Date Published: December 01, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Richard J. Archer, Andrew J. Parnell, Andrew I. Campbell, Jonathan R. Howse, Stephen J. Ebbens.
The field of active colloids is attracting significant interest to both enable applications and allow investigations of new collective colloidal phenomena. One convenient active colloidal system that has been much studied is spherical Janus particles, where a hemispherical coating of platinum decomposes hydrogen peroxide to produce rapid motion. However, at present producing these active colloids relies on a physical vapor deposition (PVD) process, which is difficult to scale and requires access to expensive equipment. In this work, it is demonstrated that Pickering emulsion masking combined with solution phase metallization can produce self‐motile catalytic Janus particles. Comparison of the motion and catalytic activity with PVD colloids reveals a higher catalytic activity for a given thickness of platinum due to the particulate nature of the deposited coating. This Pickering emulsion based method will assist in producing active colloids for future applications and aid experimental research into a wide range of active colloid phenomena.
Micrometer and nanoscale synthetic swimming devices show enhanced displacements far exceeding Brownian motion by exploiting localized catalytic reactions. Such devices have generated interest due to potential applications ranging from microfluidic transport in lab‐on‐a‐chip devices,1, 2, 3 rapid environmental decontamination,4 and directed drug delivery.5 As evidence of the growing scope for real‐world applications for these devices, a recent report showed improved delivery of a model drug in vivo.6 In addition, self‐motile swimming devices also provide a route to experimentally verify a wide range of phenomena that have been predicted for colloidal systems. One particular area of interest is the potential for high volume fractions of interacting motile colloids to display a rich variety of emergent behavior, including self‐organizing effects such as clustering.7 However, despite this attention, the current methods by which synthetic catalytic swimming devices are manufactured remain cumbersome and have significant drawbacks, which limit the viability of proposed applications, and prevent extensive experimental research effort into active colloid phenomena. Prominent, widely studied examples of synthetic swimming devices include bimetallic rods, consisting of connected catalytically active and inactive segments,8 microrockets, consisting of rolled‐up microtubes with catalyst coating the interior walls,9 and Janus particles, consisting of spherical colloids where one hemisphere is catalytically active.10 Platinum is used as the catalytically active material in the majority of the reported examples due to its ability to perform the rapid room‐temperature decomposition of hydrogen peroxide, and consequently produce motion via either phoretic11 or bubble release mechanisms.12 A key feature of the three device types is that motion production requires a specific distribution of catalyst, particularly for nanorod and Janus devices that move via self‐phoresis where the mechanism is critically reliant on gradients which can only be generated by having an asymmetric catalyst distribution. Taken together, the requirement for both metallization and asymmetry generation has resulted in the current manufacturing methods being typically cumbersome and low yielding, owing to a reliance on lab‐based physical vapor deposition (PVD) techniques to deposit the catalyst. For the aforementioned examples, bimetallic rod synthesis requires a combination of PVD and electroplating into porous membranes for each required metal, microrockets require PVD of multiple metals onto sacrificial polymer films which when etched away cause the metal films to roll into the required tubular structure, while active Janus colloids require platinum PVD onto spherical colloids.10, 13, 14 The PVD requirement restricts all these processes to 2D planar batch fabrication, and the requirement during PVD for high vacuum environments creates scalability issues and high energy demands to vaporize the source material. PVD is currently hard to implement as a continuous process as it requires transfer of material for coating from ambient to vacuum conditions. Additionally, unless scrupulously clean, vacuum chambers are subject to introducing surface hydrocarbon contamination which can impair reactivity and require cleaning stages. Moreover, PVD requires both localization of colloids onto a solid support with a degree of control (e.g., spin‐coating) to avoid shadowing, and finally transfer of colloids back into the solution phase, which is hard to accomplish efficiently and can introduce contamination. Another general drawback for PVD techniques is the difficulty in maintaining stoichiometry when evaporating compound materials such as metal oxides, often requiring the introduction of reactive gases into the chamber. These metal oxide materials have been suggested as alternative more versatile catalysts for powering self‐motile systems.15 The potential to develop an analogue of the solution‐based method we present here to also deposit metal oxides as Janus coatings overcomes the stoichiometry issues with PVD.
We have demonstrated a Pickering emulsion route to produce batches of active colloids which exhibit similar motility to those made by PVD. However, unlike the PVD method, the solution phase method is scalable, and does not require access to expensive equipment. Here, we made batches of active Janus particles on a small test scale producing 5 mL at ≈0.5 mg mL−1 mass fraction (±0.1 mg mL−1), however the Pickering emulsion approach can allow for gram‐scale production.41 The described method generates self‐motility by functionalizing silica colloids, which is advantageous due to the simplicity of producing large quantities of monodisperse colloids with selectable diameters.23, 24 Silica offers other potential advantages in controllable porosity,25 allowing use for drug storage and delivery which is a potential application area for active Janus particles.26 The methodology also leaves exposed amine groups on the catalytically inactive side of the Janus colloids, which will enable attachment of proteins, a prerequisite for the proposed mass transport applications and antibodies which could potentially allow biological recognition and specific targeting.33, 34 Despite the difference in preparation method, which is expected to produce platinum coatings with different characteristics (PVD coatings will have a gradient of thickness and reactivity from pole to equator, whereas the methods here are expected to produce a uniform coating), the observed trajectories do not reveal a significant difference in motility. This observation could help inform mechanistic understanding for active colloids. Significantly, less metal was required to produce rapid propulsion for the colloids made by Pickering masking, which is suggested to be due to the rougher, particulate platinum topography. Due to the cost of platinum, this is an additional significant benefit of the described method.
Materials: Tetraethyl orthosilane (TEOS, 98%), ammonia (25% wt), paraffin wax, sodium borohydride (99%), sodium dodecylsulfate (SDS, 98%), cetyltrimethylammonium bromide (CTAB, 98%), hexachloroplatinic acid hexahydrate (>37.5% Pt basis), hydrogen peroxide (30% wt), and methanol (99.6%) were purchased from Sigma Aldrich. APS (97%) and formaldehyde (37% wt) were purchased from Alfa Aesar. All materials were used as received. Deionized (DI) water was obtained from an Elga Purelab Option filtration system (15 MΩ cm).
The authors declare no conflict of interest.