Nanoswimmers: Navigating the Nanoscale World

Nanoswimmers are tiny, self-propelled devices or particles that can navigate through fluids at the nanoscale. These artificial swimmers, typically ranging in size from a few nanometers to a few micrometers, are designed to mimic the motion of natural microorganisms such as bacteria and sperm cells. By harnessing various propulsion mechanisms, nanoswimmers offer exciting opportunities for targeted drug delivery, biosensing, environmental remediation, and studying fundamental aspects of motion at the nanoscale.

Nanoswimmers rely on various propulsion mechanisms to overcome the challenges of swimming at low Reynolds numbers, where viscous forces dominate over inertial forces. Some of the common propulsion strategies employed by nanoswimmers include:

Chemical propulsion is one of the most widely studied mechanisms for nanoswimmers. In this approach, the nanoswimmer is typically designed as a Janus particle, with two distinct surface properties. One half of the particle is coated with a catalyst that promotes the decomposition of a fuel, such as hydrogen peroxide, while the other half remains inert. The asymmetric decomposition of the fuel creates a concentration gradient around the particle, leading to self-diffusiophoresis and propulsion.

Magnetic nanoswimmers are powered by external magnetic fields. These swimmers are usually composed of magnetic materials, such as iron oxide nanoparticles, and can be controlled by manipulating the strength and orientation of the applied magnetic field. Magnetic propulsion allows for precise control over the direction and speed of the nanoswimmers, making them attractive for targeted drug delivery and remote manipulation.

Acoustic nanoswimmers harness the energy of sound waves to generate propulsion. When exposed to ultrasound, these swimmers experience acoustic radiation forces that can induce directional motion. Acoustic propulsion is particularly useful for biomedical applications, as ultrasound can penetrate deep into biological tissues without causing damage.

Light-driven nanoswimmers convert light energy into mechanical motion. These swimmers typically incorporate photoactive materials, such as TiO2 or Au nanoparticles, which generate local thermal gradients or induce photothermal effects upon light irradiation. The resulting temperature gradients or photothermal effects can lead to self-thermophoresis or self-electrophoresis, propelling the nanoswimmer.

To optimize the performance and functionality of nanoswimmers, several design considerations need to be taken into account:

The size and shape of nanoswimmers play a crucial role in determining their swimming behavior and interactions with the environment. Smaller nanoswimmers experience stronger Brownian motion and require more efficient propulsion mechanisms to overcome random thermal fluctuations. The shape of the nanoswimmer, such as spherical, rod-like, or helical, can also influence its hydrodynamic properties and swimming efficiency.

Surface functionalization of nanoswimmers is essential for imparting specific properties and functions. For example, nanoswimmers can be functionalized with targeting ligands, such as antibodies or aptamers, to enable selective binding to specific cells or molecules. Surface coatings can also be used to improve biocompatibility, prevent aggregation, or enable triggered release of cargo.

For biomedical applications, nanoswimmers must be biocompatible and non-toxic to minimize adverse effects on living systems. Biodegradable materials, such as polymers or lipids, can be used to construct nanoswimmers that can be safely metabolized or excreted by the body after completing their intended function. Biocompatibility and degradability are crucial considerations for the safe and effective use of nanoswimmers in vivo.

Nanoswimmers have diverse applications across various fields, including:

Nanoswimmers can be used as active drug delivery vehicles to transport therapeutic agents directly to diseased sites. By functionalizing nanoswimmers with targeting moieties and loading them with drugs, they can navigate through the body and release their payload specifically at the desired location. This targeted approach can enhance the efficacy of treatments while minimizing side effects.

Nanoswimmers can be employed as mobile sensors for detecting specific biomarkers or pathogens. By incorporating sensing elements, such as fluorescent probes or electrochemical transducers, nanoswimmers can collect and transmit real-time information about their local environment. This capability can be exploited for early disease detection, monitoring of physiological processes, or environmental monitoring.

Nanoswimmers can be designed to capture and remove pollutants or contaminants from water or soil. For example, nanoswimmers functionalized with adsorbents or catalysts can actively seek out and degrade harmful chemicals or microorganisms. The self-propelled nature of nanoswimmers allows them to cover large areas and access hard-to-reach locations, making them efficient for environmental cleanup applications.

Despite the significant progress made in the field of nanoswimmers, several challenges remain to be addressed. One of the main challenges is the limited swimming efficiency and controllability of nanoswimmers in complex biological environments. The presence of obstacles, fluid viscosity, and chemical gradients can greatly influence the motion and behavior of nanoswimmers. Developing advanced control strategies and improving the robustness of nanoswimmers in real-world settings are crucial for their practical implementation.

Another challenge lies in the scalable fabrication and mass production of nanoswimmers with well-defined properties and functionalities. The development of high-throughput synthesis methods and quality control processes is essential for the commercialization and widespread adoption of nanoswimmer technologies.

Future research directions in the field of nanoswimmers include the development of multi-functional and intelligent nanoswimmers that can perform complex tasks, such as sensing, actuation, and decision-making. The integration of nanoswimmers with other nanoscale systems, such as nanoelectronics or nanophotonics, could lead to the emergence of novel hybrid devices with unprecedented capabilities. Additionally, the exploration of collaborative swarms of nanoswimmers and their collective behavior could open up new possibilities for coordinated drug delivery, parallel sensing, or cooperative manipulation at the nanoscale.

Frontiers in Life Science, Propulsion of an artificial nanoswimmer: a comprehensive review

Current Opinion in Electrochemistry, Recent progress in enzyme-driven micro/nanoswimmers: From fundamentals to potential applications

Accounts of Chemical Research, Designing Micro- and Nanoswimmers for Specific Applications