A Universal Method for the Preparation of Magnetic and Luminescent

Apr 6, 2010 - To alter the Cd/Fe ratio during the extraction, 5, 10, 15, or 20 mL of iron ... Total uncertainty in the magnetic data was calculated as...
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J. Phys. Chem. C 2010, 114, 7758–7766

A Universal Method for the Preparation of Magnetic and Luminescent Hybrid Nanoparticles Recep Kas,† Esra Sevinc,‡ Ugur Topal,§ and Havva Yagci Acar*,†,‡ Department of Chemistry, Koc UniVersity, Rumelifeneri Yolu, Sariyer 34450, Istanbul, Turkey, Graduate School of Materials Science and Engineering, Koc UniVersity, Rumelifeneri Yolu, Sariyer 34450, Istanbul, Turkey, and TUBITAK-UME, Magnetic Measurements Lab, PK.54, 41470 Gebze, Kocaeli, Turkey ReceiVed: January 12, 2010; ReVised Manuscript ReceiVed: March 21, 2010

Hybrid nanoparticles (MDOTs) composed of luminescent quantum dots (QDs) and superparamagnetic iron oxides (SPIOs) were prepared by the ligand-exchange mechanism in a simple and versatile extraction method. In this method, aqueous QDs (CdS or CdTe) coated with carboxylated ligands exchange the fatty acid (lauric acid) coating of SPIOs in a water-chloroform extraction process. QDs form a coating around SPIOs and transfer them into the aqueous phase in high efficiency. The method worked successfully with both small and polymeric coating molecules selected as cysteine, 2-mercaptopropionic acid, and a poly(acrylic acid)/ mercaptoacetic acid mixture. The original properties of the nanoparticles were well-preserved in the hybrid structures: All MDOTS showed ferrofluidic behavior and had a luminescence in the original color of the QD. Magnetic properties and the luminesence intensity of MDOTs can be easily tuned with the SPIO/QD ratio. All particles are small and show very good stability (optical and colloidal) over months. For stable MDOTs with good luminescence properties, highly luminescent aqueous QDs (CdS or CdTe) with the mentioned coatings were prepared. The first examples of CdTe coated with 2MPA emitting from green to red and CdTePAA/MAA were provided as well. 1. Introduction Quantum dots (QDs) and superparamagnetic nanoparticles (MNPs) are under heavy investigation for especially medical and biotechnology applications.1 Advancement in QD technology created new opportunities in biology, medicine, sensors, optics, solar cells, barcodes, lasers, etc.2-6 In recent years, QDs replaced organic fluorophores in the labeling because of their higher quantum yield and better photostability. Optical detection of cancer cells, DNA hybridization, and immunoassays are some of the areas where QDs are exploited for biotechnology and medicine.7-10 Unique optical properties of QDs are sizedependent based on the quantum confinement.11 Therefore, semiconductor QDs with size tunable emission color and a broad absorption/narrow emission spectrum allows excitation of different QDs at a single wavelength, which is invaluable for optical detection of multiple events or targets. Magnetic nanoparticles (MNPs) have been evaluated for a variety of applications from magnetic storage to therapy.12 Their applications heavily depend on the ability of superparamagnetic iron oxides (SPIO) to respond to an external magnetic field either to generate a signal or to drag or hold particles.13 Therefore, SPIOs are currently used for magnetic separation, cell sorting, immunoassays, and diagnostic imaging.13-18 The use of MNPs in cancer therapy, hyperthermia, drug/gene delivery, cell tracking, and magnetoresistive biosensors has been widely studied by both academia and industry.15,19-24 In many of these fields, there is a great demand for multiplexing and/or dual action. In the fast pace of nanotech* To whom correspondence should be addressed. Tel: 902123383131. Fax: 902123381559. E-mail: [email protected]. † Department of Chemistry, Koc University. ‡ Graduate School of Materials Science and Engineering, Koc University. § TUBITAK-UME.

nology research, the ability to do more with less is highly desirable. Hybrid structures harnessing properties of different materials could have a differentiating impact on the current technologies. Development of hybrid nanoparticles (MDOT) composed of magnetic nanoparticles and luminescent quantum dots, as proposed here, offers a tremendous opportunity to create multifunctional single entities addressable by a magnetic field and a light source, hence detectable both magnetically and optically. These MDOTs would lead the way to next-generation materials in many areas, but especially in the medical imaging, therapy, labeling, separation, and sensors. As an example, these MDOTs could provide dual imaging (magnetic and optical) in diagnosis, dual action of imaging and therapy, and combined action, such as detection/labeling/sensing and magnetic dragging/separation, as well as an opportunity for multiplexing. There are different approaches in preparing such hybrid nanostructures.25 Major ones are the encapsulation of magnetic nanoparticles and quantum dots in a matrix (polymeric or silica),26-31 formation of core-shell nanoparticles utilizing lattice mismatch,32,33 layer-by-layer assembly using polyelectrolytes,34 combination of oppositely charged particles through electrostatic interaction,35 or growth of QDs from the surface of iron oxide microspheres.36,37 All the methods mentioned above are relatively labor intensive and have many limitations. For example, packing particles in a carrier system, such as polymers or silica, usually creates large particles that limit the end use. On the other hand, shell growth on the initial semiconductor core is a limited approach and the lattice mismatch may have a negative influence on the particle properties. In addition, the SPIO/QD ratio is rather difficult to control, and therefore, luminescence quenching is an important problem to overcome for these hybrid systems. Besides, most of the reported methods use hydrophobic particles prepared from the decomposition of organic precursors. However, especially for biotechnology applications, transfer of

10.1021/jp100312e  2010 American Chemical Society Published on Web 04/06/2010

Magnetic Luminescent Nanoparticles

Figure 1. Preparation of magnetic iron oxide luminescent quantum dot (MDOT) hybrid nanoparticles.

the particles into water is necessary. This is usually accomplished through ligand exchange with water-soluble molecules, micelle formation using amphiphilic macromolecules, or a silica coating.38-43 Each of these methods has its own advantages and disadvantages, such as significant loss in the luminescence properties or aggregation.44 These events impact the final size and luminescence intensity of the hybrid materials.44-46 Alternatively, further problems arise from the use of incompatible solvents carrying the two particles with different solubilities in the process of hybrid formation.47 In addition to these, a limitation to form multicolor MDOT arrays is an important drawback with many of these approaches as well. Here, we demonstrate a very simple and a versatile extraction method that uses a simple ligand-exchange mechanism to prepare small, stable, aqueous hybrid nanoparticles of SPIOs and QDs, responding to a magnetic field and luminescing in different colors with good intensity. The method is very efficient in controlling the luminescence of the hybrids through the control of the SPIO/QD ratio. Mild preparation conditions provide a great advantage in maintaining good optical properties originating from the aqueous QDs. In this method, during an aqueous-organic extraction process, SPIOs coated with LA are transferred from the organic phase into the aqueous phase that has the QDs decorated with carboxylic acid groups. The transfer of nanoparticles coated with hydrophobic ligands from an organic solvent into water can be accomplished through water-soluble molecules.48,49 We applied a similar approach for the preparation of hybrid MDOTs: carboxylic-acid-functionalized aqueous QDs exchange the fatty acid coating of SPIOs dispersed in an organic solvent, which subsequently transfers SPIOs into water (Figure 1). In the heart of this approach lies the selection of an appropriate ligand for the coating of QDs. Desirable ligands should be water-soluble and contain at least two functional groups that can effectively passify the surface of QDs and iron oxides, simultaneously. We have demonstrated the idea using lauric acid (LA)-coated SPIOs and various thioacid (2-mercapto propionic acid, cysteine, mercaptoacetic acid) or polymer (poly(acrylic acid)/MAA) coated CdX (X ) S, Te). To achieve highly luminescent MDOTs, aqueous QDs with good luminescence and stability are necessary. Previously, we

J. Phys. Chem. C, Vol. 114, No. 17, 2010 7759 have demonstrated that the aq QDs with a PAA/MAA mixture or 2MPA coatings show exceptional stability and good luminescence.50,51 Therefore, for the development of multicolor MDOTs, we have prepared CdTe-2MPA QDs in different colors for the first time in the literature. The CdTe/PAA-MAA QD was also prepared for the first time to alter the coating structure. Cysteine-coated QDs were also used due to their good biocompatibility and trifunctional nature, which would broaden the scope of the ligands that are suitable for the suggested extraction method. Selection of different ligands proves that both small and macromolecular ligands with thiol, amine, and carboxylate groups can effectively support the hybrid structure. While we were waiting for the reviewer’s comments, Jana et al. reported a similar approach where ligand exchange was performed in a reverse micelle to prepare QD/iron oxide hybrid material.52 The method worked best with the multifunctional polymeric coatings, yet it created a significant amount of aggregated material that cannot be suspended in water. Besides, hybrid structures did not respond to a bar magnet unless the solubility was lowered. We clearly benefit from the mild preparation conditions and versatility of the method as well as the good quality of QDs (with 2MPA and PAA/MAA coatings) that were originated from our laboratory. Provided examples will demonstrate the formation of MDOTs with a superior ease, efficiency, and control compared with all previous methods. The ligand-exchange method was studied in detail to demonstrate the universality and the ability to control properties of the hybrid nanoparticles. These achievements are critical for the realization of hybrid nanoparticles in a practical sense. 2. Experimental Section Materials. All of the chemicals were the highest purity available or analytical grade. Cadmium acetate dihydrate (Cd(Ac)2 · 2H2O), sodium sulfide trihydrate (Na2S · 3H2O), 2-mercaptopropionic acid (2-MPA), (R)-(+)-cysteine, mercaptoacetic acid (MAA), ammonium hydroxide (26%), and reagent grade chloroform were purchased from Merck. FeCl3 · 6H2O, FeCl2 · 4H2O, and lauric acid (LA) were purchased from Fluka. Poly(acrylic acid) sodium salt (PAA, Mn ) 2100) was purchased from Aldrich. Milli-Q water (Millipore) was used as a solvent. Na2Te powder was prepared according to the procedure described in the literature by Klemm et al.53 Synthesis of CdS Quantum Dots. CdS QDs were prepared as described in the literature.50,54 CdS QDs were prepared at an initial Cd/S ratio of 2.5. For CdS-2MPA and CdS-Cysteine, a ligand/Cd ratio of 2.0 was used. All particles were washed through 5000 cutoff Amicon ultrafiltration tubes to remove the excess coating molecules. Synthesis of CdTe Quantum Dots. Briefly, a desired amount of ligand was added to a 5 mM aqueous Cd(Ac)2 · 2H2O solution, and the pH of the solution was adjusted to 9.5-10 using 1 M NaOH. This solution was deoxygenated for 15 min and heated to 60 °C. In another flask, aqueous Na2Te solution was prepared (2 or 2.5 mM based on the desired Cd/Te ratio) and added slowly to the Cd2+/ligand solution. After the addition was complete, the reaction mixture was refluxed. Sizes of the CdTe QDs were varied by the reflux time. CdTe-2MPA QDs luminescing in different colors from green to red were prepared at different Cd/Te ratios and reflux times: Yellow and orange luminescing CdTe/2MPA QDs were prepared at a Cd/Te mole ratio of 2; green and red luminescing ones were prepared at a Cd/Te ratio of 2.5. For all CdTe-2MPA QDs, the 2MPA/Cd ratio was 2.

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CdTe/PAA-MAA QDs were synthesized at a Cd/Te mole ratio of 2.5 and a total COOH/Cd ratio of 2.5 (40/60 mol/mol PAA-MAA). All particles were washed through 5000 cutoff Amicon ultrafiltration tubes to remove the excess coating molecules before hybrid formation. During the washing step, the total volume was replaced with fresh water at least three times. Synthesis of Lauric-Acid-Coated Magnetic Iron Oxide Nanoparticles. Iron salts (2.365 g of FeCl3 · 6H2O, 0.869 g of FeCl2 · 4H2O) and 1.450 g of lauric acid (LA) were added into a three-neck round-bottomed flask containing 46 mL of deoxygenated water. The flask was fitted with a reflux condenser and a mechanical stirrer. The solution was purged with nitrogen for at least 20 min and then heated to 85 °C in an oil bath. NH4OH (12.06 mL, 25%) was injected to the hot solution under vigorous stirring. The color of the solution turned to dark brown-black, which is typical for the iron oxide formation. Reaction was continued for 30 min for the crystal growth and then cooled to room temperature. This produces LA bilayer coated iron oxide nanoparticles in water. LA monolayer coated iron oxide nanoparticles were then extracted into chloroform as follows: fifty milliliters of the aqueous suspension containing LA bilayer coated iron oxide nanoparticles was shaken vigorously with 75 mL of chloroform and 1 mL of isopropyl alcohol, then sonicated at 40 °C for 15 min, and was finally transferred into a separatory funnel. The organic phase, containing the LA monolayer coated iron oxide, was separated and passed through a 200 nm syringe filter. This will be called the stock solution of LA coated iron oxide for the rest of the paper, which usually contains 2.57 mM iron (0.00257 mmol of iron). Preparation of QD-Iron Oxide Hybrid Nanoparticles. A typical procedure can be described as follows: LA coated iron oxide nanoparticles in chloroform were mixed with aqueous QDs in a 100 mL round-bottomed flask fitted with a mechanical stirrer. The mixture was both sonicated and stirred at 1000 rpm and then was transferred into a separatory funnel to separate the aqueous phase that contains the hybrid nanoparticles. The aqueous phase was subjected to a back-extraction with chloroform. The dark color originating from the iron oxide disappeared in the organic phase, yet the aqueous phase became dark brown, indicating the transfer of SPIOs from organic to aqueous phase. All hybrid solutions were filtered from a 200 nm syringe filter and washed through Amicon ultrafiltration tubes with a 30.000 cutoff value until no luminescence is detected in the filtrate. Because quantum dots can pass through the pores of this size filters, excess or unbounded quantum dots were washed off from the solution containing the hybrid nanoparticles. Usually, the stock solution of LA coated iron oxide contains 2.57 mM iron (0.00257 mmol of iron). To alter the Cd/Fe ratio during the extraction, 5, 10, 15, or 20 mL of iron oxide stock solution was mixed with 30 mL of aqueous QDs at a 2.5 mM cadmium concentration (0.075 mmol of Cd2+). This corresponds to Fe/Cd ratios of 0.171, 0.343, 0.514, and 0.685, as listed in Table 1. Characterization. Absorbance spectra were recorded with a Schimadzu model 3101 ultraviolet-visible-near infrared spectrophotometer (3101 PC). Absorbance of the samples was kept below 0.15 at the excitation wavelength. Photoluminesence (PL) spectra were recorded on a Horiba Jobin Yvon FluoroMax-3 spectrofluorometer at the excitation wavelengths of 400 and 355 nm with 450 and 380 nm long pass filters for CdTe and CdS, respectively. PL spectra were calibrated with the absorption values at the excitation wavelength. Quantum yields

Kas et al. TABLE 1: CdS-2MPAe/Iron Oxide Hybrid Nanoparticles sample ID

Fe/Cda feed

Dh(I)b (nm)

Dh(N)c (nm)

Fe/Cda hybrid

QYd (%)

MDOT-I MDOT-II MDOT-III MDOT-IV

0.171 0.343 0.514 0.685

158 154 143 144

74 82 75 81

1.143 1.617 2.149 2.578

59.3 44.4 31.9 17.5

a Cation mole ratio based on ICP. b Hydrodynamic size measured by DLS and calculated according to the intensity average. c Hydrodynamic size measured by DLS and calculated according to the number average. d Quantum yield of the MDOT calculated with respect to Rhodamine 6G. e QY of CdS-2MPA ) 69%.

(QYs) were calculated as reported in the literature using Rhodamine B (31% QY in water) for CdS and Rhodamine 6G (95% QY in ethanol) for CdTe.50 A Malvern Instruments ZetaSizer Nanoseries Nano-S was used to determine the hydrodynamic sizes of the nanoparticles. The instrument uses a He-Ne laser at 633 nm and collects the scattered intensity at 173°. All measurements were performed at 25 °C with three replicates. All samples were dried using a Labconco freeze-drier system for the characterization methods requiring the powder of the materials. Fourier transform infrared (FTIR) spectra were recorded on a JASCO FT-IR-600PLUS spectrometer using KBr pellets of the dried samples. X-ray diffraction analysis was performed using a HUBER G670 diffractometer with a germanium monochromator and Cu KR radiation (λ ) 1.5406 Å). Data were analyzed by using STOE WinXPOW software. Magnetization loops were measured at room temperature using a vibrating sample magnetometer (LDJ Electronics Inc., model 9600), which was calibrated with a reference nickel sphere, with applied fields up to 1.6 T. Measurements were repeated at least five times under the same conditions in order to ensure the repeatability of the data. Total uncertainty in the magnetic data was calculated as 2% based on the GUM and EA-4/02 documents by the coverage factor k ) 2, which corresponds to a coverage probability of approximately 95% for a normal distribution. Iron (Fe3+) and cadmium (Cd2+) concentrations of the samples were determined by a Spectro Genesis FEE ICP spectrometer. For the analysis, samples were prepared as follows: aqueous samples were heated to 80 °C, then an excess amount of HNO3, H2SO4, and H2O2 was added until the solution became colorless and no bubbles were observed. Solutions were kept at 80 °C for at least 3 h. Finally, samples were diluted before the measurements. Values reported are the average of three measurements of three replicates. 3. Results and Discussion Preparation of QDs and SPIOs. The quality of the hybrid nanoparticles will depend on the quality of its components. The stability (both colloidal and photoluminescence), coating chemistry, and the quantum efficiency of QDs are very important for the formation of stable and highly luminescent hybrid structures. It is also known that the coating of the QDs has a significant influence on such properties. Both thiols and carboxylates can stabilize CdS and CdTe QDs. We have demonstrated previously the effectiveness of the 2MPA coating in producing CdS with high QY and good colloidal stability.50 Therefore, first, high-quality aqueous CdS-2MPA QDs were prepared. In addition to CdS, CdTe QDs are of great value because of a narrower band gap and ease of multicolor QD formation. We have adopted the 2MPA coating to CdTe to

Magnetic Luminescent Nanoparticles obtain multicolor CdTe QDs with about a 23% QY (with respect to Rhodamine 6G) for the first time in the literature. To diversify the coating chemistry, we have also prepared aqueous CdScysteine and novel CdTe-PAA/MAA (40/60 mol ratio) QDs. Cysteine is attractive due to its trifunctional nature and biocompatibility. PAA is quite popular for particle coatings due to its multifunctional nature and carboxylic acid groups that can adsorb on both quantum dots and iron oxide. Yet, PAA alone does not provide a highly luminescent CdS QD.54 On the other hand, if it is mixed with mercaptoacetic acid (MAA), luminescence is enhanced dramatically in addition to the good colloidal stability. The PAA/MAA mixture at a 40/60 ratio was demonstrated as the best composition for the CdS coating based on the quantum yield.51 Therefore, a new QD with a CdTePAA/MAA composition with a 26% QY (with respect to Rhodamine 6G) was prepared. All QDs were prepared in optimized conditions to give the best quality of QDs based on our previous reports. The second component of the MDOTs is SPIOs. SPIOs were prepared in a procedure similar to our previously developed method:55 First, SPIOs were synthesized with a lauric acid (LA) coating in water. LA molecules form an interdigitated bilayer around the crystal in the aqueous solution, preventing precipitation and excessive aggregation (Supporting Information). The physically adsorbed outer LA layer can be easily stripped off, forming the hydrophobic LA monolayer coated nanoparticles (SPIO-LA). This was achieved by a very simple procedure where the aqueous SPIOs were shaken with an organic solvent, such as chloroform or toluene. In addition to our previously reported method, addition of a small amount of isopropanol increased the efficiency of this extraction. This is a very effective and gentile method that renders hydrophilic iron oxide nanoparticles hydrophobic. SPIO-LA nanoparticles obtained from this procedure usually are about (hydrodynamic size) 10 nm (Supporting Information). This method is more advantageous than other methods, such as precipitation of hydrophobic particles into polar solvents, ligand exchange, or coating bare iron oxides with LA molecules. These methods usually cause loss of material and aggregation. Preparation of MDOTs. All the QDs prepared for this study have carboxylic acid functionality on the periphery, which can adsorb on the SPIO surface effectively. Hybrid nanoparticles composed of SPIOs and QDs were formed as the LA molecules were exchanged with the carboxylate groups of the QDs in a simple extraction process where aqueous QD-COOH was shaken with SPIO-LA in chloroform. The aqueous phase contains the hybrid MDOTs at the end. In this system, ligands used for the stabilization of QDs are bound to the surface of both QDs and SPIOs simultaneously. In a way, QDs coat SPIOs during the ligand exchange and transfer iron oxide nanoparticles to the aqueous medium (Figure 1). To ensure this hypothesis, SPIOs in chloroform were shaken with plain water rigorously, yet no transfer of SPIOs to the aq phase was observed. This indicates that the only way for SPIOs to be present in the water is to be coated with QDs. It is important to note here that all unbounded QDs are removed from the aqueous MDOTs by washing through ultracentrifuge tubes until no QD was observed in the filtrate. Therefore, MDOT suspensions contain only the hybrid structures but not a mixture of SPIOs and QDs. This is important to ensure that the luminescence obtained from MDOTs originate only from the hybrid structures but not from the combination of MDOTs and free QDs. In the exchange procedures, excess QDs were used to provide an effective coating for the existing iron oxide nanoparticles.

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Figure 2. TEM images of the CdS-2MPA/iron oxide hybrid nanoparticles (MDOT-II).

To determine the feasible ratios for the effective transfer of SPIOs into water and to alter the SPIO/QD ratio, which is critical for the luminescence, different QD/SPIO ratios were studied using the CdS-2MPA as a model: the cadmium concentration was fixed at 2.5 mM (0.075 mmol), and the Fe/Cd mole ratios were changed between 0.171 and 0.685 (Table 1). As the iron oxide amount increased in the feed, the iron content of the hybrid increased as well. In all the cases, the typical black-brown color of the SPIO completely disappeared from the organic phase and appeared in the aqueous phase after the extraction, indicating the complete transfer of SPIOs into the aqueous phase. Structure of MDOTs. DLS measurements and TEM images (Figure 2) indicated the existence of small aggregates of about 80 nm, which is in the small size regime. The structure depicted in Figure 1 is an idealized picture. In reality, each hybrid nanoparticle is composed of multiple iron oxides and QDs. During the ligand-exchange mechanism, slight aggregation of the nanoparticles is possible as LA desorbs from the surface of iron oxide. Also, it should be remembered that about 3-4 nm sized QDs are capturing the surface of 10 nm iron oxide particles. Achieving this on a single particle is most likely not possible. Yet, this system still benefits from the in situ coating of each nanoparticle, which prevents excessive aggregation of the initial SPIOs and QDs and, in return, allows preparation of small hybrid nanoparticles. Small sizes are especially important for the in vivo applications, such as imaging and drug delivery, as the size influences the blood circulation time and biodistribution of the particles.56-58 The IR spectra of the CdTe-2MPA/iron oxide MDOTs show a small shift of the asymmetric and symmetric carboxylate stretching peaks (1575 and 1398 cm-1) toward lower wavenumbers (1559 and 1389 cm-1) (Figure 3). This may indicate the binding of free surface carboxylates of QDs to the iron oxide

Figure 3. IR spectra of the (a) CdTe-2MPA/SPIO MDOT and (b) the corresponding CdTe-2MPA.

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Figure 4. (a) Absorbance and (b) absorbance calibrated photoluminesence spectra of CdS-2MPA/SPIO hybrid nanoparticles.

Figure 5. Pictures of the CdS/2MPA/SPIO (MDOT-II) and the original QD (CdS-2MPA) under (a) daylight and (b) UV excitation (λexc ) 366 nm) 13 months after preparation. Figure 6. UV absorbance and PL spectra of CdS/2MPA/SPIO (MDOT-II) taken initially and 13 months after preparation.

crystal surface, which is necessary for the hybrid structure formation. XRD of the dried powders indicates the presence of both iron oxide (magnetite) and CdTe (cubic) with peaks at the 2θ values of 62.5453, 35.4388, 21.1513, and 18.2913 for iron oxide and 24.0271, 39.7411, and 46.9768 for CdTe (Supporting Information). Properties of MDOTs. In the range of the electromagnetic spectrum where these QDs emit, iron oxide nanoparticles absorb strongly; therefore, as the ratio of the iron oxide/QD increased, the observed luminescence of MDOTs decreased (Figure 4). This reflects itself in the calculated quantum yield (QY) of the hybrid nanoparticles as well. However, an advantage of the described method is the ability to control the iron oxide/QD ratio and hence the QY of the MDOT, as shown in Table 1. Starting with a CdS-2MPA having a 69% QY, MDOTs with a 59-17% QY were achieved, depending on the iron oxide content of the hybrid. Another significance of the presented method is that the optical properties originating from the QDs do not change; that is, no shift in the PL peak position was seen (Figure 4b). As a consequence, MDOT solutions show a typical brown color of the iron oxide, but there is no change in the emission color of the original QD (Figure 5). This finding along with the good luminescence of MDOTs suggests that no damage was done on the QD surface during the extraction. Because the extraction does not require any harsh conditions (no heating or additives, such as acids or bases), the surface of the QD should stay untouched, preventing loss of ligands from the surface, particle growth, or degredation. Preparation of QDs in aqueous medium with stable coatings contributed to this result to a great extent. When QDs with hydrophobic ligands (such as TOPO) are used, exchange of these coatings with hydrophilic ones to transfer QDs into water decreases the quantum yield of the QD significantly and usually causes a loss of stability.59 Stability is an important issue as well. MDOTs showed excellent colloidal and luminescent stability. Figure 6 shows

Figure 7. M-H loop generated by VSM for MDOT-II and MDOTIV.

no significant change in the absorption and luminescence spectra of the hybrid containing CdS-2MPA (MDOT-II) after 13 months, as an example. These MDOTs benefit from the excellent stability of the original QDs with these newly adopted coating systems. All the aqueous MDOTs showed ferrofluidic behavior as desired when placed next to a bar magnet. The saturation magnetization of the hybrid nanoparticles is below the value of pure iron oxide because only a fraction of the whole material is magnetic, yet their saturation moments are higher than most of the similar hybrids reported in the literature.60-62 As can be seen in Figure 7, initial magnetic susceptibilities () M/H) of these samples seem to be similar but the saturation magnetization (Msat) takes different values. Msat values for CdS/2MPA/SPIO were determined to be 7.3 and 7.5 emu/g (34.7 and 28.6 emu/g Fe, Supporting Information) for the MDOT-II and MDOT-IV, respectively. In addition, both of them show a superparamagnetic property with almost no remanence magnetization and zero coercive fields (15 Oe for MDOT-II and 35 Oe for MDOT-IV)

Magnetic Luminescent Nanoparticles at room temperature (inset of Figure 7). For these two samples, the sizes of the hybrid particles are comparable but they differ in Fe/Cd ratio, which is 1.617 for MDOT-II and 2.578 for MDOT-IV. It is natural to expect comparable Msat values in units of emu/g Fe for these samples, or there must be a significant difference on whole sample magnetizations (in units of emu/g). Yet, here we see a slightly different result. There might be several reasons for such results: Because MDOT-IV has about 50% more iron than MDOT-II at about the same overall size, the concentration of the aggregated magnetic particles throughout the whole sample may be larger in MDOTIV samples. Within these aggregates, defects or dislocations on grain boundaries, such as the nonmagnetic coatings in our case, was reported to reduce the strength of exchange interactions between the neighboring magnetic particles. This would make the magnetization of the sample harder and reduce the saturation

J. Phys. Chem. C, Vol. 114, No. 17, 2010 7763 magnetization.63 The slightly larger coercive field in MDOTIV compared with the MDOT-II supports this hypothesis. Surface pinning of the magnetic moments at interfaces between magnetic particles may also be a factor affecting the saturation magnetization.64 It was extensively investigated by Berkowitz et al. and reported that spin pinning occurs at ferrite-organic coating interfaces and cause high effective anisotropy fields and low measured saturation magnetizations.65 Altogether, the nature and amount of the coating seem to be responsible for the observed Msat values. However, MDOTs seem to have a proper concentration of magnetic Fe3O4 to yield good quality magnetic quantum dots. MDOTs in Different Colors. The ability to retain the luminescence color of the original QD can be exploited to generate MDOTs luminescing in various colors. For this purpose, an array of MDOTs was prepared with CdTe-2MPA

Figure 8. (A) Pictures of the CdTe/2MPA/SPIO MDOTs luminescing green to red under UV excitation (366 nm). (B) Concentrated samples of red luminescing MDOTs under (a) daylight and (b) UV excitation (366 nm) responding to an external magnet (0.3 T).

Figure 9. Absorbance calibrated photoluminescence spectra of the CdTe-2MPA (dashed line) and the corresponding MDOTs (solid line) luminescing (a) green, (b) yellow, (c) orange, and (d) red.

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TABLE 2: Hydrodynamic Sizes of the MDOTs composition

Dh(I)a nm

Dh(N)b (nm)

CdTe/2MPA/SPIO-Green CdTe/2MPA/SPIO-Yellow CdTe/2MPA/SPIO-Orange CdTe/2MPA/SPIO-Red CdS/Cystein/SPIO CdTe/PAA-MAA/SPIO

144 146 148 150 129 137

79 81 78 82 70 65

a Hydrodynamic size measured by DLS and calculated according to intensity average. b Hydrodynamic size measured by DLS and calculated according to number average.

QDs emitting from green to red. The ligand exchange was performed at an Fe/Cd ratio of 0.343 (based on the results obtained with CdS-2MPA) (Figure 8). All MDOTs luminesce in the same color as that of the original quantum dot with no significant shift in the original PL peak position (Figure 9). Although a 15-33% drop in the luminescence intensity with respect to the original QD was observed due to the presence of SPIOs, all of these MDOTs displayed good luminescence, as seen in Figure 8. The hydrodynamic sizes of these hybrid nanoparticles are also around 80 nm (Table 2). MDOTs with Different Coatings. The described method can easily be adapted to different QDs with various coatings (ligands) as long as functional groups that can bind to the iron oxide are available on the surface of the QD. To demonstrate this versatility, MDOTs were also prepared from cycteine-coated CdS, which is trifunctional with an additional amine unit, as well as PAA/MAA (60/40) coated CdTe, which has a polymeric component. All these extractions were done at the Fe/Cd ratio of 0.343. DLS measurements indicated the existence of small aggregates around 70-65 nm, comparable to the MDOTS prepared with 2MPA-coated QDs (Table 2). These MDOTs exhibited good luminenesence with only a 14% drop from CdScysteine and an 18% drop from CdTe-PAA/MAA (Figure 10). Figure 11 shows that the strong green luminescence of the MDOT obtained from the CdTe-PAA/MAA is comparable to that of the parent QD. In this picture, the CdTe solution is slightly yellow itself, and it should not be confused with the brown color of MDOT suspensions originating from the iron oxide. These results prove that both polymeric coatings and small molecule coatings can be effectively used to support the hybrid formation. 4. Conclusions The described extraction method using ligand exchange to form hybrid nanoparticles offers a great efficiency, simplicity, and versatility compared with previous methods.

Figure 11. Pictures of the CdTe-PAA/MAA (left) and the corresponding MDOT (CdTe/PAA-MAA/SPIO (right) under (a) daylight and (b) UV excitation (366 nm).

In this method, aqueous QDs with surface carboxylates replace the LA coating of SPIOs in a water/chloroform extraction, transferring SPIOs into water with a QD coating with high efficiency. Both polymeric coatings and small molecules were effective in stabilizing QDs and SPIOs simultaneously. The presence of additional functional groups, such as amines, in the case of cysteine, did not cause any complications. Hybrid systems displayed the characteristics of its components: they showed ferrofluid behavior with a relatively high saturation magnetization and possessed good luminescence as well as longterm colloidal and photoluminesence stability. The system benefits from several advantages: (1) Preparation of QDs in water eliminated the need to do a ligand exchange on the QD surface to render it water-soluble. This eliminated the usual drop in the quantum yield of the QD due to surface defects originating from surface perturbation. (2) The room-temperature process allows the maintenance of the original properties (color of emission) of the QDs, which enables the formation of an array of MDOTs luminescing in different colors. (3) Properties, such as the QY and Msat, can be easily tuned by the SPIO/QD ratio. MDOTs reported here have better Msat values and stronger luminescence than the previously reported examples, to the best of our knowledge. (4) The preparation method of hydrophobic SPIOs kept the size of the SPIOs small, which enables the formation of small MDOTs. (5) Absence of the precipitation for the isolation of particles prevented the loss of material, surface disturbance, and excessive aggregation, which is a valuable merit to keep the size of the hybrid structures in the small size regime (less than 100 nm). Besides, limited aggregation reduces the concentration dependent quenching, supporting strong luminescence. (6) The use of highly stable and luminescent QDs supported stable MDOTs with good luminescence even after the drop in luminescence resulting from the absorption by SPIOs. For this purpose, CdS quantum dots with the recently

Figure 10. Absorbance calibrated photoluminescence spectra of (a) (dashed line) CdS-Cysteine and (solid line) the corresponding MDOT (inset: picture of the MDOT at λexc ) 366 nm) and (b) (dashed line) CdTe-PAA/MAA and (solid line) the corresponding MDOT.

Magnetic Luminescent Nanoparticles investigated 2MPA coating as well as the more studied cysteine coating were prepared and used. In addition, aqueous CdTe/ PAA-MAA and multicolor CdTe-2MPA QDs were prepared for the first time in the literature to be used for the hybrid structures. Hybrid nanoparticles of superparamagnetic iron oxide and luminescent quantum dots are invaluable in high-tech areas. Such entities would ultimately enable dual functions, such as detection (of an analyte, cell, pathogen, receptor, etc.) and separation, detection (optical, magnetic), and therapy (magnetic, photodynamic), multiplexing in sensors, dual mode imaging (optical and magnetic), imaging and magnetic dragging (magnetofection), and many more. Small sizes and a stable aqueous colloidal form are very advantageous for the in vivo and in vitro studies. Although examples provided here contain only SPIOs and QDs, there is no reason why this method cannot be easily applied to other nanoparticles, such as gold or TiO2. Acknowledgment. This work was partially supported by MIRG-CT-2005-031072 and Koc University. The authors thank Serdar Celebi at Eindhoven Institute of Technology for the TEM images. Supporting Information Available: Schematic representation of LA bilayer and monolayer coated iron oxide nanoparticles and the formation of LA monolayer coated iron oxides; absorbance spectra of CdS-Cysteine, CdTe/PAA-MAA quantum dots, and the corresponding MDOTs; absorbance and luminescence spectra of CdTe-2MPA-Iron oxide hybrid nanoparticles; XRD pattern of the iron oxide and CdTe-2MPA/SPIO-Red; histograms from the size analysis of some nanoparticles; DLS of MDOT-II 13 months after synthesis; M-H loop for the LA monolayer coated iron oxide, MDOT-II, and MDOT-IV; and table of the calculated sizes for the QDs used. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Parak, W. J.; Gerion, D.; Pellegrino, T.; Zanchet, D.; Micheel, C.; Williams, S. C.; Boudreau, R.; Le Gros, M. A.; Larabell, C. A.; Alivisatos, A. P. Nanotechnology 2003, 14, R15. (2) Parak, W. J.; Pellegrino, T.; Plank, C. Nanotechnology 2005, 16, R9. (3) Choi, S. H.; Song, H. J.; Park, I. K.; Yum, J. H.; Kim, S. S.; Lee, S. H.; Sung, Y. E. J. Photochem. Photobiol., A 2006, 179, 135. (4) Eastman, P. S.; Ruan, W. M.; Doctolero, M.; Nuttall, R.; De Feo, G.; Park, J. S.; Chu, J. S. F.; Cooke, P.; Gray, J. W.; Li, S.; Chen, F. Q. F. Nano Lett. 2006, 6, 1059. (5) Grundmann, M. Physica E 1999, 5, 167. (6) Jorge, P. A. S.; Martins, M. A.; Trindade, T.; Santos, J. L.; Farahi, F. Sensors 2007, 7, 3489. (7) Prinzen, L.; Miserus, R.-J. J. H. M.; Dirksen, A.; Hackeng, T. M.; Deckers, N.; Bitsch, N. J.; Megens, R. T. A.; Douma, K.; Heemskerk, J. W.; Kooi, M. E.; Frederik, P. M.; Slaaf, D. W.; van Zandvoort, M. A. M. J.; Reutelingsperger, C. P. M. Nano Lett. 2007, 7, 93. (8) Gao, X. H.; Chan, W. C. W.; Nie, S. M. J. Biomed. Opt. 2002, 7, 532. (9) Smith, A. M.; Dave, S.; Nie, S. M.; True, L.; Gao, X. H. Expert ReV. Mol. Diagn. 2006, 6, 231. (10) Pinaud, F.; Michalet, X.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Iyer, G.; Weiss, S. Biomaterials 2006, 27, 1679. (11) Trindade, T.; O’Brien, P.; Pickett, N. L. Chem. Mater. 2001, 13, 3843. (12) Laurent, S.; Forge, D.; Port, M.; Roch, A.; Robic, C.; Elst, L. V.; Muller, R. N. Chem. ReV. 2008, 108, 2064. (13) Brzeska, M.; Panhorst, M.; Kamp, P. B.; Schotter, J.; Reiss, G.; Puhler, A.; Becker, A.; Bruckl, H. J. Biotechnol. 2004, 112, 25. (14) Sun, C.; Lee, J. S. H.; Zhang, M. Q. AdV. Drug DeliVery ReV. 2008, 60, 1252. (15) Graham, D. L.; Ferreira, H. A.; Freitas, P. P. Trends Biotechnol. 2004, 22, 455. (16) Barentsz, J. O.; Futterer, J. J.; Takahashi, S. Eur. J. Radiol. 2007, 63, 369.

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