IntroductionEngineered nanomaterials (ENMs) provide much-needed breakthroughs for resolving some longstanding challenges in drug delivery and personalized medicine. They exhibit pronounced interactions with biomolecules and the ability to effi ciently cross many physiological barriers due to their small size, which may also cause undesired side effects (also see the Wick et al., Hemmer et al., and Bhattacharya et al. articles in this issue). 1 -5 Spectroscopic tools can be used to determine the underlying biophysical mechanisms and the nature of nanobio interactions at the cellular, subcellular, and protein levels, which cannot be gleaned from traditional toxicological studies. For example, some ENMs release metal ions through dissolution at varying rates in different media, including blood. 6 -11 As discussed in this article, these metal ions can adversely affect the proper functioning of biomolecules and cells through an "action-at-a-distance" mechanism. Spectroscopic tools can clearly identify the net bioavailable metal ion content and help establish relationships between ion-release from ENM and ENM-induced toxicity. As outlined in Figure 1 , this article discusses how various spectroscopic tools (e.g., Raman, photoluminescence, infrared, and surface plasmon) can probe the intrinsic properties of ENMs (e.g., crystallinity and electronic states) and biomolecules (e.g., protein structure) and provide a comprehensive understanding of ENM transport within organisms (e.g., via biodistribution) and new perspectives of biological interactions (e.g., loss of protein structure upon adsorption on ENM) at the nanoscale.
Fluorescence imaging of ENM biodistributionThe ultimate prerequisite for a harmful effect from any toxicant is exposure. Therefore, tools allowing the visualization of the nanoparticles (NPs) in vivo are crucial for risk evaluation. On the other hand, the growing utilization of ENMs in potential clinical applications ranging from diagnosis to therapeutic treatment of diseases necessitates a need for understanding their in vivo transport, biodistribution, and long-term fate in the organism. 12 ENMs are known to exhibit a higher uptake by monocytes and macrophages of the mononuclear phagocyte system (a part of the immune system containing phagocytic cells) upon injection into animals, which prevents their rapid clearance through excretion and impairs therapeutic or diagnostic targeting. 2 , 12 Indeed, the clearance mechanisms for most ENMs remain unresolved due to the existing diffi culties in in vivo tracking and monitoring of the ENMs. Fluorescence spectroscopy is one of the important tools in assessing the biological interaction Engineered nanomaterials (ENMs) strongly interact with biomolecules and cells due to their similar size scales. Consequently, ENMs are beginning to emerge as new medical diagnostic tools, probes in cell biology, and delivery vehicles, compelling us to understand the interactions at the nano-bio interface. Optical spectroscopic tools are excellent probes to characterize ENMs...