Preparation and near-infrared photothermal conversion property of cesium tungsten oxide nanoparticles
© Chen and Chen; licensee Springer. 2013
Received: 21 December 2012
Accepted: 18 January 2013
Published: 5 February 2013
Cs0.33WO3 nanoparticles have been prepared successfully by a stirred bead milling process. By grinding micro-sized coarse powder with grinding beads of 50 μm in diameter, the mean hydrodynamic diameter of Cs0.33WO3 powder could be reduced to about 50 nm in 3 h, and a stable aqueous dispersion could be obtained at pH 8 via electrostatic repulsion mechanism. After grinding, the resulting Cs0.33WO3 nanoparticles retained the hexagonal structure and had no significant contaminants from grinding beads. Furthermore, they exhibited a strong characteristic absorption and an excellent photothermal conversion property in the near-infrared (NIR) region, owing to the free electrons or polarons. Also, the NIR absorption and photothermal conversion property became more significant with decreasing particle size or increasing particle concentration. When the concentration of Cs0.33WO3 nanoparticles was 0.08 wt.%, the solution temperature had a significant increase of above 30°C in 10 min under NIR irradiation (808 nm, 2.47 W/cm2). In addition, they had a photothermal conversion efficiency of about 73% and possessed excellent photothermal stability. Such an effective NIR absorption and photothermal conversion nanomaterial not only was useful in the NIR shielding, but also might find great potential in biomedical application.
Plasmonic nanomaterials could exhibit special absorption via the excitation of surface plasmon [1–3], and the maximum absorption band was highly sensitive to the particle’s size [4, 5], shape , local environment , and the coupling between near nanoparticles . Furthermore, under optical illumination, they could convert the absorbed photon energy into heat energy in approximately 1 ps and then transfer the heat to the surrounding media in tens of picoseconds [2–4, 9]. Such an efficient light-to-heat conversion property made them become useful as nanoheaters and therefore gain more and more attention in the past decade [1, 9].
Photothermal therapy is an attractive therapy technique using photosensitizers to generate heat from light absorption and then kill the cancer cells [10, 11]. To avoid the nonspecific heating of healthy cells and allow deeper penetration into tissues, near-infrared (NIR) light is usually utilized . Furthermore, because the use of plasmonic nanomaterials as photosensitizers makes this technique possess spatial selectivity, a lot of plasmonic nanomaterials with NIR photothermal conversion property have been examined. Typical examples include gold nanorods [13–15], gold nanoshells [16, 17], gold nanocages , single-walled [19–21] or multi-walled  carbon nanotubes, graphene or reduced graphene oxide , and germanium . Among them, gold-based nanomaterials received the most attention, owing to their good biocompatibility and tunable optical property. However, gold is an expensive noble metal, and the preparation of its nanostructures with NIR photothermal conversion property usually needs an accurate synthesis condition or repeated deposition. Thus, the alternatives with lower cost or simpler preparation method are still in demand .
Recently, to reduce the energy consumption for air-conditioning and decrease the emission of carbon dioxide, NIR-shielding materials have received considerable attention in the development of transparent and solar heat-shielding filters for solar control windows of automobiles and architectures [26–34]. Among various materials with the capability of shielding NIR light via reflection or absorption mechanism, cesium tungsten oxide (particularly Cs0.33WO3) nanoparticles have been regarded to be highly promising in transparent solar filter application [26–30]. Because of the strong absorption in the NIR region, owing to the free electrons or polars, they also might be efficient as a photosensitizer in NIR photothermal therapy. However, their utilization in heating the reaction media or photothermal therapy via NIR photothermal conversion has not been reported.
Until now, only limited work has been reported for the solvothermal synthesis of cesium tungsten oxide nanorods . The main method for the synthesis of cesium tungsten oxides was the solid state reaction . To obtain the nanosized powder, further grinding was necessary. Thus, in this work, Cs0.33WO3 nanoparticles were prepared by a stirred bead milling process. Although Takeda and Adachi have reported the preparation of tungsten oxide nanoparticles by milling in organic medium with a dispersant agent , for future possible biomedical application and avoiding the use of toxic organic solvent, an aqueous milling process of Cs0.33WO3 nanoparticles without extra dispersant agents which have not been reported was attempted in this work. The appropriate pH of dispersion solution for grinding was determined, and the effect of grinding time on the size of Cs0.33WO3 nanoparticles was examined. Furthermore, the NIR photothermal conversion property of the resulting Cs0.33WO3 powder after grinding for various times was studied to demonstrate the excellent NIR photothermal conversion property of Cs0.33WO3 nanoparticles.
Cesium tungsten oxide (Cs0.33WO3) coarse powder with a primary particle size of about 1 to 2 μm were obtained from the Industrial Technology Research Institute of Taiwan (ITRI). Deionized water was produced by Direct-Q3 ultrapure water system of Millipore Co., Billerica, MA, USA. Potassium hydroxide was purchased from Wako Pure Chemical Industry Co., Ltd (Osaka, Japan). Nitric acid was supplied by Merck KGaA (Darmstadt, Germany). The yttrium-stabilized zirconia (95% ZrO2, 5% Y2O3; density 6,060 kg/m3) grinding beads with a diameter of 50 μm were obtained from Toray Ind., Inc. (Tokyo, Japan). Polyethylene glycol 6000 (PEG 6000; molecular weight 7,000 to approximately 9,000 daltons) was a product of Merck KGaA.
Cs0.33WO3 nanoparticles were prepared via a stirred bead milling process using high-performance batch-type stirred bead mill JBM-B035 manufactured by Just Nanotech Co., Ltd, Tainan, Taiwan. This mill consists of a rotor, a mill chamber, and grinding beads. The rotor and mill chamber are made of highly wear-resistant materials: sintered silicon carbide. The mill chamber is cooled with water and has a net grinding charmer volume of 350 mL. The grinding beads are fluidized by the rotor in the mill chamber as the grinding medium. For the typical stirred bead milling process, Cs0.33WO3 coarse powder (10 wt.%) was added to the aqueous solution of potassium hydroxide at pH 8, and then the dispersion was put into the stirred bead mill. An agitation speed of 2,400 rpm (peripheral speed 10 m/s) was used to exert both shearing and imparting forces on the Cs0.33WO3 coarse powder and was run for different times. Samples were taken at various intervals of grinding time for particle size analysis. The filling ratio of the mill chambers by grinding beads was 60 vol.%. The mill was operated at a constant temperature of 20°C.
The zeta potential and mean hydrodynamic diameter of Cs0.33WO3 nanoparticles in the aqueous dispersion were measured using a Malvern Nano-ZS dynamic light-scattering spectrometer (Malvern Instruments Ltd., Worcestershire, UK). For the measurement of zeta potential, the concentration of Cs0.33WO3 nanoparticles was 10 mg/L, and the pH of aqueous dispersion was adjusted by the addition of potassium hydroxide or nitric acid. Transmission electron microscopy (TEM) analysis was carried out on a Hitachi model H-7500 (Hitachi High-Tech, Minato-ku, Tokyo, Japan) at 120 kV. High-resolution TEM (HRTEM) image of a single Cs0.33WO3 nanoparticle and the corresponding electron diffraction pattern were observed using a Jeol model JEM-2100F (JEOL Ltd., Akishima, Tokyo, Japan) at 200 kV. The content of the contaminant ZrO2 from the stirred bead milling process was determined using an energy dispersive X-ray (EDX) spectrometer attached to the TEM. The crystal structure was characterized by X-ray diffraction (XRD) analysis on a Shimadzu RX-III X-ray diffractometer (Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan) using CuKα radiation (λ = 0.1542 nm). The absorption spectra were measured by a Jasco V-570 UV–vis-NIR spectrophotometer (Jasco Analytical Instruments, Eaton, MD, USA).
The NIR photothermal conversion property of Cs0.33WO3 nanoparticles was investigated in deionized water at different concentrations. The aqueous dispersion of Cs0.33WO3 nanoparticles was added to a 2-mL polystyrene cell, and then the dispersion was exposed to an 808-nm diode laser (HPM (LD1202) X26, Power Technology Inc., Little Rock, AR, USA) with an irradiation area of 0.3 cm2 and an intensity of 820 mW (i.e., 2.73 W/cm2). The temperature of aqueous dispersion was detected with a thermocouple. Photothermal conversion efficiency was calculated using the method reported by Chen et al. . For the study on the photothermal stability of Cs0.33WO3 nanoparticles under NIR irradiation, the aqueous dispersion of Cs0.33WO3 nanoparticles (0.08 wt.%, obtained after grinding for 3 h) was continuously re-exposed to an 808-nm diode laser (2.73 W/cm2) for 5 cycles. For each cycle, the aqueous dispersion was irradiated for 10 min and then cooled to the initial temperature. Using a thermocouple, the variation of temperature with time was monitored.
Results and discussion
According to the method reported by Chen et al. , the photothermal conversion efficiency for the aqueous dispersion of Cs0.33WO3 nanoparticles (2 mg/mL) under NIR irradiation (808 nm, 2.47 mW/cm2) could be determined to be 73%, close to that of gold nanorods with an effective radius of 30 nm. Because the Cs0.33WO3 nanoparticles examined had a mean hydrodynamic diameter of 50 nm and the photothermal conversion efficiency increased with the decrease of particle size , this result revealed that the resulting Cs0.33WO3 nanoparticles had a photothermal conversion property comparable to gold nanorods.
Hexagonal Cs0.33WO3 nanoparticles with a mean hydrodynamic diameter of about 50 nm were prepared successfully in an aqueous solution of pH 8 by bead milling. They possessed excellent NIR photothermal conversion property and stability. With decreasing particle size or increasing particle concentration, the NIR photothermal conversion-induced temperature increase is enhanced. Such a nanomaterial not only could be used in the transparent solar heat-shielding filters, but also is useful for the development of NIR-triggered photothermal conversion materials in biomedicine.
CJC is currently a Ph.D. student of the National Cheng Kung University (Taiwan). DHC is a distinguished professor of the Chemical Engineering Department at National Cheng Kung University (Taiwan).
We are grateful to the National Science Council, Taiwan, for the support of this research under contract no. NSC 100-2221-E-006-164-MY2.
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