Development of a coumarin-furan conjugate as Zn2 + ratiometric fluorescent probe in ethanol-water system

Article abstract of DOI:10.1016/j.saa.2016.11.034

In this study, a novel coumarin-derived compound bearing the furan moiety called 7-diethylamino-3-formylcoumarin (2′-furan formyl) hydrazone (1) has been designed, synthesized and evaluated as a Zn^{2 +} ratiometric fluorescent probe in ethanol-water system. This probe 1 showed good selectivity and high sensitivity towards Zn^{2 +} over other metal ions investigated, and a decrease in fluorescence emission intensity at 511 nm accompanied by an enhancement in fluorescence emission intensity at 520 nm of this probe 1 was observed in the presence of Zn^{2 +} in ethanol-water (V: V = 9: 1) solution, which provided ratiometric fluorescence detection of Zn^{2 +}. Additionally, the ratiometric fluorescence response of 1 to Zn^{2 +} was nearly completed within 0.5 min, which suggested that this probe 1 could be utilized for sensing and monitoring Zn^{2 +} in environmental and biological systems for real-time detection.

Full text of DOI:10.1016/j.saa.2016.11.034

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 174 (2017) 214–222
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Development of a coumarin-furan conjugate as Zn2+ ratiometric
fluorescent probe in ethanol-water system
Chao-rui Li a, Si-liang Li a,b, Zheng-yin Yang a,
⁎
a
College of Chemistry and Chemical Engineering, State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, PR China
School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 August 2016
Received in revised form 18 November 2016
Accepted 19 November 2016
Available online 28 November 2016
In this study, a novel coumarin-derived compound bearing the furan moiety called 7-diethylamino-3-
formylcoumarin (2′-furan formyl) hydrazone (1) has been designed, synthesized and evaluated as a Zn2+
ratiometric fluorescent probe in ethanol-water system. This probe 1 showed good selectivity and high sensitivity
towards Zn2+ over other metal ions investigated, and a decrease in fluorescence emission intensity at 511 nm
accompanied by an enhancement in fluorescence emission intensity at 520 nm of this probe 1 was observed in
the presence of Zn2+ in ethanol-water (V : V = 9 : 1) solution, which provided ratiometric fluorescence detection
of Zn2+. Additionally, the ratiometric fluorescence response of 1 to Zn2+ was nearly completed within 0.5 min,
which suggested that this probe 1 could be utilized for sensing and monitoring Zn2+ in environmental and bio-
logical systems for real-time detection.
Keywords:
Coumarin
Ratiometric fluorescent probe
Zinc ion
Selectivity
Sensitivity
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
detection and non-destructive properties, fluorescence detection has
attracted serious concerns of chemists, biologists, environmentalists
As we all know, zinc is the second most abundant transition metal
element in the human body with its concentrations ranging from
nanomolar (nM) to millimolar (mM) [1], and it is one of the most im-
portant transition metals in living systems [2]. The research of zinc ion
(Zn2+) has drawn considerable interests among chemists, biologists,
environmentalists and pharmacologists due to its chemical and physical
properties [3–6]. Zn2+ plays a crucial role in various biological processes
such as neurotransmission [7], DNA synthesis [8], gene transcription [9],
apoptosis [10], enzyme catalysis [11], immune function [12], mammali-
an reproduction [13] and pathology [14]. It is well known that the aver-
age level of Zn2+ in adult human bodies is 2–3 g, and the intracellular
concentration of Zn2+ is 10–100 mM [15–16]. Genetic abnormalities
or environmental factors may lead to the disorder of Zn2+ metabolism
in biological systems caused by the misregulation in the concentrations
or activities of Zn2+, which is associated with a variety of diseases like
Alzheimer's disease [17], Parkinson's disease [18], epilepsy [19], cere-
bral ischemia [20], amyotrophic lateral sclerosis (ALS) [21], diabetes
[22] and infantile diarrhea [23]. Therefore, it is highly desirable to devel-
op a convenient and rapid method for detecting Zn2+ to maintain its
concentration in a suitable level in biological systems [24].
and pharmacists [25–28]. In recent years, a number of fluorescent
probes for detecting and recognizing Zn2+ have been studied by various
groups [29–32], but some of them can be applied only in organic solu-
tions, which restricts their potential applications in environmental and
biological systems [33]. Additionally, some Zn2+ fluorescent probes dis-
play relatively low selectivity and suffer from the interferences from
other metal ions, especially Cd2+ [34], which is the same group as
Zn2+ in the periodic table and has similar spectroscopic properties
with that of Zn2+ [35]. Therefore, when these two metal ions are coor-
dinated with probe molecule respectively, the similar changes of fluo-
rescence intensities as well as the shift of wavelengths can be usually
obtained [36]. Thus, it is a great challenge to design and synthesize a
fluorescent probe to sense and monitor Zn2+ with high selectivity and
sensitivity in aqueous solutions [37].
Intensity-based fluorescent probes in which the fluorescence inten-
sity at a certain wavelength is only one signal can be affected by other
variables like probe molecule concentrations, micro-environmental
conditions or differences in optical components between instruments
[38]. Comparably, ratiometric detection can overcome the limitations
of those intensity-based fluorescent probes by measuring the ratio of
the fluorescence intensities at two different wavelengths via self-cali-
bration processes, so that they can provide quantitative measurements
for a specific analyte [39]. As a result, the development of ratiometric
fluorescent probes has attracted broad interests among chemists and bi-
ologists for detecting and monitoring metal ions in environmental and
biological systems [40–44].
Owing to the relevant advantages like good selectivity, high sensitiv-
ity, inexpensive apparatus, simple sample preparation, real-time
⁎
Corresponding author.
E-mail address: yangzy@lzu.edu.cn (Z. Yang).
http://dx.doi.org/10.1016/j.saa.2016.11.034
1386-1425/© 2016 Elsevier B.V. All rights reserved.

C. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 174 (2017) 214–222
215
Coumarin and its derivatives have been extensively utilized as med-
icines, fluorescent dyes and fluorescent probes, due to their excellent
optical properties such as high fluorescence quantum yield, large Stock's
shift, good photostability and nontoxicity [45–47]. Taking all these into
consideration, we have designed and synthesized a novel coumarin-de-
rived fluorescent probe bearing the furan moiety called 7-
Diethylamino-3-formylcoumarin (2′-furan formyl) hydrazone (1)
(Scheme 1), which showed ratiometric fluorescence response to Zn2+
in ethanol-water (V : V = 9 : 1) solution. A decrease in fluorescence
emission intensity at 511 nm accompanied by an enhancement in fluo-
rescence emission intensity at 520 nm of this probe 1 was observed in
the presence of Zn2+, which provided ratiometric fluorescence detec-
tion of Zn2+, and 1 showed good selectivity and high sensitivity towards
Zn2+ over other common metal ions, especially Cd2+. Furthermore, the
reversibility and regeneration of 1 were perfect and the ratiometric
fluorescence response of 1 to Zn2+ was fast. Hence, owing to the good
selectivity, high sensitivity and perfect reversibility for detecting and
recognizing Zn2+, this probe 1 could be utilized as a Zn2+ ratiometric
fluorescent probe in environmental and biological systems for real-
time detection.
Cr(NO3)3, Cu(NO3)2, Fe(NO3)2, Fe(NO3)3, K(OAc), Mg(NO3)2,
Mn(NO3)2, NaClO4, Ni(NO3)2 and Pb(OAc)2 were obtained from com-
mercial suppliers, and used as received without further purification.
Stock solution of compound 1 (10 mM) in DMSO was prepared, and
stock solutions (10 mM) of the cationic salts of Zn2+, Al3+, Ba2+
Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, K+, Mg2+, Mn2+, Na+
,
,
Ni2+ and Pb2+ were also prepared in absolute ethanol. Distilled water
was used throughout all experiments.
2.2. Apparatus
1H NMR spectra were measured on the JNM-ECS 400 MHz instru-
ments spectrometers in CDCl3 using TMS (tetramethylsilane) as an in-
ternal standard. The ESI-MS data were obtained in ethanol from a
Bruke Esquire 6000 spectrometer. UV–vis absorption spectra were de-
termined on a Shimadzu UV-240 spectrophotometer at 298 K. Fluores-
cence spectra were recorded on a Hitachi RF-5301 spectrophotometer
equipped with quartz cuvettes of 1 cm path length at 298 K. Melting
points were determined on a Beijing XT4-100x microscopic melting
point apparatus without correction.
2. Experimental
2.3. Synthesis
2.1. Materials
Ethyl 2-furan formate (2) and 2-furan formylhydrazine (3) were
synthesized according to the reported method [48]. The synthetic
route of compound 1 was shown in Scheme 1.
Synthesis of compound 1 (7-Diethylamino-3-formylcoumarin (2′-
furan formyl) hydrazone).
2-Furan formic acid, hydrogen peroxide, concentrated sulfuric acid,
hydrazine hydrate, 4-diethylaminosalicylaldehyde, diethyl malonate,
piperidine, hydrochloric acid, glacial acetic acid, sodium hydroxide,
phosphorus oxychloride, absolute ethanol, N,N-dimethyl formamide
(DMF), dimethyl sulfoxide (DMSO) and cationic salts such as
Zn(NO3)2, Al(NO3)3, Ba(OAc)2, Ca(NO3)2, Cd(OAc)2, Co(OAc)2,
Ethyl
7-diethylaminocoumarin-3-formate
(4),
7-
diethylaminocoumarin-3-formic acid (5) and 7-diethylamino-3-
formylcoumarin (6) were prepared according to the method reported
Scheme 1. The synthetic route of compound 1.

216
C. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 174 (2017) 214–222
previously [49]. A solution of 2-furan formylhydrazine (3) (0.126 g,
1.000 mmol) in absolute ethanol (10 mL) was added dropwise to anoth-
er solution containing 7-diethylamino-3-formylcoumarin (6) (0.245 g,
1.000 mmol) in absolute ethanol (30 mL) under stirring. The reaction
mixture was stirred vigorously at room temperature for 12 h, during
which time the color of the solution turned from orange yellow to or-
ange red. Then half of the ethanol was evaporated under reduced pres-
sure and the rest of solution was cooled to room temperature. After
placing the solution into refrigerator for 4 h, an orange red solid was
separated out from the solution and filtered under reduced pressure,
washing five times with ice-cold ethanol (10 mL). The obtained crude
product was recrystallized from absolute ethanol (30 mL) to furnish
the desired product 1 as an orange yellow powder (Scheme 1). Yield:
0.11 g (31.16%). m.p. 239–242 °C, 1H NMR (400 MHz, CDCl3) (Fig. S1):
8.66 (s, 1H,\\NH\\), 7.80 (s, 1H, H4), 7.69 (s, 1H, H7), 7.02 (s, 1H, H5),
6.91 (d, 1H, J = 7.2 Hz, H3), 6.88 (s, 1H, H6), 6.31 (dd, 1H, J = 7.2 Hz,
J = 1.6 Hz, H2), 6.26 (dd, 1H, J = 2.8 Hz, J = 1.2 Hz, \\CH_ N\\),
6.20 (d, 1H, J = 1.6 Hz, H1), 3.76 (q, 4H, J = 5.6 Hz,\\CH2\\), 1.99 (t,
refluxing in absolute ethanol. Then ethyl 7-diethylaminocoumarin-3-
formate (4) was acidated in absolute ethanol to give (7-
5
diethylaminocoumarin-3-formic acid). Subsequently, 5 was reacted
with phosphorus oxychloride in N,N-dimethyl formamide (DMF) and
7-diethylamino-3-formylcoumarin (6) was obtained. Finally, the con-
densation reaction between 2-furan formylhydrazine (3) and 7-
diethylamino-3-formylcoumarin (6) in absolute ethanol furnished the
desired compound 1 as an orange yellow powder. The structure of com-
pound 1 was characterized by 1H NMR and mass spectrometry (ESI-
MS), and the details of the characterization data of compound 1 were
presented in the Supporting Information (Fig. S1–S2).
3.2. UV–vis Titration of Compound 1 with Increasing Amounts of Zn2+
In order to clarify the interaction of compound 1 with metal ions, the
spectroscopic properties of 1 towards various chemically and biological-
ly important metal ions were investigated by UV–vis and fluorescence
methods in ethanol-water (V : V = 9 : 1) solution. In order to gain in-
sight into the UV–vis response of compound 1 to Zn2+, we firstly con-
ducted the UV–vis absorption titration spectrum of compound 1 in the
presence of increasing amounts of Zn2+ in ethanol-water (V : V = 9 :
1) solution and the results were shown in Fig. 1. There were almost no
bands in the range from 230 nm to 600 nm in UV–vis absorption spec-
trum of free 1, but two new bands centered at 342 nm and 474 nm ap-
peared with increasing absorbance upon addition of various
concentrations of Zn2+ (Fig. 1), which indicated that a new complex
had been formed between compound 1 and Zn2+ in ethanol-water (V
: V = 9 : 1) solution.
6H, J = 5.6 Hz, \\CH3). MS (ESI) (Fig. S2): m/z [M + H+ +
354.3599, found 354.1438; [M + Na+ +
calcd 376.3418, found
376.1244; [M + K
+ + calcd 392.4503, found 392.0950; [2M + Na+]+
]
calcd
]
]
calcd 729.6937, found 729.2406. Anal. Calcd. for C19H19N3O4 (%): C,
64.58; H, 5.42; N, 11.89; O, 18.11. found: C, 65.24; H, 4.84; N, 8.84; O,
21.08.
2.4. General Information
Test solutions were prepared by placing 10 μL of the probe stock so-
lution into cuvettes, adding an appropriate aliquot of each metal ion
stock, and diluting the solution to 2 mL with ethanol-water (V : V = 9
: 1) solution. For all fluorescence measurements of compound 1, the ex-
citation wavelength was set at 322 nm and the fluorescence emission
spectra were recorded over the range of 470–640 nm. The excitation
and emission slit widths were 3 nm and 1.5 nm in fluorescence emission
spectra of 1, respectively.
3.3. Fluorescence Titration of Compound 1 with Increasing Amounts of
Zn2+
The quantitative nature of compound 1 for sensing Zn2+ was then
elucidated by conducting fluorescence emission spectrum of 1 in the
presence of increasing amounts of Zn2+ in ethanol-water (V : V = 9 :
1) solution as described in Fig. 2. Upon excitation at 322 nm, compound
1 alone exhibited an intense emission peak centered at 511 nm. Wher-
evers, with a continuous increase in Zn2+ concentration, a gradual de-
crease in emission intensity at 511 nm was observed. Simultaneously,
a new emission peak centered at 520 nm appeared with increasing in-
tensity and a well-defined isoemission point was obtained at 516 nm.
It was probably because that the proton of hydrazone nitrogen atom
The binding constant value for complex 1-Zn2+ was determined on
the basis of the nonlinear filtting of the fluorescence titration curve as-
suming a 2 : 1 stoichiometry by the Benesi–Hildebrand method (1)
[50–51]:
1
1
1
≡
0:5 þ
i
ð1Þ
h
F−Fmin
ðFmax−FminÞ
KðFmax−FminÞ Zn2þ
where Fmin, F, and Fmax are the emission intensities at 520 nm of the or-
ganic moiety considered in the absence of zinc ion, at an intermediate
zinc concentration, and at a concentration of complete interaction, re-
spectively, and where K is the binding constant.
The limit of detection (LOD) of compound 1 for detecting Zn2+ was
calculated from the fluorescence titration. The ratio of emission intensi-
ties at 520 nm and 511 nm (F520 nm/F511 nm) of compound 1 without any
anion was measured to determine the S/N ratios [52–53], and the stan-
dard deviation of blank measurements was calculated. The LOD value
was calculated based on 3 × σblank/k, where σblank is the standard devi-
ation of the blank solution and k is the slope of the calibration plot.
3. Results and Discussion
3.1. Synthesis and Characterization of Compound 1
Compound 1 was synthesized according to the synthetic route
outlined in Scheme 1. Ethyl 2-furan formate (2) and 2-furan
formylhydrazine (3) were synthesized according to the reported meth-
od [48]. Firstly, ethyl 7-diethylaminocoumarin-3-formate (4) was pre-
pared by reacting 4-diethylaminosalicylaldehyde with diethyl
malonate using piperidine and glacial acetic acid as catalysts under
Fig. 1. Change in UV–vis absorption spectrum of 1 (100 μM) upon addition of increasing
amounts of Zn2+ (0.2, 0.4, 0.6, 0.8, 1.0, 1.4, 1.8, 2.2, 2.6, 3.0, 3.4, 3.8, 4.2, 4.6, 5.0 equiv.,
respectively) in ethanol-water (V : V = 9 : 1) solution.

C. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 174 (2017) 214–222
217
for the quantative detection of Zn2+ in ethanol-water system with ex-
cited at 322 nm.
3.4. The Effect of Reaction Time on the Fluorescence Response of Compound
1 to Zn2+
We explored the effect of reaction time on the fluorescence response
of compound 1 to Zn2+ to determine if 1 could be used as a Zn2+
ratiometric fluorescent probe for real-time detection. For this purpose,
the ratios of fluorescence emission intensities at 520 nm and 511 nm
(F520 nm/F511 nm) were recorded at different time in ethanol-water (V :
V = 9 : 1) solution of compound 1 in the presence of 5.0 equiv. of
Zn2+. As depicted in Fig. S4, the ratio increased rapidly and nearly satu-
rated within 0.5 min, and almost no further changes were obtained with
more reaction time (Fig. S4), which demonstrated that the ratiometric
fluorescence response of compound 1 to Zn2+ was so fast that 1 could
be utilized as a Zn2+ ratiometric fluorescent probe for real-time
detection.
3.5. Selectivity of Compound 1 towards Zn2+ over Other Metal Ions
Fig. 2. Change in fluorescence emission spectrum of 1 (50 μM) upon addition of increasing
amounts of Zn2+ (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 equiv., respectively) in ethanol-
water (V : V = 9 : 1) solution with an excitation at 322 nm. Insert: Fluorescence intensity
ratio at two peaks (F520 nm/F511 nm) versus the concentration of Zn2+ over the range 0 to
50 μM.
Selectivity is an important factor that can develop a certain fluores-
cent probe for broad applications. The selectivity of compound 1 to-
wards Zn2+ over other chemically and biologically important metal
ions was then examined by adding a variety of respective metal ions
to the ethanol-water (V : V = 9 : 1) solution of 1. As illustrated in
Fig. 3 (a), with excited at 322 nm, compound 1 in the absence of any
metal ion displayed an intense emission peak centered at 511 nm,
in compound 1 was deprotonated upon binding with Zn2+, which
strengthened the electron-accepting ability of the hydrazone nitrogen
atom from the coumarin unit, and the intramolecular charge transfer
(ICT) process from the coumarin unit to the hydrazone nitrogen atom
enhanced [54–56] (Scheme 2). As a result, the fluorescence emission
was red-shifted upon addition of Zn2+. Moreover, the ratio of fluores-
cence emission intensities at 520 nm and 511 nm (F520 nm/F511 nm) in-
creased from 0.890 to 1.102 with Zn2+ equivalence up to 0.5, and
almost no changes were observed in the ratio with further addition of
Zn2+ (Fig. 2), indicating that a 2 : 1 complex was formed between com-
pound 1 and Zn2+. Interestingly, when we chose the absorption band of
complex 1-Zn2+ at 342 nm or 474 nm as an excitation wavelength, the
fluorescence emission spectra of 1 in the presence of 5.0 equiv. of Zn2+
were similar with that upon excitation at 322 nm, but a few differences
in fluorescence emission intensity were observed (Fig. S3). Therefore,
this compound 1 could be utilized as a ratiometric fluorescent probe
which was red-shifted to 520 nm in the presence of 5 equiv. of Zn2+
and the fluorescence emission intensity at 511 nm decreased to a cer-
tain degree upon addition of 5 equiv. of Co2+, Cu2+, Fe3+ and Ni2+
,
.
The reason was probably attributed to the paramagnetic properties of
these four metal ions, so when they were binding with probes, the emis-
sion would be strongly quenched via a photoinduced metal to
fluorophore electron or energy transfer mechanism [57–60]. However,
the addition of other metal ions induced slight or no changes in the fluo-
rescence emission spectrum of compound 1 (Fig. 3 (a)). In addition, the
ratios of fluorescence emission intensities at 520 nm and 511 nm
(F520 nm/F511 nm) were recorded in the mixtures of 1 and respective
metal ions, and it showed a significant increase in the presence of
Zn2+ (Fig. 3 (b)). From the results above, it was concluded that
Scheme 2. The proposed binding mechanism for the response of 1 to Zn2+
.

218
C. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 174 (2017) 214–222
Fig. 3. (a) Fluorescence emission spectrum of 1 (50 μM) in the absence and presence of various respective metal ions (5 equiv.) in ethanol-water (V : V = 9 : 1) solution with an excitation at
322 nm. (b) Ratiometric fluorescence response profile changes (F520 nm/F511 nm) of 1 (50 μM) in the presence of various respective metal ions (5 equiv.) under the identical conditions in
ethanol-water (V : V = 9 : 1) solution. ((1) 1; (2) 1 + Zn2+; (3) 1 + Al3+; (4) 1 + Ba2+; (5) 1 + Ca2+; (6) 1 + Cd2+; (7) 1 + Co2+; (8) 1 + Cr3+; (9) 1 + Cu2+; (10) 1 + Fe2+; (11)
1 + Fe3+; (12) 1 + K+; (13) 1 + Mg2+; (14) 1 + Mn2+; (15) 1 + Na+; (16) 1 + Ni2+; (17) 1 + Pb2+) (λex = 322 nm, slit: 3.0/1.5 nm).
compound 1 had good selectivity towards Zn2+ over other common
metal ions investigated with ratiometric fluorescence response.
fluorescence emission intensities at 520 nm and 511 nm (F520
/
nm
F511 nm) showed negligible changes upon addition of all the metal ions
In order to further explore the selectivity of compound 1 towards
Zn2+ in the presence of other interfering metal ions, competition exper-
iments were carried out by adding other respective metal ions (5 equiv.)
to the ethanol-water (V : V = 9 : 1) solution of 1 with Zn2+ (5 equiv.)
and the results were illustrated in Fig. 4. The fluorescence emission in-
tensity at 520 nm quenched remarkably in the solution of 1 with Zn2+
in the presence of Co2+, Cu2+, Fe3+ and Ni2+, and Al3+, Cr3+, Fe2+
also decreased the fluorescence emission intensity at 520 nm to a cer-
tain degree. However, nearly no changes were observed in the case of
other metal ions tested (Fig. 4 (a)). On the other hand, the ratio of
to the solution of 1 with Zn2+ except for Al3+, Cr3+, Cu2+, Fe2+ and
Fe3+, which slightly enhanced or decreased the value of F520
/
nm
F511 nm. Interestingly, the addition of Cd2+ also caused almost no change
in the ratio and had no influence on the ratiometric detection of Zn2+ by
1 (Fig. 4 (b)). These results suggested that the ratiometric detection had
more accurate responses that could eliminate the interferences from
other metal ions than intensity-based detection. Therefore, compound
1 had good selectivity towards Zn2+ in the presence of most coexisting
metal ions, because most metal ions investigated did not interfere with
the ratiometric detection of Zn2+ by 1.
Fig. 4. (a) Fluorescence response profile changes at 520 nm of 1 (50 μM) with Zn2+ (5 equiv.) in the presence of other respective metal ions (5 equiv.) under the identical conditions in
ethanol-water (V : V = 9 : 1) solution. (b) Ratiometric fluorescence response profile changes (F520 nm/F511 nm) of 1 (50 μM) with Zn2+ (5 equiv.) in the presence of other respective
metal ions (5 equiv.) under the identical conditions in ethanol-water (V : V = 9 : 1) solution. ((1) Zn2+; (2) Zn2+ + Al3+; (3) Zn2+ + Ba2+; (4) Zn2+ + Ca2+; (5) Zn2+ + Cd2+; (6)
Zn2+ + Co2+; (7) Zn2+ + Cr3+; (8) Zn2+ + Cu2+; (9) Zn2+ + Fe2+; (10) Zn2+ + Fe3+; (11) Zn2+ + K+; (12) Zn2+ + Mg2+; (13) Zn2+ + Mn2+; (14) Zn2+ + Na+; (15)
Zn2+ + Ni2+; (16) Zn2+ + Pb2+) (λex = 322 nm, slit: 3.0/1.5 nm).

C. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 174 (2017) 214–222
219
3.6. Binding Stoichiometry between Compound 1 and Zn2+
reached the highest at a pH range from 6 to 7, but decreased with the
pH changing from 6 to 1. Furthermore, almost no fluorescence emission
at 520 nm was observed under alkaline conditions (Fig. S7 (a)). Addi-
tionally, in the pH range of 7–11, the values of F520 nm/F511 nm of 1
with Zn2+ were higher than that of 1 alone. However, when pH de-
creased or increased, a sharp decrease was observed in compound 1 in
the presence of Zn2+, and the values of F520 nm/F511 nm of 1 with Zn2+
were lower than that of free 1 (Fig. S7 (b)). It was mainly due to the pro-
tonation process of the Schiff-base nitrogen atom in compound 1 under
acidic conditions, which weakened its binding with Zn2+. Moreover, the
hydrolysis process of Zn2+ was observed under alkaline conditions,
which reduced the concentration of complex 1-Zn2+ in the system.
From the results above, it was concluded that our synthesized com-
pound 1 could be utilized as a Zn2+ ratiometric fluorescent probe in
neutral environments and applied for detecting and recognizing Zn2+
under physiological conditions.
A Job's plot was then explored to verify the binding stoichiometry
between compound 1 and Zn2+. For this purpose, the total concentra-
tion of compound 1 and Zn2+ was kept constant at 100 μM, and the
ratio of fluorescence emission intensities at 520 nm and 511 nm
(F520 nm/F511 nm) was plotted as the molar ratio of Zn2+ in complex 1-
Zn2+. As can be seen from Fig. 5, the value of F520 nm/F511 nm reached
maximum at the molar ratio of 0.3, demonstrating a 2 : 1 binding stoi-
chiometry between compound 1 and Zn2+ in ethanol-water (V : V =
9 : 1) solution, which was consistent well with the fluorescence titration
experiments. On a basis of 2 : 1 binding stoichiometry, the binding con-
stant (K) of compound 1 with Zn2+ was estimated from fluorescence ti-
tration experiments by the Benesi–Hildebrand equation (1) and the
result was found to be 1.58 × 108 M−0.5 (Fig. S5), which was within
the range 103–109 of those reported Zn2+ fluorescent probes [61–64].
The limit of detection (LOD) is one of the most important parameters
in designing fluorescent probes for metal ions detection. For many prac-
tical purposes, it is very significant to detect analytes at low concentra-
tions. On a basis of this fact, we calculated the LOD value of compound 1
for sensing Zn2+ from fluorescence titration experiments and the result
was illustrated in Fig. S6. A linear relationship between the fluorescence
emission intensities ratio F520 nm/F511 nm and the concentration of Zn2+
was obtained in the range of 0–0.45 μM (R2 = 0.9864). Based on the re-
sults above, the LOD value was calculated to be 1.13 × 10−7 M (Fig. S6),
which was comparable to those previously reported Zn2+ fluorescent
probes [65–67] and was lower than the amount of labile Zn2+ in
human bodies [68]. Thus, this compound 1 showed high sensitivity for
Zn2+ and could be applied for sensing Zn2+ in environmental and bio-
logical systems.
3.8. Reversibility and Regeneration of Compound 1 for Sensing Zn2+
In order to broaden the applications of compound 1 in environmen-
tal and biological systems, we examined if this compound 1 had good
reversibility and regeneration for sensing Zn2+ in ethanol-water sys-
tem. For this purpose, EDTANa2, which was a good chelating agent
with metal ions, was added to a mixture of 1 and Zn2+ (5 equiv.) in eth-
anol-water (V : V = 9 : 1) solution. As illustrated in Fig. 6, a red-shift was
observed in fluorescence emission spectrum of 1 upon addition of Zn2+
,
which was blue-shifted again with introduction of 5 equiv. of EDTANa2,
and the obtained fluorescence emission spectrum was almost the same
as that of free 1 (Fig. 6). These results demonstrated that this compound
1 had good reversibility and regeneration for sensing Zn2+ in ethanol-
water system, so that it could be applied for detecting Zn2+ in environ-
mental and biological systems.
3.7. The Effect of pH on the Fluorescence Response of Compound 1 to Zn2+
In order to investigate if our synthesized compound 1 could be ap-
plied for detecting and recognizing Zn2+ under physiological condi-
tions, the effect of pH on the fluorescence response of 1 to Zn2+ was
further conducted by recording the fluorescence emission intensity at
520 nm (F520 nm) and the ratio of fluorescence emission intensities at
520 nm and 511 nm (F520 nm/F511 nm) as a function of pH in ethanol-
water (V : V = 9 : 1) solution of 1 in the absence and presence of
Zn2+ (5.0 equiv.). As can be seen from Fig. S7, in the case of compound
3.9. 1H NMR Experiments
Finally, 1H NMR spectra of compound 1 in the absence and presence
of 1.0 equiv. of Zn2+ were carried out to further explore the binding
mechanism for the response of 1 to Zn2+, and spectral changes of com-
pound 1 upon addition of Zn(NO3)2·6H2O in CDCl3 were depicted in
Fig. 7. When Zn2+ was added to the solution of 1, the proton signal of
the imino group (\\NH\\) was shifted downfield significantly from δ
1 in the absence and presence of Zn2+, the values of F520
both
nm
Fig. 5. Job's plot for determining the stoichiometry between 1 and Zn2+ in ethanol-water
(V : V = 9 : 1) solution (XZn = [Zn2+]/([Zn2+] + [1]), the total concentration of 1 and
Zn2+ was 100 μM).
Fig. 6. Change in fluorescence emission spectrum of 1 (50 μM) with Zn2+ (5 equiv.) upon
addition of EDTANa2 (5 equiv.) in ethanol-water (V : V = 9 : 1) solution with an excitation
at 322 nm.

220
C. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 174 (2017) 214–222
Fig. 7. 1H NMR spectra of 1 upon addition of Zn2+ in CDCl3 (a) 1; (b) 1 and Zn2+ (1.0 equiv.).
8.663 ppm to δ 8.747 ppm with a decrease in its integral area. Simulta-
neously, the signal of the seventh proton H7 of the furan moiety was also
shifted downfield slightly from δ 7.689 ppm to δ 7.697 ppm. Neverthe-
less, nearly no changes in chemical shifts of other protons in compound
1 were obtained in the presence of Zn2+ (Fig. 7), suggesting that the ox-
ygen atom of the carbonyl group from hydrazone and the Schiff-base ni-
trogen atom in compound 1 took part in the coordination and formed a
2 : 1 complex with Zn2+ (Scheme 2), which corresponded well with the
fluorescence titration experiments and Job's plot analysis.
Zn2+ by 1. In addition, the ratiometric fluorescence response of com-
pound 1 to Zn2+ was so fast, and 2 : 1 binding stoichiometry between
1 and Zn2+ was determined by fluorescence titration experiments,
Job's plot analysis and 1H NMR experiments. As a result, this compound
1 could be utilized as a ratiometric fluorescent probe for detecting and
monitoring Zn2+ in ethanol-water system for real-time detection and
applied for sensing Zn2+ in environmental and biological systems. Fur-
thermore, the applications of novel sensors for detecting biologically
and environmentally important metal ions in real samples are ongoing
in our laboratory.
4. Conclusion
Acknowledgments
In conclusion, a novel coumarin-derived compound 1 bearing the
furan moiety was designed, synthesized and characterized by 1H NMR
spectrum and ESI-MS spectrum. This compound 1 showed ratiometric
fluorescence response to Zn2+ with a gradual decrease at 511 nm and
a gradual enhancement at 520 nm in ethanol-water (V : V = 9 : 1) so-
lution. Moreover, compound 1 had good selectivity and high sensitivity
towards Zn2+ over other common metal ions, for the limit of detection
(LOD) of 1 for sensing Zn2+ could reach 1.13 × 10−7 M, and most metal
ions investigated did not interfere with the ratiometric detection of
This work is supported by the National Natural Science Foundation
of China (81171337). Gansu NSF (1308RJZA115).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.saa.2016.11.034.

C. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 174 (2017) 214–222
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