Two-stage metal ion sensing by through-space and through-bond charge transfer

Article abstract of DOI:10.1039/d0tc01082k

A highly selective colorimetric and ratiometric "two-stage"/"off-on"type fluorescent probe with an ability to exclude other heavy and transition metal ions has been designed and synthesized. Low concentrations of Fe3+ cause through-space charge transfer which leads to fluorescence quenching exceeds 96%, and a further increase of Fe3+ concentration results in the formation of a complex with the maximum emission peak of 1,3-bis((E)-2-(pyridin-4-yl)vinyl)-2-methoxy benzene (BPVMB) being shifted largely by 100 nm (from 416 to 516 nm), which was examined by fluorescence, absorption detection and femtosecond transient absorption measurements on the basis of intramolecular charge transfer (ICT).

Full text of DOI:10.1039/d0tc01082k

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Nosang V. Myung, Yong-Ho Choa et al.
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ARTICLE
Two-stage metal ion sensing through space and through bond
charge transfer
Guoxin Yin,†a Yu Wang,†a Yuping Yuan,†a,c Ruxue Li,b Rui Chen,b Hsing-Lin Wang,*a
Received 00th January 20xx,
Accepted 00th January 20xx
A highly selective colorimetric and ratiometric “two-stage”/“off-on” type fluorescent probe with the ability to exclude other
heavy and transition metal ions has been designed and synthesized. Low concentration of Fe3+ causes through space charge
transfer which leads to fluorescence quenching exceeds 96%, and further increase of Fe3+ concentration results in formation
of complex with the maximum emission peak of 1,3-bis((E)-2-(pyridin-4-yl)vinyl)-2-methoxy benzene (BPVMB) shifted largely
by 100 nm (from 416 to 516 nm), which was examined by the fluorescence, absorption detection and femtosecond transient
absorption measurements on the basis of intramolecular charge transfer (ICT).
DOI: 10.1039/x0xx00000x
Introduction
To date, it is still difficult for most sensing platforms to
In recent years, the synthesis and design of highly selective distinguish Cu2+ and Fe3+. For most of the reported fluorescent
and sensitive sensors for heavy and transition metal ions have sensors of Cu2+ and Fe3+, binding of the metal ion causes a
received widely attention because of its biological, quenching of the fluorescence emission due to their
environmental and industrial applications.1–6 Among various paramagnetic nature.20–24 Only a few sensors in which the
metal ions, iron is the most abundant and crucial transition binding of Fe3+ causes an increase in the fluorescence intensity
element in the human body and plays an important role in or change in wavelength have been reported. Rhodamine-B
physiological processes such as electron transfer, oxygen based sensors have been reported as “turn-on” fluorescence as
uptake, gene expression and oxygen metabolism.7 Overload or well as colorimetric sensors for Cu2+, Al3+ and Fe3+. However, the
deficiency of iron can cause a series of human diseases.8–13 The chemosensor cannot differentiate Cu2+, Fe3+ and Al3+.25,26 In this
third most abundant metal copper in the human body is also an regard, ratiometric and colorimetric measurement are more
essential trace element. Excess copper can often be toxic to attractive to achieve higher selectivity and sensitivity upon
certain biological systems and is associated with various binding of Fe3+ to the receptor. Ratiometric measurements have
neurodegenerative disorders including Wilson’s disease and important features that measure the analyte concentration and
Alzheimer’s disease.14,15 On the other hand, water and soil minimize the measurement errors.27,28 However, up to now,
contamination by the metal ions including iron, copper and few colorimetric and ratiometric fluorescent chemosensors for
other metals has attracted widespread interests as it has posed Fe3+ have been found in the literature. To our best knowledge,
threat to our daily life. Thus, it is important to detect and most of the reported Fe3+ sensors are based on an assumption
quantify those metal ions in both biological and environmental of intramolecular charge transfer (ICT) mechanism which hasn't
systems. Current detective determination methods for Fe3+ been proved yet.
such as atomic absorption spectroscopy, mass spectrometry
and electrochemical spectroscopic analysis suffer either from two-stage
In this manuscript, we present a novel sensing platform: a
ratiometric and colorimetric fluorescent
their elaborate and sophisticated nature, or from time chemosensor with high Fe3+ selectivity. By varying the
consuming experiments.16–19 In contrast, fluorescence methods concentration of Fe3+, it is possible to produce through-space
are increasingly being called upon due to their conveniences charge transfer quenching followed by fluorescence turn-on
and high sensitivity.20
with a large red shift of about 100 nm.29–33 At first, the addition
of Fe3+ induced fluorescence quenching due to the through
space charge transfer. Then binding of Fe3+ to the pyridyl group
of BPVMB induced a large red shift in fluorescent emission
spectra due to intramolecular charge transfer (ICT). Such a turn
off and then green emission will be useful in applications as it
can detect the existence of different Fe3+ concentration. An ICT
mechanism for our system is further proved by femtosecond
transient absorption measurements. Furthermore, high
sensibility for Fe3+ was achieved via replacement method.
a. Department of Materials Science and Engineering Southern University of Science
and Technology 1088 Xueyuan Avenue, Shenzhen, 518055, P.R. China
b. Department of Electrical and Electronic Engineering Southern University of
Science and Technology 1088 Xueyuan Avenue, Shenzhen, 518055, P.R. China
c. Academy for Advanced Interdisciplinary Studies，Southern University of Science
and Technology, Shenzhen, 518055, China
†
These authors contributed equally to this work
*Corresponding authors
Electronic Supplementary Information (ESI) available: Supporting information
including the synthesis and corresponding characterization data for BPVMB about
absorption titration spectra, titration profile, fluorescence intensity, 1H and 13C
spectra. See DOI: 10.1039/x0xx00000x
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Experimental section
Materials
Determination of apparent dissociation constant
Fluorescence spectroscopy was used to determine the
Acetonitrile, tetrahydrofuran and all the chloride salts of
metal ions were obtained from Aldrich. All other reagents and apparent dissociation constants (Kd) of BPVMB (50 μM) with
solvents utilized in the experiments were of reagent and Zn2+ and Cd2+, using the reported method.27 The fluorescence
spectroscopic grades and used as received without further intensity data were fitted to equation 1, and Kd was calculated:
[푀3 +
]
푓푟푒푒
purification.
F = 퐹0 +(퐹푚푎푥 ― 퐹0)
(1)
퐾푑 + [푀3 +
]
푓푟푒푒
where F is the fluorescence intensity, Fmax is the maximum
fluorescence intensity, F0 is the fluorescence intensity with no
addition of Fe3+ and Cu2+, and [M3+]free is the free Fe3+ and Cu2+
concentration.
Measurements of UV and PL
UV absorption spectra were obtained on Cary 5000 UV-vis-
NIR spectrophotometer. Fluorescence emission spectra were
obtained using HORIBA PTI-QM-8075 Luminescence
spectrometer. A 1 cm width and 3.5 cm height quartz cuvette
was used. The metal ion (Na+, Mg2+, Ca2+, Pb2+, Cd2+, Mn2+, Ni2+,
Cu2+, Zn2+ and Fe3+) stock solutions were prepared in acetonitrile
in the order of 4 mM. Ligand BPVMB stock solutions were
prepared (C = 1 mM) in acetonitrile. Working solutions of
ligands and metal ions were freshly prepared from the stock
solutions. The excitations were carried out at 320 nm for ligand
BPVMB with 3nm emission slit widths.
Results and discussion
Effect of solvent on the fluorescence of BPVMB
Measurements of femtosecond transient absorption (TA) spectrum
The TA spectra and dynamics were recorded using a
standard pump−probe configuration at 350 nm, ∼100 fs pump
pulses at a 1 kHz repetition rate, and a broad-band white-light
supercontinuum probe (18SI80466 Rev.1, Newport). The
excited power was 4 mW and the spot diameter was 300 μm.
Synthesis of BPVMB
Fig. 1 (a) Molecular structure of BPVMB. (b) Fluorescence
emission of BPVMB in different solvents (tetrahydrofuran,
dimethyl sulfoxide, methanol, chloroform, ethanol, ethyl
acetate, dimethylformamide and acetonitrile). (c) Plot of
normalized absorbance of BPVMB in different solvents. (d)
Fluorescence emission spectra of BPVMB (0.1 mM) (λex. = 320
nm) in different solvents.
1,3-diiodo-2-methoxybenzne (500mg, 1.39mmol) and 4-
vinylpyridine (0.3ml, 2.78mmol) were dissolved in the 10ml
N,N-dimethylformamide and 10ml diisopropylethylamine in a
flask. The mixture was bubbled with N2 for 20min and
palladium acetate (15.7mg, 0.07mmol) and tris(2-
methylphenyl) phosphine (30.4mg, 0.1mmol) were added into
the mixture under N2 flow. The reaction was stirred at 100oC
under N2 for 24h. The reaction was cooled to room temperature
and filtered through celite. The filtrate was collected and
evaporated in vacuum. The crude product was purified by
column chromatography with methanol/ethyl acetate (1/4,
volume ratio) as eluent. After removing the solvent, 340.5mg
white crystal was collected with yield of 78%. 1H-NMR
(400MHz, CDCl3),  8.59 (d, 4H, J=8Hz), 7.63(d, 2H, J=4Hz),
7.61(d, 2H, J=4Hz), 7.40(d, 4H, J=8Hz), 7.20(t, 1H, J=8Hz), 7.07(d,
2H, J=16Hz), 3.81(s, 3H); 13C-NMR (100MHz, CDCl3),  156.65,
150.29, 144.71, 130.37, 123.73, 127.32, 127.11, 124.84, 120.98,
62.61. The synthesis route is shown as below (Scheme 1):
The UV-absorption and fluorescence spectra of BPVMB in
tetrahydrofuran, dimethyl sulfoxide, methanol, chloroform,
ethanol, ethyl acetate, dimethylformamide and acetonitrile
were measured and the results are shown in Fig. 1. All solutions
exhibit blue emission without the addition of metal ions (Fig.
1b). The absorption spectra of all solutions exhibit a peak value
at around 305nm (Fig. 1c) and in detail the absorbance peaks of
BPVMB in tetrahydrofuran, dimethyl sulfoxide, methanol,
chloroform, ethanol, ethyl acetate, dimethylformamide and
acetonitrile are at 305, 312, 306, 306, 307, 302, 307 and 304 nm,
respectively. The emission spectra of BPVMB in these solvents
show the peak value at the wavelength from 420 to 428 nm.
The small changes in both UV absorption and fluorescence
spectra indicate a negligible solvatochromatism property of BPV
MB (Fig. 1d).
Scheme 1. Synthetic scheme of BPVMB. (1) iodine, H2O, r.t.; (2)
iodo-methane, acetone, reflux; (3) palladium acetate, triphenyl-
phosphine, 4-vinylpyridine, N,N’-dimethylformamide
Fe3+ selectivity
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Fig. 2 (a) Changes in the fluorescence emission due to BPVMB (50 μM) in the presence of 4mM of various added cations under
UV-lamp (365 nm) in acetonitrile. (b) UV−vis spectra and (c) fluorescence spectra of BPVMB (50 μM) in acetonitrile and in
presence of various added cations (4mM). (d) Changes in the fluorescence emission due to BPVMB (50 μM) in the presence of
4mM of various added cations under UV-lamp (365 nm) in tetrahydrofuran. (e) UV−vis spectra and (f) fluorescence spectra of
BPVMB (50 μM) in tetrahydrofuran and in presence of various added cations (4mM).
In order to evaluate the metal ions sensing properties of BPVMB,
In contrast, BPVMB responds differently with respect to
we first examined the optical property of BPVMB solutions with various metal ions in tetrahydrofuran solutions, see Fig. 2d.
various metal ions (including Na+, Mg2+, Ca2+, Pb2+, Cd2+, Mn2+, Quenching of fluorescence was observed after addition of Ni2+
Ni2+, Cu2+, Zn2+ and Fe3+) in acetonitrile. Fluorescence emission and Fe3+. While there is no change in emission color upon
of BPVMB upon addition of metal ions is shown in Fig. 2a. The addition of various metal ions. Of particular interest is Cu2+
Cu2+ and Ni2+ could cause fluorescence quenching, while Zn2+ which leads to fluorescence quenching in acetonitrile, yet in
and Cd2+ induce a slight red shift in fluorescence from 400 nm tetrahydrofuran, it has almost no effect on the fluorescence of
to ~460 nm (blue fluorescence), while Fe3+ causes a red-shift to BPVMB. Such phenomenon suggests that electronic coupling
530 nm (green fluorescent); such red shift enables BPVMB to between BPVMB and Cu2+ and Fe3+ to certain extent is solvent-
easily distinguish Fe3+ among other metal ions, even with naked dependent.
eye.
To validate the feasibility of using BPVMB as Fe3+ selective
fluorescent sensor, we have carried out anti-interference
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In order to understand the fluorescent responseVoiefw BArPtiVcleMOBnlitnoe
DOI: 10.1039/D0TC01082K
Fe , the absorption and fluorescence emission spectra with different
experiments in the presence of various metal ions (Fig. 3). When
BPVMB was treated with 5 equiv. of Fe3+ in the presence of the
other metal ions (Mg2+, K+, Cd2+, Ca2+, Zn2+, Cu2+, Ni2+, Pb2+, Mn2+,
Cr3+ and Al3+) with same concentration, Cd2+ and Mn2+ inhibited
about 60% of the interaction between BPVMB and Fe3+ ions
judging from the change of F516/F416. Therefore, BPVMB can be
used as a selective fluorescent sensor for Fe3+ in the presence
of these competing metal ions tested in this work.
3+
Fe3+ concentration were recorded. The absorption spectrum exhibits
a peak value at 300nm for BPVMB in acetonitrile and peak value at
240nm, 320nm, 360nm for Fe3+ in acetonitrile (Fig. S1). Upon
addition of 10uM Fe3+ to the BPVMB solution, the absorbance at
~300 nm gradually red-shifts and eventually disappears with a
concomitant increase of two new peaks at 240nm and 360nm. These
two peaks are due to the increasing concentration of Fe3+(Fig. S2).
The isosbestic points at 275nm and 325nm indicate that the spectra
change is due to relative compositional changes between BPVMB
and Fe3+. Fluorescence emission spectra of BPVMB is shown in Fig. S3
upon excitation at 350 nm. Upon addition of Fe3+, the fluorescence
intensity at 416 nm decreases until the concentration ratio of BPVMB
and Fe3+ reaching 1:2. With continuous adding aliquots of Fe3+ based
on the concentration ratio of BPVMB and Fe3+ from 1:2 to 1:4, a red-
shifted broad emission band centered at 516 nm increases gradually.
Fluorescence detection limits of BPVMB for Fe3+ was determined
from linear plot of F516/F416 versus [Fe3+] in acetonitrile solution (Fig.
S11). The detection limit is 13.109 μM calculated on the basis of
3σ/K.39
The above results suggest a charge transfer between Fe3+ and
BPVMB where BPVMB acts as an electron donor and the Fe3+ serves
as an electron acceptor. We hypothesize this to be a two stage
processes. At this first stage, which occurs at low Fe3+ concentration,
charge transfer through space between Fe3+ and BPVMB which
causes quenching of the fluorescence (Fig. 4a). With increasing
concentration of Fe3+ from 0 to 0.6 μM, a significant decrease in the
416 nm emission was observed (Fig. 4c). The upward curvature of
Stern-volmer plot (Fig. S4) is typical for situations in which both
dynamic quenching and static quenching occur.40 Such type of plot
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
S. S. S. S.
Fe Mg
S. S. S. S. S. S. S. S. S.
Cd Ca Zn Cu Ni Pb Mn Cr Al
K
Fe Fe Fe Fe Fe Fe Fe Fe Fe Fe Fe
Fig. 3 Competitive selectivity of BPVMB toward Fe3+ in the
presence of other metal ions. Emission ratio at 516 and 416 nm
(F516/F416) of PBVMB (S.) (50 μM) in acetonitrile induced by
indicated metal ions. The final concentration for Fe3+, Mg2+, K+,
Cd2+, Ca2+, Zn2+, Cu2+, Ni2+, Pb2+, Mn2+, Cr3+ and Al3+ is 250 μM.
Excitation was at 350 nm. S. = free sensor (PBVMB).
Fig. 4 (a) A schematic representation of the through-space charge transfer which leading to the fluorescence quenching at lower
Fe3+ concentrations (the first stage) (b) formation of intra-molecular charge transfer complex at higher concentrations (the second
stage); (c) Fluorescence quenching of BPVMB in the presence of low Fe3+ concentrations, from 0 - 6μM Fe3+; “turn off” (d) “turn-
on” state with a large red shift from 415 nm to 520 nm at higher Fe3+ concentrations.
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Fig. 5 TA spectra and kinetics of BPVMB and BPVMB:Fe3+ solution measured with 350 nm excitation. (a) TA spectra of BPVMB and
different proportions of BPVMB:Fe3+ solution at 2 ps. (b) Peak positions of different proportions of BPVMB:Fe3+ solution at about
418 nm. (c)-(d) TA spectra of BPVMB:Fe3+=1:3 solution within and after 20 ps.
indicates a complex interaction mechanism, which indicates the and time were monitored by a white light continuum probe pulse,
presence of specific binding interactions and more than one which providing
a complete record of charge transfer and
quenching processes: concomitant through space charge transfer recombination processes. In the Fig. 5a, the TA spectra of the
and intramolecular charge transfer in this situation.41 This is different proportions of BPVMB:Fe3+ solution at 2 ps are shown. In
consistent with our hypothesis that both complex between the the picture, it can be seen that there are four obvious ultra-fast
BPVMB and the Fe3+ ions and non-complexed Fe3+ formed during signals, namely two negative signals at about 360 and 550 nm, and
static and dynamic quenching.
two positive signals at about 420 and 650 nm, respectively. The peak
As the addition of Fe3+ to the BPVMB solution from 0.6 μM, a red- of 360 nm consists of a major band of the ground-state bleaching
shifted broad emission band centered at 530nm increases gradually (GSB) from the absorption of Fe3+ that due to it is not present in the
(Fig. 4d), which is highly likely due to the BPVMB-Fe3+ complex signal of BPVMB sample. While for the peak of 550 nm, it close to the
formation through bonding of the pyridine moiety with Fe3+. PL peak position of green light emission of the BPVMB:Fe samples
Therefore, we propose that the large red shift in the second stage is with a ratio of more than 1 : 2 according to the Fig. S3 (a little red
dominated by intramolecular charge transfer among BPVMB and Fe3+ shift may come from the thermal effect), indicating that this peak is
(Fig. 4b). The disassociation constants (Kd) of BPVMB with Fe3+ in derived from the stimulated emission (SE) of new combining state
these two stages at 416 nm and 516 nm were determined by between BPVMB and Fe (the PL emission peak of the pure Fe3+
emission spectroscopy (Fig. S5) to be 9.5×10-4 and 1.07×10-4, solution is at 415 nm and is very weak). The peak of about 650 nm is
respectively.27 The smaller dissociation constant for the second stage derived from excited state absorption (ESA) of Fe3+, while the peak
might indicate the BPVMB-Fe3+ complex formation at higher Fe3+ of about 420 nm is more important signal in the BPVMB:Fe3+ samples
concentration.
that is derived from ESA of BPVMB and charge transfer (CT) from
In order to analyse the carrier kinetics between iron ions and BPVMB to Fe3+ (this CT process will be described in detail below). In
BPVMB, the details of pump-probe TA spectra are described in Fig. 5. addition, it is worth noting that with the concentration of Fe ions
A pump pulse at 350 nm (∼100 fs and 1 kHz) was used to excite the increases in the BPVMB:Fe3+ samples, the peak position of about 420
BPVMB and different proportions of BPVMB:Fe3+ solution. The nm gradually shifts blue as shown in Fig. 5b. It implies that the CT
induced absorption changes (ΔOD) as a function of both wavelength from BPVMB to Fe3+ gradually increases with the increases of Fe3+
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Fig. 6 (a) Schematic illustration of charge carrier dynamics in the BPVMB:Fe3+ samples. (b) Kinetics of transient of the BPVMB and
BPVMB:Fe3+=1:3 solution at 418 nm and 460 nm. The solid lines are multiexponential fits to these kinetics.
concentration, and finally reaches a saturation value. It is well known 418 nm will appear, and then the carriers transfer to about 460 nm
that the CT often occurs within 20 ps. In order to focus on this through intra-band relaxation, thereby participating in the radiation
process, the BPVMB:Fe3+=1:3 sample with a more obvious transfer recombination of Fe3+ that produces 550 nm green light emission. It
signal is selected for detailed analysis, which is shown in the Fig. 5c. can be noted that with the increase of Fe3+ concentration and time
Obviously, the GSB signal less change within 20 ps implying that (within 20 ps), the peak position near 420 nm gradually blue shifts.
electrons from excited states have not yet returned to ground states. This is due to the charges accumulation before they transferred to
However, the positive signal at 418 nm is significantly reduced while the state of 460 nm. Thus, the intensity of ~418 nm will decrease and
the signal at 460 nm is significantly increased. In addition, the the ~460 nm will increase. The transient signal of the BPVMB and
positive signal at about 418 nm is gradually blue shift. In the Fig. 5d, ground state after 20 ps. This phenomenon verifies that there is CT
all of the signals gradually return to the ground state after 20 ps. This between BPVMB and Fe3+, and the charge carrier dynamics
phenomenon verifies that there is CT between BPVMB and Fe3+, and
Table 1 The values of the kinetics of transient of the BPVMB
the charge carrier dynamics in the BPVMB:Fe3+ samples have been
shown in Fig. 5.
and different proportions of BPVMB:Fe3+ samples at 418 nm
and 460 nm.
In the Fig. 6a, it shows the ground states and excited states of
BPVMB and Fe3+ and the new combining state between BPVMB and
Fe3+. After being excited by 350 nm, there are the processes of the
GSB, ESA, CT and SE in the sample and have been mentioned above.
Among them, the process of CT from BPVMB to Fe3+ will occur after
the Fe3+ was added, but it may have two cases that at low and high
concentrations, respectively. At low concentration of Fe3+
(BPVMB:Fe3+<1:2), the BPVMB and Fe3+ are not combined due to the
relative low binding constant, and the CT from BPVMB to Fe3+ is only
through space (space charge transfer : SCT, and the positions of
BPVMB and Fe3+ gradually approach with the concentration of Fe3+
BPVMB:Fe3+=1:3 solution at 418 nm and 460 nm represent the
increases). However, most of those carriers will undergo non-
radiative recombination in Fe3+, resulting in quenching of the PL.
While at high concentration of Fe3+ (BPVMB:Fe3+ >1:2), the CT from
BPVMB to Fe3+ will be transferred through the new combined state
(binding charge transfer : BCT), as shown in the Fig. 6a. When the
charges transferred from BPVMB to Fe3+, a positive signal at about
kinetic of this process. The solid lines are multiexponential fits to
these kinetics, are shown in the Fig. 6b and Table 1.
It can be seen that the BPVMB sample has two lifetimes
components that may come from the ESA and SE (0.36 and 2480 ps,
respectively). Note that those two lifetimes component, each occupy
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Fig. 7 (a) Emission spectra of (1) ligand BPVMB (blank), (2) BPVMB + Cu2+, and (3) BPVMB + Cu2+ + Fe3+. (b) Emission spectra of
BPVMB (5 µM) in acetonitrile obtained by adding aliquots of 80 µL CuCl2 (0.4 mM) solution (c) Emission spectra of BPVMB (5 µM)
in acetonitrile obtained by adding aliquots of 125 µL FeCl3 (0.4 mM) solution. Excitation was at 320 nm. (d) Visible emission
observed from samples of BPVMB, BPVMB/Cu2+, and BPVMB/Cu2+/Fe3+ under UV-lamp (365 nm).
~50% that gives rise to a very weak TA signal of BPVMB. After the Fe3+
The emission response of BPVMB against various metal ions
was added, the system has one more carrier transport channel (CT), showed a remarkable selectivity of Fe3+ binding in two stages, and
therefore it will have three part of lifetimes, namely the CT (t1), the green emission starts from the molar ratio of Fe3+: BPVMB equal
nonradiative recombination or intraband relaxation (t2), and to 2:1. However, for practical applications, ratiometric signal is
radiative recombination (t3), respectively. At low concentration of expected to be more exclusive at even lower concentration. In
Fe3+ (BPVMB:Fe3+<1:2), the value of t1 decreases with the content of addition, one of the most essential criterions for cation probe is the
Fe3+, indicating that the process of CT though space is more faster ability to detect a specific cation in the presence of other competing
due to the Fe3+ gradually combines with BPVMB. And also, the t2 and ions. Herein, we report detection of Fe3+ based on an intermolecular
t3 are faster with the increase of Fe3+ because of the carrier SCT (through space) and intramolecular (through bond) charge transfer
transport channel gradually become BCT. After the ratio of mechanism. In addition, we performed an experiment to detect Fe3+
BPVMB:Fe3+>=1:2, the green light emission appears, indicating the using a displacement strategy to achieve a highly selective “off-on”
BCT channel had become the main transmission process. The kinetics type fluorescent sensor (Fig. 7d). The displacement approach was
of transient of BPVMB:Fe3+=1:3 at 418 nm, the time of the CT process first developed to increase the emission shift of ratiometric signal of
is ~680 fs, which implying the Fe3+ sensing in this work is super Zn2+.27 To date, only a few examples of achieving such indirect cation
sensitive. While for the situation of the 460 nm, it only exists two part exchange in one complex-based chemosensor is reported in the
of lifetimes (~12 ps from intraband relaxation and ~900 ps from literature, especially for Fe3+.42,43 In our case, the addition of Cu2+
radiative recombination), which is basically consistent with the quenched the fluoresce as shown in Fig. 7b. Since the affinity of
carriers’ lifetime at 418 nm. It implies that the increase of the carriers BPVMB (Fig. S7) with Fe3+ (Kd=1.35× 10−4) is larger than BPVMB-Cu2+
of the new combining state at 460 nm is derived from the CT at 418 (Kd=79.9×10−4) (Fig. S6). When Fe3+ was added to the solution of
nm (namely the BPVMB). This result is consistent with the previous r BPVMB-Cu2+ complex, Cu2+ was displaced by Fe3+ (Fig. 7c). The
esults.
titration profile based on the emission ratio at 516 and 416 nm,
F516/F416 and the results are shown in Fig. S8. As can be seen, there is
a linear dependence of the intensity ratios of emission at 516 nm to
that of 416 nm (F516/F416) on when concentration of Fe3+/BPVMB is
Detection of Fe3+ based on the displacement approach
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within the range from one to three, suggesting, the addition of Fe3+
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into the quenched BPVMB-Cu2+ complex leads to the emergence of
green emission resulting from the formation of BPVMB-Fe3+. The
above results further validate that the non-fluorescent BPVMB-Cu2+
complex can be used as a latent off-on sensor for Fe3+. In summary,
fluorescence detection limits of BPVMB for Fe3+ by displacement
approach were tested. A plot of F516/F416 versus [Fe3+] in acetonitrile
solution gave a linear relationship (Fig. S12). The detection limit is
2.631 μM calculated on the basis of 3σ/K.39
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Conclusions
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We have synthesized a new fluorescent probe BPVMB for
ratiometric and colorimetric Fe3+ sensing. BPVMB has the
strongest affinity with Fe3+ among all heavy and transition metal
ions and a high selectivity for Fe3+ in two interaction modes:
through space and through bond charge transfer. The
fluorescent quenching was observed at low Fe3+ concentration
(below 6 µM) and then a large red-shift in emission from 416
nm to 520 nm as the Fe3+ increases from (7 µM to 20 µM)
resulting from the formation of the Fe3+-BPVMB complex. Also,
ratiometric and colorimetric detection of Fe3+ with higher
sensitivity can be achieved via a Cu2+ displacement approach.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research was funded by the Leading Talents of
Guangdong Province Program (2016LJ06N507), Shenzhen Basic
Research Fund (CYJ20170817110652558), the National Key
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Research
and
Development
Program
of
China
(2018YFB0704100), the Key-Area Research and Development
Program of GuangDong Province (2019B010941001).
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