Solution reduction synthesis of amine terminated carbon quantum dots

Article abstract of DOI:10.1039/c3ra47770c

Highly luminescent water soluble carbon quantum dots (CQDs) with narrow size distributions have been prepared via a simple room temperature, solution-phase synthesis. The CQDs, stabilised by covalently bound allylamine ligands to minimise surface oxidation, exhibit an excitation wavelength dependent blue luminescence with a quantum yield of 25%.

Full text of DOI:10.1039/c3ra47770c

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Solution reduction synthesis of amine terminated
carbon quantum dots†
Cite this: RSC Adv., 2014, 4, 12094
Keith Linehan and Hugh Doyle*
Received 18th December 2013
Accepted 18th February 2014
DOI: 10.1039/c3ra47770c
www.rsc.org/advances
Highly luminescent water soluble carbon quantum dots (CQDs) with
The use of microemulsion based synthetic methods have
narrow size distributions have been prepared via a simple room been the most promising due to their ability to control the size,
temperature, solution-phase synthesis. The CQDs, stabilised by shape and surface chemistry of the CQDs. Rhee and co-workers
covalently bound allylamine ligands to minimise surface oxidation, reported the solution-phase synthesis of CQDs using reverse
21
exhibit an excitation wavelength dependent blue luminescence with a micelles as nanoscale reactors. The CQDs were formed via
quantum yield of 25%.
condensation polymerisation and subsequent carbonization of
glucose within AOT reverse micelles at 160 C. Control of the
ꢀ
water–surfactant ratio within the micelle allowed the CQD
diameter to be tuned from 1.8 to 4.1 nm, but with increasing
polydispersity at larger diameters. They later reported a similar
hydrothermal synthesis of nearly monodisperse (1.4 Æ 0.15 nm)
Carbon quantum dots (CQDs) have attracted great interest in
recent years as a new class of uorescence nanomaterial with
attractive photophysical properties, high quantum yields, low
1–3
toxicity and biocompatibility.
Compared to conventional
22
CQDs using octanol as the surfactant. In both cases, subse-
organic dyes and semiconductor nanocrystals, CQDs possess
several advantages in terms of chemical inertness, easy func-
tionalisation, high resistance to photobleaching, an absence of
quent in situ surface passivation resulted in formation of
hexadecylamide-capped CQDs. More recently, Gao et al. repor-
ted on the hydrothermal synthesis of CQDs by oxidation of C60
by hydrogen peroxide under alkaline conditions within CTAB

uorescence intermittency and the potential for low cost
production. As a result, much attention has been paid to their
potential application in areas from biological labelling and
imaging to uorescence nanosensors and optoelectronic
ꢀ
23
reverse micelles at 150 C.
Previously, we reported the synthesis of size monodisperse
carbon quantum dots using a room temperature microemulsion
2,4,5
devices.
As CQDs may be prepared using cheap precursor
24
strategy. The surfaces of the CQDs are terminated with a
materials, moderate reaction conditions and relatively simple
equipment, numerous synthetic approaches have been reported
in the literature, which may be broadly divided into two main
covalently attached alkyl monolayer, rendering the resulting
hydrophobic quantum dots dispersible in a wide range of non-
polar solvents. However, for carbon quantum dots to be used
effectively in many biological applications, it is essential that
they are water-soluble and stable against aggregation and
precipitation within a biological system, possess a high photo-
luminescence quantum yield in the visible region and exhibit
excellent photostability under typical illumination conditions.
In this communication, we report the room temperature
synthesis of highly monodisperse, amine terminated CQDs that
form stable aqueous dispersions and exhibit strong visible
emission. The CQDs are synthesized in reverse micelles via the
reduction of carbon tetrachloride using a hydride reducing
agent; see the ESI† for further synthetic details. The hydrogen-
terminated CQDs are functionalised using a platinum-catalysed
6
categories. Physical methods include arc discharge, laser
7
,8
9
ablation/passivation
and plasma treatment. Chemical
10–12
methods include electrochemical synthesis,
combustion
hydrothermal and pyrolysis routes,
1
3,14
15
and acidic oxidation,
supported
16–18
synthesis
and
microwave/ultrasonic
19,20
synthesis.
However, all these methods possess some draw-
backs including, e.g. extensive post-synthetic purication, lack
of control of CQD surface chemistry and sample polydispersity.
In addition, many of these methods also generate relatively
broad size distributions, necessitating tedious size separation
1
processes to obtain monodisperse CQDs.
Tyndall National Institute, University College Cork, Lee Maltings, Cork, Ireland. concerted reaction to covalently attach allylamine ligands to the
E-mail: hugh.doyle@tyndall.ie; Tel: +353-21-2346300
surface, chemically passivating the surface and rendering the
CQDs dispersible in polar solvents, see Scheme 1.
†
Electronic supplementary information (ESI) available: Synthetic method,
characterisation and additional gures. See DOI: 10.1039/c3ra47770c
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Scheme 1 Chemical passivation and functionalisation of the CQDs
using a platinum-catalysed concerted reaction. CQDs are synthesized
by reduction of CCl within TOAB reverse micelles in toluene.
4
Fig. 1(a) shows a transmission electron microscope (TEM)
image of the as-synthesized CQDs. The CQDs are highly size and Fig.
2
FTIR spectra of neat allylamine, allylamine-capped and
shape monodisperse, see Fig. 1(a) and S1 and S2 of the ESI.† uncapped CQDs.
Fig. 1(b) shows a high resolution TEM image of a single CQD,
illustrating that the nanocrystals are highly crystalline and form
À1
a single contiguous crystalline phase. The lattice fringes shown 1640 and 3080 cm for the capped CQDs, which is observed in
the spectrum of neat allylamine, is consistent with successful
100) spacing reported for graphitic carbon. Fig. 1(c) shows a binding of the allylamine ligand to the CQD surface, as previ-
˚
in Fig. 1(b) correspond to a d spacing of 2.1 A, matching the
(
2
4
histogram of the CQD diameters, determined by analysis of ously reported. In contrast, the FTIR spectrum of the uncap-
TEM images of ca. 200 CQDs located at different locations on ped CQDs shows no evidence of peaks that may be assigned to
the grid. The mean diameter of the CQDs is 1.8 Æ 0.3 nm, based amine groups. The presence of peaks between 3000 and 2850
À1
on tting the histogram to a Gaussian model.
cm are assigned to C–H stretching modes at the CQD surface,
À1
The surface chemistry of the quantum dots was character- while the additional peaks below 1300 cm are due to the
ized by infrared spectroscopy; Fig. 2 shows the FTIR spectra of presence of oxygenic species, indicating signicant surface
the allylamine ligand, together with allylamine-capped and oxidation.
uncapped CQDs. Spectra of allylamine-capped CQDs and the
The optical properties of the CQDs were studied using UV-vis
absorption, photoluminescence (PL), photoluminescence exci-
À1
allylamine ligand show peaks from ca. 3700–3500 cm
,
assigned to the N–H stretching of the amine, while the peaks tation (PLE) and quantum yield measurements. The allylamine
À1
observed between 3000 and 2850 cm are attributed to C–H capped CQDs exhibit a strong absorption in the UV region, with
À1
stretching modes. The large feature centred near 1670 cm for a shoulder at ca. 320 nm and a tail extending into the visible
the allylamine-capped CQDs is consistent with amine N–H range. The PL spectra were recorded using excitation wave-
deformation modes, but also with stretching modes of carbox- lengths ranging from 300 to 400 nm in 20 nm increments. The
ylate species caused by surface oxidation. The peaks observed carbon quantum dots exhibit a primarily blue luminescence,
À1
between 1500 and 1400 cm are attributed to C–C bending clearly dependent on the excitation wavelength used, with the
modes. The absence of the characteristic CH ] CH peaks at wavelength position of the PL maximum red-shiing from
2
441 nm to 465 nm, as the excitation wavelength is increased
from 300 nm to 400 nm. This is in good agreement with liter-
7,16
ature reports for CQDs dispersed in water.
Photoluminescence from CQDs still remains a widely
debated topic despite the extensive research carried out in
recent years. A number of different interpretations explaining
1,3
the origin of PL in CQDs have been proposed. Sun et al.
suggested that the PL is due to the presence of energetic trap
7
states near the CQD surface. In contrast, Zhao et al. attributed
the dependency on excitation wavelength to size differences
10
rather than emissive trap states. Although the exact mecha-
nism still remains unresolved, evidence exists that both mech-
anisms can contribute to both CQD emission colour and
intensity. Given the high degree of size monodispersity in the
CQDs reported here, the most plausible explanation is that the
blue luminescence originates from recombination of photo-
generated excitons at shallow surface trap states. PLE spectra
recorded at the PL intensity maximum (423 nm, see Fig. 3(a))
Fig. 1 (a) Representative TEM image of the carbon quantum dots. (b)
High-resolution TEM image of an individual CQD. (c) Size histogram of
the CQDs with curve fitted to the data using a Gaussian model.
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24
alkyl-terminated surfaces, which exhibit a monotonically
decreasing luminescence intensity at increasing excitation
wavelengths, see Fig. S3† for a comparison of the excitation
wavelength dependence of amine- and alkyl-terminated CQDs.
This underlines the importance of the surface in determining
the photophysical properties of CQDs, which has been widely
observed for CQDs prepared using different preparation
1
–3
methods and surface functionalities. Carbon quantum dots
capped with either ligand show the same red-shi in the posi-
tion of the PL maximum at increasing excitation wavelengths.
The CQDs showed similar emission characteristics when
dispersed in different polar solvent media, see Fig. S4.† PL
spectra of allylamine-terminated CQDs recorded in different pH
environments (see Fig. S5†) showed uctuations in the PL
intensity, but no overall trend was observed.
Photoluminescence quantum yield of the allylamine capped
carbon quantum dots in water were determined using the
comparative method by Williams et al., see ESI† for further
details. Fig. 3(b) shows the integrated PL intensity of the amine
terminated CQDs compared to the 9,10-diphenylanthracene
emission standard used; the quantum yield (QY) was deter-
mined to be ca. 25% at an excitation wavelength of 320 nm. This
QY value is comparable to values obtained for CQDs reported in
the literature. The CQDs also exhibit excellent long-term PL
stability, decreasing by less than 3.5% aer continuous illumi-
nation for 12 hours; see Fig. 3(c).
In conclusion, a simple, room temperature synthetic route
for production of size monodisperse CQDs has been demon-
strated, with close control of internal structure and surface
chemistry. The surfaces of the CQDs are chemically function-
alised with allylamine, producing the hydrophilic CQDs that are
readily dispersible in water and polar organic solvents. The
CQDs possess an excitation wavelength dependent emission in
the visible, and exhibit excellent photostability and high (25%)
quantum yields, making them attractive candidates for biolog-
ical labelling and uorescence sensing applications.
Acknowledgements
This work was supported by the European Commission under
the FP7 Projects SNAPSUN (grant agreement no. 246310) and
CommonSense (grant agreement no. 2618309) and the Irish
Higher Education Authority under the PRTLI program (Cycle 3
Fig. 3 (a) UV-vis absorption, PLE and PL spectra of an aqueous
dispersion of the CQDs recorded at different excitation wavelengths.
“Nanoscience” and Cycle 4 “INSPIRE”).
(
b) Integrated PL intensity versus absorbance for an aqueous CQD
dispersion and a solution of 9,10-diphenylanthracene in cyclohexane.
c) Long term PL stability recorded over 12 hours. Inset: optical images
Notes and references
(
of CQD dispersions under white (left) and UV (365 nm; right)
illumination.
1
H. Li, Z. Kang, Y. Liu and S. T. Lee, J. Mater. Chem., 2012, 22,
4230–24253.
2
2
J. C. G. Esteves da Silva and H. M. R. Gonçalves, TrAC, Trends
Anal. Chem., 2011, 30, 1327–1336.
show a narrow peak centred at ca. 347 nm, considerably above
the band gap energy of the CQDs, suggesting radiationless
transfer and recombination at states close to the CQD surface.
Interestingly, the relative luminescence intensity of the amine-
terminated CQDs at different excitation wavelengths shows a
maximum at 340 nm, in contrast to similarly sized CQDs with
3 S. N. Baker and G. A. Baker, Angew. Chem., Int. Ed., 2010, 49,
6726–6744.
4 W. Kwon, S. Do, D. C. Won and S.-W. Rhee, ACS Appl. Mater.
Interfaces, 2013, 5, 822–827.
5 F. Wang, Y.-H. Chen, C.-Y. Liu and D.-G. Ma, Chem.
Commun., 2011, 47, 3502–3504.
12096 | RSC Adv., 2014, 4, 12094–12097
This journal is © The Royal Society of Chemistry 2014

View Article Online
Communication
RSC Advances
6
7
X. Xu, R. Ray, Y. Gu, H. J. Ploehn, L. Gearheart, K. Raker and 15 D. Pan, J. Zhang, Z. Li, C. Wu, X. Yan and M. Wu, Chem.
W. A. Scrivens, J. Am. Chem. Soc., 2004, 126, 12736–12737. Commun., 2010, 46, 3681–3683.
Y. P. Sun, B. Zhou, Y. Lin, W. Wang, K. A. S. Fernando, 16 R. Liu, D. Wu, S. Liu, K. Koynov, W. Knoll and Q. Li, Angew.
P. Pathak, M. J. Meziani, B. A. Harruff, X. Wang, H. Wang, Chem., Int. Ed., 2009, 121, 4668–4671.
P. G. Luo, H. Yang, M. E. Kose, B. Chen, L. M. Veca and 17 J. Zong, Y. Zhu, X. Yang, J. Shen and C. Li, Chem. Commun.,
S. Y. Xie, J. Am. Chem. Soc., 2006, 128, 7756–7757. 2011, 47, 764–766.
H. Goncalves and J. C. G. Esteves da Silva, J. Fluoresc., 2010, 18 A. B. Bourlinos, A. Stassinopoulos, D. Anglos, R. Zboril,
8
9
2
0, 1023–1028.
H. Jiang, F. Chen, M. G. Lagally and F. S. Denes, Langmuir,
009, 26, 1991–1995.
V. Georgakilas and E. P. Giannelis, Chem. Mater., 2008, 20,
4539–4541.
2
19 H. Zhu, X. Wang, Y. Li, Z. Wang, F. Yang and X. Yang, Chem.
Commun., 2009, 5118–5120.
1
1
1
1
1
0 Q. L. Zhao, Z. L. Zhang, B. H. Huang, J. Peng, M. Zhang and
D. W. Pang, Chem. Commun., 2008, 5116–5118.
1 L. Zheng, Y. Chi, Y. Dong, J. Lin and B. Wang, J. Am. Chem.
Soc., 2009, 131, 4564–4565.
2 J. Zhou, C. Booker, R. Li, X. Zhou, T. K. Sham, X. Sun and
Z. Ding, J. Am. Chem. Soc., 2007, 129, 744–745.
3 H. Liu, T. Ye and C. Mao, Angew. Chem., Int. Ed., 2007, 119,
20 X. Wang, K. Qu, B. Xu, J. Ren and X. Qu, J. Mater. Chem.,
2011, 21, 2445–2450.
21 W. Kwon and S. W. Rhee, Chem. Commun., 2012, 48, 5256–
5258.
22 W. Kwon, S. Do and S.-W. Rhee, RSC Adv., 2012, 2, 11223–
11226.
23 M. X. Gao, C. F. Liu, Z. L. Wu, Q. L. Zeng, X. X. Yang,
W. B. Wu, Y. F. Li and C. Z. Huang, Chem. Commun., 2013,
49, 8015–8017.
6593–6595.
4 X. Wang, L. Cao, F. Lu, M. J. Meziani, H. Li, G. Qi, B. Zhou,
B. A. Harruff, F. Kermarrec and Y. P. Sun, Chem. Commun.,
2009, 3774–3776.
24 K. Linehan and H. Doyle, RSC Adv., 2014, 4, 18–21.
This journal is © The Royal Society of Chemistry 2014
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