Synthesis and unique photoluminescence properties of nitrogen-rich quantum dots and their applications

Article abstract of DOI:10.1002/anie.201408422

Nitrogen-rich quantum dots (N-dots) were serendipitously synthesized in methanol or aqueous solution at a reaction temperature as low as 50 °C. These N-dots have a small size (less than 10 nm) and contain a high percentage of the element nitrogen, and are thus a new member of quantumdot family. These N-dots show unique and distinct photoluminescence properties with an increasing percentage of nitrogen compared to the neighboring carbon dots. The photoluminescence behavior was adjusted from blue to green simply through variation of the reaction temperature. Furthermore, the detailed mechanism of N-dot formation was also proposed with the trapped intermediate. These N-dots have also shown promising applications as fluorescent ink and biocompatible staining in C. elegans.

Full text of DOI:10.1002/anie.201408422

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DOI: 10.1002/anie.201408422
Quantum Dots
Synthesis and Unique Photoluminescence Properties of Nitrogen-Rich
Quantum Dots and Their Applications**
Xiuxian Chen, Qingqing Jin, Lizhu Wu, ChenHo Tung, and Xinjing Tang*
Abstract: Nitrogen-rich quantum dots (N-dots) were seren-
dipitously synthesized in methanol or aqueous solution at
a reaction temperature as low as 508C. These N-dots have
a small size (less than 10 nm) and contain a high percentage of
the element nitrogen, and are thus a new member of quantum-
dot family. These N-dots show unique and distinct photo-
luminescence properties with an increasing percentage of
nitrogen compared to the neighboring carbon dots. The
photoluminescence behavior was adjusted from blue to green
simply through variation of the reaction temperature. Further-
more, the detailed mechanism of N-dot formation was also
proposed with the trapped intermediate. These N-dots have
also shown promising applications as fluorescent ink and
biocompatible staining in C. elegans.
nanotubes, and GOD) are superior in terms of robust
chemical inertness, easy functionalization, high aqueous
solubility, low toxicity, and excellent biocompatibility as
[
6]
they do not use heavy metals and small hydrophobic
[7]
molecules. They have also attracted great interest in many
fields of science owing to their outstanding fundamental
properties and show potential exciting applications in many
[
13]
[8]
[9]
[10]
fields, such as catalysts, bioimaging agents, sensors,
[11]
and photovoltaic devices.
For C-dots, a few percent of
nitrogen doping of carbon dots have been reported to give
[13c]
excellent optical properties.
However, most of the nitrogen
atoms were induced on particle surface under harsh con-
ditions with a few percent of quantum dots by weight, which is
much less than the percent of carbon element of the same
[
13a,b,14]
quantum dots.
Therefore, the desire for new nano-
Q
1
uantum dots, such as gallium arsenide, typically contain
a few thousand atoms and have been widely studied since
990s owing to their useful electronic and photonic proper-
structured materials based on new elements (such as nitro-
gen) is still challenging.
Fluorescent nitrogen-rich quantum dots (N-dots) were
synthesized when 2-azidoimidazole aqueous or methanol
solution was inadvertently heated at 708C overnight without
the addition of acids (Figure 1a). The solution turned dark
brown and showed strong cyan-green fluorescence when
irradiated with hand-held UV lamp (365 nm). After filtration
with a 0.22 mm filter and vacuum-freeze drying, the obtained
cinnamon-colored solid residue that was later characterized
as nanoparticles was insoluble in common organic solvents
(such as ethyl acetate, acetone, chloroform), but was highly
dispersible in water. Different from the synthesis of other C-
[
1]
ties. The small size of these nanostructures results in
a quantum confinement of charge carriers (electron–hole
pairs). This causes a quantization of the energy spectra with
discrete energy levels. Based on these specific properties, the
applications of these nanostructures have ranged from
photonics, electronics, and drug delivery to biological and
biomedical imaging in vitro and in vivo. However, heavy
metals (for example, CdSe, PbTe, and CdTe) as essential
elements in semiconductor quantum dots have risks of long-
term toxicity and/or potential environmental hazards. There-
fore, the needs for more biocompatible nanomaterials with
similar properties are urgently required. Recently, new
quantum dots have been discovered and investigated, such as
BN nanosheets, BCNO nanoparticles, nanodiamonds, gra-
phene oxide (GOD), C-dots, and self-assembled nanoparti-
[2]
[
3]
[15]
dots under harsh conditions (for example: high-energy ion
bean radiation, hydrothermal carbonization with extreme
high temperature, laser ablation, large amount strong acids),
these N-dots were synthesized in a single-step reaction under
much milder conditions with 2-azidoimidazole as a starting
material. We then expanded the scope of starting materials.
However, the raw reaction solution of 3-azidopyrazole, 2,4,6-
triazido-1,3,5-triazine, and 3-azido-1,2,4-triazole did not show
any photoluminescence and had no nanoparticle formation.
The raw reaction solution of 2-azidobenzimidazole showed
very weak fluorescence emission with a negligible number of
collected nanoparticles (Supporting Information, Figure S1).
UV/Vis absorption and photoluminescence of the N-dot
aqueous solution from 2-azidoimidazole were first measured
after simple filtration with a 0.22 mm filter. However, their
complicated absorption and photoluminescence spectra (Fig-
ure 1b) indicated that the N-dots might have broad size
distribution, which is a general disadvantage of bottom-up
synthesis strategy for some nanoparticles such as carbon
[
4]
[
5]
cles. Compared to traditional semiconductor quantum dots,
carbon based nanostructures
[
12]
(such as C-dots, carbon
[*] X. Chen, Q. Jin, Prof. Dr. X. Tang
State Key Laboratory of Natural and Biomimetic Drugs
School of Pharmaceutical Sciences, Peking University
No. 38, Xueyuan Rd. Beijing 100191 (China)
E-mail: xinjingt@bjmu.edu.cn
Prof. L. Wu, Prof. C. Tung
Technical Institute of Physics and Chemistry
The Chinese Academy of Sciences (China)
[
**] This work was supported by the National Basic Research Program of
China (973 Program: 2013CB933800, 2012CB720600), National
Natural Science Foundation of China (21372015), and the Innova-
tion Team of the Ministry of Education (BMU20110263). We thank
Prof. Ivan Dmochowski for proofreading and helpful suggestions.
[16]
dots.
To further characterize these N-dots in detail, we
turned to separate these N-dots using size-exclusion chroma-
tography with Sephadex G-25, and successfully collected
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408422.
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Figure 2. a)–c) TEM images of N-dot 1, N-dot 2, N-dot 3 (scale bars
00 nm); Insets are HRTEM images (scale bars 5 nm). d)–f) AFM
1
images and size distributions of N-dot 1, N-dot 2, and N-dot 3. Height
profiles are given for the marked white line in the AFM images.
dots are monodisperse nanoparticles with good crystallinity
(inserts in Figure 2a–c). Atomic force microscopy (AFM) was
further applied to demonstrate the topographic morphology
of N-dots and identify the accurate heights of N-dot 1, N-
dot 2, and N-dot 3 (Figure 2d–f), confirming the homoge-
neous distribution and height histograms of the N-dots. By
quantifying the height of around 70 nanoparticles for each
kind of N-dot, the size distributions of these three N-dots
(
4
Supporting Information, Figure S2) were 2.07 Æ 0.61 nm,
.62 Æ 0.64 nm, and 5.39 Æ 0.80 nm in diameter for N-dot 1,
N-dot 2, and N-dot 3, which were consistent with their elution
times of SEG chromatography. And their sizes were also
consistent to the crystal cores of corresponding N-dots (insets
in Figure 2a–c). Further typical X-ray diffraction (XRD)
patterns of all three N-dots show that they had the similar
broad peak centered at 238. The XRD results, along with
HRTEM data, illustrated that these N-dots have a crystalline
core with an amorphous surface.
Figure 1. a) Synthesis of N-dots. b) UV/Vis absorption spectra of a raw
N-dot aqueous solution and photoluminescence emission spectra of
a raw N-dot aqueous solution excited at 360 nm. c) Digital image
À1
(
top) of 0.2 mgmL N-dots dispersed in pure water and their
We then applied Fourier-transform infrared (FTIR)
spectroscopy and X-ray photoelectron spectroscopy (XPS)
to further characterize the properties of chemical bonds and
functional group on the surface of these N-dots. Figure 3a
demonstrates that N-dot 1, N-dot 2, and N-dot 3 have very
similar IR spectra that revealed the existence of functional
fluorescent images (bottom) under excitation of hand-held UV lamp
365 nm); from left to right: N-dot 1, N-dot 2, N-dot 3.
(
three fractions of N-dots according to their elution time.
These three kinds of N-dots solutions showed different color
and photoluminescence when excited with a hand-hold UV
lamp (365 nm), named N-dot 1 (blue), N-dot 2 (cyan), and N-
dot 3 (cyan green) (Figure 1c). Further elemental analysis of
these N-dots indicated that the nitrogen proportion increased
from N-dot 1, N-dot 2, to N-dot 3, in which N-dot 3 had
a nitrogen component as high as 34.48%, about 10% higher
than the carbon content of the same N-dot (Supporting
Information, Table S1), which has never been reported
À1
À1
groups such as OH or NH (3415 cm ), CH (2919 cm ,
À1
À1
À1
2851 cm ), C-O-C (1082 cm ), C=N (1545 cm ), and C=O
À1
[13c,18]
(1644 cm ) on these N-dots.
In Figure 3b, the typical
XPS surveys of three N-dots indicate a high nitrogen content
of these dots, which was consistent to the results of their
element analysis (Supporting Information, Table S1). The
deconvoluted C1s XPS spectrum of N-dot 1 (Supporting
Information, Figures S3–S5 and Table S2) indicated four main
components assigned as CÀC at ca. 284.80 eV, C=N at ca.
286.31 eV, C=O at ca. 287.52 eV, and CÀN or CÀO at ca.
[13c,15a,17]
previously.
Different kinds of microscopy were applied to further
verify these N-dots. As shown in Figure 2a-c, TEM images of
N-dot 1, N-dot 2, and N-dot 3 clearly indicated that these N-
[
17]
288.66 eV.
The N1s XPS spectrum of N-dot 1 around
398.31 eV, 399.23 eV, 399.91 eVand 400.69 eV was pointed to
2
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the nitrogen atom of graphite-like structure, pyridinic-like N
[
13a,c]
and pyrrolic-like N and NH groups, respectively.
The
content of pyrrolic-like N was the most abundant (Supporting
Information, Table S3), which indicated a big amount of C=N
like groups of N-dots. The O1s XPS spectrum of N-dot 1
could be decomposed into peaks at ca. 530.68 eV, ca.
5
31.38 eV, ca. 532.24 eV and ca. 532.93 eV, indicating the
[
13c,19]
presence of ÀOH, *O=CÀO, CÀO, and O=CÀO*.
XPS
data of N-dot 2 and N-dot 3 demonstrated that they contained
the similar functional groups as N-dot 1 on particle surface
(
Supporting Information, Table S2). The presence of func-
[
7b]
tional groups (C=N, CÀO, NH, OH, etc) imparts excellent
solubility in water without the need of further chemical
modification of nanoparticle surface. XPS spectra and FTIR
spectra, along with element analysis data of N-dots, confirmed
that these N-dots contained the elements nitrogen, carbon,
oxygen, and hydrogen (Figure 3) with an even higher
percentage of nitrogen than carbon both on the surface and
in internal crystal core.
The importance of nanoparticle sizes and functional
groups for optical properties of N-dots is demonstrated in
Figure 4, presenting the optical absorption and photolumi-
nescence spectra of these N-dots. Figure 4a shows that the
UV/Vis absorption spectra of N-dot 1 and N-dot 2 have
similar absorption curves with an absorption peak around
310 nm, while N-dot 3 had an absorption peak around 310 nm
and a broad absorption shoulder peak around 400–500 nm,
suggesting a gradual red-shift of their absorption. The photo-
Figure 3. a) FTIR spectra and b) XPS survey of N-dot 1, N-dot 2, and
N-dot 3.
À1
Figure 4. a) The UV/Vis absorption spectra of three N-dots (66.7 mgmL ). b)–d) Photoluminescence emission spectra of b) N-dot 1, c) N-dot 2,
and d) N-dot 3 recorded from 300 to 500 nm in 20 nm increments.
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luminescence spectra of N-dot 1, N-dot 2, and N-dot 3
displayed an emission maximum at 435 nm, 442 nm, and
4
3
51 nm, respectively, with the excitation wavelength at
40 nm (Supporting Information, Figure S6), indicating
strong quantum confinement of nanoparticles. Figure 4b,c
show the red-shift of photoluminescence emission with the
reduction of their corresponding intensity as the excitation
wavelength increased from 340 to 500 nm in 20 nm incre-
ments. Like many C-dots, N-dot 1 and N-dot 2 showed the
wavelength-dependent photoluminescence behavior, that is,
the observed photoluminescence red-shift of N-dots was
accompanied by a significant reduction in photoluminescence
intensity that is a signature of the gradual evolution of the
confinement in quantum dots. In contrast to N-dot 1 and N-
dot 2, N-dot 3 displayed a distinctive photoluminescence
performance. Photoluminescence emission of N-dot 3 aque-
ous solution red-shifted as the excitation wavelength
increased between 340 and 400 nm. However, when the
excitation wavelength passed over 400 nm, its emission peak
centered at 515 nm without obvious decrease in photolumi-
nescence intensity. Interestingly, simply by adjusting the
reaction temperature at 50 or 1008C, we obtained blue
photoluminescence and green photoluminescence of the raw
N-dot aqueous solutions irradiated by hand-held UV lamp
Scheme 1. Proposed mechanism for the formation of N-dots.
(
365 nm) (Supporting Information, Figure S8). N-dots pro-
solvents (such as methanol and water). The involvement of
methanol in N-dots formation was also confirmed by the
duced at 508C showed similar luminescence spectra as N-
dot 3 with an emission peak centered at 515 nm, while N-dots
produced at 1008C showed similar luminescence spectra as N-
dot 1 with wavelength-dependent photoluminescence behav-
ior (Supporting Information, Figures S9 and S10). Without
any surface passivation, the photoluminescence quantum
yields of N-dot 1, N-dot 2, and N-dot 3 were 0.10, 0.14 and
1
13
methoxyl peaks on H NMR and C NMR of N-dots
(Supporting Information, Figure S15). The mechanism also
explains why no or little N-dots could be formed with other
similar starting materials (such as 3-azido-1,2,4-triazole, 3-
azidopyrazole, 2,4,6-triazido-1,3,5-triazine). Furthermore, we
also found that green photoluminescence of N-dots formed at
508C could gradually turn into blue photoluminescence if the
2-azidoimidozole in methanol continued heating at 508C for
15 days. This observation could explain the unique photo-
luminescence properties of N-dot 3 that is possibly due to
incomplete ring opening of aziridine moiety by solvents in the
formation of N-dots. This is also consistent to the high
percentage of nitrogen content in elemental analysis.
0
.09, respectively under 360 nm excitation using quinine
[
18b]
sulfate in 0.10m H SO as a reference,
higher than that of C-dots without surface passivation.
which are usually
2
4
[
5b,16,20]
According to the photoluminescence mechanism of quantum
dots, we expect the photoluminescence quantum yield would
be much higher if their surfaces were passivated by polyeth-
ylene glycol, metal, etc., as seen for C-dots previously
reported.
Although possible mechanisms have been proposed for
[
5f,6b]
The high photoluminescence quantum yields and excita-
tion-wavelength-dependent photoluminescence properties of
N-dots make them be a valuable fluorescent ink. The aqueous
solutions of N-dots obtained at different reaction temperature
(508C, 70 8C, 1008C) were chosen as a new type of fluores-
cence ink. A calligraphy of letters (N-dots and P, K, U) was
then achieved on commercially available filter paper with
[18b]
the formation of carbon dots,
no detailed information were
provided owing to the harsh synthetic conditions and/or
complicated carbonization of carbohydrates. Herein, we
proposed a mechanism for the formation of N-dots from 2-
azidoimidozole, as shown in Scheme 1. After decomposition
of azido moiety, the active intermediate, nitrene, reacted with
double bond of 2-azidoimidozole to form tricycle of aziridine
moiety and polymers were then formed through self-poly-
merization. Methanol or water may then be involved in a ring-
opening reaction and finally N-dots are formed by probable
À1
three N-dot-based fluorescent inks (0.3 mgmL ) using
a writing brush. This calligraphy was invisible in daylight,
but was clearly observed under a hand-held UV lamp
(365 nm, Figure 5a) and remained consistent in an indoor
environment, which is beneficial for practical applications.
Furthermore, these fluorescent words kept relatively sharp
after dipping in water (Supporting Information, Figure S11a),
which was not achievable with normal fluorescent ink
(fluorescein was used here; Supporting Information, Fig-
ure S11b). Not only as fluorescence ink coated on paper, these
N-dots could also stain cotton, silk, PAGE gel, and so on
(Figure 5; Supporting Information, Figures S12–S14). Fig-
ure 5b indicated the cotton was first soaked in N-dots
[
18b]
nuclear burst at supersaturation point.
To confirm the
mechanism, 2-azidoimidozole was mixed with methyl acrylate
at 508C for two days. Almost no fluorescence of the reaction
solution could be observed; instead, we isolated a new organic
compound that proved to be 2-aziridinecarboxylic acid-1-
(
1H-imidazol-2-yl) methyl ester, a trapped product of active
intermediate of self-polymerization. The aziridine moieties in
polymers were not very stable and could be attacked by
4
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biocompatible staining. Furthermore, a detailed mechanism
was proposed for the formation of these N-dots through
nitrene-mediated self-polymerization and condensation.
Future work should be focused on surface modification and
their full applications, particularly in biomedical sciences.
Received: August 21, 2014
Published online: && &&, &&&&
Keywords: bioimaging · fluorescence ink · nanomaterials ·
.
photoluminescence · quantum dots
Figure 5. a),b) Calligraphy in N-dot-based fluorescent inks with a writ-
ing brush on filter paper. “p”, “k”, and “u” were written in different
aqueous solutions (0.3 mgmL ) of N-dots that were obtained at
[1] D. Bera, L. Qian, T.-K. Tseng, P. H. Holloway, Materials 2010, 3,
2260 – 2345.
À1
5
08C, 708C, and 1008C respectively, and “N-Dot” was performed in
[2] W. L. Wilson, P. F. Szajowski, L. E. Brus, Science 1993, 262,
1242 – 1244.
the same ink as “p”. Photos were captured under the excitation of
hand-held UV lamp (365 nm). c)–f) Bright-field image and fluorescent
images of cotton fibers stained with N-dots (0.3 mgmL ). g)–
k) Bright-field image and fluorescent images of C. elegans stained with
an N-dot aqueous solution (0.5 mgmL ). Fluorescence images of
cotton fibers and C. elegans were captured under UV, blue, and green
excitation; the fluorescent ink used here was raw N-dot aqueous
solution obtained at 1008C. Scale bars: 50 mm.
[3] a) X. Michalet, F. F. Pinaud, L. A. Bentolila, J. M. Tsay, S.
Doose, J. J. Li, G. Sundaresan, A. M. Wu, S. S. Gambhir, S. Weiss,
Science 2005, 307, 538 – 544; b) C. Wu, D. T. Chiu, Angew. Chem.
Int. Ed. 2013, 52, 3086 – 3109; Angew. Chem. 2013, 125, 3164 –
3190; c) C. X. Guo, H. B. Yang, Z. M. Sheng, Z. S. Lu, Q. L.
Song, C. M. Li, Angew. Chem. Int. Ed. 2010, 49, 3014 – 3017;
Angew. Chem. 2010, 122, 3078 – 3081.
À1
À1
[
4] F. Mammeri, A. Ballarin, M. Giraud, G. Brusatin, S. Ammar,
Colloids Surf. A 2013, 416, 1 – 7.
[
5] a) W. Lei, D. Portehault, R. Dimova, M. Antonietti, J. Am.
Chem. Soc. 2011, 133, 7121 – 7127; b) D. Pan, L. Guo, J. Zhang,
C. Xi, Q. Xue, H. Huang, J. Li, Z. Zhang, W. Yu, Z. Chen, Z. Li,
M. Wu, J. Mater. Chem. 2012, 22, 3314; c) Q. Wu, X. Wang, Q.-S.
Li, R.-Q. Zhang, J. Cluster Sci. 2013, 24, 381 – 397; d) V. N.
Mochalin, O. Shenderova, D. Ho, Y. Gogotsi, Nat. Nanotechnol.
À1
aqueous solution (0.3 mgmL , N-dots obtained at 1008C) for
2
min and then was thoroughly washed. Clear excitation-
wavelength-dependent fluorescence was observed under an
inverted fluorescence microscope. Under UV, blue, and green
light excitation, the cotton fibers showed blue, green, and red
fluorescence staining (Figure 5b–e), whereas nearly no fluo-
rescence is observed in the uncoated cotton fibers under the
same conditions (Supporting Information, Figure S12). Fur-
thermore, C. elegans were also used as a model for N-dot
2
2
011, 7, 11 – 23; e) L. Shcherbyna, T. Torchynska, Physica E
013, 51, 65 – 70; f) Y. Sun, B. Zhou, Y. Lin, W. Wang, K. A. S.
Fernando, P. Pathak, M. J. Meziani, B. A. Harruff, X. Wang, H.
Wang, P. G. Luo, H. Yang, E. K. Muhammet, B. Chen, L. M.
Veca, S. Xie, J. Am. Chem. Soc. 2006, 128, 7756 – 7757; g) P.
Scheiner, J. Org. Chem. 1967, 32, 2022 – 2023.
À1
imaging. C. elegans were incubated at 0.5 mgmL raw N-dot
[
[
6] a) J. Shen, Y. Zhu, X. Yang, C. Li, Chem. Commun. 2012, 48,
solution (N-dots obtained at 1008C), and then were imaged
under an inverted fluorescence microscope. As shown in
Figure 5 f–j, cyan, green, and red fluorescence emission were
clearly visible under different excitation wavelengths.
3686 – 3699; b) P. G. Luo, F. Yang, S.-T. Yang, S. K. Sonkar, L.
Yang, J. J. Broglie, Y. Liu, Y.-P. Sun, RSC Adv. 2014, 4, 10791.
7] a) S. Liu, L. Wang, J. Tian, J. Zhai, Y. Luo, W. Lu, X. Sun, RSC
Adv. 2011, 1, 951; b) S. Liu, J. Tian, L. Wang, Y. Luo, X. Sun, RSC
Adv. 2012, 2, 411.
In summary, we serindipitously synthesized novel N-dots
by using 2-azidoimidazole as a single reactant source under
mild conditions, representing a relatively simple and low-cost
approach to produce water-soluble quantum dots. A charac-
teristic feature of N-dot synthesis is that neither harsh
conditions nor surface passivation is needed. The yield of
raw N-dots was about 33%, which was a relatively high yield
for the production of quantum dots in comparison to that of
carbon dots. By size-exclusive chromatography, we separated
three fractions of quantum dots with increasing amount of
nitrogen composition by elemental analysis (as high as
[
8] H. Zhang, H. Ming, S. Lian, H. Huang, H. Li, L. Zhang, Y. Liu,
Z. Kang, S.-T. Lee, Dalton Trans. 2011, 40, 10822.
[
9] a) L. Cao, X. Wang, M. J. Meziani, F. Lu, F. Wang, P. G. Luo, Y.
Lin, B. A. Harruf, L. M. Veca, D. Murray, S. Xie, Y. Sun, J. Am.
Chem. Soc. 2007, 129, 11318 – 11319; b) S. Qu, X. Wang, Q. Lu,
X. Liu, L. Wang, Angew. Chem. Int. Ed. 2012, 51, 12215 – 12218;
Angew. Chem. 2012, 124, 12381 – 12384; c) S. Zhu, Q. Meng, L.
Wang, J. Zhang, Y. Song, H. Jin, K. Zhang, H. Sun, H. Wang, B.
Yang, Angew. Chem. Int. Ed. 2013, 52, 3953 – 3957; Angew.
Chem. 2013, 125, 4045 – 4049.
[
10] a) C. Yu, X. Li, F. Zeng, F. Zheng, S. Wu, Chem. Commun. 2013,
9, 403 – 405; b) J. M. Liu, L. P. Lin, X. X. Wang, L. Jiao, M. L.
4
3
4.48%), as confirmed by XPS data. All these quantum
Cui, S. L. Jiang, W. L. Cai, L. H. Zhang, Z. Y. Zheng, Analyst
2013, 138, 278 – 283.
dots were then characterized by FTIR, HRTEM, and AFM,
and their optical properties were evaluated in detail. The
photoluminescence behavior can be adjusted from blue to
green simply by the change of reaction temperatures. These
results confirmed that these N-dots were indeed nitrogen-rich
quantum dots and demonstrated optimal and unique optical
properties compared to their neighboring carbon dots, and
have shown promising application as fluorescent inks and
[11] S. K. Bhunia, A. Saha, A. R. Maity, S. C. Ray, N. R. Jana, Sci.
Rep. 2013, 3, 1473.
[
12] a) X. Xu, R. Ray, Y. Gu, H. J. Ploehn, L. Gearheart, K. Raker,
W. A. Scrivens, J. Am. Chem. Soc. 2004, 126, 12736 – 12737; b) H.
Li, Z. Kang, Y. Liu, S.-T. Lee, J. Mater. Chem. 2012, 22, 24230;
c) V. Lopez, G. R. Perez, A. Arregui, E. Mateo-Marti, L.
Banares, J. A. Martın-Gago, J. M. Soler, J. Gomez-Herrero, F.
Zamora, ACS Nano 2009, 3, 3352 – 3357.
Angew. Chem. Int. Ed. 2014, 53, 1 – 7
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
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.
Angewandte
Communications
[
13] a) Y. Xu, M. Wu, Y. Liu, X. Z. Feng, X. B. Yin, X. W. He, Y. K.
Zhang, Chem. Eur. J. 2013, 19, 2276 – 2283; b) C. Hu, Y. Liu, Y.
Yang, J. Cui, Z. Huang, Y. Wang, L. Yang, H. Wang, Y. Xiao, J.
Rong, J. Mater. Chem. B 2013, 1, 39; c) Y.-Q. Zhang, D.-K. Ma,
Y. Zhuang, X. Zhang, W. Chen, L.-L. Hong, Q.-X. Yan, K. Yu, S.-
M. Huang, J. Mater. Chem. 2012, 22, 16714.
14] Y. Dong, H. Pang, H. B. Yang, C. Guo, J. Shao, Y. Chi, C. M. Li,
T. Yu, Angew. Chem. Int. Ed. 2013, 52, 7800 – 7804; Angew.
Chem. 2013, 125, 7954 – 7958.
[16] Y. Yang, D. Wu, S. Han, P. Hu, R. Liu, Chem. Commun. 2013, 49,
4920 – 4922.
[17] X. Jia, J. Li, E. Wang, Nanoscale 2012, 4, 5572 – 5575.
[18] a) H. Li, X. He, Y. Liu, H. Huang, S. Lian, S.-T. Lee, Z. Kang,
Carbon 2011, 49, 605 – 609; b) B. De, N. Karak, RSC Adv. 2013, 3,
8286; c) S. Barman, M. Sadhukhan, J. Mater. Chem. 2012, 22,
21832 – 21937.
[
[19] S. Sahu, B. Behera, T. K. Maiti, S. Mohapatra, Chem. Commun.
2012, 48, 8835 – 8837.
[
15] a) L. Zhou, B. He, J. Huang, Chem. Commun. 2013, 49, 8078 –
[20] Q. Liang, W. Ma, Y. Shi, Z. Li, X. Yang, Carbon 2013, 60, 421 –
428.
8080; b) F. Du, Y. Ming, F. Zeng, C. Yu, S. Wu, Nanotechnology
2013, 24, 1 – 9; c) H. Zhu, X. Wang, Y. Li, Z. Wang, F. Yang, X.
Yang, Chem. Commun. 2009, 5118 – 5120.
6
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 7
These are not the final page numbers!

Angewandte
Chemie
Communications
Quantum Dots
X. Chen, Q. Jin, L. Wu, C. Tung,
X. Tang*
&&&& — &&&&
Synthesis and Unique
A new member of the family: Nitrogen-
rich quantum dots were serendipitously
synthesized at low temperature. These N-
dots contain a high percentage of the
element nitrogen and have unique pho-
toluminescence properties. The photolu-
minescence behavior of N-dot solutions
Photoluminescence Properties of
Nitrogen-Rich Quantum Dots and Their
Applications
can be adjusted from blue to green simply
by variation of reaction temperature.
These N-dots show promising applica-
tions as fluorescent ink and biocompat-
ible staining.
Angew. Chem. Int. Ed. 2014, 53, 1 – 7
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7
These are not the final page numbers!