Formation and rupture of a supramolecular nanocapsule triggered on - Off - on supramolecular switch for Zn2+

Article abstract of DOI:10.1002/ejoc.201201416

A novel selective fluorescence probe that contains 2-p-phenylimidazo[4,5-f] [1,10]phenanthroline and a viologen unit linked by a flexible chain was synthesized and found to be able to select zinc ions with a clear fluorescence enhancement and a large stok

Full text of DOI:10.1002/ejoc.201201416

FULL PAPER
DOI: 10.1002/ejoc.201201416
Formation and Rupture of a Supramolecular Nanocapsule Triggered
ON–OFF–ON Supramolecular Switch for Zn2+
Song Jiang,[a] Man Liu,[a] Yanting Cui,[a] Dapeng Zou,*[a] and Yangjie Wu*[a]
Keywords: Supramolecular chemistry / Host–guest systems / Nanoparticles / Sensors / Fluorescence / Zinc
A novel selective fluorescence probe that contains 2-p-phen-
ylimidazo[4,5-f][1,10]phenanthroline and viologen unit
ment and a large stokes shift in aqueous solution. This probe
can also be designed as an ON ON molecular switch by
a
–OFF–
linked by a flexible chain was synthesized and found to be
able to select zinc ions with a clear fluorescence enhance-
the formation and rupture of a supramolecular nanocapsule
with cucurbit[8]uril (CB[8]) in aqueous solution.
is influenced by the microenvironment of the fluorophore.[8]
Moreover, the larger Stokes shift should improve image
quality because of the reduced excitation interference.
Therefore, the development of novel fluorescent Zn2+ sen-
sors based on typical ICT fluorophores is an attractive and
promising strategy.
Introduction
In recent years, selective and sensitive probes for metal
ions have been greatly exploited because of their simplicity
and high sensitivity, particularly those that are of biological
interest and environmental relevance.[1] Zinc(II), the second
most abundant transition metal ion in the human body af-
ter iron, is known to play significant roles in biological pro-
cesses such as regulators of enzymes, structural cofactors in
metalloproteins, neural signal transmission, and gene ex-
pression.[2–4] This ion is also involved in pathological pro-
cesses such as Alzheimer’s disease, epilepsy, Parkinson’s dis-
ease, amyotrophic lateral sclerosis, ischemic stroke, and in-
fantile diarrhea.[5] As a result, a great number of fluorescent
sensors have been designed to detect and analyze Zinc(II)
ions.[6] A few fluorescent sensors have also been developed
and successfully applied to the imaging of Zn2+ in living
cells or zebrafish embryos.[7] However, to the best of our
knowledge, most of these chemosensors for Zn2+ have some
limitations. For instance, some fluorescent sensors need or-
ganic solvents for Zn2+ detection because of their low
water-solubility.[6a,6f,6h,6k,7a] These reported sensors also
have a shortcoming in that they suffer from interference
Cucurbit[8]uril (CB[8]),[9] a cyclic host molecule compris-
ing eight glycoluril units bridged by 16 methylene groups,
possesses a hydrophobic inner cavity and two restrictive
portals lined with ureido carbonyl groups, which is known
to form host–guest complexes with a range of organic flu-
orophores[10] and cationic species.[11] The host–guest com-
plexes between CB[8] and organic fluorophores can be used
as fluorescent sensors for metal cations[12] or enzymes.[13]
For example, Nau and co-workers reported for the first time
a fluorescent dye displacement approach to the real-time
monitoring of gas binding.[14] A phenanthroline derivative
has recently been reported as a fluorescent sensor for
zinc(II).[15] This research encouraged us to design a water-
soluble probe for Zn2+ by introducing a viologen unit into
the 2-p-phenylimidazo[4,5-f][1,10]phenanthroline. The bi-
pyridinium unit introduced into the probe was expected be
from some heavy and transition metal ions such as Fe3+
Co2+, Ni2+, and Cu2+ [6h]
,
.
When a fluorophore contains an electron-donating group
conjugated to an electron-withdrawing group, it undergoes
intermolecular charge transfer (ICT) from the donor to the
acceptor upon excitation by light. The consequent change
of the dipole moment leads to a larger Stokes shift, which
[a] The College of Chemistry and Molecular Engineering,
Zhengzhou University,
Zhengzhou, Henan 450052, P. R. of China
Fax: +86-371-67763390
E-mail: zdp@zzu.edu.cn
wyj@zzu.edu.cn
Homepage: http://www.zzu.edu.cn/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201416.
Scheme 1. Structure of CB[n] (n = 7, 8) and 1.
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helpful to study the interactions of the sensor with CB[8]. zinc ion makes the phenanthroline moiety electron-poor;
In this paper, we report a novel selective fluorescence probe addition of Zn2+ makes intramolecular charge transfer
1 for Zn2+ that contains 2-p-phenylimidazo[4,5-f][1,10] (ICT) from phenoxyl to phenanthroline easier, resulting the
phenanthroline and a viologen unit linked by a flexible fluorescence enhancement of 1. Moreover, the 205 nm
chain (Scheme 1). Compound 1 was found to select zinc Stokes shift means that interference of the excitation light
ions in water with a clear fluorescence enhancement. It on the emission intensity can be avoided. The larger Stokes
could also be used as an on–off–on molecular switch shift in the emission of 1 after Zn2+ coordination can be
through the formation and rupture of supramolecular nan- explained in terms of the ICT mechanism, and a similar
ocapsules with CB[8] in aqueous solution. The probe could Stokes shift was also observed by Li’s group. The introduc-
be used in a potential fluorescence displacement assay, as tion of a bipyridinium unit into the probe did not change
reported previously.[13–14]
the fluorescent property of the phenanthroline derivative.[15]
Figure 2 shows the variation of the fluorescence intensity at
525 nm of 1 versus the amount of Zn2+ added to the solu-
tion. The binding stoichiometry was determined to be 1:1
by a Job’s plot of the fluorescence data (see the Supporting
Information).
Results and Discussion
1
Compound 1 was synthesized and characterized by H
NMR, 13C NMR, and mass spectra analysis (see the Sup-
porting Information). The spectroscopic properties of 1
were investigated in pure water. As shown in Figure S1, 1
exhibits a maximum absorption peak at 275 nm (ε =
3.8ϫ104 m–1 cm–1). With increasing amounts of Zn2+
(0–5.0 equiv.), the absorbance at 275 nm increased grad-
ually, accompanied by a slight redshift to 279 nm, illustrat-
ing that Zn2+ coordination led to an increase in the elec-
tron-withdrawing ability of phenanthroline and the elec-
tron-donating ability of the phenoxyl group.[16]
The fluorescent response of 1 to zinc ions was then exam-
ined. Figure 1 shows the fluorescence response of 1 to vari-
ous metal ions in aqueous solutions. Free 1 showed weak
fluorescence emission at 470 nm upon excitation at 320 nm
in aqueous solution. Addition of physiologically important
metal ions (Na+, K+, Mg2+, Ca2+) or a range of heavy and
transition metal ions (Mn2+, Cr3+, Fe3+, Ag+, Pb2+) had
little or no effect on the emission of 1. In contrast, addition
of paramagnetic metal ions (Cu2+, Co2+, Ni2+) clearly
quenched the fluorescence. Notably, selective and large flu-
orescent enhancements were observed upon addition of
Cd2+ (13 fold) and Zn2+ (25 fold) to solutions of 1 at
525 nm. The suggested mechanism of fluorescence enhance-
ment by zinc ions should be that the coordination of the
Figure 2. Fluorescence spectra of 1 (14 μm) in water in the presence
of different amounts of Zn2+ (0–10 equiv.).
The influence of pH on the fluorescence of 1-Zn2+ was
determined by fluorescence titration in aqueous solution.
As shown in Figure 3, the fluorescence of 1-Zn2+ at 525 nm
remained unaffected between pH 4 and 7.8, indicating that
the amide NH of the imidazole unit was little influenced by
pH. However, the fluorescence was quenched above pH 8.4.
Figure 1. Variation of the fluorescence intensity at 525 nm (λEx
320 nm) of 1 (14 μm) in water after addition of various metal cat-
ions: 40 equiv. of Na+, K+, Mg2+, Ca2+, 1 equiv. of Cr3+, Mn2+
Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Hg2+ and Pb2+
=
,
Figure 3. Fluorescence intensity (525 nm) of 1-Zn2+ (1:1, 14 μm) at
various pH values in water.
.
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An on–off–on Supramolecular Switch for Zn2+
This response may be due to the stronger complexation of was quenched upon addition of 1 equiv. of CB[8]. The stoi-
OH– with Zn2+ than the complexation of the deprotonated chiometry of the binding of 1-Zn2+ with CB[8] was further
N– with Zn2+. Furthermore, the almost invariable fluores- verified by a fluorescence titration curve. After addition of
cence intensity from pH 6 to 7.8 indicated that 1 is suitable an increasing number of equivalents of CB[8] into an aque-
for potential biological applications.
ous solution of 1-Zn2+ (14 μm), the corresponding fluores-
Competition experiments were performed by monitoring cence spectra changes were recorded; Figure 6 shows a plot
the change in fluorescence intensity at 525 nm (due to the of the fluorescence intensity as a function of total CB[8]
1-Zn2+ complex) upon addition of different metal ions to a concentration. The data can be fitted to a 1:1 binding
solution of 1 and Zn2+ ions (Figure 4). It can be seen that model with a binding constant of 8.26ϫ104 m–1.
Na+, K+, Mg2+, Ca2+, Al3+, Mn2+, Cr3+, Fe3+, Ag+, Pb2+
had a negligible effect on the emission of 1-Zn2+, presum-
ably because these metal ions cannot displace Zn2+ ions
from 1. Since Cu2+, Co2+ and Ni2+ also appear to bind 1,
they quench the fluorescence of the 1-Zn2+ complex. In
brief, sensor 1 has excellent selectivity over most competing
metal ions.
Figure 4. Change in fluorescence intensity at 525 nm of a mixture
of 1 (14 μm) and Zn2+ ions (1 equiv.) in water upon addition of
different metal ions (2 equiv.) (λEx = 320 nm).
It is well-known that CB[8] has strong affinity for the
viologen unit, which led us to believe that CB[8] may affect
the fluorescence of 1. To address this issue, the effect of
CB[8] on the fluorescence of 1-Zn2+ was tested. As shown
in Figure 5, the fluorescence of 1-Zn2+ in aqueous solution
Figure 6. Fluorescence spectra (Top) of 1-Zn2+ (14 μm, 1 equiv. of
Zn2+) in aqueous solution in the presence of different amounts of
CB[8] (0–1.6 equiv.). (Bottom) Plot of I525nm vs. equiv. of CB[8].
Figure 7 displays the absorption spectra of 1-Zn2+ taken
in the course of titration with CB[8]. On addition of CB[8]
to the solution of 1-Zn2+, the intensity of the absorption
maxima at 279 nm decreased, accompanied by a slight red-
shift to 282 nm; the second band at 345 nm increased
slightly with an isosbestic point at 333 nm.
The formation of the 1:1:1 inclusion complex of 1 and
Zn2+ with CB[8] was further confirmed by ESI-MS analysis.
When 1 equiv. of CB[8] was added to a solution of 1-Zn2+
in water, the ESI-MS analysis produced a quadruply
charged peak at m/z 489.6502 (calculated for [1 + Zn2+
+
CB[8] – 2Cl–]4+, 489.6498; see the Supporting Information).
1H NMR spectroscopic analysis was also used to moni-
tor binding interactions. Upon addition of 1 equiv. CB[8]
Figure 5. Visible emission observed from samples of 1, 1 + Zn2+
(14 μm, 1:1), and 1 + Zn2+ + CB[8] (1:1:1, 14 μm).
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located outside the cavity of CB[8]. This binding mode is
similar to the system reported by Sun.[17] In contrast, pro-
tons H1, H2, and H3 in the phenanthroline moiety under-
went a large downfield shift by 0.16–1.21 ppm, indicating
that the phenanthroline unit is located in the deshielding
region outside the carbonyl portal. Possibly, formation of a
supramolecular capsule influences the ICT and quenches
the luminescence of 1-Zn2+. In other words, as shown in
Scheme 2, CB[8] makes charge transfer from the phenoxyl
to the viologen moiety easier than from the phenoxyl to the
phenanthroline group, resulting in quenching of the lumi-
nescence of 1-Zn2+
.
To support the proposed mechanism of the quenching
of the fluorescence of 1-Zn2+ by CB[8], the interaction of
cucurbit[7]uril (CB[7]) with 1 was also investigated. 1H
NMR spectroscopy was used to monitor the binding inter-
actions. As shown in Figure S9, upon addition of 1 equiv.
Figure 7. Absorption spectra of 1-Zn2+ (14 μm) in the presence of
various concentrations of CB[8] (0–1.0 equiv.) in aqueous solution.
1
CB[7] to the solution of 1, in comparison to the H NMR
spectrum of 1 alone, the α1- and β1, β2-protons of the viol-
ogen unit undergo large upfield shifts by 0.03–0.66 ppm,
whereas the alkyl and phenoxyl protons shifted downfield,
indicating that the α1-, β1, and β2-protons of the viologen
unit were located inside the cavity of CB[7], whereas the
alkyl and phenoxyl units were located in the deshielding
region outside the carbonyl portal. The formation of a
stable 1:1 inclusion complex between 1 and CB[7] was fur-
ther confirmed by ESI-MS (see the Supporting Infor-
mation). These experiments indicate that CB[7] preferen-
tially binds to the viologen unit in 1, whereas CB[8] prefer-
entially encloses both the phenoxyl moiety and most parts
of viologen (Scheme 2).
to a solution of 1-Zn2+, the color of the solution turned
from bright-yellow to red. To gain further information on
the nature of the 1:1 inclusion complex, 2D-NMR tech-
niques, including COSY, HSQC and HMBC, were applied
(see the Supporting Information). As shown in Figure 8, in
comparison with the 1H NMR spectra of 1 alone (Figure 8,
A), the α1-, β1-, β2-protons of the viologen unit in 1-Zn2+
-
CB[8] shifted upfield from δ = 8.77, 8.02, 7.81 ppm to 8.76,
7.15, 7.00 ppm, whereas the α2-protons shifted downfield
from 8.22 to 8.33 ppm (Figure 8, C). The protons of the
phenyl moiety in 1-Zn2+-CB[8] shifted upfield from δ = 7.19
and 6.61 ppm to 7.00 and 5.87 ppm, respectively. This result
indicates that the viologen unit folds back to form an intra-
The fluorescence of 1-Zn2+-CB[7] was then tested; upon
molecular charge-transfer complex with the phenyl unit of addition of different amounts of CB[7], the change of fluo-
1 inside the cavity of CB[8], whereas the α2-protons were rescence intensity of 1-Zn2+ in aqueous solution was negli-
Figure 8. 1H NMR spectra of (A) 1 alone; (B) 1-Zn2+; (C) after addition of 1.0 equiv. CB[8] to the solution of 1-Zn2+; (D) after addition
of 7.0 equiv. 1-amantadine hydrochloride (AD) to the solution of 1-Zn2+-CB[8].
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An on–off–on Supramolecular Switch for Zn2+
Scheme 2. Proposed mechanism of fluorescence enhancement by zinc ions, and of quenching by cucurbit[8]uril.
gible (see the Supporting Information), indicating that the plexes of 1-Zn with CB[7] and CB[8]. We anticipate that
ICT process in 1-Zn2+ was not influenced by the addition this probe will be of great benefit to biomedical researchers
of CB[7]. This result supported the proposed mechanism of for studying the bioactivity of Zn2+ in biological systems.
the fluorescence enhancement of 1-Zn2+ and quenching of In addition, this probe might be very useful for providing
the fluorescence of 1-Zn2+-CB[8].
an opportunity to design more advanced supramolecular
To assess the applicability of the capsular complex for its fluorescent switches.
potential applications, we introduced a stronger CB[8]
binder, 1-amantadine hydrochloride (AD), which has a
higher binding constant of (8.19Ϯ1.75)ϫ108 m–1, into the
Experimental Section
solution to rupture the capsule and release 1.[18] Fluores-
Experimental Details: NMR spectra were obtained with a Bruker
cence titration of 1-Zn2+-CB[8] with AD provided the ex-
pected changes; the fluorescence intensity was enhanced
with increasing AD concentration (see the Supporting In-
AVANCE III 400 MHz spectrometer. HRMS were recorded with
an Agilent 6540 Q-TOF spectrometer. Fluorescence spectra were
measured with a JY Horiba FluoroMax-P Spectrofluorometer.
UV/Vis absorbance spectra were measured with an Agilent 8453
UV/Vis spectrometer. The pH values were measured with a Mettler
1
formation). H NMR spectroscopy was also used to moni-
tor the release of 1 (see the Supporting Information). Upon
addition of 1 equiv. AD to the solution of 1-Zn2+-CB[8], all Toledo DELTA 320 pH meter. All materials and solvents employed
were commercially available and used as supplied without purifica-
tion.
of the protons of AD were shifted upfield compared with
those of free AD. Upon addition of 7 equiv. AD to the solu-
1
tion of 1-Zn2+-CB[8] (Figure 8, see part D vs. B), the H
Supporting Information (see footnote on the first page of this arti-
cle): Copies of the 1D and 2D NMR spectra, MS, UV/Vis for in-
clusion complex.
NMR spectrum was similar to that of 1-Zn2+, indicating
that AD located inside the cavity of CB[8] and pushed 1-
Zn2+ out of the CB[8] structure. Addition of excess CB[8]
encased 1-Zn2+ into the cavity again, leading to the original
form of 1-Zn2+-CB[8] and quenching the fluorescence
(switch off). Addition of AD could regenerate the fluores-
cence (switch on). This cycle could be repeated for several
times. Thus, the above experiments clearly showed the col-
Acknowledgments
We are grateful to the National Natural Science Foundation of
China (NSFC) (grant numbers 21172200 and 21101057), the Inno-
vation Fund for Outstanding Scholar of Henan Province (grant
lapse and resumption of the capsular structure, releasing number 0621001100) and the Research Program of Fundamental
and encasing 1-Zn2+, and reaffirmed its suitability for its
projected applications as a supramolecular switch.
and Advanced Technology of Henan Province (grant number
122300413203) for financial support.
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Published Online: March 13, 2013
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