Updated insight into the characterization of nano-emulsions

ABSTRACT

Introduction

In most of the studies, nano-emulsion characterization is limited to their size distribution and zeta potential. In this review, we present an updated insight of the characterization methods of nano-emulsions, including new or unconventional experimental approaches to explore in depth the nano-emulsion properties.

Area covered

We propose an overview of all the main techniques used to characterize nano-emulsions, including the most classical ones, up to in vitro, ex vivo and in vivo evaluation. Innovative approaches are then presented in the second part of the review that presents innovative, experimental techniques less known in the field of nano-emulsion such as the nanoparticle tracking analysis, small-angle X-ray scattering, Raman spectroscopy, and nuclear magnetic resonance. Finally, in the last part we discuss the use of lipophilic fluorescent probes and imaging techniques as an emerging tool to understand the nano-emulsion droplet stability, surface decoration, release mechanisms, and in vivo fate.

Expert opinion

This review is mostly intended for a broad readership and provides key tools regarding the choice of the approach to characterize nano-emulsions. Innovative and uncommon methods will be precious to disclose the information potentially reachable behind a formulation of nano-emulsions, not always known in first intention and with conventional methods.

KEYWORDS:

• Nano-emulsion
• characterization
• fluorescence
• surface properties
• innovative methods

Article highlights

• The state-of-the-art of the characterization of nano-emulsions.

• Review of original physicochemical methods used to characterize nano-emulsions, and their complementarity with the conventional approaches.

• An update of the in vitro characterization of nano-emulsions compared with the different experimental approaches and cellular models described in the current literature.

• The review of in vivo fate of nano-emulsion, stakes, and challenges.

• A focus on the use of lipophilic fluorescent dyes solubilized in nano-emulsion droplets, and the review of the original results provided by these methods.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

The authors thank the China Scholarship Council for funding Ph.D. fellowship for X.W., (CSC No. 201706240033).