Highly Luminescent Quantum Dots: New Tools for Biological Applications

Fatemeh Mir Najafi Zadeh1,Fan Wang2,Peter Reece2 and John Arron Stride1,3* 1. School of Chemistry, University of New South Wales,Sydney, NSW 2052, Australia 2.School of Physics, University of New South Wales,Sydney, NSW 2052, Australia

3.Bragg Institute, Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia.

j.stride@unsw.edu.au

Abstract

Water soluble CdSe(S)/ZnO core/shell quantum dots (QDs) were synthesized by an aqueous hydrothermal route. They have been characterized by X-ray powder diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), UV-visible absorption and laser emission spectroscopy. The effects of the hydrothermal conditions and reaction time, in addition to the nature of the precursor, on the formation of the QDs were investigated. Key words: Cadmium, selenium, water soluble

quantum dots, biocompatible nanoparticles

1. Introduction

Semiconductor nano particles or quantum dots (QDs) are a special class of nanoparticles, having unique optical properties such as narrow emission and wide absorption bands, along with good photo stability[1].They are very good candidate materials for medical imaging, assisting, as markers, in the detection of cancer and in the active delivery of chemotherapy drugs to cancer cells[2]. In order to apply QDs to biological systems, they should be both water soluble and bio-compatible[3]. There are different methods for the synthesis of water soluble QDs, including silanization, the encapsulation of QDs by hydrophilic polymers, ligand exchange and the direct synthesis of water soluble QDs via aqueous routes[4-6]. Direct synthesises of water soluble QDs which can be achieved by using hydrothermal or microwave assisted methods, have attracted great interest due to the photo properties of QDs[7-9]. Aqueous synthesises exhibit low toxicity, reproducibility and the products have good stability, biocompatibility and water solubility[10]. In this work, water soluble CdSe QDs were synthesized using MPA as a stabilizer and the hydrothermal method used resulted in sulfur form the 3-mercaptopropionic acid (MPA) participating the growth of CdSe(S) particles. The as-prepared CdSe(S) QDs were then coated with a ZnO outer-shell to produce CdSe/ZnO core/shell QDs in a modified literature method[11], resulting in QDs having high quality optical properties.

2.Experimental procedure

Chemicals: 3-mercaptopropionic acid (MPA) was obtained from Fluka. CdCl2.2.5H2O, NaBH4, Se (powder) , NaOH, Zn(CH3COO)2, Na2SO3 and L-cysteine, were all obtained from Sigma-Aldrich. All reagents were used as supplied, without additional purification. Ultra-pure water were used in all synthesises. Synthesis of CdSe(S) and CdSe(S)/ZnO QDs: NaBH4 was synthesized according to literature methods[10,11] and used as the source of selenide in the preparation of CdSe(S) QDs: 4NaBH4 +2 Se +7H2O 2NaHSe+Na2B4O7 + 14H2

A Cd precursor solution was prepared by mixing CdCl2 and MPA; a CdSe-MPA complex formed upon addition of the NaHSe to the Cd precursor solution:

CdCl2 +NaHSe + NaOH+MPA CdSe-MPA + 2NaCl +H2O After the hydrothermal treatment, CdSe-MPA capped QDs were obtained in accord with previous reports[10,11]. The as prepared CdSe(S) QDs were then coated with ZnO resulting in CdSe(S)/ZnO core/shell QDs. In order to compare the properties and particle sizes of CdSe(S) QDs, Na2SeSO3 and L-cysteine were also used as the Se precursor and capping agent respectively and CdSe(S) QDs were prepared. Characterization of CdSe(S) and CdSe(S)/ZnO

QDs: UV-vis absorption spectra were measured with a Varian Cary UV spectrometer. Photoluminescence spectra were measured with on a custom-built microscope coupled to an Acton 2300 spectrometer using an excitation laser at 514.5 nm. X-ray powder diffraction spectra (PXRD) were taken on a X'pert PRO Multi-purpose X-ray diffraction System (MPD system). X-ray photoelectron spectroscopy (XPS) was performed usng an Escalab 250Xi spectrometer. Transmission electron micrographs (TEM) and energy dispersive X-ray spectra (EDS) were recorded by a Philips CM200 instrument.

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3.Results and discussion The PXRD results indicated that the CdSe QDs were obtained were cubic in structure and that even after passivation with the ZnO-coating, the core QDs maintained this crystallinity (Figure 1).

Figure 1. PXRD from CdSe(S) QDs (a) and CdSe(S)/ZnOcore/shell QDs (b).

HRTEM images revealed that both the CdSe and CdSe/ZnO core/shell QDs are spherical highly crystalline nanoparticles, evidenced by the direct observation of atomic planes (Figure. 2).

(a)

(b)

Figure 2. TEM images of CdSe(S) QDs (a) and CdSe(S)/ZnO core/shell QDs (b)

Optical spectroscopy showed the particles to have narrow emission and wide absorption bands (Figure. 3 and 4).

Figure 3. Photo luminescence spectrum and (inset) UV/Vis absorption of CdSe(S) QDs.

Figure 4. Photo luminescence spectrum and (inset) UV/Vis absorption of CdSe(S)/ZnO core/shell QDs.

EDS spectra of CdSe(S)/ZnO QDs confirmed the existence of both oxygen and zinc in the QDs, in agreement with the XPS results (Figure. 5 and 6).

Figure 5. EDS spectrum of CdSe(S)/ZnO core/shell QDs.

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XPS data confirmed the existence of both Zn and O in the core/shell CdSe(S)/ZnO QDs. When the CdSe(S) QDs were coated with ZnO, a new peak originating from Zn 2P3/2 linked to oxygen atoms, was observed at 1022.0 eV and confirming the formation of ZnO shells around the CdSe(S) cores( Figure. 6 and 7).

Figure 6. XPS spectra of CdSe(S) QDs.(a) XPS survey spectrum; binding energy of (b) S 2p, (c) Se 3d and (d) Cd

3d.

Figure 7. XPS spectra of CdSe(S)/ZnO core/shell QDs. (a) XPS survey spectrum; binding energy of (b) S 2p, (c) Se

3d, (d) Zn 2P and (e) Cd 3d.

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It was determined that basic pH is an effective parameter in the formation of high quality water soluble QDs. Under basic pH, carboxylate anions are formed by the hydrolysis of carboxyl groups in the MPA capping agent rendering it hydrophilic. Also, the thermal conditions were found to strongly influence the QD optical properties such that without the hydrothermal treatment, the photo luminescent properties were significantly diminished (Figure. 8).

Figure 8. .Photo luminescence spectra of CdSe(S) QDs prepared after hydrothermal treatment(a) and reflux(b).

The reaction time, thermal regime, pH, selenide precursor and the capping agent were all found to be effective parameters affecting the water solubility and particle size of the resultant QDs. The determination of the particle size of cubic CdSe QDs from the PXRD data, using the Scherrer equation is shown in Table 1.

Table 1. Determination of CdSe QD particle size.

The condition of preparation

of sample

Diameter/nm

Acidic pH, Na2SeSo3 4.3±0.02

Basic pH, Na2SeSo3 5.8±0.02

Basic pH, Na2SeSo3, using L-cysteine as capping agent

5.8±0.02

Basic pH, NaHSe 2.3±0.01

4.Conclusions

In this work we have modified literature procedures of synthesising CdSe(S) QDs in aqueous media. It was determined that the photoluminescence of CdSe(S) QDs decreases with increasing reaction time, but improves after passiviation with ZnO shells around the CdSe(S) QD cores. As such, hydrothermal conditions have been shown to lead to QDs displaying both high optical quality and water solubility.

Acknowledgments

We would like to thank the Schools of Chemistry and Physics and the Mark Wainwright Analytical Center, all of the University of New south Wales, for the use of facilities and the Australian government for financial support.

References

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