Polydiacetylenes Containing 2-Picolylamide Chemosensor for Colorimetric Detection of Cadmium Ions

Article abstract of DOI:10.1002/bkcs.12163

This article describes the preparation and analysis of Cd²⁺-sensing polydiacetylene-based chemosensors (PDA-MP). Polydiacetylenes (PDAs), a family of conjugated polymers, exhibit a strong response to external stimuli. Accordingly, we have designed a novel PDA sensor, which is linked to mono 2-picolylamine, to increase the PCDA-MP monomer content in polymer system. As a result, it enhanced sensitivity toward Cd²⁺ with the low detection limit (2.10 μM) in aqueous solution. Our sequential modification of chelate structure improved the Cd²⁺-sensing ability of PDAs. Finally, the interaction between PCDA–MP and Cd²⁺ was confirmed by Raman spectroscopy, Field Emission Scanning Electron Microscopes (FE-SEM), and Proton nuclear magnetic resonance (¹H NMR).

Full text of DOI:10.1002/bkcs.12163

Article
DOI: 10.1002/bkcs.12163
BULLETIN OF THE
Y. K. Kim et al.
KOREAN CHEMICAL SOCIETY
Polydiacetylenes Containing 2-Picolylamide Chemosensor for
Colorimetric Detection of Cadmium Ions
Yong Kyun Kim,† Thanh Chung Pham,† Jaewon Kim,‡ Chaeeon Bae,† Yeonghwan Choi,†
Min Hee Jo,‡ and Songyi Lee†,‡,
*
†Interdisciplinary Program of Biomedical, Mechanical and Electrical Engineering, Pukyong National
University, Busan 48513, South Korea
‡Department of Chemistry, Pukyong National University, Busan 48513, South Korea.
*E-mail: slee@pknu.ac.kr
Received September 25, 2020, Accepted November 19, 2020
This article describes the preparation and analysis of Cd2+-sensing polydiacetylene-based chemosensors
(PDA-MP). Polydiacetylenes (PDAs), a family of conjugated polymers, exhibit a strong response to exter-
nal stimuli. Accordingly, we have designed a novel PDA sensor, which is linked to mono 2-picolylamine,
to increase the PCDA-MP monomer content in polymer system. As a result, it enhanced sensitivity toward
Cd2+ with the low detection limit (2.10 μM) in aqueous solution. Our sequential modification of chelate
structure improved the Cd2+-sensing ability of PDAs. Finally, the interaction between PCDA–MP and
Cd2+ was confirmed by Raman spectroscopy, Field Emission Scanning Electron Microscopes (FE-SEM),
and Proton nuclear magnetic resonance (1H NMR).
Keywords: Polydiacetylenes, PDAs sensors, Colorimetric sensor, Fluorescent sensor, Cadmium sensor
Introduction
selectivity.11 The PCDA-HP monomer was deficient in a
single nitrogen compared to PCDA-CP and decreased the
chelating ability with another metal (Figure 1.). However,
the attempt to convert to PDA polymer using only PCDA-
HP monomers failed, due to the steric hindrance of the
bulky head group. Finally, PDA-HP was prepared with a
low content of chelating HP moiety (PCDA-HP and PCDA
in a ratio of 1:9). Given that the chelate content depends on
the number of detectable analytes, increasing the ratio of
monomers linked to chelate moiety can facilitate the detec-
tion of target molecules with high efficiency. Thus, we
have introduced a small head group linked to a mono 2-
picolylamine (MP) moiety, which still retains the key struc-
ture for Cd2+ chelation. The PDA-MP solutions consisting
of PCDA-MP and PCDA at a 5:1 ratio with a high content
of chelate display excellent color transition toward Cd2+.
Heavy metals are important to maintain multiple biochemi-
cal and metabolic activities, but exposure to excessive con-
centrations represents an increasingly serious threat.1
Besides, they lead to significant environmental contamina-
tion and accumulate in the human body, resulting in serious
diseases.2 Among the different heavy metals, cadmium is
one of the most dangerous and hazardous elements. The
absorption and accumulation by organisms cause severe
diseases and even cancer.3 For that reason, various analyti-
cal methods have been developed to detect Cd2+ using fluo-
rescent and colorimetric sensors.4 However, the selective
recognition of cadmium compared with zinc and mercury
ions is a challenge because of their closed-shell d10 elec-
tronic configurations.5 In this regard, we developed highly
selective and sensitive chemosensor based on poly-
diacetlyenes (PDAs) for visual detection of Cd2 +
.
Experimental
PDAs have attracted great attention as intelligent mate-
rials due to their fascinating properties.6 PDAs can be poly-
merized from self-assembled diacetylene monomers by
UV7 or plasma irradiation.8 In general, PDAs show a non-
fluorescent “blue phase,” which undergoes dramatic color
transition from blue to red (λmax ≈ 640–550 nm) by vari-
ous stimuli, resulting in fluorescent enhancement.9
Materials.
Unless otherwise noted, materials were
obtained from commercial suppliers and were used without
further purification. Flash chromatography was carried out
on silica gel (230–400 mesh) followed by determination of
1H and 13C NMR spectra using a Bruker Avance 400 MHz
NMR spectrometer. Chemical shifts were expressed in
ppm, and coupling constants (J) in Hz. Mass spectra were
obtained using a maXis-HD (Bruker, Billerica, MA). UV
absorption spectroscopy measurements were carried out on
V-730 UV–Visible spectrophotometer (Jasco) at room tem-
perature. Fluorescence emission spectra were obtained
In
a previous study, we prepared PCDA-CP, a
diacetylene monomer consisting of chelidamic acid and 2-
picolylamine, which recognize Cd2+ under specific condi-
tions with comparable low detection limit.10 We developed
another diacetylene monomer (PCDA-HP) with remarkable
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(20 mL) solution containing 10,12-pentacosadiynoic acid
(PCDA) (0.164 g, 0.84 mmol) under N2 at room tempera-
ture. After 1 h of stirring, DMF (0.1 mL) was added to the
solution. The resulting mixture was stirred for 4–6 h,
followed by concentration under reduced pressure to obtain
PCDA-Cl, which was used directly without purification by
dissolving in ACN (15 mL). It was added to MP in ACN
(15 mL) and trimethylamine (0.4 mL) was added to solu-
tion. The reaction mixture was stirred and refluxed over-
night under N2. This mixture was purified by silica gel
column chromatography (Hexane:EA = 1: 2) as eluent to
obtain a white solid (yield: 30%). 1HNMR (400 MHz,
D2O) δ 8.57–8.55 (dq, J = 4.8, 0.8 Hz, 1H), 7.92–7.89 (td,
J = 8.8, 2 Hz, 2H), 7.72–7.67 (td, J = 7.6,2 Hz, 1H), 7.66–
7.63 (t, J = 4.4, 1H), 7.35–7.33 (d, J = 7.6, 1H), 7.25–7.21
(m, 1H), 7.19–7.15 (td, J = 8.8, 2 Hz, 2H), 4.76–4.75 (d,
J = 5.2, 2H), 2.59–2.55 (t, J = 7.6, 2H), 2.27–2.22 (q,
J = 6.8, 4H), 1.79–1.71 (q, J = 7.6, 2H), 1.56–1.47 (m,
4H), 1.43–1.31 (m, 26H), 0.89–0.86 (t, J = 6.4, 3H), 13C
NMR δ (400, D2O MHz) 157.85, 155.93, 155.93, 150.29,
137.48, 135.05, 131.75, 131.36, 115.10, 115.10, 113.17,
113.17, 96.71, 77.43, 77.31, 77.11, 76.79, 53.53, 46.40,
46.40, 31.70, 22.77, 22.77, 14.24, 12.80, 12.80. ESI HRMS
m/z = 585.4051 [M + H]+, calc. for C13H12N2O2 = 584.85.
Colorimetric Response (CR%) Value Measurement. To
quantify degree of color transition, CR% value was calcu-
lated according to the following equation:12
N
N
O
N
N
N
O
O
O
O
NH
N
O
N
H
N
N
H
N
H
H
O
O
O
O
O
7
7
7
11
11
11
PCDA-CP
PCDA-HP
PCDA-MP
Figure 1. Structures of PDA monomers (PCDA-CP, PCDA-HP,
and PCDA-MP) for Cd2+ sensing.
using an F-7000 Fluorescence Spectrophotometer (Hitachi
High-Tech, Tokyo, Japan).
Preparation of PDA Micelle. The PDA micelle were
formed using ultrasonication at high temperature. A mix-
ture of PCDA-MP and PCDA with 5:1 ratio was dissolved
in Dichloromethane (DCM) (10 mL). The mixture was
stirred and heated with a stream of nitrogen gas. After
organic solvent was dried, D.W (20 mL) was added to mix-
ture. The resulting solution was ultrasonicated for 30 min at
80ꢀ ꢀC. And then solution including micelle was kept at
0 C overnight. PDA micelles were prepared by illumina-
tion of 254 nm UV for 18 min, presenting a blue-colored
solution.
PB0 −PB1
CR =
× 100
Synthesis
of
4-hydroxy-N-(pyridin-2-ylmethyl)
PB0
benzamide (MP). Oxalyl chloride (1 mL, 11.45 mmol)
was added dropwise to a THF (20 mL) solution containing
4-hydroxybenzoic acid (0.5 g, 3.62 mmol) under N2 at
where PB = Ablue/(Ablue + Ared(violet)), A means the absor-
bance at the PDA of blue (648 nm) or the red (547 nm)
color in UV/Vis spectrum. PB0 is the ratio of the A at
648 nm to that at 547 nm in the absence of Cd2+, while
PB1 is the ratio of the A at 648 nm to that at 537 nm when
various concentrations of Cd2+ were dealt.
room
temperature.
After
1
h
of
stirring,
Dimethylformamide (DMF) (0.2 mL) was added to the
solution and the resulting mixture was stirred during 4 h,
followed by concentration under reduced pressure to yield
diacyl chloride as a yellow solid, which was used directly
without purification by dissolving in Acetonitrile (ACN)
(20 mL). It was added to 10 mL ACN containing 2-
picolylamine (0.53 mL, 1.5 eq). The reaction mixture was
stirred and refluxed overnight under N2. This mixture was
purified by silica gel column chromatography (DCM:
MeOH = 20: 1) as eluent to obtain a white solid
(yield:49%). Proton nuclear magnetic resonance (1HNMR)
(400 MHz, D2O) δ 8.69–8.67 (dd, J = 6, 0.4 Hz, 1H),
8.55–8.51(td, J = 8, 0.4 Hz, 1H), 8.00–7.98 (d, J = 8 Hz,
1H), 7.96–7.92 (m, 1H), 7.79–7.75 (dt, J = 8.8, 2.8 Hz,
2H), 6.98–6.95 (dt, J = 8.8, 2.8 Hz, 2H), 4.91 (s, 2H) 13C
NMR δ (400, D2O MHz) 170.82, 159.87, 153.22, 147.11,
141.05, 129.76, 125.77, 125.59, 124.02, 115.54, 41.32. ESI
Results and Discussion
Preparation of PDA-MP. The PCDA-MP monomers
were difficult to form PDA micelles due to the steric hin-
drance of head group. We have optimized the appropriate
ratio via a series of experiments. PCDA-MP and PCDA
formed vesicles at a 5:1 ratio from assembled structures
with relatively hydrophilic head group and hydrophobic ali-
phatic chain. The PDA solutions were prepared by UV irra-
diation (254 nm, 1 mW/cm2, 18 min) (Figure 2(a)). The
PDA-MP existed as a rigid π-conjugated network along the
PDA polymer backbone in a blue-colored solution. The
addition of cadmium ion to PDA-MP solution resulted in
twisting of the well-aligned backbone structure. As a result,
the maxima absorption band shift occurred with a dramatic
color transition (Figure 2(b)).
HRMS m/z
=
227.0824 [M + H]+, calc. for
C13H12N2O2 = 228.25.
Synthesis of 4-((pyridin-2-ylmethyl)carbamoyl)phenyl
pentacosa-10,12-diynoate (PCDA-MP). Oxalyl chloride
(1 mL, 11.45 mmol) was added dropwise to a DCM
Characterization of PDA-MP Liposomes. To confirm
structural change following the addition of cadmium ion,
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Polydiacetylenes Containing 2-Picolylamide
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Figure 2. (a) Self-assembly and polymerization of PCDA-MP and
PCDA with 5:1 ratio; (b) schematic illustration of PDA colorimet-
ric transition by detection of cadmium ion.
Figure 3. Raman spectra of PDA-MP liposomes at room
emperature (blue line) and after treatment with Cd2+ (red line).
PDA-MP lioposomes were characterized by Raman spec-
troscopy, Field Emission Scanning Electron Microscopes
(FE-SEM). Raman spectra were analyzed before and after
the addition of cadmium ion at room temperature (Figure 3).
The C═C and C≡C bond stretching of blue-phase PDAs
appeared at 1451 and 2071 cm−1. Compared to double
bonds, additional strong triple bonds appeared at a higher
energy peak (2071 cm−1). Treatment with Cd2+ decreased
the original peaks and shift to 151p6 and 2120 cm−1. The
chemical shifts suggested that the rigid π-conjugated net-
work was twisted after the addition of cadmium ion, which
is a red phase.13 SEM images of PDA-MP showed struc-
tureless particles. However, the treatment with cadmium
ions resulted in uniform nano structures due to the forma-
tion of metal complexes (Supporting Information
Figure S7).
fluorescent emission intensity following excitation at
540 nm was followed by increasing the concentration of
Cd2+. Compared with other metal ions, cadmium ion only
induced color transition of PDA-MP in UV and fluores-
cence spectra (Supporting Information Figure S8).
Colorimetric Detection of Cadmium Ion. To demon-
strate improved sensitivity of PDA-MP compared to previ-
ous study, we calculated color response value (CR%) and
limit of detection (LOD) using spectroscopic data (Fig-
ure 6). The detection limit was estimated at 2.10 μM (3δ/
S), where δ represents the standard deviation of the blank
measurements, and S is the slope of the intensity versus
Cadmium Ion Detection Using PDA-MP Micelles.
Figure 4(a) displays the selectivity of PDA-MP toward Cd2
+in aqueous solutions. The PDA-MP (500 μM)-conjugated
polymer sensor turned red using perchlorate (ClO4−) as the
counter anion in the presence of 1 eq. of Ca2+ except other
metal ions such as Li+, Na+, K+, Cs+, Mg2+, Ca2+, Cr3+,
Mn2+, Fe2+, Fe3+, Ni2+, Cu2+, Ag+, Zn2+, Cd2+, and Hg2+.
This phenomenon also suggests that PDA-MP polymer can
be used to easily detect cadmium ion with the naked eye.
In Figure 4(b), PDA-MP showed a gradual color transition
by increasing the amount of cadmium ions and the resulting
PDA-MP showed dramatic color differences between blue
and red.
Spectroscopic Properties of PDA-MP in the Presence of
Cadmium Ion. The Cd2+-triggered colorimetric and fluo-
rometric changes of PDA-MP were also monitored by UV–
vis and fluorescence spectroscopy. The effect of cadmium
ion on the absorption spectra of PDA-MP is depicted in
Figure 5(a). When the amount of Cd2+ was gradually
increased, a 547 nm peak emerged as the original peak
(648 nm) decreased. As shown in Figure 5(b), the enhanced
Figure 4. Color transition of PDA-MP (500 μM) (a) in the pres-
ence of 1 eq. various metal ions in water; (b) in the presence of
various amount of Cd2+ (0, 0.03, 0.05, 0.1, 0.3, 0.5, 1 and 2 eq.)
in water (200 μM of PDA-MP).
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Polydiacetylenes Containing 2-Picolylamide
KOREAN CHEMICAL SOCIETY
Figure 6. (a) Calibration curve of PDA-MP (200 μM) versus Cd2+
concentration. (b) Quantitative colorimetric response of PDA-MP
(200 μM) in the presence of various Cd2+ concentrations.
Figure 5. Spectroscopic data (a) UV–vis absorption spectra of
PDA-MP (200 μM) in the presence of various equivalents of Cd2+
in aqueous solutions; (b) fluorescence spectroscopy titrations of
PDA-MP (200 μM) in the presence of various equivalents of Cd2+
(excitation at 540 nm, slit: 5 nm/5 nm).
that the Cd2+ ion interacted with ligand (MP) and not just
PCDA moiety.
Conclusion
sample concentration curve. In a previous study, the LOD
was 16.48 μM,10 whereas the present value (2.10 μM) was
dramatically decreased suggesting enhanced sensitivity.
Figure 6(b) explains CR%, which shows a linear increase
in Cd2+ concentration from 2 μM to and the maximal CR
value was obtained after adding 60 μM. Additionally Job’s
plots (continuous variation plots) were carried out
(Supporting Information Figure S9), and the molar fractions
at 0.65 presented a 2:1 stoichiometry.14
In the current study, we successfully designed and synthe-
sized a polydiacetylene monomer linked to a simple Cd2+
ion chelate. PDA-MP conjugated polymer sensors were
prepared via photopolymerization with PCDA-MP and
Interaction Between PCDA-MP and Cadmium Ion. 1H
NMR titration experiments were performed in CDCl3 (0 eq
of Cd2+) and CD3OD (0.1, 0.2, 0.4, 0.5, 1.0 eq of Cd2+).
The addition of Cd2+ to PCDA-MP monomer led to a
downfield shift of some aromatic proton (Hb, Hc, Hd, He,
and Hf) peaks and upfield shift of other protons (Ha) (Fig-
ure 7 and Supporting Information Table S1). Additionally,
Cd2+ treatment resulted in broadening of peaks, suggesting
Figure 7. Partial 1H NMR spectra (400 MHz) of PCDA-MP
(2 mM) upon the addition of Cd2+ in CDCl3 (0 eq) CD3OD (0.1,
0.2, 0.4,0.5, 1.0 eq).
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low detection limits. Moreover, the fluorescence emission
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Acknowledgments. This research was supported by Basic
Science Research Program through the National Research Foun-
dation of Korea(NRF) funded by the Ministry of Education
(No. 2017R1A6A3A04004954) for S. L. and by the Pukyong
National University Development Project Research Fund, 2020.
Supporting Information. Additional supporting informa-
tion may be found online in the Supporting Information
section at the end of the article.
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