Reversible Phase Transfer of Carbon Dots between an Organic Phase and Aqueous Solution Triggered by CO2

Article abstract of DOI:10.1002/anie.201800037

Carbon dots (CDs) have attracted increasing attention in applications such as bio-imaging, sensors, catalysis, and drug delivery. However, unlike metallic and semiconductor nanoparticles, the transfer of CDs between polar and non-polar phases is little un

Full text of DOI:10.1002/anie.201800037

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Angewandte
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Accepted Article
Title: Reversible Phase Transfer of Carbon Dots between Organics
and Aqueous Solution Triggered by CO2
Authors: Xiaoyan Pei, Dazhen Xiong, Huiyong Wang, Shuaiqi Gao,
Xinying Zhang, Suojiang Zhang, and Jian Ji Wang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201800037
Angew. Chem. 10.1002/ange.201800037
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10.1002/anie.201800037
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Reversible Phase Transfer of Carbon Dots between Organics and
Aqueous Solution Triggered by CO2
Xiaoyan Pei,[a] Dazhen Xiong,[a] Huiyong Wang,[a] Shuaiqi Gao,[a] Xinying Zhang,[a] Suojiang Zhang,[b]
and Jianji Wang*[a]
Abstract: Carbon dots (CDs) have attracted increasing attention in
applications such as bio-imaging, sensors, catalysis, and drug
delivery. However, unlike metallic and semiconductor nanoparticles,
the transfer of CDs between polar and non-polar phases is little
understood. In this work, a novel class of amine-terminated CDs is
developed and their phase transfer behaviour has been investigated.
It is found that these CDs can reversibly transfer between aqueous
and organic solvents by alternatively bubbling and removal of CO2 at
atmospheric pressure. The mechanism of such CO2-switched phase
transfer involves reversible acid-base reaction of amine-terminated
CDs with CO2 and the reversible formation of hydrophilic ammonium
salts. By using the CDs as catalysts, the phase transfer protocol is
applied in Knoevenagel reaction for efficient homogeneous reaction,
heterogeneous separation, and recycling of the catalysts.
of Pd nanoparticle catalyst. Peng et al.[8a] proposed a reversible
phase transfer approach for -cyclodextrin-capped gold
nanoparticles between aqueous and toluene phases switched by
UV and visible light, and the NPs could be applied to catalyze
the reduction of 4-nitrophenol. Dorokhin et al.[9a] demonstrated
that in the presence of naphthalene and adamantine derivatives,
ferrocene-modified CdSe/ZnS quantum dots could move back
and forth between aqueous and chloroform phases by
controlling host-guest complexation between ferrocene units and
cavity of -cyclodextrin.
Due to the low-cost, biocompatibility, photostability and facile
functionalization, carbon dots (CDs) have emerged as a new
class of carbon-based nanomaterials, and widely used in bio-
imaging, sensors, catalysis, drug delivery and so on.[10-13]
However, to the best of our knowledge, no reversible phase
transfer has been reported for CDs to date. Therefore, it is of
great significance to develop efficient strategies for the
reversible phase transfer of CDs in order to realize the full
potential of these NPs.
CO2 is inexpensive, nontoxic, and easily removable, thus it
can serve as a useful trigger for process switching,[14] but little is
known for CO2-switchable phase transfer of nanoparticles even
for metallic and semiconductor nanoparticles. Herein, we
developed a novel class of amine-terminated CDs. It was found
that these CDs were well dispersed in organic solvents.
Interestingly, they could transfer to water by CO2 bubbling at
room temperature and atmospheric pressure. Importantly, they
could return to organic phase by removing CO2 at 60 oC with N2
bubbling. Thus, the amine-terminated CDs could reversibly
transfer between aqueous and organic solvents by alternatively
bubbling and removal of CO2. The mechanism of the CO2-
switchable reversible phase transfer was investigated by the
measurements of interfacial tension, water contact angle and
13C NMR spectroscopy. Based on the unique phase-transfer
Nanoparticles (NPs) have attracted tremendous attention in
many applications such as catalysis,[1] electronics,[2] and energy
conversion[3] because of their size- and shape-dependent
physical and chemical properties. Usually, some NPs can be
well dispersed and used in organics but not in water, while some
others can be well dispersed and used in water but not in
organic solvents. Thus, transfer of NPs from a polar solvent to a
non-polar one (or vice versa) is generally required to maximize
the advantages provided by their environments. This makes
phase transfer an important aspect in the synthesis,
functionalization and application of NPs.[4]
In this context, a number of excellent works have been
reported on the phase transfer of NPs triggered by different
stimuli-responses such as ionic strength,[5] temperature,[5d,6] pH,[7]
light,[8] and ligand exchange.[9] For example, Wang et al.[5]
carried out outstanding work on the transfer of polymer brush-
grafted gold NPs from salty water to toluene by altering ionic
strength to develop a fundamental understanding of the behavior
of inorganic NPs at water-oil interfaces. Zhao et al.[6a,b,c] reported
the transfer of polymer brush-grafted silica NPs between water
and oil (ethyl acetate/octane/ionic liquid) driven by temperature,
and its application in the development of new phase transfer
behavior of the functionalized CDs,
a
highly efficient
Knoevenagel reaction, product separation and catalyst recycling
were achieved where the CDs were used as catalysts.
The amine-terminated CDs were prepared by pyrolysis of the
mixture of citric acid and amines in 1-octanol at 180 oC (Scheme
1 and S1), and the detailed procedures were described in the
Supporting Information. In order to examine the effect of the
structure of amines on the catalytic activity of CDs, the amines
used here consisted of primary amine-terminated diamines 1,4-
diaminobutane (DAB), 1,6-diaminohexane (DAH) and 1,8-
diaminooctane (DAO), secondary amine-terminated diamines 1-
ethyl-1,4-diaminobutane (EDAB), 1-ethyl-1,6-diaminohexane
(EDAH) and 1-ethyl-1,8-diaminooctane (EDAO), and ternary
catalysts. Yang and co-workers[7a] developed
a
novel
mesoporous silica nanocomposite shuttle, which could reversibly
transfer between water and ether (or ethyl acetate) in response
to the pH, and be used as a carrier for separation and recycling
[a]
X. Pei, Dr. D. Xiong, Dr. H. Wang, S. Gao, Prof. X. Zhang, Prof. J.
Wang
Collaborative Innovation Center of Henan Province for Green
Manufacturing of Fine Chemicals, Key Laboratory of Green
Chemical Media and Reactions, Ministry of Education, School of
Chemistry and Chemical Engineering, Henan Normal University,
Xinxiang, Henan 453007 (P. R. China)
amine-terminated
diamines
N,N-diethyl-1,4-diaminobutane
(DEDAB), N,N-diethyl-1,6-diaminohexane (DEDAH) and N,N-
diethyl-1,8-diaminooctane (DEDAO). The structure and
morphology of the as-prepared CDs were characterized by
Ultraviolet-visible (UV-Vis) absorption spectrum, FT-IR spectra,
fluorescence (FL) spectra, thermogravimetric analysis (TGA), X-
ray photoelectron spectroscopy (XPS), and transmission
E-mail: Jwang@htu.cn
[b]
Prof. S. Zhang
Beijing Key Laboratory of Ionic Liquids Clean Process, Institute of
Process Engineering, Chinese Academy of Sciences, Beijing
100190 (P. R. China)
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electron microscopy (TEM).
Figure 1. XPS spectrum (a) and TEM images (b) of the DAB-CD. Inset in (a)
is high resolution XPS spectrum for C1s of DAB-CD, and inset in (b) is the
HRTEM image of a single nanoparticle.
Scheme 1. Schematic diagram for the preparation of the primary amine-
terminated CDs.
temperature, they were present almost exclusively in the
aqueous phase (Figure 2b). When CO2 was removed by N2
bubbling at 60 oC, DAB-CD was returned to 1-octanol phase
(Figure 2c). The content of DAB-CD in 1-octanol phase was
determined quantitatively by PL spectroscopy (see Figure S16),
and it was shown that the mass percent of DAB-CD in 1-octanol
phase was 91.5% before bubbling CO2 (Figure 2d), 6.2% after
bubbling CO2 (Figure 2e), and 90.9% after the removal of CO2
(Figure 2f), respectively. Therefore, the CO2-switchable phase
transfer of the DAB-CD was reversible. Similar reversible phase
transfer process was also observed for secondary amine- and
tertiary amine-terminated CDs (Figure S17), and the contents of
the amine-terminated CDs in 1-octanol phase increased in the
order: primary amine-terminated CDs < secondary amine-
terminated CDs < tertiary amine-terminated CDs. The possible
reason is that the steric hindrance of primary amine on the CDs
is relatively small compared with that of other branched chain
amines, so its ability to form a hydrogen bond with water is
stronger than that of secondary and tertiary amines,[18] which
leads to a higher solubility in water and lower solubility in 1-
octanol. In addition, it was found that the transfer of the amine-
terminated CDs from 1-octanol to water phases could also be
completed by lowering the aqueous phase pH with aqueous HCl
(Figure S18). Importantly, benzene, toluene, dichloromethane
and chloroform were also found to be effective organic solvents
for the reversible phase transfer (Figures S19-S23).
Taking DAB-CD as an example, its UV-Vis absorption peaks
appeared at 206, 240 and 301 nm (Figure S1), respectively. The
peak at 206 nm might be assigned to π-π* transition of the
unsaturated conjugated nanocarbon structure, and the peak at
301 nm could be ascribed to the n-π* transition of the C=O bond,
while the absorption peak at 240 nm would be due to the
conjugation between the CD and DAB,[15-16] which indicates the
formation of nitrogen-doped CDs. In the fluorescence spectrum
of DAB-CD, the optimal excitation and emission wavelengths at
397 nm and 458 nm were visible (Figure S1), and the CD
showed a strong blue fluorescence under the irradiation of UV
lamp at 365 nm. Excitation-dependent photoluminescence (PL)
behavior was observed for DAB-CD (see Figure S2), which is
common in fluorescent carbon materials.[16] In the FTIR spectrum
of DAB-CD, the bending vibration of N-H at 1542 cm-1 and the
vibrational absorption band of C=O at 1641 cm-1 (Figure S3)
confirmed the formation of amido bond on the surface of the
amine-terminated CDs.[17] Furthermore, TGA was also
performed for the amine terminated-CDs (see Figures S4 and
S5), and the content of the amine-terminated CDs was given in
Table S1.
XPS spectrum confirmed that the surface of DAB-CD was
composed of carbon, nitrogen and oxygen elements (Figure 1a).
The content of each element was given in Table S1. The fine
structure spectrum of C1s exhibited three main peaks at 284.4,
285.2 and 287.7 eV (inset of Figure 1a), which were attributed to
graphitic (or aliphatic), nitrous, and oxygenated carbon atoms,
respectively.[15] Based on the data of TGA and XPS, the density
of surface amine groups on the resulting carbon dots was
calculated, which was in the range from 2.79 to 4.06 mmol/g
(Table S1).
It can be seen from TEM images of DAB-CD, well-dispersed
spherical nanoparticles with average particle diameter of 5.5 nm
were formed (Figure 1b), and most particles had a crystalline
structure composed of parallel crystal planes with the lattice
spacing of 0.20-0.22 nm (inset of Figure 1b), which are very
close to the (100) diffraction planes of graphite.[16] It was also
found that the difference in alkyl chain length of amines had little
effect on the structure and morphology of the amine-terminated
CDs (see Figures S5-S9). Similar results were also found for
secondary amine- and tertiary amine-terminated CDs (Figures
S10-S15).
The phase transfer of the amine-terminated CDs was
examined in aqueous and 1-octanol phases. Taking DAB-CD as
a representative example, it was found that in 1-octanol this CD
showed a strong blue fluorescence under the irradiation of UV
lamp at 365 nm (Figure 2a). Upon bubbling of CO2 at room
Figure 2. Photographs of the DAB-CD in 1-octanol and water phases taken
under a UV lamp before bubbling CO2 (a), after bubbling CO2 (b), and after
removal of CO2 (c). The upper layer in the vials is 1-octanol and the lower
layer is water. The PL spectra of DAB-CD in 1-octanol and aqueous phases
before bubbling CO2 (d), after bubbling CO2 (e), and after removal of CO2 (f).
The black curve is PL spectrum of the DAB-CD in 1-octanol phase and the red
curve is PL spectrum in aqueous phase.
To understand the mechanism of phase transfer of the amine-
terminated CDs between organic and aqueous phases, the
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effect of CO2 bubbling on the interfacial tension of 1-
octanol/water was determined in the absence of CDs at 25.0 C
mixture of DAB-CD-CO2 at 60 oC for 30min with N2 bubbling
(Figure 3). Moreover, no significant change in fluorescence
intensity was observed in 1-octanol within 15 cycles (Figure S28).
These results suggest that the reaction between DAB-CD and
CO2 was completely reversible.
o
(see Supporting Information). It was shown that the change in
interfacial tension caused by bubbling of CO2 was very small,
which was not the driving force for the phase transfer of the CDs
from 1-octanol to water. In addition, both temperature and N2
bubbling were found to have no influence on the phase transfer
of the amine-terminated CDs (Figures S24 and S25).
Knoevenagel condensation reaction is one of the most
important reactions for the C=C bond formation between a C=O
group and an activated methylene group, and has been used to
synthesize a number of important chemical intermediates, drugs
and polymers.[20] As a proof of concept, our amine-terminated
CDs were used in the Knoevenagel reactions to demonstrate the
coupling of homogeneous catalysis, heterogeneous separation
and recycling of the CD catalysts. As shown in Figure 4, under
the catalysis of the CDs, the reaction for the preparation of 2-(4-
bromobenzylidene)malononitrile proceeded well in 1-octanol at
45 °C, and the product was precipitated from the reaction
system and suspended in the organic phase. Then, the CDs
could be transferred to water phase upon CO2 bubbling. After
that, the product was filtered out from the organic phase. Once
new reactants were added in the organic phase and CO2 was
removed by N2 bubbling, the same reaction happened again.
Next, water contact angles of the amine-terminated CDs were
o
measured at 25.0 C before and after bubbling of CO2. As an
example, the result for the DAB-CD was shown in Figure S26a.
It was found that water contact angle of DAB-CD dramatically
decreased from 73o to 28o upon bubbling of CO2, but the value
was restored to 72o after CO2 was removed. Similar trend was
observed for the secondary amine- and tertiary amine-
terminated CDs (Figures S26b and S26c). These data suggest
that hydrophilicity of these CDs increased significantly upon
exposure to CO2.[7a],[19] We speculated that the hydrophilicity
increase may be ascribed to the acid-base reaction of amines on
the surface of CDs with CO2 and the formation of hydrophilic
products in the presence of water.
To confirm this point, 13C NMR spectroscopy was used to
study the products produced from the acid-base reaction in 1-
octanol/water biphasic system. As an example, 13C NMR spectra
of the DAB-CD before and after the reaction with CO2 were
shown in Figure 3. A new signal at 160.2 ppm was observed and
-
assigned to HCO3 anion,[15] which confirms the formation of
hydrophilic bicarbonates salts. Furthermore, the new signal at
164.5 ppm could be ascribed to carbonyl group of carbamate.[14b]
Similar results were also found in the 13C NMR spectra of the
secondary amine-terminated CDs such as EDAB-CD (Figure
S27a). For tertiary amine-terminated CDs such as DEDAB-CD,
only one new signal was found at 160.2 ppm (Figure S27b),
indicating the formation of hydrophilic bicarbonates salts. Thus,
for primary amine- and secondary amine-terminated CDs, the
ammonium salts were composed of carbamate and
bicarbonates salts. However, for tertiary amine-terminated CDs,
bicarbonate salt was the only ammonium salt. Therefore, CO2-
switchable phase transfer mechanism of the amine-terminated
CDs involves the acid-base reaction between CO2 and amines
of these CDs and the formation of hydrophilic ammonium salts,
which resulted in the phase transfer of amine-terminated CDs
from organics to aqueous solution.
Figure 4. Reaction between 4-bromobenzaldehyde and malononitrile
catalyzed by DAB-CD (a), and the homogeneous reaction and heterogeneous
separation procedures for the DAB-CD (b).
As
a
representative example, the product 2-(4-
bromobenzylidene)malononitrile produced under the catalysis of
DAB-CD was identified by 1H NMR spectroscopy (see Figure
S29), and the yield was 95.2%. After six recycles, its yield still
reached to 93.7% (Figure S30). In addition, it was found that the
yield for the same reaction catalyzed by different CDs decreased
in the order: primary amine CDs (95.2%) > secondary amine
CDs (81.1%) > tertiary amine CDs (75.3%). This suggests that
the amine-terminated CDs with stronger basicity exhibited higher
catalytic activities in Knoevenagel reactions.[21] To extend the
substrate scope, several benzaldehyde derivatives were used to
react with malononitrile under the same conditions. Except 4-
fluorobenzaldehyde, medium (71.5%) to excellent (95.0%) yields
were obtained (Table S2, entries 1-6 and 8, and Figures S31-
S36). Compared to other carbon nanomaterials,[22] the amine-
terminated CDs exhibited superior catalytic activities for the
Knoevenagel reaction. The resulting products could be easily
separated from the reaction mixture by filtration, and the CDs
could be readily recovered and recycled.
Figure 3. 13C NMR spectra of DAB-CD before bubbling CO2, after bubbling
CO2, and after the removal of CO2
Furthermore, the reversibility of the process was investigated
by monitoring 13C NMR spectrum of the DAB-CD before and
after removal of CO2. It was shown that the signals at 160.2 and
164.5 ppm disappeared after CO2 was removed by heating the
In summary, we have prepared a novel class of amine-
terminated CDs by the pyrolysis of the mixture of citric acid and
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