Application of antibacterial nanoparticles in orthodontic materials

During the orthodontic process, increased microbial colonization and dental plaque formation on the orthodontic appliances and auxiliaries are major complications, causing oral infectious diseases, such as dental caries and periodontal diseases. To reduce plaque accumulation, antimicrobial materials are increasingly being investigated and applied to orthodontic appliances and auxiliaries by various methods. Through the development of nanotechnology, nanoparticles (NPs) have been reported to exhibit excellent antibacterial properties and have been applied in orthodontic materials to decrease dental plaque accumulation. In this review, we present the current development, antibacterial mechanisms, biocompatibility, and application of antibacterial NPs in orthodontic materials.

Progress on photocatalytic semiconductor hybrids for bacterial inactivation

Due to its use of green and renewable energy and negligible bacterial resistance, photocatalytic bacterial inactivation is to be considered a promising sterilization process. Herein, we explore the relevant mechanisms of the photoinduced process on the active sites of semiconductors with an emphasis on the active sites of semiconductors, the photoexcited electron transfer, ROS-induced toxicity and interactions between semiconductors and bacteria. Pristine semiconductors such as metal oxides (TiO₂ and ZnO) have been widely reported; however, they suffer some drawbacks such as narrow optical response and high photogenerated carrier recombination. Herein, some typical modification strategies will be discussed including noble metal doping, ion doping, hybrid heterojunctions and dye sensitization. Besides, the biosafety and biocompatibility issues of semiconductor materials are also considered for the evaluation of their potential for further biomedical applications. Furthermore, 2D materials have become promising candidates in recent years due to their wide optical response to NIR light, superior antibacterial activity and favorable biocompatibility. Besides, the current research limitations and challenges are illustrated to introduce the appealing directions and design considerations for the future development of photocatalytic semiconductors for antibacterial applications.

Bio-dissolution process and mechanism of copper phosphate hybrid nanoflowers by Pseudomonas aeruginosa and its bacteria-toxicity in life cycle

Copper phosphate hybrid nanoflowers (HNF) have been widely used in chemical industries and wastewater treatment owing to its excellent catalytic activity and high stability. However, their fate and ecological risks have not received due attention after being discharged into natural environment. The significance of bacteria on the dissolution and fate of HNF and its toxicity to bacteria was evaluated from the perspective of its life cycle. Results showed that in the presence of Pseudomonas aeruginosa, HNF was gradually 'disassembled' into smaller nanoparticles (NPs), and then dissolved completely. More than half of the dissolution products (Cu²⁺) entered biological phase, and PO₄^3- was absorbed and utilized by bacteria as a phosphorus source. The mechanisms of HNF bio-dissolution are as follows: the metabolites of bacteria dissolve HNF through complexation and acidification, in which small molecular organic acids and amino acids play an important role. Bacteria toxicity experiments of HNF in its cycle life show that HNF exhibits lower cell toxicity, but its intermediate (smaller NPs) and final dissolved products (Cu²⁺) exhibit stronger cytotoxicity by increasing the level of intracellular ROS and membrane permeability of bacteria. This research is helpful to provide ecological risk assessment, develop targeted applications, and rationally design future nanomaterials.

Effects of Chloride Ions on Dissolution, ROS Generation, and Toxicity of Silver Nanoparticles under UV Irradiation

This work investigates the effect of chloride ion (Cl^-) on dissolution, reactive oxygen species (ROS) generation, and toxicity of citrate-coated silver nanoparticles (AgNPs) under UV irradiation. The dissolution rate was decreased by 0.01 M Cl^- due to AgCl passivation on the AgNP surface. By contrast, high concentrations of Cl^- (0.1 or 0.5 M) promoted dissolution due to the formation of soluble Ag-Cl complexes (AgCl _x^{1- x}). The generation of O₂^•- in the AgNPs/Cl^-/UV system was promoted by 0.01 M Cl^-, whereas it was retarded by 0.1 or 0.5 M Cl^-, which was probably because the aggregation of AgNPs at high ionic strength reduced the nanoparticles' surface areas for radical formation. Additionally, Cl^- contributed to •OH generation in the AgNPs/Cl^-/UV system, in which the produced •OH concentrations increased with increasing Cl^- concentrations. The reduction reaction between silver ions and O₂^•- resulted in lower dissolution rates of AgNPs/Cl^- mixtures under UV irradiation than those in the dark. The phototoxicity of AgNPs toward E. coli with different concentrations of Cl^- followed the order of 0.5 M > 0 M > 0.1 M > 0.01 M. Both ROS and dissolved Ag played significant role in the phototoxicity of AgNPs. This work demonstrates the potential importance of anions in the fate and biological impact of AgNPs.

Fabrication of bacterial cellulose-ZnO composite via solution plasma process for antibacterial applications

Zinc oxide (ZnO) was successfully synthesized by applying a solution plasma, a plasma discharge in a liquid phase, without the addition of a reducing agent and simultaneously deposited into a bacterial cellulose pellicle that functioned as a template. By the reasons of its nano-sized structure as well as favorable porous configuration, the BC pellicle has been proved to be a splendid upholding template for the coordination of ZnO. In addition, the ZnO-deposited BC composites demonstrated strong antibacterial activity without a photocatalytic reaction against both Staphylococcus aureus and Escherichia coli. Hence, the ZnO-deposited BC composites can be used as an antibacterial material in wound dressing and water disinfection applications.

Photocatalyzed Reduction of Bicarbonate to Formate: Effect of ZnS Crystal Structure and Positive Hole Scavenger

Zinc sulfide is a promising catalyst due to its abundance, low cost, low toxicity and conduction band position that enables the photoreduction of CO2 to formic acid. This study is the first to examine experimentally the photocatalytic differences between wurtzite and sphalerite under the parameters of size (micrometer and nanoscale), crystal lattice, surface area, and band gap on productivity in the photoreduction of HCO3(-). These photochemical experiments were conducted under air mass coefficient zero (AM 0) and AM 1.5 solar simulation conditions. We observed little to no formate production under AM 1.5, but found linear formate production as a function of time using AM 0 conditions. Compared to earlier reports involving bubbled CO2 in the presence of bicarbonate, our results point to bicarbonate as the species undergoing reduction. Also investigated are the effects of three hydroxylic positive hole scavengers, ethylene glycol, propan-2-ol (isopropyl alcohol, IPA) and glycerol on the reduction of HCO3(-). Glycerol, a green solvent derived from vegetable oil, greatly improved the apparent quantum efficiency of the photocatalytic reduction.

Radical macro spatial separation: a novel way to enhance the photocatalytic efficiency

Radical macro spatial separation can be achieved via a new photocatalytic system and significantly improved the photocatalytic efficiency.

Synthesis of poly acrylic acid modified silver nanoparticles and their antimicrobial activities

Poly acrylic acid modified silver (Ag/PAA) nanoparticles (NPs) have been successfully synthesized in the aqueous solution by using tannic acid as a reductant. The structure, morphology and composition of Ag/PAA NPs were characterized by various techniques such as X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible absorption spectroscopy (UV-vis) and thermogravimetry analysis (TGA). The results show that PAA/Ag NPs have a quasi-ball shape with an average diameter of 10 nm and exhibit well crystalline, and the reaction conditions have some effect on products morphology and size distribution. In addition, the as-synthesized Ag/PAA NPs antimicrobial activities against Escherichia coli (E. coli), Pseudomonas aeruginosa (P. aeruginosa) and Staphylococcus aureus (S. aureus) were evaluated by the methods of broth dilution, cup diffusion, optical density (OD600) and electron microscopy observation. The as-synthesized Ag/PAA NPs exhibit excellent antibacterial activity. The antimicrobial mechanism may be attributed to the damaging of bacterial cell membrane and causing leakage of cytoplasm.

Schottky or Ohmic metal-semiconductor contact: influence on photocatalytic efficiency of Ag/ZnO and Pt/ZnO model systems

The relationship between the contact type in metal-semiconductor junctions and their photocatalytic efficiencies is investigated. Two metal-semiconductor junctions, silver on zinc oxide (Ag/ZnO) and platinum on zinc oxide (Pt/ZnO) serve as model system for Ohmic and Schottky metal-semiconductor contact, respectively. Ag/ZnO, with Ohmic contact, exhibits a higher photocatalytic efficiency than Pt/ZnO, with Schottky contact. The direction of electric fields within the semiconductor is found to play a crucial role in the separation of photogenerated charges, and thus strongly influences the photocatalytic efficiency.

Antibacterial activity of ZnO nanoparticles with a modified surface under ambient illumination

In various practical applications, nanomaterials typically have functionalized surfaces. Yet, the studies of toxicity and antibacterial activity of functionalized nanoparticles are scarce. We investigated the effect of surface modifications on antibacterial activity of ZnO under ambient illumination, and we found that nanoparticles coated with different surface modifying reagents could exhibit higher or lower toxicity compared to bare ZnO, depending on the surface modifying reagent used. Different surface modifying reagent molecules resulted in differences in the release of Zn(2+) ions and the production of reactive oxygen species (ROS). However, the antibacterial activity did not correlate with the ROS levels or the Zn(2+) ion release. One of the surface-modified ZnO samples exhibited significantly lower Zn(2+) ion release while at the same time exhibiting improved antibacterial activity. In all cases, damage of the cell wall membranes and/or changes in the membrane permeability have been observed, together with the changes in ATR-FTIR spectra indicating differences in protein conformation. Mechanisms of antibacterial activity are discussed.

Influence of lattice integrity and phase composition on the photocatalytic hydrogen production efficiency of ZnS nanomaterials

Ambient S annealing was adopted to regulate the crystallinity of a ZnS microsphere, which resulted in a significant improvement in the photocatalytic hydrogen production activity (PHPA). Moreover, with S ambient treatment, wurtzite ZnS showed better PHPA than sphalerite ZnS, possibly because the inter-polar electric field of the wurtzite phase could promote the separation of photo-excited electron-hole pairs.