A real-time fluorescent sensor specific to Mg2+: Crystallographic evidence, DFT calculation and its use for quantitative determination of magnesium in drinking water

Article abstract of DOI:10.1039/c4dt03068k

An off-the-shelf fluorescence turn-on Mg²⁺ chemosensor 3,5-dichlorosalicylaldehyde (BCSA) was rationally designed and developed. This proposed sensor works based on Mg²⁺-induced formation of the 2:1 BCSA-Mg²⁺ complex. The coordination of BCSA to Mg²⁺ increases its structural rigidity generating a chelation-enhanced fluorescence (CHEF) effect which was confirmed by single crystal XRD studies of the BCSA-Mg²⁺ complex and TD/DFT calculations. This sensor exhibits high sensitivity and selectivity for the quantitative monitoring of Mg²⁺ with a wide detection range (0-40 μM), a low detection limit (2.89 × 10^-7 mol L^-1) and a short response time (<0.5 s). It can also resist the interference from the other co-existing metal ions, especially Ca²⁺. Consequently, this fluorescent sensor can be utilized to monitor Mg²⁺ in real time within actual samples from drinking water. This journal is

Full text of DOI:10.1039/c4dt03068k

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DOI: 10.1039/C4DT03068K
A real-time fluorescent sensor specific to Mg2+:
crystallographic evidence, DFT calculation and its use
for quantitative determination of magnesium in
drinking water†
Guangwen Men, Chunrong Chen, Shitong Zhang, Chunshuang Liang, Ying Wang,
Mengyu Deng, Hongxing Shang, Bing Yang and Shimei Jiang*
An “off-the-shelf” fluorescence “turn-on” Mg2+ chemosensor BCSA has been
developed for real-time quantitative monitoring of magnesium in drinking water.

Dalton Transactions
Page 2 of 9
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Cite this: DOI: 10.1039/c0xx00000x
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ARTDIOCI: 1L0.1E039/TC4YDT0P30E68K
A real-time fluorescent sensor specific to Mg2+: crystallographic
evidence, DFT calculation and its use for quantitative determination of
magnesium in drinking water†
Guangwen Men, Chunrong Chen, Shitong Zhang, Chunshuang Liang, Ying Wang, Mengyu Deng,
Hongxing Shang, Bing Yang and Shimei Jiang*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
An “off-the-shelf” fluorescence “turn-on” Mg2+ chemosensor 3, 5-dichlorosalicylaldehyde (BCSA) was
rationally designed and developed. This proposed sensor works based on Mg2+-induced formation of the 2:1
BCSA-Mg2+ complex. The coordination of BCSA to Mg2+ increases its structural rigidity generating a
chelation-enhanced fluorescence (CHEF) effect which was confirmed by single crystal XRD studies of the
BCSA-Mg2+ complex and TD/DFT calculation. This sensor exhibits high sensitivity and selectivity for the
quantitative monitoring of Mg2+ with a wide detection range (0-40 μM), a low detection limit (2.89×10-7
mol/L) and a short response time (< 0.5 s). It can also resist the interference from the other co-existing metal
ions especially Ca2+. Consequently, this fluorescent sensor can be utilized to monitor Mg2+ in real-time within
actual samples from drinking water.
etc. However, these synthetic fluorescent sensors demand long
preparation time and high cost of synthesis. As an improvement,
salicylaldehyde derivatives would be better for the design of
Mg2+ sensors because they could not only chelate various metal
ions accompanied by the spectroscopic changes but also be
purchased from commercial source without further synthesis. But,
until now, fluorescent Mg2+ sensors based on salicylaldehyde
derivatives have not been reported.
Introduction
Magnesium ion, one of the most abundant intracellular divalent
cations, has been recognized as a vital biological ion for all living
things over the past century.1 It plays an essential role in many
biological processes.2 From a health point of view, the daily
intake of magnesium for an adult human should reach ca. 300 mg,
which may come from green vegetables, dairy products, grain
and seafood.3 Deficiencies or excesses of intake of this ion can
lead to a variety of diseases.3, 4 Mineral drinking water may also
supply magnesium in fair quantities.5 The standard method for
the determination of Mg2+ in drinking water is EDTA
complexiometric titration.6, 7 However, sometimes this method is
laborious, time-consuming, reagent-consuming, and poorly
removes of the interference from several metal ions. In this
regard, the development of reliable and sensitive analytical
methods for Mg2+ is highly desirable.
On the other hand, it is known that the most important feature
of the Mg2+ sensors is their potential application in the field of
quantitative Mg2+ discrimination in drinking water. Such Mg2+
sensors require high selectivity toward the analyte over the other
co-existing species. Most of the reported Mg2+ sensors are limited
in distinguishing Mg2+ from large quantities of Ca2+ owing to the
similar chemical properties of Mg2+ and Ca2+ 16 and few examples
providing selectivity in Mg2+ recognition have been reported.15a
However, to the best of our knowledge, there is still a research
bottleneck with regard to the development of designing highly
selective Mg2+ fluorescent sensors in practical applications,
particularly for the quantitative detection of Mg2+ in drinking
water.
Recently, fluorescent chemosensors appear to be particularly
attractive on account of their highly sensitive and selective, easy
to fabricate and non-destructive properties as well as their
capability of remote and in situ monitoring of optical
measurement.8 Tremendous amounts of effort have been put forth
for the development of the new fluorescent sensors for metal ions
during the past few decades.8a, 9 Compared to several transition
metal ions, such as Hg2+, Cu2+ and Zn2+,10 only a few fluorescent
chemosensors have been developed for the detection of Mg2+ due
to its spectroscopic silence.11 Generally, previous designs for
Mg2+ fluorescent sensors are based on several metal chelating
structures that have a high affinity toward Mg2+; these sensors
Herein, we have developed a fluorescence “turn-on” Mg2+
sensor
based
on
a
simple
commercial
3,
5-
dichlorosalicylaldehyde BCSA (Chart 1). As expected, upon
adding Mg2+, the 2:1 BCSA-Mg complex producing light blue
fluorescence was formed and the emission spectra underwent an
obvious blue-shift and an enhancement. According to this
significant fluorescence signal change, BCSA could detect Mg2+
sensitively, selectively and quantitatively with a wide detection
range (0-40 μM), a low detection limit (2.89×10-7 mol/L) and a
short response time (< 0.5 s). Moreover, it could resist the
include β-diketone,11e crown ether,11c,
polyether,
12
13
calix[4]arene,14 carboxylic acid,11b salicylaldehyde Schiff base,15
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Dalton Transactions
interference from the other co-existing metal ions especially Ca2+.
Mg2+ chemosensor. In order to further study the structural
These results made the application of the quantitative and real-
time detection of Mg2+ in drinking water a reality. In order to
facilitate the applications in portable field analysis for BCSA, the
PMMA thin film doped with BCSA molecules were fabricated
and used to monitor Mg2+ in drinking water selectively.
contribution of 2-hydroxy group to Mg2+ binding, BCMB in
View Article Online
which the involved proton was substituted by a methyl group was
DOI: 10.1039/C4DT03068K
designed (Chart 1). Unlike BCSA, there was no obvious change
in the fluorescence spectra of BCMB after adding different metal
ions including Mg2+ (Fig. S1d, ESI†). The main reason is the
existence of 2-methoxy group prevents BCMB from the
deprotonated process. Hence, in this work, we preferred BCSA as
a Mg2+ fluorescent sensor and tried to explore its Mg2+ detection
properties, mechanisms and applications.
Mg2+ detection properties of BCSA.
According to previous works, 19 salicylaldehyde derivatives
exist as intramolecular hydrogen-bonded, closed conformers in
non-polar solvents. Whereas in polar protic solvents that can
break the intramolecular hydrogen bond of the closed conformer,
the strongly solvated open conformers are dominant. Hydroxyl
derivatives of aromatic compounds are generally acidic in the
excited singlet state relative to the ground state.20 Therefore, there
is a tendency for BCSA in the excited state to undergo
intermolecular proton transfer with the polar protic solvent
(Scheme 1); the resulting deprotonated form of BCSA could
possibly show a large Stokes-shifted emission.21
Chart 1. Chemical structures of SA, DBHB, BCSA and BCMB.
Results and discussion
Rational design of the Mg2+ sensor based on salicylaldehyde
derivatives and its spectral properties.
Compared to the well developed salicylaldehyde Schiff base
17
chemosensors,
their simple precursor salicylaldehyde
derivatives have rarely been used in the field of metal ions
detection through the chelation of its carbonyl O and phenol O9b.
This is probably due to the weak binding constants and poor
optical properties for their metal complexes. These factors are
known to be readily influenced by substituents on the benzene
ring of salicylaldehyde derivatives. In this regard, the following
simple, commercial compounds were selected: non-substituted
SA, DBHB substituted by 3, 5-position tert-butyl groups, and
BCSA substituted by 3, 5-position chlorine atoms (Chart 1).
These compounds were subjected to examine their responses to
different metal ions (Fig. S1, ESI†). The results showed no
fluorescent emission for SA in the fluorescence spectra. However,
this compound displayed a weak fluorescent emission after the
addition of Al3+ (Fig. S1c, ESI†). These observations are similar
to those reported by Wu et al.9b Both DBHB involving electron
donating groups and its mixture solutions with different metal
ions showed no fluorescence (Fig. S1b, ESI†). While BCSA
involving electron withdrawing groups in 3, 5-position
substituents on the phenol group exhibited a fluorescent emission
at 509 nm and it could selectively respond to Mg2+ based on a
strong light blue emission (Fig. S1a, ESI†). These results
indicated that 3, 5-position substituents on the phenol group not
only distinctly affected the fluorescence intensities of
salicylaldehyde derivatives and their metal complex,17a, 18a but
also determined the types of the analytes. This was because the
dichloro-substitution increased acidity of the phenolic part of
BCSA18b and thereby promoted the deprotonated process after
which phenol O takes part in metal ion binding. On the basis of
its excellent selectivity, BCSA was selected as a candidate for
Scheme 1. The ground state reaction and photochemical scheme of
BCSA in polar protic solvents.
In order to satisfy actual application, the sensor BCSA was
used in the HEPES buffer-ethanol solution; hence it exists
predominantly in its open conformer of the enol form in the
ground state. In this solution, BCSA shows a single large Stokes-
shifted emission band centered at 509 nm with a relatively low
quantum yield (Φf) of 0.018. So this fluorescence band mainly
belongs to the emission of the deprotonated form of BCSA (Fig.
1a). Gradual titration of Mg2+ led to a large increase in the
intensity of the emission as the fluorescence peak underwent a 28
nm blue shift from 509 to 481 nm. The intensity of the
fluorescence peak had a 22-fold enhancement and a high
quantum yield of 0.333 whenever the solution was saturated with
~3 equiv. Mg2+. As a result, the fluorescent color of the solution
turned from green to light blue (inset in Fig. 1a). This is
presumably attributed to the formation of the BCSA-Mg2+
complex which causes a chelation-enhanced fluorescence (CHEF)
effect.15c, 22
More interestingly, it can be noticed that the increasing
fluorescence intensity of BCSA had
a significant linear
correlation with the concentration of Mg2+ over a wide range of
0-40 μM (Fig. 1b). Hence, BCSA can accomplish the quantitative
analysis of Mg2+, and its wide detection range makes it possible
to meet more requirements in practical applications. The
detection limit of BCSA calculated as 2.89×10-7 mol/L from the
fluorescence titration spectra. 23 This result indicates that the
proposed fluorescence turn-on sensor shows quite high sensitivity
2
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Dalton Transactions
Page 4 of 9
toward trace amounts of Mg2+ compared to those of some
according to fluorescence spectroscopy. From the practical point
previously reported Mg2+ sensors.14b, 15b
of view, we conducted competitive experiments at the proper
concentration level (mg/L) close to thDatOIo: 1f0.r1e0a3Vl9ie/Cswa4AmDrtTipc0llee3Os0n6li8innKe
commercial mineral drinking water. Negligible interference to the
Mg2+ sensing was brought by the introduction of Zn2+, Ca2+, K+,
Li+, Na+, Sr2+, Ba2+ and NH4+ cations (Fig. 2b). Evidently, BCSA
exhibits excellent interference immunity to the common mineral
cations in drinking water, which is comparable with previous
fluorescent Mg2+ sensors.11f, 12a, 12b, 13, 14a
Fig. 1 (a) Fluorescence spectra of BCSA in a 95:5 (v/v) ethanol-HEPES
(0.05 M, pH = 7.0) solution upon addition of increasing concentration of
Mg2+ (λex = 400 nm, [BCSA] = 50 μM). The inset is a plot of intensity
change vs. concentration of Mg2+ added. (b) The fluorescence intensity at
481 nm of BCSA (50 μM) was linearly related to the concentration of
Mg2+ (0-40 μM).
The response of BCSA to Mg2+ was confirmed to be reversible
according to the titration of the Mg2+ bonding agent, EDTA
disodium (Fig. S3, ESI†). The addition of EDTA disodium to the
mixture of BCSA and Mg2+ solution resulted in a reduction of the
emission and a regeneration of free BCSA. The reversibility is
beneficial to the fabrication of reusable devices for sensing Mg2+.
These observations demonstrate BCSA to be a reversible high-
performance Mg2+ chemosensor with a fluorescence turn-on
signaling. Moreover, Fig. S4 depicts the fluorescence intensity
changes of the bands of BCSA and BCSA-Mg2+ at 481 nm in
different organic solvents-HEPES solutions. It is obvious that the
fluorescent response of BCSA to Mg2+ happened in all these
solvents. The results above manifest that this sensor can
sensitively detect Mg2+ in different water miscible organic
solutions.
Fig. 2 (a) (I-I0)/I0 ratios of BCSA (50 μM) in the presence of 8 equiv. of
various metal ions in a 95:5 (v/v) ethanol-HEPES (0.05 M, pH = 7.0)
solution. (b) Fluorescence intensity of BCSA-Mg2+ in the presence of
other competing metal ions in a 95:5 (v/v) ethanol-HEPES buffer solution.
The concentration of BCSA was 50 μM, while that of Mg2+ and other
cations were 10 mg/L ([Ca2+] = 30 mg/L) in HEPES buffer (0.05 M, pH =
7.0), respectively (λex = 400 nm).
To evaluate the selectivity of BCSA in practice, changes of the
fluorescence intensity at 481 nm of BCSA caused by Mg2+ and
some other metal ions including alkali, alkaline earth, and
transition metal ions under the same conditions were investigated.
As shown in Fig. 2a, only Mg2+ led to a prominent fluorescence
enhancement, while the other ions did not cause obvious
interference. It is worth mentioning that except for Zn2+ and Ca2+,
the other competitive metal ions displayed no further increase of
emission intensity other than a certain extent quenching of the
fluorescence of BCSA. Meanwhile, the titration of Mg2+ caused a
22-fold fluorescence enhancement that was approximately 9
times larger than the value induced by an equivalent addition of
Zn2+ and 20 times larger than that of Ca2+. These results indicate
that BCSA can easily distinguish Mg2+ from other cations
Fig. 3 Time course of the fluorescence response of BCSA (50 μM) to
Mg2+; arrows represent addition of the equivalent Mg2+. Inset: The
magnification of the region between 62.4 and 63.2 s (λex = 400 nm).
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In addition, a short response time is necessary for a fluorescent
evidence was provided by comparing the ESI mass spectra of
BCSA and BCSA-Mg2+. In absence of Mg2+, a peak at m/z =
188.9 corresponding to [BCSA-H]- in theDEOSI:I10m.1a0s3Vs9ie/sCwp4AeDrctTict0rleu3Om0n6l8inKe
under negative ion mode was clearly observed (Fig. S5, ESI†).
While adding 4 equiv. Mg2+, the formation of 2:1 complex
between BCSA and Mg2+ was further confirmed by the
chemosensor to monitor Mg2+ in real samples. So we also carried
out the time course of the response for BCSA to successive
addition of the equivalent Mg2+ (Fig. 3). Excitingly, we found the
response was extremely rapid and completed in less than 0.5 s
(inset in Fig. 3). Therefore, BCSA is likely to be used for the
real-time detection of Mg2+ because it and its immobilization
materials can be applied to in-situ, fast detection in drinking
water.
emergence of the peaks at m/z
= 494.1 attributed to
[2BCSA+MgCl2+Na]+ (Fig. S6, ESI†) and m/z = 474.7 attributed
to [2BCSA+MgCl2]- (Fig. S7, ESI†) in the ESI mass spectra
under positive and negative ion modes, respectively.
Mechanisms of Mg2+ detection.
First, a preliminary explanation of the sensing mechanism was
investigated by the UV-vis spectroscopic titration method. In the
absorption spectra of BCSA, the maximum long-wave absorption
band in the range 300-395 nm that centers at 341 nm is assigned
1
to a S0 → ππ* transition absorption and its shoulder absorption
band around 400 nm may be owing to the ground state
intermolecular hydrogen bonding between solute and solvent
21
interaction19,
(Fig. 4a). Upon the addition of increasing
concentrations of Mg2+, the original absorption band at 341 nm
decreased and the absorption band centered at 400 nm enhanced
gradually. The changes in UV-vis absorption spectra induced by
Mg2+ are probably due to the coordination interaction between
BCSA and Mg2+ ion.
Fig. 5 Partial 1H NMR spectra of BCSA (5mM in DMSO-d6) in the
presence of increasing equivalents of Mg2+.
Further efforts were put forth to study the interaction of BCSA
with Mg2+ by means of the 1H NMR titration experiment (Fig. 5).
With the addition of excess Mg2+, the signal of the hydroxyl
proton Ha experienced an obvious upfield shift from 11.26 to
11.19 ppm and a decrease in intensity. This observation was due
to the coordination between Mg2+ and 2-hydroxy O atom of
BCSA with the deprotonation of the phenolic hydroxyl group. At
the same time, the aldehyde proton Hb at 10.16 ppm was shifted
upfield toward 10.13 ppm, indicating the involvement of the
carbonyl O. These data give the conclusion that the binding of
BCSA to Mg2+ generates a rigid structure through a chelating
reaction via the carbonyl O and phenol O.
Fig. 4 (a) The absorption spectra of probe BCSA (50 μM) in a 95:5 (v/v)
ethanol-HEPES (0.05 M, pH = 7.0) solution upon addition of increasing
concentrations of Mg2+ (0-400 μM). (b) Absorbance changes of band at
400 nm of BCSA and Mg2+ with a total concentration of 50 μM in a 95:5
(v/v) ethanol-HEPES (0.05 M, pH = 7.0) solution, indicating a 2:1
stoichiometric ratio of BCSA:Mg2+.
Fig. 6 The crystal structure of the compound BCSA-Mg2+. One part of
disordered THF molecule is omitted for clarity.
Light yellow-colored, strong blue fluorescent crystals were
formed after slow evaporation of solvent from the sample that
contained BCSA and magnesium acetate in THF.24 For the sake
of a deep insight of the BCSA-Mg2+ interaction, these crystals
were analyzed by single crystal X-ray diffraction. As illustrated
in Fig. 6, the crystal structure of BCSA-Mg2+ contains one unique
The mode of coordination of BCSA with Mg2+ was revealed
by the continuous variation (Job’s plot, Fig. 4b) analysis of the
absorption spectra, which showed a maximum at 0.33 mole
fraction of Mg2+. This proved the formation of
a
2:1
stoichiometric compexation of BCSA with Mg2+. More direct
4
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Page 6 of 9
Mg2+ center, two independent BCSA and two coordinated water
Mg2+ may be calculated as the difference between hardness and
molecules, which is in accordance with the conclusion deduced
by Job’s plot and ESI mass spectra. Moreover, the Mg2+ atom is
six-coordinated in the form of a slightly distorted octahedron by
four O atoms from two bidentate BCSA anion ligands with cis-
configuration and two O atoms from two coordinated water
molecules. These crystallographic data produce solid evidence
and confirm the 1H NMR data obtained in solution.
calcium as CaCO3 if suitable inhibitors are used in the hardness
View Article Online
titration and interfering metals are present in non-interfering
DOI: 10.1039/C4DT03068K
concentrations in the calcium titration. According to our proposed
method, a 4.75 mL ethanol solution containing a spot of BCSA
was added to the 0.25 mL HEPES buffer solution that was
prepared by real water samples to keep the pH value at 7.0. As
soon as the mixture solutions were prepared, we could easily
observe their fluorescent color changes from green to light blue
by naked eye, indicating the existance of Mg2+ in these real
sample could be quickly determined. The details of the emission
signal variations in these recognition systems were monitored by
their fluorescence spectra (Fig. S9, ESI†). The data of the
fluorescence intensity at 481 nm was fed back to the external
calibration curve that was obtained above (Fig. 1b) and then the
Mg2+ concentrations of the samples were obtained from this curve.
All the measurements were performed three times. The analysis
results summarized in Table 1 shows good agreement between
the proposed optosensing method and the standard EDTA
titration method for Mg2+ concentrations in real water samples
with a relative error less than 2%. This indicates that the proposed
sensor is applicable for real-time quantificational detection of
Mg2+; it can meet the sensitivity and selectivity requirements for
practical applications involving the real water sample monitoring.
For the purpose of throwing light upon the mechanisms of the
fluorescence signal changes for Mg2+ sensing as mentioned above,
quantum calculations were carried out with the assist of
Gaussian09_B.01 package, 25 involving the natural transition
orbital (NTO)26 analysis on the excited states (Fig. 7a). Compared
to the familiar frontier molecular orbital (FMO) analyzing
method, the NTO analyzing directly considers the real transition
patterns in the excited states, instead of the base states, which is
more persuasive in describing the characteristics of the excited
states. As shown in Fig 7, from the NTO “Hole” to the “Particle”,
π-electrons of the excited states underwent a transition between 3,
5-position chlorine atoms and aldehyde group for both BCSA and
BCSA-Mg2+. Furthermore, the calculated emission spectra of
BCSA and BCSA-Mg2+ complex in gas phase were centered at
498 nm (2.49 eV) and 443 nm (2.80 eV), respectively. This
agrees with the available experimental data (centered at 500 and
481 nm, respectively). Such a result could explain the appearance
of Mg2+-induced blue shift of the emission in the fluorescence
spectra. In the DFT optimized lowest excited-state geometry of
the enol form of BCSA (Fig. S8, ESI†), there is a free proton
located between the carbonyl O and phenol O. This means that in
the excited singlet state, BCSA is generally acidic and exists in
its deprotonated form, leading to a large Stokes-shifted emission,
which is in accordance with the discussion above.
Table 1. Mg2+ concentrations found in different commercial drinking
water samples.
Fig. 7 S1→S0 natural transition orbital (NTO) and energy level diagram
for the emissions of BCSA and BCSA-Mg2+ complex calculated by
TD/DFT method. The NTO “Hole” and “Particle” represent the visualized
wave functions of the excited molecules. The properties of the transition
pattern illustrated above are both 99.9% according to the calculations.
Fig. 8 (a) Fluorescence image of BCSA-doped PMMA sheets upon
addition of different metal ions ([M] = 200 μM). (b) Fluorescence
changes of BCSA-doped PMMA sheets toward Mg2+ and EDTA-2Na. (c)
Fluorescence changes of BCSA-doped PMMA sheets toward three
drinking water samples.
Applications of the Mg2+ sensor and its PMMA thin film:
Detecting in real samples from bottled drinking water.
The practical applications of the designed fluorescent
chemosensor were first evaluated by detecting Mg2+ in real water
samples from commercial bottled drinking water (obtained from
three brands available in China), and the results were compared
with those obtained from the standard EDTA titration method.
According to the EDTA titration method, the concentration of
Functional material modified fluorescent dye probes are known
for their great potential in being the environment monitoring
device which can be convenient for applications in the future. So
we were also devoted to the design and the fabrication of
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Dalton Transactions
polymeric thin film sensors for Mg2+, using the PMMA thin film
were purchased from commercial resources and used without
as the solid support to anchor the primary dye molecule BCSA.27
This was expected to display the function of Mg2+ sensing. As
shown in Fig. 8a, the unique highly blue fluorescent response
toward Mg2+ demonstrated the high selectivity of this film.
Furthermore, this film has been proved to be reusable according
to the alternate immersion in the solutions containing Mg2+ and
EDTA disodium, respectively (Fig. 8b). Finally, the films were
dipped into the real samples from drinking water and rapidly
dried in the oven. It can be observed from Fig. 8c that the
fluorescence intensity of these films show evident enhancements.
It means that this film sensor can also achieve the fast detection
of Mg2+ in drinking water; therefore, it can reach its full potential
in the area of portable field analysis.
further purification.
View Article Online
DOI: 10.1039/C4DT03068K
Instrumentation
The UV-vis absorption spectra were taken on a Shimadzu 3100
UV-VIS-NIR recording spectrophotometer using a 3 nm slit
width. The fluorescence spectra were determined with
a
Shimadzu RF-5301PC spectrofluorophotometer. 1H-NMR (TMS)
and 13C-NMR were recorded on a Bruker UltraShield 500MHz
spectrometer. Mass spectra were measured on
a Thermo
Scientific ITQ 1100™ GC/MSn and a Q Trap MS (Applied
Biosystems/MDS Sciex, Concord, ON, Canada) which was
equipped with an electrospray ionisation (ESI) source. Elemental
analyses were carried out with a vario MICRO cube elementar.
Unless otherwise mentioned, all the measurements were
performed at room temperature and repeated at least once.
Conclusions
Spectral measurements
In summary, we designed and developed an “off-the-shelf”
fluorescence “turn-on” Mg2+ sensor BCSA that can be purchased
from a commercial source without further synthesis. It works
based on the Mg2+-induced formation of 2:1 BCSA-Mg2+
complex, producing a chelation-enhanced fluorescence effect
which has been studied by various spectroscopic methods
involving fluorescence, UV-vis, ESI mass and NMR spectra. The
binding mode of BCSA with Mg2+ was further confirmed by
single crystal XRD analysis. We also used TD/DFT calculation to
provide an in-depth explanation of the optical properties of the
BCSA and BCSA-Mg2+. In fluorescence spectra, addition of ~3
equiv. Mg2+ led to a significant quantum yield enhancement from
0.018 to 0.333 and a 22-fold fluorescence peak intensity
enhancement. Over a wide Mg2+ concentration range (0-40 μM),
BCSA can quantitatively detect Mg2+ with a low detection limit
of 2.89×10-7 mol/L. It can also accomplish the real-time
monitoring of Mg2+ with a short response time of less than 0.5 s.
Moreover, BCSA shows an excellent selectivity toward Mg2+
over other common metal ions in drinking water. As a result, this
proposed sensor BCSA can be utilized for the quantitative
detection of Mg2+ in real water samples from bottled drinking
water. For the convenience of the utilization in the future, BCSA
was doped into the PMMA film to fabricate one kind of new
Mg2+ polymeric thin film sensor. It was demonstrated to be a
smart portable device which could respond reversibly to Mg2+ in
drinking water with high selectivity.
Stock solutions of the metal ions (10 mM) were prepared in
deionized water. A stock solution of BCSA (2 mM) was prepared
in ethanol. The solution of BCSA was then diluted to 50 μM with
a
95:5 (v/v) ethanol-HEPES buffer solution. In titration
experiments, each time a 2 mL solution of BCSA (50 μM) was
filled in a quartz optical cell of 1 cm optical path length, and the
Mg2+ stock solution was added into the quartz optical cell
gradually by using a micro-pipette. Spectral data were recorded at
2 min after the addition. In selectivity experiments, the test
samples were prepared by placing appropriate amounts of metal
ion stock into 2 mL solution of BCSA (50 μM). For fluorescence
measurements, excitation was provided at 400 nm, and emission
was collected from 403 to 625 nm.
Quantum yield measurement
The quantum yield is measured at room temperature by a single
excitation wavelength (370 nm, which is coming from the Xenon
lamp of the spectrofluorophotometer) referenced to quinine
sulfate in sulfuric acid aqueous solution (3×10-5 mol/L quinine
sulfate and 0.5 mol/L sulphuric acid, Φfr = 0.546) and calculated
28
according to the equation as follow,17d,
where Φfs is the
radiative quantum yield of the sample; Φfr is the radiative
quantum yield of the standard; As and Ar are the absorbances of
the sample and standard at the excitation wavelength,
respectively; Ds and Dr are the integrated areas of the emission
for sample and standard, respectively; Ls and Lr are the lengths of
the absorption cells for the sample and standard test; and Ns and
Nr are the indexes of refraction of the sample and standard
solutions (pure solvents were assumed), respectively.
Experimental section
Materials
1−10− ArLr Ns2 Ds2
All the materials for synthesis and spectra were purchased from
commercial suppliers (analytical grade) and used without further
purification. Solutions of metal ions were prepared from the
corresponding hydrochloride. The solvents were HPLC grade (>
99%, Tianjin Guangfu Fine Chemical Research Institute) and
dried over molecular sieves prior to use. Ultrapure water (18.25
MΩ*cm) from a Millipore Milli-Q purification system was used
throughout the work. The three bottled drinking water samples
are Wahaha natural mineral drinking water, Shengshuang mineral
water and Quanyang Spring drinking water, respectively, which
Φ fs = Φ fr ×
×
×
1−10−AsLs Nr2 Dr2
Preparation of the polymeric thin films
100 mg of the polymethylmethacrylate PMMA polymer (Mw =
12000, Sigma-Aldrich) was dissolved in dichloromethane (100
mL) and doped with BCSA (1 mM). The solution was poured
onto a clean glass surface and a homogeneous, non-fluorescent
polymer sensor film was obtained via spin-coating. This thin film
6
| Journal Name, [year], [vol], 00–00
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Dalton Transactions
Page 8 of 9
was used for Mg2+ detection. For the erasing process, a solution
of EDTA disodium was sprayed onto the film after the film was
dipped into real samples and dried. The non-fluorescent thin film
was restored.
H. Kojima, Y. Urano, H. Komatsu, K. Suzuki and T. Nagano,
View Article Online
Analyst, 2003, 128, 719-723.
DOI: 10.1039/C4DT03068K
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An excess of freshly distilled methyl iodide was dropped in the
DMF (15 mL) solution of 0.1 g BCSA with 0.072 g K2CO3. The
mixture was stirred at 50 ℃ for 14 hours. The entire reaction
mixture was then poured into water (50 mL) and extracted with
diethyl ether (3 × 50 mL). All the organic extracts were collected
and washed with 0.1 N NaOH solution (2 × 25 mL) followed by
water (50 mL). The organic layer was dried over anhyd. Na2SO4
and evaporated to dryness. It was further purified by column
chromatography on silica gel. A white, crystalline solid was
obtained (68% yield). 1H NMR (500 MHz, DMSO-d6): δ 10.20 (s,
1H), 8.05 (d, J = 2.60 Hz, 1H), 7.70 (d, J = 2.60 Hz, 1H), 3.96 (s,
3H). MS: m/z calculated for C8H6Cl2O2 [M+]: 203.97, found
203.93. Elemental analysis calculated for C8H6Cl2O2: C, 46.86; H,
2.95. Found: C, 47.15; H, 3.34.
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Acknowledgements
This work was supported by the National Basic Research
Program of China (2012CB933800) and the National Natural
Science Foundation of China (21374036).
Notes and references
State Key Laboratory of Supramolecular Structure and Materials, Jilin
University, 2699 Qianjin Avenue, Changchun, 130012, P. R. China. E-
mail: smjiang@jlu.edu.cn; Fax: +86-431-85193421; Tel: +86-431-
85168474.
†Electronic Supplementary Information (ESI) available: Characterization
data of the compound BCSA, detailed properties for Mg2+ detection and
other additional data. (Fig. S1-S9, Table S1). CCDC 950175. For ESI and
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This journal is © The Royal Society of Chemistry [year]