A self-calibrating dual responsive platform for the sensitive detection of sulfite and sulfonic derivatives based on a robust Hf(iv) metal-organic framework

Article abstract of DOI:10.1039/c9cc07869j

A robust hafnium-based metal organic framework, Hf-PBTA, with sensitive and self-calibrating dual-emissive fluorescence response towards sulfite and sulfonic derivatives, including antibiotic sulfamethazine, has been developed, which shows fast detection

Full text of DOI:10.1039/c9cc07869j

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A self-calibrating dual responsive platform for the
sensitive detection of sulfite and sulfonic
derivatives based on a robust Hf(IV) metal–organic
framework†
Cite this:DOI: 10.1039/c9cc07869j
Received 7th October 2019,
Accepted 5th December 2019
Kai Xing,ab Rui-Qing Fan, *a Xiao-Yuan Liu,bc Shuang Gai,a Wei Chen,a
DOI: 10.1039/c9cc07869j
bc
Yu-Lin Yang *a and Jing Li
*
rsc.li/chemcomm
A robust hafnium-based metal organic framework, Hf-PBTA, with
Most of the reported fluorescent sensors rely on the changes
sensitive and self-calibrating dual-emissive fluorescence response of a single fluorescence signal (e.g. intensity enhancement or
towards sulfite and sulfonic derivatives, including antibiotic sulfa- quenching),10–12 which suffers from low selectivity.13 In contrast,
methazine, has been developed, which shows fast detection of self-calibrating sensors have high signal-to-background and low
sulfite ions at a concentration as low as 76 ppb. The opposite sensitivity to external factors, which not only can monitor host–
response tendency from two radiative pathways towards aromatic guest interactions through variation of the fluorescence intensity
sulfonic molecules and sulfite anions stems from the synergistic ratio of different emission centers,14 but also can improve selec-
effect of the pyridine protonation effect, p–p stacking interaction tivity and sensitivity for more accurate and reliable quantification
and intramolecular twist motion.
of specific targets.15 However, ratiometric fluorescent sensors with
a self-calibrating character for sulfonic derivatives are rare,
Drinking water has a direct and intimate impact on human life. because it remains a significant challenge to design two emissive
Contaminants in drinking water pose a serious risk to the health centers with optimal performance and to control other factors,
of living beings and ecosystems.1,2 Among them, hazardous especially in complex realistic detection systems.
pollutants such as sulfonic derivatives, widely utilized in feed
As a group of promising sensor candidates, fluorescent
additives,3 pharmaceuticals and personal care products4 are hafnium metal–organic frameworks (Hf-MOFs) made of highly
usually generated from excessive use and inefficient disposal, connected metal oxygen clusters and diverse organic ligands
and this scenario is getting worse because of the incremental are well known for their stability and durability owing to
population and global industrialization.5,6 Thus, it is impera- the high dissociation enthalpies of a typical Hf–O bond
tive to develop efficient methods to monitor and remove such (802 kJ molꢀ1),16 which is desirable for real-world applications.
pollutants from drinking water systems. Various analytical In addition, the tunable pore shape and decorated active sites
techniques have been used to detect sulfonic derivatives over also increase the modularity to control the interactions between
the past few decades.7 Besides the direct instrument analysis, the target analytes and the framework.17 Therefore, a strategy
chemical sensor based methods have also been developed.8 that fully utilizes the conjugated system of p electrons, the
Notably, the fluorescence based methods are particularly attrac- hydrogen binding sites of functional groups and the twisting
tive due to several unique merits, including simplicity, high motions within the ligand is feasible to design and construct
sensitivity and fast response.9
Hf-MOF based sensors with self-calibrating dual-emissive ratio-
metric sensing behavior.
Herein, we report the solvothermal synthesis of a Hf-MOF
(Hf-PBTA) (H4PBTA = 4,40,400,400 0-(4,40-(1,4-phenylene)bis (pyridine-
6,4,2-triyl))-tetrabenzoic acid) and its ratiometric and self-
calibrating fluorescence based sensing of sulfonic derivatives. The
overall crystal structure of Hf-PBTA is isoreticular to that of
BUT-15.18 To confirm this, the optimization of Hf-PBTA was
conducted based on the reported BUT-15 structure replacing Zr4+
by Hf4+. The result from Rietveld refinement shows an excellent
agreement between the simulated and experimental PXRD patterns
(Fig. S1, ESI†). Hafnium forms a typical Hf6 oxide cluster node
which is connected by a tetratopic ligand to form a 3D framework
a MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion
and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of
Technology, Harbin, 150001, P. R. China. E-mail: fanruiqing@hit.edu.cn,
ylyang@hit.edu.cn
b Department of Chemistry and Chemical Biology, Rutgers University,
123 Bevier Road, Piscataway, New Jersey, 08854, USA. E-mail: jingli@rutgers.edu
c Hoffmann Institute of Advanced Materials, Shenzhen Polytechnic,
7098 Liuxian Blvd, Nanshan District, Shenzhen, 518055, P. R. China
† Electronic supplementary information (ESI) available: Experimental details,
PXRD patterns, TGA, emission spectra and supplementary figures. See DOI:
10.1039/c9cc07869j
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The fluorescent sensing performance was evaluated on two
typical categories containing sulfonic group substituted derivatives:
(1) the aliphatic sulfonic molecules, such as aminomethane-
sulfonic acid (AMSA) and 1-heptanesulfonic acid sodium salt
(HSA); (2) the aromatic sulfonic derivatives, such as benzene-
sulfonic acid (BSA) and p-toluenesulfonic acid (TSA) (Scheme S2,
ESI†). For each analyte, fluorescence intensity was recorded after
adding 1 mM selected analyte aqueous solution to the sensing
suspension in an incremental manner. The Hf-PBTA crystals were
uniformly dispersed with the morphology of octahedral shape
and an average size of B500 nm (Fig. S11, ESI†).
As shown in Fig. S12 and S13 (ESI†), the addition of aliphatic
sulfonic molecules had no influence on the fluorescence
intensity even at high concentrations. Nevertheless, continuous
addition of aromatic sulfonic molecules led to a gradual
decrease in the intensity of the high energy band and increase
in the intensity of the low energy band (Fig. S14 and S15, ESI†).
Setting BSA as an example (Fig. 2a), the intensity ratio of
I480/I388 displays a linear relationship with the BSA concen-
tration in the range of 0–50 mM. The limit of detection (LOD)
Fig. 1 (a) The 3D structure of Hf-PBTA; (b) the water-drop shaped 1D
channel along the b axis; (c) the eight connected inorganic hafnium
oxygen cluster, the four connected organic H4PBTA ligand and the
simplified topological sqc-a network.
with a quadrangular shape (I: 9 ꢁ 16 Å2) and waterdrop shaped was calculated to be 0.192 ppm on the basis of 3s/m (Fig. S14,
(II: 17 ꢁ 21 Å2) channels (Fig. 1a and b), which can be simplified as ESI†). Furthermore, about 90% of the equilibrium intensity
a 4,8-c binodal sqc-a topological network (Fig. 1c). The phase purity ratio was reached in 40 seconds (Fig. S17, ESI†) without
of Hf-PBTA and its chemical stability in water and in acid/alkaline stirring, indicating a relatively fast response.20 The observed
aqueous solutions were confirmed by PXRD analysis (Fig. S2 and phenomenon may be attributed to the energy transfer because
S3, ESI†) at room temperature. The crystallinity of the framework of the strong overlap between the absorption spectrum of
was retained for the pH range of 1–10 after being soaked in the Hf-PBTA and emission spectra of aromatic sulfonic molecules
solution for 48 h. The structure was stable up to 507 1C in nitrogen (Fig. S18–S20, ESI†), while none of the aliphatic molecules
atmosphere (Fig. S5, ESI†), which is B50 1C higher than that of the exhibit such overlaps.21 In addition to the long-range order
Zr isostructural MOF because of the higher Hf–O dissociation
enthalpy.
The optical properties were investigated by UV-Vis and
photoluminescence spectroscopic analysis. As shown in Fig. S7
(ESI†), the UV-Vis absorption band of Hf-PBTA is centered at
330 nm, which is narrowed as a result of coordination with the
metal ion to form a framework (DFWHM = 84 nm). Solid state
photoluminescence spectra show that both Hf-PBTA and the
ligand display two emission bands (450 nm and 544 nm for
ligand; 426 nm and 560 nm for Hf-PBTA) under excitation of
320 nm at room temperature. The lifetime of Hf-PBTA in the
solid state was estimated to be 10.65 ns. According to the
reported studies (Table S1, ESI†), the high energy (HE) band
and low energy (LE) emission band could be assigned to the
organic ligand based on the p - p* and n - p* electron
transition, respectively, which is also supported by the calcu-
lated electron density distribution of the ligand (Fig. S9, ESI†).
Additionally, the emission intensity of Hf-PBTA was found to be
steady in aqueous solution (Fig. S10, ESI†) for one week,
implying a good photo-stability. Notably, the blue shift of the
Fig. 2 (a) 3D fluorescent emission of Hf-PBTA with the incremental
emission band in aqueous solution compared to its solid-state
can be attributed to weaker hydrogen bonding interactions and
decrease of the interchromophore coupling.19 Combining its
unique fluorescence behavior with high water stability and
permanent porosity, we considered Hf-PBTA a suitable candidate
and explored its self-calibrating ratiometric sensing behavior
towards sulfonic derivatives in aqueous medium.
addition of benzenesulfonic acid (BSA) (Ex = 330 nm); (b) 3D fluorescent
emission of Hf-PBTA with the incremental addition of sulfite anions
(SO32ꢀ) (Ex = 330 nm); (c) the influence of other anions and water on
the two emission bands of Hf-PBTA (black square and star: normalized
intensity of Em388; red circle and star: normalized intensity of Em480);
(d) the surface area percent in each electrostatic potential surface range
on the vdW surface of the H4PBTA ligand (inset: ESP mapped molecular
vdW surface of H4PBTA ligand).
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energy transfer, the intimate guest–host interactions should be As shown in Fig. S28 (ESI†), other anions gave a negligible effect
taken into consideration as well. From the structure of aromatic on the fluorescence of Hf-PBTA and only very small influence was
sulfonic molecules, the possible interacting moiety or influential observed with water (Fig. 2c). Even when other anions with equal
factors may come from the following three aspects: (1) the aromatic concentration were present in the solution, the intensity ratio of
2ꢀ
conjugation system with rich p electricity; (2) the protonation effect I388/I480 still increased after the addition of SO3 anions, which
caused by an acidic environment; (3) the potential hydrogen confirmed that sulfite anions generate stronger interaction with the
binding interaction with substituted sulfonic anion group.
framework (Fig. S28 and S29, ESI†). The above results also suggest
Starting from the delocalization of the conjugation system of that the two decay pathways of the Hf-PBTA may be influenced
Hf-PBTA, the large size and aromatic ring of the aromatic differently upon the addition of different sulfonic derivatives.
sulfonic derivatives might induce p–p stacking interaction with
Notably, the size of the sulfite anion is smaller than sulfonic
the wall of the waterdrop-shaped channels within the framework, derivatives, so the type II channel might be more preferential to
resulting in a quenching effect on the p* - p decay transition and interact with. To confirm this, we calculated the electrostatic
the enhancement of the radiative pathway of the 480 nm emission potential (ESP) on the molecular surface, which is critical for
band, which could be supported by a similar response behavior understanding and predicting the guest–host intermolecular
towards pyridine-3-sulfonic acid (PSA) (Fig. S16, ESI†) at 0–20 mM. interaction site.23 Accordingly, the maxima and minima points
Additionally, this effect is synergistic with the influence from were determined using Multiwfn software.24 From the ESP
sulfonic groups, which can be proven by the selective fluorescent mapped van der Waals (vdW) surface of the H4PBTA ligand
response to aromatic molecules without a sulfonic group and its (Fig. 2d), the pyridine ring with more negative electrostatic
deprotonated species (Fig. S21–S23, ESI†). Here, the different values was the most active moiety to interact with the guest
linear sensing range stems from various electron densities of anions upon the carboxyl group occupied by a Hf-oxygen
conjugated systems in analytes. Secondly, on account of the easy cluster. To gain better insight into the interaction and the
protonation of the nitrogen atoms of the pyridine part within the sensing behavior, density functional theory (DFT) calculation
ligand in an acidic environment,22 the contrast experiment was was applied here. As shown in Fig. 3, the introduction of the
carried out and the fluorescence was recorded in aqueous solution sulfite anion plays a key role in the configuration of H4PBTA in
with pH variation to figure out the impact on the sensitivity the ground and excited state, leading to the dihedral angles
towards protons. As shown in Fig. S24 (ESI†), the peak of the between the phenyl rings and pyridine ring (defined as +A–Py,
HE band decreases, while the intensity of the LE band increases. B–Py, C–Py, D–Py and E–Py) changing significantly. For H4PBTA
The trend of these two emissions is similar to that of aromatic with a two sulfite anion system, the + outer benzene-Py
sulfonic molecules, but the acidic environment has more exhibited an obvious decrease upon excitation. For excited
influence on the intensity ratio of I480/I388 under the same H4PBTA with a four sulfite anion system, the dihedral angles
concentration with aromatic sulfonic analytes, indicating that of +A–Py, B–Py and D–Py showed a certain increase to some
the protonation of H4PBTA ligand has a certain influence on extent, however, the whole co-planarity still improved com-
the dual-emission of the MOF, but other aspects may contribute pared with the ground state. Moreover, the significant decrease
to the emission in a more dominant way.
of +C–Py (D = 311) in the excited state with an increases in the
In order to demonstrate the influence of sulfonic anion concentration was also beneficial to the decay channel of p* -
groups, the fluorescence response of Hf-PBTA towards inorganic p transition. The twist motion could be attributed to the
sulfite anions was measured. It was noted that the emission band restriction caused by hydrogen binding interaction between
centered at 388 nm rose and that at 480 nm fell (Fig. 2b), which the protonated sulfite anion and pyridine moiety25,26 and
is an exactly opposite tendency compared with the response dipole–dipole intermolecular interactions between the analytes.27
behavior towards aromatic sulfonic species. Simultaneously, the These interactions would also reduce the transition of the n
significant increase in the relative proportion of p - p* transition electron and hinder the n–p* electronic transitions.28 Therefore,
in the emission spectrum is also in support of the crucial the ligand might adjust the configuration itself and further change the
influence of the sulfite anion. Compared with other selected
anion analytes, the Hf-PBTA displays the most efficient response
behavior and sensitivity towards sulfite anions based on a quan-
titative analysis via fluorescence titration (Fig. S25, ESI†). The
intensity ratio (I388/I480) also linearly correlates with the low
concentration range, and deviates to an exponential relationship
at higher concentrations. The LOD was calculated to be 76 ppb.
Accordingly, the solution color shifts from cyan to light-blue with
Commission Internationale d’Eclairage (CIE) coordinates varying
from (0.21, 0.28) to (0.21, 0.23), where the green (G)/blue
(B) intensity ratios of the image (taken by smartphone) changed
from 1.06 to 0.76 visually by the aid of a color-scanning evaluation
Fig. 3 (a) The molecular configuration variation of the H4PBTA ligand
after interacting with two and four sulfite anions in the ground state and
application (Fig. S26, ESI†). In addition, Hf-PBTA displays a good
selectivity and anti-interference ability towards other anions.
singlet state respectively; (b) the dihedral angle between the benzene ring
(A–E) and pyridine ring (Py) with two and four sulfite anions.
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de-excitation channels of the excited state, resulting in an increased
intensity of the HE band and decreased intensity of the LE band.
The excellent stability, sensitivity and selectivity prompted
us to evaluate the use of Hf-PBTA to quantitatively determine
a trace amount of aromatic sulfonic compounds, such as
sulfamethazine (SMA), in a water system. This antibiotic is
widely used in the treatment of bacterial infections for creatures;
however, contamination from antibiotic residues in drinking
water or even animals could lead to severe diseases,29,30 and
consequently it needs to be detected in a sensitive way. Therefore,
the emissive intensity ratio (I480/I388) of Hf-PBTA was significantly
enhanced upon exposure to SMA containing water (Fig. S30, ESI†).
The LOD of Hf-PBTA was calculated to be 0.668 ppm (Table S2,
ESI†) based on the linear relationship between the response signal
and concentration, where the p–p aromatic conjugation effect was
mainly responsible among the three synergistic aspects due to the
nearly neutral pH and another substituted aromatic part of SMA.
In summary, a highly sensitive and ratiometric sensing plat-
form towards sulfite and sulfonic derivatives has been constructed
based on robust Hf-PBTA. Notably, the Hf-MOF-based sensor
material displays opposite response towards aromatic sulfonic
derivatives and sulfite anions. By combining the pyridine proto-
nation effect, p–p stacking interaction and twist motion induced
by hydrogen binding interaction, helpful insight on the observed
fluorescence changes has been obtained. The robust Hf-PBTA also
exhibits sensitive detection of antibiotics in aqueous solution.
This work serves as a good example of a self-calibrating fluores-
cent ratiometric sensor for possible applications in monitoring
hazardous chemical species in aqueous solutions.
Kai Xing, Ruiqing Fan, Wei Chen, Shuang Gai and Yulin Yang
thank the National Natural Science Foundation of China (Grant
No. 21873025 and 21571042) for their support. Kai Xing gratefully
acknowledges the financial support of the China Scholarship
Council (CSC, 201806120319) to support his study at Rutgers
University. The RU team acknowledges the partial support from
the National Science Foundation (Grant No. DMR-1507210).
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Conflicts of interest
The authors declare no competing financial interests.
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