Preparation and Investigation of Quantum-Dot-Loaded Hollow

Article pubs.acs.org/JPCC

Preperation and Investigation of Quantum-Dot-Loaded Hollow Polymer Microspheres C. A. Hanley,†,‡ J. E. McCarthy,†,‡ F. Purcell-Milton,†,‡ V. Gerard,†,‡ D. McCloskey,‡,§ J. Donegan,‡,§ Y. P. Rakovich,∥,⊥ and Y. K. Gun’ko*,†,‡,# †

School of Chemistry, §School of Physics, and ‡CRANN, Trinity College Dublin, Dublin 2, Ireland Centro de Física de Materiales (MPC, CSIC-UPV/EHU) and Donostia International Physics Center (DIPC), Po Manuel de Lardizabal 5, Donostia-San Sebastian 20018, Spain ⊥ IKERBASQUE, Basque Fondation for Science, Alameda Urquijo, 36-5, Plaza Bizkaia, Bilbao 48011, Spain # Saint Petersburg National Research University of Information Technologies, Mechanics and Optics, 197101 Saint Petersburg, Russia ∥

S Supporting Information *

ABSTRACT: In this work, hollow poly(methyl methacrylate) (PMMA) microspheres loaded with CdSe/CdS core−shell quantum dots (QDs) were fabricated onto hydrophilic glass substrates using a spray-drying method. The PMMA microspheres were investigated using scanning electron microscopy, confocal microscopy, and fluorescent lifetime imaging microscopy (FLIM) to investigate the morphology of the spheres and confirm their hollow structure. The QDs were used as fluorophores in confocal microscopy to observe the central cavity of the microspheres and in solid-state photoluminescence spectroscopy to observe whispering gallery modes (WGMs), demonstrating the high optical quality of the hollow microspheres and their potential application as optical microresonators. diameters of 100 μm. Hollow polyaniline/Fe3O4 composite microspheres were also produced by emulsion polymerization in a multistep process, giving spheres of ∼1-μm diameter.12 Although the preparation of hollow polymer microspheres by the above methods are well-known, these techniques often involve multiport synthesis, expensive equipment, high temperatures (furnaces), long reaction times, and stabilizing ligands, resulting in expensive processing that is difficult to scale up to the industrial scale.9,13,24−27 Spray drying is a relatively simple and efficient method for producing microspheres. This method is based on accumulating solute at the sprayed droplet surface that diffuses relatively more slowly than the solvent during the drying process, resulting in the formation of a solid shell. Spray drying of polymer solutions has previously been used for the production of a range of polymer microspheres for biological applications.28,29 For example, polysaccharide microspheres were formed by spray drying a solution of chitosan, hydroxypropyl-β-cyclodextrin, and polyethelyene glycol to form spherical microspheres to study the release of bovine serum.30 Manitol, trehalose, and insulin solutions have also been used in spray-freeze-drying methods to create spherical shapes, showing that a range of solutions can

1. INTRODUCTION Over the past two decades, hollow microspheres have attracted great interest because of their unique properties and various potential applications ranging from drug delivery to gas storage.1−5 Various hollow microspheres from organic polymers and inorganic materials have been produced and investigated.6−9 Hollow polymer microspheres are of particular importance, as these materials have been envisaged for drug delivery, cosmetics preservation, catalysis, smart coatings, and various encapsulation applications. Hollow polymer microspheres can be prepared by a variety of methods including microencapsulation, liquid droplet methods, self-assembly techniques, emulsion polymerization, templating, microfluidic techniques, and a number of other approaches.8,10−21 For example, a new electrohydrodynamic atomization (EHDA) method has recently been developed to create a range of biocompatible hollow microspheres for drug delivery systems.20,22 Using a two-syringe system, microspheres with a polymethylsilsesquioxane shell and perfluorohexane core were produced, although precise conditions are needed to form a majority of hollow spheres and cross-linking of polymer spheres can occur during synthesis. In another work, the use of water− oil−water emulsions of polystyrene and acrylonitrile−butadiene−styrene/polycarbonate in combination with solvent evaporation techniques enabled the production of hollow microspheres.14,23 This method typically results in spheres with © 2013 American Chemical Society

Received: May 22, 2013 Revised: October 17, 2013 Published: October 18, 2013 24527

dx.doi.org/10.1021/jp405053k | J. Phys. Chem. C 2013, 117, 24527−24536

The Journal of Physical Chemistry C

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be used to form microspheres.31,32 To date, it has not been confirmed that these methods can enable the manufacture of polymer microspheres with hollow cavities with sufficiently uniform refractive indexes for optical applications. Spherical microparticles support high-quality electromagnetic modes known as whispering gallery modes (WGMs).33,34 These modes result in defined peaks in the fluorescence spectra from emitters on the surface or embedded in the microspheres.35 The positions of these modes are extremely sensitive to the refractive index of the local surroundings.36,37 Quantum dots (QDs) are versatile light-emitting nanomaterials with a range of potential photonic applications because of their tunable and narrow emission spectra, long fluorescent lifetimes, resistance to photobleaching, and relatively easy integration compared to organic dyes.25,38−42 So far, QDs have been incorporated into solid (nonhollow) polymer microspheres by swelling in organic solvents or by layer-by-layer (LbL) deposition onto the surface.43−47 QD-loaded microspheres have been proposed for potential applications as optical microresonators, single-photon emitters, optical sensors, and lowthreshold microlasers.47 However, the large-scale production of these microstructures is still very challenging technically, which limits further development in this area. Herein, we report a new approach for the production of uniform hollow poly(methyl methacrylate) (PMMA) microspheres of high optical quality containing embedded CdSe/CdS quantum dots that were incorporated using in situ processing. In our method, a glass substrate is treated with an oxidizing solution of sulfuric acid and hydrogen peroxide (“piranha etch”), so as to remove any organic contaminants and promote the formation of hydroxyl groups on the glass surface, making the substrate hydrophilic. As a result, PMMA spheres can be formed by simple spraying of polymer solution over the pretreated glass substrate, followed by solvent evaporation, producing spherical polymer microcavities. The size and concentration of spheres on the surface can be controlled by adjusting the solution concentration and nozzle aperture. We characterized the new hollow microsphere composites by various instrumental techniques and demonstrated their potential as optical microresonators.

became transparent. Once the CdO/ODPA/ODE mixture reached 250 °C, heating was turned off, and the Se precursor was injected rapidly using a needled syringe. To make CdSe/CdS core−shell QDs, a CdSe/TOPS (coordinated TOP and sulfur) precursor was prepared as follows: First, 0.75 mL of TOP was combined with 0.032 g of sulfur powder under an inert atmosphere. This mixture was then sonicated until a clear solution was formed. This precursor was added to 0.5 g of CdSe cores dispersed in 2 mL of TOP under argon. CdO, ODPA, trioctylphosphine oxide (TOPO), and steric acid (SA) were degassed for 1 h under a vacuum and then heated to 320 °C. The CdSe/TOPS solution was rapidly injected into the Cd solution and allowed to react for 7 min. The reaction was quenched by addition of methanol. To purify core or core−shell quantum dots, repeated extractions with a hexanes/methanol (1:1, v/v) mixture were performed. The purified QDs were stored in toluene at +4 °C until further use. 2.3. Preparation of Glass Substrates. Glass coverslips with a thickness of 0.15 mm were purchased from VWR. These glass substrates were washed with Millipore water before use and dried for 2 h in an oven before being sprayed with polymer solutions. Piranha solution was made by combining 5 mL of 98% H2SO4 with 5 mL of H2O2 (50%, w/v). Each washed glass slide was inserted into the freshly made solution of piranha etch and left for 10 min. After 10 min, the glass slide was removed, dipped in pure Millipore water, then dipped in another clean Millipore water bath, and finally rinsed with Millipore water. The slide was dried using a heat gun and then sprayed with polymer solution. Glass slides coated with SDBS (90%, Sigma-Aldrich) were prepared in a similar way, with a final step of dipping the glass slide into a 1% (w/v) aqueous solution of SDBS after it had been cleaned and dried with a heat gun. 2.4. Preparation of PMMA/QD Solution. PMMA was used without any further purification (99%, MW 120000) and dissolved in degassed toluene at concentrations (w/v) of 1%, 2.5%, 5%, 7.5%, and 10% by sonication. Each toluene/PMMA solution was sonicated in a sonic bath until it was clear and homogeneous. QDs in toluene were added dropwise to the polymer solution and further sonicated for an additional 30 min to ensure homogeneous mixing. 2.5. Preparation of Hollow Microspheres by SprayDrying Process. Glass substrates were placed on a clean flat surface and secured in place by masking tape. A commercially available air gun (Theoben Evolution) was placed directly over the substrate at a fixed distance of 15 cm. The air gun was attached to a pressurized argon gas cylinder with a constant pressure of 20 psi. The nozzle aperture was adjusted by a screw at the back of the gun and was calibrated for five settings: 1, closed; 2, 44% of the open aperture size; 3, 75% of the open aperture size; 4, 87% of the open aperture size; 5, open (A = 0.1385 mm2). Spraying for all samples was done at constant pressure and distance. The spray time for each sample was 500 ms. Samples were allowed to dry in ambient conditions. 2.6. Instrumentation and Measurements. An ultrasonic bath (Grant XB6 at 50−60 Hz) was used for sonication of the polymer solutions. A Cary 50 UV−vis spectrometer was used to record absorbance spectra of the QDs. A Perkin-Elmer LS 55 fluorescence spectrometer and a 1 cm × 1 cm quartz cuvette were used to record photoluminescence spectra. Photoluminescence decays of the QD-doped microspheres were measured using the time-correlated single-photon-counting method (TimeHarp, PicoQuant). Samples were excited using a

2. EXPERIMENTAL SECTION 2.1. Materials. All starting materials were purchased from Sigma-Aldrich and used without further purification unless otherwise stated. These include cadmium oxide (CdO, 99.99%) selenium (Se, 99.99%) trioctylphosphine (TOP, 90%), trioctylphosphine oxide (TOPO, 99%) poly(methyl methacrylate) (PMMA, 95%, MW 120000), sulfur powder (S, 99.99%), octadecene (ODE, 99%), sodium dodecylbenzene sulfonate (SDBS, technical grade), stearic acid (SA, 95%), and octadecyl phosphonic acid (ODPA, 97%). Glass coverslips (15 mm × 15 mm × 0.15 mm) were obtained from VWR. All solvents were distilled and degassed before use. 2.2. Synthesis of CdSe/CdS QDs. CdSe/CdS QDs were synthesized using a method adapted from Weller et al.48 that is based on the hot injection technique in the noncoordinating solvent 1-octadecene (ODE). Briefly, 0.057 g of cadmium oxide (CdO) and 0.29 g of octadecyl phosphonic acid (ODPA) were added to a 50 mL three-neck flask containing 5 mL of ODE. This mixture was degassed under a vacuum and then heated under a nitrogen atmosphere at 300 °C. The mixture was then cooled to 250 °C. The selenium (Se) precursor was prepared by mixing 0.016 g of Se with 0.75 mL of trioctylphoshine (TOP), under an inert atmosphere, and then sonicating the solution until it 24528



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Figure 1. (A) Diagram of experimental setup, (B) formation of hollow microspheres, and (C) SEM image of a formed microsphere with a 15° tilt.

Figure 2. Wetting angles of washed glass slides: (bottom) original before treatment and samples after treatment with (middle) piranha etching solution and (top) SDBS.

480-nm diode laser and analyzed with an Olympus IX71 inverted microscope (mag and NA). Steady-state PL of the spheres was measured using a Reinshaw micro-Raman setup, exciting the spheres with a 488-nm

Olympus FV 1000 point-scanning confocal laser microscope using a multiline argon (453, 488, and 515 nm) laser and a 405-, 543-, and 633-nm diode laser. Forty images were recorded along the z axis, with a 400-nm distance between each image. 24529



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Figure 3. SEM images: (A) Panoramic image of PMMA spheres sprayed with a 5% (w/v) solution of toluene/PMMA and (inset) closeup of a single sphere with a 15° tilt. (B) PMMA spray dried onto untreated glass surface, 15° tilt. (C) Electron beam damage of a PMMA sphere by SEM, 15° tilt. (D) Collapsed spheres formed by spraying 10% (w/v) toluene/PMMA solution, 25° tilt.

acid treatment of the glass slide, it was found that microspheres did not form (Figure 3B). The formation of the hollow cavity of the microsphere is due to quick evaporation of the outer PMMA/toluene layer when on the substrate, owing to toluene’s volatility at ambient conditions. Once the shell has formed, the remaining solvent can evaporate over time through the pores of the PMMA shell to leave a hollow center. To further increase the hydrophilicity of the glass slide, an acid-cleaned slide was dip-coated in a 0.01 M solution of SDBS in water. The contact angles of cleaned, piranha-etched, and SDBSdip-coated slides were measured by dropping 1 μL of toluene onto the slides (Figure 2). The contact angles of 50 1-μL drops were measured for each substrate, and the value was found to increase from 6.92° ± 1.24° for untreated slides to 16.32° ± 1.42° and 22.71° ± 1.56° for piranha-etched and SDBS-treated slides, respectively. The produced PMMA microspheres were characterized by a range of microscopy techniques. Microspheres were coated with gold for SEM analysis, which sometimes resulted in damage to and collapse of the spheres as a result of melting under the highenergy electron beam (Figure 3). Increasing the PMMA concentration in the solution above 7.5% resulted in the formation of collapsed and half-formed spheres (Figure 3C,D). This behavior can be attributed to the larger spheres formed because of the high viscosity of the polymer solution being sprayed. In Figure 3C, one can see that thinning of the shell can cause the hollow spheres to collapse as a result of differences in the shell thickness and drying effects. The thin shell of the spheres formed under certain conditions can also lead to buckling of the hollow spheres as they dry. This results in the formation of hemispheres with a small cavity in the center (Figure 3C).

Transmission electron microscopy images were recorded using an FEI TITAN electron microscope at 200 kV. Samples were drop cast onto lacey carbon grids and dried under a vacuum before imaging. Field-emission scanning electron microscopy (SEM) was performed using a Hitachi s-4300 instrument, operated at 5.0 kV, on gold-coated surfaces and a Zeiss Auriga focused ion beam (FIB) system with a Cobra ion column. Surface wetting of toluene droplets on glass slides was analyzed using First Ten Angstroms 125 software, and pictures were taken using a Cannon 5.5-mm telecentric lens with an APPR B/W camera.

3. RESULTS AND DISCUSSION Hollow microspheres were produced by spray drying of a PMMA solution in toluene that was blended with luminescent CdSe/ CdS core−shell QDs. A schematic of the experimental setup is shown in Figure 1A. A commercial air gun (Theoben Evolution) with pressurized argon gas was used to spray the polymer−QD solution. The argon pressure, nozzle aperture, nozzle−substrate distance, and solution viscosity were varied to find the optimum conditions for the formation of hollow microspheres between 0.7 and 5 μm in diameter. To form spheres, the glass slides were cleaned with a solution of hydrogen peroxide and sulfuric acid to remove any organic contaminants and promote the formation of −OH groups on the surface. This is a well-known treatment for glass substrates that creates a hydrophilic surface and increases the wetting angle of nonpolar solvents.49 As the solvent evaporated from the surface of the glass slide, the polymer dried at the solution−air interface, forming a hollow sphere on the substrate (Figure 1B). The hydrophilicity of the glass slide determined the shape of the microsphere formed, as well as the area of contact between the final sphere and the substrate. When the treated surface was smooth, well-shaped spherical microstructures formed (Figures 1C and 2A). Without 24530



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in the Supporting Information). The luminescence from the quantum dots can be seen at the edges of the microspheres, whereas there is no luminescence in the center, indicating a hollow cavity. Figure 4 shows the same microsphere focused at the top and then at decreasing heights of 1.6 and 3.6 μm, respectively, showing that, as we moved down along the z axis, a hollow ring was observed, indicating the absence of luminescent particles in the center of the cavity. A focused ion beam (FIB) of Ga ions in combination with an ultra-SEM instrument was used to cut a microsphere and observe the hollow cavity (Figure 5). In this experiment, the microsphere was selected under a microscope and then cut at one edge using a Ga ion beam to reveal the hollow cavity. This was done on a sphere of slightly larger diameter, as smaller spheres often collapsed or moved during the cutting process (see Supporting Information). During spraying experiments, the nozzle aperture was adjusted using a screw on the back of the spray gun, which controlled the retraction of the needle linearly, resulting in a nonlinear change in aperture area (Figure 6). Five settings for aperture size were used, and aperture size was determined by microscopy. Aperture sizes of 2−4 as described in the Experimental Section were found to give hollow spheres. Using a 5% (w/v) PMMA solution, we found that we could control the diameter of the spheres by increasing the aperture size. SEM was used to determine the average sphere size for each aperture width (Figure 6). An increase in aperture size (from 1 to 2) resulted in a change in average sphere size from 0.7 ± 0.16 to 1.04 ± 0.27 μm, a 32.8% increase. Opening the aperture further from 3 to 4 (from 75% to 87%) resulted in an average sphere size of 1.23 ± 0.24 μm, a 15.4% increase in sphere diameter from aperture size 2. Above this aperture size, sphere formation was compromised because of larger droplets depositing on the surface and a greater size distribution from the spray. The working distance from the nozzle head to the substrate is also critical for the formation of spheres when working at a

Confocal microscopy was used to image the internal structure of QD-loaded PMMA microspheres. The sample was raster scanned through the focus of the objective at different heights along the z axis taking cross-sectional slices of the fluorescence emission. Focusing down the z axis, we were able to observe the hollow centers of the microspheres (Figure 4; see also Figure S9

Figure 4. Microscopy images of PMMA microspheres (left, confocal images under UV, going down the z axis to show the hollow center; right, optical microscopy images).

Figure 5. Focused-ion-beam (FIB) lithography on a hollow microsphere to show the cavity. The sphere was imaged from above using a Ga ion beam (above, left) and then continuously during milling at a 45° tilt. The sample was coated with gold before imaging and milling. 24531



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Figure 6. (Left) Sizes of PMMA microspheres measured by SEM microscopy for spray gun aperture widths. (Right, top) Aperture size and (bottom) needle size determined using optical microscopy.

constant pressure. An optimum working distance of 20 cm from the nozzle of the gun to the glass substrate was found for sphere formation. At distances of less than 15 cm, the pressure of the spray distorted sphere formation on the substrate, leading to uneven coverage of the substrate and aggregation of the droplets on the surface. At distances of more than 35 cm, droplets were found to aggregate during spraying; combined with more solvent evaporation while the droplets were in transit, this resulted in large mounds on the substrate, as seen in Figure 3B. CdSe/CdS quantum dots were prepared by a modified hotinjection synthesis and characterized by TEM (Figure 7), emission absorbance spectroscopy in solution, and florescence lifetime imaging microscopy (FLIM) in a polymer matrix (Figure 8). According to TEM, the average size of the QDs was 5.1 ± 0.5 nm (Figure 7). FLIM of the CdSe/CdS QDs in a polymer matrix and of doped PMMA spheres showed a change in the lifetimes of the samples (Figure 8). The PL decay was fitted with a biexponential function. It was found not only that were the fluorescent lifetimes reduced by 30.5% for t1 and 19.7% for t2 compared to those for the original free CdSe/CdS QDs, but also that there was a greater contribution to the shorter lifetime (t2) for the doped spheres (Table 1 and Figure 8 top right). This is most likely due to the aggregation and partial clustering of QDs in the polymer matrix that results in energy transfer and self-quenching processes. Comparison of the emission spectra of colloidal QDs (Figure 8, bottom right) indicated that the QD emission was also redshifted by 15 nm when the QDs were incorporated in a PMMA matrix (Figure 9). A photoluminescence quantum yield (PLQY) of 58% for QDs after incorporation into the PMMA matrix was calculated using Rhodamine 6G as a reference. The polymer

Figure 7. TEM image of CdSe/CdS quantum dots on a lacey carbon substrate.

samples with QDs also showed a very bright emission when viewed under a UV lamp. Steady-state PL of the spheres was measured using a Reinshaw micro-Raman setup and exciting the microspheres with a 488-nm laser. Well-defined WGM resonant peaks were observed in multiple spray-dried particles (Figure 9; Figures S1−S8, Supporting Information). The specific structure of the WGM varied from microsphere to microsphere. A typical spectrum from a spherical particle is shown in Figure 9 (top). The observed WGM peak structure is a result of coupling of electronic states in 24532



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Figure 8. (Left) Fluorescent lifetime imaging microscopy (FLIM) images of two microspheres. (Right, top) Biexponential lifetime results for a single microsphere compared to CdSe/CdS QDs in a polymer matrix. (Right, bottom) PL and UV spectra of CdSe/CdS QDs in toluene solution.

demonstrating that these microspheres are of high optical quality and could potentially be used as optical microresonators.50−55 From additional measurements on a variety of spheres, it was clear that the size and shape of the microspheres influence the WGM spectra to a degree that no two spectra are completely similar (see Figures S1−S4, Supporting Information). This contrast can be seen in Figures S3 and S4 (Supporting Information), where the degeneracy in WGMs in Figure S3 (Supporting Information) results in a broad WGM peak, whereas the peaks are well-defined and spaced in Figure S4 (Supporting Information). After the optimization of the spray-drying procedure, approximately 65% of all produced microspheres were of good optical quality and have showed well-defined WGMs. It has also been demonstrated that the spray-drying method can be used for organic fluorophores while maintaining the WGM spectrum of the sample (Figures S5−S8, Supporting Information). We used coumarin dye 153 because it gives a broad PL emission, has a high PLQY, and emits close to the emission of our QD samples. WGM resonances for coumarin-loaded microspheres were more pronounced on the red side (longer wavelengths) of the emission maximum of the fluorophore. These experiments illustrate the versatility of the system and approach enabling the incorporatation of not only QDs, but a range of organic dyes and nanoparticles that can influence the

Table 1. Fluorescent Lifetimes of Original QDs and QDLoaded PMMA Microspheres sample

amp 1 (%)

τ1 (ns)

amp 2 (%)

τ2 (ns)

χ2

CdSe/CdS PMMA sphere

82 33

14.9 ± 0.6 10.4 ± 0.6

18 67

2.4 ± 0.1 1.9 ± 0.1

1.05 1.14

the QDs and photon states of the microsphere. The placement and spacing between WGM peaks were determined by the size and refractive index of the microsphere, whereas the spectral intensity distribution depends on the parameters of the QDs. For these samples, both transverse electric (TE) and transverse magnetic (TM) modes were observable, demonstrating a random orientation of the nanoparticles in the PMMA matrix. The FSR was shown to correspond well to the SEM-measured diameter of the particles. Particles with slight deformation break the degeneracy of the WGMs in a perfect sphere and result in more complex, broader features (Figure 9).50 These broad features are, in fact, collections of narrow high-Q modes with spacings below the spectral resolution of the spectrometer used.46 A lower bound for the quality factor of the modes can be obtained by fitting the modes and extracting the full width at halfmaximum (fwhm). The microspheres showed a quality factor (Q factor) of 5.4 × 103 limited by the spectrometer resolution, 24533



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Figure 9. (Top) Steady-state PL (left) showing WGM resonances and (right) with PL of QD subtracted (black). (Bottom, left) Laser-light and (right) bright-light images of microsphere.

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effect of the emission spectra while still maintaining the optical quality needed for WGM resonances to occur.

*Tel.: +353 1 8963543. E-mail: [email protected].

4. CONCLUSIONS In summary, hollow QD-loaded polymer microspheres have been prepared by a novel approach using a spray-drying technique. These spheres were formed by solvent evaporation on glass slides, leading to thin shelled microcavities that display WGMs at the emission band of the incorporated QDs. The Q factor for these spheres was calculated to be 5.4 × 103, and a reduction in lifetime of the QDs was observed when incorporated into the PMMA matrix. It was demonstrated that microsphere size can be controlled by adjusting the nozzle aperture to give high-quality spherical microspheres with less than ±19% deviation in the size distribution. However, buckling and deformation of microspheres larger than 3 μm was common, because of the thin shell of the hollow microspheres. We believe that our approach can be further developed and optimized for the large-scale production of new hollow microsphere structures with a range of potential applications including microencapsulation, optical sensing, and photonics.

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AUTHOR INFORMATION

Corresponding Author Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Science Foundation Ireland (Grant SFI 07/IN.1/ I1862), Trinity College Dublin, Higher Education Authority, and Ministry of Education and Science of the Russian Federation (Grant 14.B25.31.0002) for financial support. We also thank the Electron Microscopy unit (CMA) in Trinity College Dublin for SEM images.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental details, calculations, spectra, photographs, and SEM and confocal microscopy images. This material is available free of charge via the Internet at http://pubs.acs.org. 24534



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