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A facile and highly sensitive probe for Hg(II) based on metal-inducedaggregation of ZnSe/ZnS quantum dots

Jun Ke,ab Xinyong Li,*ab Yong Shi,a Qidong Zhaoa and Xuchuan Jiang*b

Received 6th March 2012, Accepted 5th June 2012

DOI: 10.1039/c2nr31238g

Sensitive and selective detection strategies for toxic heavy metal ions, which are rapid, cheap and

applicable to environmental and biological fields, are of significant importance. As a result of specific

interaction between thiol(s) used as ligands and heavy metal ions, the photoluminescence intensity of

quantum dots (QDs) in PBS buffer solution was quenched and the aggregation of QDs was formed at

the same time. Herein, we present water-soluble, low toxic QDs, ZnSe/ZnS, which were applied for

ultrasensitive Hg2+ ion detection with a low detection limit (2.5 nM). In addition, a model has been

proposed to explain the aggregation of QDs in the presence of heavy metal ions such as Hg2+ ions.

Introduction

Heavy metal ions, particularly Hg2+ ions, are one of the most

serious problems to human health due to their high toxicity,

mobility, and ability of accumulation in ecological systems.1 To

date, a variety of chemosensors and biosensors have been

developed to detect heavy metal ions.2–5 The fluorescent probe

based on semiconductor quantum dots (QDs), has received

growing attention in many fields recently. The unique photonic

and electronic properties of QDs offer many opportunities for

detecting various specific analytes, such as heavy metals, DNA,

proteins and chiral organic substances.6–8 However, the aggre-

gation of quantum dots always influence and reduce photo-

luminescence intensity due to self-absorption effect and/or

irradiative mechanism change.9,10 Metal ions are often chosen as

a linker between two nanoparticles or fluorescent chromophore

functional groups by coordination bond or metal-sulfide bond

and thus would result in drastic fluorescent quenching or fluo-

rescent resonance energy transfer.11,12 On the other hand, such a

quenching method could be used as an effective approach to

design a ‘‘turn-off’’ chemosensor. In addition, the main concern

about the use of fluorophores based on quantum dots in bio-

logical applications is represented by their inherent toxicity, as

most of them are composed of elements (e.g., Hg, Cd, Te, In, As,

and Pb) which are known to be highly poisonous for living

organisms.13–16 Zinc chalcogenide, a class of inorganic semi-

conductor materials composed of low cytotoxic elements, is a

good choice as cadmium-free QDs substituting for CdE (E ¼ S,

aState Key Laboratory of Fine Chemical, Key Laboratory of IndustrialEcology and Environmental Engineering (MOE), School ofEnvironmental Science and Technology, Dalian University ofTechnology, Dalian, 116024, China. E-mail: xyli@dlut.edu.cnbSchool of Materials Science and Engineering, University of New SouthWales, Sydney, NSW 2052, Australia. E-mail: xcjiang@unsw.edu.au

4996 | Nanoscale, 2012, 4, 4996–5001

Se, Te) QDs to assemble chemosensors in the environmental

analyst field.17–19

On the basis of the above strategy, in this study, we utilize a

mercaptopropionic acid (MPA)-coated ZnSe/ZnS core–shell

QDs to selectively detect Hg2+ ions in aqueous solution. The

possible mechanism of the aggregation of QDs in the presence of

heavy metal ions was proposed and proved by using a FT-IR

spectrum, which revealed the changes of the surface properties

and the interaction between QDs and Hg2+ ions. Especially, the

effects of ZnS shell on the sensing system are investigated by

comparing the performances of two different QDs with and

without ZnS shell in Hg2+ ion detection. Our experimental

observations confirmed that the ZnS shell could to some extent

not only protect the ZnSe core from the interference of

the external environment, but also improve the selectivity for

Hg2+ ions.

Experimental section

Materials

All reagents were analytical grade and used without further

purification. Selenium powder (Se) and mercaptopropionic acid

(MPA) were purchased from Acros Organics. Sodium borohy-

dride (NaBH4, 96+ %), oleic acid (OA), sodium oleate, zinc

acetate (Zn(Ac)2) and sodium sulfide nonahydrate (Na2S$9H2O,

99%) were purchased from Sinopharm Chemical Reagent

(Shanghai, China). All other reagents were of analytical grade

and used without further treatment.

Synthesis of oleic acid-coated ZnSe nanocrystals

A sodium hydrogen selenide solution (NaHSe) was prepared as

follows: an appropriate amount of 19.72 mg Se powder

(0.25 mmol) was mixed with 65 mg NaBH4 powder and 5 mL

deionized water in a 50 mL round-bottom flask under stirring

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and N2 flow. The mixture was stirred in an ice water bath for

around 1 h. Oleic acid-coated ZnSe NCs were prepared via a

modified liquid–solid–solution strategy.20 Briefly, 1.2179 g of

sodium oleate and 7 mL of oleic acid were dissolved in a mixture

solution of 5 mL of deionized water and 20 mL of ethanol to

form a clear solution. 0.1098 g zinc acetate (0.5 mmol) dissolved

in 5 mL of deionized water were added to this solution by stir-

ring. 5 mL of the fresh as-prepared NaHSe solution was injected

into the above mixture. After stirring with agitation for 10 min,

the solution was transferred into a 50 mL Teflon�-lined stainless

steel autoclave. The autoclave was sealed and heated at 125 �Cfor 6 h. Then the system was cooled down to room temperature

in air. Oleic acid-coated ZnSe QDs were centrifuged from the

crude solution and dispersed in 40 mL cyclohexane to form a

colorless clear solution for further use.

Synthesis of water soluble MPA-capped ZnSe QDs

Oil-soluble QDs were made water-soluble using Peng’s method.21

10 mL of the purified oleic acid-coated ZnSe QDs in a trans-

parent cyclohexane solution was injected in a 20 mL glass bottle

and excess MPA were added to form a cloudy solution. The

mixture was then shaken with ultrasonication for 30 min. The

MPA-capped ZnSe QDs were flocculated and separated out

from the free MPA. Repeated centrifugations in cyclohexane (at

least three times) were used to remove excess MPA from the

mixture. Finally, an appropriate amount of 2 M NaOH aqueous

solution or phosphate buffered saline (PBS, 10 mM, pH 7.4) was

added to the precipitate until the QDs were completely dissolved

into solution.

Synthesis of water soluble MPA-capped ZnSe/ZnS core–shell

QDs

Highly luminescent ZnSe/ZnS core–shell QDs were prepared

through successive ion layer adsorption and reaction (SILAR).22

The as-prepared water-soluble MPA-capped ZnSe QDs colloidal

solution was diluted to 30 mL with deionized water in three

flasks. And then a 10 mL fresh aqueous solution containing zinc

acetate and MPA was added into the colloidal solution under N2

flow by stirring at room temperature. The pH of the solution was

adjusted to 11 with 2 M NaOH solution. The amount of zinc

acetate and MPA (the molar ratio of Zn/MPA ¼ 1 : 5) depends

on the desirable shell thickness of ZnS and the volume of ZnSe

core. The principle of the calculation is the volume ratio between

the core and shell volumes using bulk lattice parameters of zinc

blende ZnS according to the previous report.20 After 30 min

stirring, 10 mL Na2S aqueous solution was added into the

solution (molar ratio of Zn/S¼ 1 : 1) at a rate of 1 mLmin�1. The

mixture was refluxed at 100 �C for 3 h under stirring, and then

ZnSe/ZnS core–shell QDs were obtained. The reaction was

terminated by cooling the reaction mixture down to room

temperature. The obtained MPA-capped ZnSe/ZnS core–shell

QDs was purified via the standard two-step centrifugation

process with the addition of acetone and ethanol to remove the

unreacted precursors. The as-obtained MPA-capped ZnSe/ZnS

core–shell were dispersed in the PBS buffer solution.

This journal is ª The Royal Society of Chemistry 2012

Performance of Hg2+ detection

For Hg2+ detection, 5 mL of Hg2+ ions solution (1 mM) was added

into 1mLof probe solution containingMPA-cappedZnSeQDs or

MPA-capped ZnSe/ZnS QDs. Experimental conditions of other

heavy metal ions were the same as that in the sample of Hg2+ ions.

Interfering experiments were conducted through adding 5 mL of

Hg2+ ions into probe solutions in the presence of other heavymetal

ions. The mixture was incubated for 30 seconds and then the

fluorescence emission spectra were recorded at room temperature.

Each spectrum was recorded repeatedly at least three times.

Characterizations

The UV-vis absorption spectra were obtained on Shanghai

Tianmei Tech Ltd. The room temperature photoluminescence

(PL) spectra of the QD samples were measured using a Hitch

F-4500 fluorescence spectrometer. The PL quantum yield (QY)

for both ZnSe/ZnS core–shell were calculated through refer-

encing to a standard (anthracene in ethanol, QY¼ 27%, emission

range: 360–480 nm) using the following equation.23

f ¼ f0 ��I

I 0

���A0

A

���n

n0

�2

where f and f0 are the PL QY for the QD sample and organic

dye, respectively; I (QD) and I0 (dye) are the integrated emission

peak areas at a given wavelength; A (QD) and A0 (dye) are the

absorption intensities at the same wavelength used for PL exci-

tation; n (QD) and n0 (dye) are the refractive indices of the

solvents. According to equations, we estimated the QY of ZnSe

core and ZnSe/ZnS core–shell as 0.8% and 2.0%.

A TEM image was taken on a FEI TECNAI transmission

electron microscope at an accelerating voltage of 200 kV. The

suspensions of ZnSe and ZnSe/ZnS core–shell were air-dried on a

carbon-coated copper grid for the TEM measurements. The

FT-IR spectra were recorded on a VERTEX 70-FTIR spec-

trometer. Heavy metal ions were added into the probe solution

and then QDs samples were separated from the solution,

followed by vacuum drying. The as-obtained powder samples

were characterized by FTIR spectrometer.

Results and discussion

Optical properties and structural characterization

Bright blue luminescent colloidal MPA-coated ZnSe/ZnS core–

shell QDs were fabricated successfully using the modified

conditions (Scheme 1). In comparison with bare ZnSe core, the

red-shift of the first absorption peak at 392 nm was exhibited in

the UV-Vis spectrum of the as-prepared ZnSe/ZnS core–shell

due to the epitaxial growth of ZnS shell (Fig. 1a). The band-gap

emission peak of ZnSe core is at 407 nm due to the quantum

confinement, and that of ZnSe/ZnS core–shell QDs at 422 nm

was shifted to longer wavelength (Fig. 1b). Another shoulder

peak obviously observed from both PL spectra is attributed to

the surface trap emission which could be eliminated by opti-

mizing the reaction conditions.20 Prior to the epitaxial growth of

ZnS shell on the ZnSe core, OA-coated ZnSe QDs was trans-

ferred into water solution through ligand exchange (Scheme 1).

Unfortunately, this procedure led to severe photoluminescence

Nanoscale, 2012, 4, 4996–5001 | 4997

Scheme 1 The synthesis procedures of MPA-coated ZnSe/ZnS QDs.

Fig. 1 (a) Absorption spectra and (b) photoluminescence spectra of OA-

coated ZnSe (A), MPA-coated ZnSe (B) andMPA-coated ZnSe/ZnS (C);

(c) particle-size distribution and (d) TEM and HRTEM images of MPA-

coated ZnSe/ZnS.

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quenching of the original oil-soluble ZnSe QDs due to the surface

traps being exposed to the aqueous solution.24 While the

dramatic change of fluorescent intensity was noticed before and

after the ligand exchange, the size variation of ZnSe QDs was not

observed by comparing the UV-Vis spectrum of ZnSe QDs

(Fig. 1a). In order to lower the non-radioactive possibility, the

introduction of ZnS shell is a need.25 Therefore, the fluorescence

intensity of band gap was apparently enhanced, which demon-

strates that the charge carrier is well confined within the core

region and separated from the surface because of the well-defined

epitaxis on the surface of the ZnSe core. The TEM and HRTEM

images (Fig. 1d) show the formation of ZnS shell around ZnSe

core. A typical particle-size distribution (Fig. 1c) shows that the

average diameter of ZnSe/ZnS core–shell is about 6.0 nm. The

quantum yield of ZnSe/ZnS core–shell QDs is improved to 2.0%,

far greater than the 0.8% of bare ZnSe core.

Fig. 2 Effect of pH values on the fluorescence intensity of MPA-coated

ZnSe/ZnS. pH values at the direction of arrow: 11.0, 10.0, 9.0, 8.0, 7.4,

7.0, 6.0, 5.0, 4.0, 3.0, 2.0, 1.0. Inset: TEM image of MPA-coated ZnSe/

ZnS at pH 1.0.

Effect of pH on the stability of MPA-coated ZnSe/ZnS QDs in

aqueous solution

Our aim was to explore the physicochemical factors that will

enhance the optical properties of QDs to detect heavy metal ions.

4998 | Nanoscale, 2012, 4, 4996–5001

To investigate the effect of pH of the probe solution on the PL

intensity, the fixed amount of probe solution was added into the

same volume solution at various pH values (1.0–11.0), and

measured by fluorescence spectrometer. The effects of pH values

on PL intensity of MPA-coated ZnSe/ZnS QDs are obvious

(Fig. 2). The emission intensity of PL increased with the increase

of pH values. Under alkaline conditions, the deprotonated thiol

and carboxyl could balance attractive force and repulsive force

and stabilize quantum dots in aqueous solution, which is bene-

ficial to enhancing the intensity of PL.26 However, as the pH

value is lower than 6.0, the emission intensity of probe solution is

almost quenched completely and the aggregation of QDs

appeared instantaneously. It was concluded that the aggregation

of QDs was due to the unbalance of attractive force and repulsive

force among these quantum dots, such as internanoparticle

H-bonding interaction,9 and resulted in the complete quenching

of PL. The UV-Vis spectrum (Fig. 3) demonstrates that the

lowering of the solution’s pH value resulted in the aggregation of

QDs, but did not damage the nanostructure of QDs. TEM

images (the inset in Fig. 2) are consistent with the results of UV-

Vis spectra. Below pH 7.0, the MPA ligand confers poor

protection for QDs, which could undermine the fluorescent

intensity. Although the emission intensity of QDs at pH 11.0 is

stronger than that of QDs at pH 7.4, metal hydroxide will be

easily formed under strong alkaline condition, which may be

harmful for sensing performance. In addition, the emission

intensity of ZnSe/ZnS QDs is still satisfactory for Hg(II) detec-

tion at pH 7.4. As a compromise, we chose to conduct the

detection at an intermediate pH value of 7.4.

Detection of Hg2+ ions and the mechanism of selectivity

For MPA-coated ZnSe/ZnS QDs, upon the introduction of Hg2+

ions, a significant change was observed from the photo-

luminescence spectrum. Fig. 4a depicts the fluorescence intensity

of MPA-coated ZnSe/ZnS QDs in the presence of Hg2+ ions from

0 nM to 70 nM in phosphate buffered saline (PBS, 10 mM, 7.4)

buffer solution. As the concentration of Hg2+ was increased, the

emission intensity of sensor system decreased significantly.

Meanwhile, the clear solution became turbid because of the

This journal is ª The Royal Society of Chemistry 2012

Fig. 3 Absorption spectra of MPA-coated ZnSe/ZnS QDs at different

pH values. pH values at the direction of arrow: 1.0, 2.0, 3.0, 4.0, 5.0, 6.0,

7.0, 7.4, 8.0, 9.0, 10.0, 11.0.

Fig. 4 (a) Photoluminescence spectra of MPA-coated ZnSe/ZnS QDs in

PBS buffer (pH ¼ 7.4) in the presence of different amounts of Hg2+ ions

(0–70 nM). The excitation wavelength was 360 nm. Inset: these are digital

photos of MPA-coated ZnSe/ZnS QDs in the presence of Hg2+ ions at

0 nM and 70 nM. (b) Calibration curve of the fluorescence at 423 nm of

QDs vs. [Hg2+] (0–70 nM). Inset: a linear relationship between I/I0 at 422

nm vs. [Hg2+] (0–40 nM), R2 ¼ 0.98.

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aggregation of ZnSe/ZnS QDs. The aggregation resulted in the

fluorescent quenching of probe solution.27 The colour of the

aggregation changed from white to brown over several hours

because of the oxidation of ZnSe/ZnS without surfactant

protection. Fig. 4b shows the detection relationship between the

fluorescence intensity at 422 nm and the concentration of Hg2+

ions, in particular, a linear range from 0 to 40 nM. The detection

limit of our probe solution is 2.5 nM. Limitation of detection for

Hg2+ ions, in our study, is lower than the limitation of detection

reported by other groups using QDs as sensors.31–34 For the

reasons of the aggregation, we hypothesized that MPA ligands

were separated from the surface of QDs (Fig. 5). It is often used

to keep the stability of QDs in aqueous solution because of two

different functional groups of MPA molecule.21 The MPA

ligands, however, initially interacted with Hg2+ ions due to the

formation of strong Hg–S bond.28 In order to demonstrate the

separation process, FT-IR spectra of MPA-coated ZnSe/ZnS

QDs (Fig. 6) were taken when heavy metal ions were added into

the probe solution. The absence of the S–H stretching bond

between 2682 cm�1 and 2561 cm�1 proves the attachment of the

MPA molecular via covalent bonds between thiols and surface

Zn atoms of ZnSe/ZnS QDs.30 The experimental data showed

that IR vibration peaks of MPA molecular binding on the

surface of QDs appearing at 3418 cm�1, 1567 cm�1 and 1400 cm�1

are attributed to the O–H stretching vibration, C]O asymmetric

stretching vibration of carboxylic acid and S–CH2 wagging

vibration, respectively.9,29 However, those IR peaks of MPA

ligands in the presence of Hg2+ ions disappear with respect to

other heavy metal ions. The results could support our hypothesis,

that is, the separation of MPA from the surface of QDs in the

presence of Hg2+ ions caused the aggregation of QDs.

Selectivity of MPA-coated ZnSe/ZnS QDs for Hg2+ ions

In order to study the selectivity of our probe solution for Hg2+

analysis, the luminescence features of the system were measured

in the presence of nine other heavy metal ions, including Co2+,

Cd2+, Ag+, Pb2+, Fe3+, Cr3+, Cu2+, Zn2+, and Ni2+. MPA-coated

ZnSe/ZnS QDs did not give significant response for these metal

ions (Fig. 7a, black column). This shows that these heavy metal

ions have slightly quenched the photoluminescence intensity of

the probe solution due to the weak affinity between these heavy

metal ions and the thiol with respect to Hg2+ ions. The relative

Fig. 5 Schematically illustrating the possible sensing mechanism for

Hg2+ ions based on metal-induced aggregation of QDs.

Nanoscale, 2012, 4, 4996–5001 | 4999

Fig. 6 FT-IR spectra of MPA-ZnSe/ZnS QDs in the presence of heavy

metal ions (100 mM).

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metal-sulfide bond strength is determined by their respective Ksp

values.18,27 In the presence of other heavy metal ions, the fluo-

rescence emission of MPA-coated ZnSe/ZnS could still be

completely quenched by adding Hg2+ ions to the probe solutions

(Fig. 7a, red column). This indicates that other heavy metal ions

have small influence on the detection of Hg2+ ions over the

available range of detection. To further investigate the effect of

ZnS shell on the selectivity and sensitivity of our probe solution,

control experiments were done through synthesizing MPA-

coated QDs without the ZnS shell. The data of MPA-coated

Fig. 7 The fluorescent response of 1 nM (a) MPA-coated ZnSe/ZnS

(lmax ¼ 422 nm) and (b) MPA-coated ZnSe (lmax ¼ 407 nm) in 10 mM

PBS buffer solution at pH 7.4, in the absence (black column) and pres-

ence (red column) of another 70 mL,1 mM Hg2+ solution containing a

specified metal ion of the same concentration. The excitation wavelength

was 360 nm.

5000 | Nanoscale, 2012, 4, 4996–5001

ZnSe QDs probe for these heavy metal ions show that this probe

solution still gives great response for Hg2+ ions, but simulta-

neously has obvious fluorescent intensity quenching in the

presence of other heavy metal ions (Fig. 7b). These heavy metal

ions could easily bind on the surface of QDs because of the high

surface energy of QDs and/or the attractive electrostatic inter-

actions.6 The binding of heavy metal ions to the surface of QDs

would result in increased non-radioactive recombination of free

excitons and thus quench the fluorescent intensity of QDs. A

shell can often be used as a protection layer to reduce the surface

trap and to enhance quantum yield. Additionally, in this study, it

was utilized to restrict the interaction between heavy metal ions

and QDs. Therefore, the introduction of ZnS shell reduced to

some extent the effect of heavy metal ions on the fluorescence

intensity of QDs and thus enhanced the selectivity of probe

solutions.

Conclusions

We have developed a cadmium-free ZnSe/ZnS quantum dot-

based chemosensor by taking advantage of the metal-induced

aggregation strategy. The as-synthesized chemosensor could

selectively and rapidly detect Hg2+ ions on the nanomole scale in

aqueous solution. The strong bond between thiol and Hg2+ ions

over other heavy metal ions was demonstrated to be a critical

factor influencing the quality of this selective chemosensor. The

FT-IR results proved that Hg2+ ions can specially and strongly

interact with thiol and thus cause the aggregation of QDs.

Moreover, the ZnS shell could to some extent not only protect

the ZnSe core from the interference of the external environment,

but also improve the selectivity for Hg2+ ions. These experimental

results could be a good starting point for exploring the applica-

tions of cadmium-free ZnSe/ZnS QDs in the analytical field in

the near future. Furthermore, this proposed model could be

viewed as a fundamental strategy to fabricate a specific nano-

sensing system for heavy metal ions in biomedicine and envi-

ronmental fields.

Acknowledgements

This work was supported financially by the National Nature

Science Foundation of China (no 20877013 and NSFC-RGC

21061160495), the National High Technology Research and

Development Program of China (863 Program) (no.

2010AA064902), the Major State Basic Research Development

Program of China (973 Program) (no. 2011CB936002), the

Excellent Talents Program of Liaoning Provincial University

(LR2010090) and the Key Laboratory of Industrial Ecology and

Environmental Engineering, China Ministry of Education.

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