Thermal, Mutual, and Self-Diffusivities of Binary Liquid Mixtures Consisting of Gases Dissolved in n-Alkanes at Infinite Dilution.

In the present study, dynamic light scattering (DLS) experiments and molecular dynamics (MD) simulations were used for the investigation of the molecular diffusion in binary mixtures of liquids with dissolved gases at macroscopic thermodynamic equilibrium. Model systems based on the n-alkane n-hexane or n-decane with dissolved hydrogen, helium, nitrogen, or carbon monoxide were studied at temperatures between 303 and 423 K and at gas mole fractions below 0.06. With DLS, the relaxation behavior of microscopic equilibrium fluctuations in concentration and temperature is analyzed to determine simultaneously mutual and thermal diffusivity in an absolute way. The present measurements document that even for mole gas fractions of 0.007 and Lewis numbers close to 1, reliable mutual diffusivities with an average expanded uncertainty ( k = 2) of 13% can be obtained. By use of suitable molecular models for the mixture components, the self-diffusion coefficient of the gases was determined by MD simulations with an averaged expanded uncertainty ( k = 2) of 7%. The DLS experiments showed that the thermal diffusivity of the studied systems is not affected by the dissolved gas and agrees with the reference data for the pure n-alkanes. In agreement with theory, mutual diffusivities and self-diffusivities were found to be equal mostly within combined uncertainties at conditions approaching infinite dilution of the gas. Our DLS and MD results, representing the first available data for the present systems, reveal distinctly larger mass diffusivities for mixtures containing hydrogen or helium compared to mixtures containing nitrogen or carbon monoxide. On the basis of the broad range of mass diffusivities of the studied gas-liquid systems covering about 2 orders of magnitude from about 10-9 to 10-7 m2·s-1, effects of the solvent and solute properties on the temperature-dependent mass diffusivities are discussed. This contributed to the development of a simple semiempirical correlation for the mass diffusivity of the studied gases dissolved in n-alkanes of varying chain length at infinite dilution as a function of temperature. The generalized expression requiring only information on the kinematic viscosity and molar mass of the pure solvent as well as the molar mass and acentric factor of the solute represents the database from this work and further literature with an absolute average deviation of about 11%.

Geometric asymmetry driven Janus micromotors.

The production and application of nano-/micromotors is of great importance. In order for the motors to work, asymmetry in their chemical composition or physical geometry must be present if no external asymmetric field is applied. In this paper, we present a "coconut" micromotor made of platinum through the partial or complete etching of the silica templates. It was shown that although both the inner and outer surfaces are made of the same material (Pt), motion of the structure can be observed as the convex surface is capable of generating oxygen bubbles. This finding shows that not only the chemical asymmetry of the micromotor, but also its geometric asymmetry can lead to fast propulsion of the motor. Moreover, a considerably higher velocity can be seen for partially etched coconut structures than the velocities of Janus or fully etched, shell-like motors. These findings will have great importance on the design of future micromotors.

Marangoni self-propelled capsules in a maze: pollutants 'sense and act' in complex channel environments.

Environmental remediation is a highly pressing issue in society. Here we demonstrate that autonomous self-propelled millimeter sized capsules can sense the presence of pollutants, mark sites for visible identification and remove the contamination, while navigating in a complex environment of interconnected channels, the maze. Such long-range self-powered capsules propelled by the Marangoni effect are capable of releasing chemicals to alter the pH and induce aggregation during pollutant flocculation at a faster rate than convection or diffusion. These devices are foreseen to have real-world environmental applications in the near future.

Biomimetic artificial inorganic enzyme-free self-propelled microfish robot for selective detection of Pb(2+) in water.

The availability of drinking water is of utmost importance for the world population. Anthropogenic pollutants of water, such as heavy-metal ions, are major problems in water contamination. The toxicity assays used range from cell assays to animal tests. Herein, we replace biological toxicity assays, which use higher organisms, with artificial inorganic self-propelled microtubular robots. The viability and activity of these robots are negatively influenced by heavy metals, such as Pb(2+) , in a similar manner to that of live fish models. This allows the establishment of a lethal dose (LD50 ) of heavy metal for artificial inorganic microfish robots. The self-propelled microfish robots show specific response to Pb(2+) compared to other heavy metals, such as Cd(2+) , and can be used for selective determination of Pb(2+) in water. It is a first step towards replacing the biological toxicity assays with biomimetic inorganic autonomous robotic systems.

Beyond platinum: bubble-propelled micromotors based on Ag and MnO2 catalysts.

Autonomous bubble-propelled catalytic micro- and nanomachines show great promise in the fields of biomedicine, environmental science, and natural resources. It is envisioned that thousands and millions of such micromachines will swarm and communicate with each other, performing desired actions. To date, mainly platinum catalyst surfaces have been used for the decomposition of a fuel, hydrogen peroxide, to oxygen bubbles. Here we propose Pt-free, low-cost inorganic catalysts for powering micromotors based on silver and manganese dioxide surfaces. Such Ag- and MnO2-based bubble-powered micromotors show fast motion even at very low concentrations of fuel, down to 0.1% of H2O2. These catalysts should enable unparalleled widespread use of such motors in real applications, as it will be possible to make them in large quantities at low cost.

Towards biocompatible nano/microscale machines: self-propelled catalytic nanomotors not exhibiting acute toxicity.

Recent advances in nanotechnology have led to the evolution of self-propelled, artificial nano/microjet motors. These intelligent devices are considered to be the next generation self-powered drug delivery system in the field of biomedical applications. While many studies have strived to further improve the various properties of these devices such as their efficiency, performance and power, little attention has been paid to the actual biocompatibility of nanojets in vivo. In this paper, we will present for the first time the investigation of the toxicity effects of nanojets on the viability of human lung epithelial cells (A549 cells). From the 24 h and 48 h post-exposure studies, it is clearly shown that the nanojets we used in our work has negligible influence on the cell viability across all the concentrations tested. As such, the toxicity profile of our nanojets have been shown to be neither dose- nor time-dependent. This is strongly indicative of the benign nature of our nanojets, which is of paramount significance as it is the first step towards the applications of nano/micromotors in real-world practical medical devices.

Blood proteins strongly reduce the mobility of artificial self-propelled micromotors.

Autonomous self-propelled catalytic microjets are envisaged as an important technology in biomedical applications, including drug delivery, micro/nanosurgery, and active dynamic bioassays. The direct in vivo application of these microjets, specifically in blood, is however impeded by insufficient knowledge on the in vivo viability of the technique. This study highlights the effect of blood proteins on the viability of the microjets. The presence of blood proteins, including serum albumin and γ-globulins at physiological concentrations, has been found to dramatically reduce the viability of the microjets. The reduction of viability has been measured in terms of a lower number of active microjets and a decrease in the velocity of propulsion. It is clear from this study that in order for microjets to function in biomedical applications, different modes of propulsion besides platinum-catalyzed oxygen bubble ejection must be employed. These findings have serious implications for the biomedical applications of catalytic microjets.

Blood electrolytes exhibit a strong influence on the mobility of artificial catalytic microengines.

The developments in biomedical sciences foresee the inclusion of self-propelled catalytic micromotors for in vivo therapeutic strategies in the near future. We show here that blood electrolytes, such as Na(+), K(+), Ca(2+), Cl(-), SO4(2-) and phosphates, decrease the mobility of the Pt catalyzed tubular microjets. This effect is significant and in many cases, the microjets are completely disabled at physiologically relevant concentrations of the ions. A strategy to counterbalance this negative influence is suggested. These findings have a strong influence in the field of bubble-propelled artificial micromotors, where applications in blood are often envisioned.

Corrosion of self-propelled catalytic microengines.

Here we show that rolled-up and electroplated catalytic microjet engines undergo dramatic corrosion in fuel solution.

Reynolds numbers influence the directionality of self-propelled microjet engines in the 10(-4) regime.

The motion directionality of self-propelled bubble-jet microengines is influenced by their velocities and/or viscosity of the media in which they move. The influence of the fuel concentration from 1 to 3 wt% of H2O2 in 0.5% steps and of the glycerol fraction from 0 to 64% in aqueous solution on the directionality of the microjets motions is examined systematically. We show that with decreasing Reynolds numbers of the system (that is, with increasing viscosity or decreasing velocity of the microjets), the directionality of the motion shifts from circular to linear motion. This translates to a shorter travel time towards a designated target for the microjets despite moving at a slower speed, since the movements are linear instead of circular. We show that such dependence of trajectories of microjets on Re is a general issue. This observation has a strong implication for the real-world applications of microjets.

Influence of real-world environments on the motion of catalytic bubble-propelled micromotors.

Self-propelled autonomous micromachines have recently been tasked to carry out various roles in real environments. In this study, we expose the microjets to various types of water that are present in the real world, examples include tap water, rain water, lake water and sea water, and we sought to investigate their behaviors under real world conditions. We observed that the viability and mobility of the catalytic bubble jet engines are strongly influenced by the type of environmental water sample. Amongst the four water samples tested, the seawater sample exhibits the strongest influence, completely disabling any motions arising from the microjets. The motion of the microjets is also reduced in tap water, which contains large quantities of inorganic ions that have been purposely introduced into tap water via processing in water treatment plants. Lake water and rain water samples exhibited the least influence on the microjet's motion. All of the four water samples were also characterized by determining their ion compositions and conductivities, and we will show that there is a distinct correlation between the reduced mobility of the microjets with the ion content of the water found in real environments.

Artificial micro-cinderella based on self-propelled micromagnets for the active separation of paramagnetic particles.

In this work, we will show that ferromagnetic microjets can pick-up paramagnetic beads while not showing any interaction with diamagnetic silica microparticles for the active separation of microparticles in solution.

Challenges of the movement of catalytic micromotors in blood.

Catalytic microjet bubble-propelled engines have attracted a large amount of interest for their potential applications in biomedicine, environmental sciences and natural resources discovery. One of the current efforts in this field is focused on the search of biocompatible fuels. However, only a minimal amount of effort has been made to assess the challenges facing the movement of such devices in a real world environment, especially with regards to the components of blood and their interactions with the catalytic microjets. Herein, we will show the limitations on the movement of catalytic microengines prepared via the rolled-up, as well as the templated-electrochemical deposition method, in an artificial blood sample, due to the presence of two main components of animal blood: the cellular component (red blood cells in this study) and serum. We will show that the motion of catalytic microjets is only possible in highly diluted dispersions of the red blood cells and serum. This finding has a profound implication on the development of the whole field, where the components found in real environments have to be considered carefully, and issues arising from the presence of such components have to be resolved prior to deploying these devices in real world applications.

Poisoning of bubble propelled catalytic micromotors: the chemical environment matters.

Self-propelled catalytic microjets have attracted considerable attention in recent years and these devices have exhibited the ability to move in complex media. The mechanism of propulsion is via the Pt catalysed decomposition of H2O2 and it is understood that the Pt surface is highly susceptible to poisoning by sulphur-containing molecules. Here, we show that important extracellular thiols as well as basic organic molecules can significantly hamper the motion of catalytic microjet engines. This is due to two different mechanisms: (i) molecules such as dimethyl sulfoxide can quench the hydroxyl radicals produced at Pt surfaces and reduce the amount of oxygen gas generated and (ii) molecules containing -SH, -SSR, and -SCH3 moieties can poison the catalytically active platinum surface, inhibiting the motion of the jet engines. It is essential that the presence of such molecules in the environment be taken into consideration for future design and operation of catalytic microjet engines. We show this effect on catalytic micromotors prepared by both rolled-up and electrodeposition approaches, demonstrating that such poisoning is universal for Pt catalyzed micromotors. We believe that our findings will contribute significantly to this field to develop alternative systems or catalysts for self-propulsion when practical applications in the real environment are considered.

Magnetotactic artificial self-propelled nanojets.

Self-propelled catalytic bubble-ejecting nanotubes (nanojets) are expected to perform a variety of autonomous tasks. Herein, we will show that with the introduction of a Ni segment into the Au/Ni/Pt nanotube design followed by consequent magnetization a permanent change in the magnetic domain orientation of the Ni segment can be induced. Consequently, this results in the presence of a permanent magnet within the nanojet with its north/south domains oriented along the tube axis. Such a magnetized nanojet orients itself according to the external magnetic field and propels itself toward or away from the source of the magnetic field depending on its orientation. This behavior is similar to that of the magnetotactic bacteria. The ability to sense the magnetic field is expected to have a strong impact on future applications of autonomous self-propelled nanojets.

Self-propelled nanojets via template electrodeposition.

In this paper, we present a rapid, high-yield, low-end and low-cost fabrication of nanojet motors using a template directed electrochemical deposition method. Using an electrochemical deposition method, the bubble-ejecting nanojets were grown within the alumina template, which is commercially available. These fabricated nanosized devices have typical dimensions of 300 nm (diameter) by 4.5 µm (length), and they are able to move in a hydrogen peroxide fuel solution with velocities up to approximately 40 body lengths per second. They are also capable of exhibiting various modes of movement such as straight, screw-like and circular motions, in a similar manner comparable to larger micro-sized jets. In addition, due to their small dimensions, the movements of these nanojets can be strongly influenced by colliding them with microbubbles. This highly parallel method which is of low-cost and requires the usage of low-end equipment that can be easily located in any laboratory opens up the doors for world-wide nanojet fabrication in the near future.

Micromotors with built-in compasses.

We demonstrate here that iron containing rolled-up microtubular engines can be magnetized and act as compass needles - they sense the direction of an external magnetic field from afar and align the directionalities of their movements according to the external field, in a similar fashion to magnetotactic bacteria.

Inherently electroactive graphene oxide nanoplatelets as labels for single nucleotide polymorphism detection.

Graphene materials are being widely used in electrochemistry due to their versatility and excellent properties as platforms for biosensing. However, no records show the use of inherent redox properties of graphene oxide as a label for detection. Here for the first time we used graphene oxide nanoplatelets (GONPs) as electroactive labels for DNA analysis. The working signal comes from the reduction of the oxygen-containing groups present on the surface of GONPs. The different ability of the graphene oxide nanoplatelets to conjugate to DNA hybrids obtained with complementary, noncomplementary, and one-mismatch sequences allows the discrimination of single-nucleotide polymorphism correlated with Alzheimer's disease. We believe that our findings are very important to open a new route in the use of graphene oxide in electrochemistry.

Liquid-liquid interface motion of a capsule motor powered by the interlayer Marangoni effect.

A novel thin capsule motor has been described in this report. It utilizes the Marangoni effect for the solid capsule to run at a water-oil interlayer, which has not been reported previously. Intrinsic and environment factors influencing the motion were investigated. It is also possible for the velocity, direction, and start/stop of the motion of the capsule to be manipulated.