A phosphonate coordination polymer with highly sensitive detection of ascorbic acid and the proton conductivity of its polymer composites

Article abstract of DOI:10.1016/j.poly.2020.114347

A phosphonate coordination polymer, [Cd(H₃bmpa)₂]_n (1), based on Cd²⁺ ions and H₄bmpa (H₄bmpa = N-Benzyliminobis(methylenephosphonicacid)) was synthesized under hydrothermal conditions. It is characterized by single crystal X-ray diffraction, PXRD and TGA. Crystal structure indicates that adjacent Cd²⁺ ions are linked by oxygen atoms in the phosphonate groups to form a 1D chain structure. Adjacent 1D chains are further linked by hydrogen bonds to form a 2D planar structure. Complex 1 is a bifunctional material that has both fluorescent and proton conducting properties. It can detect ascorbic acid sensitively and selectively with a detection limit of 5 μM. To further improve the proton conductivity of Complex 1, PVA (Polyvinyl alcohol) was introduced with it to prepare the composites. The proton conductivity of 1?PVA-10 is 3.05 × 10^?5 S·cm^?1 (323 K, 98% RH), which is two orders of magnitude higher than that of coordination polymer.

Full text of DOI:10.1016/j.poly.2020.114347

Journal Pre-proofs
A phosphonate coordination polymer with highly sensitive detection of ascorbic
acid and the proton conductivity of its polymer composites
Xue-Ting Liu, Tian Lin, Chen-Xi Zhang, Qing-Lun Wang
PII:
S0277-5387(20)30004-8
DOI:
https://doi.org/10.1016/j.poly.2020.114347
POLY 114347
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
15 September 2019
16 December 2019
2 January 2020
Please cite this article as: X-T. Liu, T. Lin, C-X. Zhang, Q-L. Wang, A phosphonate coordination polymer with
highly sensitive detection of ascorbic acid and the proton conductivity of its polymer composites, Polyhedron (2020),
doi: https://doi.org/10.1016/j.poly.2020.114347
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2020 Published by Elsevier Ltd.

A phosphonate coordination polymer with highly sensitive
detection of ascorbic acid and the proton conductivity of its
polymer composites
Xue-Ting Liu, Tian Lin, Chen-Xi Zhang and Qing-LunWangcd
a
a
*ab
aCollege of Chemical Engineering and Materials Science, Tianjin University of Science and
Technology, Tianjin 300457, P. R. China
bKey Laboratory of Brine Chemical Engineering and Resource Eco-utilization, Tianjin University of
Science and Technology, Tianjin 300457, P. R. China
cKey Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University,
Tianjin 300071, P. R. China
dCollege of Chemistry, Nankai University, Tianjin 300071, P. R. China.
Abstract: A phosphonate coordination polymer, [Cd(H
ions and H bmpa (H bmpa = N-Benzyliminobis(methylenephosphonicacid)) was
synthesized under hydrothermal conditions. It is characterized by single crystal X-ray
3
bmpa) ]
2 n
(1), based on Cd2+
4
4
2
+
diffraction, PXRD and TGA. Crystal structure indicates that adjacent Cd ions are
linked by oxygen atoms in the phosphonate groups to form a 1D chain structure.
Adjacent 1D chains are further linked by hydrogen bonds to form a 2D planar
structure. Complex 1 is a bifunctional material that has both fluorescent and proton
conducting properties. It can detect ascorbic acid sensitively and selectively with a
detection limit of 5 µM. To further improve the proton conductivity of Complex 1,
PVA (Polyvinyl alcohol) was introduced with it to prepare the composites. The proton
-5
-1
conductivity of 1@PVA-10 is 3.05×10 S·cm (323K, 98% RH), which is two orders
of magnitude higher than that of coordination polymer.
Keywords: phosphonate coordination polymer; fluorescence; proton conductivity;
polymer composites
1
. Introduction
Coordination polymers (CPs) are coordination compounds with 1D, 2D, or 3D
structures formed by the assembly of metal ions and inorganic or organic ligands
through coordination bonds [1, 2]. Coordination polymers have structural diversity
due to the difference between metal ions and ligands. At present, it is widely used in
catalysis [3, 4], gas adsorption and separation [5, 6] and magnetic material [7, 8]. In
recent years, the fluorescent properties [9] and proton conductivity [10, 11] of
coordination polymers are also of great interest due to its stability and crystallinity.
Ascorbic acid, abbreviated as AA, is ubiquitous in vegetables, fruits and blood
[
12]. People should consume 70-90 mg of AA per day. Excessive use of ascorbic acid

can cause diarrhea, urinary calculi and sometimes lead to adverse reactions such as
decreased phagocytosis of white blood cells. Lack of ascorbic acid can lead to the
inability of the hydroxylation process of hydroxyproline and lysine to proceed
smoothly, collagen synthesis is blocked and visceral bleeding is caused [13, 14].
Therefore, it is important to selectively and sensitively detect AA. Nowadays, various
methods for detecting AA have been explored, such as electrochemical detection [15,
1
6], colorimetric methods [17, 18], liquid chromatography [19]. However, the above
methods have poor selectivity and high detection limit. On the contrary, fluorescence
detection has the advantages of low detection limit and high sensitivity [20, 21].
In recent years, proton exchange membrane fuel cells (PEMFCs) have received
extensive attention due to their environmental protection and energy conservation [22,
2
3]. In addition, proton conductive materials are the key to achieving good
performance of PEMFCs [24]. The traditional Nafion membrane is costly and
inconvenient for practical applications [25, 26]. Therefore, it is more meaningful to
search for proton conductive materials with high proton conductivity and stability.
Compared with other proton conductive materials, such as solid acid and perovskite,
coordination polymer has been favored for its good chemical stability[27].
Doping[28], post-synthetic modification [29] and polymer composites [30] are the
most common ways to increase proton conductivity. Among them, the preparation of
the polymer composites can not only improve the proton conductivity, but also be
applied in fuel cell as proton exchange membrane [31]. Therefore, it has caused
widespread concern.
The organic ligands for the synthesis of coordination polymer are diverse.
Compared with the most common carboxylic acid ligands, the phosphonate ligands
have additional coordination sites that increase the number of coordination sites and
can form stronger bonds with metal ions, resulting in better thermal stability and
1
0
chemical stability[32]. Meanwhile, coordination polymer composed of d -metal ions
and conjugated organic ligands generally have potential as luminescent materials[33].
Therefore, a 1 D chain phosphonate coordination polymer, [Cd(H
3
bmpa)
2
] (1), was
n
2
+
synthesized based on Cd ions and H
4
bmpa, which is a bifunctional material that has
both fluorescent and proton conducting properties. Polyvinyl alcohol (PVA) is a
hygroscopic polymer with hydrophilic sites such as hydroxyl groups, which can
effectively absorb water molecules. After forming a composite film with complex 1, it
can act as a link between complex 1 to form a channel for proton transfer, thereby
effectively increasing the proton conductivity. In this work, PVA is used to prepare
polymer composites with complex 1.
2
2
. Experimental
.1 Chemicals
Phosphorous acid (purity 99%) was purchased from Energy Chemical.
Benzylamine (purity 99%) was purchased from Macklin. Arginine (purity 98%),
valine (99%), methionine (purity 99%), leucine (purity 99%) and histidine (purity
9
9%) were purchased from RYON. Formaldehyde (purity 40%), glycine (purity 99%),

phenylalanine (purity 98%), isoleucine (purity 99%) and ascorbic acid (purity 99%)
were purchased from Fuchen Chemical.
2
.2. Synthesis
The ligand H
4
bmpa (H bmpa = N-Benzyliminobis(methylenephosphonic acid))
4
was synthesized according to reference[34]. Composites were prepared by traditional
solution casting method[35].
2
.2.1. Synthesis of H bmpa
4
5
.47 mL benzylamine and 8.20 g phosphorous acid were mixed together in 10
mL HCl and 25mL H O, and refluxed at 140 ℃ for 1 h. Then 27 mL formaldehyde
solution was slowly added and refluxed for 1 h. The white powders were obtained.
Anal. Calc. for C : C, 36.61; H, 5.08; N, 4.74; found: C, 36.48; H, 5.06; N,
.71%; FTIR (KBr, cm ): 2983 (w), 2476 (br), 2296 (br), 1467 (m), 1167 (s), 930 (s),
2
9
H
15NO
6 2
P
−
1
4
8
2
39 (m), 570 (s), 472 (w).
.2.2. Synthesis of [Cd(H
3
bmpa)
2
n
] (1)
A mixed powder of Cd(NO
3
)
2
·4H
2
O (0.0771g,0.25mmol) and H bmpa (0.0590 g,
4
0
.5mmol) was placed in a 25 mL Teflon-lined autoclave, then 10 mL of distilled
water was added. The reactor was put into a programmable temperature control oven
with 140 ºC for 96 h. Then it slowly cooled to room temperature. Transparent
rod-shaped crystals were obtained and washed several times with water, finally dried
in vacuum oven. Yield: 46% (based on ligand H
4
bmpa). Anal. Calc. for
C
18
H
28
N
2
O
12 4
P
Cd: C, 30.83; H, 4.00; N, 4.00; found: C, 30.76; H, 3.96; N, 3.98%;
−
1
FTIR (KBr, cm ): 3437 (w), 2953 (br), 2575 (br), 1462 (m), 1152 (s), 917 (s), 744
m), 531 (s).
(
2
.2.3. Synthesis of composite membrane materials
A mixed of 0.5 g PVA and 8 mL H O was placed in a in a 50 mL round bottom
2
flask, stirred slowly at 95 ℃ for 1 h, and then cooled to room temperature. Different
amount of complex 1 were added, stirred for 30 minutes and sonication to removed
air bubbles. The casting solution was uniformly dispersed on a glass plate, and the
solvent was removed under a vacuum at 50 ℃. Different content of composite
membranes 1@PVA-5, 1@PVA-10, 1@PVA-15 and 1@PVA-20 were obtained in
this experiment. (5, 10, 15 and 20 are the percentage of the mass and total mass of
complex 1)
2
.3. X-ray crystallographic study
The crystal sample of complex 1 was measured by a Rigaku Saturn CCD
diffractometer at 120 K using graphite monochromated M
o
-K radiation (λ = 0.71073
α
Å) with the ω-scan technique. The non-hydrogen atoms in the crystal structure are
solved by direct method, the anisotropic thermal parameters and coordinates are
refined by the full matrix least squares method. The isotropic heat is refined by
theoretical hydrogenation and the identification of the appropriate Q peak. The
parameters obtain the coordinates of the hydrogen atoms, and all crystal structures are
resolved by the Olex2 program [36]. The crystal data details of complex 1 are shown
in Tables S1 and S2.

2
.4. Fluorescence measurements
Coordination polymer was formulated into a 2 mg/mL aqueous solution, and
then obtained a suspension by sonication for 30 min. 1 mL of the suspension and
different amino was added to the 4 mL centrifuge tube and sonication for 30 min
(The amino acids are glycine, valine, phenylalanine, methionine, leucine, histidine,
arginine, isoleucine, and ascorbic acid, the concentration of amino acid aqueous
-2
solution is 10 M). Then, the mixture solution was used for fluorescent. All other
chelating agent solutions were configured according to the method described above
(The chelating agents are tartaric acid, citric acid, oxalic acid, sodium gluconate,
ethanolamine, triethanolamine, and diethanolamine.). Meanwhile, different pH
solutions were prepared according to different concentrations of HCl and NaOH.
2
.5. Proton conduction measurement
Coordination polymer was ground into a powder and tableted to obtain a sample
wafer with a diameter of 6 mm and a thickness of 0.2 to 0.4 mm. Both sides of the
wafer have uniformly coated with carbon conductive adhesive. The wafer coated with
carbon conductive paste was suspended in a sealed environment of different humidity
(65% RH-98% RH) for AC impedance test. The proton conductivity is calculated by
the formula:
σ = d/SR
-1
where σ = proton conductivity (S·cm ), d = the thickness of the wafer (cm), S = the
2
specific surface area of the wafer (cm ), R = the resistance of the wafer (Ω).
The temperature of the wafer under certain humidity conditions is tested to
determine the proton conduction mechanism by calculating the activation energy
value. Arrhenius formula:
σT = σ
0
exp(E
a
/k
B
T)
σ
0
= pre-exponential factor, E
a
= the activation energy, k = the Boltzmann
B
constant.
3
3
. Result and discussion
.1. Crystal structure
Complex 1 belongs to monoclinic system and space group C2/c (Table S1). The
2
+
4−
asymmetric unit of complex 1 contains one Cd ion and two bmpa ligands. Cd1
atom coordinates with six phosphonate oxygen atoms (O3, O3, O4, O4, O5, O5),
forming a distorted octahedron coordination geometry. The Cd-O bond length range is
2
.2543 (14)-2.3045 (14) Å, which are comparable with those Cd(II) complexes
assembled from similar di-phosphonic acid ligand [37]. The angles of O-Cd-O are
between 83.27(5) and 167.46(7)°(Table S2). The ligand acts as tridentate metal linker
2
+
and bridged with three Cd ions and P (1) atom is chelated with O3, P (2) atom is
chelated with O4 and O5. (Figure 1(a)). The adjacent two Cd2+ sites are linked by
bmpa4− ligand to construct a 1D chain structure with the Cd···Cd distance being
5
.36Å (Figure 1(b)). The Cd···Cd distance is between those in other reported Cd2+
phosphonates (3.64 Å-6.76 Å) [38-40]. There are five kinds of hydrogen bonding in
complex 1 (Table S3). 1D chains further form a 2D planar structure by hydrogen

bonds (Figure S1).
3
.2. PXRD and TG pattern
Figure S2 shows the PXRD profiles of complex 1 and its sample placed under
high humidity conditions for 4 days, together with the simulated PXRD pattern of
complex 1. All of the profiles show high similarity, which indicates that complex 1 is
stable under high humidity conditions. The TG analysis of complex 1 is shown in
Figure S3. Among 30~296 ℃ temperature range, there is almost no weight loss of
complex 1 due to the absence of free water. After 296 °C, it suffered great weight
losses due to the decomposition of complex 1. It is proved that complex 1 has good
thermal stability, which can be stable in the temperature range of 30 ~ 296℃.
3
.3. Fluorescence properties
Coordination polymer composed of d10-metal ions and conjugated organic
ligands generally have potential as luminescent materials. Considering that complex 1
has good structural stability in water, we have explored its fluorescent properties in
aqueous solution. The 3D fluorescence spectrum shows that the emission peak
intensity at 298 nm changes with the excitation wavelength, and the optimal
excitation wavelength is 260 nm (Figure 2(a) and Figure S4). The sensing potential of
complex 1 for amino acid molecules in aqueous solution at room temperature was
studied. Different amino acid solutions were added to the complex 1 suspension. The
results shows that the addition of glycine, and methionine have almost no significant
effect on the luminescence intensity, and the change in fluorescence intensity are
caused by the addition of valine, phenylalanine, leucine, histidine, arginine, and
ascorbic acid. In particular, ascorbic acid exhibits a complete luminescence quenching
effect (Figure S5). The anti-interference ability of complex 1 was further studied by
mixing AA with various other amino acids. The results indicate that AA can be
selectively detected relatively rapidly in aqueous solution (Figure S6). It is
demonstrated that complex 1 has a relatively high selectivity for AA. To explore the
effects of other chelating agents and different pH conditions on complex 1, the effect
of chelating agents and pH on fluorescence were investigated. The result showed that
different pH conditions have certain fluorescence quenching effect, but it is not
significant (Figure S7). In addition of gluconic acid has caused the luminescence
intensity increases, other chelating agents such as tartaric acid, citric acid and oxalic
acid all have certain fluorescence quenching effect, but AA exhibits a biggest
luminescence quenching effect (Figure S8). It indicates that complex 1 has the
potential to detect AA as a fluorescent probe.
A series of different concentrations of AA solution was added to the suspension
of complex 1 to explore the sensitivity of fluorescence detection of AA. The
fluorescence intensity is increase with decreasing AA concentration and the minimum
detection limit is 5µM (Figure 2(b)). The luminescent quenching effect for AA was
calculated to further investigate the relationship between AA concentration and
luminescence intensity. In addition, the quenching effect was analyzed according to
the Stern-Volmer equation [41]: I /I =1+Ksv [AA], where [AA] is the concentration of
0

ascorbic acid, I
0
and I are the luminescent intensity of complex 1 suspension absence
4
−1
and presence of AA. The quenching constant Ksv is 1.650×10 M and R (the linear
−5
correlation coefficient) is 0.9933 at low concentration (from 0 to 10 M), it indicates
that complex 1 has an excellent quenching effect on AA (Figure 2(c)). Fluorescence
regeneration can reduce the cost of detection and is more suitable for practical
applications [42], therefore, the fluorescence reversibility of the complex 1 was
studied. The result shows that the quenching of AA by complex 1 remained relatively
stable over 5 cycles (Figure S9). It shows that the fluorescent probe detects the
potential of AA in practical applications.
To obtain the fluorescence quenching mechanism of ascorbic acid, the emission
spectrum of complex 1 and UV absorption spectrum of AA were studied. The result
shows that the absorption band of AA overlaps with the emission band of complex 1
(Figure S10). It may be that the resonance energy on the original complex 1 is
transferred to AA, resulting in fluorescence quenching. Therefore, the mechanism of
fluorescence quenching of complex 1 is probably the transfer of resonance energy [43,
4
4].
3
.4. Proton conductivity
The Nyquist plot shows that the AC impedance of complex 1 decreases with the
increase of humidity at 290 K (Figure 3(a)). It is indicated that the change in humidity
has a profound effect on the proton conductivity of complex 1. The proton
-8
-1
conductivity is 8.75 × 10 S·cm at 98% RH (Figure 3(b)). With the temperature
-8
-1
increases, the proton conductivity increases from 8.75×10 S·cm (290 K, 98% RH)
to 2.80×10-7 S·cm (312 K, 98% RH) (Figure 3(c)). To improve the proton
conductivity of complex 1, the polymer composites 1@PVA-X (X= 5%wt, 10%wt,
-1
1
5%wt, 20%wt) were prepared. Alternating current (AC) impedance spectroscopy
measurements were performed on polymers of different complex contents at 298 K
and 98% RH. The proton conductivity increases with the content of complex
increases from 5%wt to 10%wt, however, the proton conductivity decreases when the
content of complex 1 is higher than 10%wt (Figure S11). With the increase of
humidity, the impedance value decreases continuously, the proton conductivity of
@PVA-10 is 7.50×10-8 S·cm (298 K, 98% RH) (Figure 4(b)). The proton
-1
1
-5
-1
conductivity of 1@PVA-10 is 3.05×10 S·cm (323K, 98% RH), which is two orders
of magnitude higher than that of coordination polymer (Figure 4(c)).
The E
conduction mechanism is Vehicle mechanism [45]. When exotic water molecules
entered, the absorbed water molecules can aggregate around the H bmpa ligands.
a
value of complex 1 is 0.53eV (Figure 3(d)), which indicates that proton
4
Therefore, the uncoordinated phosphonate group can interact with the extraneous
+
water molecules to form H
chain [46]. The E
proton conduction mechanism is also Vehicle mechanism. In the polymer composites
@PVA-10, proton transfer within the coordination polymer follows the Vehicle
mechanism described above. The alcoholic hydroxyl group in the PVA polymer
3
O ions and thereby moving along the hydrogen bond
a
value of 1@PVA-10 is 0.50 eV (Figure 4(d)), which indicates
1

outside the coordination polymer acts as a hydrophilic site, and the protons and the
extraneous water molecules form hydronium ions and move in a certain direction
[
47].
4
. Conclusion
We have successfully synthesized a phosphonate acid bifunctional coordination
polymer, which has certain luminescence properties and proton conductivity. The
luminescence properties of complex 1 show sensitively detect AA in aqueous solution
and the detection limit is 5 µM. Furthermore, it has fluorescence regeneration
properties, which can be used as a luminescence-based probe for detect AA in
medical applications. Meanwhile, complex 1 and polyvinyl alcohol form composites,
-5
-1
the proton conductivity of 1@PVA-10 is 3.05×10 S·cm (323K, 98% RH), which is
two orders of magnitude higher than coordination polymer (298 K, 98% RH). The
method of preparing polymer composites has certain advantages in improving proton
conductivity.
Appendix A. Supplementary data
CCDC 208751 contains the supplementary crystallographic data for this paper.
These
data
can
be
obtained
free
or
of
from
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html,
the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
1
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Figure 1 (a) The asymmetry unit of complex 1.Thermal ellipsoids are drawn at 50%
probability (Dashed lines indicate symmetrical bonds, symmetry codes: (i)
-x+1,y,-z+1/2; (ii) -x+1,y,-z+3/2; (iii) x,y,z-1; (iv) x,-y,z-1/2; (v) -x+1,-y,-z+1; (vi)
x,y,z+1;), (b) 1D chain structure of complex 1 along c axis.
Figure 2 (a) Luminescence intensity of complex 1 at different excitation wavelengths
(
b) emission spectra of complex 1 with different concentrations of AA, (c) the Stern–
Volmer plot of I /I versus the concentration of AA for complex 1 at low concentration
from 0 to10 M).
0
−
5
(

Figure 3 (a) Impedance spectra of complex 1 at 290K and 65-98%RH. (b) RH
dependence of the proton conductivity at 290 K, (c) Impedance spectra of complex 1
at 98% RH and 290-312K. (d) E of complex 1 at different temperature and 98% RH
a
Figure 4 (a) 1@PVA-10 at 65%RH and 298K, (b) Impedance spectra of 1@PVA-10
at 290K and 75-98%RH, (c) Impedance spectra of 1@PVA-10 at 98% RH and

2
98-323K, (d) E
a
of 1@PVA-10 at different temperature and 98% RH
Graphical Abstract
A phosphonate coordination polymer with highly sensitive detection of ascorbic
acid and the proton conductivity of its polymer composites
Graphical Abstract
A phosphonate coordination polymer with highly sensitive detection of ascorbic
acid and the proton conductivity of its polymer composites
A bifunctional coordination polymer has been synthesized which based on ligand
N-Benzyliminobis(methylenephosphonic acid). It could detect ascorbic acid sensitively and
selectively with detection limit of 5 µM in aqueous solutions. The polymer composites
-5
-1
1
@PVA-10 has been synthesized and its proton conductivity has improved 3.05×10 S·cm
323K, 98% RH), which is two orders of magnitude higher than that of coordination
polymer.
(
Highlights
•
A phosphonate coordination polymer has been successful synthesized under
hydrothermal conditions.

•
It is an excellent luminescence-based sensor with high sensitivity and
selectively for detection of ascorbic acid (AA) in aqueous solutions. The
detection limit is 5 µM.
•
The polymer composites 1@PVA have been successful synthesized. AC
impedance analysis shows that the proton conductivity of 1@PVA-10 has
improved to 3.05×10-5 S·cm-1 (323K, 98% RH), which is two orders of
magnitude higher than that of coordination polymer.
Declaration of Interest Statement
We declare that we have no financial and personal relationships with other people or
organizations that can inappropriately influence our work, there is no professional or
other personal interest of any nature or kind in any product, service and/or company
that could be construed as influencing the position presented in, or the review of, the
manuscript entitled.
Chenxi zhang
Tianjin university of science and technology
2
019/12/17
Author Contributions Section
Xue-Ting Liu: Synthesis of compounds,fluorescence properties test,Writing
manuscript,
Tian Lin: The proton conductivity test
Chen-Xi Zhang: Data process and analyst, revised manuscript
Qing-LunWang: Writing- Reviewing and revised manuscript