A detour strategy for colloidally stable block-copolymer grafted MAPbBr3 quantum dots in water with long photoluminescence lifetime

Shuang Yang a, Feng Zhang b, Jia Tai a, Yang Li a, Yang Yang a, Hui Wang a, Jianxu Zhang c, Zhigang Xie c, Bin Xu a, Haizheng Zhong *b, Kun Liu *a and Bai Yang a
aState Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun, 130012, P. R. China. E-mail: kliu@jlu.edu.cn
bBeijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China. E-mail: hzzhong@bit.edu.cn
cState Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China

Received 21st February 2018 , Accepted 23rd February 2018

First published on 26th February 2018


Perovskite quantum dots (PQDs) exhibit remarkable photoluminescence properties; however, their use in biological applications is hindered by their extreme sensitivity to water. We report a facile and general strategy for the preparation of aqueous colloidally stable polystyrene-b-poly(ethyl oxide) (PS-b-PEO) grafted MAPbBr3 QDs (MA = methylammonium): transferring the as-synthesized PQD@PS-b-PEO from toluene into water using precipitation in hexane as a critical intermediate step. When rehydrating the precipitate in water, the PQDs can be dispersed well individually or self-assembled into well-defined vesicular nanostructures with high photoluminescence quantum yields of up to 43%, high color purity (full width at half maximum down to 18 nm), and long average photoluminescence lifetimes up to 164 ns. The resulting PQD nanostructures in water also show excellent thermo- and photo-stability, low cytotoxicity, and bright potential for cell imaging. This work highlights the future prospects of using polymer-modified PQDs with desired physicochemical properties for biomedical applications.


Introduction

Recent advances in colloidal chemistry have provided researchers with access to very high quality perovskite quantum dots (PQDs) with tunable size, shape, and composition in non-polar solvents.1–7 Because of their excellent photoluminescence quantum yield (PLQY), narrow spectral line widths, low lasing threshold, and broad optically-tunable band gap, PQDs have shown immense promise as light emitters in LEDs, lasers and high-efficiency photovoltaics.8–12 In addition, PQDs are highly attractive for sensitive or advanced fluorescence bioapplications (e.g., in flow cytometry, and time-gate and ultra-high resolution bio-imaging).13–17 However, the development of PQDs in these fields has been hindered by two major obstacles, namely, the toxicity of lead in the environment and in organisms, and the extreme instability of PQDs in water.18,19 Promising progress has been made theoretically and experimentally to overcome the first limitation by replacing lead with environmentally-friendly metal ions, which will pave the way for biological applications of PQDs.20–24 Nevertheless, perovskite nanocrystals are strongly ionic and inherently hygroscopic, making them vulnerable to traces of moisture or even polar solvents.25–31

A number of strategies have been attempted to protect PQDs against degradation in water, including the encapsulation of PQDs with inorganic silica and sodium nitrate (NaNO3) shells, amorphous alumina matrices (AlOx), branched capping ligands, cross-linkable hydrophobic ligands, or bulk and thin films of polymers.32–42 However, these methods only provided limited control over the architecture of the PQDs, and the PQDs cannot be dispersed in aqueous solution for bioapplications. Other attempts to phase transfer the as-synthesized PQDs from nonpolar solvents to water via surface passivation with a hydrophobic or an amphiphilic matrix were also unsuccessful and only led to aqueous suspensions of microscale PQD ensembles, with sizes similar to or even larger than those of cells.43–45 The preparation of individually dispersible PQDs that are colloidally and chemically stable in water remains the most challenging, yet highly rewarding goal for many bioapplications.46,47

Block copolymers (BCPs) that contain two or more chemically distinct polymer segments undergo microphase separation in the bulk phase and in thin films; in block-selective solvents, BCPs form micelles with a wide range of morphologies.48–50 These polymers are also extensively used as ligands to stabilize nanoparticles (NPs),51–53 providing improved chemical and colloidal stability compared to small molecular ligands54 and precise control of the spatial arrangements of NPs over multiple length scales.55–63 More importantly, the introduction of other functional building blocks into BCP-NP nanocomposites will allow the integration of properties complementary to those of the NPs.

Herein, we report a facile and general strategy for the preparation of aqueous colloidally stable PS-b-PEO (polystyrene-b-poly(ethyl oxide)) grafted MAPbBr3 PQDs (PQD@PS-b-PEO, MA = methylammonium) with excellent long-term stability against water, heat, and light. The hydrophobic PS block around the PQDs provides robust encapsulation on the nanoscale against water, and the hydrophilic PEO block offers colloidal stability and cytocompatibility in aqueous solutions (Fig. 1).


image file: c8nr01493k-f1.tif
Fig. 1 Schematic illustration of the synthesis and toluene-to-water phase transfer of PEO-b-PS-NH2 grafted MAPbBr3 QDs.

Results and discussion

Previously, we have reported a ligand-assisted reprecipitation (LARP) method for the fabrication of brightly luminescent and color-tunable colloidal MAPbX3 QDs (X: Cl, Br, and I) at room temperature under ambient conditions.64 The LARP synthesis involves simply mixing a DMF solution of MAPbX3 precursors with a vigorously stirred poor solvent for the PQDs (e.g., toluene or hexane) in the presence of co-ligands of n-octylamine and oleic acid, which allowed the controlled crystallization of precursors into colloidal nanoparticles.

In this work, we use an amine-terminated BCP, PEO-b-PS-NH2, as the ligand for the synthesis of MAPbBr3 QDs. To investigate the important role of the hydrophobic layer in protecting the PQDs from water, a series of PS-b-PEO diblock copolymers with three PS segment lengths and narrow molecular weight distributions were prepared by atom transfer radical polymerization of styrene with PEO-Br as the macro-initiator (Table 1, see the ESI for synthesis details). The bromide end group on the PS block chain end was converted into a primary amine via a method reported previously (ESI Fig. 3).65

Table 1 Synthesis and characterization of amphiphilic PEO-b-PS BCPs
Sample Compositiona M n/GPCb (kg mol−1) M n/NMRc (kg mol−1) PDI
a The number of styrene repeated units was obtained from 1H NMR results. b Number-average molecular weight (Mn) was determined by GPC using polystyrene standards for calibration. c Molecular weight was calculated from 1H NMR measurements.
P1 PEO45-b-PS58-NH2 11.9 8.1 1.09
P2 PEO45-b-PS69-NH2 13.1 9.2 1.10
P3 PEO45-b-PS106-NH2 16.6 13.1 1.10


In a typical synthesis of PQD@PS-b-PEO, lead bromide (PbBr2), methylammonium bromide (MABr), and oleic acid (OA) were dissolved in dimethyl formamide (DMF) to form a clear precursor solution. A fixed amount of the precursor solution was added dropwise to a vigorously stirred solution of PS-b-PEO in toluene, a good solvent for both blocks. A green colloidal solution was immediately observed, indicating the formation of MAPbBr3 QDs from the precursors. The solution was transparent with no large aggregates, indicating the formation of highly pure MAPbBr3 QDs.

Fig. 2a and b show representative transmission electron microscopy (TEM) images of PQD@PS-b-PEO, prepared by slowly drying a toluene solution on a carbon-film coated TEM grid. Remarkably, PS-b-PEO not only functioned as a surface ligand to confine the PQDs to nanoscale dimensions (average diameter of 3.5 ± 0.9 nm), but it also served as a template to precisely assemble the QDs into aligned chains (for PQD@PS58-b-PEO45, Fig. 2a) or hexagonal arrays (for PQD@PS106-b-PEO45, Fig. 2b), depending on the volume ratio between the PS and PEO blocks. The features of the PQD arrays can be tightly controlled by varying parameters such as film thickness, solvent evaporation rates, and substrate interactions along with the molecular characteristics of the BCP.


image file: c8nr01493k-f2.tif
Fig. 2 TEM images of ordered arrays of BCP grafted PQDs dried from toluene solutions: (a) PQD@PS58-b-PEO45 and (b) PQD@PS106-b-PEO45. TEM images of (c) individually dispersed PQD@PS58-b-PEO45 and (d) PQD@PS106-b-PEO45 vesicles in water. Insets in (a) and (c): HRTEM images of the corresponding nanocrystals. Inset in (d): An HRTEM image of the corresponding vesicle.

In order to analyze the phase structure of the PQDs, powder X-ray diffraction (PXRD) (ESI Fig. 5) and high-resolution transmission electron microscopy (HRTEM) were performed. The PXRD patterns were indexed to those of a well-defined cubic phase (a = 5.93 Å, space group Pm[3 with combining macron]m). HRTEM (Fig. 2a inset) confirmed that the MAPbBr3 QDs possess a high degree of crystallinity with interplanar distances of 2.95 Å, corresponding to the spacing of the (200) lattice planes.

Our initial attempts to directly transfer the as-synthesized PQD@PS-b-PEO into an aqueous solution via a toluene/water emulsion interface were unsuccessful; instant decomposition of the MAPbBr3 nanocrystals was observed, as indicated by the disappearance of their PL. This result confirmed the extreme sensitivity of PQDs to water, as the solubility of water in toluene is only 0.0266 mol dm−3 at room temperature. Although water is a poor solvent for the PS blocks, this concentration of water was too low to cause the complete collapse of the PS blocks to form a closed shell around the MAPbBr3 core, resulting in direct contact of MAPbBr3 with water. Alternatively, we have tried redispersing powdered PQD@PS-b-PEO (obtained from a toluene solution) in water. Even though a flow of nitrogen was used to evaporate toluene at room temperature, the hygroscopic PEO blocks retained a trace amount of water. The resulting moisture around the unprotected MAPbBr3 cores caused a significant decrease in the PL intensity of PQD@PS-b-PEO.

To overcome this challenge, we developed a pretreatment method to ensure the complete encapsulation of the PQD core by a PS shell prior to the transfer of PQD@PS-b-PEO to water. The pretreatment involved precipitating the PQDs into hexane, a poor solvent for both blocks. Dropwise addition of a toluene solution of the as-synthesized PQD@PS-b-PEO into hexane yielded a suspension of microscale aggregates, which were eventually precipitated as a thin film at the bottom of the container (Fig. 1). Although the solubility of water in hexane is one order of magnitude lower than in toluene at room temperature, it has been observed that trace amounts of water can be concentrated in the hydrophilic PEO block.66 Therefore, to eliminate contact between water and the MAPbBr3 nanocrystal core, we took advantage of the microphase separation of the PEO and PS blocks to ensure complete encapsulation of the PQD core by the hydrophobic PS block. The film of the PQD@PS-b-PEO aggregate was annealed overnight, where microphase separation was facilitated by the remaining toluene in the hexane solution. The film of PQDs was washed twice with hexane to remove any remaining toluene, allowing further tightening of the PS block around the PQD core. Hexane was removed and the film was dried under a stream of nitrogen before water was added to dissolve the aggregate.

Film rehydration is a method widely used for the preparation of amphiphilic BCP micelles and vesicles, as well as the assembly of amphiphilic BCP-tethered NPs.55 We adapted this method to rehydrate the PQD@PS-b-PEO films in water under sonication, which gave greenish transparent solutions regardless of the PS block length (Fig. 3a). The aqueous solutions showed very bright green PL upon irradiation with 365 nm UV light (Fig. 3b), which indicated the successful transfer of PQDs into water with the MAPbBr3 cores intact.


image file: c8nr01493k-f3.tif
Fig. 3 (a) UV-Vis absorption (solid line) and PL emission spectra (dashed line) of PQD@PS106-b-PEO45 in toluene (green) and in water (blue). Inset: Photographs of an aqueous solution of PQD@PS58-b-PEO45 in a glass vial filled with toluene (left) and the same vial illuminated with a 365 nm UV lamp (right). (b) Time-resolved PL decay of PQD@PS106-b-PEO45 in water. (c) A low temperature PL spectrum of PQD@PS106-b-PEO45 in water at 78 K. (d) Schematic illustration of a recombination process in PQD@PS-b-PEO.

Fig. 2c shows a representative TEM image of PS58-b-PEO45 stabilized PQDs after transfer into water. PQD@PS58-b-PEO45 presented as individually dispersed NPs with their MAPbBr3 core well-preserved in the aqueous solution and a block copolymer shell with a thickness of around 3 nm in the dry state (ESI Fig. 5). Both HRTEM imaging (inset in Fig. 2c) and PXRD results (ESI Fig. 5) confirmed that the MAPbBr3 cores were highly crystalline, with no noticeable changes in lattice spacing. A dynamic light scattering (DLS) study showed that the hydrodynamic diameter of PQD@PS58-b-PEO45 in aqueous solution (25.8 nm) was even smaller than that in toluene (37.0 nm) due to a shrinkage of PS blocks (ESI Fig. 4). This result also supported the presence of individually dispersed PQD@PS58-b-PEO45 in aqueous solutions.

For the BCP with the longest PS block, PQD@PS106-b-PEO45 self-assembled to form well-defined vesicles after film hydration (Fig. 2d and ESI Fig. 6). The formation of vesicles was due to the increased volume ratio between the hydrophobic and hydrophilic blocks. The vesicles are nanosized hollow spheres, composed of a hydrophobic PS membrane with hydrophilic PEO interior and exterior shells.55 The PQD nanocrystals were tightly encapsulated by the PS blocks and participated in forming the PS membrane. The formation of vesicles in aqueous solutions was confirmed by DLS (ESI Fig. 4b). An average hydrodynamic diameter of 104 nm was observed, consistent with the average size of 61 ± 13.3 nm from TEM studies (Fig. 2d and ESI Fig. 6). For the intermediate PS block length, a mixture of individual nanoparticles and tubular nanostructures was observed.

To investigate the quality of protection afforded by the PS block, we further compared the optical properties of the as-synthesized PQDs in toluene and the corresponding PQDs embedded in vesicles in aqueous solution. Fig. 3a and ESI Fig. 7 show the absorption (solid line) and emission (dashed line) spectra of PQD@PS-b-PEO in toluene and water. The as-synthesized PQDs with three PS block lengths in toluene showed a band edge at 505 nm, a sharp emission peak at 515 nm with a full width at half maximum (fwhm) of only 18 nm, and a solution PLQY of 56%. The color saturation of PQD@PS-b-PEO is comparable to those of our previously reported PQDs with small molecule ligands.64 Upon transferring to an aqueous solution, the absorption peak of PQD@PS-b-PEO became less pronounced due to scattering caused by vesicles. The strong scattering of the PQD@PS-b-PEO nanostructures in water is mainly due to the larger difference in the refractive index between the polymer shell and the solvents.67 Although the PL peak position showed a slight red-shift to 528 nm (515 nm in toluene), the fwhm values of the PL peak remained constant at 18 nm. The red-shifting of the photoluminescence peak of PQD@PS-b-PEO upon transferred into water was caused by the change of dielectric constant when the PS formed a tight shell. The PLQY was 43% for PQD@PS106-b-PEO45 vesicles, and 29% for individual nanoparticles from PQD@PS58-b-PEO45.

We also measured the time-resolved PL spectra of the as-synthesized PQD@PS-b-PEO nanostructures in aqueous solutions (Fig. 3b). The PL decay curve exhibited a long recombination tail and was further fitted by a multiexponential equation with an average lifetime of 164 ns (ESI Table 1). It is noted that the average PL lifetime is much longer than that of related systems (ESI Table 2), such as colloidal MAPbBr3 nanocrystals with small molecule ligands (∼10.3 ns).64,68 To further understand the differences in PL lifetime, the temperature dependent PL lifetime of PQD@PS106-b-PEO45 in both toluene and water was also measured (ESI Fig. 8). As shown in ESI Fig. 8a and c, with temperature increasing, the PL intensity of PQD@PS-b-PEO decreased, while the average PL lifetime prolonged (ESI Fig. 8b and d).

It has been known that the PL lifetime of perovskite nanocrystals is correlated to their particle size and defect concentrations.69–71 In our case, the particle size of as-synthesized PQD@PS-b-PEO is similar to that of the ones capped with small molecule ligands (n-octylamine and OA) (∼3.3 nm). However, PQD@PS-b-PEO in toluene has an average PLQY of 56%, which is lower than that of n-octylamine and OA capped QDs. The decreased PLQY suggests that PS-b-PEO capped QDs have more surface defects due to the relatively low grafting density of PS-b-PEO polymer chains compared to that of octylamine and OA capped PQDs. To reveal the origin of the prolonged PL lifetime, low temperature PL measurement was also conducted to explore the existence of defect states in PQD@PS-b-PEO. As shown in Fig. 3c, the PL spectrum at 78 K shows a main emission peak at 540 nm. Except for the main peak, a long emission tail with the wavelength regime of 550–560 nm was also identified. The relatively red-shifted long tail can be attributed to the recombination through trap states. Combining with the analysis of PL lifetime and low temperature PL spectra, we proposed a carrier recombination mechanism. As schematically shown in Fig. 3d, the photoexcited carriers can go through exciton recombination or be trapped by defect states. The surface trap assisted recombination is a slow process, which contributes to the observed longer PL lifetime.72,73 Moreover, it should be noted that these prolonged PL lifetimes are important for improved detection sensitivity and accuracy for time-gated (TG) bioimaging applications.

It is well known that perovskite materials suffer from rapid degradation upon exposure to moisture. This process also appears to be accelerated by heat.25–29 PS has relatively low oxygen and water diffusion coefficients among glassy polymers,74,75 which should offer good protection of the PQDs against moisture and photooxidation in aqueous solutions. When stored at room temperature and under ambient light, all aqueous solutions of PQD@PS-b-PEO appeared to be water- and photo-stable for at least one month (up to three months for PQD@PS106-b-PEO45 vesicles) regardless of the PS block length. To test the stability of PQD@PS-b-PEO with different PS block lengths, their aqueous solutions were heated from 25 to 90 °C in five-degree intervals; upon reaching a target temperature, the solutions were held for 30 minutes at that temperature before further increases (Fig. 4a). PL spectra were collected after each 30 minute hold (ESI Fig. 8a–c). The PL decay curves (Fig. 4a and ESI Fig. 9) gave extrapolated onset temperatures (i.e., the temperature at which the PL decay begins) of 46, 56, and 70 °C for the short, intermediate, and long PS block lengths, respectively. These temperatures are consistent with the surface glass transition temperature (Tg) of PS homopolymer with corresponding molecular weights.76,77 Above the Tg of PS, a dramatic increase in the free volume of the PS shell allows water molecules to penetrate through the shell and come into contact with the MAPbBr3 nanocrystals. This result suggests that the thermostability of PQD@PS-b-PEO is primarily governed by the stability of polymer shells, which can be increased by a combination of elongating the hydrophobic chain length, changing the composition of the hydrophobic block, cross-linking the hydrophobic shell, and shortening the hydrophilic block.78,79


image file: c8nr01493k-f4.tif
Fig. 4 Thermo- and photo-stability of PQD@PS-b-PEO in water. (a) The evolution of the relative PL intensity of aqueous solutions of PQD@PS58-b-PEO45 (blue), PQD@PS69-b-PEO45 (green), and PQD@PS106-b-PEO45 (red) from 25 to 90 °C. The solutions were held for 30 minutes at each temperature before heated to the next target temperature. (b) Temporal variations of the relative PL emission intensity of PQD@PS106-b-PEO45 vesicles in water. The solutions were irradiated continuously with a 365 nm LED lamp with power densities of 7.0 (red), 5.3 (green), and 3.5 (blue) mW cm−2.

Encouraged by the ultra-stable characteristic of these nanostructures, we also investigated the long-term stability of PQD@PS106-b-PEO45 vesicles which have been stored in water for over 7 months. Their photoluminescence quantum yield decreased to 33% compared to 43% of their initial value and the PL emission peak was only red-shifted for 1 nm (ESI Fig. 10). In addition, the lead ion concentration of the present solution was lower than the detection limit of the inductively coupled plasma optical emission spectrometry (<0.2 ppm), indicating that the leach of lead from the QD core to water solution is negligible.

Fig. 4b shows the temporal variations of the relative PL emission intensity of an aqueous solution of PQD@PS106-b-PEO45 vesicles, irradiated continuously with a 365 nm LED lamp with power densities of 7.0, 5.3, and 3.5 mW cm−2. The PL spectra were collected every 3 minutes. Even after 100 minutes, the relative PL intensities dropped less than 20% from their initial values for all power settings. The excellent photostability observed confirms that the tight covering formed from the PS shell conferred good protection for the PQD core from both oxygen and water.

To evaluate the potential usefulness of PQD@PS-b-PEO in different biological and industrial applications, we tested the stability of an aqueous solution of PQD@PS106-b-PEO45 vesicles at 37 °C at various pH values (ESI Fig. 11). For pH = 1, the PL intensity decreased to 52.3% from their initial values, accompanied by a blue shift of the PL emission peak from 529 nm to 509 nm caused by anion exchange of Cl to Br. For other pH values, PL emission peaks showed no observable shift. For pH = 13, the PL emission peak decreased to 72.9%. When pH varied from 3 to 10, the PL intensity remained above 80%, and the intensity decay could be caused by photobleaching when the PL emission of the sample was measured multiple times.

Due to their intrinsic hollow nanostructure and compartmentalized domains with diverse functionalities, PQD-containing polymer vesicles may find applications in biomedical fields including drug delivery, gene therapy, magnetic resonance imaging, and theragnostics. The high luminescence efficiency and color purity, appropriate size, and extended stability of the PQD@PS106-b-PEO45 vesicles in aqueous solutions also pave their way for cell imaging applications (Fig. 5). Cytotoxicity testing showed that more than 90% of the human cervical epithelial carcinoma (HeLa) cells survived when the concentrations of PQD@PS106-b-PEO45 vesicles were below 30 μg mL−1 (Fig. 5a), indicating their low cytotoxicity and suitability for cell imaging. Fig. 5b–d show confocal laser scanning microscopy images of HeLa cells after incubation with PQD@PS106-b-PEO45 vesicles for 3 hours at 15 μg mL−1. The living cells exhibited bright green PL, clearly indicating that vesicle penetration had occurred. The majority of the PQDs were in the cytoplasm. In addition, the PQD luminescence from the cell culture remained very stable under continuous UV irradiation for over an hour.


image file: c8nr01493k-f5.tif
Fig. 5 Cytotoxicity testing and HeLa cell imaging using PQD@PS106-b-PEO45 vesicles. (a) The cell viability rate for HeLa cells incubated with PQD@PS106-b-PEO45 vesicles at varying concentrations for 24 hours. Confocal fluorescence images of (b) HeLa cells with their nuclei stained with DAPI, (c) the same cell incubated with PQD@PS106-b-PEO45 vesicles (15 μg mL−1) for 3 hours, and (d) the overlap of (b) and (c). Scale bars in (b), (c), and (d) correspond to 100 μm.

Conclusions

In summary, we have developed a facile method for the synthesis of amphiphilic PEO-b-PS-NH2 grafted MAPbBr3 PQDs. Nanoencapsulation of the PQDs by the hydrophobic PS block allows for phase transfer from toluene to water with hexane as an intermediate solvent. The perovskite nanocrystals remained intact and maintained their strong photoluminescence post-transfer. Aqueous colloidally stable PQD@PS-b-PEO showed remarkable long-term thermo-, photo-, and pH-stability, as well as low cytotoxicity towards HeLa cells. This method not only paves the way for biological applications of PQDs, it can also be extended to the protection of other water- and air-sensitive nanoparticles. More importantly, this approach will allow new opportunities for the rational integration of functional building blocks with properties complementary to these nanoparticles in order to meet current and future demands in advanced nanomaterials.

Materials and methods

Materials, methods, and chemical syntheses are described in the ESI, and all experiments were performed in compliance with the relevant laws and institutional guidelines.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

K. L. gratefully acknowledge financial support from the National Natural Science Foundation of China (21474040). H. Z. thanks the National Natural Science Foundation of China/Research Grants Council Joint Research Project (51761165021). K. L. thanks the National Natural Science Foundation of China (21474040). B. Y. and K. L. thanks JLU Science and Technology Innovative Research Team 2017TD-06 for financial support.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nr01493k

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