Retracted Article: Facile fabrication of water-dispersible AgInS₂ quantum dots and mesoporous AgInS₂ nanospheres with visible photoluminescence

Abstract

A facile one-step method for preparing water-dispersible carboxylic acid-terminated AgInS₂ quantum dots (QDs) with near-infrared (NIR) emission was developed. In the presence of polyethylenimine, the as-prepared AgInS₂ QDs gathered together to form QDs-self-assemblies, followed by centrifugating to obtain QDs aggregates. After that, the resulting QDs aggregates were calcined at elevated temperature to yield AgInS₂ nanospheres. Experimental results confirmed that the AgInS₂ nanospheres exhibited a mesoporous structure and consisted of repeated units of AgInS₂ QDs. The mesoporous AgInS₂ nanospheres had photoluminescence (PL) in the visible region that was different from that of the original AgInS₂ QDs emitted in the NIR region. High PL stability and low cytotoxicity of the nanospheres were verified experimentally. These results further revealed the potential of mesoporous AgInS₂ nanospheres for biomedical application, especially serving as novel nanoprobes for in vitro and in vivo PL imaging.

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1. Introduction

High specific surface area, uniform pore microstructure and tailorable surface functionality have been markedly promoted for the further development of three-dimensional (3D) mesoporous materials. In the past two decades, great efforts have been dedicated to potential applications of 3D mesoporous materials, especially in the fields of storage, catalysis, adsorption, transportation, separation, drug delivery and optical devices.^1–9 As reported previously, silica, carbon, metal oxides and metals are the most extensively used precursors for mesoporous materials.^10–12 In contrast, tremendous studies have been focused on the preparation of silica-based mesoporous materials, while preparing colloidal semiconductor quantum dots (QDs)-based mesoporous materials with photoluminescence (PL) properties is highly desired and still remains a challenge because of the unique chemical complexity of QD-based photoluminescent mesoporous materials.^13,14

In earlier literatures, mesoporous silica nanospheres covalently bonded with luminescent materials (upconverted luminescent nanoparticles (lanthanide complexes) and photoluminescent semiconductor QDs) consisting of luminescent materials as the core and mesoporous silica as the shell have been prepared, together with their good biocompatibility and PL character for bioimaging.^15–20 Feng et al. reported magnetic mesoporous silica nanospheres covalently bonded with near-infrared (NIR) luminescent lanthanide complexes, i.e., Ln(DBM)₃phen–MMS (Ln = Nd, Yb) that exhibited potential applications in drug delivery and optical imaging.¹⁶ Gai et al. developed core–shell structured bifunctional (mesoporous, luminescent) nanocomposites by coating mesoporous silica on Y₂O₃:Eu³⁺ spheres via sol–gel process, which were used to track or monitor drug release by the change of luminescence during the release process of loaded drugs and disease therapy.¹⁵ Han et al. synthesized Ag₂S@MSN mesoporous silica core–shelled nanospheres with NIR PL properties, which may be a good candidate for bioimaging or biolabeling.¹⁹ Nevertheless, the preparation of these as-referred photoluminescent mesoporous silica nanospheres are relatively complicated, usually requiring multistep reaction processes and harsh experimental conditions. These defects partly restrain their extended applications. In this regard, to date the facile preparation of photoluminescent mesoporous nanospheres is highly expected and has become a problem to be settled urgently.

So far, QDs-based photoluminescent mesoporous nanospheres have hardly any been reported.¹³ This new type of photoluminescent mesoporous materials is promising since the nanospheres directly consist of QDs without other precursors for mesoporous materials and thus can avoid complicated surface modification and severe shell-growing process by Stöber method. Researchers have found the optical absorption and PL emission in mesoporous ZrO₂ and SiO₂ due to color centers related to oxygen vacancies.^21,22 Similarly, the sulfur-related defects accounting for the optical absorption and PL emission of mesoporous Ag₂S nanospheres have also been discovered.¹³ Except for these, a large amount of surface defects existing on the pore internal surfaces of sulfur-containing QDs-based mesoporous nanospheres could result in the formation of new energy levels in the band gap.²¹ These reported results reveal that the mesostructures with high specific surface area favor the occurrence of sulfur vacancies. Based on the above evidences, it is rational to use sulfur-containing QDs as the sole precursor to prepare photoluminescent mesoporous nanospheres.

Recently, photoluminescent Ag₂S mesoporous nanospheres were synthesized by assembling Ag₂S nanoparticles (NPs) with opposite charges. Authors respectively prepared positively and negatively charged Ag₂S NPs by using –NH₂ and –COOH terminated capping ligands and the oppositely charged Ag₂S NPs assembled under electrostatic interactions, followed by calcination of Ag₂S NPs-assembles at elevated temperature to generate Ag₂S mesoporous nanospheres.¹³ Based on this method, authors synthesized CuS mesostructures further.¹⁴ This is one typical sample of QDs-based photoluminescent mesoporous nanospheres, using Ag₂S QDs as precursors for mesoporous materials. In this method, the oppositely charged Ag₂S NPs were prepared respectively using different capping ligands, which hardly maintained the uniform Ag₂S particle diameter in QDs-assembles and would affect the ordered mesoporous structures (pore size, uniformity). Herein, a facile method to fabricate QDs-based photoluminescent mesoporous nanospheres has been put forward, using –COOH terminated AgInS₂ QDs as the sole precursors and high-branched polyethylenimine (PEI) to form QDs-aggregates (Fig. 1).

Fig. 1 Schematic illustration of the synthetic procedure of AgInS₂ QDs and mesoporous AgInS₂ nanospheres.

With a band gap of ∼1.8 eV at room temperature, low toxic AgInS₂ QDs have a large absorption in the visible-to-NIR region and can emit in the NIR region with high extinction coefficients, serving as an intriguing alternative for bioimaging applications.^23–25 Although various methods have been established for preparing AgInS₂ QDs,^26–29 most of which were prepared in organic phase or using organic ligands as capping reagents, accompanied with high temperature reaction, hydrophobic AgInS₂ QDs and tedious surface modification or ligand exchange (reducing the optical performance of QDs) to yield hydrophilic QDs for biomedical applications. A facile method to directly prepare water-dispersible (hydrophilic) AgInS₂ QDs is more appealing and facilitates their biomedical purposes. Before this work, only one literature has reported the direct synthesis of water-dispersible AgInS₂ QDs.³⁰ It utilized multidentate poly(acrylic acid)-graft-mercaptoethylamine polymers as the capping ligand and S–N₂H₄·H₂O complex as the sulfur precursor. AgInS₂ QDs with water-dispersibility could be obtained at room temperature, but a higher temperature favors the production of QDs (nucleation, growth) with high luminescence efficiency.³¹ Except for the synthesis complexity of multidentate polymers as the capping ligand, S–N₂H₄·H₂O complex as the sulfur precursor to release sulfur resource is hardly controlled, and maintains a spontaneous and fast release in aqueous solution. In general, a lower release rate of sulfur resource is preferable to the preparation of high-quality sulfur-containing QDs.³¹ In this article, we put forward a new method to synthesize water-dispersible AgInS₂ QDs with NIR PL (Fig. 1). The synthesis reaction was performed in ethylene glycol (EG) at 150 °C. We selected 3-mercaptopropionic (MPA) as the capping reagent plus the sulfur precursor because MPA is an extensively used stabilizer for QDs and also can release sulfur resource slowly at elevated temperature.³² This method is facile and the acquirement of high-quality AgInS₂ QDs with water-dispersible and NIR PL could be expected reasonably.

2. Experimental

2.1. Materials

Silver nitrate (AgNO₃, 99.9%), indium acetate (In(Ac)₃, 99.99%), 3-mercaptopropionic (MPA, 99%), ethylene glycol (EG, 98%) and polyethylenimine (PEI, M_w = 10⁴, 99%) were purchased from Sigma-Aldrich. Other chemicals with analytical grade were obtained from Shanghai Chemical Reagent Corp and all chemicals can be directly used as received without any purification. 3-[4,5-Dimethylthiazol-2-yl]-2,5-diphenyl-tetrazolium bromide (MTT) and other biological agents were bought from Invitrogen Corp. HeLa cells were provided by the cell bank of Shanghai Science Academe of China. The water used in experiments was prepared using a Milli-Q water purification system (Milli-Pore, Bedford) and phosphate buffer saline (PBS, 1 mM) with a desired pH was prepared for direct use.

2.2. Preparation of AgInS₂ QDs

In a typical synthetic procedure, 0.1 mmol of AgNO₃, 0.1 mmol of In(Ac)₃, 0.2 mmol of MPA and 50 mL of EG were mixed under argon flow. After vigorous agitation for 10 min, this mixture became a homogeneous suspension system. The resulting system was heated to 150 °C and maintained at this temperature. After several minutes, the AgInS₂ QDs with bright emission were produced in the reaction system. Afterward, the reaction system was cooled to room temperature. The products of QDs were obtained by centrifugating the reaction mixture and the resultant colloidal precipitates were dried in vacuum at 60 °C, and re-dispersed in aqueous solution.

2.3. Fabrication of AgInS₂ mesoporous nanospheres

Briefly, the aqueous solution of as-prepared AgInS₂ QDs was prepared under stirring and the concentration of QDs was 10 mg mL⁻¹. After 10 min of continuous stirring at room temperature to retain a homogeneous solution, freshly prepared aqueous solution of PEI was dropwisely added to the homogeneous solution under slightly stirring. The concentration of PEI in the mixture solution was fixed to 0.1 wt%. When the stirring was halted, the resulting colloidal solution of QDs-self-assembles from gathering under electrostatic interactions was centrifugated, washed with ethanol and dried in vacuum at 60 °C to yield QDs aggregates. The QDs aggregates were heated to different temperatures (250, 300, 350 and 400 °C) with a heating rate of 10 °C min⁻¹ in a furnace under an argon atmosphere. At each fixed temperature, the samples of QDs aggregates were kept for 2 h to yield the corresponding mesoporous AgInS₂ nanospheres.

2.4. Characterization

Transmission electron microscope (TEM, Jeol) images were acquired with a JEOL JEM-1400 TEM operating at 120 kV of acceleration voltage. High-resolution TEM (HRTEM) images were acquired by an H-600 (Hitachi) TEM operating at an acceleration voltage of 200 kV. Scanning electron microscopy (SEM, JEOL) images were obtained with a JCM-5700 SEM. Dynamic light scattering (DLS, Malven Instruments) was utilized to record hydrodynamic radius and zeta potential. X-ray photoelectron spectroscopy (XPS, Thermo Scientific) was captured with a Kratos XSAM 800 X-ray photoelectron spectrometer. N₂ adsorption–desorption isotherms were obtained with a Micro-meritics Tristar 3000 pore analyzer under continuous adsorption conditions at 77 K, equipped with Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) for analysis of the surface area, pore size and pore volume. Thermogravimetric analysis (TGA, Nicolet) was performed with a Diamond TG/DTA/DSC analyzer. UV-vis-NIR absorption spectra were recorded on a UV-2450 spectrophotometer (Shimadzu). Emission spectra were measured with a FLSP 920 fluorescence spectrophotometer (Edinburgh Instruments) with a xenon lamp used for excitation. Fluorescence lifetime was measured by an Edinburgh FL nF900 mode single-photon counting system equipped with a hydrogen lamp for excitation. Powder X-ray diffraction (XRD, Siemens) patterns were obtained by wide-angle X-ray scattering, a D5005 XRD equipped with graphite monochromated Cu K_α radiation (λ = 1.54056 Å).

2.5. Photostability and cytotoxicity assay

To examine the photostability of mesoporous AgInS₂ nanospheres, the nanospheres (50 μg mL⁻¹) were incubated with PBS (1 mM, pH 7.4) at room temperature and then continuously irradiated with a mercury lamp (100 W) for different times (0–180 min). PL intensities of the nanospheres were recorded at different time point. As a reference, PL intensities of quinine sulfate (1 μM) were measured under the same irradiation conditions.

To investigate the cytotoxicity of mesoporous AgInS₂ nanospheres, HeLa cells were cultured as subconfluent monolayers on 25 cm² cell culture plates with vent caps in a 1× minimum essential α medium, supplemented with fetal bovine serum (10%) in a humidified incubator containing CO₂ (5%) at 37 °C. When the cells had grown to subconfluence, they were dissociated from the surface with a solution of trypsin (0.25%) plus EDTA for 30 s, and aliquots of the mixture solution (100 μL) were seeded (1 × 10⁴ cells) in a 96-well plate. After incubation for 24 h at 37 °C, the medium was replaced with 100 μL of serum-free DMEM medium containing different concentrations of mesoporous AgInS₂ nanospheres (0–1 mg mL⁻¹). The treated cells were incubated for 24 h and 48 h in the dark. Cell viabilities were quantitated by a standard MTT assay.

3. Results and discussion

3.1. Synthesis and characterization

TEM images in Fig. 2a exhibit that the as-prepared products (AgInS₂ QDs) are spherically monodisperse NPs that possess an average diameter of ∼7.4 nm through measuring 100 particles in TEM visual field. Lattice fringes of AgInS₂ QD were observed in the HRTEM image (as inserted in Fig. 2a). The interplanar spacing is approximately 0.35 nm, related to the characteristic of (002) facets for the orthorhombic AgInS₂.³⁰ The crystal microstructure of AgInS₂ QDs was characterized by XRD. The XRD patterns (Fig. S1 in ESI†) also indicate that the as-synthesized AgInS₂ QDs exhibit an orthorhombic crystal structure (JCPDS card no. 25-1328).²⁸ These results indicate that the products should be AgInS₂ nanocrystals. Fig. 2b (SEM images) depicts the surface morphology of as-prepared mesoporous AgInS₂ nanospheres (Fig. 2b–e, Sample I, calcined at 250 °C). These nanospheres consist of spherical secondary particles with the average diameter of about 109 nm (based on size statistics of 100 observable particles in visual field) and they are made up of densely packed smaller primary NPs, which induces the mesoporous structure. As indicated in Fig. 2c, TEM images of these nanospheres were recorded, showing spherical and monodisperse NPs of ∼100 nm. TEM images with high magnification (Fig. 2d) reveal that these nanospheres consist of dense and consolidated NPs, and well-ordered structure can be seen. In other words, there is the dense packing of primary NPs (AgInS₂ QDs, ∼7 nm) as observed in Fig. 2d, which implies that an assembly process was conducted for as-prepared mesoporous AgInS₂ nanospheres. After addition of positively charged PEI, the negative charged primary NPs stabilized by MPA were attracted each other under electrostatic interactions.^33,34 The subsequent calcination of resultant NPs-self-assembles resulted in mesoporous AgInS₂ nanospheres comprising densely and compactly packed NPs (AgInS₂ QDs). Size distribution of as-prepared AgInS₂ QDs and mesoporous AgInS₂ nanospheres were examined based on DLS. In Fig. 2e, the size distribution profile of QDs shifted to a dramatically larger value since the self-assembly process of QDs yields larger nanospheres. Both the two size distributions are narrow and suggest the average diameters of QDs and nanospheres are 7.6 nm and 112 nm respectively, in good agreement with the results of corresponding TEM and SEM images (Fig. 2a–c).

Fig. 2 (a) TEM images and inserted HRTEM images of AgInS₂ QDs. (b) SEM images, (c) TEM images and (d) amplifying TEM images of mesoporous AgInS₂ nanospheres. (e) Diameter distribution of AgInS₂ QDs and mesoporous AgInS₂ nanospheres.

We employed XPS technique to analyze the surface chemical composition and element valence state of products. The XPS spectrum of mesoporous AgInS₂ nanospheres (Fig. 3a) shows the Ag, In and S signals from AgInS₂ QDs, but the signals of C, N and O from MPA ligand around the surface of QDs are not observed (Fig. 3a–c, Sample I). The XPS spectrum of AgInS₂ QDs indicates the simultaneous presence of not only the Ag, In and S signals from AgInS₂ QDs, but also the C, N and O signals from MPA (Fig. S2 in ESI†). These results should be attributed to the calcination process of QDs to form nanospheres, during which MPA is calcinated and evaporated. High-resolution XPS spectra in Fig. 3b present a peak at 161.5 eV that is assigned to the 2p electrons of S. The peaks located at 367.5 eV and 374.1 eV are ascribed to Ag 3d_5/2 and Ag 3d_3/2, respectively. In addition, the binding energies of In 3d_5/2 (444.1 eV) and In 3d_3/2 (451.2 eV) well match with the previously reported results for AgInS₂ nanocrystals.³⁵ To character porous structures of products, the N₂ adsorption–desorption isotherms of mesoporous AgInS₂ nanospheres were analyzed. Fig. 3c shows the typical IV curve that is the characteristic of mesoporous solid.³⁶ The corresponding BET surface area is 44.3 m² g⁻¹ and the BJH pore-size distribution suggests an average pore diameter of ∼8.7 nm. The narrow distribution of pore sizes demonstrates almost uniformly agglomerated particles.^10,37 To investigate the formation process of nanospheres under calcination treatment, AgInS₂ QDs were self-assembled to generate QDs-aggregates and then they were treated at elevated temperature. As shown in Fig. 3d, TG analysis was conducted for the QDs-aggregates. From room temperature to 150 °C, the weight loss of QDs-aggregates is neglectful (only ∼1.5%) due to the loss of water molecule and solvents. A distinct weight loss between 150 °C and 350 °C can be found, which is attributed to the combustion and volatilization of the capping ligands (MPA and PEI), accompanied by ∼31.0% of weight loss. At beyond 350 °C, no dramatic weight-loss is found, which implies the nearly wholly consuming of capping ligands and the resulting mesoporous nanospheres comprising almost only AgInS₂ QDs.

Fig. 3 (a) Full XPS spectra of mesoporous AgInS₂ nanospheres. (b) S 2p, Ag 3d and In 3d signals recorded for the mesoporous AgInS₂ nanospheres. (c) N₂ adsorption–desorption isotherms and pore-size distribution of mesoporous AgInS₂ nanospheres. (d) TG curve of the AgInS₂ QDs-aggregates.

3.2. Absorption and emission spectra

The as-prepared AgInS₂ QDs were studied by UV-vis-NIR absorption and PL emission spectra. As displayed in Fig. 4a, the QDs show an absorption peak ∼630 nm and a band edge is found at around 790 nm. Under excitation at 630 nm, the QDs present a sharp PL emission peak at 820 nm. There is a large Stokes shift (190 nm) that facilitates the bioimaging application of low toxic AgInS₂ QDs because PL emission disturbing from excitation light will be minimized. The PL quantum yield (QY) was close to 35.7%, calculated by using indocyanine green as a reference standard (13% of QY in dimethylsulfoxide).³⁸ This PLQY is a relatively higher value for water-dispersible AgInS₂ QDs prepared in aqueous solution,³⁰ and even it is comparable to that of the hydrophobic QDs synthesized in high-temperature organic phase.^26–29 At different calcination temperatures, the absorption spectra and emission spectra of resultant mesoporous AgInS₂ nanospheres were recorded respectively (Fig. 4b and c, Sample I–IX, calcined at 250, 300, 350, 400 °C). Under ultrasonication, the each group of nanospheres were dispersed in PBS (pH 7.4) prior to their optical measurements. As demonstrated in Fig. 4b, the mesoporous AgInS₂ nanospheres show a significant difference from their precursors (AgInS₂ QDs). There is a new absorption band appeared at ∼360 nm for the nanospheres calcinated at 250 °C. With the increase of calcination temperature, the excitonic absorption peak of nanospheres indicates a slight blue shift from 360 to 350 nm. The PL emission spectra of nanospheres were measured in Fig. 4c, and suggest a clear emission centered at 480 nm under 360 nm of light excitation. In comparison with the absorption peaks, the PL emission peaks have a much slight shift due to the increase of calcination temperature. It means that the calcination temperature has an extra little impact on PL emission of as-prepared mesoporous AgInS₂ nanospheres. In addition, we measured the average diameters of nanospheres obtained from 2 h of calcinations treatment at different temperatures (Fig. S3 in ESI†). With the increase of calcination temperature, there was little difference (< 5%) in the diameter of nanospheres at 250 °C and other temperatures (300, 350, 400 °C). It indicated that the main structures had been formed when the calcination temperature increased to 250 °C.

Fig. 4 (a) vis-NIR absorption spectrum and PL emission spectrum of AgInS₂ QDs. (b) UV-vis absorption spectra and (c) PL emission spectra of mesoporous AgInS₂ nanospheres calcinated at different temperatures. (d) PL emission lifetime results for AgInS₂ QDs and mesoporous AgInS₂ nanospheres based on biexponential fitting. The excitation wavelength for PL emission and PL lifetime measurements was 365 nm.

The AgInS₂ QDs are just closely packed together with clear interfaces to form mesoporous AgInS₂ nanospheres between QDs, rather than a bulk crystal. Thus, the QDs in nanospheres should keep own optical characteristics. We measured the UV-vis-NIR absorption spectrum of nanospheres (Fig. S4 in ESI†), and found the characteristic absorption of nanospheres in the wavelength region of the exciton peak of AgInS₂ QDs (∼630 nm). The marked difference between the absorption of AgInS₂ QDs and mesoporous AgInS₂ nanospheres was that the nanospheres had a new absorption band in UV region. The absorption peak at 350–360 nm of nanospheres should originate from other stuff, which was produced during heat treatment. PL emission of nanospheres in visible region is not related to the band gap of AgInS₂ QDs. Previous reports have observed the similar optical absorption and PL emission in mesoporous oxides and sulfides,^13,22 which were related to oxygen and sulfur vacancies or defects. In mesoporous AgInS₂ nanospheres, so the sulfur-related defects account for the new optical absorption and PL emission. A large amount of surface defects existing on the pore internal surfaces of mesoporous AgInS₂ nanospheres could result in a new energy level in the band gap. Also, the mesostructures with high specific surface area favor the occurrence of sulfur vacancies. These results mentioned above demonstrate that the new absorption band and PL emission of mesoporous AgInS₂ nanospheres are reasonable.

Based upon the biexponential fitting, the PL decay data of nanospheres indicate a second emission lifetime of 58.9 ns (Fig. 4d, Sample III, calcined at 350 °C), which should be assigned to the sulfur-related defects.^13,21 The first emission lifetime (a shorter characteristic time) might be from the defects on the pore surface of nanospheres. Additionally, as inserted in Fig. 4d, the PL decay data of QDs present PL lifetimes of 131.0 ns and 508.7 ns, which are basically accordance with that of the AgInS₂ QDs reported in earlier literatures,³⁰ but they are significant higher than that of the mesoporous AgInS₂ nanospheres. On the basis of potentially biomedical applications, the absorption and emission mechanism of photoluminescent QDs-based mesoporous nanospheres may be a research focus in our further work.

3.3. Photostability and cytotoxicity

Relevant experiments were conducted to examine the photostability of mesoporous AgInS₂ nanospheres. As can be seen in Fig. 5a, the PL intensity of nanospheres retained over 80% of the original PL intensity after 180 min of continuous irradiation with a mercury lamp at a power output of 100 W (Fig. 5a and b, Sample III). As a comparison, quinine sulfate used as a commercial dye was subjected to the same irradiation conditions. The PL intensity of quinine sulfate decreased to only 10% of the original value, which suggests an almost entire photobleaching. These results indicate the excellent PL stability of as-prepared mesoporous AgInS₂ nanospheres when compared with commercial dyes. Moreover, the cytotoxicity of nanospheres incubated in HeLa cells was investigated by using a standard cell viability method.^38,39 Fig. 5b exhibits the viability of HeLa cells treated with the nanospheres at different concentrations and incubation times. At the nanosphere concentration up to 1.0 mg mL⁻¹, the nanospheres generate negligible impacts on the cell viability because the cell viability of more than 76% remains after 48 h incubation with the nanospheres. Therefore, the mesoporous AgInS₂ nanospheres exhibit good PL stability and low cytotoxicity. Although more and further studies on newly prepared nanomaterials are necessary before their biomedical applications, the applied potential of these proposed nanospheres in this work is significant and could be expected rationally. When uptake by tumor cells, these nanospheres could be further developed as a new and promising nanoprobe used for in vitro and in vivo PL imaging application.

Fig. 5 (a) PL stability of mesoporous AgInS₂ nanospheres and quinine sulfate under irradiation of a mercury lamp (100 W). (c) Cell viability data of HeLa cells incubated with mesoporous AgInS₂ nanospheres of different concentrations.

4. Conclusions

In summary, NIR-emitting AgInS₂ QDs stabilized by MPA were facilely synthesized. In the presence of PEI, the as-synthesized MPA-stabilized QDs were self-assembled into QDs aggregates under electrostatic interactions. Then, the resultant aggregates were calcinated at elevated temperature to yield AgInS₂ nanospheres. Relevant experiments were conducted to character and investigate the physicochemical properties. Experimental results suggested that the AgInS₂ nanospheres consisting of AgInS₂ QDs showed mesoporous structure, visible PL, high PL stability and low cytotoxicity. These confirmed properties of the mesoporous AgInS₂ nanospheres implied their potential application in biomedical PL imaging.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21475071, 21405086, 21275082 and 21203228), the Natural Science Foundation of Shandong (ZR2014BQ001 and BS2014YY009), the National Key Basic Research Development Program of China (2012CB722705), the Taishan Scholar Program of Shandong and the Natural Science Foundation of Qingdao (13-1-4-128-jch and 13-1-4-202-jch).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11545k

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