Home
Search
Collections
Journals
About
Contact us
My IOPscience
Facile synthesis and characterization of highly fluorescent and biocompatible N-acetyl-lcysteine capped CdTe/CdS/ZnS core/shell/shell quantum dots in aqueous phase
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2012 Nanotechnology 23 495717 (http://iopscience.iop.org/0957-4484/23/49/495717) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 182.88.216.103 The article was downloaded on 20/11/2012 at 02:11
Please note that terms and conditions apply.
IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 23 (2012) 495717 (10pp)
doi:10.1088/0957-4484/23/49/495717
Facile synthesis and characterization of highly fluorescent and biocompatible N-acetyl-L-cysteine capped CdTe/CdS/ZnS core/shell/shell quantum dots in aqueous phase Qi Xiao1,2,3 , Shan Huang1,2,4 , Wei Su1 , W H Chan3,4 and Yi Liu2,4 1
College of Chemistry and Life Sciences, Guangxi Teachers Education University, Nanning 530001, People’s Republic of China 2 College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, People’s Republic of China 3 Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Kowloon, Hong Kong E-mail:
[email protected],
[email protected] and
[email protected]
Received 11 September 2012, in final form 22 October 2012 Published 19 November 2012 Online at stacks.iop.org/Nano/23/495717 Abstract The synthesis of water-soluble quantum dots (QDs) in aqueous phase has received much attention recently. To date various kinds of QDs such as CdTe, CdSe, CdTe/CdS and CdSe/ZnS have been synthesized by aqueous methods. However, generally poor-quality QDs (photoluminescent quantum yield (PLQY) lower than 30%) are obtained via this method and the 3-mercaptopropionic acid stabilizer is notorious for its toxicity and awful odor. Here we introduce a novel thiol ligand, N-acetyl-L-cysteine, as an ideal stabilizer that is successfully employed to synthesize high-quality CdTe/CdS/ZnS QDs via a simple aqueous phase. The core/shell/shell structures of the CdTe/CdS/ZnS QDs were verified by x-ray photoelectron spectroscopy, energy dispersive x-ray spectroscopy, x-ray powder diffraction and transmission electron microscopy. These QDs not only possess a high PLQY but also have excellent photostability and favorable biocompatibility, which is vital for many biological applications. This type of water-dispersed QD is a promising candidate for fluorescent probes in biological and medical fields. (Some figures may appear in colour only in the online journal)
1. Introduction
Sun et al 2007, Xiao et al 2008, 2009, 2012 and 2013, Huang et al 2008, 2009 and 2013 and Lei et al 2011). All kinds of highly quality QDs can be prepared using high boiling point solvents such as trioctylphosphine (TOP), trioctylphosphine oxide (TOPO), and so on in an organic phase, e.g., CdSe, CdSe/ZnS core/shell and CdSe/CdS/ZnS core/shell/shell QDs (Dabbousi et al 1997, Talapin et al 2004, Qu and Peng 2002). However, the high toxicity of the precursors, the high time consumption to convert them into water-soluble dispersions and the high capital cost
Quantum dots (QDs) have attracted considerable interest in the past two decades because of their excellent properties such as size-tunable emission, broad photoexcitation combined with narrow photoemission, symmetric emission spectra and better photostability than traditional fluorescent labels (Alivisatos 1996, Bruchez et al 1998, Chan and Nie 1998, 4 Authors to whom any correspondence should be addressed.
0957-4484/12/495717+10$33.00
1
c 2012 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 23 (2012) 495717
Q Xiao et al
Scheme 1. Schematic illustration of the synthesis of NAC-capped CdTe/CdS/ZnS core/shell/shell QDs.
biocompatibility (non-cytotoxic to K562 cells at 3 mM and up to 48 h incubation). Green et al (2009) prepared CdTe/CdS/ZnS quantum dots which appeared stable in aqueous solution and could be easily used in biological labeling in a one-pot reaction. Moreover, they suggested that the CdS shell induces type II behavior and that the deposition of the ZnS shell results in the growth of slightly anisotropic particles. Yan et al (2010) investigated a facile way to directly and continuously synthesize CdTe/CdS/ZnS core/shell/shell QDs using a modified chemical aerosol flow method. Samanta et al (2012) demonstrated the aqueous synthesis of colloidal nanocrystal heterostructures consisting of CdTe cores encapsulated by CdS/ZnS or CdSe/ZnS shells. By tuning the core size and thickness of the shell layers in the CdTe/CdSe/ZnS nanocrystals, NIR emissions with peak wavelengths of up to 730 nm were obtained. These methods can provide a simple, ultrafast and continuous approach to prepare core/shell/shell quantum dots with enhanced anti-oxide ability and stability. Optimization of the synthetic procedure, using MPA as the stabilizer, is usually very time-consuming, and MPA is notorious for its toxicity and awful odor. NAC, a therapeutic drug (Prescott et al 1979), is used as a mucolytic reagent (Boman et al 1983) in the treatment of acetaminophen hepatotoxicity, and is of interest for antioxidant/radical-scavenging activity (S¨arnstrand et al 1995). As a derivative of L-cysteine, NAC has excellent biocompatibility, is environmentally friendly, inexpensive, stable, nonvolatile, inodorous, and has good water-solubility (Choi et al 2007, Laura et al 2003, Lovric et al 2005). Up to now, the direct application of NAC in the synthesis of ligand-capped CdTe/CdS/ZnS core/shell/shell QDs in aqueous solution is unknown. Herein, we describe an extension of our previous NAC-capped CdTe/CdS synthesis and the simple deposition of a ZnS shell. In this paper, we present the first example of core/shell/shell structure of NAC-capped CdTe/CdS/ZnS QDs which are synthesized in aqueous phase (scheme 1).
limit their wide application in biological and medical fields. Direct synthesis of water-soluble QDs via an aqueous method is an alternative strategy, and the procedure is simpler, cheaper, less toxic, much more environmentally friendly and suitable for biological studies. Zou et al (2008) reported an ultrafast and facile aqueous phase route under atmospheric pressure to prepare high-quality green- to NIR-emitting CdTe nanocrystals (NCs) with mercaptopropionic acid (MPA) as the capping reagent. In contrast to previous reports, redto NIR-emitting CdTe NCs with emission efficiency up to 50% can be obtained within 1 h reflux time under optimized experimental conditions. Deng et al (2010) have developed a facile low-temperature aqueous method for synthesizing stable, bright, water-soluble CdTe/CdS magic-core/thick-shell NCs that are tetrahedral-shaped and whose NIR emission can be tuned by varying the shell thickness. We have previously described the synthesis of high-quality water-soluble NIRemitting CdTe/CdS QDs utilizing N-acetyl-L-cysteine (NAC) as an ideal stabilizer, and the application of the as-synthesized particles in imaging human nasopharyngeal carcinoma cells (HK-1) and African green monkey kidney cells (COS-7) (Zhao et al 2009, 2010). There are, however, limitations on the application of NAC-capped CdTe/CdS nanoparticles in biological studies. The as-synthesized particles do not possess an external ZnS shell, which has been found to reduce the toxic character and the level of cadmium leached into the biological system (Liu et al 2006, Cho et al 2007). Until now, there have been only few studies on the production of high-quality CdTe/CdS/ZnS core/shell/shell QDs in an aqueous phase by microwave irradiation, hydrothermal or chemical aerosol flow methods. He et al (2008) demonstrated for the first time the synthesis of CdTe/CdS/ZnS core/shell/shell QDs in aqueous phase assisted by microwave irradiation. As-prepared core/shell/shell QDs not only possess a high photoluminescent quantum yield (PLQY) (40–80%) but also exhibit excellent photostability (17-fold more stable than bare CdTe QDs) and favorable 2
Nanotechnology 23 (2012) 495717
Q Xiao et al
These NAC-capped CdTe/CdS/ZnS core/shell/shell QDs have been systematically characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), x-ray powder diffraction (powder XRD), x-ray photoelectron spectroscopy (XPS) and energy dispersive x-ray spectra (EDX). UV–vis absorption spectra, fluorescence spectra and fluorescence decay measurements are also applied to investigate their photophysical properties. Due to the NAC ligands and the core/shell/shell structure, these QDs not only possess high quantum yields (QYs) (40–50%), longer fluorescence lifetimes (43.2 ns) and super-photostability in neutral or alkaline solutions, but also exhibit excellent photostability (20-fold more stable than NAC-capped CdTe core QDs) and favorable biocompatibility (non-cytotoxic to HK-1 cells at 1 µM and up to 48 h incubation), which is of crucial importance for many biomedical applications.
when the temperature had cooled down to room temperature. To remove the excess NAC–Cd complexes at the end of the synthesis, cold 2-propanol was added to the reaction mixture to precipitate the NAC-capped CdTe core QDs. The as-prepared product was dried overnight under vacuum at 30 ◦ C and stored in a refrigerator for further experiments. 2.3. Synthesis of NAC-capped CdTe/CdS core/shellQDs The colloidal precipitate was used for NAC-capped CdTe core QDs in subsequent steps. The CdTe/CdS precursor solution was prepared by adding the as-prepared NAC-capped CdTe core QDs to a N2 -saturated solution containing 1.0 mmol CdCl2 , 0.2 mmol Na2 S and 5.0 mmol NAC. The CdTe/CdS precursor solution (40 ml) was placed in a three-necked flask. The air in the system was pumped off and replaced with N2 . Then the mixture was heated to 100 ◦ C and reacted at this temperature for 5–90 min. Aliquots of the sample were taken at different time intervals and used to record their UV–vis absorption and fluorescence spectra. The strong size-dependent optical property of the NAC-capped CdTe/CdS core/shell QDs was monitored to reflect the mean particle size and size distribution through the temporal evolution of UV–vis absorption and fluorescence spectra. NAC-capped CdTe/CdS core/shell QD samples were taken when the temperature had cooled down to room temperature. Cold 2-propanol was added to precipitate the NAC-capped CdTe/CdS core/shell QDs, which were dried overnight under vacuum at 30 ◦ C and stored in a refrigerator for further experiments.
2. Experimental details 2.1. Materials Tellurium (powder, 200 meshes, 99.8%), sodium borohydride (NaBH4 , 99.8%), CdCl2 ·H2 O (99.99%), NAC (≥99%) and rhodamine 6G were purchased from Sigma. Na2 S, ZnCl2 and 2-propanol were obtained from Beijing Chemical Reagents Company. All chemicals were of the highest commercially available purity and used as received without further purification. Ultrapure water was prepared by a Milli-Q-RO4 water purification system (Millipore).
2.4. Synthesis of NAC-capped CdTe/CdS/ZnS core/shell/shellQDs
2.2. Synthesis of NAC-capped CdTe core QDs The synthesis procedure was according to the literature method (Zou et al 2008) with minor modifications. Briefly, 0.2 mmol of tellurium powder and 1.0 mmol of NaBH4 were loaded into a 25 ml two-necked flask equipped with a constant pressure funnel which contained 5.0 ml of ultrapure water. The air in this system was pumped off and replaced with N2 . The reaction mixture became dark red after heating at 80 ◦ C for 30 min under N2 flow protection. The obtained NaHTe solution was stored under N2 protection for further use at room temperature. Then, 0.2 mmol of CdCl2 and 0.34 mmol of NAC solution were mixed in a 40 ml solution. The pH of the mixture was adjusted to 12.0 by adding 1.0 M NaOH solution dropwise under stirring. The mixture was transferred into a three-necked flask. The air in the system was pumped off and replaced with N2 . Under stirring, 1 ml of NaHTe solution (0.04 mmol) was added into the Cd precursor solution by syringe at room temperature. The molar ratio of [Cd]:[Te]:[NAC] was fixed at 1.0:0.2:1.7. Then the mixture was heated to 100 ◦ C and reacted at this temperature for 5–120 min. Aliquots of the sample were taken at different time intervals and used to record their UV–vis absorption and fluorescence spectra. The strong size-dependent optical property of the NAC-capped CdTe core QDs was monitored to reflect the mean particle size and size distribution through the temporal evolution of UV–vis absorption and fluorescence spectra. NAC-capped CdTe core QD samples were taken
The CdTe/CdS/ZnS precursor solution was prepared by adding the as-prepared NAC-capped CdTe/CdS core/shell QDs to a N2 -saturated solution containing 1.0 mmol ZnCl2 , 0.2 mmol Na2 S and 5.0 mmol NAC. The CdTe/CdS/ZnS precursor solution (40 ml) was placed in a three-necked flask. The air in the system was pumped off and replaced with N2 . Then the mixture was heated to 60–70 ◦ C and reacted at this temperature for 1–90 min. Aliquots of the sample were taken at different time intervals and used to record their UV–vis absorption and fluorescence spectra. The strong size-dependent optical property of the NAC-capped CdTe/CdS/ZnS core/shell/shell QDs was monitored to reflect the mean particle size and size distribution through the temporal evolution of UV–vis absorption and fluorescence spectra. NAC-capped CdTe/CdS/ZnS core/shell/shell QD samples were taken when the temperature had cooled down to room temperature. Cold 2-propanol was added to precipitate NAC-capped CdTe/CdS/ZnS core/shell/shell QDs, which were dried overnight under vacuum at 30 ◦ C and stored in a refrigerator for further experiments. 2.5. Characterization The pH values of all the solutions were measured using a Sartorius PB-10 pH meter. UV–vis absorption spectra were obtained on a Varian Cary 300 UV–visible 3
Nanotechnology 23 (2012) 495717
Q Xiao et al
Figure 1. (a) Representative UV–vis absorption and fluorescence spectra of NAC-capped CdTe core QDs (λem = 545 nm, QY ∼ 25%), NAC-capped CdTe/CdS core/shell QDs (λem = 590 nm, QY ∼ 35%) and the corresponding NAC-capped CdTe/CdS/ZnS core–shell–shell QDs (λem = 625 nm, QY ∼ 45%). (b), (c) Photographs of the emission colors of the corresponding QDs under the radiation of room light (b) and a UV lamp (c).
CdTe/CdS core/shell QDs and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs, respectively. Finally, the diluted samples were irradiated with a 350 W Xe lamp (365 nm) for different time intervals and the photostabilities of the QDs were recorded.
absorption spectrophotometer. Fluorescence spectra were recorded on a Perkin Elmer LS50B spectrometer. All optical measurements were performed at room temperature under ambient conditions. Fluorescence decay curves were captured on a PTI TimeMasterTM Model C-720 lifetime spectrofluorometer. Transmission electron microscopy (TEM) and high-resolution (HR) TEM images were recorded on a JEOL JEM 2100F electron microscope. The TEM sample was prepared by dropping an aqueous QDs solution onto an Agar carbon-coated copper grid (400 meshes) and the solvent was left to evaporate. FT-IR spectra over the range of 400–4000 cm−1 were recorded on a Perkin Elmer Paragon 1000 FT-IR spectrometer. The free NAC ligand, NAC-capped CdTe core QDs, NAC-capped CdTe/CdS core/shell QDs or NAC-capped CdTe/CdS/ZnS core/shell/shell QDs mixed with solid KBr were ground to a fine powder. The powder was pressed into a pellet at 15 000 psi for IR measurement. Powder XRD was recorded on a Bruker D8 Advance powder x-ray diffractometer with Cu (Kα) radiation and a Ni filter. XPS measurements were acquired with a Leybold Heraeus SKL 12 x-ray photoelectron spectrometer. EDX spectra were captured by an LEO 1530 field emission scanning electron microscope equipped with an energy dispersive x-ray spectrometer. The QYs of the QDs were measured according to the literature by using rhodamine 6G as the reference standard (QY ∼ 95% in ethanol) (Bao et al 2004).
2.7. Comparison of the cytotoxicities of CdTe core QDs, CdTe/CdS core/shell QDs and CdTe/CdS/ZnS core/shell/shell QDs The HK-1 nasopharyngeal carcinoma cell line was used in this study. Cells were routinely cultured in RPMI 1640 supplemented with 10% fetal bovine serum (Gibco BRL) and antibiotics (50 IU ml−1 penicillin and 50 µg ml−1 streptomycin). The cells were incubated at 37 ◦ C in a humidified 5% CO2 incubator. Cells in the exponential growth phase were used for the experiment (1 × 104 cells/well) and seeded into 96-well plates overnight. Serial dilutions of the NAC-capped CdTe core QDs, NAC-capped CdTe/CdS core/shell QDs and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs were prepared in ultrapure water and added into each well. The concentrations of the QDs were calculated following a previously published method (Yu et al 2003). The cells were further incubated at 37 ◦ C in a humidified atmosphere with 5% CO2 for 48 h in the dark, after which the cytotoxicities of the QDs were evaluated using an MTT reduction assay and the results were expressed as the mean standard deviation of three separate trials.
2.6. Comparison of the photostabilities of CdTe core QDs, CdTe/CdS core/shell QDs and CdTe/CdS/ZnS core/shell/shell QDs
3. Results and discussion 3.1. UV–vis absorption and fluorescence spectral properties of NAC-capped CdTe/CdS/ZnS core/shell/shell QDs
In order to exclude the influences of residual NAC, Cd2+ , Te2− , S2− and Zn2+ , the comparison was carried out after post-treatment as described previously. Briefly, 2-propanol was added dropwise into the QD solutions under stirring until the solutions became turbid. The turbid solutions were further stirred for 30 min and the QDs were collected by centrifugation. The QDs were redispersed in ultrapure water to give an absorption value of −0.1 at the excitation wavelength of the NAC-capped CdTe core QDs, NAC-capped
Figure 1 displays representative UV–vis absorption and fluorescence spectra of NAC-capped CdTe core QDs, NAC-capped CdTe/CdS core/shell QDs and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs fabricated by this aqueous solution route. In both spectra, a wavelength red-shift is observed for the NAC-capped CdTe/CdS/ZnS core/shell/shell QDs (λem = 625 nm) compared with the 4
Nanotechnology 23 (2012) 495717
Q Xiao et al
Table 1. Optical properties and size distributions of synthesized NAC-capped CdTe/CdS/ZnS core/shell/shell QDs for different reaction times. Reaction PLQY time (min) λabs (nm) λem (nm) (%)
FWHM (nm)
Diameter (nm)
1 10 20 30 40 50 60 90
46 48 48 50 55 60 64 68
3.19 3.24 3.32 3.39 3.46 3.51 3.58 3.70
545 550 558 566 574 581 590 604
588 595 603 610 620 627 635 656
37.4 39.2 40.5 39.3 40.2 39.5 39.3 39.0
NAC-capped CdTe core QDs (λem = 545 nm) and the NAC-capped CdTe/CdS core/shell QDs (λem = 590 nm), which indicates the formation of the expected NACcapped CdTe/CdS/ZnS core/shell/shell structure instead of Cdx Zn1−x Tey S1−y alloy structure (Mekis et al 2003, Pan et al 2005). It is reported that the temperature is most important for the formation of a ZnS shell on the surface of a CdTe/CdS core/shell QD. Higher temperatures will cause broad size distributions and more surface defects, and result in poor spectral properties of the QDs. Therefore, relatively low temperatures (60–70 ◦ C) were used in this research for better epitaxial growth of the ZnS shell which could effectively reduce the surface defects of the CdTe/CdS core/shell QDs and enhance the spectral properties of the as-prepared NAC-capped CdTe/CdS/ZnS core/shell/shell QDs significantly. Figure 2 shows the evolution of the UV–vis absorption and fluorescence spectra of the NAC-capped CdTe/CdS/ZnS core/shell/shell QD samples taken for various reaction times. With increase of the reaction time, a red-shift appears in the band-edge emission of the NAC-capped CdTe/CdS/ZnS core/shell/shell QDs in both the UV–vis absorption and fluorescence spectra. The fluorescent peak positions can reach 588, 595, 605, 610, 620, 627, 635 and 656 nm for reaction times of 1, 10, 20, 30, 40, 50, 60 and 90 min, respectively; this indicates that the emission wavelength can be tuned by simply controlling the reaction time, and the reaction can be completed within 90 min (see table 1). Our synthetic method is much faster and more convenient compared with some reported synthesis methods for core/shell QDs and core/shell/shell QDs in aqueous solutions which often take several days and need expensive instruments. In addition, the full width at half maximum (FWHM) is extremely narrow and the FWHM values are changed from 46 to 68 nm, which indicates that the size range of the as-prepared NAC-capped CdTe/CdS/ZnS core/shell/shell QDs is quite narrow and these QDs can be applied directly in biomedical regions without further purification. Meanwhile, the QYs of the as-prepared NAC-capped CdTe/CdS/ZnS core/shell/shell QDs are 40–50%. Moreover, the QDs are quite stable in the dark under ambient conditions. No obvious precipitation appeared in solution and no significant changes were observed in either the UV–vis absorption or fluorescence spectra after 6 months stored in a refrigerator.
Figure 2. UV–vis absorption and fluorescence spectra of NAC-capped CdTe/CdS/ZnS core/shell/shell QDs for different reaction times.
3.2. Characterization of the NAC-capped CdTe/CdS/ZnS core/shell/shell QDs Figure 3 shows the TEM images of the as-prepared NACcapped CdTe core QDs (λem = 545 nm, QY ∼ 25%), NACcapped CdTe/CdS core/shell QDs (λem = 590 nm, QY ∼ 35%) and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs (λem = 625 nm, QY ∼ 45%). It is fairly obvious that these QDs are nearly spherical with good size distribution and excellent monodispersity. The average sizes of the NACcapped CdTe core QDs, NAC-capped CdTe/CdS core/shell QDs and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs are 2.7, 3.5 and 4.0 nm, respectively, which indicates that the observed increments in size are due to the growth of the first CdS shell (0.8 nm) and the second ZnS shell (0.5 nm), respectively. Since the diameters of the as-prepared NAC-capped CdTe/CdS/ZnS core/shell/shell QDs are significantly smaller than 10 nm, they could be easily absorbed by cells and further incorporated into biomedical regions. In order to confirm the presence of NAC on the surface of the as-prepared NAC-capped CdTe core QDs, NAC-capped CdTe/CdS core/shell QDs and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs, the FT-IR spectra of free NAC and the three types of QDs were compared and the results are shown in figure 4. The FT-IR spectra of the three types of QDs are quite similar but are distinctively different from that of the free NAC. The typical characteristic peak of the S–H bond (−2552 cm−1 ) in the three types of QDs disappears while the other characteristic peaks do not change at all. These results indicate that the NAC combines onto the surfaces of the three types of QDs by the same pattern as that by which the S–H group of NAC binds onto the QD surface through the Cd–S bond. On the other hand, the abundant carboxylate groups of NAC mean that these QDs have super water-solubility and higher fluorescence stability. Figure 5 shows the powder XRD analysis results for NAC-capped CdTe core QD, NAC-capped CdTe/CdS core/shell QD and NAC-capped CdTe/CdS/ZnS core/shell/shell QD solid samples that were precipitated from 5
Nanotechnology 23 (2012) 495717
Q Xiao et al
Figure 3. Transmission electron microscopy (TEM) images of the as-prepared (a) NAC-capped CdTe core QDs (λem = 545 nm), (b) NAC-capped CdTe/CdS core/shell QDs (λem = 590 nm) and (c), (d) NAC-capped CdTe/CdS/ZnS core/shell/shell QDs (λem = 625 nm) synthesized in aqueous phase. The scale bars are 20 nm in (a)–(c) and 2 nm in (d). The insets display the size distributions of the corresponding QDs.
Figure 5. Powder x-ray diffraction (XRD) patterns of NAC-capped CdTe core QDs (λem = 545 nm), NAC-capped CdTe/CdS core/shell QDs (λem = 590 nm) and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs (λem = 625 nm). The standard diffraction lines for cubic CdTe, cubic CdS and cubic ZnS are shown for comparison.
Figure 4. FT-IR spectra of free NAC, NAC-capped CdTe core QDs (λem = 545 nm), NAC-capped CdTe/CdS core/shell QDs (λem = 590 nm) and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs (λem = 625 nm).
the aqueous solution with an excess of 2-isopropanol. The diffraction pattern of the NAC-capped CdTe core QDs is quite close to that of bulk cubic CdTe. However, because of the growth of the CdS and ZnS shells, the diffraction patterns of the NAC-capped CdTe/CdS core/shell QDs and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs shift slightly to higher angles, while both the peak widths and shapes are nearly unchanged. In addition, the emerging diffraction patterns of the CdS and ZnS phases are close to those of bulk cubic CdS and ZnS. These results further demonstrate the formation of core/shell and core/shell/shell structures rather than the alloyed structure that would show a narrow XRD peak width after increasing the particle size (He et al 2007, 2008). XPS results could further evidence the NAC-capped CdTe/CdS core/shell and the NAC-capped CdTe/CdS/ZnS core/shell/shell structures of the QDs. More detailed information on the nature and type of the constituent elements present in these QDs can be provided through the XPS technique. As shown in figure 6, the coordination of Cd–SR in NAC-capped CdTe core QDs is different from that of
Cd–S in NAC-capped CdTe/CdS core/shell QDs (Borehet et al 2003). The typical characteristic S2p3 peak of the NAC-capped CdTe core QDs at 164 eV is shifted to 162 eV for the NAC-capped CdTe/CdS core/shell QDs, which confirms the proposed core/shell structure in which CdS acts as the first shell encapsulating the CdTe core QDs (Pan et al 2005). Moreover, compared with the NAC-capped CdTe/CdS core/shell QD XPS spectrum, new Zn2p, Zn3p3 and Zn Auger peaks that are attributed to Zn from the ZnS shell appear in the NAC-capped CdTe/CdS/ZnS core/shell/shell QD XPS spectrum, which further validates the expected core/shell/shell structure of the QDs in which the ZnS acts as the second shell encapsulating the CdTe core QDs, and the results are consistent with previous reports. In addition, the energy dispersive x-ray spectroscopy (EDS) technique was employed to investigate the surface composition of the NAC-capped CdTe core QDs, NAC-capped CdTe/CdS core/shell QDs and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs. By comparing the EDS 6
Nanotechnology 23 (2012) 495717
Q Xiao et al
Figure 6. X-ray photoelectron spectra (XPS) of NAC-capped CdTe core QDs (λem = 545 nm), NAC-capped CdTe/CdS core/shell QDs (λem = 590 nm) and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs (λem = 625 nm).
Figure 7. EDX spectra of synthesized NAC-capped CdTe core QDs (λem = 545 nm), NAC-capped CdTe/CdS core/shell QDs (λem = 590 nm) and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs (λem = 625 nm).
spectrum of the NAC-capped CdTe/CdS/ZnS core/shell/shell QDs with those of the NAC-capped CdTe core QDs and NAC-capped CdTe/CdS core/shell QDs, the formation of the first CdS shell and the second ZnS shell on the surfaces of the CdTe QDs is fairly obvious, as shown in figure 7. These three QDs all display the S, Cd and Te peaks at 2.317, 3.145 and 3.767 keV, respectively, but new peaks of Zn at 1.020, 8.620 and 9.560 keV are present in the spectrum of the NAC-capped CdTe/CdS/ZnS core/shell/shell QDs. In contrast, the surface S content of the CdTe core QDs (39.43%) is much lower than those of the CdTe/CdS core/shell QDs (55.62%) and CdTe/CdS/ZnS core/shell/shell QDs (51.82%), which is because the CdS shell together with the NAC ligand is attached to the CdTe/CdS core/shell QD and the CdTe/CdS/ZnS core/shell/shell QD while only the NAC ligand is capped on the CdTe core QD. Moreover, the surface content of Te decreases from 10.69% to 1.89% and the surface content of Cd changes from 49.88% to 7.98%, indicating the growth of a ZnS shell on the surface of the CdTe/CdS core/shell QD. Figure 8 displays the fluorescent time-resolved spectra of the NAC-capped CdTe core QDs, NAC-capped CdTe/CdS core/shell QDs and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs with fluorescence emission wavelengths of 545, 590 and 625 nm, respectively. Their corresponding average lifetimes are 22.5, 34.3 and 43.2 ns, respectively. Obviously, the average lifetime of the CdTe/CdS/ZnS core/shell/shell QDs is notably longer than those of the CdTe core QDs and CdTe/CdS core/shell QDs. For the core/shell and core/shell/shell structure QDs, their lifetimes will be prolonged by an increase of their diameters (Patrock et al 2007).
Figure 8. Fluorescence decay curves of NAC-capped CdTe core QDs (λem = 545 nm), NAC-capped CdTe/CdS core/shell QDs (λem = 590 nm) and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs (λem = 625 nm).
higher photostability under strong UV light irradiation. As shown in figure 9, the integrated fluorescence intensity of CdTe core QDs decreases rapidly and CdTe core QDs easily aggregate and precipitate out of solution under UV irradiation in only 25 min. By contrast, the photostabilities of CdTe/CdS core/shell QDs and CdTe/CdS/ZnS core/shell/shell QDs were significantly improved under the same experimental conditions. For the CdTe/CdS core/shell QDs with λem = 590 nm, the emission intensity stabilized at about 55% of its initial value after up to 120 min irradiation under the same conditions. For the CdTe/CdS/ZnS core/shell/shell QDs with λem = 625 nm, only a very slight decrease in emission intensity was observed after 120 min, which indicated that these CdTe/CdS/ZnS core/shell/shell QDs were quite photostable under UV irradiation. It has been reported that the unsaturated Te atoms on the surface of the CdTe core QDs may be oxidized under UV irradiation which may lead to possible fluorescence quenching of the QDs (Borehet et al 2003). Furthermore, Te is a toxic
3.3. The photostability of NAC-capped CdTe/CdS/ZnS core/shell/shell QDs The improved characteristics of the NAC-capped CdTe/CdS/ ZnS core/shell/shell QDs are well demonstrated by their 7
Nanotechnology 23 (2012) 495717
Q Xiao et al
Figure 9. Photostability of NAC-capped CdTe core QDs (λem = 545 nm), NAC-capped CdTe/CdS core/shell QDs (λem = 590 nm) and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs (λem = 625 nm) under strong UV light irradiation.
metal to humans and will be released into the solution after the QD degradation under UV irradiation. However, when double shells are coated on the surface of CdTe core QDs, the Te atoms will be effectively protected from UV irradiation and the optical properties of the CdTe/CdS/ZnS core/shell/shell QDs do not change even after several months when stored at ambient conditions. Figure 10 displays the influence of the pH on the fluorescent intensity of the NAC-capped CdTe core QDs, NAC-capped CdTe/CdS core/shell QDs and NACcapped CdTe/CdS/ZnS core/shell/shell QDs. The NACcapped CdTe/CdS core/shell QDs and CdTe/CdS/ZnS core/shell/shell QDs show poor stability at lower pH but exhibit higher stability in neutral or basic aqueous solution. It has been reported that protonation of the thiol moieties of QDs under acidic conditions can lead to detachment of the surface agents from QDs, which could consequently decrease the fluorescent intensity of the QDs and lead to the aggregation of QDs (Aldana et al 2005). However, in neutral and alkaline solutions, the negative charges of the carboxylate groups of NAC on the QD surface can repel each other which will lead to better stability and higher fluorescent efficiency of the QDs. The fluorescent intensities of the NAC-capped CdTe/CdS core/shell QDs and CdTe/CdS/ZnS core/shell/shell QDs reach their maxima at a pH of around 8 and stay constant to pH 12. However, the fluorescence of the NAC-capped CdTe core QDs has a relatively lower maximum at pH 7 and decreases dramatically with increase of the pH. Undoubtedly, the formation of double shells on the surfaces of CdTe core QDs can prevent the attack of the hydroxyl ion at higher pH and greatly enhance the photostability of the QDs.
Figure 10. pH dependent fluorescence intensity of NAC-capped CdTe core QDs (λem = 545 nm), NAC-capped CdTe/CdS core/shell QDs (λem = 590 nm) and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs (λem = 625 nm).
cells. As shown in figure 11, the NAC-capped CdTe/CdS/ZnS core/shell/shell QDs were nearly non-cytotoxic to HK-1 cells in concentrations of up to 1 µM after 48 h incubation, while the cell cytotoxicity significantly increased when the cells were incubated with 0.25 µM CdTe QDs or 0.5 µM CdTe/CdS QDs, respectively. Therefore, we can deduce that the core/shell/shell structure QDs are much more biocompatible than the core structure QDs and core/shell structure QDs. The reason is that the double shells can reduce the toxicity of the QDs efficiently by controlling the release of toxic Cd2+ ions and reducing the tendency of QD aggregation. Notably, the as-prepared NAC-capped CdTe/CdS/ZnS core/shell/shell QDs exhibit excellent biomedical compatibility and are suitable for future biomedical applications.
4. Conclusions
3.4. Comparison of the cytotoxicities of NAC-capped CdTe core QDs, CdTe/CdS core/shell and CdTe/CdS/ZnS core/shell/shell QDs
In conclusion, we have demonstrated for the first time the facile synthesis of highly fluorescent and biocompatible CdTe/CdS/ZnS core/shell/shell QDs by using NAC as the stabilizer in aqueous phase. The core/shell/shell structure of the as-prepared QDs was adequately characterized by TEM, FT-IR, XRD, XPS and EDS techniques. The double
To demonstrate the future biomedical applications of these QDs, the biocompatible properties of the QDs were investigated by incubating the three kinds of QDs with HK-1 8
Nanotechnology 23 (2012) 495717
Q Xiao et al
Chan W C W and Nie S M 1998 Quantum dot bioconjugates for ultrasensitive nonisotopic detection Science 281 2016–8 Cho S J, Maysinger D, Jain M, R¨oder B, Hackbarth S and Winnik F M 2007 Long-term exposure to CdTe quantum dots causes functional impairments in live cells Langmuir 23 1974–80 Choi A O, Cho S J, Desbarats J, Lovri´c J and Maysinger D 2007 Quantum dot-induced cell death involves Fas upregulation and lipid peroxidation in human neuroblastoma cells J. Nanobiotechnol. 5 1–13 Dabbousi B O, Rodriguez-Viejo J, Mikulec F V, Heine J R, Mattoussi J R, Ober R, Jensen K F and Bawendi M G 1997 Synthesis and characterization of a size series of highly luminescent nanocrystallites J. Phys. Chem. B 101 9463–75 Deng Z T, Schulz O, Lin S, Ding B Q, Liu X W, Wei X X and Ros R 2010 Aqueous synthesis of zinc blende CdTe/CdS magic-core/thick-shell tetrahedral-shaped nanocrystals with emission tunable to near-infrared J. Am. Chem. Soc. 132 5592–3 Green M, Williamson P, Samalova M, Davis J, Brovelli S, Dobsond P and Cacialli F 2009 Synthesis of type II/type I CdTe/CdS/ZnS quantum dots and their use in cellular imaging J. Mater. Chem. 19 8341–6 He Y, Lu H T, Sai L M, Su Y Y, Hu M, Fan C H, Huang W and Wang L H 2008 Microwave synthesis of water-dispersed CdTe/CdS/ZnS core–shell–shell quantum dots with excellent photostability and biocompatibility Adv. Mater. 20 3416–21 He Y, Sai L M, Lu H T, Hu M, Lai W Y, Fan Q L, Wang L H and Huang W 2007 Microwave-assisted synthesis of water-dispersed CdTe nanocrystals with high luminescent efficiency and narrow size distribution Chem. Mater. 19 359–65 Huang S, Xiao Q, He Z K, Liu Y, Tinnefeld P, Su X R and Peng X N 2008 A high sensitive and specific QDs FRET bioprobe for MNase Chem. Commun. 44 5990–2 Huang S, Xiao Q, Li R, Guan H L, Liu J, Liu X R, He Z K and Liu Y 2009 A simple and sensitive method for L-cysteine detection based on the fluorescence intensity increment of quantum dots Anal. Chim. Acta 645 73–8 Huang S, Xiao Q, Su W, Li P Y, Ma J Q and He Z K 2013 Simple and sensitive determination of papain by resonance light-scattering with CdSe quantum dots Colloids Surf. B 102 146–51 Laura P J, Sandra S M, Laura W L and Jack A H 2003 Effect of N-acetylcysteine on acetaminophen toxicity in mice relationship to reactive nitrogen and cytokine formation Toxicol. Sci. 75 458–67 Lei Y, Xiao Q, Huang S, Xu W S, Zhang Z, He Z K, Liu Y and Deng F J 2011 Impact of CdSe/ZnS quantum dots on the development of zebrafish embryos J. Nanopart. Res. 13 6895–906 Liu Y, Chen W, Joly A G, Wang Y, Pope C, Zhang Y, Bovin J O and Sherwood P 2006 Comparison of water-soluble CdTe nanoparticles synthesized in air and in nitrogen J. Phys. Chem. B 110 16992–7000 Lovric J, Bazzi H S, Cuie Y, Fortin G R A, Winnik F M and Maysinger D 2005 Differences in subcellular distribution and toxicity of green and red emitting CdTe quantum dots J. Mol. Med. 83 377–85 Mekis I, Talapin D V, Komowski A, Haase M and Weller H 2003 One-pot synthesis of highly luminescent CdSe/CdS core–shell nanocrystals via organometallic and ‘greener’ chemical approaches J. Phys. Chem. B 107 7454–62 Pan D C, Wang Q, Jiang S C, Ji X and An L 2005 Synthesis of extremely small CdSe and highly luminescent CdSe/CdS core–shell nanocrystals via a novel two-phase thermal approach Adv. Mater. 17 176–9 Patrock T K C, Donega C M, Svetlana S B, Stefan C J M, Nico A J M S and Rne A J J 2007 Highly luminescent CdTe/CdSe colloidal heteronanocrystals with temperature-dependent emission color J. Am. Chem. Soc. 129 14880–6
Figure 11. The cytotoxicities of different concentrations of NAC-capped CdTe core QDs, NAC-capped CdTe/CdS core/shell QDs and NAC-capped CdTe/CdS/ZnS core/shell/shell QDs in HK-1 cells. The cells were incubated with various concentrations of QDs (62.5–1000 nM) for 48 h in the dark. The cells’ viability was determined by MTT reduction assay and the results are expressed as the mean ± S.D. of three separate trials.
shells can efficiently protect the CdTe core QDs from the influence of the outer environment. These QDs not only possess high QYs and excellent photostability but also exhibit favorable biocompatibility and low cytotoxicity, which are most important for future biomedical applications. As such, our highly fluorescent and biocompatible NAC-capped CdTe/CdS/ZnS core/shell/shell QDs have a great number of potential applications in various biomedical fields.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 21203035, 20921062) and the National Science Fund for Distinguished Young Scholars of China (Grant No. 21225313).
References Aldana J, Lavelle N, Wang Y J and Peng X G 2005 Size-dependent dissociation pH of thiolate ligands from cadmium chalcogenide nanocrystals J. Am. Chem. Soc. 127 2496–504 Alivisatos A P 1996 Semiconductor clusters, nanocrystals, and quantum dots Science 271 933–7 Bao H B, Gong Y Y, Zhen L and Gao M Y 2004 Enhancement effect of illumination on the photoluminescence of water-soluble CdTe nanocrystals—toward highly fluorescent CdTe/CdS core–shell structure Chem. Mater. 16 3853–9 Boman G, B¨acker U, Larsson S, Melander B and W˚ahlinder L 1983 Oral acetylcysteine reduces exacerbation rate in chronic bronchitis: report of a trial organized by the Swedish Society for Pulmonary Diseases Eur. J. Respir. Dis. 64 405–15 Borehet H, Talapin D V, Gaponik N, McGinley C, Adam S, Lobo A, Moller T and Weller H 2003 Relations between the photoluminescence efficiency of CdTe nanocrystals and their surface properties revealed by synchrotron XPS J. Phys. Chem. B 107 9662–8 Bruchez M P, Moronne M, Gin P, Weiss S and Alivisatos A P 1998 Semiconductor nanocrystals as fluorescent biological labels Science 281 2013–6 9
Nanotechnology 23 (2012) 495717
Q Xiao et al
Xiao Q, Huang S, Su W, Li P Y, Ma J Q, Luo F P, Chen J and Liu Y 2013 Systematically investigations of conformation and thermodynamics of HSA adsorbed to different sizes of CdTe quantum dots Colloids Surf. B 102 76–82 Xiao Q, Qiu T, Huang S, Liu Y and He Z K 2012 Preparation and biological effect of nucleotide capped CdSe/ZnS quantum dots on Tetrahymena thermophila Biol. Trace Elem. Res. 147 346–53 Xiao Q, Zhou B, Huang S, Tian F F, Guan H L, Ge Y S, Liu X R, He Z K and Liu Y 2009 Direct observation of the binding process between protein and quantum dots by in situ surface plasmon resonance measurements Nanotechnology 20 325101 Yan C M, Tang F Q, Li L L, Li H B, Huang X L, Chen D, Meng X W and Ren J 2010 Synthesis of aqueous CdTe/CdS/ZnS core/shell/shell quantum dots by a chemical aerosol flow method Nanoscale Res. Lett. 5 189–94 Yu W W, Qu L H, Guo W Z and Peng X G 2003 Experimental determination of the extinction coefficient of CdTe, CdSe, and CdS nanocrystals Chem. Mater. 15 2854–60 Zhao D, He Z K, Chan P S, Wong R N S, Mak N K, Lee A W M and Chan W H 2010 NAC-capped quantum dot as nuclear staining agent for living cells via an in vivo steering strategy J. Phys. Chem. C 114 6216–21 Zhao D, He Z K, Chan W H and Choi M M F 2009 Synthesis and characterization of high-quality water-soluble near-infrared-emitting CdTe/CdS quantum dots capped by N-acetyl-L-cysteine via hydrothermal method J. Phys. Chem. C 113 1293–300 Zou L, Gu Z Y, Zhang N, Zhang Y L, Fang Z, Zhu W H and Zhong X H 2008 Ultrafast synthesis of highly luminescent green- to near infrared-emitting CdTe nanocrystals in aqueous phase J. Mater. Chem. 18 2807–15
Prescott L F, Illingworth R N, Critchley J A J H, Stewart M J, Adam R D and Proudfoot A T 1979 Intravenous N-acetylcysteine the treatment of choice for paracetamol poisoning Br. Med. J. 2 1097–100 Qu L and Peng X G 2002 Control of photoluminescence properties of CdSe nanocrystals in growth J. Am. Chem. Soc. 124 2049–55 Samanta A, Deng Z T and Liu Y 2012 Aqueous synthesis of glutathione-capped CdTe/CdS/ZnS and CdTe/CdSe/ZnS core/shell/shell nanocrystal heterostructures Langmuir 28 8205–15 S¨arnstrand B, Tunek A, Sj¨odin K and Hallberg A 1995 Effects of N-acetylcysteine stereoisomers on oxygen-induced lung injury in rats Chem. Biol. Interact. 94 157–64 Sun Q J, Wang Y A, Li L S, Wang D Y, Zhu T, Xu J, Yang C H and Li Y F 2007 Bright, multicoloured light-emitting diodes based on quantum dots Nature Photon. 1 717–22 Talapin D V, Mekis I, Gotzinger S, Kornowski A, Benson O and Weller H 2004 CdSe/CdS/ZnS and CdSe/ZnSe/ZnS core–shell–shell nanocrystals J. Phys. Chem. B 108 18826–31 Xiao Q, Huang S, Ma J Q, Su W, Li P Y, Cui J G and Liu Y 2012 Systematically investigation of interactions between BSA and different charge-capped CdSe/ZnS quantum dots J. Photochem. Photobiol. A 249 53–60 Xiao Q, Huang S, Qi Z D, Zhou B, He Z K and Liu Y 2008 Conformation, thermodynamics and stoichiometry of HSA adsorbed to colloidal CdSe/ZnS quantum dots Biochim. Biophys. Acta, Proteins Proteomics 1784 1020–7 Xiao Q, Huang S, Su W, Li P Y, Liang Z C, Ou J Z, Ma J Q and Liu Y 2012 Evaluate the potential environmental toxicity of quantum dots on ciliated protozoa by microcalorimetry Thermochim. Acta 547 62–9
10