Nanotechnology in Water Research E-Book

Bin Gao

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 386

Veröffentlichungsjahr: 2025

Beschreibung

Holistic perspective on environmental nanotechnology and its impact on water quality, focusing pollution control, water quality, and hydrologic pathways

Nanotechnology in Water Research delves into the intersection of nanotechnology and environmental science, exploring the transformative potential of nanotechnology in addressing environmental challenges. The book discusses the characterization, stability, transport, and fate of nanomaterials in water systems, particularly in hydrologic pathways, the applications of nanotechnology in water pollution control, and significant scientific problems and advancements in nanotechnology’s role in water research.

This title includes information on:

• Nanotechnology and nanoparticle concepts, with many examples related to water quality technologies
• Improving water treatment methods while ensuring environmental sustainability
• Sensor, remediation, adsorption, and membrane processes that detect, monitor, remove, reduce, or neutralize water contaminants
• Analytical technologies, stability theory, filtration theory, and fate and transport of nanoparticles in water to help reduce risks to humans and aquatic systems

Equally valuable as a reference, handbook, textbook, and general learning resource, this essential guide is an excellent read on the subject for students, educators, researchers, professionals, and stakeholders in environmental engineering, nanotechnology, and environmental science.

Leseprobe

Nanotechnology in Water Research

Understanding Pollution Control, Water Quality, and Hydrologic Pathways

Bin GaoProfessor, Department of Civil and Environmental EngineeringRensselaer Polytechnic InstituteTroy, NY, USA

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Preface

This book stems from the class, “Nanotechnology in Water Research,” which I have been offering to both graduate students and senior undergraduates for over a decade, first at the University of Florida and then at Rensselaer Polytechnic Institute. When I began my teaching career, nanotechnology emerged as one of the most promising technologies in the environmental field, particularly with respect to water research, opening many new frontiers for improving water quality and sustainability. Finding no textbook that could provide a comprehensive exploration of the intersection between nanotechnology and water research, I developed my own course materials. These materials cover not only the fundamentals of environmental nanotechnology but also the applications and impacts of nanoparticles on water quality. Part of my own research also focuses on environmental nanotechnology and water quality. During course development and delivery, I continually updated the materials by integrating the findings from my research group into the class. I feel it necessary to organize and refine these materials and contents into a book on nanotechnology in water research, hoping it will serve as a valuable resource for students, educators, researchers, engineers, and stakeholders interested in improving water quality and sustainability using nanotechnology in environmentally friendly ways. This book would not have been possible without the incredible support from my family, students, collaborators, and colleagues. To my wonderful wife and daughters, thank you for your patience and understanding during the countless hours I spent writing and revising the manuscript. The unwavering support from my students, including assistance with copyright requests, is truly appreciated. I would also like to extend my gratitude to my collaborators and colleagues, including visiting scholars, for their inspiring encouragement.

1Nanotechnology and Environmental Nanotechnology

1.1 Nanoscale

Nano is derived from the Greek word v (meaning “dwarf”) and is often used as a prefix to denote “one‐billionth,” or a factor of 10−9. For example, one nanometer (nm) is one‐billionth of a meter (m) and one nanogram (ng) is one‐billionth of a gram (g). In the International System of Units (SI), the base unit for length is the meter (m).

We are familiar with commonly used SI length units such as meter (m), decimeter (dm, 10−1 m), centimeter (cm, 10−2 m), and millimeter (mm, 10−3 m), which are mainly used to describe the macroworld (Figure 1.1). In contrast, for the microworld, however, SI units such as micrometer (μm, 10−6 m), nm, and picometer (pm, 10−12 m) are rarely used in our daily life (Figure 1.1).

In chemistry, a commonly used length unit to describe atoms and molecules is angstrom (Å), which is 10−10 m or 0.1 nm. For example, the diameter of a hydrogen atom is about 1 Å or 0.1 nm. This tells us that one nanometer is roughly the length of ten connected hydrogen atoms in a row, which can help us understand just how small the nanoscale is.

Table 1.1 gives examples of the dimensions of some commonly known objects in both the macro‐ and microworld. Most things in the macroworld such as the Golden Gate Bridge and ants are visible and tangible, allowing us to observe them and “feel” their existence. The finest objects visible to the naked eye are often larger than 50 μm, even for those with the best eyesight. Therefore, the dimension scale of the macroworld is typically larger than 10 μm (Figure 1.1).

Figure 1.1 Dimension scales of macro‐, micro‐, and nanoworld.

Table 1.1 Examples of the dimensions in macro‐ and microworld.

Macroscale

Microscale

Golden Gate Bridge

2.74 

km

Blood cell

6.2–8.2 

μm

Empire State Building

443 

m

E. coli

∼2 

μm

Alligator

3–4.6 

m

Coronavirus

80–120 

nm

Ant

0.75–52 

mm

Glucose molecule

∼1 

nm

Paper thickness

∼0.1 mm

Water molecule

0.275 nm

On the other hand, the dimension scale of the microworld is less than 10 μm. In the microworld, therefore, things are often smaller than the resolution of human eyes. That is why we do not “see” the living organisms in the microworld such as Escherichia coli (E. coli) bacterium, which is about 2 μm long and about 0.25 μm in diameter (Table 1.1). Nowadays, we know that microorganisms are everywhere on the Earth and account for a large percentage of its biodiversity. Before the invention of the microscope, it was hard to imagine that there are living organisms at the microscale. In some Buddhist texts and tales, Buddha once told his students and followers that there are eighty‐four thousand (means “many” in Buddhism) beings in a drop of water, which might be one of the earliest perceptions of the microworld. With the invention of tools such as microscopes, people have made great progress in exploring and understanding the microworld. Antonie Van Leeuwenhoek, a Dutch businessman and scientist, designed a single‐lensed microscope and was the first to observe bacteria in 1676 (Robertson, 2015). He also determined their size and thus is considered to be the father of microbiology.

Later developments in visualization and other detection technologies further expand the capacity to explore the microworld at an even smaller scale, the nanoworld (the dimension scale is less than 100 nm). For example, modern microscopes such as the atomic force microscope (AFM) can detect and reveal objects at atom levels, exposing phenomena at the nanoscale (1–100 nm). The US National Nanotechnology Initiative (NNI) defines nanotechnology as “a science, engineering, and technology conducted at the nanoscale (1 to 100 nm), where unique phenomena enable novel applications in a wide range of fields, from chemistry, physics and biology, to medicine, engineering and electronics” (https://www.nano.gov). This definition has also been extended to include nanomaterials that have at least one dimension smaller than 100 nm. In practice, the boundary of 100 nm could be relaxed (to several hundred nm) as long as the properties of the materials are inherently size dependent.

At the nanoscale, it is possible to manipulate matter and create “new” materials by changing individual atoms and molecules, which may lead to dramatic changes in the physical, chemical, biological, and optical properties of the materials. Unlike their larger counterparts (in bulk form), nanoparticles can exhibit unique properties in reactivity, conductivity, strength, flexibility, or reflectivity. Nanoparticles, particularly engineered nanoparticles (ENPs), thus have attracted increasing research attention in various fields including water research, which is also the focus of this book.

1.2 Nanotechnology: A Short History

The history of man‐made nanomaterials and nanostructures can be traced back to more than one thousand years ago (Bayda et al., 2020). Even without understanding or being aware of the concept of nanotechnology, skilled craftsmen in ancient times were able to use their empirical experience to manipulate and create nanomaterials and nanostructures. In the 4th century, the Romans had already developed technologies to use nanosized/colloidal gold and silver particles to control the color of glass. Those technologies further developed and applied in the stained‐glass windows in European cathedrals in the 6th–15th centuries and in glowing ceramics in the Islamic world in the 9th–17th centuries. Modern nanotechnology, however, only has a relatively short history. In fact, the term “nanotechnology” was first introduced by a Japanese scientist Norio Taniguchi in 1974 during a conference of Japan Society of Precision Engineering. When describing nanoscale semiconductor processes, Taniguchi pointed out: “‘Nanotechnology’ mainly consists of the processing of separation, consolidation, and deformation of materials by one atom or one molecule.” (Taniguchi, 1974).

It is widely accepted in the scientific communities that the concept of nanotechnology was first formally outlined by Richard Phillips Feynman, an American theoretical physicist and Nobel Prize winner. At an American Physical Society meeting in 1959, Prof. Feynman gave a famous lecture “There's Plenty of Room at the Bottom” (Feynman, 1960). Many would like to set this as the beginning of modern nanotechnology.

In the lecture, Prof. Feynman pointed out that: “The principles of physics, as far as I can see, do not speak against the possibility of maneuvering things atom by atom.” This provides the theoretical support for the “bottom‐up” approach, which is now widely used in the fabrication of nanomaterials. Nowadays, nanomaterials or nanostructures can be produced by way of either a top‐down approach (i.e., reducing the size of a bulk material to nanoparticles) or a bottom‐up approach (i.e., nanoparticles are built atom by atom or molecule by molecule) (Figure 1.2). At the end of his talk, Prof. Feynman also posed two challenges with a prize of 1000 dollars each to promote the development of nanotechnology. The first one was on a tiny motor and the second one was on fitting the entire encyclopedia on the head of a pin. The two changes were achieved in 1960 and 1985, respectively. In addition to Prof. Feynman, others have also made great contributions to the conceptualization of nanotechnology. As mentioned previously, Prof. Norio Taniguchi coined the term “nanotechnology” (Taniguchi, 1974). As the founder of molecular nanotechnology (Drexler, 1981), Dr. K. Eric Drexler, an American engineer, further developed and popularized the concept of nanotechnology.

Figure 1.2 “Top‐down” and “bottom‐up” approaches in nanotechnology.

Source: Rawat (2015)/IOP Publishing/CC BY 3.0.

The breakthroughs in supramolecular chemistry in the 1960s–1980s also contributed greatly to the further development of nanotechnology, particularly with respect to the synthesis of nanomaterials (Toma and Araki, 2009). Unlike traditional chemical that centers on individual atoms or molecules, supramolecular chemistry deals with organized entities of molecules, which are conceptually similar to nanomaterials. Some of the important concepts of supramolecular chemistry, such as molecular self‐assembly and noncovalent interactions, are also crucial to the nanofabrication processes (Figure 1.3). In fact, most of the bottom‐up syntheses of nanomaterials in nanotechnology are based on supramolecular chemistry (Schubert et al., 2003). In other words, supramolecular chemistry provides a powerful tool for researchers and scientists to develop novel nanomaterials from concept to reality.

Figure 1.3 Self‐assembly of supramolecular complexes.

Source: Ariga (2016)/with permission of Elsevier.

Because nanomaterials are in the tiny nanoworld, it is critical to have an instrument that can observe and characterize these materials, like the optical microscopes in the microworld. In the 1980s, the inventions of the scanning tunneling microscope (STM) and then the AFM helped researchers and scientists “see” and experience the materials and phenomena in the nanoworld at the atomic level (Toumey, 2012). Drs. Gerd Binnig and Heinrich Rohrer at IBM Zurich Research Laboratory were awarded the Nobel Prize in Physics for the invention of the STM in 1986 (Binnig and Rohrer, 1986). Both STM and AFM use atomic tips to closely scan the surface of an object to provide the atomic‐scale visualization (Figure 1.4). Details about STM and AFM are discussed in Chapter 8. These powerful visualization tools can provide insights into the synthesis, characterization, and applications of nanomaterials and thus have promoted the further development of nanotechnology.

Figure 1.4 Schematic description of STM.

Source: Michael Schmid/Wikimedia Commons/CC BY-SA 2.0.

It seems that all the essential components were ready for the booming of nanotechnology. Great minds have pointed out the directions; supramolecular chemistry has enabled the synthesis and production, and STM and AFM have allowed the visualization and characterization. The boom of nanotechnology is a pile of dry wood that only needs a match to ignite the fire. The discovery of the buckminsterfullerene (C60, buckyball, Figure 1.5) by Drs. Harry Kroto, Richard Smalley, and Robert Curl in 1985 is the match because of the unique and promising properties of carbon nanomaterials (Kroto et al., 1985). The three professors received the Nobel Prize in Chemistry for this discovery in 1996. The discovery of carbon nanotubes (CNTs, Figure 1.5) further fueled the boom in nanotechnology. Although CNTs have been reported previously, Dr. Sumio Iijima of NEC in Japan is often cited as the inventor of CNT. His 1991 Nature paper, “Helical microtubules of graphitic carbon” (Iijima, 1991), generated unprecedented interest in carbon nanomaterials and thus has attracted much research and public interest in nanotechnology. Using a mechanical exfoliation process called the scotch tape technique, Drs. Andre Geim and Kostya Novoselov at the University of Manchester extracted single‐layer graphene (Figure 1.5) from bulk graphite in 2004 (Novoselov et al., 2004). For their pioneering research on graphene, Drs. Geim and Novoselov were awarded the Nobel Prize in Physics in 2010. Although research on graphene can be traced back to the beginning of the last century (Kohlschütter and Haenni, 1919), the emergence of graphene and its derivatives (e.g., graphene oxide) in the 2000s has fueled intense research in nanotechnology.

Figure 1.5 Schematics of buckminsterfullerene, single‐walled carbon nanotube, and graphene.

Source: Hong et al. (2015)/with permission of American Chemical Society.

1.3 Nanotechnology in Water Research

Based on the NNI definition, nanotechnology deals with matter with at least one dimension sized from 1 to 100 nm, at which the matter may demonstrate special properties. This novel technology has attracted much interest from both the scientific community and the public because of its promising applications in improving quality of life. It is widely accepted that nanotechnology could create new materials and products to advance a variety of fields including medicine, energy, defense, electronics, and consumer products. In fact, engineered nanomaterials such as nanosized metal, metal oxides, and carbonaceous materials are already used in various industries and consumer products in our daily lives (Figure 1.6). For example, silver, titanium dioxide, and zinc oxide nanoparticles, as well as CNTs, are used in personal care products because of their unique properties including color, transparency, solubility, chemical reactivity, and biological activity (Gupta and Xie, 2018; Iavicoli et al., 2018).

Figure 1.6 Examples of nanomaterials in daily life.

Source: Iavicoli et al. (2018)/MDPI/CC BY 4.0.

As one of the most promising technologies, nanotechnology has shown advantages in many industrial sectors such as electronics, health, agriculture and food, energy, and environment (Figure 1.7). In the semiconductor industry, engineered nanomaterials and nanofabrication play a crucial role in controlling the size and efficiency of electronic devices. Nanotechnology also has strong impacts on the development of biomedical field by creating new medicines, devices, and methods for more effective and precise, faster, and safer cures. Engineered nanomaterials such as nanosized silver and copper have been widely used in agriculture and food industry for a long time because of their antimicrobial activities (Chen, 2018). Nanotechnology is also the backbone of the recent development of clean and renewable energy and has many potential applications in the energy sector to improve efficiency and sustainability of energy sources, conversion, distribution, storage, and usage. All these developments obviously would greatly benefit the environment. As a double‐edged sword, however, nanotechnology may also introduce unintended negative impacts. There are increased public concerns over the emission, exposure, and toxicity of engineered nanomaterials. Nevertheless, the great promise held by nanotechnology to the environment makes it especially attractive to environmental scientists and engineers. Several books have been published on environmental applications and impacts of nanotechnology.

Figure 1.7 Novel applications of nanotechnology.

Nanotechnology in Water Research: Understanding Pollution Control, Water Quality, and Hydrologic Pathways is also a book on environmental nanotechnology; however, it mainly focuses on nanotechnology in water research. The goal of this book is to provide an overview as well as examples for students, researchers, and environmental professionals on the applications and implications of nanotechnology in water research. After introducing the basic knowledge and potential benefits of engineered nanomaterials in Chapter 2, four applications of nanotechnology in water research including water quality monitoring (nanosensors, Chapter 3), groundwater remediation (Chapter 4), membrane filtration (Chapter 5), and adsorption (Chapter 6) are presented. The rest of the book is more focused on the implications of engineered nanomaterials in water research. Chapter 7 introduces the basic characterization techniques of nanoparticles in water, while Chapters 8 and 9 focus on the stability and removal of nanoparticles from water, respectively. The last two chapters (10 and 11) are on the fate and transport of nanoparticles in surface and subsurface flow.

References

Ariga, K., 2016. Supermolecules. in: Ebara, M. (Ed.). Biomaterials Nanoarchitectonics. Elsevier Inc.

Bayda, S., Adeel, M., Tuccinardi, T., Cordani, M., Rizzolio, F., 2020. The history of nanoscience and nanotechnology: from chemical‐physical applications to nanomedicine. Molecules 25, 112.

Binnig, G., Rohrer, H., 1986. Scanning tunneling microscopy. IBM J Res Dev 30, 355–369.

Chen, H., 2018. Metal based nanoparticles in agricultural system: behavior, transport, and interaction with plants. Chem Spec Bioavailab 30, 123–134.

Drexler, K.E., 1981. Molecular engineering: an approach to the development of general capabilities for molecular manipulation. Proc Natl Acad Sci U S A 78, 5275–5278.

Feynman, R.P., 1960. There's plenty of room at the bottom. Eng Sci 23, 22–36.

Gupta, R., Xie, H., 2018. Nanoparticles in daily life: applications, toxicity and regulations. J Environ Pathol Toxicol 37, 209–230.

Hong, G., Diao, S., Antaris, A.L., Dai, H., 2015. Carbon nanomaterials for biological imaging and nanomedicinal therapy. Chem Rev 115, 10816–10906.

Iavicoli, I., Leso, V., Fontana, L., Calabrese, E.J., 2018. Nanoparticle exposure and hormetic dose–responses: an update. Int J Mol Sci 19, 805.

Iijima, S., 1991. Helical microtubules of graphitic carbon. Nature 354, 56–58.

Kohlschütter, V., Haenni, P., 1919. Zur Kenntnis des Graphitischen Kohlenstoffs und der Graphitsäure. Z Anorg Allg Chem 105, 121–144.

Kroto, H.W., Heath, J.R., Obrien, S.C., Curl, R.F., Smalley, R.E., 1985. C

60

: Buckminsterfullerene. Nature 318, 162–163.

Novoselov, K.S., Geim, A.K., Morozov, S.V., Jiang, D., Zhang, Y., Dubonos, S.V., Grigorieva, I.V., Firsov, A.A., 2004. Electric field effect in atomically thin carbon films. Science 306, 666–669.

Rawat, R.S., 2015. Dense plasma focus‐from alternative fusion source to versatile high energy density plasma source for plasma nanotechnology. J Phys Conf Ser 591, 012021.

Robertson, L.A., 2015. van Leeuwenhoek microscopes—where are they now? FEMS Microbiol Lett 362, fnv056.

Schubert, U.S., Lohmeijer, B.G.G., Gohy, J.F., 2003. Engineering with macromolecules: from supramolecular chemistry to defined nanomaterials. Abstr Pap Am Chem Soc 225, U596.

Taniguchi, N., 1974. On the Basic Concept of ‘Nano‐Technology’. Proc. Intl. Conf. Prod. Eng. Tokyo, Part II, Japan Society of Precision Engineering.

Toma, H.E., Araki, K., 2009. Exploring the supramolecular coordination chemistry‐based approach for nanotechnology. Prog Inorg Chem 56, 379–485.

Toumey, C., 2012. Probing the history of nanotechnology. Nat Nanotechnol 7, 205–206.

2Overview of Engineered Nanoparticles

2.1 Nanoparticle Basics

Nanotechnology mainly deals with nanomaterials or nanostructures. Nanoparticles, particularly engineered nanoparticles, thus become the focal point of nanotechnology. While engineered nanomaterials can be produced at nanoscale in one dimension (e.g., membranes and films), in two dimensions (e.g., wires and tubes), or in all three dimensions (e.g., dots and cubes), they are often called two‐dimension, one‐dimension, and zero‐dimension nanoparticles, respectively. At nanoscale, properties of nanoparticles are controlled by the quantum effect that describes the physics of electron properties in solids with great reductions in particle size (Milburn and Woolley, 2008). Because this effect only appears within the nanoscale, nanoparticles have many unique properties that differ from their bulk and microscale forms. The quantum effect on nanoparticles is size dependent and increases with decreasing particle size, i.e., particles of a few nanometers would be affected the most. When their sizes are reduced to the nanoscale, materials can suddenly change their physicochemical properties such as optical, electrical, magnetic, and reactive properties. For example, gold nanoparticles show unique, size‐dependent optical properties (Figure 2.1). It is well known that bulk gold is yellow, and nanoscale gold can have various colors such as red or purple (Figure 2.1), which can adapt to various applications such as biomedical sensing and treatment and environmental screening (Sabela et al., 2017). At the nanoscale, the motion of gold's electrons is confined. Because this movement is restricted, gold nanoparticles react differently with light compared to larger‐scale gold particles. Many other metals may dramatically change their properties when their sizes are reduced to the nanoscale, at which copper may become transparent, aluminum may turn combustible, gold may melt at room temperature, etc.

Figure 2.1 Size‐dependent color change of gold nanoparticles: (a) ‘Solutions’ of different colors, and (b) Corresponding particle sizes.

Source: Ajdari et al. (2017)/with permission of Elsevier.

As we know, quantum forces affect every atom in a material, so why are there no quantum effects on the bulk? This is because there are so many atoms in a bulk material that its physicochemical properties are merely the average of all the quantum forces of the atoms that make up the material. When the size of the material reduces, the number of atoms also dramatically decreases. For example, when the diameter of a solid sphere reduces by 10 times, its mass and number of atoms decrease by 1000 times. At the nanoscale, the number of atoms in a nanoparticle are “countable” and the averaging no longer works. Therefore, the specific behaviors of individual atoms or molecules come to the forefront, making the properties of nanoparticles size‐dependent and dramatically different from the bulk materials. Because of the quantum effects, scientists and researchers are able to fine‐tune the properties of nanoparticles by changing their sizes.

2.2 Two Important Properties

Due to the differences in elemental compositions, nanoparticles of different kinds may have distinctive properties. For example, gold and silver nanoparticles of the same size and shape may have dramatically different physicochemical properties. However, all the nanoparticles regardless of their compositions have two common properties that are essential to their applications. These two important properties are also critical to the environmental applications and impacts of nanotechnology, especially with respect to nanotechnology in water research.

The first one is that nanoparticles have an extremely high surface area to mass/volume ratio (i.e., high specific surface area). In comparison to their bulk forms, nanoparticles have a much larger exposure of surface per unit mass or volume. When a bulk material is turned into nanoparticles, the total mass would remain unchanged, but the total surface area would increase dramatically. In other words, nanotechnology may not create mass but greatly increase the surface area. Figure 2.2 gives an example of how specific surface area increases with decreasing size. As shown in the figure, a solid cube with sides of 1 cm has a total surface area of 6 cm2, which can only cover two‐quarter coins. If we have a tool that breaks or cuts the cube into smaller cubes of 1 mm sides, there would be no changes in mass or volume. However, the total surface area of the smaller cubes becomes 60 cm, which can barely cover a smart phone. If we go further to take the top‐down approach to turn the cube into 1 nm cubes, the total surface area becomes 60,000,000 cm2, enough to cover the whole floor area of the White House (∼5,100 m2). Let's take a general example of a cube with a side of L. From geometry, we know the surface area and volume of the cube are 4L2 and L3, respectively. The ratio between the surface area and the volume should be 4L2/L3, which is equal to 4/L. At a fixed volume, the total surface area thus increases as the side of the cube decreases. This means that when a bulk material is divided into smaller particles, the total surface area of the material increases. It also mathematically explains why nanoparticles have extremely high specific surface areas.

Figure 2.2 Relationship between particle size and total surface area.

For the nanoparticles of several nanometers, almost all the atoms are exposed to the surface and that can increase chemical reactivity. For example, a 3 nm iron particle has 50% of its atoms on the surface, which are not bonded on one side and thus are far more active than the ones bonded inside. This leads to materials in nanoparticle form being more chemically reactive than the bulk form. This means that materials that are inert in their bulk form are reactive when produced in their nanoparticle form. For example, engineered nanoparticles with improved chemical activities have been used in many green energy applications such as solar panels, batteries, and fuel cells. A large specific surface area is also a key property of adsorbents for the removal of environmental contaminants. Nanoparticles, such as carbon nanotubes, graphene, and metal oxide nanoparticles, have demonstrated strong sorption ability to a variety of contaminants in the environment (Suthar and Gao, 2017).

The surface area of nanoparticles mainly refers to their external surface area. Many porous materials such as activated carbon also have very high specific surface area due to the contributions from their internal pores (internal surface area). Most of those internal pores are nanostructures; however, they are often called macro‐, meso‐, and micropores. This is because the International Union of Pure and Applied Chemistry (IUPAC) has defined porous materials based on the pore diameters with the diameter of macropores, mesopores, and micropores being >50 nm, 2–50 nm, and < 2 nm, respectively. Nevertheless, many of the porous materials, particularly microporous materials, are nanostructured and thus have a large specific surface area even though they are not nanoparticles.

The second important property of nanoparticles is related to their “solubility” or dispersion in liquid and gas phases. In terms of particle dispersion in a fluid, the gravity law of bulk materials does not apply to nanoparticles. The law of gravity indicates that, without external forces, a stone would sink to the bottom of a river because its density is higher than water, while a piece of wood with lower density would float to the surface. Similarly, density also affects the movement of bulk objects in the atmosphere, such as floating hot air balloons. At the nanoscale, however, the gravity law becomes less important. The effect of gravity on nanoparticles actually diminishes with decreasing particle size. In other words, if the stone and wood are turned into nanoparticles, they will not sink or float in water. Instead, they will suspend in water to form “solutions,” in which the stone and wood nanoparticles are well dispersed.

The dispersion of fine particles in a fluid was first reported by botanist Robert Brown in 1827. He used a microscope to observe plant pollen in water and found that the particles were randomly moving in the water (Figure 2.3). This motion is named after him as “Brownian motion,” which is used to describe the dispersion of colloids and nanoparticles in a fluid (Figure 2.3). The famous physicist Albert Einstein developed a mathematical model to describe the motion of particles in water due to their collisions with water molecules in 1905. Based on his theory, Einstein first determined that the average random displacement (L) of a nanoparticle is controlled by its diffusivity (D):

(2.1)

where t is time. This equation also means the Brownian travel distance of a nanoparticle in a fluid should be proportional to square root of time. In addition, Einstein's theory also led to a famous equation to calculate the diffusivity of a spherical nanoparticle in a fluid:

Figure 2.3 Brownian motion of nanoparticles in a fluid.

(2.2)

where kb is the Boltzmann's constant, T is the absolute temperature, d is the diameter of the particle, and η is the dynamic viscosity. This equation has been widely applied in the determination of the dynamics and sizes of colloidal and nanosized particles.

2.3 Prime Nanoparticles

There are too many types of nanoparticles to enumerate; however, they can be divided into two categories: engineered and natural. The focus of nanotechnology is mainly on engineered nanoparticles, which mainly include carbons, metals, and metal oxides (Table 2.1). Natural minerals such as silica and clay have a wide size range and their nanosized forms belong to the natural nanoparticles. In addition to abiotic nanoparticles, viruses are also nanoparticles, which can spread airborne diseases due to their small sizes.

Table 2.1 Engineered nanoparticles and their main applications.

Type

Example

Main applications

Carbon

Fullerenes (Cn)

Biomedical

Carbon nanotubes

Electrical, mechanical, energy, biomedical, environmental

Graphene/Graphene oxide

Electrical, mechanical, energy, biomedical, environmental

Metal

Silver

Antimicrobial, biomedical, food, textile

Iron

Environmental

Gold

Biomedical

Platinum

Catalytic, biomedical

Metal oxide

Titanium dioxide (TiO

2

)

Cosmetic, paint, coating, photocatalytic

Zinc oxide (ZnO)

Cosmetic, paint, coating, photocatalytic

Copper oxide

Antimicrobial, biomedical

Other

Quantum dot

Biomedical, energy, electric

Silicon oxide (silica)

Paint, coating, additive

Organic nanoparticle

Biomedical, food, agricultural

2.3.1 Carbon Nanoparticles