Chandrakanth Reddy Chandraiahgari

Zinc oxide microrods and nanorods: Different antibacterial activity and their mode of action against Gram-positive bacteria

The development of antibiotic resistance among pathogenic bacteria combined with increased implant-associated infections have determined a great interest towards new bactericidal materials containing various organic and inorganic substances. Among them, zinc oxide (ZnO) derived materials have received considerable attention due to their unique antibacterial, antifungal, and UV filtering properties as well as high catalytic and photochemical activities. In the present work, we investigate the antimicrobial properties against two Gram-positive bacteria (Staphylococcus aureus and Bacillus subtilis) of ZnO microrods (MRs) and nanorods (NRs), produced in bulk quantities through simple and inexpensive methods. We demonstrate that the antimicrobial effect is strongly dependent on the rod size and dose. Scanning electron microscopy analysis revealed that the two investigated microbial types interact differently with the ZnO-MRs and NRs due to their different morphology. This resulted in different outcomes as reported by their respective Colony Forming Unit (CFU) capabilities. Moreover, Fourier Transform Infrared (FT-IR) spectroscopy revealed that changes in cell outer structures, i.e. membrane and exopolysaccharides (EPS), produced by the interaction with the ZnO structures, are responsible for the antimicrobial mechanism without the accumulation of reactive oxygen species. This was further strengthened by the increased survival observed in the case of bacterial cells treated in the presence of an osmotic support, like glycerol. In addition, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analysis showed that reduced cell viability is not strictly correlated to increased zinc ion release in the suspension. We then concluded that ZnO-NRs have a superior antimicrobial effect against both S. aureus and B. subtilis at much lower doses when compared to ZnO-MRs. This is mainly due to the smaller diameter of the NRs, which promotes surface damaging and protein alteration of the cell wall. Finally, the lack of toxicity and the antimicrobial properties of ZnO-NRs versus S. aureus, validated in vivo using the nematode Caenorhabditis elegans as host infection model, confirm the promising exploitation of ZnO-NRs in biomedical applications.

Synthesis and characterization of ZnO nanorods with a narrow size distribution

The development of novel materials for energy harvesting applications or strain sensing has generated great interest towards zinc oxide (ZnO) nanostructures, and in particular towards the synthesis of ZnO nanowires or nanorods with well controlled morphology and properties. The high-yield mass production of such nanostructures by catalyst-free methods is a crucial aspect to enable a cost-effective large-scale development of new ZnO-based piezoelectric devices and materials. In the present work, we propose a method for the mass-production of high-purity ZnO-nanorods with a uniform size distribution, based on the combination of thermal decomposition of zinc acetate dihydrate and probe sonication in acetone. The quality of the produced ZnO nanorods is assessed through multi-technique characterization using field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and photo-luminescence spectroscopy (PL). The adopted synthesis method is simple, cost effective and feasible for large-scale production. Various process parameters such as precursor amount and growth time have been found to play an important role in controlling the formation of the as grown nanostructures with high uniformity in size and morphology. Size distribution curves were employed to depict the effect of various process parameters for tailoring the morphology, homogeneity and aspect ratio of the nanorods. Our results reveal that the high crystallographic quality of ZnO nanorods grown by a long-time thermal decomposition method is not affected by probe sonication, which is proposed as a post-synthesis step necessary to produce ZnO nanorod powder with a uniform distribution of diameters and lengths.

Zinc Oxide Nanorods-Decorated Graphene Nanoplatelets: A Promising Antimicrobial Agent against the Cariogenic Bacterium Streptococcus mutans

Nanomaterials are revolutionizing the field of medicine to improve the quality of life due to the myriad of applications stemming from their unique properties, including the antimicrobial activity against pathogens. In this study, the antimicrobial and antibiofilm properties of a novel nanomaterial composed by zinc oxide nanorods-decorated graphene nanoplatelets (ZNGs) are investigated. ZNGs were produced by hydrothermal method and characterized through field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) techniques. The antimicrobial activity of ZNGs was evaluated against Streptococcus mutans, the main bacteriological agent in the etiology of dental caries. Cell viability assay demonstrated that ZNGs exerted a strikingly high killing effect on S. mutans cells in a dose-dependent manner. Moreover, FE-SEM analysis revealed relevant mechanical damages exerted by ZNGs at the cell surface of this dental pathogen rather than reactive oxygen species (ROS) generation. In addition, inductively coupled plasma mass spectrometry (ICP-MS) measurements showed negligible zinc dissolution, demonstrating that zinc ion release in the suspension is not associated with the high cell mortality rate. Finally, our data indicated that also S. mutans biofilm formation was affected by the presence of graphene-zinc oxide (ZnO) based material, as witnessed by the safranin staining and growth curve analysis. Therefore, ZNGs can be a remarkable nanobactericide against one of the main dental pathogens. The potential applications in dental care and therapy are very promising.

Evaluation of the antibacterial power and biocompatibility of zinc oxide nanorods decorated graphene nanoplatelets: new perspectives for antibiodeteriorative approaches

BackgroundNanotechnologies are currently revolutionizing the world around us, improving the quality of our lives thanks to a multitude of applications in several areas including the environmental preservation, with the biodeterioration phenomenon representing one of the major concerns.ResultsIn this study, an innovative nanomaterial consisting of graphene nanoplatelets decorated by zinc oxide nanorods (ZNGs) was tested for the ability to inhibit two different pathogens belonging to bacterial genera frequently associated with nosocomial infections as well as biodeterioration phenomenon: the Gram-positive Staphylococcus aureus and the Gram-negative Pseudomonas aeruginosa. A time- and dose-dependent bactericidal effect in cell viability was highlighted against both bacteria, demonstrating a strong antimicrobial potential of ZNGs. Furthermore, the analysis of bacterial surfaces through Field emission scanning electron microscopy (FESEM) revealed ZNGs mechanical interaction at cell wall level. ZNGs induced in those bacteria deep physical damages not compatible with life as a result of nanoneedle-like action of this nanomaterial together with its nanoblade effect. Cell injuries were confirmed by Fourier transform infrared spectroscopy, revealing that ZNGs antimicrobial effect was related to protein and phospholipid changes as well as a decrease in extracellular polymeric substances; this was also supported by a reduction in biofilm formation of both bacteria. The antibacterial properties of ZNGs applied on building-related materials make them a promising tool for the conservation of indoor/outdoor surfaces. Finally, ZNGs nanotoxicity was assessed in vivo by exploiting the soil free living nematode Caenorhabditis elegans. Notably, no harmful effects of ZNGs on larval development, lifespan, fertility as well as neuromuscular functionality were highlighted in this excellent model for environmental nanotoxicology.ConclusionsOverall, ZNGs represent a promising candidate for developing biocompatible materials that can be exploitable in antimicrobial applications without releasing toxic compounds, harmful to the environment.

Control of the size and density of ZnO-nanorods grown onto graphene nanoplatelets in aqueous suspensions

Zinc oxide nanorods (ZnO-NRs) with high density and chemical purity were grown onto unsupported graphene nanoplatelets (GNPs) in aqueous suspensions, using two different growth approaches namely: a hydrothermal method and ultrasonic probe sonication. The size and density of the ZnO-nanorods grown onto graphene nanoplatelets were controlled through seed layer deposition and through the proper setting of the process parameters, in particular through the control of the fluidodynamics of the colloidal suspension during the growth. The highest growth density of the ZnO nanorods having a diameter of ∼45 nm was obtained onto GNPs seeded by the probe sonication method and through the hydrothermal method in dynamic conditions. XRD and XPS investigations confirmed that all produced ZnO-GNP composites are characterized by high crystallinity and purity, although solution dynamics affected their UV luminescence. The proposed approaches enable the controlled high-density growth of crystalline ZnO-NRs onto GNPs in an aqueous suspension, at a low cost, and are suitable for large scale production.

Nanofiller induced electroactive phase formation in solution derived poly(vinylidene fluoride) polymer composites

Electroactive self-standing flexible composite poly(vinylidene fluoride) ( PVDF) films were prepared by simple solution casting method. In this study we investigated the effect of two different nanofillers, namely GNPs and ZnO NRs and the use of five different hexahydrate salts (HS) on the nucleation of the electroactive phase in the solution derived PVDF films. FT-IR analysis revealed that both HS dissolution and nanofillers dispersion in the PVDF matrix induced the electroactive phase formation without any electrical poling ascribed to their strong surface interaction with the polymeric chains. Moreover, we observed that the electroactive phase is still present when HS dissolution is combined with nanofiller dispersion, thus opening the route to the possibility of tailoring the operating frequency of the resulting electroactive material. This versatile electroactive phase induction by the nanofillers is a prominent way for production of large-area flexible piezoelectric films.

Piezoelectric Effect and Electroactive Phase Nucleation in Self-Standing Films of Unpoled PVDF Nanocomposite Films

Novel polymer-based piezoelectric nanocomposites with enhanced electromechanical properties open new opportunities for the development of wearable energy harvesters and sensors. This paper investigates how the dissolution of different types of hexahydrate metal salts affects β-phase content and piezoelectric response (d33) at nano- and macroscales of polyvinylidene fluoride (PVDF) nanocomposite films. The strongest enhancement of the piezoresponse is observed in PVDF nanocomposites processed with Mg(NO3)2⋅6H2O. The increased piezoresponse is attributed to the synergistic effect of the dipole moment associated with the nucleation of the electroactive phase and with the electrostatic interaction between the CF2 group of PVDF and the dissolved salt through hydrogen bonding. The combination of nanofillers like graphene nanoplatelets or zinc oxide nanorods with the hexahydrate salt dissolution in PVDF results in a dramatic reduction of d33, because the nanofiller assumes a competitive role with respect to H-bond formation between PVDF and the dissolved metal salt. The measured peak value of d33 reaches the local value of 13.49 pm/V, with an average of 8.88 pm/V over an area of 1 cm2. The proposed selection of metal salt enables low-cost production of piezoelectric PVDF nanocomposite films, without electrical poling or mechanical stretching, offering new opportunities for the development of devices for energy harvesting and wearable sensors.

SEM tomography for the investigation of hybrid structures

The morphological investigation at the micrometric scale of a graphene - ZnO nanorods hybrid structure is performed by scanning electron microscopy. When operated in the scanning-transmission imaging mode, the detection strategy allows implementation of a tomographic approach to recover the three dimensional spatial arrangement of the sample constituents. This tomographic approach complements the serial-sectioning imaging methods and is suitable for thin, self-standing specimens.

PVDF composite films including graphene/ZnO nanostructures and their antimicrobial activity

Poly(vinylidene fluoride) (PVDF) is a biocompatible polymer commonly used for biomedical applications, food packaging and hygiene products. Within these contexts, the polymer surface is typically exposed to microorganisms and bacteria, with consequent biofilm formation. In this paper, self-standing flexible composite poly(vinylidene fluoride) (PVDF) films with antimicrobial properties were prepared by simple solution casting method. We investigated the effect of three different types of nanofillers, namely graphene nanoplatelets (GNPs), zinc oxide nanorods (ZnO-NRs) and ZnO-NR-decorated GNPs (ZNGs), on the antimicrobial activity of PVDF composite films against Pseudomonas aeruginosa. ZnO-NRs were also grown directly over the surface of PVDF composite films filled with ZnO nanoparticles, acting as nucleation seeds. In all cases, vitality tests performed in static conditions demonstrated a strong antimicrobial property of the produced specimens. This effect can be mainly ascribed to the bacteria/nanomaterials strong surface interaction, which results in the biofilm integrity disruption.