Review
Small is beautiful: Surprising nanoparticles

https://doi.org/10.1016/j.ijpharm.2016.02.016Get rights and content

Abstract

In the preparation of nanoparticles for drug delivery, it is well known that their size as well as their surface decorations can play a major role in interaction with living media. It is less known that their shape and internal structure can interplay with cellular and in vivo fate. The scientific literature is full of a large variety of surprising terms referring to their shape and structure. The aim of this review is to present some examples of the most often encountered surprising nanoparticles prepared and usable in the pharmaceutical technology domain. They are presented in two main groups related to their physical aspects: 1) smooth surface particles, such as Janus particles, “snowmen”, “dumbbells”, “rattles”, and “onions” and 2) branched particles, such as “flowers”, “stars” and “urchins”. The mode of preparation and potential applications are briefly presented. The topic has a serious, wider importance, namely in opportunity these structures have to allow exploration of the role of shape and structure on the utility (and perhaps toxicity) of these nanostructures.

Introduction

Nowadays, the comparison in the literature of nanoparticles (NPs) decorated with specific targeting moieties, biocompatible polymers, cell penetrating, stealth, stimulus-sensitive or imaging agents, to objects such as Swiss army knifes equipped with several blades, corkscrews, scissors, saws, tweezers or toothpicks, is no longer surprising (Fig. 1). On the other hand, naming NPs as Janus particles, dumbbells, snowmen, Mickey Mouse, rattles, yolk-shells, raspberries, onions, stars, flowers or sea urchins, is still rather surprising, at least in the domain of drug delivery, drug targeting or theranostics.

It is well known that the biological activity of NPs is significantly dependent on their size (Shang et al., 2014) and surface decoration. It is less known that the cellular and in vivo fate of NPs is influenced by their internal structure and external shape (Ma et al., 2013).

The aim of this article is not to provide an extensive review of the different types of “surprising” NPs, because this could be the topic for a voluminous book. Its aim is to present some examples of the most often encountered surprising NPs, prepared and usable in the pharmaceutical technology domain. These NPs will be presented in two main groups related to their shape: 1) smooth surface particles, such as Janus particles, “snowmen”, “dumbbells”, “rattles”, and “onions”, 2) branched particles, such as “stars”, “flowers” and “urchins”.

Section snippets

Janus nanoparticles

These NPs have been named after the Roman god Janus whose head is represented bearing two opposite faces (Fig. 2A). Thus, true Janus NPs are constituted by asymmetric spheres with two incompatible or different faces, for example hydrophilic (polar) and hydrophobic (non-polar). A number of papers have been devoted to their preparation (Perro et al., 2005, Wurm and Kilbinger, 2009, Tran et al., 2014) that determine their constitution.

Based on recent developments regarding the synthesis and design

Branched nanoparticles

Three main types of NPs are described here: stars, flowers and urchins (Fig. 7, Fig. 8, Fig. 9 ). In some cases, it is difficult to make a clear distinction between them and some authors describe them as “star/flower” NPs (Pandey et al., 2014). It can be considered that the structural complexity increases from “stars” to “flowers” and “urchins”. Anyhow, there are clear flower structures with “petals”, and sometime they have a star-shaped heart.

Conclusion

This review of the “surprising nanoparticles” is not an exhaustive treatise. In fact, nothing has presented here about other fancifully named forms such as “fingerprint” (Wang et al., 2014), “worm” (Christian et al., 2009, Geng and Discher, 2006, Loverde et al., 2012, Wang et al., 2014), “caterpillar” (Deng et al., 2014), “cabbage” and “sea-anemone” (Darbandi et al., 2014) NPs. The imagination of scientists in discovering and naming such curious, often amusing structures invisible to the naked

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