Assessing cypermethrin penetration in Pinus sylvestris wood products by immuno-electron microscopy

The preservative efficacy of organic biocidal products is strongly related to their capacity of penetration and retention within wood tissues. The specific detection of the pyrethroid insecticide cypermethrin is currently obtained after extraction followed by chemical analysis by chromatography techniques. However, visualizing the insecticide molecule within the wood structure requires specific probes together with microscopy techniques. Therefore, the aim of the present work was to prepare a polyclonal antibody directed against cypermethrin and to implement it on Pinus sylvestris wood samples treated with technical cypermethrin. The antibody was tested on cypermethrin-impregnated wood, and the specific recognition of the insecticide was visualized by transmission electron microscopy (TEM). The immunogold-TEM assay evidenced the capacity of the synthetic biocide active substance to penetrate in the wood. The depth of penetration was measured on sections taken at increasing distances from the coated surface of the wood. Under the present conditions of application, cypermethrin was shown to penetrate up to the 6- to 9-mm zone below the surface. Such results correlated with chemical analyses carried out by GC-ECD after extraction. In addition, the immuno-TEM investigation allowed visualizing, for the first time at the ultrastructure scale of resolution, that cypermethrin was able to diffuse within the secondary wood cell walls. The scarce labeling of the compound middle lamella shows that the chemical does not diffuse freely in this part of the cell walls. The results suggest that the adsorption within the cell walls is an essential factor for the retention of cypermethrin and for its permanence in treated wood products.