A highly selective chemosensor for nickel(II) based on fluorescence quenching of a bispyrazole derivative

Article abstract of DOI:10.1007/s11164-016-2621-9

A novel bispyrazole, 2,6-bis(5-phenyl-1H-pyrazol-3-yl)-pyridine, was designed and synthesized. The bispyrazole showed extreme selectivity for Ni²⁺ over other metal ions such as Ag⁺, Al³⁺, Ba²⁺, Ca²⁺,

Full text of DOI:10.1007/s11164-016-2621-9

Res Chem Intermed
DOI 10.1007/s11164-016-2621-9
A highly selective chemosensor for nickel(II) based
on fluorescence quenching of a bispyrazole derivative
Lu Lin1 Sheng-Ting Hu1 Yao-Chao Yan1
•
•
•
Dun-Jia Wang1 Ling Fan1 Yan-Jun Hu1
Guo-Dong Yin1
•
•
•
Received: 14 February 2016 / Accepted: 22 June 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract A novel bispyrazole, 2,6-bis(5-phenyl-1H-pyrazol-3-yl)-pyridine, was
designed and synthesized. The bispyrazole showed extreme selectivity for Ni2? over
other metal ions such as Ag?, Al3?, Ba2?, Ca2?, Cd2?, Co2?, Cr3?, Cu2?, Fe3?, Hg2?
,
K?, Mg2?, Na?, Pb2?, and Zn2? in MeOH–H2O solution. The sensing sensitivity and
binding properties of the bispyrazole for nickel(II) ion were investigated by fluorescent
spectroscopic techniques. It was found that the bispyrazole possessed high selectivity
and sensitivity toward Ni2? over other metal ions. The binding stoichiometry was
confirmed as 1:1 (bispyrazole/Ni2?) by Job plot experiment and the linearity of the
Benesi–Hildebrand plot. The binding constants K and thermodynamic parameters (DH,
DG, and DS) at different temperatures were obtained. Meanwhile, the detection limit of
the bispyrazole for Ni2? was determined to be 3.25 9 10-7 mol L-1
.
Keywords Nickel(II) Á Chemosensor Á Fluorescence quenching Á Binding
constants Á Thermodynamic parameters Á 2,6-bis(5-Phenyl-1H-pyrazol-3-yl)-
pyridine
Introduction
Fluorescent chemosensors for heavy- and transition-metal ions have received consid-
erable attention due to their useful applications in environmental sciences, biology, and
chemistry [1–3]. In recent years, many highly selective and sensitive fluorescent probes
Electronic supplementary material The online version of this article (doi:10.1007/s11164-016-2621-9)
contains supplementary material, which is available to authorized users.
&
Dun-Jia Wang
dunjiawang@163.com
1
College of Chemistry and Chemical Engineering, Hubei Collaborative Innovation Center for
Rare Metal Chemistry, Hubei Key Laboratory of Pollutant Analysis and Reuse Technology,
Hubei Normal University, Huangshi 435002, China
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L. Lin et al.
for metal ions have been reported [4–10]. Among these metal ions, nickel(II) ion is an
essential micronutrient for animals and plants. Loss of nickel homoeostasis is harmful to
prokaryotic and eukaryotic organisms [11]. In excess, it can lead to serious problems
such as asthma, pneumonitis, respiratory system cancer, and disorders of the central
nervous system [12]. Therefore, it is necessary and important to develop an effective
method for determining the amount of nickel(II) ion. Conventional analytical methods
such as atomic absorption spectrometry, plasma emission spectroscopy, inductively
coupled plasma mass spectrometry, and X-ray fluorescence have been used for nickel
assay. Most of these approaches are sophisticated and time consuming, not being
suitable for quick, online detection. The fluorimetric chemosensor method for detecting
nickel is expected to be more desirable because it is relatively easier, cheaper, and highly
sensitive. Recently, a few studies have reported determination of nickel(II) ion based on
chemical optical sensors. Yari et al. [13] developed a new nickel(II) ion-selective optode
based on 2-amino-1-cyclopentene-dithiocarboxylic acid. Gupta et al. [14] reported a
nickel(II)-selective sensor based on dibenzo-18-crown-6 in a poly(vinyl chloride) (PVC)
matrix. Aksuner et al. [15] reported a novel optical chemical sensor for determination of
nickel(II) based on fluorescence quenching.
In this study, we designed a new bispyrazole derivative, 2,6-bis(5-phenyl-1H-
pyrazol-3-yl)-pyridine (BPP), to act as a fluorescent chemosensor for metal ion
detection. The chemosensor BPP was synthesized by condensation of the
corresponding bis-b-diketone with hydrazine hydrate (Scheme 1). The metal ions
Ag?, Al3?, Ba2?, Ca2?, Cd2?, Co2?, Cr3?, Cu2?, Fe3?, Hg2?, K?, Mg2?, Na?,
Ni2?, Pb2?, and Zn2? were tested for metal ion binding selectivity using the
chemosensor BPP. Fortunately, nickel(II) ion exhibited strong fluorescent quench-
ing upon binding with the chemosensor BPP. Thus, the chemosensor BPP can be
used as an excellent sensor for detecting nickel(II) ions.
Experimental
Materials and instrumentation
Acetophenone, diethyl 2,6-pyridinedicarboxylate, sodium, hydrazine hydrate, and
4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) were purchased from
Scheme 1 Synthesis of bispyrazole BPP
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A highly selective chemosensor for nickel(II) based on…
Sun Chemical Technology (Shanghai) Co., Ltd. Other reagents used were of
analytical grade and were used without further purification.
Melting points were measured on an X-4 digital melting point apparatus and are
uncorrected. Elemental analysis (C, H, N) was determined on a PerkinElmer 2400
elemental analyzer. Infrared (IR) spectra were recorded on a Nicolet FTIR 5700
spectrophotometer with KBr pellets. 1H nuclear magnetic resonance (NMR) spectra
were measured on an Advance IIITM 300 MHz NB digital NMR spectrometer in
dimethyl sulfoxide (DMSO)-d6 solution with tetramethylsilane (TMS) as internal
standard. Electrospray ionization mass spectra (ESI–MS) were obtained with a
Finnigan LCQ advantage max spectrometer. Ultraviolet–visible (UV–Vis) spectra
were obtained on a Hitachi U-3010 spectrometer. Fluorescence emission spectra
were obtained with a Varian Cary Eclipse fluorescence spectrometer. pH
measurements were carried out using a PHS-3C pH-meter (Shanghai, China).
Synthesis of 2,6-bis(5-phenyl-1H-pyrazol-3-yl)pyridine (BPP)
The bis-b-diketone, 3,30-pyridin-2,6-diyl-bis(1-phenylpropane-1,3-dione), was pre-
pared by Claisen condensation of diethyl 2,6-pyridinedicarboxylate with acetophe-
none in toluene using sodium as condensing agent according to literature [16].
3,30-Pyridin-2,6-diyl-bis(1-phenylpropane-1,3-dione) (0.28 g, 0.75 mmol) was
dissolved in chloroform (30.0 mL), and hydrazine hydrate (0.32 g, 6.4 mmol) was
added dropwise under stirring. The reaction mixture was refluxed for 1 h. Then the
solvent was removed, and white precipitate was obtained. The crude products were
recrystallized from absolute ethanol to give the bispyrazole (BPP) as pale crystals.
Yield 75 %, m.p. 162–163 °C; IR (KBr): m 3432(s), 3199(s), 3021(m), 1568(s),
1
1457(s), 1284(m), 1180(m), 1078(m), 1052(m), 961(m), 803(s), 763(s) cm-1; H
NMR (300 MHz, DMSO-d6): d 13.60 (bs, 2H, NH), 7.94–7.82 (m, 5H, Ar–H), 7.83
(d, J = 7.6 Hz, 2H, Ar–H), 7.53 (s, 2H, C = CH), 7.49–7.44 (m, 4H, Ar–H), 7.35
(t, J = 7.2 Hz, 2H, Ar–H) ppm; ESI–MS m/z: 364.39 [M ? 1]?; Anal. calcd. for
C23H17N5: C 76.01, H 4.71, N 19.27; found C 76.42; H 4.69; N 19.36; UV–Vis
(CHCl3): k = 264 nm (e = 23,225 L mol-1 cm-1), 312 nm (e = 9325 L mol-1
cm-1) (a figure showing the UV–Vis results for BPP is provided in the Electronic
Supplementary Material).
Procedures for metal ion sensing
Stock solution of bispyrazole BPP (1.0 9 10-3 mol/L) was prepared in methanol.
Stock solution for metal ions was prepared in 1:1 (v/v) MeOH–H2O solution with
concentration of 1.0 9 10-3 mol/L. HEPES buffer solution (1.0 9 10-2 mol/L, pH
7.0) was prepared using HEPES and sodium hydroxide. Working solution of BPP
(1.0 9 10-5 mol/L) was prepared in methanol by adding HEPES buffer solution to
adjust the pH to 7.0.
In the experiments, 3.0 mL solution of BPP (1.0 9 10-5 mol/L) was placed in a
quartz cell (1 cm width), and the solution of metal ions was added into the quartz
cell gradually by microsyringe to obtain the working solution. The volume for the
solution of metal ions added was \75 lL to prevent noticeable change of the total
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L. Lin et al.
volume of the testing solution. Fluorescence emission spectra of BPP were recorded
in the kem range of 300–500 nm at kex = 258 nm with 5.0/5.0 nm slit width and
500 V photomultiplier tube (PMT) voltage.
Results and discussion
Metal ion sensing properties
The sensing ability of the chemosensor (BPP) for various metal ions was
investigated by adding tenfold molar excess of metal ions (Ag?, Al3?, Ba2?
,
Ca2?, Cd2?, Co2?, Cr3?, Cu2?, Fe3?, Hg2?, K?, Mg2?, Na?, Ni2?, Pb2?, and
Zn2?) into BPP, a neutral 10:1 (v/v) MeOH–H2O solution. The changes in the
fluorescence spectrum of BPP upon addition of different metal ions are presented in
Fig. 1. It was observed that the fluorescence intensity of BPP showed negligible
variation in the presence of Ag?, Al3?, Ba2?, Ca2?, Co2?, Cr3?, Fe3?, K?, Mg2?
,
and Na?. However, metal ions Cd2?, Cu2?, Hg2?, Pb2?, and Zn2? caused
fluorescence quenching to some extent. Interestingly, nickel(II) ion revealed very
distinct quenching activity over other metal ions, as it could quench the fluorescence
of BPP completely. For practical applicability, competitive experiments of the
chemosensor BPP for Ni2? ion were carried out. The changes in the fluorescence
intensity for BPP were determined on treatment with 10 equiv. Ni2? ion in the
presence of the same equiv. of other metal ions (Fig. 2). From Fig. 2, all of the
tested interfering metal ions showed no observable interference with the detection of
Ni2? ion. Therefore, bispyrazole BPP can be used as a highly selective chemosensor
for nickel(II) ion. In addition, the pH value had a great influence on the detection
procedure. We determined the fluorescence intensity of BPP in absence and
presence of nickel(II) ion in the pH range from 3.0 to 10.0 (Fig. 3). From Fig. 3, the
florescence OFF–ON system worked well in the pH range from 6.5 to 7.5 in 10:1 (v/
v) MeOH–H2O solution. Acidic medium (pH \6.5) caused decreased emission
intensity, probably due to a weakened conjugate system by formation of BPP
ammonium salt. At higher pH values (pH [7.5), BPP competed with OH- ion for
Ni2? ion, increasing metal ion precipitation and accordingly weakening the
fluorescence quenching effect.
To elicit the chemosensing properties of BPP toward nickel(II) ion, fluorescence
titrations with Ni2? in 10:1 (v/v) MeOH–H2O were carried out at two different
temperatures (288 and 298 K). In absence of nickel(II) ion, the fluorescence
maximum of BPP (10 lM) was centered at about 356 nm and its fluorescence
intensity was strongest. As shown in Fig. 4, upon gradual addition of Ni2?
(0.52–5.24 lM), the fluorescence intensity gradually decreased with increasing
Ni2? concentration, forming the basis for fluorimetric determination of nickel(II)
ion. In addition, when ethylenediaminetetraacetic acid (EDTA, 0–7.0 lM) was
added to BPP–Ni2? solution, the fluorescent intensity at 356 nm was gradually
enhanced due to chelation of Ni2? ion with EDTA, releasing the chemosensor BPP
(Fig. 5). These results indicate that BPP could be developed as a reversible
fluorescence chemosensor for Ni2? ion.
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A highly selective chemosensor for nickel(II) based on…
(a)
(b)
Fig. 1 a Fluorescence spectra of BPP (10 lM) upon addition of different metal ions (0.1 mM) in 10:1
(v/v) MeOH–H2O (pH 7.0) solution. b Histogram of fluorescence selectivity experiment (kex = 258 nm)
Fluorescence quenching data are presented as plots of F0/F versus [Q] at different
temperatures (288 and 298 K) in Fig. 6. These results show that this quenching
process is in agreement with the Stern–Volmer equation [17],
F0=F ¼ 1 þ KSV½QŠ;
ð1Þ
where F0 and F are the fluorescence intensities in absence and presence of quencher,
respectively, KSV is the dynamic quenching constant, and [Q] is the concentration of
quencher.
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L. Lin et al.
Fig. 2 Competitive selectivity of BPP (10 lM) in 10:1 (v/v) MeOH–H2O (pH 7.0) solution toward Ni2?
(10 equiv.) in presence of other metal ions (10 equiv.) with excitation at 258 nm
Fig. 3 Fluorescence intensity of BPP at various pH values in absence and presence of Ni2? (5.0 equiv.)
at kem = 356 nm
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A highly selective chemosensor for nickel(II) based on…
(a)
(b)
Fig. 4 Fluorescence spectra of BPP (10 lM) in 10:1 (v/v) MeOH–H2O (pH 7.0) solution upon addition
of different concentrations of Ni2? (0.52–5.24 lM) at two different temperatures (a T = 288 K,
b T = 298 K). The insets show the emission intensity at kem = 356 nm as a function of [Ni2?
(kex = 258 nm)
]
The dynamic quenching constants (KSV) calculated for the interaction between
BPP and Ni2? were 5.73 9 105 (R = 0.9900) at 288 K and 4.75 9 105
(R = 0.9911) at 298 K. Evidently, KSV decreased with increasing temperature. It
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L. Lin et al.
Fig. 5 Changes in fluorescence spectrum of BPP–Ni2? in 10:1 (v/v) MeOH–H2O (pH 7.0) solution upon
gradual addition of EDTA (0–7.0 lM) (kex = 258 nm)
288 K
298 K
Fig. 6 Stern–Volmer plots for quenching between BPP and Ni2? at 288 and 298 K
can be concluded that the interaction between BPP and the Ni2? quencher can
probably be described by a static quenching model [18].
The binding stoichiometry of the chemosensor BPP–Ni2? complex was also
deduced based on Job plot experiments [19]. The fluorescence intensity at 356 nm
was plotted against the molar fraction of nickel(II) ion under a constant total
concentration of BPP and Ni2? in 10:1 (v/v) MeOH–H2O solution. Maximum
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A highly selective chemosensor for nickel(II) based on…
fluorescent quenching occurred when the mole fraction of nickel(II) ion was close to
0.5 (Fig. 7), indicating a 1:1 stoichiometric ratio between BPP and nickel(II) ion.
Binding constants and detection limit
The binding constants for the chemosensor BPP–Ni2? complex were determined by
the Benesi–Hildebrand equation [20],
1
1
1
¼
þ
;
ð2Þ
n
ðFmax À F0ÞK½Ni2þ
Š
F À F0 Fmax À F0
where [Ni2?] is the nickel(II) ion concentration, F is the fluorescence intensity at
356 nm for given Ni2? concentration, F0 is the fluorescence intensity at 356 nm in
absence of Ni2?, and Fmax is the maximum fluorescence intensity at 356 nm in
presence of Ni2? in solution. The binding constant K is determined from the ratio of
the intercept and slope of the Benesi–Hildebrand plot. If the plot of 1/[F - F0]
versus 1/[Ni2?] (n = 1) gives a straight line, then the complex stoichiometry is 1:1,
whereas if the plot of 1/[F - F0] versus 1/[Ni2?]2 (n = 2) gives a straight line, then
the stoichiometry of the complex between chemosensor BPP and nickel(II) ion will
be 1:2.
Analysis of the fluorescence titration curves at two different temperatures
(Fig. 3) by Benesi–Hildebrand plots (Fig. 8) revealed that plots of 1/(F - F0)
versus 1/[Ni2?] were both a straight line, indicating a 1:1 complex between BPP and
nickel(II) ion. These results are in good agreement with the corresponding values
obtained from the molar ratio method. On the basis of plots of 1/(F - F0) versus
Fig. 7 Job plot of 1:1 complex between BPP and Ni2?, where the 356-nm emission is plotted against
mole fraction of Ni2? under constant total concentration of 10 lM in 10:1 (v/v) MeOH–H2O (pH 7.0)
solution
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L. Lin et al.
(a)
(b)
Fig. 8 Benesi–Hildebrand plots of BPP with Ni2? at two different temperatures (a T = 288 K,
b T = 298 K). The excitation wavelength was 258 nm, and the monitored emission wavelength was
356 nm
1/[Ni2?] (Fig. 4), the binding constants at 288 and 298 K were calculated to be
1.55 9 104 M-1 (R = 0.99951) and 1.77 9 104 M-1 (R = 0.99953), respectively.
According to the above results, the bispyrazole BPP could be used as a reversible
fluorescence chemosensor for Ni2? ion. The possible binding mode between BPP
and Ni2? ion is presented in Scheme 2.
To obtain the detection limit of Ni2? ion using the chemosensor BPP, BPP
(10 lM) was treated with various concentrations of Ni2? (0.52–5.24 lM). At room
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A highly selective chemosensor for nickel(II) based on…
Scheme 2 Proposed binding mode between BPP and Ni2?
temperature, the fluorescence intensity at 356 nm was plotted as a function of Ni2?
concentration. As shown in Fig. 9, the fluorescence intensity of BPP was linearly
proportional to the Ni2? concentration in the range of 0.52–5.24 lM. The linear
regression equation was determined to be F0 - F = 18.25 ? 6.89 9 107[Ni2?
]
(n = 9, R = 0.99794, SD = 7.47064). The detection limit was determined from the
following equation [21, 22]: detection limit = 3 9 SD/B, where SD is the standard
deviation of blank measurement and B is the slope of the fluorescence intensity
versus the Ni2? concentration. The calculated detection limit for Ni2? ion was
3.25 9 10-7 M, which is sufficiently low to enable detection of micromolar
amounts of nickel(II) ion.
Thermodynamic parameters
To further elucidate the interaction between BPP and nickel(II) ion, the thermo-
dynamic parameters were determined according to the Van’t Hoff equations (3–5)
[23, 24]. The enthalpy change (DH) can be regarded as a constant if the temperature
Fig. 9 Fluorescence intensity (F0 – F) versus Ni2? concentration at room temperature. The excitation
wavelength was 258 nm, and the monitored emission wavelength was 356 nm
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L. Lin et al.
Table 1 Binding constants and thermodynamic parameters of BPP with Ni2?
T (K)
K (M-1
)
DG (kJ mol-1
)
DS (J mol-1 K-1
)
DH (kJ mol-1
)
288
1.55 9 104
1.77 9 104
-23.10
-24.23
113.21
113.22
9.51
0298
does not vary significantly. The free energy change (DG) can be calculated from
Eq. (3) based on the binding constants at different temperatures (288 and 298 K).
DG ¼ ÀRT ln K;
ð3Þ
where R is the gas constant, T is the experimental temperature, and K is the binding
constant at the corresponding temperature. Then, the enthalpy change (DH) and
entropy change (DS) can also be obtained from the following equations:
ꢀ
ꢁ
K2
K1
1
1
DH
ln
¼
À
;
ð4Þ
ð5Þ
T1 T2
R
DG ¼ DH À TDS:
The calculated values of the thermodynamic parameters (DH, DG, and DS) are
listed in Table 1. Among these thermodynamic parameters, the negative DG value
implies that the interaction process between BPP and nickel(II) ion is spontaneous
[25]. The positive DH value indicates that their interaction is an endothermic
process, and the positive DS value also reveals the chelating characteristic of the
chemosensor BPP with nickel(II) ion [26].
Conclusions
A new chemosensor, 2,6-BPP, was synthesized and characterized by Fourier-
1
transform infrared (FTIR), H NMR, elemental analysis, and mass spectrometry.
The sensing selectivity and sensitivity, and binding properties of BPP for nickel(II)
ion were investigated in neutral 10:1 (v/v) MeOH–H2O solution. The chemosensor
BPP exhibited high selectivity and sensitivity toward Ni2? over other metal ions.
The fluorescence emission of BPP could be effectively quenched upon addition of
Ni2? (0.52–5.24 lM) in MeOH–H2O solution, and the corresponding quenching
process can probably be described by a static quenching model. The binding ratio of
BPP and Ni2? was determined to be 1:1, as confirmed by Job plot experiment and
the linearity of the Benesi–Hildebrand plot. The binding constants K and
thermodynamic parameters (DH, DG, and DS) were also calculated, and the
detection limit of BPP for Ni2? was found to be 3.25 9 10-7 mol L-1. Thus,
bispyrazole BPP can be used as a highly selective, sensitive, and reversible
chemosensor for sensing micromolar concentrations of nickel(II) ion.
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A highly selective chemosensor for nickel(II) based on…
Acknowledgments We gratefully acknowledge financial support from the National Natural Science
Foundation of China (nos. 21542009 and 21273065) and the Educational Commission of Hubei Province
(no. B2015132).
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