Graphene oxide (GO) is a promising and remarkable nanomaterial that exhibits antimicrobial activity due to its specific surface–interface interactions. In the present work, for the first time, we have reported the antibacterial activity of GO-coated surfaces prepared by two different methods (Hummers’ and improved, i.e., GOH and GOI) against bacterial biofilm formation. The bacterial toxicity of the deposited GO-coated surfaces was investigated for both Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) models of bacteria. The mechanism of inhibition is different on the coated surface than that in suspension, as determined by measurement of the percentage inhibition of biofilm formation, Ellman’s assay, and colony forming unit (CFU) studies. The difference in the nature, degree of oxidative functionalities, and size of the synthesized GO nanoparticles mitigates biofilm formation. To better understand the antimicrobial mechanism of GO when coated on surfaces, we were able to demonstrate that beside reactive oxygen species-mediated oxidative stress, the physical properties of the GO-coated substrate effectively inactivate bacterial cell proliferation, which forms biofilms. Light and atomic force microscopy (AFM) images display a higher inhibition in the proliferation of planktonic cells in Gram-negative bacteria as compared to that in Gram-positive bacteria. The existence of a smooth surface with fewer porous domains in GOI inhibits biofilm formation, as demonstrated by optical microscopy and AFM images. The oxidative stress was found to be lower in the coated surface as compared to that in the suspensions as the latter enables exposure of both a large fraction of the active edges and functionalities of the GO sheets. In suspension, GOH is selective against S. aureus whereas GOI showed inhibition toward E. coli. This study provides new insights to better understand the bactericidal activity of GO-coated surfaces and contributes to the design of graphene-based antimicrobial surface coatings, which will be valuable in biomedical applications.
Temperature-dependent photoluminescence (PL) of titanium oxide (TiO2) shows an evolution of blue emission when exposed to 50 keV Ar+ ions. The origin of observed PL has been examined by X-ray absorption near-edge spectroscopy (XANES) at Ti-K,L and O-K edges, revealing the reduction of ligand field splitting owing to the formation of oxygen vacancies (OVs) by destroying TiO6 octahedral symmetry. Detailed PL and XANES analyses suggest that the fluence (ions/cm2) dependent increase in OVs not only boosts the conduction electrons but also increases the density of holes in localized self-trapped exciton (STE) states near the valence band. Based on these observations, we propose a model in which doped conduction electrons are recombining radiatively with the holes in STE states for blue light emission.
Transformation of self-assembled crystalline TiO2 nanorods to amorphous layer, and the corresponding impact on optical-bandgap (Eg) on Si pyramids are investigated by irradiating with 50 keV Ar+-ions. Initially, Eg is found to be reduced from 3.23 to 2.94 eV up to a fluence of 1 × 1016 ions/cm2, and discussed in terms of the rise in oxygen vacancies (VO). However, a sudden increase in Eg to 3.38 eV is detected at a fluence of 1 × 1017 ions/cm2 through evolution of voids by over-saturating VO, manifesting the appearance of degenerate states by shifting the Fermi level above the conduction band minimum via Burstein-Moss effect.
Towards developing next generation scalable TiO2-based resistive switching (RS) memory devices, the efficacy of 50 keV Ar+-ion irradiation to achieve self-organized nano-channel based structures at a threshold fluence of 5 × 1016 ions/cm2 at ambient temperature is presented. Although x-ray diffraction results suggest the amorphization of as-grown TiO2 layers, detailed transmission electron microscopy study reveals fluence-dependent evolution of voids and eventual formation of self-organized nano-channels between them. Moreover, gradual increase of TiO/Ti2O3 in the near surface region, as monitored by x-ray photoelectron spectroscopy, establishes the upsurge in oxygen deficient centers. The impact of structural and chemical modification on local RS behavior has also been investigated by current-voltage measurements in conductive atomic force microscopy, while memory application is manifested by fabricating Pt/TiO2/Pt/Ti/SiO2/Si devices. Finally, the underlying mechanism of our experimental results has been analyzed and discussed in the light of oxygen vacancy migration through nano-channels.
Oxygen vacancy (OV) controlled hydrophobicity of self-assembled TiO2 nanorods (NRs) on chemically etched Si pyramids is investigated by irradiating with 50 keV Ar+-ions at room temperature. Apparent contact angle (CA) is found to increase from 122° to 141° up to a fluence of 1 × 1015 ions/cm2, followed by a gradual reduction to 130° at 1 × 1017 ions/cm2. However, the drop in apparent CA is found to be associated with the decrease in fractional surface area via transformation of NRs to an amorphous layer above 1 × 1015 ions/cm2, though it is still higher than that of as-grown one. Detailed X-ray photoelectron spectroscopy and electron paramagnetic resonance measurements suggest that the control of hydrophobic behavior is related to the suppression of surface free energy via migration of OVs into the voids in TiO x layers.
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