Progress towards standardized and validated characterizations for measuring physicochemical properties of manufactured nanomaterials relevant to nano health and safety risks

•Identified intrinsic and extrinsic properties of engineered nanomaterials that are related to nano-EHS.•Significant progress has been made to standardize and validate methods for intrinsic NP properties.•Effort is needed to standardize and validate characterization methods for system-dependent (extrinsic) properties.•Factors influencing the reliability and reproducibility of characterization methods have not been well characterized.•Improvement in reporting data and associated meta-data is needed, especially for extrinsic properties of nanomaterials.

Managing the risks of manufactured nanomaterials requires the ability to accurately and reproducibly measure the physical and chemical properties of these materials that are relevant to their risk assessment. However, many properties of manufactured nanomaterials and their non-equilibrium system-dependent behaviors challenge many of the conventional characterization methods used to characterize them. New methods and modifications to conventional methods are being developed to address some of these shortcomings. Here, we critically review progress towards standardization and validation of methods used to characterize manufactured nanomaterials to assess their health and safety risks. The limitations and accessibility of each method are presented, and recommendations towards improving method standardization are made where appropriate. The properties considered include intrinsic (system independent) nanomaterial properties; particle size distribution, specific surface area, particle shape, hydrophobicity, chemical composition, redox potential and band gap, and extrinsic (system dependent) properties; density, dustiness, zeta potential, agglomeration rate and surface affinity, dissolution rate and solubility, and reactive oxygen species generation. The methods evaluated include organisation for economic cooperation and development (OECD) test guidelines, NanoValid standard operating procedures, NANoREG protocols and methods being proposed by peer-reviewed publications. Compared with the characterization of intrinsic properties, the characterization of extrinsic properties is generally less validated and less reproducible. This is mainly because the medium properties may have significant effects on the result. Thus, reporting of extrinsic properties will need to include associated meta-data to make them comparable across studies. While some methods are becoming standardized and even automated, the full range of factors influencing the reliability and reproducibility of those measurements had not yet been well characterized.

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