High Quality CdHgTe Nanocrystals with Strong Near-Infrared

Article pubs.acs.org/Langmuir

High Quality CdHgTe Nanocrystals with Strong Near-Infrared Emission: Relationship between Composition and Cytotoxic Effects Wenjing Cai,†,⊥ Liming Jiang,‡,⊥ Dongmei Yi,† Haizhu Sun,*,†,‡ Haotong Wei,§ Hao Zhang,§ Hongchen Sun,*,‡ and Bai Yang§ †

College of Chemistry, Northeast Normal University, Changchun 130024, P. R. China Department of Pathology, School of Stomatology, Jilin University, Changchun 130021, P. R. China § State Key Laboratory of Supramolecular Structure & Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China ‡

S Supporting Information *

ABSTRACT: High quality CdHgTe quasi core/shell nanocrystals (NCs) were prepared via the one-step method. The relationship between the composition, structure, and property was systematically investigated by the combination of X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission (ICP), and the photoluminescence (PL) measurements. The quantum yield (QY) was ∼50% when the feed ratio of Cd2+ to Hg2+ was equal to 1. The PL property was further polished, and the QY was improved to ∼80% through the variance of the prepared conditions such as the ratio of ligand to metal ion and HTe− to metal ion, pH value, and temperature. In addition, the cytotoxic effects of CdHgTe NCs were systematically studied. The results showed that, for Cd0.21Hg0.79Te NCs, its quasi core/shell structure was very stable and little cadmium ions were released. As a result, such NCs showed little cytotoxicity and would find applications in tissue imaging or detection.

1. INTRODUCTION Semiconductor nanocrystals (NCs) with near-infrared (NIR) emission in the spectral region of 800−1500 nm have attracted much interest due to their potential applications in the field of biological tissue imaging,1−3 solar cells,4−7 and telecommunication, etc.8,9 For example, there is little signal interference coming from the autofluorescence and absorbance of the tissues within the spectral window of 900−1300 nm, which makes these NCs find applications in biological imaging and detection.10,11 Another advantage is that the NIR light can penetrate the deep tissues such as liver and spleen while the fluorescence imaging using visible fluorescent tags is hampered.12 Because NIR light occupies 50% of the total energy in the solar spectrum, it is significant to take advantage of this energy in fabricating solar cells.13 Other attractive applications of NIR NCs involve up-conversion and optical amplification in telecommunication systems at the wavelengths of 1.3 and 1.55 μm as well as conceal materials, etc.14−18 Hg chalcogenides are the most attractive among the NCs with NIR emission.19−25 For instance, some binary semiconductors such as lead chalcogenides and group III antimonides possess band gaps higher than 0.18 eV, while Hg chalcogenides have zero band gap.26 Consequently, they have promising application in NC−polymer solar cells, infrared optical devices, and electroluminescence devices. CdxHg1−xTe © 2013 American Chemical Society

is one of the most important mercury compounds due to the wide range of tunable optical, electronic, and magnetic properties, obtained through compositional tuning (x) of this material.27 However, some difficulties are encountered during the preparation of such NCs. As we know, NCs grown at high temperature, which facilitates the crystallization process, can exhibit a high degree of crystallinity and high photoluminescence (PL) quantum yield (QY). It is quite difficult for the Hg-based NCs because of their high reactive activity and instability at high temperature. NCs obtained at low temperature normally result in low PL QY and poor properties. In order to solve this contradiction, Weller, Gaponik, and Rogach et al. prepared CdTe NCs first and then added mercury salt to the CdTe solution.28−30 The HgTe shell was formed by the substituting Cd2+ with Hg2+ at the surface of CdTe core. They got the CdHgTe NCs with PL QY ∼40% and then realized the application of these NCs in electroluminescence field.30 In addition, we took advantage of large difference in the solubility between CdTe and HgTe to successfully prepare ternary quasi core/shell CdHgTe NCs via one step in aqueous solution.31 However, it still remains a challenge to make clear how the Received: December 15, 2012 Revised: February 2, 2013 Published: March 4, 2013 4119

dx.doi.org/10.1021/la3049696 | Langmuir 2013, 29, 4119−4127

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Table 1. Feed Ratios of Cd2+/Hg2+ and the Actual Compositions of CdHgTe NCs Determined by ICP and XPS Measurements

composition of the NCs influences their properties, especially, the cytotoxic effects of CdHgTe NCs since they have promising applications in biological tissue imaging.32−36 Herein, we prepared gradient CdHgTe quasi core/shell NCs by an efficient one-step synthetic process. The composition of the NCs was precisely characterized by X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma atomic emission (ICP) instruments. The highest PL QY ∼80% was obtained by varying the experimental conditions. The cytotoxic effect of CdHgTe NCs was systematically investigated, and the mechanism how the CdHgTe NCs influenced the metabolism of cells was proposed.

feed ratios of Cd2+/Hg2+

determined by ICP

determined by XPS

5 2.5 1.25 1 0.8

Cd0.83Hg0.17Te Cd0.46Hg0.54Te Cd0.29Hg0.71Te Cd0.23Hg0.77Te Cd0.14Hg0.86Te


Cd0.30Hg0.70Te, Cd0.21Hg0.79Te, and Cd0.15Hg0.85Te (Table 1), indicating that the determination of the NC composition is precise. The change in the composition of the CdHgTe NCs results in the variance of their structure and hence brings important influence on their properties. Figure 1a gives the UV absorption and PL spectra of the Cd0.85Hg0.15Te NCs (Cd2+/Hg2+ = 5). The 1s−1s exciton peak is hardly seen in the UV spectrum, which suggests a wide distribution of NC diameter. The Stoke shift is as large as 300 nm, implying a strong electron−phonon coupling in the interior of the NCs, which leads to low QY (only 2.7%). The distribution of Cd2+ in the NCs is random, resulting in many hanging bonds in the NCs, and consequently the NCs are easily photooxidated. This is confirmed by Figure 2 in which four peaks are clearly observed in the XPS spectrum of Te element for the Cd0.85Hg0.15Te NCs. The peaks at 572 and 583 eV come from the Te element in bulk Cd0.85Hg0.15Te. The peaks at 576 and 586 eV normally result from the oxidized state Te element (TeO2), which indicates partial Te element in the NCs has been oxidized. In general, only two peaks are observed for the NCs with excellent surface construction while four peaks appear for the NCs with oxidized Te element (please refer to the Supporting Information for XPS spectra of other elements in pure CdTe and HgTe NCs). As is clearly observed from Figure 1b−d, the 1s−1s exciton peaks become more and more obvious with decreasing the ratios of Cd2+ to Hg2+. The narrowing of the absorption and emission bands suggests a narrow distribution of the NC size. The Stoke shifts change from 300 to 181 nm for Cd 0 .5 8 Hg 0. 42 Te, Cd 0. 30 Hg 0 .7 0 Te, Cd 0.2 1 Hg 0 .79 Te, and Cd0.15Hg0.85Te NCs, corresponding to the feed ratios of Cd2+/Hg2+ equal to 2.5, 1.25, 1, and 0.8, respectively. The QY of Cd0.58Hg0.42Te NCs abruptly increases to 38%, implying the large change in NC structure occurred. In theory, because the solubility (Ksp) of HgTe is ∼20 orders of magnitude lower than that of CdTe in water, the nucleation speed of HgTe is much faster than that of CdTe when the concentration of Hg2+ and Cd2+ is comparable. Therefore, the core rich in Hg is formed first. With the decrease of Hg2+ concentration, the nucleation speed of CdTe becomes faster than that of HgTe. Consequently, the shell rich in Cd forms on the surface of HgTe core due to the similar crystallographic constants of CdTe and HgTe. NCs with such structure are called quasi core/shell NCs or gradient core/shell NCs (Scheme 1).31,37,38 This structure can be proved by using the ICP method. First, changes in the Cd2+ content during the NC growth process with different feed ratios of Cd2+/Hg2+ were analyzed. The ICP data of Cd0.21Hg0.79Te NCs are summarized in Table 2. The Cd content gradually increases from 5.79 wt % at the beginning of the reaction to 8.88 wt % after 2 h. Second, the NCs were etched with acid and then analyzed for their breakdown products as a function of time (Table 2). Within a short etching

2. EXPERIMENTAL SECTION 2.1. Materials. Tellurium powder (200 mesh, 99.8%) was purchased from Aldrich Chemical Corporation. 3-Mercaptopropionic acid (MPA) and IR125 were purchased from Acros Chemical Corporation. CdCl2 (99+%), HgCl2 (99+%), and NaBH4 (99%) were all commercially available. All of the solvents were analytical grade and used as received. 2.2. Synthesis of CdHgTe NCs. To a N2-saturated aqueous solution of CdCl2 and HgCl2 at pH 9 in the presence of MPA as a stabilizing agent was added the freshly prepared NaHTe solution. The concentration of metal ion (M2+) was 3.75 × 10−3 M. The solution was then placed in an oven at 40 °C for 30 min. The resulting CdHgTe NCs in aqueous medium were precipitated by addition of isopropanol and isolated by centrifugation for the analyses by transmission electron microscopy (TEM), XPS, and ICP measurements. 2.3. Cytotoxic Effect of CdHgTe. 3-(4,5-Dimethylthiazol-2-yl)2,5-diphenyltetrasodium bromide (MTT) assay was used to detect the vitality of the cells (mouse fibroblast cell line). L929 cells at a density of 5 × 103 cells cm−3 were seeded into a 96-well plate in Dulbecco’s Modified Eagle Medium (DMEM) supplemented with 10% fetal calf serum (FBS) and 1% penicilin−streptomycin. Then, different concentrations or kinds of NCs were added as the experimental groups and the same volume of phosphate buffered saline (PBS) as the control group. The color of the solution was orange, and little change occurred before and after the addition of NC samples. At each time point, 20 μL of MTT solution (5 mg mL−1) was added into each well, and the cells were further incubated for 4 h at 37 °C. After that, the medium was discarded, and then 200 μL of dimethyl sulfoxide (DMSO) was added to each well to dissolve purple formazan granules. The optical density of the solution was then read at 570 nm with microplate reader. 2.4. Characterizations. UV−vis spectra were acquired on a Shimadzu 3600 UV−vis−NIR spectrophotometer. Fluorescence experiments were performed on a PTI Fluorescence Master System. The QYs of CdHgTe NCs were estimated using IR125 as a reference. The absorbance of the samples and IR125 at the excitation wavelength (765 nm) was below 0.1 in order to avoid any significant reabsorption. XPS was done on a VG ESCALAB MK II spectrometer with Mg K excitation (1253.6 eV). ICP was carried out with a PerkinElmer OPTIMA 3300DV analyzer. TEM and High-resolution TEM (HRTEM) imaging was implemented by a JEM-2100F electron microscope at 300 kV. X-ray diffraction (XRD) was carried out using a Siemens D5005 diffractometer.

3. RESULTS AND DISCUSSION 3.1. Composition of CdHgTe NCs. CdHgTe NCs with different compositions were prepared by varying the feed ratios of Cd2+ to Hg2+. The actual compositions determined by ICP method are Cd0.83Hg0.17Te, Cd0.46Hg0.54Te, Cd0.29Hg0.71Te, Cd0.23Hg0.77Te, and Cd0.14Hg0.86Te, for the feed ratios of Cd2+/Hg2+ equal to 5, 2.5, 1.25, 1, and 0.8, respectively (Table 1). These results are very close to the ones obtained via XPS measurement, which are Cd0.85Hg0.15Te, Cd0.58Hg0.42Te, 4120


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Figure 1. UV absorption and PL spectra of Cd0.85Hg0.15Te, Cd0.58Hg0.42Te, Cd0.30Hg0.70Te, Cd0.21Hg0.79Te, and Cd0.15Hg0.85Te NCs, corresponding to the feed ratios of Cd2+/Hg2+ equal to (a) 5, (b) 2.5, (c) 1.25, (d) 1, and (e) 0.8.

Figure 2. XPS spectra for the feed ratios of Cd2+/Hg2+ equal to 5, 2.5, 1.25, 1, and 0.8.

Scheme 1. Formation Process of CdHgTe Quasi Core/Shell NCs

Figure 3. (a) TEM image of Cd0.21Hg0.79Te NCs. Insets are their HRTEM image and SAED. (b) XRD pattern of Cd0.21Hg0.79Te NCs. (c) EDX of Cd0.21Hg0.79Te NCs, showing that they contain cadmium, mercury, and tellurium elements.

have a gradient core/shell structure. For Cd0.85Hg0.15Te NCs, however, its ICP data show that the Cd wt % almost keeps the same whatever during the NC growth or etched process (Table S1). This suggests that Cd2+ indeed randomly distributes in the Cd0.85Hg0.15Te NCs, and they do not have the quasi core/shell structure.31,37,38 The formation of quasi core/shell structure leads to increase of CdHgTe NCs QY compared with pure HgTe NCs (only 7%). The QYs of the Cd0.30Hg0.70Te, Cd0.21Hg0.79Te NCs are 52% and 55%, which implies the excellent surface construction due to the formation of the shell with proper thickness. Figure 2 exhibits that the oxidated Te element peaks become smaller and smaller, indicating that NC structure changes from Cd2+ randomly distributed in the NCs to the formation of quasi core/shell structure. Figure 1e gives the UV absorption and PL spectra of Cd0.15Hg0.85Te NCs (Cd2+/Hg2+ = 0.8). Although the oxidated Te element peaks in the XPS spectra continue

Table 2. Content of Cd wt % for Cd0.21Hg0.79Te NCs at Growth and Acid Etched Process growth time (s)

10

20

30

80

3600

7200

content of Cd (wt %) etched time (min) content of Cd (wt %)

5.79 1 34.34

6.40 2 10.59

8.06 3 10.14

8.37 4 8.84

8.56 5 3.77

8.88 180 0.82

time (about 2 min), the Cd wt % abruptly decreases, from 34.34 to 10.59 wt %, indicating that the shell of the NCs is rich in Cd. With increasing the etching time (from 2 to 4 min), the Cd content gradually decreases. After that, the Cd wt % abruptly decreases again. Especially, after 180 min etching, the Cd wt % is only 0.82 wt %, suggesting that the core is rich in Hg. Thus, the results strongly indicate that these ternary NCs 4121


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Figure 4. UV absorption and PL spectra for the ratios of ligand to metal ion (M2+) equal to (a) 1, (b) 2.5, (c) 2.9, (d) 3.5, and (e) 4.

Figure 5. UV absorption and PL spectra for the ratios of HTe− to metal ion (M2+) equal to (a) 0.2, (b) 0.4, (c) 0.5, (d) 0.6, and (e) 0.8.

investigate their influence on the PL properties. Note that the feed ratio of Cd2+/Hg2+ is set to 1. a. Ratios of Ligand to Metal Ion. As we know, aqueous NCs possess electric double-layer structure, the overlap of which makes NC disperse in solution. According to this structure, entire aqueous NCs have inorganic core, ligand layer, adsorbed layer, and diffuse layer from inside to outside. The nature of diffuse layer determines the NC growth rate while adsorbed layer inside diffuse layer dominates PL QY.39−41 Figure 4 exhibits the UV absorption and PL spectra of CdHgTe NCs with different ratios of ligand to metal ion. It is observed that the highest QY (71%) appears when the ratio is equal to 1. In solution, the complexes of the metal ions and ligands compose the ligand layer of CdHgTe NCs. At the feed ratio of Cd2+/Hg2+ equal to 1, the metal ion formed these complexes with MPA is most of Cd2+ because Hg2+ has reacted with HTe−. There are several forms of the complex including Cd(MPA), Cd(MPA)22−, Cd(MPA)34−, etc. Normally, the close arrangement of these complexes can guarantee less surface defect and hence lead to high PL QY. In order to realize such arrangement, the concentration of Cd(MPA) is required to be the highest because the spatial obstruction of Cd(MPA) is low when coordinating with NC. Therefore, the Cd(MPA) complexes can form a closed layer on the surface of NCs, which improves their PL property. It is obvious that the ratio of ligand to metal ion equal to 1 can lead to the highest concentration of Cd(MPA) complexes. As a result, the highest QY appears at such ratio.40 When the ratios continuously increase to 2.5 and 2.9, the QYs decrease to 63% and 67%. On one hand, the decrease is

Table 3. Actual Compositions and S/Te Ratios for the Ratios of HTe− to Metal Ion Equal to 0.2, 0.4, 0.5, 0.6, and 0.8 ratios of HTe− to metal ion

actual composition

ratios of S/Te

0.2 0.4 0.5 0.6 0.8


0.98 1.15 2.73 1.31 0.83

decreasing (Figure 2), the QY of these NCs decreases to 48%. Because the content of Hg2+ increases, the rich-Hg core becomes bigger. Simultaneously, the rich Cd shell becomes thinner, and hence leads to the decrease in QY. The Cd0.21Hg0.79Te NCs were further characterized by TEM, HRTEM, and XRD measurement. The TEM and HRTEM images show the size of NCs is ∼3 nm in diameter (Figure 3a and inset). The presence of cadmium, mercury, sulfur, and tellurium species in Cd0.21Hg0.79Te NCs is confirmed by EDX results. Selected-area electron diffraction (SAED) and XRD patterns show that Cd0.21Hg0.79Te NCs have a cubic zinc blende structure. As discussions above, for all of the NCs with quasi core/shell structures, their QYs are much higher than that of pure HgTe. Therefore, the formation of such structure is important for the improvement of the NC PL properties. 3.2. Optimizing PL Properties of CdHgTe Quasi Core/ Shell NCs. Since the experiment conditions including ratios of ligand to metal ion, metal ion to HTe−, pH value, and temperature are dominant parameters which significantly govern the quality of the resultant NCs, it is necessary to 4122


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oxide element peak is observed, which suggests a good passivation of the NC surface. Therefore, the best ratio of ligand to metal is set to 2.9.40 b. Ratios of HTe− to Metal Ions. Figure 5 gives the UV absorption and PL spectra of CdHgTe NCs with different ratios of HTe− to metal ion. The emission peak position shifts from 1305 to 1075 nm with the ratios changing from 0.2 to 0.8. Table 3 shows the compositions of the NCs are Cd0.09Hg0.99Te, Cd 0 .1 8 Hg 0. 82 Te, Cd 0. 28 Hg 0 .7 2 Te, Cd 0.4 3 Hg 0 .57 Te, and Cd0.53Hg0.47Te. Therefore, the change of the peak position results from the variance of the composition. The increase of Cd2+ in the NCs leads to the blue shift of the spectra. From the dynamic view, we can draw the same conclusion. NCs normally present faster growth with low Te source/metal ratio because it results in excess metal−ligand complexes in solution and therefore high ionic strength. This variation reduces the surface potential and thickness of NC diffuse layer and as a result facilitates NC growth.39,40 The QYs are 2.7%, 17%, 52%, 23%, and 5.1% when the ratio is adjusted to 0.2, 0.4, 0.5, 0.6, and 0.8, respectively. The different QY results from the different surface reconstruction. High S/Te ratio corresponds to high QY because more S element coming from the MPA ligands takes part in the surface reconstruction. This can remove the nonradiative recombination from the surface defects. The S/Te ratios determined by XPS increase from 0.98 to 1.15 and 2.73 for the ratios of 0.2, 0.4, and 0.5, and decrease to 1.31 and 0.83 for the ratios of 0.6 and 0.8. The change trend is in accordance with that of QY. Another reason resulting in the change of the QY is the varying of Cd(MPA) concentration. When the ratio is 0.2, the concentrations of Cd(MPA), Cd(MPA)22−, and Cd(MPA)34− are high in the system. Therefore, it is disadvantaged for the arrangement of ligand on the surface of NCs. Consequently, the QY is low (∼2.7%). When the ratio is 0.8, the excess HTe− introduces defects on the surface of NCs. The defects lead to the oxidation and result in the deposition of NCs at last. Figure S4 gives the XPS spectra of Te element with different ratios of HTe− to metal ion. Te element has been oxidated and four peaks are observed at the ratio of 0.8. Consequently, the QY is only ∼5%. When the ratio is 0.5, the concentration of Cd(MPA) is proper to form a closed layer on the surface of NCs. As a result, the QY is highest, ∼52%. Therefore, the best ratio of HTe− to metal ion is 0.5.39,40 c. pH Value and Temperature. The highest QY is about ∼82% with the pH value of 8. When pH value is 10, the QY abruptly decreases to 42%. The QY is 48% with pH value of 11. It has been reported that NCs possess low PL QY when they are prepared with high pH. Moreover, increasing pH value also increases ionic strength and reduces PL QY. Both NCs and ligands in solution are negatively charged, and the electrostatic repulsion between them is low at near neutral pH, which facilitates the traverse of free ligands through the adsorption layer. Consequently, low pH value (8 or 9) is proved to benefit the adsorption of free ligands on NCs surface, thus increasing the PL QY of NCs. The proper temperature synthesized CdHgTe NCs ranges from 40 to 80 °C. High temperature leads to the precipitation of the NCs.39,40 3.3. Cytotoxic Effect of CdHgTe Quasi Core/Shell NCs. HgTe, Cd0.21Hg0.79Te, Cd0.85Hg0.15Te, and CdTe NCs which have large differences in their composition and structure are selected to investigate their cytotoxicity by using MTT assay. Statistical analysis was carried out for all MTT results with a two-sample test, comparing each sample group to the related

Figure 6. L929 cell proliferation incubated with different concentrations of HgTe, Cd0.21Hg0.79Te, Cd0.85Hg0.15Te, and CdTe NCs at (a) 1 day, (b) 5 days, and (c) 1 week.

because Cd(MPA) concentration declines. On the other, high ratios increase the ionic strength of solution. The general relation between PL QY and ionic strength is that high ionic strength usually reduces PL QY. This is because high ionic strength makes the diffuse layer thinner, which leads to the poor charge selectivity. Consequently, complexes including Cd(MPA)22− and Cd(MPA)34− are all absorbed on the surface of the NCs. The spatial obstruction of Cd(MPA)22− and Cd(MPA)34− complexes is too high to form a closed protective layer on the surface of NCs. Therefore, when the ratios increase to 3.5 and 4.0, the QYs decrease to 44% and 39%. However, this does not mean that the ratio of 1 is the best. There exists a balance between improving the QY by increasing the amount of Cd(MPA) and the necessity for an effective amount of stabilizer to provide stability and surface construction. The stabilizer molecules occupy the surface sites instead of Te atom, remove the dangling bonds resulting from Te, and hence prevent the oxidation of NCs. Figure S3 gives the CdHgTe XPS spectra of Te element with different ratios of ligand to metal ion. The presence of tellurium oxide has been detected for the ratios of 1 and 2.5. When the ratio increases to 2.9, no 4123


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Figure 7. Morphology of L929 cell in (a) control group and incubated with (b) HgTe, (c) Cd0.21Hg0.79Te, and (d) Cd0.85Hg0.15Te NCs at 5 days. The concentration of the NCs is 4 μg mL−1.

unpaired hole in the NCs. These unpaired holes further induce ligand oxidation and cleavage. The corrosion of the NCs outer surface can generate much more cadmium ions, which promotes the formation of reactive oxygen species (ROS) via Cd2+-specific cellular pathways. The generated ROS can damage the proteins, DNA, and lipids, leading to severe cell functional impairments and eventually to cell death. Therefore, the cytotoxicity of NCs in our system mainly depends on their ability of releasing cadmium ions.33 For Cd0.21Hg0.79Te NCs, its quasi core/shell structure is very stable, and little cadmium ions can be released. For Cd0.85Hg0.15Te NCs in which Cd2+ randomly distributes, however, their structure is not as stable as the core/shell one, and consequently, they exhibit higher cytotoxicity. In addition, with respect to the HgTe NCs, although Hg2+ belongs to high toxic ion, the solution activity of HgTe is too low to release Hg2+. As a result, they almost show no cytotoxicity. Whether the heavy metal ions such as Cd2+ and Hg2+ exist or not can be confirmed by testing the sample solution using ICP method after the cells had been incubating with NCs for 1 week. The results showed the existence of Cd2+ in the sample containing Cd0.85Hg0.15Te and CdTe NCs while no heavy metal ions were detected for the sample containing Cd0.21Hg0.79Te and HgTe NCs. In addition, ICP results also showed the composition of the quasi core/shell Cd0.21Hg0.79Te NCs changed to Cd0.19Hg0.81Te, but that of Cd0.85Hg0.15Te changed to Cd0.58Hg0.42Te. Therefore, the composition and surface chemistry are very important in determining their ability of releasing toxic ion and hence their cytotoxicity. Figure 7 gives the morphology of the cells incubated with HgTe (b), Cd0.21Hg0.79Te (c), and Cd0.85Hg0.15Te(d) NCs for 5 days (the concentration is 4 μg mL−1).The cells in the control group (a) and HgTe NCs show good spread morphology, and little dead cells are observed. For the cells incubated with Cd0.21Hg0.79Te NCs, the dead cells are more than those

group. The mean difference was significant at the 0.05 level; namely, the comparison was effective when P < 0.05. Figure 6 gives the MTT results of the control group, HgTe, Cd0.21Hg0.79Te, Cd0.85Hg0.15Te, and CdTe NCs with different concentrations. The L929 cells have been incubated with the solution of NCs for 1 day, 5 days, and 1 week. From Figure 6a− c, it is observed that the cytotoxicity of Cd0.21Hg0.79Te NCs is almost the same with that of Cd0.85Hg0.15Te NCs at 1 μg mL−1. The P values of statistical analysis between Cd0.21Hg0.79Te and Cd0.85Hg0.15Te at 1 μg mL−1 are 0.355, 0.974, and 0.339 for 1 day, 5 days, and 1 week, respectively, which is higher than 0.05. These results indicate that cytotoxicities of Cd0.21Hg0.79Te and Cd0.85Hg0.15Te are similar at low concentration. With the concentration increasing from 1 to 4 μg mL−1, the cytotoxicity of Cd0.85Hg0.15Te NCs becomes much higher than that of Cd0.21Hg0.79Te NCs. The P values of statistical analysis between Cd0.21Hg0.79Te and Cd0.85Hg0.15Te at 4 μg mL−1 are 0, 0.05 and 0, for 1 day, 5 days, and 1 week, respectively, indicating the difference is significant. At high concentration (8 μg mL−1) of NCs, the cytotoxicity of Cd0.21Hg0.79Te NCs is comparable with that of Cd0.85Hg0.15Te NCs again. Their P values are higher than 0.05. This implies that such concentration is too high to apply as biological tissue imaging. The cytotoxicity of CdTe NCs is higher than that of Cd0.85Hg0.15Te NCs. To our surprise, the cytotoxicity of HgTe is the lowest among all of the four kinds of NCs. The cytotoxicity of NCs mainly comes from cadmium ions (Cd2+) released from NCs. On one hand, these released cadmium ions themselves have cytotoxicity, which can directly lead to the death of cells. On the other, during the process of in vitro cell imaging under the aqueous aerobic condition, the irradiation of light normally results in the photo oxidation of the NCs in live cells. This process leads to electron transfer from the excited NCs to O2 to produce superoxide O2− and an 4124


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Cd0.21Hg0.79Te NCs are candidates for use in biological tissue imaging or detection. To further decrease the cytotoxicity, the prepared Cd0.21Hg0.79Te NCs were precipitated by using isopropanol to remove surplus Cd2+ in solution. Then, the obtained powder was redistributed in water and incubated with cells. From Figure 8a−c, it is observed that the MTT results of Cd0.21Hg0.79Te NCs powders show little cytotoxicity. The morphology of cells keeps very well with the use of Cd0.21Hg0.79Te NCs powders at the fifth day, and little dead cells are observed for the concentration of 4 μg mL−1 (Figure S5). For the Cd0.21Hg0.79Te NCs without removing the surplus Cd2+ in solution, however, the dead cells are much more than those for the powders. According to the results above, the proper concentration which can be used as tissue imaging or detection is about 4 μg mL−1. The Cd0.21Hg0.79Te NCs can be directly used after synthesis or purification. The cells incubated with Cd0.21Hg0.79Te NCs by using such optimized condition can keep good spread morphology, which indicates they are in good state (Figure 9).

4. CONCLUSION In summary, we prepared CdHgTe quasi core/shell NCs via one-step reaction by making use of the great difference in the solubility of CdTe and HgTe in solution. This method was facile, and above all, the resultant NCs possessed high-quality PL properties without size-selective precipitation. The highest QY ∼80% was obtained through optimizing the experimental conditions. The cytotoxic effects of CdHgTe NCs showed that the quasi core/shell structure of Cd0.21Hg0.79Te NCs was very stable and little cadmium ions could be released, and they could find applications in tissue imaging or detection.

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Figure 8. L929 cell proliferation incubated with different concentrations of Cd0.21Hg0.79Te NCs solution and powders at (a) 1 day, (b) 5 days, and (c) 1 week.

ASSOCIATED CONTENT

S Supporting Information *

Purification of NCs, measurements of QYs, XPS spectra of CdTe and HgTe NCs, the Cd wt % change in the Cd0.85Hg0.15Te NCs at the growth and etched process, XPS spectra of Te element in CdHgTe NCs, and the morphology of L929 cell incubated with CdHgTe NC powder and solution. This material is available free of charge via the Internet at http://pubs.acs.org.

incubated with HgTe NCs. For the cells incubated with Cd0.85Hg0.15Te NCs, many cells are dead, and the living cells do not show spread morphology, indicating their state is not very well. Although the cytotoxicity of HgTe is the lowest, its PL QY is much lower than that of Cd0.21Hg0.79Te NCs, and therefore,

Figure 9. SEM image of L929 cell in (a) control group and incubated with (b) Cd0.21Hg0.79Te NCs at 4 days. 4125


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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (Haizhu Sun), hcsun@mail. jlu.edu.cn (Hongchen Sun); Tel +86-431-85099667; Fax +86431-85099667. Author Contributions ⊥

Wenjing Cai and Liming Jiang contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by NSFC (20804008, 30830108) and the Science Technology Program of Jilin Province (201201064).

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High Quality CdHgTe Nanocrystals with Strong Near-Infrared

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