Simple and efficient rhodamine-derived VO2+ and Cu2+-responsive colorimetric and reversible fluorescent chemosensors toward the design of multifunctional materials

Article abstract of DOI:10.1039/c8tc06296j

A rhodamine-based derivative, (Z)-2-((3,5-dibromo-2-hydroxybenzylidene)amino)-3′,6′-bis(diethylamino)spiro-[isoindoline-1,9′-xanthen]-3-one (RhDBS), has been chosen as a representative dual chemosensor for VO²⁺ and Cu²⁺ among twenty-

Full text of DOI:10.1039/c8tc06296j

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Fine-tuning of gold nanorod dimensions and plasmonic properties using
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DOI: 10.1039/C8TC06296J
Simple and efficient rhodamine-derived VO2+ and Cu2+-responsive colorimetric
and reversible fluorescent chemosensors toward the design of multifunctional
materials
Daoyong Jianga, Xingying Xuea, Guoning Zhangb, Yucheng Wangb, Han Zhanga, Chao Fengc, Zhifei
Wanga and Hong Zhaoa , *
a School of Chemistry and Chemical Engineering, Southeast University, Nanjing, 211189, China
bInstitute of Medicinal Biotechnology, Chinese Academy of Medical Sciences and Peking Union Medical College,
Beijing, 100050, China
cSchool of Materials and Chemical Engineering, Bengbu University, Bengbu 233030, PR China
*Corresponding author E-mail address: zhaohong@seu.edu.cn
Abstract:
A
rhodamine-based
derivative,
(Z)-2-((3,5-dibromo-2-hydroxybenzylidene)amino)-3',6'-
bis(diethylamino)spiro-[isoindoline-1,9'-xanthen]-3-one (RhDBS) has been chosen as
a
representative dual chemosensor for VO2+ and Cu2+ among the twenty-five Schiff-base receptors. It
has been synthesized by Schiff-base condensation and characterized by standard techniques
including single crystal X-ray crystallography. RhDBS can hypersensitively detect VO2+ via
fluorescence “off-on” type with high quantum yield. Meanwhile, the chemosensor exhibits
Cu2+-selective chromogenic behavior and can detect Cu2+ ions by naked-eye. The detection limits
for VO2+ and Cu2+ have been determined to be 3.65 nM and 0.84 μM respectively, which are much
lower than the allowable maximums set by WHO, demonstrating the reliability and high efficiency
of this chemosensor. Job’s plot and mass spectral analysis show 1:1 stoichiometry between RhDBS
and metal ions. RhDBS has also shown distinct responses to VO2+ through electrochemical (CV)
signals. Additionally, the RhDBS·Cu2+ and RhDBS·VO2+ adducts can act as two secondary
colorimetric and fluorescent sensors for pyrophosphate (PPi) anions. Advanced molecular
memory devices have been constructed through these properties and interpreted. Notably, the
analytical applicability of RhDBS can be further achieved by using test paper strips and silica gel
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2+
DOI: 10.1039/C8TC06296J
plates. More excitingly, RhDBS can be resoundingly employed to trace out the intracellular VO
ions in living cells and it is the first reported smart chemosensor capable of detecting VO2+ in vitro.
In the solid state, RhDBS displays distinguished VO2+ and Cu2+ responsiveness along with obvious
colour changes and different emission wavelengths which lead to large red shifts of the CIE-diagram.
More importantly, some meaningful regular patterns are first discovered through theoretical and
experimental studies toward the design of multifunctional materials.
Introduction
Over the past few decades, the design of optical sensors for selective recognition and sensing of
desired metal ions has been an active research field owing to its importance in physiological
processes.[1] In recent times, numerous optical sensors including rhodamine-modified chemosensors
have been developed for heavy- and transition-metal ions[2] like Cu(Ⅱ),[3] Zn(Ⅱ),[4] Hg(Ⅱ),[5]
Fe(Ⅱ/Ⅲ)[6] due to the extensive use of these metal ions and their subsequent impact on the
environment and human health. Amongst these metal ions, copper is a trace element for sustaining
all known life, and is important in scores of basal physiological processes in the human body.[7] The
deficiency of copper ions can cause varieties of dysfunctions, and can also increase the risk of
coronary heart diseases.[8] However, elevated Cu2+ ions in human body can cause oxidative stress
and leads to neurodegenerative disorders,[9] such as Menkes syndrome, Alzheimer’s disease,
Parkinson’s disease, Wilson’s disease, Prion disease, and toxicity.[10] These potential risk factors
make it indispensable to monitor the amount of copper in environmental and biological samples.[11]
To ensure safety, a detection limit as low as 20 μM (~1.3 ppm) is recommended by the
Environmental Protection Agency (EPA).[12]
Vanadium is an essential ultra-trace bio-element that has captured considerable attention owing to
its fascinating structural characteristics in bioinorganic chemistry as well as in other areas.[13] Its
presence is prevalent in water samples as well as in organs and tissues.[14] Vanadium serves as a
nutrient at low concentrations, but can be toxic at higher concentrations. The National Institute for
Occupational Safety and Health (NIOSH) has set a permissible exposure limit of ~0.7 μM for
vanadium for the excessive vanadium can be immediately hazardous to life and health. As a
biocompatible element, vanadium has numerous excellent biological and physiological functions.
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Varieties of enzymes, including nitrogenases, haloperoxidases and phosphomutases, involve
vanadium, and many of the effects of vanadium are due to inhibition or stimulation of enzymes that
participate in phosphate metabolism.[15] Furthermore, vanadium can replace other divalent metal
ions in enzymes owing to its compatibility with varieties of potential ligand systems. Recently, it has
been disclosed that the oxidovanadium(IV) complex is bioactive[16] and can play the role of
antioxidants.[17] On the other hand, the biological effects of vanadium convert its compounds into
potential therapeutic agents to be used in the treatment of a number of diseases, like heart disease
and malignant tumor.[18] In particular, vanadyl(IV) derivatives show insulin-mimetic activity,[19] thus
they have the potential as medicines to cure diabetes. Therefore, development of an efficient
analysis method for the detection of vanadyl(IV) ions is very important. The detection technique of
chemosensor combined with the fluorescence spectrometry or UV–Vis spectrophotometry is the
most favorable method used for the determination of ionic species because of the adequate
beneficial features such as operational simplicity, quick response, cost-effectiveness and naked-eye
detection.[20] Unfortunately, the cases of independent and efficient colorimetric or fluorometric
chemosensors for the detection of vanadyl ions have scarcely been reported till now. Khajehsharifi
et al[21] have reported an Eriochrome Cyanine R-based colorimetric chemosensor for vanadyl ions in
some real samples in the micromolar range, whereas Selvakumar’s group[22] have described two
Schiff-base fluorescent chemosensors for vanadyl ions with the detection limit in the range of 10-5 to
5 × 10−8 M. Hence the research of available fluorescent chemosensors for vanadyl(IV), especially in
vitro and in vivo is still in its infancy.
As is known to all, rhodamine B derivatives are splendid fluorophores and chromophores, which
show exceptional photophysical properties like long emission and absorption wavelengths and high
fluorescence quantum yield (ΦF).[23] Specifically, the spirolactam ring-closed form is
non-fluorescent and colourless, but it can be converted to the ring-open state in the presence of
external stimuli such as proton acid, metal ion and mechanical force giving rise to strong
fluorescence and pink color.[24] In this content, rhodamine B-based molecular motifs are often
employed in “off-on” fluorescent or colorimetric switch chemosensors. Dual sensor is based on a
single fluorogen that can independently monitor two guest analytes and can provide distinct spectral
responses via the same or different channels;[25] they have already emerged and have gradually
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become an intriguing research focus in the area of fluorescence chemosensors.[26] To date, only a
few rhodamine-based “off-on” chemosensors for dual metal ions (Cu2+/Mn+) have been developed
and reported, including combinations of Cu2+/(Zn2+, Al3+, Cr3+, Ce4+, Fe3+, Hg2+, Pb2+).[8, 27] There
have been very few reports of rhodamine-based chemosensors for dual sensing of VO2+ and Cu2+.
We know that only Yin’s group[28] has first reported a rhodamine-derived salicylidene Schiff-base
for dual sensing of VO2+ and Cu2+ in aqueous methanol solution. Unfortunately, there exists no
previous work that has conducted an in-depth study of the relationship between coordinate structure
and sensing performance and almost all of these reported chemosensors have been rarely examined
in vitro or in vivo. Thus, the development of fluorescence chemosensors with excellent performance
remains a tremendous challenge.
DOI: 10.1039/C8TC06296J
Given the above challenges and our interest in developing new chemosensing platforms, we have
now proposed a series of Schiff-base receptors and a novel rhodamine B-based derivative
conjugated with
a
salicylaldehyde fragment to form
a
dual-function chemosensor,
(Z)-2-((3,5-dibromo-2-hydroxybenzylidene)amino)-3',6'-bis(diethylamino)-spiro-[isoindoline-1,9'-
xanthen]-3-one (RhDBS) for the detection of a metal ion couple Cu2+/VO2+, via absorption and
fluorescence measurements in aqueous medium. Density functional theory (DFT) has been
employed to optimize the geometry of the sensor and determine the structure-coordination of the
sensor towards VO2+ and Cu2+. Furthermore, the RhDBS-based ion responsive test paper strips and
silica gel plates have been prepared and the fluorescent behaviour of the RhDBS towards VO2+ ions
has been successfully exploited for living cells imaging and effectively used in molecular memory
device, which all make it more versatile and widen its fields of applicability. Additionally, RhDBS
can be potentially used as photoluminescent materials for anti-counterfeit printing[29], optical data
storage (ODS)[30], dye lasers[31], and organic light emitting diodes (OLEDs)[32] owing to the dramatic
photophysical properties in solution and the multiple metal ions-responsive behavior in the solid
state.
Experimental section
Materials and instrumentation
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The details about materials and instrumentation used throughout the experiment are displayed in the
ESI.
Scheme 1. Synthetic routes of sensor RhDBS and other rhodamine derivatives
Scheme 2. Model receptors: Schiff-base imines
Synthesis of RhDBS
RhDBS is prepared through the condensation reaction of rhodamine B hydrazide (M1) with
2-hydroxy -3,5-dibromobenzaldehyde in 30 mL absolute ethanol. To an ethanolic solution of
2-hydroxy-3,5-dibromobenzaldehyde (0.21 g, 1.1 mmol), M1 (0.46 g, 1.0 mmol) is added dropwise
and then three drops of glacial acetic acid. The resulting mixture is kept at 25°C with continuously
stirring for 2 h in a three-necked 50 mL RBF under N2 atmosphere. Reaction is monitored by TLC
(petroleum ether/CH2Cl2, 10:1, v/v). After completion of the reaction, the precipitate is filtered off
and then washed with cold ethanol for several times. The product is recrystallized and dried under
1
vacuum to give a yellowish solid in 86% yield. RhDBS: H NMR (DMSO-d6, 300 MHz) δ (ppm):
1.10 (t, 12H, NCH2CH3, J = 6 Hz), 3.35 (q, 8H, NCH2CH3, J = 6 Hz), 6.37 (dd, 2H, Xanthene-H,
J1= 1.8 Hz, J2= 9 Hz ), 6.47 (d, 4H, Xanthene-H, J = 12 Hz), 7.15 (d, 1H, Phen-H, J = 9 Hz ), 7.55
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(d, 1H, Phen-H, J = 2.1 Hz ), 7.68 (q, 2H, Phen-H, J = 9 Hz), 7.71 (d, 1H, Phen-H, J = 3 Hz), 7.96
13
(d, 1H, Phen-H, J = 6 Hz), 9.04(s, 1H, NCH), 11.40(s, 1H, OH); C NMR (300 MHz, DMSO-d6):
163.65, 153.05, 152.60, 150.83, 148.70, 148.62, 135.71, 134.26, 131.93, 128.90, 127.90, 127.42,
123.79, 123.12, 121.05, 113.97, 111.13, 110.43, 108.26, 104.21, 97.47, 43.51, 12.22; FT-IR (KBr,
ν/cm−1): 3420 (OH); 1720 (C=O, carbonyl); 1633 (C=N); 1406, 1375(C-N); 1234, 1216(C-O);
ESI-MS, m/z: calcd for C35H34Br2N4O3, 718.462; found, [M+H]+ 719.10321. The other Schiff-base
receptors are also easily prepared by condensation reaction and fully characterized (for the detailed
procedures and characterization data, see the experimental section as described in ESI).
X-ray crystallography of RhDBS
The detailed experimentation procedures of X-ray crystallographic data collection and refinement
kindly consult ESI section.
Fig. 1. Single crystal X-ray structure of RhDBS
The qualified single crystal of RhDBS is grown in acetonitrile-acetone (v/v, 4:1) mixed solution.
The crystal structure clearly discloses the unique spirolactam ring, with the lactam unit and xanthene
moiety form a vertical plane (Fig.1), breaking the conjugation of the whole molecular structure
which leads to the colorlessness and non-fluorescence of the free chemosensor RhDBS.
Cell culture and fluorescence imaging
MCF-7 cells (human breast carcinoma) are maintained in confocal dishes (1 × 104 cell/dish) at
37 °C in 95 % air with 5 % CO2 for 24 hours at dark. After being washed twice with PBS buffer, the
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chemosensor RhDBS is added to the cells at the final concentration of 20 μM and theDnOiI:n1c0.u10b3a9/tCe8dTCf0o6r296J
1 h and washed with PBS buffer three times to remove excess RhDBS. Afterwards, the fluorescence
imaging is carried out by a laser-scanning confocal fluorescent microscope (Leica, TCS-SP8) and
the excitation wavelength is set to 520 nm.[33]
Results and discussion
A proper testing platform is prerequisite for a selective and sensitive chemosensor towards the
metal ions. Aiming to this target, we investigate the effects of the solvent media and pH values on
the spectral response of RhDBS (Fig. S2-S4, ESI). The data suggests that chemosensor RhDBS can
be an appropriate candidate for the potential use under physiological condition. Thus, the
fluorescence and UV–vis data are recorded in dimethyl sulfoxide (DMSO)/Tris-HCl (7:3, v/v, 0.01
M, pH 7.4) mixed solution of RhDBS (20 μM) with proper amounts of metal ions throughout the
experiment. In addition, the blank RhDBS solution does not show any absorption peaks at 560 nm
and the ignorable fluorescence signal at 586 nm confirmed the predominant existence of the
ring-closed spirolactam form of RhDBS in this platform.
Assessments of the chemosensors for VO2+
Fig. 2. Fluorescence spectra of the RhDBS and other rhodamine derivatives in the presence of VO2+ (10 Eq.) in
DMSO-Tris-HCl (7:3, v/v, pH 7.4) solution (λex/em= 520/586 nm; slit: 2.5 nm/5 nm). Inset: Representative bar chart
showing the relative fluorescence intensity upon addition of 10 Eq. VO2+ ions.
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To investigate the effects of various substituents and steric configurations on sensing performance
of rhodamine-derived chemosensors for VO2+, we initially design and prepare eleven rhodamine
derivatives (Scheme 1) such as RhDBS, ReDBS, and RhGDM (Scheme S3, ESI), and conduct the
sensing experiments (Fig. 2). RhS[34] and RhHS[35] are two chemosensors for Cu2+, and here we have
studied the performance of sensing of VO2+ because they have the non-substituent group and typical
electron-donating group (hydroxyl) on the salicyl of the Schiff base moiety respectively, whereas the
other rhodamine derivatives have one or two electron-withdrawing groups (-F, -Br, -I) except that
ReDBS, ReBS, and RhGDM possess the different coordination spatial configurations. The results
indicate that ReDBS and ReBS are both fluorescent chemosensors for the selective and sensitive
detection of Fe3+ with Al3+ as a main interference, but they have no obvious response to VO2+ (Fig.
S5, ESI). RhGDM can simultaneously detect VO2+, Hg2+, and trivalent cations (Fe3+, Ga3+, Cr3+,
Al3+) (Fig. S6, ESI), hence, it can not be an ideal candidate for the detection of VO2+ due to the poor
selectivity. RhS can also detect VO2+ with 195-fold fluorescence enhancement, however, the
fluorescence intensity of RhHS undergone 38-fold increase upon reaction with VO2+ ions. RhFS and
RhBS both carry one electron-withdrawing group (-F or -Br) and show a better sensing performance
than that of RhS with 573-fold and 446-fold fluorescence enhancement respectively, whereas the
RhIS with one halogen group (-I) causes the negligible fluorescence which increases upon the
addition of VO2+ ions. RhDFS and RhDBS with two electron-withdrawing groups (double -F or
double -Br) show much better sensing performance than that of above rhodamine derivatives with
3230-fold and 2554-fold fluorescence enhancement respectively; in contrast, the relative
fluorescence intensity of RhDIS containing two halogen groups (double -I) is very weak upon
adding of 10 equiv. VO2+ ions. In addition, it should be noted that the exception of Al3+ as an
obvious interference, the selectivity of RhDFS to VO2+ is excellent (Fig. S7, ESI). On the basis of
these results, we have concluded that the relative fluorescence intensity of the chemosensors for
VO2+ follows the order: RhDFS > RhDBS > RhFS > RhBS > RhS > RhDIS (Inset Fig. 2). The
introduction of the electron-withdrawing group (-F, -Br) enhances the sensitivity of the sensor to
VO2+, and with the decrease of electronegativity, the selectivity and sensitivity should be decreased
further. On the other hand, the number of the electron-withdrawing group is another important factor
that affects the sensitivity of the sensor to VO2+. These phenomena can be explained as follows: the
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electron-withdrawing substituents (-F, -Br) can increase the acidity of phenolic hydroxyl group
which makes it easier to lose proton and then combine with VO2+ ions. Moreover, the geometry
optimization for these free receptors is done by DFT calculations. Because of the introduction of the
electron-donating group (-OH), the LUMO energy of sensor RhHS is higher than that of other VO2+
sensors, whereas the introduction of the electron-withdrawing groups (double -Br) causes much
lower LUMO energy of RhDBS and it also has the minimum energy gap which makes it easier to
activate (Table S2, ESI). These experimental and theoretical findings can be probably offered an
important guidance on the development of rhodamine-based receptors for efficiently recognizing
VO2+. To better understand the laws, we have also successfully prepared fourteen simple Schiff-base
ligands with different functional substituents and coordination spaces (Scheme 2) and studied their
photophysical properties and sensing performance (Fig. S8-S9 and Table S3-S4, ESI). These results
indicate that fluorogens L1, L2 and L3 feature typical aggregation-induced emission (AIE)
properties,[36] and can act as three chemosensors for Cu2+ (Fig. S10-S12, ESI); A2 is a dual
chemosensor for Ca2+/Mg2+ (Fig. S13, ESI). Frustratingly, all these Schiff-base motifs emit no
fluorescence in response to VO2+, thus we consider that the rhodamine based frameworks like
RhDBS exhibit the special features which are beneficial to the detection of VO2+. Overall, the
RhDBS is a highly selective and sensitive fluorescent turn-on chemosensor for VO2+, therefore, we
prefer to select RhDBS as a quintessential example to expound in the further discussion.
Spectroscopic properties
Primarily, we investigate the spectral response of RhDBS in the presence of VO2+ and Cu2+ and
other competitive metal ions, such as alkali and alkaline-earth ions (Mg2+, Ca2+, Ba2+), heavy- and
transition-metal ions (Cr3+, Fe3+, Fe2+, Mn2+, Co2+, Ni2+, Zn2+, Cd2+, Hg2+, Ag+), rare-earth metal
ions (La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Dy3+, Er3+, Th4+), and main group metal ions (Al3+, Sn2+,
Pb2+). The absorbance and fluorescence spectra are recorded in Fig. 3. It is observed that only the
introductions of Cu2+ and VO2+ ions (10.0 equiv.) separately to the solution of RhDBS promote
dremarkable changes in color and absorbance peak under identical conditions while blank and other
metal ions checked lead to negligible changes except very slight interference from Fe3+ (Fig. 3a and
Fig. S14, ESI). The binding of VO2+ and Cu2+ ions cause a distinct pink colour, and a new and
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strong absorption peak at 560 nm in aqueous DMSO medium, due to the chemical equilibrium
shifted to the rhodamine spirolactam ring-opened form. Interestingly enough, under the same
conditions, the chemosensor RhDBS exhibite an intense fluorescence emission at 586 nm upon
addition of 10 equiv. VO2+ ions. Notably, except VO2+ ions the addition of metal cations results in
negligible fluorescence increases (Fig. 3b). These above results indicate that RhDBS has significant
“turn-on” fluorescence selectivity toward VO2+. Therefore, RhDBS can successfully serve as
“naked-eye” and colorimetric chemosensor for Cu2+ as well as the fluorescence chemosensor for
VO2+. What is more, to find the stoichiometry of the RhDBS with Cu2+ and VO2+ ions, we perform
Job’s method. The observed results reveal that the RhDBS-Cu2+ and RhDBS-VO2+ complexes are
both 1:1 in the binding stoichiometry (Fig. S15, ESI). The stoichiometries are further confirmed by
ESI mass spectra of chemosensor in the presence of metal ions. The observed mass peaks at m/z
843.0755, 798.2 corresponding to [RhDBS+VO+H2O+CH3CN]+ (calculated: 843.46),
[RhDBS+Cu+H2O]+ (calculated: 799.02) respectively (Fig. S16-S17, ESI).
Additionally, outstanding selectivity is one of the momentous features for a sensor toward metal
ions. Thus, the tolerance of RhDBS in the presence of Cu2+/VO2+ and other miscellaneous cations is
also investigated (Fig. S18, ESI). These results indicate that the competing metal cations, including
Ag+, Sn2+, Pb2+, Mg2+, Ca2+, Ba2+, Fe2+, Mn2+, Co2+, Ni2+, Zn2+, Cd2+, Hg2+, Cr3+, Al3+, Fe3+, La3+,
Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Dy3+, Er3+ and Th4+ do not display any obvious interferences for Cu2+
ions detection (Fig. S18a, ESI). Thus, RhDBS exhibits high selectivity for Cu2+ in absorbance mode.
In the same way, negligible variations in the fluorescence intensity of RhDBS with VO2+ (10 equiv.)
in the presence of the metal ions mentioned above (20 equiv., except the Cu2+) is observed (Fig.
S18b, ESI). So it is inferred that the co-existence analytes have no marked influence on the detection
of VO2+ and Cu2+ ions.
The sensitivity of the chemosensor RhDBS for Cu2+ and VO2+ is better apprehended by the
absorbance spectra of RhDBS upon titration with Cu2+ and the fluorescence spectra of RhDBS upon
titration with VO2+ in DMSO/Tris-HCl buffer (7:3, v/v, 10 mM, pH=7.4) solution, respectively (Fig.
4). Upon addition of increasing amounts of Cu2+ ions, a new absorption band centered at 560 nm
appears with increasing absorption intensity (Fig. 4a). Similarly, when the VO2+ was added to the
solution, RhDBS reveals a significant emission enhancement at 586 nm (Fig. 4b). The absorption
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2+
2+
DOI: 10.1039/C8TC06296J
and emission intensities are positive correlation to the concentrations of Cu and VO , respectively
and the addition of Cu2+ and VO2+ ions (0 to 400 μM) follows a sigmoidal pattern (Inset Fig. 4).
Plotting fluorescence intensity at 586 nm verse concentration of VO2+ (8-100 μM) afford a good
linear relationship (R2 = 0.9964) (Fig. S19a, ESI). According to the equation DL = 3δ/S, the
detection limit of RhDBS for VO2+ is 3.65 nM. Similarly, the detection limit of RhDBS for Cu2+ is
found to be 0.84 μM (8-100 μM, R2 = 0.9957) (Fig. S19b, ESI). Besides the normal ring-opening,
the fluorescence enhancement phenomenon in the presence of VO2+ ions can be attributed to
chelation which enhances fluorescence (CHEF) process. Furthermore, the RhDBS-VO2+ adduct with
a high fluorescence quantum yield (Ф) of 70.114%, which is nearly 2504-fold to that of free RhDBS
(Ф = 0.028%) (Fig. S20, Table S5, ESI). Contrary to the result, the fluorescence of the spirolactam
ring-opened form of RhDBS is quenched by Cu2+ ions because the chelation quenches fluorescence
(CHQF) effect.[27d]
Fig. 3. (a) UV-vis and (b) fluorescence spectra of RhDBS (20 μM) in the presence of various metal ions (30 Eq.
except Cu2+/VO2+ that are 10 Eq.) in DMSO-Tris-HCl (7:3, v/v, pH 7.4) solution (λex/em= 520/586 nm; slit: 2.5
nm/5 nm). Inset: Photograph of RhDBS (a) in the presence of Cu2+ under day light; (b) in the presence of VO2+
under UV light (365 nm).
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Fig. 4. (a) Changes in the absorption spectra of RhDBS (20 μM) with increasing concentrations of Cu2+ (0-20 Eq.)
in DMSO/Tris-HCl (7:3, v/v, pH 7.4) solution. Inset: the corresponding absorbance at 560 nm as a function of the
Cu2+ concentration. (b) Changes in the fluorescence intensity of RhDBS (20 μM) with increasing concentrations of
VO2+ (0-20 Eq.) in DMSO/Tris-HCl (7:3, v/v, pH 7.4) solution (λex/em= 520/586 nm; slit: 2.5 nm/5 nm). Inset: the
relative fluorescence intensity at 586 nm as a function of the VO2+ concentration.
In addition to high selectivity and sensitivity, a short reaction time is other one fundamental
parameter for RhDBS to monitor Cu2+ and VO2+. To investigate the detection capability of RhDBS
to Cu2+ and VO2+, we perform the kinetic profiles of the reaction systems. Through the naked-eye
observation, the response time is < 3 s. All the interactions of RhDBS with Cu2+ are all completed in
less than 1 min (Fig. S24, ESI) and in these cases, the equilibriums are all reached in less than 3 min
(Fig. S25, ESI). These observations indicate that Cu2+ and VO2+ can rapidly induced ring-opening of
RhDBS spirolactam. In addition, as the reactions are completed in short times, the absorbance and
emission intensity remain unchanged with more reaction time, indicating that the sensor RhDBS is
photostable. Therefore, this chemosensor can be used for the real-time monitoring of Cu2+ and VO2+.
Finally, compared with the previous work, the proposed chemosensor RhDBS has many merits
which are superior to those of the recently reported dual sensors including rhodamine-based
chemosensors (Table S6, ESI).
Sensing mechanism
In order to strengthen the binding mechanism, we have well established the mechanistic route of
coordination of RhDBS with VO2+ in DMSO-H2O solution via the opening of spirolactam ring by
1H-NMR study (Fig. 5). The peaks related to protons in the imine and xanthene unit are deshielded
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2+
DOI: 10.1039/C8TC06296J
upon addition of 1.0 equiv. of VO ions and the proton (Hb) of the N=CH group shifts downfield
from δ = 9.039 ppm to δ = 9.58 ppm which indicates that VO2+ ions are bound to the “N” atom,
thereby reducing the electronic density of the imine group hydrogen. In addition, the downfield shift
of the OH group proton (Ha) occurs and finally it disappears, supporting deprotonation of RhDBS.
The methylene protons (Hc) of diethylamino group is shifted downfield by 0.272 δ(3.347→3.619)
forming an broad peak after the spirolactam ring is opened by the VO2+ ion due to the delocalization
on the xanthene moiety. The above results allow one to conclude that a single VO2+ ion is attached to
the oxygen atom of the spiro ring, and the imine nitrogen atom and the oxygen atom of the hydroxyl
group on the benzene ring are responsible for the electron deficiency of the xanthene system.
Unfortunately, due to the paramagnetic intrinsic of Cu2+, 1H NMR spectrum of the Cu2+ complex is
not available.[37]
Fig. 5. 1H NMR spectra of RhDBS in the pr esence of 1.0 eqv. of VO2+ in DMSO-d6.
To get a further insight into the coordination mode of VO2+ and Cu2+ with the chemosensor
RhDBS, we carry out energy-optimized structures, spatial distributions and orbital energies of the
HOMO and LUMO of free RhDBS and its corresponding ensembles from DFT calculations at the
B3LYP level and the 6-31G** basis set for free RhDBS and LANL2DZ basis set for metal
complexes using the Gaussian 03 program.[38] As shown in Fig. 6, the electron density in the HOMO
is spread over the xanthene unit, whereas for the LUMO it is distributed to the substituted
spirolactam ring of RhDBS. On the contrary, the π electrons of HOMO orbitals of RhDBS-VO2 are
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mainly distributed to the bromo substituted aromatic ring, and the π*-type LUMO is principally
located over the xanthene moiety. Hence, the electronic transitions (π→π*) should happen between
the oxidovanadium-centered HOMOs and the LUMOs located in the rhodamine unit. In
RhDBS-Cu2+, the spatial distribution is almost similar to the RhDBS-VO2+. The energy gaps
between the HOMO and LUMO of the free RhDBS, Cu2+-complex, and VO2+-complex are found to
be 3.782 eV, 2.363 eV and 2.731 eV, respectively. The results disclose that the binding of Cu2+ and
VO2+ to free RhDBS lowers the energy gap and also stabilized the systems.[39] All the theoretical
observations are in consistent with those of the experimental findings. Moreover, the DFT
calculations of the other receptors are also investigated and the detail results were tabulated (Table
S2, ESI). If we go through the table very carefully, we can find that RhDBS has the lowest LUMO
orbital and the minimum value of the HOMO-LUMO energy gap among these rhodamine
derivatives except the RhDGM. Thus, the results may provide us a theoretical basis for designing
the rhodamine-derived chemosensors for VO2+.
Fig. 6. Energy diagram of HOMO and LUMO orbitals of RhDBS and its corresponding metal complexes
calculated at the DFT level using a B3LYP/6-31G** basis set.
In addition, the consequence of VO2+ on the fluorescence decay behaviour of RhDBS is studied
by using a TCSPC method (Fig. 7). The time-resolved fluorescence spectrum of free RhDBS shows
bi-exponential decay, suggesting the existence of two emitting species.[40] Their lifetimes are found
to be τ1 (0.95 ns, 86.96%) and τ2 (2.44 ns, 13.04%) respectively, and the average lifetime value is
1.03 ns. However, following the addition of VO2+ ions, bi-exponential decay disappears, manifesting
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the existence of only one emitting species having elevated lifetime of τ (1.84 ns, 100D%O)I:,10c.l1e03a9r/lCy8TdCu06e296J
to the ring-opened form of RhDBS and the newly formed VO2+ complex as proposed in Scheme 3.
The result is in accordance with their band gap. Afterwards, we have also calculated the radiative
rate constant (kr) and the total non-radiative rate constant (knr) of RhDBS and its VO2+ complex
(Table S5, ESI). These data indicate that the kr : knr ratio has increased owing to the reasonably
reduction in knr in support of the fluorescence enhancement of VO2+ complex which is attributable to
the CHEF process.[41]
Fig. 7. Fluorescence lifetime decay of RhDBS, RhDBS-VO2+ in aqueous DMSO solution (λex = 560 nm , λem
=
586 nm).
On the other hand, rhodamine-derived chemosensors have been broadly employed as
electrochemical sensors for metal cations.[42] Further evidence of the binding of VO2+ with the spiro
ring and Schiff-base moiety is seen with cyclic voltammetry, in the presence and absence of VO2+
(Fig. 8). In the absence of VO2+, a typical voltammogram is seen for an rhodamine derivative, with
two irreversible reduction peaks (Epc1 = -0.92 V, Epc2 = 0.845 V) observed between -1.5 and 1.5 V vs
SCE, assigning to the formation of two successive anion radicals of RhDBS. After RhDBS interacts
with VO2+ ions, it causes a distinct behavior in the CV. Intriguingly, with the addition of one equiv.
of VO2+, both the reduction peaks occuring on the RhDBS are shifted to a significantly more
positive potential (Epc1 = -0.545 V, Epc2 = 0.925 V), meanwhile a new one-electron sharp oxidation
peak is appeared at -0.917 V for RhDBS due to the binding of VO2+ ions. The result reveals that the
RhDBS is irreversibly oxidised in presence of the VO2+ ions. This also favors the mechanism of the
ligand to metal ion electron transfer (LMTC) and indicates impossibility of the back-donation of
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[43]
electron from vanadium to the antibonding orbital of π-conjugated system of RhDBSD.OI: 10M.103o9r/Ce8oTvCe06r,296J
it can be concluded that this anodic shift can be possibly attributed to the opening of the spiro ring
upon VO2+-complexation with free RhDBS, which lowers the electron density on the xanthene
system.[44] Almost no credible changes in CV curves of the chemosensor RhDBS are observed upon
addition of other metal cations (Fig. S26 ESI). All these observations suggest that RhDBS can be
also exploited for electrochemical sensing of VO2+ ions.
Fig. 8. Cyclic voltammograms of RhDBS(c = 100 μM) at scan rate 50 mV sec-1 in DMSO-H2O (7:3, v/v)
and after adding 1 equiv. of VO2+ to RhDBS.
Repeatability of RhDBS for sensing VO2+ and Cu2+
Reversibility is one of the fundamental criteria for most chemosensors and it is defined generally
as the capability of the sensors developed to show consistent performances and give similar
sensitivity properties when the sensor or method is repeatedly used.[45] Thus, in order to qualitatively
investigate the reproducibility of detection process of VO2+ and Cu2+, we add up to seventeen kinds
2-
-
-
2-
of important physiological and environmental anions: F-, Cl-, Br-, I-, AcO-, S2O3 , NO3 , NO2 , CO3 ,
2-
2-
-
-
2-
3-
4-
S2-, SO4 , SO3 , H2PO4 , H2PO2 , HPO4 , PO4 , and P2O7 (PPi) into RhDBS-Cu2+ and
RhDBS-VO2+ mixed systems (DMSO/H2O, 7/3, v/v, pH 7.4), respectively. As shown in Fig. 9a, the
considerable changes are observed when the addition of 20 equiv. of PPi to the solution of
RhDBS-Cu2+ complex, while other competitive anion species induce negligible absorbance spectral
changes except S2O32- under the same conditions. However, of these anions, only PPi shows full
restoration of the original absorbance intensity at 560 nm given by free sensor RhDBS, thus
achieving a selective “ON-OFF” behavior of absorption. In contrast, upon interaction with
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competing anions ensemble RhDBS-VO2+ show reliable changes in fluorescenceDsOpI:e10c.t10ra39/iCn8TCth06e296J
presence of PPi (400 μM) with F- and S2- as the main interferences but only at a much higher
concentration (up to 1 mM) (Fig. 9b). Moreover, the fluorescence “ON-OFF” switching property of
RhDBS-VO2+ species is further corroborated by fluorescence titration experiments. As shown in Fig.
9c, with the incremental addition of PPi (0-20 equiv.), the fluorescence of RhDBS-VO2+ at 586 nm
is gradually decreased and reaches a plateau when 20 equiv. of PPi is added and more than 95%
fluorescence intensity of the system is quenched. Thus, the results confirm that sensor RhDBS is
restored during the detection procedure of PPi.
Fig. 9. (a) Absorption spectra and (b) Fluorescence spectra of the [RhDBS-Cu2+] whole (20 μM in
DMSO-Tris-HCl (7:3, v/v, pH 7.4)) was added with various external anions (1 mM) including F-, Cl-, Br-, I-, AcO-,
2-
-
-
2-
2-
2-
-
-
2-
3-
4-
S2O3 ,NO3 , NO2 ,CO3 , S2-, SO4 , SO3 , H2PO4 , H2PO2 , HPO4 , PO4 and P2O7 (PPi, 400 μM). (c)
4-
Fluorescence spectra: upon addition of [P2O7 ] = 0, 20, 40,... 200, 300, 400, 500, 600, μM to DMSO-Tris-HCl (10
mM, 7:3, v/v, pH 7.4) buffer. (λex =520 nm; slit: 2.5/5 nm).
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2+
DOI: 10.1039/C8TC06296J
Additionally, to verify the stability and reversibility of the RhDBS-VO ensemble, we conduct
PPi-addition experiments. As shown in Fig. 10a, the periodically alternating addition of a constant
amount of VO2+ and PPi to the solution of RhDBS leads to a switchable change in the fluorescence
intensity at 586 nm. Such an ‘off-on-off’ fluorescence switching behaviour of RhDBS can be
repeated at least four cycles without much loss in sensitivity by the modulation of VO2+/PPi addition
(Fig. 10b) and this phenomenon exhibits that RhDBS alone does not interact directly with the
anions. Therefore, PPi anions bind to copper ion and oxovanadium(IV) ions through breaking the
corresponding complexes to ligands and metal ions as illustrated in Scheme 3.[46] Thus, reversibility
in case of VO2+ ions can be displayed with the assistance of PPi ions. Nevertheless, these dramatic
changes could be negligible when VO2+ ions are in excess. Their corresponding fluorescence colour
changes under a UV-lamp are also shown in Fig. 10b inset. All these experimental results indicate
that RhDBS is an reversible “off-on-off” chemosensor toward VO2+, Cu2+, and PPi.
Fig. 10. (a) Emission spectra of RhDBS (20 μM) by alternated adding VO2+ and PPi in DMSO-Tris-HCl (7:3, v/v,
pH 7.4). (b) Fluorescence intensity of RhDBS (20 μM) at 586 nm upon alternate addition of a constant amount of
VO2+ and PPi. Inset: photographs of fluorescence reversibility of RhDBS-VO2+ and RhDBS-VO2+ plus PPi in
DMSO/H2O (7:3, v/v; pH 7.4) solution (λex = 520 nm, Slit: 2.5/5 nm).
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Scheme 3. Possible proposed sensing mechanism of metal complexes (RhDBS-Cu2+ and RhDBS-VO2+) towards
PPi anion.
Molecular logic gates
Fig. 11. (a) The sequential logic circuit of the memory machine and its truth table.(b) Feedback loop showing the
reversible logic operations for the memory element with “Reading-Erasing-Reading-Writing” functions.
Molecular logic based on the conversion of chemically encoded input into output signal, is an
interdisciplinary research field, and has gained much attention in recent years, especially INHIBIT
logic because of the noncommutative behaviour.[47, 48] Target analytes generally are acted as the
inputs and here VO2+, Cu2+, and PPi are considered as the three chemical inputs for the logic gates.
In the present system, two different types of monomolecular logic circuits have been constructed, i.e.
INHIBIT logic circuit with the combination of AND and NOT logic gates and the combination of
NOT and OR logic gates, respectively (Fig. S27, ESI). Base on the truth tables (Fig. S27 truth table
a and b, ESI) these logic circuits have been achieved. The INHIBIT circuit has single output based
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on the fluorescence signal changes and the second one is with two outputs from theDaObI:s1o0r.1p03t9io/Cn8TaCn06d296J
emission channels.
In addition, sequential logic circuits are of great importance in creating memory devices to store
information and perform operation through feedback loops, in which one of the outputs of the
memory device functions as an input and is memorized as a ‘memory element’.[49] Thus, basing on
the above observations, we are able to design sequential logic circuits displaying
‘Reading-Writing-Erasing-Reading’ behavior with the help of binary logic (Fig. 11). In the concrete
system, the ‘ON’ state (Output = 1) corresponds to the strong fluorescence at 586 nm, while the
‘OFF’ state (Output = 0) refers to the quenched fluorescence emission signal at the same wavelength.
Here, the two chemical inputs of VO2+ and PPi are designated as (In VO2+) and (In PPi) for the Set
(S) and Reset (R) inputs respectively, their existance is defined as “1” and their absence is defined as
“0”. The operation of this memory unit is as follows:[50] whenever the Set input is high (S = 1), the
system writes and memorizes the binary state “1”. By utilizing reset input (R=1), the stored
information is erased, i.e. the binary state will be “0” with fluorescence quenching dramatically. As
depicted in Fig. 11a, this sort of reversible and reconfigurable sequences of Set/Reset logic
operations can be expressed in the form of a feedback loop, which demonstrates a memory feature
analogous to ‘Reading–Writing–Erasing–Reading’ functions.[51] Fig. 11b describes the bistability
behavior, ‘OFF-ON’ state, of the sensor RhDBS and lets slip the non-volatile nature of the memory
effect. This behavior can be widely employed in microprocessors for memory elements of integrated
logic circuits.[52] It is incredibly proud that the logic device developed here shows its great
superiority over early designed related systems at least in regard to the reversible and reproducible
features as well as the promising potentials for successive sensing of heavy- and transition-metal
ions and Pi.
Practical applications of the proposed sensor
In order to check the practical application of chemosensor RhDBS, we put common filter paper
test strips and silica gel plates (GF254) forward for rapid in situ detection of Cu2+, VO2+ and PPi
ions. For the goal, filter paper strips (Whatman) are soaked with a 100 μM DMSO/H2O (7:3, v/v; pH
7.4) solution of RhDBS for several minutes and then dried in vacuo. The distinct visible colour
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changes are observed instantly upon spraying the test strips with corresponding metal ions aqueous
solutions (20 μM) in day light (Fig. 12a). The colourless strips are converted to pink, and the
RhDBS-VO2+ strip displays an intense red fluorescence while the strip containing RhDBS-Cu2+
shows a quenched fluorescence under UV light. These pink coloured test strips containing
RhDBS-VO2+ and RhDBS-Cu2+ are then sprayed with aqueous PPi solution, respectively, all the
pink colours subsequently fade and bright red fluorescence of the test strips disappears due to the
restoration of RhDBS.[53] Additionally, the fluorescence and colours are enhanced with the increased
concentration of VO2+ (Fig. S28, ESI). Thus, the naked-eye detection with low detection limit can
achieve by the solid state paper film. These observations resemble the changes occur in solution.
Therefore, the chemosensor can quite conveniently for rapid on-site detection of Cu2+, VO2+ and PPi
ions, exhibiting a low-cost detection approach without using any other expensive instruments.
As shown in Fig.12b, the prepared TLC strips are colourless in the absence of VO2+ or Cu2+ (20
μM), however, when writing on the silica gel plates containing RhDBS with a writing brush dipped
in the aqueous solution of VO2+ and Cu2+ respectively, the changes in colour (colourless to pink) are
observed rapidly and the brilliant tangerine fluorescent of the writing images appear on the plates
containing RhDBS-VO2+ under UV illumination. Therefore, the dilute aqueous solution of VO2+ can
be functioned as the fluorescent security ink and word, design or some specific pattern can be drawn
on to these solid support surfaces.[54] It is worth mentioning that other tested metal ions were unable
to induce any reliable colour changes on TLC plates as well as on their respective paper strips. Thus,
the silica gel plates containing RhDBS can also serve as a multifunctional smart material for the
in-field detection of VO2+ and Cu2+ ions in non-solution environment. All the above experimental
observations also offer high potentiality of the near future chemsensors to adsorb on suitable solid
supports allowed prototypes of applicative dipstick assays to be developed for quick and easy visual
detection of the target analytes.[55]
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Fig. 12. Photographs of the test strips of RhDBS for the detection of Cu2+, VO2+ and PPi in dayligDhOtI:a1n0d.10u3n9/dCe8rTC30665296J
nm UV light. A solution of RhDBS with a concentration of 100 μM was used to develop the strips: (a) paper strips
and (b) silica gel plates.
Owing to the astonishing selectivity and the excellent performance for rapid fluorescence
detection of VO2+ under physiological pH, and very low-toxicity (Fig. S29, ESI) of sensor RhDBS,
we are promoted to test its potential use for bioimaging in MCF-7 cancerous cells (Fig. 13). Cells
incubated with RhDBS alone exhibits no fluorescence, whereas a bright red fluorescence signal is
captured after the RhDBS-loaded MCF-7 cells is treated with 100 μM of VO2+ confirming the
fluorescence “OFF-ON” behavior of RhDBS in the presence of VO2+ in solution. However, the
fluorescence signal is significantly quenched by PPi (100 μM) supporting the fluorescence
“ON-OFF” phenomenon. The results indicate intracellular permeability of the chemosensor RhDBS
and its potential to detect intracellular VO2+ and PPi in living cells. Therefore, RhDBS can be
employed for monitoring VO2+ ions in biological samples.
Fig. 13. Confocal fluorescence imaging representing ‘OFF-ON-OFF’ fluorescence mechanism in the presence of
RhDBS-VO2+-Pyrophosphate in living MCF-7 cells. a), e) bright-field and b), f) fluorescence image of RhDBS
(10 μM) ; c) the fluorescence image of cells initially treated with RhDBS (20 μM) followed by treatment with
VO2+ (100 μM) for 30 min; g) cells initially treated with RhDBS (20 μM) followed by VO2+ (100 μM) and further
incubated with pyrophosphate; d) the merged image of a) and c); h) the overlay of e) and g); λex = 520 nm ,
emission (540-700 nm); scale bar = 20 μm.
Solid state luminescent properties
To further expand the potential application realms, we examine solid state luminescent properties
of RhDBS and other rhodamine derivatives. The solid state samples showed distinct colour changes
from yellowish to dark pink (Fig. S30a, ESI) after mechanical grinding due to the amorphous
characteristics.[27d] The original form of RhDBS exhibits bright yellowish green light with an
emission peak at 529 nm (Fig. 14). The colour changes can be observed rapidly upon mechanical
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grinding of RhDBS with the metal salts of Cu2+ and VO2+ under the 365 nm UV light (Fig. 14a).
Delightfully, as the introduction of Cu2+ and VO2+, the “turn-on” fluorescence signals are both
obtained and the emission wavelengthes of RhDBS-VO2+ and RhDBS-Cu2+ are 615 nm and 626 nm
respectively, causing the quite large red shifts (86 nm and 97 nm) (Fig. 14b). It should be noted that
the RhDBS-Cu2+ phosphor has the longest wavelength and there is negligible emission signals can
be found among the other copper complexes (Fig. S30b, ESI). All the colour changes and different
fluorescence behaviours demonstrate the admirable sensitivity of RhDBS toward Cu2+ and VO2+
ions in the solid state. In general, CIE chromaticity diagram is used to better express the color
variations in the emission spectrum of photoluminescent materials.[56] In solid state, the chromaticity
coordinates of RhDBS are found to be 0.3462 and 0.5108 in the ‘yellowish green’ region of the
corresponding CIE diagram, however, the colour shifted toward reddish orange upon chelation with
the Cu2+ and VO2+ ions as depicted in Fig. 15. The corresponding ‘x’ and ‘y’ values for the Cu2+ and
VO2+ metal complexes are ( 0.6597, 0.34) and (0.6388, 0.3608) respectively. To our delight, we find
the color coordinates of RhDBS-Cu2+ phosphor are very near to “ideal red” chromaticity (0.68, 0.32)
for the National Television Standard Committee (NTSC) system. The observed high
photoluminescent intensity along with a satisfactory color purity implies that the RhDBS-Cu2+
phosphor may be appropriate for lighting and display uses. Moreover, the thermal property of an
emitter is the fundamental factor involving device stability and the good thermal stability is a
prerequisite for the application in OLEDs.[57] The thermal decomposition temperature (Td) of
chemosensor RhDBS is 322ꢀ°C (Fig. S31) indicating the high morphological stability of the
amorphous phase in a deposited film. Therefore, the above results indicate that the RhDBS-Cu2+
may be a suitable candidate material as a red phosphor for OLEDs.
DOI: 10.1039/C8TC06296J
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Fig. 14. Colour changes upon mechanical grinding receptor RhDBS and other rhodamine derivatives with the salts
of VO2+ and Cu2+(RhDBS) under a 365 nm UV lamp (a) and their solid-state emission spectra (λex= 400 nm) (b).
Fig. 15. CIE coordinates for RhDBS, RhDBS-Cu2+, RhDBS-VO2+, RhDFS-VO2+, RhDIS-VO2+, RhHS-VO2+,
RhS-VO2+, ReDBS-VO2+ and RhGDM-VO2+ in the solid state and RhDBS-VO2+ in solution.
Conclusions
In summary, we have successfully designed and synthesized twenty-five Schiff-base ligands
including eleven rhodamine derivatives. Among these compounds, a new rhodamine-based
derivative (RhDBS) is picked out as the best difunctional chemosensor which exhibits different
selectivity at submicromolar levels for Cu2+ from chromogenic measurements and at subnanomolar
levels for VO2+ from fluorogenic measurements. Thus, easy to synthesize rhodamine B derivative is
capable of detecting Cu2+ and VO2+ via “naked eye” and fluorescence spectral studies and it can also
act as a electrochemical sensor for VO2+ ion. Moreover, the RhDBS-Cu2+ and RhDBS-VO2+
complexes can be used as chemosensors for relay recognition of PPi. Particularly, the fluorescence
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2+
“off-on-off” responding circle can be repeated over four times by the sequential adDdOitI:i1o0n.103o9f/CV8TOC06296J
and PPi. Several INHIBIT logic gates have been constructed and understood. Especially, the
chemosensor is also efficiently applied in both in-field analysis and the bioimaging. To the best of
our knowledge, this is the first reported rhodamine B derivative capable of detecting VO2+ in living
cells. RhDBS also displays the VO2+ and Cu2+ ions-responsive behavior in the solid state and can be
used as photoluminescent solid-state materials. More importantly, the introduction of electron
withdrawing group (e.g. -F, -Br, -I) into the Schiff base imine moiety can dramatically affect the
sensing performance of the chemosensor toward VO2+ while the other Schiff-base motifs show the
unsatisfactory performance in determining the VO2+ ion. We believe that both the regularity between
structure and properties and a dual rhodamine-derived chemosensor such as that developed here, can
be a novel strategy in developing other remarkable chemosensors for VO2+ in vitro and in vivo.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research was financially supported by the Fundamental Research Funds for the Central
Universities (3207047405), the National Natural Science Foundation of China (81771976), the
National Key Basic Research Program of China (2014CB744501), and the Key Research Project of
Natural Science from Provincial Bureau of Education, Anhui, China (KJ2017A572).
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