Bifunctional Superparticles Achieved by Assembling Fluorescent

Article pubs.acs.org/JPCC

Bifunctional Superparticles Achieved by Assembling Fluorescent CuInS2@ZnS Quantum Dots and Amphibious Fe3O4 Nanocrystals Xiaoyu Sun,† Ke Ding,† Yi Hou,† Zhenyu Gao,‡ Wensheng Yang,‡ Lihong Jing,† and Mingyuan Gao*,† †

Institute of Chemistry, The Chinese Academy of Sciences, Bei Yi Jie 2, Zhong Guan Cun, Beijing 100190, China College of Chemistry, Jilin University, Changchun 130012, China

‡

S Supporting Information *

ABSTRACT: Monodispersed superparticles (SPs) consisting of hydrophobic CuInS2@ ZnS quantum dots (QDs) were prepared through a self-assembling process induced by introducing ethanol into the cyclohexane solution of the QDs stabilized by 1dodecanethiol. The solvent polarity-dependent size and size distribution of the resultant SPs were carefully characterized by using scanning electron microscopy, transmission electron microscopy, and dynamic light scattering. The formation and the growth mechanism of the SPs were investigated. By manipulating the nucleation and the growth kinetics through the addition rate of ethanol, the size was effectively tuned without broadening the size distribution of the SPs. By overcoating the SPs with amphibious Fe3O4 nanocrystals (NCs) stabilized by poly(4-vinylpyrollidone), aqueous dispersible SPs were obtained. Inheriting the fluorescent and magnetic properties from the corresponding mother particles, the resultant SPs presented excellent fluorescent and magnetic properties and, thus, may potentially be useful for bioseparation, detection, etc.

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INTRODUCTION Owing to the remarkable size-dependent electrical, optical, magnetic, and catalytic properties, inorganic nanocrystals (NCs) are often used as “artificial atoms” for fabricating superparticles (SPs) to magnify the physical properties of single NCs and, on the other hand, to integrate the unique properties of the subunits. Therefore, assembling NCs into uniform SPs has become a novel approach for achieving advanced materials potentially useful for photonics, sensors, catalysis, and bioanalysis.1−7 The preparations of SPs can roughly be classified into two groups, i.e., self-assembly of NCs formed in situ2,8−11 and selfassembly of NCs preformed.3−5,12−34 Contrasting the former approach, the latter one offers the opportunity to tailor the properties of the SPs by using NCs with clearly defined physical properties. In addition, it holds the flexibly to achieve SPs through different approaches, e.g., soft-template directed assembly and interparticle interaction guided assembly. The soft-template approach generally consists of two steps, i.e., the formation of microemulsions containing hydrophobic NCs and the subsequent removal of the oil phase therein, which gives rise to SPs of the NCs glued together.12−14 Through this strategy, SPs of quantum dots (QDs),5,15 NaLnF4 NCs,16,17 magnetic iron oxide NCs,18,19 noble metal NCs,4,20 and multicomponent NCs,21,22 have been fabricated. Nevertheless, to achieve SPs with controllable size and narrow size distribution remains difficult due to the complex thermodynamic and kinetic behaviors of the template. Furthermore, removing the oil phase also affects the morphology of the resultant SPs.14 The interparticle interaction guided self-assembly of preformed NCs, however, provides an alternative approach © 2013 American Chemical Society

with better control over the morphology of the resultant SPs. The main driving forces include hydrogen-bonding,23 electrostatic interaction,24,25 and dipolar interaction,26 apart from multidentate molecule-mediated interactions.27 The sizes and size distributions of the SPs can normally be tuned by varying the ratio of particle surface ligands to the core NCs or mediator to nanoparticles. Recently, by partially removing the detergent molecules overcoated on hydrophobic Fe3O4 nanoparticle in aqueous system, Cao and co-workers reported supercrystals of Fe3O4 NCs with different shapes through the solvophobic interaction;28,29 the sizes of the SPs were successfully controlled by the ratio of the detergent to Fe3O4 NCs. The hydrophobic−hydrophobic interactions between NCs can also be used for preparing SPs of preformed NCs. For example, by introducing a bad solvent into a thermodynamically stable dispersion of hydrophobic NCs, the stability of the dispersion is disturbed, which leads to coagulation of the preformed NCs.30 If the coagulation is induced in a controlled manner, SPs can consequently be obtained upon the hydrophobic−hydrophobic interactions. Very recently, Liz-Marzán and co-workers demonstrated the solvent-induced self-assembly of Au NCs coated with polystyrene by using an amphiphilic block copolymer (PS-b-PAA) to terminate the particle aggregation and obtained Au SPs with narrow size distribution.31 Although the solvent-induced self-assembly provides the possibility to fabricate differently sized SPs by controlling the kinetics of the self-assembling process,31−34 the content of inorganic NCs in SPs is limited by the thick polymer corona Received: August 11, 2013 Revised: September 10, 2013 Published: September 11, 2013 21014

dx.doi.org/10.1021/jp408036p | J. Phys. Chem. C 2013, 117, 21014−21020

The Journal of Physical Chemistry C

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°C for 1 h under nitrogen protection, ZnS-coated CuInS2 (CuInS2@ZnS) QDs were precipitated by ethanol, washed with ethanol for 3 times, and finally dissolved in cyclohexane for further experiments. The ZnS-coating was carried out herein for enhancing the fluorescence efficiency of the CuInS2 nanocrystal core. In addition, the current QDs are more environmentally friendly than QDs of cadmium chalcogenides, though they are simply used as model particles in the current investigations. Synthesis of Amphibious Fe3O4 NCs. The amphibious Fe3O4 NCs stabilized by PVP were synthesized according to a previous report.38 Typically, Fe(acac)3 (1.2 mmol) was first dissolved in NVP (30 mL). The resultant solution was purged with nitrogen for 30 min at room temperature to remove oxygen, and then heated to 200 °C. After the reaction took place for 2 h, 30 mL of ethanol was introduced at room temperature, followed by 150 mL of ether as precipitant. The resultant black precipitates were isolated by a permanent magnet and redissolved in 30 mL of ethanol, followed by a precipitation process using 150 mL of ether. The abovementioned purification procedures were repeated for 3 times to obtain amphibious Fe3O4 NCs. Preparation of SPs of CuInS2@ZnS QDs. In brief, a certain amount of ethanol was introduced by using a peristaltic pump into a cyclohexane solution of CuInS2@ZnS QDs under vigorous stirring. With addition of ethanol, the solution turned from transparent red to turbid pink. After the addition of ethanol, the mixture was kept under stirring for 30 min. After that, the as-prepared SPs of QDs were collected by centrifugation and then washed 4 times with ethanol. Coating the Hydrophobic SPs with Amphibious Fe3O4. The as-prepared SPs were first dispersed in ethanol, and then a certain amount of ethanol solution of the amphibious Fe3O4 NCs was added dropwise under stirring. The molar ratio between the Fe3O4 NCs and CuInS2@ZnS QDs was around 0.14:1. The resultant SPs@Fe3O4 particles were collected by centrifugation, washed by ethanol and deionized water, and then redispersed in deionized water for further characterizations. Characterizations. Fluorescence and UV−vis absorption spectra were recorded with a Cary Eclipse fluorescence spectrometer and a Cary 50 UV−vis spectrometer, respectively. The transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns were recorded with a JEM-100CXII microscope operating at an accelerating voltage of 100 kV. The scanning electron microscope (SEM) images were recorded with a Hirachi S4800 SEM operating at a beam energy of 5 kV. The hydrodynamic size of SPs was determined with the help of a Malvern Zetasizer Nano ZS. The CuInS 2 @ZnS QDs concentration was determined by using inductively coupled plasma atomic emission spectroscopy (ICP-AES) produced by Jiangsu Skyray Instrument Co., Ltd. The concentration of Fe was determined by the 1,10-phenanthroline spectrophotometric method. The addition rate of ethanol was controlled by a peristaltic pump produced by LongerPump (BT100-1J), China Co., Ltd.

around the NC cores. Moreover, the complexity of solventinduced conformation variation of the polymer coating layer, interactions between polymers from different particles, and interactions between polymer and solvent makes it difficult to control the size and size distribution of the resultant SPs. In comparison with the polymer-coated NCs, the small molecule-coated ones are more sensitive to the changes of the dispersion media. On the other hand, the inorganic content of the NCs in SPs can largely be increased by using the latter ones as a building block. To show the solvent-induced self-assembly of NCs capped by small molecular ligands, we herein report SPs of CuInS2@ZnS quantum dots (QDs) capped by 1dodecanethiol. The effect of the solvent polarity on the selfassembly process of the hydrophobic QDs was investigated. On the basis of the kinetic studies, size tunable SPs with narrow size distribution were obtained. In addition, amphibious Fe3O4 NCs stabilized by poly(4-vinylpyrollidone) (PVP) were used to overcoat the hydrophobic SPs so as to achieve water-dispersible fluorescent/magnetic bifunctional SPs, as depicted by Scheme 1. Scheme 1. General Procedures for Preparing Hydrophobic SPs of CuInS2@ZnS QDs and the Water-Dispersible SPs Coated with Amphibious Fe3O4 Nanocrystals

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EXPERIMENTAL SECTION Chemicals. Iron(III) acetylacetonate (Fe(acac)3, Aldrich, 97%) was used after recrystallization two times. 1-Vinyl-2pyrrolidone (NVP, Jiaozuo Meida Fine Chemical Co., Ltd., medical grade) was purified by distillation under reduced pressure in the presence of phenothiazine. Indium(III) acetate (In(OAc)3, Alfa Aesar, 99.99%), 1-dodecanethiol (SigmaAldrich, 97%), copper(I) iodide (CuI, Aladdin, 99.995%), and zinc stearate (ZnSt2, Aladdin, 90%) in the current investigations were used as received. Analytical grade chemicals such as ethanol and cyclohexane were obtained from Sinopharm Chemical Reagents, Beijing, Co., Ltd. and used as received. Synthesis of CuInS2@ZnS QDs. CuInS2@ZnS QDs were synthesized as follows.35−37 Typically, In(OAc)3 (0.5 mmol) and CuI (0.5 mmol) were added to a flask containing 1dodecanethiol (30 mL) under vigorous stirring at room temperature. After being purged with N2 at room temperature for 30 min, the mixture was heated to 200 °C and kept for 2 h under nitrogen to form CuInS2 nanocrystal core. After the solution was cooled to room temperature, ZnSt2 (1 mmol) was added to the flask. The mixture was stirred for 30 min under nitrogen and then heated to 230 °C. After maintaining at 230

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RESULTS AND DISCUSSION The representative TEM images and particle size histograms of CuInS2@ZnS QDs and Fe3O4 NCs are shown in Figure 1. The as-prepared CuInS2@ZnS QDs shown in Figure 1a are of 2.9 ± 0.6 nm. The SAED pattern shown as the inset of Figure 1a 21015



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Figure 2. Temporal hydrodynamic sizes of the hydrophobic SPs formed in the ethanol/cyclohexane mixtures with different REC ratios.

Figure 1. TEM images, electron diffraction patterns, and particle size histograms of the as-prepared CuInS2@ZnS QDs (a) and the amphibious Fe3O4 NCs (b). The scale bars correspond to 50 nm.

principally be used for discussing the growth of the SPs. According to the LaMer model, the nucleation process of colloidal particles is determined by the supersaturation degree of the system. Higher supersaturation degree is in favor of fast nucleation, leading to a larger number of nuclei, followed by a particle growth process without further nucleation. According to the results shown in Figure 2, the size of the initially formed SPs is inversely correlated to the REC ratio, which strongly supports that the nucleation of the SPs process is determined by the supersaturation degree of the QDs in the mixed solvent. With respect to SPs formed by REC ratios higher than 2.5:1, the following growth of SPs is greatly suppressed, which implies that the solubility of hydrophobic QDs becomes rather limited. However, the SPs particles formed by REC ratios below 2:1 did not reach equilibrium sizes within 5 days. Therefore, it can be deduced that the solubility of the hydrophobic QDs remains high. Consequently, Ostwald ripening drives the SPs to further grow in size. To further disclose the growth kinetics of the SPs, the following samples were prepared. First, an ethanol/cyclohexane solution was prepared, into which a cyclohexane solution of QDs was quickly injected and the final REC ratio was set to 1.5:1. Then, the resultant solution of the primary SPs was divided into three portions. In 2 min, an ethanol/cyclohexane mixture with the REC ratio of 1.5:1 was injected into the first portion to achieve sample A. In parallel, an equal volume of ethanol was injected either quickly into the second portion to prepared sample B or slowly into the third portion to generate sample C. The final REC ratio of the mixed solvent for samples B and C was of 4:1. The DLS results of samples A−C are shown in Figure 3. Sample A presents a single scattering peak at 348.7 nm with a PDI (polydispersity index) value of 0.18. In difference, the fast addition of ethanol into the solution of the primary SPs greatly increased the particle size to 1115.2 nm; in the meantime, a big portion of smaller SPs of 114.7 nm in sample B were generated, which strongly suggests that the fast increase of the solution polarity quickly decreases the solubility of the hydrophobic QDs. Consequently, part of the free QDs chooses to grow on the primary SPs, and the rest is consumed by repetition nucleation. In difference, slow addition of ethanol into the solution of the primary SPs did not lead to repetition nucleation. Consequently, the hydrodynamic size of sample C was remarkably increased to 795.5 nm. Meanwhile, the PDI

indicates that the QDs possess a chalcopyrite (JCPDS Card No. 32-0339) crystalline structure. The ICP-AES analysis revealed that the ratio of Cu/In/Zn was 0.48:0.62:1. The as-prepared amphibious Fe3O4 NCs shown in Figure 1b are of 5.5 ± 0.6 nm. The SAED pattern placed aside suggests that the resultant particles are magnetite nanocrystals. In fact, the long alkyl chain of 1-dodecanthiol enables the cyclohexane to be a good solvent and ethanol to be a bad solvent for the 1-dodecanthiol-capped QDs. Consequently, 1dodecanthiol-capped QDs present a long-term colloidal solubility in cyclohexane or even in an ethanol/cyclohexane mixture with the REC ratio (ethanol-to-cyclohexane v/v ratio) below 1:1. However, when the REC ratio was increased to 1.25:1, QDs precipitates appeared in the solution with a small fraction of QDs remaining suspended in the supernatant within 24 h, which suggests that ethanol can effectively decrease the energy barrier between QDs to induce the coagulation of QDs. To disclose the solvent polarity-dependent self-assembling process of the hydrophobic QDs, a series of ethanol/ cyclohexane mixtures with different REC ratios were prepared and the cyclohexane solutions containing equal amounts of CuInS2@ZnS QDs were injected into the above systems. The final REC ratios were set as 1.5:1, 1.75:1, 2:1, 2.5:1, and 4:1, respectively. It was observed that when the REC ratio was higher than 1.5:1, the QDs quickly aggregated in the mixed solvents. To show the temporal evolutions of the self-assembling process of the QDs, dynamic light scattering (DLS) was used to continuously monitor the hydrodynamic size of the resultant SPs. As shown in Figure 2, the hydrodynamic size of the initially formed SPs and their growth tendency are inversely correlated to the REC ratio. When the REC was set as 2.5:1, the SPs reach their equilibrium size of 165.3 nm in 24 h. In contrast, 10 min of aging gives rise to SPs of 96.2 nm when the REC was set as 4:1, while 24 h of aging slightly increases the size of SPs to 107.1 nm. It should be mentioned that the results shown in Figure 2 were obtained by quickly injecting QDs solution into premixed solvents in order to disclose the selfassembling process of the hydrophobic QDs in media with defined polarities. Since spherical SPs dominate the aggregated QDs when the REC ratio was higher than 1.5:1, as shown in Figure S1 in the Supporting Information (SI), the classic LaMer model can 21016



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Figure 3. Hydrodynamic size profiles of SPs obtained by REC ratios of 1.5:1 (sample A) and 4:1 (sample B and C), respectively.

value of sample C was slightly decreased to 0.12 in comparison with that for sample A. Even though sample B and sample C were eventually formed in the mixed solvent with identical REC ratio, the addition rate of the bad solvent strongly determines the particle size and PDI values of the resultant SPs. These results together with those shown in Figure 2 strongly suggest that the formation and the following growth of the SPs obey the classic LaMer model established for colloidal particles. If this deduction holds, the size and size distribution of the SPs should be controllable by manipulating the supersaturation degree of the QDs in the mixed solvent. To verify this hypothesis, in combination with all aforementioned results, the SPs were prepared in the following experiments by introducing ethanol into the cyclohexane solution of the hydrophobic QDs. In this way, the supersaturation degree of QDs can better be controlled by the introduction rate of ethanol so as to suppress the repetition nucleation for achieving differently size SPs with narrow particle size distributions. In detail, three ethanol addition rates were adopted, i.e., 46.3 mL min−1, 2.67 mL min−1, and 0.27 mL min−1, for producing SPs at an optimized REC ratio of 3.4:1. As shown in Figure 4, the addition rate of ethanol is very effective for achieving differently sized hydrophobic SPs, i.e., 183.0 ± 25.0 nm (46.3 mL min−1), 496.7 ± 41.8 nm (2.67 mL min−1), and 1024.1 ± 107.7 nm (0.27 mL min−1). The SPs are fairly monodispersed and structurally stable. They slightly decreased in size by less than 12% over 6 months, as shown in SI Figure S2, which can be attributed to further elimination of organic solvent molecules entrapped in the interstitial regions between QDs within the SPs.11 Water-dispersibility is essentially required for the biological applications of functional particles. Different from the classic methods for transferring the hydrophobic nanoparticles into an aqueous system by using detergents, amphibious Fe3O4 NCs were used to overcoat the hydrophobic SPs shown in Figure 4, on the one hand for achieving water-dispersible SPs, and on the other hand for forming magnetic/fluorescent bifunctional SPs. The SEM results provided in Figure 5 suggest that the surface coating procedures did not alter the particle shape. But the particle sizes are slightly decreased to 169.8 ± 21.4 nm, 489.9 ± 32.4 nm, and 990.5 ± 127.1 nm, respectively, which is probably caused by the compression of the loosely packed hydrophobic

Figure 4. SEM images of the SPs prepared through addition of ethanol into the cyclohexane solutions of CuInS2@ZnS QDs by rates of 46.3 mL min−1 (a), 2.67 mL min−1 (b), and 0.27 mL min−1 (c), respectively. The scale bars correspond to 2 μm. The size histograms of the resultant SPs are placed aside.

QDs within the SP core induced by the surface tension at the particle/water interface. Further DLS results, as provided in SI Figure S3, revealed that the hydrodynamic sizes of the SPs shown in Figure 5 were of 214.3 nm, 598.2 nm, and 1023.0 nm, respectively, rather in consistence with SEM results, which suggests that the resultant SPs@Fe3O4 particles can well be dispersed in aqueous media. Apart from the excellent water-dispersibility, the SPs@Fe3O4 particles also exhibit excellent colloidal stability, as shown in Figure 6a. Most importantly, they inherited both fluorescent and magnetic properties of the corresponding mother nanocrystals. As demonstrated by the results shown in Figure 6b, the SPs@Fe3O4 particles are highly fluorescent and can be magnetically collected by a permanent magnet. To further characterize the optical properties of the SPs, the fluorescence spectra of SPs (in ethanol) and SPs@Fe3O4 (in either ethanol or water) were recorded for comparing with that of the mother QDs. As shown in Figure 7, the emission peak of the SPs shifts from 620 nm for the mother QDs to 661 nm, which can be explained by the Förster resonance energy transfer in the closely packed nanoparticles.39−42 In contrast, the following surface coating of the amphibious Fe3O4 NCs slightly shifts the emission peak further to 670 nm, which can partly be attributed to the stronger reabsorption of the photoemission at the shorter wavelength side by Fe3O4 NCs. By using Rhodamine 6G as a fluorescence standard,43 the fluorescence quantum yield (QY) of the CuInS2@ZnS QDs 21017



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Figure 7. Fluorescence spectra of SPs (ca. 500 nm) in ethanol, and the corresponding SPs@Fe3O4 in ethanol and water for comparing with that of the mother QDs. The excitation wavelength is 475 nm. The absorption spectrum of the mother QDs is also presented.

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CONCLUSIONS In summary, superparticles of fluorescent CuInS2@ZnS QDs have successfully been prepared through a facile approach by introducing bad solvent into the solution of QDs in good solvent. Systematic investigations reveal that the formation and the following growth of the SPs follows the LaMer model developed for conventional colloidal particles. Consequently, the ratio of bad solvent to good solvent plays an important role in determining the supersaturation degree of the system. Therefore, by manipulating the supersaturation degree through the controlled addition rate of the bad solvent, the SPs with narrow size distributions are obtained in the size range of 180− 1000 nm. By overcoating the SPs with amphibious Fe3O4 NCs instead of conventional detergent, aqueous dispersible SPs are prepared. Since both the magnetism and fluorescence of the mother nanocrystals are largely preserved, magnetic field responsive fluorescent SPs are obtained in aqueous media. From a large perspective, the current investigations may offer a facile and universal approach for developing functional superparticles by integrating the unique properties of different kinds of preformed nanocrystals.

Figure 5. SEM images and particle size histograms of the SPs@Fe3O4 prepared by coating the SPs shown in Figure 4. The scale bars correspond to 2 μm.

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ASSOCIATED CONTENT

* Supporting Information S

Figure 6. (a) Temporal hydrodynamic size of SPs@Fe3O4 in water. (b) Photographs of an aqueous dispersion of SPs@Fe3O4 (ca. 500 nm) taken under UV light and ambient light, respectively, before and after it was placed close to a permanent magnet of 0.5 T.

SEM images of the hydrophobic SPs formed in the ethanol/ cyclohexane mixed solvents with different REC ratios; SEM images of the hydrophobic SPs captured 6 months after they were prepared; and hydrodynamic size profiles of differently sized SPs@Fe3O4 particles in deionized water. This material is available free of charge via the Internet at http://pubs.acs.org.

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was estimated to be around 23% based on a literature method.44−46 Due to the interference of the scattering effect, the conversion rate of the QDs for forming SPs was determined by the ICP-AES method. In combination with the fluorescence results shown in Figure 7, the fluorescence QY of SPs was estimated to be around 13% in ethanol, while it drops to ∼9% for SPs@Fe3O4 in ethanol and to ∼7% for SPs@Fe3O4 in water. Although the fluorescence QY drops by approximately 60% after the QDs are assembled into the final SPs@Fe3O4 particles, the fluorescence brightness remains high, since each SP contains approximately 106−108 QDs.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 10 8261 3214. Fax: +86 10 8261 3214. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Basic Research Program of China (2011CB935800), NSFC (81090271, 21203210, 21203211, 21021003) for financial support. 21018



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