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Smart Pickering Emulsions

Periodic Reporting for period 1 - SPicEs (Smart Pickering Emulsions)

Reporting period: 2016-06-01 to 2018-05-31

This action focused on the preparation and characterization of Pickering-type emulsions (PEs) using functionalized micro- and nano-particles, along with their use as confined spaces that can be programmed to perform an action. PEs are emulsions stabilized by solid particles (powders) rather than traditional molecular surfactant and we postulated that using particles with specific properties could impart such properties to the emulsion itself, leading to technological applications.
To this end, we first developed a protocol based on the so-called Stöber method for the production of silicon dioxide particles, to precisely control the size of the particles, load them with fluorescent dyes allowing for their visualization and modify their surfaces with functional moieties such as fullerenes and short nucleic acid sequences.
Next, we compared discontinuous and continuous-flow approaches to the preparation of the emulsions. Comparison of these different approaches, leading to emulsions with different properties, is interesting beyond the scope of this project and could in principle be applied in different fields such as cosmetics and food, where emulsions are obiquitous. It should be mentioned that, while the use of flow-focusing microfluidic devices operated in a continuous fashion allowed to produce highly monodispersed droplets, the use of solid particles as emulsifiers put the devices under intense stress and often caused their failure on the long run.
Characterization protocols for our emulsions needed to be developed, since emulsified systems are tipically observed as a whole, but we were more interested in the behaviour of the single droplets and the fate of the emulsification agent. For example we can easily describe the physico-chemical properties of creams or foams, but the observation of the single droplets/bubbles in an emulsified system is tricky. Moreover, in this action we delved deeper into the problem, by trying to observe directly the emulsifier – micro and nanoparticles – on the surface of the droplets.
Finally, we tested one of the emulsions we developed, consisting of silicon dioxide particles decorated with [60]fullerene moieties, as a chemical reactor for the light-induced generation of singlet oxygen (a powerful, eco-friendly oxidant) and the oxidation of a model substrate. In this context, each droplet in the emulsion acts as a stand-alone chemical reactor, meaning that the oxidation can be easily scaled by de facto numbering up of the reaction vessels. This, in turn, does not change the geometry of the single reaction units – a welcome feat in a chemical process such as the one considered.
1. Development of a protocol based on the Stöber method to prepare silica nanoparticles with controlled size, surface features and fluorescent doping. Preparation of silica nanoparticles decorated with different moieties.
Silica nanoparticles with sizes in the 80-400 nm range were prepared. Loading of the nanoparticles with fluorophores (notably fluorescein and rhodamine-type dyes) was carried out by employing silanes modified to bear the dyes directly in the reaction mixtures. Surface modification was attained by delayed addition of suitable silanes at the end of the synthesis, right before purification, typically by using custom-made trialkoxysilanes with the fourth substituent on the silicon atom bearing the desired structure. Explored moieties were [60]fullerenes, beta-cyclodextrines and short DNA strands.
2. A study was carried out on the synthesis of silicon nanoparticles according to a well-known method described in literature, that turned out not to yield the claimed products.
A 2014 publication claimed that silicon nanoparticles could be prepared by citrate-prompted reduction of alkoxysilanes in water. Failure to achieve the expected results for the characterization of Si(0) particles, lead us to a study that confirmed how the products consisted of silica mixed with carbon quantum dots, the latter resulting from the thermal trearment of citrate.
3. Comparison of discontinuous and continuous emulsification processes, carried out with an homogenizer or a commercially-available microfluidic chip.
Continous-flow processing, an interesting alternative to traditional techniques, has been evaluated for the preparation of Pickering-type emulsions. Commercially-available flow-focusing junctions have been tested to this end, in comparison with an homogenizer equipped with a mixing element suitable for emulsification. As demonstrated by image 1, the microfluidic device allowed for a very narrow size distribution of droplets coated with rhodamine-doped silica, even though the startup and the shutdown phases of the process were often subject to clogging problems.
4. Preparation and observation of model emulsions using fluorescent particles in order to study their fate.
The use of laser scanning confocal microscopy to image fluorescent particles on the surface of Pickering-emulsified droplets was essayed, in order to observe how particles with different surface functionalization adsorb on the liquid-liquid surface (see image 2 for a 3D reconstruction of a single oil droplet in water stabilized by rhodamine-doped silica). However, a similar study was published by another group during the action, which prompted us to try a different approach. We therefore used dynamic light scattering to assess the absorption of particles with a broad size distribution and managed to develop a protocol that could be further refined to shed light on the kinetic of adsorption of particles in correlation to their sizes.
5. Use of a PE as a reactor for singlet oxygen oxidation of a model substrate
Oil-in-water emulsions stabilized by silica nanoparticles surface modified with [60]fullerene were used to oxidize a model substrate (diphenylisobenzofuran) with singlet oxygen. [60]fullerene is a well known, stable singlet oxygen sensitizer and its presence on the surface of the organic phase turned each droplet in a catalytic photochemical chemical reactor with an extremely high surface-to-volume ratio, that optimized oxygen transport, yielding better results in comparison with a system in which the same particles were just suspended in a homogeneous solution of the substrate.
The study on the preparation of silicon nanoparticles allowed to identify a possible mistake in a published paper with more than 150 citations. This is obviously an important progress, in that it allows a better comprehension of the process leading to the formation of silicon and silicon dioxide nanoparticles, as well as pointing out the importance of proper characterization of nanomaterials (an aspect that is too often overlooked in favour of quicker quality checks).
The use of microfluidics to handle solid suspensions is in general far from trivial. In our case, we were able to extend the lifespan of the flow focusing devices we employed by developing a peculiar system for the startup and – especially – the shutdown phases of the process. This approach can be considered generally advantageous for continuous-flow approaches.
Indirect observation of the asdorption of particles on the liquid-liquid interface by DLS measurements on samples of the continuous phase represent a new approach to the study of PEs, alternative to direct visualization by confocal microscopy. It goes past the state of the art in that it gives an alternative quantitative point of view of the emulsification process.
Oil-in-water droplets stabilized with rhodamine-doped silica, prepared with a flow focusing device
3D reconstruction of a single oil droplet in water stabilized by rhodamine-doped silica