Highly selective fluorescent turn-on-off sensing of OH?, Al3+and Fe3+ions by tuning ESIPT in metal organic frameworks and mitochondria targeted bio-imaging

Article abstract of DOI:10.1039/d1ra03078g

Herein we report a multifunctional high performance metal organic framework (Zn-DHNDC MOF) based chemosensor that displays an exceptional excited state intramolecular proton transfer (ESIPT) tuned fluorescence turn-on-off response for OH^?, Al³⁺and Fe³⁺ions along with mitochondria targeted bio-imaging. Properly tuning ESIPT as well as the hydroxyl group (-OH) allows Zn-DHNDC MOF to optimize and establish chelation enhanced fluorescence (CHEF) and chelation enhanced quenching (CHEQ) based sensing mechanisms. The MOF benefits from acid-base interactions with the ions which generate a turn-on bluish green fluorescence (λ_Em492 nm) for OH^?, an intense turn-on green fluorescence (λ_Em528 nm) for Al³⁺and a turn-off fluorescence quenching response for Fe³⁺ions. The aromatic -OH group indeed plays its part in triggering CHEF and CHEQ processes responsible for the turn-on-off events. Low limits of detection (48 nM of OH^?, 95 nM for Al³⁺, 33 nM for Fe³⁺ions), high recyclability and fast response time (8 seconds) further assist the MOF to implement an accurate quantitative sensing strategy for OH^?, Al³⁺and Fe³⁺ions. The study further demonstrates the MOF's behaviour in cellular medium by subjecting it to live cell confocal microscopy. Along with a bio-compatible nature the MOF exhibited successful accumulation inside the mitochondria of MCF7 cancer cells, which defines it as a significant bio-marker. Therefore the present work successfully represents the multidisciplinary nature of Zn-DHNDC MOFs, primarily in sensing and biomedical studies.

Full text of DOI:10.1039/d1ra03078g

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Highly selective fluorescent turn-on–off sensing of
OHꢀ, Al3+ and Fe3+ ions by tuning ESIPT in metal
organic frameworks and mitochondria targeted
bio-imaging†
Cite this: RSC Adv., 2021, 11, 27787
a
b
*
*
and Arijit Ghosh
Soumya Bhowal
Herein we report a multifunctional high performance metal organic framework (Zn-DHNDC MOF) based
chemosensor that displays an exceptional excited state intramolecular proton transfer (ESIPT) tuned
fluorescence turn-on–off response for OHꢀ, Al3+ and Fe3+ ions along with mitochondria targeted bio-
imaging. Properly tuning ESIPT as well as the hydroxyl group (–OH) allows Zn-DHNDC MOF to optimize
and establish chelation enhanced fluorescence (CHEF) and chelation enhanced quenching (CHEQ)
based sensing mechanisms. The MOF benefits from acid-base interactions with the ions which generate
a turn-on bluish green fluorescence (lEm 492 nm) for OHꢀ, an intense turn-on green fluorescence (lEm
528 nm) for Al3+ and a turn-off fluorescence quenching response for Fe3+ ions. The aromatic –OH
group indeed plays its part in triggering CHEF and CHEQ processes responsible for the turn-on-off
events. Low limits of detection (48 nM of OHꢀ, 95 nM for Al3+, 33 nM for Fe3+ ions), high recyclability
and fast response time (8 seconds) further assist the MOF to implement an accurate quantitative sensing
strategy for OHꢀ, Al3+ and Fe3+ ions. The study further demonstrates the MOF's behaviour in cellular
medium by subjecting it to live cell confocal microscopy. Along with a bio-compatible nature the MOF
exhibited successful accumulation inside the mitochondria of MCF7 cancer cells, which defines it as
a significant bio-marker. Therefore the present work successfully represents the multidisciplinary nature
of Zn-DHNDC MOFs, primarily in sensing and biomedical studies.
Received 20th April 2021
Accepted 4th August 2021
DOI: 10.1039/d1ra03078g
rsc.li/rsc-advances
biological system. Excess Al3+ may lead to various diseases such
as Parkinson's, Alzheimer's encephalopathy etc.6 Since preven-
tive measurements are always rst priority, reliable and accu-
rate techniques for detecting these vital metal ions were highly
necessary.
Introduction
Aluminium (Al) and iron (Fe) are the most abundant (Al ꢁ 8.1%
& Fe ꢁ 5.6%) metals in the earth's crust. These metals in their
ionic forms (Al3+, Fe3+) are not only omnipresent but also vital
for the survival of living beings.1,2 Fe3+ ions mediate several
active processes in both plant and animal biological systems
such as haemoglobin and myoglobin formation, oxygen trans-
port, mitochondrial electron transport, chlorophyll synthesis,
chloroplast maintenance, etc.3,4 Deciency of Fe3+ ions in the
human body can cause reduction of healthy red blood cells or
anaemia, whereas excess Fe3+ in blood may cause liver and heart
diseases, diabetes, etc.5 Al3+ ions, although being present in our
food, water and surroundings, do not actively inuence the
In this report we elaborate on the ESIPT based optical
properties of –OH functionalised Zn-DHNDC MOF to create an
explicit detection strategy for these highly inuential metal
ions. MOFs are crystalline, rigid, porous, mono or multidi-
mensional, organic–inorganic hybrid polymers that are con-
structed with inorganic metal ions and multidentate organic
ligands.7–10 In recent years applications of MOFs have evolved
magnicently, showing potential in various elds such as gas
storage11,12 and separation,13,14 sensing,15–18 catalysis,19,20 drug
delivery,21,22 magnetic studies,23 proton conductivity,24–26 bio-
labelling27 etc. Introducing new functional groups in a frame-
work immensely alters the characteristic properties as well as
applications.28 Optical properties especially uorescence
behaviours are one such aspect that may comprehensively get
modied by incorporating chromophoric functional groups.29
Such kind of uorescent MOFs are widely utilized as potential
chemical sensors,30 photoactive LEDs,31 aromatic explosive
detectors32 etc. Particle size of the MOFs can be further
aSchool of Chemical Sciences, IACS Jadavpur, Kolkata, West Bengal 700032, India.
E-mail: bhowalsoumya@gmail.com; ghosh.arijit.1234@gmail.com
bSchool of Biological Sciences, IACS Jadavpur, Kolkata, West Bengal 700032, India.
E-mail: ghosh.arijit.1234@gmail.com
† Electronic supplementary information (ESI) available: Experimental and
material section, ligand characterisation, optical properties of MOF (UV-VIS, PL,
TCSPC), particle size distribution (TEM, DLS) UV-VIS, PL, TCSPC titrations of
MOF in presence of metal ions and anions, calculation of LOD, recyclability,
exposure time etc. See DOI: 10.1039/d1ra03078g
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stabilized in nano region to form nanoscale MOFs (NMOFs).33 deactivating one, needs higher temperature and greater time
Fluorescent NMOFs having both particle size and optical (24 h) to produce the desired disulpho derivative. The second
properties can be manoeuvred successfully in bio-medical step involving hydroxylation was performed by fusing the
applications as luminescent markers.34 High thermal stability, disulpho derivative with NaOH at around 310–330 C in solid
ꢂ
porous geometry and optical properties of MOFs dene them as state (Fig. 1A). Normal solvo-thermal methods were not appli-
prominent carriers for delivering drugs inside cancer cells.35 cable for this reaction mainly for two reasons. First, the sulpho
ꢀ
Progressive research further demonstrates the capability of group in presence of NaOH became deprotonated (–SO3
)
MOFs acting as contrasting agents in Magnetic Resonance which was highly co-ordinated by solvent medium, making it
Imaging (MRI).36 In photodynamic therapy MOF induced cyto- almost impossible for OHꢀ nucleophile to replace –SO3Na
toxic ROS (reactive oxygen species) was exploited to obtain early group. Second, the highly stable C–S bond needed greater
cancer cell apoptosis.37 MOF based selective labelling of temperature (around 300–320 ꢂC) to cleave which was not
organelles such as mitochondria38 is one of the current trending possible to obtain in normal solvo-thermal conditions.
topics due to its signicant contribution in understanding the Although the ligand possesses two carboxylic acid (–COOH) and
proper functioning of the organelles in cellular system.39,40 two hydroxyl functional groups (–OH), construction of the Zn-
Several MOFs with ESIPT based uorescent characteristics have DHNDC framework involves –COOH group chelating with
been previously reported.41 Although MOF based ESIPT tuned Zn2+ to form the cubic framework leaving the –OH group free in
sensing mechanisms of metal ions42 were rarely explored. To the the pores. In general the presence of proton containing group
best of our knowledge this is the rst report of a MOF demon- such as –OH, –NH2 etc. alongside a proton accepting group such
strating various ESIPT tuned mechanisms for sensing multiple as –C]C, –C]N generates keto–enol tautomerism. Therefore
ions (OHꢀ, Al3+ and Fe3+) and mitochondria targeted bio- upon irradiating such compounds, rapid proton transfers
imaging.
between the –OH group and aromatic –C]C group takes place
Here we report the successful design and applications of which lead to the ESIPT properties.
–OH functionalized Zn-DHNDC framework synthesized with
The synthetic procedure yielded stable cubic orange-red
Zn2+ ion and 4,8-dihydroxynaphthalene-2,6-dicarboxylic acid. crystals. The structure was conrmed by matching the experi-
Structural analysis with powder XRD conrms the cubic IRMOF mentally obtained as-synthesized and activated PXRD data with
topology as well as high phase purity and stability of the the simulated diffraction patterns.43 The PXRD data presented
framework.43 Optical properties of the MOF were measured with in Fig. 1C presenting a well matched diffraction pattern further
absorption, emission and uorescence decay revealed the conrms the classical IRMOF-8 topology. The MOF crystals
presence of inherent ESIPT based optical properties. Further possess a highly symmetrical cubic geometry with space group
investigation of ESIPT in various polar and non-polar solvents Fm3m.43 The SBU Zn4O(ꢀCO2)6 is composed of four tetrahedral
ꢀ
exhibited a facilitated ESIPT behaviour in water due to extended Zn2+ metal ions linking with one m4 oxygen and 6 ligand
polar and H bonding interactions between MOF and solvent carboxylates groups. Thus the Zn4O metal ion cluster act as a 6
medium. Investigation of optical properties in pH 4–10 range point connected node while the ligand acts as a 4 connected
demonstrated an ESIPT suppressed intense bluish green uo- node. The (6, 4) bonding between ligands and SBU nodes
rescence in higher pH solutions which also represented the pH propagate along all three axes resulted the 3D framework. The
responsive characteristics of the MOF. A similar ESIPT sup- framework contained cubic channels as pores lled with
pressed intense turn on bluish uorescence was also observed solvents, moisture and free –OH group. The stability of the MOF
selectively for OHꢀ ions. When subjected to various metal ions in various chemical environments were investigated by per-
on the other hand the MOF showed a CHEF mediated intense forming PXRD of the MOF recovered from hot water, NaOH,
turn on green luminescence for Al3+ ions and a CHEQ assisted HCl, AcOH, NH4OH solutions. Fig. 1D constitutes a similar
uorescence quenching for Fe3+ metal ions. With high recy- curve nature, good stability of the peaks and intensity of all the
clability, low LOD and fast responsiveness the Zn-DHNDC MOF recovered MOF samples which further conrmed the structural
implements an accurate selective and quantitative detection robustness of the MOF. Thermo gravimetric analyses of the as-
strategy for these ions. The MOFs potential as a bio-marker was synthesised and activated MOF revealed their corresponding
exploited by successful cellular uptake manifesting least cyto- thermal stabilizations. The as-synthesized MOF showed a step-
toxicity both at pH-7 and pH-9. Further investigation with live wise weight reduction with increasing temperature. At rst
ꢂ
cell confocal microscopy exhibited localization of the MOF in a weight loss of 15% at around 60–95 C, 14% at around 170–
mitochondrial region. Therefore the scope of application for the 230 ꢂC was observed. These two steps represent the evaporation
Zn-DHNDC MOF is not only limited to naked eye detection of of ethanol, moisture and DMF from the pores of the MOF. A
OHꢀ, Al3+ and Fe3+ ions but it also exceeds in bio-medical eld. considerable weight loss of around 24% was observed at 380–
ꢂ
482 C which signied the degradation of the framework. The
activated MOF on the other hand showed a plot with a single
Results and discussion
step in the region of 360–500 ꢂC signifying the framework
The synthetic procedure of the ligand can be divided into two degradation. Therefore the stability of the framework is up to
segments, rst sulphonation and then hydroxylation. Sulpho- 360 ꢂC (Fig. 3S†). The FT-IR data of the as-synthesized, activated
nation with oleum 65% in 140 ꢂC is a reaction of high yield MOF are exhibited in Fig. 4S.† The gure represents two wide
although being hazardous. The reaction being
a
ring peaks at around 3135 and 3460 cmꢀ1 which can be assigned as
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Fig. 1 (A) Synthetic scheme of 4,8-dihydroxynaphthalene-2,6-dicarboxylic acid; (B) three dimensional structures of the –OH functionalized Zn-
DHNDC MOF drawn from literature report;43 the red balls represent the oxygen, grey balls represent carbon and blue balls represent Zn2+ atoms;
(C) the powder XRD plots of simulated, as-synthesised and activated MOF; (D) PXRD plot of MOF collected from hot water (80 ꢂC), NaOH,
NH4OH, HCl, AcOH solutions.
the O–H stretching frequency of the phenolic hydroxyl group unscathed crystalline MOF particles in nano region. The
and solvent molecules respectively.
Energy-Dispersive X-ray spectroscopy (EDX) of such nano range
The H-bonding interaction between the solvent molecules, MOF particles constitutes the distribution of all the elements of
moisture with linker –OH group broadens the peak represent- the MOF (Fig. 5S†) and thus conrming the stability of the MOF
ing O–H stretching. Other important peaks characterizing the in nano region. The particle size in a solution phase was
ligand of the MOF are the peak at 1670 cmꢀ1 representing the determined with dynamic light scattering (DLS). The hydrody-
symmetric stretching frequency of carboxylate, 1410 cmꢀ1 rep- namic diameter was found around 78–105 nm (Fig. 6S(E)†). A
resenting the asymmetric stretching of carboxylate, 1202 cmꢀ1 high solvent-MOF interaction may attribute for higher particle
representing O–H bending, 1047 cmꢀ1 representing C–OH size observed from DLS compare to TEM. The zeta potential of
stretching frequency, 797 cmꢀ1 representing C]C bending etc. the MOF was observed around ꢀ8 mV (Fig. 20S†).
The sharp peak at around 628 cmꢀ1 can be assigned to metal–
oxygen stretching frequency which represents a successful
formation of the framework (Fig. 3S†). Transmission electron
Optical properties of MOF
microscopy (TEM) micrographs exhibited in Fig. 5S† represents
agglomerated cubic crystalline MOF particles in nano region.
The particle size of the MOF resides in the range of 25–50 nm.
Moreover HRTEM images display an arrangement of lattice
fringes representing the overlapping of crystal lattice planes.
The selected area electron diffraction (SAED) image displays
a denite array of white spots representing the diffraction of the
Spectroscopic methods are the most prominent techniques for
measuring the ESIPT based optical properties. These methods
involve absorbance (UV-VIS), emission (Photoluminescence,
PL), time-correlated single photon counting (TCSPC) etc. The
absorbance of the MOF showed a strong peak around 248 nm,
three small humps at 287, 299, 312 nm and a broad peak at
363 nm respectively. The ligand also displayed a similar curve
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nature with a slight shi towards red region (Fig. 6S†). The solutions were sonicated for 20 min and rested for 30 min
initial four peaks residing at 248, 287, 299, 312 nm represent the before estimating of the optical properties. In 365 nm light the
p–p* transition while the broad peak at 360 nm can be assigned MOF showed a faint bluish green uorescence in water while in
as the n-p* transition.15 The TCSPC was used to determine the the rest of the solvent solutions displayed slight bluish or no
uorescence decay and average lifetime of the MOF. The curve uorescence at all (Fig. 2(C)). The PL data of the MOF in
displayed a bi exponential decay nature with average lifetime of different solvents exhibit diverse emission spectra nature
around 2.97 ꢃ 0.119 ns (Fig. 6S†). The emission of the ligand (Fig. 2(D)). The MOF showed a wide curve with a single peak
showed two broad peaks with almost similar width and inten- having lEm at around 430 nm in dichloromethane. In presence
sity having lEm around 414 and 536 nm situated over a wide of water, acetone, acetonitrile, DMA, MeOH, EtOH, DMF, DMSO
(364–600 nm) range (exited with 350 nm light). The MOF also the MOF displayed two headed curves which indicated the
exhibited a two headed broad emission curve having lEm existence of enol and keto tautomer. MOF in acetone and
around 410 and 503 nm situated at the same wide range (lEx
¼
acetonitrile solutions showed a similar type of curve nature with
350 nm). But for MOF the peak on 410 nm possess a higher a slight shi in position. The lEm of enol tautomer was found
intensity than the other one (Fig. 6S(B)†). The MOF also show- around 423 and 425 nm and keto was around 407 and 410 nm
cased a considerable Stokes shi of 50 nm. Therefore observa- respectively. DMF and EtOH solution of MOF also showed
tions such as a Stokes shi along with a wide two headed curve a similar curve nature and situated in an almost same wide
nature situated over blue to red range only suggested the pres- region having lEm around 410 and 430 nm respectively. In DMA
ence of additional exited state activities or ESIPT phenomenon. the MOF produces two distinctly separate peaks situated at 408
The peak situated at lower (lEm 410 nm) and higher wavelength and 430 nm. In this case the two peaks representing enol and
region (lEm 503 nm) can be assigned to the enol and keto keto are situated 22 nm apart from each other. The MOF in
tautomer respectively.44,45 The presence of ligand to metal DMSO showed a slight red shied curve with two less separated
charge transfer (LMCT) between ligand to d10 metal ion (Zn2+) is peaks resembling the nature of EtOH and DMF. In DMSO the
the reason behind the enhancement of the 410 nm peak. For d10 lEm of the MOF was observed at 419 and 436 nm respectively. In
metal ions such as Zn2+, Cd2+ based MOFs, such LMCT between MeOH, the MOF showed a broad peak with lEm at 428 nm and
carboxylate oxygen orbitals (p) to vacant metal ion orbital (s) is a wide hump having lEm around 540 nm.
a common occurrence. Although d10 metal ion based MOFs were
As discussed earlier the MOF in water exhibits two prom-
previously reported to show such LMCT behaviour,46 the ESIPT inent peaks having lEm at around 410 and 503 nm. The MOF
phenomenon is associated with –OH group modication.47 showing a blue shied obscure peak nature in solvents such as
Therefore observations conclude that optical properties of the acetone, acetonitrile, DMF and ethanol only suggest a weak
Zn-DHNDC MOF are heavily inuenced by the –OH inclusion in interaction between MOF and solvent medium due to which
the framework.
ESIPT characteristics gets hindered. The slight red shied curve
The ESIPT phenomenon was further investigated in different in DMSO along with well dened curve natures in methanol,
polar and non-polar solvents. The experiments was conducted water and DMA are due to presence of internal H bonding and
by dispersing 1 mg of the MOF in 3 mL of solvents such as increasing polar interactions between MOF and solvents that
water, methanol (MeOH), ethanol (EtOH), acetone, dime- stabilize the ESIPT behaviour. Both ligand and MOF showing
thylformamide (DMF), dimethyl sulphoxide (DMSO), dichloro- two properly separated peaks only reects the stability of these
methane (DCM), dimethylacetamide (DMA), acetonitrile. The components in water medium due to such interactions. In
Fig. 2 (A) Fluorescence of MOF (365 nm light) in different solvents such as MeOH, DMA, EtOH, DMSO, water, DCM, DMF, ACN, acetone; (B) the
PL spectra of MOF in the abovementioned solvents; (C) fluorescence of MOF (365 nm light) in pH 4–10 solutions; (D) the PL spectra of MOF in pH
4–10. (E) Energy level diagram representing ESIPT and keto–enol tautomers of the MOF.
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MeOH, the wider peak created at 540 nm suggested the pres- reason a comparatively blue shied intense PL was recorded in
ence of intermolecular H bonding but compare to water it is basic medium which was the reason for the bluish green uo-
weak. MOF in DCM fails to show the traditional behaviours of rescence. In acidic medium on the other hand uorescence
ESIPT due to absent of such interactions. The MOF in ground intensity decreased due to the quenching nature of H+ ions. The
state reside in enol form. When irradiated the ligand also form ESIPT on the other hand get facilitated in acidic conditions,
the keto tautomer due to rapid proton transfer between the two shiing the curve towards red region. Therefore in lower pH
isomers which produce the two headed emission curve. The solutions and acidic medium the MOF showed a faint yellowish
keto tautomers in water are further stabilized by severe inter- green uorescence. In pH-10 due to presence of strong basic
molecular interactions. This coupled with the stability of MOF conditions the MOF dissociates which is responsible for the
found with PXRD (Fig. 1D) makes it a suitable medium for sudden drop in uorescence intensity. The PXRD of MOF
further detection experiments.44,45,47
collected from pH-9 and pH-10 buffer solutions also project
a similar outcome (Fig. 25S†). Therefore the pH responsive
uorescence property of the MOF is highly inuenced by ESIPT
characteristics. The optimum stability of MOF is found to be at
pH responsiveness of the MOF
In a similar method the pH responsiveness of the MOF was pH-9. The zeta potential of the MOF at pH-7 is ꢀ8.10 mV while
measured by subjecting the MOF (1 mg) to different pH solu- at pH-9 it is ꢀ19.2 mV. This phenomenon also hints the pres-
tions ranging from pH 4–10. In 365 nm light (Fig. 2(C)) as the ence of deprotonation in presence of higher pH solutions
pH increased the uorescence colour of the MOF in these (Fig. 36S†).
mediums changed from faint yellowish green to intense bluish
green. The PL curves of the MOF in pH 4–10 range (Fig. 2D)
demonstrated a rapid increase in intensity in pH 4–9 range
Inuence of anions
while a sudden drastic reduction of intensity at pH-10. In acidic The inuence of anionic components on the optical properties
pH solutions (pH ¼ 4, 5) the intensity of the MOF was the lowest was measured by subjecting the MOF (1 mg) in 10ꢀ3 M aqueous
and the overall curve was comparatively red shied with the solution of tetra butyl ammonium (TBA) salts (Fꢀ, Brꢀ, Clꢀ, Iꢀ,
keto lEm around 550 nm. In pH 6 and 7 the overall uorescence OHꢀ, ClO4ꢀ, OAcꢀ, NO3ꢀ, H2PO4ꢀ) of different anions in
intensity increases with the curve shiing a bit towards blue aqueous medium. In 365 nm light the MOF showed an intense
region. For pH solution 8 and 9 the two headed curve nature bluish green uorescence selectively in presence of hydroxyl
change into a wide curve with single peak. Therefore overall (OHꢀ) ions (Fig. 3A). The rest of the solutions did not exhibit
a blue shi of curve was observed with increasing pH medium any alterations of the usual uorescence of the MOF. The
along with the enhancement of intensity. To observe the inu- primary investigation was carried out by obtaining absorbance
ence of acidic and basic medium on the optical properties of MOF in presence of different anions (Fig. 9S†). Apart from
separately the Zn-DHNDC MOF was subjected to PL titration OHꢀ ion, none of the other anions cause any effective change in
with 10ꢀ3 N NaOH and HCl solution were obtained (Fig. 8S†). UV-VIS nature of the MOF. In presence of OHꢀ a red shi of the
For titration with NaOH, the uorescence intensity of the MOF curve was observed as seen in Fig. 3C.
enhanced quite a fold. With gradual addition of NaOH the two
UV-VIS titration with OHꢀ ion displayed a gradual red
headed curve changes into a single headed peak and shis shiing of the curve until saturation point is reached. At satu-
slightly towards the blue region at saturation point. With ration point the pH of the solution remain in 9.2, the p–p* peak
gradual addition of HCl on the other hand the uorescence shied up to 257 nm and the n-p* peak stabilized at around
intensity of the MOF decreased rapidly. In this case gradual 393 nm with addition of around 500 mL of 10ꢀ3 M OHꢀ ions.
addition of HCl caused the overall PL curve to move toward red Equivalent amount of OHꢀ ions are added individually to all the
region as the two headed curve nature remained intact.
anion@MOF solutions. The absorbance plots of all the solu-
Certain conclusions can be made from these experiments. tions exhibit a similar bathochromic shi resembling the
The PXRD represented a good stability of the MOF in HCl and OHꢀ@MOF solution. Next the emission properties were ob-
NaOH medium which was previously established. The acidic tained by conducting PL titrations of the MOF with all the
protons of –OH group are the key component in bestowing the anionic solutions (10ꢀ3 M) (Fig. 11S†). In a similar scenario the
ESIPT phenomenon in the MOF. ESIPT oen leads to reduction MOF only showed a rapid change only in presence of OHꢀ ions.
of uorescence intensity. In high pH solutions and base titra- Titration with Fꢀ, OAcꢀ solutions shows a slight enhancement
tions the acidic protons interact with the base medium only to in intensity (Fig. 11S†). As observed earlier during the PL titra-
form deprotonated MOF.48 Due to such deprotonation the tion with NaOH the OHꢀ titration also showcased an
ESIPT and keto–enol tautomerism was suppressed.48 Therefore enhancement of uorescence intensity along with trans-
the two headed curve nature transforms into a curve with single formation of ESIPT induced two headed curve nature (Fig. 3E).
peak. The deactivation of ESIPT causes reduction of Stokes shi At saturation point (pH-9.2) the MOF displayed around 6 fold
which furthermore resulted in the blue shi of the emission enhancement of uorescence intensity with addition of 500 mL
curves. Due to the deprotonation the delocalised p electron of 10ꢀ3 M OHꢀ solution. Selectivity was obtained by adding
density inside naphthalene aromatic ring increased quite a fold. equivalent amount of OHꢀ to all the anion@MOF solutions.
This phenomenon also increased the charge transfer from lled The PL spectra of all the solutions showed a similar curve nature
ligand orbitals to vacant Zn2+ metal orbitals.46 Due to this to OHꢀ@MOF. The Fluorescence decay measurements in
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Fig. 3 The optical properties of MOF in presence of various anions. (A) The fluorescence of the MOF in presence of anions in 365 nm light; (B)
represents the emission data of the MOF in presence of various anions; (C) UV-VIS titration of the MOF with OHꢀ ions; (D) the PL titration of the
MOF with OHꢀ ions; (E) the fluorescence decay of MOF and the MOF in presence of OHꢀ ions; (F) fluorescence intensity of MOF in anions and
anion-OHꢀ ion mixture.
presence of OHꢀ recorded an increase of average lifetime of the limit of detection (LOD) is measured by using the equation 3s/K
MOF. With addition of 500 mL of 10ꢀ3 M OHꢀ the average life- where s is the standard deviation and K is the slope of the
time of the MOF increased up to 6.7 ꢃ 0.376 ns (Fig. 3D). The uorescence titration curve. The LOD is calculated to be 4.83 ꢄ
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10ꢀ8 M (48 nM) (Fig. 33S†). To obtain the recyclability of the
MOF it is subjected to OHꢀ in 5 cycles. The rst cycle consisted
of obtaining the emission of MOF suspended in OHꢀ ion
solutions. The MOF is then recollected with centrifugation and
washed with water to remove the OHꢀ ions from the surface of
the MOF. Aer measuring the emission of the recovered MOF it
is then again subjected to OHꢀ ion solutions for the second
cycle. All the 5 cycles showcased uorescence enhancement
(Fig. 34S†) although the h cycle featured intensity reduction
of around 10%. The loss of intensity could be due to partial
degradation of the MOF. The PXRD of the MOF recovered from
the 5th cycle showed a good stability of the peaks. The exposure
time was obtained by conducting a time dependent sensing
with 300 mL of OHꢀ anion. Almost 4–4.5 fold enhancement of
uorescence intensity was observed within 5 second time range.
Also within 750 second around 5 fold increase of intensity was
recorded (Fig. 35S†).
Al3+ sensing
At rst the UV-VIS of the MOF in presence of all the above
mentioned metal ions were obtained. In presence of Al3+ the UV-
VIS curve exhibited an overall red shi, especially the n-p* peak.
The UV-VIS titration with 10ꢀ3 M Al3+ displayed a slight bath-
ochromic shi of the p–p* peak up to 250 nm with a slight
decrease of intensity. The n-p* peak on the other hand stabilized
at 367 nm (Fig. 4B) in saturation point. In presence of other metal
ions except Fe3+ the MOF did not show any alterations of UV-VIS
nature. Therefore to obtain the selectivity equivalent amount of
Al3+ ion was added to all the metal ion solutions of the MOF. The
absorbance of all the solutions showed a bathochromic shi and
curve nature similar to Al3+@MOF solution. The PL of the MOF in
presence of all metal ions also provided a similar outcome. In
presence of Al3+ ions, the PL intensity of the MOF enhances quite
a fold (lex – 350 nm). The overall curve shied toward red region
but in this case also the ESIPT induced two headed curve nature
transformed into a curve with single peak (Fig. 4C) at saturation
point.
Higher pH solutions generate OHꢀ ions in aqueous medium.
The OHꢀ ions being strong base, easily deprotonated the MOF
and increase the LMCT between ligand and Zn2+ ions. Other
anions such as OAcꢀ and Fꢀ although possessing strong base
nature could not alter the emission nature of the MOF apart form
a slight enhancement of intensity. The rest of the anions with
comparatively weaker base nature did not cause any effect on
emission of the MOF. Therefore the selectivity lies in the basicity
of the OHꢀ ions as it is the only ion strong enough to suppress
ESIPT. The zeta potential of the MOF in presence of different
anion solutions also exhibit deprotonation. A greater negative
zeta potential than the MOF in neutral water was observed in
presence of anionic solutions. In presence of anions with
stronger base nature such as OHꢀ, OAcꢀ the potential is lowest
(ꢀ24.6 & ꢀ19.6 mV) while for anions with weaker base properties
such as Brꢀ, Iꢀ, ClO4ꢀ ions the potential stays at –10 to ꢀ13 mV
range. Therefore observations conclude that most of the anions
are capable of deprotonating the MOF. The degree of deproto-
nation for anions with weaker base strength is very less to almost
nothing. Anion with stronger base nature however had a higher
degree of deprotonation which lead to the potential becoming
greatly negative. To the best of our knowledge this is the rst
report on an ESIPT based MOF selectively sensing hydroxide ions
in aqueous medium along with high pH mediums.
The PL titration with 10ꢀ3 M Al3+ solution displayed an initial
bathochromic shi of 24 nm and slight enhancement of the keto
peak with addition of 50 mL of Al3+ solution. Then gradual
addition of Al3+ solution caused the intensity of the enol peak to
reduce while the keto peak increases prominently. With addition
of around 650 mL of 10ꢀ3 Al3+, the MOF showed around 3 fold
increase of intensity. At saturation point the lEm of the
Al3+@MOF solution is stabilized at 528 nm. In this case also apart
from Fe3+ ions other metal ions did not cause any alterations in
emission nature of MOF. To obtain the selectivity, equivalent
amount of Al3+ was added to all the MOF-metal ion solutions and
subjected for PL. The PL for all the solutions except Fe3+ featured
a turn on uorescence similar to Al3+@MOF (Fig. 17S†). The
uorescence decay of the MOF in presence of Al3+ showed an
increase of average lifetime. Addition of 500 mL of Al3+ exhibited
an increase in average lifetime up to around 7.52 ꢃ 0.822 ns
(Fig. 4D). The Stern–Volmer constant (Ksv) was calculated
according to the literature.49 The Stern–Volmer constant (Ksv) is
1.65 ꢄ 105 Mꢀ1 for the enhancement of uorescence (Fig. 26S†).
The LOD is calculated as 9.52 ꢄ 10ꢀ8 M (95 nM) (Fig. 31S†) which
fall among one of the lowest reported LODs for Al3+ ions.49–52
The particle size and potential were also altered by the pres-
ence of Al3+ ion. The hydrodynamic diameter of MOF is observed
around 78–105 nm with an average diameter of 89 nm. With
addition of 400 mL of 10ꢀ3 M Al3+ the average diameter of the
MOF particles increased up to around 530 nm (Fig. 4E). The
aqueous dispersed MOF showed a zeta potential of around
ꢀ8.1 mV. With addition of 400 mL of 10ꢀ3 M Al3+ the potential
increased up to 13.7 mV (Fig. 4F). Again the reusability for Al3+
sensing was obtained by conducting the recyclability test for 5
cycles. The uorescence intensity of Al3+@MOF remained stable
throughout the 5 cycles (Fig. 34S†). The PXRD of the MOF
recovered from the h cycle also showed good peak stability
(Fig. 25S†) also corroborates with the recyclability experiment.
The exposure time measured with 300 mL of Al3+ solution showed
1.7–1.9 fold enhancement of uorescent intensity within 8
Metal ion detection properties
While interactions with anions represents the acidic properties
of the –OH group, the MOF also possess a Lewis base character.
The basic properties of the –OH group were further exploited by
subjecting the MOF in various metal ion solutions. 1 mg of MOF
was added to 1 mL of 10ꢀ3 M aqueous solutions of salts con-
taining different metal ions such as Fe3+, Cr3+, Co2+, Ni2+, Mn2+,
Cd2+, Mg2+, Cu2+, Ag+, Al3+, Ba2+, Na+ (all chloride salts) and pH-
7 solution in glass vials. At 365 nm light the MOF exhibited an
intense green uorescence in presence of Al3+ ions while for
Fe3+ ions the MOF showed no uorescence at all. Rest of the
metal ion solutions of MOF did not project any alterations in
optical properties (Fig. 4A). All the following sensing experi-
ments were conducted in pH-7 to maintain a uniform medium.
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Fig. 4 Optical properties of MOF in presence of metal ions and Al3+ ion sensing. (A) Fluorescence of MOF in presence of different metal ions in
365 nm light; (B) UV-VIS titration of MOF by Al3+ ions; (C) PL titration of MOF by Al3+ ions; (D) fluorescence decay titration of MOF by Al3+ ions. (E)
Dynamic light scattering (hydrodynamic diameter size) titration of MOF by Al3+ ions. (F) Zeta potential titration of MOF by Al3+ ions; (G) fluo-
rescence intensity of MOF in presence of Al3+ ions; (H) fluorescence intensity of MOF in presence different metal ions and metal ion-Al3+
solution.
second. In 850 second the Al3+@MOF solution showed up to 2 2 nm) p–p* peak along with a decrease of absorbance. But the
fold enhancement of uorescence (Fig. 35S†).
discrete p–p* and n-p* peaks are heightened in such a way that
the separate discreet peaks merged to become a single peak
with broad shoulders. To observe the alteration of the curve
nature closely, the MOF was subjected to UV-VIS titration with
10ꢀ3 M Fe3+ solution (Fig. 5B). The titration also showed
Fe3+ sensing
The UV-VIS in presence of Fe3+ ions featured some heavy
alterations. The Fe3+@MOF showed a slight red shied (around
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a gradual heightening of discrete p–p* and n-p* peaks only to phenomenon (Fig. 21S†). All the solutions exhibited curve
form the single wide peak (Fig. 5B) situated in a region of 225– natures similar to Fe3+@MOF solution (Fig. 21S†). The PL of the
500 nm range. Again equivalent amount of Fe3+ ions were added MOF in presence of Fe3+ solutions displayed complete
to all the metal ion solutions of MOF to measure the UV-VIS quenching of uorescence (Fig. 5C). PL titration with 10ꢀ3
M
Fig. 5 Fluorescence quenching phenomenon of MOF in presence of Fe3+ ions. (A) Fluorescence of MOF in presence of different metal ions in
365 nm light; (B) UV-VIS titration of MOF by Fe3+ ions; (C) PL titration of MOF by Fe3+ ions; (D) fluorescence decay titration of MOF by Fe3+ ions.
(E) Dynamic light scattering (hydrodynamic diameter size) titration of MOF by Fe3+ ions. (F) Zeta potential titration of MOF by Fe3+ ions; (G)
fluorescence intensity of MOF in presence of Fe3+ ions; (H) fluorescence intensity of MOF in presence different metal ions and metal ion-Fe3+
solution.
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Fig. 6 Possible mechanism of MOF-ion interaction triggering CHEF and CHEQ.
Table 1 TCSPC of MOF and MOF-ions
Amp (a1) ¼
Amp (a2) ¼
Lifetime
(s1/nS)
Lifetime
(s2/nS)
Avg. lifetime
T (nS)
Medium
B1/(B1 + B2)
B2/(B1 + B2)
c2
MOF
0.9256
0.1008
0.3179
0.7076
0.0743
0.9016
0.6821
0.2923
2.528
0.108
3.553
0.459
8.485
8.195
9.379
3.204
1.172
1.150
1.146
1.173
2.971
7.381
7.527
1.261
MOF-OHꢀ
MOF-Al3+
MOF-Fe3+
Fe3+ solution also showed gradual quenching of luminescence 400 mL of 10ꢀ3 M Fe3+ solution the average diameter size
along with a slight red shi (Fig. 5C) whereas the lEm shied increases up to around 1110 nm (Fig. 5E) while the zeta
upto 546 nm. Again the binding selectivity was obtained by potential increases up to 4.75 mV (Fig. 5F). In this case also the
adding equivalent amount of Fe3+ in all the metal ion solution recyclability was observed for 5 cycles. The intensity of the MOF
of MOF. The PL of all the solutions showed an effective showed a slight decline in uorescence intensity. Around 10%
quenching (Fig. 22S†). The corresponding Stern–Volmer curve intensity of MOF reduced in the 5th cycle which could be due to
was found to be linear at lower concentration and nonlinear at partial degradation of the framework (Fig. 33S†) although the
higher concentration which suggested the presence of both corresponding PXRD of the recovered MOF showcased good
dynamic and static quenching mechanisms. The Stern–Volmer stability. Time dependent uorescence measurements with 300
constant (Ksv) is 7.131 ꢄ 106 Mꢀ1 (Fig. 26S†). The LOD is mL 10ꢀ3 M Fe3+ solution exhibited a 65% of intensity reduction
calculated as 3.3 ꢄ 10ꢀ8 M (33 nM) (Fig. 32S†). The calculated within 8 second time. It further reduces up to 72% within 850
LOD for Fe3+ ions are also one of the lowest reported in seconds (Fig. 35S†).
literature.50,53–55
The uorescence decay measurements displayed a slight
Mechanism
reduction in average lifetime (1.26 ns) of the MOF in presence of
lower Fe3+ concentration where in higher Fe3+ concentration the
uorescence intensity reduced rapidly (Fig. 5D). This phenom-
enon indicated formation of a non-uorescent complex in
ground state. The average hydrodynamic diameter of the MOF
in presence of Fe3+ increased rapidly. With addition of around
Our approach to explore the mechanism of MOF-ion interac-
tions are based on the complexation between MOF and ionic
components. These interactions severely affected the ESIPT
based optical properties. Fig. 6 represents the proposed binding
mechanism between MOF and ions. In neutral water the MOF
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Fig. 7 Delivery of –OH functionalized Zn-DHNDC (green) inside mitochondria and partially in cytoplasm shows no cytotoxicity. (A) Repre-
sentative Live cell confocal images showcasing localization of MOF (10 mM) in MCF7 cells as observed by the green fluorescence of the MOF
(green channel). As a mitochondrial marker MitoTracker red (red channel) was used. Right panel exhibits the merged and enlarged images; (B)
quantification of the pixel intensities of the MOFs fluorescence and MitoTracker red along the indicated white line in the merged and enlarged
image (left panel). The white line was drawn arbitrarily along the region of interest (ROI). a.u., arbitrary units; (C) survival curves (MTT assays) of
MCF7 cells treated with MOF at pH-7 for 72 h. The standard deviation (n $ 3) was represented by the error bars. (D) Survival curves (MTT assays) of
MCF7 cells treated with MOF at pH-9 for 72 h. The standard deviation (n $ 3) was represented by the error bars.
showed a less intense faint bluish green uorescence. The lone titration of MOF with OHꢀ and Al3+ ions projected a large
pair of electron on –OH functional groups is involved in bathochromic shi which represent the deprotonation of the
photoinduced electron transfer (PET) process which can reduce –OH functional group.48,56 The hydroxyl proton is the key
the emission intensity quite a bit. It is also noteworthy that component in keto–enol tautomerism and ESIPT. Therefore for
ESIPT can also decrease uorescence intensity.63 The UV-VIS OHꢀ ions the deprotonation lead to suppression of ESIPT which
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enhances the uorescence intensity. Al3+ ions however rst turn on uorescence emission. Rapid aggregation MOF parti-
deprotonated and then successfully chelated with the –OH cles shown in Fig. 4E in presence of Al3+ render it unsuitable for
group.
labelling cancer cells. Therefore all imaging was performed as
With increasing amount of Al3+ the hydroxyl lone pair of the MCF7 cells were incorporated separately with MOF in pH-7
electrons got blocked which evidently resulted in suppression of (neutral pH) and pH-9 (high pH) buffer solutions for 3 hours.
PET and ESIPT simultaneously. Such interaction also activated The cells were also costained with the mitochondrion-specic
CHEF57 causing the enhancement of uorescence intensity. The dye MitoTracker red. The particle size of the MOF in pH-7 and
deactivation of ESIPT with increasing concentration of Al3+ and pH-9 remained in the range of 80–120 nm. The cell survival
OHꢀ ions could also be observed from the PL titrations which assay (Fig. 7C and D) performed to validate cytotoxicity of the
displayed a gradual transformation of the two headed peak MOF revealed no cell killing activity. The uorescent images
nature of MOF. For Fe3+ ions, the mechanism of binding is certainly exhibit an unaltered nuclear morphology without any
similar but the optical phenomenon is quite different due to its signicant deformation conrming the bio-compatibility of the
paramagnetic quenching nature. The UV-VIS titration with Fe3+ MOF inside cellular medium. Upon excitation, the uorescent
showcasing broadening of the plot represented the ligand to particles of MOF were found exclusively inside the mitochon-
metal charge transfer (LMCT).58 The LMCT occurs from electron dria and partially in the cytoplasm. Surprisingly, the MOF
rich orbitals (p) of –OH oxygen to vacant orbitals (s) of Fe3+. Due localization completely excluded the nucleus (Fig. 7A). The
to LMCT a reduction of p electron density can be observed which correlation of the pixel intensities along a line in the two
lead to the drastic reduction of uorescence intensity. The PXRD channels (green and red) clearly substantiated the colocaliza-
collected from the recycled MOF showed good stability of the tion of the MOF inside mitochondria.
peaks. The EDX data of thoroughly washed MOF (Fig. 37S†)
Mitochondria specic labelling mechanisms usually follow
recovered from the sensing experiments showed no presence of two routes. The rst route consists of labelling agents utilizing
Al3+ or Fe3+ ions which rules out the possibility of ion exchange. the negative membrane potential (up to ꢀ180 mV) of the
Therefore the uorescence enhancement or quenching does not mitochondrial matrix. In this case lipophilic type cations such
occur from any kind of ion exchange or framework degradation. as triphenylphosphonium (TPP) were attached with the label-
For Fe3+, the TCSPC data and Stern–Volmer plot suggested the ling agent to bestow a cationic character.61 Furthermore due to
presence of static quenching mechanism where the MOF forms this cationic nature the MOF can easily target the outer
a non-uorescent ground state complex with Fe3+ ions. The membrane of the mitochondria. The second route consists of
TCSPC data is summarised in Table 1.
the labelling agent targeting and binding with the mitochon-
Such results also indicated the presence of CHEQ mediated drial receptors or lipids such as benzodiazepine, cardiolipin
quenching behaviour for Fe3+ ions.59 Metal ions in water reside etc.62 Since the MOF at pH 7 or 9 possess negative zeta potential
in a heavily hydrated solvent sphere which can interfere in therefore we propose that mitochondria labelling by Zn-
interactions between –OH and metal ions. But metal ions pos- DHNDC follow the second route in which the metal ions of
sessing high ionic potential such as Al3+ and Fe3+ ions the MOF binds with these receptor in pH-7 creating a perfect co-
successfully approach and chelate with the –OH groups which localization between MOF and mitotracker. However at higher
also affected the potential of the MOF. Therefore zeta potential pH (pH-9) as the zeta potential of the MOF solution becomes
of MOF turning positive in presence of Al3+ and Fe3+ ions also further negative it could affect the degree of co-localization
hinted complexation between MOF and metal ions. The MOF is observed in Fig. 7B. Together these data suggest that the MOF
also capable of differentiating between Al3+ and Fe3+ ions. is completely non-toxic and retained exclusively inside the
Selectivity between these two metal ions was measured by per- mitochondria and partially in the cytoplasm while excluding the
forming PL titrations of Al3+@MOF with 10ꢀ3 M Fe3+ solution nucleus.
and Fe3+@MOF solution with 10ꢀ3 M Al3+ solution.
The titration of Al3+@MOF with Fe3+ solution showed a rapid
decline of uorescence intensity with the gradual addition of
Conclusions
Fe3+ ions (Fig. 29S†). But for the titrations of Fe3+@MOF with In summary, we developed a novel strategy for ESIPT tuned
Al3+ solution no uorescence enhancement was observed turn-on-off sensing of OHꢀ, Al3+ and Fe3+ ions and mitochon-
(Fig. 30S†). Therefore the MOF showed a higher selectivity for dria targeted bio imaging by hydroxyl functionalized Zn-
Fe3+ ions. The MOF in presence of Cu2+ also showcased para- DHNDC MOF. The exceptional sensing capacity of ZnDHNDC
magnetic quenching60 although compare to Fe3+ it is less.
MOF originates from the –OH group interacting with ionic
Therefore observations conclude the reason behind turn-on-off components and trigger processes such as CHEF and CHEQ.
optical phenomenon is the acid-base complexation based mech- The interactions also lead to suppression of ESIPT which
anism between –OH group of the MOF and ionic components.
especially facilities the turn on uorescence phenomenon for
OHꢀ and Al3+ ions. For Fe3+ ions, the CHEQ assisted quenching
follow the static mechanism where the MOF forms non uo-
rescent ground state complex with Fe3+ ions and trigger LMCT.
Biological studies
Finally the MOF was subjected for live cell confocal microscopy The selectivity and sensitivity is not only limited to versatile
to explore its potential for the subcellular localization. The MOF uorescence phenomenon for each of these ions but also
in presence of high pH solution and Al3+ showed an enhanced expand to low LOD values (48 nM of OHꢀ, 95 nM for Al3+, 33 nM
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for Fe3+ ions), fast response time (8 second) and high recycla- 14 J.-R. Li, J. Sculley and H.-C. Zhou, Chem. Rev., 2012, 112, 869–
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rescent MOFs, ESIPT tuned sensing mechanisms of Al3+ and 15 M. D. Allendorf, C. A. Bauer, R. K. Bhaktaa and
OHꢀ ions by MOFs are rarely reported. In fact to the best of our
R. J. T. Houka, Chem. Soc. Rev., 2009, 38, 1330–1352.
knowledge this is the rst report on sensing of hydroxide ion by 16 D. J. Wales, J. Grand, V. P. Ting, R. D. Burke, K. J. Edler,
an ESIPT based MOF.
C. R. Bowen, S. Mintova and A. D. Burrows, Chem. Soc.
Rev., 2015, 44, 4290–4321.
In cellular medium the MOF showing a selective labelling of
important organelles such as mitochondria also establishes its 17 Z. Hu, B. J. Deibert and J. Li, Chem. Soc. Rev., 2014, 43, 5815–
relevance in bio medical eld. When subjected to live cell 5840.
confocal microscopy with MCF7 cells the MOF showed high 18 Z. Hao, G. Yang, X. Song, M. Zhu, X. Meng, S. Zhao, S. Song
mitochondria accumulation in pH-7 medium along with a bio- and H. Zhang, J. Mater. Chem. A, 2014, 2, 237–244.
compatible nature. Therefore the Zn-DHNDC MOF can be used 19 L. Zeng, X. Guo, C. He and C. Duan, ACS Catal., 2016, 6,
as a long term sensor and bio-marker as it successfully 7935–7947.
demonstrate trace level uorescent turn-on–off sensing of OHꢀ, 20 L. Ma, C. Abney and W. Lin, Chem. Soc. Rev., 2009, 38, 1248–
Al3+, Fe3+ ions in aqueous medium and mitochondria targeted
bio-imaging.
1256.
21 W. Cai, J. Wang, C. Chu, W. Chen, C. Wu and G. Liu, Adv.
Sci., 2019, 6, 1801526.
22 K. M. L. Taylor-Pashow, J. D. Rocca, Z. Xie, S. Tran and
W. Lin, J. Am. Chem. Soc., 2009, 131, 14261–14263.
23 G. Huang, G. Fernandez-Garcia, I. Badiane, M. Camarra,
S. Freslon, O. Guillou, C. Daiguebonne, F. Totti, O. Cador,
T. Guizouarn, B. L. Guennic and K. Bernot, Chem.–Eur. J.,
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Conflicts of interest
There is no conict of interest.
Acknowledgements
SB would like to thank UGC for fellowship. AG would like to 24 F.-M. Zhang, L.-Z. Dong, J.-S. Qin, W. Guan, J. Liu, S.-L. Li,
thank IACS for fellowship. SB and AG also thank Dr Raju
Mondal and Dr Benu Brata Das for their constant help and
support.
M. Lu, Y.-Q. Lan, Z.-M. Su and H.-C. Zhou, J. Am. Chem.
Soc., 2017, 139, 6183–6189.
25 A.-L. Li, Q. Gao, J. Xu and X.-H. Bu, Coord. Chem. Rev., 2017,
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26 W. J. Phang, H. Jo, W. R. Lee, J. H. Song, K. Yoo, B. Kim and
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