Nanomotor: Definition, Propulsion Mechanisms, and Applications

The term is used in two closely related ways. In the strict sense, nanomotors are a few tens to a few hundreds of nanometers across. In much of the experimental literature, however, they are discussed together with submicrometer and micrometer-sized swimmers under the broader term "micro/nanomotors". Many important platforms – Janus particles, bimetallic nanorods, tubular microjets, and helical microswimmers – therefore sit near the boundary between nanotechnology and colloidal microrobotics.

A working nanomotor must break symmetry in shape, composition, surface chemistry, or applied field so that energy input produces persistent translation, surface slip, or field-induced torque rather than isotropic agitation. Without symmetry breaking, the same energy input generally produces heating, local mixing, or random fluctuations rather than directed motion against Brownian motion.

The field of synthetic nanomotors opened in 2004 when Paxton and coworkers at Penn State reported bimetallic nanorods, 370 nm in diameter with 1 µm long platinum and gold segments, that swam autonomously in dilute hydrogen peroxide solution at speeds of up to ten body lengths per second. The thrust on a single rod was on the order of 10−14 N, generated by the asymmetric decomposition of H2O2 at the platinum end. Since then, the field has expanded to encompass hundreds of motor designs and a range of energy sources, and overlaps closely with the broader categories of nanoswimmers, nanomachines, and nanobots.

The Reynolds number compares inertial to viscous forces in a fluid flow. For a 1 µm particle moving at 10 µm/s through water, the Reynolds number is roughly 10−5. At such low values, inertia is irrelevant: the moment a nanomotor stops generating thrust, it stops moving in less than a microsecond and travels a distance much shorter than its own diameter while coming to rest. Swimming strategies that work for fish or submarines are useless here, because they rely on coasting between strokes.

A second constraint, known as Purcell's scallop theorem, states that any swimmer at low Reynolds number that performs only time-reversible (reciprocal) shape changes produces no net displacement over a cycle. Biological microorganisms evade this constraint with non-reciprocal motions: rotating helical flagella in bacteria, beating cilia in protozoa, or the bending waves of E. coli and sperm flagella. Synthetic nanomotors must do the same, either by carrying onboard chemistry that breaks time-reversal symmetry (catalytic Janus nanoparticles) or by being driven externally in non-reciprocal patterns (rotating magnetic helices).

A third constraint is thermal noise. The Stokes–Einstein equation gives a 100 nm sphere in water a translational diffusion coefficient near 4 × 10−12 m2/s, and rotational diffusion randomizes its heading on a timescale of milliseconds to seconds depending on size. Useful directed motion is therefore possible only when propulsion speed and persistence time produce a displacement that is large compared to the diffusion length over the same interval.

A handful of physical mechanisms dominate the field. Most chemically powered nanomotors rely on self-phoresis: an asymmetric reaction on the particle surface generates a local gradient of a solute, temperature, or electric potential, and the fluid slip that this gradient drives across the particle surface produces motion in the opposite direction.

In self-diffusiophoresis, demonstrated in 2007 by Howse and coworkers using polystyrene microspheres half-coated with platinum, local concentration gradients drive surface slip around the particle and move it through the fluid. Self-electrophoresis is similar but is driven by a self-generated electric field; it is the accepted mechanism for many bimetallic catalytic nanorods, where the two metals act as the anode and cathode of a short-circuited galvanic cell. In bubble propulsion, the catalytic reaction is so vigorous that gas bubbles nucleate, grow, and detach from one face of the motor, kicking it forward with each release. Tubular microjets fabricated by rolling up strained thin films exploit this mechanism most efficiently because bubbles are confined and ejected through the tube opening.

Externally powered mechanisms break symmetry by an applied field rather than by chemistry. Rotating magnetic fields drive helical or flexible nanostructures that translate along their axis because their shape couples rotation to translation, in the same way a corkscrew advances when turned. Acoustic radiation forces in standing ultrasound waves push asymmetric metallic rods along their long axis. Light-driven motors use thermophoresis from local heating, photochemical reactions on plasmonic or semiconductor surfaces, or radiation pressure. Biohybrid motors mount a synthetic cargo on a swimming bacterium, sperm cell, or algal cell and inherit its motility entirely.

No single propulsion mechanism is universally best. The choice depends on the working fluid, the available fuel or field, the required speed and persistence, and biocompatibility constraints. The table below summarizes the main families.

A catalytic Janus nanoparticle is the simplest architecture: a sphere or rod with one hemisphere or end coated by a catalytic material, often platinum or an enzyme such as urease, glucose oxidase, or catalase. Mesoporous silica spheres functionalized with surface-bound enzymes have become a workhorse for biomedical studies because their pores can be loaded with drug cargo and the enzymatic reaction proceeds on physiologically tolerated fuels. The original Pt–Au bimetallic nanorod remains a model system for studying self-electrophoresis.

Tubular microjets, fabricated by rolling up strained thin films into hollow tubes, are inner-coated with platinum and propel by ejecting oxygen bubbles from one tube opening; speeds of hundreds of micrometers per second are routine in dilute peroxide. Helical microswimmers fabricated by glancing-angle deposition or two-photon lithography have a corkscrew shape that converts rotation in an applied magnetic field into translation, in direct imitation of bacterial flagella. DNA origami has been used to assemble molecular-scale motors and rotors, and bioinspired biohybrid motors mount magnetic helices, polymer beads, or drug-loaded particles onto live E. coli, sperm cells, or microalgae.

Generating thrust is only half the problem. A nanomotor that swims in a random direction is rarely useful, so most designs incorporate a steering strategy. Magnetic motors are the easiest to control: an embedded ferromagnetic segment, often nickel, cobalt, or an iron oxide nanoparticle layer, orients the swimmer along the applied field, and rotating-field protocols can both propel and steer the motor along arbitrary trajectories. Magnetic nanoparticles incorporated into otherwise chemically powered motors give the same external handle without giving up onboard propulsion.

Chemotaxis, the ability to follow a chemical gradient, is more difficult to engineer but more autonomous. Catalytic motors moving in a fuel-concentration gradient tend to accumulate in regions of high fuel availability because their speed increases with fuel concentration; this produces a statistical drift up the gradient rather than a deterministic guidance, but it can localize swarms of motors at a target. Pre-defined chemical patterns on surfaces, fuel-releasing beads, and pH-responsive surface chemistries are all used to bias trajectories. Phototaxis is similarly available for light-driven motors. Despite a decade of work, true closed-loop nanoscale chemotaxis comparable to that of bacteria remains an open research goal.

Active motion can in principle improve targeted drug delivery by enhancing mixing, accelerating tissue penetration, and helping particles cross biological barriers that passive nanoparticles traverse only by slow diffusion. Demonstrations include nanomotors that swim through mucus barriers in the stomach and bladder, magnetic helical swimmers that penetrate the vitreous of the eye, and enzyme-powered Janus particles that deliver cargoes into cells.

One of the most prominent in vivo results to date is a 2024 study showing that urease-powered mesoporous silica nanobots labelled with radioactive iodine accumulated in orthotopic bladder tumours in mice with about eightfold higher uptake than passive controls and reduced tumour size by roughly 90 percent after a single intravesical dose. Other preclinical studies have explored antibiotic-loaded motors for bacterial biofilms and magnetically guided systems for crossing biological barriers, including the blood–brain barrier in rodent models, but these remain early-stage demonstrations rather than clinically validated therapies.

Self-propelled motors can act as mobile reactors that mix and clean polluted water more efficiently than static catalysts. Tubular microjets carrying onboard catalysts have been used to degrade organic dyes, oxidize heavy-metal pollutants such as Cr(VI), and capture oil droplets in spilled fuel. Hybrid motors that combine a photocatalyst with magnetic guidance can be collected and reused after a remediation run, which is impractical for free catalyst suspensions. Light-driven motors made of titanium dioxide and bismuth-based oxides extend this approach to visible-light remediation.

Nanomotors actively transport receptor-modified surfaces through a sample, increasing the rate at which target molecules encounter the recognition layer relative to a static sensor. Motion-based sensing schemes detect targets through changes in motor velocity, trajectory, or aggregation state. Beyond sensing, nanomotors have been used to capture and isolate cells, transport single biomolecules to defined locations, and assemble structures by ferrying building blocks – pointing toward an active form of directed self-assembly that does not exist for static colloids.

Nanomotors should not be confused with every small machine-like system in nanotechnology. The word usually refers to a self-propelled colloidal object moving through a surrounding fluid, not simply to any nanoscale component that changes shape or position.

Despite two decades of progress since the first catalytic nanorod demonstrations, nanomotors remain at the proof-of-concept stage for almost every proposed application. The most active chemical fuels – concentrated hydrogen peroxide above all – are not biocompatible, while truly biocompatible fuels such as urea and glucose produce only modest speeds. Externally powered motors avoid this fuel trade-off, but require field-generating hardware; depending on the modality, practical limits may include tissue penetration, spatial resolution, gradient strength, heating, and imaging feedback. Manufacturing at the scales relevant to therapy is unsolved for most designs, and long-term stability, immune response, biodistribution, clearance from the body, and the regulatory pathway for an active in vivo device that consumes a "fuel" are all open questions.

Fundamental science also has unfinished business. The relative weight of self-electrophoresis, self-diffusiophoresis, and bubble-driven propulsion in catalytic motors is still debated for individual systems. Collective behaviours such as swarming and clustering in dense nanomotor populations are a frontier of active-matter physics with direct implications for how a therapeutic dose would behave inside a patient. Distinguishing nanomotors from neighbouring categories – nanoactuators that produce motion but are fixed in place, molecular motors at the single-molecule scale, and biological motor proteins that already perform many of the tasks researchers aspire to recreate – helps frame what remains genuinely new and where the field is reinventing nature.

A nanomotor is a nanoscale or near-nanoscale device that converts energy from its surroundings into directed motion in a fluid. Strictly nanoscale motors are a few tens to a few hundreds of nanometers across, but the term is often used together with submicrometer and micrometer-scale devices under the broader label micro/nanomotors. The energy source can be a chemical fuel decomposed at the motor's surface, an applied magnetic field, light, ultrasound, or a biological cell tethered to the device. Nanomotors are distinct from molecular motors, which are single molecules or molecular assemblies that produce motion through conformational changes.

At the nanoscale, random thermal collisions with solvent molecules constantly kick particles around, producing Brownian motion. Directional motion becomes observable when the motor's propulsion speed and persistence length exceed the displacement expected from Brownian diffusion over the same time interval. In practice, a catalytic Janus nanoparticle a few hundred nanometers in size moves ballistically over micrometer distances before Brownian rotation randomizes its heading, after which its motion looks like enhanced diffusion rather than steady swimming.

A molecular motor is a single molecule or small molecular assembly, such as the rotaxane- and catenane-based devices recognized by the 2016 Nobel Prize in Chemistry, that produces motion through controlled conformational changes. A nanomotor is a colloidal-scale device, usually nanoscale or near-nanoscale, made up of many molecules organized into an asymmetric structure that self-propels in fluid. Biological molecular motors such as kinesin, myosin, and the bacterial flagellar motor are typically classed with molecular motors rather than with synthetic nanomotors.

Hydrogen peroxide is by far the most studied fuel, decomposed at a platinum, manganese oxide, or silver surface to produce oxygen and water and to drive self-diffusiophoresis or bubble propulsion. Other fuels include hydrazine, halogen-based oxidants, acidic or alkaline media that drive zinc or magnesium dissolution, and biocompatible substrates such as urea, glucose, and triglycerides that are decomposed by enzymes immobilized on the nanomotor surface. Enzymatic fuels are preferred for biomedical applications because they avoid the toxicity of hydrogen peroxide at the concentrations needed for fast motion.

Nanomotors remain at the preclinical stage. One of the most advanced biomedical demonstrations uses urease-powered mesoporous silica nanobots that consume urea as fuel inside the bladder. A 2024 study in Nature Nanotechnology by the Sánchez group showed that radioiodine-labelled urease-powered nanobots accumulated in orthotopic bladder tumours in mice and reduced tumour size by about 90 percent after a single intravesical dose. Translation to clinical use will require safety data, biodistribution and clearance control, closed-loop monitoring or targeting strategies, and reproducible sterile manufacturing at clinically relevant dose volumes and particle quantities.

Yes, but the steering strategy depends on the propulsion mechanism. Magnetic helical microswimmers and nanorods with a ferromagnetic segment can be steered precisely by external magnetic fields, including rotating fields that produce corkscrew-like motion. Catalytic Janus motors can be biased by chemical gradients, by structured fuel patterns, or by coupling them to a magnetic guidance system. Light-driven motors can be steered by gradients of illumination, and ultrasound-driven motors can be focused into acoustic traps. Truly autonomous chemotaxis, where the motor follows a gradient toward a target without external control, remains an active research goal.

Journal of the American Chemical Society, Catalytic nanomotors: autonomous movement of striped nanorods

Physical Review Letters, Self-motile colloidal particles: from directed propulsion to random walk

Chemical Reviews, Fabrication of micro/nanoscale motors

Angewandte Chemie International Edition, Chemically powered micro- and nanomotors

Chemical Society Reviews, Advanced materials for micro/nanorobotics

Nature Nanotechnology, Urease-powered nanobots for radionuclide bladder cancer therapy

ACS Nano, Micro- and nanomotors: engineered tools for targeted and efficient biomedicine