“Pulling” Nanoparticles into Water: Phase Transfer of Oleic Acid

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NANO LETTERS

“Pulling” Nanoparticles into Water: Phase Transfer of Oleic Acid Stabilized Monodisperse Nanoparticles into Aqueous Solutions of r-Cyclodextrin

2003 Vol. 3, No. 11 1555-1559

Yong Wang,† Jian Feng Wong,† Xiaowei Teng,† Xue Zhang Lin,† and Hong Yang*,†,‡ Department of Chemical Engineering and Laboratory for Laser Energetics, 253 GaVett Hall, UniVersity of Rochester, Rochester, New York 14627 Received September 3, 2003; Revised Manuscript Received September 28, 2003

ABSTRACT This paper describes a general method to drastically improve the dispersity of oleic acid stabilized nanoparticles in aqueous solutions. We use oleic acid stabilized monodisperse nanoparticles of iron oxides and silver as model systems, and have modified the surface properties of these nanoparticles through the formation of an inclusion complex between surface-bound surfactant molecules and r-cyclodextrin (r-CD). After the modification, the nanoparticles of both iron oxide and Ag can transfer from hydrophobic solvents, such as hexane, to r-CD aqueous phase. The efficiency of the phase transfer to the aqueous solutions depends on the initial r-CD concentration. The r-CD/oleic acid complex stabilized nanoparticles can be stable for long periods of time in aqueous phase under ambient atmospheric conditions. Transmission electron microscopy (TEM), ultraviolet−visible (UV−vis) spectroscopy, Fourier transform-infrared (FT-IR) spectroscopy, and colorimetric methods have been used in the characterization of these nanoparticles.

Introduction. This paper describes a simple and effective way to increase the dispersity of oleic acid stabilized nanoparticles in aqueous phase. By adding R-cyclodextrin (CD) in aqueous solution, the oleic acid stabilized nanoparticles of iron oxide and silver can be irreversibly transferred from hexane into CD aqueous solutions. Magnetically and optically active nanoparticles of metal and metal oxides, such as Fe2O3, Fe3O4, and Ag, have been actively investigated in part because of their applications in sensing, imaging, delivery, and separations of various chemical and biological entities.1-14 Magnetic nanoparticles of iron oxides have been explored for biological applications both as tags in sensing and imaging and as activity agents in hyperthermia therapy.2-6 Surface plasmon-based detection methods using silver and gold nanoparticles8 have shown impressive femtomolar sensitivity and single-site selectivity for biological molecules.7 The function-specific biological applications require uniform particles at the nanometer length scale. Recently, highly monodisperse magnetic nanoparticles of these types such as Fe2O3, FePt, and Co have been made often in organic solutions.15-20 This nonaqueous method offers control over the particle size and shape with close to atomic layer precision-two important parameters that affect * Corresponding author. E-mail: [email protected] † Department of Chemical Engineering. ‡ Laboratory for Laser Energetics (LLE). 10.1021/nl034731j CCC: $25.00 Published on Web 10/18/2003

© 2003 American Chemical Society

the chemical and physical properties of the nanoparticles. In almost all of these colloidal systems, a layer of surfactant molecules is essential to prevent the nanoparticles from aggregation. The long hydrocarbon chains of the surfactants are responsible for the hydrophobicity. These long aliphatic chains also cause the nanoparticles to be immiscible in the aqueous solution, and the biological applications of these nanoparticles are greatly restricted because of their poor solubility in aqueous solution. To realize their potentials in biological applications and to understand the environmental implications of these nanoparticles, it is therefore, important to develop a generic method that can be used to transfer the particles from organic phases to aqueous solutions. It is feasible to increase the aqueous phase dispersity of nanoparticles by modification of the soft surfaces, i.e., the surface-bound surfactant layers, of the nanoparticles. One such strategy is to use ligand exchange with shorter chain stabilizing agents to increase the dispersity of nanoparticles in water. Caruso and co-workers have used 4-(dimethylamino)pyridine (DMAP) to conduct the phase transfer of gold nanoparticles from toluene to aqueous phase.21 Alternatively, the surface can also be altered via the formation of surfactant bilayers with charged hydrophilic ions exposed to the liquid phase. The latter method shows an improved dispersity of nanoparticles in aqueous phase.22

Figure 1. Chemical structures of (a) oleic acid, (b) R-CD molecules, and (c) a schematic illustration of transfer of oleic acid stabilized nanoparticles from organic into aqueous phase by surface modification using CD.

Herewith, we present a surface modification method that is based on the formation of a host-guest complex between the surfactant molecules bound to the surfaces of nanoparticles and the hydrophilic polycyclic macromolecules, and it is expected that the hydrophilic macromolecular hosts could effectively increase the dispersity of the nanoparticles in aqueous phase. Iron oxide and silver nanoparticles used in this research17,23,24 were protected by oleic acid molecules, one of the surfactants that have been proved to be an effective protective agent in the nonaqueous synthesis of nanoparticles of several classes of materials. It is difficult to change the surface structural property of those particles because the exposed groups of oleic acids are aliphatic, Figure 1a. To increase the hydrophilic properties of these nanoparticles, we used R-CD as host molecule to generate inclusion complexes with surface-bound oleic acid molecules. Cyclodextrins are cyclic oligosaccharides that consist of six, seven, and eight glucopyranose units for R-, β-, and γforms, respectively, based on the number of repeating units, Figure 1b.25 They are composed of hydrophobic cavities that can form complexes with various organic molecules, and the hydrophilic rims of hydroxyl groups. CDs, thiolated CDs, and their derivatives have been used in the modification of gold, platinum, and palladium nanoparticles.26-30 The phase transfer of gold particles from aqueous to organic solutions could be facilitated through the formation of inclusion complexes with the CD and its derivatives such as per-6thio-β-cyclodextrin.27,28 CD can also interact with various short-chain carboxylic acid molecules.31 It is therefore quite possible that the complex could form between R-CD and oleic acid used in stabilization of nanoparticles. We show 1556

that these CD-oleic acid complexes could effectively change the surface hydrophobic (or hydrophilic) properties and facilitate the phase transfer of nanoparticles into aqueous solutions, as illustrated in Figure 1c. Experimental Details. Synthesis of Oleic Acid Stabilized 10 nm Iron Oxide Nanoparticles. Thermal decomposition of iron pentacarbonyl (99.999+%, Aldrich, 1.53 mmol, 200 µL) in octyl ether (99+%, Aldrich, 33.2 mmol, 10 mL) in the present of oleic acid (99+%, Aldrich, 4.6 mmol, 1.47 mL) was used to make iron oxide nanoparticles following a method described elsewhere.23 Such nanoparticles could be readily oxidized and form Fe2O3 under either ambient atmosphere conditions or using trimethylamine N-oxide as oxidation agent.18,23,32 The oxidation by using trimethylamine N-oxide can substantially increase the crystallinity of the nanoparticles of Fe2O3.32 Synthesis of Oleic Acid Stabilized 8 nm Ag Nanoparticles. Silver nanoparticles were prepared by thermal reduction of silver trifluoroacetate in isoamyl ether in the presence of oleic acid.24 Silver trifluoroacetate (99.99+%, Aldrich, 0.18 mmol, 0.04 g), oleic acid (99+%, Aldrich, 0.72 mmol, 230 µL), and isoamyl ether (99%, Aldrich, 3 mL) were mixed under argon protection. Ag nanoparticles formed after the reaction at ∼160 °C for 30 min., and were precipitated out from the solution using ethanol and stored in hexane. Phase Transfer. The phase transfer was conducted by vigorously stirring the mixtures of hexane suspensions of either iron oxide or silver nanoparticles and equal volume of R-CD (Aldrich) aqueous solution under room temperature. The typical concentration of nanoparticle in hexanes was ∼0.5 mg particle/mL. The concentration of R-CD aqueous solution was 5 mM, unless indicated otherwise. After stirring for 20 h, the top hexane layer became colorless and was discarded; while the bottom aqueous layer was collected and centrifuged twice to obtain a transparent yellowish nanoparticle suspension which was used in the characterization. Characterizations. The TEM images were recorded on a JEOL JEM 2000EX at an accelerating voltage of 200 kV. The TEM specimens were made by placing a drop of hexane or water suspension of nanoparticles on a carbon coated copper grid. The TEM specimen of water-dispersible nanoparticles was dried under vacuum for 5 h at room temperature prior to the characterizations. A UV-vis spectrometer (Perkin-Elmer, Lambda 900) was used to measure the absorption of the nanoparticles in both hexane and R-CD aqueous solutions. FT-IR spectra of the iron oxide nanoparticles after phase transfer were collected on a Nicolet 20 SXC spectrometer. Those particles could be precipitated from corresponding solutions by adding small amount of salt such as KBr in the transparent yellowish suspensions and further mixed with KBr solid to form a pellet. The nanoparticleKBr pellets were dried for 2 h at 110 °C prior to the FT-IR measurements. Results and Discussion. The phase transfer of the nanoparticles was conducted by vigorous stirring of the mixture of nanoparticle-hexanes solution (0.5 mg/mL) and R-CD aqueous solution (5 mM). During this process, the transport of iron oxide particles from organic phase to Nano Lett., Vol. 3, No. 11, 2003

Figure 2. Photographs of a two-phase mixture of 10 nm iron oxide nanoparticles (a) before and (b) after phase transfer, and (c) aqueous suspension after centrifuge. The initial concentration of iron oxide nanoparticles was 0.5 mg/mL. The top layers were hexane and bottom layers were R-CD aqueous solutions.

Figure 3. TEM images of iron oxide nanoparticles (a) before and (b) after phase transfer.

aqueous phase can be followed by the change of the colors of the mixtures. Figure 2 shows iron oxide nanoparticles in hexane, in aqueous solution after phase transfer, and in aqueous solution after further centrifuge. The yellowish color gradually disappeared from the top hexane phase during the mixing, while at the same time the aqueous layer became yellowish. At the end of the phase transfer process, the bottom layer ended up as a translucent suspension, Figure 2b. This opaque color could be due to the aggregation of nanoparticles, and solid could be recovered after centrifuge at 6000 rpm, leaving a transparent yellow suspension, Figure 2c. In a control experiment, the same hexane solution of iron oxide nanoparticles was stirred together with only deionized water for 20 h, and no obvious color change was observed in either hexane or aqueous phases, indicating that CD is required for the phase transfer of oleic acid stabilized nanoparticles of iron oxide in this process. The nanoparticles after transfer to the CD aqueous solution were inspected using TEM. Figure 3a,b shows the TEM images of the nanoparticles prepared from the hexane suspension before phase transfer and those from aqueous suspension after phase transfer, respectively. No obvious shape or size change was found between the samples before and after the phase transfer. This observation suggests that there are negligible effects on the hard surface of iron oxide nanoparticles by using R-CD; it is most likely that the change of the organic layer made the phase transfer possible through the formation of inclusion complexes between R-CD and the Nano Lett., Vol. 3, No. 11, 2003

Figure 4. FT-IR spectra of iron oxide nanoparticles (a) before and (b) after phase transfer.

surface-bound oleic acid molecules. The absence of ordered assembly for the particles from aqueous phase shown in the Figure 3b could be due to the modified surfaces and/or the slow evaporation of the solvent.33 The resulting nanoparticles in aqueous solution showed a broad absorption peak around 470 nm in the UV-vis spectra similar to that in the hexane solution before phase transfer, Figure S1 in Supporting Information, which indicates the presence of iron oxide nanoparticles. An interesting property of this nanoparticle aqueous solution is that the particles can precipitate by adding salts, such as NaCl, in the aqueous suspensions of nanoparticles (∼5 mg NaCl/mL suspension). Other salts including KCl, KBr, KI, and NaBr also worked. After the addition of these salts, a brownish solid could be observed with naked eye on the bottom of the vial, and the yellowish solution became colorless. These solids responded to a disk magnet with the field strength of ∼0.2 T. The brownish precipitations resulting from the addition of salts were washed several times with deionized water, dried at 110 °C for 3 h, and mixed with KBr to make a pellet specimen for FT-IR inspection. Figure 4 shows the FT-IR spectra of surfactant stabilized iron oxide nanoparticles before and after the phase transfer. Compared with the as-synthesized oleic acid stabilized particles, there are several new IR bands presented in the oleic acid/CD stabilized nanoparticles that can be assigned to R-CD. The strong band at 1150 cm-1 corresponds to the antisymmetric glycosidic vibration, Va(C-O-C), and the bands at 1080 and 1030 cm-1 correspond to the coupled stretch vibration, V(C-C/C-O). The broad band at 3400 cm-1 arises from the O-H vibrations and has been observed previously in the CD films.28 Although water molecules could not be ruled out due to the formation of a CD complex even after drying at 110 °C and could contribute to the observed IR absorption at 3400 cm-1, the appearance of this IR band and those in the fingerprint regions clearly indicated the presence of CD molecules that could not be removed from the solid materials by extensive washing with deionized water. The IR band at 1710 cm-1 observed in the sample from aqueous solution is from a CdO stretch, which indicates the presence of oleic acid in the samples. The 1557

complexion between inorganic ions and CD molecules31 could be the main reason for the observed salt effect, since the oleic acid capped nanoparticles were transferred into aqueous phase most likely through the formation of an inclusion complex between oleic acid and R-CD. When the salt was added into the aqueous solution, the corresponding ions interacted with CD and destabilized the oleic acid-CD complex,31 which led to the decrease in dispersity of iron oxide nanoparticles in water and caused the aggregation of nanoparticles. We noted that the precipitates cannot be redispersed in either hexane or water, which is another indication of the complex formation between surface stabilizing agents and CD. Details of this formation mechanism and the salt effects are currently under investigation. The obtained nanoparticle aqueous suspension is very stable; no obvious change was found after about a year under ambient atmosphere conditions. This aqueous suspension of iron oxide nanoparticles was vigorous stirred together with hexane for 20 h; the nanoparticles remained in aqueous phase, and no reverse transfer to hexane was observed. The nanoparticle suspension is thermally stable; no precipitation or color change was observed in this aqueous suspension after being heated at 95 °C for 12 h. The high thermal stability suggests that the formation of mini- and macroemulsions is most likely not the reason for the stabilization of nanoparticles in CD aqueous solutions. After this thermal treatment, the Fe2O3 nanoparticles could still precipitate by adding the salts at the mentioned concentration (∼5 mg NaCl/mL suspension). We examined if this phase transfer method is a generic method by using oleic acid stabilized 8 nm Ag nanoparticles, Figure 5.24 These Ag nanoparticles were relatively monodisperse with a typical standard size deviation of < ∼10% and exhibited a narrow absorbance peak at 412 nm, which is due to the surface plasmon of Ag nanoparticles. The absorption at 259 nm most likely comes from the colorless dimers of silver trifluoroacetate, (AgOOCCF3)2.12 To examine the effect of R-CD concentration on the phase transfer of Ag nanoparticles in aqueous solution, we mixed Ag nanoparticle suspension in hexane (∼0.5 mg Ag nanoparticles/mL hexane) with equal volumes of R-CD aqueous solutions at the concentrations of 0.5, 1, 3, 5, 7, and 10 mM, respectively. Figure 6a shows the photograph of Ag nanoparticles in CD aqueous suspension at the various CD concentrations after phase transfer. At the low CD concentrations (0.5, 1, and 3 mM), only slight color changes were found in the aqueous phase. In these cases, although some Ag nanoparticles were transferred into the aqueous phase and the bottom aqueous layer became translucent, large parts of the transferred particles were not stable. After centrifuge at 6000 rpm, most of those particles precipitated, and only light yellowish solutions resulted, Figure 6a. As the CD concentration was increased to ∼5 mM, stable aqueous Ag suspensions were obtained. This concentration dependent effect further suggests that the complexation between oleic acid and CD is essential for the phase transfer of Ag nanoparticles to aqueous phase. When the CD concentration is too low, only small portions of surface-bound oleic acid 1558

Figure 5. (a) TEM image and (b) UV-vis spectrum of oleic acid stabilized 8 nm Ag nanoparticles in hexane.

molecules on the surfaces of Ag nanoparticles formed complexes with the CD, which is not sufficiently hydrophilic for the stabilization of nanoparticles in aqueous phase. At the relatively high CD concentrations, substantial amounts of oleic acid molecules could form host-guest complexes with CD, which lead to the increase of the stability of Ag nanoparticles in the hydrophilic media. There was an optimum concentration of CD to conduct the phase transfer judging by the absorbance at 410-420 nm by Ag nanoparticles, Figure 6b. The most intense peak was at 416 nm for the sample with the initial CD concentration of 5 mM. Figure 6b summarized the UV-vis curves of the silver particles in CD aqueous solutions at the various CD concentrations. The absorbance peaks of Ag nanoparticles shifted from 412 nm in hexane to 415-417 nm after the phase transfer to CD aqueous solutions. Further studies are necessary to elucidate this concentration-dependent effect. As a control experiment, Ag nanoparticles in hexane were mixed with equal volume of pure deionized water, and the mixture was stirred vigorously for 20 h. No color change was observed in either hexane or aqueous phase, and no UV-vis absorption peak was found for aqueous samples. A salt effect similar to that for the above Fe2O3 nanoparticles was observed for CD-oleic acid stabilized Ag nanoparticles. Conclusion. Oleic acid capped nanoparticles of iron oxide and silver have been successfully transferred into R-CD aqueous phase from hexane. Inclusion complexes between Nano Lett., Vol. 3, No. 11, 2003

phase. This material is available free of charge via the Internet at http://pubs.acs.org. References

Figure 6. (a) Photographs and (b) UV-vis spectra of silver nanoparticles in R-CD aqueous solution obtained by mixing 0.5 mg Ag nanoparticle/mL hexane solution with 0.5, 1.0, 3.0, 5.0, 7.0, and 10.0 mM R-CD aqueous solutions, respectively.

surface-bound oleic acid and CD appear to change the hydrophobic surface to a hydrophilic surface. The obtained aqueous suspensions of nanoparticles have very good stability, even at the temperature close to the boiling point of water. We believe this approach can be a generic method for the stabilization of oleic acid capped nanoparticles in aqueous solutions, which is an important aspect for the exploration of biological applications and environmental impacts of various oleic acid or other surfactant stabilized nanoparticles that are made in organic solvents. Acknowledgment. This work is supported in part by the University of Rochester and by the U. S. Department of Energy (DE-FC03-92SF19460). We appreciate the generous help from Mr. Hongwei Liu on the operation of UV-vis spectrometer. We thank Professor Matthew Yates for lending us the UV-vis spectrometer. The support of the DOE does not constitute an endorsement by the DOE of the views expressed in this article. Supporting Information Available: UV-vis spectra of nanoparticles of iron oxide in hexane and in R-CD aqueous

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