Green synthesis and potential application of low-toxic Mn...
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Green synthesis and potential application of low-toxic Mn :ZnSe/ZnS
core/shell luminescent nanocrystalsw
Dong Zhu,ab Xiaoxing Jiang,a Cuie Zhao,a Xiaolian Sun,a Jianrong Zhang*a and
Jun-Jie Zhu*a
Received 6th April 2010, Accepted 3rd June 2010
First published as an Advance Article on the web 23rd June 2010
DOI: 10.1039/c0cc00791a
A microwave-assisted synthetic procedure is presented for the
preparation of low-toxic Mn :ZnSe/ZnS core/shell nanocrystals
to label antibodies for selective detection of human immunoglobulin
G (IgG) based on fluorescence resonance energy transfer
(FRET) between the Mn :ZnSe/ZnS and Au nanoparticles
(AuNPs).
Recently, the synthesis of low-toxic quantum dots (QDs) and
their application in biomedicine have attracted considerable
attention.1 ZnSe nanocrystals doped with Mn ions which do
not contain any Class A element (Cd, Pb, and Hg) can be used
as a new generation of luminescent nanocrystals due to their
strong dopant emission.2 In 2001, Norris et al.3 presented an
organometallic synthetic route for the preparation of Mn-doped
ZnSe (Mn :ZnSe) nanocrystals and it was confirmed that the
Mn impurities were embedded inside the nanocrystals. Peng
et al.4 introduced nucleation-doping and growth-doping, two
new synthetic strategies in high-temperature organometallic
synthesis. Pure and strong dopant emission was observed due
to the Mn2+ 4T1(4G) - 6A1(
6S) transition. The Mn : ZnSe
nanocrystals prepared by the organometallic method had high
quantum yield (QY), high crystallinity and monodispersity.
However, some organic reagents used in this procedure are
environmentally unfriendly, and long reaction time and
limited operation conditions were also necessary. Recently,
an inorganic shell material with a wider band gap was used to
passivate a cadmium chalcogenides quantum dot to reduce the
bio-toxicity and improve the quantum yield.5 ZnS is a suitable
shell material with a wide band gap (3.67 eV for bulk ZnS6a)
for the formation of the core/shell nanostructure. To the best
of our knowledge, no report has been published on the
synthesis and optical properties of Mn : ZnSe/ZnS core/shell
nanocrystals. Herein, a green and rapid route for the synthesis
of low-toxic Mn :ZnSe/ZnS core/shell nanocrystals in the
aqueous phase is presented. A sensing system for the detection
of human IgG is established based on the FRET between the
Mn :ZnSe/ZnS core/shell nanocrystals and AuNPs.
Fig. 1 depicts the synthetic route for the Mn :ZnSe/ZnS
core/shell nanocrystals. The Mn :ZnSe core nanocrystals with
oleate capping ligands were firstly prepared via a microwave-
assisted hydrothermal reaction for 40 min, and then the core
nanocrystals reacted with mercaptopropionic acid (MPA).
Zn2+ ion is inclined to be a soft Lewis acid, and the RCOO�
group of oleate ligand is a hard Lewis base, while the RSH
group in MPA is a soft Lewis base. The RSH groups prefer to
bind to the Zn2+ ions compared with RCOO� because hard
acids tend to associate with hard bases and soft acids with soft
bases.6b So the surface ligands replacement of oleate capping
ligands by MPA succeeded in the procedure, and the polar
carboxylic groups renderd the nanocrystals water-soluble. An
additional ZnS shell was deposited on the outer layer of the
Mn :ZnSe to form the core/shell nanostructure. The detailed
experiment for the preparation is elaborated in the Electronic
Supplementary Information (ESI).wIn a traditional aqueous synthesis, the growth rate of ZnSe
QDs with MPA capping ligands was very slow by refluxing at
100 1C.7a,c However, microwave irradiation was fast and
highly efficient for transferring energy into the reaction system
and the temperature increased uniformly throughout the
reactants.7b In our microwave irradiation reaction, high
temperature (170 1C) to the advantage of doping Mn into
the ZnSe nanocrystals lattice could be easily obtained in 5 min,
and a fast and homogeneous nucleation process could be
achieved, which improves the crystallinity of the Mn :ZnSe
nanocrystals. The selection for using oleate capping ligands is
appropriate at the high temperature, and conventional MPA
can be partially decomposed at the temperature. In the
procedure, the time required to attain good crystallinity and
uniform size (about 5 nm) of Mn : ZnSe core nanocrystals was
within one hour.
Fig. 1 Schematic illustration for the synthesis of the Mn :ZnSe/ZnS
core/shell nanocrystals.
a Key Laboratory of Analytical Chemistry for Life Science (Ministryof Education of China), School of Chemistry and ChemicalEngineering, Nanjing University, Nanjing 210093, P. R. China.E-mail: [email protected], [email protected];Fax: +86 25 83594976; Tel: +86 25 83686130
b Jiangsu Institute of Metrology, P. R. Chinaw Electronic supplementary information (ESI) available: Chemicals,apparatus and experimental details. See DOI: 10.1039/c0cc00791a
5226 | Chem. Commun., 2010, 46, 5226–5228 This journal is �c The Royal Society of Chemistry 2010
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The crystallinity of the Mn :ZnSe core nanocrystals was
demonstrated by powder X-ray diffraction (XRD) as shown in
Fig. 2(A). All of the XRD peaks of the Mn :ZnSe core
nanocrystals could be indexed as the cubic zinc blende
structure, which was consistent with the values in the standard
card of ZnSe. The incorporation of Mn2+ ions into the ZnSe
nanocrystal lattice was confirmed by electron paramagnetic
resonance (EPR) spectroscopy as shown in Fig. 2(B). Since the
hyperfine splitting is strongly dependent on the local environment,
EPR is a good technique for determining the locality and
distribution of Mn2+ ions in nanoparticles. A six line
spectrum owing to the hyperfine interaction with the 55Mn
nuclear spin (I = 5/2) is shown in Fig. 2(B). We extracted the
hyperfine splitting constant of 62.6 � 10�4 cm�1 from
Fig. 2(B). The value of the hyperfine splitting constant for
the Mn substituted at Zn sites in the cubic ZnSe lattice is
61.7 � 10�4 cm�1.3 The current experimental value was close
to this value, which indicated that the Mn was substitutionally
incorporated into the ZnSe host lattice. The incorporated
Mn2+ concentrations in the Mn :ZnSe core nanocrystals
can be determined by inductively coupled plasma atomic
emission spectroscopy (ICP-AES). The obtained Mn :Zn
molar ratio in the Mn :ZnSe core nanocrystals was 0.74%.
The morphology of the Mn : ZnSe core nanocrystals was
characterized with high resolution transmission electron
microscopy (HRTEM). As demonstrated in Fig. 2(C), the
nanocrystals showed high crystallinity and monodispersity.
The average size of the Mn :ZnSe core nanocrystals was
about 5.0 nm. The HRTEM image of one individual
Mn :ZnSe core nanocrystal indicated the distances between
the adjacent lattice fringes to be 0.33 nm, corresponding with
the literature value for the (111) d spacing, 0.324 nm (JCPDF
No. 800021).
The diffraction peak shift from cubic ZnSe to cubic ZnS
phase (Fig. 2(A)) was due to the proposed ZnS shell around
the outer layer of the Mn : ZnSe core nanocrystals. As shown
in Fig. 2(D), the average size of the Mn :ZnSe/ZnS core/shell
nanocrystals was 6.0 nm, larger than that of the core nano-
crystals. The difference means that the shell thickness is
around 1.0 nm. The HRTEM image of the Mn :ZnSe/ZnS
core/shell nanocrystal showed interplanar distances of 0.33 nm,
agreeing well with those of 0.33 nm for the Mn :ZnSe core
nanocrystal. However, because the core and the shell have
similar electron densities and lattice parameters, the image
contrast cannot be used to distinguish the shell and the core.
The X-ray photoelectron spectra (XPS) results confirmed the
proposed Mn :ZnSe/ZnS core/shell structure. As shown in
Fig. 2(E), the coordination of Zn–S in the Mn :ZnSe/ZnS
core/shell nanocrystals is different from that of Zn–SR (i.e.,
thiols) in the Mn :ZnSe core nanocrystals. Therefore, the
binding energy assigned to S 2p shifted from 164 eV for the
Mn :ZnSe to 162 eV for the Mn :ZnSe/ZnS nanocrystals,
which verified the proposed core/shell structure and was
consistent with previous reports.8 The molar ratio of S/Se
increased remarkably from 1.1 : 1 in the Mn :ZnSe core nano-
crystals to 10 : 1 in the Mn : ZnSe core/shell nanocrystals,
which further confirmed the growth of the ZnS shell.
As shown in Fig. 2(F), a small peak at 430 nm was
attributed to the formation of an exciton of the intrinsic ZnSe
nanocrystals. The subtle red shift in the absorption spectra of
the Mn :ZnSe/ZnS core/shell nanocrystals revealed the formation
of a bigger particle size with the growth of the shell of ZnS
around the Mn :ZnSe core. The inset of Fig. 2(F) shows
corresponding photoluminescence (PL) spectra of the nano-
crystals. The PL spectra exhibited two peaks, a strong peak
around 585 nm from an internal electronic transition of the
Mn (4T1-6A1), and a weak blue peak around 450 nm from
exciton recombination in the ZnSe. When an ZnSe host
nanocrystal was excited by photons with energy higher than
its band gap, an exciton (an electron–hole pair) could be
generated. The direct recombination of the electron–hole pair
causing the semiconductor nanocrystals to emit photons,
typically being quantum confined in the case of nanocrystals,
gave the well-known band edge emission. However, the orange
emission around 585 nm of the Mn-doped ZnSe nanocrystals
was fundamentally different. The energy of a photogenerated
electron–hole pair could be transferred into the electronic d–d
levels of the Mn2+ ions. The internal electronic transition of
Fig. 2 (A) XRD patterns of the Mn :ZnSe core nanocrystals (a) and
the Mn :ZnSe/ZnS core/shell nanocrystals (b). Diffraction lines for
cubic phases of bulk ZnSe and ZnS are shown for guidance. (B) EPR
spectrum of the as-prepared Mn :ZnSe core nanocrystals. (C)
HRTEM image of the Mn :ZnSe core nanocrystals. (D) HRTEM
image of the Mn :ZnSe/ZnS core/shell nanocrystals. Top inset: the size
distribution histogram. Bottom inset: HRTEM image of one individual
nanocrystal. (E) XPS spectra of the Mn :ZnSe core and the
Mn :ZnSe/ZnS core/shell nanocrystals. (F) UV-Vis spectra of the
Mn :ZnSe core (a) and Mn :ZnSe/ZnS core/shell nanocrystals (b).
Inset: the corresponding PL spectra (lex = 350 nm).
This journal is �c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 5226–5228 | 5227
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the Mn (4T1-6A1) led to the characteristic dopant emission
around 585 nm.4
Obviously, the Mn :ZnSe/ZnS core/shell nanocrystals
exhibited increased PL intensity compared with the Mn : ZnSe
core nanocrystals, and the PL quantum yield (QY) of the
Mn :ZnSe/ZnS core/shell nanocrystals was up to 25%.
Because a large number of surface defects exist in a nanoparticle
surface, nonradiative recombination paths could be generated
for the excitation energy in the Mn :ZnSe core nanocrystals; as
a result, quenching of the emission of Mn2+ takes place to
some extent. By the growth of an additional ZnS shell on the
Mn :ZnSe nanocrystals, the surface defects could be greatly
reduced; thus, emission intensity of the Mn :ZnSe/ZnS core/
shell nanocrystals could be enhanced greatly. Furthermore, an
additional ZnS shell could make the dopant Mn2+ as far as
possible from the surface of the nanocrystals, which resulted in
emission centers away from the surface trap states of the
nanocrystals, and thereby improved the optical performance
of the nanocrystals.
To explore the application of biomedical labeling for the
Mn :ZnSe/ZnS core/shell nanocrystals, a preliminary test was
carried out. A sensing system was fabricated for the detection
of human IgG based on FRET between the Mn :ZnSe/ZnS
core/shell nanocrystals and the AuNPs as shown in Fig. 3. The
Mn :ZnSe/ZnS linked with goat anti-human IgG (Mn :ZnSe/
ZnS-Ab1) acted as fluorescence donors. The AuNPs linked
with rat anti-human IgG (AuNPs-Ab2) acted as acceptors,
mostly because of their exceptional quenching ability.9
Then FRET occurred through the conjugation between the
Mn :ZnSe/ZnS-Ab1 and the AuNPs-Ab2 in the presence of
human IgG. The detailed experiment is elaborated in ESI.wThe calibration graph for human IgG is linear over the range
of 0.2–3.2 mM (Fig. 4). The FRET behaviors confirmed the
accessibility of biolabel for the Mn :ZnSe/ZnS core/shell
nanocrystals and the potential biosensing application.
In summary, the low-toxic luminescent Mn :ZnSe/ZnS core/
shell nanocrystals were successfully synthesized by a green and
rapid route. The obtained Mn :ZnSe/ZnS core/shell nano-
crystals have good crystallizability and favorable monodispersity.
The emission intensity of the Mn :ZnSe/ZnS core/shell nano-
crystals was considerably increased compared with the bare
core materials. The nanocrystals show promise for application
in sensitive biosensing and cell imaging.
We greatly appreciate the support of the National Natural
Science Foundation of China for the Key Program
(20635020), the Creative Research Group (20821063), and
General Program (Nos. 20975048, 50972058). This work is
also supported by the National Basic Research Program of
China (2006CB933201).
Notes and references
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2 D. J. Norris, A. L. Efros and S. C. Erwin, Science, 2008, 319, 1776.3 D. J. Norris, N. Yao, F. T. Charnock and T. A. Kennedy, NanoLett., 2001, 1, 3–7.
4 (a) N. Pradhan, D. Goorskey, J. Thessing and X. G. Peng, J. Am.Chem. Soc., 2005, 127, 17586–17587; (b) P. Narayan andX. G. Peng, J. Am. Chem. Soc., 2007, 129, 3339–3347;(c) N. Pradhan, M. David, Battaglia, Y. C. Liu and X. G. Peng,Nano Lett., 2007, 7, 312–317.
5 (a) X. G. Peng, M. C. Schlamp, A. V. Kadavanich andA. P. Alivisatos, J. Am. Chem. Soc., 1997, 119, 7019;(b) H. B. Bao, Y. J. Gong, Z. Li and M. Y. Gao, Chem. Mater.,2004, 16, 3853.
6 (a) S. Sapra, A. Prakash, A. Ghangrekar, N. Periasamy andD. D. Sarma, J. Phys. Chem. B, 2005, 109, 1663; (b) A. Vogel, inTextbook of Quantitative Chemical Analysis, ed. G. H. Jeffery,J. Bassett, J. Mendham and R. C. Denney, Longman Scientific &Technical, London, 5th edn, 1989, ch. 2, pp. 53–54.
7 (a) C. Wang, X. Gao, Q. Ma and X. G. Su, J. Mater. Chem., 2009,19, 7016; (b) H. F. Qian, X. Q. L. Li and J. C. Ren, J. Phys. Chem.B, 2006, 110, 9034–9040; (c) A. Shavel, N. Gaponik andA. Eychmiiller, J. Phys. Chem. B, 2004, 108, 5905–5908.
8 Y. He, H. T. Lu, L. M. Sai, Y. Y. Su, M. Hu, C. H. Fan, W. Huangand L. H. Wang, Adv. Mater., 2008, 20, 3416–3421.
9 T. Pons, I. L. Medintz, K. E. Sapsford, S. Higashiya, A. F. Grimes,D. S. English and H. Mattoussi, Nano Lett., 2007, 7, 3157–3164.
Fig. 3 (A) Schematic illustration of antibody immobilized on the
surface of the Mn :ZnSe/ZnS core/shell nanocrystals with EDC/NHS.
(B) Schematic illustration of the FRET system between the Mn : ZnSe/
ZnS core/shell nanocrystals and AuNPs.
Fig. 4 FRET-based sensing of human IgG. Relative PL intensity
(P/P0) of Mn :ZnSe/ZnS-Ab1 in the presence of AuNPs-Ab2 with
different concentrations of human IgG. Inset: fluorescence spectra of
Mn :ZnSe/ZnS-Ab1 in the system.
5228 | Chem. Commun., 2010, 46, 5226–5228 This journal is �c The Royal Society of Chemistry 2010
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