Abstract
The spreading of phytosanitary inputs in fields comprise a significant part of preventive treatment. Indeed, the symptomatic dead spot on plant leaves only appears one or two weeks after primary infection. In order to reduce unnecessary use of hazardous chemistry, efforts for early detection of plant diseases are demanded. Scientific research can challenge the design of fungi spores micro-sensors, detecting the apparition of phytopathogens and monitoring the epidemic development of the disease. The sensor we propose is constructed of Ge-Se-Te chalcogenide RIB waveguide : the analytes falling on the detector surface interact with the evanescent field of the propagating light, and therefore alter it. The main difficulty of such a detection strategy is to provide selectivity. How could we immobilized pathogenic spores on the chalcogenide waveguides? How to detect spores among all particles from aerosol?To provide selectivity to the sensing system, we aimed to functionalize the Ge-Se-Te waveguides with a new surface chemistry providing high affinity towards fungi spores. We identified two routes of functionalization : with peptides and with organosilane precursors. Peptides bond the surface thanks to several weak interactions, while organosilane bind the Ge-Te-Se by forming a Te-O-Si covalent group. We demonstrated specific capture of the analyte thank to the biotin-streptavidin complex.The literature search indicates, the wettability as a key parameter for spores immobilization on artificial surfaces. Hydrophobic spores rather immobilize onto hydrophobic surfaces, such as plants leaves. Therefore, in a mimetic approach we used the silanization route to modify the surfaces, with two new precursors. Tetraethoxysilane provide hydrophilic behavior to the chalcogenide, whereas Octyl-trimethoxysilane provide hydrophobic behavior to the chalcogenide. We were able to demonstrate a higher rate of spores’ immobilization on the hydrophobic chalcogenide, as expected.To go further and increase sensitivity, we also propose a new strategy for selective functionalization of the waveguide surfaces. Indeed, not all of the spores are immobilized inside the evanescent field. We achieved a selective functionalization of the sensor components, between active and non-active areas. We’ve rendered more hydrophobic the waveguides than the surrounding non active area. The spores test immobilization demonstrates higher concentration near the waveguide, where the evanescent field is located, suggesting higher sensitivity.We conclude our study by the optical investigation of spores of Magnaporthe oryzae, also known as the rice blast fungus. The difficulty is that in free space, such micrometric fungi cells cause intense scattering in the near-infrared region (1-2.5µm). This phenomema, well explained by the Mie theory, could hinder the spore detection. Nevertheless, we still could demonstrate the spores detection with the chalcogenide waveguide and the optical set-up build for the near-infrared region (1-2.5µm), although, the optical signature of spores is not yet optimized. We further propose to work in the mid-infrared spectrum region (2.5-15µm), where numerous specific absorption events occur. Detection with such wavelengths should provide information enough for bio-particles identification.