A dual functional probe for "turn-on" fluorescence response of Pb2+ and colorimetric detection of Cu2+ based on a rhodamine derivative in aqueous media

Article abstract of DOI:10.1039/c5dt02731d

A dual functional probe L based on rhodamine was devised and synthesized. Probe L can sense Pb²⁺ and Cu²⁺ in aqueous solution through two approaches: a significant fluorescence enhancement caused by Pb²⁺ and a visible color change from colorless to orchid induced by Cu²⁺. Competitive experiments showed that probe L had high fluorescence sensitivity for Pb²⁺ and excellent colorimetric selectivity for Cu²⁺ over many environmentally relevant ions. The mechanisms of L for sensing Pb²⁺ and Cu²⁺ have been well demonstrated by ESI-MS, ¹H NMR titration, IR, the crystal structure of L-Pb²⁺ and density functional theory calculation of L-Cu²⁺. In addition, fluorescence image detection of Pb²⁺ in living cells displayed an enhanced fluorescence effect.

Full text of DOI:10.1039/c5dt02731d

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DOI: 10.1039/C5DT02731D
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ARTICLE TYPE
A dual functional probe for “turnꢀon” fluorescence response of Pb2+ and
colorimetric detection of Cu2+ based on rhodamine derivative in aqueous
media
Min Li,a Xiu-Juan Jiang,a Hui-Hui Wu,a Hong-Lin Lu,a Hai-Yang Li,a Hong Xu,*,a Shuang-
5
Quan Zang,*,a and Thomas C. W. Maka,b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
First published on the web Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000xl
A dual functional probe L based on rhodamine was devised and synthesized. Probe L can sense Pb2+ and Cu2+ in aqueous solution
10 through two approaches: a significant fluorescence enhancement caused by Pb2+ and a visible color change from colorless to orchid
induced by Cu2+. Competitive experiments showed that probe L had high fluorescence sensitivity for Pb2+ and excellent colorimetric
selectivity for Cu2+ over many environmental relevant ions. The mechanisms of L for sensing Pb2+ and Cu2+ have been well
1
demonstrated by ESIꢀMS, H NMR titration, IR, the crystal structure of LꢀPb2+ and density functional theory calculation of LꢀCu2+. In
addition, fluorescence image detection of Pb2+ in living cells displayed an enhanced fluorescence effect.
15
of views.
50
Fluorescent8 and colorimetric9 means provide good
alternatives due to their low cost, easyꢀoperation, simple
instrumental implementation, fast response and excellent
selectivity, but many reported probes for the two metal ions
generally undergo fluorescence quenching owing to their inherent
Introduction
Selective and sensitive detection of toxic heavy metal and
transition ions is a significant issue because pollution caused by
metal ions may have adverse impacts on organisms and the
20 environment.1 Particularly, Pb2+ ion is regarded as one of the
most toxic metal ions, even very low levels of lead exposure can
severely damage the body’s brain, kidneys, liver and central
nervous system, especially to children.2 On the other hand, Cu2+
ion, as the third most abundant essential transition metal ion,
25 plays a critical role in physiological and biochemical functions.3
Nevertheless, at high concentration levels, copper becomes a
dangerous element and has potentially toxic effects on living
organisms.4 It causes oxidative stress and related symptoms,
which may lead to many neurodegenerative diseases such as
30 Parkinson’s, Prion and Alzheimer's diseases.5 Traditional
methods for detection Pb2+ and Cu2+ are commonly utilizing
atomic absorption spectrometry (AAS)6 and inductively coupled
plasma mass spectrometry (ICPꢀMS)7, but these methods often
require tedious sampleꢀpreparation steps and highly skilled
35 individuals. Therefore, the development of new and simple
analytical methods for motoring Pb2+ and Cu2+ is of utmost
importance from both environmental and biological point
55 quenching nature.10 As we all know, when interacting with
analytes, a turnꢀon fluorescence signal is more credible and
preferable in applications than a turnꢀoff signal because the turnꢀ
off response usually suffers from a high background.11,12 The
rhodamine framework has been proven to be a classic mode to
60 construct turnꢀon fluorescent and colorimetric probes because of
its particular spirolactam structure and excellent photophysical
properties such as long absorption and emission wavelength,
large absorption coefficient and good photostability.13 During the
last decades, although much work has been devoted to develop
65 probes based on rhodamine, only several turnꢀon fluorescent
probes for detecting Pb2+ based on rhodamine derivatives were
reported as a result of high fluorescence quenching ability of the
heavy metal ion. For example, in 2005, Yoon group reported a
highly selective sensor based on rhodamine B for Pb2+ examined
70 in acetonitrile.14a In 2010, Hu et al. reported a fluorescence
enhancement sensor based on rhodamineꢀphenylurea conjugate,
which displayed an excellent selectivity toward the detection of
Pb2+ in acetonitrile and Hg2+ in aqueous solution.14b On the other
hand, although a large number of rhodamineꢀbased probes for
75 Cu2+ have been reported,14c,d,e probes for detecting Pb2+ and Cu2+
simultaneously were relatively few. It is still of high importance
to construct single probes capable of the simultaneous detection
of multiple targets. And most fluorescent probes operated in
organic solvents or in mixed organic–aqueous media because the
80 preparation of them often requires laborious multistep organic
synthesis. The incompatibility with aqueous environments may
____________________________________________________
a College of Chemistry and Molecular Engineering, Zhengzhou
40 University, Zhengzhou, 450001, P. R. China
Fax: (+86) 371− 6778 0136; Eꢀmail: zangsqzg@zzu.edu.cn
b Department of Chemistry and Center of Novel Functional Molecules,
The Chinese University of Hong Kong, Shatin, New Territories, Hong
Kong SAR, People's Republic of China
45 †Electronic supplementary information (ESI) available: Details about
fluorescence spectra and characterization data including NMR, MS, IR,
crystallographic data and DFT/TDꢀDFT calculations data.
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restrict application of these probes to some extent.15 Therefore,
developing probes with good water solubility remains another
challenge.
absorbance and fluorescence spectra of the mixtures were
measured. All fluorescence spectra were measured at 483 nm as
an excitation wavelength, both excitation and emission slit widths
Recently, Tae et al. group reported two cyclenꢀconjugated 60 were 8 nm.
rhodamine hydroxamate derivatives, which could bind
5
Synthesis of probe L
specifically with Pd2+ ion16a and selectively sensing Cd2+ ion16b
both in aqueous solution, respectively. Inspired by the reports, in
this work, we introduced an acyclic diethyl iminodiacetate
receptor designed to satisfy the size and charge requirements of
Compound 1 was prepared according to literature method as
depicted in scheme 1.16 To a solution of compound 1 (180 mg,
0.28 mmol) in acetonitrile (12 mL) was added potassium
65 carbonate (77.6 mg, 0.56 mmol) and potassium iodide (46.7 mg,
0.28 mmol). Diethyl iminodiacetate (50 ꢁL, 0.28 mmol) was
added to the above solution under the reflux condition, and the
reaction mixture was refluxed for 12 h. The mixture was filtered
and concentrated. The crude product was purified by column
70 chromatography on silica gel (CH2Cl2/MeOH: 15:1, v/v)
affording colorless oil L (126.5 mg, 60%). Mp: 46–47 °C. 1H
NMR (400 MHz, CDCl3, ppm) δ = 7.91 (d, 1H, J = 7.6 Hz), 7.54
(t, 1H, J = 15.2 Hz), 7.38 (m, 4H, J = 76.8 Hz), 7.05 (d, 1H, J =
6.4 Hz), 6.53 (d, 2H, J = 8.8 Hz), 6.36 (s, 2H), 6.25 (d, 2H, J =
75 8.8 Hz), 4.88 (s, 2H), 4.11 (m, 4H, J = 21.2 Hz), 3.93 (s, 2H),
3.53 (s, 4H), 3.31 (m, 8H, J = 20.4 Hz), 1.21 (t, 6H, J = 14.4 Hz),
1.14 (t, 12H, J = 13.6 Hz). 13C NMR (400 MHz, CDCl3, ppm) δ =
171.08, 164.57, 157.59, 155.49, 153.61, 150.54, 148.81, 136.96,
133.07, 128.80, 128.65, 128.30, 123.86, 122.94, 121.87, 121.20,
80 107.97, 104.95, 97.81, 79.98, 65.46, 60.43, 59.72, 54.76, 44.33,
14.20, 12.61. ESIꢀMS: m/z calcd for [M+H+]+ C43H52N5O7:
750.3862, found: 750.3879; calcd for [M+Na+]+ C43H51N5O7Na:
772.3681, found: 772.3680. Anal. calcd for C43H51N5O7: C, 68.87;
H, 6.85; N, 9.34%. Found: C, 68.92; H, 6.90; N, 9.38%. IR (KBr
85 plate, cm−1): 1717, 1635, 1616, 1547, 1516, 1465, 1399, 1384,
1328, 1305, 1266, 1219, 1193, 1150, 1118, 1077, 1025, 818, 784,
756, 698, 685, 577, 536, 457.
10 the Pb2+ cation17 to a rhodamine hydroxamate derivative. The
synthetic route to probe L was shown in Scheme 1. Just as we
expected, the probe L displayed a selective recognition of Pb2+
via
a significantly fluorescence enhancement over other
environmental relevant species in aqueous media and also
15 exhibited a sensitive response of Cu2+ through a visible color
change from colorless to orchid. Fluorescence enhancement
mechanism of probe L for sensing Pb2+ was directly proved by
single crystal structure of complex LꢀPb2+. ESIꢀMS and DFT
calculation results confirmed complexation of L with Cu2+.
20 Experimental section
Materials and methods
All materials were purchased and used without further
1
purification. All solvents were analytical reagent grade. H NMR
and 13C NMR were carried out on a Bruker 400 spectrometer.
25 Chemical shifts were reported in ppm using tetramethylsilane
(TMS) as the internal standard. Mass spectra were recorded on a
WatersꢀQꢀTOF spectrometer (ESI). Fluorescence spectra were
determined on a FluoroMaxꢀ4 spectrofluorometer. UV−visible
absorption spectra were recorded on a TUꢀ1901 doubleꢀbeam
30 UV−vis Spectrophotometer. Melting point was measured on an
XT4A microscope electron thermal apparatus. Element analyses
(C, H, N) were conducted using a PerkinꢀElmer 240 elemental
analyzer. The Fourier transform infrared (FTꢀIR) spectra were
obtained in the range of 400−4000 cm−1 as KBr pellets on a
35 Bruker VECTOR 22 spectrometer. All pH measurements were
made with a Model PHSꢀ3C meter (Dapu, China). Timeꢀresolved
fluorescence spectra were measured on a LifeSpec picosecond
TRF spectrometer (Edinburgh Instruments Ltd.). Fluorescence
quantum yields were determined by comparing the corrected
40 spectra with that of pure rhodamine B (Ф = 0.97 in ethanol)18
taking the total area under the curve and using Equation 1. Where
Фu and Фs are the fluorescence quantum yields of the corrected
spectra for the sample and standard, Au and As are the absorbance
of the sample and standard at the excitation wavelength (483 nm),
45 Iu and Is are the corresponding integrated emission intensities, and
η
u and
and standard solutions.
Фu = Фs(IuAs/IsAu)(ηu/ηs)2
ηs are the refractive indexes of the solvents for the sample
(1)
Fluorescence and UV−vis studies
50 A stock solution of L was prepared in CH3CN−H2O (1:1, v/v).
Solutions of metal ions were prepared in distilled water from their
chloride salts except AgNO3 due to the low solubility of its
chloride salt. In the titration experiment, a solution of L was
taken on a quartz cuvette (3.0 mL; path length, 1 cm), and diluted
55 stock solutions of the metal ions were added gradually using a
micropipette. After equilibrium at room temperature for 1 min,
Scheme 1. Synthesis of probe L.
90 Synthesis of L-Pb2+
A solution of Pb(NO3)2 (11 mg, 0.03 mmol) in water (0.5 mL)
was added to a solution of L (5 mg, 0.006 mmol) in methanol
(2.5 mL). The mixture left undisturbed at room temperature and
2
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golden stripꢀshaped crystals were obtained by slow evaporation
after about one week. Anal. Calcd for C82H110N14O34Pb2: C: 55 led to a notably fluorescence enhancement centered at 576 nm,
containing 1% CH3CN (v/v). A titration experiment of L for Pb2+
43.77, H: 4.93, N: 8.71%. Found: C: 43.62, H: 4.99, N: 8.60%.
accompanying with the solution emission color change from dark
to jacinth, indicating a Pb2+ꢀselective turnꢀon fluorescence signal
behavior. In comparison to L, an 18ꢀfold significant fluorescence
enhancement (quantum yield Ф = 0.152) could be observed when
60 10 equiv of Pb2+ was added. The fluorescence [1/(F − F0)] at 576
nm varies as a function of 1/[Pb2+] in a linear relationship (R =
0.9995), demonstrating a 1:1 stoichiometry of LꢀPb2+ complex
with a binding constant 2.43 × 104 M−1 (Fig. S5). The method of
continuous variation (Job’s plot) was also used to prove the 1:1
65 stoichiometry (Fig. S6). The detection limit of probe L as a
fluorescent sensor for Pb2+ detection was determined from a plot
of fluorescence intensity as a function of Pb2+ concentration. A
good linear relationship was observed between the fluorescence
intensity at 576 nm and the concentrations of Pb2+ in
70 concentration range of 0 and 10 × 10−6 M, implying Pb2+ could be
quantitatively detected in a wider concentration range (Fig. S7).
The detection limit for Pb2+ was found to be 2.5 × 10−7 M based
on LOD = 3σ/s, where σ was the standard deviation of blank
measurements, and s was the slope between fluorescence
75 intensity and Pb2+ concentration.22
Single crystal X-ray diffraction analyses
5
Singleꢀcrystal data of complex LꢀPb2+ was collected on a Bruker
SMART APEX CCD diffractometer equipped with graphiteꢀ
monochromated Mo Kα radiation (λ = 0.71073 Å) using the ωꢀ
scan technique at room temperature. The empirical absorption
corrections were applied to the intensities using the SADABS
10 program.19 The structure was solved using the program SHELXSꢀ
9720 and refined with the program SHELXLꢀ97.21 All nonꢀ
hydrogen atoms were subjected to anisotropic refinement. The
hydrogen atoms of L were included in the structure factor
calculation at idealized positions using a riding model and refined
15 isotropically. Crystallographic data have been deposited with the
Cambridge Crystallographic Data Center. CCDC reference
number: 1064106.
Theoretical calculations
Theoretical investigations on L and complex LꢀCu2+ were
20 performed by Gaussian 09 program package and B3LYP method.
The geometries of L and LꢀCu2+ complex were fully optimized at
the DFTꢀB3LYP level. The calculations were carried out with a
mix basis set, i.e., 6ꢀ31 G (d) basis set for C, H, O, N atoms,
LanL2DZ for Cu atom.
25 Cytotoxicity assay
The cytotoxicity effects of probe L and its complex LꢀPb2+ were
studied by a standard methylthiazolyltetrazolium (MTT) assay in
HepG2 cell lines. HepG2 cells were seeded in 96ꢀwell plates.
Various concentrations (10, 20, 30, 40, 50 ꢁM) of L as well as its
30 complex LꢀPb2+ were added into cells and incubated for 24 h at
37 °C under 5% CO2. Solvent control samples (cells treated with
DMSO alone) were also included in parallel sets. After
incubation, the growth media was removed. The MTT solution
was added to each well of 96ꢀwell plate, and the plate was
35 incubated for another 2 h. The insoluble purple formazan product
was dissolved in DMSO. A microtiter plate reader (BioꢀRad
iMark microplate reader) was used to measure absorbance of each
well. Each experiment was repeated for three times.
Fig. 1. Fluorescence titration spectra (λex = 483 nm) of L (10 ꢁM) upon
addition of Pb2+ (0−10 equiv) in aqueous HEPES buffer (10 mM, pH 6.5)
containing 1% CH3CN (v/v). Inset: photographs of the solution of L
before and after addition of 10 equiv of Pb2+ under 365 nm UV lamp.
Cell imaging studies
40 HepG2 cell lines were cultured in modified Roswell Park
Memorial Institute (RPMI) 1640 medium. Cells were incubated
with 30 ꢁM L for 30 min at 37 °C. After being washed with
phosphate buffer saline (PBS) three times to remove the
remaining L, the cells were then incubated with 30 ꢁM PbCl2 for
45 another 30 min. The incubated cells were washed with PBS and
mounted onto a glass slide. Fluorescence images of mounted cells
were acquired on the olympas fluorescence microscope.
80 Fig. 2. Fluorescence responses (λex = 483 nm) of L (10 ꢁM) at 576 nm in
aqueous HEPES buffer (10 mM, pH 6.5) containing 1% CH3CN (v/v).
Red bars: a free probe or a probe treated with the marked metal ions; blue
bars: a probe treated with the marked metal ions (10 equiv) followed by
10 equiv of Pb2+.
Results and discussion
Fluorescence spectra studies of detection of Pb2+
50 The change in fluorescence spectra of L upon titration with Pb2+
was displayed in Fig. 1. Upon excitation at 483 nm, probe L
alone displayed very weak fluorescence with a low quantum yield
(Ф = 0.036) in aqueous HEPES buffer (10 mM, pH 6.5)
85
Common metal ions, Na+, K+, Ca2+, Mg2+, Zn2+, Cd2+, Fe3+,
Fe2+, Al3+, Mn2+, Co2+, Ni2+, Ag+, Cr3+, Hg2+, and Cu2+ were used
to evaluate the selectivity of probe L (10 ꢁM). However, no
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significant variation in the fluorescence spectra of L (Fig. 2) was
observed with any other metal ion except Cd2+ and Hg2+ (Fig. 3)
which induced similar fluorescence emission but to a small extent.
To check the practical ability of L as a Pb2+ selective fluorescent
An optimized solvent system of aqueous HEPES buffer (10 mM,
pH 7.2) containing 1% CH3CN (v/v) was selected for the
spectroscopic investigations. As shown in Fig. 4 and Fig. S27, the
UV–vis titration spectra of free L displayed three obvious
5
probe, we carried out competitive experiments in the presence of 35 absorption peaks at 246 nm (ε = 7.69 × 104 M−1 cm−1)), 275 nm
10 equiv of Pb2+ mixed with the above cations (10 equiv each).
As shown in Fig. 2, most of the above cations didn’t produce a
perceived change in the fluorescence intensity except Cr3+, Hg2+
and Cu2+. When competed with these three cations, the
and 317 nm, but there was almost no absorbance peak in the
visible region, indicating that L existed as a spirocylic form.
Upon addition of Cu2+ gradually, the peaks at 246, 275 and 317
nm decreased and a new absorption band centered at 567 nm
10 fluorescence intensity of LꢀPb2+ was 30–50% lower than in the 40 increased, indicating LꢀCu2+ complex formation and this new
absence of them. So these three cations would have little
influence on Pb2+ determination.23 Besides, various anions such
absorption band may attribute to ringꢀopening process of
spirolactam ring triggered by Cu2+.27 In addition, the absorption
behavior changed the color of the solution from colorless to
orchid, allowing nakedꢀeye detection of Cu2+. These changes
−
−
−
2−
as F−, Cl−, Br−, I−, NO2 , S2−, NO3 , SO32−, SO42−, HCO3 , CO3
,
−
−
−
S2O32−, SCN−, ClO4 , AcO−, HSO3 , HSO4 and organic citrate
15 anion (10 equiv each) have been tested and most of them 45 were likely due to the coordination of L with Cu2+.27c The
displayed no obvious effect on fluorescence of LꢀPb2+ complex
except a few sulphides and the citrate anion, which might capture
Pb2+ from LꢀPb2+ complex due to a much larger stability constant
of citrate and Pb2+ (K = 2.0 × 1012) (Fig. S8),24 but the
absorbance [1/(A − A0)] at 567 nm varies as a function of 1/[Cu2+]
in a linear relationship (R = 0.9968), suggesting a 1:1 ratio of Lꢀ
Cu2+ complex with a binding constant 1.20 × 105 M−1 (Fig. S9).
Job’s plot also proved the 1:1 stoichiometry (Fig. S10). Fig. S11
20 interference was of little importance at the low concentration in 50 showed the absorption intensity changes of L as the concentration
nature.25 Therefore, these results suggested that probe L had good
of Cu2+ in the low micromolar range. From this linear calibration
Pb2+ selectivity in the presence of a wide range of coexistent ions.
graph, the detection limit for Cu2+ was found to be 5.8 × 10−7
based on LOD = 3σ/s.22
M
Fig. 5. UV−vis absorption spectra of L (10 ꢁM) with various metal ions
55 (10 equiv) in aqueous HEPES buffer (10 mM, pH 7.2) containing 1%
CH3CN (v/v).
Fig. 3. Fluorescence emission spectra (λex = 483 nm) of L (10 ꢁM) in the
presence of metal ions (10 equiv) in aqueous HEPES buffer (10 mM, pH
25 6.5) containing 1% CH3CN (v/v).
UV−vis spectra studies of detection of Cu2+
Fig. 6. Photographs of L (10 ꢁM) upon addition of marked metal ions (10
equiv) under visible light.
An important feature of probe L was its high selectivity
60 towards the Cu2+ over the other competitive species such as Na+,
K+, Ca2+, Mg2+, Mn2+, Ni2+, Zn2+, Co2+, Cd2+, Fe2+, Fe3+, Cr3+,
Hg2+, Pb2+, Ag+, and Al3+ (10 equiv each). No obvious responses
were observed in the UV–vis spectra of L upon addition of 10
equiv of the above species. However, addition of 10 equiv Cu2+
65 induced a large absorption band centered at 567 nm (ε = 1.509 ×
105 M−1 cm−1) appeared (Fig. 5). In addition, a competitive
experiment revealed that all of the tested foreign metal ions (Fig. 7)
−
−
2−
as well as anions (F−, Cl−, Br−, I−, NO2 , NO3 , SO32−, SO4
,
HCO3 , CO32−, S2O32−, SCN−, ClO4 , AcO−, HSO3 and HSO4 )
70 (Fig. S12) exhibited almost no change in absorbance behavior,
except Pb2+, Ag+, Na+, K+ and Ca2+ triggered fairly slight
−
−
−
−
Fig. 4. UV−vis titration spectra of L (10 ꢁM) upon addition of Cu2+
(0−1.2 equiv) in aqueous HEPES buffer (10 mM, pH 7.2) containing 1%
CH3CN (v/v). Inset: photographs of the solution of L before and after
30 addition of 10 equiv of Cu2+ under visible light.
absorption intensity enhancements of LꢀCu2+ (A<1.2 A
)
LꢀCu2+
which might due to the competitive coordination of the metal ions
4
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− +
to Cu2+, and S2− showed obvious interference which could be
to the species [L – C2H4 + Pb2+ + NO3 ] appeared (Fig. S13).
attributed to the fact that S2− reacted with Cu2+ to form CuS with 50 The spirolactum ring in the complex LꢀPb2+ was opened because
the low solubility product constant (Ksp = 1.27 × 10−36).26 Notably,
when a high concentration of Pb2+ (2 mM, 200 equiv of L) was
used, a mild absorption band centered at 563 nm was observed
(Fig. S30), which might attribute to the Pb2+ꢀtriggered opening of
spirolactam ring. It would have little influence on Cu2+ detection,
as was evident from a strong interaction between L and Cu2+ and a
large binding constant value for LꢀCu2+ (120000 M–1). All of these
of the chelation of Pb with O1 (carbonyl oxygen of rhodamine B)
and O3 (hydroxamate oxygen), which induced fluorescence
enhancement. The Pb−O band lengths were comparable to related
complexes in the literature.30
5
10 results proved that probe L displayed remarkably specificity
toward Cu2+ and could serve as a sensitive nakedꢀeye indicator for
Cu2+ in HEPES buffer solution.
55 Fig. 8. The Xꢀray crystal structure of LꢀPb2+. All hydrogen atoms, solvent
molecules and counter nitrate anions are omitted for clarity. Symmetry
code: #1 −x + 1, −y, −z + 1.
Fig. 7. Absorbance responses of L (10 ꢁM) at 567 nm in aqueous HEPES
buffer (10 mM, pH 7.2) containing 1% CH3CN (v/v). Red bars: a free
15 probe or a probe treated with the marked metal ions; black bars: a probe
treated with the marked metal ions (10 equiv) followed by 10 equiv of
Cu2+.
Furthermore, 1H NMR titration in the mixed solvent of
CD3OD and D2O was performed. As shown in Fig. S14,
60 significant shifts were observed for the signals of protons Hk and
Hn (Hk: from peak centered at 4.832 ppm to 4.966 ppm, ∆δ =
0.134 ppm; Hn: from peak centered at 3.554 ppm to 3.934 ppm,
∆δ = 0.38 ppm), suggesting the two “N” atoms on the pyridyl
group and diethyl iminodiacetate moiety participated in the
65 coordination with Pb2+ and induced the decrease of the electric
density in Hk and Hn. One equivalent of Pb2+ was enough to
induce the final shift of the Hk and Hn protons of L, CD3OD/D2O
as coꢀsolvent might promote complex association. Besides, the
downshift peaks of L near 6.35 ppm were assigned to the proton
70 signals of xanthenes. The addition of Pb2+ into the solution led to
an obvious downfield shift. This was due to the decrease in
electron density of the aromatic ring, which also proved the
delocalization of xanthenes, that is, Pb2+ induced the opening of
spirocycle.
Sensing mechanism of L-Pb2+
As we all know, rhodamineꢀbased derivatives generally undergo
20 a spirolactam ring opening process upon coordination with
recognition species, resulting in fluorescence and color changes.28
The spiroringꢀopening mechanisms of rhodamine derivatives are
usually deduced by mass spectrum analysis, NMR titration, IR
and so on. However, the reports of ringꢀopened mechanism
25 directly proved by crystal structures are still relatively few to our
knowledge.13a,29 In this work, crystals suitable for singleꢀcrystal
Xꢀray analysis of complex LꢀPb2+ were obtained by reacting
Pb(NO3)2 with probe L in waterꢀmethanol (0.5 mL/2.5 mL)
mixed solution. Xꢀray structure analysis of complex LꢀPb2+
30 clearly showed the ringꢀopening mechanism of the rhodamine
lactam ring when L coordinated with Pb2+ (Fig. 8). Detailed
crystallographic data and selected bond lengths and angles for
complex LꢀPb2+ were listed in Tables S1 and S2.
75 Sensing mechanism of L-Cu2+
To explore the mechanism, we attempted to obtain the crystal
structure of complex LꢀCu2+ but failed. While, formation of
complex LꢀCu2+ was supported by its [L+Cu2+]2+ peak at
406.1514 in the electrospray ionization (ESI) mass spectrum (Fig.
80 S15). Besides, UV–vis absorption spectra properties of L toward
Cu2+ could also prove the formation of complex LꢀCu2+, as
discussed in section of UV−vis spectra studies of detection of
Xꢀray structure analysis revealed that L and Pb2+ formed a
35 dinuclear brigded complex LꢀPb2+, which brigded by O atoms
and crystallized in a triclinic system with the space group Pī. As
depicted in Fig. 8, each Pb2+ center was coordinated by six O
−
atoms (O1, O3, O4, O7, O7#1, and O12 from NO3 ) and two N
atoms (N4 and N5). It deserved to be mentioned that one ethoxy
40 group (–OC2H5) of diethyl iminodiacetate moiety decomposed
into hydroxyl group (–OH) after Pb2+ binds with L. It was likely
that Pb2+ coordinated with hydroxamate oxygen generating a
good nucleophile toward esters and induced the ester hydrolysis.
A schematic represented the transformation process of L structure
45 upon complexation to Pb2+ was showed in Scheme S1. The ester
hydrolysis induced by Pb2+ coordination was also confirmed by
ESIꢀTOF mass spectrum analysis result. When Pb(NO3)2 was
added to L methanol solution, a signal at 991.3148 (m/z) assigned
1
Cu2+. Unfortunately, the H NMR titration spectrum of LꢀCu2+
did not provided useful information due to the paramagnetic
85 nature of Cu2+ ion,31 and the signals of aromatic protons became
greatly broader after addition of Cu2+ (Fig. S16). IR analysis was
also carried out to further research the coordination mode (Fig.
S17). The carbonyl peak of the spirolactum ring (1716 cm−1)
disappeared after adding Cu2+, which suggested that carbonyl “O”
90 atom was involved in the coordination with Cu2+.32 This is key to
the spiro ringꢀopening and absorbance enhancement of the
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rhodamine dyes.14e The peak at 1120 cm−1 corresponding to the
Reversibility studies
stretching vibration of N–O– bond shifted to higher frequency
(1138 cm−1) upon interaction with Cu2+, indicating the
hydroxmate “O” atom coordinated with Cu2+.
45 Reversibility is an important factor for evaluating the
practicability of a probe. Both fluorescence response of L for
Pb2+ and absorption response of L for Cu2+ were reversible,
which were verified by the reversible titration using EDTA for
Pb2+ and Na2S for Cu2+ (Figs. S22 and S26). It was noticed that
50 adding EDTA disodium to the solution of LꢀPb2+ species resulted
in the diminution of the fluorescence intensity at 576 nm and
addition of Na2S to the solution of LꢀCu2+ species led to the
absorbance decrease along with orchid color fading, implying the
reversible binding of L with Pb2+ or Cu2+. Besides, the reversible
55 chelation of L with Pb2+ was also supported by the fluorescence
lifetime data. The fluorescence decay curves for L, LꢀPb2+ and Lꢀ
Pb2+ꢀEDTA complexes were fitted by biꢀexponential function
with acceptable χ2 values and all the data were presented in Fig.
S23–S25 and Table S4. The lifetime of probe L decayed in a biꢀ
60 exponential manner with time constants τ1 = 0.384 ns (50%) and
τ2 = 0.384 ns (50%) with χ2 = 1.024, indicating that the
fluorescence spectra consisted of two contributions from free L
(Fig. S23 and Table S4). After 10 equiv of Pb2+ was added into
buffer solution of L, the average fluorescence lifetime of the Lꢀ
65 Pb2+ system increases to 2.004 ns (Fig. S24 and Table S4), which
was attributed to the introduction of Pb2+ and the chelation
enhanced fluorescence (CHEF) process occurred with L.
Addition of 100 equiv of EDTA to the LꢀPb2+ solution reduces
the value to 0.467 ns (Fig. S25 and Table S4). These results
70 clearly demonstrated high reversibility of L toward Pb2+ and the
potential applications in realꢀtime monitoring.
5
On the basis of the above analysis results, DFT computational
studies were actualized to further understand the binding
interaction of L with Cu2+ in water solvent. As shown in Fig. 9,
the optimized structure of complex LꢀCu2+ showed that Cu center
was in a distorted octahedral geometry geometry with two axially
10 Cl– anions (Cl1 and Cl2) and two N atoms (N4 and N5) and two
O atoms (O1, O3) in a slightly distorted equatorial plane.
Fig. 9. DFTꢀoptimized geometries of complex LꢀCu2+ in water solvent
(grey atom, C; red atom, O; blue atom, N; green atom, Cl; coral atom, Cu).
To interpret the absorption enhancement phenomenon
15 induced by Cu2+, timeꢀdependent DFT calculation was carried out
in water solvent. The relevant frontier molecular orbitals of L and
LꢀCu2+ complex were shown in Figs. S18 and S19. Upon Cu2+
binding with L, HOMO–10 electron density of LꢀCu2+ was
majorly focused on the carbonyl oxygen atom and two chloride
20 atoms that coordinated with Cu2+, whereas that on LUMO+1 of
LꢀCu2+ was majorly focused on the spirolactum moieties (Fig.
S19). The orbital electron transition was relevant to MLCT (from
Cu center to L moiety) characteristic, which was responsible for
the significant absorption enhancement. As shown in Table S3,
25 the lowest energy transition of LꢀCu2+ came from HOMO−10 →
LUMO+1 (f = 0.4619) orbital transition, and its contribution to
the lowest energy excitation was 17%.
Cell imaging studies
Effect of pH
For practical applicability, the optimal pH condition of probe L
30 was estimated. As depicted in Figs. S20 and S21, nearly no
changes in the fluorescence and absorption intensity were
observed within the pH range from 5.0 to 11.0, indicating that the
Fig. 10. MTT assay to determine the cytotoxic effect of L and LꢀPb2+ on
HepG2 cells.
free L was stable and insensitive over this wide span of pH. Upon 75 To demonstrate the potential application of the probe L in
treatment with 10 equiv of Pb2+, the strong fluorescence emission
35 was observed in the pH range of 5.0 to 6.5, which indicated that
L could recognize Pb2+ in the weak acidic media (Fig. S20).
Besides, when we decreased pH value to 6.5, an excellent
fluorescence imaging of cells, live cell imaging experiments were
performed in HepG2 cells. The cytotoxic effect of L and LꢀPb2+
on living HepG2 cells was first assessed. As shown in Fig. 10,
more than 70% cells were viable up to 30 ꢁM concentration of L
selective response to Pb2+ was obtained in fluorescence spectra of 80 and LꢀPb2+. Therefore, the concentration of 30 ꢁM was chosen as
L, so pH 6.5 was selected as the detection medium toward Pb2+
40 throughout the fluorescence experiments. The absorption became
remarkable enlargement after addition of 10 equiv of Cu2+ when
the solution pH was between 6.0 and 8.0 (Fig. S21), revealing L
could detect Cu2+ in neutral pH values.
an optimum concentration for imaging study. HepG2 cells which
were grown on cover slips were incubated with 30 ꢁM L in 10
mM HEPES (pH = 6.5, 0.9% NaCl) for 30 min at 37 °C, only
very weak fluorescence was observed (Fig. 11b). After being
85 washed with PBS three times to remove the remaining L, the
6
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cells were then incubated with 30 ꢁM Pb2+ under the same
experiment condition. The incubated cells were mounted onto a
glass slide, intense jacinth fluorescence was clearly observed
through a fluorescence microscope (Fig. 11d). The results
indicated that L was cell permeable and could be used as an
effective intracellular Pb2+ imaging agent.
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Fig. 11. Fluorescence microscopic images of HepG2 cells. (a) brightfield
transmission image of HepG2 cells incubated with L; (b) fluorescence
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fluorescence image of (c).
Conclusions
In summary, we synthesized a dual functional probe L by
introducing an acyclic diethyl iminodiacetate unit into
a
15 rhodamine hydroxamate derivative. Probe exhibited
L
fluorescence enhancement response to Pb2+ and colorimetric
response to Cu2+ along with a color change from colorless to
orchid in aqueous HEPES buffer with only 1% CH3CN. The
probe showed a highly binding affinity only toward Pb2+ and
20 Cu2+ ions even in the presence of other biologically relevant
species with low detection limits of 2.5 × 10−7 M and 5.8 × 10−7
M, respectively. The mechanisms of L for sensing both Pb2+ and
Cu2+ were based on rhodamine spirolactam ringꢀopening, but it
was interesting that Pb2+ coordination could induce the ester bond
25 hydrolysis of probe L, while Cu2+ didn’t. Furthermore, the
fluorescence imaging experiment showed the application of probe
L as an imaging reagent for monitoring Pb2+ in living cells. We
anticipate that the present probe could serve as a new tool in
Pb2+ꢀrelated chemical and biological studies.
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30 Acknowledgments
This work was supported by the National Natural Science
Foundation of China (No. 21371153) and Program for Science & 100
Technology Innovation Talents in Universities of Henan Province
(13HASTIT008) and Key Scientific and Technological Project of
35 Henan Province (132102210411) and Zhengzhou University (P.
R. China).
105
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75
Table of Contents
A dual functional probe for “turnꢀon” fluorescence response of Pb2+ and colorimetric detection of Cu2+
based on rhodamine derivative in aqueous media
Min Li, Xiu-Juan Jiang, Hui-Hui Wu, Hong-Lin Lu, Hai-Yang Li, Hong Xu,* Shuang-Quan Zang,* and Thomas C. W. Mak
80 A dual functional probe L based on rhodamine hydroxamate derivative exhibited highly selective fluorescence enhancement response to
Pb2+ and sensitively colorimetric response to Cu2+ in aqueous media. Fluorescence image detection of Pb2+ in living cells displayed an
enhanced fluorescence effect.
8
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