Stephen J. Ebbens

In pursuit of propulsion at the nanoscale

This review describes recent developments in self-propelling nano- and micro-scale swimming devices. The ability of these devices to transport nano-scale components in a fluidic environment is demonstrated. Furthermore, the adaptations needed for these devices to meet biological transport challenges such as targeted drug delivery are highlighted. Particular emphasis is placed on describing autonomously powered devices driven by asymmetrical chemical reactions. Methods to control the speed and direction of such swimming devices using external fields are described, and contrasted to recent demonstrations of statistical autonomous migrations and organisations driven by chemical gradients, inter swimmer interactions and external photo-stimulus. Finally the challenges and advantages of converting other nature inspired swimming mechanisms into realistic artificial self-powered devices are considered.

Electrokinetic effects in catalytic platinum-insulator Janus swimmers

The effect of added salt on the propulsion of Janus platinum-polystyrene colloids in hydrogen peroxide solution is studied experimentally. It is found that micromolar quantities of potassium and silver nitrate salts reduce the swimming velocity by similar amounts, while leading to significantly different effects on the overall rate of catalytic breakdown of hydrogen peroxide. It is argued that the seemingly paradoxical experimental observations could be theoretically explained by using a generalised reaction scheme that involves charged intermediates and has the topology of two nested loops.

Direct Observation of the Direction of Motion for Spherical Catalytic Swimmers

Nonconductive Janus particle swimmers made by coating fluorescent polymer beads with hemispheres of platinum have been fully characterized using video microscopy to reveal that they undergo propulsion in hydrogen peroxide fuel away from the catalytic platinum patch. The platinum coating shadows the fluorescence signal from half of each swimmer to allow the orientation to be observed directly and correlated quantitatively with the resulting swimming direction. The observed swimmer direction is consistent with both the bubble release and diffusiophoretic propulsion mechanisms.

Boundaries can steer active Janus spheres

The advent of autonomous self-propulsion has instigated research towards making colloidal machines that can deliver mechanical work in the form of transport, and other functions such as sensing and cleaning. While much progress has been made in the last 10 years on various mechanisms to generate self-propulsion, the ability to steer self-propelled colloidal devices has so far been much more limited. A critical barrier in increasing the impact of such motors is in directing their motion against the Brownian rotation, which randomizes particle orientations. In this context, here we report directed motion of a specific class of catalytic motors when moving in close proximity to solid surfaces. This is achieved through active quenching of their Brownian rotation by constraining it in a rotational well, caused not by equilibrium, but by hydrodynamic effects. We demonstrate how combining these geometric constraints can be utilized to steer these active colloids along arbitrary trajectories.

A study of single drug particle adhesion interactions using atomic force microscopy

This paper aims to use Atomic Force Microscopy (AFM) to characterise the interaction forces between micronized salbutamol particles, an active ingredient frequently used in metered dose inhalers, and also to glass, lactose and a fluoropolymer. The methodology used involves challenging a salbutamol functionalized AFM tip to the surfaces of interest and measuring the force experienced by the cantilever as a function of tip-sample separation. Analysis of this force-distance data allows quantification of the particle-substrate adhesion. This study yields a ranking of adhesion as glass>lactose>salbutamol>polytetrafluoroethylene (PTFE). An increase in the interaction force between the salbutamol particle and PTFE on repeated contact due to tribocharging is also observed.

Importance of particle tracking and calculating the mean-squared displacement in distinguishing nanopropulsion from other processes.

In this paper we show that processes such as Brownian motion, convection, sedimentation, and bacterial contamination can cause small particles to move through liquids in a fashion which may be mistaken as nanopropulsion. It is shown that particle tracking and subsequent statistical analysis is essential to ascertain if small particles actually propel themselves, or if they are propelled by another process. Specifically we find that it is necessary to calculate the mean-squared displacement of particles at both short and long time intervals, to show that the direction of propulsion changes coincident with rotation of the particle by Brownian motion, as this allows nanopropulsion to be differentiated from Brownian motion, convection and sedimentation. We also find that bacteria can attach themselves to particles and cause them to be propelled. This leads to motion which appears very similar to nanopropulsion and cannot be differentiated using particle tracking and therefore find that carefully designed control experiments must be performed. Finally, we suggest an experimental protocol which can be used to investigate the motion of small objects and prove if they move due to nanopropulsion.

Identifying and mapping surface amorphous domains.

PurposeUndesirable amorphous material generation during formulation is implicated in a growing number of pharmaceutical problems. Due to the importance of interfacial properties in many drug delivery systems, it seems that surface amorphous material is particularly significant. Consequently, this study investigates a range of methods capable of detecting and mapping surface amorphous material.MethodsA micron-sized localized surface domain of amorphous sorbitol is generated using a novel localized heating method. The domain is subsequently investigated using atomic force microscopy (AFM) imaging, nanomechanical measurements, and Raman microscopy 3-D profiling.ResultsAFM phase and height images reveal nanoscale-order variations within both crystalline and amorphous sorbitol domains. Nanomechanical measurements are able to quantitatively distinguish the amorphous and crystalline domains through local Young’s modulus measurements. Raman microscopy also distinguishes the amorphous and crystalline sorbitol through variations in peak width. This is shown to allow mapping of the 3-D distribution of the amorphous phase and is hence complementary to the more surface sensitive AFM measurements.ConclusionsAFM and Raman microscopy map the distribution of amorphous material at the surface of a sorbitol crystal with submicron spatial resolution, demonstrating surface analysis methods for characterizing semicrystalline solids generated during pharmaceutical processing.

In situ imaging and height reconstruction of phase separation processes in polymer blends during spin coating

Spin coating polymer blend thin films provides a method to produce multiphase functional layers of high uniformity covering large surface areas. Applications for such layers include photovoltaics and light-emitting diodes where performance relies upon the nanoscale phase separation morphology of the spun film. Furthermore, at micrometer scales, phase separation provides a route to produce self-organized structures for templating applications. Understanding the factors that determine the final phase-separated morphology in these systems is consequently an important goal. However, it has to date proved problematic to fully test theoretical models for phase separation during spin coating, due to the high spin speeds, which has limited the spatial resolution of experimental data obtained during the coating process. Without this fundamental understanding, production of optimized micro- and nanoscale structures is hampered. Here, we have employed synchronized stroboscopic illumination together with the high light gathering sensitivity of an electron-multiplying charge-coupled device camera to optically observe structure evolution in such blends during spin coating. Furthermore the use of monochromatic illumination has allowed interference reconstruction of three-dimensional topographies of the spin-coated film as it dries and phase separates with nanometer precision. We have used this new method to directly observe the phase separation process during spinning for a polymer blend (PS-PI) for the first time, providing new insights into the spin-coating process and opening up a route to understand and control phase separation structures.

Gravitaxis in Spherical Janus Swimming Devices

In this work, we show that the asymmetrical distribution of mass at the surface of catalytic Janus swimmers results in the devices preferentially propelling themselves upward in a gravitational field. We demonstrate the existence of this gravitaxis phenomenon by observing the trajectories of fueled Janus swimmers, which generate thrust along a vector pointing away from their metallically coated half. We report that as the size of the spherical swimmer increases, the propulsive trajectories are no longer isotropic with respect to gravity, and they start to show a pronounced tendency to move in an upward direction. We suggest that this effect is due to the platinum caps asymmetric mass exerting an increasing influence on the azimuthal angle of the Janus sphere with size, biasing its orientation toward a configuration where the heavier propulsion generating surface faces down. This argument is supported by the good agreement we find between the experimentally observed azimuthal angle distribution for the Janus swimmers and predictions made by simple Boltzmann statistics. This gravitaxis phenomenon provides a mechanism to autonomously control and direct the motion of catalytic swimming devices and so enable a route to make autonomous transport devices and develop new separation, sensing, and controlled release applications.

Towards nanoscale metrology for biomolecular imaging by atomic force microscopy

Atomic force microscopy (AFM) provides three-dimensional images with resolution at or near the atomic level. Many researchers have addressed the metrological aspects of AFM imaging of physical samples, producing a range of recognized standards for calibrated spatial measurements. However, because of the complex interplay of forces between an AFM tip and biological samples, the dynamic environment in which such imaging takes place, and difficulties in immobilization, no satisfactory equivalents exist for the biological AFM community. Here an exploration of the effects of AFM imaging parameters on apparent biomolecular dimensions in aqueous environments is carried out for a three-component system comprising GroEL protein, plasmid DNA and gold nanoparticles. The biomolecules in this system have been chosen to present differing imaging challenges, while gold nanoparticles serve as an internal reference for tip performance. Concurrent immobilization of these entities requires optimization of sample preparation methodology, described here as it is hoped the system may act as a template for future bio-metrological standards. Investigation of differences in measured heights and lateral dimensions of DNA, GroEL and gold spheres under a range of imaging conditions and the implications of these results for accurate imaging of biological samples and as a potential bio-metrological standard are discussed.