Synthesis, microscopy and manipulation of colloidal matter

Abstract

In the first part, synthetic routes to model systems of spherical silica nanoparticles with diameters in the range 30-360 nm are presented. The general method uses nanoparticles formed by an arginine catalysed synthesis as seeds for growth of Stöber-type silica. Fluorescently dyed and uniformly sized particles could be made with diameter as low as 30 nm (polydispersity <7%). By growing the particles to diameter 360 nm (polydispersity <2%), the possibility of imaging single particles in 3D with super-resolution optical microscopy was opened up, while keeping the gravitational length of the particles more than 50 times greater than the particle diameter.

The second synthesis investigated was that of colloidal droplets. Droplet microfluidics is well known to be capable of producing emulsion droplets of low polydispersity, albeit with low throughput. Parallelisation has been successfully applied to scale up the production of monodisperse droplets but this has not yet been achieved for the full range of droplet sizes which are accessible in single droplet junctions. Crucially, the colloidal size range with droplet diameters around 1 µm (or less) is still missing. In this work, a new device is tested which is based on the downscaling of a recently presented parallel design known as the ‘millipede’. This ‘nanopede’ was found to yield crystallising droplets of close to 1 µm with polydispersity below 7%.

The focus then moves to microscopy. Imaging deep inside thick specimens is problematic in all forms of microscopy, and stimulated emission depletion (STED) is no exception. A challenge unique to this technique is engineering the depletion spot shape such that it remains effective at focal planes away from the cover glass. A test sample was developed to characterise the morphing depletion pattern as a function of depth inside the sample and measure the resulting 3D STED point spread functions (PSFs) at focal depths up to 100 µm. The sample consisted of three types of silica particle: gold-core (to reflect the incident laser spots), fluorescent-core (to measure PSFs) and a scaffold of unfunctionalised particles to disperse and immobilise the probe particles in 3D. The depletion spot shape was maintained by adjusting the correction collar of the objective lens for each imaging height. Thus a compact 3D STED PSF was maintained while imaging deep inside the sample.

Finally, attention is given to manipulation of colloidal particles. Direct measurement of diffusiophoretic velocities of colloidal silica particles in electrolyte gradients is demonstrated. The gradients were generated using a microfluidic Ψ-shaped channel and the motion of the fluorescent particles imaged with confocal microscopy. Analysis of single particle trajectories confirmed the action of diffusiophoretic transport and observations were compared to theoretical predictions. The measured diffusiophoretic mobilities were consistently higher than those predicted by theory and this is attributed to a complication of the solute profile after investigation using a fluorescent salt. Despite this, the measured velocity profiles have shapes in close agreement with theory and it is proposed that small changes to the experimental design will reconcile the observations with predictions.