Nanomotors: From Construction to Applications

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Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

Photocatalytic Micro/Nanomotors: From Construction to Applications Published as part of the Accounts of Chemical Research special issue “Fundamental Aspects of Self-Powered Nano- and Micromotors”. Renfeng Dong,†,‡ Yuepeng Cai,† Yiran Yang,§ Wei Gao,§ and Biye Ren*,‡

Acc. Chem. Res. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/29/18. For personal use only.

†

School of Chemistry and Environment, Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, Guangdong Provincial Engineering Technology Research Center for Materials for Energy Conversion and Storage, South China Normal University, Guangzhou 510006, China ‡ School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China § Department of Medical Engineering, Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125, United States S Supporting Information *

CONSPECTUS: Synthetic micro/nanomotors (MNMs) are a particular class of micrometer or nanometer scale devices with controllable motion behavior in solutions by transferring various energies (chemical, optical, acoustic, magnetic, electric, etc.) into mechanical energy. These tiny devices can be functionalized either chemically or physically to accomplish complex tasks in a microcosm. Up to now, MNMs have exhibited great potential in various fields, ranging from environmental remediation, nanofabrication, to biomedical applications. Recently, light-driven MNMs as classic artificial MNMs have attracted much attention. Under wireless remote control, they can perform reversible and repeatable motion behavior with immediate photoresponse. Photocatalytic micro/nanomotors (PMNMs) based on photocatalysts, one of the most important light-driven MNMs, can utilize energy from both the external light source and surrounding chemicals to achieve efficient propulsion. Unlike other kinds of MNMs, the PMNMs have a unique characteristic: photocatalytic property. On one hand, since photocatalysts can convert both optical and chemical energy inputs into mechanical propulsion of PMNMs via photocatalytic reactions, the propulsion generated can be modulated in many ways, such as through chemical concentration or light intensity. In addition, these PMNMs can be operated at low levels of optical and chemical energy input which is highly desired for more practical scenarios. Furthermore, PMNMs can be operated with custom features, including go/stop motion control through regulating an on/off switch, speed modulation through varying light intensities, direction control through adjusting light source position, and so forth. On the other hand, as superoxide radicals can be generated by photocatalytic reactions of activated photocatalysts, the PMNMs show great potential in environment remediation, especially in organic pollutant degradation. In order to construct more practical PMNMs for future applications and further extend their application fields, the ideal PMNMs should be operated in a fully environmentally friendly system with strong propulsion. In the past decade, great progress in the construction, motion regulation, and application of PMNMs has been achieved, but there are still some challenges to realize the perfect system. In this Account, we will summarize our recent efforts and those of other groups in the development toward attractive PMNM systems. First, we will illustrate basic principles about the photocatalytic reactions of photocatalysts and demonstrate how the photocatalytic reactions affect the propulsion of PMNMs. Then, we will illustrate the construction strategies for highly efficient and biocompatible PMNMs from two key aspects: (1) Improvement of energy conversion efficiency to achieve strong propulsion of PMNMs. (2) Expansion of the usable wavelengths of light to operate PMNMs in environment-friendly conditions. Next, potential applications of PMNMs have been described. In particular, environment remediation has taken major attention for the applications of PMNMs due to their photocatalytic properties. Finally, in order to promote the development of PMNMs which can be operated in fully green environments for more practical applications, an outlook of key challenges and opportunities in construction of ideal PMNMs is presented.

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INTRODUCTION Utilizing micro/nanomachines to perform complicated tasks in a microcosmic world is an exciting challenge. Synthetic micro/ nanomotors (MNMs) have attracted much attention in the © XXXX American Chemical Society

Received: May 31, 2018

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DOI: 10.1021/acs.accounts.8b00249 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research past decades due to their controllable motion behavior and tunable functionality. A variety of MNMs have been developed, including bimetal nanowires,1,2 flexible nanowires,3,4 Janus microspherical motors,5,6 tubular microrockets,7,8 helical swimmers,9,10 star-like rotors,11,12 and supermolecule-based nanomotors.13,14 Their excellent propulsion, efficient direction control, and abundant functions hold great promise in a number of fields ranging from environmental,15,16 biomedical,17,18 to nanoengineering applications.19 According to energy sources utilized, MNMs can be divided into two categories: chemically20,21 and physically driven.22 Chemically driven MNMs can convert chemical energy to mechanical energy by local chemical reactions with the presence of fuels in the solution. In general, if chemically driven MNMs show strong propulsion, high concentrations of fuels are usually required, which can impose toxic and harmful effects to the environment and human body. In addition, for the chemically driven MNMs, once the motors come in contact with fuels, the reactions will continue until the fuels or the motors are depleted. This can be reflected by the slowingdown of MNMs over time. Physically propelled MNMs are powered by external physical stimuli, such as optical,23 acoustic,24 magnetic,9 or electrical fields.25 However, strong propulsion output usually requires high physical energy input, which is not economic and may even be harmful to human body or environment in some cases, such as high intensity ultraviolet (UV) light. With these constraints, constructing excellent MNMs with precise motion control and low external chemical and physical input requirements for various applications has become critically important. Light, as one of the most common external physical stimuli, has been widely used for driving MNMs. Light-driven micromotors can be designed and fabricated by photoresponsive materials, such as photocatalytic,26 photolytic,27 photothermal,28 photochromic29 or photoisomerized30 materials. Among these, photocatalytic micro/nanomotors (PMNMs) and photoinduced thermophoretic MNMs are the most studied light-driven MNMs since they can achieve stronger propulsion than other light-driven micromotors. Thermophoretic MNMs are propelled by local temperature gradient, but high light energy is necessary to induce dramatic temperature increase. However, high local temperature may cause cell damage due to thermal radiation, which may limit their biomedical applications.31 PMNMs based on photocatalytic materials, can efficiently utilize photocatalytic reactions to convert both optical and chemical energy to the motor’s mechanical movement. Up to now, the propulsion mechanisms of PMNMs can be classified as light-induced selfdiffusiophoretic propulsion, light-induced bubble propulsion, and light-induced self-electrophoretic propulsion. Figure 1 takes Janus spherical motors to illustrate the propulsion mechanisms of PMNMs. Light-induced self-diffusiophoretic propulsion results from the asymmetrically generated gradients of phocatalytic electrolytes and nonelectrolyte products, while light induced self-electrophoretic propulsion is enabled by the self-generated electric field (E) from the asymmetric distribution of ions across the bipolar motor system; bubble propulsion is caused by the ejection of a jet of bubbles from PMNMs. The details of the mechanisms have been described in Guan’s review.23 Different kinds of PMNMs have showed their great potentials in various fields. However, there are still challenges to be addressed, such as low efficiency, high energy

Figure 1. Propulsion mechanisms of the PMNMs.

requirement, and harmful energy input, which have greatly restrained their operability and application scenarios. We have summarized the recent advances in PMNMs in Supporting Information Table S1, including the species of the photocatalysts, the structures, activation light, fuel type, propulsion mechanisms, and their highest speeds. The main idea of this Account is focused on recent progress, future challenges, and possibilities in developing highly efficient, biocompatible, and functional photocatalytic micro/nanomotors for practical applications in a broad horizon. Particular attention will be given to the choice of photocatalysts, construction strategies, and potential applications. First, we will illustrate basic principles of the photocatalytic reactions of photocatalysts and demonstrate how the photocatalytic reactions affect the propulsion of PMNMs. Then, we will illustrate the construction strategies for highly efficient and biocompatible PMNMs from two key aspects: improvement of energy converting efficiency and expansion of the usable wavelengths of light. Finally, we will summarize the challenges of developing attractive PMNMs ranging from photocatalysts, fuels to lights and present an outlook for the future of ideal PMNMs. We hope this Account can further promote the development of future intelligent PMNMs toward wide range practical applications.

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PHOTOCATALYSTS FOR PMNMs Photocatalysts have been extensively investigated for several years, motivated by the fascinating applications in pollution remediation, energy innovation, and chemical synthesis due to their attractive photocatalytic properties.32 Taking the advantages of the photocatalytic reactions, photocatalystbased PMNMs which can convert the light energy to motors’ mechanical energy have been successfully developed. PMNM propulsions are attributed to the gradient or asymmetrical distribution of photocatalytic products (such as ions, molecules or gases) generated by the photocatalysts. When photocatalysts are exposed to light of a particular wavelength, charge separation occurs within the photocatalysts; electrons will be generated and transferred to the conduction bands (CBs), leaving holes in the valence bands (VBs). The generated electrons and holes will participate in the cycles of redox reactions with the surrounding reactants and enhance the reaction. Theoretically, all kinds of photocatalysts can be used to construct PMNMs. However, considering the efficiency, cost, stability and current fabrication methods, there are still many challenges to construct PMNMs by photocatalysts. The first reported PMNMs are plain TiO2 microspheres. They can be propelled under 2.5 W/cm2 UV light intensity at the speed of 10 ± 3 μm/s in pure water by self-diffusiophoresis.26 This B

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For fuel-free PMNMs, light is the only concern for propulsion. In order to decrease the energy consumption and lower the harm of UV light, TiO2−Au Janus photocatalytic micromotors with light-induced self-electrophoretic propulsion have been designed toward to the photocatalytic efficiency improvement strategy42 (Figure 2A). On one hand, TiO2 can reserve sensitive and flexible light-controlled properties into the system; on the other hand, Au composition can greatly enhance the photocatalytic properties and the light energy conversion efficiency of TiO2, which will cause strong

work has opened new possibilities for light-driven MNMs. Later on, bubble propulsion of plain TiO2 micromotors have been developed.33 Up to now, due to the low cost, nontoxicity and high chemical stability of TiO2, TiO2-based PMNMs are still the most reported so far (Table S1). Recent researches on PMNMs are mainly focused on two aspects: (1) improving the photocatalytic efficiency of photocatalysts and further promoting the propulsion of PMNMs with low energy consumption; (2) constructing different PMNMs by various photocatalysts, such as BiOI,34 ZnO,35 WO3,36 Fe3O4,37 Cu2O,38 Si,2 C3N4,39 and so forth. With the unique properties of each photocatalyst, these PMNMs exhibit special motion performances.

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CONSTRUCTION OF ATTRACTIVE PMNMs BASED ON PHOTOCATALYSTS The basic requirements for attractive PMNMs should include highly efficient propulsion and green energy input. That means that strong propulsion of PMNMs should be converted by low intensity light and low concentration fuel, even under fuel-free conditions. In addition, the input energy should be a harmless light resource or biocompatible fuel species. Improvement of Energy Converting Efficiency

As for the aspect of improving the energy converting efficiency and further lowering the energy consumption of PMNMs, highly efficient photocatalytic reactions are necessary for strong propulsion of PMNMs. In principle, higher photocatalytic efficiency corresponds to more products under a fixed light intensity, which will lead to a larger concentration gradient and thus a stronger motion behavior of PMNMs. The photocatalytic efficiency is usually described by an apparent quantum efficiency (QE%) and can be calculated according to eq 1: no. of reacted h+ or e− × 100% no. of incident photons no. of product molecules = × 100% no. of incident photons

QE% =

(1)

Generally, under the same light illumination, PMNMs with higher QE exhibit stronger propulsion behavior. For example, the QE% values of TiO2−Au and plain TiO2 are 0.04%40 and 4.14%,41 respectively. Therefore, TiO2−Au has much stronger photocatalytic efficiency than TiO2 for hydrogen production under UV irradiation. Accordingly, TiO2−Au micromotors42 have much stronger propulsion than the plain TiO2 particles.26 The entire photocatalytic process mainly includes charge separation, consumption and recombination. Effective photoexcited charge carriers that are consumed by redox reactions are beneficial to the photocatalytic propulsion, whereas the remaining carriers recombine to form luminescence or heat, significantly weakening the photocatalytic properties. Thus, accelerating charge consumption and restricting recombination of the photoexcited charge carriers are effective strategies to increase the photocatalytic activity, such as improving the crystallinity, constructing heterogeneous structures or expanding the surface area. Doping of metallic or nonmetallic elements on the surface of the photocatalysts or constructing heterostructured composites with noble metals (Au, Ag, Pt, etc.) is also a conventional approach to enhance the photocatalytic properties.23 Therefore, according to the strategies above, choosing or designing highly efficient photocatalysts to construct PMNMs can realize strong propulsion with low energy consumption.

Figure 2. (A) (a) Schematic of catalytic TiO2−Au Janus micromotors powered by UV light in water. (b, c) Track lines and velocities of micromotors with various UV intensities over 1 s. Scale bar, 10 μm. Panel (A) is reproduced with permission from ref 42. Copyright 2016, American Chemical Society. (B) (a) Schematic of propulsion of the ZnO-based microrocket in H2O2 solution under UV light. (b) Light intensity dependence of the velocity of ZnO−Pt and PEDOT−Pt microrockets in 6% H2O2. (c) Fuel concentration dependence of the velocity of ZnO−Pt microrockets with UV light on/off. 77 mW/cm2 UV light, 2% SDS. Panel (B) is reproduced with permission from ref 35. Copyright 2017, the Royal Society of Chemistry. C

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Figure 3. (A) (a) Schematic of visible-light-driven BiOI-metal Janus micromotors. (b, c) Track lines of the micromotors under 43 900 and 100 lx light over 30 s, respectively. Scale bar: 5 μm. (B) Average MSD of micromotors under green and blue light (insets are the corresponding veloctities). Panels (A) and (B) are reproduced with permission from ref 34. Copyright 2017, American Chemical Society.

by the normal catalytic reactions between Pt inner layer and H2O2. When the UV light is turned on, the charge separation is generated within the photocatalyst ZnO shell. Such additional charge carriers enhance the catalytic reaction of H2O2 and generate more O2 bubbles. The motors can reach up to 300 μm/s in 6% H2O2 solution under only 4.3 mW/cm2 UV light and 200 μm/s even in 3% H2O2 solution under 77.7 mW/cm2 UV light35 (Figure 2B(b, c)). This case indicates that such photocatalyst-metal design strategy of microrockets can greatly improve their propulsion, meanwhile also efficiently lower both the light intensity and the fuel concentration required for PMNMs compared to those of the plain photocatalyst microrockets. It is clear from Table S1 that most of the recently reported motors designed by the photocatalysts-metal strategies, decrease their light energy requirement to the mW/ cm2 level and still preserve their strong propulsion, Other than these approaches, for single component PMNMs, crystallinity is the most important factor which significantly influence the photocatalytic efficiency, thereby affecting the propulsion of PMNMs. Taking TiO2 as an example, it is well-known that crystalline TiO2 has much stronger photocatalytic efficiency than amorphous TiO2. Accordingly, under similar UV light intensities, amorphous TiO2-based micromotors exhibit no photocatalytic motion in pure water, and they can be only propelled in an H2O2 environment,47 while crystalline TiO2 micromotors can be operated well in pure water.26 Preserving the crystalline structure of photocatalysts and further increasing the surface area (e.g., porous or rough surface) can greatly improve the contact between photocatalyst and the reactant. Theoretically, this strategy can further enhance the photocatalytic properties and generate stronger propulsion of PMNMs. Constructing heterogeneous structures is also an efficient approach which can improve the photocatalytic efficiency. We have done some investigations on Cu2O@N-CNTs composite micromotors with good heterogeneous structures, under the same light intensity, the speed of Cu2O@N-CNTs motors is 3 times higher than that of plain Cu2O in the same fuel environment. The details of this work will be published soon.

propulsion of motors. As a result, such motors can lower the activation UV light intensity to 2.5 mW/cm2 in pure water, such a light intensity is 100 times lower than that of plain TiO2 micromotors propelled by self-diffusiophoresis26 (Figure 2A(b,c)). Various controls from several aspects have confirmed the light-induced self-electrophoresis mechanism, including different metal coatings, the moving direction, ionic environment tests, and electrochemical measurements. All of these results are highly consistent with the self-electrophoretic propulsion of bimetallic nanowire motors. They are also of high speed comparable to common Pt-based chemically induced self-electrophoretic Janus micromotors under a very low UV light intensity (40 mW/cm2). These photocatalytic motors display attractive “stop and go” propulsion behavior and their speed can be precisely controlled by applied light intensity. Similar to TiO2−Au micromotors, recently reported TiO2−Pt micromotors also demonstrate efficient photocatalytic propulsion.43 The development of these TiO2-based Janus motors paves a new way to light-driven MNMs. A number of new photocatalytic TiO2-based motors have been developed by modifying the crystal structures, surface structures and shapes of TiO2, including full visible lightdriven black-TiO2−Au Janus micromotors,44 highly efficient mesoporous TiO2−Au Janus micromotors,45 and plasmonic photocatalytic caps-like double layer TiO2−Au nanomotors.46 These motors are all based on the photocatalyst−metal Janus system and propelled by light-induced self-electrophoresis. Analogously, such a design strategy is also suitable for lowering both of the light intensity and fuel concentration requirement of chemical-fueled PMNMs. TiO2-based PMNMs with bubble propulsion have attracted much attention due to their efficient propulsion behavior. For example, under 200 mW/cm2 UV light exposure, the velocity of amorphous TiO2− Au Janus micromotors is around 30 μm/s in 8 wt % H2O2 solution;47 the speed of plain TiO2 microrockets is 62 μm/s in 15 wt % H2O2 solution.33 However, UV light is harmful to the environment and H2O2 is toxic for the human body. As a result, not only for energy conservation and environment protection purposes but also for extending practical applications of PMNMs, lowering the harmful light intensity, toxic fuel concentration, and improving the utilization efficiency of fuels are key challenges that must be overcome. We have incorporated photocatalysts ZnO and metal Pt into a hybrid ZnO-Pt photocatalytic microrocket which combines both of the photocataytic and chemocatalytic properties35 (Figure 2B(a)). Without UV illumination, the rockets can be propelled

Expansion of the Usable Wavelengths of Light

In order to promote the practicality of PMNMs, converting environmentally friendly light energy (visible light, low energy near infrared (NIR), etc.) and biocompatible fuels energy (water) to PMNMs propulsion is the best option. The activation light wavelengths are determined by the band gaps (Eg) of the photocatalysts which are subjected to the positions D

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methyl orange (MO) solutions are approximately 1.7, 1.5, and 1.4 times faster, respectively, than those observed in pure water under the same UV light intensity (25 μm/s) (Figure 4A(a)).

of CBs and VBs and can be calculated by the UV−vis absorption spectrum. Usually, the visible light-activated (400− 800 nm) photocatalysts have the band gap between 1.5 and 3.1 eV. As a result, we can regulate the activation light wavelength to an environmentally friendly range by choosing or designing photocatalysts with suitable band gaps for PMNMs. In addition, the photocatalytic reaction happens only when the input light energy E is larger than the Eg of the photocatalysts. The relationship can be illustrated by the eq 2. E = hν =

hc > Eg λ

(2)

where c is the speed of light, ν is the frequency of light, h is the Planck constant, and λ is the wavelength in the light absorption range. Accordingly, visible light-driven micromotors have been developed. Bismuth oxyiodide (BiOI) is a common photocatalyst with a 1.7 eV band gap which can be activated by visible light. Therefore, following our previous work about UV light driven TiO2−Au in pure water, we have designed and fabricated BiOI-based micromotors which can be powered by two of the most environment-friendly energies in the world: visible light and water34 (Figure 3A). This is the first demonstration of micromotors activated under a broad visible-light spectrum range (green and blue light) in pure water. The velocities of such motors can be regulated not only by varying light intensities but also by adjusting light wavelength. Such motors exhibit stronger propulsion under blue light than under green light due to their stronger blue light absorption (Figure 3B). Many other photocatalysts-based visible light driven micromotors have been developed, such as Cu2O-based Janus microspherical motors,38 Si-based nanotreelike motors48 and Fe3O4-based nanowire motors.37 According to the different band gaps of photocatalysts, the motor can be activated by different wavelengths. However, the main drawback of such visible-light driven PMNMs is their weak propulsions. Table S1 clearly illustrates that the reported maximum velocity of visible light driven PMNMs is 37.5 μm/s in toxic H2O2 solutions.2 For the pure water-fueled visible-light driven PMNMs, the maximum velocity is only 2.1 μm/s.34 This is in stark contrast to the UV light-driven PMNMs which exhibit maximum velocities of 471 and 25 μm/s in H2O235 and H2O,42 respectively. The results obviously illustrate that the energy converting efficiency of visible light-driven PMNMs is much lower than those of UV light-driven ones. Since high speed is significant for practical applications of PMNMs, developing visible light-driven PMNMs with strong propulsion is still a great challenge.

Figure 4. (A) (a) and (b) are the velocities of the TiO2−Au micromotors in different solutions and at a range of MB dye concentrations, respectively. Scale bar: 50 μm. Panel (A) is reproduced with permission from ref 49. Copyright 2016 Springer Ltd. (B) (a) Speed dependence of Au-WO3@C Janus micromotors upon the DCIP concentration. (b )UV−vis spectra variation of DCIP solution treated by active Au-WO3@C micromotors. Panel (B) is reproduced with permission from ref 36. Copyright 2017, American Chemical Society. (C) (a, b) UV−vis spectra variation of MP and bNPP treated by active TiO2/Au/Mg micromotor, respectively. (c) Spore destruction efficiency by the different treatments. (d) SEM images of the intact spores and ruptured spores after treated by moving TiO2/Au/Mg motors. Scale bar, 1 μm. Panel (C) is reproduced with permission from ref 50. Copyright 2014, American Chemical Society.

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APPLICATIONS As highly reactive oxygen species are generated by photocatalytic reactions, photocatalyts are usually used to purify the contaminated water. PMNMs, a class of active motors, can exhibit continuous photocatalytic propulsion, efficiently promote fluid transport and mixing, and improve the contact surface area between photocatalysts and pollutant, resulting in promoted decontamination efficiency.49 Taking the advantages of both photocatalysts and active motors, PMNMs have shown great potential in environmental applications. TiO2−Au micromotors exhibit light-induced dye-enhanced motion through self-electrophoretic effects in dye solutions under UV irradiation.49 The velocities of the motors in 10−5 g/L methyl blue (MB), 10−4 g/L cresol red (CR), and 10−4 g/L

Obviously, their velocities strongly depend on the dye concentration. Therefore, the dye concentration can be detected through measuring the speed of motors49 (Figure 4A(b)). In addition, such active motors can also more rapidly degrade the dyes (MB, CR and MO) when compared with static motors. This is the first example of PMNMs which can E

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Accounts of Chemical Research make “waste” (dyes) profitable, and this work provides a great importance for enhancing the efficiency of light-driven Janus micromotors based on photocatalytic reactions and developing green technology for environmental remediation. Based on the same consideration, the photocatalytic Janus micromotor WO3−Au also exhibits attractive potential in pollutant (sodium-2,6-dichloroindophenol, DCIP) detection (Figure 4B(a)) and degradation (Figure 4B(b)) under UV light.36 In addition, taking advantage of the photocatalytic reactions, the efficient degradation of the biological and chemical warfare agents by MNMs have also been demonstrated.50 The photocatalysts-based TiO2/Au/Mg micromotors can rapidly complete in situ mineralization of highly persistent organophosphate nerve agents into harmless products; the selfpropelled micromotors display ∼7-fold and 6-fold faster degradation of methyl paraoxon (Figure 4C(a)) and bis(4nitrophenyl)phosphate (Figure 4C(b)), respectively, compared to the static micromotors. This clearly indicates that the new self-propelled photocatalytic platform offers greatly improved destruction of the nerve agents. Furthermore, they can also efficiently destroy cell membranes of the anthrax simulant Bacillus globigii spore and kill them (Figure 4C(c, d)). Since PMNMs are also a kind of classic catalytic MNMs, other applications as normal catalytic MNMs, including efficient cargos transportation abilities,51 micropumps functionare26 and drug loading and delivery capacity,45 are also well demonstrated.

No NIR activated PMNMs has been reported so far. Although NIR photocatalysts do exist, such as WS2,53 Cu2(OH)PO4,54 β-NaYF4: Yb3+, Tm3+@ TiO2 composites,55 and so forth. Owing to the lack of species and low photocatalytic efficiency, there are still challenges to fabricate PMNMs via these materials. As a result, exploring efficient photocatalytic MNNs activated by fully green environment is still on the way. Fuel is the other key aspect of the motors’ operating environment. Currently, the fuels for PMNMs mainly are water and H2O2. Although water is the ideal fuel, water-fueled motors usually cannot achieve good propulsion; H2O2-fueled motors have strong propulsion, but their toxicity is a disadvantage. As a result, either enhancing the propulsion in water environment, or exploring novel and biocompatible fuels for PMNMs with strong propulsion will be the significant concerns for future investigation works on fuels. In addition, the fuel options are extremely limited. It is clear from Table S1 that there are few kinds of fuels developed so far; other than water, only the toxic or harmful fuels H2O2 and organic dyes were used. As a result, more biocompatible or harmless fuels should be developed. The best candidates are nutrients necessary in the human body, such as glucose, proteins, and vitamins. We have already done studies on glucose-fuelled PMNMs, and the details will soon be published. Functionalized MNMs have made great progress in recent years. A number of applications have been demonstrated with MNMs ranging from cargo delivery to environmental remediation and bio sensing; many examples have been reviewed in detail by Katuri et al.56 Drawing from the advantages of MNMs, such as tiny scale and active motion, and combining them with the unique properties of PMNMs, like photocatalytic performance, PMNMs have already shown great potential in cargo delivery,51 environment remediation,57 and drug loading and delivery.45 However, most of the applications of PMNMs are focused on the environmental field. This situation can be attributed to the lack of functionalization on PMNMs. As a kind of MNM, the controllable active motion of PMNMs is the most impressive feature compared with normal micronano particles. Current applications of PMNMs are mostly based on their inherent photocatalytic properties. Therefore, extending the applications of PMNMs by modifying motors with organic functional groups or combining different materials is of great importance and potential. The ideal PMNMs should be biocompatible, highly efficient, and multifunctional. In addition, they should be operated in a friendly environment, no matter the light or the fuel. Therefore, further efforts for the next few years should be focused on three key points: first, efficiency and functionalization of biocompatible motors will be the most critical issue; second, an environmentally friendly activation light source will enable substantial advances for practical applications; finally, biocompatible fuels will be the best options for any situations. The ultimate challenge for PMNMs is to operate such tiny devices efficiently in fully green real world environments.

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OUTLOOK AND CONCLUSIONS PMNMs, owing to their outstanding photocatalytic properties and light controllable photocatalytic propulsion, have shown great potential in future practical applications. However, there are still many challenges that hinder the wide applications of PMNMs. Photocatalysts are playing key roles in the PMNMs. Currently, on one hand, most of PMNMs are based on metal oxide photocatalysts, and their effective absorption spectrum range is not broad enough to cover all the visible light and/or NIR range. On the other hand, the materials for constructing PMNMs are not biocompatible, which limit their applications in biological fields. Actually, organic photocatalysts have recently attracted much attention due to their structural diversity, variety of photochemical mechanisms, and tunable optical-electronic properties.52 Therefore, organic photocatalysts can be one of the most promising candidates for the next generation of PMNMs. Still, how we can further improve the photocatalytic efficiency is always the key issue for enhancing propulsion of PMNMs. Therefore, designing excellent photocatalysts and constructing highly efficient photocatalysts-based MNMs are still a great challenge. In practical applications, environmentally friendly and biocompatible energy input are important factors that need to be considered. The energy aspect includes two parts: light and fuels. For the light, visible light and low power NIR are obviously the ideal light resources for PMNMs. Although many visible-light driven motors have been reported, as listed in Table S1, it is obvious that most of the visible light-driven PMNMs are fueled by toxic H2O2. However, those that are fueled only by water have extremely weak propulsion. For NIR driven MNMs, they are usually based on photoinduced thermophoresis and need very high light energy, which will cause dramatic temperature increases in the target local area and could cause damage to the surroundings to some degree.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.accounts.8b00249. Summary of the classic photocatalytic micro/nanomotors (PDF) F

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Biye Ren: 0000-0003-0131-8750 Funding

The project receives financial support from the National Nature Science Foundation of China (21805096, 21674039, 21471061, and 21671071); Natural Science Foundation of Guangdong Province (2018A030313358, 2017A030310432); Applied Science and Technology Planning Project of Guangdong Province (2015B010135009, 2017B090917002). Notes

The authors declare no competing financial interest. Biographies Renfeng Dong received his Ph.D. in 2016 from South China University of Technology and is now an associate professor at this University. His research interests cover synthetic micro/nanomachines, photocatalytic materials, and functional polymers. Yuepeng Cai received his Ph.D. in 2002 from Sun Yat-Sen University and continued his postdoctoral work at Chinese University Hong Kong. He is currently a professor of Chemistry at South China Normal University. His interests are functional micro/nanomaterials, energy storage and conversion materials, and functional crystalline porous materials. Yiran Yang received her B.S. degree in Bioengineering from Rice University in 2017. She is currently pursuing her Ph.D. degree in Medical Engineering at Caltech. Her research interests include wearable electronics, biosensors, and nanorobotics. Wei Gao received his Ph.D. degree in 2014 from University of California, San Diego, and performed his postdoctoral work at University of California, Berkeley between 2014 and 2017. He is currently an assistant professor of medical engineering at California Institute of Technology. His research focuses on wearable biosensors and synthetic micro/nanomachines. Biye Ren received his Ph.D. in Materials Science in 1999 from South China University of Technology and became a professor of materials at this university in 2006. His research focuses on the artificial micro/ nanoscale systems, macromolecular self-assembly, and polymer materials.

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DOI: 10.1021/acs.accounts.8b00249 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.accounts.8b00249 Acc. Chem. Res. XXXX, XXX, XXX−XXX