Characterization and Optimization of the Fluorescence of Nanoscale

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Characterization and Optimization of the Fluorescence of Nanoscale Iron Oxide/Quantum Dot Complexes Minjung Cho, Elizabeth Quevedo, Seung Soo Lee, Christopher J Jones, Wonhee Jang, and Vicki L. Colvin J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 13 Jun 2014 Downloaded from http://pubs.acs.org on June 19, 2014

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The Journal of Physical Chemistry

Characterization and Optimization of the Fluorescence of Nanoscale Iron Oxide/Quantum Dot Complexes Minjung Cho 1,2, Elizabeth Q. Contreras 1, Seung Soo Lee 1, Christopher J. Jones 1, Wonhee Jang ,1,3, Vicki L. Colvin 1* 1

Department of Chemistry, Rice University, Houston, TX 77005, USA

2

Department of Translational Imaging & Nanomedicine, Houston Methodist Research Institute,

Houston, TX 77030, USA 3

Department of Life Science, Dongguk University, Seoul, South Korea

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

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ABSTRACT

In this paper, nanoscale iron oxide/quantum dot (QD) complexes were formed in an efficient and versatile reaction that relied on the nucleation of chalcogenides on preformed iron oxide nanocrystals. Iron oxide nanocrystals acted as seeds for the growth of CdSe quantum rods (QRs), CdSe QDs, and CdSe@ZnS QDs. A zinc sulfide shell was added to protect the CdSe core in the complex chemically and provide a reasonable fluorescence quantum yield (~5%). Highresolution transmission electron microscopy revealed that QDs shared an interface with iron oxide, yielding structures that resemble pincushions with QDs or QRs studding the surface of the iron oxide. These complexes only formed under specific conditions of temperature, injection rate, and surfactant composition that minimized the formation of unbound QDs. As a superparamagnetic material, iron oxide provided a high purity (~89%) of complexed materials without unbound QDs. The quantitative photoluminescence quantum yields of the purified complexes correlated with the number of QDs per iron oxide. These nanoscale complexes retained the size-dependent optical and magnetic properties of each component.

KEYWORDS: iron oxide, quantum dot, iron oxide/quantum dot (QD) complexes, magnetic capture efficiency (MCE), and photoluminescence quantum yield (PLQY)


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INTRODUCTION Multifunctional nanostructures consisting of two or more functional components represent a new class of materials with potential applications, including multimodal imaging and simultaneous detection and therapy.1-13

Specifically, magnetic-fluorescent hybrid nanostructures are

recognized as a powerful class of multifunctional nanostructures because of their combination of magnetic and fluorescent properties.5,7,10,11,14,15 Magnetic iron oxide nanoparticles possessing super paramagnetic properties have a variety of applications, such as magnetic resonance imaging (MRI) contrast agents,16-18 magnetic cell sorting and separation,19,20 and magnetic drug delivery.21,22 Fluorescent materials, such as quantum dots (QDs), offer size-tunable optical properties and high fluorescence quantum yield.23 As fluorescent probes, QDs have important biomedical applications, including in vivo cell targeting, imaging, and multiplexed biological detection.24-28 These magnetic-fluorescent hybrid nanostructures can open new capabilities, such as optical imaging combined with MRI29-35 and visible multiple cell tracking and separation.36-38 However, progress in the development of these hybrid nanostructures is limited by many demands that biomedical applications place on the materials, including stability, product yield, and size tunability. Ideally, these assemblies would be prepared in high yield, with a limited hydrodynamic size (d < 50 nm) and controlled stoichiometry. Three strategies have been explored to date: polymer encapsulation, ligand cross-linking, and direct seed nucleation.11 The first strategy relies on the encapsulation of the magnetic nanoparticles, including iron oxide and QDs or fluorescent dyes inside larger, often submicron, polymeric capsules or porous silica beads.39-51 This method is relatively simple and suitable for in vitro applications, such as multimodal imaging and drug delivery.33,35,52-56 However, the resulting materials are relatively large (>100 nm) and easily aggregated. The second strategy examines the use of direct linking



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agents to coordinate magnetic particles and QDs or fluorescent dye.57-66 Iron oxide nanoparticles are simply covalently linked to the fluorescent dye or QDs via an m-2,3-dimercaptosuccinic acid (DMSA) ligand or dodecylamine (DDA) cross-linkers.57,59 The cross-linkers can directly affect conjugation so that particle size remains small. However, this strategy is limited by its low product yield and the complicated functionality of linkers. The third strategy exploits the observation that QDs can nucleate directly on some kinds of magnetic nanoparticles, including iron oxide nanocrystals.67-79 This method can be extended to many kinds of hybrid nanostructures, including metal-metal, metal-semiconductor, metal oxide–metal, and metal oxide–semiconductor, and offers products with a small size, homogeneous structure, and tunable stoichiometry.80-89 In this paper, we focus on this direct nucleation route, specifically, magneticsemiconductor heterostructures that provide small and tunable properties. The direct nucleation of QDs on an iron-based magnetic nanostructure can reach high purity and high yields of magnetic–QD complex materials through the use of the conventional high temperature metal precursor decomposition method.67-79,90-93 More specifically, prior studies have examined the formation of magnetic-semiconductor heterostructures by using a magnetic nanoparticle as a seed for the nucleation of semiconductor materials. Kwon et al. described the synthesis of iron oxide/CdS heterostructures and examined the size dependence on growth rate and the effects of lattice mismatch on the heterojunctions.90-93 Gu et al. developed FePt-CdS, FePt-CdSe, and FePt-ZnS heterodimer nanoparticles and explored the morphology of the structures by controlling various experimental conditions.68,94,95 Recently, they demonstrated intracellular manipulation of FePt-CdSe nanoparticles.96 Selvan et al. developed Fe2O3-CdSe nanocomposites and, by fluorescence imaging, showed internalized silica-coated magnetic QDs in a live cancer cell.13,97-99


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Generally, direct nucleation produces both complexed materials and self-nucleation of secondary materials as excess. Ideally, a reaction should produce mostly complexed materials as opposed to isolated nanoparticles when the efficiency of the complex formation is optimized. In order to achieve optimal conditions, a quantitative method should be developed to understand favorable conditions that minimize the formation of unbound QDs. Moreover, an unanswered question is the extent to which the complex reduces the photoluminescence quantum yield (PLQY), and a related issue of strategies to minimize quenching. The PLQYs of magnetic/QD complexes through direct seed nucleation are generally estimated to be 3–38%.67,71,96-99 Because the QDs are in direct contact with the metal surface of magnetic nanoparticles, the PLQY of the complex decreases compared with stand-alone QDs.67,71,74,96-98,100,101 Moreover, excess unbound QDs affect the PLQY of the complex so that it should be removed to attain an exact value. Precise measurement is needed to determine an accurate PLQY value of the complex due to the possible presence of excess unbound QDs. Here, we optimized the synthesis of nanoscale iron oxide/QD complexes that possess both good responsiveness to external magnetic fields and a PLQY value in excess of 5%. The optimized synthesis permits control of the type and number of QDs. Because iron oxide nanoparticle is a superparamagnetic material, the complexes can be separated from unbound QDs with an external magnetic field. This magnetic capture allows for measurement of reaction yields and optimization of conditions to minimize excess unbound QDs; thus, the PLQY of complexes can be accurately determined. Although the photoluminescence of iron oxide/QD complexes is lower than that found for pure QDs, it is nonbleaching and high enough for easy visualization. Additionally, the PLQY correlates well with the number of QDs per iron oxide nanoparticle. This paper illustrates an efficient method to control the synthesis of magnetic



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fluorescent complexes that possess variable morphology, improved fluorescent emission, and high quantum yield.

EXPERIMENTAL SECTION Chemicals. Iron oxide, hydrate (FeO(OH), catalyst grade 30–50 mesh 99%), oleic acid (OA, technical grade 90%), 1-octadecene (ODE, technical grade 90%), cadmium oxide (CdO, powder 99.99%), selenium (Se, 100 mesh 99.5%), sulfur (S, powder 99.98%), zinc oxide (ZnO, powder 99%), trioctylphosphine (TOP, technical grade 90%), octadecylamine (ODA, 97%), hexadecylamine (HDA, 99%), trioctylphosphine oxide (TOPO, 99%), hexadecane (reagent plus, 99%), and phenyl ether (reagent plus, 99%) were all purchased from Sigma-Aldrich. All nanocrystals were synthesized under ultra-high purity nitrogen (N2, 99.99%). Methanol (99.8%), acetone (99.5%), hexanes (98.5%), sodium bicarbonate (99.7%), dimethylformamide (DMF, 99.8%), nitric acid (HNO3, 70%), and hydrogen peroxide (H2O2, 30%) were purchased from Fisher Scientific (USA). All chemicals were used without further purification. Synthesis of Iron Oxide Nanocrystals. Iron oxide nanocrystals were synthesized following our previously published method.102 First, 0.178 g FeO(OH), 2.26 g OA, and 5 g 1-ODE were mixed in a 50 mL three-neck flask and initially heated to 120 °C for 2 hours to remove residual water and then to 240 °C for 30 minutes to synthesize iron oleate. Then, the reaction mixture was heated at 320 °C for 2 hours under N2 flow. To purify the resulting black colloidal nanocrystals, 20 mL methanol and 20 mL acetone were added to 5 mL of colloidal solution that was centrifuged at 4,150 rpm for 30 minutes. The precipitates at the bottom of the solution were redispersed using hexanes; this process was repeated six times. Finally, the 10 nm iron oxide nanocrystals were purified and dispersed in hexanes. A series of iron oxide nanocrystals with


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diameters ranging from 8 to 17 nm was synthesized by controlling the ratio between FeO(OH) and OA from 1:3 to 1:6. The purified iron oxide nanocrystal solution was stored in hexanes. Synthesis of Iron Oxide/CdSe QD Complexes. To synthesize the iron oxide/CdSe QD complexes, CdO (0.0386 g, 0.3 mmol), ODA (3 g, 11 mmol), and 1-ODE (10 g, 40 mmol) were mixed in a three-neck flask and heated to 200 °C for 1 hour. After cooling the solution down to room temperature, a 1 mL iron oxide nanocrystal solution stored in hexanes at a concentration of 6,000 mg/L Fe was injected into the mixture. The mixture was then heated to 100 °C for 30 minutes to evaporate the hexanes completely. Afterwards, the reaction mixture was heated to reflux at 280 °C for 1 hour under N2 flow. After the mixture cooled to 220 °C, Se (0.9 mmol) dissolved in 2 mL TOP was slowly added at a rate of 0.4 mL/min. The mixture was kept at 250 °C to allow for the growth of CdSe QDs at different time intervals (1–25 minutes). Aliquots of the growth solution were quenched in 10 mL hexanes. Preparation of Precursor Solutions for Shell Growth. The preparation of precursors and shell growth for QDs was modified from the previously published procedure.103 The zinc precursor stock solution (0.04 M) was prepared by dissolving ZnO (0.39 g) in OA (10.83 g) and ODE (108 mL) at 250 °C. The sulfur precursor stock solution (0.04 M) was prepared by dissolving sulfur powder (0.128 g) in ODE (100 mL) at 200 °C. Both precursor solutions were prepared under N2 flow and clear solutions were acquired. The Zn precursor solution was preheated to ensure complete dissolution before injection. To make the shell, a specific amount of stock solution was introduced via syringe into the hot reaction mixture. Synthesis of Iron Oxide/CdSe@ZnS QD Complexes. To synthesize iron oxide/CdSe/ZnS core/shell QD complexes, the solution of iron oxide/CdSe QD complexes, which were grown for 20 minutes, without purification was allowed to first cool down to room temperature and then



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reheated to 220 °C to deposit a ZnS shell onto the CdSe QDs. At 220 °C, 1 mL of the Zn precursor (0.04 M ZnO/OA in ODE) was slowly injected into the solution and stirred until complete deposition for 10 minutes. The same volume of sulfur in 1-ODE was sequentially injected into the reaction solution and stirred for 20 minutes. Afterwards, the reaction mixture was cooled down to 60 °C and dispersed in hexanes. Synthesis of Iron Oxide/CdSe QR Complexes. To synthesize iron oxide/CdSe QR complexes, CdO (0.0386 g, 0.3 mmol), ODA (3 g, 11 mmol), and ODE (10 g, 40 mmol) were mixed and heated to 200 °C for 1 hour. After cooling down to 60 °C, a purified iron oxide nanocrystal (0.023 g, 0.1 mmol) solution was added and heated to 100 °C for 30 minutes to remove the hexanes. Afterwards, the reaction mixture was heated and refluxed at 300 °C for 1 hour under N2 to decompose the precursors. After cooling down to 200 °C, Se (0.9 mmol) dissolved in TOP (2 mL) was slowly injected into the mixture solution at a rate of 0.4 mL/min. After injection, the solution remained at 250 °C for 1 hour to synthesize the iron oxide/CdSe QR complexes. Finally, the solution was cooled down and dispersed in hexanes. Magnetic Capture Purification. To determine the product yield, i.e., how much Cd converted to the QDs attached to iron oxide nanoparticles, iron oxide/QD complexes were separated and purified from unbound QDs with the help of a magnetic force. First, iron oxide/QD complexes were purified of the excess QD precursor, surfactant, and solvent with three cycles of centrifugation by adding methanol and acetone. Afterwards, a strong handheld magnet (NdFeB rod magnet with 1/8 inch diameter and length, respectively; United Nuclear Scientific Equipment & Supplies, USA) was placed for 12 hours in proximity to the purified solution in hexanes. In this magnetic capture process, the iron oxide/QD complexes were separated from the excess unbound QDs. This process was repeated three times. Inductively coupled plasma optical


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emission spectroscopy (ICP-OES) analyzed the Cd concentrations of the captured, supernatant, and nontreated original solution. The magnetic capturing efficiency (MCE) was calculated by the following equation: MCE % =

Ultraviolet-Visible

Absorption

Cd

Cd

× 100 1

Spectroscopy. A Varian Cary 5000 UV-VIS-NIR

spectrophotometer was used to measure ultraviolet-visible (UV-Vis) absorption spectra. Photoluminescence Spectroscopy and Photoluminescence Quantum Yield Determination. A Jobin Yvon Spex Fluorolog 3 fluorescence spectrophotometer (USA) measured the photoluminescence (PL) spectra of the iron oxide/QD complexes. The absolute value of the iron oxide/QD complexes was determined by comparing the PL integrated intensity of the QDs at the same Cd concentration. The Cd concentration was determined by ICP-OES. The absolute PLQY was calculated by the gradient method by comparing the PL integrated intensity of the CdSe QDs with that of the solution of rhodamine 6G (R6G) in ethanol. The absorption spectra of iron oxide/QD complexes, CdSe QDs, and R6G were recorded at 480 nm excitation, with the optical intensities of all samples below 0.10 absorbance. The following equation was used to calculate the quantum yield:104 Φ = Φ ∙ ∙ ∙

! !

∙

" 2 "

At room temperature, Φx and Φr are the absolute quantum yield of the CdSe QDs and R6G. The Φr of R6G in ethanol is 95%. Ar and Ax are compared to the absorption value at the excitation wavelength, e.g., 480 nm in this study. Ir and Ix are the intensities of excitation; those of the sample in our experiment are equal to the values of the standard sample. nr and nx are the



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refractive indices of the solvents. Ethanol (n = 1.359) and hexanes (n = 1.372) are used in this study. Dx and Dr are the PL integrated intensities of CdSe QDs and R6G excited at 480 nm. For the calculation of PLQY of the iron oxide/QD complexes, Equation 2 was modified. The variables Φx and Φr are the absolute quantum yields of the iron oxide/QD complexes and the calculated quantum yield of CdSe QDs compared with R6G. The calculated value of CdSe QDs in hexanes was 10%. Ar and Ax are the absorption values of iron oxide/QD complexes and CdSe QDs with an equivalent Cd concentration at the same excitation wavelength. Ir and Ix are the intensities of excitation; those of the sample in our experiment are equal to the values of the standard sample. nr and nx are the refractive indices of the solvents. All iron oxide/QD complexes and CdSe QDs are dispersed in hexanes. The refractive indexes nr and nx are equal in this calculation. Transmission Electron Microscopy. A JEOL 2100 field emission transmission electron microscope operating at 200 kV with a single tilt holder examined the transmission electron microscopy (TEM) specimen. The TEM sample was made by evaporating one drop of purified solution in hexanes onto ultrathin carbon type-A 400 mesh copper grids (Ted Pella Inc., USA). Energy-filtering transmission electron microscopy (EFTEM) provided high-resolution images showing a crystalline structure, and a gatan imaging filter (GIF) provided the quantitative elemental maps (Fe and Cd) of a sample within narrow energy ranges. Energy-dispersive X-ray spectrometry (EDS) detected the chemical compositions of the complexes. The average diameter was obtained by counting over 1,000 individual nanocrystalline particles using Image-Pro Plus 5.0 (Media Cybernetics, Inc., USA). Inductively Coupled Plasma Optical Emission Spectroscopy. Perkin Elmer inductively coupled plasma optical emission spectroscopy (ICP-OES) equipped with auto samplers measured


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the concentration of Cd to determine the product and quantum yields of the iron oxide/QD complexes. A purified iron oxide/QD complex solution was digested by using nitric acid (HNO3, 70%) and hydrogen peroxide (H2O2, 30%) in the analysis. X-ray Diffraction. The X-ray diffraction (XRD) patterns were recorded using a Rigaku D/Max Ultima II with a zero-background sample holder and analyzed by JCPDS card. The 2θ range was measured from 10 to 80 ° with Cu Kα radiation. The X-rays were generated at 40 kV and 40 mA.

RESULTS AND DISCUSSION Nanoscale iron oxide/QD complexes were synthesized by the nucleation and growth of QDs on preformed iron oxide nanocrystals. Monodispese iron oxide nanocrystals were used as seeds for the nucleation sites of QDs. As shown in Figure 1a, the monodispese iron oxide nanocrystals with a narrow size distribution (

Characterization and Optimization of the Fluorescence of Nanoscale

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