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Nanotechnology 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...