Applied Materials Today
Engineered nanomaterials for antimicrobial applications: A review
Graphical abstract
Introduction
The high rate of infectious disease outbreaks and illnesses stemming from disease carrying pathogens have and will always be of great concern. Consequences of these outbreaks lead to high costs of resolution, disruption in daily activities, economic recession of regions and countries and in severe cases death. Eradication of some of these infectious diseases has been achieved. However, a comeback has been observed for some diseases thought to be already eradicated, with resistant strains to modern day medicine [1].
Antibiotics and antimicrobial resistant microbes (“superbugs”) come about because of rampant use and misuse and incomplete dosage of antimicrobials such as antibiotics, antifungals, antivirals or anti-parasitics, leading to these pathogens recovering with resistance the specific antimicrobials. Vaccinations have been developed to combat the spread of diseases by increasing immunity of individuals towards various diseases. However, a significant amount of the population either react severely to these vaccines or refuse taking it due to person believes and/or stereotypes. Hence, reducing the wide spread use, poses a risk to the general population and efficiency [2,3]. This has become a serious problem in public health, leading to millions of dollars of ineffective pharmaceuticals. Likewise, spread of these diseases is prolific especially with the lack of proper antimicrobials to tackle the problem.
Antibiotics, chemical disinfectants and other forms of microbial biocides are highly effective against microorganisms. However, these effects are temporal as they wear off fairly quickly or diminish over a short period of time. Therefore, constant reapplication is required to keep up with disinfection [4]. This can be cumbersome, expensive and sometimes logistically inefficient. Similarly, the efficiency of these agents is limited to how well and long they are applied.
In this day and age of widespread advanced industrialization, manufacturing, and commercialization of all sorts of products with the potential to reach all corners of the globe, an avenue for the potential spread of microbes through goods and products contaminated especially at manufacturing facilities can be problematic. Therefore, it is of utmost importance to curb, if not eliminate the spread and growth of microbes on manufacturing facilities, equipment and products, hospital equipment and devices, consumable clinical materials and especially processed and packaged foods. Therefore, researchers and pharmaceutical companies have been driven to investigate new potent antibacterial agents to curb the spread of diseases and antibiotics resistant bacteria strains.
Of recent, controlling bacteria using antimicrobial materials such as nanoparticles have caught the attention of researchers across the globe. Interests in the use of these materials stem from the morphological and physicochemical properties such as high surface area to volume ratio, physical and chemical properties which have been successfully used in other applications [5,6], dissociation response stimulated by different environments over time and potency towards a wide spectrum of pathogens [7,8]. Also, the surface charge on these nanoparticles can facilitate binding to the opposite surface charge of the bacteria, leading to effective antimicrobial activities [9]. Likewise, the effective longevity and durability of these antimicrobial nanoparticles when utilized in antimicrobial applications are promising due to their insolubility [10] as well as close interaction with microbial membranes [11].
It has been shown that a nanoparticle with a size of around 1–10 nm has outstanding antimicrobial activity efficiency due to the increased contact area with bacteria [7]. Therefore, nanoparticles with increasingly smaller dimension, for example, metal nanoparticles with biocidal properties including silver, copper oxide, zinc oxide, titanium dioxide, zero-valent iron; carbon nanotubes, and bio-nanoparticles like chitosan nanocomposites are part of a class of antimicrobial nanoparticles which are currently under extensive investigation [12]. These nanoparticles show excellent biocidal activities toward a range of microorganisms without the concern of developing resistant strains [9].
The application of antimicrobial nanoparticles and their mechanism against microbes have been well investigated from a biological and healthcare viewpoint and extensively reported in the literature. Here, we reviewed the current advances in antimicrobial nanoparticle applications from a materials engineering standpoint in very relevant application platforms such as healthcare, food packaging and water treatment. Furthermore, niche areas such as coatings for architectural paints, antifouling, clothing, personal care (soaps and cosmetics), and air filtration/quality are also reviewed. The review extracted current advances in antimicrobial nanoparticle engineering and their applications while identifying knowledge gaps and indicating future research directions.
Bacteria constitute an outer cell wall structure which is used to determine their classification; they are either classified as gram-positive or -negative. Gram-positive bacteria have a thicker layer of peptidoglycan in their cell walls than gram-negative bacteria do, while lacking an outer membrane of lipopolysaccharide which gram-negative bacteria have. This means that gram-negative bacteria have an extra layer (periplasmic) sandwiched between the outer layer and the plasma membrane as shown in Fig. 1a. However, this layer is thinner in comparison to that of gram-positive bacteria as mentioned earlier and therefore, susceptible to penetration and ultimately destruction of the cell when exposed to antimicrobial nanoparticles [[13], [14], [15]].
In the following sections, bactericidal mechanisms of antimicrobial nanoparticles and their effectiveness, specific to the various applications used are discussed in detail. A summary of the likely bactericidal mechanism of antimicrobial nanoparticles, especially those of metal ions is schematically represented in Fig. 1b. Similarly, the general modes of action and possibility of multiple simultaneous antimicrobial activities occurring are summarized through the following mechanisms described below:
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Antibacterial effects by reactive oxygen species (ROS) generation: ROS (e.g. superoxide anions, hydroxyl radical, and hydrogen peroxide) are produced after exposure to nanomaterials such as metal oxides. These ROS can induce the peroxidation of the polyunsaturated phospholipids in the bacterial cells to damage DNA, and subsequently, cell death [16,17].
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Antimicrobial effects by physical damage: Bacterial cell wall membranes can be damaged when interacting with sharp edges of the nanostructured material [18].
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Antimicrobial effects by binding: Binding materials on the bacterial cell wall can cause loss of cell membrane integrity and efflux of cytoplasmic materials [19].
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Antimicrobial effects by release of metal ions: Metal ions released from the nanomaterials into culture media can inhibit the ATP production and DNA replication to destroy the cells [20].
Section snippets
Antimicrobial nanoparticles (ANPs) in healthcare applications
One of the greatest concerns in the health sector is the spread and prevention of diseases from pathogens especially from bacteria and viruses. Pathogenic contamination can be difficult to curtail and can become fatal in some cases by the time it is realized. Whether it be in medical facilities such as hospitals, laboratories or at pharmaceutical industries, there is always the possibility of contamination or disease transmission through various means due to the disease-causing microbes’ size
Antimicrobial nanomaterial applications in food packaging and processes
Microbial food spoilage is a major global concern that can reduce the shelf life of food while increasing the risk of foodborne diseases. According to world health organization (WHO), foodborne pathogens cause up to 600 million illnesses and 420,000 deaths worldwide every year [48]. In order to improve the safety and shelf life of food, there are various food preservation strategies that are extensively employed in the industry including pasteurization, thermal sterilization, chemical
Nanomaterials for water disinfection
Water is a fundamental resource for society both for human consumption and industrial processes. This vital resource has shown increasing scarcity in recent years with prolonged droughts becoming a common phenomenon around the globe, which has posed a serious threat to the sustainable development of nations. Although our planet possesses vast amounts of water, only a limited amount of this resource is available for human consumption. Most of this water source is not apt for its direct usage due
Nanomaterials for antimicrobial coatings and other niche applications
Antimicrobial agents are typically incorporated into most cleaning agents and household products. These agents in the products are highly effective and can inhibit up to 99.9% of microbes upon application [224]. However, the antimicrobial action has a relatively short effective life span as microbes find their way back to treated surfaces straightaway. Therefore, for surfaces requiring constant inhibition of microbes such as medical devices, food contact surfaces etc. a permanent treatment
Challenges, future trends and concluding remarks
Nanoparticles have shown great potential for antimicrobial activities and applications especially due to their high surface area and size, leading to increased contact with microbes. Moreover, the antibiotic resistance of microorganisms is irrelevant for nanoparticles as the modes of action of antimicrobial nanoparticles end in contact with the microorganisms’ cell wall and destruction, which is less prone to promote antibiotics resistance. The utilization of nanotechnology in targeting and
Acknowledgements
Emmanuel O. Ogunsona, Ewomazino Ojogbo, and Tizazu H. Mekonnen acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) for the financial support in carrying out this research. We will also like to thank our colleagues in the same group for the insightful discussions on the subject matter.
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