the available solar cells having effectiveness of 10–25% are made up of silicon [13]. The cost of a solar cell can be reduced by improving its efficiency, which makes it more economically competitive [14]. By using nanostructured surface for reflectivity in solar cell devices, the antireflection ability may be enhanced effectively using NMs having diameter less than the wavelength of incident light. To increase the usage of solar light, antireflection NMs are patterned with active components, and thus the cost can be reduced [15]. Also, light‐capturing capacity can be additionally increased by light trapping, so it requires less amount of material to absorb solar light, and the cost can be reduced [16].
NMs can be used to solve various challenges in the environment, e.g. it can be used to restore the polluted soil with cleaning of water and air, and it also can reduce the impact of chemical manufacturing using nanoscale catalyst [17, 18]. Therefore, various opportunities are offered by nanotechnology in manufacturing novel NMs that improves living conditions using advanced techniques in various fields.
Despite significant development and increased utilization in many marketed products, NMs and their technology are still facing challenges in energy, environment, health, and safety (EHS). One of the principal challenges that comes up with nanotechnology is its influence on environment and its toxicity to humans. The cause for toxicity in humans is due to the availability of NMs with different properties that might lead to adverse drug reaction. NMs undertake biodegradation in their working atmosphere. This might lead to intracellular changes and gene modifications. If the gene alteration is undesirable, then it might prove hazardous for human beings. NMs are not always environmentally safe. They sometimes enter the ecology's food chain and cause changes. So, it is necessary first to understand the life cycle of NMs with their activity in the environment. It is necessary to focus on the size, structure, and reactivity of NMs in environmental systems [19].
2.2 Road to Sustainability in Nanotechnology
To have sustainability in nanotechnology, there is a need to (Figure 2.1)
1 address the accountability of NMs and their related toxicity.
2 strengthen large‐scale manufacturing of NMs and minimize possible risks involved in it.
3 develop new capabilities for sustainable environment and health.
4 have a robust regulatory guideline and support.
2.2.1 Accountability of Nanomaterials and Related Toxicity
The developments in NMs are evolving as an inevitable part of daily life. With increase in the use of nanostructured material, it will eventually augment its exposure to the environment and to all those living and nonliving things that are made up of natural NMs called atom. Although the overall exposure by NM will be very less currently, in future with surge in the acceptance, the likelihood of exposure will increase. The effects of nanocomponents on environment are unknown as it largely depends on exposure and toxicity. Even after the selection of relatively less toxic material, the chance of harmful effects is unpredictable. Ironically, the unique properties of nanoparticles (NPs) that convey significant characteristics bring toxicity as well [20]. For instance, a naturally occurring oxide of titanium (TiO2), which is frequently used in the preparation of NMs, in the presence of sunlight, these TiO2 NPs can be toxic to some freshwater organisms even at parts‐per‐billion levels due to uptake and in situ photogeneration of reactive oxygen species [21, 22]. The severity of toxicity of NMs depends on the physicochemical properties, viz., size, surface area, structure, shape, stability, and surface energy [20]. These make an imperative need to assess the toxic effect based on composition and physicochemical properties of NMs.
Figure 2.1 Road to sustainability in nanotechnology.
2.2.1.1 Size
The characteristic that brings the unique performance of NMs is its nanoscale structured engineering. At the same time, this physicochemical property creates opportunity for increased interaction with biological tissues at molecular level having similar size and structure. This same principle has caused pharmaceutical companies to formulate highly efficient and intelligent drug delivery systems for several diseases, which was not possible in their conventional size formulation. Not only in health care but also in all other sectors, these NMs have improved their characteristics such as strength, weight, appearance, efficiency, and durability. A research for pharmacological action based on NP size shows that NPs < 50 nm diffuse quickly to tissues of living things and bring potential toxicity to those tissues, while NPs >50 nm taken up by a cleaning system of mammalians, reticuloendothelial system (RES), and the RES organs, viz., liver, spleen, and lymph nodes, will become the target of oxidative stress [23]. Few other researchers show that particles <10 nm get deposited in the tracheobronchial of the lung, while all the other <100 nm particles are deposited all over the lungs and cause respiratory adverse effects [24–26]. Several other toxic effects, such as mitochondrial perturbation by silica [27], damage to nervous system [28], endothelial dysfunction [29], generation of neoantigens [30], and immune toxicity [31], are reported with limited clinical evidence. The uptake and interaction in biological tissues observed previously and substances generally regarded as safe now show adverse responses. The NPs generated during manufacturing may get inhaled, and ultrafine particles (<100 nm) induce pulmonary inflammation, oxidative stress, and distal organ involvement or get absorbed through the lungs and can create toxicity in vital systemic circulation. As the size reduces, it increases surface area and finally enhances capability to react with oxygen. Due to increased reaction with oxygen, it enhances inflammation, fibrosis, cytotoxicity, oxidative injury, and carbon deposition in lungs [32]. It is the sole responsibility of the researcher to involve toxicology scientists and closely monitor the toxicity of NPs during each development stage until robust regulatory guidelines based on size and surface area become available. One also needs to take care until the airborne NP hazard has been appropriately assessed; this risk should be managed by taking steps to avoid large quantities of these NPs becoming airborne.
2.2.1.2 Surface Area
The surface area increases with the reduction in the size of the same quantity of any material. NPs, although made from nontoxic materials, become hazardous, as the material developed reactivity at molecular level. The toxic effect of few such particles does not seem hazardous, but if more surface area becomes available, it will further add on risk. Thus, surface area also requires attention and monitoring for the toxicity study along with the size. As size reduces, it increases surface area and finally enhances capability to react with oxygen. Due to increased reaction with oxygen, it increases inflammation, fibrosis, cytotoxicity, oxidative injury, and carbon deposition in lungs of mineral particles, quartz, titanium dioxide, asbestos, and carbon black despite the same materials being inert at macro or higher size range[32]. Toxicity in biological systems generated by NPs is predominantly through the formation of oxidative responses and consequent formation of free radicals. Free radicals are hazardous as they oxidize the lipid and damage the DNA with inflammatory responses. The data also shows that the smaller the size, the more able it is toward the formation of reactive oxygen species [20]. Every NP thus closely needs to be monitored for its new developed chemical reactivity, and it should be done from its development phase.
2.2.1.3 Surface
The surface of NMs is a unique property, and the surface phenomenon seems to be a new direction in developing nontoxic NPs. The groups present on the surface or coating can change the physical, chemical, magnetic,
