Water-soluble polymeric probe with dual recognition sites for the sequential colorimetric detection of cyanide and Fe (III) ions

Article abstract of DOI:10.1016/j.dyepig.2019.04.010

A water-soluble polymeric probe derived from an azo-Schiff base was developed for the consecutive colorimetric sensing of CN^? and Fe³⁺ ions in water. Herein, the synthesis of a random copolymer of N,N-dimethylacrylamide (DMA) and 3-vinylbenzaldehyde (VBA) via reversible addition-fragmentation chain transfer polymerization, [p(DMA-co-VBA, P1] is described. A post-polymerization modification reaction between the aldehyde groups of P1 and (E)-2-amino-4-((4-nitrophenyl)diazenyl)phenol in ethanol led to the target polymer, P2, with azo-Schiff base moieties. The colorimetric detection of CN^? ions occurred because of the abstraction of phenolic proton of P2 by the CN^? ions to form hydrazone, which was accompanied by the color change from brick-red to purple. The selective colorimetric detection of Fe³⁺ ions with P2 over other cations in water was also observed via the formation of a coordination complex between Fe³⁺ ions and azo-Schiff base ligands of P2. After the individual detection of CN^? and Fe³⁺ was carried out, consecutive sensing studies were performed. Although Fe³⁺ was sequentially observed after the presence of CN^? ions was noted, detection using a reverse sequence from Fe³⁺ to CN^? was not possible because of the intense color of the resulting P2[sbnd]Fe³⁺ complexes, which suppressed any observable color changes typically caused by the formation of hydrazone.

Full text of DOI:10.1016/j.dyepig.2019.04.010

DyesandPigments167(2019)174–180
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Water-soluble polymeric probe with dual recognition sites for the sequential
colorimetric detection of cyanide and Fe (III) ions
Moumita Gupta, Hyung-il Lee∗
Department of Chemistry, University of Ulsan, Ulsan, 680-749, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
Dual sensor
Iron (III) sensor
Cyanide sensor
Polymeric probe
Colorimetric
A water-soluble polymeric probe derived from an azo-Schiff base was developed for the consecutive colorimetric
sensing of CN− and Fe3+ ions in water. Herein, the synthesis of a random copolymer of N,N-dimethylacrylamide
(DMA) and 3-vinylbenzaldehyde (VBA) via reversible addition-fragmentation chain transfer polymerization, [p
(DMA-co-VBA, P1] is described. A post-polymerization modification reaction between the aldehyde groups of P1
and (E)-2-amino-4-((4-nitrophenyl)diazenyl)phenol in ethanol led to the target polymer, P2, with azo-Schiff base
moieties. The colorimetric detection of CN− ions occurred because of the abstraction of phenolic proton of P2 by
the CN− ions to form hydrazone, which was accompanied by the color change from brick-red to purple. The
selective colorimetric detection of Fe3+ ions with P2 over other cations in water was also observed via the
formation of a coordination complex between Fe3+ ions and azo-Schiff base ligands of P2. After the individual
detection of CN− and Fe3+ was carried out, consecutive sensing studies were performed. Although Fe3+ was
sequentially observed after the presence of CN− ions was noted, detection using a reverse sequence from Fe3+ to
CN− was not possible because of the intense color of the resulting P2eFe3+ complexes, which suppressed any
observable color changes typically caused by the formation of hydrazone.
Sequential detection
1. Introduction
[15–18]. Though various small molecular colorimetric probes have
been developed for multi-ion sensing studies, they are not soluble in
Development of chemosensors for the efficient detection of Fe3+
and toxic CN− ions has attracted considerable attention [1–4]. Fe3+
ions not only provide the oxygen-carrying capacity of heme but also act
as a cofactor in many enzymatic reactions in human body [5,6]. Defi-
ciency or overdoses of this biologically important cation lead to Alz-
heimer's and Parkinson's diseases and various disorders in organs [7–9].
On the other hand, CN− ions released by many industrial and chemical
processes are easily absorbed by the human body through the lungs,
gastrointestinal tract, and skin, thereby causing many health problems
[10–12]. The strong binding of CN− ions with iron in cytochrome c
oxidase can result in premature death [13]. Thus, it is very important to
develop molecular probes for the selective detection of Fe3+ and CN−
ions individually and sequentially.
The detection of these ions relies heavily on instrumental methods,
including atomic absorption spectroscopy and inductively coupled
plasma mass spectrometry [14]. Unfortunately, these techniques have
numerous disadvantages, such as high costs, no portability, and lengthy
times required for analysis. With this in mind, colorimetric and
fluorometric chemosensors have been developed for the detection of
various ions, thus widening the range of analytes that can be utilized
water [19,20]. Hence, evolution of the polymeric probes used for the
quantitative and qualitative detection of multi-ions is very important
for sensing the presence of toxic analytes in aqueous waste [21,22].
Moreover, polymers with sensor moieties in their polymeric backbones
are more advanced systems than the small molecules typically used in
such studies as they can be prepared and subsequently transferred into
films and coatings, or designed into distinct geometries, such as linear,
spherical, and crosslinked networks that enable the detection analytes
both in liquid and vapor states [23].
Recently, a few polymeric probes have been developed for the
colorimetric detection of Fe3+ ions [24]. Visual color change due to the
formation of co-ordination complexes between the sensing moiety and
the analyte was observed. There are very few published reports for the
detection of Fe3+ ions, which are based on phosphonate-functionalized
polyfluorene and acrylic polymers with different sensing units (such as
kojic acids) [25,26]. On the other hand, numerous strategies have been
implemented for the colorimetric detection of CN− ions, including H-
bonding interactions [27], complex formations [28], nucleophilic at-
tacks on activated C]O [29] and C]C [30], boron center [31], and
deprotonation reactions [32,33].
∗ Corresponding author.
E-mail address: sims0904@ulsan.ac.kr (H.-i. Lee).
https://doi.org/10.1016/j.dyepig.2019.04.010
Received 28 February 2019; Received in revised form 5 April 2019; Accepted 5 April 2019
Availableonline13April2019
0143-7208/©2019PublishedbyElsevierLtd.

M. Gupta and H.-i. Lee
DyesandPigments167(2019)174–180
Here, we report the synthesis of an azo-Schiff base-derived single
polymeric probe, P2, that could be utilized for the colorimetric detec-
tion of both trivalent iron (Fe3+) and CN− ions in aqueous media both
individually and sequentially. The aldehyde functionalized polymer
was post-modified with an amine-functionalized azo chromophore via
imine bond chemistry. The imine bond and the adjacent phenolic–OH
group are known to form coordination bonds with metals and undergo
deprotonation and/or addition reactions with basic/nucleophilic an-
ions. Thus, there is a degree of versatility in the application of said
probes for the colorimetric detection of cations (Fe3+ ions) and anions
(CN− ions) by dual channel. Moreover, in sequential detection studies,
the addition of CN− ions followed by Fe3+ ions in this order allows for
better detection of both ions, whereas addition in the reverse sequence
leads to the Fe3+ ions masking the subsequent detection of the CN−
ions. Although different probes for individual detection of CN− and
Fe3+ ions have been documented previously, we report a novel poly-
meric chemosensor for the one-pot detection of Fe3+ and CN− ions
individually and sequentially.
aqueous solution (2 mL) of NaNO2 (300 mg, 4.34 mmol) was added
dropwise and stirred for another 15 min at 0 °C. In another RB flask,
BOC-protected aminophenol (840 mg, 4.34 mmol) and sodium bi-
carbonate (1.15 g, 10.8 mmol) were dissolved in MeOH:H2O (1:1, v/v,
500 mL) and stirred in an ice bath. After 30 min, the diazonium salt
solution was added dropwise to the basic solution of aminophenol at
0 °C. The reaction was allowed to stir for 3 h before being poured into
water. The pH of the resultant solution was adjusted to neutral to allow
for precipitation. A brown-colored solid product was observed, which
was collected via filtration. Yield = 1.12 g (90.4%). 1H-NMR
(DMSO‑d6, 300 MHz, δ in ppm): 11.14 (1H, s, AreOH), 8.42–8.39 (3H,
d, AreH), 8.04–8.01 (2H, d, AreH), 7.68–7.64 (1H, d, AreH),
7.04–7.01 (1H, d, AreH), 1.49 (9H, s, eCH3).
2.5. Synthesis of (E)-2-amino-4-((4-nitrophenyl)diazenyl)phenol (Azo)
In a 25 mL round bottomed flask, (E)-tert-butyl(2-hydroxy-5-((4-ni-
trophenyl)diazenyl)phenyl)carbamate (0.1 g, 2.92 mmol) was loaded in
1 mL of anhydrous DCM (dichloromethane). TFA (225 μL) was added
dropwise to the solution at 0 °C; after which, the reaction was stirred at
room temperature for 4 h. Upon completion, the solvent was evapo-
rated and the residue was extracted using ethyl acetate. The collected
organic layer was evaporated under vacuum. Yield = 0.069 g (92%).
1H-NMR (DMSO‑d6, 300 MHz, δ in ppm): 11.5 (1H, s, AreOH), 8.34
(2H, d, AreH), 7.93 (2H, d, AreH), 7.25 (2H, d, AreH), 6.87 (1H, d,
AreH), 5.06 (2H, br, AreNH2).
2. Materials and methods
2.1. Materials
3-Vinylbenzaldehyde (VBA, 99.0%) and dimethyl acrylamide
(DMA, 99.0%) were purchased from Aldrich and purified by passing
through a column filled with basic alumina in order to remove any
impurities. 2-Aminophenol, 4-nitroaniline, 2-dodecylsulfanylthio-
carbonylsulfanyl-2-methylpropionic acid (DMP), tert-butyloxycarbonyl
(BOC) anhydride, TFA (trifluoro acetic acid), all metal salts, HEPES (4-
(2-hydroxyethyl)-1-piperazineethanesulfonic acid) buffer solution, and
tetrabutyammonium salts were purchased from Aldrich at the highest
purity available and were used as received. The solvents were obtained
from commercial suppliers and were used as received. 2,2′-Azobis(iso-
butyronitrile) (AIBN, Aldrich, 98%) was recrystallized from ethanol.
2.6. Poly(dimethylacrylamide-co-benzophenoneacrylamide-co-
vinylbenzaldehyde) [p(DMA-co-VBA)] (P1)
To synthesize this random copolymer via controlled radical poly-
merization, DMA (3 g, 30.26 mmol), VBA (166 mg, 1.26 mmol), AIBN
(2.58 mg, 0.03 mmol), DMP (0.11 g, 0.32 mmol), and DMF (8 mL) were
added to Schlenk flask and purged for 30 min under argon atmosphere
to remove any dissolved oxygen. Then, the reaction mixture was heated
at 60 °C for 12 h. After solvent evaporation, precipitation was en-
couraged using diethyl ether. 1H-NMR (DMSO‑d6, 300 MHz, δ in ppm):
10.01 (1H, s, eCHO), 7.75–7.56 (13H, m, AreH), 3.1–2.6 (21H, s,
aliphatic H). GPC: Mn = 6650, Mw = 7200, and PDI = 1.1.
2.2. Instrumentation
1H-NMR spectra were collected in DMSO‑d6 using a Bruker Avance
300 MHz NMR spectrometer. The apparent molecular weight and mo-
lecular weight distributions were measured via gel permeation chro-
matography (GPC, Agilent Technologies 1200 series) using a poly-
methylmethachrylate standard with DMF (dimethyl formamide) as the
eluent at 30 °C and at a flow rate of 1.00 mL/min. The UV–Vis ab-
sorption spectra were recorded using a Varian Cary-100 UV–Vis spec-
trophotometer.
2.7. Synthesis of P2 via post-modification of P1
P1 and (E) -2-amino-4-((4-nitrophenyl)diazenyl)phenol were taken
into ethanol, and Et3N was added. The reaction mixture was heated at
80 °C for 48 h. After solvent evaporation, precipitation was encouraged
using diethyl ether. The precipitate was dried in vacuum to afford P2.
1H-NMR (DMSO‑d6, 300 MHz, δ in ppm): 8.34 (2H, d, AreH), 7.94–7.52
(15H, br, AreH), 7.22 (2H, d, AreH), 6.86 (1H, d, AreH), 3.2–2.0
(21H, s, aliphatic H). GPC: Mn = 7000, Mw = 7300, and PDI = 1.0.
2.3. Synthesis of tert-butyl (2-hydroxyphenyl)carbamate (S1)
To the solution of 2-aminophenol (1) (4 g, 36.6 mmol) in 60 mL
THF–water (1:1 v/v) triethylamine (Et3N) (9.8 mL, 44.1 mmol) was
added. Then, BOC anhydride (8.4 mL, 36.6 mmol) was added and
stirred overnight. The reaction mixture was diluted with water and
subsequently extracted with ethyl acetate. The organic layer was dried
over Na2SO4, and the solvent was evaporated. The crude mixture was
purified via column chromatography (hexane:EtOAc, 3:1) to get desired
2.8. Sensing studies
A 170 mM stock solution of P2 was prepared in water. Solution
samples (10 mM) of the metal ions Fe3+, Li+, K+, Na+, Co2+, Cu2+
,
Cd2+, Fe2+, Al3+, Hg2+, Zn2+, Ni2+, Mg2+, and Pb2+ were prepared
by dissolving the corresponding salts in water. For the anion studies,
stock solutions of the corresponding tetrabutylammonium salts
product in
a
light orange color. Yield = 3 g, (42.4%). 1H-NMR
(DMSO‑d6, 300 MHz, δ in ppm): 9.74 (1H, s, AreOH), 7.79 (1H, d,
eNHCCH), 7.60–7.57 (1H, d, eOHCCH), 6.87–6.74 (1H, m,
eCHCHCH-), 1.46 (9H, s, eCH3).
(100 mM) of the anions F−, Cl−, Br−, I−, CN−, ClO4−, HSO4−, CN−
,
S2−, SH−, PO43−, and SCN− in water were prepared. A 10 μL aliquot of
each metal ion and a 5 μL aliquot of each anion's stock solution were
added using a microsyringe to a 0.35 mL sample solution of P2 placed in
a cuvette. The absorption spectral changes were monitored.
2.4. Synthesis of (E)-tert-butyl(2-hydroxy-5-((4-nitrophenyl)diazenyl)
phenyl)carbamate (S2)
4-Nitroaniline (0.5 g, 3.62 mmol) was added to
a
solution of
3. Results and discussion
MeOH:H2O (1:1). Concentrated HCl (2.0 mL) was added slowly, and the
solution was stirred at 0 °C. In order to generate the diazonium salt, an
Azo-1 (Figure S2) was synthesized via a diazotization reaction
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DyesandPigments167(2019)174–180
Scheme 1. Synthesis of water-soluble polymeric probe (P2) with azo-Schiff base moieties.
Fig. 1. 1H NMR spectra of (a) P1 and (b) P2.
between BOC-protected 2-aminophenol (Figure S1) and diazonium salt
of 4-nitroaniline at 0 °C. Deprotection of the BOC groups in the presence
of TFA led to Azo-2. The formation of Azo-2 was monitored using 1H-
NMR spectroscopy (Figure S3). The appearance of aromatic protons in
the region of 8.39 to 6.61 ppm and the disappearance of the eCH3
protons on the BOC groups at 1.5 ppm confirmed the successful
synthesis of Azo-2.
The design strategy for the synthesis of the azo-Schiff base-derived
polymeric probe, P2, is depicted in Scheme 1. A random copolymer, p
(DMA-co-VBA) (P1), with aldehyde functionalities in the side chains
was prepared as reported previously (Fig. 1a) [16]. The incorporation
ratio of DMA and VBA along the P1 backbone was 96:4. P1 had a
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DyesandPigments167(2019)174–180
Fig. 2. (a) UV–Vis absorption spectra of P2 (1.52 × 10−5 M) upon the gradual addition of CN− ions at pH 7, (b) plausible mechanism of P2 for the detection of the
CN− ions, (c) selectivity bar diagram of P2 with various anions of tetrabutylammonium (TBA) salts, and (d) reversibility of P2 (1.52 × 10−5 M) upon the alternating
addition of CN− ions and 0.75 (N) HCl in H2O.
Mn = 6650 g/mol and a polydispersity of Mw/Mn = 1.1 (Figure S4). P1
was subjected to a single-step, post-polymerization modification reac-
tion with Azo-2 to obtain P2. The aldehyde group (eCHO) of P1 reacted
with amine group of Azo-2 to form P2. The aldehyde peak of P1 at
10.01 ppm disappeared, and the imine proton (eCH]N) of P2 ap-
peared at 5.08 ppm, thus demonstrating complete transformation
(Fig. 1b).
Having examined the colorimetric detection of CN− ions, pH-de-
pendent sensing studies were conducted (Figs. S9 and S10). P2 showed
high sensitivity towards CN− ions at low to neutral pH where the
neutral azo form was dominant. Negligible response was observed at
high pH (pH 10 to 12) where the hydrazone form was already generated
by deprotonation of the phenolic protons, thereby preventing the de-
tection of CN− ions. This pH-dependent detection behavior was em-
ployed to provide reversibility in the sensing system (Fig. 2d and Fig.
S11). The reversible detection process was achieved by decreasing the
pH of the solution after the detection of CN− ions occurred. Once CN−
ions were detected, the absorption band at 530 nm reverted to its ori-
ginal position (411 nm) upon the addition of 0.75 N HCl into the so-
lution. This cycle was repeated four times.
The detection of CN− ions by P2 (1.52 × 10−5 M of azo-chromo-
phore units, assuming 4% incorporation along the backbone) was tested
using UV–Vis absorption spectroscopy in aqueous HEPES buffer solu-
tion at pH 7 (Fig. 2a). The addition of increasing concentration of CN−
ions up to 28.0 mM caused P2's characteristic absorption maximum to
red-shift from 411 to 530 nm. This drastic shift could be observed with
the naked eye as a color change from brick-red to purple. This phe-
nomenon could be explained through the abstraction of phenolic pro-
tons by CN− ions, thus leading to the formation of a quinoid structure
in presence of electron-withdrawing nitro groups [Fig. 2b] [31]. The
lower detection limit for CN− ions was determined to be 0.1 mM
(Figure S5). In order to evaluate the selectivity of P2 towards CN− ions
over the other anions present, the spectral responses of P2 were mon-
The detection of Fe3+ ions by P2 (1.52 × 10−5 M) was also ex-
amined using UV–Vis absorption spectroscopy in aqueous HEPES buffer
solution at pH 7. Upon the gradual addition of Fe3+ ions, a new band
appeared at 304 nm with a distinct color change from brick-red to
yellow (Fig. 3a). The reason for the large blue-shift of 107 nm in the
absorption spectra is due to the formation of a Fe3+ ion coordination
complex with the lone pair of the O-atom of the hydroxyl group in azo
moiety and the N-atom of the imine bond of P2. This inhibited the intra-
molecular charge transfer process by affecting the electronic environ-
ment of the azo-chromophore (Fig. 3b) [33–35]. The detection limit of
P2 with Fe3+ ions was found to be 0.1 mM (Fig. 3c). The selectivity of
P2 was also examined by screening various alkalis, alkalines, and
transition metal cations under the same physiological conditions em-
ployed for Fe3+ ion detection. Fig. 3d represented the selectivity bar
diagram of P2 revealed that P2 was highly selective towards Fe3+ ions
itored by screening the other anions, such as Cl−, F−, Br−, I−, CN−
,
NO2−, HSO4−, S2−, SH−, PO43−, and SCN−, using aqueous solutions of
their tetrabutylammonium salts (up to 28 mM). The diagram below
clearly indicated that P2 selectively detected CN− ions over all other
anions, especially those that did not show any spectral changes (Fig. 2c,
Figs. S6-S8). Even though F− ions were thought to be direct competitors
of CN−, our results demonstrated that P2 showed excellent selectivity
towards CN− ions without any interference with F−
.
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M. Gupta and H.-i. Lee
DyesandPigments167(2019)174–180
Fig. 3. (a) UV–Vis absorption spectra of P2 (1.52 × 10−5 M) upon the addition of Fe3+ ions (up to 0.8 mM) in H2O at pH 7, (b) schematic representation for the
formation of the coordination complex of Fe3+ ions with P2, (c) a linear regression curve for the calculation of lower detection limit for Fe3+ ions, and (d) selectivity
bar diagram for different cations (up to 0.8 mM).
Fig. 4. UV–Vis absorption spectra of P2 (1.52 × 10−5 M) upon the addition of (a) CN− ions followed by Fe3+ ions, and (b) Fe3+ ions followed by CN− ions in water.
over other metal cations. The UV–Vis absorption spectral responses of
individually, consecutive sensing studies were performed by monitoring
the changes in absorbance spectra with the sequential addition of each
ion. The addition of CN− ions to P2 solution has exhibited a red-shift
from 411 to 530 nm along with an intense color change from brick-red
to purple. The sequential addition of Fe3+ ions into the solution of
P2 + CN− ions prepared in situ showed a blue-shift from 530 to 304 nm
with a color change from purple to yellow (Fig. 4a). In the alternative
sequence, the blue-shift appeared with a color change from brick-red to
P2 towards metal cations Li+, K+, Na+, Co2+, Cu2+, Cd2+, Fe2+, Al3+
,
Hg2+, Zn2+, Ni2+, Mg2+, and Pb2+ were shown in Figures S12 - S14.
Metal binding studies of P2 for Fe3+ ions were performed under dif-
ferent pH conditions (Figure S15). No significant change was found in
absorption spectra of P2 upon the addition of Fe3+ ions at various pH
values.
After investigating the proclivity of P2 towards CN− and Fe3+ ions
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DyesandPigments167(2019)174–180
Fig. 5. Schematic representation for the plausible mechanism of the consecutive sensing study of P2 with CN− and Fe3+ ions in water.
yellow upon the addition of Fe3+ ions. However, no further shifts were
found in the absorbance spectra of the aqueous solution of P2 + Fe3+
prepared in situ after the addition of CN− ions (Fig. 4b). These results
revealed that P2 acted as a dual sensor with distinct color changes only
when CN− ions were added followed by Fe3+ ions. The plausible me-
chanism behind these phenomena is depicted in Fig. 5. After the de-
tection of CN− ions, the quinoid chromophore formed in situ was ex-
pected to coordinate with Fe3+ ions through the lone pair of O-atom
and N-atom of the imine bond. Thus, the addition of Fe3+ ions to the
P2 + CN− solution resulted in further color changes from purple to
yellow, which in turn enabled the consecutive detection of CN− and
Fe3+ ions. However, after the detection of Fe3+ ions via the co-
ordination complex, addition of CN− ions to the P2eFe3+ complex
formed in situ did not lead to a distinct color change. Though regardless
of the ability of the CN− ions to abstract the phenolic proton of the P2-
Fe3+ complex, the intense color of the P2-Fe3+ complex present in the
medium suppressed any color changes caused by elongated conjuga-
tion.
complexes between Fe3+ ions and azo Schiff-base ligands of P2. Lastly,
the sequential sensing studies revealed that the detection of CN− ions
followed by Fe3+ ions was possible via the change in color from brick-
red to purple to yellow. Addition of the ions in the reverse sequence was
unsuccessful because the yellow-colored solution after the formation of
P2eFe3+ complex suppressed any color changes caused by the quinoid
structure after the addition of CN− ions.
Acknowledgements
This work was supported by the Basic Science Research Program
(NRF-2017R1A2B4003861) administered by the National Research
Foundation of Korea, funded by the Ministry of Science, ICT, and Future
Planning of Korea.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.dyepig.2019.04.010.
4. Conclusions
References
A polymeric probe, P2, based on the Schiff-base unit with azo
moiety was developed for the colorimetric detection of Fe3+ ions and
CN− ions in aqueous media. P2 showed dual sensing for Fe3+ ions and
CN− ions in water with high selectivity and sensitivity. For the CN−
ions sensing studies, P2 showed a red-shift of 119 nm with a distinct
color change from brick-red to purple. CN− ions could be reversibly
detected by decreasing the pH of the solution after the detection of CN−
ions. The colorimetric sensing of the Fe3+ ions was accomplished by a
107 nm blue-shift in the absorption band with a visible color change
from brick-red to yellow due to the formation of coordination
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