Nanoscience has consistently been a developing and advancing field with a great diversity of applications in medicine, energy, electronics, biotechnology, materials. Engineered nanomaterials (ENMs) and nanoparticles (NPs) are simply defined as materials with at least one dimension of 1–100 nm in size. This definition means that their various chemical and physical properties allow them to be altered and changed to perform their targeted functions and tasks. Additionally, due to their vast diversity in multiple research fields, nanoparticles have now been incorporated into common everyday products such as food preservatives, cosmetics, clothes; this constant unseen contact with nanoparticles has promoted the field of nano toxicology to study the greater impact these ENMs have on both biological and environmental systems. However, due to limitations in analytical instrumentation and analytical test methods directly applicable to measuring ENMs in the environmental and biological matrices, nanotoxicity remains an underdeveloped field as it struggles to keep up with the advancing research and development of nanoparticles and nanoparticle-based materials actively being developed.

Surface coatings of ENMs can alter their toxicity by providing additional electrostatic forces, molecular adhesion, and atomic layer deposition, contributing to cell death. Furthermore, the elemental composition of ENMs contributes to their overall toxicity to both biological and environmental systems. Such elements can range from transition metals (gold, silver, copper, iron, etc.) to non-metals (silica, carbon). They can greatly alter the previously listed size, morphology, coating, and physical and chemical properties.

NPs and ENMs are primarily introduced into the environment through consumer products. This problem has many arising concerns due to low detection concentrations, usually ng/L, and the current limits of detection of analytical instruments. NPs can also be integrated into the human body in a multitude of ways, but most commonly through inhalation, ingestion, and skin absorption, while environmental exposure is usually through the air, water, and soil integration. For biological matrices, ENMs can impact the mitochondrial function of cells and produce reactive oxidative species (ROS). The analytical measurement of mitochondrial function, damage, and ROS levels in biological systems remains a primary tool in assessing toxicity. ROS levels greatly impact cell metabolism as they are natural byproducts of cell metabolism and contribute to cell survival, death, signaling, inflammation, and differentiation. An imbalance of ROS leads to disrupted redox homeostasis in cells, which ultimately interferes with the cell’s overall function in relation to DNA/RNA breakage, membrane destruction, protein carbonylation, and other means. However, ROS compounds have been looked at previously as an alternative to chemotherapy for cancer treatment. Radical compounds such as superoxide (O₂^•−), hydroxyl (HO^•), hydroperoxyl (HO₂^•), peroxyl (RO₂^•), alkoxyl (RO^•), carbon dioxide (CO₂^•−), carbonate (CO₃^•−) and singlet oxygen (¹O₂) are involved in key cell reactions that revolve around signaling and homeostasis processes. However, high levels of ROS compounds can result in oxidative damage to healthy cells and interfere with cell metabolism. ROS accumulation contributes to normal cells turning into cancer cells. The introduction of NPs into biological systems can interfere with ROS generation in several ways depending on the characteristics of the NPs.

Due to the variety of factors that impact ENMs and NPs toxicity, as previously mentioned, there is not a singular method for accurate detection of ENMs and NPs toxicity. Rather, there are several methods that are commonly used in conjunction to help identify the characteristics of ENMs and NPs and their overall toxicity. Dynamic light scattering (DLS) is most commonly used to determine hydrodynamic particle size, and zeta potential (also determined by the DLS instrument) determines particle surface charge. At the same time, methods such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) allow for the visual detection of ENMs and NPs that can then be measured for their size distribution. Although these methods give insight into the characteristics of the ENMs and NPs, they do not give toxicity analyses. For this, researchers turn to in vitro and in vivo examinations.

To examine in vitro toxicity first, one standard measurement technique to measure ENMs and NPs toxicity in vitro studies is MTT (3-[4, 5-dimethylthiazole-2-yl]-2, 5-diphenyltetrazolium bromide) assays, which assess the cells’ mitochondrial function by detecting mitochondrial dehydrogenase through an enzymatic reduction. Another standard measurement technique for toxicity determination is examining ROS formation within the cells, which indicates oxidative stress and interference in cell function. In order to measure intracellular ROS, a fluorescent ROS indicator is typically utilized. When in the presence of ROS, this indicator will chemically change and thus yield a different fluorescent signal.