Journal of Nanoparticle Research

, Volume 11, Issue 5, pp 1087–1098

Microdistribution of copper-carbonate and iron oxide nanoparticles in treated wood

Authors

    • Forestry and Forest Products Research Institute
  • Makoto Kiguchi
    • Forestry and Forest Products Research Institute
  • Philip D. Evans
    • Centre for Advanced Wood ProcessingUniversity of British Columbia
Research Paper

DOI: 10.1007/s11051-008-9512-y

Cite this article as:
Matsunaga, H., Kiguchi, M. & Evans, P.D. J Nanopart Res (2009) 11: 1087. doi:10.1007/s11051-008-9512-y
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Abstract

Aqueous dispersions of copper-carbonate nanoparticles and microparticles have just begun to be exploited commercially for the preservative treatment of wood. The success of the new systems will depend, in part, on the uniform distribution of the preservative in wood and the ability of copper to penetrate cell walls. We examined the distribution of copper in wood treated with a nano-Cu preservative. Copper particles are not uniformly distributed in treated wood, but they accumulate in voids that act as the flow paths for liquids in wood. Particles are deposited on, but not within cell walls. Nevertheless, elemental copper is present within cell walls, but at a lower level than that in wood treated with a conventional wood preservative. These findings suggest that nano-Cu preservatives are able to deliver bioactive components into wood cell walls even though the majority of copper particles are too large to penetrate the cell wall’s nanocapillary network.

Keywords

Copper carbonateNanoparticlesMicroparticlesWoodPreservativeMicro-distributionSEMEDX

Introduction

Nanoparticles of copper-carbonate and iron oxide in aqueous systems have just begun to be exploited commercially for the preservative treatment of wood (Leach and Zhang 2004, 2005; Chen et al. 2006). In this application, preservatives containing nanoparticles compete with aqueous wood preservatives that contain dissolved or complexed copper compounds (Preston 2000; Evans 2003). Seventy-nine thousand tons of copper salts and oxides were used in North America in 2004 for the manufacture of wood preservatives (Vlosky 2006). Hence, wood preservatives have the potential to become one of the largest end uses for nanoparticles. The successful exploitation of nanoparticles in wood preservatives will depend on the ability of the new treatments to match the performance of current aqueous systems. Previous studies have demonstrated that the effectiveness of wood preservatives is related to their uniform distribution in wood and capacity of bioactive components such as copper to penetrate cell walls and react with cellulose, hemicellulose and lignin, the polymeric structural components of wood (Arsenault 1973; Hulme and Butcher 1977, Drysdale et al. 1980; Zhang and Kamdem 2000, Matsunaga et al. 2000, 2001, 2002, 2004). Lignified wood cell walls contain a nanocapillary network that is 1–10 nm in size (Kerr and Goring 1975; Flournoy et al. 1991; Fujino and Itoh 1998), but it is not known whether cell walls can be penetrated by nanoparticles. Furthermore, preservatives containing nanoparticles are so new that there have been no previous studies of their microdistribution in treated wood. The possibility exists, however, that their microdistribution will differ from that of wood treated with conventional aqueous preservatives because some of the particles may not be able to penetrate wood cell walls, unlike copper ions in conventional aqueous preservatives.

In this study we compared the microdistribution and deposition characteristics of wood treated with a preservative containing copper-carbonate and iron oxide nanoparticles and microparticles with that of wood treated with the conventional copper-based wood preservative, ACQ (alkaline copper quaternary), which accounted for approximately half of the total amount of copper-based preservatives used in North America in 2004 (Vlosky 2006). Conventional scanning electron microscopy in combination with energy dispersive analysis of X-rays (EDX) was used to examine and map the deposition of copper nanoparticles and microparticles within the porous microstructure of wood. The level of copper in the cell walls of thick-walled latewood fibres was quantified using point analysis of X-rays. High-resolution mapping of particles required the use of a field-emission-scanning electron microscope (FE-SEM) in combination with EDX and drift-correction software. Our findings are discussed in terms of their relevance to the performance and environmental impact of wood treated with preservatives containing nanoparticles.

Materials and methods

Preparation of wood samples

Four air-dried Southern pine (Pinus sp.) boards, 38 mm (thickness) × 140 mm (width) × 1500 mm (length) in size, that had been commercially pressure treated with an aqueous dispersion of copper carbonate and iron oxide particles, hereafter referred to as nano-Cu preservative, were purchased commercially. The sizes of the copper particles in the nano-Cu preservative have a Gaussian distribution from 1 to 25,000 nm, and a mean of 190 nm (Leach and Zhang 2004, 2005). A similar number of Southern pine boards that had been treated with the conventional amine-modified aqueous water-borne wood preservative ACQ were purchased at the same time. A strip measuring 38 × 140 × 10 mm was sawn from the centre of each board. Small specimens measuring 10 × 10 × 1.5 mm, and located 5 or 17 mm from the radial and tangential surfaces of the treated boards were cut from each board. Hence the total number of specimens that was prepared was 16 (4 boards × 2 treatments × 2 specimens per treatment). These specimens were either split using a single-edged razor blade to expose their radial surfaces, or cut in the dry condition using a sledge microtome to reveal their transverse surfaces.

Particle deposition and micro-scale mapping

Uncoated specimens were attached to aluminium stubs using double-sided tape and examined using a Hitachi S-2600N variable pressure SEM (VP-SEM) equipped with a XOne energy dispersive X-ray detector (EDX). Specimens were examined at a pressure of 10 Pa, using an accelerating voltage of 15 kV, an illuminating current of 0.3 nA, a working distance of 15 mm, and take-off and tilt angles of 20º and 0º, respectively. The contrast between copper nanoparticles and wood was greatest when images were formed from backscattered electrons and therefore these were used in preference to secondary electron images when examining the deposition of nanoparticles in the porous structure of wood. A second set of treated specimens was coated with carbon and examined using a high-vacuum Jeol JSM-5600LV SEM equipped with a Jeol JED-2140 EDX. This microscope was used to map the distribution of copper and iron in treated wood samples, and employed an accelerating voltage of 15 kV, an illuminating current of 1.5 nA, a working distance of 20 mm, take off and tilt angles of 30º and 0º, respectively, and dead and dwell (digital map) times of 15–20% and 0.2 ms/point, respectively. The elemental peaks detected were Fe-Kα and Cu-Kα using window widths of 6.33–6.48 keV and 7.96–8.13 keV, respectively.

High-resolution mapping

Specimens containing prepared transverse and radial faces were attached to aluminium stubs and coated with iridium. A Hitachi S4800 FE-SEM equipped with an Apollo EDX system was used to map the distribution of copper at high resolution and differentiate copper from iron particles. Images were viewed in backscattered electron mode and a YAG detector and/or upper detector were used to capture backscattered electrons. The operating conditions were as follows: accelerating voltage, 1, 5 or 15 kV; illuminating current of 0.7 nA; working distance, 15 mm; take off angle, 30º; tilt angle, 0º; dwell time (digital map), 0.2 ms/point; pixels, 256 × 200. The elemental peaks detected were Fe-Lα and Cu-Lα using window widths of 0.650–0.740 keV and 0.880–0.970 keV, respectively. Drift correction system and software were used to help prepare sharp high resolutions maps of copper and iron particles.

Quantitative analysis of copper concentration in wood samples

The concentration of elemental copper in the latewood of wood samples subjected to EDX analyses was subsequently analyzed using inductively coupled plasma (ICP) atomic emission spectroscopy. Latewood samples from the eight boards and two specimens were digested separately in a mixture of HNO3/H2O2 as described by Haraguchi (1992). The resulting solution was filtered through a 0.45-μm disk filter unit. The copper content of the resulting filtrate was analyzed using a Perkin-Elmer Optima 4300DV ICP atomic emission spectrometer operating at a wavelength of 324.752 nm. Copper concentration was expressed as CuO/dry weight of sample (g/kg).

Levels of copper in cell walls of latewood fibres

The level of copper in cell wall layers of latewood fibres was obtained by 200 live second point analyses using the EDX system attached to the HITACHI S-2600N VP-SEM (mentioned above). The levels of copper are expressed as the ratio of the Cu-Kα peak to background obtained using SEM–EDX. The experiment was designed to examine the effects of the two preservatives, sample depth (5 vs. 17 mm from treated surface) and cell wall layer (secondary wall vs. the interfacial layer between cells, the middle lamella (ML)) on the level of copper in cell walls. Figure 1 shows the location of the point analyses and presents representative spectra obtained from the secondary cell wall layer and the ML. The eight different boards (four for each preservative treatment) provided replication at the higher level. Total replication was 320 (4 boards for each of the 2 treatments (8) × 2 treatment depths × 10 cells × 2 analyses per cell, one for each wall layer).
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Fig. 1

Backscattered electron image of Southern pine treated with a preservative containing copper and iron nanoparticles and microparticles. (A), image of thick walled latewood fibres showing location of point analyses in (a) secondary cell wall and (b) the interfacial layer between cells, the middle lamella. Shown below are EDX spectra for secondary wall (S2) and middle lamella (ML)

Statistical analysis of copper in cell walls

Statistical analysis of the levels of copper in the cell wall layers of latewood fibres in wood treated with the nano-Cu preservative or ACQ was performed using analysis of variance for a balanced hierarchical design. The analysis examined the effect of preservative type, treatment depth, cell wall layers and random factors (variation between boards, specimens and cells) on the level of copper in wood cell walls. We also performed a second analysis of variance using concentration of copper in latewood as a covariate, because it is reasonable to assume that levels of copper in the cell walls of latewood fibres, obtained using EDX, will be related to the overall concentration of copper in latewood. Statistical computation was performed using Genstat 5 (Genstat 2000). Before the final analysis, diagnostic checks were performed to determine whether data conformed to the underlying assumptions of analysis of variance, i.e. normality with constant variance. Significant results (p < 0.05) are presented graphically and least significant difference (error) bars (p < 0.05) on graphs can be used to compare differences between individual means.

Results and discussion

Deposition of nanoparticles in the porous microstructure of wood

The porous microstructure of wood is optimized to transport fluids longitudinally up the tree stem and, to a lesser extent, radially across the stem. Conduction of fluids in the longitudinal direction in conifer wood occurs in hollow fibres (~20–40 μm in diameter and 2–4 mm in length) that are joined end-to-end and connected by valve-like structures (pits) that allow fluids to pass from one fibre to another (Siau 1984). Conduction of fluids in the radial direction occurs in structures called rays that are aggregates of parenchyma cells that also contain connecting pits (Siau 1984). These flow paths that facilitate fluid flow in trees are essentially the same ones that allow preservatives to penetrate wood’s microstructure (Nicholas and Siau 1973). SEM photomicrographs of transverse sections of wood treated with the nano-Cu preservative revealed the presence of white deposits in the lumens of some fibres (Fig. 2a, b), particularly near large resin canals (Fig. 2a, b), which are an important additional flow path in pine (Wardrop and Davies 1961). These deposits were also prominent in the rays, running from top to bottom in Fig. 2a, but were absent from wood treated with the conventional wood preservative, ACQ (Fig. 2c). Rays and pits can be most easily observed at radial longitudinal surfaces and examination of such surfaces in wood treated with the nano-Cu preservative confirmed the presence of white deposits in the rays, particularly on the pits connecting ray cells (Fig. 3a). These deposits were also concentrated on the bordered pits that connect fibres (Fig. 3b, arrowed), and they were also occasionally observed within the fibres themselves, where they were located on the tertiary wall layer adjacent to cell lumens (Fig. 3c, arrowed). The deposits appeared to be absent from subsurface wall layers exposed during specimen preparation (Fig. 3c). All of these deposits contrasted strongly with the wood cells walls when they were viewed using backscattered electron imaging. Backscattered electrons increase with increasing atomic number and this is the basis for atomic number contrast (also called compositional contrast or Z contrast). The strong contrast between the deposits and the wood cell wall, which has an elemental composition of 53% C, 40% O, 6.5% H and 0.5% N, strongly suggested that the deposits consisted of inorganic elements. EDX mapping confirmed that the deposits contained copper, as expected (Fig. 4). Mapping also revealed iron (Fig. 4b) in the same regions of the wood microstructure as copper (Fig. 4c), but it was not possible to differentiate between the two elements within the wood microstructure using conventional SEM–EDX.
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Fig. 2

Backscattered electron images of transverse surfaces of wood treated with preservative containing copper and iron nanoparticles and microparticles (a, b) and the conventional preservative ACQ (c). Latewood fibres in the centre of (a) contrast with the band of thinner walled earlywood fibres at the top of the photomicrograph. Note inorganic deposits in (a), particularly in lumens of latewood fibres and epithelial cells around a resin canal in latewood (b)

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Fig. 3

Backscattered electron images of radial longitudinal surfaces from wood treated with preservative containing copper and iron nanoparticles and microparticles. Note heavy deposits of inorganic material in (a) rays; (b) bordered pits (arrowed); and c lumens of fibres (arrowed)

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Fig. 4

Conventional SEM-EDX colour mapping of a radial section of southern pine treated with copper and iron nanoparticles and microparticles: (a) Backscattered electron image; (b) Fe-Kα X-ray image; (c) Cu-Kα X-ray image. Both iron and copper were detected on bordered pits (left of centre) and in rays (top right)

High-resolution FE-SEM revealed more clearly the deposition of copper and iron nano and microparticles in treated wood and also their sizes and shapes. The particles formed aggregates on or around pits that connect fibres with rays, but they were not able to penetrate through such pits (Fig. 5a). The nanoparticles also aggregated in the chamber of the bordered pits that connect fibres (Fig. 5b), but in contrast to the ray pits, the nanoparticles appeared to be able to pass through bordered pits (Fig. 5b, c). The micro and nanoparticles were rectangular in shape and varied in size from ~50 to 700 nm (Fig. 6a, b). The average diameter of the openings in the membrane of bordered pits in pine and other coniferous species is ~300 nm (Petty 1970; Ohgoshi et al. 1982a, b) and hence the openings are large enough to allow some of the copper and iron nanoparticles observed here to pass through the pits. Furthermore, during the pressure treatment of wood with preservatives, pit membranes are often ruptured and the effective diameter of the pit then becomes that of the pit opening, which is ~4000 nm in pine (Siau 1984). In such cases, some of the micron-sized Cu and Fe particles from the nano-Cu preservative could penetrate the porous microstructure.
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Fig. 5

Backscattered FESEM images showing: (a) particles aggregating on pits in rays; and (b, c) within pits that connect fibres

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Fig. 6

Backscattered FESEM images showing heterogeneous mix of nanoparticles and microparticles (a, b) and their presence on the wood cell wall

High-resolution mapping of copper and iron

Field-emission-scanning electron microscope in combination with EDX was used to differentiate between copper and iron nanoparticles and help to determine if nanoparticles could penetrate wood cell walls. Figure 7a is a backscattered electron image of a bordered pit chamber containing numerous inorganic deposits. Figures 7b, c are digital colour EDX maps of iron and copper, respectively, in the same pit chamber. From these maps, it is apparent that iron and copper existed as separate particles. Copper appeared to show a greater tendency to aggregate than iron.
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Fig. 7

Field Emission SEM-EDX colour mapping of a pit showing the aggregation of copper and iron nanoparticles and microparticles: (a) backscattered electron image; (b) Fe-Lα X-ray image; (c) Cu-Lα X-ray image

Backscattered electrons that are detected during scanning electron microscopy are formed deeper within specimens than secondary electrons. When higher-accelerating voltages, are used, backscattered electrons can provide information on the subsurface (~100 nm) structure of materials depending upon their atomic number (Goldstein et al. 2003). This has been used previously to reveal the structural architecture of the phosphorus and potassium skeleton in plankton (Suzuki et al. 1995, Suzuki and Toda 1996, Suzuki et al. 1998, Kasahara et al. 1999, Matsugo et al. 1999). Therefore, if nanoparticles are able to penetrate wood cell walls, then it may be possible to see them within walls in images formed from backscattered electrons generated using high-accelerating voltages and beam currents. Figure 8 is an image of a cell wall of a latewood fibre in a treated wood sample formed from backscattered electrons using a high-accelerating voltage (15 kV) and a large beam current. The resolution of the FE-SEM used to obtain this image is at least ~10–20 nm, however, there was no obvious indication of the presence of heavy element (iron or copper) particles in this cell wall or any of the others that were examined. Checks in the cell wall in Fig. 8 do not appear to have provided a means for copper and iron particles to penetrate the cell wall, possibly because the checks developed after the wood was treated with the nano-Cu preservative.
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Fig. 8

Backscattered electron image of a cell wall of wood treated with nanoparticles and microparticles. Image was obtained using a FESEM at 15 kV

Point analysis of copper in cell walls of latewood fibres

Point analysis using EDX was used to determine if copper was present in cell walls of latewood fibres in wood treated with the nano-Cu preservative. A full factorial experiment examined the effects of preservative type, sampling depth and cell wall layer on the level of copper in wood cell walls. One might expect that some of the variability in cell wall copper would be explained by the overall retention of copper in the different samples, as well as by the aforementioned experimental variables. We explicitly took account of this by using sample concentration of copper in latewood as a continuous explanatory variable (covariate) in the analysis of variance that was used to examine the effects of experimental factors and random effects on cell wall copper (peak to background ratio). ICP–atomic emission spectroscopy of latewood samples treated with the nano-Cu preservative and ACQ revealed that the average concentration of copper was lower in the former (2.32 g/kg) than in those treated with ACQ (4.67 g/kg). Accordingly, analysis of variance of levels of cell wall copper in samples treated with the two types of preservatives showed that the level of copper in latewood cell walls was significantly (p = 0.036) lower in wood treated with the nano-Cu preservative than in wood treated with ACQ (Fig. 9a). Differences in the levels of copper in the cell wall of latewood fibres in wood treated with the two preservatives can be compared using the least significant difference (error) bar in Fig. 9a. This bar is quite large reflecting the underlying variability of data, possibly due to the use of unmatched commercially purchased samples. We also performed an analysis of variance using concentration of copper in latewood as a covariate. This analysis showed that there was no significant difference (p = 0.95) in the level of copper in latewood cell walls in samples treated with the two preservatives. The levels of copper in the interfacial zone between cell walls (ML) was greater than in the secondary cell wall layer (S2) in samples treated with the nano-Cu preservative or ACQ (Fig. 9b), but the difference was significantly greater in the latter. This difference was independent of the overall concentration of copper in latewood.
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Fig. 9

Comparison of copper levels in cell walls of latewood fibres in wood treated with preservative containing copper and iron micro and nanoparticles (N-Cu) and the conventional preservative ACQ: (a) levels of copper in cell walls; (b) levels of copper in different cell wall layers (middle lamella, ML, and secondary wall, S2). Levels of copper were determined by EDX and the statistical significance of differences between means can be compared using the LSD (error) bars

Clearly, our findings indicated that copper was present in the cell walls of latewood fibres in wood treated with the nano-Cu preservative. Cell walls in wood contain a nanocapillary network that can accommodate silver particles that are 80 nm in size (Rudman 1966). Particles of this size, however, were generated in vivo from ionic silver by impregnating wood with silver nitrate, followed by a second solution that precipitated silver in cell walls (Rudman 1966). Hence, this previous research does not demonstrate that nanoparticles of this size can penetrate wood cell walls. For the latter to occur, they would need to be small enough to pass through the wood cell walls nanocapillary network. Estimates of the size of the capillaries in this network vary from 1.2 nm for wood that has previously been dried to a maximum of 10 nm in wet (never dried) ‘green’ wood (Tarkow et al. 1966; Stone and Scallan 1968; Kerr and Goring 1975; Grethlein 1985; Flournoy et al. 1991). The wood that was analyzed here was dried before preservative treatment and, therefore, we may assume that the sizes of the nanocapillaries were at the lower end of previous estimates. Flournoy et al. (1991) used a series of molecular probes with diameters ranging from 0.4 to 61 nm to determine the accessibility and diameters of the cell wall’s nanocapillary network in wood that had been dried. Their results indicated that molecules larger than 1.2 nm were excluded from 80% of the cell wall. Nevertheless, it is possible that penetration of cell walls by nanoparticles smaller than 1 nm, which was well beyond the resolution of the FE-SEM used here, could account for the presence of some copper in cell walls of treated wood. Alternatively, the copper could have penetrated cell walls as Cu2+ ions, which have a diameter of ~0.087 nm (Martišius and Giraitis 2006). Numerous studies have shown that wood cell walls are accessible to ionic copper, which can then bind to wood’s molecular components (Petty and Preston 1968; Chou et al. 1973; Greaves 1974; Zhang and Kamdem 2000; Petrič et al. 2000; Matsunaga et al. 2004; Cao and Kamdem 2005). Copper carbonate, which was used in the nano-Cu wood preservative here, is insoluble in water. Nanoparticles of copper, however, are more readily solubilised than microparticles (Xia et al. 2006). Hence, it is possible that conversion of copper carbonate nanoparticles into Cu2+ ions could have produced sufficient ionic copper to account for the concentration and distribution of copper in cell walls of treated wood examined here. Further research, however, would be needed to confirm this, and also whether differences in the distribution and availability of copper in nano-Cu treated wood affects its resistance to attack by Basidiomycota, Ascomycota and Fungi Imperfecti, the fungi most commonly responsible for wood decay (Preston et al. 2008; Stirling et al. 2008).

The use of nanoparticles for the preservation of wood is in its infancy and the commercial systems consist of a heterogeneous mix of nanoparticles and microparticles. The size of the particles in the preservatives varies from 1 nm to 25 μm (Leach and Zhang 2004, 2005). The patent literature mentions the desirability of having a more truncated size distribution from 5 nm to 10 μm or even more preferably from 50 nm to 1 μm (Leach and Zhang 2005). The exclusion of large micro-particles (>1 μm) from the preservatives is mentioned as being desirable because they can block flow paths and prevent the penetration and uniform distribution of the preservative in the wood (Leach and Zhang 2005). However, there is little other published information on the effect of particle size and its distribution on the characteristics of the nano-Cu treated wood or its performance. Clearly, there is great potential for further optimization of the systems to improve their efficacy. Reduction in particle size could help cell wall penetration as suggested by our findings, but the presence of larger particles, which could provide a reservoir of active compounds, might also be desirable, because wood preservatives are required to prevent decay for very long periods of time (>30 years). Over such periods of time, there is the potential for wood preservatives to leach into the environment (Waldron et al. 2005), and this has led to restrictions on the use of many wood preservatives (Evans 2003). Concern has also been expressed about the leaching of nanoparticles from materials, and the potential adverse environmental effects resulting from the accumulation of nanoparticles in groundwater or soil, and also their impacts on the health of people who are exposed to them (Borm et al. 2006). Colvin (2003) recommended a proactive approach to understanding the environmental impacts of nanomaterials. This recommendation is particularly germane to wood containing nanoparticles because, as mentioned above, treated wood could become a very significant end use for nanoparticles and, furthermore, there are manifold opportunities during the use of treated wood for the release of nanoparticles into the environment.

Conclusions

Aqueous dispersions of copper carbonate and iron oxide nanoparticles and microparticles impregnated into wood under pressure are deposited in the void spaces, which act as the main flow paths for liquids in wood. The particles were rectangular in shape and varied in size from ~50 to 700 nm. The size of the particles and their propensity to aggregate in pits that connect cellular elements in wood and on cell walls suggested that they were too big to penetrate wood cell walls. Furthermore, we were unable to observe nanoparticles in wood cell walls using an FE-SEM with a resolution of at least ~10–20 nm. Nevertheless, we detected elemental copper in cell walls of latewood fibres in wood treated with the nano-Cu preservative, but its concentration was lower than that in wood treated with the conventional wood preservative ACQ. This difference may be explained by the lower overall concentration of copper in the latewood of wood samples treated with the nano-Cu preservative compared to those treated with ACQ. In both cases, however, the concentration of copper was higher at the interface between cells than in the secondary wall, although the difference was more pronounced in samples treated with ACQ. It is possible that conversion of copper carbonate nanoparticles into ionic copper may explain the presence and micro-distribution of copper in cell walls of wood treated with the nano-Cu preservative. Such a mechanism for the delivery of copper into wood cell walls could be exploited more generally in nanoparticulate wood preservative systems by changing the size of the particles to obtain more controlled release of bioactive agents. This would be desirable because wood preservatives need to remain biologically active for long periods of time, while at the same time minimizing leaching of biocides into the environment.

Acknowledgements

The authors gratefully acknowledge the expert technical assistance of Derrick Horne (BioImaging Facility, UBC) and Bill Roth (Hitachi High Technologies America, Pleasanton, CA). We are very grateful for the financial support for this research provided by The OECD (Award of a Co-operative Research Program Fellowship to HM), Canadian Foundation for Innovation, BC Knowledge Development Fund and Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant-in-Aid for Scientific Research no. 19780138). We would like to thank Dr. Kazuyuki Oda and Dr. Junji Matsumura for their suggestions and Dr. Yuko Ito for ICP analysis.

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