Micro/nanomotor technology: the new era for food safety control

Abstract Food poisoning caused by eating contaminated food remains a threat to global public health. Making the situation even worse is the aggravated global environmental pollution, which poses a major threat to the safety of agricultural resources. Food adulteration has been rampant owing to negligent national food safety regulations. The speed at which contaminated food is detected and disposed of determines the extent to which consumers’ lives are safeguarded and agricultural economic losses are prevented. Micro/nanomotors offer a high-speed mobile loading platform that substantially increases the chemical reaction rates and, accordingly, exhibit great potential as alternatives to conventional detection and degradation techniques. This review summarizes the propulsion modes applicable to micro/nanomotors in food systems and the advantages of using micro/nanomotors, highlighting examples of their potential use in recent years for the detection and removal of food contaminants. Micro/nanomotors are an emerging technology for food applications that is moving toward mass production, simple preparation, and important functions.


Introduction
The food production chain is an intricate network from farm to table.Any errors or omissions in the steps has the potential to serious food contamination and food safety incidents (Nerín, Aznar, and Carrizo 2016).Contaminated food is not only a waste of agricultural resources (Li 2019) that results in economic losses to farmers and producers (Ramirez-Hernandez et al. 2020) but is also a health hazard than can cause serious food safety incidents when it reaches the hands of consumers (He and Shi 2021).To prevent contaminated food in the market from posing threats to consumer health, the receiving of raw materials, finished product inspection, and market supervision should be performed timely and accurately (Shen, Xu, and Li 2021;Zwietering et al. 2016).Furthermore, if some technologies can be applied to remove contaminants from contaminated food in situ, a path of technological advancement in line with the concept of green and sustainable development can be achieved (Chen et al. 2019;Probstein and Hicks 1993).Nanotechnology is currently one of the most attractive and promising technologies because nanomaterials have high surface areas and novel physicochemical properties that are different from those of their macroscale counterparts (Rizwan et al. 2014;Roduner 2006;Singh et al. 2017).Engineered nanomaterials are used for both food industry development, such as food antimicrobials (Handford et al. 2014), food ingredients, or additives, and food safety monitoring, for example, biosensors for diagnostics (Lv et al. 2018;Srivastava, Dev, and Karmakar 2018).However, how to fully mix the substances in the trace system is a problem to be solved in the current trace detection.As a result, micro/nanomotors that move autonomously in solution have emerged.
Devices capable of energy conversion are generally referred to as motors.For example, heat engines use internal energy to do work and convert other forms of energy into mechanical energy at the macroscopic level.At the microscopic scale, enzyme proteins act as molecular motors to convert chemical energy directly into mechanical energy in the activities of living organisms.Inspired by natural microorganisms, researchers have designed and manufactured micro/nanomotors that can move autonomously (Chang et al. 2022;Tu, Peng, and Wilson 2017).The emergence of self-propelled micro/nanomotors has changed the passive motion state of nanomaterials in solutions.After nearly a decade of development, the motion velocity of micro/nanomotors has been further increased, and a variety of new propulsion methods have been expanded to enable the spatiotemporal motion control of micro/nanomotors.The self-driven energy of the motors originates from the conversion of externally supplied energy, i.e., conversion of chemical or physical energy.However, the low Reynolds number environment and Brownian motion pose a major challenge to micro/nanomotors at the microscale (Sánchez, Soler, and Katuri 2015).To date, the predominant use of hydrogen peroxide (H 2 O 2 ) as a fuel for propelling the motion of micro/nanomotors is mainly because of its strong bubble-recoil-based mechanism (Wrede et al. 2021), stability of the catalytic layer (Ludwig and Schindler 2017), and simple design and fabrication of the motor structure (Zhou et al. 2020).However, the use of H 2 O 2 is limited in biomedicine owing to its poor biocompatibility (Ou et al. 2020).Therefore, magnetic fields (Chen et al. 2017), acoustic field (Xu, Xu, and Zhang 2017), and light (Wang et al. 2018) are utilized as non-chemical external energy sources for energy conversion to propel micro/nanomotor motion.The concept of micro/nanomotors has been used to design applications for environmental monitoring, contaminant removal (Jurado- Sánchez and Wang 2018;Parmar et al. 2018), biomedicine delivery (Xu et al. 2020), medical diagnostics (Feng et al. 2021;Kong et al. 2019), and so on.To the best of our knowledge, an overview of contaminant removal and detection applications in food has not yet been reported.
In recent years, considerable progress has been made in food contaminant detection and food industry production applications through the functional transformation of mobile platforms based on micro/nanomotors (Maria-Hormigos, Jurado -Sánchez, and Escarpa 2018;Molinero-Fernandez et al. 2017;Pacheco, Jurado-Sanchez, and Escarpa 2018).A comprehensive review of this topic is necessary to facilitate further development in this exciting research area.In this review, we focus on the application of micro/nanomotors in food safety control and contamination detection (Scheme 1).The framework of this review is organized into four parts.First, we summarize the micro/nanomotor driving mechanism applicable to food systems.Second, the benefits of using micro/nanomotors in food systems is systematically elucidated.Third, the contaminant removal/detection in food systems using micro/nanomotors is briefly discussed.Finally, the existing challenges and prospects of micro/nanomotors in food systems are presented.

Micro/nanomotors suitable for food systems
In virtue of excellent solution stirring, continuously moving motors have been studied as proof-of-concept for environmental remediation and monitoring, biomedical diagnosis and therapy (Wan et al. 2021;Zarei and Zarei 2018).The sporadic papers reporting the conceptual application of micro/nanomotors in food has led us to focus on food systems.However, due to the diversity and complexity of food components, the designs, properties, and functions associated with motor propulsion methods are not quite the same as those applied to environmental and biomedical sections.It needs to be reclassified and summarized according to the nature of food.Firstly, the liquid viscosity from the abundant proteins and polysaccharides in the food increases the demand for micro/nanomotor power.Secondly, the large amounts of organic acid contained in fruits and vegetables require that the materials used to manufacture the motors be stable and corrosion-resistant.Finally, we put higher requirements on the propulsion method of micro/ nanomotors used for in situ work, which cannot use or produce harmful substances.Hence, we classify the propulsion mechanisms applied to motors in food according to their purpose of operation.In general, the classification is consistent with the conventional method into chemical and external physical propulsion, with slightly different subordinate classifications, as described in detail later.The applicability of each type of micro/nanomotor-affiliated propulsion mechanism in food, including the advantages and limitations are listed in Table 1.The following sections will discuss the classification for the different propulsion mechanism of micro/nanomotors in detail.

Micro/nanomotors driven by chemical fuels
A liquid environment is necessary for micro/nanomotor motion, and ideally, it will be most energy efficient and environmentally friendly if it can be "fueled" for energy conversion by in situ application of chemicals present in the original environment.In this section, we present three possible fuels for food contamination detection or in situ removal applications, namely, H 2 O 2 , water, and glucose, and provide an outlook on the potential development directions.

Hydrogen peroxide
H 2 O 2 is widely used as a disinfectant in the food industry because of its simple chemical composition (Linley et al. 2012).Its decomposition produces only two cleaning products, namely, water and oxygen, which acts as reactive oxygen itself.Compared to other catalytic motor-powered fuels, such as enzymes, H 2 O 2 is widely used by researchers for fueling fast-moving motors because of its low cost, ease of access, and high bubble recoil.However, the velocity of micro/nanomotors is proportional to the concentration of H 2 O 2 within a certain range.To obtain greater power in samples with higher viscosity, researchers often add higher than safe doses of H 2 O 2 , which causes potential toxicity to the food matrix.Therefore, the micro/nanomotors with H 2 O 2 as the power source is suitable for rapid detection of trace Scheme 1. schematic overview of a micro/nanomotor-based platform for food safety control and detection applications of different contaminants.food samples with high concentration of H 2 O 2 that can significantly improve the detection efficiency.The catalytic materials of H 2 O 2 are asymmetrically modified on the motor surface as power conversion units.When bubbles are continuously generated from one side of the surface, the reverse propulsion force triggers the movement of micro/nanomotors.Examples of common designs that use H 2 O 2 to propel motors include asymmetric structure known as Janus, or a cavity structure design in a tubular, bowl-like shape.Compared with the Janus structure, the cavity structure can effectively collect and release the generated bubbles, thereby maximizing the energy conversion efficiency and considerably increasing the motor velocity.Reports have demonstrated that catalytic material-coated microspheres can move automatically in H 2 O 2 solutions and reach a distinct speed under different H 2 O 2 concentrations, but their motion speeds are generally low.For example, Zhang et al. (2015) deposited a platinum layer with a thickness of approximately 20 nm on top of silica microspheres to fabricate an asymmetrically structured micromotor capable of operating at 9 μm s −1 in a 5-wt% H 2 O 2 solution.Ren et al. (2019) designed a Janus micromotor with a moon-like shape and a concave surface on one side.The motion of the motor depends on the amount of MnO 2 loaded on the concave surface of the motor and the concentration of H 2 O 2 in the solution.The data showed that the micromotor could reach a speed of approximately 30 μm s −1 in a 5-wt% H 2 O 2 solution.Surprisingly, asymmetric micromotors can be controllably fabricated with various structures in large quantities using a capillary microfluidic device, which is beneficial for industrial mass production.Hollow tubular motors with nano/microscale are known as jets because of their small sizes and large amounts of power (Maric et al. 2019).Gao et al. (2011) used a commercial template with a built-in symmetrical double-cone pore structure to synthesize the outer layer of polyaniline and the inner wall of a Pt coating to obtain a conical bilayer microtube structure that was applied after template dissolution (Figure 1A).Further studies showed that the opening size, surfactant concentration, and fuel concentration could affect the speed of this micromotor.Its speed reached approximately 300 μm s −1 in a 1-wt% H 2 O 2 solution containing 1.6% sodium cholate.It is worth noting that the motion of the microtubule motor was dependent on the addition of surfactants such as Triton X-100, sodium cholate, and sodium dodecyl sulfate.These surfactants adsorb onto the catalytic layer surface of the motor and promote bubble generation and separation by reducing the interfacial free energy (Wang, Zhao, and Pumera 2014).Pijpers et al. (2020) used biodegradable copolymers polyethylene glycol and poly(D, L-lactide) to self-assemble into a shell from the bottom up and synthesize a MnO 2 catalyst in situ inside a cavity to form a complete stomatocyte nanomotor (Figure 1B).The oxygen bubbles generated by the fuel are released from the neck channel of a 94-nm single path to generate the driving force.When the concentration of H 2 O 2 is 50 mM (~0.15 wt%), the nanomotor speed is 20 μm s −1 .In particular, the inorganic material MnO 2 can be degraded in a glutathione solution, which has established safety and health prospects.

Water
The reaction of active metal such as Na/Mg/Al and the H + ionized in water can generate H 2 bubbles, forming asymmetric momentum distribution on the surface of the micro/nanomotor for generating propulsion.Compared with the propulsion manner that requires additional H 2 O 2 , the Mg/Zn/Al-water reaction has attracted widespread attention owing to its relative safety, and has been applied to the design and fabrication of micro/nanomotors.In general, in addition to the formation of H 2 bubbles in the reaction of Mg/Zn/Al with water, a passivation layer of hydroxide is formed, which affects the continuity of the reaction.To extend the lifetime of water-driven motors, Wang's group (Gao, Pei and Wang 2012) first proposed the use of Al-Ga alloys to solve the problem of passivation films that are formed during the reaction process.This micromotor can move at a speed of 3 mm s −1 in water and maintain a similar speed even in a wide pH range of 4-10.It was also tested in cell culture media, phosphate-buffered saline (PBS), and human serum samples with different viscosities.Its speed decreased with increasing viscosity compared to that of pure water but the micromotor was able to operate normally (Figure 1C).Wang et al. (2020c) fabricated water-driven biocompatible micromotors using Mg microspheres as the original core and coated them sequentially with biodegradable components, such as poly(lactic-co-glycolic acid), sodium alginate, and chitosan.To prevent the generation of a passivation layer, NaHCO 3 was further added to convert the passivation layer component Mg(OH) 2 into soluble MgCO 3 .
Similarly, zinc, a comparatively reactive metal, can react with acidic solutions to produce hydrogen bubbles with a power source (Gao et al. 2015).Apart from active metals, carbonate, as an easily available material, can react in acidic environment and simultaneously produce carbon dioxide gas and ions with a certain concentration, which can be the power source of asymmetric micromotors (Saad, Kaur, and Natale 2020).The micromotors are able to move in situ due to the acidic micro-environment formed by the natural organic acid content of fruits and vegetables.
Here, the micromotor used is disposable because it will gradually lose its power source and eventually stop moving with the rapid decomposition of the active metals.Consequently, prolonging their lifetime and how to recycle them after usage become the current issues to be solved.

Glucose
Although enzymes with high biocompatibility, including trypsin, glucose oxidase, glucose oxidase coupled with catalase, acetylcholinesterase, urease, and catalase (Patiño et al. 2018), have been used as power conversion components for the fabrication of micro/nanomotors, in view of the presence or absence of these enzyme substrates in food, we only present glucose as the power source for in situ motion in food.Glucose is widely used as a raw material not only for food and pharmaceutical products but also for many bulk fermented biochemical substances, such as amino acids, antibiotics, and sugar alcohols, which are vital in production.Glucose oxidase can specifically and efficiently catalyze the oxidation of glucose to produce gluconic acid and H 2 O 2 , causing a change in the concentration gradient of the solution, which can be developed to manufacture micro/nanomotors based on the self-diffusiophoresis mechanism (Ma et al. 2016;Yuan, Liu, et al. 2021).Here, self-diffusiophoresis is one of the self-propelling forms produced by asymmetric momentum distribution on the surface of particles.He's team (Ji et al. 2019) designed a Janus nanomotor using gold nanoparticles as carriers.They grafted polymer brushes on one side of the gold sphere and covalently immobilized glucose oxidase on the other, powered by the translational diffusion of glucose fuels to the nanomotor (Figure 1D).This polymer brush, mimicking bacterial flagella, significantly improved the translational diffusion of the nanomotor.The motor achieved a speed of 1.2 μm s −1 in the presence of a 5-mM glucose solution in a viscous glycerinum/water mixed solvent (v/v: 1:1), which was increased to 9.1 μm s −1 when the glucose solution concentration was increased to 80 mM.However, H 2 O 2 released during the reaction poses a risk to the food environment.Abdelmohsen et al. (2016) fabricated a nanomotor that can be powered using both glucose and H 2 O 2 fuels by entrapping glucose oxidase and catalase in the lumen of stomatocytes.Both enzymes maintain high activity in the lumen and are less affected by the external environment.The combination of catalase and glucose oxidase in 1:3 (w/w) cascade can indirectly convert glucose into oxygen and water.In the presence of glucose at concentrations as low as 5 mM, the nanomotor reached a speed of 6 µm s −1 (Figure 1E).Future research efforts must be devoted to improving the effective driving force of enzyme-catalyzed micro/nanomotors and finding alternative materials that maintain efficient enzyme activity in harsh environments.

Micro/nanomotors driven by external energy
Precise propulsion control of micro/nanomotors is of great importance in practical applications.Although fuel-driven motors have a higher power, they are in a disorderly motion in solutions and cannot attain precise propulsion control.External physical fields with high degrees of freedom, such as light, electricity, magnetism and sound, can realize the precise control of the velocity and spatial orientation of micro/nanomotors as well as provide a continuous movement lifetime.This section will summarize micro/nanomotors driven by light (visible light and near infrared light), magnetic field and ultrasound are expected to be applied in various food environments.

Light
Light energy is one of the most common and readily available external physical stimuli.Two light-driven mechanisms, namely, photocatalysis and photoinduced thermophoresis, have been extensively studied because of their relatively strong driving force.The mechanism of light-driven motor motion involves inducing asymmetric generation of ions, molecules, or gases on the surface of a photocatalyst by one-sided illumination, resulting in energy transfer and motion.Villa et al. (2019) designed a one-step synthesis of star-shaped BiVO 4 micromotors that are motile upon exposure to visible light (Figure 2A).The star micromotors can reach speed of 5 μm s −1 in 0.1 wt% H 2 O 2 solution with 2500 mW cm −2 light intensity and can also be propelled in solutions with a viscosity (6 cP at 20 °C).In addition, by turning the light source on and off, the micromotor enabled the capture-transport-release of yeast cells.Tong et al. (2020) reported mass-produced regular spherical micromotors with fast movement under visible light using an emulsion method with iron phthalocyanine and gelatin as the active material and excipient, respectively, in a solution.Upon exposure to visible-light irradiation, iron phthalocyanine, as a micromotor component, reduces oxygen and water to superoxide radical anions (•O 2 -) and protons (H + ) and hydroxyl radicals (•OH), respectively.The different ionic products generated by the reaction exhibit different diffusion rates, while the negatively charged micromotor on the surface tends to move toward the light source.Under a light irradiation of 1200 mW cm −2 , the micromotor reaches a speed of up to 47 μm s −1 , which increases with an increase in light intensity.Moreover, because the material density is similar to that of water, the motors can move faster in water.This micromotor also has application potential in pollutant degradation.
In addition to the use of visible-light sources, the excellent penetration and safety of near infrared (NIR) light sources are attractive.Photothermal materials can produce thermal gradients through photothermal effects in the presence of NIR light.Combining a photothermal material with micro/nanomotors allows the motors to generate a self-thermophoretic force to advance in a liquid under NIR irradiation.For example, Xuan et al. (2016) used half gold-coated mesoporous silica nanoparticles to fabricate NIR-driven motors, which can easily adjust their velocity and go/stop by controlling the power and on/off switching of the NIR laser.Gold nanoshells were introduced using the thermal gradient caused by surface plasmon resonance heat generation with NIR light as a power source.With the power of 3 W cm −2 NIR light, the velocity of 80-nm motor in PBS and fetal bovine serum solution can reach 5.86 and 1.76 μm s −1 , respectively.Zhou et al. (2020) designed isotropic reduced graphene oxide aerogel microspheres with controllable diameters that have ultralow density and strong adsorption capacity (Figure 2B).These micromotors generate extremely high velocity (17.60 mm s −1 ) when irradiated by a simple NIR light source and are expected to be a breakthrough in overcoming countercurrent.In addition, the residual carboxyl groups on the surface of reduced graphene oxide, which have application prospects in the food industry, can be further chemically modified.

Magnetic field
Magnetic separation is generally more efficient and specific than separation methods such as centrifugation and filtration (Kumar and Mohammad 2011).The micro/nanomotors can be accurately navigated by applying an external magnetic field by introducing magnetic components into their structure.As shown by Sun et al. (2019), pine pollen-based micromotors (PPBMs) with vacuum loading of Fe 3 O 4 nanoparticles can perform finish rolling, tumbling, and spinning motion modes by different magnetic actuations.These three rolling modes are controlled by three magnetic field strengths, namely, H R , H T , and H S , respectively, and the motors demonstrate excellent maneuverability in different liquid environments (Figure 2C).It can be seen that PPBM was able to achieve a velocity of 108.25 μm s −1 by rolling motion when a magnetic field of H R =10 mT, 20 Hz was applied in deionized water.Similarly, applying a field with H T = 10 mT, 15 Hz, the PPBM was able to achieve a tumbling velocity of 175.19 μm s −1 , indicating that the tumbling motion allowed the motors to achieve higher velocity.Similarly, in addition to the use of pollen as a biological template for micromotors, the deposition of magnetic materials on the surface of plant spores has potential for designing micromotors (Zhang et al. 2018;Zhang et al. 2019b).

Ultrasound wave
As medical auxiliary diagnostic imaging devices, typical diagnostic sonographic scanners operate in the frequency range of 2-18 MHz (Carovac, Smajlovic, and Junuzovic 2011), in which the ultrasound power is not harmful to humans.Wang et al. (2012) first demonstrated that a rod-like or spherical material made of metals with an asymmetric composition or shape can move directionally (~200 μm s −1 ) under ultrasonic treatment.Thereafter, an increasing number of micro/nanomotors have been designed to be propelled by ultrasound, often using gold nanowires as substrates, and the purposeful functionalization of their surfaces for a variety of biomedical applications have been explored.For example, by modifying the carboxyl groups on the surface of gold nanowires, ovalbumin (model protein antigen) can be wrapped around the surface of gold nanowires via amine-carboxyl interactions, enabling an effective intracellular antigen delivery strategy using ultrasound (Wang et al. 2021).Furthermore, the gold nanowire motors were biocompatible and their internalization had no effect on cell morphology.The discovery of ultrasonic propulsion can avoid the limitations of fuels or high ionic strength environments and lays a foundation for expanding the driving mode and adapting to a wider range of applications.

Hybrid propulsion
To further improve the velocity conversion efficiency of the micro/nanomotors, it has been reported to combine the propulsion manners described in 2.1 and 2.2 to obtain a higher velocity which are suitable for the medium with high viscosity.The combination of acoustic and magnetic fields is the most common double hybrid field, where the magnetic field is used for precise navigation and the acoustic field provides power, which is widely used in precision medicine.For instance, He's team ( (Gao et al. 2019) packed iron oxide nanoparticles into healthy red blood cells (RBCs) inspired by natural cells to develop an RBC-mimicking micromotor for the active delivery of oxygen and photosensitizers.The motor designed with a biconcave RBC structure and high density allows the ultrasonic energy to be converted into motion, enabling high-speed propulsion in fresh whole blood.As shown in Figure 2D, a magnetic field of 1 mT can guide the movement direction of the RBC-mimicking micromotor.Concurrently, the velocities of micromotors in PBS, serum, and blood were 15.39, 10.13 and 7.15 μm s −1 , respectively, under the ultrasonic conditions of 0.8 W and 2.15 MHz.It should be noted that even in a highly viscous environment of 15.5 mPa s, the micromotor can still move at a speed of 2.34 μm s −1 , which implies potential application in food media.
An alternative strategy is an external energy-assisted chemical fuel propulsion.For example, Lu et al. (2020) used ultrasound to construct tubular micromotor swarms driven by oscillating self-generated bubbles, which are called dandelion-like microswarms (Figure 2E).Such micromotor swarms can be efficiently propelled in highly ionic environments and thus hold promise for more complex locomotion.Through numerical simulations, the authors confirmed that the bubble-based swarming mechanism is dominated by a localized acoustic field caused by bubble oscillation.Under the applied conditions of 10 V and 101 kHz ultrasound, as well as the presence of 2% H 2 O 2 in the solution, the average velocity of the dandelion-like microswarm movement is approximately 50 mm s −1 , which is 100 times higher than that of conventional bubble-propelled tubular motors.In addition, there are micromotors that use chemical fuels with an applied magnetic field to guide the direction of movement (Wang et al. 2019a) and dual-propulsion motors using H 2 O 2 and NIR light, which can be used for active cargo delivery (Xing et al. 2019).Micromotors designed to provide hybrid power using combinations of acoustic fields, magnetic fields, and/or chemical fuels have also been developed and are expected to play an even greater role in the near future (Valdez-Garduño et al. 2020).Although glucose is safe as a fuel, it is slightly inferior to H 2 O 2 in terms of propulsive power.Wang et al. (2019b) demonstrated a Cu 2 O@N-doped carbon nanotube micromotor for glucose-fueled photocatalytic propulsion, which exhibited conventional propulsion capabilities (18.71 μm s −1 ) compared with classic Pt-based Janus catalytic micromotors.
Recent investigations have demonstrated that the green and safe evolution of micro/nanomotor propulsion methods will greatly promote their application in the food industry.Nevertheless, the applicability and work efficiency of micro/ nanomotors in food systems depend on their movement mechanism, velocity, and lifetime.Note that the complexity and diversity of food ingredients affect the properties of liquids.For instance, the sugar contained in the food itself or the sugar added during processing increases the viscosity.
Sour substances cause a lower pH in foods, resulting in a corrosive solution or reduction in inherent polyphenols and vitamins in the food.The barrier needs to be tackled by more researchers working on food research.

Superiority of micro/nanomotors in the food field
Apple juice has been widely used for validating the practical application of micro/nanomotors because of its importance in food production (Campuzano et al. 2012;Gao et al. 2013;Wang 2012).We illustrated the potential advantages of micro/ nanomotors in food industry by taking the application of micro/nanomotor in apple juice sterilization as an example.
Thermal sterilization is the main sterilization method in food factories.It includes pasteurization, high-temperature short-time sterilization, and ultrahigh-temperature sterilization in the case of apple juice production.The thermal sterilization method has a better result, but high temperatures have adverse effects on the quality of the juice, such as browning, change in flavor, and nutrient loss.In view of these shortcomings, non-heat sterilization technology has become a research hotspot in recent years.The major research topics in fruit juice sterilization are pulsed electric fields, ultrasound and high hydrostatic pressure processing (Khandpur and Gogate 2015).Non-thermal sterilization technology can sterilize and inactivate enzymes at a lower temperature and help maintain the required juice color, flavor, taste, nutrients, and freshness.However, the existing non-thermal sterilization technologies require several instruments and equipment, which cannot be efficiently applied to the industrial production of apple juice at present.In commercial pasteurization, the Alicyclobacillus sterilization effect is greatly reduced because Alicyclobacillus can survive at low pH values and high temperatures.In addition, some strains release chemical substances during the growth period, which cause a peculiar smell in fruit juices and greatly damage the economic value of commodities (Al-Qadiri et al. 2006).A rapid, convenient, and efficient technology for the separation and removal of harmful microorganisms from apple juice can not only eliminate microbial contamination in the juice but also ensure its quality and safety after processing.
Currently, there are many studies on the separation and enrichment of microorganisms in food samples using ads or pt ion met ho ds, w hich main ly include bacteriophage-based, lectin-based, metal hydroxide-based, and antibody-based specific adsorption and separation methods (Benoit and Donahue 2003).The immunomagnetic separation technology constructed with magnetic nanospheres can retain the biological activity of the targe with the maximum extent during the separation process; it has been extensively studied in food safety evaluation and hazard control (Wang et al. 2020b).However, owing to the high cost of using immunomagnetic particles, their standardization, routinization, and mass production still need to be further explored.In addition, liquid food systems, such as apple juice, are characterized by high viscosity and ionic strength.When conventional nanomaterials or microparticles are dispersed in these systems, their Brownian motion is significantly hindered, resulting in their inability to move autonomously and efficiently.Immunomagnetic nanomaterials in apple juice cannot function fully or it may take a longer time to achieve their microorganism capture effect.As a result, a technology that is capable of both efficiently moving in an environment of high viscosity and high ionic strength as well as identifying, capturing, and isolating microorganisms needs to be developed urgently.
Whitesides's group first proposed a self-moving component based on the impact force of catalytic materials that catalyzes the decomposition of H 2 O 2 solutions to produce oxygen bubbles (Ismagilov et al. 2002).Thereafter, many studies have reported the use of different shapes and sizes of micro/nanomotors, as well as enzyme-based catalytic fuels other than H 2 O 2 , such as urea and glucose, for the development of a wider range of biomedical applications.Currently, a study has shown that micro/nanomotors can be effectively propelled in apple juice and can accomplish complex assignments, such as capturing, collecting, and releasing a category of microorganisms (such as gram-negative bacteria) through surface functionalization of the motors (Campuzano et al. 2012).However, despite the advantages of the fuel-driven motor with sufficient power, there are also disadvantages that need to be addressed, such as rapid fuel consumption and the harm caused by excessive H 2 O 2 concentrations in the food system (Xu, Xu, and Zhang 2017).Therefore, fuel-free micro/nanomotor models, including light-, magnetic field-, and ultrasound-driven as well as multimode hybrid propulsion units, are continually emerging.Thus far, the increased motor speed has made it an attractive platform for enhancing chemical reactions in food safety control, either in microfluidic channels or in large liquid reactors.In the following sections, we provide an in-depth discussion of the potential micro/nanomotor applications in the food field, along with the current challenges and future trends in micro/nanomotor strategies.

Application in food safety control and processing
Once the motor is successfully moving, we need to further functionalize it to perform specific tasks, which will further increase the difficulty of the design and fabrication.Combining existing specific recognition components with high-speed mobile platforms, we now face the challenge of firmly attaching these "working units" to the motor.In addition, the in situ operation of the motor in the food system needs to be considered during the designs and operating mechanisms of functional motors developed in the biomedical field to ensure that the motor has a negligible impact on the food system after operation.In this section, we discuss some typical cases of harmful substance removal during food production and processing.The rest of the cases with potential applications in food systems are summarized in Table 2.

Isolation and killing of pathogens
Pathogenic microorganisms that exist in food or those that use food as a transmission medium are known as foodborne pathogens.Such microorganisms can cause foodborne diseases when they directly or indirectly contaminate the food.In the food industry, rapid analysis and timely removal of pathogenic microorganisms are of great importance to food producers (Ripolles-Avila et al. 2020).
As an artificial tool with a motion speed that causes liquid agitation, functionalization of the micro/nanomotor surface modified with various receptors (e.g., lectins and antibodies) can enable it to attain efficient in situ capture ability.When modified with antimicrobial agents, it can show excellent bactericidal ability.Lectins, as recognition molecules, are often used in constructing sensors for foodborne pathogens because of their ability to bind to specific bacteria (Mi et al. 2021).Wang's group (Campuzano et al. 2012) first constructed a lectin-modified self-propelled microtubule motor for direct isolation of Escherichia coli from apple juice and drinking water samples.Lectin modification on the micromotor surface was achieved through the medium of a self-assembled monolayer, which is attached to the outer surface of the motor at one end and coupled to the lectin at the other.Researchers showed that micromotors modified with lectins could effectively identify and isolate target bacteria from different environments, including low-and high-sugar concentrations (as in drinking water and apple juice, respectively) as well as high-salt (seawater and urine) environments.In addition, E. coli was released by destroying the structure of the sugar-lectin complex in a low pH glycine solution.Later, they showed another anti-Bacillus globigii (a Bacillus anthracis surrogate) antibody-modified micromotor based on immunoassay, which was aimed at the selective capture and destruction of anthrax spores (Orozco et al. 2015).The stable physical and chemical properties of bacterial spores indicate more rigorous procedures such as larger doses of disinfectants, longer disinfection times, and higher temperatures (Hilgren et al. 2007).The specific recognition of antibodies compensates make up for the deficiency that traditional disinfectants need a large amount to kill spores.The antibody that specifically recognizes and captures anthrax spores in this example binds covalently to the exposed carboxyl groups on the surface of the motor through amination, allowing the micromotor to efficiently capture anthrax spores.As shown in the above two examples, motors for the specific capture of pathogenic bacteria require specific recognition elements for the motor unit to perform the capture and transfer functions.
There are three bactericidal mechanisms of micro/nanomotors.First, the micro/nanomotor is loaded with substances possessing antibacterial properties, and the antibacterial properties are greatly improved with the help of a high-speed mobile platform.Second, micro/nanomotors are designed to produce reactive oxygen/nitrogen species, the bactericidal mechanism of which can prevent antimicrobial resistance.Finally, micro/nanomotors are capable of mechanical bacterium cell lysis through kinematic friction or make use of external energy shock for killing bacteria  (Gong et al. 2022;Gu et al. 2020).We focus on the first two common mechanisms of bactericidal action.Antimicrobial peptides are known as "natural antibiotics" and have attracted the attention of researchers because of their potential to overcome the growing problem of antibiotic resistance.Nisin is the most common class of bacterial antimicrobial peptides, which can specifically kill the target bacteria without harming the host itself.In addition, it is the only bacteriocin that can be applied to a wide range of food products including beverages, dairy, baked goods, oils, and meat (Bahrami et al. 2019).Escarpa's team (Yuan, Jurado-Sánchez and Escarpa 2021) used nisin, which has natural antibacterial properties, in combination with a micromotor for selective killing of gram-positive bacteria (Figure 3A).The asymmetric growth of Pt and Fe 2 O 3 provides the motor with both catalytic and magnetic propulsion engines, and nisin is covalently immobilized on the thioglycolic acid-modified GO layer for highly specific binding to gram-positive bacteria.Nisin kills bacteria by disrupting the lipid II unit form of the bacterial membrane; therefore, gram-positive bacteria without lipid membranes are the targets of nisin's selective action.Experiments using Staphylococcus aureus as representative gram-positive bacteria showed that the capture/killing efficiency of the moving motor was approximately 85% in juice, serum, and tap water, and the germicidal efficiency was two times higher than that of a static motor.It also exhibited high selectivity in environments where E. coli was present.
Because of its high efficiency, safety, and environmental friendliness, photocatalytic degradation has been extensively studied in the degradation, sterilization, and disinfection of organic pollutants in water bodies (Kumar, Travas-Sejdic, and Padhye 2020).Villa et al. (2022) fabricated an enzyme/ photocatalytic tandem microrobot, whose body was composed of TiO 2 as a nanotube bundle decorated with CdS nanoparticles, which were functionally assembled using urease (Figure 3B).In particular, the coupling of TiO 2 with CdS enhances the visible-light response capability of the materials.This enables the microrobots to produce reactive oxygen species under visible-light irradiation, causing damage to the cell membrane of E. coli, leading to their death.The loading of urease can drive the microrobots by catalyzing the decomposition of urea, and the motor was able to reach a speed of approximately 3.3 µm s −1 in a 50 mM urea solution.

Heavy metal ions
Heavy metals are not readily biodegradable and they can enter into the food chain and ultimately into the human body.In the human body, they interact strongly with proteins and enzymes, rendering them inactive, and accumulate in certain organs, causing chronic toxicity.Adsorption is the most widely used mechanism for removing heavy metal ions from contaminated water, and some natural biomaterials with porous structures are excellent adsorbents.The adsorption rate of an adsorbent can be increased significantly by stirring the solution.Zhang's team (Zhang et al. 2018) used natural Ganoderma lucidum spores as templates to synthesize micromotors that were controlled and recycled by a magnetic field.In particular, multitudinous magnetic micromotors can be regarded as macroscopic robots when moving collectively in a magnetic field, which is conducive to the processing of a large number of samples.The micromotors in collective motion were able to remove nearly half (48.7%) of the lead in the contaminated solution within 15 min, and by 50 min, they were able to achieve 81.1% lead removal (Figure 3C).The proof-of-concept established that, under the action of a magnetic field, lead ions in contaminated water can be degraded by approximately half within 15 min.This achievement provides a strategy for the removal of metal ions with high biocompatibility.

Toxins
Methicillin-resistant Staphylococcus aureus (MRSA), also known as the "superbug," is harmful because of its ability to secrete.Serious infections caused by MRSA are a huge global public health threat; consequently, the removal of MRSA and its toxins is an urgent scientific problem that needs to be solved (Algammal et al. 2020).Wang's group (Ávila et al. 2018) reported a nanomotor with gold nanowires coated with RBCs and platelets (PLs) for the concurrent removal of MRSA and toxins.The RBC-PL-robots were fabricated by fusion of RBCs and PL membranes to 3-mercaptopropionic acid-modified gold nanowires during ultrasonication, which endows the nanorobot with diverse biological functions.In addition, the fusion process allowed retention of the bilayer structure of the hybrid membranes and preservation of their protein functions.The top part of Figure 3D shows the dual detoxification capability of biomimetic nanorobots in this example in terms of rapid bacterial isolation and efficient neutralization of pore-forming toxins.The red columns in Figure 3D show the changes in optical density at 600 nm and hemolysis percentage calculated before and after 5 min of robotic treatment for aliquots taken from both MRSA samples.

Organic pollutants
Adsorption and photocatalysis have been used for the simultaneous removal of organic pollutants from water (Gusain et al. 2019).Microplastics have been reported to be detected in food and drinking water, which may compromise food security, food safety, and human health (Liu et al. 2021).Inspired by the strong adhesive properties of muss el feet, Mar tin Pumera's group (Zhou, Mayorga-Martinez and Pumera 2021) used self-polymerized polydopamine coated on Fe 3 O 4 nanoparticle surfaces to produce magnetic microrobots that provide a viable method for capturing, transporting and recycling microplastic contaminants under an external rotating magnetic field (Figure 4A).Furthermore, they demonstrated that lipase immobilized on microrobots can degrade polycaprolactone microplastics while capturing them.Rhodamine dye is one of the most commonly used synthetic dyes.Previously, pepper powder, beef jerky, and preserved fruit were used as food additive, however, its use as a food additive or colorant was prohibited after being classified as a category III carcinogen (Gusain et al. 2019).Chen et al. (2022) prepared magnetic microswimmers for the degradation of Rhodamine B (RhB) using the inherent helical structure of banana leaves as a biological template (Figure 4B).The magnetic nickel layers modified on the template by simple chemical methods were utilized to obtain power and directional guidance in a magnetic field, whereas Fe 3+ -TA (tannic acid) films were used to provide a catalyst for the Fenton reaction, resulting in the production of •OH.It was demonstrated that the microswimmer could reach a speed of 51 μm s −1 in a magnetic field (5 Hz and 5 mT).Moreover, it could degrade RhB up to 90% in a 0.03-mM RhB contaminated solution at pH 2.

Food technology and processing
The only approved use of H 2 O 2 in the dairy industry is in certain types of cheese and cheese byproduct applications at a maximum level of 0.05% in the United States (Martin et al. 2014).Therefore, a small amount of H 2 O 2 can be used as a disinfectant in the dairy industry, and it has been shown that 1% H 2 O 2 can be used in skim milk to drive micromotors (Maria-Hormigos, Jurado-Sánchez, and Escarpa 2018).Lactose intolerance refers to the inability of patients to fully digest lactose from dairy products.Diarrhea, abdominal distension, abdominal pain, and other digestive system symptoms will occur when lactose-intolerant patients consume dairy products (Misselwitz et al. 2019).Degradation of lactose before intake is the best solution for lactose intolerance.Surfactant-free β-galactosidase micromotors have been shown as moving biocatalyst for highly efficient lactose hydrolysis from raw milk (Maria-Hormigos, Jurado-Sánchez, and Escarpa 2018).The hydrolysis characteristics of the enzyme were combined with the effective movement of carbon nanotube micromotors to make the hydrolysis rate of lactose close to 100% (Figure 4C).Experiments have shown that the removal efficiency of the mobile platform is higher than that of the free enzyme and static platform (twice the lactose hydrolysis efficiency is generated in 25 min), and the application of micromotors in a food-related field is demonstrated for the first time.

Application in food contamination detection
Contamination related to food safety can be divided into exogenous and endogenous contamination.Exogenous contaminants include pesticides, veterinary drugs, and heavy metal residues, whereas endogenous pollutants include toxins carried by raw food materials and hazardous substances produced during storage and processing.A rapid and accurate analysis of food safety is essential for food production and consumer safety.The conventional detection method carries a high-speed motion platform, which can significantly improve the efficiency of contaminant detection.Obviously, the mobile platform based on micro/nanomotors is one of the most promising strategies for enhancing chemical reactions at trace volumes (1 μL).Table 3 summarizes the proof-of-concept applications and detection effect of typical motor sensors.

Toxins
Direct or indirect exposure to toxin-contaminated food poses a great threat to the health of humans and livestock (Gupta et al. 2021).Thus, it is necessary to develop sensitive and specific detection and quantification methods for different toxins to avoid risks to human health.The fluorescence-based quantitation platform enables a more sensitive detection of food contaminants through signal amplification of the nanomaterials (Sharma et al. 2015).
When the fluorescence is mounted on the high-speed mobile platform with the micro/nanomotor, on the one hand, the detection volume can be reduced from 1 mL to 1 μL, so that the purpose of trace detection is achieved.On the other hand, compared with a static micro/nanomotor, the time required for fluorescence detection will be greatly shortened, thereby improving the detection efficiency.

Bacterial toxins
Endotoxins and exotoxins are produced by bacteria and are closely related to their pathogenicity.Exotoxins are mainly toxic metabolites secreted by gram-positive bacteria during growth and reproduction, whereas endotoxins cause toxicity through bacterial lysis in the host.The main toxic component of exotoxins is protein, whereas that of endotoxins is lipid A in lipopolysaccharides (LPS).Escarpa's team (Jurado-Sanchez et al. 2017) reported phenylboronic acid-modified graphene quantum dot (GQD) based Janus micromotors for endotoxin detection.The detection strategy relies on the combined action of phenylboronic acid as a specific recognition receptor of the core polysaccharide region of LPS and the GQDs used to quench the fluorescence with the target endotoxin.In addition, the micromotors can use two propulsion methods, catalytically powered by Pt nanoparticles and magnetically actuated via Fe 3 O 4 nanoparticles.According to the relationship between the degree of fluorescence quenching and concentration of LPS, also working in the presence of an excess concentration of glucose, fructose, and galactose, the sensitivity and selectivity of micromotors for detecting endotoxins were demonstrated.Subsequently, researchers extended the previous approach and used it in food control applications for lethal endotoxin detection for the first time (Pacheco, Jurado-Sanchez, and Escarpa 2018).The micromotors allowed the detection of Salmonella enterica endotoxin in unprocessed milk, egg, and mayonnaise samples within 15 min, and the viscous food samples did not hamper their practical application (Figure 5A).In addition, the detection efficiencies of the micromotor at mixing and static conditions were also compared.From the time lapse microscopy images and the corresponding fluorescence decay plot vs time profile, it could be seen that in the solution contaminated with 1 ng mL −1 of the LPS, the moving motor could completely achieve fluorescence quenching within 15 min, while the static motor still showed fluorescence.It is proved that the moving motor can obviously improve the detection efficiency.Through this example, the application of micromotors in the detection of food toxins has achieved initial success, and it has provided inspiration and motivation for using more mobile platform-based detection methods in food contamination screening.Clostridium difficile is a gram-positive anaerobe that exists in the human intestinal tract and can produce enterotoxins and cytotoxins.Covering with magnetic Fe 3 O 4 and fluorescent carbon dots, spores were designed to detect C. difficile toxins (Figure 5B) (Zhang et al. 2019b).The chemical modification of the spores did not change their natural appearance.They still maintained a complete structure, rough surface, and hollow cavity, thus allowing toxin molecules to diffuse in between for subsequent fluorescence quenching.The mobile fluorescence platform exhibited ultrahigh performance; the LOD was comparable to that of high-performance ELISA in stool samples (1.73 ng ml −1 ), but the detection time was eight times shorter than that of conventional ELISA.

Mycotoxins
Mycotoxins are secondary metabolites produced by toxin-producing fungi under suitable conditions.Fumonisin (FB) is a toxin with a wider distribution than aflatoxin and is mostly found in corn and its products.Ochratoxin is another mycotoxin that has attracted worldwide attention after aflatoxin.In particular, ochratoxin A (OTA) is the most harmful to agricultural products and human health.Escarpa's team ((Molinero-Fernandez et al. 2017) used the excellent aptamer fluorescence quenching ability of graphene to develop tubular micromotors made of reduced graphene oxide for efficient detection of mycotoxins (FB and OTA) in food.The main detection method is based on fluorescence quenching caused by the binding of the free dye-labeled aptamers to the graphene surface, and the presence of toxins facilitates binding to the aptamers to show fluorescence (Figure 5C).Compared with static micromotors and shaking procedures, moving motors can achieve a 100% fluorescence quenching rate within 2 min.Micromotors have the same test results in food samples (beer and wine) and standard products.

Phytotoxins
Ricin is a potent natural toxin that causes cell death by inhibiting protein synthesis.Ingesting a small amount of food containing ricin can be fatal.The lethal dose of human inhalation and injection of ricin is approximately 5-10 µg kg −1 (Bradberry et al. 2003).Wang's group (Esteban-Fernández de Ávila et al. 2016) demonstrated dye-labeled aptamer-modified tubular micromotors for ricin detection.The ricin B aptamer exhibited strong stability over a wide range of pH (2-7) and temperature (4-63 °C), which is conducive to detection in various harsh environments.In the presence of non-target protein detection substances, such as bovine serum albumin (BSA) and saponin toxin, it also has the ability to recognize ricin, which establishes its specificity.The detection performance of the dye aptamer-modified micromotors in liquid samples containing low (ng mL −1 ) ricin concentrations can be estimated quantitatively using fluorescence intensity (Figure 5D).

Heavy metal ions
Mercury pollution in water caused by various natural resources and industrial wastes is a serious threat to the environment and human health.Thus, monitoring of food and water contaminated by heavy metals is particularly important to protect human health.The combination of the fluorescence enhancement of acridine orange with a Mn 2 O 3 /γ-AlO(OH) substrate and fluorescence quenching by Hg 2+ inspired Li et al. (2021) to establish a colorimetric detection platform for Hg 2+ (Figure 6A).By adding Hg 2+ to the moving micromotors, a significant decrease in the fluorescence intensity was observed within 3 min.According to the calculation, the linear range of Hg 2+ detected by the micromotor platform is 5 × 10 −8 to 5 × 10 −5 M, and the LOD can reach 1.75 × 10 −8 M. It is worth noting that up to 30 μM Hg 2+ can be detected visually.
Colorimetric detection methods have received considerable attention because they can be visible to the naked eye through color changes.Yang et al. (2021) reported a micromotor detection platform loaded with a nanozyme that enables faster colorimetric determination of copper ion concentrations while exhibiting high-speed motion characteristics (Figure 6B).The authors used selected carbonized kapok fibers as carriers for tubular motors.Furthermore, they synthesized NiCo 2 O 4 @ MnO 2 nanosheets and fabricated them with BSA functionalization.The added H 2 O 2 not only serves as a driving fuel for the micromotor but also as a system component for colorimetric detection.The linear range of copper ion detection can be divided into two segments, i.e., 0.005-0.1 μM and 0.5-50 μM, with an LOD of a lower linear range of 2 nM.
In addition to being directly used for detection, nano/micromotors can be used as tools to assist in food rapid detection.The accumulated heavy metals in soil and water can migrate to crops such as rice.Arsenic is a heavy metal that readily accumulates in rice plants.Autonomously moving micromotors were used for the first time to enable the digestion of rice samples under mild conditions and onsite determination of arsenic in rice using a paper-based analytical device (Figure 6C) (Luo et al. 2021).The authors used MnFe 2 O 4 as a Fenton-like nanocatalyst, which was able not only to catalyze the production of oxidative radicals to effectively degrade the rice samples but also to maintain an efficient radical yield propelled by bubbles.The concentration range of arsenic can be obtained from the color of the HgBr 2 impregnated paper visually.The observable arsenic concentration range of the rice samples in the experiment was 40-1000 μg kg −1 , which was fully compliant with the requirements of GB 2762-2017.The presence of Mn 2+ , Fe 3+ , and NO 2 -after dissolution of the micromotor itself did not interfere with the assay results, which is particularly important for the feasibility of micromotor-assisted sample processing in this case.

Persistent organic pollutants
Phthalate esters are used as plasticizers in food packaging materials and have been shown to be associated with lower sperm quality in males and attention deficit/hyperactivity disorder in children after exposure (Chang et al. 2021).
Rojas, Jurado-Sanchez, and Escarpa (2016) described a new Janus micromotor strategy for the direct determination of diphenyl phthalate (DPP) as a model compound in a wide variety of food and biological samples.The disposable analysis platform based on micromotors can degrade non-electroactive DPP into electroactive phenol, and the DPP content can be measured directly using differential pulse voltammetry.It was established that this method could analyze foods containing viscous media, such as milk and whiskey, within 5 min.By constructing a calibration plot of peak current versus DPP concentration, a linear relationship was observed in different samples, such as water, milk, and whiskey, in the ranges of 0.12-1, 0.12-1, and 0.50-2 mM, respectively.Concurrently, according to the calculated results, the LOD of the samples (S/N = 3) were 0.039, 0.040, and 0.15 mM, respectively.

Conclusions and prospects
In this review, we summarize the propulsion mechanisms, advantages, and potential applications of micro/nanomotors for food safety control in contaminated food.The aim is to present effective strategies for minimizing economic losses and threats to human safety caused by food contamination.These artificial micro/nanomotors have been designed to display high velocities in different liquids at the microscopic scale.Such continuous movements at high velocities lead to built-in sample solution mixing, enhancing the speed of chemical reactions.To date, two main power sources, namely, chemical fuels (H 2 O 2 , glucose, etc.) and external energy sources, for driving micro/ nanomotors to higher power levels have been developed.By exploring the applicability of the viscosity properties of food to the motor's rate of motion, we present the challenges and possible countermeasures for manufacturing micro/nanomotors.On the one hand, for applications where micro/nanomotors need to work in situ, such as removal of pathogenic bacteria or harmful substances and production and processing in the food industry, the biocompatibility of material components and the relative safety of propulsion methods for micro/nanorobots in food environments need to be considered primarily.On the other hand, it is critical for researchers of micro/nanomotors, which need to achieve rapid detection applications by enhancing solution mixing, to focus on increasing their energy conversion efficiency and lifetimes.In addition, the cost of micro/nanomotors produced by different materials and processes varies (Table S1), and although methods have been reported for the manufacture of kilogram quantities micro/ nanomotors which need further functionalization steps before real applications.Notably, there is still room for development in proof-of-concepts of micro/nanomachines for chemical/biological sensing and removal.Although the simultaneous detection and removal of heavy metal ions and phenols have been developed, the integrated detection and removal of more complex food hazards such as pathogenic bacteria, pesticides, and veterinary drugs have not been reported.Therefore, future micro/ nanomotor research should be devoted to tackle the challenges of integrated micro/nanomotors.In general, we hope that the broad prospects of micro/nanomotors in food applications will stimulate multidisciplinary research and further development.For example, multidisciplinary collaboration between food science and chemistry, physics, biology, and materials science is more promising to develop micro/nanomotors for food samples, and the pilot plant required for continuous large-scale production is an essential part of real-world industrial applications.We envision that future materials for micro/nanomotors will be taken from food or biological bodies rather than synthetic materials on behalf of safety and environmental friendliness.Moreover, the technical means have reached the requirement that the motor can be mass-produced, and the development has progressed to the pilot stage.The micro/nanomotors will be a highly anticipated new star in food industry as we believed in the near future.

Figure 1 .
Figure 1.schematic of autonomous propelled nano/micromotors by chemical fuels.(a) Preparation process of polyaniline/Pt microtube engines and their speed upon the hydrogen peroxide concentration.© 2011.american Chemical society.reproduced with permission (Gao et al. 2011).(B) the preparation process of stomatocytes shaped nanomotors and the degradation of the catalyst in glutathione within their lumen.© 2020 american Chemical society.reproduced under the terms of Creative Commons public use license (CC-BY-nC-nd) (Pijpers et al. 2020).(C) water-driven micromotors.© 2012.american Chemical society.reproduced with permission (Gao, Pei and wang 2012).(d) the polymer-brush-grafted, glucose-oxidase-powered Janus gold nanoswimmers with a positive, macroscale chemotactic behavior.© 2019.wiley-vCH verlag GmbH & Co. KGaa, weinheim.reproduced with permission (Ji et al. 2019).(e) the bowl-shaped structure nanomotors powered via a one-enzyme or two-enzyme system under a single fuel (hydrogen peroxide or glucose).© 2016.american Chemical society.reproduced with permission (abdelmohsen et al. 2016).

Figure 3 .
Figure 3. (a) (i) nisin modified micromotors are used for selective capture/inactivation of gram-positive bacteria units and biofilms.(ii) left part shows the fluorescent images of total bacteria (in green) and inactivated bacteria (in red) with the corresponding plots showing the inactivation efficiency on the right part of the image, where: (a) initial conditions without contact with micromotors, (b) unmodified moving micromotors, (c) unmodified micromotors with free peptide under magnetic stirring, (d) static modified micromotors and (e) moving modified micromotors.© 2020.wiley-vCH GmbH.reproduced with permission (Yuan, Jurado-sánchez and escarpa 2021).(B) schematic illustration of the fabrication of hybrid photocatalytic tio 2 /Cds microrobots functionalized with urease for the removal of E. coli biofilm and their false-colored seM images of after the microrobots treatment.© 2022.the authors.small published by wiley-vCH GmbH.reproduced with permission (villa et al. 2022).(C) remediation ability using static and swarming porous spore@Fe 3 o 4 biohybrid adsorbents.© 2018.wiley-vCH verlag GmbH & Co. KGaa, weinheim.reproduced with permission (Zhang et al. 2018).(d) schematic of rBC-Pl-robots for bacteria targeting and pore-forming toxin neutralization.© 2018.reproduced under the terms of the Creative Commons attribution-nonCommercial license (CC BY-nC) (Ávila et al. 2018).

Figure 4 .
Figure 4. (a) schematic image of microplastic removal by adhesive Pda@Fe 3 o 4 /lipase Magrobot.© 2021.wiley-vCH GmbH.reproduced with permission (Zhou, Mayorga-Martinez and Pumera 2021).(B) (i) spiral microswimmers from plant template preparation for degradation of rhB.(ii) uv-vis absorbance spectra of rhB aqueous solutions at different pH values following 60 min treatment and the degradation efficiency of microswimmers at different magnetic field frequencies.© 2022.elsevier B.v. all rights reserved.reproduced with permission (Chen et al. 2022).(C) (i) schematic of the preparation of carbon nanotubes micromotors and the real images for micromotors.(ii) effect of time and number of β-galactosidase micromotors for lactose removal and lactose removal efficiency under different conditions.© 2017.wiley-vCH verlag GmbH & Co. KGaa, weinheim.reproduced with permission (Maria-Hormigos, Jurado-sánchez and escarpa 2018).

Figure 5 .
Figure 5. Micromotors for toxin detection.(a) schematics for the quenching mechanism by lPs from S. enterica bound to GQds on the microsensors.lPs detection on food samples and their images taken from videos.© 2018.american Chemical society.reproduced with permission (Pacheco, Jurado-sanchez and escarpa 2018).(B) schematic illustration of the preparation and potential application of fluorescent magnetic spore-based microrobots and the fluorescence response of them to C. difficile toxins.© 2019.reproduced under the terms of the Creative Commons attribution-nonCommercial license (CC BY-nC 4.0) (Zhang et al. 2019b).(C) schematic illustration of mycotoxins detection using unmodified rGo/Pt micromotors and their calibration of ota and FB.© 2017.american Chemical society.reproduced with permission (Molinero-Fernandez et al. 2017).(d) the images and calibration plot for correlation between the ricin B concentration and the fluorescence intensity (top).Fluorescence recovery and micromotor movement in different ricin B toxin-spiked samples (a-e: PBs, tap water, saliva, serum, and urine, respectively) and their corresponding digitization (bottom).© 2016.american Chemical society.reproduced with open access (esteban-Fernández de Ávila et al. 2016).

Table 1 .
Comparison of applicability, advantage and limitation of micro/nanomotor propulsion methods in food environment.

Table 2 .
application of micro/nanomotors in food safety control and processing.

Table 3 .
application of micro/nanomotors in food contamination detection.