Switchable control of hydrophilicity and hydrophobicity in conjugated polymer nanoparticles by carbon dioxide

Article abstract of DOI:10.1080/15421406.2019.1645464

We synthesized yellow-emissive, fluorine-based conjugated polymer to fabricate CO₂-responsive conjugated polymer nanoparticles (CPNPs). The CPNPs were functionalized with tertiary amine to have responsiveness to CO₂. The amine-functi

Full text of DOI:10.1080/15421406.2019.1645464

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Switchable control of hydrophilicity and
hydrophobicity in conjugated polymer
nanoparticles by carbon dioxide
Hyunchul Kim & Taek Seung Lee
To cite this article: Hyunchul Kim & Taek Seung Lee (2019) Switchable control of hydrophilicity
and hydrophobicity in conjugated polymer nanoparticles by carbon dioxide, Molecular Crystals and
Liquid Crystals, 685:1, 78-86, DOI: 10.1080/15421406.2019.1645464
To link to this article: https://doi.org/10.1080/15421406.2019.1645464
Published online: 04 Oct 2019.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
019, VOL. 685, NO. 1, 78–86
2
https://doi.org/10.1080/15421406.2019.1645464
Switchable control of hydrophilicity and hydrophobicity in
conjugated polymer nanoparticles by carbon dioxide
Hyunchul Kim and Taek Seung Lee
Organic and Optoelectronic Materials Laboratory, Department of Advanced Organic Materials and
Textile System Engineering, Chungnam National University, Daejeon 34134, Korea
ABSTRACT
KEYWORDS
conjugated polymer
nanoparticles; fluorescence;
switchable hydrophilicity
We synthesized yellow-emissive, fluorine-based conjugated polymer
to fabricate CO
CPNPs). The CPNPs were functionalized with tertiary amine to have
responsiveness to CO . The amine-functionalized CPNPs became
2
-responsive conjugated polymer nanoparticles
(
2
2
hydrophilic by CO , bubbling because the bubbling led to formation
of cationic ammonium ions at the side chains of the hydrophobic
CPNPs. This resulted in high dispersion stability in aqueous phase
even after vigorous mixing in the presence of organic phase
(1-octanol). Subsequent N2 bubbling was done to remove CO2
present in water, leading to deprotonation of the side chains of
CPNPs. The CPNPs became hydrophobic and moved to the organic
phase. The CO -responsive property was based on the amine groups
2
in the side chain of polymer that reversibly interacted with bicarbon-
-
3
ate ion (HCO ), formed by dissolving CO in water, generating
2
switchable hydrophilicity and hydrophobicity.
1. Introduction
CPNPs are attracting much attention in fluorescent sensing and optoelectronic devices
due to their high quantum efficiency and high conductivity, bright fluorescence [1, 2].
Thus, they can be applied to various fields including biomedical technology [3] and
photocatalysis [4] because of their low toxicity and cost-efficiency compared with metal
nanoparticles. For the applications in the biomedical fields of CPNPs, the use of
biocompatible solvents is essentials. In this regard, water is a cheap, safe, biocompatible,
environmentally-friendly solvent [5]. Moreover, the use of water as a solvent can open
up potential possibilities for various applications such as water splitting [6], anti-
bacterial treatment [7], photodynamic therapy [8], and water treatment [9].
To use hydrophobic CPNPs in water, dispersion stability of CPNPs in water is highly
required for the biomedical application. However, unmodified pure hydrophobic CPNPs
often show aggregation in aqueous phase, in which they have moderate ionic strength
or contain divalent metal ions [10]. Several investigations have shown that hydrophilic
modification of the surface of polymer nanoparticles improves colloidal stability [11].
However, if the formation of CPNPs are mainly driven by hydrophobic interactions,
CONTACT
tslee@cnu.ac.kr
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
ß 2019 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
79
hydrophilic side chains can cause considerable interference in hydrophobic assembly
among polymer chains [12]. Previous reports reported that stable CPNPs were formed
when low number of hydrophilic side chains were introduced. The more hydrophilic
side chains were introduced, the more unstable CPNPs were formed [13]. Therefore, it
is important to control the introduction of hydrophilic side chains to obtain compact
and stable CPNPs.
Recently, CO is known for its biocompatibility and abundancy in environment, it
2
has been used as an external stimulus for manipulating the wettability of materials [14].
The CO -responsiveness of polymers containing tertiary amines was reported by Zhao
2
and his co-workers [14]. The pK of conjugated acid of tertiary amines ranges from
a
about 6.0 to 7.0 [15]. It means that they can be easily protonated by bicarbonate anion
(
pH ꢀ 4.0-5.0), simply formed by dissolving CO in water [16]. In addition, the result-
2
ant ammonium ions can be deprotonated by the removal of CO . Therefore, the tertiary
2
amines reversibly form hydrophilic ammonium salts and deprotonated amine upon
addition and removal of CO , respectively. Using such a property, we synthesized
2
yellow-emissive conjugated polymer and fabricated stable CPNPs by controlling the
degree of functionalization of tertiary amines as hydrophilic side chains in CPNPs. They
showed high colloidal stability in water and switchable wettability by CO2.
2. Experimental
Instruments and materials
1
H NMR spectra were obtained from a Bruker Fourier-300 spectrometer (Korea Basic
Science Institute). UV-vis absorption spectra were recorded on a PerkinElmer Lambda
3
5 spectrometer. Photoluminescence spectra were taken using a Varian Cary Eclipse
spectrometer. The molecular weight was determined by gel-permeation chromatography
GPC), with tetrahydrofuran (THF) as an eluent with a polystyrene standard. Zeta-
(
potentials and size distributions were measured by dynamic light scattering (DLS,
Zetasizer Nano ZS, Malvern). Elemental analysis was performed with a CE Instruments
EA-111- elemental analyzer. All chemicals were purchased from Sigma-Aldrich (U.S.A.)
and solvents were purchased from Samchun Chemicals (Korea). All reagents were used
without further purification unless otherwise noted.
Synthesis of 2,7-Dibromo-9,9-bis(6-bromohexyl)fluorene (1) [17]
2
2
,7-Dibromofluorene (2 g, 6.17 mmol) and tert-butylammonium bromide (TBAB,
00 mg, 0.62 mmol) were dissolved in 1,6-dibromohexane (10 mL) and 50% KOH solu-
ꢁ
tion (20 mL) was then injected under nitrogen. The solution was stirred at 75 C for 2 h.
The reaction mixture was extracted with dichloromethane and the resulting solution
was evaporated. The reaction mixture was crystallized from methanol to remove the
1,6-dibromohexane. The product was further purified by column chromatography using
hexane and dichloromethane. The final product was a white solid (yield 1.96 g, 49%).
1
H NMR (300 MHz, CDCl , ppm) 7.51 (d, 2H, J ¼ 8Hz), 7.45 (dd, 2H, J ¼ 2Hz, 2Hz),
3
7
1
.43 (d, 2H, J ¼ 1.5Hz), 3.27 (t, 4H, J ¼ 7Hz), 1.97 (m, 4H), 1.64 (m, 4H), 1.17 (m, 4H),
.05 (m, 4H), 0.59 (m, 4H).

8
0
H. KIM AND T. S. LEE
2.1.2. Synthesis of conjugated polymer (2) [17]
1
1
(650 mg, 1 mmol) and 2,1,3-benzothiadiazole-4,7-diboronic acid pinacol ester (388 mg,
mmol) were dissolved in dry toluene (15 mL). Tetrakistriphenylphosphine palla-
dium(0) (59 mg, 0.05 mmol) and 2M potassium carbonate aqueous solution were added
to the solution. The mixture was purged with nitrogen under stirring and heated at
ꢁ
8
5 C for 48 h. After cooling down to room temperature, the reaction mixture was
precipitated in methanol (200 mL). The obtained orange powder was purified in a
Soxhlet apparatus and dried (yield 376mg, 60%).
1
H NMR ð300 MHz; CDCl ; ppmÞ : 8:06 ð8HÞ; 3:32 ð4HÞ; 2:17 ð4HÞ; 1:73 ð4HÞ;
3
1
:3 ð8HÞ; 0:94 ð4HÞ:
Synthesis of amine-functionalized conjugated polymer (3) [18]
To a solution of 2 (100 mg) in dry THF (8 mL), diethylamine (DEA, 2 mL) was added
ꢁ
dropwise. The mixture was purged with nitrogen and heated at 50 C for 24 h. After the
reaction, the solution was filtered to isolate ammonium bromide salt and the solvent
Scheme 1. Synthesis of 3.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
81
was further evaporated under reduced pressure. The product was washed in water
overnight, isolated by filtration, and washed with water and methanol. The red orange
powder was obtained (yield 87mg, 92%).
1
Figure 1. H NMR spectra of (a) 2 and (b) 3 in CDCl .
3

8
2
H. KIM AND T. S. LEE
Preparation of amine-functionalized conjugated polymer nanoparticles (CPNPs)
(2 mg) was dissolved in THF (20 mL). 1 mL of the solution was injected quickly into
3
mixed solution (12 mL) of deionized water and methanol (9: 3, v/v) during sonication.
The THF was removed by CO blowing, followed by filtration through a syringe filter
2
(0.22 mm).
CO -responsiveness of CPNPs
2
CPNPs in water (5 mL) was added into a 20 mL vial. An equal amount of 1-octanol
(
5 mL) was added to the vial. Then CO was bubbled to the mixture and mixed
2
vigorously. After that, CPNPs was uniformly dispersed in aqueous phase. CO was
2
substituted by N , followed by vigorous mixing, resulting in CPNPs in 1-octanol phase.
2
3. Results and discussion
The synthetic route to the amine-functionalized conjugated polymer 3 was shown in
Scheme 1. The monomer with alkyl halides (1) was synthesized according to previous
report [17]. The conjugated polymer containing benzothiadiazole (2) was synthesized
Figure 2. (a) UV-vis and (b) fluorescence spectra of 2 in chloroform (᭿) and CPNPs in water ( ).
᭺

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
83
via the Suzuki cross-coupling reaction in the presence of Pd catalyst. The number- and
weight-average molecular weights of 2 were determined by GPC and found to be 6520
and 22830, respectively. The CO -responsive conjugated polymer 3 was obtained via
2
modification of 2 with DEA. Elemental analysis of 3 indicated that the functionalization
of amine groups was performed with 61%. Thus copolymer 3 was obtained, in which
m: n was 61: 39 as mol%. In the NMR spectra of 2, the peak corresponding to the
Scheme 2. (a) Preparation of CPNPs by reprecipitation and (b) phase transfer of CPNPs by
N2 bubbling.
ꢁ
Table 1. DLS data of CPNPs in water at 25 C.
Size (nm)
Zeta potential (mV)
After CO
After N
2
bubbling
bubbling
150 ± 0.2
170 ± 10
þ35 ± 3
þ14 ± 2
2

8
4
H. KIM AND T. S. LEE
Figure 3. Fluorescence spectra of layers and photographs of the location of CPNPs (a) before CO2
bubbling, (b) after CO bubbling and shaking, followed by (c) N bubbling and shaking. Upper and
2
2
lower phases in the vial correspond to 1-octanol (oil) (
᭺
) and water (᭿) layers, respectively.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
85
carbon protons adjacent to the bromine was shown at 3.3 ppm (Fig. 1a). The peak was
decreased and a new peak was observed at 2.4 ppm upon the reaction of 2 with DEA
(Fig. 1b). The peak of 3.3 ppm did not completely disappear after the reaction with
DEA, and the degree of amination was calculated by peak integration. The peak integra-
tion of g: h were measured as 1: 2 (Figure 1b), resulting in the degree of amination was
calculated to be 66%. Therefore, it was concluded that the amination was performed by
about 60%.
3
exhibited UV-vis absorption at 314 nm and 444 nm (Figure 2a) and had a strong yel-
low fluorescence at 542 nm in chloroform solution (Figure 2b). CPNPs were prepared by
conventional reprecipitation method (Scheme 2a). The UV-vis absorption and fluores-
cence emission of CPNPs were similar to those of 3 with slight red shift. The particle size
and zeta potential of CPNPs from DLS are tabulated in Table 1. The average particle size
of CPNPs after CO bubbling was found to be 150 nm with zeta potential of þ35 mV. A
2
larger size and less positive zeta potential of CPNPs were obtained after N bubbling.
2
This is because of the protonation of tertiary amines by bicarbonate anions that was
formed when CO was dissolved in water confirming that the CPNPs were hydrophilized
2
,
after CO bubbling with positive charge introduced on the surface of CPNPs. Tertiary
2
-
3
amine is easily protonated with an aid of HCO ion. (below equations) [19].
ꢂ
þ
CO þ H O $ HCO þ H
(1)
(2)
2
2
ꢂ
3
þ
þ
ꢂ
3
R3N þ HCO3 H $ R NH HCO
þ
3
To elucidate switchable hydrophilicity and hydrophobicity of CPNPs by CO , disper-
2
sion stability was investigated on water (water)-1-octanol (organic) bilayer. After CO2
bubbling, most CPNPs were uniformly located in aqueous phase with stable dispersion
(photograph in Figure 3a), even after vigorous shaking (photograph in Figure 3b). The
location of CPNPs can be verified by fluorescence spectra of CPNPs in 1-octanol and
water after vigorous shaking, exhibiting higher fluorescence in aqueous phase
(Figure 3b). Therefore, it was confirmed that CPNPs became hydrophilic by the
presence of CO and preferred locating in aqueous phase rather than in organic phase
2
(
1-octanol). N bubbling and shaking was conducted to remove the CO charged in
2
2
aqueous phase. After the N bubbling, the CPNPs dispersed in water was transferred to
2
the 1-octanol phase, and the fluorescence in aqueous phase almost disappeared and a
strong intensity of CPNPs in 1-octanol was observed (Figure 3c). This was because
ammonium ions on the surface of CPNPs were deprotonated as CO was substituted by
2
N , so that CPNPs became hydrophobic, preferring the location in the organic phase
2
rather than in the water phase.
4. Conclusion
We fabricated tertiary-amine functionalized conjugated polymer nanoparticles by simple
reprecipitation method and investigated their CO -responsive properties. After CO2
2
bubbling, CPNPs became hydrophilic and uniformly dispersed in water and they stayed
in aqueous phase even after vigorous mixing in a water/1-octanol bilayer. When the
CO dissolved in the aqueous phase was removed by N , CPNPs became hydrophobic
2
2
and phase transfer took place, in which they moved to organic phase. Based on these

8
6
H. KIM AND T. S. LEE
results, switchable hydrophilicity and hydrophobicity control of CPNPs by CO and N2
2
was successfully elucidated. It is expected that CPNPs with enhanced dispersion stability
in water can be used for various applications in water as a solvent.
Acknowledgment
Financial support from the National Research Foundation (NRF) of Korean government through
Basic Science Research Program (2018R1A2A2A14022019) and Nuclear R&D Project
(2016M2B2B1945085) is gratefully acknowledged.
References
[1] D. T. McQuade, A. E. Pullen, & T. M. Swager. Chem. Rev., 100, 2537–2574 (2000).
[2] S. W. Thomas, G. D. Joly, & T. M. Swager. Chem. Rev., 107, 1339–1386 (2007).
[3] X. Feng, L. Liu, S. Wang, & D. Zhu. Chem. Soc. Rev, 39, 2411–2419 (2010).
[4] G. Zhang, Z. A. Lan, & X. Wang. Angew. Chem. Int. Ed., 55, 15712–15727. (2016).
[5] G. J. ten Brink, I. W. Arends, & R. A. Sheldon. Science, 287, 1636–1639 (2000).
[6] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, & M. Antonietti.
Nat. Mater., 8, 76, (2009).
[7] B. C. Ma, S. Ghasimi, K. Landfester, & K. A. Zhang. J. Mater. Chem. B, 4, 5112–5118
(
2016).
[
[
8] C. Zhu, L. Liu, Q. Yang, F. Lv, & S. Wang. Chem. Rev., 112, 4687–4735 (2012).
9] S. Ghasimi, S. Prescher, Z. J. Wang, K. Landfester, J. Yuan, & K. A. Zhang. Angew. Chem.
Int. Ed., 54, 14549–14553 (2015).
[
[
10] Y. Jin, F. Ye, C. Wu, Y. H. Chan, & D. T. Chiu. Chem. Commun., 48, 3161–3163 (2012).
11] C. Wu, T. Schneider, M. Zeigler, J. Yu, P. G. Schiro, D. R. Burnham, & D. T. Chiu.
J. Am. Chem. Soc., 132, 15410–15417 (2010).
[
12] C. Wu, & D. T. Chiu. Angew. Chem. Int. Ed., 52, 3086–3109 (2013).
[
13] X. Zhang, J. Yu, C. Wu, Y. Jin, Y. Rong, F. Ye, & D. T. Chiu. Acs Nano, 6, 5429–5439
(
2012).
[
[
[
[
[
14] H. Liu, S. Lin, Y. Feng, & P. Theato. Polym. Chem., 8, 12–23 (2017).
15] H. Liu, Y. Zhao, C. A. Dreiss, & Y. Feng. Soft Matter, 10, 6387–6391 (2014).
16] S. Lin, P. Theato. Macromol. Rapid Commun., 34, 1118–1133 (2013).
17] B. Liu, & G. C. Bazan. Nat. Protoc., 1, 1698 (2006).
18] J. Byun, W. Huang, D. Wang, R. Li, & K. A. Zhang. Angew. Chem. Int. Ed., 57,
2
967–2971 (2018).
19] J. Zhao, J. Ye, N. Zhu, H. Hong, L. Ma, J. Yang, & J. Zhang. ACS Sustain. Chem. Eng., 6,
105–7108 (2018).
[
7