Facile synthesis and optical properties of polymer-laced ZnO-Au hybrid nanoparticles
© Wang et al.; licensee Springer. 2014
Received: 30 December 2013
Accepted: 27 February 2014
Published: 7 March 2014
Bi-phase dispersible ZnO-Au hybrid nanoparticles were synthesized via one-pot non-aqueous nanoemulsion using the triblock copolymer poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) as the surfactant. The characterization shows that the polymer-laced ZnO-Au nanoparticles are monosized and of high crystallinity and demonstrate excellent dispersibility and optical performance in both organic and aqueous medium, revealing the effects of quantum confinement and medium. The findings show two well-behaved absorption bands locating at approximately 360 nm from ZnO and between 520 and 550 nm from the surface plasmon resonance of the nanosized Au and multiple visible fingerprint photoluminescent emissions. Consequently, the wide optical absorbance and fluorescent activity in different solvents could be promising for biosensing, photocatalysis, photodegradation, and optoelectronic devices.
Multi-constituent nanomaterials with diverse compositions and tailorable morphology exhibit multiple functionalities and novel properties, showing prospective potentials in biological detection and sensing, drug delivery, hyperthermia, cell separation, magnetic data storage, strong catalysis, photoelectric conversion, and many other areas [1–3]. Syntheses of such nanoparticles and investigating their properties are hence of general interest. On one hand, gold nanoparticles as a typical noble metal product, because of their chemical stability, original biocompatibility, and prevailing effects of surface plasmon resonance in the visible region, offer excellent, versatile opportunities in immunoassay, biosensing, and optimal catalysis [4–8]. On the other hand, ZnO nanoparticles with a wide energy bandgap are an excellent, well-studied semiconductor, accompanied by shifting of the intrinsic band due to quantum confinement [3, 9–11]. Strong, tunable absorption and emission bands revealed in ZnO nanostructure, characterized by the particle size and the surrounding medium, have found uses in biosensing technology, electronics, photoelectronics, catalysis, and chemical degradation.
By nanoengineering these two materials into a single entity, the ensuing nanostructure would not only exercise the unique properties of gold and the semiconductor, but also generate novel collective phenomena based on the interaction between Au and ZnO [12–15]. Such a structural nanoassembly can have the extra advantages of biocompatibility and low toxicity and afford an easy, effective contact between biological tissue and the nanoparticles, anticipated to be benign for biological detection, photocatalysis, and dye-sensitized solar cells. Ranking in a variety of interesting structural forms, the synthesis of ZnO-Au nanoparticles has been performed for various purposes [16–21]. In addition, the natural coating of nanoparticle surfaces by an ultrathin film of biocompatible molecules is highly desirable for future biomedical applications, especially if done in situ during the synthesis process of the nanoparticles [3, 17]. We here report the preparation of ZnO-Au hybrid nanoparticles by one-pot non-aqueous nanoemulsion with the triblock copolymer poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEO-PPO-PEO) as the surfactant. The copolymer has proved many distinctive merits, such as aqueous solubility, biocompatibility, non-charging, and non-toxicity, and is often used in a number of fields [22–26]. In nanoemulsion processes, the PEO-PPO-PEO molecules principally participate in the reactions as a surfactant, playing a role in stabilizing the nanoparticles formed and even acting as a reducing agent, as attested in our reports on long-term stable, highly crystalline, monosized Fe3O4/Ca3(PO4)2, Fe3O4/ZnO, Fe3O4/Au, and FeAu nanoparticles [3, 8, 27, 28]. In this work, the ZnO-Au nanoparticles prepared without a secondary surface modification were bi-phase dispersible. The characterization shows that such polymer-laced ZnO-Au nanoparticles are monosized and of high crystallinity and possess excellent dispersibility and optical performance in both organic and aqueous medium.
Bi-phase dispersible polymer-laced ZnO-Au nanoparticles were prepared by one-pot non-aqueous nanoemulsion, using gold acetate and zinc acetylacetonate as the precursors, the triblock copolymer PEO-PPO-PEO as the surfactant, and 1,2-hexadecanediol as the reduction agent. Typically, 0.25 mmol of gold acetate, 0.25 mmol of zinc acetylacetonate, 0.1358 mmol of PEO-PPO-PEO, and 2.5 mmol of 1,2-hexadecanediol were mingled in 10 ml octyl ether in a 250-ml flask under vigorous stirring. The reaction mixture was first slowly heated to 125°C within 2 h and homogenized at this temperature for 1 h under vigorous stirring, then rapidly heated to 280°C within 15 min and refluxed at the temperature for 1 h. After cooling down to room temperature, ethanol was added to the reacted solution to precipitate the PEO-PPO-PEO-laced ZnO-Au nanoparticles by centrifugation. The precipitated product was washed with ethanol/hexane (2:1) several times. The resultant nanoparticles prepared in such a process can be re-dispersed in hexane, ethanol, and distilled water directly, without a secondary surface modification which is usually required . For comparison, Au and ZnO nanoparticles were prepared similarly using only gold acetate or zinc acetylacetonate as the precursor.
The morphology of the ZnO-Au nanoparticles was analyzed by transmission electron microscopy (TEM, JEM-100CX), whereas the structure was characterized by X-ray diffractometry (XRD, X'Pert Pro, PANalytical B.V., Almelo, The Netherlands; λ = 1.54056 Å) using Cu Kα radiation. An Avatar 360 Fourier transform infrared spectroscopy (FTIR) spectrometer (Nicolet Company, Madison, WI, USA) was applied to perform the Fourier transform infrared spectroscopy investigation. In the FTIR studies, the washed ZnO-Au nanoparticles and the pure PEO-PPO-PEO polymer employed in the preparation were crushed with a pestle in an agate mortar, the individually crushed material was mixed with potassium bromide (IR spectroscopy grade) (Merck, Darmstadt, Germany) in about 1:100 proportion. The mixture was then compressed into a 2-mm semitransparent disk by applying a force of 10 t for 2 min. The FTIR spectra were recorded at the wavelength range of 400 to 4,000 cm-1. Moreover, the optical properties of the ZnO-Au nanoparticles separately dispersed in hexane, ethanol, and water, together with the Au and ZnO nanoparticles in hexane, were characterized by an UV-visible spectrophotometer (UV-vis near IR spectrophotometer, Hitachi U4100; Hitachi, Shanghai, China) and a photoluminescence (PL) spectrophotometer (Hitachi F7000, Japan).
Results and discussion
where and ϵ = ϵ2/ϵ1.
In the expression, Eg(R) and Eg(bulk) represent the bandgap energies of the nanoparticles of radius R and the bulk material with a dielectric constant ϵ2 surrounded in a medium of dielectric constant ϵ1. The parameters me and mh indicate the effective masses of the electron and the hole of the exciton, whereas e is the electron charge and ħ the Planck constant divided by 2π. The bracket <> means average over a wave function of position r. In addition to the change observed in the band positions from the ZnO nanoparticles to the Au-ZnO nanoparticles, comparing the shapes of the bandgap absorption in Figure 4a,e further sheds light on the impact of Au on ZnO, in which the Au-ZnO nanoparticles show increased absorption intensity with the decreasing wavelength against the almost flat absorption of the ZnO nanoparticles. As revealed in the multiple domain nanostructure from the TEM analysis above, moreover, the Au nanocrystallites in the hybrid nanoparticles produce more surface and interface defects, i.e., imperfect lattices and oxygen vacancies that are expected to generate a defect level in the energy band, resulting in likely contributions of more induced excitons and increased exciton density to the moderate enhancement in the absorption intensity in the UV range. Furthermore, the SPR action induced by the Au nanocrystallites, which is to be addressed below, offers additional channels to absorb the incident electromagnetic waves and thus probably augment the UV absorption of the hybrid nanoparticles.
The second well-defined absorption between 520 and 550 nm features the optical property of surface plasmon resonance in consequence of Au nanostructuring [27, 28, 33, 34]. Dependent on the solvent, the peak position of the plasmon band in the solution of the Au-ZnO nanoparticles varies from approximately 533 nm in hexane, approximately 550 nm in water, to approximately 542 nm in ethanol, in comparison to the Au nanoparticles in hexane which has an absorption peaking at approximately 525 nm. Nominally, the peak position and band shape of the plasmon resonance may be subject to factors of composition, dimension, nanostructure shape, dielectric medium, and nanostructuring of the nanoparticle system [33–35]. The distinctly broadening and red-shifting of the surface plasmon spectra of the ZnO-Au hybrid nanoparticles could be due to the fact that the strong interfacial coupling between Au and ZnO results in electron transfer from Au to ZnO taking account of the formation of ZnO-Au nanocomposites [12, 33, 34]. It is useful to point out that the Au atoms sitting on the surface of the ZnO-Au nanoparticles covered by PEO-PPO-PEO, which is observed as a result of the plasmon resonance addressed above and tested in the experiment, enable thiolation linkage to other molecules .
In summary, we have synthesized the amphiphilic ZnO-Au hybrid nanoparticles by the one-pot non-aqueous nanoemulsion process adopting the biocompatible and non-toxicity triblock copolymer PEO-PPO-PEO as the surfactant. The FTIR assessment substantiates the lacing of the PEO-PPO-PEO macromolecules onto the surface of the nanoparticles. The morphology and structural analyses show the narrow particle size distribution and high crystallinity of the polymer-laced nanoparticles. Moreover, the optical measurements present the well-defined absorption band of the nanoparticles dispersed in different polar and non-polar solvents, manifesting both the ZnO bandgap absorption and the surface plasmon resonance of the nanosized Au, whereas the fluorescent properties reveal multiple fingerprint emissions. Such bi-phase dispersible ZnO-Au nanoparticles could be applicable in biological detection, solar cells, and photocatalysis.
This work was supported partly by the Scientific and Technological Development Projects, Science and Technology Department of Henan Province, China (No. 112300410011), the National Natural Science Foundation of China (No. 51172064), Research Center Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology, South Korea (No. 2009-0081506) and the Industrial Core Technology Development Program funded by the Ministry of Knowledge Economy, South Korea (No. 10033183).
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