Olivier did his PhD at the Department of Chemistry of the University of Hull, 2001-2004. He also did post-docs at the University of Hull in 2005, NCSU 2005-6 and the University of Leeds, 2007-2011. He joined the University of Leeds as a lecturer in 2012 and was promoted to associate professor in 2017. PhD supervisor: Prof Vesselin Paunov

Research project 1 (PhD thesis):

Fabrication of Janus microparticles by microcontact printing

 We developed a novel method for preparation of asymmetrically coated colloid particles by using a microcontact printing technique [1-2] – see Fig. 1. Films of water-insoluble ionic surfactants deposited on PDMS stamps were printed onto latex particle monolayers of opposite surface charge in order to produce spherical latex particles of dipolar surface charge distribution (Fig. 2). We studied the effects of salt on the aggregation of such dipolar particles in aqueous suspensions. Upon addition of salt, dipolar colloid particles were found to give ‘‘linear’’ aggregates. The method works with both cationic and anionic surfactants. The half-coated Janus particles exhibit orientational interactions resulting in the formation of linear chain aggregates at high salt concentration. The directional, electrolyte-controlled assembly of these particles in linear aggregates could be used for making photonic crystals with novel symmetries, electrolyte sensitive gels and in electrorheological fluids. We have also extended this method to print colloid monolayers onto colloid monolayers. For monolayers of particles of similar size, the printing produced a high yield of particle doublets, while for monolayers of particles of very different size, complex structures such as ‘‘raspberry-like’’ particles have been fabricated. This method may find further applications for directed assembly of other colloidal structures.

Fig. 1. Scheme of the preparation of dipolar colloid particles by microcontact printing of water-insoluble cationic surfactant on a monolayer of sulfate latex particles.

Fig. 2 (C) and (D) High magnification fluorescence microscopy images of 9.6 μm sulfate PS latex particles stamped with a film of Neuro Dio (hydrophobic fluorescent cationic dye) deposited on an elastomer stamp, and re-dispersed in milliQ water. The anionic and cationic amphiphilic molecules stick equally well on the PS latex particles predominantly due to hydrophobic interactions with the PS surface.

References

1. Cayre, O., Paunov, V.N., Velev, O.D., “Fabrication of Asymmetrically Coated Colloid Particles by Microcontact Printing Techniques“, J. Mater. Chem., 10 (2003) 2445-2450.
2. Cayre, O., Paunov, V.N., Velev, O.D., “Fabrication of Dipolar Colloid Particles by Microcontact Printing“, Chem. Comm., 18 (2003) 2296-2297.

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Research project 2 (PhD thesis):

Fabrication of Colloidosomes with Hydrogel Cores

We report a novel technique for the preparation of colloidosomes based on an aqueous gel core encapsulated with a monolayer of colloid particles [1]. This was achieved by templating water-in-oil emulsions stabilized by latex particles followed by gelling of the aqueous phase with agarose, dissolution of the oil phase in ethanol and redispersion of the obtained colloidosome microcapsules in water (Fig. 1). Two different strategies for strengthening the colloid monolayer shell of the colloidosome microcapsules were explored based on: (i) using glutaraldehyde to cross-link the particles (amine-functionalized) within the colloidosome membrane and (ii) swelling the colloidal monolayer in tricaprylin oil. It was found that even without strengthening, the latex particle monolayer can sustain the transfer of the capsules from the oil to the water phase due only to the particle adhesion to the aqueous gel core. However, the use of glutaraldehyde as a cross-linking agent for the particles was found to improve significantly the quality and the integrity of the monolayer shells and the retention of particles upon transfer (Fig. 2). We discovered that if tricaprylin is used as an oil phase, the latex particles in the colloidosome membrane partially swell and form ‘‘capillary bridges’’ of swollen polystyrene on the capsule surface (Fig. 3). We found ways to control this process by varying the temperature and the degree of exposure of the template to tricaprylin, which allows the pore size of the colloidosome membrane to be controlled independently of the size of the colloidal particles. Such microcapsules made from biodegradable colloid particles may find applications as delivery vehicles and for controlled release of drugs and cosmetic or food supplements. We also developed for the first time [2]: (a) emulsions stabilized solely by microrods (microfibers) and (b) “hairy” colloidosomes whose shells consist of microrod particles. We have designed and fabricated novel colloidosome capsules that consist of aqueous gel cores and shells of polymeric microrods. This was done by templating water-in-oil emulsions stabilized by rodlike particles followed by gelling of the aqueous phase, dissolution of the oil phase in ethanol, and redispersion of the obtained colloidosome microcapsules in water. The microrod shell around the colloidosomes may impart superior mechanical stability compared to the previously used microspheres.

Fig. 1 Schematics of the method for preparation of the gelled colloidosomes of spherical microparticles [1].

Fig. 2 Optical images of colloidosomes with gelled cores covered by a monolayer of 3.9 μm amine-functionalized latex microparticles after being inter cross-linked at the surface. The dimples of the microcapsule surface indicate the previous positions of the voids between the original latex particles [1].

Fig. 3 Fully encapsulated aqueous gel beads of ‘‘golf ball’’ appearance after heating the colloidosomes with PS latex particle shell at 75 °C in tricaprylin for 15 min. The colloidosome shell consists of a fully-fused ‘‘swollen’’ monolayer of 3.9 μm latex microparticles. The dimples of the microcapsule surface indicate the previous positions of the voids between the original latex particles [1].

Fig. 4 Schematics of the method for preparation of the gelled colloidosomes of polymeric microrods [2].

Fig. 5 Optical microscope image (3D reconstruction) of “hairy” colloidosome microcapsule produced by transferring the micro-rod coated agarose hydrogel  beads in water [2].

References

1. Noble, P.F., Cayre, O.J., Alargova, R.G., Velev, O.D., Paunov, V.N., “Fabrication of ‘Hairy’ Colloidosomes by Using Polymeric Microrods”, J. Amer. Chem. Soc., 126 (2004) 8092-8093.
2. Cayre, O.J., Noble, P.F, Paunov, V.N., “Fabrication of Novel Colloidosome Microcapsules with Gelled Aqueous Cores”, J. Mater. Chem., 14 (2004) 3351-3355.

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Research project 3 (PhD thesis):

Gel Trapping Technique for Measuring Contact Angles of Microparticles Adsorbed at Liquid Surfaces

We have used the recently developed gel trapping technique (GTT [1]) to determine the three-phase contact angles of submicrometer silica particles partially coated with octadecyl groups. The particles were spread at air-water and decane-water surfaces, and the aqueous phase was subsequently gelled with a nonadsorbing polysaccharide. The particles trapped at the surface of the aqueous gel were lifted by molding with curable poly(dimethylsiloxane) and imaged with scanning electron microscopy (SEM) to determine the particle contact line diameter which allows their contact angle at the original air-water or oil-water interface to be estimated [1,2]. We report for the first time the use of the GTT for characterizing the contact angle of individual submicrometer particles adsorbed at liquid interfaces (see Fig. 1). The SEM images also reveal the structure of the particle monolayer at the interface and the structure of adsorbed particle aggregates. We have recently developed a version of the GTT where the imaging of the PDMS matrix with the particles is done by AFM which allows the three-phase contact angle (wettability) of individual nanoparticles of diameters as small as several tens of nanometers to be characterized at both air-water and oil-water interface [3].

Fig.1 Scheme of the GTT adapted for (A) air-water and (B) oil-water interfaces. The particle equatorial diameter, d, can be measured directly (C) or by extrapolation after fitting the particle profile with a circle (D). The particle contact angle at the respective liquid interface can be calculated from the SEM images of the PDMS replica after measuring the particle contact line diameter, dc; that is, sinθ=dc/d. The SEM images in parts C and D are given here only for reference [1,2].

References

1. Paunov, V.N., “Novel Method for Determining the Three-Phase Contact Angle of Colloid Particles Adsorbed at Air-Water and Oil-Water Interface”, Langmuir, 19 (2003) 7970-7976.
2. Cayre, O.J., Paunov, V.N. “Three-Phase Contact Angles of Colloid Gold and Silica Particles at Air-Water and Decane-Water Surfaces Studied with the Gel Trapping Technique“, Langmuir, 20 (2004) 9594-9599.
3. Arnaudov, L.A., Cayre, O.J., Stoyanov, S.D., Cohen-Stuart, M., Paunov, V.N., “Nanoimprinting Method for Characterization of the Wettability of Individual Nanoparticles Adsorbed at Liquid Surfaces”, Phys. Chem. Chem. Phys., 12 (2010) 328-331.