Synthesis of a novel hyperbranched polymer and its application in multi-channel sensing Fe3+

Article abstract of DOI:10.1007/s11164-019-04042-5

A novel hyperbranched polymer was designed and synthesized as a spectral probe for Fe³⁺. The polymer showed high selectivity and sensitivity to Fe³⁺ in CH₃CN/H₂O (75/25, v/v). Fe³⁺ caused a new peak a

Full text of DOI:10.1007/s11164-019-04042-5

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-04042-5
Synthesis of a novel hyperbranched polymer and its
application in multi‑channel sensing Fe3+
Tengxuan Tang1 · Jing Wang1 · Dongmei Xu1
Received: 14 August 2019 / Accepted: 6 November 2019
© Springer Nature B.V. 2019
Abstract
A novel hyperbranched polymer was designed and synthesized as a spectral probe
for Fe3+. The polymer showed high selectivity and sensitivity to Fe3+ in CH3CN/
H2O (75/25, v/v). Fe3+ caused a new peak at 560 nm in UV–Vis absorption, a 31 nm
fuorescence redshift, a 35-fold enhancement in fuorescence intensity at 575 nm and
an 8.3-fold enhancement in fuorescence quantum yield, accompanied by a visual
color change from colorless to pink and a fuorescence from dark to bright orange.
The colorimetric and fuorescent detection limits were 1.29 and 1.88 µM, respec-
tively. The detection was almost not interfered by other common metal cations. The
polymer could be applied in assaying Fe3+ in real sample with similar precision to
that of atomic absorption spectroscopy.
Keywords Hyperbranched polymer · Rhodamine · Probe · Fe3+
Introduction
Trivalent iron (Fe3+) plays a vital role in the environment and organisms. Its def-
ciency or overload will cause diseases like anemia, liver dysfunction, diabetes,
hemochromatosis and cancer [1–3]. Therefore, the monitoring of Fe3+ in envi-
ronment and organisms is of great importance. In recent years, spectral probe has
received much attention in the detection of environmental and biological Fe3+
because of its rapid response, high sensitivity, specifcity, low-cost and real-time
monitoring [4–13].
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-019-04042-5) contains supplementary material, which is available to authorized users.
* Dongmei Xu
xdm.sd@163.com
1
College of Chemistry, Chemical Engineering and Materials Science, Soochow University,
Suzhou 215123, Jiangsu, China
Vol.:(0123456789)
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T. Tang et al.
Most turn-on spectral probes for Fe3+ were based on the rhodamine scafold
[14–21]. However, the reported small molecule probes had some problems such as
poor selectivity and easy to lose. Recently, some researchers devoted into studying
macromolecular fuorescence probe to overcome the inadequacy [22, 23] because
the microenvironment of the polymer chains is in favor of improving the probe’s
selectivity and the macromolecular backbone of the polymer can resist the probe’s
loss.
Reversible addition–fragmentation chain transfer (RAFT) polymerization has
been considered as one of the most practical polymerization methods for controlling
the molecular weight and distribution. Moreover, RAFT polymerization can be car-
ried out by one-pot reaction and relatively easily generate hyperbranched polymer
with multiple terminal groups which benefts further modifcation [24–27].
In this work, we designed and synthesized a hyperbranched polymeric spectral
probe by using a rhodamine derivative to modify the epoxy groups in a RAFT poly-
mer. The probe showed excellent sensitivity and selectivity to Fe3+ through multiple
channels.
Experimental
Synthesis of PGMN2
The reagents and instruments could be found in Supplementary material. Hyper-
branched polymeric spectral probe PGMN2 was synthesized by the reaction between
the small molecular rhodamine derivative (RE1N2) and the RAFT polymer PGM as
illustrated in Scheme 1.
The rhodamine derivative (RE1N2) and the inimer (MCP) were synthesized
according to the references [25, 28]. To a stirred solution of rhodamine B (0.5 g,
1.13 mmol) in ethanol (15 mL), ethylenediamine (0.5 mL, 7.5 mmol) was added
and the mixture was refuxed overnight under N2 protection. After cooled to the
room temperature, the solvent was removed under reduced pressure and then the
residue was dissolved in 20 mL dichloromethane (DCM). The organic solution was
washed with water twice, and DCM was removed by evaporation. Earthy yellow
solid RE1N2 (0.4703 g, yield 86.3%) was obtained after dried in vacuo. To a stirred
solution of 4-cyanopentanoic acid dithiobenzoate (4-CPDB) (0.5 g, 1.79 mmol) in
dry DCM (25 mL), hydroxyethyl methylacrylate (HEMA) (235 µL, 1.93 mmol)
and 4-dimethylamino pyridine (DMAP) (0.045 g, 0.37 mmol) were added and then
N,N′-Dicyclohexylcarbodiimide (DCC) (1 g, 4.86 mmol) was dissolved in 50 mL
dry DCM and dropped into the solution. The mixture was reacted in the ice-water
bath for 24 h under N2 protection. The crude product was obtained by fltration,
water-washing and Na2SO4-drying. The crude product was further purifed through
silica gel chromatography with petroleum ether/ethyl acetate (10/1, v/v) as eluent.
1
MCP was gotten as a pink liquid (0.5726 g, yield 69.2%). The H NMR spectra of
RE1N2 (Fig. S1) and MCP (Fig. S2) were in line with the references.
The RAFT polymer PGM was synthesized in a similar way to the literature
[26]: MCP (0.2 g, 0.5 mmol) was dissolved in 10 mL dioxane and dropped into a
1 3

Synthesis of a novel hyperbranched polymer and its application…
Scheme 1 Synthetic route of PGMN2
100 mL round fask, and then glycidyl methacrylate (GMA) (1.3 mL, 10 mmol)
and 2,2′-azobis(isobutyronitrile) (AIBN) (16 mg, 0.1 mmol) were added. The
mixture was reacted at 70 °C for 12 h under N2 protection. After precipitation
in cold methanol and dried in vacuum, 0.8792 g (54.2% total monomer conver-
sion) PGM was gotten. Its number average molecular weight (Mn) was 10,200
1
and the polydispersity (PDI) was 1.71 (Fig. S3). H NMR (400 MHz, CDCl3, δ/
ppm): 0.93 (s, C(CH3)COO and C(CN)(CH3) from MCP), 1.09 (s, C(CH3)COO
from GMA), 1.18–1.26 (m, C(CH3)(COO)CH2 and C(CN)(CH3)CH2 from MCP),
1.50 (s, CH2=CCH3 from MCP), 1.86–1.92 (t, J = 7.2 Hz, C(CN)(CH3)CH2CH-
2COO from MCP), 1.97 (s, C(CH3)(COO)CH2 from GMA), 2.64 and 2.85 (s,
CH2O from GMA), 3.24 (m, CH from GMA), 3.69–3.73 (t, J = 6.8 Hz, COOCH-
2CH2OCO from MCP), 3.80 and 4.31 (s, COOCH2 from GMA), 5.64 and 6.20 (s,
1 3

T. Tang et al.
Fig. 1 1H NMR spectrum of PGM (CDCl3, 400 MHz)
C=CH2), 7.40–7.91 (m, ArH) (Fig. 1). FT-IR: 3001 (Ar), 2937, 2885 (CH3, CH2),
1723 (C=O), 1484, 1452, 1393 (Ar), 1258, 1148 (C–O–C), 907 (epoxy) (Fig. S4).
PGM (0.2 g) was dissolved in 12 mL dioxane in a 100 mL round fask, and
RE1N2 (0.03 g, 0.06 mmol) was added. The mixture was protected by N2 and
reacted at 60 °C for 8 h, then precipitated in 20 mL cold methanol, washed with
cold methanol (10 mL×3) to remove unreacted RE1N2 and dried in vacuum, and
0.195 g (yield 85.0%) faint yellow solid PGMN2 was achieved. The number average
molecular weight and polydispersity of PGMN2 were 12,200 and 1.92 (Fig. S3). 1H
NMR (400 MHz, CDCl3, δ/ppm): 0.93–1.98 (m, CH3 and CH2 except those speci-
fed below), 2.64 and 2.84 (s, CH2O from GMA, NHCH2CH(OH) from the reac-
tion of PGM and RE1N2), 3.24 (s, CH from GMA, NHCH2CH(OH) from the reac-
tion of PGM and RE1N2), 3.31–3.35 (m, NCH2CH2NH from RE1N2), 3.52–3.56
(t, J=6.8 Hz, COOCH2CH2OCO from MCP), 3.80 and 4.31 (s, COOCH2 from
GMA, COOCH2CH(OH) from the reaction of PGM and RE1N2), 5.64 and 6.01 (s,
C=CH2), 6.24–6.48, 7.10 (m, ArH from RE1N2), 7.40–7.91 (m, ArH from PGM
and RE1N2) (Fig. 2). FT-IR: 3440 (OH, NH), 3064 (Ar), 2967, 2851 (CH3, CH2),
1728(C=O, ester), 1618 (C=O, amide), 1548, 1489, 1451,1398 (Ar), 1264, 1163
(C–O–C), 1121 (C–N), 908 (epoxy) (Fig. S4).
Spectra study of PGMN2
PGMN2 was dissolved in 10 mL CH3CN to get a 5 mg/mL stock solution. Metal
salts were dissolved in deionized water to get 1.0×10−2 mol/L stock solutions.
1 3

Synthesis of a novel hyperbranched polymer and its application…
Fig. 2 1H NMR spectrum of PGMN2 (CDCl3, 400 MHz)
When studying the selectivity of PGMN2 to metal ions, 100 µL stock solution of
PGMN2 and one of the metal ion stock solutions were added to a 10 mL volumet-
ric fask. To demonstrate the anti-interference of other metals, one of the metal ion
stock solutions except Fe3+ (250 µL) was individually added to a 10 mL volumetric
fask containing PGMN2 (100 µL) and Fe3+ (250 µL). In the fuorescence titration
experiment, 100 µL PGMN2 solution was mixed with a certain amount of Fe3+ stock
solution. All samples were diluted to volume with CH3CN and H2O. The medium
for the UV–Vis absorption and fuorescence spectra measurement was CH3CN/H2O
(75/25, v/v). The pH was adjusted by 0.1 mol/L HCl and 0.1 mol/L NaOH aqueous
solutions.
Detecting Fe3+ in real sample
1 mL pond water (after fltration) or tap water from Dushu Lake Campus of Soo-
chow University was added into a 10 mL volumetric fask, and then PGMN2 (100
µL) and Fe3+ stock solutions (100 or 180 µL) were added. PGMN2 method: The
mixture was eventually diluted to volume with CH3CN and H2O, and the fuores-
cence spectra were recorded. The concentration of Fe3+ in the samples was obtained
from the linear relationship between the maximal fuorescence intensity of the sam-
ple and the Fe3+ concentration. AAS method: The mixture was diluted to volume
1 3

T. Tang et al.
with 0.1 mol/L HCl, and the Fe3+ concentration in the samples was determined by
an AA240FS-GTA120 atomic absorption spectrometer.
Results and discussion
Structure parameters of the hyperbranched PGM and PGMN2
The hyperbranched polymer PGM was generated by RAFT copolymerization of
GMA and the intermediate MCP which was from 4-CPDB esterifed with HEMA.
1
In the H NMR spectrum of PGM (Fig. 1), the integrals (I) of the peaks rep-
resenting the linear element (L), branching element (D) and terminal element (T)
were IL =Ib′ +Ic′ +Id′ +Im′ +In′ =(6.33+5.73)+5.56+(5.56+5.87)+6.76+10.56=
46.37, ID =Ie′ +Ii′ +(Ih′ +Ik′)+(Ig′ +Il′)=13.43+6.78+(6.47 +4.52)+15.75=46.95
and IT =Ia′ =1.00. Based on the Ref. [29], the degree of branching (DB)=(ID +IT)/
(ID +IT +IL)≈0.50.
1
From H NMR spectrum (Fig. 1) and GPC (Fig. S3) of PGM, the signal at 7.87
was from the phenyl of MCP and the peak at 3.24 was from the epoxy group of
PGM. The number average chain transfer agent functionality (Fn) in per hyper-
branched PGM was 1.71 based on the equation Fn=MnPGM ×I7.87/(MwMCP ×I7.87
+
2MwGMA ×I3.24), in which MWMCP and MWGMA were molecular weight of MCP and
GMA. By comparing the integration of the peaks at 7.87 (I7.87) and 3.24 (I3.24) ppm,
the molar ratio (R) of GMA to MCP was determined to be 39.52 (R=2I3.24/I7.87).
Hence, each hyperbranched PGM contained about 67.6 (Fn×R) GMA units [26].
The spectral probe PGMN2 was synthesized through PGM functionalized with
rhodamine unit RE1N2. The number of RE1N2 in PGMN2 based on GPC was calcu-
lated by the number average molecular weight (Mn) of PGMN2 and PGM as well as
the molecular weight of RE1N2 according to the equation m=[MnPGMN2 −MnPGM]/
1
MRE1N2. The result was 4.12. Alternatively, in the H NMR spectrum of PGMN2
(Fig. 2), the chemical shift at 3.35 was the signal of hydrogen in the ethanediamine
of rhodamine unit and the chemical shift at 3.24 was belonged to the epoxy unit of
PGM. According to the integral (I) of these two peaks, 6.54% [I3.35/(I3.35 +4I3.24)] of
GMA units was estimated to take part in the epoxy ring opening reaction. Therefore,
the number of RE1N2 grafted to PGM was 4.42 (6.54%×67.6) [26] which was near
to 4.12 from GPC.
Selectivity and sensitivity of PGMN2 to Fe3+
Solubility of PGMN2 was studied, and it was found that PGMN2 was poorly dis-
solved in methanol, ethanol and deionized water (H2O), moderately dissolved in
CH3CN and DMF, and well dissolved in solvents with medium polarity such as
DCM, THF, acetone and dioxane. Both solubility, practicality and environment
amity are considered, the mixture of CH3CN and H2O was selected as the sensing
medium.
1 3

Synthesis of a novel hyperbranched polymer and its application…
The selectivity of hyperbranched spectral probe PGMN2 to common metal ions
such as Ca2+, Mg2+, K+, Na+, Fe2+, Cu2+, Fe3+, Zn2+, Co2+, Mn2+, Pb2+, Ni2+,
Cr3+, Cd2+, Hg2+ and Ag+ was investigated by UV–Vis absorption and fuores-
cence spectroscopy in CH3CN/H2O (75/25, v/v). As shown in Fig. 3, only when
Fe3+ was added into the PGMN2 solution, an apparent peak at 565 nm accompa-
nied by a naked-eye observed color change from colorless to pink appeared in the
UV–Vis absorption spectra (Fig. 3a). Similarly, a 31 nm fuorescence redshift, a
35-fold fuorescence enhancement at 575 nm and an 8.3-fold enhancement in fu-
orescence quantum yield (using rhodamine B in ethanol as the reference standard,
Φ = 0.97) with the emerging of a bright orange fuorescence color after Fe3+ were
added (Fig. 3b). All these results exhibited that PGMN2 was highly selective and
sensitive to Fe3+ and had the potential to be utilized as a turn-on highly selective
and sensitive multi-channel spectral probe for Fe3+.
UV–Vis absorption and fuorescence titration with Fe3+
In order to investigate the possibility of PGMN2 used as the spectral probe for
Fe3+, titration experiment was done and the results are shown in Fig. 4. The
absorbance at 565 nm in UV–Vis absorption spectra showed a linearly propor-
tional relationship with the concentration of Fe3+ ranging from 40 to 240 µM
(Fig. 4a), and the correlation coefcient was 0.993. In addition, the fuorescence
intensity at 575 nm had a linear response toward Fe3+ with a correlation coef-
fcient of 0.992 (Fig. 4b). The DLs evaluated from colorimetric and fuores-
cent titrations [19] were 1.29 and 1.88 µM, respectively. The results proved the
hypothesis that PGMN2 could quantitatively trace Fe3+ in CH3CN/H2O (75/25,
v/v) by multiple channels.
0.6
PGMN2
+Fe3+
PGMN2
a
4x105
PGMN
2
b
PGMN2
3+
+Fe
blank,K+,Na+,Mg2+
,
Fe3+
Ca2+,Cu2+,Zn2+,Cr3+
,
Ag+,Hg2+,Co2+,Ni2+
Cd2+,Mn2+,Fe2+
,
0.3
0.0
2x105
blank,K+,Na+,Mg2+
Ca2+,Cu2+,Zn2+,Pb2+
,
,
Fe3+
Ag+,Fe2+,Cr3+,Hg2+
,
Co2+,Ni2+,Cd2+,Mn2+
0
500
600
Wavelength /nm
700
300
400
500
600
Wavelength /nm
Fig. 3 UV–Vis absorption (a) and fuorescence (b) spectra of PGMN2 in the absence and presence of
various cations. Solvent: CH3CN/H2O (75/25, v/v). Concentration: 50 µg/mL for PGMN2, 200 µM for
metal ions. λex: 467 nm, slit width: 5 nm
1 3

T. Tang et al.
1.2x106
8.0x105
0.3
0.2
0.1
0.0
1.2x106
8.0x105
a
b
0.06
0.03
0.00
4.0x105
0.0
0
100
[Fe3+
200
300
/10-6
M
0
100
[Fe3+
200
300
]
]
/10-6
M
4.0x105
0.0
500
300
400
500
600
600
Wavelength /nm
700
Wavelength /nm
Fig. 4 UV–Vis absorption (a) and fuorescence (b) spectra of PGMN2 with various concentrations of
Fe3+. Solvent: CH3CN/H2O (75/25, v/v), concentration: 50 µg/mL for PGMN2, from bottom to top, Fe3+
:
0, 10, 20, 30, 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, 260, 280, 300 and 320 µM. Insets: the
relationship between the absorbance at 560 nm/fuorescence intensity at 575 nm and the concentration of
Fe3+. λex: 467 nm, slit width: 5 nm
Efect of coexisting ions
In order to fgure out if other common metal cations will infuence the detection
of Fe3+ by PGMN2, K+, Na+, Mg2+, Cu2+, Zn2+, Cr3+, Fe2+, Ca2+, Pb2+, Hg2+,
Ni2+, Mn2+, Co2+, Cd2+ and Ag+ were introduced into the solution of PGMN2-
Fe3+ individually, and the absorption and fuorescence spectra were recorded,
respectively (Fig. 5). The experiments showed that the addition of these cations
had negligible infuence on the absorption and fuorescence maxima of PGMN2-
Fe3+ solution, which indicated the excellent anti-interference performance of
PGMN2 for monitoring Fe3+.
6
0.08
0.04
0.00
1.2x10
a
b
5
8.0x10
5
4.0x10
0.0
+
+
+
+
+
+
+
+
+
+
+
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+
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2
K
a
+
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2+i
3
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+
+
+
+
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+
+
+
+
+
+
+
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2
2
2
2
2
2
2
2
2
2
2i
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3
a
g
A
+
+
3
e
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2
2
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2
2
2
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g
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C +
3
e
n
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+
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d
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+
+
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n
b
P +
3
e
g
e
+
n
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A
+
+
C +
F
N 3
+
3
o 3+
C
N
C +
F
C3+
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+
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o 3+
C
+
3
e
C3+
M+
M3
+
H3+
+
+
+
+
+
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+
M+
M3
+
+
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F
F
F
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Fig. 5 Efects of coexisting ions on the UV–Vis absorbance at 560 nm (a) and fuorescence intensity at
575 nm (b) of the PGMN2-Fe3+ solutions. Solvent: CH3CN/H2O (75/25, v/v). Concentration: 50 µg/mL
for PGMN2, 300 µM for ions. λex: 467 nm, slit width: 5 nm
1 3

Synthesis of a novel hyperbranched polymer and its application…
a
b2.1x10
6
0.15
0.08
0.01
PGMN2
PGMN2
3+
PGMN2+Fe
PGMN2+Fe3+
6
1.4x10
5
7.0x10
4
6
8
10
4
6
8
10
pH
pH
Fig. 6 Efects of pH on the absorbance at 560 nm (a) and fuorescence intensity at 575 nm (b) of
PGMN2 and PGMN2-Fe3+. Solvent: CH3CN/H2O (75/25, v/v), concentration: 50 µg/mL for PGMN2,
300 µM for Fe3+, pH: 4.3, 5.0, 5.5, 5.8, 6.0, 6.5, 7.0, 7.5, 8.0, 9.0 and 10.0. λex: 467 nm, slit width: 5 nm
1.2x106
a
b
PGMN2+Fe3+
0.06
PGMN2+Fe3+
8.0x105
0.03
4.0x105
PGMN2
PGMN2
120
0.00
0.0
0
40
80
120
160
0
40
80
Time /min
160
Time /min
Fig. 7 Kinetics of PGMN2 sensing Fe3+ by UV–Vis absorption (a) and fuorescence (b) spectra. Sol-
vent: CH3CN/H2O (75/25, v/v), concentration: 50 µg/mL for PGMN2, 300 µM for Fe3+. λex: 467 nm, slit
width: 5 nm
Efect of pH
To know the acid and alkali media suitable for PGMN2, the UV–Vis absorption
and fuorescence spectra of PGMN2 and PGMN2-Fe3+ in the pH range of 4.3–10.0
(Fig. 6) were studied. The absorbance at 560 nm (A560) (Fig. 6a) and the fuores-
cence intensity at 575 nm (F575) (Fig. 6b) of the solutions before and after introduc-
ing Fe3+ coincided at pH 4.7 and pH 8.0, respectively. However, when pH increased
from 5.0 to 7.5, A560 and F575 of the PGMN2-Fe3+ solutions were obviously larger
than those of the PGMN2 solutions with the same pH. Therefore, PGMN2 could be
applied to monitor Fe3+ in the pH range of 5.0–7.5.
Kinetics of PGMN2 sensing Fe3+
The time dependence of PGMN2 sensing Fe3+ by UV–Vis absorption and fuores-
cence spectra was investigated, and the results are given in Fig. 7. It could be seen
1 3

T. Tang et al.
Table 1 Determination of Fe3+ in environmental water samples
Method
Sample
Fe3+ added (10−6 Fe3+ found (10−6 Recovery (%)
RSD (%)
mol/L)
mol/L)
PGMN2
Tap water
Pool water
Tap water
Pool water
100
180
100
180
100
180
100
180
102.6
178.8
97.8
183.9
101.1
181.3
99.6
102.6
99.3
97.8
102.1
101.1
100.7
99.6
0.45
0.86
1.46
0.73
1.99
1.83
1.41
1.42
AAS
180.8
100.4
Solvent: CH3CN/H2O (75/25,v/v), concentration: 50 µg/mL for PGMN2, RSD: Relative standard devia-
tion. Each test was repeated three times
that the reaction between PGMN2 and Fe3+ fnished almost after 90 min they was
mixed.
Real sample assay
We tried to use PGMN2 as a fuorescent probe to determine the amount of Fe3+
in spiked tap water and pool water and compared the result with that from AAS
(Table 1). The concentration of Fe3+ found by PGMN2 and AAS was very closed
and both consistent with the Fe3+ added. The recovery from PGMN2 was between
97.8% and 102.6% with the relative standard deviation (RSD) of three measure-
ments less than 1.5% which was similar to that from AAS. Therefore, PGMN2 can
be applied in the efective detection of Fe3+ in environmental water.
Conclusion
In summary, this paper discussed a novel rhodamine-based hyperbranched poly-
meric turn-on colorimetric and fuorescent probe PGMN2 for Fe3+. With high selec-
tivity and sensitivity to Fe3+ over other common cations, PGMN2 can sense Fe3+
by six channels including absorbance increase, fuorescence intensity strengthening,
fuorescence quantum yield enhancement, fuorescence redshift, visual color change
and fuorescence color change. Moreover, the probe can monitor Fe3+ in a wide con-
centration range, with low detection limit under physiological pH. The probe can
also be applied in environmental water sample assay, and its accuracy is similar to
that of AAS.
Acknowledgements This work was supported by the National Natural Science Foundation of China
(21074085), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
1 3

Synthesis of a novel hyperbranched polymer and its application…
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