Design and synthesis of a squaraine based near-infrared fluorescent probe for the ratiometric detection of Zn2+ ions

Article abstract of DOI:10.1016/j.dyepig.2013.06.020

A squaraine based ratiometric near-infrared fluorescent probe with bipyridyl binding arms for Zn²⁺ (SQ-1) has been designed and synthesized. SQ-1 exhibits a selective fluorogenic response to Zn²⁺ in acetone by a 22 nm red-shift and a remarkable enhancement of the fluorescence intensity. Further study shows that the calculated detection limit for Zn ²⁺ is 6.1 × 10^-8 M (3σ/k) and the identification for Zn²⁺ will not be impacted by competing ions such as Na⁺, K⁺, Mg²⁺, Ca²⁺, Pb ²⁺, Mn²⁺, Ba²⁺ and Hg²⁺. The binding mode between SQ-1 and Zn²⁺ has also been discussed.

Full text of DOI:10.1016/j.dyepig.2013.06.020

Dyes and Pigments 99 (2013) 699e704
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Design and synthesis of a squaraine based near-infrared fluorescent
probe for the ratiometric detection of Zn2þ ions
,
,b,
Yangwei Huang a b, Qian Lin b, Jiaming Wu b, Nanyan Fu a
*
a State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou 350002, PR China
b Key Laboratory of Analysis and Detection for Food Safety, Ministry of Education & Fujian Provincial Key Laboratory of Analysis and Detection Technology
for Food Safety, and Department of Chemistry, Fuzhou University, Fuzhou 350108, PR China
a r t i c l e i n f o
a b s t r a c t
A squaraine based ratiometric near-infrared fluorescent probe with bipyridyl binding arms for Zn2þ (SQ-
1) has been designed and synthesized. SQ-1 exhibits a selective fluorogenic response to Zn2þ in acetone
by a 22 nm red-shift and a remarkable enhancementꢁo8f the fluorescence intensity. Further study shows
s
/k) and the identification for Zn2þ will not
Article history:
Received 25 August 2012
Received in revised form
26 April 2013
Accepted 13 June 2013
Available online 25 June 2013
that the calculated detection limit for Zn2þ is 6.1 ꢀ 10 M (3
be impacted by competing ions such as Naþ, Kþ, Mg2þ, Ca2þ, Pb2þ, Mn2þ, Ba2þ and Hg2þ. The binding
mode between SQ-1 and Zn2þ has also been discussed.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Squaraine
Bipyridyl ionophore
Ratiometric detection
Fluorescent sensor
Near-infrared functional dye
Zinc ion
1. Introduction
into human tissue to allow subcellular detection owing to the low
absorptivity and scattering [10]. Compared to other fluorescent
Zinc is one of the essential trace elements in the human body,
widely distributed in human cells and body fluids, and plays an
important role in many biological processes, such as gene expres-
sion, immune function and neurotransmission [1]. It is also associ-
ated with some pathological processes, including, for example, the
onset of Alzheimer’s disease, ischemic stroke, and epilepsy [2]. Due
to its d10 electron configuration, Zn2þ does not display any spectro-
scopic or magnetic signals. Therefore it is a major challenge to design
sensors excited by UV light, NIR sensors are ideal due to good
transmission and low autofluorescence in the living cells. To the best
of our knowledge, there are only a few NIR fluorescent sensors for
the detection of Zn2þ [11], which were mostly based on BODIPY [12],
tricarbocyanine [13,14], naphthalenediimide (NDI) [15] and spi-
robenzopyran [16] fluorophores. Moreover, ratiometric fluorescent
sensors involve signal rationing to detect the fluorescence intensities
at two different wavelengths and may avoid the influences of other
factors. These promise to be preferred to the conventional method of
monitoring the fluorescence intensity at a single wavelength [17].
Consequently, it is an attractive goal to design novel ratiometric and
NIR fluorescent probes for Zn2þ based on a new fluorophore.
Squaraine derivatives show intense absorption and emission
from the visible to the near-infrared waveband due to their donore
acceptoredonor type of charge transfer and their extensively con-
jugated structures [18]. These favorable optical properties are
highly suitable for designing fluorescent probes [19e23]. In 2000, a
2,3,3-trimethyl-3H-indole-derived squaraine dye was synthesized
by Dilek and Akkaya, which on binding to Zn2þ resulted in a three-
state fluorescence response. It provided an example of a molecular
ternary logic gate but not a real fluorescent probe for Zn2þ [24]. Due
to the strong zinc ion binding capacity of bipyridyl, we therefore
and synthesize fluorescent chemosensors for the detection of Zn2þ
.
Although some promising results have been reported in the past
decade [3e8], there is still a great need for improved methodology
for this purpose. In recent years, near-infrared (NIR) fluorescent
sensors have attracted increasing attention for their potential in vivo
imaging applications [9]. In the near-infrared region around 650e
900 nm, biological samples have low background fluorescence sig-
nals. However, near-infrared radiation can penetrate deep enough
* Corresponding author. Key Laboratory of Analysis and Detection for Food Safety,
Ministry of Education & Fujian Provincial Key Laboratory of Analysis and Detection
Technology for Food Safety, and Department of Chemistry, Fuzhou University,
Fuzhou 350108, PR China. Tel.: þ86 13459108842.
E-mail addresses: nanyan_fu@fzu.edu.cn, nanyan_fu@hotmail.com (N. Fu).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.06.020

700
Y. Huang et al. / Dyes and Pigments 99 (2013) 699e704
sough to use a squaraine dye as a receptor by the linkage of con-
jugated double bonds, and a novel squaraine dye (SQ-1) has been
designed and synthesized. To the best of our knowledge, SQ-1 is the
first ratiometric fluorescent probe for zinc ion based on squaraine.
2.4.2. Synthesis of 1
NaH (0.058 g, 2.4 mmol) was added to a solution of 6 (0.12 g,
0.59 mmol) in dry THF (15 mL) in a round-bottom flask. The sus-
pension was stirred for 6 min. A solution of 2 (0.18 g, 0.59 mmol) in
dry THF (5 mL) was added dropwise to the flask with stirring. The
solution was heated at 50 ꢂC over night. The mixture was poured
into ice water (20 mL). The aqueous layer was extracted with EtOAc
(10 mL ꢀ 3) and the organic portions were dried over Na2SO4,
filtered, and followed by solvent removal under vacuum. The crude
product was purified by column chromatography on silica gel
(petroleum ether/ethyl acetate, v/v ¼ 3/1) to afford a yellow oil
(0.15 g), yield 75%.
2. Experiment procedures
2.1. Materials and general methods
All the solvents were redistilled before use. Except where spe-
cifically noted, other chemicals and reagents were obtained from
commercial suppliers and used without further purification.
4-Methyl-2,20-bipyridine (4) was synthesized according to the liter-
ature [25]. 1-(2-(2-Methoxy ethoxy)ethyl)-1H-pyrrole (7) was syn-
thesized according to the literature [26] (see p S1, Supplementary
data). The syntheses and manipulations of squaraine dye were
carried out under dry N2 atmosphere. 1H NMR and 13C NMR spectra
were recorded in CDCl3 using a Bruker AV-400 spectrometer
(400 MHz) which the chemical shifts given relative to TMS
(0.00 ppm). Mass spectra (ESI) were measured by a ThermoFinnigan
DECAX-30000 LCQ Deca XP ion trap mass spectrometry. Elemental
analyses were carried out with a Vario MICRO elemental analyzer.
Melting points of compounds were determined with a SGW X-4 in-
strument without correction. Absorbance spectra were measured
with a Perkin Elmer Lambda750 UV spectrophotometer. Fluorescent
emission spectra were collected on a Cary Edipse fluorescence
spectrophotometer.
1H NMR (400 MHz, CDCl3)
d 3.32 (s, 3H), 3.48e3.50 (m, 2H),
3.54e3.56 (m, 2H), 3.76 (t, J ¼ 5.8 Hz, 2H), 4.23 (t, J ¼ 5.8 Hz, 2H),
6.20e6.21 (m, 1H), 6.61e6.62 (m, 1H), 6.80e6.81 (m, 1H), 6.88 (d,
J ¼ 16.0 Hz, 1H), 7.30e7.35 (m, 2H), 7.37 (d, J ¼ 16.0 Hz, 1H), 7.82 (td,
J ¼ 7.8, 1.8 Hz, 1H), 8.41 (d, J ¼ 8.0 Hz, 2H), 8.59 (d, J ¼ 5.2 Hz, 1H),
8.70e8.71 (m,1H); 13C NMR (100 MHz, CDCl3)
d 46.7, 59.1, 70.8, 71.2,
71.9, 108.6, 109.2, 117.7, 120.3, 121.3, 121.6, 122.8, 123.7, 124.4, 130.8,
136.9,146.4,149.1,149.4,156.3,156.4; ESI-MS: m/z 350.5 ([M þ H]þ).
2.4.3. Synthesis of 2
4-Methyl-2,20-bipyridine (4) (0.20 g, 1.2 mmol), N-bromosucci-
nimide (0.21 g, 1.2 mmol), and 2,20-azobis(isobutyronitrile) (5 mg)
were added to CCl4 (20 mL). The solution was refluxed under N2 for
6 h. The mixture was cooled to room temperature, filtered, and
evaporated to near dryness. The residue was purified by flash
column chromatography on silica gel (petroleum ether/triethyl-
amine, v/v ¼ 97/3) to afford pyridinium salt 3, which is too reactive
to store [27], and 3 was immediately dissolved in P(OEt)3 (5 mL).
The mixture was heated at 80 ꢂC for 12 h in the dark. Excess P(OEt)3
was removed under high vacuum after the reaction. The crude
product was isolated by column chromatography on silica gel (ethyl
acetate) to afford 2 as yellow oil (0.10 g). The total yield was 55%.
2.2. Preparation of stock solutions
Metal salt solution (0.01 M) of Agþ, Ca2þ, Cd2þ, Co2þ, Cu2þ, Hg2þ
,
Kþ, Mg2þ, Mn2þ, Naþ, Ni2þ, Pb2þ, Ba2þ, Al3þ, Fe3þ and Zn2þ were
prepared in doubly deionized water. A stock solution of SQ-1
(1 ꢀ 10ꢁ3 M) was prepared in acetic acid.
1H NMR (400 MHz, CDCl3)
d
1.27 (t, J ¼ 7.1 Hz, 6H), 3.24 (d,
2.3. Details of Zn2þ ion sensing studies
J ¼ 22.2 Hz, 2H), 4.04e4.11 (m, 4H), 7.28e7.33 (m, 2H), 7.81
(td, J ¼ 7.8, 1.8 Hz, 1H), 8.35 (s, 1H), 8.40 (d, J ¼ 8.0 Hz, 1H), 8.61 (d,
The squaraine dye SQ-1 was dissolved in acetone (3
ion consecutive titrations were carried out by sequentially adding
L of the Zn2þ solutions into 2 mL acetone in a quartz cuvette. The
mM). Metal
J ¼ 5.0 Hz,1H), 8.66e8.68 (m, 1H); 13C NMR (100 MHz, CDCl3)
d 16.4
(d, J ¼ 6.0 Hz), 33.7(d, J ¼ 136 Hz), 62.4 (d, J ¼ 6.7 Hz),121.2, 122.4 (d,
J ¼ 6.9 Hz), 123.8, 124.9 (d, J ¼ 5.6 Hz), 136.9, 142.2 (d, J ¼ 8.6 Hz),
149.2, 149.3 (d, J ¼ 2.7 Hz), 155.9, 156.4 (d, J ¼ 2.4 Hz); ESI-MS: m/z
307.6 ([M þ H]þ).
6
m
solution was mixed well on a lab dancer and was left to stand for
2 min before recording the absorption and emission spectra.
2.4. Synthesis
2.4.4. Synthesis of 6
To DMF (0.34 g, 4.7 mmol) in a round bottom flask was added
POCl3 (0.67 g, 4.4 mmol) dropwise at 0 ꢂC. The cooling bath was
removed and stirring was continued for another 15 min at room
temperature. The solution was diluted with 1,2-dichloroethane
(5 mL) and again cooled to 0 ꢂC. A solution of 7 (0.63 g,
3.7 mmol) in 1,2-dichloroethane (5 mL) was added dropwise. The
mixture was heated to reflux for 40 min and cooled to room tem-
perature, and a solution of Na2CO3 (2.86 g) in water (15 mL) was
added under vigorous stirring. Then the mixture was refluxed for
40 min and allowed to cool to room temperature. The aqueous layer
was separated and exacted with CH2Cl2 (10 mL ꢀ 3). The combined
organic layers were washed with saturated Na2CO3 solution twice,
and dried with Na2CO3 and filtered, followed by solvent removal
under vacuum. The crude product was purified by column chro-
matography on silica gel (petroleum ether/ethyl acetate, v/v ¼ 4/1)
to afford 6 as a light yellow oil (0.35 g), yield 48%.
2.4.1. Synthesis of SQ-1
Squaric acid (27 mg, 0.24 mmol) and 1 (163 mg, 0.48 mmol)
were added to a three-necked flask equipped with a DeaneStark
trap. A mixture of n-butanol (20 mL) and toluene (20 mL) were
added and the reaction mixture was refluxed under N2 over night.
The solvents were removed under reduced pressure and the crude
product was purified by column chromatography on silica gel (ethyl
acetate/methanol, v/v ¼ 20/1e5/1) to afford SQ-1 as a green solid
(26 mg), yield 30%, m.p 199e201 ꢂC.
1H NMR (400 MHz, CDCl3)
d 3.17 (s, 6H), 3.41e3.43 (m, 4H),
3.54e3.57 (m, 4H), 3.90 (t, J ¼ 4.4 Hz, 4H), 5.02 (d, J ¼ 4.6 Hz, 4H),
6.97 (d, J ¼ 4.7 Hz, 2H), 7.25 (d, J ¼ 16.0 Hz, 2H), 7.33e7.36 (m, 2H),
7.48 (dd, J ¼ 5.1, 1.4 Hz, 2H), 7.70 (d, J ¼ 16.1 Hz, 2H), 7.84 (td, J ¼ 7.8,
1.7 Hz, 2H), 7.89 (d, J ¼ 4.6 Hz, 2H), 8.43 (d, J ¼ 7.9 Hz, 2H), 8.50 (s,
2H), 8.68 (d, J ¼ 5.1 Hz, 2H), 8.72 (d, J ¼ 4.1 Hz, 2H); 13C NMR
(100 MHz, CDCl3)
d
47.4, 58.9, 70.8, 71.8, 72.0, 114.7, 119.1, 120.2,
FTIR (KBr): 3111, 2877, 2818, 2724, 1661, 1528, 1481, 1404, 1370,
120.9, 121.2, 124.0, 124.5, 130.6, 132.0, 137.0, 144.5, 147.2, 149.2,
149.6, 155.7, 156.8, 169.7, 176.6; ESI-MS: m/z 777.3 ([M þ H]þ); Anal.
Calcd for C46H44N6O6: C 71.12, H 5.71, N 10.82. Found: C71.07, H
5.75, N 10.79.
1321,1246, 1116, 764, 610 cmꢁ1; 1H NMR (400 MHz, CDCl3)
d 3.34 (s,
3H), 3.45e3.48 (m, 2H), 3.51e3.53 (m, 2H), 3.75 (t, J ¼ 5.3 Hz, 2H),
4.51 (t, J ¼ 5.3 Hz, 2H), 6.21 (dd, J ¼ 4.0, 2.5 Hz, 1H), 6.93 (dd, J ¼ 4.0,
1.7 Hz, 1H), 7.07 (s, 1H), 9.51 (s, 1H); 13C NMR (100 MHz, CDCl3)

Y. Huang et al. / Dyes and Pigments 99 (2013) 699e704
701
Scheme 1. Synthesis of SQ-1.
d
48.8, 59.0, 70.4, 70.6, 71.8, 109.5, 125.1, 131.1, 132.8, 179.3; ESI-MS:
3.2. Physicochemical properties and absorption experiments
m/z 198.0 ([M þ H]þ).
The photophysical properties of SQ-1 in different solvents are
summarized in Table 1. Due to the large -extended conjugated
p
3. Results and discussion
system, a dramatic red-shift of about 50 nm compared to BDPSQ (bis
[4-(dimethylamino)phenyl]-squaraine, 627 nm in CH2Cl2 [18]) is
observed. Compared to other solvents, the absorption maximum of
SQ-1 is obviously red-shifted in formic acid and acetic acid because
of the protonation of oxygen atom in the central four membered ring
of SQ-1 [28], which also induces quenching of the emission. Due to
the formation of H-type aggregates induced by H2O, the maximum
absorption of SQ-1 is remarkably blue-shifted in water with a
decrease of the molar extinction coefficient and the fluorescent
emission is quenched (see Fig. S16, Supplementary data).
3.1. Synthesis
SQ-1 was prepared through a multi-step process as shown in
Scheme 1. In the presence of NaOH and tetrabutylammonium
bromide, pyrrole was reacted with 1-bromo-2-(2-methoxyethoxy)
ethane to form 7. Then the pyrrole derivative 7 was formylated by
Vilsmeier reagent to give the corresponding pyrrole carbaldehyde
6. The phosphonate 2 was prepared by a four-step synthetic
sequence, which was started with 2-acetylpridine in the presence
of iodine to give an activated pyridinium salt 5. The Kröhnke re-
action between 5 and crotonaldehyde in the presence of ammo-
nium acetate afforded bipyridyl derivative 4, which was
brominated by NBS with initiation by AIBN, to give 3. The Arbuzov
reaction between 3 and P(OEt)3 afforded phosphonate 2. Then
WittigeHorner condensation of 2 and 6 produced 1. SQ-1 was
prepared by the condensation of 1 and squaric acid. This new
compound was fully characterized by 1H NMR, 13C NMR and ESI
mass spectroscopy (see pp S2eS10, Supplementary data).
3.3. Ratiometric fluorescence response of SQ-1 toward Zn2þ
To explore the chemosensing behavior of SQ-1 toward Zn2þ, the
fluorescent titration of SQ-1 with Zn2þ in acetone were performed
as shown in Fig. 1. The sensor SQ-1 exhibits an emission in the near-
infrared range around 690 nm in acetone. Upon the addition of
increasing amounts of Zn2þ, a 22 nm red-shift from 698 to 720 nm
of fluorescent emission with a well-defined isosbestic point at
Table 1
Absorption and emission parameters for SQ-1 in different solvents.
Solvent
CH3CN
EtOH
Acetone
CHCl3
THF
DMF
HAc
HCOOH
H2O
lmax (nm)
3
675
677
680
684
685
686
697
725
624
(Mꢁ1 cmꢁ1
)
2.8 ꢀ 105
697
3.2 ꢀ 105
689
2.6 ꢀ 105
698
2.8 ꢀ 105
697
3.2 ꢀ 105
698
2.8 ꢀ 105
699
3.0 ꢀ 105
1.9 ꢀ 105
1.4 ꢀ 105
lem (nm)
N
N
N

702
Y. Huang et al. / Dyes and Pigments 99 (2013) 699e704
1000
800
600
400
200
0
[SQ-1 þ 2Zn2þ þ 2CH3COOꢁ þ 4CH3COCH3]2þ (calculated for 627.2)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
in the ESI mass spectrum (see Fig. S18, Supplementary data). A peak
at m/z 899.4 assignable to [SQ-1 þ Zn2þ þ CH3COOꢁ]þ (calculated
for 899.3) was also found, which may be formed by losing one zinc
ion from the 1:2 complex. Certainly, this MS result can not rule out
the 1:1 (SQ-1:Zn2þ) stoichiometric manner absolutely. To investi-
gate the binding mode further, a 1H NMR titration was also con-
ducted. As shown in Fig. 3, the aromatic signals of SQ-1 become
broadened with observable chemical shift changes after the coor-
dination of SQ-1 with Zn2þ. This provided direct evidence for the
formation of metal-organic coordination oligomers or polymers
[29], and indicated that the bipyridyl moieties on both sides of SQ-1
participated in the coordination with Zn2þ. According to the re-
ported binding stoichiometry between bipyridine and Zn2þ [30],
the 1:1 complex may exist simultaneously in the solution. There-
fore, the stoichiometry of the SQ-1-Zn2þ complex requires further
justification.
0
2
4
6
8
10
[ Zn ] / [SQ]
680
700
720
740
760
780
800
Wavelength (nm)
Fig. 1. Change in fluorescence intensity of SQ-1 (3 mM) with increasing addition of
Zn2þ (0e30
mM) in acetone. Inset: ratiometric calibration curve F720/F698 as a function
of Zn2þ concentration. (lex ¼ 670 nm, slit: 5 nm/5 nm, PMT Volts: 650.).
3.4. Selectivity investigation
To investigate the selectivity of SQ-1 toward Zn2þ, the fluores-
cence response to other environmentally relevant metal ions were
measured. The emission spectra upon exposure to various metal
about 710 nm was observed. The intensity ratio between 720 and
698 nm increased linearly with the concentration of Zn2þ up to a
mole ratio (SQ-1/Zn2þ) of 1:4, and then leveled off (I720/I698 ¼ 3.5).
Correspondingly, as shown in Fig. S17 (Supplementary data), a
15 nm red-shift from 680 nm to 695 nm of the absorption band was
also observed. Thus, SQ-1 can positively respond to Zn2þ, demon-
strating that it is a novel ratiometric fluorescent probe for zinc in
the near-infrared region based on a squaraine fluorophore.
ions (60 m
M, 20 equiv) including alkali (Naþ and Kþ), alkaline earth
(Mg2þ, Ca2þ and Ba2þ) and transition and heavy metal ions (Al3þ
,
Mn2þ, Fe3þ, Pb2þ, Agþ, Cd2þ, Cu2þ, Co2þ, Ni2þ and Hg2þ) in acetone
solution are shown in Fig. 4(A). The fluorescence response of SQ-1
with alkali and alkaline earth metal ions Naþ, Kþ, Mg2þ and Ca2þ
keep silent due to the low affinity of SQ-1 chelator toward these
metals. While Agþ, Al3þ, Fe3þ, Ba2þ, Pb2þ and Mn2þ partly
Preliminary studies on the detection limit were also conducted
on the titration of SQ-1 with Zn2D. A series of fluorescence mea-
surements versus Zn2þ ion concentrations ranging from 0.6 to
quenched the fluorescence, and Co2þ Cu2þ Ni2þ and Cd2þ
, ,
completely quenched the fluorescence. Upon addition of Hg2þ, an
8 nm red-shift compared to other ions was observed. Interestingly,
only when Zn2þ ions were added to SQ-1 in the solution, the
fluorescent emission showed a remarkable red-shift and a great
enhancement compared to the above mentioned metal ions.
Although SQ-1 does not show obvious binding discrimination
among most transition metal ions, it has a high fluorescent selec-
tivity toward Zn2þ, which is similar to other probes for Zn2þ based
on the bipyridine receptor [31,32]. Significantly, by measuring the
changes in the ratio of the fluorescence intensities at 720 nm and
698 nm, only Al3þ and Hg2þ gave a slight interference to the
3.6
m
M were obtained and primarily linear-fitted (R2 ¼ 0.99,
k ¼ 6.90 ꢀ 107 au/M). The detection limit was calculated as
6.1 ꢀ 10ꢁ8 M (based on 3
s/k, where s is the standard deviation of
the blank measurements, and k is the slope of the intensity ratio
versus sample concentration plot).
To determine the binding stoichiometry of SQ-1 and Zn2þ, a
Job’s plot for the emission was measured. As shown in Fig. 2, it
exhibits a maximum fluorescence at 0.67 mol fraction of Zn2þ
,
suggesting that a 1:2 (SQ-1:Zn2þ) complex is formed. This was also
confirmed by the appearance of a peak at m/z 627.3 assignable to
200
160
120
80
40
0
0.0
0.2
0.4
0.6
0.8
1.0
2+
Mole fraction of Zn
Fig. 2. Job’s plot for the complexation of SQ-1 with Zn2þ in acetone. F0: the fluores-
cence intensity of SQ-1 in acetone. F: the fluorescence intensity of SQ-1 with corre-
sponding Zn2þ in acetone. (lex ¼ 670 nm, slit: 5 nm/5 nm, PMT Volts: 650.).
Fig. 3. 1H NMR spectrum of SQ-1 (4 mM) in CDCl3 with increasing concentration of
Zn(CH3COO)2. The mole ratio of Zn(CH3COO)2 to SQ-1 is (1) 0, (2) 1.0, (3) 11.0,
respectively.

Y. Huang et al. / Dyes and Pigments 99 (2013) 699e704
703
4.0
A
700
600
500
400
300
200
100
0
SQ-1 only
3.5
+
+
2+
2+
K
,
Na
,
Ca , Mg
3.0
2.5
2.0
1.5
1.0
0.5
0.0
2+
Hg
3+
2+
3+
2+
2+
Al
,
Mn
,
Fe
,
Pb
,
Ba
+
Ag
2+
2+
2+
2+
Cd
,
Cu , Co , Ni
2+
680
700
720
740
760
780
800
2+
2+
Ba
2+
Pb
2+
Zn
2+
+
+
2+
Ca
Hg
Mg
Mn
Na
K
Wavelength (nm)
only
Metal ions
Fig. 5. The competition experiments of Zn2þ (60
(60
M, 20 equiv) in acetone. (lex ¼ 670 nm, slit: 5 nm/5 nm, PMT Volts: 650.).
m
M, 20 equiv) to other metal ions
3.5
B
m
3.0
2.5
2.0
1.5
1.0
0.5
0.0
program at semiempirical PM3 levels. Without taking the sol-
vent effect into account, we found that the two pyridyls at the
binding arms of SQ-1 are twisted with a 22ꢂ angle in the opti-
mized geometry of the dye (Fig. 6(A)). In the optimized geom-
etry of the complex, a tetrahedryl coordinated zinc is proposed,
and the two pyridyls are pulled in the same plane by the central
metal (Fig. 6(B)).
Based on the calculations, the possible binding mode between
SQ-1 and Zn2þ is depicted in Fig. 7. Upon binding with Zn2þ, the
twist angle between the two pyridyl moieties is decreased, leading
to a more planar conformation of the SQ-1-Zn2þ complex. The
conformational restriction and intramolecular charge transfer (ICT)
produce a strong red-shift and a remarkable enhancement of
fluorescent emission [33].
Mg
SQ-1
Ca
Al
Fe Ba Pb Ag
Co
Cu
Ni
Na
Mn
K
Hg
Cd
Zn
only
Metal ions
Fig. 4. (A) The fluorescence spectra of SQ-1 (3 mM) in the presence of different metal
ions (60 mM, 20 equiv) in acetone. (B) The ratiometric response of F720 to F698 toward
Zn2þ. (lex ¼ 670 nm, slit: 5 nm/5 nm, PMT Volts: 650.).
ratiometric response of F720 to F698 toward Zn2þ (Fig. 4(B)).
The ratiometric fluorescent responses toward excess amount of
these metal ions (300 mM, 100 equiv and 600 mM, 200 equiv) have
also been recorded (see Fig. S19, Supplementary data). These re-
sults suggest that SQ-1 has high selectivity for the detection of Zn2þ
and can be used as a ratiometric and turn-on fluorescent chemo-
sensor for Zn2þ compared to other environmentally and biologi-
cally relevant ions.
To further explore the tolerance of SQ-1 to Zn2þ over other metal
ions, competition experiments were conducted in the presence of
Zn2þ (20 equiv) mixed with metal ions (20 equiv and 100 equiv)
respectively. As displayed in Fig. 5 and Fig. S20(A) (Supplementary
data), the amplitude of Zn2þ induced fluorescence enhancement
was not interfered with biologically ubiquitous ions such as Naþ,
Kþ, Mg2þ, Ca2þ and some transitional metal ions like Pb2þ, Mn2þ
,
.
Ba2þ, Hg2þ, showing SQ-1 has a pronounced selectivity for Zn2þ
However, high ionic strength and some transition metal ions such
as Fe3þ, Al3þ, Ni2þ, Cd2þ, Cu2þ, Co2þ and Agþ could quench the
fluorescence of SQ-1, interfering the Zn2þ detection (see Figs,
S20(B) and S21, Supplementary data).
3.5. Hypothesis of the binding mode
To further study the coordination geometry of SQ-1 and Zn2þ
theoretical calculations were performed with the Gaussian 03
,
Fig. 6. Proposed geometry-optimized structures of SQ-1 (A) and its Zn2þ complex (B)
(Geometry is optimized using semiempirical PM3 calculation.).

704
Y. Huang et al. / Dyes and Pigments 99 (2013) 699e704
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4. Conclusion
In conclusion, SQ-1, a novel ratiometric near-infrared fluores-
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is calculated as 6.1 ꢀ 10ꢁ8 M (3
s
/k) and the identification for Zn2þ
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Acknowledgments
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This work was financially supported by the Fujian Provincial
Natural Science Foundation of China (No. 2008J0148), the National
Natural Science Foundation of China (No. 20702005, 21105013), the
Scientific Research Foundation for the Returned Overseas Chinese
Scholars, State Education Ministry (No. LXKQ0802), and Funding
(Type A) from Fujian Education Department (No. JA12038). We
thank Fujian Institute of Research on the Structure of Matter, Chi-
nese Academy of Sciences for the MS, Elemental Analysis and NMR
measurements.
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Appendix A. Supplementary data
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Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2013.06.020.
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