Principle, Fabrication and Emerging Applications of Nanobottle Motor

Abstract

Micro/nano-motors play an important role in energy, environment, and biomedicines. As a new type of nano-motors, nanobottles attract great attention due to their distinct advantages of a large cavity, high specific surface area, bionic streamline structure, and chemotactic motion. Here, we systematically review the development of nanobottle motors from aspects of propulsion mechanisms, fabrication methods and potential applications. Firstly, three types of propulsive modes are summarized, with focus on chemical propulsion, light driving and magnetic actuation. We then discuss the fabrication methods of nanobottles, including the soft-template-based hydrothermal method and the swelling-inducement and wet-chemistry methods. The potential applications of nanobottle motors are additionally highlighted in energy, environmental, and biomedical fields. Finally, the future challenges and outlooks of nanobottle motors are discussed for the further development of this technology.

1. Introduction

Since the invention of the steam engine, various motors with macro size, shape and composition have brought tremendous changes to industry and daily life. To have a better understanding of our environments, human beings have also started to explore the micro-world. As nanotechnology advances, self-propelled motors with a size small enough to voyage into organisms have attracted extensive research interests. Inspired by biological motors (e.g., cells or certain bacteria) [1], researchers proposed the concept of micro/nano-motors. Typical sizes of the micro/nano-motors are in the range from tens of nanons to dozens of microns [2], commonly referred to as micro-swimmers, micro-machines, and micro-robots. Different from nanoparticles that only perform Brownian motion based on passive diffusion, the micro/nano-motors can convert various external energy into driving force to perform specific mechanical motions as well [3,4,5,6]. Owing to the advantages of microscale, autonomous motion, surface functionalization, adjustability and diverse shapes, micro/nano-motors have become an important tool for scholars to explore the microscopic world [7,8,9,10] and have been used in energy, environment and biomedicine fields [11,12,13,14,15,16].

Different geometries of micro/nano-motors, such as helical, tubular, Janus sphere, nanowire, and other biomimetic shapes have been developed to meet the requirement for practical applications [17,18,19,20,21,22]. Among them, nanobottles have a large cavity, high specific surface area and bionic streamline structure, as well as the characteristics of chemotactic motion of perceived gradient, and adjust direction trend gradient [23,24], showing great potential for energy, environmental, and biomedical applications [25,26,27]. However, the construction of nanobottles is challenged because of difficulties in forming open structures and inadaptability of macroscopic principles. Various strategies have been proposed to manufacture such nanobottles. Early composited nanobottles derived from hydrothermal methods do not have a lid to be opened or closed, which is referred to as inorganic nanobottles [28]. The advanced technologies have fabricated nanobottles with thermo-sensitive phase changed material lid that could only be used once [29]. A few studies have summarized the preparation methods of nanobottles [30,31,32,33], and their fabrication techniques have potential for practical applications.

Although nanobottles are promising for nano-motors, the technology in this area is still at primary stage. Existing studies were mostly aimed at how to form nanobottles, especially in the formation of open structures. The topic of the nanobottle motor has not been systematically studied and summarized. To accelerate the development of nanobottle technology, this work introduces the concept of nanobottle motors from aspects of the propulsion mechanisms, fabrication methods and their emerging applications (Figure 1). The propulsion mechanism is first discussed, including chemical propulsion, light-driving and magnetic actuation. We then summarize the fabrication methods of nanobottles that involve the soft-template-based hydrothermal method and the swelling-inducement and wet-chemistry methods. The emerging applications of nanobottle motors are additionally highlighted in energy, environmental and biomedical fields. Finally, a summary and prospects are given for the future development of this technology. This work provides basic knowledge and encourages researchers to develop new types of nanobottles for diverse applications.

2. Propulsion Mechanisms

The propulsion mechanisms of nanomotors relies on the asymmetric field of chemical products or energy to break the static equilibrium and achieve autonomous motion. The general expression is described by

$$U=-b\nabla \psi$$

(1)

where U is the motor velocity, b is the velocity coefficient, and ∇ψ is the gradient of potential function ψ including pressure, potential, concentration, temperature [44]. Based on these factors, nanobottle motors can be classified as three common types of gradient field derived from chemical sources, light or magnetic field.

2.1. Chemical Propulsion

Chemical propulsion is a dominant driving mode due to its advantages of a simple principle, strong power, facile preparation, and common equipment. In such a mode, it requires the introduction of chemical fuels and catalysts into the environment to promote autonomous motion by chemical reaction. Chemical propulsion can be further classified as self-diffusiophoresis, self-electrophoresis and bubble propulsion [45,46,47,48]. Phoretic motor is generally perceived as a separate particle suspended in an infinite fluid containing a diluted dissolved solute. In the circumstances, the solute concentration is described through the species conservation equation:

$$\frac{\partial c_i}{\partial t}+\nabla ·{\mathit{j}}_i=0$$

(2)

where c_i is the species concentration, and j_i is the flux that expressed by generalized Nernst–Planck equation including advection (in the dilute limit):

$${\mathit{j}}_i=c_i\mathit{u}-D_i\left[\nabla c_i+\frac{c_i}{k_{B}T}\nabla {\phi }_i\right]$$

(3)

where u is the fluid velocity, D is the solute diffusivity, k_B is the Boltzmann’s constant, T is the temperature, and φ_i is a generalized interaction potential that represents overall mutual effect between solute species and moving motors. The three terms on the right side of Equation (3) express the advection, diffusion and interaction caused by great gradient of φ, respectively [49]. Furthermore, at the micro/nano scale, inertia force is ignored, whereas viscous force acts a dominant role. Thus, the micro/nano-motors move in low Reynolds number flows, and the momentum balance of fluid is described by Navier–Stokes Equations:

$$\nabla ·\mathit{u}=0,\eta {\nabla }^2\mathit{u}=\nabla p+{\mathit{F}}_b$$

(4)

where u is the principal part velocity, η is the fluid viscosity, p is the pressure, and F_b is the generalized body force. Equations (2)–(4) describe the basic system of equations governing self-electrophoresis and self-diffusiophoresis.

Since the current research on nanobottle motors by chemical propulsion only includes self-diffusiophoresis and bubble propulsion, we mainly expound the above two methods described in this section.

Self-diffusiophoresis refers to the chemical reaction on nanobottle surface which forms asymmetrical concentration gradient to propel the motor [50,51]. Since different molecules interact differently with particles, the potential is defined as −∇ψ ≡ F_s, where F_s is the net force experienced through solute molecule. This force can be integrated and eventually converted into a force expressed as F_b = −c∇ψ, which corresponds to the third term of Equation (4) [44]. The chemical catalyst reaction can occur inside the nanobottle, in which the chemical gradient is the maximum near the opening (Figure 2a). Nanobottle motors have the motion with axis of symmetry because of the self-diffusiophoresis. Self-diffusiophoretic force (expressed as F_d = −α∂c) mainly acts on the opening around the motor surface. The bottom diameter of nanobottle is larger than the opening, indicating that the fluid friction (F_f = v/μ) is mainly applied to the bottom of nanobottle (Figure 2b). Hydrophilic nanobottle applies a negative force on the surrounding fluid and form a puller-like flow field (Figure 2c). However, the hydrophobic nanobottle motor exerts a positive force on the surrounding fluid and generates a pusher-like field (Figure 2d) [35].

Bubble propulsion is less affected by electrolyte and has the advantages of a large driving force and high energy utilization rate to sustain movement [53]. The mechanism begins with a chemical reaction between fuels (e.g., H₂O₂, active metals, CaCO₃, etc.) and catalysts to form gas molecules.

$$2{\mathrm{H}}_2{\mathrm{O}}_2=2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2$$

(5)

With the reaction, gas molecules gradually increase in the cavity. Due to the limitation of cavity, small bubbles are easier to aggregate and produce large bubbles. Under the pressure, large bubbles are ejected from the opening of nanobottle. The reverse driving force formed by bubble separation leads to the motor having the opposite motion (Figure 2e) [52,54]. For example, a nanobottle loaded in Pt nanoparticles can swim in H₂O₂ solution via O₂ detachment (Equation (5)) [55]. Compared with self-diffusiophoresis, bubble propulsion needs a higher reactant concentration to produce bubbles as well as lower surface tension in solution to form bubble molecules. However, it is not sensitive to salt ions and, thus, can be implemented in complex environments.

2.2. Light Driving

As an external stimulus, light is capable of adjusting parameters such as light intensity, light frequency, polarization, propagative direction, as well as precise input tuning energy or select on/off mode to achieve remote control [56]. Light-driven motors are thus considered as a facile propulsion mode, due to the advantages of high controllability, good programmability and easy operation [57,58,59]. The principle of light-driven motors depends on breaking the symmetry of pressure upon light irradiation, leading to the motion of micro/nano-motors [50]. Near-infrared light is often used for propulsion because its characteristics of strong penetration, ideal transmittance, and controllable irradiated area [60,61,62]. Photothermic materials (e.g., precious metals, carbon, etc.) of motors absorb near-infrared light that leads to fluid temperature in cavity rises rapidly and produces local temperature gradient. The temperature gradient increases the internal pressure of the motor and forces the heated fluid to be ejected out from the bottleneck. Hereby, it is the inside bottle, under more pressure than the outside bottle, that creates a net force at the bottom. Nanobottle motors can thus move in the direction of temperature gradient. In addition, the open/close motion of carbonaceous nanobottle motor can be controlled by the near-infrared switch state (Figure 3) [36]. The light-driven thermophoresis force can be expressed as [63]:

$$F=-\frac{9\pi d_p{\eta }^2{k}_a}{2{\rho }_{g}T{k}_p}\nabla T\left(r,t\right)$$

(6)

where d_p is the motor diameter, η is the fluid viscosity, k_a is the thermal conductivity of fluid, k_p is the thermal conductivity of motor, and T (r, t) is the temperature increment as a function of special and time coordinates.

2.3. Magnetic Actuation

Magnetic field features the qualities of high penetration, noninvasive and high controllability [64], so it is an attractive method to control the motion of motors. The mechanism is to exert magnetic force or torque to magnetized objects. The magnetic force (F) and magnetic torque (T) on the magnetic motors can be described as [65]:

$$\mathit{F}=v\left(\mathit{M}\times \nabla \right)\mathit{B}$$

(7)

$$\mathit{T}=v\mathit{M}\times \mathit{B}$$

(8)

where v is the volume of magnetic motors, M is the magnetization, and B is the magnetic flux density. We can see that the magnetic force on a magnetic nanobottle motor is zero even in a magnetic field. The magnetic torque relies on the direction of the magnetic dipole moment and magnetic field. The magnetic field must be either time varying (e.g., rotating, oscillating and pulsed magnetic fields) or space varying (e.g., gradient magnetic field) to realize the continuous motion of nanobottle motors [66]. When a magnetic field gradient is imposed, nanobottle motors are subjected a magnetic force that pulls them along the direction dictated by the magnetic field gradient [67]. In such a process, nanobottles can be propelled in the direction of the magnetic field (Figure 4).

3. Fabrication Methods

Nanobottles refer to hollow colloidal particles with a single opening on the surface. The hollow cavity has high load capacity as warehouse for various cargos and can also be used as a place for reaction to generate driving force. Compared with the hollow particles with entirely sealed shell structure, the hollow particles with holes have large internal and external specific surface areas, which provide more reaction or adsorption sites for functional cargos and channels for the release of functional cargos. The key parameters such as radii and lengths of nanobottle have great impact on their motion, and thus, controllable preparation of nanobottles is critically important for practical application. Nanobottles of different materials (e.g., SiO₂, Mg, carbon and precious metals), sizes and shapes have been developed for practical application. The fabrication methods generally include soft-template, swelling-inducement, and wet-chemistry methods. Meanwhile, we briefly list main references of fabrication methods in

Table 1. The main references of nanobottles growth.

Method	Materials	Variables	Basic Steps
Soft- template	Ribose Oleic acid Poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide)	Temperature Reaction time	Produce nanoemulsion Nanoemulsion swelling, the shell cracked by tensile force Nanoemulsion flows out, forming the neck
Swelling- inducement	Polystyrene microspheres SiO2 Ethanol	Pressure Volumetric percentage of solute in tetrahydrofuran/water	After absorbing tetrahydrofuran, the swollen microsphere pokes a hole in the shell The swollen polystyrene is squeezed through opening Ethanol quenching
Wet-chemistry synthetic	Acetate Thioacetamide Cetyltrimethyl ammonium bromid Au PbS Weak acids	Au/PbS ratio	Obtain the edge PbS nanooctahedron Au deposition, formation of Au/PbS nanostructure Adding weak acids to dissolve and remove sulfide

.

3.1. Soft-Template Method

Hydrothermal method refers to that in a closed system under high temperature and high pressure to grow crystals in aqueous solution or water vapor. This method has been broadly used in preparation of nanobottles. To control the nanobottle geometry, the hydrothermal method contains hard template and soft template. Soft-template method is carried out through self-assembly between core templates (e.g., organic surfactants, block copolymers, nanoemulsion drops) and precursor molecules to form shape confinement through weak noncovalent bonds (e.g., hydrogen bonds, Van der Waals forces, electrovalent bonds) [68]. Due to the chemical interaction between core templates and precursor molecules, this method relies on self-assembly, nucleation, and growth process, and can be used to control the geometry parameter. These are the major steps to fabricate carbon nanobottle with ribose as the carbon source (as a carbon precursor under hydrothermal condition), oleic acid as the core, and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) as surfactant [42]. With temperature increasing, core nanoparticles aggregate and sedimentate to form nanoemulsion. As the reaction is progressed, carbon precursor polymerizes at the emulsion interface, and the polyethylene oxide blocks in surfactant become more hydrophobic to generate swelling of nanoemulsion. The tensile stress is caused by volume expansion of nanoemulsion and eventually leads to polymer shell splitting when the pressure exceeds the critical pressure. At the final stage, continuous filling process leads to the outflow of nanoemulsion to form a new template surface. Precursor continues to aggregate on the surface of the new template to form asymmetric carbon nanobottle (Figure 5).

Soft templates generally in thermodynamics unstable liquid/gas are formed with high deformation ability, which can easily affected by various external parameters (e.g., PH value, additives concentration, solvent and temperature). The soft templates can be easily removed through evaporation or calcination without complicate template removal process, but the uniformity of nanoparticles is often affected. Furthermore, it is necessary to use a large amount of organic substances (e.g., ionic liquids) as reaction medium, which is easy to pollute the environment and not conducive to mass production.

3.2. Swelling-Induced Method

Swelling-induction method relies on swelling polymer microspheres with organic solvent, making organic solvent diffuse from the interior of nanoparticles and then quenching with ethanol to obtain the opening structure [41,69,70,71,72]. The main steps with polystyrene microspheres serving as the starting point [73]. After coating the surface of polystyrene microspheres with SiO₂ thin shell, introduce tetrahydrofuran/water emulsion to swell the polystyrene spheres. After absorbing tetrahydrofuran, the swollen polystyrene creates pressure in the shell. When the pressure surpasses a threshold to poke a hole in the shell, the swollen polystyrene will be squeezed out through the opening and reduce the pressure in the shell to ensure only one hole is created. After being quenched with ethanol to from Janus structure (Figure 6a), the swollen extent and the size of Janus particle can be adjusted through increasing the volumetric percentage of solute in tetrahydrofuran/water emulsion (Figure 6b–f). When volume percentage of solvent is less than lower limit, the pressure caused by swelling will not be enough to break the SiO₂ shell; while the volume percentage is above the upper limit, dissolution of polystyrene has occurred in shell. Increasing the volume percentage of tetrahydrofuran to 20% causes more polystyrene to protrude through the hole. After quenching through ethanol, the extruded polystyrene spheres have a diameter of 280 nm. When volume percentage of tetrahydrofuran is increased to 30%, the diameter of polystyrene spheres is increased to 333 nm. Further increasing the volume percentage of tetrahydrofuran to 50% would lead to polystyrene being partially dissolved in SiO₂ shell. In the pure tetrahydrofuran solution, polystyrene swells to obtain SiO₂ nanobottles (Figure 6f).

During swelling induction, the sizes, shapes and structure of nanobottles can be tailored by controlling swell extent through different types or percentage of solvents. In this process, each swollen particle can maintains the spherical shape by quenching the swelling with ethanol. Different from prior methods that rely on Janus particles, this method is based on commercial polystyrene nanospheres, and the hole is automatically punctured through swelling, making it possible to fabricate uniform nanobottles on a large scale.

3.3. Wet-Chemistry Method

Wet chemistry is a general method to synthesize nanobottles by controlling reaction dynamics and thermodynamic parameters to tune the sizes and shapes of nanobottle [74]. Asymmetric Au nanobottle can be prepared with such method (Figure 7a) [40,75]. Firstly, lead acetate and thioacetamide are heat-treated in the solution of cetyltrimethyl ammonium bromide to obtain PbS nanooctahedron with uniform edge lengths and slightly truncated vertex. The PbS nanooctahedron is used as sacrifice template, which can determine the scale parameters of Au nanobottles, and then disperse sulfide nanooctahedron into the growth solution. Au preferentially deposits at one vertex of each octahedron with the resultant core, then grow along four adjacent facets to produce Janus Au/PbS nanostructure. Finally, added weak acids dissolve and remove sulfide components in Au/PbS Janus nanostructures to produce the Au nanobottle. The opening size (d) and the diameter in the direction perpendicular to its symmetrical axis (D) can be adjusted by such a method, which influences the plasma properties of Au nanobottles. Keeping the opening size constant, the cavity volume and overall size of Au nanobottles will expand with the increase in edge length of PbS nanooctahedron. Furthermore, the opening size can be adjusted through controlling the ratio of Au/PbS. With the increase in Au/PbS ratio, the opening size of Au nanobottle first increases, and then it decreases after reaching the maximum, while the overall size and the cavity volume continue to increase in the Au nanobottle (Figure 7b).

In wet-chemistry method, the edge length of PbS nanooctahedron and Au/Pb ratio are crucial factors during Au overgrowth process to adjusting sizes of the Au nanobottles. Moreover, there are three advantages to choosing PbS as sacrificial template for synthesizing Au nanobottles. Firstly, PbS nanocrystals of various shapes can be easily prepared by wet-chemistry method. Secondly, PbS has Fermi level higher than Au, which facilitate the selective deposition of Au on specific surfaces of PbS nanocrystals. Thirdly, PbS is readily dissolved in weak acids.

4. Potential Applications

4.1. Energy

Solar water splitting has been considered as one of the most eco-friendly technologies for clean and renewable energy [76,77,78]. The current solar-to-fuel conversion rate in water splitting is 5–10% or even lower [79]. A lot of efforts have been made in the research of photocatalysts [80]. The photocatalysts usually present themselves in two forms: suspension powders and immobilized film [81]. The absorption and scattering caused by the former result in uneven illumination distribution and, thus, low photon transfer. Photocatalysts film has good photon harvesting properties. However, the internal mass transfer is slow for the immotile film [82,83]. Therefore, it is urgent to design motile photocatalytic carriers. Nano-motors with self-propulsion capability provide an advanced method to solve these problems such as delivery, mass transfer in microcomplex systems. Nano-motors can accelerate the mass transfer rate through the dynamic working mode to improve photocatalytic degradation performance [84,85,86]. Furthermore, light-driven motors can convert light energy into mechanical energy without any pollution, which is quite often applied to photocatalytic systems [87].

Nanobottle motors have large specific surface area and low density, and they can also greatly accelerate the mass transfer rate through motion to improve the photocatalytic degradation. The near infrared light-driven nanobottle motors has been proved to improve the photocatalytic rate [37]. Compared with pure graphitic carbon nitride and TiO₂ P25, in the ultraviolet range of 240–280 nm, the samples containing carbon nanobottle motors have strong visible-light absorption ability. The participation of carbon nanobottle motors expands the absorption range in the visible region and improves the photocatalytic activity. By use of trolamine aqueous solution as sacrificial agent, the photocatalytic hydrogen productions are further studied under full spectral light at room temperature. The photocatalytic hydrogen precipitation rate of graphitic carbon nitride is 37.1 μmol mg⁻¹ h⁻¹. An appropriate amount of carbon motors is beneficial to photocatalytic performance. The optimal usage of carbon nanobottle motors is determined to be 10 wt%; the hydrogen precipitation rate of carbon nanobottle/graphitic carbon nitride motors is 78.9 μmol mg⁻¹ h⁻¹, which is 2.1 times higher than that of pure graphitic carbon nitride (Figure 8a). Additionally, the hydrogen production rate of carbon nanobottle/TiO₂P25 motors is 158.7 μmol mg⁻¹ h⁻¹, which is 2.3 times larger than that of pristine TiO₂ P25 (Figure 8b). Overuse of carbon nanobottles can block part of illumination, leading to a reduction in the photocatalytic efficiency. In addition, the catalytic nanobottle motors containing precious metals are also attractive, due to the catalyst not being easily consumed for continuous motion [43]. To prove the effective motion of nanobottles, several studies have selected solid Au spheres with similar size and the same composition, but with gold-loaded nanobottle motors. The results show that hydrogen production of nanobottle motors is obviously higher than Au spheres. It also confirms that Au spheres exhibit short Brownian motion, and nanobottle motors show long motion trajectory (Figure 8c). UV–Vis data further disclosed that the catalytic efficiency of nanobottle motors was 1.8 times higher than that of the Au sphere. When the content of nanobottle motors was 0.1 wt%, the reduction rate of para-nitrophenol reached 96.5% within 6 min (Figure 8d). These results express the catalytic performance of nanobottle motors is superior to solid spheres because of the super automaticity and high loading capability.

Although nanobottle motors can effectively improve the photocatalytic efficiency and water splitting, there are still some aspects to be improved. Firstly, photocatalytic nanobottle motors have a low speed of motion, with the speed of nanobottle motors ranging from only a few to hundreds of microns per second [34,35]. Secondly, the material of motors driven by infrared light often includes precious metals (e.g., Au, Pt and Ag) as cocatalysts, which makes the fabrication of nanobottle motors a cumbersome and expensive process. Thirdly, light-driven motors generally use the heat generated by photothermic conversion of near-infrared light as a driving force to increase photocatalytic efficiency. This method is generally limited to laboratorial studies. Therefore, future research needs to demonstrate the feasibility of efficient propelled nanobottle motors upon sunlight and their applications in practical production.

4.2. Environment

Water pollution is one of the major forms of environmental pollution. It is estimated that 1.8 million global deaths in the year 2016 can be attributed to water pollution [88]. The major agents of water pollution are heavy metals, dyes, oil, bacteria, microplastics, etc. [89,90,91,92,93]. It is very urgent to conduct monitoring and remediation of the polluted environment. The conventional techniques of wastewater remediation are precipitation, adsorption, membrane separation and chemical methods [94,95]. These methods have several shortcomings. For example, precipitation leads to the formation of secondary sludge. Membrane separation is an energy-intensive and costly process [96]. Furthermore, most disposal methods translate one pollutant into another, for instance, the use of activated carbon for adsorption, which is then discarded without processing. Chemical methods use harmful reagents such as peroxide, coagulants and flocculants [97]. Therefore, the advanced techniques for water cleaning should adhere to two standards: decontamination capacity and reduce additional pollutants [98,99]. Nanomaterials have a more significant filtration effect as compared with larger particles with the same chemicals. The unique structures of nanoparticles make them strong adsorbents (especially for organics), such as dendritic, spherical, tubs and nanobottle [100,101]. The autonomous motion micro/nano-motors can accelerate the mass transfer rate, so the sewage disposal processes will be significantly superior to traditional remediation [102]. Micro/nano-motors have been used for environmental monitoring and remediation due to the remediation process having a more significant performance and higher efficiency [103,104,105].

The fast motion of nanobottle motors in the solution can actively grab the target molecule or ion to improve the adsorbed and separated efficiency. Additionally, with the large specific surface area of nanobottle motors having a strong affinity for organic pollutants, they can be used in environmental remediation. It is a special way to monitor the target molecules in the environment by observing the motion state of nanobottle motors. As a proof of concept, enzyme-driven nanobottle motors were used to monitor heavy medal ions in waste water, while enzymes were rapidly inactivated in a solution containing heavy metal ions and changed the motion velocity of nanobottle motors. Therefore, the existence of heavy metal ions in the solution can be determined through monitoring the motion velocity of nanobottle motors, and one can even roughly estimate the concentration of heavy metal ions (Figure 9a) [106]. Magnetic mesoporous silica nanobottle motors have a high specific surface area and large porosity, and they can also be used for adsorbing and removing heavy metal ions in polluted water. It was shown that the removal rate of Cu²⁺ in water solution was 60% in the first 5 min, and then it increased slowly over time, eventually reaching more than 80% after 1 h (Figure 9b). Since the removal of Cu²⁺ is a surface adsorption process, when the concentration of silica nanobottle motors increased from 0.5 to 3 mg mL⁻¹, the removal rate of Cu²⁺ increased to more than 90% after 30 min (Figure 9c). In addition, nanobottle motors can also be used for oil removal. A magnetic modulate micromotor was developed with monolayer MnFe₂O₄ nanobottle motor to remove contaminated oil from water (Figure 9d) [38]. The catalytic decomposition of hydrogen peroxide on the inner surface of MnFe₂O₄ nanobottle motor generates oxygen, which promotes the nanobottle to sustain autonomous motion. Owing to the hydrophobic long oleic acid chains on the surface of nanobottles, oil droplets can be adsorbed through the interaction between the hydrophobic surface and the oil. The nanobottles can thus capture oil droplets under the action of an external magnetic field and autonomous movement to implement oil–water separation (Figure 9e).

Nanobottle motors have the characteristics of autonomic motion that can used for observing the motion state to realize environmental detection. The autonomic motion of nanobottles also improves the mass transfer of solution to raise the degradation efficiency of pollutants. However, there are still some problems in the research of nanobottle motors in environmental application. First, fuel consumption is a major obstacle in the environmental remediation of nanobottle motors. Furthermore, most reports on sewage treatment only use nanobottle motors to treat the virtual sewage prepared in laboratory, rather than the real sewage from sewage plant. Therefore, the motion and the treatment effect of nanobottle motors are unknown in real polluted environment.

4.3. Biomedicine

Biomedicine is one of the most influential areas of research in human society. Various diseases have been found to threaten human life and health, such as malignant tumor (namely cancer), thrombus, gastroenteritis, etc. [107]. Among them, the death rates associated with malignant tumor are second after cardiovascular diseases, and malignant tumor has become the most concerned of global public healthcare [108]. The current treatment methods of malignant tumor including surgery, radiotherapy, chemotherapy and immunotherapy [109]. Chemotherapy is one of the main cures for cancer. However, this treatment is to passively deliver the drug to the diseased site. The drugs stay on or around the surface, unable to penetrate into deeper areas, and leads to a clinical therapeutic effect that is not ideal. The major constraint is the limited ability of drugs to reach the diseased site, whereby only 0.7% of drug particles can penetrate tumors [110], leading to an insufficient supply of the drug at tumor sites, systemic toxicity, and chemo resistance [111,112,113]. To solve these difficulties, X-ray irradiation, ultrasound, electroporation and magnetic conductivity have been used to deliver drugs to heighten penetration. Nevertheless, these methods can cause inevitable side effects, such as radioactive brain necrosis and low endocrine function. Therefore, it is urgently needed to develop a safe, efficient and targeted drug carrier for clinical treatment. Inspired by the self-propulsion of human cells such as sperm, the self-propelled micro/nano-motors are considered to provide high-efficiency drug delivery for disease treatment. Micro/nano-motors have the characteristics of remote control, high drug loading, autonomous motion and on-demand therapeutic release, which can eliminate the dependence on blood flow and random diffusion, overcome biological barriers to penetrate tissue, and improve the rate of drug target acquisition to minimize the required dose [114,115,116,117]. These advantages of micro/nano-motors promise new prospects for cancer treatment.

The typical example of targeted therapy is hydrogen and chemotherapeutics to synergistically treat the cancer, which are loaded in Mg-based nanobottle motors [39]. Hydrogen is an endogenous gas with physiologic/pathological regulatory functions. Active hydrogen produced by Mg-based nanobottle motors can eliminate overexpressed reactive oxygen species in tumor cells (Figure 10a). CellROX Green reagent is non-fluorescent in reduced status while it combines with DNA and becomes fluorescent after oxidization, which was used to determine the reactive oxygen species level in control group and experimental groups. The results are shown in Figure 10b,c, wherein Mg-based nanobottle motors have the highest reactive oxygen species scavenging ability, which is over 12 times higher than that of passive nanoparticles. Then, they assessed the curative effect of this therapeutic method on colon cancer cell line and breast cancer cell line (Figure 10d,e). When the concentration of Mg–nanobottle-motors–doxorubicin increased from 50 to 100 μg·mL⁻¹, the survival rate of breast cancer cell line decreased sharply from 92.37% to 31.00%, while the viability of Mg-poly (lactic-co-glycolic acid)-doxorubicin decreased slightly from 91.34% to 75.43%. Apart from drug delivery, trapping pathogens is an important therapy. Researchers fabricated an inert parylene nanobottle motor with internal chemoattractant and therapeutic layers, which is capable of self-propulsion through alternating counterclockwise (run) and clockwise rotation (tumble) of flagella [118]. It can attract, trap and destroy pathogens by controlling sequential dissolution and autonomous release of chemical attractants and treatment drugs. The dissolved chemical attractants reduce the propulsion speed, resulting in an increase in the concentration gradient (Figure 10g) [119]. This local concentration distribution leads to chemotactic attraction that causes pathogens to move towards the cavity inlet and converge within the hollow cavity (Figure 10f). The release of fungicides (Ag⁺) leads to the dissolving of pathogens in the cavity. In contrast, the nanobottle motors consisting of chemical attractants and fungicides have over four times the pathogen killing efficiency (94%) than the microtraps that loaded fungicides alone (20%).

The propulsion mode has been improved via the use of cytotoxic fuel to biocompatible alternative fuels or external field. The motion has been designed from non-directionality and uncontrollable speed to realize precise control of motion. These breakthroughs give nanobottle motors great potential application in biomedicine. Although there has been great progress in biomedicine research by use of nanobottle motors, there are still many challenges from laboratory to clinical treatment. The first point is how to ensure that the motion of nanobottle motors is not affected in the complicated biological environment of the human body. The second is that the biocompatibility and degradability of nanobottle motors need to be improved. Most current research has been conducted at the cellular or animal level, so further studies are needed on circulation metabolism in vivo to ensure the biosafety of nanobottle motors in drug delivery. The third point is that the motion of nanobottle motors in current research is mostly performed by artificial control, so they cannot work independently, and they are vulnerable to environmental forces.

5. Summary and Outlook

Micro/nano-motors are an important research field within nanotechnology, and various geometries have been developed to execute applications. As a new type of motor, the research on nanobottles is still in the initial stage, but it attracts wide attention because of its unique advantages of large cavity, high specific surface area, bionic streamline structure and chemotactic motion. This work summarizes the propulsion mechanisms, fabrication methods and potential applications of nanobottle motors. The propulsion mechanisms include chemical propulsion, light driving and magnetic actuation. Chemical propulsion is divided into bubble propulsion and self-diffusiophoresis. Bubble propulsion generates reverse driving force by the bubble’s separation. The driving force of self-diffusiophoresis and light driving originate from their asymmetric concentration gradient and asymmetric temperature gradient, respectively. Magnetic actuations apply magnetic force or torque to magnetized motors. Three methods including the soft-template-based hydrothermal method and the swelling-inducement and wet-chemistry methods are then summarized for the fabrication of nanobottles. Soft-template-based hydrothermal method is a method of self-assembly between precursor molecules and core templates. Swelling inducement is performed through swelling polymer microspheres with organic solvent and then quenching with ethanol. Wet chemistry tunes the sizes and shapes of nanobottles by controlling reaction dynamics and thermodynamic parameters. Moreover, the potential applications of nanobottle motors in energy, environmental and biomedical fields are additionally discussed in consideration of their structure and motion properties. We constructed a simple table to describe the typical examples of nanobottle motors in three application fields above (

Table 2. Some typical applications of nanobottle motors.

Field	Represent Examples	Materials Involved	Principle
Energy	Solar water splitting	Carbon	Large specific surface area, low density, accelerates the mass transfer rate through motion [37]
	Photocatalytic	Silica, Au, Fe3O4	Introduces chemically active ingredients, accelerates the mass transfer rate through motion [43]
Environment	Environment monitoring	MnFe2O4	The motion of nanobottle motors in the solution can grab the target molecule or ion [38]
	Environment Remediation	Magnetic silica	Large specific surface area of nanobottle motors has affinity for organic pollutants [106]
Biomedicine	Targeted therapy	Mg	Great performance of drug loading, tissue penetration and biocompatibility [39]
	Trapping pathogens	Mg	The Mg core is depleted, the exposed inner surface dissolves, releasing chemoattractant [118]
	Drug delivery	MgTiO2	Bubble propulsion [120]
	Photothermal therapy (PTT)	Au	Photothermal conversion [40]
	Treat gastroenteric diseases	Silica	The reaction of CaO2 with gastric juice neutralizes gastric acid [52]
	Drug loading and controlled release	Phase-change material (PCM)	Controlling the temperature causes the phase-change material blocking the hole to change the state of the phase [29,121]

). Overall, nanobottles have irreplaceable advantages for potential applications. However, the current research achievements cannot meet the practical requirements. Here, we list propulsion mechanisms, fabrication methods, potential applications advantages and challenges of nanobottle motors in

Table 3. The propulsion mechanisms, fabrication methods and potential applications of nanobottles as well as their major advantages and limitations.

Propulsion	Mechanism	Fabrication Method	Feature	Potential Applications	Advantages	Challenges
Chemical propulsion	Bubbles separated from motor	Soft-template method	Self-assembly between precursor molecules and core templates. The fabrication makes it easy to control morphology.	Solar water splitting/Photocatalytic	Large surface area Low density Enhances the mass transfer	Low speed of motion Light utilization is difficult Requires photocatalysts
	Asymmetrical concentration gradient
Light driving	Asymmetric temperature gradient	Swelling- inducement method	Swelling polymer microspheres with organic solvent and then quenching with ethanol. Opening structure is obtained through diffusion of emulsion solvent from inside of nanoparticle.	Environment monitoring/ Remediation	Fast motion in solution Strong affinity for organic pollutants	Fuel consumption The motion and post-treatment of nanobottles have unknown effect on real environment
Magnetic actuation	Apply magnetic force or torque to magnetized motors.	Wet-chemistry synthetic method	Controlling reaction dynamics and thermodynamic parameters to control the sizes and shapes of noble metal nanobottles. A facile and robust method.	Targeted therapy/Trapping pathogens	Asymmetrical cavity structure Autonomic motion High impermeability	Achieves functional nanobottles in complex environment Biocompatibility/degradability Difficult to work independently

. The advantages of a large surface, low density and enhancement of the mass transfer rate give nanobottle motors great photocatalytic performance. The fast motion and strong affinity for organic pollutants of nanobottle motors makes them useful for water cleaning. Nanobottles motors promise new prospects for cancer treatment due to the characteristics of an asymmetrical cavity structure, autonomic motion and high impermeability. Although the research on nanobottle motors has seen huge efforts, much of this field remains to be explored. Future research may aim at the speed increase, fuel consumption, biocompatibility and intelligence of nanobottles.

(1)

In energy applications, motor speed is crucial for photocatalytic efficiency. The current propulsion methods of nanobottles use a single driving source. Multiple-fields propulsion may provide an effective way to increase the speed of nanobottles. The first is choosing one propulsion method (such as light driving) as the main role, and the other one (e.g., magnetic-field pulsion) as an auxiliary means. The other way is to place the nanobottle in the coupling field to achieve better propulsion. In addition, the prices and properties (e.g., light absorption capacity, surface reaction kinetics) of nanobottle motors materials should be comprehensive and researched in depth, so as to obtain fabrication materials with low cost and high solar energy conversion efficiency. On the other hand, machine learning can design, predict and screen materials without experimentation, which can effectively save time and materials costs [122,123,124,125]. Furthermore, it is important to strengthen the research on catalytic efficiency in sunlight to promote the application of nanobottle motors in practical industrial production.

(2)

In terms of environment applications, the hug hurdle is fuel consumption. Improving the motion efficiency as well as precise controlling the structure and performance of nanobottle motors may overcome the problems. Another significant attention is development of eco-friendly nanobottles or design of appropriate recycling methods to avoid secondly pollution in the process of environment monitoring and remediation. The last is the need to be further tested in practical applications (such as the sewage from sewage plant) by using eco-friendly fuels.

(3)

For biomedical applications, toxic chemical fuels are strictly forbidden due to the constraints of the human environment. Combination cell derivatives with biocompatible materials could be a possible way to improve the biocompatibility of nanobottles. Moreover, strengthening the research will be focused on how the blood constituents affect the motion of nanobottle motors in the blood stream, as well as through changing the driving source and designing innovative materials with nanobottle motors to improve the mobility in complex physiological environments. Finally, the intellectualization of multiple nanobottles is expected to be achieved for targeted therapy, imaging and diagnosing the location of the disease in vivo, and cleaning of a tumor or microthrombus.