UiO-68-ol NMOF-Based Fluorescent Sensor for Selective Detection of HClO and Its Application in Bioimaging

Article abstract of DOI:10.1021/acs.inorgchem.7b02012

Fluorescent probes are powerful tools for the investigations of reactive oxygen species (ROS) in living organisms by visualization and imaging. As one of the most important of the natural reactive oxygen species (ROS), hypochlorous acid (HClO) plays a crucial role in various physiological and pathological processes. We report herein a new redox-switchable NMOF of UiO-68-ol via a direct ligand modification approach. The obtained UiO-68-ol NPs, which contains organic-based molecular redox switches, exhibit excellent photophysical properties for biological application and can be highly sensitive and selective fluorescent probes to detect HClO species in living cells.

Full text of DOI:10.1021/acs.inorgchem.7b02012

Article
pubs.acs.org/IC
UiO-68-ol NMOF-Based Fluorescent Sensor for Selective Detection of
HClO and Its Application in Bioimaging
Yan-An Li,§,† Song Yang,§,† Qian-Ying Li,† Jian-Ping Ma,† Shaojun Zhang,*,‡ and Yu-Bin Dong*,†
†College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for
Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong
Normal University, Jinan 250014, People’s Republic of China
‡Shandong Qianfoshan Hospital, Jinan 250014, People’s Republic of China
S
* Supporting Information
ABSTRACT: Fluorescent probes are powerful tools for the
investigations of reactive oxygen species (ROS) in living
organisms by visualization and imaging. As one of the most
important of the natural reactive oxygen species (ROS),
hypochlorous acid (HClO) plays a crucial role in various
physiological and pathological processes. We report herein a
new redox-switchable NMOF of UiO-68-ol via a direct ligand
modification approach. The obtained UiO-68-ol NPs, which
contains organic-based molecular redox switches, exhibit
excellent photophysical properties for biological application
and can be highly sensitive and selective fluorescent probes to
detect HClO species in living cells.
probes that are able to quickly detect some target analytes in
living cells with high selectivity and sensitivity.7
INTRODUCTION
■
As one of the most important of the natural reactive oxygen
species (ROS), hypochlorous acid (HClO) plays a crucial role
in various physiological and pathological processes. Endoge-
nous HClO in living cells, such as neutrophils, macrophages,
and monocytes, is known to be generated by myeloperoxidase
(MPO)-catalyzed peroxidation of chloride ions.1 Although it
might be able to injure some invasive microorganisms during
biomolecular chemical modification processes, excessive HClO
can also result in a variety of human diseases such as
inflammatory diseases, cardiovascular diseases, cancer, and
neurodegeneration.2 Therefore, monitoring the level of intra-
cellular HClO species is of great interest and significance for
elucidating the role of HClO in various biological processes.
As a promising method, fluorescence imaging technology is
widely used in monitoring HClO species in living cells due to
its high spatial and temporal resolution.3 So far, small organic
fluorescent molecules might be the most widely used probes to
detect HClO in living systems.4 These reported fluorescent
probes contain HClO recognition functional groups which can
respond to HClO and, furthermore, differentiate it from other
kinds of ROS species. However, some known HClO
fluorescent probes do not have good performance in real-
time monitoring of the HClO level in living systems due to
their delayed response time.5 Therefore, the development of
new types of fluorescent bioprobes in living cells is imperative.
We have initiated a synthetic program for the preparation of
bioprobes based on nanoscale metal−organic frameworks
(NMOFs),6 in which the concepts of organic-based molecular
sensors (OMSs) and NMOFs were combined to yield NMOF
In this contribution, a new redox-switchable UiO-type of
NMOF, UiO-68-ol, which was generated from the function-
alized bicarboxylic acid ligand H2L-ol and Zr(IV) salt (Scheme
1), is reported. It can be a highly sensitive and specific
fluorescence probe to detect HClO in living cells. The design
and synthesis of UiO-68-ol are based on the following reasons.
First, UiO-type MOFs are known to be very stable in aqueous
media, and they possess large internal pores which can facilitate
the accommodation of the OMS and the contact between the
OMS and the target analyte.8 Second, UiO-type NMOFs have
low toxicity9 and possess good cellular biocompatibility and
membrane permeability, which allows them to be suitable for
bioimaging.9 Third, the incorporation of OMSs into periodic
and porous NMOFs via covalent bonding interactions would
effectively prevent not only aggregation-caused quenching
(ACQ)10 but also dye leaching. Hopefully, these types of
NMOF-based probes can exhibit higher sensing and bioimaging
performance in comparison to the corresponding small organic
molecular sensors.
EXPERIMENTAL SECTION
■
Materials and Instrumentation. All of the starting materials were
purchased from commercial sources, unless otherwise noted, and used
without further purification. Infrared (IR) samples were prepared as
KBr pellets, and spectra were obtained in the 400−4000 cm−1 range
Received: August 5, 2017
© XXXX American Chemical Society
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a
Scheme 1. Synthesis of H2L-ol (Top) and UiO-68-ol (Bottom)
a
A photograph of the UiO-68-ol single crystals is given as an insert.
1
using a PerkinElmer 1600 FTIR spectrometer. H NMR data were
8.04−8.02 (d, 4H, −C6H4−), 7.72−7.70 (d, 4H, −C6H4−), 7.18 (s,
2H, −C6H2−).
collected using an AM-400 spectrometer. Chemical shifts are reported
in δ relative to TMS. Fluorescence spectra were obtained with an FLS-
920 Edinburgh Fluorescence Spectrometer with a xenon lamp. The
scanning electron microscopy (SEM) micrographs were recorded on a
Gemini Zeiss Supra TM scanning electron microscope. The X-ray
diffraction (XRD) experiments were obtained on a D8 ADVANCE X-
ray powder diffractometer with Cu Kα radiation (λ = 1.5405 Å). The
Brunauer−Emmett−Teller (BET) surface area was measured on an
ASAP 2020/TriStar 3000instrument (Micromeritics) using nitrogen
adsorption at 77 K. Confocal fluorescence imaging studies were
performed with a TCS SP5 confocal laser scanning microscope (Leica
Co., Ltd. Germany) with an objective lens (×20). The microscope
images of crystals were collected on a Leica DMI3000B inverted
microscope. The UV−vis absorption spectra were recorded on a
Shimadzu UV-3600 spectrophotometer. The crystal data were
obtained with an Agilent SuperNova X-ray single-crystal diffractom-
eter.
The obtained UiO-68-one (10 mg) was further immersed in a PBS
solution of ascorbic acid (VC; 2.5 mg/mL) for 15 min, and the yellow
crystals turned colorless to generate UiO-68-ol′ quantitatively. IR
(KBr pellet, cm−1): 3455 (m), 2671 (w), 1674 (s), 1617(s), 1419 (s),
1395 (s), 1286 (s), 1211 (w), 1180 (m), 1009 (w), 851 (w), 765 (m),
1
562 (w). H NMR (400 MHz, DMSO-d6, 25 °C, TMS, ppm): 12.95
(s, 2H, −COOH), 9.26 (s, 2H, −OH), 8.00−7.98 (d, 4H, −C6H4−),
7.71−7.69 (d, 4H, −C6H4−), 6.93 (s, 2H, −C6H2−).
Synthesis of UiO-68-ol Nanocrystals and Their Redox
Reactions. ZrCl4 (9.6 mg, 0.040 mmol), H2L-ol (14.56 mg, 0.040
mmol), and CH3COOH (240 μL) were dissolved in DMF (3.2 mL).
The tightly capped flasks were kept in an oven at 120 °C under static
conditions. After 24 h, the reaction system was cooled to room
temperature and nanocrystals were obtained (41% yield). The
nanocrystals were suspended in fresh DMF (10 mL) overnight.
After they were soaked in methylene chloride for 3 days, the
nanocrystals were centrifuged and dried at 80 °C. IR (KBr pellet,
cm−1): 3412 (m), 1722 (s), 1614 (s), 1477 (m), 1305 (s), 1286 (s),
1H NMR Measurement on the Digested MOFs. In a typical
procedure, the activated MOF sample (ca. 10 mg) was digested with
sonication in 1 mL of DMSO and 10 μL of 30% HF aqueous solution.
Upon addition of water (ca. 5 mL), the precipitate was washed and
1
1217 (m), 1192 (s), 1055 (m), 801 (s), 709 (s), 517 (w). H NMR
(400 MHz, DMSO-d6, 25 °C, TMS, ppm): 12.97 (s, 2H, −COOH),
9.50 (s, 1H, −OH), 8.02−7.98 (d, 2H, −C6H4−), 7.96−7.93 (d, 2H,
−C6H4−), 7.81−7.79 (d, 2H, −C6H4−), 7.63−7.61 (d, 2H, −C6H4−),
7.08 (s, 1H, −C6H2−), 6.96 (s, 1H, −C6H2−), 3.77 (s, 3H, −CH3).
When a PBS solution (900 μL) of UiO-68-ol (1 mg) was combined
with a PBS solution of NaClO (10−3 M, 100 μL), the mixture changed
from colorless to yellow immediately. The obtained yellow solution
promptly changed back to colorless once a PBS solution of VC (10−3
M, 150 μL) was added. This redox process was further demonstrated
by the emission spectra.
Cell Culture and Treatments. The RAW264.7 cell lines were
provided by the Institute of Basic Medicine, Shandong Academy of
Medical Sciences (People’s Republic of China). The cells were grown
in DMEM (Invitrogen, CA, USA) containing 10% heat-inactivated
newborn calf serum, 100 U/mL penicillin, and 100 mg/mL
streptomycin under an atmosphere of 5% CO2 and 95% air at 37
°C. For confocal fluorescence imaging, cells were incubated in glass-
bottom dishes for 24 h. Cells were incubated at 37 °C with 10 mg/L
UiO-68-ol in PBS (phosphate buffered saline) for 1 h and washed with
PBS, and fluorescence images were captured. In a control experiment,
RAW264.7 cells were pretreated with PMA (phorbol-12-myristate-13-
acetate; 1.5 mg/L) for different times under an atmosphere of 5% CO2
and 95% air at 37 °C and then washed with PBS. These cells were
1
collected for H NMR study.
Synthesis of UiO-68-ol Bulk Crystals and Their Redox
Reactions. ZrCl4 (11.2 mg, 0.048 mmol), H2L-ol (25 mg, 0.07
mmol), and C6H5COOH (222.6 mg, 1.82 mmol) were dissolved in
DMF (3 mL). The tightly capped flasks were kept in an oven at 120
°C under static conditions. After 24 h, the reaction system was cooled
to room temperature and the bulk colorless crystals were obtained in
43% yield. The crystals were suspended in fresh DMF (10 mL)
overnight. After they were soaked in methylene chloride for 3 days, the
crystals were centrifuged and dried. IR (KBr pellet, cm−1): 3412 (m),
1722 (s), 1614 (s), 1477 (m), 1305 (s), 1286 (s), 1217 (m), 1192 (s),
1055 (m), 801 (s), 709 (s), 517 (w). 1H NMR (400 MHz, DMSO-d6,
25 °C, TMS, ppm): 12.97 (s, 2H, −COOH), 9.50 (s, 1H, −OH),
8.02−7.98 (d, 2H, −C6H4−), 7.96−7.93 (d, 2H, −C6H4−), 7.81−7.79
(d, 2H, −C6H4−), 7.63−7.61 (d, 2H, −C6H4−), 7.08 (s, 1H,
−C6H2−), 6.96 (s, 1H, −C6H2−), 3.77 (s, 3H, −CH3).
When UiO-68-ol (10 mg) was immersed in a phosphate buffered
saline (PBS) solution of NaClO (10−3 M, 10 mL, pH 7.4) for 15 min,
the colorless crystals turned yellow to generate UiO-68-one in 95%
yield. IR (KBr pellet, cm−1): 3001 (m), 1702 (s), 1644 (s), 1402 (m),
1286 (s), 1199 (m), 1128 (s), 861 (s), 774 (s), 542 (w). H NMR
(400 MHz, DMSO-d6, 25 °C, TMS, ppm): 13.17 (s, 2H, −COOH),
1
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Figure 1. (a) Redox reactions based on H2L-ol. Photographs of H2L-ol, H2L-one, and H2L-ol′ in PBS under ambient light are given as an insert. (b)
UV−vis spectra of H2L-ol, H2L-one, and H2L-ol′. (c) Fluorescence spectra of H2L-ol, H2L-one, and H2L-ol′ (λex 405 nm, λem 482 nm).
Photographs of H2L-ol, H2L-one, and H2L-ol′ in PBS under UV light are given as an insert.
further loaded with 10 mg/L UiO-68-ol for 1 h and washed with PBS.
In another control experiment, RAW264.7 cells were pretreated with
PMA (1.5 mg/mL) for 30 min under an atmosphere of 5% CO2 and
95% air at 37 °C and then washed with PBS; these cells were loaded
with 10 mg/L UiO-68-ol for 1 h and washed with PBS. These cells
were further loaded with 2 mg/L VC for different times and washed
with PBS. Samples were excited at 405 nm and observed between 500
and 550 nm.
H2L-ol was conbined with HClO (0.1 M, 1 mL of NaClO; 12
M, 5 μL of HCl) in phosphate buffer (PBS, 0.2 M) for 20 min;
H2L-one was generated in ca. 95% yield on the basis of H
1
NMR measurements (Figure 1a and the Supporting
Information). After further treament of the obtained H2L-one
by ascorbic acid (VC) in DMSO at room temerpature for 15
min, the reduced product of H2L-ol′ was quantitatively
1
X-ray Structural Determination.11 X-ray intensity data were
measured at 100 K on an Agilent SuperNova CCD-based
diffractometer (Cu Kα radiation, λ = 1.54184 Å). After a
determination of crystal quality and initial tetragonal unit cell
parameters, a hemisphere of frame data was collected. The raw data
frames were integrated with CrysAlisPro (compiled Aug 2, 2013,
16:46:58). Empirical absorption correction used spherical harmonics,
implemented in the SCALE3 ABSPACK scaling algorithm. Analysis of
the data showed negligible crystal decay during data collection. The
structure was solved by a combination of direct methods and
difference Fourier syntheses and refined by full-matrix least squares
against F2, using the SHELXTL software package. A large void space is
present in the framework, which contains many significant electron
density peaks. The species in this region were too severely disordered
to be modeled and were treated with SQUEEZE/PLATON. The
functional groups at the central benzene ring are statistically
disordered and subject to large thermal vibration and disorder. They
were not resolved in the electron map and have been omitted from the
structural model. The sum formula given is for the actual compound
including the substituents at the central benzene ring, but unresolved
solvate molecules were ignored. A crystal data and structure
determination summary for UiO-68-ol are given in Table S1 in the
Supporting Information. CCDC 1564073 contains the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
obtained on the basis of a H NMR spectrum (Figure 1a and
1
the Supporting Information). In the H NMR spectra, the
single peak for −OH at 9.49 ppm (1H) of H2L-ol disappeared
after the HClO treament, and it reappeared at 9.26 ppm (2H)
after the addition of VC (Supporting Information). In
comparison to H2L-ol and H2L-ol′, the IR spectrum of H2L-
one showed a 1650 cm−1 adsorption derived from the >CO
group, which is reflected in the oxidation restricted to the −OH
and −OCH3 moieties on the central benzene ring (Supporting
Information).
Interestingly, this redox process was accompanied by a visual
colorless to yellow to colorless color change (Figure 1a), which
is auite consistent with the UV−vis spectra (Figure 1b). More
importantly, the fluorescence intensity also went through a
dramatic change during this redox process. As shown in Figure
1c, the strong fluorescence of H2L-ol (λex 405 nm, λem 482 nm)
was almost quenched after treatment with HClO. On the other
hand, the blue emission was largely recovered (ca. 92%) after
the reduction of H2L-one to H2L-ol′ by VC (Figure 1c).
Synthesis and Characterization of UiO-68-ol and Its
Redox Properties. With this redox-switchable ligand H2L-ol
in hand, we designed and prepared the Zr(IV)-MOF based on
it. The combination of H2L-ol and ZrCl4 under solvothermal
conditions (DMF, C6H5COOH, 120 °C, 24 h) afforded bulk
UiO-68-ol colorless crystals in 43% yield (Scheme 1). In
RESULTS AND DISCUSSION
1
■
addition to the H NMR spectrum and thermogravimetric
analysis (TGA, Supporting Information), single-crystal X-ray
diffraction was also used to characterize UiO-68-ol, and this
revealed that UiO-68-ol (C63H48O30Zr3) crystallized in the
Synthesis and Characterization of H2L-ol and Its
Redox Property. As shown in Scheme 1, the p-methylox-
yphenol-containing redox-switchable ligand H2L-ol was pre-
pared through a multistep synthetic processure (Supporting
Information). For examination of its redox-switchable hehavior,
cubic space group Fm3m. It is isostructural with the pristine
̅
UiO-68.
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1
Figure 2. (a) p-Methyloxyphenol/quinone/hydroquinone redox reactions on the bulk crystals monitored by the H NMR (DMSO-d6) spectra
performed on the digested crystals. (b) PXRD patterns of UiO-68-ol, UiO-68-one, UiO-68-ol′, and UiO-68-ol NMOFs in PBS for 24 h. (c)
Dynamic light scattering (DLS) measurement of UiO-68-ol and its SEM image. (d) Photographs of PBS solutions of UiO-68-ol, UiO-68-one, and
UiO-68-ol′ NMOFs under ambient light and on excitation with a mercury lamp. Photographs of UiO-68-ol, UiO-68-one, and UiO-68-ol′ samples
are given as an insert. (e) Fluorescence change of UiO-68-ol PBS solution upon successive addition of HClO and VC (λex 405 nm, λem 475 nm).
Due to the redox-switchable H2L-ol involved, we anticipated
that the generated UiO-68-ol bulk crystals could also feature a
similar fluorescence switching behavior in the solid state. As
expected, UiO-68-ol is indeed a redox-switchable MOF, and its
redox-switching process was monitored on the digested bulk
(δ 3.80 ppm) with respect to the loss of the methyl group,
indicating the generation of the reduced product of UiO-68-ol′
in nearly quantitative yield. This observation confirmed that
UiO-68-ol bulk crystals indeed exhibited redox-switching
properties and the chemical transformation occurred on the
central p-methyloxyphenol and p-benzoquinone rings through
the p-methyloxyphenol/quinone/hydroquinone redox reac-
tions,12 which are the same as those of H2L-ol.
For detecting HClO species in living cells, UiO-68-ol
NMOFs were prepared by using acetic acid instead of benzoic
acid to perform the solvothermal synthetic reaction (DMF,
CH3COOH, 120 °C, 24 h). As shown in Figure 2b, the
obtained UiO-68-ol NMOFs feature the same structural
pattern as that of bulk single crystals on the basis of the
1
crystals by the H NMR spectra. As indicated in Figure 2a, the
1H NMR spectra showed that the −OH proton at δ 9.50 ppm
in UiO-68-ol gradually disappeared (ca. 15 min) upon addition
of NaClO (10−3 M, 100 μL), indicating that UiO-68-one had
formed (ca. 95% yield). After treament of the generated UiO-
68-one by VC (10−3 M, 150 μL) for 15 min, the −OH proton
peak appeared again (δ 9.30 ppm), but the integral area
corresponded to two −OH protons. Meanwhile, basically no
proton resonance for −OCH3 was found in the upfield region
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Figure 3. (a) Time-dependent fluorescence spectra of UiO-68-ol with sequential addition of HClO and VC. (b) N2 adsorption isotherms for UiO-
68-ol, UiO-68-one, and UiO-68-ol′ at 77 K.
Figure 4. (a) Color change of UiO-68-ol (1 mg, 0.2 M PBS, 900 μL) in the presence of various representative ROSs, from left to right: blank, HClO,
1
−
•OH, O2, O2 , H2O2, ONOO−, and NO. [ROS] = 10−3 M. (b) Luminescence spectra of UiO-68-ol (0.1 mg/mL) upon addition of HClO at
different concentrations in PBS, respectively. The emission maximum was observed at 475 nm (λex 405 nm). Ksv = 1.21 × 106 L mol−1. (c)
Luminescence spectra of H2L-ol (0.073 mg/mL) upon addition of HClO at different concentrations in PBS, respectively. The emission maximum
was observed at ca. 482 nm (λex 405 nm). Ksv = 1.30 × 102 L mol−1. Each individual spectrum was recorded after incubation of the probes with the
HClO analyte within 5 s.
PXRD patterns. A scanning electron microscopy (SEM) image
demonstrated that the generated UiO-68-ol NMOF particles
were uniformly distributed (Figure 2c), and their size is
centered at ca. 170 nm, which is well supported by a dynamic
light scattering (DLS) measurement (Figure 2c). As shown in
Figure 2d, when a solution of UiO-68-ol NPs (1 mg) in PBS
(900 μL, 0.2 M) was added to a PBS solution of NaClO (10−3
M), the system turned from colorless to yellow immediately.
Considering that the pKa of HClO is 7.46, we concluded that
UiO-68-ol herein responded to HClO rather than ClO−,
similar to the behavior of other organic molecular fluorescence
probes reported previously.4a,b After addition of VC (10−3 M),
the system promptly turned back to colorless. The redox
transformation on UiO-68-ol NPs is also accompanied by a
drastic fluorescence change. As indicated in Figure 2d,e, UiO-
68-ol NPs in PBS solution showed a bright blue emission at
475 nm, whereas UiO-68-one NPs were basically non-
fluorescent. After further reduction of UiO-68-one, the UiO-
68-ol′ NPs had a blue emission again (recovered by 82%).
Therefore, UiO-68-ol NMOF exhibited the same redox-
switchable phenomenon as those of H2L-ol and UiO-68-ol
bulk crystals and the structural integrity of UiO-68-ol was well
preserved during the redox process, which is well demonstrated
by the PXRD patterns (Figure 2b) and their TEM images
(Supporting Information). In addition, the PXRD pattern
further confirms that the UiO-68-ol NMOF is very stable in
PBS solution (Figure 2b). Such an NMOF would be a good
candidate as a fluorescence probe for bioimaging in living cells.
To our delight, the redox-switching processes on the UiO-
68-ol and UiO-68-one NPs are very fast, and the whole redox
response time is within only ca. 3 s (Figure 3a). We believe this
quick response can be ascribed to the large surface areas of the
NMOFs. As shown in Figure 3b, all three NMOFs exhibited
microporous material features. The Brunauer−Emmett−Teller
(BET) surface areas were found to be 2517.3, 2198.8, and
2302.6 m2/g, respectively, indicating their high porosity.
Logically, the nanosize of the NMOFs, together with their
porous nature, should dramatically increase the probe/analyte
contacting probability and consequently the quick respone.
In addition, the selectivity of UiO-68-ol for other kinds of
ROSs was also investigated. We were pleased to find that UiO-
68-ol was stable toward most other common ROS species, such
•
1
as OH, O2, H2O2, ONOO−, and NO, as presented in Figure
4a. Thus, the oxidation of UiO-68-ol to UiO-68-one is specific
for HClO. As mentioned above, it is a ligand-based redox
process, and only HClO instead of other ROS species can be an
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Figure 5. (left) Overlaid DIC confocal fluorescence images of living RAW264.7 cells. The cells were incubated with 10 mg/L UiO-68-ol in PBS for 1
h (a−c). RAW264.7 cells were pretreated with PMA (1.5 mg/L) for 5 (d−f) and 30 min (g−i) and further incubated with 10 mg/L UiO-68-ol for 1
h. RAW264.7 cells (g−j) were incubated with 2 mg/L VC for 5 (j−l) and 30 min (m−o). Incubation was performed at 37 °C under a humidified
atmosphere containing 5% CO2. The emission wavelengths are 500−550 nm (λex 405 nm). Scale bars are 25 μm. (right) The corresponding mean
fluorescence intensity (MFI) of RAW264.7 cells. The observation indicates that the emission of UiO-68-ol in living cells was quenched by HClO
and further recovered after treatment with VC.
Figure 6. RAW264.7 cells pretreated with NONOate (1,1-diethyl-2-hydroxy-2-nitrosohydrazine; 30 μM)/Cys (30 μM) for 30 min under an
atmosphere of 5% CO2 and 95% air at 37 °C and then washed with PBS. These cells were further loaded with 10 mg/L UiO-68-ol for 1 h and
washed with PBS. Samples were excited at 405 nm and observed between 500 and 550 nm. Scale bars are 10 μm.
effective oxidant to trigger the −OH to >CO transformation
on the ligand, which resulted in the high selectivity of UiO-68-
ol NMOF for HClO (Supporting Information). We next
evaluated the detection limit of UiO-68-ol for HClO under
simulated physiological conditions on the basis of luminescence
spectra. The corresponding emission spectra toward HClO
based on UiO-68-ol are shown in Figure 4b. The reactions of
the UiO-68-ol NMOF with HClO at different concentrations
were carried out, and good linearity between the relative
fluorescent intensities and the concentrations of the HClO
species in the range of 10−7−10−4 M was observed (linear
coefficient 0.9930, Supporting Information), indicating that the
detection limit for HClO on the basis of the proposed method
is ca. 10−7 M. In contrast, the HClO detection limit on the basis
of H2L-ol is only ca. 10−3 M (with a linear coefficient of 0.9914
within (1−6) × 10−3 M, Figure 4c and the Supporting
Information). The much higher sensitivity exhibited by the
NMOFs rationally resulted from the ordered arrangement of
the HClO-reactive groups in the rigid frameworks in the solid
state, which could effectively prevent their aggregate formation
and consequently the ACQ effect. Additionally, the redox
reactions directly occurred on the host framework rather than
the encapsulated dye guest; thus, the dye leaching problem can
be completely avoided. Interestingly, the UiO-68-ol NMOF
can be reused, but according to the UiO-68-ol → UiO-68-one
→ UiO-68-ol′ → UiO-68-one → UiO-68-ol′ → UiO-68-one
cycle (Supporting Information).
Bioimaging of HClO in Living Cells Based on UiO-68-
ol. By taking advantage of the high selectivity and sensitivity,
we finally employed UiO-68-ol NMOFs to image HClO in
living cells. When RAW264.7 cells were incubated with UiO-
68-ol NPs for 1 h, a bright fluorescence in the green channel
was clearly observed, indicating that UiO-68-ol NPs have good
membrane permeability. Notably, the emission intensity of
UiO-68-ol significantly decreased upon stimulation with
phorbol-12-myristate-13-acetate (PMA, 1.5 mg/L). As shown
in Figure 5, the bright green emission gradually dimmed as time
went on, and the emission was almost completely quenched at
30 min, indicating that UiO-68-ol is able to respond to
intracellular HClO and, furthermore, can detect the change in
HClO at different levels under ambient conditions. On the
other hand, the luminescence in the green channel can also be
largely recovered by reduction back to hydroquinone (Figure
5). This redox-based fluorescence “turn-off-on” process was
further demonstrated by the mean fluorescence intensity
(MFI). As shown in Figure 5, the mean fluorescence intensity
F
DOI: 10.1021/acs.inorgchem.7b02012
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(MFI) in RAW264.7 cells decreased by 50 and 91% after
treament with PMA for 5 and 30 min, respectively.
Furthermore, the fluorescence intensity in RAW264.7 cells
was effectively recovered after addition of VC, and it increased
by 50 and 83% at 5 and 30 min, respectively. Such observation
is consistent with that of the redox-switching phenomenon
exhibited by H2L-ol ligand and UiO-68-ol bulk crystals.
To further confirm the selectivity of UiO-68-ol for HClO
species in living cells, the fluorescence response of UiO-68-ol
to RNS and RSS, which are two types of important bioactive
species in living cells, was also examined. NO and cysteine
(Cys) were chosen to perform the bioimaging experiments. As
shown in Figure 6, NO and Cys could not cause an intensity
decrease of green fluorescence with excitation at 405 nm, which
is drastically distinct from the case for HClO. Therefore, NO
and Cys cannot disturb the HClO detection and the UiO-68-ol
MOF probe shows very high specificity to the detection of
HClO in living cells.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the NSFC (Grant
Nos. 21671122 and 21475078) and the Taishan Scholar’s
Construction Project.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02012.
Ligand synthesis, additional characterization data for the
redox reactions, detection limit measurements, and an
ORTEP figure and single-crystal data for CCDC
1564073 (PDF)
Accession Codes
CCDC 1564073 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail for S.Z.: vickie5454@163.com.
*E-mail for Y.-B.D.: yubindong@sdnu.edu.cn.
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ORCID
Yu-Bin Dong: 0000-0002-9698-8863
Author Contributions
§Y.-A.L. and S.Y. contributed equally.
Notes
The authors declare no competing financial interest.
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