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Highlights • A novel methodology using grazing-incidence optical reflectivity as a sensor, to detect and quantify unicellular organisms. • Optical technique for the rapid evaluation of the density of biological cells adsorbed on a flat and transparent surface. • Possibility for monitoring in real time the detachment of spores from a surface by an air. 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. [ABSTRACT FROM AUTHOR]

• Optical determination of gold-nanofilms thickness. • Adhesive layer and nanodefects analysis. • Measurement of the internal TM reflectivity as a function of the angle of incidence. We propose and investigate an alternative optical method to determine the thickness of metallic nanofilms thinner than about 50 nm deposited on glass substrates. The method consists of measuring the internal TM reflectivity as a function of the angle of incidence and fitting the curve with Fresnel formulae for a bilayer system to find the film thickness. Gold nanofilms were fabricated by electron beam evaporation (nominal thicknesses of 20 nm, 30 nm and 50 nm) and characterized by common methods (SEM, AFM, Profilometer). The reflectivity curve of TM (or p) polarized light presents a clear peak around the critical angle with air and then a dip due to the excitation of a surface plasmon-polariton mode at the gold/air interface. Fitting the theoretical model around the reflectivity peak readily gives the film's thickness, and fitting other sections of the curve away from the critical angle with air without altering the fit around the peak, provides the thickness and effective refractive index of the adhesive layer (Cr, Ti). The present approach to gold nanofilm's thickness measurement is robust and expeditious. It is less sensitive to surface roughness, contamination and nanofabrication defects such as nanometer sized holes. [ABSTRACT FROM AUTHOR]

The aim of this study was to trace contacts of coronavirus disease 2019 (COVID-19) hospitalised patients and determine the risk factors of infection in urban areas. Longitudinal analysis of contacts identified from index cases. A contact tracing study was carried out in the Northern Metropolitan area of Barcelona, Spain, during the inter-epidemic lapse of May to July 2020, a period of low SARS-CoV-2 incidence. Index cases were notified from the referral hospital. Contacts were traced and followed up for 14 days. Reverse transcription polymerase chain reaction was performed on day 0 and day 14 for contacts. In total, 368 contacts were identified from 81 index cases (median of seven contacts per index case), from which 308 were traced successfully. The median age of contacts was 28 years, 62% (223 of 368) were men. During the follow-up period, 100 contacts tested positive for COVID-19 (32.5% [95% confidence interval {CI} = 27.3–38.0]), with a secondary infection rate of 48.3% (95% CI = 40.8–55.9) among housemates. Clusters of index and respective contacts tended to aggregate within disadvantaged neighbourhoods (P < 0.001), and non-national index cases (N = 28, 34.1%) resulted in higher secondary infection rates compared with nationals (51.0% [95% CI = 41.0–60.9] vs 22.3% [95% CI = 16.8–28.8]; P < 0.001). Disadvantaged communities experience a disproportionate burden of COVID-19 and may act as infection reservoirs. Contact tracing with a cross-cutting approach among these communities is required, especially during inter-epidemic periods. [ABSTRACT FROM AUTHOR]