Optical method for sensing surface coverage by unicellular organisms

Abstract

A theoretical and experimental assessment of an optical technique for the rapid quantification of the density of biological cells adsorbed on a flat surface, as well as for monitoring in real time the surface coverage is presented. The method relies on the high sensitivity of the optical reflectivity of a surface near grazing incidence to the presence of particles large compared to the wavelength of light, a fact mostly unknown and not yet utilized in optical sensing. First, we perform a theoretical assessment of the method for sensing surface-coverage by biological-cell type particles. For this, we use a recently developed analytical model for the coherent reflection of light from a flat interface partially covered by a disordered monolayer of particles. Then, we investigate the viability of the technique experimentally with spores of Penicillium digitatum (PD) deposited on microscope slide. These fungus spores, along with those of Penicillium italicum (PI) are the main cause of losses in the citrus industry. We illustrate the possibility of monitoring in real time, the detachment of spores from a surface by an air flow. Experimental measurements are found to follow qualitatively theoretical predictions, showing that the theoretical model can be used for general sensor design purposes. From the experimental tests performed, we conclude that resolutions better than 1% in surface coverage of a flat surface by unicellular organisms are achievable without difficulty.