Porous Silicon-Zinc Oxide Nanocomposites Prepared by Atomic Layer Deposition for Biophotonic Applications.

In the current research, a porous silicon/zinc oxide (PSi/ZnO) nanocomposite produced by a combination of metal-assisted chemical etching (MACE) and atomic layer deposition (ALD) methods is presented. The applicability of the composite for biophotonics (optical biosensing) was investigated. To characterize the structural and optical properties of the produced PSi/ZnO nanocomposites, several studies were performed: scanning and transmission electron microscopy (SEM/TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance, and photoluminescence (PL). It was found that the ALD ZnO layer fully covers the PSi, and it possesses a polycrystalline wurtzite structure. The effect of the number of ALD cycles and the type of Si doping on the optical properties of nanocomposites was determined. PL measurements showed a "shoulder-shape" emission in the visible range. The mechanisms of the observed PL were discussed. It was demonstrated that the improved PL performance of the PSi/ZnO nanocomposites could be used for implementation in optical biosensor applications. Furthermore, the produced PSi/ZnO nanocomposite was tested for optical/PL biosensing towards mycotoxins (Aflatoxin B1) detection, confirming the applicability of the nanocomposites.

Graphene as Barrier to Prevent Volume Increment of Air Bubbles over Silicone Polymer in Aqueous Environment.

The interaction of air bubbles with surfaces immersed in water is of fundamental importance in many fields of application ranging from energy to biology. However, many aspects of this topic such as the stability of surfaces in contact with bubbles remain unexplored. For this reason, in this work, we investigate the interaction of air bubbles with different kinds of dispersive surfaces immersed in water. The surfaces studied were polydimethylsiloxane (PDMS), graphite, and single layer graphene/PDMS composite. X-ray photoelectron spectroscopy (XPS) analysis allows determining the elemental surface composition, while Raman spectroscopy was used to assess the effectiveness of graphene monolayer transfer on PDMS. Atomic force microscopy (AFM) was used to study the surface modification of samples immersed in water. The surface wettability has been investigated by contact angle measurements, and the stability of the gas bubbles was determined by captive contact angle (CCA) measurements. CCA measurements show that the air bubble on graphite surface exhibits a stable behavior while, surprisingly, the volume of the air bubble on PDMS increases as a function of immersion time (bubble dynamic evolution). Indeed, the air bubble volume on the PDMS rises by increasing immersion time in water. The experimental results indicate that the dynamic evolution of air bubble in contact with PDMS is related to the rearrangement of surface polymer chains via the migration of the polar groups. On the contrary, when a graphene monolayer is present on PDMS, it acts as an absolute barrier suppressing the dynamic evolution of the bubble and preserving the optical transparency of PDMS.

Interaction of the Helium, Hydrogen, Air, Argon, and Nitrogen Bubbles with Graphite Surface in Water.

The interaction of the confined gas with solid surface immersed in water is a common theme of many important fields such as self-cleaning surface, gas storage, and sensing. For that reason, we investigated the gas-graphite interaction in the water medium. The graphite surface was prepared by mechanical exfoliation of highly oriented pyrolytic graphite (HOPG). The surface chemistry and morphology were studied by X-ray photoelectron spectroscopy, profilometry, and atomic force microscopy. The surface energy of HOPG was estimated by contact angle measurements using the Owens-Wendt method. The interaction of gases (Ar, He, H2, N2, and air) with graphite was studied by a captive bubble method, in which the gas bubble was in contact with the exfoliated graphite surface in water media. The experimental data were corroborated by molecular dynamics simulations and density functional theory calculations. The surface energy of HOPG equaled to 52.8 mJ/m2 and more of 95% of the surface energy was attributed to dispersion interactions. The results on gas-surface interaction indicated that HOPG surface had gasphilic behavior for helium and hydrogen, while gasphobic behavior for argon and nitrogen. The results showed that the variation of the gas contact angle was related to the balance between the gas-surface and gas-gas interaction potentials. For helium and hydrogen the gas-surface interaction was particularly high compared to gas-gas interaction and this promoted the favorable interaction with graphite surface.