www.rsc.org/dalton | Dalton Transactions
PAPER
Quinoline-based molecular clips for selective fluorescent detection of Zn2+ †
Da-Yu Wu, Li-Xia Xie, Chang-Li Zhang, Chun-Ying Duan,* Yong-Gang Zhao and Zi-Jian Guo*
Received 20th April 2006, Accepted 8th June 2006
First published as an Advance Article on the web 22nd June 2006
DOI: 10.1039/b605653a
New selective Zn2+ fluorescent sensors, di(2-quinoline-carbaldehyde)-2,2 -bibenzoyl-hydrazone (QB1)
and di(2-quinolinecarbaldehyde)-6,6 -dicarboxylic acid hydrazone-2,2 -bipyridine (QB2), have been
designed and prepared. Both QB sensors exhibit an emission band centered at 405 nm (excitation at
350 nm) with low quantum yield. Zinc binding not only red-shifts the emission band to 500 nm, but
also enhances the fluorescence intensity by an order of magnitude based on the deprotonization
strategy via self-assembly. These probes are highly selective for Zn2+ over biologically relevant alkali
metals, alkaline earth metals and the first row transition metals such as Mn2+ , Fe2+ , Co2+ and Ni2+ in
buffered aqueous DMSO solution.
Introduction
The design of chemosensors—molecules that can selectively
recognize and signal the presence of a specific analyte through
the naked eye, electrochemical and optical response—is one of
the main achievements of supramolecular chemistry.1 On account
of its simplicity and high sensitivity, fluorescence is increasingly
important for trace chemical detection.2 Since the pioneering work
on the cation crown ether3 using fluorescence for monitoring
low concentrations of metal cations, considerable effort has been
devoted to developing fluorescent chemosensors for cations,4
neutral guests and anions. After Fe ions, the Zn(II) ion is the
most abundant transition metal ion in the human body.5 The
emerging importance of Zn2+ in neurological signalling and some
proposed functions6 in biological systems have generated an urgent
demand for the development of Zn2+ -specific molecular probes,7,8
and many Zn2+ fluorescent sensors have been reported, exhibiting
high selectivity and sensitivity over other biologically essential
metal ions in specific ranges of concentration.9–12
The most widely used fluorescent probes for Zn2+ are TSQ (N-(6methoxy-8-quinolyl)-p-toluenesulfonamide) and its congeners,13
which are quinoline derivatives bearing sulfonamide groups that
give substantial fluorescence enhancement upon Zn(II) bonding.
Recently, we have initiated a program aimed at developing Zn2+
quinoline-based fluorescent sensors by coupling 8-aminoquinoline
with dansyl-sulfonamide and successfully shifted the excitation
wavelength up to 395 nm, with the emission blue-shift ca 80 nm
based on the photo-induced charge transfer (PCT) mechanism.14
In this paper, we transfer the sensing strategy to self-assembly
chemistry and report on a new type of easily accessible quinolinebased probes QB (Scheme 1) that bind Zn2+ in a 2 : 2 molar
ratio, in which the quinoline groups act as both fluorophore and
binding units. It is expected that in the presence of metal ions,
the imide-groups are deprotonated and the chemosensors act as
negative chelators, from which the participation of deprotonated
State Key Laboratory of Coordination Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University, 210093 Nanjing, P. R. China.
E-mail: [email protected]
† Electronic supplementary information (ESI) available: Fluorescence
data. See DOI: 10.1039/b605653a
3528 | Dalton Trans., 2006, 3528–3533
Scheme 1
The chemical structures of chemosensors QB1 and QB2.
imide groups in an extensive p-system results and red-shifts of both
excitation and emission bands are achieved.15,16 Furthermore, the
signal transduction occurs via CHEF (chelation-enhanced fluorescence) or self-assembling fluorescent enhancement (SAFE),17
resulting fron the formation of a helicate structure upon chelation
of Zn(II).
Experimental
Materials and methods
Unless otherwise stated, materials were obtained from commercial
suppliers and were used without further purification. Diphenic
dihydrazide was prepared according to the procedure reported
in the literature,18 as were binicontinic acid and biniconitinic
ester.19 The 1 H NMR spectra were recorded on a DRX 500
Bruker spectrometer at 298 K with TMS as internal standard.
The ESI-MS spectra were performed on a LCQ system (Finnigan
MAT, USA) using CH3 OH–CH3 CN as the mobile phase. For free
chemosensors QB1 and QB2, before the measurements, several
drops of acetic acid (HAc) were added to the CH3 OH–CH3 CN
solution (5 lM) to measure the protonated QB1 and QB2. Element
analysis of C, N and H were performed with a Perkin-Elmer 240
analyzer. IR (KBr pellet) spectra was recorded on a Vector 22
Bruker spectrophotometer.
Fluorescence emission spectra were obtained using the ABseries2 luminescence spectrometer. Stock solution (2 × 10−2 mol
L−1 ) of the aqueous salts of K+ , Li+ , Na+ , Ca2+ , Mg2+ , Co2+ ,
Ni2+ , Cu2+ , Mn2+ , Zn2+ , Cd2+ , Fe2+ , Cr3+ and Hg2+ were prepared.
This journal is © The Royal Society of Chemistry 2006
Stock solutions of host QB1 (1.0 × 10−4 mol L−1 ) and QB2
(1.0 × 10−5 mol L−1 ) were prepared in buffered aqueous DMSO
solutions. 2 ml host solutions were placed into a test tube,
adding an appropriate aliquot of each metal stock into the host
solution. A starting aqueous solution of 50 lM QB prepared in
50 mM TRIS (or 50 mM HEPES), 50 mM KNO3 , was used
for detecting fluorescence dependence on pH upon addition of
0.75 mM Zn(ClO4 )2 , and the pH of the solutions was adjusted by
addition of 0.2 mol L−1 HCl (or 0.1 mol L−1 NaOH). The quantum
efficiency of metal-free or metal-bound ligands was measured by
using the diluted aqueous solutions of QB (50 lM) in 50 mM
HEPES, pH = 7.25, 50 mM KNO3 . Optically matching solutions
of Ru(2,2-bpy)3 (ClO4 )2 (U = 0.059)20 were selected as the reference
in spectroscopic grade CH3 CN at an excitation wavelength of
450 nm. The concentration of the reference was adjusted to
match the absorbance of the test sample. For all the fluorescent
measurements, the excitation wavelength was 420 nm except for
the free QB sensors, which were excited at ∼350 nm. Excitation
and emission slit widths were 8 nm and 4 nm, respectively.
Optical absorption spectra were collected on a Shimadzu 3100
spectrophotometer in an aqueous DMSO solution (H2 O–DMSO,
30 : 70, v/v, 50 mM HEPES buffer, 50 mM KNO3 , pH = 7.25)
at room temperature. All the spectroscopic measurements were
performed at least in triplicate and averaged.
Di(2-quinolinecarbaldehyde)-2,2 -bibenzoylhydrazone (1, QB1).
A solution of 2,2 -bibenzoylhydrazide (2 mmol, 0.54 g) in methanol
(15 mL) was added to a methanol solution (10 mL) containing
2-quinoline-carbaldehyde (4 mmol, 0.63 g). After 5 drops of
acetic acid was added, the yellow mixture was heated at boiling
temperature under magnetic stirring for 2 h. During the reaction,
a pale yellow precipitate was formed, which was collected by
filtration, washed with methanol–ether, and dried in vacuo. Yield:
0.98 g, 90%. Anal calc. for QB1 (C34 H24 N6 O2 ): H 4.41, C 74.43, N
15.33. Found: H 4.35, C 74.95, N 15.69; ESI-MS: m/z 549.1 for
[QB1 + H]+ , calc. for C34 H25 N6 O2 , 549.2. 1 H NMR (DMSO-d 6 )
d H 12.41(1 H, s, NH), 8.33(1 H, d, J = 8.65 Hz), 8.20(1 H, s),
7.97(1 H, d, J = 8.60 Hz), 7.90(1 H, d, J = 7.55 Hz), 7.66(1 H,
d, J = 7.50 Hz), 7.61(1 H, t, J = 15.3 Hz), 7.54(1 H, 3 t + 1 d,
J = 7.2 Hz, 1 H, J = 7.2 Hz), 7.35(1 H, d, J = 7.60 Hz); IR (solid
KBr pellet, cm−1 ): 3495 (m), 3189 (m), 3057 (m), 1694 (s), 1596 (s),
1553 (s), 1504 (m), 1429 (w), 1289 (s), 1160 (m), 752 (m).
Di(2-quinolinecarbaldehyde)-6,6 -dicarboxylic acid hydrazone-2,
2 -bipyridine (2, QB2). Biniconitinic ester (2.72 g, 10 mmol) was
dissolved in methanol, 10 mL 80% N2 H4 ·H2 O (0.94 g, 15 mmol)
was added dropwise. The solution was refluxed for 2 h and the
white product was produced in a high yield (2.45 g, 90%). 6,6 Dicarboxylic acid hydrazide-2,2 -bipyridine (1.1 mmol, 0.3 g) and
2-quinolinecarbaldehyde (2.2 mmol, 0.35 g) were mixed in boiling
methanol with 3 drops of acetic acid. After 2 h of stirring,
pale yellow precipitates obtained were filtered off, washed with
methanol and dried over P2 O5 under vacuum. Yield: 0.56 g, 92%.
Anal calc. for QB2 (C32 H22 N8 O2 ): H 4.03, C 69.79, N 20.36. Found:
H 4.10, C 69.94, N 19.85; ESI-MS: m/z 551.2 for [QB2 + H]+ ,
calc. for C32 H23 N8 O2 , 551.6. 1 H NMR (DMSO-d 6 ) d H 12.63 (1 H,
s, NH), 8.68 (1 H, d, J = 13.1 Hz), 8.40 (1 H, s), 8.14 (1 H,
d, J = 8.60 Hz), 8.05 (1 H, t, J = 8.91 Hz), 7.95 (1 H, d, J =
8.56 Hz), 7.89(1 H, d, J = 8.15 Hz,), 7.81 (1H, t, J = 9.12 Hz),
7.57 (1 H, 3t + 1d, J t = 9.31 Hz, J d = 7.85 Hz), 7.45 (1 H, d,
This journal is © The Royal Society of Chemistry 2006
J = 10.02); IR (solid KBr pellet): 3424(m), 3182(m), 3056(m),
1692(s), 1561(m), 1505(m), 1459(w), 1429(w), 1418(w), 1358(w),
1296(m), 1217(w), 1159(m), 1119(w), 1016(m), 954(w), 923(w),
902(w), 880(w), 833(w), 754(m), 682(w), 628(w).
Complex Zn–QB1, (3). Zn(ClO4 )2 ·6H2 O (0.1 mmol, 0.037 g)
dissolved in 15 mL methanol–H2 O (v/v, 8 : 2) was added to a
suspension of QB1 (0.1 mmol, 0.055 g) in 10 mL methanol. The
solution was stirred at boiling temperature for 20 min to obtain
a clear solution and allowed to stand at room temperature. After
several days, needle-like crystals were obtained by filtration. Anal
calc. for Zn2 (C34 H23 N6 O2 )2 (ClO4 )2 (CH3 OH)2 : H 3.47 C 57.02 N
11.57; Found: H 3.74, C 57.25, N 11.66; ESI-MS: m/z 1223.4
(calc.. for [Zn2 (QB1)2 ]+ , 1223.9) and 611.3 (calc. for [Zn(QB1)]+ ,
611.1). 1 H NMR (DMSO-d 6 ): d H 8.6(1H, d, J = 6.15 Hz), 8.20
(1H, s), 8.05(1H, s), 8.03(1H, s), 7.86(1H, s), 7.65(1H, d, J = 7.3
Hz), 7.62(1H, s), 7.57(1H, s), 7.52(1H,s), 7.42(1H, s), 7.36(1H, s).
IR (solid KBr pellet, cm−1 ): 3442(m), 1635(s), 1592(m), 1569(s),
1551(s), 1509(s), 1306(s), 1151(s), 1083(s), 939(w), 836(w), 756(m),
688(w).
Complex Zn–QB2, (4). Zn(ClO4 )2 ·6H2 O (0.1 mmol, 0.037 g)
dissolved in 10 mL EtOH–H2 O (v/v, 8 : 2) was added dropwise to
QB2 dissolved in 15 mL EtOH–H2 O (v/v, 8 : 2). The solution was
stirred for 20 min at boiling temperature, after which it was filtered
and allowed to stand at room temperature. The cubic crystal
suitable for single-crystal X-ray diffraction was obtained after
several days. Anal calc. for Zn2 (C32 H21 N8 O2 )2 (ClO4 )2 (C2 H5 OH)2
(H2 O)2 : H 3.75, C 52.46, N 14.39; Found: H 3.21, C 52.95, N
13.98; ESI-MS: m/z 1249.5 (calc. for [Zn2 (QB2)2 (H2 O)]+ , 1249.9).
1
H NMR (DMSO-d 6 ): d H 8.76 (1H, d, J = 5.5 Hz), 8.42 (1H, s),
8.24(1H, s), 8.21(1H, s), 8.18(1H, s), 8.08 (1H, d, J = 6.5 Hz),
7.88(1H, s), 7.58 (1H,s), 7.68(1H, s), 7.31(1H, s). FTIR (solid
KBr pellet, cm−1 ): 3422(m), 1647(m), 1571(m), 1507(m), 1437(w),
1304(m), 1120(s), 916(w), 832 (w).
Caution! Perchlorate salts are potentially explosive in the presence
of organic compounds. Only a small amount of the materials should
be prepared and handled with care.
Crystallography
The crystallographic data for QB1 and Zn–QB2 are presented
in Table 1. Intensities were collected on a Siemens SMARTCCD diffractometer with graphite-monochromated Mo-Ka (k =
0.71073 Å) using the SMART and SAINT programs.21 45 frames
of data were collected at 298 K with an oscillation range of 1◦ per
frame and an exposure time of 10 s per frame. Indexing and unit
cell refinement were based on all observed reflections from those 45
frames. The structure was solved by direct method and refined on
F 2 by full-matrix least-squares methods with SHELXTL version
5.1.22 All the non-hydrogen atoms, except the disordered solvent
molecules, were refined with anisotropic thermal displacement
coefficients. Hydrogen atoms were located geometrically, whereas
those of solvent molecules were found on Fourier difference maps,
and all the hydrogen atoms were refined in the riding model. The
ethanol molecules were disordered into three parts with the site
occupancy factor (s.o.f) each being fixed at 13 . The oxygen atoms in
the perchlorate anion were also disordered into two parts, the s.o.f
for all the oxygen atoms were fixed at 12 .
CCDC reference numbers 604880 and 604881.
Dalton Trans., 2006, 3528–3533 | 3529
Table 1 Crystallographic data for QB1 and zinc complex 4
Empirical formula
Mr
Crystal system
Space group
a/Å
b/Å
c/Å
b/◦
V /Å3
Z
Dcald /g cm−3
l/mm−1
Rint
R1
wR2
Goodness of fit
QB1
Zn–QB2
C36 H32 N6 O4
612.68
Orthorhombic
P21 21 21
10.930(2)
12.558(2)
22.710(4)
Zn2 C68 H60 N16 O17 Cl2
1574.96
Monoclinic
C2/c
24.676(6)
19.217(4)
15.276(4)
94.850(4)
7218(3)
4
1.449
0.818
0.0364
0.060
0.175
1.063
3117.2(1)
4
1.302
0.09
0.1220
0.0393
0.0995
1.019
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b605653a
Results and discussion
The chemosensor QB1 was obtained by reaction of diphenic
dihydrazide and 2-quinolinecarbaldehyde in methanol solution
and identified by IR, elemental analyses, 1 H NMR and X-ray
crystal structure analyses. The chemosensor QB2 was synthesized
according to the procedure shown in Scheme 2. Binicontinic
acid and ester were prepared according to the literature.19 The
reaction of binicontinic ester with N2 H4 ·H2 O in CH3 OH at
boiling temperature produced 6,6 -dicarboxylic acid hydrazide2,2 -bipyridine.
Scheme 2 Synthesis of the chemosensor QB2. Reagents and conditions:
3 drops of acetic acid, C2 H5 OH, 90 ◦ C, 92%.
same side. The two phenyl rings sit in a twisted formation with
the dihedral angle about 53◦ . Two quinoline-containing arms are
almost parallel to each other with the dihedral angle about 2.8◦
and the plane–plane separation about 3.53 Å, respectively. Such
an inter-planar separation is shorter than the sum of the van der
Waals radii of two aromatic rings, indicating the presence of p–p
stacking interactions between the two arms. The dihedral angles
between the phenyl ring and the quinoline-containing side chain in
each arm are about 45◦ in average. Bond distances and angles are
within the normal ranges. Each arm adopts a unique configuration
in which N(1) [or N(6)] is trans to N(2) and O(1) [or N(5) and O(2)].
Driven by metal-coordination, rotation of the quinoline fragment
by 180◦ around N–N bond orients the nitrogen atom N(1) [or
N(6)] of the quinoline ring, imino nitrogen atom N(2) [or N(5)]
and the oxygen atom O(1) [or O(2)] in the same direction, thus
enabling each arm to function as a planar tridentate N2 O chelator,
as found for most of the carbohydrazone and thiosemicarbazone
compounds.23
QB1 exhibits a weak fluorescent peak at 405 nm when dissolved
in a DMSO–H2 O (80 : 20) solution (1.0 × 10−4 M) and exciting
at 350 nm. After addition of Zn2+ , a new emission peak appears
at 500 nm with a shoulder positioned at 520 nm by exciting the
DMSO–H2 O solution (1.0 × 10−4 M) at 420 nm (Fig. 2). Similar
results were obtained for QB1 and QB1–Zn complex (3) in their
solid state. The presence of a new peak with the separation of about
100 nm in the maximum emission wavelength from the original
ligand indicates no interference from QB1 in the detection of Zn2+
at this wavelength. The enhancement of luminescent intensity
of the new peak, upon binding to analyte, is usually preferred
to promote the sensitivity of the sensor. During the titration of
zinc(II), the fluorescence intensity shows a linear increase at low
concentration followed by a smooth increase until a plateau is
observed (see ESI†). The binding curve shows a 1 : 1 binding
stoichiometry. Since the two chelating arms of the ligand cannot
coordinate to one metal center in an octahedral geometry, a 2 :
2 metal–ligand complex is proposed4e with dissociation constant,
pK d , for Zn–QB1 calculated as 9.6 through least-square linear
fitting.24
As shown in Fig. 1, molecules of QB1 crystallized in a chiral
space group P21 21 21 with both quinoline rings positioned in the
Fig. 2 Fluorescence emission spectra at 293 K in DMSO–H2 O (80 : 20)
for QB1 (1.0 × 10−4 mol L−1 ) only (---) and after Zn(ClO4 )·6H2 O (2.0 ×
10−3 mol L−1 ) was added ( ).
Fig. 1 Molecular structure of QB1 showing the atomic-numbering
scheme. Thermal ellipsoids are shown at 30% probability. Selected bond
distances (Å): O(1)–C(11) 1.210(5), O(2)–C(24) 1.222(6), N(3)–C(11)
1.347(6), N(4)–C(24) 1.363(6).
3530 | Dalton Trans., 2006, 3528–3533
As shown in Fig. 3, upon addition of Zn2+ , a peak at 12.41 ppm
in the original 1 H NMR spectrum of the ligand QB1, which is
assigned to a signal of the imino proton, disappears, indicating
the potential deprotonation of the ligand in the presence of Zn2+
This journal is © The Royal Society of Chemistry 2006
Fig. 3 Partial 1 H NMR (500 MHz) of QB1 (3 mM) in DMSO-d 6 . (a)
QB1 only; (b) QB1 + four eq. of Zn(ClO4 )·6H2 O.
QB2 exhibits similar fluorescent behavior, with an excitation
wavelength at 343 nm and emission wavelength at 410 nm. Upon
addition of Zn2+ , a new emission appears at 500 nm with the
excitation wavelength at 420 nm (see ESI†). Further addition of
Zn2+ also causes a typical titration curve for the formation of a 1 :
1 metal–ligand species (Fig. 5) with a larger dissociation constant
(pK d = 14.10) than that of Zn–QB1 (pK d = 9.6).25 It is suggested
that the presence of the nitrogen atom on the bipyridyl ring favors
the conjugation of the whole ligand26 and stabilizes the metal–
ligand complex. Only one peak at m/z 1249.5 is observed in the
ESI-MS spectrum of the ligand QB2 in the presence of equivalent
Zn2+ ions, corresponding to the species [Zn2 (QB2)2 (H2 O)]+ (calc..
1249.9), which agrees well with the results of the luminescent
titrations.
ions. Meanwhile, the chemical shifts of H8 (Dd = 0.12 ppm) and
H4 (Dd = 0.28 ppm) on quinoline rings exhibit relative downshifts,
which could be assigned to the coordination of quinoline ring
to the Zn2+ center. Furthermore, the small but obviously upfield
shifts of protons on the phenyl rings confirm the deprotonation.
Fig. 5 Luminescent titration of QB2 (1.0 × 10−5 mol L−1 ) with increasing
concentration of Zn2+ . Excitation wavelength was 420 nm.
Crystals of the Zn–QB2 compound suitable for structural analyses were obtained by evaporating the solution in air. As shown
in Fig. 6, the complex has crystallographic twofold symmetry
with two identical Zn centers bridged by two ligands to form a
double helical cationic species, with the Zn · · · Zn separation about
6.30 Å. Each octahedral zinc center is coordinated by two identical
N2 O tridentate chelating units from different ligands. These two
Fig. 4 ESI-MS spectra of QB1 (a) and QB2 (b) in the presence of one eq.
of Zn(ClO4 )·6H2 O.
ESI-MS spectra (Fig. 4) of the ligand in the presence of Zn2+
ions exhibit a peak at m/z 1223.4, corresponding to [Zn2 (QB1)2 ]+
(calc.. 1223.9), and a peak at m/z 611.3, assigned to the species
[Zn(QB1)]+ (calc.. 611.1), indicating the formation of 2 : 2
metal–ligand complexes in solution and partial deprotonation
of the ligand upon coordination. In the IR spectra, the strong
peak at 1694 cm−1 of the receptor QB1, which corresponds to
the characteristic amide carbonyl absorption,4a was shifted to
1635 cm−1 for the Zn2+ compound, suggesting that a strong binding
of the carbonyl group occurs with Zn.
This journal is © The Royal Society of Chemistry 2006
Fig. 6 Molecular structure of Zn2 (QB2)2 helicate. Hydrogen atoms
and anions are omitted for clarity. Selected bond distances (Å):
Zn(1)–N(7) 2.052(2), Zn(1)–N2 2.068(2), Zn(1)–N(1) 2.148(2), Zn(1)–N(8)
2.175(2), Zn(1)–O(1) 2.177(2), Zn(1)–O(2) 2.177(2), O(1)–C(11) 1.236(3),
O(2)–C(22) 1.254(3), N(3)–C(11) 1.363(3), N(6)–C(22) 1.321(3).
Dalton Trans., 2006, 3528–3533 | 3531
chelating units coordinate to a Zn atom in a mer configuration
with pairs of carbonyl O atoms and quinoline N atoms in a cis
relationship, whereas the imino N atoms are trans to each other,
as found in related metal complexes synthesized recently in our
laboratory. The C–O bond distances of 1.234(3) and 1.254(3)
Å for C(11)–O(1) and C(22)–O(2), respectively, together with
1.363(3) for C(11)–N(3) and 1.321(3) Å for C(22)–N(6) suggest
that the proton on N(3) is lost and QB2 acts as a monoanionic
ligand during the coordination. Unfortunately, many attempts to
crystallize QB1 with Zn2+ in a variety of solvents failed to yield
X-ray quality crystals. The fact that the Zn–QB1 complex failed
to crystallize in solution, unlike Zn–QB2, may be attributed to the
reversible complexation that disfavors crystallization.
From a mechanistic viewpoint, the ligands QB1 and QB2 are set
as bis-tridentate pocket-like coordination sites bearing a simple
fluorophore at both terminals, which prompts Zn2+ having a
flexible coordination configuration to self-assemble as a double
helical structure. In detail, the dihedral angles between the pyridine
ring and quinoline-containing arm in the Zn–QB2 metal–ligand
complex are 12◦ and 22◦ for deprotonated and protonated forms,
respectively (cf. 45◦ in QB1 itself). Bond distances within the two
arms are intermediate between the formal single and double bonds.
Clearly, the metal-induced deprotonation of the QB sensors leads
to the nearly planar conformation of each arm, from which a
more extensive p-system, favoring the electronic delocalization
and lowering the energy of the emission band, is achieved.15
To determine whether QB1 and QB2 function as highly selective
chemosensors for Zn2+ , the fluorescent spectra are recorded in
the presence of various metal ions. As the representative data
for QB2 shown in Fig. 7, using these metal ions (15 eq.), QB
sensors showed large CHEF effects only with Zn2+ . The fluorescent
intensities are not increased by addition of various metal ions
found at high concentrations in cells, such as K+ , Na+ , Mg2+ and
Ca2+ . Addition of transition metal ions such as Cr3+ , Mn2+ , Fe2+ ,
Co2+ , Ni2+ , Cu2+ , Cd2+ and Hg2+ does not increase the fluorescent
intensity. Thus, although QB sensors may bind to transition metal
ions, the resulting complexes do not fluoresce due to a quenching
mechanism.27
One challenge in the development of Zn(II) sensors is to achieve
Zn(II) selectivity over divalent first-row metal ions while maintaining the adequate fluorescent intensity upon metal bonding.
The widely used di(2-picolyl)amine (DPA)-containing ligand is a
valuable sensor for biological systems due to its high affinity for
Zn(II) over Ca(II), however, DPA-based dyes readily bind other
divalent transition metal ions and block Zn(II) coordination, which
compromises the Zn-induced fluorescence. To further explore
the utility of QB as an ion-selective fluorescence chemosensor
for Zn2+ , we investigated the selectivity of QB1 and QB2 for
Zn(II) over first-row transition metals and Cd(II), in addition
to biologically relevant alkali and alkaline earth metals. These
competitive experiments were performed in buffered aqueous
DMSO solutions (H2 O–DMSO, 70 : 30, v/v, 50 mM HEPES
buffer, 50 mM KNO3 , pH = 7.25), since the fluorescent signalling
of a sensor in aqueous solution is crucial to practical application.
As the representative data for QB2 shows (Fig. 8), QB1 and QB2
(50 lM) readily detect Zn(II) (50 lM) in the presence of 500 lM
of K+ , Na+ , Mg2+ and Ca2+ , indicating that these physiologically
relevant components will not interfere with the detection of Zn(II)
in such systems. Of the divalent first-row transition metal ions
(50 lM) examined, only Cu(II) compromises the Zn(II)-induced
fluorescence enhancement of the QB sensors due to a quenching
mechanism.28,29 Furthermore, the chemosensors also differentiate
Zn(II) from Cd(II). Addition of Cd2+ results in negligible fluorescence enhancement, and the competitive experiment shows that
Zn(II) readily displaces Cd(II) from the metal ion coordination
sphere.30
Fig. 8 Selectivity of QB2 for Zn2+ over the metal ions of interest in an
aqueous DMSO solution (H2 O–DMSO, 70 : 30, 50 mM HEPES buffer,
50 mM KNO3 , pH = 7.25). In order: Black bars: QB2 + 15 eq. cation of
interest. Light gray bars: addition of 1 eq. Zn(II) to the solution containing
1 eq. QB2 and 10 eq. cation of interest. Dark gray bars: addition of 1 eq.
of Zn(II) to the solution containing 1 eq. QB2 and 1 eq. cation of interest.
Gray bars: addition of 10 eq. of Zn(II) to the solution containing 1 eq.
QB2 and 1 eq. cation of interest. Samples were excited at 420 nm.
Fig. 7 Fluorescent emission changes of QB2 (1.0 × 10−5 mol L−1 ) upon
addition of Li+ , Na+ , K+ , Mg2+ , Ca2+ , Cr3+ , Mn2+ , Fe2+ , Co2+ , Ni2+ , Cu2+ ,
Zn2+ , Cd2+ and Hg2+ (15 eq.) in aqueous DMSO solution (H2 O–DMSO,
70 : 30, v/v, 50 mM HEPES buffer, 50 mM KNO3 , pH = 7.25). Excitation
wavelength was 420 nm.
3532 | Dalton Trans., 2006, 3528–3533
Fluorescence spectroscopy was used to monitor the viable pH
range of QB1 and QB2 in the presence of 15 eq. of Zn2+ under
aqueous conditions. As shown in Fig. S6†, under the biological
pH window of 6.8–7.7, addition of Zn2+ to the solution of QB
sensors showed very intense emission centered at 500 nm, with a
quantum yield of 0.03 and 0.06 for QB1 and QB2, respectively,
though some quenching is observed below pH ∼ 6.78 and above
pH ∼ 8.06 (for details, see ESI†) Furthermore, addition of Zn2+ in
This journal is © The Royal Society of Chemistry 2006
the presence of K+ , Na+ , Mg2+ and Ca2+ in the pH range of 6.8–7.7
hardly alters the emission intensity. These results unambiguously
prove that QB sensors can, at biological pH, selectively detect
Zn2+ over most other metal ions that exist at high concentrations
in living cells, and, particularly QB1, which can bear a wider pH
range, meet the requirement of a Zn2+ sensor candidate.
Conclusions
We have presented the self-assembly strategy31 for the design
of highly selective Zn sensors by introducing a carboxylic acid
hydrazone group into quinoline-based fluorescent dyes. This approach has been utilized to obtain high selectivity for Zn2+ over the
first-row transition metals (except Cu2+ ) by modulating the N2 O
chelator with a suitable binding strength. The binding-induced
deprotonation of the sensors causes electronic delocalization
within the more extensive p-system and results in a red-shift
of the luminescence (about 100 nm), at which the fluorescence
intensity is enhanced significantly due to the formation of a
fluorescent molecular helicate via self-assembly. After examination
of the fluorescence in the presence of the competing metal ions,
it was concluded that QB could be a good Zn2+ sensor based
on the mechanism of the CHEF or self-assembling fluorescent
enhancement. In conclusion, the sensor QB could self-assemble to
form a bimetallic molecular helicate in the presence of Zn2+ , which
showed strong emission in both solid state and solution.32 Efforts
to utilize current QB sensors and prepare new ones for imaging
applications are in progress.
Acknowledgements
This work is supported by the National Natural Science Foundation. The authors thank the referees for their helpful suggestions.
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