Miniemulsion

INORGANIC CHEMISTRY IN A NANOREACTOR: ROOM TEMPERATURE MINIEMULSION APPROACH TO CRYSTALLINE INORGANIC NANOSTRUCTURES

The research on this topic is mainly focussed on exploiting the colloidal method of miniemulsion for the synthesis, in confined space, of transition metals- and lanthanides-doped metal oxides[20-25], sulphides[26-29], halogenides[30] and hydroxides[31], as well as complex oxides and nanocomposites[32] (see Fig. 3), for potential applications in the field of optical bioimaging and catalysis[33]. 

Fig. 3 Miniemulsions are really versatile colloidal systems which can be exploited coupled with a wide variety of chemical processes (adapted from [34])

Due to their characteristics (see Fig. 4), miniemulsions (30-500 nm droplet size) are perfectly suited to obtain monodispersed nanoparticles, with a good control on both size and morphology of the final material (nanostructure size: 10-200 nm). Additionally, the confinement induced by the miniemulsions strongly influences the crystallisation processes, for example allowing the obtainment, already at room temperature, of crystalline phases usually produced at high temperatures.

Fig. 4 Miniemulsion are stabilised through the use of high intensity ultrasounds (adapted from [30])

Crystallisation of the target nanostructures is accomplished by mixing two miniemulsion separately containing the precursors(two-miniemulsions approach), or by adding a precipitating agent to the pre-formed miniemulsion containing the metal precursor (diffusion approach). Both approaches allow the confinement of the precipitation within the droplets of the final miniemulsion, thus allowing control on size and morphology of the final material. 

By exploiting the precipitation approach, it is also easy to endow the crystalline matrices with functional properties, by incorporating luminescent doping ions (in particular MnII, SmIII, EuIII, TbIII) in a quantitative way. The selected matrices are specifically chosen to maximise biocompatibility, so that the monodispersed, luminescent nanoparticles might be used as nanoprobes in bioimaging applications[21,26,28,30-31].

Alternatively, we have also pioneered the use of tailor-made single-source precursors which, upon UV irradiation, decomposed in the confined space of the droplets to yield a Au-TiO2 nanocomposite[32].

Part of the activity is also focussed on the surface engineering of these particles in order to enhance redispersibility in physiological media and biocompatibility. For more details see Surface Chemistry and Functionalisation

To better understand the crystallisation process in confined environment, we are currently developing a continuous-flow setup which allows us to follow in situ and in a time-resolved fashion the crystallisation of our target materials within the droplets produced by miniemulsion. Such experiments were performed at synchrotron facilities (Elettra sincrotrone, Swiss Light Source – Paul Scherrer Institute) with complementary techniques (SAXS/WAXS and XAS). 

Moreover, our experimental investigations (inorganic synthesis and analytical investigations of crystallisation phenomena within miniemulsion droplets) are backed-up by theoretical modelling in collaboration with Prof. A. Polimeno. In particular, the aim of the collaboration is to rationalise and correlate mesoscopic properties at the fluid dynamic and thermodynamic levels and to understand the key parameters which determine the relative thermodynamic stability of different polymorphs in the confined conditions of miniemulsion droplets.

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