Fluorescent salicylaldehyde hydrazone as selective chemosensor for Zn 2+ in aqueous ethanol: A ratiometric approach

Article abstract of DOI:10.1002/bio.1175

4-N,N-diethylaminosalicylaldehyde hydrazone Schiff base (1) and its analogues (2-6) were synthesized and characterized by NMR, MS and elemental analysis. Compound 1 exhibited ratiometric fluorescent response to Zn ²⁺ over other metal ions in aqueous ethanol solution with neutral buffer. The complexation ratio, site and constant and the effect of pH value and water fraction on its fluorescent response to Zn²⁺ were investigated. Copyright

Full text of DOI:10.1002/bio.1175

Research Article
Received: 27 April 2009,
Revised: 10 July 2009,
Accepted: 16 August 2009,
Published online in Wiley Online Library: 13 October 2009
(wileyonlinelibrary.com) DOI 10.1002/bio.1175
Fluorescent salicylaldehyde hydrazone as
selective chemosensor for Zn2+ in aqueous
ethanol: a ratiometric approach
Na Li, Weixin Tang, Yu Xiang, Aijun Tong,* Peiyuan Jin and Yong Ju
ABSTRACT: 4-N,N-diethylaminosalicylaldehyde hydrazone Schiff base (1) and its analogues (2–6) were synthesized and char-
acterized by NMR, MS and elemental analysis. Compound 1 exhibited ratiometric fluorescent response to Zn2+ over other metal
ions in aqueous ethanol solution with neutral buffer. The complexation ratio, site and constant and the effect of pH value and
water fraction on its fluorescent response to Zn2+ were investigated. Copyright © 2009 John Wiley & Sons, Ltd.
Keywords: ratiometry; fluorescent chemosensor; zinc ion; salicylaldehyde hydrazone
derivatives were found to display rather strong fluorescence, so
they might be used for constructing ratiometric fluorescent che-
Introduction
Zinc ion with concentration as high as 10 μM in serum (1) is the
second most abundant transition heavy metal ion after iron in
human body. It is an essential component of many enzymes and
plays critical roles in many biological activities. (2,3) On the other
hand, zinc, especially at high concentrations, is an important pol-
lutant in water or soil. Thus, the quantitive determination of zinc
in physiological and environmental conditions is an essential
subject of analytic and environmental chemistry. (4)
mosensors. Compound 6 was then investigated (Scheme 1).
Unfortunately, its fluorescence quantum yield (Φ) in the absence
of Zn2+ (0.012) was only one-sixth of that in the presence of Zn2+
(0.066), which denied its quality as a ratiometric fluorescent che-
mosensor, which required nearly equal Φ for both species.
In our previous work, rhodamine-based chemosensors were
investigated. (40–44) Most of them exhibit fluorescence enhance-
ment of metal ions complex. (40–43) However, rhodamine B
4-N,N-dimethylaminobenzaldehyde hydrazone displayed a ratio-
metric fluorescent response to Cu2+ in acetonitrile through metal-
induced oxidation mechanism, (44) and its oxidation product
was found to be slowly hydrolyzed to rhodamine B by trace water
from hydrated salts or solvent. However, this chemosensor did
not exhibit any absorption and fluorescence change induced by
Zn2+. In this work, 4-N,N-diethylaminosalicylaldehyde hydrazones
containing a spirolactam moiety (1–3) were synthesized, and
they proved to be a highly selective and sensitive ratiometric
fluorescent chemosensor for Zn2+.
Fluorescent chemosensors for metal ion have been of exten-
sive interest for years. (5–10) Among them, ratiometric fluores-
cent sensors have attracted more and more attention for their
advantages. A ratiometric fluorescent sensor detects the pres-
ence or concentration of the guest by measuring the ratio of fluo-
rescence intensities at two different wavelengths. (11,12)
Therefore, they are insensitive to probe concentration or back-
ground fluorescence, which means the influence of environment
or photobleaching can be largely reduced in practical applica-
tion. At the same time, the perceived color change would make
the rapid visual sensing possible and bring higher sensitivity.
(13,14) Although many successful approaches to fluorescent
sensing of Zn2+ have been reported, (11,15–35) few have the
ability of a ratiometric fluorescent response to Zn2+. (11,21–35)
Furthermore, most of these ratiometric fluorescent probes for
Zn2+ suffer from shortcomings, such as low water solubility, dis-
turbance from Cd2+, large fluorescence increase or decrease in
the Zn2+-bound form. Therefore, developing novel ratiometric
fluorescent chemosensors for Zn2+ is still of extensive concern.
It has been reported that salicylaldimines (36,37) underwent
selective fluorescence enhancement upon chelating Zn2+, (38,39)
in which salicylaldimine moiety served as both receptor and
reporter. However, to our best knowledge, few ratiometric fluo-
rescent chemosensor for Zn2+ have been developed using this
strategy. The weak fluorescence of most salicylaldimines makes
them ideal fluorescent enhancement chemosensors but not
ratiometric ones. Recently, in our effort to develop fluorescent
salicylaldimines, 4-N,N-diethylaminosalicylaldehyde hydrazone
Experimental
Materials and apparatus
DMF, absolute ethanol (analytical grade) and deionized water
(distilled) were used throughout the experiment as solvents. All
the materials for synthesis were purchased from commercial sup-
pliers (Alfa, Acros, Beijing Chemical Co.) and used without further
* Correspondence to: Aijun Tong, Key Laboratory of Bioorganic Phosphorus
Chemistry & Chemical Biology (Ministry of Education), Department of
Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China.
E-mail: tongaj@mail.tsinghua.edu.cn
Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology
(Ministry of Education), Department of Chemistry, Tsinghua University,
Beijing 100084, People’s Republic of China
Luminescence 2010; 25: 445–451
Copyright © 2009 John Wiley & Sons, Ltd.

N. Li et al.
Fluorescein hydrazide. A similar procedure for rhodamine B
hydrazide was applied. The product was obtained as a yellow
solid (82%) by removal of ethanol under reduced pressure
and then washed with acetonitrile. ESI mass spectrometry:
m/z 347.2 (77% [M + H]+), 369.2 (23% [M + Na]+); M+ calculated
346.1.
4-N,N-diethylaminosalicylaldehyde rhodamine B hydrazone
(1). A 0.46 g aliquot of rhodamine B hydrazide (1 mmol) was
dissolved in 20 mL absolute ethanol. A 0.29 g aliquot of 4-N,N-
diethylaminosalicylaldehyde (1.5 mmol) was added. The mixture
was refluxed for 6 h. After cooling to room temperature, the reac-
tion mixture was concentrated to ca 10 mL and allowed to stand
at room temperature overnight. The precipitate formed was fil-
tered and washed with cold ethanol (3 × 10 mL). After drying
under reduced pressure, 0.37 g 1 was obtained as a yellow solid
(57%). Crystals of 1 were obtained by recrystalization in DMSO–
ethanol (1:3, v/v). ESI mass spectrometry: m/z 632.6 (96% [M +
H]+), 654.6 (4% [M + Na]+); M+ calculated 631.3. 1H-NMR (CDCl3), δ
(ppm): 1.11 (m, 18H, NCH2CH3), 3.27 (m, 12H, NCH2CH3), 6.09 (m,
2H, Phen–H), 6.23 (dd, 2H, xanthene–H), 6.47 (d, 2H, xanthene–
H), 6.49 (d, 2H, xanthene–H), 6.90 (d, 1H, Phen–H), 7.13 (d, 1H,
Ar–H), 7.46 (dd, 2H, Ar–H), 7.93 (d, 1H, Ar–H), 9.19 (s, 1H, N=C–H),
10.96 (s, 1H, Phen–OH). 13C-NMR (CDCl3) δ (ppm): 12.7, 12.8, 44.4,
44.6, 66.6, 98.1, 98.3, 103.4, 106.0, 107.6, 108.0, 123.0, 124.1, 128.2,
128.5, 130.4, 132.9, 133.0, 149.0, 150.5, 150.6, 153.8, 154.8, 160.7,
163.7. Elemental analysis: C 73.7%, H 7.32%, N 11.3%, calculated:
C39H45N5O3, C 74.1%, H 7.18%, N 11.1%.
Scheme 1. Chemical structures of compounds 1–6.
purification. The solutions of metal ions were prepared from their
nitrate salts, except for MnCl2. HEPES (purchased from Alfa) buffer
solutions (pH = 5–9) were prepared using 10 mM HEPES and
proper amount of NaOH under adjustment by a model pHS-3C
pH meter (Shanghai, China).
Absorption spectra were determined on a Jasco V-550 UV–vis
spectrophotometer. Fluorescence spectra measurements were
performed on a Jasco FP-6500 spectrofluorimeter equipped with
a xenon discharge lamp, 1 cm quartz cells. Fluorescence lifetime
test was applied on a LP920-KS spectrofluorimeter. NMR spectra
were recorded using a Joel JNM-ECA300 spectrometer operated
at 300 MHz. ESI MS spectra were obtained on a HP 1100 LC-MS
spectrometer without using the LC part. Elemental analysis result
was obtained from a Flash EA1112 elemental analyzer. All of the
measurements were operated at room temperature of 290 K.
4-N,N-diethylaminosalicylaldehyde rhodamine 6G hydra-
zone (2). A similar procedure was applied with rhodamine 6G
hydrazide instead of rhodamine B hydrazide and ethanol–dichlo-
romethane (1:1, v/v) as solvent. The product was obtained as
yellow solid (63%). ESI mass spectrometry: m/z 604.5 (90% [M +
H]+), 629.5 (10% [M + Na]+); M+ calculated 603.3. 1H-NMR (CDCl3),
δ (ppm): 1.09 (t, 6H, NCH2CH3), 1.28 (t, 6H, NHCH2CH3), 1.86 (s, 6H,
xanthene–CH3), 3.18 (q, 4H, NCH2CH3), 3.26 (q, 4H, NCH2CH3), 3.47
(s, 2H, NH), 6.08 (m, 2H, Phen–H), 6.29 (dd, 2H, xanthene–H), 6.41
(d, 2H, xanthene–H), 6.88 (d, 1H, Phen–H), 7.10 (d, 1H, Ar–H), 7.46
(dd, 2H, Ar–H), 7.95 (d, 1H, Ar–H), 9.09 (s, 1H, N=C–H), 10.99 (s, 1H,
Phen–OH). 13C-NMR (CDCl3) δ (ppm): 12.7, 14.9, 16.9, 38.4, 44.6,
66.5, 96.9, 98.4, 103.4, 106.2, 107.5, 118.0, 123.1, 124.0, 127.9,
128.5, 130.6, 132.8, 133.0, 147.6, 150.5, 151.1, 152.0, 154.1, 160.7,
163.8. Elemental analysis: C 73.8%, H 7.13%, N 11.2%, calculated:
C37H41N5O3, C 73.6%, H 6.84%, N 11.6%.
Metal ion sensing using 1
A 1 mM stock solution of 1 was prepared in DMF–ethanol (1:9,
v/v). To a 5 mL glass tube was added 4.0 mL 90% (v/v) ethanol–
water (containing 10 mM HEPES buffer), 50 μL stock solution of
1. Then the proper amount of metal ion stock solutions [1 mM in
90% (v/v) ethanol–water (containing 10 mM HEPES buffer)] was
injected into the tubes. The mixtures was shaken for 5 min and
followed by standing at room temperature for 2 min. A 3.0 mL
aliquot of the solutions in the tubes was then transferred into
a
1 cm quartz cell for the absorption or fluorescence
measurement.
In selectivity/interference measurement, metal ions were first
added to buffer solution before 1 was added, and then the
mixture was well mixed and allowed to stand at room tempera-
ture for 2 min before testing.
4-N,N-diethylaminosalicylaldehyde fluorescein hydrazone
(3). Compound 3 was synthesized by a similar method to 1
using fluorescein hydrazide instead of rhodamine B hydrazide.
The product was obtained as yellow solid (61%). ESI mass spec-
trometry: m/z 522.6 (92% [M + H]+), 544.6 (8% [M + Na]+); M+
calculated 521.3. 1H-NMR (DMSO-d6), δ (ppm): 1.04 (t, 6H,
NCH2CH3), 3.28 (q, 4H, NCH2CH3), 5.98 (s, 1H, Phen–H), 6.17 (d, 1H,
Phen–H), 6.52 (m, 6H, including: 4H, xanthene–H, 2H, xanthene–
OH), 6.67 (d, 2H, xanthene–H), 7.03 (d, 1H, Phen–H), 7.14 (d, 1H,
Ar–H), 7.61 (s, 2H, Ar–H), 7.91 (d, 1H, Ar–H), 9.09 (s, 1H, N=C–H),
10.99 (s, 1H, Phen–OH). 13C-NMR (DMSO-d6) δ (ppm): 12.9, 40.6,
65.8, 97.9, 103.0, 104.3, 106.8, 110.2, 113.0, 123.5, 124.3, 128.7,
129.6, 130.1, 132.7, 134.1, 150.6, 150.0, 152.8, 155.4, 159.2, 160.3,
163.3. Elemental analysis: C 71.1%, H 5.45%, N 7.88%, calculated:
C31H27N3O5, C 71.4%, H 5.22%, N 8.06%.
Synthesis of compounds 1–6
Rhodamine B hydrazide. Rhodamine B hydrazide was pre-
pared through a reported procedure. (41)
Rhodamine 6G hydrazide. Rhodamine 6G hydrazide was pre-
pared using a similar procedure for rhodamine B hydrazide. After
filtration and washed with ethanol, the product was obtained
as light pink solid (81%). ESI mass spectrometry: m/z 429.3 (88%
[M + H]+), 451.2 (12% [M + Na]+); M+ calculated 428.2.
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Luminescence 2010; 25: 445–451

Fluorescent salicylaldehyde hydrazone as selective chemosensor for Zn2+
4-N,N-diethylaminobenzoaldehyde rhodamine B hydrazone
Compound 1 (10 μM in H2O: EtOH = 1: 9, v/v, buffered by 10 mM
HEPES at pH 7.0) exhibited an absorption band at 385 nm
(ε = 0.37 × 105 cm/M) in the absence of Zn2+. Upon the addition
of Zn2+ up to 5 equiv., a 37 nm red-shift (ε = 0.39 × 105 cm/M) in
absorption spectra was observed, with isobestic points at 400,
332 and 301 nm (Fig. 1). Job’s plot analysis clearly revealed
that the maximum absorbance at 430 nm was achieved when
molecular fraction of 1 was 0.5, suggesting a 1 : 1 metal-to-ligand
complex formed. (45)
Excited at 400 nm, the emission peak of 1 showed red shift up
to17 nm with an isosmissive point at 500 nm upon the addition
of Zn2+ (Fig. 2). There was also an obvious color change of the
fluorescence from dark cyan to greenish yellow which could be
monitored easily by the naked eye. The ratio of fluorescence
intensity at 530 and 475 nm was used for ratiometric measure-
ment; it increased from 1.25 to 3.75 as the concentration of Zn2+
increased from 0 to 30 μM. Further addition of the Zn2+ had little
effect on this ratio.
(4). Compound 4 was synthesized by a similar method to 1
using 4-N,N-diethylaminobenzoaldehyde instead of 4-N,N-
diethylaminosalicylaldehyde. The product was precipitated by
adding equal volume of water and then filtered. After washing
three times with 1:1 (v/v) water–ethanol, 4 was obtained as a
yellow solid (65%). ESI mass spectrometry: m/z 616.5 (88% [M +
H]+), 638.5 (12% [M + Na]+); M+ calculated 615.3. 1H-NMR (CDCl3),
δ (ppm): 1.11 (m, 18H, NCH2CH3), 3.29 (m, 12H, NCH2CH3), 6.22 (m,
2H, Phen–H), 6.42 (d, 2H, xanthene–H), 6.52 (m, 4H, xanthene–H),
7.07 (d, 1H, Ar–H), 7.40 (dd, 2H, Ar–H), 7.43 (m, 2H, Phen–H), 7.97
(d, 1H, Ar–H), 8.46 (s, 1H, N=C–H). 13C-NMR (CDCl3) δ (ppm): 12.6,
12.7, 44.3, 44.4, 65.8, 97.9, 106.3, 108.0, 110.8, 122.2, 123.1, 123.6,
128.1, 128.1, 129.3, 132.9, 148.6, 148.8, 148.9, 150.1, 153.0, 164.6.
Elemental analysis: C 76.2%, H 7.59%, N 11.2%, calculated:
C39H45N5O2, C 76.1%, H 7.37%, N 11.4%.
Salicylaldehyde rhodamine B hydrazone (5) (41). Compound
5 was synthesized by a similar method as reported. (41)
4-N,N-diethylaminobenzoaldehyde benzoic hydrazone (6).
Compound 6 was synthesized by a similar method as 1 using
benzoic hydrazide instead of rhodamine B hydrazide.The product
was obtained by filtration as a yellow solid (58%). ESI mass spec-
trometry: m/z 312.4 (85% [M + H]+), 334.4 (15% [M + Na]+); M+
calculated 311.2. 1H-NMR (CDCl3), δ (ppm): 1.11 (t, 6H, NCH2CH3),
3.34 (q, 4H, NCH2CH3), 6.15 (s, 1H, Phen–H), 6.26 (d, 1H, Phen–H),
7.20 (d, 1H, Ar–H), 7.55 (m, 3H, including: 1H, Phen–H, 2H, Ar–H),
7.93 (d, 2H, Ar–H), 8.45 (s, 1H, N=C–H), 11.52 (s, 1H, Phen–OH),
11.83 (s, 1H, NH). 13C-NMR (CDCl3) δ (ppm): 13.1, 42.2, 98.1, 104.2,
107.0, 128.0, 129.0, 132.2, 132.2, 133.7, 150.6, 150.7, 160.3, 162.7.
Elemental analysis: C 73.5%, H 7.44%, N 13.9%, calculated:
C18H21N3O, C 73.2%, H 7.17%, N 14.2%.
Results and Discussion
Binding of 1 with Zn2+
Figure 1. Absorption spectra of 1 (10 μM) in the presence of 0–50 μM Zn2+ in 90%
(v/v) ethanol–water (10 mM HEPES, pH 7.0). Inset: Job plot analysis of 1 and Zn2+ at
a total concentration of 33 μM, indicating the formation of a 1:1 metal-to-ligand
complex.
For a better understanding of the interaction between 1 and
Zn2+, absorption, emission, ESI-MS and H-NMR spectra of 1 in
1
the absence and presence of Zn2+ were studied.
Figure 2. Fluorescence excitation (a) and emission (b) spectra of 1 (10 μM) in the presence of different concentrations of Zn2+ in 90% (v/v) ethanol–water (10 mM HEPES,
pH 7.0). Inset: fluorescence intensity at 530, 475 nm and ratio of F530:F475 as a function of Zn2+ concentration added to 1.
Luminescence 2010; 25: 445–451
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N. Li et al.
Figure 3. ESI-MS spectra of 1 (1 mM) in the absence and presence of 1.5 equiv.
Zn(NO3)2 in ethanol.
Figure 5. Fluorescence intensity ratio F530:F475 of 1 (10 μM) in the absence and
presence of 10 μM Zn2+ in 90% ethanol–water (10 mM HEPES) at different pH. Here
pH means the pH of HEPES buffers before adding to ethanol.
rhodamine B xanthene moiety of 1 upon the addition of 2 equiv.
Zn2+, while significant down-field shifts for Hg (Δδ = 0.31 ppm),
Hh (Δδ = 0.33 ppm), Hd (Δδ = 0.04 ppm), Hb (Δδ = 0.05 ppm) and
Hc (Δδ = 0.05 ppm), and up-field shifts for Ha (Δδ = 0.33 ppm)
and He (Δδ = 0.05 ppm) were observed, indicating that the 4-N,N-
diethylaminosalicylaldehyde hydrazone moiety was responsible
for the complexation of 1 to Zn2+.
Based on the above results, the proposed binding mechanism
of 1 and Zn2+ is illustrated in Fig. 4.
An association constant of log Ka = 5.22 was calculated for the
1:1 1–Zn2+ complex according to the absorption spectral titra-
tion data. (47) When excess EDTA was added to a solution of 1
(10 μM) in the presence of 5 equiv. Zn2+, the absorption spectra
were the same as those of 1 in the absence of Zn2+, suggesting
the reversibility of the interaction between 1 and Zn2+.
Effect of buffer pH and ethanol fraction
The effect of pH and water volume fraction of the solvent on the
recognization was also investigated. It was found that the ratio
F
little affected when the pH value was between 5.0 and 9.0, indi-
cating that the deprotonation of 1 in the absence of Zn2+ would
not occur, and the 1–Zn2+ complex was stable within this pH
range (Fig. 5).
Figure 4. 1H-NMR spectra of 1 (1 mM, sodium salt) in the absence (up) and pres-
ence (bottom) of 2.0 equiv. Zn(NO3)2 in CD3CN–D2O (9:1, v/v). [The reason for using
sodium salt (equal amount of 1 and NaOH in CD3CN–D2O) is to avoid the broaden-
ing of 1H-NMR peaks induced by the proton produced upon the binding of Zn(NO3)2
to 1.]
530:F475 of 10 μM 1 or 1–Zn2+ in aqueous ethanol solution was
On the other hand, water volume fraction played a more sig-
nificant role (Fig. 6). The change of ratio F530:F475 upon the addi-
tion of 0–20 μM Zn2+ to 10 μM 1 became mild with increasing
water volume fraction. Nevertheless, the sensing of Zn2+ in higher
water volume fraction solution could still be achieved with some
reduction in sensitivity.
ESI-MS spectra analysis of 1 in the absence and presence of 1.5
equiv. Zn2+ also supported the above 1:1 metal-to-ligand
complex and the high affinity (Fig. 3). (46) Only an intense peak
at m/z = 632.5 ([1 + H]+, calc m/z = 632.4) was observed for free
ligand 1, while a new peak at m/z = 694.9 ([1 + Zn − H]+, calcd
m/z = 694.3) appeared upon the addition of Zn2+. The observed
isotopic patterns of the peak of [1 + Zn − H]+ were in accordance
with the calculated ones, and when metal ions such as Na+, Mg2+
or Ag+ were present, only the peak of free ligand 1 was found.
The complex site was determinated by 1H-NMR (Fig. 4). It could
be seen that there was nearly no change for Hi, Hj and Hk on
Selectivity and tolerance of 1 to Zn2+ over other metal ions
The selectivity of 1 toward Zn2+ over other metal ions was notable.
Among the metal ions of interest, only Cu2+, Co2+ and Ni2+ could
cause obvious changes on the absorption spectra of 1. According
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Luminescence 2010; 25: 445–451

Fluorescent salicylaldehyde hydrazone as selective chemosensor for Zn2+
Figure 6. Fluorescence intensity ratio F530:F475 of 1 (10 μM) in the absence and
presence of 10 μM Zn2+ in ethanol–water (10 mM HEPES, pH 7.0) with different water
volume fraction.
Figure 7. Fluorescence spectra of 1 (10 μM) in the absence and presence of dif-
ferent metal ions (Na+, K+, Mg2+ and Ca2+: 1 mM; Zn2+, Pb2+, Mn2+, Fe3+, Co2+, Ni2+, Ag+,
Cd2+ and Hg2+: 10 μM) in 90% ethanol–water (v/v, 10 mM HEPES, pH 7.0).
analysis) except for 4. This result indicated the important role of
salicylaldimino moiety for these probes to bind with Zn2+. As
expected, compounds 2 and 3 showed similar ratiometric fluo-
rescence response to Zn2+ as 1, while 5 and 6 displayed only
fluorescence enhancement upon the addition of Zn2+ (Fig. 9).
It could be concluded that both 4-N,N-diethylaminosalicylaldi-
mino and spirolactam hydrazide units were essential for the
ratiometric fluorescence response of 1 to Zn2+ discussed in this
paper.
Analytical figures of merit
According to the above optimized condition of 10 μM 1 in 90%
ethanol–water (v/v, 10 mM HEPES, pH 7.0), the calibration curve
for the determination of Zn2+ by 1 was constructed. The relation-
ship between the fluorescence intensity ratio of 530 vs 475 nm
(F530:F475) and Zn2+ concentration in μM (C) was F530:F475 = 0.1233C
+ 1.4161. The linear range of the method was found to be at least
0–10.0 μM Zn2+ with a correlation coefficient of R2 = 0.9975 (n =
Figure 8. Fluorescence intensity ratio (F530:F475) of 1 (10 μM) and 1 with Zn2+
(10 μM) in the absence and presence of different metal ions (Na+, K+, Mg2+ and Ca2+
:
1 mM; Pb2+, Mn2+, Fe3+, Co2+, Ni2+, Ag+, Cd2+ and Hg2+:10 μM) in 90% (v/v) ethanol–
water (10 mM HEPES, pH 7.0).
21). The detection limit, based on the definition by IUPAC (CDL
=
3Sb/m), (48) was found to be 0.05 μM from 10 blank solutions. The
relative standard deviation (RSD) for three repeated measure-
ments of 5.0 μM Zn2+ was 3.1%.
to the ESI-MS result of 1 in the absence and presence of these
metal ions, they can also form 1:1 metal-to-ligand complexes
with 1. However, they can only quench the fluorescence of 1 fairly
but not affecting the intensity ratio of F530:F475 obviously. The
Application of 1 for Zn2+ analysis in water samples
result is shown in Figs 7 and 8. Common anions, such as Cl−, SO4
,
2−
Tap water (no Zn2+) and synthesized water (with Zn2+ and other
metal ions in tap water) were analyzed using 1 under optimized
condition. The results are summarized in Table 1, and suggest the
successful quantification of Zn2+ by 1, even in the presence of
Na+, K+, Mg2+, Ca2+, Pb2+, Ag+ and Cd2+.
−
−
NO3 , ClO4 , showed no interference to the interaction between
1 and Zn2+.
Zn2+-binding studies of control compounds
Zn2+ sensing properties of control compounds 2–6 (Scheme 1)
were also investigated to understand intensively the effect of
chemical substitution on the interaction between 1 and Zn2+.
According to their absorption spectra in the absence and pres-
ence of Zn2+ in aqueous ethanol (H2O:EtOH = 1:9, v/v, buffered
by 10 mM HEPES at pH 7.0), all these compounds exhibited some
affinity to Zn2+ (by 1:1 metal-to-ligand ratio based on Job plot
Conclusion
In summary, we find that 4-N,N-diethylamino-salicylaldehyde
rhodamine B hydrazone (1) exhibited selective and ratiometric
fluorescent response toward Zn2+ over other metal ions in
aqueous ethanol (H2O: EtOH = 1: 9, v/v, buffered by 10 mM HEPES
Luminescence 2010; 25: 445–451
Copyright © 2009 John Wiley & Sons, Ltd.
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N. Li et al.
Figure 9. Fluorescence spectra of 2 (10 μM), 3 (10 μM), 6 (10 μM) and 5 (10 μM) in the presence of different concentrations of Zn2+ in 90% ethanol–
water (v/v, 10 mM HEPES, pH 7.0).
Acknowledgment
Table 1. Determination of Zn2+ by 1 in water samplesa
The authors gratefully acknowledge the financial support from
the National Natural Science Foundation of China (NSFC, No.
20875054 and 90813014).
Zn2+ added Zn2+ found
Sample
Recovery RSDb
(%)
(%)
(μm)
(μm)
Tap water
0.00
2.50
0.00
2.50
0.00
2.60
3.89
6.72
104
99
103
5.8%
4.8%
5.9%
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