This study tests the hypothesis that copper nanoparticles can penetrate the cell walls of southern pine wood treated with a micronised preservative. We examined the nanodistribution of particles in tracheid and ray parenchyma cell walls using state-of-the-art HR (High Resolution)-TEM and HR-STEM (Scanning Transmission Electron Microscope)-EDX. These devices are capable of atomic-scale resolution. FIB (Focused Ion Beam) processing was used to make ultra-thin sections for electron microscopy. Our results show that FIB sectioning in combination with HR-TEM and HR-STEM is a powerful tool for observing the penetration of wood cell walls by nanoparticles or clusters of metal atoms. HR-STEM with a Cs corrector revealed that copper carbonate nanoparticles could not penetrate the cell walls of tracheids. Copper, however, is present in cell walls of tracheids as atoms or ions. In contrast, small copper nanoparticles (2.5 nm in diameter) were able to penetrate ray parenchyma cell walls. These particles were identified as copper carbonate by HR-TEM lattice image analysis. We conclude that the cell walls of unlignified ray parenchyma tissue in southern pine are accessible to copper nanoparticles whereas nanoparticles are excluded from lignified tracheid walls.

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