A highly selective turn-on
fluorescent sensor for Cu(
II
)
based on an NSe
2chelating moiety and its application in
living cell imaging
†
Cho-Yen Chou, Shi-Rong Liu and Shu-Pao Wu*
In this study, a boron-dipyrromethene (BODIPY)-basedfluorescent chemosensor CBS was developed for
metal ion sensing. It was found that CBS containing an NSe2moiety exhibited high selectivity for Cu2+
detection while CBS in the presence of Cu2+displayed significant fluorescence enhancement. However,
the metal ions Ag+, Ca2+, Co2+, Cr3+, Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Ni2+, Pb2+, and Zn2+produced only
minor changes in thefluorescence values of the system. The binding constant (Ka) of Cu2+binding to
CBS was found to be 7.28 103M1. The binding ratio of CBS–Cu2+complexes was determined from
the Job plot to be 1 : 1. The maximumfluorescence enhancement caused by Cu2+binding to CBS was
observed over the pH range 5.0–9.0. Additionally, the methyl thiazolyl tetrazolium (MTT) assay
demonstrated the CBS to have low cytotoxicity. Confocal fluorescence microscopy imaging using
RAW264.7 cells showed that CBS could be used as an effective fluorescent probe for detecting Cu2+in
living cells.
Introduction
Copper is third in abundance (aer iron and zinc) among the essential transition metal ions in the human body and plays an important role in various physiological processes.1 Many
proteins contain copper ions in the catalytic center. Copper ions also catalyze the formation of reactive oxygen species (ROS) that can damage lipids, nucleic acids, and proteins. Several studies have related the cellular toxicity of copper ions to serious diseases including Alzheimer's disease,2 Indian childhood
cirrhosis (ICC),3 prion disease,4 and Menkes and Wilson
diseases.5Due to the extensive use of copper in modern society,
copper ions have also become a signicant metal pollutant. The limit of copper in drinking water as set by the US Environmental Protection Agency (EPA) is 1.3 ppm (20 mM).
Several methods have been developed for the detection of copper ions in a sample, such as atomic absorption spectrom-etry,6inductively coupled plasma mass spectroscopy (ICPMS),7
inductively coupled plasma-atomic emission spectrometry (ICP-AES),8and voltammetry.9 Most of these methods require
expensive instruments and cannot be used for on-site detection. As a consequence, more attention is being focused on the
development of uorescent copper sensors that can visualize the cellular distribution of copper ions.10–28
A general strategy used in developing metal ionuorescent sensors is to combine a metal-binding unit with auorophore. The presence of target metal ions is indicated by changes in emission intensity or wavelength when the ions interact with the binding units. Because Cu2+ is known as a uorescence quencher, most uorescent sensors detect Cu2+ through a uorescence quenching process that undergoes an energy or charge transfer mechanism.13,29Due to problems with
sensi-tivity,uorescent sensors that detect metal ions using uores-cence enhancement (turn-on) are easier to be monitored than those using uorescence quenching (turn-off). This paper reports on a newly designed BODIPY-based uorescent enhancement sensor for Cu2+ based on the photoinduced electron transfer (PET) mechanism. Binding Cu2+to the sensor blocks the PET mechanism and greatly enhances the uores-cence of BODIPY.
In this study, a BODIPY-based uorescent sensor was designed for metal ion detection. BODIPY, which stands for boron-dipyrromethene, is auorescent dye with a high molar absorption coefficient and uorescence quantum yield.30It was
used as the signal transduction unit in this study while the NSe2
moiety behaves as a chelator for the metal ion (Scheme 1). The chemosensor CBS (copper binding sensor) exhibits weak uorescence due to uorescence quenching by photoinduced electron transfer from the lone pair electrons on the nitrogen atom to the BODIPY. Binding of metal ions to the chemosensor blocks the PET mechanism and results in a great enhancement
Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 300, ROC. E-mail: [email protected]; Tel: +886-3-5712121 ext. 56506 † Electronic supplementary information (ESI) available:1H and13C NMR spectra
of chemosensor CBS, ESI mass spectra of the CBS–Cu2+ complexes and
DFT-optimized structure of the CBS–Cu2+ complexes calculated with the
B3LYP/LanL2DZ method are included. See DOI: 10.1039/c3an00286a Cite this: Analyst, 2013, 138, 3264
Received 6th February 2013 Accepted 28th March 2013
DOI: 10.1039/c3an00286a
www.rsc.org/analyst
PAPER
Published on 28 March 2013. Downloaded by National Chiao Tung University on 28/04/2014 02:02:36.
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in theuorescence of the BODIPY. The metal ions Ag+, Ca2+,
Cr3+, Co2+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Ni2+, Pb2+, and
Zn2+were tested for metal ion binding selectivity with CBS, but
only Cu2+caused a green emission upon binding with CBS.
Experimental
Chemicals
AgClO4$cH2O and Pb(ClO4)2$3H2O were purchased from
Acros. Cr(ClO4)3$6H2O was purchased from Alfa Aesar.
Ca(ClO4)2$4H2O, Cd(ClO4)2$cH2O, CoCl2$6H2O, Cu(BF4)2$cH2O,
Cu(ClO4)2$cH2O, Fe(BF4)2$6H2O, FeCl3$6H2O, Hg(ClO4)2$cH2O,
Mg(ClO4)$6H2O, Ni(O2CCH3)$4H2O, Zn(BF4)2$cH2O and KBr
were purchased from Sigma-Aldrich. MnSO4$H2O was purchased
from Riedel-de Haen. For all aqueous solutions, deionized water puried by a Millipore system was used.
Apparatus
UV/Vis spectra were recorded on an Agilent 8453 UV/Vis spec-trometer. NMR spectra were obtained on a Bruker DRX-300 NMR and an Agilent Unity INOVA-500 NMR spectrometer. Fluorescence spectra measurements were performed on a Hitachi F-7000 uorescence spectrophotometer. Fluorescent images were taken on a Leica TCS-SP5-X AOBS Confocal Fluo-rescence Microscope.
Synthesis of chemosensor CBS
A mixture of 8-[chloromethyl]-4,4-diuoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene31(1.0 mmol),
bis[2-(phenylselenyl)-ethyl]amine32(1.0 mmol), potassium iodide (0.25 mmol) and
potassium carbonate (1.0 mmol) in 30 mL THF was stirred for 8 h at room temperature under N2atmosphere. Aer ltration, THF
was evaporated under reduced pressure. The crude product was puried by column chromatography (hexane : CH2Cl2¼ 3 : 1) to
give compound CBS as a red solid. Yield: 226 mg (35%); mp: 125– 126C.1H NMR (300 MHz, CDCl3): 7.38 (t, J¼ 2.7 Hz, 4H), 7.22
(m, 6H), 6.05 (s, 2H), 3.94 (s, 2H), 2.96 (d, J¼ 3.9 Hz, 4H), 2.89 (d, J¼ 3.6 Hz, 4H), 2.55 (s, 6H), 2.44 (s, 6H);13C NMR (125 MHz, CDCl3): 155.2, 141.9, 139.3, 133.0, 132.5, 129.6, 129.1, 126.9,
122.2, 52.6, 49.5, 29.6, 29.1, 25.1, 17.2, 14.6; MS (ESI) found 646.1 [M + H]+; HRMS (ESI) calcd for C
29H34BF2N3Se2646.1223
[M + H]+; found, 646.1252 [M + H]+.
Metal ion binding study by UV-vis anduorescence spectroscopy
Chemosensor CBS (15.0 mM) was added with different metal ions (15.0 mM). All spectra were measured in 1.0 mL acetoni-trile–water solution (v/v ¼ 7 : 3, 10 mM Hepes, pH 7.0). The light pathlength of the cuvette was 1.0 cm.
The pH dependence on Cu2+binding in chemosensor 1 studied byuorescence spectroscopy
Chemosensor CBS (15.0mM) was added with Cu2+(15.0mM) in 1.0 mL acetonitrile–water solution (v/v ¼ 7 : 3, 10 mM buffer). The buffers were: pH 3–4, CH3COOH/NaOH; pH 4.5–7.0,
MES/NaOH; pH 7.0–10, Hepes.
Determination of the binding stoichiometry and the apparent association constantsKaof Cu(II) binding in chemosensor
CBS
The binding stoichiometry of CBS–Cu2+ complexes was
deter-mined by Job plot experiments.33Theuorescence intensity at
516 nm was plotted against the molar fraction of CBS under a constant total concentration (30.0 mM). The uorescence approached a maximum intensity when the molar fraction was 0.5. These results indicate that the chemosensor CBS forms a 1 : 1 complex with Cu2+. The association constant (Ka) of CBS–Cu2+
complexes was determined by the consequent equation (1):34
1/(I I0) ¼ 1/{Ka (Imax I0) [Cu2+]} + 1/(Imax I0), (1)
where I is theuorescence intensity at 516 nm at any given Cu2+ concentration and I0is theuorescence intensity at 516 nm in
the absence of Cu2+. The association constant Kawas evaluated
graphically by plotting 1/(I I0) against 1/[Cu2+]. Typical plots
{1/(I I0) vs. 1/[Cu2+]} are shown in Fig. 5. Data were linearly
tted according to eqn (1) and the Kavalue was obtained from
the slope of the line.
Quantum yield determination
The uorescence quantum yields were determined using rhodamine B as a reference with a known F value of 0.49 in EtOH.35The sample and the reference were excited at the same
wavelength (lex ¼ 520 nm), maintaining nearly equal
absor-bance (0.1) and emission spectra. The quantum yield was calculated according to the following eqn (2):
FS/FR¼ (AS/AR) (AbsS/AbsR) (hS2/hR2), (2)
whereFSandFRare theuorescence quantum yields of the
sample and the reference, respectively; ASand ARare the
emis-sion areas of the sample and the reference, respectively; AbsSand
Scheme 1 Synthesis of chemosensor CBS.
AbsRare the corresponding absorbance of the sample and the
reference solution at the wavelength of excitation;hSandhRare
the refractive indices of the sample and the reference, respectively.
Cell culture
RAW264.7 cells were grown in H-DMEM (Dulbecco's Modied Eagle's Medium, high glucose) supplemented with 10% FBS (Fetal Bovine Serum) in an atmosphere of 5% CO2at 37C.
Cytotoxicity assay
The methyl thiazolyl tetrazolium (MTT) assay was used to measure the cytotoxicity of CBS in RAW264.7 cells. RAW264.7 cells were seeded into a 96-well cell-culture plate. Various concentrations (10, 25, 50mM) of CBS were added to the wells. The cells were incubated at 37C under 5% CO2for 24 h. 10mL
MTT (5 mg mL1) was added to each well and incubated at 37C under 5% CO2 for 4 h. The MTT solution was removed and
yellow precipitates (formazan) observed in plates were dissolved in 200mL DMSO and 25 mL Sorenson's glycine buffer (0.1 M glycine and 0.1 M NaCl). A Multiskan GO microplate reader was used to measure the absorbance at 570 nm for each well. The viability of cells was calculated according to the following equation:
Cell viability (%) ¼ (mean of absorbance values of the treatment group)/(mean of absorbance values of the control group).
Cell imaging
The cells cultured in DMEM were treated with 10 mM solution of Cu2+(2 mL; nal concentration: 20 mM) dissolved in steril-ized PBS (pH 7.4) and incubated at 37 C for 30 min. The treated cells were washed with PBS (2 mL 3) to remove the remaining metal ions. DMEM (2 mL) was added to the cell culture, which was then treated with a 10 mM solution of chemosensor CBS (2mL; nal concentration: 20 mM) dissolved in DMSO. The samples were incubated at 37C for 30 min. The culture medium was removed, and the treated cells were washed with PBS (2 mL 3) before observation. Fluorescence imaging was performed with a Leica TCS-SP5-X AOBS Confocal microscope. The cells were excited with a white light laser at 488 nm, and the emission was collected at 530 10 nm.
Computational methods
Quantum chemical calculations based on density functional theory (DFT) were carried out using a Gaussian 09 program. The ground-state structures of CBS and the CBS–Cu2+
complexes were computed using the density functional theory (DFT) method with the hybrid-generalized gradient approximation (HGGA) functional B3LYP. The 6-31G basis set was assigned to nonmetal elements (C, H, N, B, F, and Se). For the CBS–Cu2+ complex, the LANL2DZ basis set was used
for Cu2+, whereas the 6-31G basis set was used for other atoms.
Results and discussion
Synthesis of CBS
The synthesis of the uorescent probe, CBS, is outlined in Scheme 1. CBS was obtained by the reaction of 8-[chloromethyl]-4,4-diuoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene and bis[2-(phenylselenyl)ethyl]amine under weak base condi-tions in THF (tetrahydrofuran). CBS exhibits weakuorescence (F ¼ 0.006) compared to BODIPY.30This is due touorescence
quenching by photoinduced electron transfer from the lone pair electrons on the nitrogen atom to the BODIPY.
Cation-sensing properties
The sensing ability of CBS was tested by mixing it with the metal ions Ag+, Ca2+, Cr3+, Co2+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Ni2+, Pb2+, and Zn2+. Cu2+was the only ion that caused a green emission from CBS (Fig. 1). During Cu2+titration with CBS, a new emission band centered at 516 nm was formed (Fig. 2). Aer adding 1.0 equivalent of Cu2+, the emission intensity
reached a maximum. The quantum yield of the emission band was 0.11, which is 18-fold that of CBS at 0.006. These observa-tions indicate that Cu2+is the only metal ion that readily binds
with CBS, causing signicant uorescence enhancement and permitting highly selective detection of Cu2+.
To study the inuence of other metal ions on Cu2+binding
with CBS, competitive experiments were performed with other metal ions (15.0mM) in the presence of Cu2+(15.0mM) (Fig. 3). It was found that uorescence enhancement caused by the mixture of Cu2+with most metal ions was similar to that caused by Cu2+alone. Smalleruorescence enhancement was observed
Fig. 1 Fluorescence change of CBS (15.0mM) upon addition of various metal ions (15.0mM) in acetonitrile–water (v/v ¼ 7 : 3, 10 mM HEPES, pH 7.0) solutions.
only when Cu2+was mixed with Hg2+, which indicates that Hg2+ competes with Cu2+for binding with CBS. None of the other metal ions were found to interfere with the binding of CBS with Cu2+.
In order to understand the binding stoichiometry of CBS–Cu2+complexes, Job plot experiments were carried out. In
Fig. 4, the emission intensity at 516 nm is plotted against the molar fraction of CBS under a constant total concentration (30.0 mM). Maximum emission intensity was reached when the molar fraction was 0.50. Results indicated a 1 : 1 ratio for
CBS–Cu2+complexes, in which one Cu2+ion was bound to one
CBS. The formation of a 1 : 1 CBS–Cu2+complex was conrmed
by ESI-MS; the peak at m/z¼ 707.8 indicated a 1 : 1 stoichi-ometry for the CBS–Cu2+complex (see Fig. S4 in the ESI†). The
association constant Ka was evaluated graphically by plotting
1/(I I0) against 1/[Cu2+] (Fig. 5). The data were linearlyt and
the Kavalue was obtained from the slope and intercept of the
line. The association constant (Ka) of Cu2+binding in CBS was
found to be 7.28 103M1. The detection limit of CBS as a uorescent sensor for the analysis of Cu2+was determined from
the plot ofuorescence intensity as a function of the concen-tration of Cu2+(see Fig. S5 in the ESI†). CBS was found to have a detection limit of 0.87mM, which means it is able to detect Cu2+concentrations in the micro-molar range.
Fig. 2 Fluorescence response of CBS (15.0mM) to various equivalents of Cu2+in
acetonitrile–water (v/v ¼ 7 : 3, 10 mM HEPES, pH 7.0) solutions. The excitation wavelength was 500 nm.
Fig. 3 Fluorescence response of the chemosensor CBS (15.0mM) to Cu2+
(15.0mM) or 15.0 mM of other metal ions (black bars) and to the mixture of other metal ions (15.0mM) with 15.0 mM of Cu2+(gray bars) in acetonitrile–water
(v/v¼ 7 : 3, 10 mM HEPES, pH 7.0) solutions.
Fig. 4 Job plot of the CBS–Cu2+complexes in an acetonitrile–water (v/v ¼ 7 : 3,
10 mM HEPES, pH 7.0) solution. The total concentration of CBS and Cu2+was
30.0mM. The monitored wavelength was 516 nm.
Fig. 5 Benesi–Hildebrand plot of CBS with Cu2+in CH
3CN/H2O (v/v¼ 7 : 3,
10 mM HEPES, pH 7.0). The excitation wavelength was 500 nm and the observed wavelength was 516 nm. The binding constant was 7.28 103M1for Cu2+
binding in CBS.
Fig. 6 1H NMR 300 MHz spectra of CBS (10.0 mM) upon titration with various
equivalents of Cu2+in CDCl3/DMSO-d6(v/v¼ 9 : 1).
To gain a clearer understanding of the structure of CBS–Cu2+
complexes,1H NMR spectroscopy was employed (Fig. 6). Cu2+is a paramagnetic ion and can therefore affect the proton signals that are close to the Cu2+binding site. The1H NMR spectra of
CBS recorded with increasing amounts of Cu2+show that the
proton (Hd, He, and Hf) signals shied upeld as more and
more Cu2+ was added. This indicates that Cu2+ binds to CBS
mainly through the nitrogen atom and two selenium atoms. The proton signals (Hg, Hh, and Hi) in the phenyl ring also
showed upeld shis upon the addition of Cu2+. This indicated
that Cu2+binds to the selenium atoms that are attached to the phenyl ring and Cu2+ binding affects the ring current at the phenyl ring.
To elucidate the structures of CBS and CBS–Cu2+complexes,
density functional theory (DFT) calculations were undertaken using the Gaussian 09 soware package. Chemosensor CBS and CBS–Cu2+complexes were subjected to energy optimization by
using B3LYP/6-31G and B3LYP/LANL2DZ, respectively. The global minima structures for CBS and CBS–Cu2+complexes are
shown in Fig. 7. The distances of Cu2+ from the two selenium
atoms were 2.47 ˚A and 2.41 ˚A, and from the nitrogen atom was 2.11 ˚A.
Density functional theory (DFT) calculation was also applied to determine the detecting mechanism of CBS for Cu2+. As shown in Scheme 2, the highest occupied molecular orbital (HOMO) of the bis[2-(phenylselenyl)ethyl]amine moiety (elec-tron donor) matches that of theuorophore BODIPY (electron
acceptor); the HOMO energy level (5.18 eV) of the bis[2-(phe-nylselenyl)ethyl]amine moiety is higher than that of the uo-rophore BODIPY (5.73 eV). Consequently, when the BODIPY moiety is excited by light, the intramolecular electron transfer from the bis[2-(phenylselenyl)ethyl]amine moiety to the BOD-IPY moiety is energetically allowed. Hence, the uorescence of the BODIPY moiety is quenched through a PET process (F < 0.01). In contrast, upon the complexation of CBS by Cu2+,
the HOMO energy level of the bis[2-(phenylselenyl)ethyl]amine
Fig. 7 DFT-optimized structures of (a) CBS and (b) CBS–Cu2+complexes. Blue
atom, N; red atom, Cu; yellow atom, Se.
Scheme 2 Energy diagram for the reaction of CBS with Cu2+.
Fig. 8 Fluorescence response (516 nm) of free chemosensor CBS (15.0mM) and after addition of Cu2+(15.0mM) in CH
3CN/H2O (v/v¼ 7 : 3, 10 mM buffer) solution
as a function of different pH values. The excitation wavelength was 500 nm.
Fig. 9 Reversibility of the interaction between CBS and Cu2+by the introduction
of S2to the system.
Fig. 10 Cell viability values (%) estimated by an MTT assay versus incubation concentrations of CBS. RAW264.7 cells were cultured in the presence of CBS (0–50 mM) at 37C for 24 h.
moiety (7.68 eV) is lower than that of the BODIPY unit; therefore, the PET process is restricted and theuorescence of BODIPY is restored.
We performed pH titration of CBS to investigate a suitable pH range for Cu2+sensing. As depicted in Fig. 8, the emission intensities of metal-free CBS were very low. Aer mixing the chemosensor CBS with Cu2+, the emission intensity at 516 nm suddenly increased at pH 5.0 and reached a maximum in the pH range of 5.0 to 9.0. The emission intensity decreases at pH > 9.0. This indicates poor stability of the CBS–Cu2+complexes at high
pH. For pH < 5, the emission intensity is very low due to the protonation of the amine group, which prevents the formation of CBS–Cu2+complexes.
For a chemosensor to be extensively used in the detection of specic targets, the reversibility is an important issue. To explore whether the binding process of the chemosensor CBS with Cu2+ was reversible, an excess amount of S2was added into the solution of the chemosensor CBS with Cu2+. In Fig. 9, the emis-sion peak at 516 nm decreases signicantly aer the addition of S2. When Cu2+was added to the system, theuorescence of CBS was enhanced again. This observation indicated the reversible binding character of the chemosensor CBS with Cu2+.
Living cell imaging
The potential of CBS for imaging Cu2+ in living cells was investigated next. First, an MTT assay with a RAW264.7 cell line was used to determine the cytotoxicity of CBS. In Fig. 10, the cellular viability was estimated to be greater than 80% aer 24 h, which indicates that CBS (<50mM) has low cytotoxicity. Furthermore, the images of cells were obtained using a confocal uorescence microscope. When RAW264.7 cells were incubated with CBS (10mM), no uorescence was observed (Fig. 11a). Aer the treatment with Cu2+, bright green uorescence was observed in the RAW264.7 cells (Fig. 11b). An overlay of uo-rescence and bright-eld images shows that the uouo-rescence
signals are localized in the intracellular area, indicating a subcellular distribution of Cu2+ and good cell-membrane permeability of CBS.
Conclusion
In conclusion, we developed a BODIPY-based uorescent chemosensor for Cu2+ sensing. We observed signicant uo-rescence enhancement with CBS in the presence of Cu2+. However, adding Ag+, Ca2+, Cr3+, Co2+, Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Ni2+, Pb2+, or Zn2+to the chemosensor solution caused
only minimal changes inuorescence emission. The optimal pH range for Cu2+detection by CBS is 5.0–9.0. In addition, the
chemosensor CBS has low cytotoxicity and therefore can be applied for detecting Cu2+in living cells.
Acknowledgements
We gratefully acknowledge the nancial support from the National Science Council (ROC) and National Chiao Tung University.
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