Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data

Plasmonic semiconductor nanoparticles showing nonlocal response

We predict that localized surface plasmons (LSP) in semiconductor particles exhibit spatial nonlocal response effects as the geometry enters the nanometer scale. To investigate these nonlocal effects, we first apply the hydrodynamic model (HDM) to nanospheres of two different semiconductor materials: intrinsic InSb and n-doped GaAs. Our results show that the semiconductors indeed display nonlocal effects, and that these effects are even more pronounced than in metals, and more tunable as well. We also present a two-fluid hydrodynamic model for semiconductors containing electrons and holes (from thermal or external excitation) or light and heavy holes (in p-doped materials). The two-fluid model predicts the existence of two longitudinal modes, an acoustic and an optical, whereas only an optical mode is present in the HDM. By extending nonlocal Mie theory to two plasmas, we simulate the optical properties of two-fluid nanospheres and predict that the acoustic mode gives rise to peaks in the extinction spectra that are absent in the HDM. And from a numerical study, we predict that by considering dimers rather than monomers of nanowires, the extinction cross section and field enhancement of the acoustic localized surface plasmon resonances can increase substantially. In this conference proceedings, we present calculations of the two-fluid GNOR model, which show that acoustic surface plasmon modes are surprisingly robust against size-dependent broadening

X ray microcopy of polymer photonic structures.

The last decades were beneficial for nanofabrication techniques but validation of the increasingly complex nano-structures became more challenging. Recent advancements of X-ray tomography permit 3D reconstruction down to 20 nm resolution. We propose the use of this non-invasive technique to quantify the fabrication quality of 3D structures made with direct laser writing (DLW). The comparison of X-ray density function with the designed fabrication density map brings into light information of how polymerization takes place throughout the fabricated volume and also gives the opportunity to estimate the impact on the optical response of such media