Novel aqueous synthesis methods for ZnTe/ZnSe and Mn²⁺-doped ZnTe/ZnSe Type-II core/shell quantum dots

Abstract

In this paper, novel approaches for the synthesis of Type-II core/shell quantum dots (ZnTe/ZnSe QDs) and Mn²⁺-doped Type-II core/shell quantum dots (Mn : ZnTe/ZnSe QDs) with mercaptopropionic acid (MPA) as stabilizer were proposed. To our knowledge, it is the first time that both core/shell and doping synthesis methods are employed together to prepare photoluminescent Type-II core/shell Mn : ZnTe/ZnSe QDs in aqueous phase. The results show that the photoluminescence (PL) character of ZnTe/ZnSe QDs significantly changed with the doping of Mn²⁺. Furthermore, the fluorescence color of the QDs ranged over an extended emitting wavelength from blue to orange. Also, an increased PL quantum yield (from 5.3% to 7%) was obtained. The effects of the experimental conditions for synthesis and the PL emission properties were investigated in detail. The simple synthetic routes and improved optical properties make these QDs excellent probes for various strategies in chemo/biosensing and bioimaging.

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1. Introduction

Semiconductor nanocrystals or quantum dots (QDs) have a variety of applications in photonic devices and biological probes for fluorescent whole-body imaging,^1–3 cancer therapy⁴ and other fields because of their optical and electronic properties including broad absorption, narrow emission spectra, large extinction coefficients, long fluorescence lifetime, resistance to photobleaching and size-tunable emission.^5–8 In the past decades, considerable research efforts have been directed on QDs, such as CdE (E = S, Se, Te), as fluorescent markers and particularly, their applications in the biological field. However, the toxicity of QDs with heavy metal ions presents a challenge in the fields of biomedical pharmacy, medicines and chemical reagents. Many physicochemical modifications have been applied to reduce the toxicity of the CdE (E = S, Se, Te) QDs, such as coating them with silica shell or organics.^9,10 Nevertheless, these efforts cannot entirely solve the problem. In order to prevent the leakage of heavy metal ions, other types of QDs have been reported. Among them, the preparation of Zn-based QDs without heavy metal elements, such as ZnS, ZnSe and ZnTe QDs, is an ideal choice for low-toxicity applications. A key outstanding problem is that these materials have large band gaps in general. As a result, they can hardly emit optical signals in visible ranges, which severely limits their applicability.

Until now, two impactful synthesis routes for ZnE (E = S, Se, Te) QDs have been reported: core/shell modification and doping preparation. On the one hand, due to their effective band gap engineering and tunable optical properties, core/shell heterostructured QDs composed of two semiconductor materials have attracted much attention. By taking advantage of the type-II band alignment and quantum confinement effects, type-II core/shell QDs (ZnTe/ZnSe) with strong photoluminescence have been reported.¹¹ In Type-II QDs, the valence and conduction bands in the cores are lower (or higher) in energy than those in the shells, and the effective band gap is smaller than that of either the constituent core or shell. In addition, the effective band gap of Type-II QDs is governed by the band offsets of the cores and shells.^12–14 As a result, Type-II core/shell QDs can emit longer wavelengths that are beyond the bulk band gaps of either core or shell. This consequence can be successfully used for in vivo imaging and photovoltaic applications.^15,16 So far, many Type-II core/shell systems have been reported, including CdTe/CdSe and CdSe/CdS.^17,18 ZnTe offers an attractive alternative to CdTe as a core material because of its low ionization potential (high valence band edge) for effective hole localization. In recent studies, ZnTe/ZnSe QDs have been synthesized as the first cadmium-free Type-II QDs by the organometallic route.^19,20 Bang et al. addressed comprehensive studies on the synthetic method for colloidal ZnTe/ZnSe QDs. Solution–liquid–solid synthesis of ZnSe/ZnTe quantum wires also has been reported by Buhro and co-workers.²¹ However, organometallic methods are usually carried out in harsh conditions, under high temperatures maintained at 200–250 °C. The organometallic precursors used also involve some toxic and expensive reagents, such as trioctylphosphine and tributylphosphine.

On the other hand, QDs doped with transition–metal impurities (e.g. Mn : ZnSe QDs) have attracted considerable attention lately as a new generation of luminescent material. The most important motivations for incorporation of intentional impurities in these QDs were to improve and control their physical properties, including optical properties, magnetic properties, longer fluorescence lifetime and higher thermal stability.^22,23 Doped QDs can potentially retain the distinct advantages that the undoped QDs possess, such as narrow and symmetric emission with tunable colors, broad and strong absorption, and reasonable stability. In addition, doped QDs can avoid the self-quenching problem due to their substantial ensemble Stokes shift. Diverse transition–metal and lanthanide ions have been doped into QDs. Mn ions stand out among the various transition–metal ions for their acceptable toxicity under industrial standards.²⁴ Mn²⁺-doped ZnSe QDs with good fluorescence properties have been synthesized successfully in aqueous solution.²⁵

In this study, we employed both core/shell and doping synthesis methods to prepare ZnTe/ZnSe QDs (Type-II core/shell QDs) and Mn²⁺-doped ZnTe/ZnSe QDs (Mn : ZnTe/ZnSe QDs) in the aqueous phase. Compared with organometallic routes, aqueous synthesis of hydrophilic QDs shows great promise, with biocompatibility, less toxic reagents, lower reaction temperature, and comparable photoluminescence (PL) quantum yield (QY). The optimal precursor ratio and the synthesis conditions for obtaining ZnTe/ZnSe QDs and Mn²⁺-doped ZnTe/ZnSe QDs with good PL emission properties were studied in detail. Compared with ZnSe QDs synthesized in aqueous solution, ZnTe/ZnSe Type-II core/shell QDs and Mn²⁺-doped ZnTe/ZnSe QDs have more reliable optical properties. These results indicate that they have the potential for application in biological assays and imaging investigations as outstanding fluorescent labels.

2. Experimental

2.1 Chemicals

All chemicals were of analytical reagent grade and used without further purification. Mercaptopropionic acid (MPA) (99%), tellurium powder (∼200 mesh, 99.8%), selenium powder (∼200 mesh, 99.9%), Zn(NO₃)₂·6H₂O (99.9%), MnCl₂·4H₂O (99.9%), and NaBH₄ (99%) were purchased from Aldrich Chemical Co. The water used in all experiments had a resistivity higher than 18 M Ώ cm⁻¹.

2.2 Synthesis of ZnTe/ZnSe quantum dots

ZnTe/ZnSe QDs were synthesized in aqueous solution as follows: sodium hydrogen tellurium (NaHTe) and sodium hydrogen selenium (NaHSe) were prepared in aqueous solutions by the reaction of NaBH₄ according to the sodium hydrogen telluride synthesis method.²⁶ The molar ratios of tellurium powder to NaBH₄ and selenium powder to NaBH₄ all were 1 : 2. 5 mL of Zn(NO₃)₂ stock solution (0.5 × 10⁻² M) was injected into a 250 mL three-necked flask and degassed for 30 min by bubbling with nitrogen. Fresh NaHTe solution was added to the N₂-saturated Zn(NO₃)₂ solution at pH 11 in the presence of MPA as the stabilizing reagent. Zn-to-MPA precursor ratio was 1 : 6. The pH of the mixture was adjusted to 11 by dropwise addition of a 1 M NaOH solution with stirring. The reaction was then switched from nitrogen bubbling to nitrogen flow and subjected to reflux at 100 °C for 1 h with a condenser attached. Then we added fresh NaHSe into the three-necked flask and heated it for 4 h. Finally, we cooled down the reaction mixture to room temperature and used the as-prepared ZnTe/ZnSe QDs directly without any post-treatments.

2.3 Synthesis of Mn : ZnTe/ZnSe quantum dots

In aqueous solution, Mn : ZnTe/ZnSe QDs were synthesized under different conditions with different addition orders and reaction conditions. In a typical experiment, 0.2 mL MnCl₂ solution (1.25 × 10⁻² M), 5 mL of Zn(NO₃)₂ stock solution (0.5 × 10⁻² M) and MPA were mixed in a 250 mL three-necked flask and adjusted to pH 11 by the addition of 1 M NaOH solution with stirring. Zn-to-MPA precursor ratio was 1 : 12. The solution was degassed for 30 min with nitrogen; then the freshly prepared NaHTe solution was added into the mixture. The reaction was subjected to reflux at 100 °C for 1 h in nitrogen atmosphere. Afterwards, fresh NaHSe was added into the three-necked flask and heated for 6 h. After cooling to room temperature, the obtained Mn : ZnTe/ZnSe QDs were used directly without any post-treatments. The QD solution was precipitated by ethanol, and the precipitate was centrifuged, washed with ethanol, and dried in vacuum. The obtained powder was used for subsequent characterization.

2.4 Characterization

Fluorescence measurements were performed on a Shimadzu RF-5301 PC spectrofluorophotometer. UV-vis absorption spectra were obtained using a GBC Cintra10e UV-visible spectrometer. In both experiments, a 1 cm path-length quartz cuvette was used. Transmission electron microscopy (TEM) images were obtained using a Hitachi electron microscope operating at 200 kV. TEM samples were prepared by dropping the aqueous QD solution onto carbon-coated copper grids and allowing the excess solvent to evaporate. FT-IR spectra were recorded with a Bruker IFS66V FT-IR spectrometer equipped with a DGTS detector (32 scans). Powder X-ray diffraction (XRD) was carried out with a SIEMENS D5005 diffractometer with Cu Kα radiation.

3. Results and discussion

3.1 Characterization of ZnTe/ZnSe QDs and Mn : ZnTe/ZnSe QDs

In developed biosensing and bioimaging systems, water-soluble QDs are in high demand for their ligand exchange with bifunctional small molecules. To make the QDs water soluble, we chose MPA as a small molecule stabilizer to accomplish QD decoration. In this study, water-soluble ZnTe/ZnSe QDs and Mn : ZnTe/ZnSe QDs with MPA as capping reagent were prepared directly in aqueous solutions. We studied their PL properties by comparing the spectra for the emission intensities of QDs, as shown in Fig. 1. The emission peak of ZnTe/ZnSe QDs was at 480 nm (Fig. 1b). It was obviously different from the ZnSe QD emission peak at 445 nm (Fig. 1a). Moreover, a red-shift of about 100 nm of the band-edge emission features can be observed with the Mn²⁺-doped ZnTe/ZnSe QDs (Fig. 1c). This transformation can be counted as a signature, distinct feature to indicate that the incorporation of Mn²⁺ into the ZnTe/ZnSe QDs was successful. Fig. 1 also shows a photograph of the ZnSe QDs, ZnTe/ZnSe QDs and Mn : ZnTe/ZnSe QDs under UV light. From the photograph, we can see that under UV light, ZnTe/ZnSe QDs and Mn : ZnTe/ZnSe QDs in aqueous solution emit strong blue and yellow fluorescence, respectively. This corresponds to the fluorescence emission spectra observed.

Fig. 1 The fluorescence spectra of (a) ZnSe QDs, (b) ZnTe/ZnSe QDs and (c) Mn : ZnTe/ZnSe QDs. The inset shows a photograph of (a) ZnSe QDs, (b) ZnTe/ZnSe QDs and (c) Mn : ZnTe/ZnSe QDs under UV light.

FT-IR spectra of the purified and dried ZnTe/ZnSe QDs and Mn : ZnTe/ZnSe QDs with MPA as capping reagent are given in Fig. 2. The most pronounced IR absorption bands of ZnTe/ZnSe QDs and Mn : ZnTe/ZnSe QDs occurred at 3413.14 cm⁻¹ (Fig. 2a) and 3403.91 cm⁻¹ (Fig. 2b), respectively, representing O–H stretching vibration. The symmetric and asymmetric stretching vibrations of the carboxylate groups appeared at 1577.56 cm⁻¹ (sνCOO⁻) and 1403.14 cm⁻¹ (mνCOO⁻) in Fig. 2a and 1577.56 cm⁻¹ (sνCOO⁻) and 1405.06 cm⁻¹ (mνCOO⁻) in Fig. 2b. The spectrum of ZnTe/ZnSe QDs and Mn : ZnTe/ZnSe QDs also displayed bands at 2925.62 cm⁻¹ (νCH₂) and 990.34 cm⁻¹ (δOH) in Fig. 2a and 2921.77 cm⁻¹ (νCH₂) and 991.28 cm⁻¹ (δOH) in Fig. 2b. However, the vibration peak of S–H (2550–2670 cm⁻¹, wνS—H), which should exist in MPA, was not present in the FT-IR spectra. This resulted from the covalent bonds between thiols and Zn atoms on the surface of QDs. This indicates that the ZnTe/ZnSe QDs and Mn : ZnTe/ZnSe QDs have been capped by the MPA stabilizer.

Fig. 2 FT-IR spectrum of (a) ZnTe/ZnSe QDs and (b) Mn : ZnTe/ZnSe QDs.

3.2 Optimization of the synthesis of ZnTe/ZnSe QDs

In the synthesis process, nucleation of host occurs after the addition of NaHTe and Zn precursors. The ZnTe host nucleation was not accompanied by an emission increase. Considering the solubility and reactivity of Se powder under basic conditions, NaBH₄ was used to assist the dissolution of Se powder at room temperature. Then, the ZnSe shell formed following the introduction of Se precursor NaHSe. Fig. 3 shows the wide-angle X-ray diffraction (XRD) patterns (Fig. 3A) and TEM images (Fig. 3B) of the undoped ZnTe/ZnSe QD powder sample. Selected area electron diffraction (SAED) patterns are recorded as shown in the inset of panel B of Fig. 3. The micrograph shows ZnTe/ZnSe QDs of about 4–5 nm diameter. Bars on the top and bottom of Fig. 3A represent bulk cubic structures of ZnSe and ZnTe, respectively. The XRD pattern of the QDs exhibits characteristic peaks at ca. 27.5, 46.5, and 53.4°, corresponding to the (111), (220) and (311) reflecting planes of the cubic zinc blende. Additionally, the main strong peaks observed are for (111), which clearly matches both ZnTe and ZnSe bulk materials that belong to the cubic phase structure. The crystal structure demonstrated by the XRD diffractogram in the sample was zinc blende. It was in accordance with the structure that Bang and Park reported.²⁰

Fig. 3 (A) XRD pattern and (B) TEM images of ZnTe/ZnSe QDs. Inset: Selected electron diffraction pattern of the corresponding particles.

According to previous study, the ZnTe core-only structures showed no PL. This is presumably due to the unstable surfaces, which are prone to nonradiative traps. Meanwhile, the ZnSe that deposited as shell material can passivate the trap sites of ZnTe core QDs. This could go a step further to make the ZnTe/ZnSe QDs emissive.²⁰ The Se-to-Te ratio would play an important role in the PL emission of ZnTe/ZnSe QDs during the reaction process. Fig. 4 shows the changes in PL intensity for a series of ZnTe/ZnSe QDs of different Se-to-Te ratios. When the ratio changed between 5 : 1 and 20 : 1, the PL intensity increased markedly with increased Se-to-Te ratio. When the amount of Se increased, the shell grew thicker. At the ratio of 20 : 1, PL intensity reached its maximum value. A 9-fold increase in PL intensity was obtained. However, the much thicker ZnSe shells can enhance Type-II spatial separations between electrons and holes, which resulted in decreased PL intensity (Fig. 4c).^27–29 As a result, the brightest sample among the series studied was the one in which Se-to-Te ratio was 20 : 1.

Fig. 4 The fluorescence spectra of ZnSe/ZnSe QDs with different ratios of Se-to-Te: (a) 5 : 1, (b) 10 : 1, (c) 30 : 1, (d) 20 : 1.

As Fig. 5A shows, the PL emission spectrum of ZnTe/ZnSe QDs with 350 nm excitation showed the emission peaks at about 480 nm in various pH conditions, which were different from the emission peak of ZnSe QDs (445 nm). This indicates that the ZnTe/ZnSe QDs have been synthesized successfully. pH below 5.9 was unsuitable for the synthesis of ZnTe QDs. Only in weakly acidic and in alkaline matrices could ZnTe QDs be successfully prepared.³⁰ Besides, the usual synthetic condition of ZnSe is alkalescent, according to a previous report.²⁰ The PL intensity of QDs was quenched in acidic conditions such as pH 6 (Fig. 5A(a)). The PL intensity in pH 6 was similar to that in pH 7 (Fig. 5A(b)). The reason was that the low pH environment does not favor ZnSe shell deposition. In addition, a portion of ZnTe QDs with a thin ZnSe shell deposition oxidized to insoluble Te powder upon exposure to air. When the solution pH was increased to alkalinity, PL was enhanced. It reached a 6–8 fold enhancement compared with the PL intensity in pH 7 solution. A series of experiments showed that the pH 11 alkaline solution is the optimum synthetic pH condition for ZnTe/ZnSe QDs. The influence of heating time was also studied, and the results are shown in Fig. 5B. The time was varied from 1 h to 4 h. The deposition of ZnSe shell progressed with increased heating time. At the beginning of the reaction, the PL intensity of small-sized QDs was weak. With increased heating time, the nanocrystal grew, and PL intensity sharply increased to 91.9% in 3 h. The PL intensity reached maximum at 3.5 h after the addition of NaHSe. From 3.5 h to 4 h, the PL intensity became stable, with only 2% increment. Therefore, 3.5 h was selected as the optimum synthesis time. The final quantum yield (QY) of the ZnTe/ZnSe QDs was 5.3%. It is competitive with the organometallic route, whose QY was 6%.²⁰

Fig. 5 (A) The influence of pH on the PL intensity of ZnTe/ZnSe QDs (a) pH 6, (b) pH 7, (c) pH 9, (d) pH 10. Inset: a plot of normalized intensity against the pH. (B) The influence of heating time on the PL intensity of ZnTe/ZnSe QDs: (a) 1.5 h, (b) 2 h, (c) 2.5 h, (d) 3 h, (e) 3.5 h, (f) 4 h. Inset: a plot of normalized intensity against the heating time.

3.3 Optimization of the synthesis of Mn : ZnTe/ZnSe QDs

The introduction of doped QDs immediately attracted much attention. The main reason was that both electronic states and electromagnetic fields could be modified, and the optical properties of impurities were reduced. In order to improve the wavelength properties of ZnTe/ZnSe QDs, we doped metal ions with ZnTe/ZnSe QDs. We chose Mn²⁺ as dopant for ZnTe/ZnSe QDs because of its acceptable toxicity and good possibility of altering and controlling QDs' physical properties. The dopant Mn²⁺ led to some interesting results, such as emission red-shift and PL stabilization. Mn²⁺-doped QDs exhibited a typical photoluminescence. A pure and strong dopant emission was observed at about 575 nm due to the Mn^{2+ 4}T₁ → ⁶A₁ transition. These phenomena were not found in the bulk state because their electronic states were confined in a very small volume. In comparison to the absorption band gap of the ZnTe/ZnSe QDs, Mn²⁺-doped QDs have a large Stokes shift resulting from their relatively small emission energy gap. Mn²⁺-doped ZnTe/ZnSe QDs with substantial ensemble Stokes shift can avoid the self-quenching problem usually caused by the intramolecular ground-state dimer complex or energy transfer between adjacent QDs. Additionally, as a dopant, an amount of Mn²⁺ would diffuse into the host layer during the annealing process of the synthesis. The ions in each individual d-dot are clustered together in close proximity.³¹ The emission centers away from the surface trap states of the nanocrystals, thereby improving the optical performance of the nanocrystals. Thus, the Mn ions in the ZnTe/ZnSe QD host crystal field increased the PL intensity.

There are three synthesis strategies with different orders of adding precursors, according to Peng and Yan's report.³² Following the different orders, several different PL spectra of QDs were exhibited, as shown in Fig. 6. The inset shows the synthesis process. The first strategy (Fig. 6a) formed ZnTe before adding Mn²⁺. This growth-doping process formed the nuclei with pure dopant cations followed by the growth of a host shell on the core. After dopant growth, a new layer of shell was coated onto the surface-doped nucleus. The PL intensity of QDs increased slowly upon heating at 100 °C and reached its maximum value (PL QY = 1%) after 5 h. The maximum value was too weak to reach a satisfactory PL degree. The second strategy (Fig. 6b) introduced both dopant and host ions into the reaction system at the same time. After heating for 5 h, the QD spectrum had two peaks at 483 nm and 566 nm. Their intensities were similar. This result may be from undoped ZnTe/ZnSe QDs and surface-adsorbed dopants, which are often mixed with the doped QDs. What is more, the peak at 483 nm was stronger than the one at 566 nm; to some extent, this would influence the PL intensity measurement for Mn : ZnTe/ZnSe QDs. This indicates that the doping efficiency, which is also related to the types of Mn²⁺ precursor and particle size, was low. The PL QY was 1.5%. For these reasons, this strategy was not appropriate. In the last strategy (Fig. 6c), Mn²⁺ and NaHTe were mixed at the beginning. Then, Zn²⁺ and NaHSe were successively added. The Mn²⁺-doped ZnTe/ZnSe QDs were prepared through a nucleation-doping strategy, which was realized by mixing dopant and host precursors during nucleation. Mn²⁺ was well doped in the host during this process. As a result, MnTe was formed. After nucleation, the reaction condition was tuned to a sufficiently mild degree, which induced the inactivation of the precursors. Then the growth of the ZnSe shell became the main process. Besides the number of ZnSe that deposited on the core as shell, some excess also remained in solution, forming ZnTe/ZnSe with an observable fluorescence signal (EM 475 nm). Thus, the product had two emissions at 475 nm and 570 nm, respectively. The peak intensity at 475 nm was much lower than the one at 570 nm. This result indicates that in the process of nucleation-doping, most core/shell QDs were doped successfully. Also, the PL intensity at 570 nm was 9-fold greater than the product of growth-doping under the same reaction condition. Compared with the other two synthetic approaches, the PL intensity from the last strategy was the strongest, under the same conditions. The reason was that nucleation-doping products cannot be quenched by lowering the temperature.^33–36 According to the result, we chose the last synthesis method for Mn : ZnTe/ZnSe QDs synthesis.

Fig. 6 The fluorescence spectra of Mn : ZnTe/ZnSe QDs with different synthesis methods: (a) dopant Mn²⁺ introduction after ZnTe nucleation (growth doping), (b) dopant Mn²⁺ introduction along with host precursors Zn²⁺ and NaHTe, (c) dopant Mn²⁺ introduction before nucleation of ZnTe (nucleation doping). Inset: the schematic illustration of (a), (b) and (c).

In later experiments to optimize conditions, the concentrations of the dopant Mn²⁺ in the nuclei were tuned by varying the Zn-to-Mn precursor ratio from 3% to 10%. Just as Fig. 7A shows, PL intensity changed with Zn-to-Mn ratio and reached maximum at 5%. Compared with 5%, a higher Zn-to-Mn ratio, which represents fewer Mn²⁺ ions doped into the ZnTe/ZnSe QDs, can cause quenching of PL intensity. This phenomenon provides the evidence for the decrease in doping concentration of Mn²⁺ ions when the Zn-to-Mn ratio was further increased. However, too much Mn²⁺ ions did not produce satisfactory PL intensity. This was clarified by the effect of MnTe nuclei. When Zn-to-Mn ratio was lower than 5%, PL intensity decreased with decreasing ratio. The reason was that larger nuclei formed during the nucleation–doping process, which were not as uniform as smaller ones.²³ It also can be seen that the influence of emission at 475 nm was weakened by adjusting the Zn-to-Mn ratio, thereby reaching 32% of 570 nm. Thus, the optimum Zn-to-Mn ratio was 5%. Fig. 7B shows the influence of heating time. PL intensity exhibited a steadily increasing trend with increasing refluxing time and reached maximum by 6 h. It was consistent with the growth of the ZnSe shell. However, the increasing trend for PL intensity of Mn²⁺-doped ZnTe/ZnSe QDs reversed under a heating time longer than 6 h. This may be due to the effect of Ostwald ripening. In this process, larger nanocrystals have been formed at the expense of smaller ones, thus quenching the PL intensity. The best quantum yield (QY) of the Mn : ZnTe/ZnSe QDs was obtained at 7%. The QY was higher than Mn : ZnSe, whose QY was 2.4%, and similar to the core/shell ZnSe : Mn/ZnS QDs with a PL QY of about 9% in water.^25,37

Fig. 7 The fluorescence spectra of ZnTe/ZnSe QDs with (A) different ratios of Mn-to-Zn: (a) 10%, (b) 3%, (c) 8%, (d) 5%, and (B) different heating times (a) 1 h, (b) 2 h, (c) 3 h, (d) 7 h, (e) 4 h, (f) 6 h.

3.4 The stability of ZnTe/ZnSe QDs and Mn : ZnTe/ZnSe QDs

In order to test the potential application value of ZnTe/ZnSe QDs and Mn : ZnTe/ZnSe QDs, we compared their optical stability. The PL emission intensity of Mn²⁺ (⁴T₁ → ⁶A₁)-doped ZnTe/ZnSe QDs remained high under room temperature and daylight for a week (Fig. 8), while the ZnTe/ZnSe QD PL intensity decreased after two days, and only about 15% remained after a week. This result demonstrates that the PL stability of Mn : ZnTe/ZnSe QDs was much better than that of ZnTe/ZnSe QDs in air. This may be the result of the unique structure of Mn : ZnTe/ZnSe QDs. After doping with Mn²⁺ by the nucleation–doping method, the host was overcoated by the ZnSe shell, which can protect the nucleus with the dopant. Thus, stability has been largely improved. Although the stability of ZnTe/ZnSe QDs was not as good as Mn : ZnTe/ZnSe QDs, the successful aqueous synthesis of minimally toxic ZnTe/ZnSe QDs provides a new synthesis method for nanomaterials that can be used for detection. For the potential application of Mn : ZnTe/ZnSe QDs in biosensing and bioimaging, their stability in phosphate (2 mmol L⁻¹), TRIS (10 mmol L⁻¹) and borate (0.2 mol L⁻¹) buffered saline systems were also investigated. A pH sequence of the different buffer solutions was mixed with QDs under the volume ratio of 1 : 1 (as shown in Fig. 9). There was no obvious quenching in any buffer solution. The good stability, low toxicity and water solubility are helpful for the application of Mn : ZnTe/ZnSe QDs in biomedical assays and imaging of cell tissues, and even for in vivo investigations.

Fig. 8 Stability of ZnTe/ZnSe QDs and Mn : ZnTe/ZnSe QDs in daylight.

Fig. 9 Stability of Mn : ZnTe/ZnSe QDs in (a) phosphate, (b) TRIS and (c) borate buffered saline system. I₀ represents the original PL intensity of QDs; I represents the PL intensity of QD-buffer solution after 30 min.

4. Conclusions

In summary, the aqueous synthesis of MPA-capped ZnTe/ZnSe Type-II core/shell QDs was successfully carried out by a facile route. In order to improve the emission and stability of ZnTe/ZnSe QDs, Mn²⁺-doped ZnTe/ZnSe QDs were obtained using a nucleation–doping strategy in aqueous solution for the first time. Extensive efforts were made to explore the properties and optimize the synthesis and luminescence performance of the ZnTe/ZnSe and Mn : ZnTe/ZnSe QDs. The obtained QDs were extremely water soluble and photostable. Moreover, they all had lower toxicity than the traditional QDs in aqueous solution. Due to their unique and stable optical properties, the non-cadmium ZnTe/ZnSe and Mn : ZnTe/ZnSe QDs can be used in biomedical applications as a novel type of fluorescent nanoprobe.

Notes and references

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