Shahrom Mahmud

Review on Zinc Oxide Nanoparticles: Antibacterial Activity and Toxicity Mechanism

Antibacterial activity of zinc oxide nanoparticles (ZnO-NPs) has received significant interest worldwide particularly by the implementation of nanotechnology to synthesize particles in the nanometer region. Many microorganisms exist in the range from hundreds of nanometers to tens of micrometers. ZnO-NPs exhibit attractive antibacterial properties due to increased specific surface area as the reduced particle size leading to enhanced particle surface reactivity. ZnO is a bio-safe material that possesses photo-oxidizing and photocatalysis impacts on chemical and biological species. This review covered ZnO-NPs antibacterial activity including testing methods, impact of UV illumination, ZnO particle properties (size, concentration, morphology, and defects), particle surface modification, and minimum inhibitory concentration. Particular emphasize was given to bactericidal and bacteriostatic mechanisms with focus on generation of reactive oxygen species (ROS) including hydrogen peroxide (H2O2), OH− (hydroxyl radicals), and O2−2 (peroxide). ROS has been a major factor for several mechanisms including cell wall damage due to ZnO-localized interaction, enhanced membrane permeability, internalization of NPs due to loss of proton motive force and uptake of toxic dissolved zinc ions. These have led to mitochondria weakness, intracellular outflow, and release in gene expression of oxidative stress which caused eventual cell growth inhibition and cell death. In some cases, enhanced antibacterial activity can be attributed to surface defects on ZnO abrasive surface texture. One functional application of the ZnO antibacterial bioactivity was discussed in food packaging industry where ZnO-NPs are used as an antibacterial agent toward foodborne diseases. Proper incorporation of ZnO-NPs into packaging materials can cause interaction with foodborne pathogens, thereby releasing NPs onto food surface where they come in contact with bad bacteria and cause the bacterial death and/or inhibition.

Physical properties of fish gelatin-based bio-nanocomposite films incorporated with ZnO nanorods

Well-dispersed fish gelatin-based nanocomposites were prepared by adding ZnO nanorods (NRs) as fillers to aqueous gelatin. The effects of ZnO NR fillers on the mechanical, optical, and electrical properties of fish gelatin bio-nanocomposite films were investigated. Results showed an increase in Youngs modulus and tensile strength of 42% and 25% for nanocomposites incorporated with 5% ZnO NRs, respectively, compared with unfilled gelatin-based films. UV transmission decreased to zero with the addition of a small amount of ZnO NRs in the biopolymer matrix. X-ray diffraction showed an increase in the intensity of the crystal facets of (10ī1) and (0002) with the addition of ZnO NRs in the biocomposite matrix. The surface topography of the fish gelatin films indicated an increase in surface roughness with increasing ZnO NR concentrations. The conductivity of the films also significantly increased with the addition of ZnO NRs. These results indicated that bio-nanocomposites based on ZnO NRs had great potentials for applications in packaging technology, food preservation, and UV-shielding systems.

High-performance dye-sensitized solar cells based on morphology-controllable synthesis of ZnO-ZnS heterostructure nanocone photoanodes.

High-density and well-aligned ZnO–ZnS core–shell nanocone arrays were synthesized on fluorine-doped tin oxide glass substrate using a facile and cost-effective two-step approach. In this synthetic process, the ZnO nanocones act as the template and provide Zn2+ ions for the ZnS shell formation. The photoluminescence spectrum indicates remarkably enhanced luminescence intensity and a small redshift in the UV region, which can be associated with the strain caused by the lattice mismatch between ZnO and ZnS. The obtained diffuse reflectance spectra show that the nanocone-based heterostructure reduces the light reflection in a broad spectral range and is much more effective than the bare ZnO nanocone and nanorod structures. Dye-sensitized solar cells based on the heterostructure ZnO–ZnS nanocones are assembled, and high conversion efficiency (η) of approximately 4.07% is obtained. The η improvement can be attributed primarily to the morphology effect of ZnO nanocones on light-trapping and effectively passivating the interface surface recombination sites of ZnO nanocones by coating with a ZnS shell layer.

Nanotripods of Zinc Oxide

We report the discovery of two-dimensional (2-D) nanotripods of ZnO, a new member of the ZnO nanostructure family. These planar nanotripods are synthesized via a novel approach known as catalyst-free combust-oxidized mesh (CFCOM) process that we developed using a ZnO factory furnace. At about 1200 °C, high velocity zinc vapor is instantly oxidized and captured in a steel mesh for 20s and then air-quenched. From this subminute synthesis process, three types of polycrystalline 2-D tripodal nanostructures are discovered. The ZnO tripods are composed of three planar arms that appear as rectangular nanoplates. We propose two growth routes for these planar tripods, namely base-arm and tripodal-core routes. For the former route, growth begins with the base arm in [[unk] [unk] 20 ] direction. During quenching, the other two arms grow from newly formed tapered facets, ([unk]110) and (1[unk]10). The tripodal-core growth route involves the formation of a hexagonal disc with ±( 0002 ) larger surfaces. From this core, three arms grow simultaneously in [ 11[unk]0 ], [ [unk]10 ] and [ 1[unk]10 ] directions, while the core transforms into a Y-shaped configuration with ±( 10[unk]0), ±( 01[unk]0 ) and ±( [unk]100 ) planes. Morphological analyses are performed using FESEM, EDS, TEM and XRD. Photoluminescence test detects the presence of structural defects associated with green and red peak emissions.

Fabrication of nanogap electrodes via nano-oxidation mask by scanning probe microscopy nanolithography

In this study, a simple technique was introduced for the fabrication of nanogap electrodes by using nano-oxidation scanning probe microscopy lithography with a Cr/Pt coated silicon tip. Silicon electrodes with a gap of sub-31 nm were fabricated successfully by this technique. The current-voltage measurements (I-V) of the electrodes demonstrated excellent insulating characteristics. This technique is simple, controllable, inexpensive, and faster than common methods. The results showed that silicon electrodes have a great potential for the fabrication of single molecule transistors, single electron transistors, and other nanoelectronic devices.

Controlling the shape and gap width of silicon electrodes using local anodic oxidation and anisotropic TMAH wet etching

A simple method for fabricating silicon electrodes with various shapes and gap widths was designed using the special properties of anisotropic tetramethylammonium hydroxide (TMAH) wet etching and local anodic oxidation (LAO). A statistical system was used for the optimization of the parameters of the LAO process to facilitate a better understanding and precise analysis of the process. Analyses of the interaction effects among the significant factors of LAO showed that the relative humidity and applied voltage were interdependent. They had the strongest interaction effect on the dimensions of the oxide mask. TMAH with a concentration of 25% was used as an etchant solution in (1 0 0) silicon with a rectangular oxide mask. The observed undercutting at convex corners, tip shape of emitters and gap widths of electrodes were exactly consistent with theoretical studies. Combination of the LAO method and anisotropic TMAH wet etching was successfully used to fabricate Si nano-gap electrodes. This fabrication method of sharp and round tip emitters was simple, controllable and faster than common techniques. These results indicate that the method can be a new approach for studying the electrical properties of nano-gap electrodes.

Increase in Upturn Power Dissipation of Surge Suppressors Due to Highly Defective Nanostructure of Zinc Oxide

Nanodefects are probable root causes for the observed high power dissipation of ZnO‐based surge suppression devices (SSDs). In this work, nanodefects are introduced by overgrinding ZnO for 100 hours via wet milling. Using FESEM, ZnO nanostructures are found to contain fine cracks, chipped‐off surfaces and nanofragments. For the defective ZnO, the zinc relative atomic % (EDS analysis) is observed to be much larger accompanied by higher oxygen vacancy concentration as revealed by PL green emission. Average particle size drops from 0.24 µm to 0.19 µm and specific surface area increases from 4.72 m2/g to 5.67 m2/g. Fabricated SSDs with defective ZnO exhibits higher power dissipation and bigger grain resistivity. A model is proposed to provide a correlation between nanodefects and power dissipation.

Growth Model for Nanoplates and Nanoboxes of Zinc Oxide from a Catalyst‐Free Combust‐Oxidized Process

A novel growth model is proposed for ZnO nanoplates and nanoboxes that are produced via a catalyst‐free combust‐oxidized (CFCO) process. In the CFCO process, molten zinc is vaporized and instantaneously oxidized in normal atmosphere to produce zinc oxide crystals that are transported into a 100–180 m cooling duct. FESEM analyses show clear images of quasi‐rectangular nanoplates and nanoboxes. The plates and boxes can be differentiated by the width‐to‐height ratio (W/H ratio) whereby a W/H ratio of (0.5–1.5) refers to nanoboxes while other ratios classify the nanoplates. These rectangular nanostructures generally have ⟨101¯0⟩ plane growth direction. We propose growth starts from two kinds of nucleation planes parallel to the six‐sided {101¯0} facets of nanorods that provide the nucleation sites for the nucleation planes.

Fabrication and characterization of novel semolina-based antimicrobial films derived from the combination of ZnO nanorods and nanokaolin

This study aimed to provide novel biopolymer-based antimicrobial films as food packaging that may assist in reducing environmental pollution caused by the accumulation of synthetic food packaging. The blend of ZnO nanorods (ZnO-nr) and nanokaolin in different ratios (1:4, 2:3, 3:2 and 4:1) was incorporated into semolina, and nanocomposite films were prepared using solvent casting. The resulting films were characterized through field-emission scanning electron microscopy and X-ray diffraction. The mechanical, optical, physical, and antimicrobial properties of the films were also analyzed. The water vapor permeability of the films decreased with increasing ZnO-nr percentage, but their tensile strength and modulus of elasticity increased with increasing nanokaolin percentage. The UV transmittance of the semolina films were greatly influenced by an increase in the amount of ZnO-nr. The addition of ZnO-nr: nanokaolin at all ratios (except 1:4) into semolina reduced UV transmission to almost 0%. Furthermore, the ZnO-nr/nanokaolin/semolina films exhibited a strong antimicrobial activity against Staphylococcus aureus. These properties suggest that the combination of ZnO-nr and nanokaolin are potential fillers in semolina-based films to be used as active packaging for food and pharmaceuticals.

Synthesis of needle-shape ZnO-ZnS core-shell heterostructures and their optical and field emission properties

Well-aligned ZnO-ZnS core-shell nano-needle arrays were synthesized using a simple aqueous solution approach to investigate the optical and field emission properties of heterostructure materials. The photoluminescence of the core-shell nano-needles exhibits a distinct enhancement compared with that of uncoated ZnO nano-needles. The UV-vis spectra show that the ZnS shell layer enhances the optical absorption of ZnO nano-needles by decreasing the interface band gap, which indicates the potential application of heterostructures in photovoltaic and solar cells. The core-shell nano-needles exhibit a remarkably high field enhancement factor of 3.74 × 103, a low turn-on field of 2.31 V/μm, and a high time stability. These findings show that the construction of core-shell heterostructure can efficiently improve the field emission performance of ZnO nano-needles, which is a promising route for the development of novel nanoemitters with controllable morphology and as suitable shell materials for heterostructures.