PCR-Free Innovative Strategies for SARS-CoV-2 Detection.

The pandemic outbreak caused by SARS-CoV-2 coronavirus brought a crucial issue in public health causing up to now more than 600 million infected people and 6.5 million deaths. Conventional diagnostic methods are based on quantitative reverse transcription polymerase chain reaction (RT-qPCR assay) and immuno-detection (ELISA assay). However, despite these techniques have the advantages of being standardized and consolidated, they keep some main limitations in terms of accuracy (immunoassays), time/cost consumption of analysis, the need for qualified personnel, and lab constrain (molecular assays). There is crucial the need to develop new diagnostic approaches for accurate, fast and portable viral detection and quantification. Among these, PCR-free biosensors represent the most appealing solution since they can allow molecular detection without the complexity of the PCR. This will enable the possibility to be integrated in portable and low-cost systems for massive and decentralized screening of SARS-CoV-2 in a point-of-care (PoC) format, pointing to achieve a performant identification and control of infection. In this review, the most recent approaches for the SARS-CoV-2 PCR-free detection are reported, describing both the instrumental and methodological features, and highlighting their suitability for a PoC application.

Visualization of Directional Beaming of Weakly Localized Raman from a Random Network of Silicon Nanowires.

Disordered optical media are an emerging class of materials that can strongly scatter light. These materials are useful to investigate light transport phenomena and for applications in imaging, sensing and energy storage. While coherent light can be generated using such materials, its directional emission is typically hampered by their strong scattering nature. Here, the authors directly image Rayleigh scattering, photoluminescence and weakly localized Raman light from a random network of silicon nanowires via real-space microscopy and Fourier imaging. Direct imaging enables us to gain insight on the light transport mechanisms in the random material, to visualize its weak localization length and to demonstrate out-of-plane beaming of the scattered coherent Raman light. The direct visualization of coherent light beaming in such random networks of silicon nanowires offers novel opportunities for fundamental studies of light propagation in disordered media. It also opens venues for the development of next generation optical devices based on disordered structures, such as sensors, light sources, and optical switches.

Optical Trapping, Optical Binding, and Rotational Dynamics of Silicon Nanowires in Counter-Propagating Beams.

Silicon nanowires are held and manipulated in controlled optical traps based on counter-propagating beams focused by low numerical aperture lenses. The double-beam configuration compensates light scattering forces enabling an in-depth investigation of the rich dynamics of trapped nanowires that are prone to both optical and hydrodynamic interactions. Several polarization configurations are used, allowing the observation of optical binding with different stable structure as well as the transfer of spin and orbital momentum of light to the trapped silicon nanowires. Accurate modeling based on Brownian dynamics simulations with appropriate optical and hydrodynamic coupling confirms that this rich scenario is crucially dependent on the non-spherical shape of the nanowires. Such an increased level of optical control of multiparticle structure and dynamics open perspectives for nanofluidics and multi-component light-driven nanomachines.