Siti Khadijah Mohd Bakhori

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.

Characterization of ZnO Nanopowder and Antibacterial Response against Staphylococcus aureus under UVA Illumination

In this study, we study the physical property and antibacterial bioactivity of ZnO nanopowder towards Staphylococcus Aureus. Transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), FTIR, Raman spectroscopy and UV-Vis were used to characterize the ZnO nanopowder. The major morphology consists of nanorods which have length 70-200 nm and width 30-120 nm. FTIR spectra performed a well-synthesis of ZnO that has Zn-O stretching bond (482 cm-1). UV-Vis absorption spectra showed an intense UV absorption at 387 nm, corresponding to optical bandgap 3.24 eV. Raman spectroscopy exhibited a prominent peak in E2high mode located at 435 cm-1. The antibacterial response of ZnO was performed toward Staphylococcus aureus. Higher concentration of ZnO had caused higher inhibition of the bacteria. Besides, the increment of capability of ZnO towards the bacteria was observed under UV radiation. It was believed that the irradiation had induced oxygen to be released from the surface of the ZnO and caused the increasing of reactive oxygen species, which enhance the bacteria inhibition.

In-vitro efficacy of different morphology zinc oxide nanopowders on Streptococcus sobrinus and Streptococcus mutans

ZnO with two different morphologies were used to study the inhibition of Streptococcus sobrinus and Streptococcus mutans which are closely associated with tooth cavity. Rod-like shaped ZnO-A and plate-like shaped ZnO-B were produced using a zinc boiling furnace. The nanopowders were characterized using energy filtered transmission electron microscopy (EFTEM), X-ray diffraction (XRD), photoluminescence (PL) spectroscopy, Raman spectroscopy and dynamic light scattering (DLS) to confirm the properties of the ZnO polycrystalline wurtzite structures. XRD results show that the calculated crystallite sizes of ZnO-A and ZnO-B were 36.6 and 39.4nm, respectively, whereas DLS revealed particle size distributions of 21.82nm (ZnO-A) and 52.21nm (ZnO-B). PL spectra showed ion vacancy defects related to green and red luminescence for both ZnO particles. These defects evolved during the generation of reactive oxygen species which contributed to the antibacterial activity. Antibacterial activity was investigated using microdilution technique towards S. sobrinus and S. mutans at different nanopowder concentrations. Results showed that ZnO-A exhibited higher inhibition on both bacteria compared with ZnO-B. Moreover, S. mutans was more sensitive compared with S. sobrinus because of its higher inhibition rate.

Toxicity evaluation of ZnO nanostructures on L929 fibroblast cell line using MTS assay

ZnO has wide applications in medical and dentistry apart from being used as optoelectronic devices such as solar cells, photodetectors, sensors and light emitting diodes (LEDs). Therefore, the toxicity evaluation is important to know the toxicity level on normal cell line. The toxicity of two grades ZnO nanostructures, ZnO-4 and ZnO-8 have been carried out using cytotoxicity test of MTS assay on L929 rat fibroblast cell line. Prior to that, ZnO-4 and ZnO-8 were characterized for its morphology, structure and optical properties using FESEM, X-ray diffraction, and Photoluminescence respectively. The two groups revealed difference in morphology and exhibit slightly shifted of near band edge emission of Photoluminescence other than having a similar calculated crystallite size of nanostructures. The viability of cells after 72h were obtained and the statistical significance value was calculated using SPSS v20. The p value is more than 0.05 between untreated and treated cell with ZnO. This insignificant value of p>0.05 can be summarized as a non-toxic level of ZnO-4 and ZnO-8 on the L929 cell line.

Structural morphology of zinc oxide structures with antibacterial application of calamine lotion

In this study, we report the structural morphology of a zinc oxide (ZnO) sample and antibacterial application of the ZnO structures in calamine lotion. Antibacterial activities of the calamine lotion towards Staphylococcus aureus and Pseudomonas aeruginosa were investigated. The structural morphology of ZnO sample was studied using a transmission electron microscope (TEM) and a field-emission scanning electron microscope (FESEM). The morphologies of the ZnO structure consisted of many rod and spherical structures. The particle sizes of the sample ranged from 40 nm to 150 nm. A calamine lotion was prepared through mixing the ZnO structures with other constituents in suitable proportion. The energy-dispersive x-ray spectroscopy (EDS) revealed the presence of large amount of ZnO structures whiles the X-ray diffraction (XRD) results showed a good crystalline property of ZnO in the calamine lotion mixture. The morphological structures of ZnO were found to remain unchanged in the calamine lotion mixture through FESEM imaging. In the antibacterial test, prepared calamine lotion exhibited a remarkable bacterial inhibition on Staphylococcus aureus and Pseudomonas aeruginosa after 24 h of treatment. The bactericidal capability of calamine lotion was largely due to the presence of ZnO structures which induce high toxicity and killing effect on the bacteria.

Enhanced photoconductivity and antibacterial response of rubber-grade ZnO upon UVA illumination

Ultraviolet-A (UVA) radiation is present in sunlight and have been associated with various types of human skin cancers. In this study, rubber-grade zinc oxide (ZnO) powder was used as the targeted materials to study its UVA photoresponse as well as its antibacterial function. ZnO powder was synthesized using French process. The morphological structures of the samples were investigated using field emission scanning electron microscopy and transmission electron microscopy. The dominant morphology of the sample was micro/nanoplate. The optical bandgap of the ZnO sample is 3.19 eV based on the UV-Visible measurement. Current-voltage measurement was conducted to study the effect of UVA (390 nm) illumination on the photoconductivity of the ZnO pellet. Photoconductivity was observed to increase significantly under UVA exposure due to light absorption on the surface of ZnO to raise the electrons across the bandgap. The current response of the UVA-induced also revealed the small persistent photoconductivity after the UVA light was turned off. Besides, higher voltage bias would lead to higher current flow under the same intensity of UVA exposure. According to the antibacterial test towards Staphylococcus aureus, the percentage inhibition of the bacterial after 24 h incubation increase when the concentration of ZnO suspension increases. The UVA illumination had improved the inhibition of the bacterial growth. This is due to the excitation of ZnO and increasing of free charge carriers in the solution, leading to potential distortion to the membrane surface of the bacteria. ZnO powder performed high absorption of UVA and they are not only can be used to block the UVA sunlight, but also have higher antibacterial capability under UVA excitation.

Effects of Annealing Treatment on Photoluminescence and Structural Properties of ZnO Nanostructures

Zinc oxide (ZnO) pellets were prepared by agglomeration of ZnO nanopowder with distilled water in this study. Annealing at 800oC in different ambient was done on the surface of the pellets in order to change the photoluminescence and structural properties of the sample. The ZnO pellets that were annealed in air, oxygen and nitrogen ambient reduce the intensity of near-band-edge emission in photoluminescence spectra. The observed change in green luminescence after the annealing process suggests that the oxygen vacancies are the recombination centers for green luminescence in the samples. Red luminescence was also observed and it is attributed to oxygen vacancies in different energy level. Different annealing ambient had significantly modified the green and red luminescence. Structural investigation carried out by X-ray diffraction (XRD) revealed the polycrystalline nature for ZnO pellets with hexagonal phase and annealing process increased the cystallinity of the ZnO pellets.

Photoluminescence and Raman Studies of Annealed ZnO Nanostructures

The influence of annealing on the optical properties of as-grown ZnO nanostructures prepared in pellets has been investigated by photoluminescence (PL) and Raman spectroscopy. The annealing temperatures of ZnO nanostructure at 600°C, 650°C and 700 °C were conducted in oxygen (O2) and nitrogen (N2) ambient. The near band edge emission (NBE) of samples recorded in the PL spectra demonstrates significant changes on optical signal whereby the NBE is redshifted after O2 annealed and became slightly higher in N2 annealed. Apart from that, weak green luminescence (GL) namely deep band emission (DBE) is observed centre at 532.95 nm (2.23 eV) and 511.00 nm (2.42 eV) for annealed in O2 and N2 respectively, whereas lower DBE observed in as-grown ZnO. On the other hand, Raman shift reveal the phonon mode of the ZnO nanostructures and the E2 (high) mode were downshifted as annealed in O2 ambient, and upshifted in N2 ambient. The downshift and upshift of the E2 (high) mode are correlated to tensile and compressive stress. Moreover the crystallite sizes were calculated from FWHM of XRD and TEM microscopy reveals the nanoplates structure of ZnO nanostructures.

Effects of Annealing Treatment on Structural, Optical and Morphology Characteristics of ZnO Nanostructures

The ZnO nanostructure produced by CFCO or French process were undergone annealing treatment at 700°C in oxygen and nitrogen ambient. Subsequently, the characteristics of structural, optical and morphology of ZnO nanostructures were investigated using X-ray diffraction (XRD), photoluminescence (PL) and transmission electron microscopy (TEM) respectively. The crystallite size of the nanostructures were calculated from full width half maximum (FWHM) of (101) peak in XRD patterns and the size is around 42 nm. PL measuremment were carried out and the near band edge emission (NBE) is increase in wavelength or namely redshifted. Moreover, deep band emission (DBE) is observed at 520 nm for ZnO annealed in nitrogen, which commonly regarded as the defect level of oxygen vacancies. The investigation continues with conducting transmission electron microscopy (TEM) to demonstrate the mophology of as grown nanostructures and annealed in both nitrogen and oxygen ambient.

Cytotoxicity evaluation of ZnO-eugenol (ZOE) using different ZnO structure on human gingival fibroblast

Application of ZnO is widely used in many industries, such as in optoelectronic devices, automotive, textile, cosmetics, medical and dentistry. In this study, emphasis was given on ZnO-eugenol (ZOE) that has been used in dental restoration. ZOE contained 80% ZnO and 20% eugenol. ZOE exhibited selective toxicity that could kill bacteria but safe on human cells. The safety of ZOE on humans is critically important. Two types of ZnO with different morphology, namely ZnO-A and ZnO-K were used to make ZOE (ZOE-A and ZOE-K) and the cytotoxicity level on human gingival fibroblast (HGF) cell line were evaluated. Both ZnO were characterized for its morphology and structural using Field Emission Scanning Electron Microscopy (FESEM) and X-ray Diffraction (XRD), respectively. The cytotoxicity level was evaluated using CCK-8 assay where the percentage of viable cells after 72 h were observed. The result showed ZnO-A, containing mostly rod-like shape with a crystallite size of 37.5 nm, had a higher percentage of viable ...