N-Acetylcysteine effects on extracellular polymeric substances of Xylella fastidiosa: A spatiotemporal investigation with implications for biofilm disruption.

The matrix of extracellular polymeric substances (EPS) present in biofilms greatly amplifies the problem of bacterial infections, protecting bacteria against antimicrobial treatments and eventually leading to bacterial resistance. The need for alternative treatments that destroy the EPS matrix becomes evident. N-acetylcysteine (NAC) is one option that presents diverse effects against bacteria; however, the different mechanisms of action of NAC in biofilms have yet to be elucidated. In this work, we performed microscopy studies at micro and nano scales to address the effects of NAC at single cell level and early-stage biofilms of the Xylella fastidiosa phytopathogen. We show the physical effects of NAC on the adhesion surface and the different types of EPS, as well as the mechanical response of individual bacteria to NAC concentrations between 2 and 20 mg/mL. NAC modified the conditioning film on the substrate, broke down the soluble EPS, resulting in the release of adherent bacteria, decreased the volume of loosely bound EPS, and disrupted the biofilm matrix. Tightly bound EPS suffered structural alterations despite no solid evidence of its removal. In addition, bacterial force measurements upon NAC action performed with InP nanowire arrays showed an enhanced momentum transfer to the nanowires due to increased cell mobility resulting from EPS removal. Our results clearly show that conditioning film and soluble EPS play a key role in cell adhesion control and that NAC alters EPS structure, providing solid evidence that NAC actuates mainly on EPS removal, both at single cell and biofilm levels.

Wurtzite Gallium Phosphide via Chemical Beam Epitaxy: Impurity-Related Luminescence vs Growth Conditions.

The metastable wurtzite crystal phase in gallium phosphide (WZ GaP) is a relatively new structure with little available information about its emission properties compared to the most stable zinc-blend phase. Here, the effect of growth conditions of WZ GaP nano- and microstructures obtained via chemical beam epitaxy on the optical properties was studied using power- and temperature-dependent photoluminescence (PL). We showed that the PL spectra are dominated by two strong broad emission bands at 1.68 and 1.88 eV and two relatively narrow peaks at 2.04 and 2.09 eV. The broad emissions are associated with the presence of carbon and a small number of extended crystal defects, respectively. For the sharp emissions, two main radiative recombination channels were observed with ionization energies estimated in the range of 50-80 meV and lower than 10 meV. No variation of the low-temperature PL spectra was observed for samples grown at different P precursor flows, while increasing Ga content enhanced the dominant broad emission at around 1.68 eV, suggesting that the group III organometallic precursor is the main source of impurities. Finally, Be-doped samples were grown, and their characteristic optical emission at 2.03 eV was identified. These results contribute to the understanding of impurity-related luminescence in hexagonal GaP, being useful for further crystal growth optimization required for the fabrication of optoelectronic devices.

Author Correction: Optical Absorption Exhibits Pseudo-Direct Band Gap of Wurtzite Gallium Phosphide.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Optical Absorption Exhibits Pseudo-Direct Band Gap of Wurtzite Gallium Phosphide.

Definitive evidence for the direct band gap predicted for Wurtzite Gallium Phosphide (WZ GaP) nanowires has remained elusive due to the lack of strong band-to-band luminescence in these materials. In order to circumvent this problem, we successfully obtained large volume WZ GaP structures grown by nanoparticle-crawling assisted Vapor-Liquid-Solid method. With these structures, we were able to observe bound exciton recombination at 2.14 eV with FHWM of approximately 1 meV. In addition, we have measured the optical absorption edges using photoluminescence excitation spectroscopy. Our results show a 10 K band gap at 2.19 eV and indicate a weak oscillator strength for the lowest energy band-to-band absorption edge, which is a characteristic feature of a pseudo-direct band gap semiconductor. Furthermore, the valence band splitting energies are estimated as 110 meV and 30 meV for the three highest bands. Electronic band structure calculations using the HSE06 hybrid density functional agree qualitatively with the valence band splitting energies.