Hirotaka Koga

In situ synthesis of silver nanoparticles on zinc oxide whiskers incorporated in a paper matrix for antibacterial applications

Silver nanoparticles (AgNPs) were successfully synthesized in situ on a paper matrix composed of ceramic fibers as the main framework and zinc oxide (ZnO) whiskers as a selective support for AgNPs. Paper-like ceramic fiber/ZnO whisker composites were prepared in advance using a high-speed, low-cost papermaking technique, then immersed in an aqueous solution of silver nitrate for 6 h. AgNPs with particle size 5–20 nm were spontaneously formed on the ZnO whiskers through selective ion-exchange between Ag and Zn species, and simultaneous ZnO-mediated photo-reduction under natural light irradiation. As-prepared material (AgNPs@ZnO paper) was subjected to antibacterial assay by the disk diffusion method using Escherichia coli. The AgNPs@ZnO paper exhibited excellent antibacterial activity and durability for repeated use, as compared with paper composites containing either ionic Ag components or commercial crystalline Ag microparticles. The facile and direct synthesis of AgNPs on a paper-like matrix is a unique approach for the immobilization of highly active metal NPs onto easy-to-handle support materials, and the AgNPs@ZnO paper is expected to be a promising bioactive material having both antibacterial function and paper-like utility.

On-paper synthesis of Au nanocatalysts from Au(III) complex ions for low-temperature CO oxidation

Au n nanoparticles (AuNPs) were successfully synthesized in situ on a microstructured paper matrix composed of ceramic fibers as the main framework and ZnO whiskers as a preferential support for AuNPs. The paper-like ceramic fiber/ZnO whisker composites were prepared using a papermaking technique, then soaked in an aqueous solution of the Au(III) complex HAuCl4. AuNPs with size <10 nm were spontaneously formed on the ZnO whiskers in the absence of reducing agents, possibly due to electron transfer from Zn(II) in ZnO whiskers to Au(III) species through Zn–O–Au bonds. As-prepared AuNPs@ZnO whisker-containing paper (AuNPs@ZnO paper) is much like ordinary paper products in being flexible, lightweight and easy to handle. AuNPs@ZnO paper demonstrated excellent catalytic performance in low-temperature CO oxidation. Complete conversion of CO to CO2 was achieved at 20 °C, 140 K lower than the reaction temperature for conventional Au/ZnO catalyst powders. This facile technique has potentially broad wide applications in ‘on-paper’ synthesis of a diverse array of metal NPs. The metal NPs@ZnO paper composites with paper-like flexibility are able to fit various reactor configurations and are thus expected to be promising catalytic materials for improving the practical utility and catalytic performance of these systems, for a wide range of industrial chemical processes.

In Situ Synthesis of Platinum Nanocatalysts on a Microstructured Paperlike Matrix for the Catalytic Purification of Exhaust Gases

The successful in situ synthesis of platinum nanoparticles (PtNPs) on a microstructured paperlike matrix, comprising ceramic fibers as main framework and zinc oxide whiskers as selective support for the PtNPs, is reported. The as-prepared hybrid material (PtNPs@ZnO paper) resembles ordinary paper products because it is flexible, lightweight, and easy to handle. In the catalytic reduction of nitrogen oxide (NO(x)) with propene for exhaust gas purification, the PtNPs@ZnO paper demonstrates a high catalytic performance at a low reaction temperature, with one-third the dosage of precious platinum compared to conventional platinum-loaded honeycomb catalysts. These results imply that the combination of easily synthesized PtNPs and a unique fiber-network microstructure can provide excellent performances, promoting the effective transport of heat and reactants to the active sites of the platinum nanocatalysts. Thus, PtNPs@ZnO materials with paperlike practical aspects are promising catalytic materials for efficient NO(x) gas purification.

On-Paper Synthesis of Silver Nanoparticles for Antibacterial Applications

Recent years have seen remarkable progress in research and development of metal nanoparticles (NPs) that takes advantage of their unique optical, magnetic, electronic, catalytic and other physicochemical properties, in a wide range of practical and potential applications such as energy, environmental, biomedical and chemical engineering (Feldheim & Foss, 2002; Ma et al., 2006; Jena & Raj, 2008; Zotti et al., 2008). For example, gold (Au) is a chemically inert metallic element in bulk form; however, AuNPs possess colorful plasmon resonances useful for bio-sensing (Shukla et al., 2005; Yokota et al., 2008), and can catalyze chemical reactions (Bond et al., 2006; Ishida & Haruta, 2007), due to their quantum size effects. NPs of inexpensive base metals, such as copper (CuNPs) have recently attracted much attention as innovative nanomaterials for applications such as highly-active gas reforming catalysts for hydrogen production (Gadhe & Gupta, 2007) and effective marine antifouling coatings (Anyaogu et al., 2008). In general, however, practical utilization of nanosized materials involves considerable difficulties since metal NPs are hard to handle directly, and easily aggregate to minimize their surface area. The inevitable aggregation of metal NPs often nullifies their unique functionalities, and eventually yields ordinary bulk metals. For that reason, an area of ongoing research has focused on effective immobilization of metal NPs on easily handled supports such as porous membranes (Dotzauer et al., 2006) and nanostructured inorganic sheets (Wang et al., 2008). Of the various metal NPs, silver NPs (AgNPs) are of increasing interest because of their high conductivity (Li et al., 2005) and tunable optical responsiveness (McFarland & Van Duyne, 2003). Moreover, it is well known that Ag exhibits potent antibacterial properties with low toxicity for humans and animals by comparison with other heavy metals (Alt et al., 2004; Shah et al., 2008). Ag and Ag-compounded materials are effective for both Gram-negative and Gram-positive bacteria, whereas the efficacy of conventional antibiotics varies with the species of bacteria (Shah et al., 2008). Many researchers have recently reported that AgNPs demonstrate excellent antibacterial activity (Sondi & Salopek-Sondi, 2004; Gogoi et al., 2006; Pal et al., 2007; Navarro et al., 2008). However, effective methods for immobilization of AgNPs for practical use are insufficiently advanced, and incorporation of AgNPs into various matrices is being actively investigated. For instance, it was reported that AgNPs were synthesized on poly(ethylene glycol)-polyurethane-titanium dioxide (TiO2) films via 14

On-Paper Synthesis of Nickel Nanoparticles and Catalytic Propane Steam Reforming for Efficient Hydrogen Production

Nickel nanoparticles (NiNPs) were successfully synthesized in situ on a microstructured paper matrix composed of inorganic fibers as the main framework and zinc oxide (ZnO) whiskers as a preferential support for NiNPs. The paper-like inorganic fiber/ZnO whisker composites were prepared using a high-speed and low-cost papermaking technique, and then simply immersed in an aqueous solution of Ni(NO3)2. After reduction in hydrogen flow, NiNPs with a fine size about 20 nm in diameter were selectively formed on the ZnO whiskers in the paper composites. As-prepared NiNPs@ZnO paper is much like an ordinary paper product, being flexible, lightweight, and easy to handle. The NiNPs@ZnO paper composites exhibited excellent catalytic performance in the steam reforming of propane, and produced hydrogen more efficiently than commercial pellet-type nickel catalysts. These results are possibly attributed both to highly active NiNPs synthesized in situ and to the uniform flow of reactants inside a microporous paper structure.

Ionic Liquid Mediated Dispersion and Support of Functional Molecules on Cellulose Fibers for Stimuli-Responsive Chromic Paper Devices

Functional molecules play a significant role in the development of high-performance composite materials. Functional molecules should be well dispersed (ideally dissolved) and supported within an easy-to-handle substrate to take full advantage of their functionality and ensure easy handling. However, simultaneously achieving the dissolution and support of functional molecules remains a challenge. Herein, we propose the combination of a nonvolatile ionic liquid and an easy-to-handle cellulose paper substrate for achieving this goal. First, the photochromic molecule, i.e., diarylethene, was dissolved in the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([bmim]NTf2). Then, diarylethene/[bmim]NTf2 was supported on cellulose fibers within the paper, through hydrogen bonding between [bmim] cations of the ionic liquid and the abundant hydroxyl groups of cellulose. The as-prepared paper composites exhibited reversible, rapid, uniform, and vivid coloration and bleaching upon ultraviolet and visible light irradiation. The photochromic performance was superior to that of the paper prepared in the absence of [bmim]NTf2. This concept could be applied to other functional molecules. For example, lithium perchlorate/[bmim] tetrafluoroborate supported within cellulose paper acted as a flexible electrolyte to provide a paper-based electrochromic device. These findings are expected to further the development of composite materials with high functionality and practicality.