Chemical Transformation of Lead Halide Perovskite into Insoluble

Jun 17, 2019 - The as-prepared CsPbBr3/CsPb2Br5 composite NCs not only retain the photoluminescence quantum yield (up to 80%) and a narrow full width ...
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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24241−24246

Chemical Transformation of Lead Halide Perovskite into Insoluble, Less Cytotoxic, and Brightly Luminescent CsPbBr3/CsPb2Br5 Composite Nanocrystals for Cell Imaging Sunqi Lou,† Zhi Zhou,‡ Tongtong Xuan,*,†,# Huili Li,∥ Ju Jiao,† Hongwu Zhang,⊥ Romain Gautier,*,§,† and Jing Wang*,†

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Ministry of Education Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, School of Materials Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China ‡ Hunan Provincial Engineering Technology Research Center for Optical Agriculture College of Science, Hunan Agricultural University, Changsha 410128, China § Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, 2 rue de la Houssinière, BP 32229, 44322 Nantes cedex 3, France ∥ Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, School of Physics and Materials Science, East China Normal University, Shanghai 200062, China ⊥ Key Lab of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, Fujian 361021, China S Supporting Information *

ABSTRACT: Lead halide perovskite nanocrystals (NCs) have been widely investigated owing to their potential applications as optoelectronic devices. However, these materials suffer from poor water stability, which make them impossible to be applied in biomedicine. Here, insoluble CsPbBr3/CsPb2Br5 composite NCs were successfully synthesized via simple water-assisted chemical transformation of perovskite NCs. Water plays two key roles in this synthesis: (i) stripping CsBr from CsPbBr3/Cs4PbBr6 and (ii) modifying the coordination number of Pb2+ (six in CsPbBr3 and Cs4PbBr6 vs eight in CsPb2Br5). The as-prepared CsPbBr3/CsPb2Br5 composite NCs not only retain the photoluminescence quantum yield (up to 80%) and a narrow full width to half-maximum of 16 nm, but also present excellent water stability and low cytotoxicity. With these properties, the CsPbBr3/CsPb2Br5 composite NCs were demonstrated as efficient fluorescent probes in live HeLa cells. We believe that our finding not only provides a new method to prepare insoluble, narrow-band, and brightly luminescent CsPbBr3/CsPb2Br5 composite NCs, but also extend the potential applications of lead halides in biomedicine. KEYWORDS: perovskite, nanocrystals, water stability, photoluminescence, cell imaging

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INTRODUCTION In bioimaging, core/shell nanocrystals (NCs), such as II−VI based NCs, have been widely investigated as luminescent probes owing to narrow-band and intense luminescence, high stability, and relatively low toxicity.1−4 However, synthesizing core/shell structures requires a complicated and high cost procedure including high reaction temperature (>200 °C).5,6 In addition, unfavorable reabsorption between different size NCs, and strong quantum effects lead to reduced photoluminescence quantum Yields (PLQY) and relatively broad emission.5,7−10 Recently, lead halide perovskite NCs, APbX3 (A = Cs, CH3NH3, X = Cl, Br, I), have also been investigated owing to simple synthesis methods and excellent photoelectric properties, such as high PLQY and inappreciable influence of self-absorption in comparison to traditional core/shell NCs.11−26 Nonetheless, the perovskite NCs suffer from poor © 2019 American Chemical Society

water-stability that prevents them from being used in bioimaging applications.27−38 Thus, many researchers have been bent their mind to improve the water stability of perovskite NCs. Moisture-impermeable polymer embedding, waterproof organic ligand coating, inorganic encapsulation, and superhydrophobic framework structures have been introduced to form composites with improved stability to resist water from moist air or solution.28,29,36,38−41 For example, Zhong et al. reported the in situ synthesis of CsPbBr3 NCs@polymer composites with bright emission and excellent water stability,11,42 and Fu and Lin et al. synthesized the perovskite−polystyrene composite beads for cellular labeling Received: March 27, 2019 Accepted: June 17, 2019 Published: June 17, 2019 24241

DOI: 10.1021/acsami.9b05484 ACS Appl. Mater. Interfaces 2019, 11, 24241−24246

Research Article

ACS Applied Materials & Interfaces agents.41,43 However, despite extensive work, the composite particles remain too large to be internalized into the cytoplasm and are, consequently, not suitable for cell-imaging applications.43 In parallel to this work on lead halide perovskite for cell imaging, reducing the dimensionality of lead halides has been reported as a method to improve the water stability, considering their applications in light-emitting diodes, lasers, and photodetectors.44−61 For example, Balakrishnan et al. presented moisture-stable CsPb2Br5 nanosheets using a treatment by dodecyl dimethylammonium bromide.54 However, these lead halide materials exhibit large size particles or/ and poor stability when directly immersed in water, and, as a consequence, could not currently meet the requirements for cell imaging. In this context, we present a facile water-assisted transformation strategy that enables the synthesis of insoluble CsPbBr3/CsPb2Br5 composite NCs with bright and narrowband green emission and tested them successfully as fluorescent probes in cell imaging.

by powder X-ray diffraction (XRD). Figures S1a and S4 showed that the CsPbBr3/Cs4PbBr6 composite NCs were quickly transformed into CsPb2Br5 with the trace of CsPbBr3. When the transformation process prolongs to 2 h, the precipitates were identified as the CsPbBr 3 /CsPb 2 Br 5 composite NCs. As depicted in Figure S5, the XRD pattern of the crystals in supernatant water solution consists mainly of some residual Cs4PbBr6, CsPb2Br5, and CsBr. Based on the TEM and XRD results, a CsBr-stripping mechanism can be proposed for the water treatment (Figure 1). Cs4PbBr6 crystals can be regarded as a CsBr-rich material. During the treatment of water, stripping of CsBr occurs due to the high water-solubility of CsBr and the good ion diffusion in Cs4PbBr6.62 This leads to the shrinking of the Cs4PbBr6 crystal lattice and subsequent transformation into the CsPbBr3 NCs. However, the CsPbBr3 NCs are not stable in water and CsBr is further stripped from CsPbBr3. In addition, water promotes the transformation of [PbBr6]4− into [PbBr8]6− in solution.63 [PbBr6]4−-based CsPbBr3 NCs initially decomposed and immediately transformed into [PbBr8]6−-based CsPb2Br5 NCs (Figure S6). Owing to the low solubility of CsPb2Br5 in water,47 further dissolution of our composite NCs is not observed, and the orthorhombic CsPbBr3 appears during the formation of CsPb2Br5 NC and survives in water because of the protective CsPb2Br5 framework. Figure 2 exhibits the photographs and the photoluminescence (PL) spectra of the CsPbBr3/CsPb2Br5 composite NCs,

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RESULTS AND DISCUSSION CsPbBr3-embedded Cs4PbBr6 NCs were first prepared through a room temperature precipitation method (see the Experimental Section in the Supporting Information).37 The assynthesized NCs were carefully tested by transmission electron microscopy (TEM). As shown in Figures S1c and S2, the TEM image of the original CsPbBr3/Cs4PbBr6 composite NCs exhibits a hexagon shape with an average crystallite dimension of about 195 nm (the maximum diagonal length). The highresolution TEM (HR-TEM) image (Figure S1d) clearly exhibits the lattice spacing values of 2.29 and 3.95 Å, corresponding to the (211) plane of CsPbBr3 and the (300) plane of Cs4PbBr6, respectively. The selected-area electron diffraction (SAED) further proves the coexistence of CsPbBr3 and Cs4PbBr6 (Figure S1e). After treatment with deionized water (Experimental Section), the CsPbBr3/Cs4PbBr6 composite NCs were transformed into CsPbBr3/CsPb2Br5 composite NCs (Figures 1 and S1a). The new products show a rectangle-

Figure 2. (a) Photographs of the CsPbBr3/CsPb2Br5 composite NCs dispersed in toluene and water under UV light (365 nm), respectively. (b) PL spectra of the NCs dispersed in toluene and water.

which were dispersed in toluene and water solutions under UV light (365 nm). The NCs show bright green emission in both toluene and water used as solvents (Figure 2a). The absorption spectra of the CsPbBr3/CsPb2Br5 composite NCs in different solutions also were characterized and are shown in Figure S7. The results show that the characteristic absorption values are ∼515.0 nm (toluene) and ∼520.0 nm (water) and the strong PL of the CsPbBr3/CsPb2Br5 composite NCs comes from the CsPbBr3 NCs instead of the CsPb2Br5 NCs, due to an indirect bandgap of ca. 3.0 eV for CsPb2Br5.64,65 Very interestingly, the as-prepared CsPbBr3/CsPb2Br5 composite NCs dispersed in water shows excellent photoluminescence properties. They retain high PLQY (80%) and a narrow full width to halfmaximum (16 nm), which are similar to the as-prepared CsPbBr3/CsPb2Br5 composite NCs (83%) or pure CsPbBr3 NCs dispersed in toluene (∼90%) (Figure 2b).66 High ater stability is the prerequisite for the CsPbBr 3 /CsPb 2 Br 5 composite NCs to be successfully used for cell imaging. To get a better understanding of the water stability of CsPbBr3/ CsPb2Br5 composite NCs, about 0.05 mg/mL NCs were dispersed in deionized water for different periods (Figure 3).

Figure 1. Schematic illustration of the water-assisted transformation process from CsPbBr3/Cs4PbBr6 composite NCs to CsPbBr3/ CsPb2Br5 composite NCs.

like structure with a crystal of around 70 nm size (Figures S1f and S3). The HR-TEM image of the NCs exhibit a lattice spacing of 4.73 Å, which can be identified as the (112) plane of the CsPb2Br5 crystals (Figure S1g). This result agrees well with the SAED analysis (Figure S1h). To follow the transformation process, the products in the supernatant water solution and the precipitates obtained after different durations of the water treatment were characterized 24242

DOI: 10.1021/acsami.9b05484 ACS Appl. Mater. Interfaces 2019, 11, 24241−24246

Research Article

ACS Applied Materials & Interfaces

(Figure S9b) of O 1s in both composite NCs can be fitted to two peaks, and the two peaks are at 532.0 and 533.5 eV, corresponding to organic C−O and organic CO, respectively, which indicates that the amorphous PbOx did not form surrounding the composite NCs, and the excellent water stability of the CsPbBr3/CsPb2Br5 composite NCs may come from the high stability of CsPb2Br5. To our knowledge, the amphipathic ability and stability of CsPbBr 3 /CsPb 2 Br 5 composite NCs, which make it possible for them to be used as fluorescent probes in photodynamic therapy, drug transport, etc., have never been reported for lead halide NCs. Because the heavy metal Pb2+ ions are toxic to the cells, the cytoxicity assessment of the CsPbBr3/CsPb2Br5 composite NCs is indispensable for cell-imaging applications. To further evaluate the cytotoxicity of the CsPbBr3/CsPb2Br5 composite NCs, cell viability assay using Cell Counting Kit-8 (CCK-8) and apoptosis assay were performed on the HeLa cells.7,71 The composite NCs show a relatively low toxicity after exposure to 0−200 μg/mL concentration of the NCs for 24 h (Figure 4a).

Figure 3. (a) PL spectral mapping of the CsPbBr3/CsPb2Br5 composite NCs dispersed in water for different time periods. (b) The relative PL intensity of 0.05 mg/mL NCs in water for different time periods; the inset pictures show the photographs of the NCs dispersed in water for different time periods under day light and UV light (365 nm), respectively.

Figure 3b shows that the relative PL intensity of the NCs remains at 90 and 45% of the original intensity after being immersed in water for 21 and 110 h, respectively. These results indicate that as-prepared CsPbBr3/CsPb2Br5 composite NCs exhibit high stability against hydrolytic degradation.66In addition, the improved water stability may also result from the surface passivation during the water-assisted transformation process.62 Thus, the surface ligands of the CsPbBr3/CsPb2Br5 composite NCs were analyzed by Fourier transform infrared (FT-IR) spectra. As shown in Figure S1b, the FT-IR spectrum of the CsPbBr3/CsPb2Br5 composite NCs shows absorption peaks similar to those of the original CsPbBr3/Cs4PbBr6 composite NCs. The peaks at 1591 and 1849 cm−1 belonged to the N−H stretching vibration and the CN−H stretching of 2-methylimidazole (MeIM).67 A peak at 1042 cm−1 can be attributed to the C−N stretching of the oleylamine.68 And the absorption bands at 2919 and 2869 cm−1 are ascribed to the asymmetric CH2 stretching and the symmetric CH2 stretching of oleic acid.69 Additionally, 1H NMR was employed to further verify and distinguish the surface ligand environment of the CsPbBr3/Cs4PbBr6 and CsPbBr3/CsPb2Br5 composite NCs, especially for oleic acid and oleylamine. The molecular structural formulas of oleic acid, oleylamine, and MeIM with the numbered hydrion in different coordination environments are shown in Figure S8. We can find that the characteristic resonances 4, 6, and 7 of oleic acid and oleylamine are in the same position, which cannot be distinguished.70 The resonances of −COOH and −NH2 need to be further found to distinguish the existence of oleic acid and oleylamine, and the characteristic resonances 1 and 3 are the chemical shifts of oleic acid, which belong to −COOH and −CH2−CO, respectively.70 The resonances α and β are ascribed to the −NH2 and α-CH2 of oleylamine. Notably, the chemical shift of the CCH−N (the resonances γ) of MeIM is also detected (the inset picture in Figure S8c). Based on these results, the MeIM, oleylamine, and oleic acid were capped on the surface of CsPbBr3/CsPb2Br5 composite NCs and contribute to the amphipathic ability of CsPbBr3/ CsPb2Br5 composite NCs that can be uniformly dispersed not only in toluene and hexane but also in water solutions. Meanwhile, the X-ray photoelectron spectra (XPS) of O 1s of the CsPbBr3/Cs4PbBr6 and CsPbBr3/CsPb2Br5 composite NCs were characterized and compared to check if an amorphous PbOx passivating layer was surrounding a core of composite NCs (Figure S9a). The high-resolution XPS spectra

Figure 4. Cell viability (a) and cell apoptosis (b, c) assay of the CsPbBr3/CsPb2Br5 composite NCs.

Even when the concentration of NCs was 200 μg/mL, the cell viability is still maintained at 80%, which is similar to CdSeS/ ZnS core/shell NCs (81%, Figure S10) and higher than Zn− In−S:Ag/ZnS8 or CdS/Cu+ NCs.7 These results indicate that the CsPbBr3/CsPb2Br5 composite NCs exhibit relatively low toxicity for the cells. To further confirm these measurements, apoptosis assay was put in practice. When the HeLa cells were exposed to the CsPbBr3/CsPb2Br5 composite NCs with concentrations of 0, 10, 50, and 100 μg/mL for 24 h, the percentage of viable cells was about 98, 94, 93, and 89%, respectively (Figure 4b,c). Furthermore, after the CsPbBr3/ CsPb2Br5 composite NCs were immersed in water (50 μg/mL) for 24 h, the concentration of Pb2+ ions in supernatant water solution after centrifugal treatment was examined by inductively coupled plasma mass spectrometer. The concentration of Pb2+ was estimated to be only 76.9 ppm in the supernatant, which means 0.878% of CsPbBr3/CsPb2Br5 composite NCs was dissolved during this experiment (the stoichiometry of the CsPbBr3/CsPb2Br5 composite NCs was calculated similar to the stoichiometry of CsPb2Br5). Based on these results, we can safely summarize that CsPbBr3/CsPb2Br5 composite NCs have low cytotoxicity, which is essential to guarantee their applications in cell imaging. 24243

DOI: 10.1021/acsami.9b05484 ACS Appl. Mater. Interfaces 2019, 11, 24241−24246

Research Article

ACS Applied Materials & Interfaces

high water stability and low toxicity, and these cesium lead halide NCs can be used for future biomedical applications.

The potential cell-imaging application of the as-prepared CsPbBr3/CsPb2Br5 composite NCs was further investigated. As shown in Figure 5, the fluorescence images of HeLa calls

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05484. Experimental section, XRD spectra, TEM images, particle size distribution, XPS spectra, absorption spectra, 1H NMR spectra, FT-IR spectra, and cell viability (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.X.). *E-mail: [email protected] (R.G.). *E-mail: [email protected] (J.W.). ORCID

Zhi Zhou: 0000-0002-0547-506X Tongtong Xuan: 0000-0002-9176-7826 Hongwu Zhang: 0000-0001-7730-4270 Jing Wang: 0000-0002-1246-991X

Figure 5. Bright-field images, fluorescence images, nuclei of cell (dyed in blue by 4,6-diamidino-2-phenylindole dihydrochloride), and the overlay images of HeLa cells incubated with 50 μg/mL CsPbBr3/ CsPb2Br5 NCs for 2, 8, 12, and 24 h at 37 °C, respectively.

Present Address #

Currently works at the College of Materials, Xiamen University, Xiamen, Fujian 361005, China (T.X.).

were cultured at 37 °C with 50 μg/mL concentration composite NCs for different durations.71 The composite-NCtreated HeLa cells exhibit green fluorescence around the nucleus, which indicates that the NCs pass across cell membranes and enter into the cells. Large CsPbBr3 quantum dots@polystyrene (CPB@PS) was previously used as cellular labeling agents.43 However, the luminescence intensities of the labeled cells remained weak with prolonging time. Compared to the CPB@PS, the HeLa cells incubated with our prepared CsPbBr3/CsPb2Br5 composite NCs exhibit increasing green luminescence intensities. The differences of performances between CPB@PS and our CsPbBr3/CsPb2Br5 composite NCs may be accounted for two reasons. First, a larger quantity of the CsPbBr3/CsPb2Br5 composite NCs were internalized into the cytoplasm owing to the nanoscale size of as-prepared CsPbBr3/CsPb2Br5 composite NCs, compared to the largesized CPB@PS. Second, the NCs show highly stable luminescence in water solution. All of these results demonstrate that as-prepared CsPbBr3/CsPb2Br5 composite NCs may work as a possible and alternative fluorescence probe for the application of biomedicine.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key R&D Program of China (2018YFB0406800 and 2018YFB0406801), National Natural Science Foundation of China (Nos. 51702373, 51772336 and 51572302), “973” programs (2014CB643801), Guangdong Provincial Science & Technology Project (2015B090926011, 2017A050501008 and 2013B090800019), Teamwork Projects of Guangdong Natural Science Foundation (S2013030012842), and Guangzhou Science & Technology Project (201807010104). R. G. acknowledges the support from the National Agency for Research (ANR Young Researchers, ANR-16-CE08-0003-01, Combi-SSL project) and Etoiles Montantes en Pays de La Loire.

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CONCLUSIONS In summary, we demonstrate a novel water-assisted transformation from cesium lead halide CsPbBr 3 /Cs 4 PbBr 6 composite NCs to CsPbBr3/CsPb2Br5 composite NCs. The transformation occurs in water solution by dissolving CsBr and precipitating CsPbBr3/CsPb2Br5 composite NCs. Based on these results and analyses, a reasonable mechanism of CsBrstripping was proposed. The as-prepared CsPbBr3/CsPb2Br5 composite NCs in water solution retain a high PLQY of 80% and narrow-band emission. The composite NCs also present high stability and low cytotoxicity. The cell imaging studies verify that the CsPbBr3/CsPb2Br5 composite NCs exhibit excellent fluorescence for identifying HeLa as cellular labeling agents. We expect that our finding can provide a new strategy to synthesize brightly luminescent cesium lead halide NCs with 24244

DOI: 10.1021/acsami.9b05484 ACS Appl. Mater. Interfaces 2019, 11, 24241−24246

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