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1 Introduction

Rapid development of the nanoscience and technology has produced numerous nanomaterials that offer revolutionary benefits in electronics, energy, medical, and health applications, but unfortunately also lead to environmental, health, and safety concerns [1]. For example, Au nanoparticles (NPs) have been explored as nanopharmaceuticals for the treatment of cancer [2], and Ag NPs have been established as superior antibacterial materials [3]. However, the wide use of nanomaterials has raised concerns regarding their potentially hazardous effects on biological systems, and the associated short- and long-term risks are not well understood. A variety of nanomaterials can generate reactive oxygen species (ROS) under certain experimental conditions [4–9]. Among various toxic responses, nanomaterial-induced oxidative stress mediated by ROS has been studied most extensively [10–12].

ROS, e.g., superoxide, hydroxyl radical, singlet oxygen, and hydrogen peroxide, are powerful oxidants that can damage cellular targets nonselectively. Free radicals, including ROS, are short lived and represent a broad range of chemically distinct entities; consequently, these species are difficult to detect in dynamic environments such as biological systems. The use of fluorescent probes (e.g., dichlorodihydrofluorescein, hydroethidine, and dihydrorhodamine) and chemiluminescent assays is a simple and easy way of detecting free radicals and ROS in cellular systems, but there are inherent limitations and many sources of artifacts [13,14]. Electron spin resonance (ESR) spectroscopy has become a powerful and direct method to detect free radicals generated chemically or formed in biological systems. We have a longstanding interest in employing ESR techniques to identify and quantify free radicals in biological systems, and study the mechanisms of interactions between biologically relevant systems and nanomaterials, metal ions, and organic molecules [4,5,7,9,15–43]. We have also published several book chapters on this subject [44–46]. In this special issue, we demonstrate that ESR spectroscopy is a powerful tool for exploring the capability of NPs to generate ROS. The ESR spin-trapping techniques used to detect ROS (including hydroxyl radicals, superoxide radical anion, and singlet oxygen) and the ESR oximetry methodology employed for monitoring oxygen and the formation of lipid peroxidation are also discussed briefly.

2 ESR spectroscopy
2.1 Principle of ESR spectroscopy

ESR, also called electron paramagnetic resonance, is a powerful technique for studying chemical species or materials that have one or more unpaired...

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