Nanotoxicology

Experimental and Computational Perspectives

Alok Dhawan, Diana Anderson, Rishi Shanker, Alok Dhawan, Diana Anderson, Rishi Shanker

• Verlag: Royal Society of Chemistry
• Jahr: 2017
• ISBN:
  9781788012591
• Auflage: 1
• Thema: Medicine
• Thema: Toxicology

The rapid expansion of the nanotechnology field raises concerns, like any new technology, about the toxicity and environmental impact of nanomaterials. This book addresses the gaps relating to health and safety issues of this field and aims to bring together fragmented knowledge on nanosafety. Not only do chapters address conventional toxicity issues, but also more recent developments such as food borne nanoparticles, life cycle analysis of nanoparticles and nano ethics. In addition, the authors discuss the environmental impact of nanotechnologies as well as safety guidelines and ethical issues surrounding the use of nanoparticles. In particular this book presents a unique compilation of experimental and computational perspectives and illustrates the use of computational models as a support for experimental work. Nanotoxicology: Experimental and Computational Perspectives is aimed towards postgraduates, academics, and practicing industry professionals. This highly comprehensive review also serves as an excellent foundation for undergraduate students and researchers new to nanotechnology and nanotoxicology. It is of particular value to toxicologists working in nanotechnology, chemical risk assessment, food science, environmental, safety, chemical engineering, the biological sciences and pharmaceutical research.

CHAPTER 1

Nanotoxicology: Challenges for Biologists

ASHUTOSH KUMAR,^*a SANJAY SINGH,^a RISHI SHANKER^b AND ALOK DHAWAN^b

^a Division of Biological & Life Sciences, School of Arts & Sciences, Ahmedabad University, University Road, Ahmedabad 380009, Gujarat, India

^b CSIR-Indian Institute of Toxicology Research, Vishvigyan Bhavan, 31, Mahatma Gandhi Marg, PO Box 80, Lucknow 226001, Uttar Pradesh, India

^*Email: [email protected]

1.1 Introduction

The manufacture of nanoscale materials with novel physicochemical properties has led to powerful nanotechnology in the 21st century, which enables the potential of existing technologies to be realised. The uniqueness in the properties of these nanoscale materials continues to provide almost unlimited applications worldwide across engineering, medicine, agriculture, food industries and biotechnology. Today, there are more than 1800 nano-enabled consumer products are available in the public domain.¹ The application of nanobased consumer products has also increased their inadvertent release into the environment during their production, usage, disposal and recycling. Living organisms including humans are exposed to these nanomaterials (NMs) throughout their life-cycle.^2,3 Unfortunately, the information about human exposure and possible adverse health effects of NMs is still meagre. How properties of NMs define their interactions with cells, tissues and organs is a scientific challenge that must be addressed for the safe use of NMs.⁴

Toxicity testing of NMs using existing in vitro and in vivo methods and models is a difficult task as there are so many different classes of NMs with various characteristics that can contribute to toxicity by diverse mechanisms. The characteristics such as NM size, shape, surface properties, composition, solubility, aggregation/agglomeration, particle uptake, the presence of mutagens and transition metals affiliated with the particles, etc.^5–8 can influence the fate of NMs in biological systems.⁹ The most common underlying mechanisms of NM-induced toxicity are oxidative stress, inflammation, immunotoxicity and genotoxicity.¹⁰ NMs interact with the cells, tissues and organs of biological systems as they have a higher potential to move across the whole organism compared to bulk materials.¹¹ Accumulation of NMs in their target organs can lead to cytotoxicity or genotoxicity.¹² NMs can cross the blood–brain barrier, enter the blood or the central nervous system, with immense potential to directly affect cardiac and cerebral functions. The NMs also have the ability to redistribute in the biological system from their site of deposition and cause harmful effects.¹³ Therefore, it is prudent to understand the fate of NMs in biological systems. At present, the methods used for assessing the toxicity of chemicals in living systems, are used to evaluate the toxicity of NMs. However, several novel properties associated with the NMs make it imperative to develop new methods for measuring the toxicity of NMs. Therefore, in this chapter, an attempt has been made to address the different challenges in the toxicity assessment of NMs.

1.2 The Hurdles in Toxicity Evaluation of NMs

It is now well established that the properties of NMs are the combined function of their size, shape, surface area, surface-to-volume ratio, chemical composition, solubility and others. Hence, to study NMs’ effects in living organisms and environments, the study design should be multipronged, and address NM characterization using validated protocols and hazard identification in humans and the environment. It is also important to mention that surface properties of NMs affect their biological behaviour. In order to measure the risk/toxicological endpoints associated with NMs, the material needs to be fully understood and characterized. Otherwise, the possible risk/toxic effects cannot be easily attributed to a certain property of the NMs or even the NM itself. For example, impurities and other components could be responsible for the observed effects.¹⁴ Therefore, a critical assessment of the biological behaviour of NMs without a careful physicochemical characterization is not meaningful.

The physicochemical properties characterization of NMs includes a range of parameters such as the analysis of purity, crystallinity, solubility, chemical composition, surface chemistry, reactivity, size, shape, surface area, surface porosity, roughness and morphology. Changes in the elemental composition, size or surface properties of NMs can result in a transformation in physical and chemical properties:

• Size: based on the material used in precursor solutions to produce NMs, properties such as solubility, transparency, absorption or emission wavelength, conductivity, melting point, colour and catalytic behaviour are changed by varying the particle size of NMs. Nanomaterials possess unique physicochemical properties due to their size; which also affects the mobility and transport behaviour of NMs.
• Composition effects: it is clear that different particle compositions lead to different physical and chemical behaviours of the material.
• Surface effects: the smaller the diameter of a spherical particle, the higher the surface-to-volume ratio and the specific surface area. This is accompanied by properties such as dispersity, conductivity, catalytic behaviour, chemical reactivity and optical properties. Therefore, more attention has to be paid to the surface material of a nanoparticle (NP) rather than its core material. When bare NMs come in contact with a heterogeneous environment, the smaller structures such as atoms, molecules or macromolecules attach to the surface of the NMs either by strong or weak interaction forces. In a biological environment, molecules such as proteins and polymers interact with the NM surface layer and form a “NM–protein corona”. It has also been shown that it is not the NMs alone, but also the corona that defines the properties of the “particle-plus-corona” compound.^15,16 This makes it necessary to understand not only the behaviour of NMs but also the biological interaction environment.
• Agglomeration: agglomeration affects the surface properties of NMs and their bioavailability to the cells.
• Solubility: some NMs are reported to produce ions in soluble form, which may be toxic to the cells e.g., ZnO, CuO.
• Surface charge and dispersity: surface charge of the NMs affects the particle solubility in suspension, whereas the dispersity of NMs provides information about their tendency to agglomerate.
• Dose metric: the exposure metric for NMs has been expressed, based on mass, number or surface area. The National Institute for Occupational Safety and Health (NIOSH) recommends that the “exposure metrics other than airborne mass concentration may be a better predictor of certain lung diseases, but it was decided that existing sampling methods will report in mass concentration because the toxicological effects observed are based on a mass dose”.¹⁷ The issue of the proper metric for enumerating NPs in workplaces is still a debatable issue. As mentioned, surface area concentration has been found to correlate well, regardless of particle size, with pulmonary response. However, this may not be true for all particle types and may also be a function of the agglomeration state.

In brief, to assess the risk/toxicity of NMs, the primary criterion is to have full knowledge of the NMs to be tested. Considering the novel characteristics of NMs, unlike their chemical counterparts, it is imperative to undertake comprehensive characterization prior to risk/toxicity evaluation.

1.3 ENM Interference with Toxicity Test Methods

1.3.1 Interference of NPs with Metabolic Activity Detection Assays

The MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay is a colorimetric test to determine the activity of cellular enzymes by the reduction of tetrazolium dye into its insoluble formazan crystals, which upon addition of dimethyl sulfoxide (DMSO) give a purple colour. The solubilized formazan absorbs at ∼590 nm. Similarly, other related tetrazolium dyes such as 2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT), 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) and the 8 (2-(2-methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium (WSTs), are used in conjunction with the intermediate electron acceptor, 1-methoxy PMS. WST-1, a cell-impermeable dye. Reduction occurs outside the cell through the plasma membrane electron transport system and produces water-soluble formazan. These tests measure cellular metabolic activity via NAD(P)H-dependent cellular oxidoreductase enzymes, which under defined conditions, reflect the number of viable cells present in the test system. Toxicity tests of engineered nanomaterials (ENMs) have been frequently carried out by using these test systems. The interference of ENM dispersions with the optical detection of MTT-formazan have been observed with many ENM systems such as TiO₂, ZnO NPs and carbon nanotubes (CNTs).¹⁸ ENMs having absorbance around the 500–600 nm (such as gold, silver and copper NPs) range are most likely to affect the absorbance by MTT-formazan in the ENPs-treated cells, whereas, MTT-formazan absorbance from untreated cells would not be affected, as they do not contain ENMs. Alternatively, ENMs having redox activity¹⁹ might undergo one-electron transition from many redox molecules (such as NAHP/NADPH, NAD/NADH and ADP/ATP), which ultimately may lead to the reduction of the MTT dye into MTT-formazan. Sometimes, it has been observed that certain lower concentrations of ENMs gives higher absorbance than corresponding controls. This may lead to the misinterpretation that exposure to ENMs can cause cell proliferation. However, the observed increase in absorbance is actually due to the reduction of more MTT-formazan dye by increased activity of mitochondrial dehydrogenase and other cellular oxidoreductase enzymes in the stressed cells on exposure to low concentrations of ENMs. Smaller ENMs (4–15 nm) composed of Au, Ag, AgO, Fe₃O₄, CeO₂ and CoO, have shown light absorption at the wavelengths used in most biological cytotoxicity test readouts: 340, 380, 405, 440, 540 and 550 nm.²⁰ Thus, if these ENMs are toxic to cells, the decreased formazan formation (due to reduced cell metabolism) could be masked by the absorbance of these NMs due to their ...