Preparations of Silver/Montmorillonite Biocomposite Multilayers and Their Antifungal Activity

A Review on Montmorillonite-Based Nanoantimicrobials: State of the Art

One of the crucial challenges of our time is to effectively use metal and metal oxide nanoparticles (NPs) as an alternative way to combat drug-resistant infections. Metal and metal oxide NPs such as Ag, Ag₂O, Cu, Cu₂O, CuO, and ZnO have found their way against antimicrobial resistance. However, they also suffer from several limitations ranging from toxicity issues to resistance mechanisms by complex structures of bacterial communities, so-called biofilms. In this regard, scientists are urgently looking for convenient approaches to develop heterostructure synergistic nanocomposites which could overcome toxicity issues, enhance antimicrobial activity, improve thermal and mechanical stability, and increase shelf life. These nanocomposites provide a controlled release of bioactive substances into the surrounding medium, are cost effective, reproducible, and scalable for real life applications such as food additives, nanoantimicrobial coating in food technology, food preservation, optical limiters, the bio medical field, and wastewater treatment application. Naturally abundant and non-toxic Montmorillonite (MMT) is a novel support to accommodate NPs, due to its negative surface charge and control release of NPs and ions. At the time of this review, around 250 articles have been published focusing on the incorporation of Ag-, Cu-, and ZnO-based NPs into MMT support and thus furthering their introduction into polymer matrix composites dominantly used for antimicrobial application. Therefore, it is highly relevant to report a comprehensive review of Ag-, Cu-, and ZnO-modified MMT. This review provides a comprehensive overview of MMT-based nanoantimicrobials, particularly dealing with preparation methods, materials characterization, and mechanisms of action, antimicrobial activity on different bacterial strains, real life applications, and environmental and toxicity issues.

Antifungal activity of montmorillonite/peptide aptamer nanocomposite against Colletotrichum gloeosporioides on Stylosanthes

Chemical agents are effective treatment methods for anthracnose induced by pathogenic Colletotrichum gloeosporioides on Stylosanthes. However, excess consumption of chemical agents destroys the environment, synthetic biology was capable of conquering the issue. The antifungal agent is developed by enclosing a bio-synthesized peptide aptamer with layered montmorillonite via electrostatic interaction. Compared with free peptide aptamer, the nanocomposite exhibits higher antifungal activity against Colletotrichum gloeosporioides, further improving the utilization of peptide aptamer. The nanocomposite killed Colletotrichum gloeosporioides by releasing peptide aptamer after they entered the spore. Moreover, montmorillonite enhances the adhesion ability of peptide aptamer via hydrophobic interactions between nanomaterials and leaves, prolonging the extension time of nanocomposite on leaves. Consequently, 0.1 mg of nanocomposite demonstrates a comparable effect to commercial carbendazim (1 %) to prevent anthracnose on leaves of Stylosanthes induced by HK-04 at room temperature. This work demonstrates an alternative to commercial antifungal agents and proposes a versatile approach to preparing environmental-friendly antifungal agents to inhibit fungal infections on crops.

Investigation of Spin Coating Cerium-Doped Hydroxyapatite Thin Films with Antifungal Properties

In this study, the cerium-doped hydroxyapatite (Ca10−xCex(PO4)6(OH)2 with xCe = 0.1, 10Ce-HAp) coatings obtained by the spin coating method were presented for the first time. The stability of the 10Ce-HAp suspension particles used in the preparation of coatings was evaluated by ultrasonic studies, transmission electron microscopy (TEM), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The surface morphology of the 10Ce-HAp coating was studied by SEM and atomic force microscopy (AFM) techniques. The obtained 10Ce-HAp coatings were uniform and without cracks or unevenness. Glow discharge optical emission spectroscopy (GDOES) and X-ray photoelectron spectroscopy (XPS) were used for the investigation of fine chemical depth profiling. The antifungal properties of the HAp and 10Ce-HAp suspensions and coatings were assessed using Candida albicans ATCC 10231 (C. albicans) fungal strain. The quantitative antifungal assays demonstrated that both 10Ce-HAp suspensions and coatings exhibited strong antifungal properties and that they successfully inhibited the development and adherence of C. albicans fungal cells for all the tested time intervals. The scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) visualization of the C. albicans fungal cells adherence to the 10Ce-HAp surface also demonstrated their strong inhibitory effects. In addition, the qualitative assays also suggested that the 10Ce-HAp coatings successfully stopped the biofilm formation.

Chitosan-Hydroxyapatite Composite Layers Generated in Radio Frequency Magnetron Sputtering Discharge: From Plasma to Structural and Morphological Analysis of Layers

Chitosan-hydroxyapatite composite layers were deposited on Si substrates in radio frequency magnetron sputtering discharges. The plasma parameters calculated from the current-voltage radio frequency-compensated Langmuir probe characteristics indicate a huge difference between the electron temperature in the plasma and at the sample holder. These findings aid in the understanding of the coagulation pattern of hydroxyapatite-chitosan macromolecules on the substrate surface. An increase in the sizes of the spherical-shape grain-like structures formed on the coating surface with the plasma electron number density was observed. The link between the chemical composition of the chitosan-hydroxyapatite composite film and the species sputtered from the target or produced by excitation/ionization mechanisms in the plasma was determined on the basis of X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and residual gas mass spectrometry analysis.

Antimicrobial Properties of Samarium Doped Hydroxyapatite Suspensions and Coatings

Post-implant infections are a major health problem, and it is well-known that treating them with conventional drugs is accompanied by many disadvantages. The development of new biomaterials with enhanced antimicrobial properties are of major interest for the scientific world. The aim of this study was to synthesize and characterize hydroxyapatite doped with Samarium (Ca10−xSmx(PO4)6(OH)2, xSm = 0.05, 5Sm-HAp) suspensions, pellets and coatings. The 5Sm-HAp coatings on Si substrates were obtained by rf magnetron sputtering technique. The different techniques such as ultrasound measurements, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Glow Discharge Optical Emission Spectroscopy (GDOES), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to examine the obtained coatings. The results showed that the doped Sm ions entered the structure of hydroxyapatite successfully and Sm ions was uniformly doped onto the surface of the support. The depth profile curves of Ca, P, O, H, Ce and Si elements exhibit their presence from a surface to substrate interface as function of sputtering time. XPS analysis indicated as calcium-phosphate structures enriched in Sm3+ ions. Furthermore, the antimicrobial properties of the 5Sm-HAp suspensions, targets and coatings were assessed against Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 25923 and Candida albicans ATCC 10231. The results of the antimicrobial assays highlighted that that the samples presented a strong antimicrobial activity against the tested microbial strains. The results showed that the coatings after 48 h of incubation inhibited the growth of all tested microbial strains under the value of 0.6 Log CFU/mL. This study shows that the 5Sm-HAp samples are good candidates for the development of new antimicrobial agents.

A Refined Theory for Characterizing Adhesion of Elastic Coatings on Rigid Substrates Based on Pressurized Blister Test Methods: Closed-Form Solution and Energy Release Rate

Adhesion between coatings and substrates is an important parameter determining the integrity and reliability of film/substrate systems. In this paper, a new and more refined theory for characterizing adhesion between elastic coatings and rigid substrates is developed based on a previously proposed pressurized blister method. A compressed air driven by liquid potential energy is applied to the suspended circular coating film through a circular hole in the substrate, forcing the suspended film to bulge, and then to debond slowly from the edge of the hole as the air pressure intensifies, and finally to form a blister with a certain circular delamination area. The problem from the initially flat coating to the stable blistering film under a prescribed pressure is simplified as a problem of axisymmetric deformation of peripherally fixed and transversely uniformly loaded circular membranes. The adhesion strength depends on the delamination area and is quantified in terms of the energy released on per unit delamination area, the so-called energy release rate. In the present work, the problem of axisymmetric deformation is reformulated with out-of-plane and in-plane equilibrium equations and geometric equations, simultaneously improved, and a new closed-form solution is presented, resulting in the new and more refined adhesion characterization theory.