Turn-on fluorescent sensor for Hg2+ via displacement approach

Article abstract of DOI:10.1021/ic702494s

A new Cu²⁺ compound Cu-NB₁ (where H₂NB is bis(2-hydroxyl-naphthalene-carboxaldehyde) benzil dihydrazone) was synthesized as a highly selective fluorescence chemosensor for the detection of Hg ²⁺ in aqueous media through a displacement turn-on signaling strategy. Whereas the coordination of Cu²⁺ resulted in a considerable quenching of the typical luminescence of the naphthol rings in Cu-NB, the addition of Hg²⁺ ion led to a dramatic increase in the emission intensity of Cu-NB at about 530 nm (excitation at 430 nm). The competitive fluorescent experiments showed that alkali, alkaline earth metal ions, the group 12 metals Zn²⁺, Cd²⁺, the first-row transition-metal ions such as Mn²⁺, Fe²⁺, Co²⁺, and Ni²⁺, as well as Pb²⁺ could not inhibit the Hg ²⁺-binding fluorescent enhancement. It is postulated that the existence of Cu²⁺ in the luminescent probe Cu-NB could turn away the interferences of other metal cations from Hg²⁺ detection. The optical responses of the free ligand upon addition of Cu²⁺ ion, and of the Hg-H₂NB compound upon the addition of Cu²⁺ were also investigated for comparisons.

Full text of DOI:10.1021/ic702494s

Inorg. Chem. 2008, 47, 5169-5176
“Turn-On” Fluorescent Sensor for Hg2+ via Displacement Approach
Guangjie He, Yonggang Zhao, Cheng He,* Yang Liu, and Chunying Duan*
State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology, Dalian, 116012, China
Received December 24, 2007
A new Cu2+ compound Cu-NB, (where H2NB is bis(2-hydroxyl-naphthalene-carboxaldehyde) benzil dihydrazone)
was synthesized as a highly selective fluorescence chemosensor for the detection of Hg2+ in aqueous media
through a displacement “turn-on” signaling strategy. Whereas the coordination of Cu2+ resulted in a considerable
quenching of the typical luminescence of the naphthol rings in Cu-NB, the addition of Hg2+ ion led to a
dramatic increase in the emission intensity of Cu-NB at about 530 nm (excitation at 430 nm). The competitive
fluorescent experiments showed that alkali, alkaline earth metal ions, the group 12 metals Zn2+, Cd2+, the
first-row transition-metal ions such as Mn2+, Fe2+, Co2+, and Ni2+, as well as Pb2+ could not inhibit
the Hg2+-binding fluorescent enhancement. It is postulated that the existence of Cu2+ in the luminescent
probe Cu-NB could turn away the interferences of other metal cations from Hg2+ detection. The optical
responses of the free ligand upon addition of Cu2+ ion, and of the Hg-H2NB compound upon the addition
of Cu2+ were also investigated for comparisons.
Introduction
of primary concern.5 Despite a reduction in its industrial use
as a result of stricter regulations, high concentrations of
mercury are still present in many environmental compart-
ments, and it can still be found in many products of daily
life such as paints, electronic equipment, and batteries.6,7
These environmental and health problems have prompted the
development of methods for the detection and the quantifica-
tion of mercury applicability, especially in situations where
conventional techniques are not appropriate. Small-molecule
fluorescent chemosensors that selectively bind to and report
on the target metal ion constitute one practical route toward
this goal, and these probes showing fluorescence enhance-
Recently, the development of selective and sensitive
imaging tools capable of rapidly monitoring heavy and
transition metal ions (HTM), such as Pb2+, Cd2+, and Cu2+,
ions has attracted considerable attention due to the environ-
mental and biological relevance of such metal ions.1–4 In
this regard, the Hg2+ ions is considered highly dangerous
because the Hg2+ ion is one of the environmentally most
important metal ions whose toxicity, even at very low
concentrations, has long been recognized and is a problem
* To whom correspondence should be addressed. E-mail: cyduan@
dlut.edu.cn.
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10.1021/ic702494s CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 12, 2008 5169
Published on Web 05/15/2008

He et al.
Scheme 1. Potential Displacement Signaling Mechanism for Sensing
Experimental Section
Hg2+
Materials and Measurements. All chemicals used were
of reagent grade or obtained from commercial sources and
used without further purification except that the solvents for
physical measurements were purified by classical methods.
Elemental analyses (carbon , hydrogen, and nitrogen) were
carried out on a PerkinElmer 240 analyzer. UV-vis spectra
were obtained on a Shimmadzu 3100 spectrophotometer at
1
room temperature. H NMR and 13C NMR spectra were
measured on a Bruker DRX-500 NMR spectrometer at room
temperature. Differential pulse voltammetry results were
recorded on an EG & G PAR model 273 instrument. The
solution-state measurements were performed in a three-
electrode cell with a pure argon gas inlet and outlet, which
has a 50 ms pulse width with current samples 40 ms after
the pulse was applied. A sweep rate of 20 mV s-1 was used
in all pulse experiments. The cell comprises a platinum wire
working electrode, a platinum auxiliary electrode, and an
Ag/AgCl wire reference electrode.
Ligand H2NB. A methanol solution of 2-hydroxy-1-
naphthalene-carboxaldehyde (1.7g, 10 mmol) and benzil
dihydrazone (1.2g, 5.0 mmol) were mixed and refluxed for
4 h. Yellow precipitates formed were filtered and washed
with methanol and dried under vacuum. Yield: 73%. Crystals
suitable for X-ray diffraction determination were obtained
by slowly evaporating a dichloromethane solution of H2NB
at room temperature. Anal. Calcd for C36H26N4O2: C, 79.1;
H, 4.8; N, 10.3. Found: C, 79.3; H, 4.9; N, 10.1%. 1H NMR
(500 MHz, CDCl3): δ 12.57 (s, 2H, -OH), 9.74 (s, 2H,
-CH)N-), 8.13 (d, 2H, Np), 8.00(d, 4H, Ph), 7.72(d, 2H,
Np), 7.69(d, 2H, Np), 7.52(m, 4H, Ph), 7.48(m, 2H, Ph),
7.47(m, 2H, Np), 7.32 (t, 2H, Np). 7.03(d, 2H, Np). 13C NMR
(75 MHz, CDCl3), δ: 164.5, 160.8, 159.9, 134.9, 132.6,
131.8, 131.5, 129.1, 128.6, 127.8, 127.7, 127.2, 123.5, 121.4,
118.3, 108.3.
Cu-NB. A dichloromethane solution (15 mL) of H2NB
(0.10 mmol, 0.055 g) and a methanol solution (15 mL) of
Cu(ClO4)2 · 6H2O (0.10 mmol, 0.037 g) were mixed and
refluxed for 3 h. A dark-red solid formed was filtered, washed
with methanol (20 mL), and dried under vacuum. Yield:
80%. Crystals suitable for X-ray diffraction determination
were obtained by slowly evaporating an ethanol/acetonitrile
(1:1 v/v) solution of the copper compound at room temper-
ature. Anal. Calcd for C36H24N4O2Cu · CH3CN: C, 70.4; H,
4.2 N, 10.8. Found: C: 70.3; H, 4.1; N, 10.9%.
Hg-H2NB. A dichloromethane solution (15 mL) of
H2NB(0.10 mmol, 0.055 g) and the ethanol solution (15
mL) of Hg(ClO4)2 · 3H2O (0.10 mmol, 0.045 g) were mixed
and refluxed for 3 h. A deep-orange solid formed was
filtered, washed with methanol, and dried under vacuum.
Yield: 82%. Anal. Calcd for C36H26N4O10HgCl2 · C2H5OH:
C, 46.0; H, 3.2; N, 5.6; Found: C, 46.3; H, 3.1; N, 5.8%.
1H NMR (500 MHz, CDCl3), δ: 12.90 (broad, 2H, -OH)
9.84 (s, 2H, -CH)N-), 8.19 (d, 2H, Np), 8.04(d, 4H,
Ph), 7.96(d, 2H, Np), 7.69(d, 2H, Np), 7.63(m, 4H, Ph),
7.57(m, 2H, Ph), 7.54 (m, 2H, Np), 7.42 (t, 2H, Np),
ment as a result of the binding event are to be favored over
those that exhibit fluorescence quenching upon cation com-
plexation8 (Scheme 1).
However, the design of sensors that give fluorescence
enhancement upon Hg2+ binding is an intriguing challenge
because Hg2+ like many other HTM cations is known as a
fluorescent quencher.9,10 In particular, one common limitation
for heavy-metal detection in the environment is the low
quantum efficiency of metal bound dyes in aqueous media
compared to that in organic solvents. Consequently, the
receptor molecules with fluorescent enhancements for Hg2+
need to be carefully designed so that the responsible
mechanism for fluorescence quenching is maximized in the
receptor and minimized in the metal-bound state of recep-
tor.11,12 On the basis of this design strategy, herein we
describe a displacement approach for a Hg2+-specific off-on
fluorescence chemosensor. The Cu2+ coordination compound,
Cu-NB (H2NB ) bis(2-hydroxyl-naphthalene-carboxalde-
hyde) benzil dihydrazone), exhibits a poor fluorescence (off
state) due to the coordination of Cu2+ quenching the
luminescence of the ligand. Upon the addition of the Hg2+
ions, the Cu2+ ion is displaced and a Hg2+ complex is formed,
turning on the fluorescence (on state). Meanwhile, the
coordination of Cu2+ in the metal-complexation chemosensor
Cu-NB avoids the cross-sensitivities toward other metal
ions, such as Na+, K+, Mg2+, Ca2+, and the first-row
transition metal ions.
(8) (a) Zheng, L.; Miller, E. W.; Parlle, A.; Isacoff, E. Y.; Chang, C. J.
J. Am. Chem. Soc. 2006, 128, 10–11. (b) Peng, X.; Du, J.; Fan, J.;
Wang, J.; Wu, Y.; Zhao, J.; Sun, S.; Xu, T. J. Am. Chem. Soc. 2007,
129, 1500–1501. (c) Nolan, E. M.; Ryu, J. W.; Jaworski, J.; Feazell,
R. P.; Sheng, M.; LippardS, J. J. Am. Chem. Soc. 2006, 128, 15517–
15528.
(9) (a) McClure, D. S. J. Chem. Phys. 1952, 20, 682–686. (b) Kavalli-
eratos, K.; Rosenberg, J. M.; Chen, W. Z.; Ren, T. J. Am. Chem. Soc.
2005, 127, 6514–6515. (c) Praveen, L.; Ganga, V. B.; Thirumalai,
R.; Sreeja, T.; Reddy, M. L. P.; Varma, R. L. Inorg. Chem. 2007, 46,
6277–6282.
(10) (a) Kemlo, J. A.; Shepherd, T. M. Chem. Phys. Lett. 1977, 47, 158–
162. (b) Ros-Lis, J. V.; Mart´ınez-Ma´n˜ez, R.; Rurack, K.; Sancenon,
F.; Soto, J.; Spieles, M. Inorg. Chem. 2004, 43, 5183–5185. (c) Moon,
S.-Y.; Youn, N. J.; Park, S. M.; Chang, S.-K. J. Org. Chem. 2005,
70, 2394–2397.
(11) Chae, M.-Y.; Czarnik, A. W. J. Am. Chem. Soc. 1992, 114, 9704–
9705.
(12) (a) Mello, J. V.; Finney, N. S. J. Am. Chem. Soc. 2005, 127, 10124–
10125. (b) Ono, A.; Togashi, H. Angew. Chem., Int. Ed. 2004, 43,
4300–4302. (c) Ros-Lis, J. V.; Marcos, M. D.; Mart´ınez-Ma´n˜ez, R.;
Rurack, K.; Soto, J. Angew. Chem., Int. Ed 2005, 44, 4405–4407.
5170 Inorganic Chemistry, Vol. 47, No. 12, 2008

“Turn-on” Fluorescent Sensor for Hg2+
7.20(d, 2H, Np).13C NMR (75 MHz, CDCl3), δ:165.1,
162.1, 160.3, 135.4, 132.7, 132.1, 129.7, 129.5, 129.0,
128.3, 127.8, 127.6, 124.0, 121.9, 118.6, 108.6.
Table 1. Crystal Data for H2NB and Cu-NB
compound H2NB
molecular formula
Cu-NB
C38H32Cl2N4O3 C38H27CuN5O2
fw
cryst syst
space group
a/Å
b/Å
c/Å
663.58
649.19
Preparation of Fluorometric Metal-Ion Titration
Solutions. Emission spectra were recorded on an AMINCO
Bowman Series 2 luminescence spectrometer. Fluorometric
titration method was similar to that of UV-vis titration, and
for all measurements excitation was at 430 nm. Both
excitation and emission slit widths were 8 nm. Stock
solutions (1.0 × 10-2 mol/L) of the perchlorate or nitrate
salts of Mn2+, Fe2+, Co2+, Ni2+, Zn2+, Cd2+, Pb2+, Hg2+,
Cu2+, and the alkali and alkaline earth metal ions were
prepared in aqueous solutions. Stock solutions of H2NB (1.0
× 10-5 M) or Cu-NB (2.5 × 10-5 M) were prepared in
DMSO/H2O (4:1 v/v), respectively. Fluorescence quantum
yield was determined using optically matching solution of
Ru(2,2-bpy)3(ClO4)2 (Φf ) 0.059 in CH3CN)13 as standard
at an excitation wavelength of 450 nm, and the quantum yield
is calculated using eq 1,
triclinic
triclinic
j
j
P1
P1
9.764(3)
12.480(5)
14.503(6)
81.77(2)
89.37(3)
76.03(2)
1696.8(11)
2
293
1.299
0.234
692
10.909(2)
11.702(3)
13.553(3)
99.904(4)
91.971(4)
111.356(4)
1578.5(6)
2
293
1.366
0.734
670
7924
5451 (0.0867)
2492
0.0655
0.1573
1.055
R/°
ꢀ/°
γ/°
V/Å3
Z
T/K
Dcalcd /Mgm-3
µ/mm-1
F(000)
no. reflns measured
no. unique reflns (Rint
10 092
5742 (0.0484)
)
no. observed. reflns with I g 2σ(I) 2958
R1a
0.0769
0.2397
1.030
wR2a
GOF
a R1 ) Σ|F0| - |Fc|/| Σ|F0|. wR2 ) [Σw(F0 - Fc )2/Σ(F0)2]1/2
.
2
2
2
I
unk ⁄ Aunk ηunk
(
)
Φunk ) Φstd
(1)
(
)
I
std ⁄ Astd ηstd
(
)
where Φunk and Φstd are the radiative quantum yields of the
sample and standard, Iunk and Istd are the integrated emission
intensities of the corrected spectra for the sample and
standard, Aunk and Astd are the absorbances of the sample
and standard at the excitation wavelength (450 nm in all
cases), and ηunk and ηstd are the indices of refraction of the
sample and standard solutions, respectively. Excitation and
emission slit widths were modified to adjust the luminescent
intensity in a suitable range. All of the spectroscopic
measurements were performed at least in triplicate and
averaged.
Crystallography. Intensities of H2NB and Cu-NB were
collected on a Siemens SMART-CCD diffractometer with
graphite-monochromated Mo KR (λ ) 0.71073 Κ) using the
SMART and SAINT programs.14 Forty-five frames of data
were collected at 298 K with an oscillation range of 1
deg/frame and an exposure time of 15 s/frame. Indexing
and unit-cell refinement were based on all observed
reflections from those 45 frames. The crystal data of H2NB
and Cu-NB were listed in Table 1. The structures were
solved by direct methods and refined on F2 by full-matrix
least-squares methods with SHELXTL version 5.1.15 Non-
hydrogen atoms were refined anisotropically with the
exception of disordered solvent molecules. The acetonitrile
molecules in Cu-NB were refined disordered into two
parts with the site occupancy factors of the atoms being
fixed at 0.5. Hydrogen atoms were fixed geometrically at
calculated distances and allowed to ride on the parent non-
hydrogen atoms with the isotropic displacement being
fixed at 1.2 and 1.5 times that of the aromatic and methyl
carbon atoms attached, respectively.
Figure 1. Molecular structure of ligand H2NB with the atomic numbering
scheme. The solvent molecules were omitted for clarity. Distance and angles
of selected bonds: C(1)-O(1) 1.348(3); C(36)-O(2) 1.333(3); C(11)-N(1)
1.278(3); N(1)-N(2) 1.403(3); N(2)-C(12)1.283(3); C(19)-N(3)1.288(3);
N(3)-N(4)1.401(3); N(4)-C(26) 1.276(3). C(11)-N(1)-N(2) 114.2(2);
N(1)-N(2)-C(12) 112.8(2); C(19)-N(3)-N(4) 112.8(2); N(3)-N(4)-C(26)
111.9(2).
Results and Discussion
Optical Properties and Cu2+-Specific Responses of
H2NB. The Schiff-base ligand H2NB was prepared by the
reaction of benzil dihydrazone with 2-hydroxy-1-naph-
thalene-carboxaldehyde in a methanol solution and identi-
fied by the single crystal X-ray structural analysis. As
shown in Figure 1, the two naphthol rings position on
opposite sides of the N(2)-C(12)-C(19)-N(3) fragment,
giving rise to a helix similar to that observed in the related
ligand, N,N′-bis[1-(pyrazine-2-yl)ethylidene]benzil dihy-
drazone.16 The helix twisted in the ligand mainly arises
(13) Juris, A.; Balzani, V. Coord. Chem. ReV. 1988, 84, 85–277.
(14) SMART and SAINT, Area Detector Control and Integration Software;
Siemens Analytical X-ray Systems, Inc.: Madison, WI, 1996.
(15) Sheldrick, G. M. SHELXTL V5.1, Software Reference Manual; Bruker,
AXS, Inc.: Madison, WI, 1997.
(16) Bai, Y.; Duan, C. Y.; Cai, P.; Dang, D. B.; Meng, Q. J. Dalton Trans.
2005, 2678–2680.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5171

He et al.
Figure 3. Fluorescence emission spectra of the ligand H2NB (10 µM) in
2 mL DMSO/H2O (4:1 v/v) with successive addition of Cu2+. The curves
of 0, 0.2, 0.4, 0.6, 0.8, and 1.0 equiv of added Cu2+ were shown. The insert
exhibits a fluorescence titration profile at 530 nm upon the addition of Cu2+
(excitation at 430 nm).
Figure 2. UV-vis titration of H2NB (10 µM) in DMSO/H2O (4:1 v/v)
solution upon addition of Cu2+. Inset: UV-vis titration profile of H2NB
upon addition of Cu2+; the absorption is recorded at 460nm.
from the torsion around the N-C-C-N bond with an
angle of 103°.
The UV-vis absorption spectrum (Figure 2) of the ligand
H2NB in a DMSO/H2O (4:1 v/v) solution exhibited a broad
naphthol-based π-π* transition band around 393 nm. Upon
the addition of a Cu(ClO4)2 · 6H2O aqueous solution, a new
band centered around 460 nm appeared. The absorption
increased almost linearly with the ratio of [Cu2+]/[ligand]
until the first stoichiometry. Meanwhile, the absorption band
of 393 nm fell down to a limit value and had a slight red-
shift. The presence of well-defined isosbestic points at 425
and 325 nm indicated that only two species coexisted in the
equilibrium. Job’s plot evaluated from the absorption spectra
of the titration solution (Supporting Information Figure S1)
exhibited the inflection point at 0.5, indicating the formation
of a 1:1 H2NB-Cu(II) complexation speices in the solution;
and the linear fitting17 of the titration curve at 460 nm based
on a 1:1 stoichiometry host-guest mode (Supporting Infor-
mation Figure S2) gave an association constant (log Kass) of
5.20 ( 0.1. Similar measurements for several other metal
ions were carried out. However, the changes of the UV
spectra could scarcely be observed even if these metal cations
were added up in 10-fold amounts of Cu2+ under the same
conditions, indicating that the UV-vis response of H2NB
is Cu2+ specific.
H2NB exhibited an emission band centered about 530 nm
(quantum yield Φ ≈ 0.080) in DMSO/H2O (4:1 v/v) (Figure
3) when excited at 430 nm (near one of the two isosbestic
points in the absorption spectrum).18 Upon the addition of 1
equiv of Cu2+ ions, the fluorescence was quenched signifi-
cantly. The association constant of the Cu2+-ligand com-
plexation species (log Ks ) 5.44 ( 0.1) calculated from the
fluorescence titration measurements was consistent well with
the value derived from UV-vis titration study, indicating a
strong tendency to form the Cu2+-ligand complexation
species. When 0.25 µM of Cu2+ was added to the solution
of H2NB (1 µM), a fluorescence decrease of 10% is observed,
Figure 4. Fluorescence responses of H2NB (10 µM) to various cations in
DMSO-H2O (4:1 v/v) solution. The blue bars represent the emission
intensities of H2NB in the presence of 0.10 mM of alkali (IA) and alkaline
earth (IIA) metals and for other cations of interest. The orange bars represent
the change of the emission that occurs upon the subsequent addition of
0.01 mM of Cu2+ to the above solution. The intensities were recorded at
530 nm, excitation at 430 nm.
indicating that the detection limit of H2NB to Cu2+ is at the
nanomolar level.
Compound Cu-NBwas isolated from the reaction of
ligand H2NB with copper perchlorate. An X-ray structural
investigation revealed that Cu-NB exhibited a mononuclear
structure with the Cu2+ center coordinated by two imine
nitrogen atoms and two naphthol oxygen atoms in a distorted
square geometry (Figure 5). The twisted angle of the two
chelating rings was calculated as 139°. The ligand lost two
protons from the two naphthol groups and coordinated to
Cu2+ as a dianionic N2O2 tetrandentate chelator, which was
confirmed by the C-O bond distances of 1.28 Å. The
N(2)-C(12)-C(19)-N(3) torsion angle is 82.6°. Such a
special twisted conformation of H2NB in the molecule,
combined with a high thermodynamic affinity to typical N,O-
chelate ligands and fast metal-to-ligand binding kinetics of
Cu2+, would be expected to lead a larger association of the
Cu-NB constant than that of most the first-row transition-
metal ions.19
(17) Lin, Z.-H.; Xie, L.-X.; Zhao, Y.-G.; Duan, C.-Y.; Qu, J.-P. Org.
Biomol. Chem. 2007, 5, 3535–3538.
The fluorescent spectra of H2NB using the other divalent
metal ions instead of Cu2+ were recorded. The results showed
that the fluorescent intensity of H2NB was hardly affected
(18) Zhang, B.-G.; Xu, J.; Zhao, Y.-G.; Duan, C.-Y.; Cao, X.; Meng, Q.-
J. Dalton. Trans. 2006, 1271–1276.
5172 Inorganic Chemistry, Vol. 47, No. 12, 2008

“Turn-on” Fluorescent Sensor for Hg2+
Figure 6. UV-vis titration of Cu-NB(10µM) in DMSO/H2O (4:1 v/v)
solution upon the addition of Hg2+. Inset: Titration curve of the absorbance
intensity at 393 nm as a function of the ratio of [Hg2+]/[Cu-NB].
Figure 5. Molecular structure of compound Cu-NB with the atomic
numbering scheme. Hydrogen atoms and the acetontrile molecule were
omitted for clarity. Distance (Å) and angles(°) of selected bonds: Cu(1)-O(2)
1.886(4), Cu(1)-O(1) 1.865(4); Cu(1)-N(1) 1.956(5), Cu(1)-N(4) 1.932(5),
N(1)-C(11) 1.282(7); N(4)-C(26) 1.276(7); C(36)-O(2) 1.279(6);
C(1)-O(1) 1.280 (7). O(1)-Cu(1)-N(1) 91.8(2). O(1)-Cu(1)-O(2)
91.8(2),O(1)-Cu(1)-N(4)150.4(2),O(2)-Cu(1)-N(4)90.8(2),O(1)-Cu(1)-N(1)
91.8(2), O(2)-Cu(1)-N(1) 155.6(2), N(4)-Cu(1)-N(1) 97.9(2).
by these ions even up to the 10-fold amounts of the ligand,
which was also in good agreement with the UV-vis results.
The high selectivity of H2NB for Cu2+ over other metal ions
should be in part contributed to the strong coordination ability
of Cu2+ and its large association constant. To further
investigate the selectivity of H2NB as a luminescence sensor
for the detection of Cu2+, competitive experiments were
carried out. As shown in Figure 5, upon the addition of 0.1
mM of Mn2+, Fe2+, Co2+, Ni2+, Zn2+, Cd2+, Pb2+, Hg2+,
and alkali and alkaline earth metal ions to the solutions of
H2NB (10 µM), no significant fluorescence changes were
observed. Upon the addition of Cu2+ (10µM) to the above-
mentioned solutions, fluorescence intensities of these solu-
tions except Hg2+ immediately dropped down to the level
of that for the H2NB solution containing only a Cu2+(10 µM)
ion. Thus, the coexistence of 0.1 mM of Mn2+, Fe2+, Co2+,
Ni2+, Zn2+, Cd2+, Pb2+, and the alkali and alkaline earth
metal ions also did not impact the Cu2+-binding quenching.
Only the presence of Hg2+could block the quenching process
of Cu2+. It was thus expected that, upon the displacement
of the Cu2+ by Hg2+, Cu2+ would no longer participate in
the quenching process; the recovery of the fluorescence of
the ligand would be achieved.
Optical Properties and Hg2+-Specific Responses of
Cu-NB. The UV-vis absorption spectrum of Cu-NB in
DMSO/H2O (4:1 v/v) exhibited a metal-to-ligand charge
transfer (MLCT) band around 460 nm. Upon the addition
of a aqueous solution of Hg(ClO4)2, the MLCT band
decreased continuously accompanied with the appearance of
a new peak at 393 nm, corresponding to the π-π* transition
of naphthol groups (Figure 6). The presence of two well-
defined isosbestic points coupled with the sigmoidal shape
of the titration curve suggested a one-step reaction. Mean-
while, the addition of other metal ions such as Co2+, Ni2+,
Zn2+, Cd2+, Pb2+, and alkali and alkaline earth metal ions
did not cause any obvious spectral changes, even in 10-fold
excess. The high selectivity of the Cu-NB for Hg2+ over
Figure 7. Luminescent titration of compound Cu-NB (25 µM) upon
addition of Hg2+. Excitation at 430nm in DMSO/H2O (4:1 v/v) solution.
The insert exhibits the fluorescence titration profile of Cu-NB at 530 nm
upon addition of Hg2+
.
other metal ions should be in part contributing to the strong
coordination ability of Cu2+ and its large association constant.
When excited at 430 nm, Cu-NB exhibited a very weak
luminescence band at about 530 nm corresponding to the
typical emission of the naphthol rings.18 The low luminescent
intensity is likely to result from the quenching of emission
by Cu2+ through a PET mechanism and/or a paramagnetic
quenching mechanism.20 As shown in Figure 7, the
addition of Hg(ClO4)2 caused a dramatic immediate
increase in emission intensity up to the maximum (Φ ≈
0.081). These results combined with the sigmoidal shape
of the titration curve suggested that of the Cu2+ ion in
the Cu-NB complex was substituted by Hg2+. According
to the two titration curves, the association constant log
Kass of the mercury coordination species was calculated
as 6.12 ( 0.1, larger than that of the Cu2+ complex. In
addition, the profiles of UV-vis and luminescent spectra
of Cu-NB in the presence of Hg2+ were quite similar to
that of the free ligand. The emission intensity at 530 nm
(excited at 430 nm) was enhanced to 10% of the original
intensity of Cu-NB (1 µM) when 250 nM (or 50 ppb)
Hg2+ was added to the solution, indicating that the limit
(19) (a) Kra¨mer, R. Angew. Chem., Int. Ed 1998, 37, 772–773. (b) Fabbrizzi,
L.; Licchelli, M.; Pallavicini, P.; Perotti, A.; Taglietti, A.; Sacchi, D.
Chem.sEur. J. 1996, 2, 75–82. (c) Fabrizzi, L.; Licchelli, M.;
Pallavicini, P.; Perotti, A.; Sacchi, D. Angew. Chem., Int. Ed. Engl.
1994, 33, 1975–1977.
(20) Varnes, A. W.; Dodson, R. B.; Wehry, E. L. J. Am. Chem. Soc. 1972,
94, 946–950.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5173

He et al.
The Mechanism Study of the Displacement Ap-
proach. From a mechanistic viewpoint, the d10 configuration
of Hg2+ with tetrahedral coordination geometry, and a large
ionic radius is likely to take advantage of the specific binding
to the ligand having two closed naphthol rings. Whereas the
tetrahedrally distorted coordination geometry constrained by
the ligand prevented the first-row transition-metal ions from
more strongly binding to the ligand than Cu2+ does, the Hg2+
ions could be readily substituted for d9 electronic conforma-
tion Cu2+, leading to the blocking of the quenching channel
and achieving the recovery of the initial luminescence in the
on state with high selectivity.25 A solid evidence for the
displacement reaction came from the ESI-MS spectrum
(Figure 9). Whereas the neutral Cu-NB itself did not
exhibit any observable positive peak at m/z ranging from
500 to 1000 in ESI-MS spectrum, upon the addition of
Hg2+ a peak at m/z ) 746.7 corresponding to the
[Hg(HNB)]+ species appeared. The isotopic peak of the
original peak fit very well to the isotope distribution
pattern calculated with the IsoPro 3.0 program for
[Hg(HNB)]+.
To further look into the nature of the interaction between
H2NB and the Hg2+ ion, pure Hg2+-H2NB complex was
isolated and characterized. Compared to the 1H NMR
spectrum of the free ligand H2NB, the Hg2+ binding caused
the small but significant downfield shifts of almost all of
the proton signals (Figure 10). Especially for the protons in
the naphthol ring, 0.25 ppm for H5 and 0.17 for H3, and so
forth, strongly suggested the participation of the naphthol
rings in the coordination.26 The presence of broadband at
about 12.9 ppm comparing to that of 12.5 ppm in the
spectrum of H2NB demonstrated the existence of OH protons
during Hg2+ binding.
Electrochemical Responses of Cu-NB to Hg2+. Cu-NB
exhibited a Cu2+/Cu+ redox couple at about -595mV of a
differential pulse voltammetric (DPV) measurement in a
DMSO/H2O (4:1 v/v) solution (Figure 11). Such a revisable
redox response could provide another opportunity for Cu-
NB to act as a multisignaling sensor for Hg2+. Adding Hg2+
caused the current of the peak at -595 mV decreased with
the potential anodically shifted, indicating that the displace-
ment reaction could occur with the concentration of the
copper compound Cu-NB decreased and the free Cu2+
increased. Meanwhile, new peaks at about -90 and 45 mV
appeared and developed. The former peak was assigned to
the Hg2+ complex couple, which increased and aniodically
shifted with the increasing of Hg2+, the latter was attributed
to the redox potential of the Cu2+/Cu+ couple without binding
to the ligand. Clearly, the presence of Hg2+ caused the
potential of Cu2+/Cu+ anodically shifted about 640 mV.
Consequently, both the current intensity and the potential
of the peak around -595 mV could be used to monitor the
concentration of Cu-NB. More interestingly, no perturbation
Figure 8. Fluorescence responses of Cu-NB (25 µM) to various cations
in DMSO/H2O (4:1 v/v) solution. The blue bars represent the emission
intensities of Cu-NB in the presence of 0.25 mM of alkali (IA) and alkaline
earth (IIA) metals and for other cations of interest. The orange bars represent
the change of the emission that occurs upon the subsequent addition of
0.05 mM of Hg2+ to the above solution. The intensities were recorded at
530 nm, excitation at 430 nm.
of detection of Cu-NB to Hg2+ met the discharge limit
for industrial wastewater according to the U.S. EPA
standard,21 or China SA standard.22
To further explore the availability of Cu-NB as a highly
selective probe for Hg2+, the fluorescent spectra of Cu-NB
coexisting with the other metal ions that could probably affect
the fluorescence were examined. When solutions of Cu-NB
(25 µM) were respectively added, 10 equivalents of Mn2+,
Fe2+, Co2+, Ni2+, Zn2+, Cd2+, and the alkali or alkaline earth
metal ions (0.25 mM), no obvious changes in the fluorescent
spectra were observed other than Pb2+ showing a marginal
alteration. Adding 50 µM equivalent of Hg2+ to the above
solutions gave rise to drastic increments of their fluorescence
intensities, revealing that Hg2+ could have specific effects
on the luminescence spectra, and the Hg2+-specfic responses
were not disturbed by the competitive ions (Figure 8). The
results strongly suggested that Cu-NB could be used as a
selective sensor for mercuric ion. In principle, this method
offers a general approach for the fluorescence detection of
metal ions, which show poor selectivity when the metal ions
are directly attached to the sensor.
Of course, the presence of Cu2+ could influence the
detection of Hg2+ in the solution. By adding an incremental
amount of Cu2+ ions, the UV-vis and luminescent spectra
of the Hg-ligand system were gradually recovered into the
profiles of the spectra of the Cu-NB system. However, the
reaction did not reach its full equilibrium even when 10 times
excess of Cu2+ were added, which confirmed that Hg-H2NB
complexation species has higher stability than the Cu-NB
complexation species. Meanwhile, the competitive Hg2+/Cu2+
off-on-off-type fluorescent signal control is reminiscent of
the Ca2+/K+ induced chromogenic behavior of a calix-
crown-5 derivative containing an indoaniline chromophore23
and has been observed in a fluoroionophore-based cyclam
system.24
(24) Kim, S. H.; Kim, J. S.; Park, S. M.; Chang, S. K. Org. Lett. 2006, 8,
371–374.
(21) Regulatory Impact Analysis of the Clean Air Mercury Rule; U.S. EPA:
Research Triangle Park, NC, 2005; EPA-452/R-05-003.
(25) Valeur, B.; Leray, I. Coord. Chem. ReV. 2000, 205, 3–40.
(26) (a) Wang, J. B.; Qian, X. H. Chem. Commun. 2006, 109–111. (b)
Shiraishi, Y.; Maehara, H.; Ishizumi, K.; Hirai, T. Org. Lett. 2007, 9,
3125–3128.
(22) Standardization Administration (SA) of the People’s Republic of China,
Integrated Wastewater Discharge Standard, GB 8978, 1996.
(23) Kubo, Y.; Obara, S.; Tokita, S. Chem. Commun. 1999, 2399–2400.
5174 Inorganic Chemistry, Vol. 47, No. 12, 2008

“Turn-on” Fluorescent Sensor for Hg2+
Figure 9. ESI-MS spectrum of the Cu-NB (2 mM) in DMSO/H2O (4:1 v/v) solution upon addition of Hg2+ (5 mM). Inserts are (a) the result of the IsoPro
3.0 ESI-MS spectrum simulation program and (b) the experimental measurement.
Figure 10. 1H NMR spectra solution of the free H2NB ligand, the hydrogen atoms with various chemical shifts are ascribed (a) and the Hg2+ complex (b),
in CDCl3.
could also be operated as a highly selective and sensitive
electrochemical sensor for Hg2+ over other metal ions,
besides the fluorometric response for Hg2+. The redox
titration of the free ligand with Hg2+ exhibited a peak at
about -80 mV, which was developed and aniodically shifted.
It was assigned to the Hg2+/Hg+ couple of the ligand-Hg2+
complexation species.
Conclusions
In summary, we have demonstrated a new and broadly
applicable displacement approach to identify new fluorescent
Figure 11. DPV of Cu-NB (1 mM in DMSO solution, 0.1 M n-Bu4NClO4)
chemosensors. The experiments show that using the metal
complex rather than organic dyes as a luminescent probe
for cations could effectively avoid the interference of other
metal cations with weaker binding capability and that the
naphtholate/naphthol-based off-on signaling mode is
more beneficial than the on-off-type transductions. The
discovery of this new Hg2+-responsive chemosensor with
(a) and in the presence of 1 mM ratio of Hg(ClO4)2 in DMSO/H2O (4:1
v/v) solution.
of the DPV of Cu-NB was observed upon the addition of
Na+, K+, Ca2+, Mg2+, Ni2+, Co2+, Zn2+, Cd2+, and even Pb2+
metal ions; such that the larger separation (640 mV) of the
redox potential of Cu2+/Cu+ clearly reveals that the Cu-NB
Inorganic Chemistry, Vol. 47, No. 12, 2008 5175

He et al.
high selectivity even in the presence of excess mono/
divalent ions and other contaminants reflects the validity
of this approach and proves the hypothesis that the
displacement signaling mechanism is suitable to such an
exploration. Hopefully, this preliminary understanding of
this cation-sensing mechanism would be helpful to find
new practical probes that show excellent cation-sensing
capacity in water or at least in water-containing organic
solvents through structural modification.
Acknowledgment. This work is supported by the National
Natural Science Foundation of China and the Start-Up Fund
of Dalian University of Technology.
Supporting Information Available: Additional spectroscopic
data and CIFs of ligand H2NB and Cu-NB. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC702494S
5176 Inorganic Chemistry, Vol. 47, No. 12, 2008