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.

We demonstrate a procedure for the separation of enzymes based on their chemotactic response toward an imposed substrate concentration gradient. The separation is observed within a two-inlet, five-outlet microfluidic network, designed to allow mixtures of active (ones that catalyze substrate turnover) and inactive (ones that do not catalyze substrate turnover) enzymes, labeled with different fluorophores, to flow through one of the inlets. Substrate solution prepared in phosphate buffer was introduced through the other inlet of the device at the same flow rate. The steady-state concentration profiles of the enzymes were obtained at specific positions within the outlets of the microchannel using fluorescence microscopy. In the presence of a substrate concentration gradient, active enzyme molecules migrated preferentially toward the substrate channel. The excess migration of the active enzyme molecules was quantified in terms of an enrichment coefficient. Experiments were carried out with different pairs of enzymes. Coupling the physics of laminar flow of liquid and molecular diffusion, multiphysics simulations were carried out to estimate the extent of the chemotactic separation. Our results show that, with appropriate microfluidic arrangement, molecular chemotaxis leads to spontaneous separation of active enzyme molecules from their inactive counterparts of similar charge and size.

Synthetic nano- and microscale machines move autonomously in solution or drive fluid flows by converting sources of energy into mechanical work. Their sizes are comparable to analytes (sub-nano- to microscale), and they respond to signals from each other and their surroundings, leading to emergent collective behavior. These machines can potentially enable hitherto difficult analytical applications. In this article, we review the development of different classes of synthetic nano- and micromotors and pumps and indicate their possible applications in real-time in situ chemical sensing, on-demand directional transport, cargo capture and delivery, as well as analyte isolation and separation.

The directed transport of microparticles in microfluidic devices is vital for efficient bioassays and fabrication of complex microstructures. There remains, however, a need for methods to propel and steer microscopic cargo that do not require modifying these particles. Using theory and experiments, we show that catalytic surface reactions can be used to deliver microparticle cargo to specified regions in microchambers. Here reagents diffuse from a gel reservoir and react with the catalyst-coated surface. Fluid density gradients due to the spatially varying reagent concentration induce a convective flow, which carries the suspended particles until the reagents are consumed. Consequently, the cargo is deposited around a specific position on the surface. The velocity and final peak location of the cargo can be tuned independently. By increasing the local particle concentration, highly sensitive assays can be performed efficiently and rapidly. Moreover, the process can be repeated by introducing fresh reagent into the microchamber.

Antimicrobial activity of experimental formulations of two structurally different nano-zinc oxide materials, plate-like Zinkicide SG4 and particulate Zinkicide SG6, was evaluated against Xanthomonas citri subsp. citri, the cause of citrus canker. In vitro assay demonstrated Zinkicide SG4 had a twofold lower minimum inhibitory concentration (MIC) against Escherichia coli and X. alfalfae subsp. citrumelonis (62.5 to 250 µg/ml) compared with copper sulfate (250 µg/ml), copper hydroxide (250 to 500 µg/ml), or cuprous oxide/zinc oxide (125 to 250 µg/ml). Zinkicide SG6 had a sevenfold to eightfold lower MIC against Escherichia coli and X. alfalfae subsp. citrumelonis (31 to 250 μg/ml). Leaves of sweet orange (Citrus sinensis) and fruit of ‘Ruby Red’ grapefruit (C. paradisi) were evaluated for citrus canker disease control. A greenhouse assay with foliage demonstrated that spray treatment with Zinkicide reduced citrus canker lesion development after injection-infiltration of X. citri subsp. citri into the leaf intercellular space. In field trials conducted in Southeast Florida in 2014 and 2015, Zinkicide SG4 and SG6 reduction of grapefruit canker incidence exceeded that of cuprous oxide and cuprous oxide/zinc oxide bactericides. Zinkicide formulations were also effective against the fungal diseases, citrus scab (Elsinoe fawcetti) and melanose (Diaporthe citri), on grapefruit. No sign of phytotoxicity to the fruit rind was observed during either season. Antimicrobial activity of Zinkicide for protection of leaves and fruit against X. citri subsp. citri was comparable or exceeded that for commercial copper and zinc oxide formulations which may be attributed to translaminar movement of Zinkicide.