The design of nanocapsules with a controlled architecture remains a major scientific and technological objective to this day (Liu, 2020). In particular, in the field of drug delivery it is desirable that the therapeutic molecule forms a dense core, to optimize its concentration per particle. Size issues are also critical (Wilhelm, 2016). Hence, for systemic delivery, particles should preferably be in between 10 and 200 nm, smaller particles being too rapidly excreted and larger ones more easily recognized and eliminated by the immune system (Hoshyar et al., 2016, Mitchell, 2021). The protecting shell is generally needed to provide dispersability and colloidal stability and sometimes more elaborate features such as stealth, targeting, responsiveness to stimuli… Polymers and polymeric surfactants have been widely used for this purpose, as they can be designed to play all these roles. Capsule shells made of inorganic nanoparticles, where the nanoparticles impart an additional specific therapeutic function such as MRI or photoacoustic imaging, radiosensitization or photothermal therapy, have also been described (Huang, 2013, Liu, 2015, Sciortino, 2016, Goubault, 2022).

Nanoprecipitation and emulsion-evaporation are the techniques allowing the highest payloads, since in these cases the core can be made of 100 % of the compound of interest. In particular, drug nanoprecipitation is widely used for the preparation of therapeutic nanoparticles. Among the various methods of nanoprecipitation, the solvent displacement method is often preferred because it requires very little energy. In this method, a solute initially dissolved in a good solvent precipitates on addition of a non-solvent. The solvent and non-solvent should be miscible. A protective shell of polymer or nanoparticles can also be deposited by co-nanoprecipitation, either simultaneously or sequentially (Liu, 2020, Sciortino, 2016, Ramos, 2021, Yan et al., 2021, Goubault, 2020). The supersaturation rate, which depends both on the solvent composition and on the solute concentration, plays a major role in nanoprecipitation by solvent displacement. Several situations are observed depending on the zone of the compositions space where the nanoprecipitation is performed (Fig. 1) (Vitale and Katz, 2003, Beck-Broichsitter et al., 2010, Pucci et al., 2018, Beck-Broichsitter, 2016). (1) Small and weakly polydisperse particles are formed in the Ouzo domain, a domain generally quite restricted near the solubility limit or “binodal” line. As this is a metastable domain, the particles evolve more or less slowly, possibly via coalescence or Ostwald ripening (Bassani et al., 2007, Roger et al., 2013, Vratsanos et al., 2023). (2) At higher solute or non-solvent concentrations (higher supersaturation) a polydisperse population of fast-growing particles is observed. (3) Particles also form in a region of the single-phase domain, called Surfactant-Free Micro-Emulsion (SFME). These are very homogeneous in size and not evolving over time. Note that the existence of this micro-emulsion cannot be explained in the framework of nanoprecipitation, since in this domain, the solute concentration is lower than its solubility limit. An explanation is not available yet, but we see it as analogous to micelles and microemulsions of surfactant systems. In a recent paper (Iglicki et al., 2023), we presented a methodology to elaborate phase diagrams of Ouzo systems using a combination of Static Multiple Light Scattering (SMLS, to probe stability) and Nanoparticle Tracking Analysis (NTA, to quantify polydispersity). This allows to discriminate unambiguously between the SFME, Ouzo, and polydisperse Ouzo “states”.

Both the SFME and Ouzo domains are of interest to prepare nanocapsules (Ramos, 2021, Yan et al., 2021, Ganachaud and Katz, 2005, Yan, 2014, Aschenbrenner, 2013). Thus, Bernard and Ganachaud showed that it was possible to form polymer-coated oil capsules by using a polymer whose precipitation domain overlaps either the SFME or the Ouzo domain of the oil (Yan, 2014, Yan et al., 2021). In their work, the solvent mixture used is water/acetone and the oil is either hexadecane or Miglyol, and the polymer is a rather water-soluble glycopolymer. The polymer does not appear to alter the water/acetone/oil diagram.

More recently, our team prepared nanocapsules coated with nanoparticles by nanoprecipitating a hydrophobic solute, butylated hydroxytoluene (BHT), in water/THF mixtures in the presence of hydrophobic nanoparticles. Although this solute is solid in the pure state at room temperature, the objects formed in the Ouzo domain contain solvent and remain partially liquid (Iglicki et al., 2023). Interestingly, the presence of nanoparticles limits the coalescence of Ouzo emulsions and can significantly focus their distribution (Goubault, 2021). This phenomenon, initially described in the case of Pickering emulsions (Bago Rodriguez and Binks, 2019, Hazt, 2023), induces a stabilization of the droplets, and consequently a widening of the Ouzo domain, towards high solute concentrations. The cross-linking of the nanoparticle shell by a polymer confers a very high mechanical stability to the capsules thus formed, which were named “hybridosomes” (Sciortino, 2016, Sciortino, 2017).

This strategy is very attractive for the elaboration of nanocapsules of nanoparticles, containing drugs, for therapeutic applications (Lepeltier et al., 2014). It is possible to dissolve the therapeutic molecule of interest in the oil forming the core of the capsule (Yan, 2017). However, since many therapeutic molecules are hydrophobic, it seems more favorable that the core of the capsule is entirely made of them. In addition to a very high stability, nanoparticles can bring additional specific properties to the nanocapsules. We have previously demonstrated that hybridosomes made of superparamagnetic iron oxides are MRI contrast agents as efficient as commercial agents such as FeraspinTM (Sciortino, 2016). When their shell contains gold nanoparticles, the hybridosomes are effective radiosensitizers, allowing to potentiate radiotherapy (Goubault, 2022).

Herein, we investigated the formulation of capsules using the cargo of interest as the organic core. Four hydrophobic drugs with various physico-chemical properties: paclitaxel, sorafenib (both molecular solids), sorafenib tosylate (ionic solid), and α-tocopherol (molecular liquid). The concentrations were varied to cover both the SFME and Ouzo domains and the cytotoxic effect of sorafenib-loaded hybridosomes was quantified in vitro.