Green synthesis and potential application of low-toxic Mn ...hysz.nju.edu.cn/jjzhu/userfiles/files/fblw/2010-7-Green synthesis.pdf · Recently, the synthesis of low-toxic quantum

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

1 (a) X. Michalet, F. F. Pinaud, L. A. Bentolila, J. M. Tsay, S. Doose,J. J. Li, G. Sundarensan, A. M. Wu, S. S. Gambhir and S. Weiss,Science, 2005, 307, 538; (b) R. G. Xie, D. Battaglia and X. G. Peng,J. Am. Chem. Soc., 2007, 129, 15432; (c) R. G. Xie and X. G. Peng,Angew. Chem., Int. Ed., 2008, 47, 7677.

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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