Nanotechnology for Disinfection and Sterilization
David W. Hobson
Naturally occurring mineral, liquid, and organic particles come in a wide range of sizes, shapes, and compositions in the universe, and living things require particles of many types, including particles with dimensions in the nanometer range, nanoparticles (NPs), to support normal biological functions and for survival. In marine, desert, forest, polar, and other free tropospheric environments, natural processes such as secondary aerosol formation, dust storms, and breaking waves are major sources of natural NPs.1,2 The atmosphere of the earth is so laden with particles, including NPs, that each breath contains roughly 50 million, give or take a few million, particles of 50 nm or less in dimension.3 The light scattering effects of these particles are largely responsible for the beautiful sunrises and sunsets that we observe.1,4 In the upper parts of the atmosphere (stratosphere, mesosphere, and thermosphere), meteoritic smoke is also a source of NPs.5 Regardless of particle size and composition, each different type of NP exhibits distinctive chemical, physical, and optical properties; biotransformation mechanisms; biological deposition; and elimination pathways that are often distinct from the elements that compose the NP and from larger, micron-sized particles of the same composition.1,2 It is clear then that living with NPs is not exactly new. The NPs that are ever present in our atmosphere such as ocean spray, volcanic ash, airborne soil sediment, and even man-made emissions such as industrial smoke, motor vehicle exhaust, dust and vapor have been around living things for a long time and, in some cases, even since life began on our planet. In fact, many cellular structures and particles with nanometer dimensions are essential for life.1 Some types of NPs with elemental compositions demonstrate antimicrobial properties such as silver (Ag), gold (Au), copper (Cu), zinc NPs, as well as other nanomaterials of various compositions and molecular structures including NP “functionalized” single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT).6,7
The ability to manufacture and engineer nanomaterials, or to control matter at the nanoscale at dimensions between approximately 1 and 100 nm, is termed nanotechnology and is a relatively new and rapidly developing and vast technology platform finding application in essentially every area of industry including a growing number of applications in medicine including the development of new, technologically advanced antimicrobial technologies. Some of these new nanotechnological antimicrobial materials have applications in disinfection and sterilization products and processes.8,9 In the 1 to 100 nm dimensional range, unique phenomena that enable the novel applications occur and represent a technological dimension and tool in our ability to design and develop new products in essentially every area of human endeavor.10 Nanotechnology encompasses nanoscale science and nanoengineering that involves imaging, measuring, modeling, and manipulating matter at the nanoscale. With the development of current and emerging technology to manipulate matter in the nanoscale and even into the atomic range, humans have arrived at a point where we possess the technological capability to engineer and construct at will essentially any feasible molecular structure using an atomic palette that includes a vast number and variety of natural and synthetic nuclides and an almost unlimited molecular structural potential.1
Even though there is great potential to engineer and develop novel nanomaterials, there can be limits to what applications can be commercially successful and at what point a given nanomaterial may pose a potential risk to human, animal, or environmental safety. Therefore, in the application of nanotechnology for disinfection or sterilization, there is a rapidly growing number of engineered nanoproducts in development and commerce that includes effective antimicrobial products that can be safe and others that are of greater concern for potential human, animal, or environmental toxicological effects.1 Those of greatest concern for safety typically require contact with
biological systems that would include uses in antimicrobial technologies as well as pharmaceuticals, medical devices, cosmetics, etc. Conversely, items such as electronics use polymers and coatings (including those that are antimicrobial) applied to metals, glass, and plastics, where the nanomaterial is permanently or very strongly bound to the larger molecular structure or are otherwise not generally available for biological contact, the likelihood of significant toxicological effect, and risk is typically expected to be of lesser concern.1
biological systems that would include uses in antimicrobial technologies as well as pharmaceuticals, medical devices, cosmetics, etc. Conversely, items such as electronics use polymers and coatings (including those that are antimicrobial) applied to metals, glass, and plastics, where the nanomaterial is permanently or very strongly bound to the larger molecular structure or are otherwise not generally available for biological contact, the likelihood of significant toxicological effect, and risk is typically expected to be of lesser concern.1
 FIGURE 74.1 Nanometer-sized materials dimensions relative to the dimensions of different biological structures and microbes. (Based on Hobson et al.1) |
A size-oriented, microbial and cellular (as well as toxicological) dimensional perspective of nanomaterial exposure to biological systems is shown in Figure 74.1. From this figure, it can readily be seen that materials within the critical nanosize range of 1 to 100 nm are also within the size range where cellular structures and biomolecules occur. Therefore, the efficacy and even safety of any given nanomaterial is dependent not only on its chemical composition but also its molecular size and structure.1 There is increasing toxicological evidence that some types of nanomaterials have both chemical and structural features that could lead to a potential for adverse biological effects.11 Nanomaterials can be naturally occurring or man-made and are most generally identified as materials having at least one dimension <100 nm.12 This chapter focuses primarily on the use of man-made, engineered nanomaterials that are products of nanotechnology rather than the effects of naturally occurring nanomaterials.
DESIGN, CHEMISTRY, MOLECULAR STRUCTURE, AND NOMENCLATUREAt the nanometer dimensional level, scientists and engineers endeavor to control individual atoms and molecules to do amazing things. Researchers are employing nanotechnology to advance technology toward solving major challenges in health, energy, agriculture, water quality, materials science, and other areas. Scientists and engineers engaged in the development of nanotechnology are, for the first time in human history, able to advance nanotechnology by making molecular structures at will to include an almost infinite variety of potential molecules and molecular structures and polymers. Essentially, any reasonably stable, nanomolecular structure and composition is now potentially feasible by design. This advanced materials fabrication capability is translating rapidly into new products in essentially every business sector and so, it should not be surprising that nanotechnology is being used to develop and advance new, safe, and effective sterilization and disinfection products. The use of nanotechnology in these products is especially of interest because it represents a new and potentially useful technology platform to help combat an increasing array of antibioticresistant bacteria. So, even in the world of infection prevention as well as the development of novel antimicrobials for other applications, it is imperative that scientists and clinicians working to advance these nanotechnologies have some understanding of their chemistry and structural characteristics and nomenclature in order to stay abreast of technological advances of interest.
The chemistry of nanomaterials covers a broad and increasing landscape and is also diverse. At the simplest level, there are NPs made from pure elements such as NPs of Ag, Au, iron (Fe), silicon, etc. Then, at a slightly higher level of chemical and molecular structural sophistication, there are particles composed of combinations of atoms such as titanium dioxide (TiO2), Fe oxide, zinc oxide (ZnO), zirconium oxide, etc. For example, zirconium oxide NPs (ZrO2), like many other elemental NPs, can be fabricated and made available for a variety of applications as nanodots, nanofluids, and nanocrystals. Zirconium oxide NPs are also often doped with yttrium oxide, calcia, or magnesia to impart different antimicrobial properties for a variety of applications including use in dental infection control.13
Elemental AgNPs represent the simplest and oldest major use of an engineered NP as an antimicrobial agent that has been used with success since ancient times and into the present in modern antimicrobial products as engineered AgNPs.14,15,16 The AgNPs have been shown to possess several mechanisms involved in their bactericidal efficacy. These mechanisms include attachment to the bacterial cell surface and penetration of the bacterial cell membrane, interrupting permeability and disrupting metabolic pathways of the cell.14,17 In addition, AgNPs can bind to bacterial DNA and prevent its replication and/or through a lethal interaction with bacterial ribosomes.18,19 The AgNP damage to the structure of the bacterial cell membrane results in leakage and reduces the activity of some membranous enzymes that has been shown to be lethal to Escherichia coli bacteria.20 The AgNPs also typically exhibit greater toxicity to microorganisms and lower toxicity to mammalian cells making them effective and potentially safe antimicrobial candidates that have found their way into commercial antimicrobial products.21
 FIGURE 74.2 Nomenclature based on nanoscale dimensions useful for classification of different types of engineered nanomaterials. (Based on Hobson et al.1) |
Combinations or different metals or metals such as Cu, palladium (Pd) and their bimetallic palladium@ copper (Pd@Cu) NPs have been synthesized.22 The synthesized Cu, Pd, and Pd@Cu NPs were evaluated for antimicrobial activity using specific different microorganism strains that included Proteus mirabilis, Bacillus thuringiensis, Shigella flexneri, Staphylococcus aureus, Klebsiella pneumoniae, E coli, Pseudomonas aeruginosa, and Salmonella Typhimurium. The antibacterial activities of bimetallic Pd@Cu NPs were found to inhibit the growth of all microorganisms tested at a maximal level of efficacy as compared with the inhibition observed with the standard control antibacterial drug ofloxacin.
Beyond the level at which different atoms are combined into the nanomaterial structure, there are more advanced chemical and polymeric structures such as fullerenes, nanotubes of many types, nanorods, polymer nanocomposites, etc. As an example, polymer nanocomposites consist of a polymer or copolymer having NPs or nanofillers dispersed in a polymer matrix that can be made into different shapes (eg, nanoplatelets, nanofibers, nanospheroids), where at least one dimension is in the range of 1 to 100 nm. Figure 74.2 provides an example of useful nomenclature for classification of most types of nanomaterials based on nanoscale dimensions.
Historically, the technological and commercial advancement of nanotechnology from the first generation to increasingly more complex and potentially profitable next generation nanomedical products has occurred to include the progressive development of four different generations of nanotechnology.23,24 These began with the first-generation passive nanostructures and then in a time progressive manner the second (active nanostructures), third (nanosystems), and fourth (molecular nanosystems) generations. This concept has been reasonably accurate both in the progression and predicted timeline over the past few decades and is useful in understanding the levels of complexity of nanomedical technologies that have been, are now emerging, and that may emerge in the future from invention, basic research, and into commercial development.
ANTIMICROBIAL MODES OF ACTION OF NANOMATERIALSSome NP-antimicrobial agents act as potent broadspectrum antimicrobial agents, disinfection and woundhealing agents, and sustained inhibitors of intracellular pathogens. Like AgNPs, these may employ more than one mechanism in their antimicrobial action. Novel nanomaterials with several antimicrobial properties are not only possible but also likely, and this multiple functionality is beginning to revolutionize clinical medicine and play a significant role in disinfection.19,25,26,27 Traditionally, most antimicrobial agents inhibit microbial growth through several
mechanisms such as cell wall inhibition and lysis, inhibition of protein synthesis, alteration of cell membranes, inhibition of nucleic acid synthesis, and antimetabolite activity.28 The NP antimicrobials, on the other hand, have a vast array of possible physiochemical properties with respect to size, shape, surface area, surface energy, charge, crystallinity, agglomeration, aggregation, and chemical composition.29,30,31 Although for many NP-antimicrobial materials the precise mechanisms of action are still unknown and are currently under investigation, studies show that NPs often act by causing bacterial cell membrane degradation.32,33,34,35 For example, Li et al35 found that cell membrane degradation of S aureus occurred following exposure to catechin-Cu NPs. Catechin is a crystalline, natural phenol, and antioxidant compound from catechu, the tannic juice or boiled extract of the Mimosa catechu plant (Acacia catechu L.). Catechin-Cu NPs were also found to exert different mechanisms of action that resulted in E coli cell wall degradation. This is thought to be an indication of potentially different impacts on gram-negative and gram-positive bacteria by these NPs.35 Similar types of multiple effects have also been observed with CuNPs that exhibit antimicrobial actions, which, in addition to cell membrane damage, include the generation of reactive oxygen species (ROS) and lipid peroxidation.33
mechanisms such as cell wall inhibition and lysis, inhibition of protein synthesis, alteration of cell membranes, inhibition of nucleic acid synthesis, and antimetabolite activity.28 The NP antimicrobials, on the other hand, have a vast array of possible physiochemical properties with respect to size, shape, surface area, surface energy, charge, crystallinity, agglomeration, aggregation, and chemical composition.29,30,31 Although for many NP-antimicrobial materials the precise mechanisms of action are still unknown and are currently under investigation, studies show that NPs often act by causing bacterial cell membrane degradation.32,33,34,35 For example, Li et al35 found that cell membrane degradation of S aureus occurred following exposure to catechin-Cu NPs. Catechin is a crystalline, natural phenol, and antioxidant compound from catechu, the tannic juice or boiled extract of the Mimosa catechu plant (Acacia catechu L.). Catechin-Cu NPs were also found to exert different mechanisms of action that resulted in E coli cell wall degradation. This is thought to be an indication of potentially different impacts on gram-negative and gram-positive bacteria by these NPs.35 Similar types of multiple effects have also been observed with CuNPs that exhibit antimicrobial actions, which, in addition to cell membrane damage, include the generation of reactive oxygen species (ROS) and lipid peroxidation.33
Other CuNP-antibacterial actions include protein oxidation and DNA degradation within E coli cells.33 Another study by Xie et al36 showed that ZnO NPs exerted a bactericidal effect by disruption of the cell membrane and oxidative stress in Campylobacter jejuni. The NP-antimicrobials such as AgNPs have also been shown to bind to lipopolysaccharides, surface proteins (ie, porin, enzymes, etc), causing microbial cell wall collapse and altering the membrane potential.36 Similarly, AgNPs have been found to induce efflux of phosphate, reduction of cellular adenosine triphosphate level, interacting with sulfhydryl (or thiol) groups and altering cytoplasmic components as well as inhibiting respiratory enzymes and blocking DNA replication in both gram-negative and gram-positive bacterial pathogens.36 These studies show that different NPs have very different physiochemical properties and thus exhibit different antimicrobial mechanisms of action.
That the antimicrobial actions of NPs include biocidal destruction of cell membranes, blockage of enzyme pathways, alterations of microbial cell wall, and nucleic materials pathway as well as other, yet to be elucidated modes of action has been observed by many researchers.37 The NP antimicrobial agents, in general, appear to have excellent potent and low tendency for inducing microbicidal resistance when compared to non-NP-antimicrobial agents.38 These multiple possible molecular mechanisms by which metal-based NPs have been shown to kill multidrugresistant (MDR) bacteria via disturbance in respiration, disruption of DNA synthesis, membrane damage, inhibition of cellular growth, etc, have been extensively reviewed.8,39,40 The microbial toxicity of metal-based NPs as well as other nanomaterials has been attributed to ROS production such as hydroxyl radicals (•OH), superoxide anions, and hydrogen peroxide (H2O2) that inhibit DNA replication as well as amino acid synthesis and damage the bacterial cell membranes via lipid peroxidation, compromising membrane semipermeability and repressing oxidative phosphorylation. The •OH formation has been observed with AgNPs, and H2O2 production has been shown with ZnO NPs.39,40,41,42 The TiO2 NPs have been observed to act via a photocatalytic process.43 Free Cu++ from Cu-containing NPs and Mg halogen (MgX2)-containing NPs also induce formation of ROS.33,44
Different microorganisms have varying sensitivities to metal ions. The Ag and ZnO NPs have been reported to exert antibacterial activity by release of Ag++ and Zn++ that disrupt the cell membrane.39,42,45 Interaction of Ag+ with sulfhydryl groups in enzymes and other cellular constituents, making them dysfunctional, is an important antibacterial action of Ag NPs.8 The Ag+ also inhibits cell wall synthesis in gram-positive bacteria, Cu++ has been shown to interact with amine and carboxyl groups on the surfaces of microbial cells, such as Bacillus subtilis and Au NPs have also been shown to act by bacterial membrane disruption.8,46
Table 74.1 shows examples the variety of antimicrobial nanomaterials as well as examples of the types of microorganisms that have shown susceptibility and reported mode(s) of action. Many different types of antimicrobial nanomaterials are possible, and their modes of action often involve several molecular targets on and within a variety of microorganisms. This is only snapshot because this list is ever expanding at a rapid pace.
Nanotechnology provides an excellent platform for the development of molecularly targeted nanomaterials. The development of targeted antimicrobial nanomaterials for use in disinfection to include health care applications as well as others shows that nanomaterials of many different types and compositions are possible and that nanomaterials, especially NPs, in general can facilitate:
Uptake into cells and transcytosis across cells
Distribution into the blood and lymph circulation to reach potentially sensitive target sites (eg, bone marrow, lymph nodes, spleen, liver, kidneys, and heart)
Brain entry via nasal nerves (eg, polio virus)
Recognition and processing by the immune system
Entry into the cell nucleus
Skin, inhalation and gastrointestinal absorption
A growing list of additional antimicrobial processes
In addition to the development of NP-antibacterial technologies, NP-microbicides including those of dendrimer-containing nanoscale microbicides also hold potential safety efficacy against viruses.59,60,61 VivaGel® (SPL7013 Gel or astodrimer sodium) is an example of this type of NP use that shows antiviral and antibacterial properties. Several clinical studies have successfully tested
the safety and efficacy of VivaGel®, which is formulated as a mucoadhesive gel and delivered vaginally to relieve the signs and symptoms of bacterial vaginosis (BV) and to reduce risk of recurrence of BV in clinical studies. The gel dendrimer is formulated against human immunodeficiency virus (HIV) and herpes simplex virus and has been reported not to interfere with vaginal or rectal physiological pH.59,60,61,62 The microbicide is meant to disrupt and block viral attachment and/or prevent the viral adsorption from targeting cells of the rectum or vagina and is an example of an innovative nanotechnology-enabled product platform being applied to develop safe and effective new antiviral nanoproducts. These include applications to address a variety of sexual and women’s health concerns including the treatment and prevention of BV, prevention of sexually transmitted infections, and incorporation into condoms.
the safety and efficacy of VivaGel®, which is formulated as a mucoadhesive gel and delivered vaginally to relieve the signs and symptoms of bacterial vaginosis (BV) and to reduce risk of recurrence of BV in clinical studies. The gel dendrimer is formulated against human immunodeficiency virus (HIV) and herpes simplex virus and has been reported not to interfere with vaginal or rectal physiological pH.59,60,61,62 The microbicide is meant to disrupt and block viral attachment and/or prevent the viral adsorption from targeting cells of the rectum or vagina and is an example of an innovative nanotechnology-enabled product platform being applied to develop safe and effective new antiviral nanoproducts. These include applications to address a variety of sexual and women’s health concerns including the treatment and prevention of BV, prevention of sexually transmitted infections, and incorporation into condoms.
TABLE 74.1 Examples of different types of engineered nanomaterials and their antimicrobial action | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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