An indole-derived porous organic polymer for the efficient visual colorimetric capture of iodine in aqueous media: Via the synergistic effects of cation-π and electrostatic forces

Article abstract of DOI:10.1039/c9cc08699d

We have successfully synthesized a novel type of recyclable indole-based porous polymer, which possesses highly effective capture behavior for iodine in aqueous solution via the synergistic effects of cation-π and electrostatic forces. More interestingly,

Full text of DOI:10.1039/c9cc08699d

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An indole-derived porous organic polymer for efficiently visual
colorimetric capture of iodine in aqueous media via the
synergistic effects of cation-π and electrostatic forces
eceived 00th January 20xx,
Accepted 00th January 20xx
Min Huang, Li Yang*, Xiuyun Li and Guanjun Chang*
DOI: 10.1039/x0xx00000x
We have successfully synthesized a new type of recyclable indole- been less explored.[19] Thus, designing new materials for
based porous polymer, which possessed a high effective capture operatively absorb iodine in aqueous solutions is important
behavior for iodine in aqueous solution via the synergistic effects and necessary.
of cation-π and electrostatic forces. More interestingly, the
It is well-known that iodine uptake in a porous material is
absorption performance can be efficiently detected by a visual dependent on the pore environments, which means there
colorimetric assay through using a smartphone.
should be existed active sites such as coordinative sites, Lewis
acid-base sites, and electrostatic sites to interact with
iodine.[20] Nevertheless, these pores can be occupied by iodine
at maximum accessibility of only 50% because of the
coagulative and non-polar properties of iodine,[21,22] which will
inevitably lead to the poorer adsorption capacity. Recently, we
Nowadays, there is a large amount of radioactive waste caused
by the development of the nuclear industry and medical
diagnosis,[1,2] which has created a serious threat to the livings
within the ecosystem. Radioiodine (129I and 131I), a typical
radioactive pollutant, has already attracted public concerns as
the volatile iodine can quickly spread into air and the
environment resulting in radiological contamination would last
over 10 million years, due to the 1.57×107 year radioactive
half-life of 129I.[3] Indeed, radioiodine capable of eliciting a
series of cytotoxic effects can cause serious health problems
such as thyroid cancer.[4] Thus, there is a challenging and
fascinating task for the exploitation of readily accessible
materials to effectively treat radioactive iodine waste.
Currently, a wide range of adsorbents have been designed
and fabricated for iodine capture, including activated carbon,[5]
inorganic porous materials,[6,7] metal-organic frameworks
(MOFs)[8] and porous organic polymers (POPs),[9-16] etc. As one
of the most promising candidates for the adsorption of iodine,
POPs are an intriguing class of porous materials owing to their
low densities, high surface areas, good thermal and chemical
stabilities as well as tunable skeletons.[10-12,17-18] Although POPs
materials have been extensively investigated on the
adsorption of iodine in volatile gas states or organic
solvents,[13,15-16] the construction of analytical methods for
efficient detection and capture of iodine in water by POPs has
described
a series of indole-derived polymers with π-
electronic-rich frameworks as outstanding adsorbents for
electron-deficient analytes.[23,24] Particularly, we have
described a new concept for constructing of indole-based POPs
as lithium-doped hydrogen adsorption materials via cation-π
interactions.[25] Enlightened by these studies mentioned above,
we postulated
a novel strategy for visual colorimetric
extraction of iodine in aqueous media by using a smart phone
via the synergistic effects of cation-π and electrostatic
interactions within a new indole-based POP.
Herein, we performed the following concept. First, we
established a new class of indole-derived POP (PTIBBL) via a
facile Friedel-Crafts reaction (Fig. 1). Second, the obtained POP
treated with sodium iodide would be particularly more
attractive to absorb iodine, as the indole rings could be
available for binding Na+ via the well-known cation-π
interactions, and meanwhile, the Na+-indole complex could
efficiently capture polyiodide anions[26] through the
electrostatic forces in aqueous solution. More interestingly,
the yellow color of aqueous solution was fade when PTIBBL
extracted iodine, allowing for visual detection. The present
visual colorimetric assay is simple, portable and suitable for
on-site detection in real water samples. This underlying design
principle is illustrated schematically in Fig. 1.
State Key Laboratory of Environment-friendly Energy Materials, School of
Material Science and Engineering, Southwest University of Science and
Technology, Mianyang, 621010, P. R. China. E-mail: yanglichem628@126.com,
gjchang@mail.ustc.edu.cn.
The PTIBBL was rationally synthesized from a Friedel-Crafts
alkylation reaction of 1,3,5-tris-(4-indolylbenzoyl)benzene
Electronic Supplementary Information (ESI) available: Details of the synthesis and (TIBB) with formaldehyde dimethylacetal (FDA). Detailed
characterization of PTIBBL; main materials and measurements; the analysis of I2
absorption behaviour; visual colorimetric detection of iodine. See
DOI: 10.1039/x0xx00000x
experimental procedures are provided in the ESI†. The as-
obtained PTIBBL was a dark-yellow powder (Fig. 2) and the
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successful preparation of it was confirmed by Fourier- in NaI aqueous solution. The adsorption amount increased
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transform infrared (FT-IR), 13C CP/MAS NMR as well as rapidly in the initial 30 min, and theDnOiIn: 1c0r.e10a3s9e/dC9sCloCw08l6y99tDo
elemental analysis, and the results were in good agreement achieve equilibrium at about 6.5 h with an equilibrium
with the proposed structures (Scheme S3, Fig. S2 and Table S1). adsorption capacity (qe,exp) of 401.68 mg g−1 (Fig. S4a and Table
Thermogravimetric analysis (TGA) displayed an apparent S2). These results suggested that the adsorption rate of PTIBBL
weight loss began at temperature as high as 523 °C and still on iodine was very fast and these advantages would make
had high char yields at 800 °C, indicating the excellent thermal PTIBBL be a potential material for iodine uptake. Furthermore,
stability of PTIBBL (Fig. S3a). Furthermore, powder X-ray the correlation coefficient R2 of the pseudo second-order
diffraction (XRD) of the sample was performed. PTIBBL showed kinetic model (R2 = 0.999) was much higher than that of the
a broad peak in the XRD pattern, suggesting that it was pseudo-first-order kinetic model (R2 = 0.858) for adsorption of
amorphous in nature (Fig. S3b).
I2 (Fig. S4 and Table S2). And the calculated equilibrium
adsorption capacity (qe,cal) of 408.16 mg g−1 was reached. It
was clear that the adsorption of I2 on PTIBBL was a
chemisorption process.
Fig. 2 (a) SEM image; inset: photograph of PTIBBL. (b) TEM image. (c) N2 adsorption-
desorption isotherm and (d) pore-size distribution of PTIBBL.
Fig. 1 Synthesis of PTIBBL and schematic illustration of the visual colorimetric capture
of I2 in aqueous solution via the synergistic effects of cation-π and electrostatic
interactions.
The fitting results of the adsorption isotherm models were
displayed in Fig. S5. The Langmuir model (R2 = 0.999) had a
better correlation than the Freundlich model (R2 = 0.973),
demonstrating that the experimental adsorption process fitted
well with the Langmuir equation. The results implied that the
adsorption of I2 was mainly monolayer adsorption and
occurred on the surface of adsorbents. The maximum uptake
capacity was 666.7 mg g-1 (Table S3). It was worth noting that
the iodine loading capacity of PTIBBL was much higher than
that of reported adsorbents shown in Table S4.[13,15,21,28] More
significantly, in order to assess the I2 capture ability of PTIBBL,
other ordinary porous materials such as activated carbon and
molecular sieve were used for comparison. The same weights
of different adsorbents (zeolite 13X, activated carbon and
PTIBBL) were immersed in a solution of iodine (300 mg L-1) at
room temperature. Fig. S6 revealed the different extraction
rates and capability of PTIBBL in comparison with zeolite 13X
and activated carbon. The uptake of iodine on PTIBBL was very
efficient, and obviously exceeded that of zeolite 13X and
activated carbon in the aqueous media. It was inferred that
the excellent affinity of PTIBBL for iodine resulted in the
exceptional uptake of iodine compared to conventional iodine
capture materials lacking an accessible interaction between
the host and I2. As expect, these characteristics made PTIBBL a
We quantified the porosity of PTIBBL by scanning electron
microscopy (SEM), transmission electron microscopy (TEM),
and sorption analysis. The SEM image (Fig. 2a) exhibited that
PTIBBL was consisted of aggregated particles with nonuniform
pores. Moreover, a TEM image (Fig. 2b) revealed the porous
structure, which is an essential requirement for iodine capture.
At 77 K, the N2 isotherms showed rapid uptake at a low
pressure (0-0.1 bar), indicating a microporous nature (Fig. 2c).
At a relatively high pressure (~0.9 bar), there was an increase
in the N2 sorption because of the interparticulate void
associated with the meso- and macrostructures of the sample.
The Brunauer-emmett-teller (BET) specific surface area of
PTIBBL was calculated to be 399.1 m2·g-1. The pore size
distribution of this polymer was calculated from the N2
adsorption isotherms by the nonlocal density function theory
(NLDFT) method, suggesting that PTIBBL possessed micropores
centred at 1.23 nm and the pore volume was 0.367 cm3·g-1 (Fig.
2d).
-
As we know, I2 could form I3 ion with binding I- to improve
its solubility in water[27,28] and the color of solution would be
altered to yellow. Thus, the kinetics was first studied to
evaluate the adsorption rate of indole-based POP PTIBBL for I2
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promising and competitive candidate for radioactive iodine The spectrum distinctly disclosed a negative band at 228 nm
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+
DOI: 10.1039/C9CC08699D
capture in aqueous media.
and a weak positive band at 257 nm, ascribable to a Na -TIBB
As displayed in Fig. 3a, the adsorption capacity of I2 by cation-π interaction (Fig. 4a). The difference spectrum (dashed
PTIBBL increased as the initial concentration of I2 increased line) reflected a small red-shift of the strong Bb transition of
and the color of the solution gradually changed from orange to the TIBB indole ring upon complexation with Na+.[23,24,29] More
light yellow or colorless after 48 hours (Fig. 3c), which importantly, upon addition with I2 in the NaI solution, the λmax
suggested that iodine was encapsulated in indole-POP of TIBB-Na+ continuously underwent a red shift to 291 nm
networks to continuously make iodine captured in solution. along with
a dramatic increase in absorbance, and a
The removal efficiency of PTIBBL was calculated by the concomitant regular increase in absorbance at 354 nm was
maximum uptake amount. In Fig. 3b, the highest removal also observed (Fig. S11). This meant the formation of a
-
efficiency reached as high up to 90.7%. With this uptake value, complex between I3 and TIBB-Na+ in the ground state. To get
PTIBBL could be considered as outstanding absorbent for more insight into the formation of complexes and the main
iodine solution than that of other porous materials (Table binding interactions, the energy levels of the frontier orbitals
S4).[13,15,21,28] By contrast, the uptake capacity of iodine for the were further calculated by using the DFT method. As can be
polymer PTIBBL was much superior to that of the monomer seen from Fig. 4b and 4c, TIBB-Na+ structure unit showed a low
TIBB (Fig. S7). This might be due to the richer porosity of unoccupied molecular orbital (LUMO) level of -3.61 eV, hinting
-
PTIBBL for I2 capture. To further confirm the form of iodine its great electron affinities. The results indicated that I3 may
loaded in PTIBBL, the XPS spectrum was measured. It was transfer a portion of the charge to the electrically deficient
turned out that the iodine existed as both neutral I2 and I3− in indole-Na+ moiety and be more easily bound as they came to
PTIBBL[19] (Fig. 3d). We considered that some adsorbed I3− ions closer. In addition, due to the specific role of ketones in
were unstable in air and would be quickly transformed to I2 electron transport,[30] we speculated that the carbonyl
molecules.[26]
presumably promoted the capture of iodine on POP via the
electrostatic forces.
Fig. 4 (a) UV absorption spectra of TIBB (30 mM) and its 1:3 complex with NaI. The
difference spectrum for TIBB-Na+ minus TIBB is doubly magnified for clarity. The
interfacial plots of the LUMO orbital for TIBB structural unit (b) and TIBB-Na+ structural
unit (c), respectively.
More gratifyingly, since the I2 concentration directly
correlated to the color of the aqueous solution, the
corresponding optical images were collected by the camera of
the smart phone and the RGB component change of the
pictures was applied to quantitatively determine the iodine.
Hence, the colorimetric response of the detection system
toward iodine at different concentrations was investigated. Fig.
5a displayed the color tonality of the solution with various
amounts of I2. As depicted in Fig. 5b, the standard curve was
established as a linear relationship through the adjusted
intensity and the logarithm of I2 concentration in the range
from 8 to 210 mg L-1. The detection limit of this method was
calculated to be 3.6 μg L-1 at a signal-to-noise ratio (S/N) of 3.
This detection assay was comparable to or even better than
that of previously reported methods for I2 adsorption such as
UV-vis spectra.[13,28,31] Here, it is noted that the present
method is cost-effective and does not involve any bulky
equipment. Consequently, this assay is undoubtedly promising
for on-site detection of radioiodine by using a portable smart
phone.
Fig. 3 (a) The adsorption of PTIBBL for iodine solution with different concentration from
100 to 500 mg L−1. (b) The removal efficiency of PTIBBL for iodine solution. (c) Color
change of iodine solution with different concentrations after adsorption for 48h. (d)
XPS spectrum of iodine-loaded PTIBBL.
Subsequently, the iodine loaded on PTIBBL could be
removed in ethanol. The reversibility of the extracting process
was conducted for five times. We found that the samples that
were subjected to multiple recycling treatments still showed
nearly the same absorbing behavior as the original POP
material (Fig. S8), suggesting that the indole-based POP could
be reused and there were no secondary pollution problems.
Meanwhile, the used material also had almost the same
structure characterization as the original sample (Fig. S9-S10).
To shed light on the mechanism of interaction between
PTIBBL and I2, the UV-vis titration of TIBB with I2 in the NaI
solution was carried out. Fig. 4a exhibited the absorption
spectra of TIBB in the absence and presence of 3 equiv. Na+,
together with the difference spectrum of TIBB-Na+ minus TIBB.
Lastly, to evaluate the practical application of this
colorimetric detection system, we applied it for I2 capture in
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J. Name., 2013, 00, 1-3 | 3
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natural water samples. Different concentrations of I2 (50, 100
and 150 mg L-1) were spiked to 100-fold diluted tap water and
lake water with NaI, respectively, and analyzed by the present
colorimetric method. As listed in Table 1, the removal rates
ranged from 92.6% to 70.4% in the iodine lake wastewater.
Besides, the I2-loaded wastewater samples were analyzed by
this visual assay and UV spectra. The detection results from
the presenting assay were well consistent with those tested by
UV spectra (Table 2). All the results unambiguously attested
that PTIBBL was qualified for practical application in efficient
treatment of radioiodine pollutants by utilizing the present
colorimetric assay.
Notes and references
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DOI: 10.1039/C9CC08699D
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Fig. 5 Optical images of the detection aqueous solution after addition of different
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Table 1 Adsorption results of iodine in natural water samples
Sample
C0 (mg L-1)
50
Ce (mg L-1)
3.9
qe (mg g-1)
184.4
Removal rate (%)
Tap water
92.2
90.3
67.9
92.6
91.1
70.4
100
9.7
361.2
150
48.1
3.7
407.6
Lake water
50
185.1
100
8.9
364.4
150
44.5
422.2
Table 2 Comparison of detection results obtained in the analysis of iodine wastewater
by this assay and UV spectra
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I2 capture by UV spectra(mg L-1)
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2.91
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In summary, we have rationally proposed a new type of
recyclable indole-based porous polymer for effective
extraction of iodine in aqueous media through the synergistic
effects of cation-π and electrostatic interactions. More
significantly, this capture behavior could be efficiently
detected via a visual colorimetric assay by taking advantage of
a smartphone. We anticipate that our strategy of developing
this innovative adsorption system could be greatly extended to
design and construct of advanced materials for the treatment
of radioactive wastes or removing pollutants from aqueous
environments.
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Conflicts of interest
There are no conflicts to declare.
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An indole-derived porous organic polymer for efficientlyDOvI:i1s0u.10a39l/C9CC08699D
colorimetric capture of iodine in aqueous media via the
synergistic effects of cation-π and electrostatic forces
Min Huang, Li Yang*, Xiuyun Li and Guanjun Chang*
Recyclable indole-based POP is successfully achieved, capable of visual colorimetric iodine
capture in water via cation-π and electrostatic interactions.