidene)) diphenol (ADMPA), for Cd²⁺ was successfully developed based on 2,9-dimethyl-1,10-p

Full text of DOI:10.1039/c9ra06356k

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A novel and fast responsive turn-on fluorescent
probe for the highly selective detection of Cd2+
based on photo-induced electron transfer†
a
Cite this: RSC Adv., 2019, 9, 36011
b
Meng-Xia Huang,a Cai-Hua Lv,a Qing-Da Huang,a Jia-Ping Lai
*
*
and Hui Sun
A
novel, highly sensitive and fast responsive turn-on fluorescence probe, 2,20-((1E,10E)-((1,10-
phenanthroline-2,9-diyl)bis(methanylylidene)) bis(azanylylidene)) diphenol (ADMPA), for Cd2+ was
successfully developed based on 2,9-dimethyl-1,10-phenanthroline and o-aminophenol. ADMPA
showed a remarkable fluorescence enhancement toward Cd2+ against other competing cations, owing
to the suppression of the photo-induced electron transfer (PET) and CH]N isomerization. A good linear
relationship (R2
¼
0.9960) was obtained between the emission intensity of ADMPA and the
concentration of Cd2+ (0.25–2.5 mM) with a detection limit of 29.3 nM, which was much lower than that
reported in literature. The binding stoichiometry between ADMPA and Cd2+ was 2 : 1 as confirmed by
the Job's Plot method, which was further confirmed by a 1H NMR titration experiment. Moreover, the
ADMPA probe was successfully applied to detect Cd2+ in real water samples with a quick response time
of only 6.6 s, which was about 3–40 times faster than the reported cadmium ion probe.
Received 14th August 2019
Accepted 15th October 2019
DOI: 10.1039/c9ra06356k
rsc.li/rsc-advances
accurate method for the qualitative and quantitative detection
of cadmium ions in environmental samples.
Introduction
To date, the primary means for detecting cadmium ions are
atomic absorption spectrometry (AAS),7,8 inductively coupled
plasma mass spectrometry (ICP-MS),9,10 electrochemical
methods11–14 and microuidic technologies.15 These detection
methods have the advantages of high sensitivity, low detection
limit and wide linear range. However, these methods have high
maintenance costs and complicated sample pretreatment
methods. In this regard, a uorescent probe/sensor is a prom-
ising alternative route for metal ion and anion detection due to
their advantages of an excellent selectivity, high sensitivity, low
cost, handy operation and biological compatibility compared
with other detection methods. Therefore, the uorescent probe/
sensor has attracted increasing attention for its design and
development.16–22 Although cadmium ion uorescent probes/
sensors have been reported in literature,23–28 there are still
some difficulties that need to be overcome such as the
complicated synthesis procedure, poor water solubility and slow
response speed.
In the present work, a simply synthesized, highly sensitive
and fast responsive turn-on uorescence probe for Cd2+ based
on a photo-induced electron transfer (PET) and CH]N isom-
erization was successfully synthesized. The uorescence char-
acterization of the obtained probe towards metal ions was
investigated in detail. The synthetic probe exhibited an evident
uorescence enhancement and quick response to Cd2+. Mean-
while, the binding stoichiometry and binding mechanism were
also explored through the Job's Plot method, FT-IR character-
ization and 1H NMR titration experiment.
Cadmium is a highly toxic heavy metal that has been classied
as a category I human carcinogen by the International Agency
for Research on Cancer (IARC).1 Nowadays, cadmium contam-
ination in the environment, as we all know, mainly comes from
the emission from the industrial production of batteries,
fertilizers, pigments, metallurgy and electroplates. In partic-
ular, in recent years, an increasing number of quantum dots
containing cadmium have been synthesized and used widely in
various laboratories. The pollution to the environment due to
these quantum dots should not be neglected. However, the
cadmium that enters the soil and water is absorbed by animals
and plants, which can nally enter humans body through
intake, thus posing a serious potential threat to human
health.2–4 Moreover, the long-term exposure to cadmium-
containing environments for humans or animals not only
impedes the normal deposition of calcium in the bones and
causes osteoporosis, but also impairs the normal function of
the immune, nervous, renal and reproductive systems.5,6
Therefore, it is of great signicance to develop a rapid and
aSchool of Chemistry & Environment, South China Normal University, Guangzhou
510006, Guangdong, China. E-mail: laijp@scnu.edu.cn; Fax: +86-20-39310187; Tel:
+86-20-39310257
bCollege of Environmental Science & Engineering, Guangzhou University, Guangzhou
510006, Guangdong, China. E-mail: cherrysunhui@aliyun.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra06356k
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8.25 (s, 2H). ESI-mass m/z calculated value for 237.06; experi-
mental value was 237.05.
Experimental section
Materials and apparatus
Synthesis of 2,20-((1E,10E)-((1,10-phenanthroline-2,9-diyl)
bis(methanylylidene)) bis(azanylylidene))diphenol (ADMPA).
ADMPA was synthesized based on a reported procedure30 with
a slight modication. Generally, 0.1180 g (0.5 mmol) of 1,10-
phenanthroline-2,9-di-carbaldehyde and 10 mL of methanol
were successively added into a 50 mL round-bottom ask. The
mixture was stirred at room temperature for 30 min. Then,
0.1310 g (1.2 mmol) of an o-aminophenol methanol solution (15
mL) was slowly added dropwise into the ask. The solution
started to become turbid 5 min later. The reaction process was
monitored by TLC. The reaction was stopped when the spot for
the raw material disappeared. Aer removing the solvent, an
ADMPA yellow product was obtained and washed with meth-
anol for three times. Yield: 85%. 1H NMR (400 MHz, DMSO-d6):
d (ppm) 9.29 (s, 2H), 9.13 (s, 2H), 8.80 (d, 2H), 8.65 (d, 2H), 8.13
(s, 2H), 7.46 (s, 2H), 7.17 (t, 2H), 6.97 (d, 2H), 6.91 (t, 2H). 13C
NMR (100 MHz, DMSO-d6): d (ppm) 159.58, 155.20, 152.27,
145.58, 137.47, 136.98, 130.05, 129.16, 128.15, 121.37, 120.43,
120.15, 116.96. Anal. calcd. for C26H18N4O2: C 74.63, H 4.34, N
13.39, found: C 74.26, H 3.96, N 13.68.
All reagents and solvents employed for the synthesis were
purchased from commercial suppliers and used without further
purication. ADMPA was dissolved in methanol at a concentra-
tion of 2 mM as a stock solution and stored in the refrigerator at
4 ꢀC. All of the metal ions, including Ag+, Al3+, Ba2+, Be2+, Ca2+,
Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Ni2+,
Pb2+ and Zn2+ were in the chloride form or nitrate form. All the
other reagents and solvents used in the synthesis and analysis
were of analytical reagent grade and distilled water was used
throughout the experiment.
1H NMR and 13C NMR experiments were conducted on
a Varian AS 400 MHz NMR system in DMSO-d6 with TMS as an
internal standard. Elemental analyses for C, H and N were
performed on a Vario EL III Organic Element Analyzer (Ele-
mentar, GER). The emission spectra were recorded on a FL-4600
spectrometer (Hitachi, Japan). The time-resolved uorescence
lifetime measurement was measured using a FLS920 transient
uorescence spectrometer (EI, UK). The FT-IR spectra were
measured in the 4000–400 cmꢁ1 range with KBr pellets on
a Spectrum Two FT-IR spectrometer (PerkinElmer, USA). The
mass spectral data for DMP-CHO was measured on a LCQ DECA
XP MAX mass spectrometer (Thermo, USA).
Fluorescence characterization
Procedures for uorescence measurement. Firstly, 8.4 mg of
ADMPA was dissolved in 10 mL of methanol to prepare a 2 mM
Synthesis
Synthesis of 1,10-phenanthroline-2,9-dicarboxaldehyde
(DMP-CHO). As shown in Scheme 1, 10-phenanthroline-2,9-
dicarboxaldehyde (DMP-CHO) was synthesized via a mildly
modied synthetic procedure that was previously reported.29
Briey, selenium dioxide (2.77 g, 0.025 mol) was dissolved in
75 mL of 1,4-dioxane and heated to reux for 30 min. Aer-
wards, 2,9-dimethyl-1,10-phenanthroline (2.61 g, 0.012 mmol)
in 1,4-dioxane (50 mL) was added dropwise into the ask and
the reaction was stirred at 85 ꢀC for 6 h. The solid–liquid
mixture was subsequently ltered by diatomite while hot. A
pale-yellow precipitate formed in the ltrate aer cooling to
ambient temperature. Aer removing the solvent, the obtained
crude product was further puried by recrystallization in DMF
Fig. 1 Emission spectral changes for ADMPA (5 mM) induced by the
addition of various metal ions (5 mM) in a DMF-water (v/v, 3 : 7) solution
at room temperature.
1
and washed with hot water several times. Yield: 85%. H NMR
(400 MHz, DMSO-d6): 10.32 (s, 2H), 8.77 (d, 2H), 8.27 (d, 2H),
Scheme 1 The synthetic route for the ADMPA probe.
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FT-IR characterization
For the FT-IR characterization, ADMPA powder, cadmium
nitrate powder and potassium bromide powder were thoroughly
mixed and ground at a mass ratio of 1 : 0.5 : 100, 1 : 1 : 100, and
1 : 2 : 100. Aer grinding, these powders were dried in a vacuum
oven at 50 ꢀC overnight. Finally, ADMPA and the different mass
ratios of ADMPA-Cd2+ were measured using a Spectrum Two FT-
IR spectrometer (PerkinElmer, USA).
1H NMR titration experiments
For the 1H NMR titration experiments, 0, 0.25, 0.5 and 1.0 equiv.
of Cd2+ were added into the ADMPA (4.18 mg, 0.01 mmol)
DMSO-d6 solution to obtain different mole fractions of the
ADMPA-Cd2+ complex. Then, the 1H NMR spectra of these
solutions were recorded on a Varian 400 MHz NMR system.
Fig. 2 The influence of single metal ions (10 mM) on the interaction
between ADMPA (5 mM) and Cd2+ (5 mM) in a DMF-water (v/v, 3 : 7)
solution.
Recovery investigation
Tap water, Yanhu water and Zhujiang River (Guangzhou, China)
water were ltered with a 0.22 mm microporous membrane and
then mixed with DMF at a volume ratio of 7 : 3. Owing to the low
concentration level of Cd2+ in the real sample, the addition
recovery method was adopted. The concentration of ADMPA
and Cd2+ were xed to 5 mM and 1 mM, respectively.
ADMPA stock solution for subsequent use. The concentration of
all the metal ion stock solutions was 10 mM. The test solution
was obtained by adding 25 mL of the ADMPA stock solution and
a xed volume of each metal ion stock solution in a 10 mL
disposable plastic centrifuge tube, and then diluting the solu-
tion to 10 mL via the addition of a DMF-water (v/v, 3 : 7) solu-
tion. Aer mixing the uorescent probe with metal ions
properly, the uorescence spectra of ADMPA was obtained on
a FL-4600 spectrometer at room temperature. The excitation
wavelength of the uorescence spectrometer was set at 275 nm.
The lex slit width was 2.5 nm, except for the quantitative
determination experiments (5 nm). The lem slit width was 5 nm
and the voltage was 700 V.
Job plot method. For the Job's plot measurement, ADMPA (2
mM) in methanol and Cd2+ (1 mM) in water were prepared. A
series of test solutions that contained different mole ratios of
Cd2+ and ADMPA were prepared and measured. The Job's plot
was drawn by plotting F versus the mole fraction of Cd2+, where
F was the uorescence intensity of each test solution at 377 nm.
Results and discussion
Selectivity of the ADMPA probe towards Cd2+
The selectivity of the uorescent probe is very important for the
selective determination of target analytes. For this purpose, the
recognition ability of the ADMPA probe was investigated via the
uorescence characterization towards metal ions, including
Ag+, Al3+, Ba2+, Be2+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+,
Hg2+, K+, Mg2+, Mn2+, Ni2+, Pb2+ and Zn2+. As shown in Fig. 1,
the ADMPA probe presented a uorescence emission at 377 nm
under an excitation wavelength of 275 nm. However, only Cd2+
ion produced a noticeable turn-on uorescence response to
ADMPA (Fig. 1, black line) when the same amount of ADMPA
was added into above mentioned cations. This was attributed to
the formation of a ADMPA-Cd2+ complex with a more rigid
Fig. 3 Time-resolved fluorescence decay of ADMPA and ADMPA- Fig. 4 On-line monitoring of the reaction velocity process for ADMPA
Cd2+ in a DMF-water (v/v, 3 : 7) solution.
(20 mM) and Cd2+ (20 mM) in a DMF-water (v/v, 3 : 7) solution.
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Table 1 Comparison of the detection limit, detection media and response time for Cd2+ between the proposed method and those reported in
literature
Probe
Solvent
lex/lem
LOD (mM)
Response time
Ref.
DMSO/Tris–HCl (1 : 9)
620/639
—
20 s
23
HEPES
418/611 653
0.032
4 min
24
DMSO/H2O (1 : 1)
341/473
365/418
0.314
0.128
—
25
26
ACN
—
ACN/HEPES (1 : 5)
400/561
0.010
0.3 min
27
MeOH/H2O (3 : 7)
400/485 560
460/567
—
—
—
33
34
H2O
0.01
HEPES
315/419
0.60
—
35
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Table 1 (Contd.)
Probe
Solvent
ACN
lex/lem
LOD (mM)
Response time
Ref.
36
402/519
0.32
—
ACN/HEPES (2 : 8)
308/590
0.010
—
37
38
MeOH/H2O (1 : 4)
360/470 585
0.27
—
DMSO/HEPES (1 : 4)
460/511
0.077
—
39
DMF/H2O (3 : 7)
275/377
0.029
6.6 s
This work
conjugated structure, which hindered the rotation of CH]N from Cu2+ and Pb2+, which produced a certain uorescence
and the suppression of the photo-induced electron transfer quenching aer continuous treatment with 1 equiv. of Cd2+
(PET) quenching process that resulted in the chelation- This was attributed to the fact that Cu2+ and Pb2+ were in the
enhanced uorescence (CHEF) effect.31,32 same group of elements as Cd2+, which possessed the same
.
In order to investigate if the probe/sensor could be applied to charge and a similar electronic layer structure. Nevertheless, the
detect target analytes in complicated environmental samples, quenching degree of the uorescence intensity at 377 nm was
the selectivity and anti-interference capability were critical relatively acceptable when the 18 mixed metal ions were added
factors used to evaluate the properties of the chemosensor. into the ADMPA-Cd2+ complex (Fig. S5†). All of the above
Thus, to further investigate the anti-interference capability of mentioned phenomena strongly certied that ADMPA could be
the ADMPA probe towards Cd2+ against 18 other competitive used as a highly selective and sensitive uorescent ‘turn-on’
metal ions, binding competition experiments were performed. chemosensor for Cd2+.
It can be seen from Fig. 2 that the uorescence emission
The uorescence lifetime of the uorescent molecular probe
intensity changes for ADMPA in a DMF-water (v/v, 3 : 7) solution was closely related to its own structure and the micro-
upon the addition of 2 equiv. of the other single metal ions environment of the uorescent probe. Researchers can
(black bars). The results showed that most of the competing directly understand the changes in the studied system by
ions had no inuence on the emission spectra of ADMPA apart measuring the time-resolved uorescence spectrum. Herein,
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Table 2 The recovery of Cd2+ in Tap water/Yanhu water/Zhujiang
River water
the time-resolved uorescence spectra of ADMPA and ADMPA-
Cd2+ were measured. As shown in Fig. 3, the lifetime decays t
a single exponential decay prole with a lifetime of 3.78 ns (c2 ¼
1.031) and 3.31 ns (c2 ¼ 1.072) for ADMPA and the ADMPA-Cd2+
complex, respectively, which were attributed to the inhibition of
CH]N isomerization and the rearrangement of the charge
rendered by the suppression of the photo-induced electron
transfer (PET).
RSD/%
Real sample
Added/mM Detected/mM Recovery/% (n ¼ 3)
Tap water
Yanhu water
Zhujiang River water 1.0
1.0
1.0
1.004
1.011
0.9772
100.4
101.1
97.72
3.7
2.6
4.8
The response time for the probe towards the target is also an
essential criteria for judging its performance, which determines
whether it is suitable for on-site or/and on-line detection.
Therefore, the response time for ADMPA towards Cd2+ was
investigated by monitoring the uorescence intensity change of
ADMPA aer the addition of Cd2+ on a FL-4600 uorescence
spectrophotometer. As shown in Fig. 4, it was clear that the
uorescence intensity of ADMPA sharply increased aer the
addition of an equal amount of Cd2+. Then, the signal remained
roughly unchanged. It is worthwhile to note that the quick
response time of ADMPA towards Cd2+ was only 6.6 s, which was
3–40 times faster than the reported cadmium ion probe (Table
1). The fast response time allows for the possibility of the on-site
of ADMPA towards Cd2+ (6.6 s) was also much faster than re-
ported literature (Table 1). Finally, the ADMPA probe was
applied for the detection of Cd2+ in tap water, lake water and
Zhujiang River water. Satisfactory recoveries between 97.72%
and 101.1% with a RSD (n ¼ 3) under 4.8% were obtained,
which suggested that the proposed method possessed a good
accuracy (Table 2 and S2†).
Binding affinity of ADMPA to Cd2+
In order to explore the binding mode for the coordination of
ADMPA and Cd2+, a Job's plot analysis was performed. As shown
or on-time detection of Cd2+
.
Quantitative determination of Cd2+
The uorescence sensing properties of ADMPA towards Cd2+
were also explored by uorescence titration experiments in
a DMF-water (v/v, 3 : 7) solution. As shown in Fig. 5, the uo-
rescence intensity of ADMPA (5 mM) at 377 nm gradually
increased with the increase in the Cd2+ concentration. A good
linear relationship (R2 ¼ 0.9960) between the uorescence
spectra of ADMPA (5 mM) and the Cd2+ concentration was ob-
tained from 0.025 to 2.5 mM with a detection limit of 29.3 nM
according to the equation, DL ¼ 3SD/S, where SD is the stan-
dard deviation of ten times the blank measurements and S is
the slope of the calibration curve. The detection limit of ADMPA
towards Cd2+ in this work was much lower than most values
reported in literature (Table 1). Furthermore, the response time
Fig. 6 Job's plot for determining the stoichiometry of ADMPA bound
with Cd2+ in a DMF-water (v/v, 3 : 7) solution. The total concentration
was 10 mM.
Fig. 5 The linear relationship between the fluorescence intensity of
ADMPA and the concentration of Cd2+. Inset: fluorescence wave-
length scanning of the ADMPA solution (5 mM) upon the addition of
various equivalents of Cd2+
.
Fig. 7 FT-IR of ADMPA and the ADMPA-Cd2+ complex.
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Fig. 8 1H NMR titration of ADMPA and ADMPA with Cd2+ (DMSO-d6) for (a) ADMPA only, (b) ADMPA with 0.25 equiv. of Cd2+, (c) ADMPA with 0.5
equiv. of Cd2+ and (d) ADMPA with 1.0 equiv. of Cd2+
.
in Fig. 6, it was clear that the uorescence intensity reached ADMPA-Cd(NO3)2 with the increase in the Cd(NO3)2 mass.
a maximum value when the molar ratio of [Cd2+]/[Cd2+] + Meanwhile, new peaks at 2397 cmꢁ1, 1764 cmꢁ1, 1383 cmꢁ1 and
[ADMPA] was about 0.33. This clearly suggested that the ratio of 825 cmꢁ1 appeared. Among them, the peak at 1764 cmꢁ1 was
ADMPA bound with Cd2+ was 2 : 1. The binding constant (Ka) obviously the stretching vibration peak of C]O and theꢁother
for the ADMPA-Cd2+ complex was calculated with uorescence three peaks were the characteristic peaks of CO2, NO3 and
titration spectra from Fig. 5 using a revised Benesi–Hildebrand NO2ꢁ, respectively. These changes clearly implied that the
equation:40,41 I0/I ꢁ I0 ¼ (a/b ꢁ a)(1/Ka[C] + 1), where I0 and I are phenolic hydroxyl and imine may have been involved in the
the uorescence intensities of ADMPA at 377 nm in the absence
and existence of Cd2+, respectively, a and b are constants, Ka is
the association constant and [C] is the concentration of Cd2+
.
42
Table 3 1H NMR shifts for ADMPA and ADMPA with Cd2+
The calculated binding constant was 3.15 ꢂ 105 Mꢁ1 from the
plot of 1/(I ꢁ I0) against 1/[C] for Cd2+.
ADMPA +
Downeld
shi
Cd2+ (mole ratio 2 : 1)
To further elucidate the mechanism for the binding modes
between the ADMPA probe and Cd2+, FT-IR titration experi-
ments for ADMPA towards Cd(NO3)2 were also performed. The
infrared spectra for the ADMPA probe and different mass ratios
of the ADMPA-Cd(NO3)2 complex were obtained. As shown in
Fig. 7, there were apparent changes in the IR spectra between
free ADMPA and ADMPA-Cd(NO3)2. Three distinct peaks for
ADMPA at 3212 cmꢁ1, 1619 cmꢁ1 and 1251 cmꢁ1 represented
a O–H vibration, CH]N vibration and C–O vibration, respec-
tively. Those peaks gradually disappeared in the spectrum of
H atom
ADMPA
H1
H2
H3
H4
H5
H6
H7
H8
H9
8.62
8.80
8.13
9.13
9.29
6.97
7.17
6.91
7.46
8.30
8.76
7.93
8.56
Disappear
5.59
6.27
5.72
5.92
0.32
0.04
0.20
0.57
—
1.38
0.90
1.19
1.54
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Fig. 9 Proposed binding mode for ADMPA with Cd2+
.
complex reaction between ADMPA and Cd2+. Owing to the aminophenol. The binding mode of ADMPA and Cd2+ was
electron transfer from ADMPA to Cd2+, the double bond in C]O conrmed by a Job's plot analysis and 1H NMR titration
was formed and the uorescence was enhanced.
experiments with a ratio of 2 : 1. The obtained ADMPA probe
For a better understanding, H NMR titration experiments was applied for the detection of Cd2+ at a concentration as low
were performed by adding various amounts of Cd2+ to ADMPA as 29.3 nM in DMF-water. The proposed method was applied to
in DMSO-d6. As shown in Fig. 8, the peak for OH5 at 9.29 ppm detect Cd2+ in an actual sample with a satisfactory recovery.
gradually disappeared with the increase of Cd2+, while the More importantly, the excellent response velocity provided
signal for other hydrogen atoms at a different downeld shi a possibility for the real-time detection of Cd2+. Thus, the
changed from 0.04 ppm to 1.54 ppm (Table 3). These changes proposed method could be a promising alternative route for
suggested that OH5 participated in the coordination of ADMPA Cd2+ detection in environmental samples.
towards Cd2+ and the formation of the complex changed the
1
conjugated structure of ADMPA, causing a shi in the hydrogen
atoms of the benzene ring and phenanthroline skeleton.
Conflicts of interest
Based on the above results, a possible binding mode for
ADMPA towards Cd2+ was proposed, as shown in Fig. 9. In the
There are no conicts to declare.
absence of Cd2+, the nitrogen atom of imine transferred an
electron to the phenanthroline ring (PET ON) and the C]N
group could rotate freely. When Cd2+ was added, the phenolic
Acknowledgements
hydroxyl and neighboring imine nitrogen reacted with Cd2+
,
This work was supported and sponsored by the National
Natural Science Foundation of China (21677053 and 21876033).
resulting in the restriction of the PET process and C]N iso-
merisation. Meanwhile, there was a full rearrangement in the
charge distribution of the entire molecule, which increased the
rigidity and turned the uorescence on.
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In conclusion, a highly-sensitive and fast responsive turn-on
uorescence ADMPA probe for Cd2+ detection was successfully
developed based on 2,9-dimethyl- 1,10-phenanthroline and o-
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