Volum

e19|Num

ber15|2009Journal of M

aterials Chem

istry Pages2089–2260

0959-9428(2009)19:15;1-U

www.rsc.org/materials Volume19|Number15|21April2009|Pages2089–2260

ISSN0959-9428PAPERTakuyaKitaokaet al.In situsynthesisofsilvernanoparticlesonzincoxidewhiskersincorporatedinapapermatrixforantibacterialapplications

PAPERJiang-JenLinet al.Synthesisofimmobilizedsilvernanoparticlesonionicsilicateclayandobservedlow-temperaturemelting

Registered Charity Number 207890

As featured in:

SeeRui-XuanDong,Chih-ChengChouandJiang-JenLin,J. Mater. Chem.,2009,19,2184.

www.rsc.org/materialsRegistered Charity Number 207890

Silvermetalparticlesofnarrowsizedistributionatca.25nmwerepreparedandspreadonthesurfaceofinorganicsilicateclays.Lowtemperaturemeltingat110oCwasdirectlyobservedbyFE-SEM.

Showcasing research from JJ Lin Polymer Labs at National Taiwan University, Taiwan

Showcasing research from JJ Lin Polymer Labs at National Taiwan University of Taiwan

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www.rsc.org/materials Volume 19 | Number 15 | 21 April 2009 | Pages 2089–22602089–2260

ISSN 0959-9428PAPERTakuya Kitaoka et al.In situ synthesis of silvernanoparticles on zinc oxide whiskersincorporated in a paper matrix forantibacterial applications

PAPERJiang-Jen Lin et al.Synthesis of immobilized silvernanoparticles on ionic silicate clay andobserved low-temperature melting

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PAPER www.rsc.org/materials | Journal of Materials Chemistry

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

Hirotaka Koga, Takuya Kitaoka* and Hiroyuki Wariishi

Received 13th November 2008, Accepted 21st January 2009

First published as an Advance Article on the web 23rd February 2009

DOI: 10.1039/b820310e

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.

Introduction

Nanosized metal particles have attracted increasing attention

due to a wide range of potential applications in optoelectronic,1

catalytic2 and biomedical3 fields. However, practical imple-

mentation of each original nanostructured form involves

considerable difficulties because metal nanoparticles (NPs) very

easily aggregate to minimize their surface area. Such inevitable

aggregation of metal NPs lessens their inherent functionality,

and eventually yields ordinary bulk metals. Consequently, an

area of ongoing research has focused on effective immobilization

of metal NPs on easy-to-handle supports such as porous

membranes4 and nanostructured inorganic sheets.5

Of various metallic elements, silver (Ag) is well known as

exhibiting antibacterial properties with low toxicity for humans

and other animals.6,7 Ag and Ag-compounded materials are

effective for both Gram-negative and Gram-positive bacteria,

whereas the efficacy of general antibiotics depends on the type of

bacteria.6 Many researchers have reported that Ag nanoparticles

(AgNPs) have significant antibacterial activity;8–10 but practical

methods for AgNP immobilization have been insufficiently

advanced. Incorporation of AgNPs into various matrices has

recently been investigated to extend their utility in practical,

biomedical applications.5,6,11–13 For example, it was reported that

AgNPs were synthesized on polyethyleneglycol-polyurethane-

titanium dioxide (TiO2) films via TiO2-mediated photocatalysis

under UV light irradiation, which facilitated photo-reduction of

silver nitrate (AgNO3) to form AgNPs on the polymer-inorganic

hybrid matrix.6 Ag-nanocoated cotton fabric was also

Department of Forest and Forest Products Sciences, Graduate School ofBioresource and Bioenvironmental Sciences, Kyushu University, 6-10-1Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan. E-mail: tkitaoka@agr.kyushu-u.ac.jp; Fax: +81 92 642 2993; Tel: +81 92 642 2993

This journal is ª The Royal Society of Chemistry 2009

successfully developed by an ion-exchange method.11 Such types

of Ag-organic polymer complexes are generally sensitive to

external stimuli (e.g. heat, light and pH), leading to insufficient

material stability. Thus Ag-doped antibacterial inorganics such

as Ag-hydrogen titanate nanobelt sheets5 and thin silica films of

ionic Ag-incorporated tetraethyl orthosilicate12 have been

developed using various approaches, e.g. sol-gel processing, ion-

exchange and surface modifications. Despite these efforts,

however, there is still a need to find more facile ways of using

AgNPs, without sacrificing their excellent bioactive functionality,

in antibacterial applications. Thus one of the present challenges is

to develop a novel method for immobilization of highly-active

AgNPs on multipurpose, familiar support materials.

In a previous study we carried out direct in situ synthesis of

copper nanoparticles (CuNPs) on an inorganic paper matrix

composed of ceramic fibers and zinc oxide (ZnO) whiskers,

through selective ion-exchange between Cu and Zn species.14 As-

prepared CuNPs@ZnO paper was lightweight, flexible and easy

to handle (similar to cardboard), and demonstrated excellent

catalytic performance in the methanol reforming process for

producing hydrogen for fuel cell applications. This facile tech-

nique has great possibilities for potential applications in the ‘on-

paper’ synthesis of metal NPs such as Ag, Pt and Au, which have

lower ionization tendency than Cu. Paper-like materials on

which functional metal NPs are supported would be expected to

have wide applications.

In this study, in situ synthesis of AgNPs on a ZnO whisker-

containing matrix was investigated both for facile immobiliza-

tion of AgNPs on a paper-like material, and for development of

a bioactive material with antibacterial activity. Paper-like

ceramic fiber/ZnO whisker composites (ZnO paper) as a sup-

porting matrix for the AgNPs synthesis were prepared by our

established papermaking technique.14 Direct synthesis of AgNPs

on ZnO paper was performed in a simple and cost-effective

J. Mater. Chem., 2009, 19, 2135–2140 | 2135

procedure as follows. ZnO paper composites were immersed in

an aqueous solution of AgNO3, followed by pick-up, washing

and drying. Antibacterial properties, including activity and

durability in repeated use, were investigated with respect to (1)

Ag-free ZnO paper as control, (2) AgNP-supported ZnO paper

(AgNPs@ZnO paper), and two ZnO whisker-free paper

composites containing either (3) ionic Ag components (AgNO3-

impregnated ZnO-free paper) or (4) commercial crystalline Ag

microparticles (Ag powder-containing paper), by disk diffusion

assay using the typical Gram-negative bacterium Escherichia coli

(E. coli).

Experimental

Materials

Ceramic fibers and ZnO whiskers were purchased from IBIDEN,

Ltd. and Matsushita Amtec, Ltd., respectively. Pulp fibers as

a matrix component in the paper fabrication process were

obtained by refining commercial bleached hardwood kraft pulp

(>90% Eucalyptus grandis natural hybrids, Brazil) to a Canadian

Standard Freeness of 300 mL with a Technical Association of the

Pulp and Paper Industry standard beater. Two types of floccu-

lants were used as retention aids: cationic poly-

diallyldimethylammonium chloride (PDADMAC; molecular

weight: ca. 3 � 105 g mol�1; charge density: 5.5 meq g�1; Aldrich,

Ltd.) and anionic polyacrylamide (A-PAM, HH-351; molecular

weight: ca. 4 � 106 g mol�1; charge density: 0.64 meq g�1; Kurita,

Ltd.). An alumina sol (Snowtex 520, Nissan Chemicals, Ltd.) was

used as a binder to improve the physical strength of the paper

composites after calcination. AgNO3 and fine Ag powders

(particle size: ca. 1 mm) were obtained fromWako Pure Chemical

Industries, Ltd. and Soekawa Chemical, Ltd., respectively. Other

chemicals were reagent grade and were used without further

purification.

Papermaking procedure

The preparation details of paper composites using organic and

inorganic fibers, through a dual polyelectrolyte retention system,

have been described in previous reports.14–18 In summary, a water

suspension of ceramic fibers and ZnO whiskers was mixed with

PDADMAC (0.5 wt% of total solids), an alumina sol binder and

A-PAM (0.5 wt% of total solids), in that order. The mixture was

added to a pulp fiber suspension, and solidified by dewatering

using a 200-mesh wire. The wet-state handsheets were pressed at

350 kPa for 3 min, then dried in an oven at 105 �C for 1 h. The

resulting paper composite (2 � 104 mm2) consisted of ceramic

fibers (5.0 g), ZnO whiskers (0.0 or 3.1 g), alumina sol (0.50 g)

and pulp fibers (0.25 g). Ag powder-containing paper composites

were prepared by substituting Ag powder (0.6 g) for ZnO whis-

kers. The paper composites obtained were calcined at 350 �C for

12 h to remove pulp fibers and to improve the physical strength

by binder sintering.14–18

Preparation of AgNPs@ZnO paper and AgNO3-impregnated

ZnO-free paper

The preparation procedure for AgNPs@ZnO paper was as

follows. As-prepared ZnO whisker-containing paper composite

2136 | J. Mater. Chem., 2009, 19, 2135–2140

was cut into disk-shaped pieces (8 � 102 mm2) and immersed in

an aqueous solution of AgNO3 (1.3 � 102 mM, 100 mL) for 6 h.

The treated disks were removed from the AgNO3 solution using

tweezers, thoroughly washed with deionized water, then dried at

105 �C for 2 h. AgNPs@ZnO whiskers (not a paper shape) were

prepared in a similar manner. In the case of AgNO3-impregnated

paper, disk-shaped ZnO whisker-free paper composite (8 � 102

mm2) was immersed in aqueous AgNO3 (73 mM, 30 mL), fol-

lowed by evaporation of water to dryness at 105 �C for 30 min to

force precipitation of Ag components on the paper composite.

Characterization

Ag and Zn contents were determined by atomic absorption

spectrophotometric analysis using a Shimadzu AA-6600F

instrument. The concentrations of Ag+ or Zn2+ extracted from

the samples with 35% nitric acid were quantified by flame atomic

absorption. Transmission electron microscopy (TEM) was

carried out using a JEM1010 instrument (JEOL, Ltd.) at an

accelerating voltage of 80 kV. The chemical states of the

component elements were analyzed by X-ray photoelectron

spectroscopy (XPS, AXIS-HSi spectrometer, Shimadzu/Kratos,

Ltd.). X-Ray diffractometry (XRD) was performed using an

XD-D1 X-ray diffractometer (Shimadzu, Ltd.) with Ni-filtered

CuKa radiation (l ¼ 1.5418 A) with scanning angle (2q) range

30–60� at 30 kV voltage and 40 mA current. The Scherrer

formula was used to calculate the Ag crystallite size on the basis

of the full width at half maximum of the Ag(111) reflection (2qz38�).12 Surface observation of paper samples was conducted

using a scanning electron microscope (SEM, JSM-5600, JEOL,

Ltd.).

Antibacterial assay

The antibacterial activity of paper composites was determined by

disk diffusion assay using E. coli (JM109, a Gram-negative

bacterium) as a model pathogenic bacterium.5,6,11 Luria-Bertani

(LB) agar (20 mL) was poured into a sterilized Petri dish, and

solidified within 10 min. E. coli bacterial suspension (LB

medium, 100 mL, 3.5 � 105 colony forming units per mL) was

uniformly inoculated on the solidified agar gel. Each disc-shaped

piece (10 mm in diameter and 1 mm in thickness) of Ag-free ZnO

paper, AgNPs@ZnO paper, AgNO3-impregnated paper or Ag

powder-containing paper was placed on the LB agar plate, then

incubated at 37 �C for 24 h. The Ag content of the latter three

samples was 2.0 mg per disc. The antibacterial activities were

compared by the diameter of the zone of inhibition around each

paper disk. In addition, the antibacterial durability for repeated

use was evaluated by the variations in the diameter of the zone of

inhibition during five-cycle antibacterial tests.

Results and discussion

AgNPs synthesis and characterization

AgNPs synthesis on ZnO whiskers was successfully achieved by

a simple technique whereby the ZnO whiskers were suspended

in an aqueous solution of AgNO3, followed by continuous

stirring for 6 h, filtration, washing with deionized water, and

drying. Fig. 1 displays TEM images of the original and the

This journal is ª The Royal Society of Chemistry 2009

Fig. 1 TEM images of original ZnO whiskers (a, b) and AgNO3-treated ZnO whiskers (c).

Fig. 3 XRD patterns of original ZnO whiskers (a) and AgNO3-treated

ZnO whiskers (b). ,: Ag, -: ZnO.

AgNO3-treated ZnO whiskers. The ZnO whiskers possessed

a tetrapod-like nanostructure composed of four ZnO needles

(Fig. 1a and b). After the soaking treatment with AgNO3 solu-

tion, a number of NPs of 5–20 nm diameter were clearly

observable on the surface of the ZnO needles (Fig. 1c). The

chemical states of the component elements were analyzed by XPS

(Fig. 2). The ZnO whiskers treated with AgNO3 solution dis-

played a well defined Ag3d5/2 peak at ca. 368 eV which was

assigned to Ag0.6,19 The Zn2p peak at ca. 1022 eV was derived

from the divalent Zn2+ component of ZnO whiskers. No nitrogen-

related species were found. XRD analysis was conducted to

elucidate the crystal structure of the component elements (Fig. 3).

The XRD pattern of ZnO whiskers treated with AgNO3 showed

typical peaks of crystalline ZnO and two weak peaks (2q¼ 38 and

44�) which correspond to the characteristic (111) and (200)

reflections, respectively, of crystalline Ag.11,19 This result indicated

successful formation of Ag nanocrystals (with crystallite size ca.

16 nm, calculated by the Scherrer formula). The TEM, XPS and

XRD data strongly suggested that the NPs spontaneously formed

on the ZnO whiskers were AgNPs. This phenomenon presumably

arises from the difference in ionization tendency between Ag and

Zn, similar to the finding reported in a previous study.14 In

essence, monovalent Ag+ ions in acidic nitrate solution were

substituted by a portion of the Zn species in the ZnO whiskers.

However, in this case, the atomic absorption analysis determined

that the amount of Zn2+ ions eluted from ZnO whiskers was

ca. 4.6 mmol mol-ZnO�1, whereas that of Ag adsorbed on the

ZnO whiskers was ca. 15.1 mmol mol-ZnO�1. Hence, a simple

ion-exchange mechanism does not adequately explain the obser-

vations. Furthermore, Ag components present on the ZnO

whiskers were neither ionic nor oxidized species, and had metallic

Ag0 crystal structure, as shown in Fig. 2b and 3b. ZnO-mediated

photocatalysis has recently become the center of attention in

relation to optical20 and photoelectronic21 applications. The band

gap of ZnO crystals is 3.37 eV,20 similar to that of the anatase-

type TiO2 crystals which can reduce Ag ions to AgNPs. Thus it is

Fig. 2 XPS spectra of original ZnO whiskers

This journal is ª The Royal Society of Chemistry 2009

presumed that rapid ion-exchange between Ag and Zn occurred

on ZnO whiskers, and simultaneously photo-reduction of adsor-

bed Ag species proceeded on the ZnO crystal surface to form

AgNPs under natural light irradiation. The molar ratio of Ag to

Zn estimated from XPS and atomic absorption analyses was

ca. 0.049 and ca. 0.015, respectively, indicating that a lot of AgNPs

preferentially existed on the surface of ZnO whiskers.

In situ synthesis of AgNPs on ZnO paper

The direct synthesis of AgNPs on a paper composite was per-

formed by using ZnO whiskers as a scaffold for AgNPs. ZnO

whiskers and ceramic fibers were first fabricated into a paper

composite by a papermaking technique;14 the retention of inor-

ganic materials was approximately 100%. Subsequently, as-

prepared ZnO paper was immersed in an aqueous solution of

AgNO3. Fig. 4 shows optical images of original Ag-free ZnO

(a) and AgNO3-treated ZnO whiskers (b).

J. Mater. Chem., 2009, 19, 2135–2140 | 2137

Fig. 5 XPS spectra of (a) Ag-free ZnO paper; (b) AgNO3-treated ZnO

paper; (c) AgNO3-impregnated ZnO-free paper; (d) Ag powder-con-

taining paper.

Fig. 6 XRD patterns of (a) Ag-free ZnO paper; (b) AgNO3-treated ZnO

paper; (c) AgNO3-impregnated ZnO-free paper; (d) Ag powder-con-

taining paper. ,: Ag, B: AgNO3, -: ZnO.

Fig. 4 Optical images of (a) Ag-free ZnO paper; (b) AgNO3-treated

ZnO paper; (c) AgNO3-impregnated ZnO-free paper; (d) Ag powder-

containing paper. The size of each paper composite is 8 � 102 mm2.

Fig. 7 SEM images of the surfaces of (a) Ag-free ZnO paper; (b)

AgNO3-treated ZnO paper (AgNPs@ZnO paper); (c) AgNO3-impreg-

nated ZnO-free paper; (d) Ag powder-containing paper.

paper, ZnO paper treated with AgNO3 solution, AgNO3-

impregnated ZnO-free paper and Ag powder-containing

ZnO-free paper. These paper composites were cardboard-like

materials, and were lightweight, flexible and easy to handle.

Fig. 5 and 6 show the XPS spectra and XRD patterns, respec-

tively, of each paper composite. The profiles of AgNO3-treated

ZnO paper shown in Fig. 5b and 6b were similar to those of

AgNPs@ZnO whiskers in Fig. 2b and 3b, respectively. AgNO3-

treated paper without ZnO whiskers was prepared in a similar

manner, and then the amount of Ag adsorbed on the paper

composite was less than 2.8%, as compared with AgNO3-treated

ZnO paper, suggesting that AgNPs were synthesized selectively

on the ZnO whiskers incorporated in the paper composite. In the

case of AgNO3-impregnated paper, the formation of crystalline

AgNO3 precipitates was confirmed by the XPS data (Ag3d5/2: ca.

369 eV, N1s: ca. 407 eV)19 and the XRD assignment (reflections

with 2q values approximately 36 and 49�),22 as shown in Fig. 5c

and 6c. Ag powder-containing paper proved to contain Ag0

crystals (Fig. 5d and 6d). Fig. 7 displays SEM images of the

surface of each paper composite, revealing a characteristic porous

fiber network microstructure composed of ceramic fibers.14–17 The

ZnO whiskers are fine fillers, and they were entangled with and

scattered on the ceramic fiber networks, although they were not

2138 | J. Mater. Chem., 2009, 19, 2135–2140

easily identified in the SEM images (Fig. 7b). From the result of

XRD analysis (Fig. 6b), Ag crystallite size of AgNPs@ZnO

paper was ca. 17 nm, which was similar to that of AgNPs@ZnO

whiskers (ca. 16 nm). Consequently, it was presumed that as-

synthesized AgNPs with their original nanometer sizes existed

in the macro-scale paper composite. AgNO3-impregnated paper

and Ag powder-containing paper exhibited relatively large

aggregates, possibly originating from AgNO3 precipitates and

Ag powders, respectively. Thus direct in situ synthesis of AgNPs

on easy-to-handle paper composites was successfully achieved by

using ZnO whiskers as a selective support in a facile and cost-

effective manner.

This journal is ª The Royal Society of Chemistry 2009

Fig. 9 Antibacterial activity for repeated use: AgNPs@ZnO paper

(circles), AgNO3-impregnated ZnO-free paper (squares), and Ag powder-

containing paper (triangles).

Antibacterial properties of AgNPs@ZnO paper

The antibacterial activity of AgNPs@ZnO paper was compared

with that of Ag-free ZnO paper, AgNO3-impregnated paper and

Ag powder-containing paper. The Ag content was adjusted to be

identical (ca. 2.0 mg) for each sample. Fig. 8 displays the optical

images of the zone of inhibition against E. coli for each paper

composite. Ag-free ZnO paper had no antibacterial activity

(Fig. 8a). By contrast, the three paper composites containing Ag

species showed a clear zone of inhibition around each paper disc

(Fig. 8b–d), indicating significant antibacterial activity. Of the

test samples, AgNPs@ZnO paper demonstrated the largest zone

of inhibition, i.e. the highest antibacterial activity, although the

same amounts of Ag components were retained in each paper

sample. The antibacterial mechanism of Ag species has been

a matter of debate for decades. Recently, researchers have

reported credible rationales for the antibacterial activity of Ag,

as follows. (1) Ag+ ions interact with phosphorus moieties in

DNA, resulting in inactivation of DNA replication, and/or (2)

they react with sulfur-containing proteins, leading to inhibition

of enzyme functions.23,24 Though the mechanism of antibacterial

action of AgNPs is still insufficiently understood, many

researchers have reported that AgNPs could be toxic because

they release Ag+ ions which play an essential role in antibacterial

effects.8,25–27 On the other hand, Pal et al. reported that spherical

AgNPs had greater antibacterial activity against E. coli than Ag+

ions in the form of AgNO3, and proposed that the nanometer size

and the presence of Ag(111) crystal faces synergistically

promoted the antibacterial effect of AgNPs.28 Lok et al. also

suggested that spherical AgNPs were significantly more efficient

against E. coli than Ag+ ions (AgNO3) in mediating their

Fig. 8 Optical images of the zone of inhibition for (a) Ag-free ZnO

paper; (b) AgNPs@ZnO paper; (c) AgNO3-impregnated ZnO-free paper;

(d) Ag powder-containing paper. Each paper composite is 10 mm in

diameter. Ag content: 0.0 mg (a) or ca. 2.0 mg (b, c, d). Incubation

condition: 37 �C, 24 h.

This journal is ª The Royal Society of Chemistry 2009

antimicrobial activities.29 Besides, Navarro et al. have recently

reported that the toxicity of AgNPs appeared to be much higher

than that of AgNO3 as a function of the Ag+ concentration.30

Hence, the elution of Ag+ ions from and the nanomorphology of

AgNPs would synergistically contribute to the excellent anti-

bacterial activity of AgNPs@ZnO paper.

Fig. 9 compares antibacterial behavior for repeated use of

AgNPs@ZnO paper, AgNO3-impregnated paper and Ag

powder-containing paper. Initially AgNPs@ZnO paper was

clearly superior to the other paper composites. In all of the test

cases, the diameter of the zone of inhibition gradually decreased

in successive test cycles. The Ag contents also decreased due to

gradual release from each paper composite during the antibac-

terial tests, in accordance with the decrease in antibacterial

effects. However, although the final Ag contents after the five-

cycle test were very similar, namely 0.2, 0.2 and 0.3 mg for

AgNPs@ZnO paper, AgNO3-impregnated paper and Ag

powder-containing paper, respectively, AgNPs@ZnO paper

maintained considerably higher antibacterial activity than the

other samples. For AgNPs@ZnO paper the diameter of the zone

of inhibition was ca. 17 mm after the fifth cycle, thus almost the

same as those for fresh AgNO3-impregnated paper and Ag

powder-containing paper. Such good durability of performance

of AgNPs@ZnO paper can possibly be attributed both to the

nanometer size and to the exposure of an active crystal face of

AgNPs, leading to efficient antibacterial activity. Thus, the

AgNPs@ZnO paper with paper-like flexibility and convenience

in handling is expected to be a promising bioactive material.

Conclusions

Bioactive AgNPs were successfully synthesized in situ on ZnO

whiskers as a selective support, which were pre-incorporated into

a ceramic paper matrix, via ion-exchange between Ag and Zn

species and simultaneous ZnO-mediated photo-reduction. The

easily fabricated AgNPs@ZnO paper demonstrates excellent

antibacterial activity and durability against the bacterium E. coli.

J. Mater. Chem., 2009, 19, 2135–2140 | 2139

The inorganic AgNPs@ZnO paper composites with a paper-like

porous structure and practical utility are expected to be prom-

ising antibacterial materials, and this novel technique has great

potential for wide applications in the ‘on-paper’ synthesis of

other metal NPs.

Acknowledgements

This research was supported by a Research Fellowship for

Young Scientists from the Japan Society for the Promotion of

Science (H. K.). The authors sincerely thank Dr H. Ichinose for

his technical support with E. coli assay.

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