Probability of aerosol transmission of SARS-CoV-2

Transmission of SARS-CoV-2 leading to COVID-19 occurs through exhaled respiratory droplets from infected humans. Currently, however, there is much controversy over whether respiratory aerosol microdroplets play an important role as a route of transmission. By measuring and modeling the dynamics of exhaled respiratory droplets we can assess the relative contribution of aerosols in the spreading of SARS-CoV-2. We measure size distribution, total numbers and volumes of respiratory droplets, including aerosols, by speaking and coughing from healthy subjects. Dynamic modelling of exhaled respiratory droplets allows to account for aerosol persistence times in confined public spaces. The probability of infection by inhalation of aerosols when breathing in the same space can then be estimated using current estimates of viral load and infectivity of SARS-CoV-2. In line with the current known reproduction numbers, our study of transmission of SARS-CoV-2 suggests that aerosol transmission is an inefficient route, in particular from non or mildly symptomatic individuals.

Tuning the oriented deposition of gold nanorods on patterned substrates.

The controlled patterning of anisotropic gold nanoparticles is of crucial importance for many applications related to their optical properties. In this paper, we report that gold nanorods prepared by a seed-mediated synthesis protocol (without any further functionalization) can be selectively deposited on hydrophilic parts of hydrophobic-hydrophilic contrast patterned substrates. We have seen that, when nanorods with lengths much smaller than the width of the hydrophilic stripe are used, they disperse on these stripes with random orientation and tunable uniform particle separation. However, for nanorods having lengths comparable to the width of the hydrophilic stripes, confinement-induced alignment occurs. We observe that different interactions governing the assembly forces can be modulated by controlling the concentration of assembling nanorods and the width of the hydrophilic stripes, leading to markedly different degrees of alignment. Our strategy can be replicated for other anisotropic nanoparticles to produce well-controlled patterning of these nanoentities on surfaces.

Shape and size effects in the optical properties of metallic nanorods.

The influence of size and geometrical shape on the optical properties of randomly oriented metallic nanorods is investigated using the discrete dipole approximation (DDA). Our calculations provide a benchmark for an accurate characterisation of nanorod suspensions by frequently used optical spectroscopic techniques. Our DDA results confirm the longitudinal plasmon resonance to be primarily affected by the nanorod aspect ratio, and also verify that the quasi-static (dipole) approximation for ellipsoidal particles is only valid for very small sizes. For prolate ellipsoidal and cylindrical nanorods with an identical aspect ratio, the latter exhibit a longitudinal resonance at significantly longer wavelengths. The importance of phase retardation and multipole contributions for larger nanorod dimensions is discussed. Also, we investigate the influence on the optical spectra of electron surface scattering, which arises from the limited size of the nanorods in comparison to the electron mean free path.