Nanometrics goes beyond the size: Assessment of nanoparticle concentration and encapsulation efficiency in nanocarriers

Nanoparticle concentration and encapsulation efficiency (EE) are critical attributes for the efficacy/toxicity of nanoformulations for human use. Consequently, their assessment at developmental stages is mandatory. However, the complexity and heterogeneity of nanoparticles along with the technical limitations of available methods makes unfeasible the use of a single technique, transversal to different nanoparticle types, for assessing each of these parameters. Different techniques can be applied: measurement accuracy relies on the adequacy of the selected method/assay conditions to the nanoformulation under study. Thus, consolidation of the knowledge gathered so far on EE and concentration measurements is required, to guide researchers tailoring the characterization methods. This review provides a critical overview on the different strategies for evaluating these attributes, presenting their fundamentals, challenges, limitations and the solutions found for reliable measurements. This aims to provide guidance for selecting nanoparticles characterization methods and for method development, towards the future establishment of standardized procedures.