Carbon and Nitrogen Based Nanosheets as Fluorescent Probes with Tunable Emission

Article abstract of DOI:10.1002/smll.201800516

2D carbon and nitrogen based semiconductors (CN) have attracted widespread attention for their possible use as low-cost and environmentally friendly materials for various applications. However, their limited solution-dispersibility and the difficulty in p

Full text of DOI:10.1002/smll.201800516

CommuniCation
Bioimaging
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Carbon and Nitrogen Based Nanosheets as Fluorescent
Probes with Tunable Emission
Jingwen Sun, Ravit Malishev, Adi Azoulay, Jonathan Tzadikov, Michael Volokh, Raz Jelinek,
and Menny Shalom*
bioimaging.[9] The facile tunability of their
2D carbon and nitrogen based semiconductors (CN) have attracted wide-
spread attention for their possible use as low-cost and environmentally
friendly materials for various applications. However, their limited solution-
dispersibility and the difficulty in preparing exfoliated sheets with tunable
photophysical properties restrain their exploitation in imaging-related applica-
tions. Here, the synthesis of carbon and nitrogen organic scaffolds with
highly tunable optical properties, excellent dispersion in water and DMSO,
and good bioimaging properties is reported. Tailored photophysical and
chemical properties are acquired by the synthesis of new starting monomers
containing different substituent chemical groups with varying electronic prop-
erties. Upon monomer condensation at moderate temperature, 350 °C, the
starting chemical groups are fully preserved in the final CN. The low conden-
sation temperature and the effective molecular-level modification of the CN
scaffold lead to well-dispersed photoluminescent CN thin sheets with a wide
range of emission wavelengths. The good bioimaging properties and the tun-
able fluorescence properties are exemplified by in situ visualization of giant
unilamellar vesicles in a buffered aqueous solution as a model system. This
approach opens the possibility for the design of tailor-made CN materials
with tunable photophysical and chemical properties toward their exploitation
in various fields, such as photocatalysis, bioimaging, and sensing.
photophysical properties together with
their biocompatibility, position CN as suit-
able and rapidly emerging materials for
imaging and biobased applications such
as sensing and drug delivery.[10–12] Nev-
ertheless, for some of these applications,
the CN materials should be well-dispersed
in a given solvent in order to form a thin
sheet structure that can interact with an
organism (cell, membrane, etc.).[13] For
mimicking real body conditions, as well
as protecting the imaged organism, a fluo-
rescence indicator should disperse well in
water (buffer condition, pH ≈7).[14] More-
over, a tunable and wide range of fluo-
rescence wavelengths is desired to allow
better detection.[15]
The traditional solid-state synthesis of
bulk-CN usually yields materials with low
dispersibility and poor fluorescence prop-
erties due to the formation of defects and
grain boundaries.[16,17] A great progress in
the exfoliation of bulk-CN to several layers
was acquired hitherto by mechanical treat-
ment[18] and by chemical means such as
protonation,[19] noncovalent interaction,[17]
Carbon and nitrogen based materials (CN) have attracted wide-
spread attention during the last years owing to their unique
structural, chemical, photophysical, and electronic properties,
as well as their low price and easy synthesis.[1–4] CN materials
have shown promising performance in multiple fields such as
water splitting,[3,5] CO2 reduction,[6] solar cells,[7] sensors,[8] and
and modification of the CN network with other groups. Conse-
quently, the exfoliated sheets could be further utilized in several
areas such as biosensing,[11] DNA detection,[17] drug delivery,[12]
and cell imaging.[20] However, albeit the continuous progress,
CN dispersibility in solvents is limited, and the CN emis-
sion is usually restricted to short wavelengths due to its band
structure.[20]
Recently, we and others showed that the modification of CN
structures with a phenyl group leads to a substantial enhance-
ment of its photophysical properties.[20–22] Consequently, the
phenyl-modified CN was utilized in several (bio)imaging appli-
cations.[20] However, additional alteration of the fluorescence
properties of the phenyl-modified CN are highly restricted due
to the lack of suitable starting monomers with versatile elec-
tron-donation capabilities. Moreover, its exfoliation in water
requires long sonication time and results in rather low concen-
trations. Hence, despite the great progress in this area, a facile
and sustainable approach to synthesize CN materials with a
broad and tunable emission range, as well as good dispers-
ibility in water is still required for further development of this
field.[23]
Dr. J. Sun, R. Malishev, A. Azoulay, J. Tzadikov, Dr. M. Volokh,
Prof. R. Jelinek, Prof. M. Shalom
Chemistry Department and Ilse Katz Institute
for Nanoscale Science and Technology
Ben-Gurion University of the Negev
Beer-Sheva 8410501, Israel
E-mail: mennysh@bgu.ac.il
Dr. J. Sun
Key Laboratory of Soft Chemistry and Functional Materials
Nanjing University of Science and Technology
Nanjing 210094, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.201800516.
DOI: 10.1002/smll.201800516
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Figure 1. a) Schematic illustration of the CN-Rs and CNN-Rs. TEM images of b) CNN-Ph, c) CNN-Cl, d) CNN-CH3, and e) CNN-OCH3. The TEM
images of all CN-Rs are given in Figure S8 (Supporting Information).
Here we report on the synthesis of the modified-CN materials
(CN-Rs, R = Ph, Ph-Cl, Ph-CH3, Ph-OCH3) with tunable optical
properties, spontaneous dispersion in a DMSO/H2O mixture,
and good bioimaging properties. For the synthesis of the CN-Rs,
we synthesized new starting monomers with different phenyl-
based groups that were subsequently calcinated at relativity low
temperature (350 °C). We show that a very mild protonation
treatment with 0.5 m HCl leads to a structural cleavage of the
obtained CN materials and to thin layer formation with reduced
surface charge. The good bioimaging properties are exemplified
here by visualizing giant unilamellar vesicles (GUVs) in a buff-
ered aqueous solution as a model system.[24]
The starting monomers, with different functional groups
were synthesized by the reaction of 4-chlorobenzonitrile,
p-tolunitrile, and 4-methoxybenzonitrile with dicyandiamide
under microwave irradiation.[25] Gas chromatography mass
spectrometry and Fourier-transform infrared spectroscopy
(FTIR) (Figures S1 and S2, Supporting Information) confirmed
the successful synthesis of 2,4-diamino-6-chlorophenyl-1,3,5-
trazine (DPT-Cl), 2,4-diamino-6-tolyl-1,3,5-trazine (DPT-CH3),
and 2,4-diamino-6-methoxyphenyl-1,3,5-trazine (DPT-OCH3).
The CN-Rs were prepared by the calcination of DPT-X (X = Cl,
CH3, OCH3) monomers at 350 °C under N2 atmosphere. Ther-
mogravimetric analysis indicates that the condensation occurs
in the range of 200–350 °C for each precursor (Figures S3–S6,
Supporting Information), therefore we chose to synthesize the
CN-Rs at 350 °C (Figure 1a). The milder reaction conditions are
advantageous compared to traditional synthesis[10] and it results
in not fully condensate CN structure as shown in Figure S7 of
the Supporting Information (a short discussion is given in the
Supporting Information). The latter opens the possibility to
further “cut” the CN scaffold into smaller units, subsequently
improving their dispersibility and surface properties as shown
later in this paper.
Elemental analysis (EA) results of the precursors and the
CN-Rs are summarized in Table S1 (Supporting Informa-
tion). The EA results revealed that the starting precursors
(DPT, DPT-Cl, DPT-CH3, and DPT-OCH3) lost a notable
amount of N and H during their calcination, probably due to
the release of N H groups.[26] Moreover, the Cl group in CN-Cl

was preserved during the calcination process. A small decrease
of oxygen in DPT-OCH3 was obtained after calcination, owing
to partial deoxygenation at 350 °C.
X-ray difraction (XRD) and FTIR analyses were used to con-
firm the formation of CN materials at 350 °C. The XRD pat-
terns (inset of Figure 2a) of all materials show a distinguished
stacking peak centered around 27° (002).[3] However, the addi-
tional X group leads to a slight shift toward lower angles,
speaking for the increased distance between the sheets. FTIR
spectra of the corresponding CN-Rs (Figure S8, Supporting
−1

Information) affirmed the existence of N H at 3400 cm ,
−1 [16]


and C N and C N at 1000 to 1700 cm . Additionally, the
characteristic breathing mode of the triazine units at 800 cm−1
was observed. For CN-CH3 and CN-OCH3, the characteristic
−1


vibrations of CH3 and C O at 1380 and 1260 cm , respec-
tively[27,28] were detected, accounting for the successful incorpo-
ration of the new groups within the CN-R scaffolds. In good
agreement with the XRD analysis, the stacked lamellar textures
of the CN-Rs were further identified by TEM images (Figure S9,
Supporting Information) and for all materials a sheet-like struc-
ture comprised of several CN layers was observed.
The exfoliation of CN-Rs to thin layers was achieved by very
gentle protonation (Figure 1a) with 0.5 m HCl (Figure 1b–e).
XRD measurements indicate a successful exfoliation as a broad-
ened stacking peak (002) is obtained (Figure 2a).[19] EA results
(Table S1, Supporting Information) of the protonated CN-Rs
(referred here as CNN-Rs – N stands for nanosheets) show
an increase in the hydrogen content compared to the CN-Rs,
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Figure 2. a) XRD patterns of CNN-Rs and CN-Rs (inset) with the same ordinate. b) FTIR spectra of CNN-Rs. c) Solid-state 13C NMR spectrum of CNN-
Ph. High-resolution XPS spectra of N 1s for d) CN-Ph, e) CN-Cl, f) CN-OCH3, and g) CN-CH3 before (green curves) and after protonation (blue curves).
indicating a successful protonation of the CN scaffolds.[19] In
addition, the protonation alters the surface charge as indicated
by zeta potential measurements (Table S2, Supporting Infor-
mation), where the protonated sheets’ charge is more positive
compared to the CN-Rs. Importantly, FTIR spectra of CNN-Rs
(Figure 2b) and CN-Rs (Figure S8, Supporting Information)
revealed that the basic motives were not chemically affected by
the mild acid treatment.
To further investigate the chemical composition and states
of CN-Rs and CNN-Rs, 13C solid-state NMR (Figure 2c;
Figures S10–13, Supporting Information) as well as X-ray pho-
toelectron spectroscopy (XPS) analyses of CN-Rs and CNN-Rs
were performed (Figure 2d–g; Figure S14, Supporting Informa-
tion). The 13C NMR spectrum of the starting DPT monomer
(Figure S10a, Supporting Information) compared to the spectra
of CN-Ph (Figure S10b, Supporting Information) and CNN-Ph
(Figure 2c) confirmed the existence of melem-unit (signals at
≈164 (C4) and ≈156 ppm (C5)) together with the preservation
of the phenyl signal at ≈128 ppm (C3).[26] Similarly, the melem-
like signals were also observed in other CN-Rs and CNN-Rs
(Figures S11–S13, Supporting Information). Characteristic sig-
nals of CH3 (21.7 ppm)[29] in the CN-CH3 and CNN-CH3 spectra
imply the preservation of the functional groups after calcination
and acid treatment. In accordance with the EA data, low signal
intensities of both OCH3 (54.8 ppm)[30] and CH3 (21.7 ppm)
were detected in the CN-OCH3 and CNN-OCH3 spectra, further
supporting the partial deoxygenation of DPT-OCH3 during the
condensation.
expected, new protonated terminal amino groups (N H) could

 
be detected at 401.2 eV together with a decrease in the C N C
group (BE of 398.6 eV).[32] Additionally, two more peaks at the
higher BEs (402.5 and 404.2 eV) for CNN-Cl can be attributed to
oxidized nitrogen, pyridine-N species, and positive charge local-
ization in the heterocycles, respectively.[33] The C 1s analysis is
shown in Figure S14a–d (Supporting Information), further con-
firming the condensation of the precursors to CN scaffold and
its structural conservation after protonation. Moreover, the Cl
2p and O 1s XPS spectra (Figure S14e,f, Supporting Informa-
tion) in CNN-Cl and CNN-OCH3 affirm that the mild acid treat-
ment did not lead to removal of the functional groups.
Interestingly, all the CN-Rs and CNN-Rs demonstrated
excellent dispersibility in DMSO (3 mg mL−1), resulting in the
formation of highly stable colloidal suspension. As shown in
Figure 3a, the colloidal suspensions exhibit great fluorescence
tunability and a wide range of emitted light was detected under
excitation from a UV lamp (365 nm). The influence of the new
groups on the optical properties of CN-Rs and CNN-Rs can
be reflected in their photophysical properties. The quantum
yields (QYs) of the dispersed CN-Rs are given in Table S3 (Sup-
porting Information). The CN-Ph exhibits a high QY of 18.6%
(Table S4, Supporting Information), compared to the bulk
carbon nitride (4.8%).[9,34] After protonation, the CNN-Rs QY
is further enhanced compared to the CN-Rs (Table S5, Sup-
porting Information). Figure 3b and Figure S15a (Supporting
Information) display the solid and liquid UV–vis absorption of
CN-Rs and CNN-Rs. The introduction of new groups enhanced
the optical density of all CN-Rs and the absorption edge was
red-shifted according to the electron-donating properties of the
new groups (Figure 3b). The absorption edge of the protonated
materials was slightly blue-shifted compared to the CN-Rs,
The XPS spectra in Figure 2d–g show the N 1s of CN-Rs and
CNN-Rs. In all cases, the N-(C)3 group (binding energy (BE) of
400.3 eV) could be identified, indicating the condensation of the
starring monomers and the formation of a heptazine unit.[31] As
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Figure 3. a) Digital photos of CN-Rs and CNN-Rs dispersed in DMSO without and under UV light (365 nm) excitation. b) Solid-state UV–vis absorp-
tion spectra of CN-Rs and CNN-Rs. Normalized PL spectra of CN-Rs and CNN-Rs under excitation at 405 nm, recorded in c) DMSO and d) DMSO/
buffer mixture (v/v = 1/4), respectively.
implying the successful cleavage and exfoliation of the CNN-Rs
(Figure 3b; Figure S15a, Supporting Information).[35]
(Figure S16, Supporting Information). In order to solve this
problem, we employed a colloidal suspension in DMSO/
H2O mixture (v/v = 1/4) with sustainable fluorescent proper-
ties (Figure S17, Supporting Information). Normalized PL
spectra of CN-Rs and CNN-Rs recorded in the DMSO/buffer
(v/v = 1/4) mixture are given in Figure 3d. In comparison to
the PL spectra recorded in DMSO, a broader emission range
can be observed for all CN materials owing to the solvent inter-
action.[37] Figure 4a and Figure S18 (Supporting Information)
present the confocal microscopy images of GUVs before and
after incubation with CN-Rs upon excitation at 405 nm. As
shown above, CN-Rs surface and the GUVs are both nega-
tively charged, thus, most CN-R particles did not penetrate
well into the GUVs. Hence, only the less negatively charged
CN-Ph demonstrated good imaging properties with very strong
luminescence.[20]
Normalized photoluminescence (PL) spectra of the CN-Rs
and CNN-Rs suspensions (in dimethyl sulfoxide (DMSO))
were collected under excitation at 405 nm (Figure 3c). The
emission peaks of CN-Cl, CN-CH3, and CN-OCH3 were red-
shifted compared to CN-Ph.[20] A similar trend was also
observed for the CNN-Rs, indicating the maintenance of the
optical properties after the protonation. The tunable optical
properties of the all the new CN materials open new possibili-
ties toward their utilization in imaging-related applications.[13]
As a proof of concept, here we opted for bioimaging of
GUVs comprising 1,2-dioleoyl-sn-glycero-3-phosphoethanol-
amine and 1,2-dioleoyl-sn-glycero-3-phospho-(1′-rac-glycerol),
designed to mimic negatively charged bacterial membranes.[36]
For GUVs imaging applications, good dispersion/solubility
of the obtained CN materials in aqueous solution is essential.
However, most carbon nitrogen based materials are difficult to
disperse in water, including the prepared CN-Rs and CNN-Rs
After protonation, pronounced labeling with different detec-
tion wavelengths was obtained for all materials (Figure 4b–e)
due to the reduced surface charge and the thinner CNN-Rs layer
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Figure 4. Fluorescence imaging of GUVs labeled with a) CN-Ph, b) CNN-Ph, c) CNN-Cl, d) CNN-OCH3, and e) CNN-CH3. f–j) Fluorescence imaging
of the sequential transformation of GUVs following the addition of 20% Triton X-100 by using CNN-Ph. Bright field microscopy (top), and confocal
fluorescence microscopy (bottom) images recorded upon excitation at 405 nm.
compared to the CN-Rs (Figure 1). To further investigate the
interaction between the GUVs and CNN-Rs, in situ emission
analysis was performed. Interestingly, the interaction between
the CNN-Rs and the GUVs leads to a blue fluorescence shift
(Figure S19, Supporting Information), for example, from green
to blue for the CNN-Cl before and after adsorption within the
GUVs, as shown in the insets of Figure S19a (Supporting Infor-
mation).[14] Moreover, a significant fluorescence enhancement
occurred in the hybrid materials. This intriguing phenomenon
can be attributed to the hydrophobic environment, induced by
the GUVs.[38] Moreover, similar wide emission wavelength is
shown in the in situ emissions of CN-Ph and CNN-Rs together
with the GUVs (Figure S19b, Supporting Information), which
is quite different from their spectra without GUVs (Figure 3d).
The difference in the photoluminescence profiles reflects the
influence of the vesicle environment on the CNN-Rs’ optical
properties and is indicative of CNN-Rs insertion into the lipid
bilayer.
the imaging capabilities of the reported modified CN
materials.[13]
In summary, we presented a simple and general low-temper-
ature synthetic method of modified carbon and nitrogen based
materials with tunable photophyscial properties, good dispers-
ibility in water, and good bioimaging abilities. The new CN-Rs
were obtained by the rational design of their starting monomers
with different electron-donating groups (phenyl, phenyl-Cl,
phenyl-CH3, phenyl-OCH3). The relatively low calcination
temperature allows the preservation of the different groups
in all CN materials. Furthermore, the cleavage of their struc-
ture and subsequently their exfoliation to very thin sheets can
be achieved by very mild acid treatment. The mild protonation
reduced the negative surface charge of the CN-Rs, while the
optical properties remained unchanged. The good bioimaging
properties were demonstrated by in situ imaging of GUVs and
their destruction in the presence of 20% Triton X-100. The
strong interaction between the modified CN materials with the
membrane leads to a strong shift of their fluorescence emis-
sion to shorter wavelengths. This simple approach opens many
possibilities for the design of carbon–nitrogen nanosheets with
tunable photophysical and chemical properties toward their uti-
lization in various applications such as bioimaging, sensing,
and photocatalysis.
More advanced real-time visualization of membrane dis-
tortion processes was performed with CN-Ph and CNN-Ph
by reacting the labeled membrane with 20% Triton X-100
(Figure 4f–j; Figure S20, Supporting Information). The
transformation of the spherical GUV to deformed small
vesicles can be nicely followed, further demonstrating
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We thank Prof. Lemcoff’s group for their help. The authors thank
the financial support of the Israel Science Foundation (ISF, Grant
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