Porphyrin-Based Metal-Organic Framework Probe: Highly Selective and Sensitive Fluorescent Turn-On Sensor for M3+(Al3+, Cr3+, and Fe3+) Ions

Article abstract of DOI:10.1021/acs.inorgchem.0c03268

The development of porphyrin-based metal-organic frameworks (MOFs) has attracted significant interest in the scientific community in recent years because of their versatile applications particularly in optical and electronic fields. In this study, a highly selective and sensitive fluorescent turn-on sensor using a porphyrinic MOF, Tb-TCPP, is presented, which displays a 10-fold fluorescence enhancement in the presence of Al3+, Cr3+, and Fe3+ ions. The detection limit is in the nM region. For the Al3+ ion, it could be visually detected at concentrations as low as 5 mM within 15 min. Tb-TCPP could also be used as an indicator for acidic or alkaline solutions at pH values of >9 and <3. The studies on the detection mechanism illustrate that cation exchange proceed between Tb-TCPP and these M3+ ions, and consequently, energy transfer from TCPP to Tb3+ is suppressed and π*-πenergy transfer of the porphyrin ligand is significantly enhanced.

Full text of DOI:10.1021/acs.inorgchem.0c03268

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Article
Porphyrin-Based Metal−Organic Framework Probe: Highly Selective
and Sensitive Fluorescent Turn-On Sensor for M3+ (Al3+, Cr3+, and
Fe3+) Ions
Chenchen Fu, Xinran Sun, Guoda Zhang, Pengfei Shi,* and Ping Cui*
Cite This: Inorg. Chem. 2021, 60, 1116−1123
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ABSTRACT: The development of porphyrin-based metal−
organic frameworks (MOFs) has attracted significant interest in
the scientific community in recent years because of their versatile
applications particularly in optical and electronic fields. In this
study, a highly selective and sensitive fluorescent turn-on sensor
using a porphyrinic MOF, Tb−TCPP, is presented, which displays
a 10-fold fluorescence enhancement in the presence of Al3+, Cr3+,
and Fe3+ ions. The detection limit is in the nM region. For the Al3+
ion, it could be visually detected at concentrations as low as 5 mM
within 15 min. Tb−TCPP could also be used as an indicator for
acidic or alkaline solutions at pH values of >9 and <3. The studies
on the detection mechanism illustrate that cation exchange proceed between Tb−TCPP and these M3+ ions, and consequently,
energy transfer from TCPP to Tb3+ is suppressed and π*−π energy transfer of the porphyrin ligand is significantly enhanced.
microcytic hypochromic anemia.6 Considering the extensive
INTRODUCTION
■
use of these metal ions in industry and consumer products and
Nowadays, environment pollution from industrial, agricultural,
their non-biodegradability, clearly, selective, sensitive, and
and domestic wastes has caused adverse harm to human health.
rapid detection of these ions is quite a importanct topic in
Among the pollutants, metal ions have received significant
industrial wastewater treatment and biological research.
attention. As is well known, many of them play crucial roles in
Until now, various detection methods for trace amounts of
the life, food, and environment systems. Nevertheless, they can
metal ions have been developed, such as electrochemical
become harmful at high levels of exposure. For example, the
techniques, ion mobility spectrometry, inductively coupled
Fe3+ ion has emerged as one of the most essential elements in
plasma−mass spectrometry (ICP−MS), fluorescent methods,
and so forth. Among them, the fluorescent method has
gathered immense attention because of its unique advantages
of simplicity, cost effectiveness, high sensitivity, and prompt
response. Metal−organic frameworks (MOFs) are outstanding
candidates for fluorescence sensors owing to their structural
uniformity, high inherent porosity, designable structures, and
tunable physicochemical properties.7 Many analytes could be
efficiently detected using MOF-based fluorescent sensors, such
as small molecules, ions, solvents, gases, biomolecules, pH, and
temperature.8 Most of the MOFs reported to show sensing
behaviors are based on the electron/energy transfer mecha-
nism, which is closely related to the weak interactions
(coordination, hydrogen bonds, and π−π stacking) between
living systems because low concentrations of Fe3+ perform a
crucial function for electron transfer, oxygen uptake,
hemoglobin formation, and so forth.1 Its overloading in the
body might give rise to serious physiological disorders such as
endotoxemia, gastrointestinal disturbance, and declining
immunity.2 Similarly, another essential trace element,
chromium (Cr), possesses the important function in
promoting the glucose metabolism, and the imbalance of
Cr3+ ingestion might cause diabetes and even malignant cells.3
Although aluminum (Al) is an unessential trace element of the
human body, as the third most abundant element on earth, it
has been extensively involved in industry and daily life to
become one of the most toxic heavy metal pollutants.4 The
tolerable daily and weekly intakes for Al recommended by the
World Health Organization (WHO) are 3−10 mg/day and 7
mg/kg body weight, respectively.5 Study reported that long-
term exposure of humans to high concentrations of Al3+
pollution will cause permanent damage to the central nervous,
skeletal, and hematopoietic systems. It has also been proven
that Al3+ is the main cause of Alzheimer’s disease,
Parkinsonism dementia, low-turnover bone disease, and
Received: November 4, 2020
Published: January 6, 2021
https://dx.doi.org/10.1021/acs.inorgchem.0c03268
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MOFs and analytes.7 At present, MOF-based luminescent
probes predominately show a fluorescence “turn-off” signal,
that is, the emission is quenched. However, “turn-off” sensors
feature low sensitivities and even give the false response
because many other species besides the targeted analytes may
also result in quenching phenomena. These “turn-off” sensors
are hardly employed in real-time applications. Comparatively,
for “turn-on” probes, when analytes are captured, either a
stronger intensity is sparked or wavelength shifts or even new
emission peaks are generated. Analytes can turn on the
fluorescence by limiting the free rotation of the linkers to
enhance the conjugation or by breaking the coordination
bonds in MOFs to prevent the ligand-to-metal charge transfer
or energy transfer.9 Therefore, development of robust and
efficient MOF-derived fluorescence “turn-on” probes for the
recognition of metal ions is more meaningful and highly
demanded.
Porphyrins are excellent near-infrared-emissive fluorescent
materials but nevertheless easily suffer from fluorescence
quenching caused by the aggregation-caused quenching effect.
In order to solve this problem, porphyrin-based MOFs have
been designed and developed. The porphyrin moieties serve as
inherent components in the frameworks, which would
significantly weaken the self-aggregation. Until now, only a
few of the porphyrin-based MOFs have been exploited for
fluorescence detection,10 exhibiting their potential in the field
of sensors. In this context, to exploit “turn-on” luminescent
probes for M3+ (Al3+, Cr3+, and Fe3+) ions, 5,10,15,20-tetrakis
(4-carboxyphenyl) porphyrin (H2TCPP) was selected to
construct a lanthanide MOF Tb−TCPP.11 As shown in
Scheme 1, when Tb−TCPP comes into contact with these M3+
an F-4600 Hitachi fluorescence spectrophotometer at room temper-
ature, and the emission spectra were recorded ranging from 450 to
800 nm with 428 nm excitation. PXRD data were collected on an
Analytical Empyrean instrument using Cu Kα radiation at room
temperature. Inductively coupled plasma atomic emission spectros-
copy (ICP) was measured on the Agilent ICP-OES730 (Agilent,
American). Energy-dispersive spectrometry (EDS) was conducted on
an Oxford instrument.
Synthesis of Tetrakis(4-carboxyphenyl)porphyrin (H2TCPP).
It was synthesized according to the previously reported literature.13
Typically, a 500 mL three-necked flask was equipped with 4-
carboxybenzaldehyde (18.48 g, 0.1236 mol) and propionic acid (300
mL). The mixture was stirred and heated to 100 °C until 4-
carboxybenzaldehyde was completely dissolved. Then, freshly distilled
pyrrole (8.4 mL, 0.12 mol) was added dropwise to the reaction
solution and the resulting dark mixture was stirred and refluxed for 12
h in the dark. After being cooled down to room temperature, the
mixture was placed at −4 °C in the freezer overnight to aid
precipitation. The precipitate was collected by filtration, followed by
carefully washing with H2O and CH2Cl2 and finally drying in vacuum
1
at 60 °C to give a purple solid (7 g, 30% yield). The H NMR of
H2TCPP is shown in Figure S1.
Synthesis of Tb−TCPP. Tb−TCPP was prepared according to a
reported method.11 Typically, to a 50 mL vial were charged H2TCPP
(100 mg, 0.12 mmol), 2-fluorobenzoic acid (10 g, 71.4 mmol), and 20
mL of N,N-dimethylformamide (DMF), and then, the mixture was
ultrasonically dissolved. After that, Tb(NO3)3·6H2O (453 mg, 0.1
mmol) and 5 mL of deionized water were added to the
abovementioned solution and further ultrasonic treatment was
utilized to form a clear solution. The resulting mixture was distributed
to 10 Pyrex vials and heated at 120 °C for 3 days and subsequently
cooled to room temperature. Dark purple hexagonal prism-shaped
crystals were harvested by filtration, followed by washing several times
with DMF until the filtrate became colorless.
Fluorescence Measurements. A suspension of Tb−TCPP was
prepared by adding 2 mg of the powder sample to 8 mL of DMF.
Then, it was ultrasonicated for 30 min, diluted with DMF to 0.05 mg/
mL, and was set aside for subsequent use. Aqueous solutions of metal
nitrates of K+, Na+, Li+, Mg2+, Cu2+, Cd2+, Zn2+, Fe2+, Ca2+, Al3+, Fe3+,
and Cr3+ were prepared, with a concentration of 0.01 M.
Scheme 1. Schematic Representation of Luminescent Al3+,
Cr3+, and Fe3+ Detection Based on Tb−TCPP
Fluorescence Response to Different Metal Ions. A total of 10 μL
0.01 M M(NO3)x (M = K+, Na+, Li+, Mg2+, Cu2+, Cd2+, Zn2+, Fe2+,
Ca2+, Al3+, Fe3+, and Cr3+, x = 1−3) aqueous solution was added to
400 μL of Tb−TCPP suspension prepared above. After 10 s, the
fluorescence of the solution was measured.
Fluorescence Response to M3+ (Al3+, Cr3+, and Fe3+) Ions with
Different Concentrations. A total of 10−80 μL of 0.001 M M3+ (Al3+,
Cr3+, and Fe3+) aqueous solution was added to the Tb−TCPP
aqueous suspension. Then, the fluorescence was measured.
Fluorescence Response to Different Anions. A total of 10 μL of
0.01 M sodium salt (PO43−, SO42−, CO32−, NO3 , F−, Cl−, Br−, I−,
−
and OAc−) aqueous solutions was added to 400 μL of 0.05 mg/mL
Tb−TCPP suspension. After 10 s, the fluorescence of the solution
was measured.
ions, the fluorescence increased immediately. In sharp contrast,
the presence of other metal cations do not result in significant
fluorescence response of Tb−TCPP. The detection limit of
Tb−TCPP for M3+ (Al3+, Cr3+, and Fe3+) ions is in the nM
range, which is lower than that of other MOF-based sensors.12
Millimolar concentration levels of Al3+ could even be visually
detected by Tb−TCPP. EDS, ICP, and powder X-ray
diffraction (PXRD) experiments confirm that the detection
mechanism involves exchange between Tb3+ with respective
M3+ ions.
Fluorescence Response to Different pH. Solutions of 0−12 pH
were prepared with HCl/NaOH. A total of 10 μL of 2 mg/mL Tb−
TCPP was added to 400 μL of aqueous solution with different pH
values prepared above, and the fluorescence of the solution was
measured.
Selectively to Detect Al3+, Cr3+, and Fe3+. A total of 10 μL of 0.01
M M(NO3)x (M = K+, Na+, Li+, Mg2+, Cu2+, Cd2+, Zn2+, and Ca2+; x
= 1−3) aqueous solution was added to 400 μL of Tb−TCPP
suspension prepared above. After 10 s, the fluorescence of the solution
was measured. Then, 10 μL of Al3+, Cr3+, and Fe3+ aqueous solution
was added to the mixed solutions mentioned above, and the
fluorescence of the solution was measured.
EXPERIMENTAL SECTION
■
Materials and Instrumentation. All chemicals were commer-
cially purchased and used without further purification. The scanning
electron microscopy (SEM) images were taken on an S-7800f
scanning electron microscope (Hitachi, Japan). Absorption spectra
were recorded on a Cary 50 UV/Vis spectrophotometer (Perki-
nElmer, Malaysia). All fluorescence experiments were performed on
Fluorescence Response of H2TCPP to Different Metal Ions. A
total of 10 μL of 0.01 M M(NO3)x (x = 1−3) aqueous solution was
added to 400 μL of 0.22 mg/mL H2TCPP suspension prepared
above. After 10 s, the fluorescence of the solution was measured.
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Figure 1. Characterizations of Tb−TCPP. (A) As-synthesized and simulated PXRD patterns of Tb−TCPP; and (B) SEM image of Tb−TCPP.
Fluorescence Response of H2TCPP to Different Anions. A total of
transition of S3 → S0, S2 → S0, and S1 → S0, respectively
(Figure 2).
−
10 μL of 0.01 M sodium salt (PO43−, SO42−, CO32−, NO3 , F−, Cl−,
Br−, I−, and OAc−) aqueous solutions was added to 400 μL of 0.22
mg/mL H2TCPP suspension. After 10 s, the fluorescence of the
solution was measured.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The three-dimensional
MOF Tb−TCPP with free-base porphyrin has been synthe-
sized via a solvothermal reaction between H2TCPP and
Tb(NO3)3·6H2O in DMF and H2O at 120 °C for 72 h,
applying 2-fluorobenzoic acid as a modulator. The structure of
the obtained crystalline material is constructed by 12-
connected Tb9 nonanuclear clusters and multitopic ligands
TCPP, showing a shp-a topology, large channels (1.2 and 0.9
nm), high BET surface area (∼1600 m2 g−1), and significant
chemical and thermal stability.11 The PXRD pattern exhibited
characteristic peaks indexed to the simulated one obtained
from single-crystal data (Figure 1A), thus being indicative of its
structural type and high-phase purity of bulk samples. Next,
their size and morphology were characterized successively by
the SEM method and are shown in Figure 1B. The as-prepared
Tb−TCPP exhibited a prism-like structure with an average
particle size of 100 μm.
Photophysical Properties. MOFs with π-conjugated
aromatic structures and rare earth metal ions have been well
exploited for detection of various analytes.7 Tb−TCPP is
assembled by the emissive Tb3+ ion and fluorescent ligand
H2TCPP, which makes it a promising luminescent material.
Additionally, the remarkable water stability of Tb−TCPP
encouraged us to study its photoluminescence properties for
ion fluorescence sensors. For comparison, the photophysical
property of the organic linker H2TCPP was also examined
(Figure S2). One can see that H2TCPP in DMF solution
featured a typical metal-free porphyrin absorption with a
strong Soret band at 420 nm and four weak absorption Q
bands at 515, 561, 596, and 648 nm. Compared to the
absorption spectrum of H2TCPP, a slight red shift occurred for
the adsorption spectrum of Tb−TCPP, which can be
attributed to the strong coordination between TCPP and
Tb3+.14 On the basis of the absorbance data, the photo-
luminescence property of Tb−TCPP was enumerated by
fluorescence spectroscopy and the emission spectrum dis-
played signature peaks at 610, 652, and 718 nm, respectively,
upon excitation at 428 nm, corresponding to the radiative
Figure 2. Fluorescence of Tb−TCPP with various metal ions in
aqueous solution and (B) relative fluorescence intensity of Tb−TCPP
in the presence of different metal ions at 652 nm (λex = 428 nm).
Fluorescence Detection of Metal Ions. To explore the
fluorescence response of Tb−TCPP toward various metal ions,
Tb−TCPP was first dispersed in DMF solution. Then, 10 μL
of 0.01 M Na+, Li+, K+, Mg2+, Cu2+, Cd2+, Zn2+, Fe2+, Ca2+,
Al3+, Fe3+, and Cr3+ was separately added to each Tb−TCPP
solution. The fluorescence intensity was measured immediately
thereafter under the same experimental conditions. Notably,
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Figure 3. Fluorescence intensity of Tb−TCPP with various concentrations of Al3+ (A), Cr3+ (B), and Fe3+ (C) (λex = 428 nm) and relationship
between fluorescence intensity and the concentration of Al3+ (D), Cr3+ (E), and Fe3+ (F) at concentrations ranging between 0 and 180 μM at 652
nm.
the addition of trivalent M3+ (Al3+, Cr3+, and Fe3+) ions
resulted in approximately 10-fold fluorescence enhancement.
In comparison, the addition of other cations did not result in a
significant fluorescence response. This result indicated that
Tb−TCPP exhibited suitable selectivity and responsiveness to
detect M3+ (Al3+, Cr3+, and Fe3+) ions with a turn-on
luminescent signal.
To further study the sensitivity of Tb−TCPP for sensing
M3+ ions, the quantitative detection for these three metal ions
was investigated. As shown in Figure 3, the fluorescence
intensity increased in an obvious fashion with increasing
concentrations of M3+ (Al3+, Cr3+, and Fe3+) ions. All the
fluorescence measurements can be performed within several
seconds. Moreover, the luminescent emission peak at 652 nm
exhibited an exponential relationship at a concentration range
of 0−70 μM for Al3+ and Cr3+ ions. For these two metal ions,
the luminescent emission peak at 652 nm exhibited a linear
relationship at a concentration range of 90−170 μM and R2 >
0.99. For Fe3+ ions, the luminescent emission peak at 652 nm
showed a linear relationship at a concentration of 110−180
μM and R2 > 0.99. On the basis of the 3σ/slope, Tb−TCPP
exhibited superior sensitivity with a detection limit as low as
7.79 nM for Al3+, 9.94 nM for Cr3+, and 16.4 nM for Fe3+. To
the best of our knowledge, MOF-based sensors designed for
the detection of M3+ (Al3+, Cr3+, and Fe3+) ions with a turn-on
luminescent signal are quite rare. More importantly, the limit
of detection for the Tb−TCPP sensor is significantly lower
than that for the other sensor systems.15 Furthermore, the
recyclable performance of Tb−TCPP to detect M3+ (Al3+,
Cr3+, and Fe3+) was investigated. After detection of M3+ ions,
Tb−TCPP−M3+ was collected by centrifugation and washed
thoroughly with DMF. As shown in Figures S3−S5, Tb−TCPP
can be reused at least three times.
Highly Selective Detection of M3+ (Al3+, Cr3+, and
Fe3+) Ions. To evaluate the selectivity of Tb−TCPP for
sensing M3+ (Al3+, Cr3+, and Fe3+) ions, the fluorescence
intensity of Tb−TCPP in mixed cation aqueous solution (0.1
mM; Na+, Li+, K+, Mg2+, Cu2+, Cd2+, Zn2+, Fe2+, and Ca2+) was
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Figure 4. Fluorescence intensity of Tb−TCPP, Tb−TCPP + M, and Tb−TCPP + M + Al3+ (A), Cr3+ (B), and Fe3+ (C); relative fluorescence
intensity of Tb−TCPP + M and Tb−TCPP + M + Al3+, Cr3+, and Fe3+ at 652 nm (λex = 428 nm, M represents the mixed cations Na+, Li+, K+,
Mg2+, Cu2+, Cd2+, Zn2+, Fe2+, and Ca2+).
measured. As shown in Figure 4, the fluorescence intensity
exhibited slightly changes after addition of any of the listed
metal ions. Comparatively, a significant increase in fluores-
cence intensity was observed after the addition of respective
M3+ (Al3+, Cr3+, and Fe3+) ions. In addition, the intensity
increase was completed within several seconds. All these results
indicated that Tb−TCPP is a highly responsive, selective, and
“turn-on” sensor for detecting M3+ (Al3+, Cr3+, and Fe3+) ions.
Anti-interference Studies for Anions. In addition to the
abovementioned studies, further investigations on anion
sensing were performed. Common anions such as F−, Cl−,
observed, most likely because of protonation of the pyrrole
ring. High acidity (pH = 0) destroys the π-electron-conjugated
double-bond system of porphyrin by protonation of the
nitrogen atoms and results in fluorescence quenching. This
result and mechanism are consistent to those of the pH sensor
based on PCN-225,8e and the pH values and fluorescent
intensity do not follow a linear relationship. At pH > 9, Tb−
TCPP resulted in a strong fluorescence signal. Therefore, the
fluorescence enhancement of Tb−TCPP in the presence of
CO3 and PO4 may be due to their alkaline conditions.
Additionally, the PXRD patterns of Tb−TCPP samples were
tested after soaking in water solution with different pH values
(Figure S9). The characteristic PXRD peaks of Tb−TCPP did
not obviously change in the pH range of 3−9, indicating that
its framework was maintained.
Fluorescence-Sensing Mechanism of the Tb−TCPP
Probe. The fluorescence detection of M3+ (Al3+, Cr3+, and
Fe3+) ions arises from the emission spectra of TCPP. For
comparison, the fluorescence intensity of H2TCPP after
addition of metal cations was screened in a similar fashion.
As shown in Figure S10, all cations did not lead to significant
fluorescence intensity changes to H2TCPP. Hence, the free
H2TCPP ligand cannot be used to detect M3+ (Al3+, Cr3+, and
Fe3+) ions.
To demonstrate the mechanism of Tb−TCPP as a “turn-on”
sensor for M3+ (Al3+, Cr3+, and Fe3+) ions, control experiments
were carried out. PXRD measurements were performed to
study the structures of Tb−TCPP before and after addition of
metal ions. As shown in Figure S11, the characteristic PXRD
peaks of Tb−TCPP do not change after addition of M3+ ions,
2−
3−
Br−, I−, NO3 , OAc−, SO42−, CO32−, and PO43− were added to
−
the solutions of Tb−TCPP for fluorescence testing. As shown
in Figures S6 and S7, except for F−, CO32−, and PO43−, the
other anions do not result in a significant fluorescence
response. Most likely, the enhanced fluorescence of Tb−
TCPP toward CO32− and PO43− could probably be ascribed to
the instability of Tb−TCPP under high-alkaline conditions.
Tb−TCPP disintegrates with coordination bond cleavage,
resulting in the fluorescence enhancement. For F−, the
enhanced fluorescence of Tb−TCPP may be ascribed to the
high affinity between F− and Tb3+ ions (Lewis acid−base
interactions), which reduces the energy transfer from TCPP to
Tb3+ and π*−π energy transfer of the porphyrin ligand is
significantly enhanced.16 Next, the fluorescence intensity
changes of Tb−TCPP were evaluated at various pH values
(0−12). As shown in Figure S8, the most acidic solution led to
the weakest fluorescence changes, while the highest fluo-
rescence intensity could be observed in solution at pH = 12. At
pH = 3, a very slight increase in emission intensity could be
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indicating that the framework was maintained. UV−visible
absorption spectra of Tb−TCPP with different cations were
also recorded (Figure S12). For Al3+, the absorption exhibited
a slight blue shift within 10 nm. Presumably, some Tb−TCPP
bonds cleaved and Al−TCPP coordination bonds formed
through cation exchange, which reduces the energy transfer
from TCPP to Tb3+ and thus enhances the absorption of
TCPP. For Cr3+ and Fe3+, the absorption peaks of TCPP were
disturbed by the UV absorption of Cr(NO3)3 and Fe(NO3)3.
EDS analysis and SEM mapping were conducted to certify if
the fluorescence enhancement of Tb−TCPP is caused by
metal cation exchange. As shown in Figures 5 and S13−S15,
Figure 6. Color changes of Tb−TCPP after addition of different
concentrations of Al3+ in DMF solution.
Moreover, the naked eye recognition of Al3+ ions was also
conducted when excited by 365 nm with a UV lamp. As shown
in Figure S18, red fluorescence occurred when Al3+ was added
to the Tb−TCPP solutions. Because of the intense color of
Fe(NO3)3 and Cr(NO3)3, the naked eye recognition to Fe3+
and Cr3+ could not be realized.
CONCLUSIONS
■
To exploit “turn-on” fluorescence probes for M3+ (Al3+, Cr3+,
and Fe3+) ions, we constructed a lanthanide MOF bearing
TCPP as the ligand in this work. The resulting Tb−TCPP
material has been proved to be highly responsive and selective
for the detection of M3+ ions. This MOF-based sensor features
a nM detection limit and a fast response time (in seconds).
Interestingly, the Al3+ solutions in the mM concentration range
could be visually detected by Tb−TCPP without using any
equipment within 15 min. PXRD, UV, EDS, and ICP analyses
illustrated that the underlying fluorescence mechanism
involves cation exchange of M3+ with Tb3+. These results
indicated that Tb−TCPP is an excellent material for rapid
detection of M3+ ions in aqueous solution.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03268.
■
sı
Figure 5. SEM image of Tb−TCPP−Al3+, Tb−TCPP−Cr3+, and
Tb−TCPP−Fe3+ and elemental mapping of Al, Cr, and Fe.
1H NMR, UV−vis, PXRD, fluorescence detections, and
EDS and ICP analysis (PDF)
EDS and mapping of Tb−TCPP−M3+ demonstrated the
presence of Al, Cr, and Fe. Moreover, ICP measurements were
conducted, as shown in Table S1. After addition of 0.05 M
Al(NO3)3, Fe(NO3)3, or Cr(NO3)3 to the corresponding Tb−
TCPP solution, the Tb3+ content was measured to be 35.28,
31.71, and 14.04 M, respectively. The increased Tb3+
concentrations definitely confirmed that the cation exchange
took place. Therefore, the mechanism could be explained as
follows: the fluorescence of the TCPP component in Tb−
TCPP was weak because of the coordination interaction and
energy transfer from TCPP to Tb3+ ions. After contact with the
M3+ ions, cation exchange reduced the energy transfer and
enhanced the π*−π transfer of TCPP, leading to the
fluorescence enhancement.
AUTHOR INFORMATION
Corresponding Authors
■
Pengfei Shi − Shandong Provincial Key Laboratory of
Detection Technology for Tumor Markers, School of
Chemistry and Chemical Engineering, Linyi University, Linyi
276000, Shandong, P. R. China; College of Chemistry and
Chemical Engineering, Qingdao University, Qingdao,
Shandong 266071, China; Email: shipengfei913@163.com
Ping Cui − School of Chemistry and Chemical Engineering,
Key Lab of Colloid and Interface Chemistry of Ministry of
Education, State Key Lab Crystal Materials, Shandong
University, Jinan 250100, P. R. China; orcid.org/0000-
0002-6918-3103; Email: cuiping@sdu.edu.cn
Interestingly, when Tb−TCPP came into contact with Al3+
in DMF solution, the color changed. As shown in Figure 6, the
color of the solution changed from colorless to red with
increasing Al3+ concentration. The color change could be
visually detected within 15 min after addition of 10 μL of 5
mM Al(NO3)3. Comparatively, the visual detection of Fe3+ and
Cr3+ could be realized for large concentrations, and the color is
ascribed to the Fe3+ and Cr3+ ions (Figures S16 and S17).
Authors
Chenchen Fu − Shandong Provincial Key Laboratory of
Detection Technology for Tumor Markers, School of
Chemistry and Chemical Engineering, Linyi University, Linyi
276000, Shandong, P. R. China
Xinran Sun − Shandong Provincial Key Laboratory of
Detection Technology for Tumor Markers, School of
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Chemistry and Chemical Engineering, Linyi University, Linyi
276000, Shandong, P. R. China
Guoda Zhang − Shandong Provincial Key Laboratory of
Detection Technology for Tumor Markers, School of
Chemistry and Chemical Engineering, Linyi University, Linyi
276000, Shandong, P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03268
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Natural Science Foundation
of China (grant no. 21775062 and 21701188), the China
Postdoctoral Science Foundation (no. 2018M642606), the
Key R&D Program of Shandong Province (2019GSF108158),
and Provincial Innovation and Entrepreneurship Training
Program for Undergraduate (S202010452057).
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