Microwave-assisted synthesis of high-quality CdTe/CdS@ZnS–SiO2 near-infrared-emitting quantum dots and their applications in Hg2+ sensing and imaging

https://doi.org/10.1016/j.snb.2014.10.031Get rights and content

Highlights

  • N-acetyl-l-cysteine capped CdTe/CdS@ZnS–SiO2 near-infrared (NIR)-emitting quantum dots (QDs) were designed and synthesized.

  • The CdTe/CdS@ZnS–SiO2 QDs showed strong NIR emission, excellent colloidal- and photo- stability, low toxicity and good water-solubility.

  • The CdTe/CdS@ZnS–SiO2 QDs presented high sensitivity and selectivity for detection of Hg2+.

  • The CdTe/CdS@ZnS–SiO2 QDs was successfully used for imaging of Hg2+ in living cells.

Abstract

Highly luminescent, good stable and low toxic N-acetyl-l-cysteine (NAC) capped CdTe/CdS@ZnS–SiO2 near-infrared (NIR)-emitting quantum dots (QDs) were successfully fabricated in aqueous solution via a microwave irradiation reduction route, in which thiol-capped CdTe/CdS QDs were employed as core templates and ZnCl2, NAC and tetraethyl orthosilicate as shell precursors. This presented ZnS-like clusters filled hybrid SiO2 model not only greatly improved the brightness and stability of original CdTe/CdS QDs, but also tremendously decreased the cytotoxicity towards HeLa cells. Furthermore, it was found that Hg2+ could effectively selective quench the QD NIR emission based on electron transfer process. On the basis of this fact, a simple, rapid and specific method for trace Hg2+ determination was proposed. Under optimal conditions, the fluorescence intensity decreased linearly with the concentration of Hg2+ ranging from 5.0 × 10−9 to 1.0 × 10−6 M and the limit of detection for Hg2+ was 1.0 × 10−9 M (S/N = 3). As practical applications, the novel NIR sensor has been demonstrated to monitor and image Hg2+ level in milk power and HeLa cells respectively with satisfactory results obtained.

Graphical abstract

Highly luminescent, good stable and low toxic N-acetyl-l-cysteine (NAC)-capped CdTe/CdS-ZnS@SiO2 quantum dots were successfully synthesized via a promising microwave strategy, and then applied in the detection and imaging of Hg2+.

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Introduction

Near-infrared (NIR) fluorescence-based bio-sensor and -imaging applications have significantly contributed to diagnostic and biomedical fields because at the NIR wavelength (650–900 nm) light has deep tissue penetration and induces minimal autofluorescence [1], [2], [3], [4]. In comparison with organic dye, semiconductor nanocrystals (quantum dots, QDs) as one kind of the most promising NIR lumophores have been widely studied in the past decade benefiting from their unique size-tunable optical properties [5]. In spite of their widespread use, the QD-based NIR fluorescence techniques have remained a challenging research objective due to the following reasons: (1) the most routine preparation of NIR-emitting QDs through high-temperature organometallic approaches may involve relatively complicated multistep processes and intrinsic hydrophobic property restricted their direct applications in biosystems [6], [7]. On the other hand, when using aqueous methods, almost the obtained NIR-emitting QDs have inferior luminescence efficiency due to the poor nucleation environment [8], [9], [10], [11]. (2) For application the water-soluble thiols play an important role and contribute greatly to the stability and functionality of the resulting QDs [12]. Unfortunately, these QDs are readily subjected to the photo-oxidation and photo-bleaching when used to sensing in complex biological media such as living cells [13]. (3) QDs are difficult to functionalize in a controlled manner and the release of heavy metal ions from the particle surface produces potential cytotoxicity [14], [15].

Thus it can be seen that the surface structure of QDs seriously affect the nature of these nanocrystals, thereby hindering their further developments in active fields. Recently, epitaxial growth of an inorganic shell (mainly including CdS, ZnS, CdSe and SiO2) on the surface of initial QD cores to prepare core/shell QDs has been explored for improving the quality of visible-emitting QDs in aqueous solution [16], [17], [18]. Among these core/shell structures, SiO2 is one superior hydrophilic coating layer regarding their high stability, good biocompatibility and convenient processability in aqueous media [19], [20]. Nevertheless, the existence of a SiO2 shell has its own limitation such as the decrease of fluorescence intensity [21], [22]. To solve this problem, Yang and Murase first presented the fabrication of hybrid SiO2-coated CdTe nanocrystals [23]. In this model, a hybrid SiO2 shell with CdS-like clusters was formed on a CdTe core. The SiO2 layer prevented a lattice constant mismatch between the core and clusters, while the CdS-like clusters donated energy to the core, resulting in the improvement of fluorescence efficiency. In order to further decrease the toxicity of Cd2+, Zhu and co-workers recently reported the preparation of ZnS-like clusters filled hybrid SiO2-coated CdSeTe QDs via a microwave-assisted approach [24]. The obtained CdSeTe@ZnS–SiO2 QDs possessed higher fluorescence and lower cytotoxicity, and have been successfully applied in the detection of Cu2+. However, to our knowledge, the high-quality water-soluble NIR-emitting QDs have remained elusive, despite pioneering efforts devoted to the rational modification of visible-emitting QDs.

Much attention has been paid to the development of fluorescent sensors for the rapid, sensitive and selective detection of chemically and biologically significant ionic species. Mercury, is considered highly toxic and widespread pollutant, and it exists in a variety of different forms (metallic, ionic, and as a part of organic salts and complexes) [25]. Mercuric ion (Hg2+), as one of the most stable inorganic forms of mercury, can accumulate in organisms and interact with the thiol groups in protein to cause serious threat to human health and natural environment [26]. Therefore, development of fluorescent Hg2+ probes is of vital importance. Up to now, a variety of fluorescent chemodosimeters for trace amounts of Hg2+ detection have been developed and mainly located in the UV and visible range [27], [28], [29], [30], [31], [32], [33]. To develop the application of Hg2+-selective fluorescent sensors in complex biological systems, NIR fluorescent nanosensors should be a more suitable and reliable choice because the fact that the NIR window it offers in the sensor design of lower background interference, and commonly leading a wider linear range and lower detection limit. However, the reports concerning Hg2+ fluorescent nanoprobes reliant on NIR emission were rather rare [34], [35].

With these insights, here we have presented the design and preparation of ZnS-like clusters filled hybrid SiO2-coated CdTe/CdS NIR-emitting QDs. As shown in Scheme 1, the mercaptopropionic acid (MPA)-capped NIR-emitting CdTe/CdS QDs was first synthesized using a previous hydrothermal method with small changes [36]. Then, the CdTe/CdS QDs were coated with a very thin SiO2 shell containing Zn2+ and N-Acetyl-l-cysteine (NAC) molecules in an alkaline condition. In this case, NAC was selected as the precursors of ZnS and the later stabilizers of QDs. It is worth mentioning that NAC as alternative stabilizers of MPA for NIR-emitting QDs that are safer and more environmentally friendly, which are beneficial to decrease the toxicity [36]. After that, the SiO2 coated QDs were refluxed under microwave irradiation. Compared with CdTe/CdS QDs, the obtained CdTe/CdS@ZnS–SiO2 QDs possessed higher NIR fluorescence, stronger stability and lower cytotoxicity. Furthermore, we found that the NIR fluorescence of QDs can be selectively quenched by Hg2+ based on electron transfer process. These QDs have been demonstrated to accord with demands for the nanoprobes of NIR in detecting and imaging of trace Hg2+ in milk powder and living cells, respectively.

Section snippets

Reagents

All the starting materials of the CdTe/CdS and CdTe/CdS@ZnS–SiO2 NIR-emitting QDs synthesis were used without further purification. CdCl2·2.5H2O (99.0%), NaBH4 (96.0%), tellurium powder (99.9%) and ZnCl2 were obtained from Sinopharm Chemical Regent Co. Ltd. (Shanghai, China). Tetraethoxysilane (TEOS), N-Acetyl-l-cysteine (NAC), mercaptopropionic acid (MPA, 99%), 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) and dimethylsulfoxide (DMSO) were purchased from Sigma–Aldrich

Preparation and characterization of NIR-emitting CdTe/CdS@ZnS–SiO2 QDs

The fabrication procedure and condition of CdTe/CdS@ZnS–SiO2 QDs are schematically illustrated in Scheme 1. Briefly, the MPA-capped NIR-emitting CdTe/CdS QDs were first prepared and purified. Afterwards, the CdTe/CdS QDs were coated with a thin silica layer at room temperature in the presence of Zn2+ and NAC (Step 1). It is worth noting that Zn2+ and NAC were the precursors of ZnS-like clusters, because Zn2+ can gradually incorporate with S2− released from the gradual thermal decomposition of

Conclusion

In summary, coating of ZnS-like clusters filled hybrid SiO2 shell has been proven to be an effective strategy for producing high-quality NIR-emitting QDs in aqueous solution for the first time. The as-prepared CdTe/CdS@ZnS–SiO2 QDs showed strong NIR emission, excellent colloidal- and photo- stability, low toxicity and good water solubility. Such QDs have been further used as a novel sensing probe for label-free, sensitive detection of Hg2+ with a detection limit as low as 1 nM. This sensing

Acknowledgments

We gratefully acknowledge the financial support from National Natural Science Foundation of China (21375043, 21175051 and 21405139), and scientific research start-up funding of Zhejiang University of Technology (101010629).

Jing Wang was born in Zhejiang Province, China, in 1986. He received his BS and Ph.D. degree in 2008 and 2013, respectively in Huazhong Agricultural University under the direction of Professor Heyou Han. Presently he is working as a lecturer at Zhejiang University of Technology. His scientific interests focus on functionalized quantum dots for fluorescent and electrochemiluminescent applications.

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      This can not only improve the photoluminescent quantum yield, stability, and light resistance, but also reduce the toxicity of cadmium. Nanocomposites based on core/shell QDs coated with SiO2 have been reported, and these QDs exhibit high quantum yield and excellent photostability [30–36]. Although this type of nanocomposite shows excellent results, there are no related reports concerning the temperature dependence of their PL spectra.

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    Jing Wang was born in Zhejiang Province, China, in 1986. He received his BS and Ph.D. degree in 2008 and 2013, respectively in Huazhong Agricultural University under the direction of Professor Heyou Han. Presently he is working as a lecturer at Zhejiang University of Technology. His scientific interests focus on functionalized quantum dots for fluorescent and electrochemiluminescent applications.

    Na Li was born in Hubei Province, China, in 1986. She received her BS degree in 2010 and now she is a Ph.D. student in Huazhong Agricultural University under the direction of Professor Heyou Han. Her scientific interests focus on functionalized quantum dots for fluorescent sensing and imaging applications.

    Feng Shao was in Shandong Province, China, in 1989. He received his MS degree in 2014 in Huazhong Agricultural University under the direction of Professor Heyou Han. His research interests focus on synthesis and modification of plasmonic nanostructures for biosensing and bioimaging.

    Heyou Han was born in Anhui Province, China, in 1962. He received his Ph.D. degree in Wuhan University in 2000 and he was a postdoctor in Jeckson State University (America) from 2000 to 2004. He has been a Professor of Huazhong Agricultural University since 2004. He has published over 80 papers in international journals. His research interests focus on functionalized nanomaterials for bioanalytical, food safety and energy applications.

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