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A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Inorganic Crystalline Materials".

Deadline for manuscript submissions: closed (30 November 2022) | Viewed by 44233

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Special Issue Editors

Prof. Dr. Yamin Leprince-Wang

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Guest Editor

ESYCOM Laboratory, Université Gustave Eiffel (UGE), 77420 Champs sur Marne, France
Interests: ZnO nanostructures; synthesis; piezoelectric nanogenerator; photocatalysis; water purification; air purification
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Guangyin Jing

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Guest Editor

School of Physics, Northwest University, Xi’an 710127, China
Interests: soft matter; bio-medical microfluidics; active matter; microswimmer
Special Issues, Collections and Topics in MDPI journals

Dr. Basma El Zein

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Guest Editor

Governance and Sustainability Center, University of Business and Technology, Jeddah 21451, Saudi Arabia
Interests: nanomaterials; synthesis; energy conversion; energy storage
Special Issues, Collections and Topics in MDPI journals

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Dear Colleagues,

ZnO is a multifunctional material possessing unique electrical, optical, acoustic, and mechanical properties. Nanostructured ZnO is one of the most fascinating nanomaterials because of its outstanding properties, including its wide direct bandgap, high electron mobility, piezoelectricity, chemical and thermal stability, and biocompatibility. With easy and low-cost growth techniques, ZnO nanomaterials present a wide variety of geometrical shapes, such as nanoparticles, nanorods, nanowires, nanobelts, nanosprings, nanocombs, etc.

Many promising applications have been developed around the ZnO nanostructures, such as transparent electronics, smart windows, piezoelectric devices, UV-lasers, UV photodetectors, gas sensors, chemical sensors, optofluidic devices, and biosensors. In addition, the ferromagnetic properties of ZnO doped with “rare-earth metals” show potential for spintronic-based devices.

In recent years, ZnO has been considered one of the best photocatalysts for water and air purification and the photoelectrochemical (PEC) water splitting process. Furthermore, due to its biocompatibility, in addition to its antibacterial and other novel properties, ZnO has been utilized in a variety of applications, such as drug delivery, cancer treatment, bio-imaging, medical products, sunscreens, skin lotions, etc.

As an n-type semiconductor, ZnO is also widely used in various solar cell structures, including dye-sensitized, organic, hybrid, and solid-state solar cells. Furthermore, the nanostructured forms lead to an increased light absorption, while also increasing the electron pathways for carrier collection. Both factors favour light harvesting and, hence, the solar cell efficiency.

It is our pleasure to invite you to submit original research papers, as well as review papers, within the scope of this Special Issue.

Prof. Dr. Yamin Leprince-Wang
Prof. Dr. Guangyin Jing
Dr. Basma El Zein
Guest Editors

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Crystals is an international peer-reviewed open access monthly journal published by MDPI.

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Keywords

ZnO nanostructures
synthesis
photocatalysis
water slitting
water purification
air purification
solar cells
piezoelectric generator
bio-application
non-wetting and self-cleaning
antimicrobial
opto-fluidics

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Published Papers (14 papers)

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Editorial

Jump to: Research, Review

4 pages, 187 KiB

Open AccessEditorial

Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications

by Yamin Leprince-Wang, Guangyin Jing and Basma El Zein

Crystals 2023, 13(2), 338; https://doi.org/10.3390/cryst13020338 - 16 Feb 2023

Viewed by 1810

Abstract

The Special Issue “Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications” is a collection of 13 papers, including 3 review papers and 10 original articles dedicated to both experimental research works and numerical simulations on ZnO nanostructures [...] Full article

(This article belongs to the Special Issue Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications)

Research

Jump to: Editorial, Review

10 pages, 36084 KiB

Open AccessArticle

Tuning Growth of ZnO Nano-Arrays by the Dewetting of Gel Layer

by Ziqian Li, Ningzhe Yan, Yangguang Tian and Hao Luo

Crystals 2023, 13(1), 30; https://doi.org/10.3390/cryst13010030 - 24 Dec 2022

Cited by 1 | Viewed by 1249

Abstract

The classical two-step sol-gel hydrothermal method enables the growth of nanoarrays on various substrates via a seed layer. The morphology of the nanoarrays is often tuned by changing the composition ratio of the seed solution. It is taken for granted that the number density and size of seeds will increase with the proportion of precursors. However, in this work, we found novel two-stage dependencies between the concentration of the precursor (ZnAC) and the geometric parameters (number density and diameter) of ZnO seed particles. The completely opposite dependencies illustrate the existence of two different mechanisms. Especially when the proportion of precursors is low (

ϕ_{Z n A C} : ϕ_{P V A} < 0.22

), the seed number density and diameter decrease with the increasing precursor concentration. This counterintuitive phenomenon should be caused by the destabilization and dewetting process of the thin film layers during annealing. Based on this new mechanism, we demonstrate the tuning growth of the ZnO seed layer and the nanowire array by annealing time. The number density of the nanorod array can be changed by 10 times, and the diameter of the nanorods can be changed by more than 8 times. The new mechanism we proposed can not only help people deepen their understanding of the formation and evolution of the seed layer but also provide a new way for the controllable growth of nanomaterials. Full article

(This article belongs to the Special Issue Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications)

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12 pages, 10113 KiB

Open AccessArticle

Controlled Synthesis and Growth Mechanism of Two-Dimensional Zinc Oxide by Surfactant-Assisted Ion-Layer Epitaxy

by Chunfeng Huang, Qi Sun, Zhiling Chen, Dongping Wen, Zongqian Tan, Yaxian Lu, Yuelan He and Ping Chen

Crystals 2023, 13(1), 5; https://doi.org/10.3390/cryst13010005 - 20 Dec 2022

Cited by 2 | Viewed by 2036

Abstract

Two-dimensional (2D) zinc oxide (ZnO) has attracted much attention for its potential applications in electronics, optoelectronics, ultraviolet photodetectors, and resistive sensors. However, little attention has been focused on the growth mechanism, which is highly desired for practical applications. In this paper, the growth mechanism of 2D ZnO by surfactant-assisted ion-layer epitaxy (SA-ILE) is explored by controlling the amounts of surfactant, temperature, precursor concentration, and growth time. It is found that the location and the number of nucleation sites at the initial stages are restricted by the surfactant, which absorbs

{Zn}^{2 +}

ions via electrostatic attraction at the water-air interface. Then, the growth of 2D ZnO is administered by the temperature, precursors, and growth time. In other words, the temperature is connected with the diffusion of solute ions and the number of nucleation sites. The concentration of precursors determines the solute ions in solution, which plays a dominant role in the growth rate of 2D ZnO, while growth time affects the nucleation, growth, and dissolution processes of ZnO. However, if the above criteria are exceeded, the nucleation sites significantly increase, resulting in multiple 2D ZnO with tiny size and multilayers. By optimizing the above parameters, 2D ZnO nanosheets with a size as large as 20 μm are achieved with 10 × 10⁻⁵ of the ratio of sodium oleyl sulfate to

{Zn}^{2 +}

, 70 °C, 50 mM of precursor concentration, and 50 min of growth time. 2D ZnO sheets, are confirmed by scanning electron microscope (SEM), energy-dispersive X-ray spectrometer (EDS), X-ray photoelectron spectroscopy (XPS), and Raman spectrum. Our work might guide the development of SA-ILE and pave the platform for practical applications of 2D ZnO on photodetectors, sensors, and resistive switching devices. Full article

(This article belongs to the Special Issue Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications)

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15 pages, 3044 KiB

Open AccessArticle

Green Synthesis of ZnO Nanostructures Using Pyrus pyrifolia: Antimicrobial, Photocatalytic and Dielectric Properties

by Zainal Abidin Ali, Iqabiha Shudirman, Rosiyah Yahya, Gopinath Venkatraman, Abdurahman Hajinur Hirad and Siddique Akber Ansari

Crystals 2022, 12(12), 1808; https://doi.org/10.3390/cryst12121808 - 12 Dec 2022

Cited by 8 | Viewed by 1881

Abstract

In this study, zinc oxide nanostructures (ZnO NS) were synthesized using Pyrus pyrifolia fruit extract. Biophysical characterization results confirmed that the synthesized materials are crystalline wurtzite ZnO structures. Field emission scanning electron microscopy (FESEM) revealed that the ZnO NS are cubical, and the sizes range 20–80 nm. Transmission electron microscopy (TEM) and XRD results revealed a crystal lattice spacing of 0.23 nm and (101) the crystalline plane on ZnO NS. UV-Visible spectrophotometer results showed an absorbance peak at 373 nm. The ZnO NS demonstrated significant antibacterial activity analyzed by metabolic activity analysis and disc diffusion assay against Escherichia coli and Staphylococcus aureus. FESEM analysis confirmed the bacterial membrane disruption and the release of cytoplasmic contents was studied by electron microscopy analysis. Further, ZnO NS achieved good photocatalytic activity of decolorizing 88% of methylene blue (MB) in 60 min. The dielectric constant and loss of ZnO were found to be 3.19 and 2.80 at 1 kHz, respectively. The research findings from this study could offer new insights for developing potential antibacterial and photocatalytic materials. Full article

(This article belongs to the Special Issue Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications)

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6 pages, 1754 KiB

Open AccessArticle

Local Three-Dimensional Characterization of Nonlinear Grain Boundary Length within Bulk ZnO Using Nanorobot in SEM

by Feiling Shen, Ning Cao, Hengyu Li, Zhizheng Wu, Shaorong Xie and Jun Luo

Crystals 2022, 12(11), 1558; https://doi.org/10.3390/cryst12111558 - 1 Nov 2022

Cited by 4 | Viewed by 1742

Abstract

Aiming at the problems of lack of data on the nonlinear morphology to divide uneven grain boundary in bulk ceramics, a unique approach of nanorobot-based characterization of three-dimensional nonlinear structure length can be creatively proposed under scanning electron microscope to quantify the actual morphology of local micro-area grain boundary in bulk ZnO. Contour shapes of the targeted grain boundaries in plane X-Y can be imaged using SEM. Z-directional relative height differences at different positions can be sequentially probed by nanorobot. Experiments demonstrate that it is effective to characterize three-dimensional length structures of nonlinear grain boundaries in bulk materials. By quantifying Z-directional relative height differences, it can be verified to show that irregular characteristics exist in three-dimensional grain boundary length, which can extend the depth effect on nonlinear bulk conductance. Furthermore, this method can also obtain nonlinear quantitative topographies to divide grain boundaries to uneven structure in the analysis of bulk polycrystalline materials. Full article

(This article belongs to the Special Issue Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications)

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18 pages, 8405 KiB

Open AccessArticle

Enhanced Broadband Metamaterial Absorber Using Plasmonic Nanorods and Muti-Dielectric Layers Based on ZnO Substrate in the Frequency Range from 100 GHz to 1000 GHz

by Ahmed Emara, Amr Yousef, Basma ElZein, Ghassan Jabbour and Ali Elrashidi

Crystals 2022, 12(10), 1334; https://doi.org/10.3390/cryst12101334 - 21 Sep 2022

Cited by 3 | Viewed by 2549

Abstract

A broadband thin film plasmonic metamaterial absorber nanostructure that operates in the frequency range from 100 GHz to 1000 GHz is introduced and analyzed in this paper. The structure consists of three layers: a 200 nm thick gold layer that represents the ground plate (back reflector), a dielectric substrate, and an array of metallic nanorods. A parametric study is conducted to optimize the structure based on its absorption property using different materials, gold (Au), aluminum (Al), and combined Au, and Al for the nanorods. The effect of different dielectric substrates on the absorption is examined using silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), titanium dioxide (TiO₂), and a combination of these three materials. This was followed by the analysis of the effect of the distribution of Al, and Au nanorods and their dimensions on the absorption. The zinc oxide (ZnO) layer is added as a substrate on top of the Au layer to enhance the absorption in the microwave range. The optimized structure achieved more than 80% absorption in the ranges 100–280 GHz, 530–740 GHz and 800–1000 GHz. The minimum optimized absorption is more than 65% in the range 100 GHz to 1000 GHz. Full article

(This article belongs to the Special Issue Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications)

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12 pages, 3111 KiB

Open AccessArticle

Influence of ZnO Morphology on the Functionalization Efficiency of Nanostructured Arrays with Hemoglobin for CO₂ Capture

by Alberto Mendoza-Sánchez, Francisco J. Cano, Mariela Hernández-Rodríguez and Oscar Cigarroa-Mayorga

Crystals 2022, 12(8), 1086; https://doi.org/10.3390/cryst12081086 - 3 Aug 2022

Cited by 6 | Viewed by 2301

Abstract

In this study, nanostructured ZnO arrays were synthesized by an accessible thermal oxidation (TO) methodology. The Zn films were chemically etched with nitric acid (HNO₃) and then oxidized in a furnace at 500 °C for 5 h. Two different morphologies were achieved by modifying the HNO₃ concentration in the etching process: (a) ZnO grass-like nanostructures and (b) rod-like nanostructures, with an etching process in HNO₃ solution at 2 and 8 M concentration, respectively. The physical and chemical properties of the samples were analyzed by X-ray diffraction (XRD), scanning (SEM) and transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and Raman spectroscopy. Both morphologies were functionalized with hemoglobin, and a difference was found in the efficiency of functionalization, which was monitored by UV–Vis spectroscopy. The sample with the highest efficiency was the ZnO grass-like nanostructures. Afterward, the capture of carbon dioxide was evaluated by monitoring a sodium carbonate solution interacting with the as-functionalized samples. The evaluation was analyzed by UV–Vis spectroscopy and the results showed a CO₂ capture of 98.3% and 54% in 180 min for the ZnO grass-like and rod-like nanostructures, respectively. Full article

(This article belongs to the Special Issue Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications)

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12 pages, 2971 KiB

Open AccessFeature PaperArticle

Motility Suppression and Trapping Bacteria by ZnO Nanostructures

by Ningzhe Yan, Hao Luo, Yanan Liu, Haiping Yu and Guangyin Jing

Crystals 2022, 12(8), 1027; https://doi.org/10.3390/cryst12081027 - 23 Jul 2022

Cited by 3 | Viewed by 1908

Abstract

Regulating the swimming motility of bacteria near surfaces is essential to suppress or avoid bacterial contamination and infection in catheters and medical devices with wall surfaces. However, the motility of bacteria near walls strongly depends on the combination of the local physicochemical properties of the surfaces. To unravel how nanostructures and their local chemical microenvironment dynamically affect the bacterial motility near surfaces, here, we directly visualize the bacterial swimming and systematically analyze the motility of Escherichia coli swimming on ZnO nanoparticle films and nanowire arrays with further ultraviolet irradiation. The results show that the ZnO nanowire arrays reduce the swimming motility, thus significantly enhancing the trapping ability for motile bacteria. Additionally, thanks to the wide bandgap nature of a ZnO semiconductor, the ultraviolet irradiation rapidly reduces the bacteria locomotion due to the hydroxyl and singlet oxygen produced by the photodynamic effects of ZnO nanowire arrays in an aqueous solution. The findings quantitatively reveal how the combination of geometrical nanostructured surfaces and local tuning of the steric microenvironment are able to regulate the motility of swimming bacteria and suggest the efficient inhibition of bacterial translocation and infection by nanostructured coatings. Full article

(This article belongs to the Special Issue Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications)

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10 pages, 4106 KiB

Open AccessFeature PaperArticle

ZnO Nanowire-Based Piezoelectric Nanogenerator Device Performance Tests

by Linda Serairi and Yamin Leprince-Wang

Crystals 2022, 12(8), 1023; https://doi.org/10.3390/cryst12081023 - 23 Jul 2022

Cited by 12 | Viewed by 2485

Abstract

Over the past two decades, the quick development of wireless sensor networks has required the sensor nodes being self-powered. Pushed by this goal, in this work, we demonstrated a ZnO nanowire-array-based piezoelectric nanogenerator (NG) prototype, which can convert mechanical energy into electricity. High-quality single crystalline ZnO nanowires, having an aspect ratio of about 15, grown on gold-coated silicon substrate, were obtained by using a low-cost and low-temperature hydrothermal method. The NG-device fabrication process has been presented in detail, and the NG’s performance has been tested in both compression and vibration modes. Peak power of 1.71 µW was observed across an optimal load resistance of 5 MΩ for the ZnO nanowires-based NG, with an effective area of 0.7 cm², which was excited in compression mode, at 9 Hz, corresponding to ~38.47 mW/cm³ volume-normalized power output. The measured voltage between the top and bottom electrodes was 5.6 V. In vibration mode, at 500 Hz, the same device showed a potential of 1.4 V peak-to-peak value and an instantaneous power of 0.04 μW, corresponding to an output power density of ~0.9 mW/cm³. Full article

(This article belongs to the Special Issue Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications)

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13 pages, 2545 KiB

Open AccessArticle

Effects of Waste-Derived ZnO Nanoparticles against Growth of Plant Pathogenic Bacteria and Epidermoid Carcinoma Cells

by Titiradsadakorn Jaithon, Jittiporn Ruangtong, Jiraroj T-Thienprasert and Nattanan Panjaworayan T-Thienprasert

Crystals 2022, 12(6), 779; https://doi.org/10.3390/cryst12060779 - 27 May 2022

Cited by 17 | Viewed by 2903

Abstract

Green synthesis of zinc oxide nanoparticles (ZnO NPs) has recently gained considerable interest because it is simple, environmentally friendly, and cost-effective. This study therefore aimed to synthesize ZnO NPs by utilizing bioactive compounds derived from waste materials, mangosteen peels, and water hyacinth crude extracts and investigated their antibacterial and anticancer activities. As a result, X-ray diffraction analysis confirmed the presence of ZnO NPs without impurities. An ultraviolet–visible absorption spectrum showed a specific absorbance peak around 365 nm with an average electronic band gap of 2.79 eV and 2.88 eV for ZnO NPs from mangosteen peels and a water hyacinth extract, respectively. An SEM analysis displayed both spherical shapes of ZnO NPs from the mangosteen peel extract (dimension of 154.41 × 172.89 nm) and the water hyacinth extract (dimension of 142.16 × 160.30 nm). Fourier transform infrared spectroscopy further validated the occurrence of bioactive molecules on the produced ZnO NPs. By performing an antibacterial activity assay, these green synthesized ZnO NPs significantly inhibited the growth of Xanthomonas oryzae pv. oryzae, Xanthomonas axonopodis pv. citri, and Ralstonia solanacearum. Moreover, they demonstrated potent anti-skin cancer activity in vitro. Consequently, this study demonstrated the possibility of using green-synthesized ZnO NPs in the development of antibacterial or anticancer agents. Furthermore, this research raised the prospect of increasing the value of agricultural waste. Full article

(This article belongs to the Special Issue Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications)

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16 pages, 6890 KiB

Open AccessFeature PaperArticle

Hydrothermally Grown ZnO Nanostructures for Water Purification via Photocatalysis

by Marie Le Pivert, Nathan Martin and Yamin Leprince-Wang

Crystals 2022, 12(3), 308; https://doi.org/10.3390/cryst12030308 - 22 Feb 2022

Cited by 15 | Viewed by 5153

Abstract

Semiconductor-based photocatalysis is a well-known and efficient process for achieving water depollution with very limited rejects in the environment. Zinc oxide (ZnO), as a wide-bandgap metallic oxide, is an excellent photocatalyst, able to mineralize a large scale of organic pollutants in water, under UV irradiation, that can be enlarged to visible range by doping nontoxic elements such as Ag and Fe. With high surface/volume ratio, the ZnO nanostructures have been shown to be prominent photocatalyst candidates with enhanced photocatalytic efficiency, owing to their being low-cost, non-toxic, and able to be produced with easy and controllable synthesis. Thus, ZnO nanostructures-based photocatalysis can be considered as an eco-friendly and sustainable process. This paper presents the photocatalytic activity of ZnO nanostructures (NSs) grown on different substrates. The photocatalysis has been carried out both under classic mode and microfluidic mode. All tests show the notable photocatalytic efficiency of ZnO NSs with remarkable results obtained from a ZnO-NSs-integrated microfluidic reactor, which exhibited an important enhancement of photocatalytic activity by drastically reducing the photodegradation time. UV-visible spectrometry and high-performance liquid chromatography, coupled with mass spectrometry (HPLC-MS), are simultaneously used to follow real-time information, revealing both the photodegradation efficiency and the degradation mechanism of the organic dye methylene blue. Full article

(This article belongs to the Special Issue Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications)

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Review

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26 pages, 10612 KiB

Open AccessReview

Recent Advances in ZnO-Based Nanostructures for the Photocatalytic Degradation of Hazardous, Non-Biodegradable Medicines

by K. M. Mohamed, J. John Benitto, J. Judith Vijaya and M. Bououdina

Crystals 2023, 13(2), 329; https://doi.org/10.3390/cryst13020329 - 15 Feb 2023

Cited by 58 | Viewed by 6681

Abstract

Antibiotics are pervasive contaminants in aqueous systems that pose an environmental threat to aquatic life and humans. Typically, antibiotics are developed to counteract bacterial infections; however, their prolonged and excessive use has provoked unintended consequences. The presence of excessive amounts of antibiotics and anti-inflammatory, anti-depressive, and contraceptive drugs in hospital and industrial wastewater poses a significant threat to the ecosystem, with groundwater containing drug concentrations of <1 mg/L to hundreds of µg/L. According to the literature, 33,000 people die directly from drug-resistant bacterial infections in Europe annually, which costs EUR 1.5 billion in health care and productivity loss. Consequently, the continuous spread of antibiotics in the ecosystem has led to greater interest in developing a sustainable method for effective antibiotic removal from wastewater. This critical review aims to present and discuss recent advances in the photocatalytic degradation of widely used drugs by ZnO-based nanostructures, namely (i) antibiotics; (ii) antidepressants; (iii) contraceptives; and (iv) anti-inflammatories. This study endows a comprehensive understanding of the degradation of antibiotics using ZnO-based nanomaterials (bare, doped, and composites) for effective treatment of wastewater containing antibiotics. In addition, the operational conditions and mechanisms involved during the photocatalytic degradation process are systematically discussed. Finally, particular emphasis is devoted to future challenges and the corresponding outlook with respect to toxic effects following the utilization of ZnO-based nanomaterials. Full article

(This article belongs to the Special Issue Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications)

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18 pages, 894 KiB

Open AccessReview

Mitochondrial Dysfunction Induced by Zinc Oxide Nanoparticles

by Leslie Patrón-Romero, Priscy Alfredo Luque-Morales, Verónica Loera-Castañeda, Ismael Lares-Asseff, María Ángeles Leal-Ávila, Jorge Arturo Alvelais-Palacios, Ismael Plasencia-López and Horacio Almanza-Reyes

Crystals 2022, 12(8), 1089; https://doi.org/10.3390/cryst12081089 - 4 Aug 2022

Cited by 16 | Viewed by 5290

Abstract

The constant evolution and applications of metallic nanoparticles (NPs) make living organisms more susceptible to being exposed to them. Among the most used are zinc oxide nanoparticles (ZnO-NPs). Therefore, understanding the molecular effects of ZnO-NPs in biological systems is extremely important. This review compiles the main mechanisms that induce cell toxicity by exposure to ZnO-NPs and reported in vitro research models, with special attention to mitochondrial damage. Scientific evidence indicates that in vitro ZnO-NPs have a cytotoxic effect that depends on the size, shape and method of synthesis of ZnO-NPs, as well as the function of the cells to which they are exposed. ZnO-NPs come into contact with the extracellular region, leading to an increase in intracellular [Zn²⁺] levels. The mechanism by which intracellular ZnO-NPs come into contact with organelles such as mitochondria is still unclear. The mitochondrion is a unique organelle considered the “power station” in the cells, participates in numerous cellular processes, such as cell survival/death, multiple biochemical and metabolic processes, and holds genetic material. ZnO-NPs increase intracellular levels of reactive oxygen species (ROS) and, in particular, superoxide levels; they also decrease mitochondrial membrane potential (MMP), which affects membrane permeability and leads to cell death. ZnO-NPs also induced cell death through caspases, which involve the intrinsic apoptotic pathway. The expression of pro-apoptotic genes after exposure to ZnO-NPs can be affected by multiple factors, including the size and morphology of the NPs, the type of cell exposed (healthy or tumor), stage of development (embryonic or differentiated), energy demand, exposure time and, no less relevant, the dose. To prevent the release of pro-apoptotic proteins, the damaged mitochondrion is eliminated by mitophagy. To replace those mitochondria that underwent mitophagy, the processes of mitochondrial biogenesis ensure the maintenance of adequate levels of ATP and cellular homeostasis. Full article

(This article belongs to the Special Issue Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications)

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15 pages, 1262 KiB

Open AccessReview

Review of the Nanostructuring and Doping Strategies for High-Performance ZnO Thermoelectric Materials

by Suraya Sulaiman, Izman Sudin, Uday M. Basheer Al-Naib and Muhammad Firdaus Omar

Crystals 2022, 12(8), 1076; https://doi.org/10.3390/cryst12081076 - 31 Jul 2022

Cited by 30 | Viewed by 4340

Abstract

Unique properties of thermoelectric materials enable the conversion of waste heat to electrical energies. Among the reported materials, Zinc oxide (ZnO) gained attention due to its superior thermoelectric performance. In this review, we attempt to oversee the approaches to improve the thermoelectric properties of ZnO, where nanostructuring and doping methods will be assessed. The outcomes of the reviewed studies are analysed and benchmarked to obtain a preliminary understanding of the parameters involved in improving the thermoelectric properties of ZnO. Full article

(This article belongs to the Special Issue Novel ZnO-Based Nanostructures: Synthesis, Characterization and Applications)

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