1 © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com Engineered Nanomaterials Quantifying and Imaging Engineered Nanomaterials In Vivo: Challenges and Techniques Xiao He,* Yuhui Ma, Meng Li, Peng Zhang, Yuanyuan Li, and Zhiyong Zhang* Quantifying and imaging the engineered nanomaterials (ENMs) in vivo can provide information on the bio-distribution and fate of ENMs in living systems. A necessary amount of in vivo quantitative data is indispensable to verify the extrapolation from in vitro tests, to modify the predictive models of ENM exposure, and to underpin the risk management strategy for ENMs. However, it remains a challenge to quantitatively assess the bio-distribution of ENMs under realistic exposure, their long-term deposition (especially in non-targeted tissues), their passage across the natural barriers, and the impacts of nano–bio interactions on their in vivo behaviors. Some commonly used techniques for in vivo ENM quantification, such as electron microscopy, fluorescence- based detection, atomic spectroscopy, radiotracing, and techniques basing on synchrotron radiation are reviewed, and their technical characteristics, the state of the art, limitations, and future prospects are addressed. Introduction 1. ............................................... 2 Abbreviations Used 2. ................................... 2 Challenges in In Vivo Quantification 3. of ENMs ...................................................... 2 Techniques for In Vivo Quantification 4. of ENMs ...................................................... 3 Summary and Outlook 5. ................................8 From the Contents small 2012, DOI: 10.1002/smll.201201502