Simple quinoline-based "turn-on" fluorescent sensor for imaging copper (II) in living cells

Article abstract of DOI:10.1139/cjc-2014-0011

In this work, a novel fluorescent sensor based on a quinoline derivative was designed and synthesized for detecting copper ions in a near-aqueous media. According to its extra-nuclear structure, copper (II) normally exhibits effectiveness for quenching the singlet excited state of organic chromophores through a fast electron transfer mechanism. However, this molecule displayed a strong fluorescence "turn-on" phenomenon upon addition of Cu²⁺, which was rarely reported in previous research. In addition, density functional theory calculations were adopted to investigate the molecular orbitals as well as the spatial structure. Furthermore, this sensor was applied into living SGC-7901 cells to extend its application in biological systems.

Full text of DOI:10.1139/cjc-2014-0011

1092
ARTICLE
Simple quinoline-based “turn-on” fluorescent sensor for
imaging copper (II) in living cells
Chen Zhou, Ning Xiao, and Yapeng Li
Abstract: In this work, a novel fluorescent sensor based on a quinoline derivative was designed and synthesized for detecting
copper ions in a near-aqueous media. According to its extra-nuclear structure, copper (II) normally exhibits effectiveness for
quenching the singlet excited state of organic chromophores through a fast electron transfer mechanism. However, this
molecule displayed a strong fluorescence “turn-on” phenomenon upon addition of Cu2+, which was rarely reported in previous
research. In addition, density functional theory calculations were adopted to investigate the molecular orbitals as well as the
spatial structure. Furthermore, this sensor was applied into living SGC-7901 cells to extend its application in biological systems.
Key words: fluorescent sensor, copper, quinoline derivative, DFT, cell-imaging.
Résumé : Dans la présente étude, un nouveau capteur fluorescent a` base d’un dérivé de la quinoléine a été élaboré et synthétisé
dans le but de détecter les ions cuivre dans un milieu quasi aqueux. En raison de sa structure extranucléaire, le cuivre (II) s’avère
normalement efficace vis- a` -vis de l’extinction de l’état excité singulet de chromophores organiques par l’intermédiaire d’un
mécanisme de transfert électronique rapide. Cependant, cette molécule a présenté un phénomène important « d’allumage » de
la fluorescence après addition de Cu2 , ce qui a été rarement décrit dans la littérature jusqu’ a` présent. Par ailleurs, les calculs de
densité de la fonctionnelle de la densité (TFD) ont été adoptés pour étudier les orbitales moléculaires et la structure spatiale. En
outre, ce capteur a été utilisé dans des cellules vivantes SGC-7901 en vue d’étendre son champ d’application aux systèmes
biologiques.[Traduit par la Rédaction]
+
Mots-clés : capteur fluorescent, cuivre, dérivé de la quinoléine, TFD, imagerie cellulaire.
9
Introduction
tial value in practical analytical application. Due to the extensive
use of copper and the growing awareness of environmental pro-
tection in modern society, the development of new Cu2 selective
In recent years there has been a growing desire to construct
optical chemosensors for fast and accurate monitoring of haz-
ardous heavy metals. Among different kinds of chemosensors,
a fluorescence-based one is a very advantageous motif for prac-
tical applications on account of its sensitivity, selectivity, and
+
“
turn-on” fluorescent sensor is in great demand.
In the present study, 2-(quinoline-8-yloxy) acetohydrazide (HQ),
a novel simple-structured fluorescent sensor, was successfully
synthesized. The fluoroionophoric properties of HQ were studied
in combination of density functional theory (DFT) and applied in
living SGC-7901cells. This work had elucidated that HQ is a
highly selective and sensitive turn-on fluorescence sensor for
1
rapid response. A common fluorescent sensor would involve the
covalent linking of a “receptor” domain with a fluorescent fragment.
The two components are intramolecularly connected together, such
that the binding of the target analyte causes significant changes
to the photophysical properties of the fluorescent fragment, achiev-
2+
Cu in near-aqueous media. It would provide useful guidance
for the selective recognition of Cu ions based on PET (photo-
2+
2
ing the purpose of identification.
induced electron transfer) mechanism.
Copper, the third most abundant transition metal ion in the
human body, plays a vital role in many fundamental physiolog-
Results and discussion
3
,4
ical processes. Many cytosolic, mitochondrial, and vesicular
oxygen-processing enzymes require copper as a redox cofactor,
but uncontrolled reactions of copper ions with oxygen result in
the formation of reactive oxygen species (ROS), which trigger ox-
idative damage to proteins, nucleic acids, and lipids. A variety of
serious neurodegenerative diseases such as Alzheimers, Parkin-
son’s, Prion, Menkes, and Wilson’s diseases are also closely related
Compound HQ (m.p. 140–142 °C) was easily synthesized with
isolated yield of 69% through the reaction of 8-hydroxyquinoline,
ethyl bromoacetate, and hydrazine (Scheme 1). The structure of
HQ (m.p. 140–142 °C) was confirmed by H NMR (Supplementary
data, Fig. S1), C NMR (Supplementary data, Fig. S2), and LC-Mass
Supplementary data, Fig. S3). The binding mode between HQ and
Cu was investigated by fluorescence spectra, Job’s plot, Benesi–
Hildebrand expression, H NMR titration, and DFT calculations.
1
13
(
2+
5
to the disorder of copper metabolism. As a transition metal ion
known for its efficient fluorescent quenching character, most of
the previous reported cation sensors generally display fluorescent
quenching upon binding with Cu under an electron or energy
transfer mechanism. In terms of sensitivity concerns, sensors ex-
hibiting fluorescence enhancement as a result of metal-ion bind-
1
Cell imaging experiments were carried out to prove the biocom-
patibility of HQ. The sensitivity and selectivity of HQ were dem-
onstrated by means of titration experiments.
2
+
Cu2+-titration and spectral responses
6
–8
ing are favored over those showing fluorescence quenching.
The fluorescent enhanced phenomenon also contributes to poten-
To gain insight into the signaling properties of HQ toward
Cu2 , titration experiments were conducted in a mixed solvent
+
Received 11 January 2014. Accepted 23 May 2014.
C. Zhou, N. Xiao, and Y. Li. Department of Chemistry, Jilin University, Changchun 130021, P. R. China.
Corresponding author: Yapeng Li (e-mail: liyapeng@jlu.edu.cn).
Can. J. Chem. 92: 1092–1097 (2014) dx.doi.org/10.1139/cjc-2014-0011
Published at www.nrcresearchpress.com/cjc on 24 June 2014.

Zhou et al.
1093
Scheme 1. Synthesis of HQ.
Fig. 1. UV–vis absorption response of HQ (0.1 (mmol/L)/L) upon
addition of different concentrations of Cu2+ (0.005 (mmol/L)/L) in
Tris-HCl solution [v(C H OH)/v(H O) = 1:9, pH = 7.2].
2
5
2
(
0.1 mol/L Tris-HCl buffer solution at pH = 7.2, [v(C H OH)/
2 5
v(buffer) = 1:9]). As illustrated in Fig. 1, HQ displayed an absorption
maximum at 319 nm in the absorption spectrum; with the addi-
tion of Cu2 , the absorption band at 319 nm increased until the
first stoichiometry. Moreover, according to the Benesi–Hildebrand
+
1
0–12
(
Fig. 2) expression,
a linear response of HQ as a function of
Cu concentration at 319 nm was observed in the range 5 ␮mol/L:
10 ␮mol/L. The association constant (K ) calculated from UV–vis
2+
–
a
2
+
4
−1
titration of HQ with Cu was 5.716 × 10 mol (R = 0.99407). In the
fluorescence emission, HQ (0.1 (mmol/L)/L) alone exhibited negli-
gible fluorescence (excitated at 319 nm, ⌽HQ = 0.019), whereas
evident enhanced fluorescence intensity was displayed at 384 nm
2
+
Fig. 2. Benesi Hilderbrand plot of HQ with Cu2+
.
(
excitated at 319 nm, ⌽Cu = 0.21, Fig. 3) after adding Cu . Further-
more, as depicted in Scheme 2, the turn-on process could be ob-
served under the irradiation of a UV lamp. In our design of HQ,
there was no separate binding site and fluorophore, so we be-
lieved that the whole molecule was involved in the emission
process. Hence we hypothesized that the obvious fluorescence
enhancement was due to the prevention of nonradiative relax-
2
+
ation pathways of N-lone electron pairs by Cu binding, and the
2
+
1
:1 binding mode between HQ and Cu was determined by the
2
+
Job’s plot evaluated from the fluorescence spectra of HQ and Cu
with a total concentration of 0.2 mmol/L (Fig. 4); this supported
our hypothesis for a 1:1 binding stoichiometry between them.
Remarkably, Fig. 5 illustrated that the fluorescence enhancement
2
+
of HQ increased linearly with the addition of Cu , with a coeffi-
cient R = 0.99853, so we might consider that HQ had a potential
application for the quantitative determination of Cu2 . In addi-
+
−
6
tion, the detecting limit was calculated to be 4.23 × 10 mol/L
from this linear relation (based on DL = KSb /S).
1
2
+
The fluorescence quantum yields of HQ and HQ-Cu were de-
F
A
A
U
S
termined according to the equation: Y ϭ Y ×
×
, where FU
u
S
F
S
U
and F denoted the integral fluorescence intensity of determinand
S
and standard substance, AU and AS denoted the relevant maxi-
mum absorbance of determinand and standard substance, respec-
tively (quinine sulphate was used as reference quantum yield
standard, ␭ex = 410 nm, quantum yield = 0.54 in 0.1 mol/L H SO ).
Fig. 3. Fluorescence spectra of HQ (0.1 (mmol/L)/L) upon addition of
different concentrations of Cu2+ (0.01 (mmol/L)/L) in Tris-HCl
solution [v(C H OH)/v(H O) = 1:9, pH = 7.2] (␭ = 319 nm).
2
5
2
ex
2
4
2
+
The quantum yields for HQ and HQ-Cu calculated from the equa-
tion in aqueous ethanol were 0.019 and 0.21, respectively.
1
To obtain further details of the coordination, we carried out H
NMR titration experiments in DMSO-d . As shown in Fig. 6, upon
6
addition the Cu2 dissolved in D O, the chemical shift of protons
+
2
in HQ changed, especially in the acceptor moiety, with a dramatic
change between ␦ 4.51 and ␦ 4.39 in the spectrum demonstrating
2
+
the existence of proton H (amino proton) during Cu binding.
a
The proton H and H displayed a shift from ␦ 4.76 to ␦ 4.86 and ␦
c
i
8
.91 to ␦ 9.05 with ⌬␦ up to 0.1 and 0.14, respectively. Simultane-
ously, other aromatic protons showed a weak shift. These data
further confirmed that both amino moiety and fluorophore are
involved in the coordination with Cu2 as envisaged.
+
Response of HQ to various metal ions and metal-ion
competition experiments
An important feature of HQ is high selectivity toward Cu2+ in
comparison with other competitive metal ions, the changes of the
fluorometric behavior of HQ (1.0 × 10−4 mol/L) caused by Cu and
2+
Published by NRC Research Press

1094
Can. J. Chem. Vol. 92, 2014
Scheme 2. Solutions of HQ without (left) and with (right) Cu2+
illuminated with a UV lamp and the binding mode of HQ-Cu2+ system.
changes. The competition experiments were also performed for
further study of the interference from other cations in the deter-
2
+
mination of Cu , and they were conducted with addition of
2
+
1
.5 equiv. Cu in HQ solution to induce fluorescence enhance-
ment before mixing 10 equiv. interferential metal ions, as shown
in red bars. The mixtures of HQ with most mentioned of the above
3
+
metal ions still exhibited strong fluorescence, except that Fe
induced a certain degree of quenching. In general, the fluo-
rometric analysis has proven that the flourescence change of HQ
2
+
at 384 nm was specific to Cu
.
The reversibility of the chemosensor is a significant for practi-
cal applications. HQ was alternately exposed to the Cu2 and EDTA
aqueous solution, and the corresponding fluorescence emission
was measured, as illustrated in Fig. 8. Unfortunately, the experi-
ment showed that the emission of HQ could not be restored. The
irreversibility might be attributed to the stronger complexation
+
2
+
Fig. 4. Job’s plot for determining the stoichiometry of HQ and Cu in
2+
Tris-HCl solution [v(C H OH)/v(H O) = 1:9, pH = 7.2] [Cu ] + [HQ], the
total concentration of HQ and Cu was 0.2 (mnol/L)/L (␭ex = 319 nm).
2
5
2
2+
2
+
capability of HQ to Cu
.
DFT calculations
To understand the recognition ability of HQ towards Cu2+, the
HOMO (highest occupied molecular orbitals) and LUMO (lowest
unoccupied molecular orbitals) distributions of HQ were deter-
mined by DFT calculations with B LYP, using the Gaussian 09
3
1
3–19
package.
The 6-31G basis set was used for the H, C, N, and O
atoms; for the Cu atom, the LANL2DZ effective core potential was
2
0,21
employed.
As illustrated in Fig. 9, the DFT calculations showed
the oscillator strength (f), molecular orbital energy levels in the
ground (Fig. 9A), and excited states (Fig. 9B) of HQ and HQ-Cu2
+
,
respectively. An obvious structural difference between HQ and
2+
HQ-Cu is shown in Fig. 10. For HQ, ␲ electrons on both the HOMO
and LUMO in the ground (Fig. 9A) and excited states (Fig. 9B) were
mainly located on the quinoline group in a non-coplanar geometry.
2+
However, when the nitrogen atom lone pair interacts with Cu , the
energy levels of both HOMO and LUMO in the ground and excited
states were lower than those of free HQ, while the ␲ electrons on the
Fig. 5. Normalized response of the fluorescence signal to changing
Cu concentrations.
2+
HOMO and LUMO of the HQ-Cu were located on the whole frame-
2+
work. Furthermore, the lone-pair-carrying N atom exhibited a nearly
2+
planar geometry with the quinoline group in HQ-Cu (Fig. 10), which
prevented the same out-of-phase ␲-interaction conjugation in HQ
(
prevented the nonradiative relaxation pathways of N-lone electron
2+
pairs by binding Cu and blocked photoinduced electron transfer
PET) between 8-hydroxyquinoline and hydrazide moiety). The mo-
(
lecular orbital energy levels lay well below the frontier orbitals of
the fluorophore, and it could not serve as an electron donor for
the purposes of quenching. It was possible to offer a qualitative
rationale for the experimentally observed fluorescence turn-on
phenomenon. Meanwhile, it was comparable to the experimental
results that the maximum absorption wavelength in simulated
2
+
absorption spectra of HQ–Cu was 327 nm. As for the simulated
emission wavelength at 384 nm, it was also in good agreement
with the experimental results.
Detection of Cu2+ in living cells
To explore the potential biological application of this sensor,
2
+
we researched the capability of HQ to track Cu in live SGC-7901
human gastric cancer cell) cell lines. The images were obtained
(
upon irradiation at 365 nm with a band path from 300 to 380 nm
under identical exposure conditions. The living cells were first
incubated with 1 mmol/L HQ in DMF for 30 min at 37 °C in 5% CO2
miscellaneous competing species (1.0 × 10−4 mol/L) in C H OH–Tris
2
5
buffer solution (1: 9, volume ratio, pH = 7.2) were investigated and
the results are shown in Fig. 7. The selectivity experiment is illus-
atmosphere, then washed with phosphate buffered saline (PBS,
2
+
pH = 7.4) 3 times, which was sufficient for the intracellular accu-
mulation of HQ, judged from weak self-fluorescence inside the
living cells (Fig. 11a). However, as shown in Fig. 11b, bright intra-
cellular fluorescence was observed in the cells supplemented with
trated as black bars, only Cu caused a strong flourescence en-
hancement at 384 nm in emission spectrum among common
metal ions (excitated at 319 nm). Meanwhile the introduction of
+
+
2+
2+
2+
2+
2+
3+
other metal ions such as Na , K , Ca , Mg , Ba , Co , Ni , Fe
,
2+
2+
2+
2+
2+
3+
2+
Mn , Hg , Pb , Cd , Zn , Al did not cause any significant
HQ followed by adding 2 mmol/L Cu under the same loading
Published by NRC Research Press

Zhou et al.
1095
Fig. 6. Partial H NMR (300 MHz) spectral changes of HQ (10 ␮mol/L) in DMSO-d upon addition of Cu2+.
1
6
Fig. 7. Fluorescence intensities of HQ (0.1 (mmol/L)/L) in the
presence of various metal ions (black bar) and competition
Conclusion
We have developed a simple turn-on fluorescent chemosensor
experiment (red bar) in Tris-HCl solution [v(C H OH)/v(H O) = 1:9,
2
5
2
2+
for Cu by the conjugation of quinoline, ethyl bromoacetate, and
pH = 7.2] (␭ex = 319 nm, ␭em = 384 nm).
hydrazine. The titration experimental results demonstrated that
HQ had excellent selectivity and sensitivity towards Cu2 over
other metal ions in near-aqueous media, and the specific recogni-
tion of HQ towards Cu was supported by the DFT calculations.
Moreover, the cell-permeable experiment indicated that this sen-
sor could indeed visualize the change of intracellular Cu2 in liv-
ing cells. Thus, it is expected that HQ could serve as a valuable
turn on fluorescent sensor in biomedical and environmental de-
tections.
+
2
+
+
Experimental section
Materials and instruments
All the materials for synthesis were purchased from commer-
cial suppliers and used without further purification. Solvents for
spectra detection was HPLC reagent without fluorescent impurity.
1
13
H and C NMR spectra were taken on a Varian mercury-400
spectrometer with TMS as an internal standard and DMSO as sol-
vent. Melting point was determined using an XT-4 digital melting
point apparatus. LC-MS spectra was analysed on an Agilent 1290-
micro TOF QII. Fluorescence spectra measurements were performed
on a Hitachi F-4500 spectrofluorimeter. The pH measurements were
made with Metteler-Toledo Instruments DELTE 320 pH. Cell experi-
ment were applied on an inverted fluorescence microscope (Olym-
pus IX-70) connected to a digital camera (Olympus, c-5050).
Fig. 8. Curves of fluorescence intensity change for HQ after treated
by aqueous solutions of Cu and Cu +EDTA.
2
+
2+
UV–vis and fluorometric analysis
In titration and selectivity experiments, the stock solution of
HQ was prepared in ethanol. Stock solutions of various cations
were prepared in Tris-HCl buffer solution at pH 7.2, the test sam-
ples were prepared by placing appropriate amounts of ions stock
into matching concentration solution of HQ [v(C H OH)/v(H O) =
2
5
2
1:9, pH = 7.2]. For fluorescence measurements, excitation was pro-
vided at 319 nm, and emission was collected from 320 to 600 nm;
both the excitation and emission slit widths were 5 nm and 5 nm,
respectively.
Preparation of HQ
A mixture of (2.0 g, 14 mmol) 8-hydroxyquinoline and (3.8 g,
2
8 mmol) K CO was added to 50 mL acetonitrile with stirring for
2 3
30 min, then (2.5 g, 15 mmol) ethyl bromoacetate was added. The
mixture was kept stirred for 8 h at room temperature and then
extracted it with CH Cl and H O 3 times, and the organic layer
2
2
2
conditions. The bright-field transmission image of SGC-7901 cells
incubated with HQ (1 mmol/L) were shown in (Fig. 11c). Fig. 11
displays the fluorescence images for the SGC-7901 stained with
was dried on MgSO4 for 12 h before distilling the solvent. The
crude product compound 1 was purified by column chromatogra-
phy [silica gel, v(EtOAc)/v(petroleum ether) = 1:2] and obtained as
red oil with a yield of 80%.
Compound 1 (0.50 g, 2.2 mmol) in 5 mL MeOH was added drop-
wise into 1 mL hydrazine and stirred for 2 h, then filtered the
2
+
22
the sensor before and after being treated with Cu , revealing that
HQ could be a valuable molecular sensor for studying biological
2
+
processes involving Cu within living cells.
Published by NRC Research Press

1096
Can. J. Chem. Vol. 92, 2014
Fig. 9. Frontier molecular orbitals at ground (A) and excited (B) states of HQ and HQ-Cu2+
.
Fig. 10. Calculated energy-minimized structure of free HQ and HQ-Cu2+
.
Fig. 11. Fluorescence microscope imaging of SGC-7901 cells stained with 1 mmol/L HQ before (a) and after treating with 2 mmol/L of Cu2+ (b).
Bright-field transmission image of SGC-7901 cells incubated with HQ (1 mmol/L) are shown in (c).
generated precipitates and washed them with H O and CH Cl
2
(2) Wang, W.; Wen, Q.; Zhang, Y.; Fei, X. L.; Li, Y. X.; Yang, Q. B.; Xu, X. Y. Dalton.
Trans. 2013, 42, 1827. doi:10.1039/c2dt32279j.
2
2
3
times, respectively. The residue was purified through column
(
3) Kaur, N.; Singh, P.; Kaur, S. Tetrahedron 2007, 3, 11724.
chromatography [silica gel, v(C H OH)/v(CH Cl ) = 2:1]. The prod-
2
5
2
2
(4) Hansen, L. M.; Smith, D. J.; Reneker, D. H. J. Appl. Polym. Sci. 2005, 95, 427.
doi:10.1002/app.21117.
uct HQ was collected as white solid with a yield of 69% (Scheme 1).
1
(5) Wang, W.; Yang, Q. B.; Sun, L.; Wang, H. G.; Zhang, C. Q.; Fei, X. L.;
Sun, M. D.; Li, Y. X. J. Hazard. Mater. 2011, 194, 185. doi:10.1016/j.jhazmat.2011.
H NMR (300 MHz DMSO, 25 °C, TMS): ␦ 4.39 (s, 2H), 4.76 (s, 2H),
7
3
.26 (d, J = 6.0 Hz, 1H), 7.53 (d, J = 3.0 Hz, 2H), 7.59 (s, 1H), 8.36 (d, J =
.9 Hz, 1H), 8.91 (d, J = 1.6 Hz, 1H), 9.46 (s, 1H). 13C NMR (75 MHz
DMSO, 25 °C, TMS): ␦ 68.77, 112.19, 121.42, 122.48, 127.23, 129.6,
07.083.
(6) Li, J.; Uversky, V. N.; Fink, A. L. Biochemistry. 2001, 40, 11604. doi:10.1021/
bi010616g.
+
(7) Karr, J. W.; Szalai, V. A. J. Am. Chem. Soc. 2007, 129, 3796. doi:10.1021/
ja068952d.
(8) Jackson, R. K.; Yu, S.; Yao, X. D.; Burdette, S. C. Dalton. Trans. 2010, 39, 4155.
doi:10.1039/c000248h.
136.51, 140.33, 149.88, 154.42, 167.32. LC-M. calcd. [(M+1)] m/z for:
+
C H N O = 218.09, found [(M+1)] m/z = 218.1
1
1
11
3
2
Supplementary data
(9) Loving, G.; Imperiali, B. Bioconjugate. Chem. 2009, 20, 2133. doi:10.1021/
bc900319z.
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Supplementary data are available with the article through the
journal Web site at http://nrcresearchpress.com/doi/suppl/10.1139/
cjc-2014-0011.
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