Recent advances of light-driven micro/nanomotors: toward powerful thrust and precise control

3.1.1 Photothermal effect propulsion

Upon NIR irradiation on the face of micro/nanomotors, the electromagnetic energy absorbed is dissipated as heat and then forms a thermal gradient around the surrounding liquid [82]. This increased temperature gradient results in the motion of the self-thermophoresis of photothermal materials, and therefore, the motion of motors can be controlled by the NIR laser illumination intensity. Higher light intensity leads to a higher temperature gradient and subsequently results in a higher actuation force of the micro/nanomotors. The thermophoretic force can be described as follows [59]:

where d_p is the particle diameter, η is the viscosity of fluid, k_α is the thermal conductivity of fluid, k_p is the particle thermal conductivity, r is the fluid density and t is time. It can be found that the thermophoretic force is concerned with the real-time temperature gradient, the particle diameter and the fluid mechanical properties.

Early in 2014, Wu and Lin have found an approach to modulating the on-demand motion of catalytic polymer-based microengines via NIR laser irradiation [55]. As obtained through the calculation according to the heat diffusion equation, the temperature shows a gradient distribution around the micromotor. While the half-metal coated colloidal particles were under NIR light irradiation, the metal side would transform the light energy to heat and lead to a sharp decline of temperature distribution at the surface of motor. The motion speed of the NIR light that actuated the Janus motor can reach up to 10 μm/s (as shown in Figure 3A). Just several years later, some researches figured out that the microsphere structure based on NIR irradiation can have a higher driving force, the maximum velocity of which goes up to 950 bodylength/s [60], far higher than the other motors. As shown in Figure 3B and C, the microsphere is half coated with Au, converting the absorbed energy to heat, and therefore, the hot Au nanoshells act as a heating source and generate local thermal gradients across the micromotors. Closer to the center, the temperature gets higher and declines more rapidly, and then the motor moves toward the opposite direction of the declining temperature at a speed as high as 42 μm/s.

Figure 3:

Micro/nanomotors with photothermal-effect propulsion. (A) Theoretical photos of the photothermal effect and the motion speed with respect to the NIR laser power of Au-silica Janus particles [82]. Copyright 2010, American Physical Society. (B) The measured temperature changes with respect to time and the calculated temperature distribution of a type of Janus mesoporous silica nanoparticle motor [60]. Copyright 2016, American Chemical Society. (C) Simulated temperature profile, working mechanism schematic and motion behavior characterization result of photothermal effect propelled Janus micromotor under NIR irradiation [58]. Copyright 2016, Springer.

In summary, the photothermal-effect propulsion mechanism for micro/nanomotors makes use of the temperature gradient under the laser irradiation to stimulate the motion of motors in liquid surroundings. Advantages of this actuation mechanism include the ease in moving in a high speed under a strong laser irradiation, as the reaction is vigorous. However, this kind of motors may be fatal to biological samples because of the local high temperature and the direction of motion cannot be controlled very precisely as the control of the temperature field distribution is a great challenge.

3.1.2 Bubble-induced propulsion

Bubble propulsion, as the name suggests, propels the motor by generating bubbles through chemical reactions [83], [84]. As bubbles are released from the surface of motors, it results in a driving force away from the surface of motors. The driving force is assumed to be balanced with the viscous drag force if the vertical forces are ignored, and the viscous drag force can be evaluated by employing the following equation [85]:

where μ is the liquid dynamic viscosity, r means the equivalent radius of the micro/nanomotors and V represents the stable motion speed of micro/nanomotors. Thus, the motion of micro/nanomotors based on the bubble propulsion is directed away from the side of the bubble generator.

In early studies, Oliver G. Schmidt and his colleagues engineered a rolled-up microtube with an inner catalytic surface, serving both as the chemical reaction chamber and as the gas-collecting cavity, from which the bubbles were then released that propelled the microtubular to move in an opposite direction [86]. In order to gain a higher driving force and velocity, it is necessary to increase the concentration of the chemicals involved in the reaction. However, it is well known that with the increased concentration of chemicals, the toxicity is expected to be higher. Another issue bothering the bubble propulsion mechanism is that the motion is very difficult to control because of the random bubble release.

Li and Mou et al. put up a new kind of light-controlled bubble propulsion of amorphous TiO₂/Au Janus micromotor, in which the UV light energy was transformed into chemical energy and subsequently into mechanical energy for micromotor fast motion in water [64]. The power conversion efficiency (η_c) of such light-driven micromotor is defined as follows [87]:

where 6πμrV is the drag force defined in Equation 2, I is the intensity of the light irradiation, N is the O₂ evolution rate from the micromotor in unit of mol/s and ΔG^θ is the Gibbs free energy of H₂O₂ decomposition reaction (−260 kJ per mole of O₂ produced). With the light powering the H₂O₂ decomposition catalyzed with Pt, the concentration of H₂O₂ can be much lowered to generate O₂ bubbles. According to the calculation, the power conversion efficiency is about 10 times higher than that of the catalytic bubble propelled micromotors based on pure H₂O₂ decomposition on Pt [87]. Besides, the state and speed of motion can be reversibly, remotely and wirelessly controlled by regulating the “on/off” switch and the intensity of light irradiation. In Figure 4, in H₂O₂ solution, Pt [88] and amorphous TiO₂ [64], [65] produce peroxide complexes on the surface of the microstructures under white light or UV illumination. The separated electrons are transferred to the peroxide complexes, which leave behind the accumulated holes. This effect enhances the transportation of the electrons and holes in the decomposition of H₂O₂ to generate oxygen bubbles and speed up the reaction significantly.

Figure 4:

Light-driven micro/nanomotors with bubble-induced propulsion. (A) Ti/Cr/Pt micromotor with propulsion force originated from O₂ bubbles. Its motion can be controlled by white light intensity [88]. Copyright 2011, Wiley-VCH. (B) Schematic of the TiO₂/Au micromotor working with oxygen bubbles, which were generated under UV irradiation [65]. Copyright 2016, The Royal Society of Chemistry. (C) Snapshots of the light-controlled motion of TiO₂ microtube micromotor in H₂O₂ solution [64]. Copyright 2015, Wiley-VCH.

Bubble propulsion can produce a high driving force, and the velocity is found to be higher than photothermal-effect propulsion. However, there are still some questions that need to be taken into consideration, such as the environment-unfriendly fuels, lack of precise motion direction control and undesired bubble generation.

3.1.3 Self-electrophoretic propulsion

Some chemical reactions induced by light irradiation rely on the properties of specific materials such as metals and semiconductors. In order to modify the chemical bonds by light, its energy content has to account for the bond energy [89], [90]. Micro/nanomotors designed with a bulk-heterojunction structure will generate ions through photocatalytic reactions in some salt solutions with incident light. Self-electrophoresis refers to the actuation of charged micro/nanomotors through local electric field around them. The local electric field is the resultant of asymmetric distribution of charged ions, which are generated from the photocatalytic reactions on the surface of the micro/nanomotors, to be specific, on the surface of the different kinds of materials. As the electric field is generated under light irradiation, the charged micro/nanomotors are driven to move in response to the field [91], [92].

There has been some representative work on self-electrophoretic propelled micro/nanomotors reported in the past decade. Figure 5A displays the schematic diagram of a nanomotor with the form of a core-shell silicon nanowire, i.e. the inside p-type Si covered with a thin layer of n⁺–Si. Under NIR or visible irradiation, the core-shell silicon nanomotor works as a solar cell, generates photovoltage along the nanomotor in H₂O₂ or Q/QH₂ solution and then forms an asymmetric distributed positive and negative charges on the p-type and n-type parts, respectively [93]. Another Janus motor made by TiO₂ and SiO₂ is illustrated in Figure 5B. e⁻ is transferred from the valence band to the conduction band inside the motor, the generated electron-hole pairs will help the photocatalytic decomposition of H₂O₂ on the surface of motor, and the concentration of H⁺ differs with each other on the TiO₂ side and SiO₂ side, leading to the self-electrophoretic propulsion toward the TiO₂ side under UV light irradiation [73]. In Figure 5C, at the end of Si, oxidation reaction occurs between Si and SiO₂, resulting in the generation of protons. Thus, the resulting proton gradient forms an electric field, which provides a driving force on particles [75]. While in Figure 5D, the electron-hole pairs are generated in the iron oxide and separated at the iron oxide/solution interface when the motor is under Vis light irradiation. Electrons are transferred from the n-type iron oxide to the gold side, and holes are driven to the iron oxide side through band bending. Therefore, the decomposition of H₂O₂ will be catalyzed at the gold and iron oxide sides, respectively, leading to a proton concentration gradient around the opposite side of the motor [76].

Figure 5:

Micro/nanomotors with self-electrophoretic propulsion. (A) Working principle schematic of n⁺-Si/p-Si micromotor decorated with Pt nanoparticles [93]. Copyright 2017, Wiley-VCH. (B) Schematic of the working mechanism of SiO₂/TiO₂ micromotor in H₂O₂ solution [73]. Copyright 2017, Wiley-VCH. (C) Motion snapshots and the propulsion mechanism of Si/Au micromotor [75]. Copyright 2017, The Royal Society of Chemistry. (D) Schematic of the Fe₂O₃/Au micromotor working mechanism driven by Vis light in H₂O₂ solution [76]. Copyright 2017, The Royal Society of Chemistry.

To sum up, this kind of micro/nanomotors can be activated by UV or Vis light at a very low concentration of chemical fuel, the speed of which can be modulated by the light intensity easily, and the whole process is smoother than the other driving mechanisms introduced above. However, the driving force cannot reach to a high value, so the velocity is lower compared with the other mechanisms.

3.1.4 Osmotic propulsion

Just like the self-electrophoretic propulsion mentioned above, osmotic propulsion can also generate some “substances” through the chemical reaction under light irradiation, but this kind of substance is neutral, which can form a zone with high product concentration. Therefore, fluid flows from one side to another, resulting in the self-diffusiophoresis of motors [42], [94].

According to the study by C. Chen, the motion velocity (U) of the micro/nanomotors driven by the osmotic propulsion can be described as follows [78]:

in which k is the Boltzmann constant, T represents the solution temperature, η is the viscosity of the liquid, K represents the Gibbs absorption strength, L is the motor length and ∇C is the molecule concentration gradient. In common, the diffusivity of the neutral substance is very slow because the magnitude of ∇C is much lower than the other gradient, so the velocity cannot reach much higher.

Taking the WO₃/Au Janus micromotor as an example in Figure 6A [43], UV light irradiation can activate photochemical reactions around the micromotor and lead to the formation of different species, such as ·OH and O₂⁻, and a molecule concentration gradient. As a consequence, the micromotor can be propelled based on the self-osmotic mechanism. O₂ molecules can be generated through the decomposition of H₂O₂ around TiO₂ micromotors [95] and dissolve into water. The O₂ concentration gradient will actuate the micromotor movement (Figure 6B) [78]. The self-assembly effect of platinum nanoparticle decorated graphite-like carbon nitride nanomotors was revealed to be the result of osmosis under light irradiation, as illustrated in Figure 6C [96], and in Figure 6D, by catalytically decomposing N₂H₄, gas molecules are generated near the Ir/SiO₂ micromotor and form a concentration gradient, which drives the motion of micromotor through osmosis effect [79].

Figure 6:

Light-driven micro/nanomotors with osmotic propulsion. (A) A novel light-driven Au-WO₃ Janus micromotor based on colloidal carbon WO₃ nanoparticle composite spheres, which displays efficient propulsion in pure water activated by UV light [43]. Copyright 2017, American Chemical Society. (B) Phototaxis of isotropic TiO₂ micromotor due to oxygen molecules’ gradient as a result of asymmetrical chemical reaction under UV irradiation [78]. Copyright 2016, Wiley-VCH. (C) Schematic illustrating the self-assembly effect of nanomotors due to osmosis under light irradiation [80]. Copyright 2017, The Royal Society of Chemistry. (D) Schematic of the Ir/SiO₂ Janus micromotors propelled by osmosis due to N₂, H₂ and NH₃ molecules’ concentration gradient generated around the micromotors [79]. Copyright 2014, American Chemical Society.

Comparing with the other three kinds of actuation mechanisms for light-driven micro/nanomotors, osmotic propulsion can be controlled more easily under light irradiation because of the stable osmotic gradient and is friendlier to the environment. However, its notable weakness is that the actuation force of this type of motor is smaller than the others, leading to a considerably humble velocity.

3.2.1 Microsphere structures

Microspheres are small spherical microparticles, with diameters typically ranging from 0.1 to 100 μm. With the streamlined structure, microspheres can reduce the viscous resistance effectively while moving in the fluid. The driving force can be balanced by the drag force approximately calculated in Equation (2) [85].

Microspheres can be applied into all kinds of situations including those four actuation mechanisms introduced above because of their wide applicability and are always made of some metal or metal oxide materials. For example, in Figure 7A, by depositing Pt thin film on Au microspheres, a type of conductive Au/Pt Janus nanomotors is synthesized [103]. Such bimetallic Au/Pt micromotors can move fast in H₂O₂ solution and repair cracks on conductive thin film autonomously. Both gold and platinum are relatively expensive as they are notable metals. Besides, some cheap semiconductors materials, such as SiO₂ and TiO₂, have been employed to fabricate the microspheres as well. Figure 7B presents a scanning electron microscopy (SEM) image of TiO₂ microsphere half coated with Au as well as its energy-dispersive X-ray spectroscopy images for Ti, O, Au. Because of the self-electrophoresis actuation mechanism, the motion speed of the TiO₂/Au micromotor is dependent on not only the light intensity but also the thickness of Au [71], and as displayed in Figure 7C, the Au/WO₃@C micromotors, shaped in microsphere, have an average diameter of 1 μm. As aforementioned, the as-prepared WO₃@C microspheres have the WO₃ nanoparticles, clearly indicated from SEM characterization, decorated on the surface of carbon microspheres through solution process [43].

Figure 7:

Structure of the microspheres employed in light-driven micro/nanomotors. (A) Schematic of the microsphere Au/Pt micromotor [103]. Copyright 2015, American Chemical Society. (B) SEM image and energy dispersive X-ray images for Ti, Au and O elements for the Au/TiO₂ Janus micromotor. The scale bar is 0.5 μm [71]. Copyright 2015, American Chemical Society. (C) SEM characterization results for Au/WO₃@C microsphere motors, whose average diameter is evaluated to be about 1 μm [43]. Copyright 2017, American Chemical Society.

Most of the microorganisms on Earth evolved into microsphere shape to adapt to the environment; therefore, the artificial microsphere motors are expected to have a better performance than other structures. The maximum speed of microspheres reaches up to 50 bodylength/s, much faster than the other structures because of the simple and easy-activated model. However, as demonstrated in Figure 7C, the fabricated micro/nanosphere structure is also faced with the issue of weak uniformity, which is critical for large scale application of micro/nanomotors.

3.2.2 Nanorod structures

Nanorods are often synthesized from metals or semiconducting materials and have an aspect ratio (length divided by width) of 3–5 [104]. A combination of ligands acts as shape control agents and bond to different facets of the nanorod with different strengths, and this allows different faces of the nanorod to grow at different rates, producing an elongated object [105]. In most reported work, nanorods are synthesized with templates, which include anodic alumina [76] and some organometallic polymer materials [75].

The motion of nanorods usually makes use of the self-electrophoretic propulsion. As the two opposite sides of the nanorod are made of different photolytic materials, oxidation reaction and reduction reaction occur separately, and charges travel from one side to another, causing a self-generated electric field to propel the micro/nanomotors. For example, Figure 8A presents both the SEM and transmission electron microscopy images of gold nanorods [57]. Under UV-Vis irradiation, the light-driven photo-electrochemical reaction generates anions and cations at opposite ends of the nanorod, resulting in an asymmetric distribution of the generated ions, which propels the nanorod by self-electrophoresis. To control the motion of nanorods better, Wang and his group have investigated the dependence of motion on the nanorod structure and revealed that the motion behavior changes with the end morphology of the nanorod (Figures 8B and 11B) [93]. Figure 8C shows the SEM image of a heterogeneous Au/Fe₂O₃ nanorod micromotor, whose motion speed and direction can be tuned by light intensity and external magnetic field, respectively [76].

Figure 8:

Various nanorod structures of light-driven micro/nanomotors. (A) SEM and transmission electron microscopy characterizations results of gold nanorods used in light-driven micro/nanomotors [57]. Copyright 2015, Elsevier. (B) Motion behaviors change with the end structure of Si/Au micromotors [93]. Copyright 2017, Wiley-VCH. (C) SEM characterization of gold/iron oxide nanorods used in self-electrophoretic micromotor [76]. Copyright 2017, The Royal Society of Chemistry.

Figure 9:

Various nanotubular structures of light-driven micro/nanomotors. (A) Preparation procedures of ZnO/Pt microrockets using polycarbonate membrane templates and the SEM image of ZnO/Pt microrocket [107]. Copyright 2017, The Royal Society of Chemistry. (B) SEM image of TiO₂ micromotor with nanotubular structure [64]. Copyright 2015, Wiley-VCH. (C) Scheme of a single conical nanotube and a microarray of TiO₂ nanotubes; oxygen bubbles are released from the larger side of the tubular [61]. Copyright 2016, Wiley-VCH.

Figure 10:

Various asymmetric branches structure of light-driven micro/nanomotors. (A) Schematic of the Janus Si/TiO₂/Pt micromotor with TiO₂ branches [70]. Copyright 2016, Nature Publishing Group. (B) Improved Si(trunk)/TiO₂(branch) asymmetric branches structure with the branches modified with dyes to enhance the photon absorption efficiency [108]. Copyright 2017, Nature Publishing Group.

Figure 11:

Wavelength and frequency of light [109].

3.2.3 Nanotubular structures

Different from the nanorod, nanotubular structures mostly utilize bubble-induced propulsion to promote the motion by taking advantage of the hollow structure [106]. Chemical reactions occur on the inner surface of the tubular and thus generate gas bubbles, which would be released from the tubular after growing into larger bubbles. Therefore, the size of both the ends is often designed a little different, helping bubbles to be released from the wider side. The recoil force during continuous expulsion of bubbles will propel the tubular to move at a high speed far away from the end bubbles releasing.

A type of Pt/ZnO micromotor with nanotubular structure is presented in Figure 9A [107]. As shown in Figure 9B, TiO₂ acts as the catalyst to help the decomposition of H₂O₂ to generate gas bubbles [64]. Because of the different width inside the tubular, bubbles will grow larger and be released from the wider side, which ensures the moving direction of nanotubulars. Figure 9C displays another type of micromotors based on TiO₂ nanotubes. Under UV light irradiation, their motion speed can get high, but the directions are difficult to control in 15% H₂O₂ solution [61].

3.2.4 Asymmetric branches’ structures

Asymmetric branches are always consisting of different materials in the opposite sides; under illumination, the light-driven photoelectrochemical reaction generates anions and cations at opposite ends, resulting in an asymmetric distribution of generated ions, which propels the micro/nanomotors by self-electrophoresis. Different from the nanorod, the structure of asymmetric branches can have many little branches at one end, which will benefit the chemistry reaction and generate a higher driving force.

Tang and his group created a schematic illustration of a microswimmer based on a Janus nanotree structure, in which TiO₂ nanowire branches are grown on a p-type silicon nanowire trunk, and in such nanotrees, the TiO₂ branches and the silicon nanowires work as the photoanode and the photocathode, respectively [70].

The transformation mechanisms are generally based on some typically reversible reactions including pericyclic reactions, cis-trans isomerization, dissociation processes, intramolecular hydrogen transfers and electron transfers (oxidation-reduction), which cause the alteration of physical and chemical properties involving wettability, surface free energy and liquid crystal alignment. In Figure 10, typical branched micromotor (Figure 10A) and its upgraded version (Figure 10B) are presented [70], [108]. With the shading effect of the large branched TiO₂ structures, asymmetric charge concentration will be formed around the branched structure and lead to a positive or negative phototaxis motion behavior of the micromotor.

In conclusion, compared with the single nanorod, asymmetric branches have a higher driving force in common because of a huge reaction area on the surface. In contrast, higher requirements are in need to produce such a complicated structure.

3.3.1 NIR light

Unlike lower-frequency radio and microwave radiation, NIR electromagnetic radiation commonly interacts with dipoles present in single molecules, which changes as atoms vibrate at the ends of a single chemical bond. It is consequently absorbed by a wide range of substances, causing them to increase in temperature as the vibrations dissipate as heat, and the same process, if run in reverse, causes bulk substances to radiate in the infrared spontaneously [110], [111], [112], [113].

Thus, NIR is always used in photothermal-effect propulsion because of the heat generated during the reaction. Heat can not only propel micro/nanomotors to move (Figure 12A) but also provide a possibility of inactivation of cells. As we all know, most of cells cannot be alive under high temperature, so diseased cells can be killed through NIR-driven micro/nanomotors. Wu and his colleagues have put up with such a micromotor, to better observe the photothermal treatment effects [55], [59] (Figure 12B, C), with the cells labeled using propidium iodide (PI, red fluorescence labeled dead cells) after laser irradiation [58]. After being exposed to a NIR laser with enough dose, cells in contact with the micromotors appeared to be killed and undergo apoptosis, and the cytoplasm was stained red, while cells away from the motors still retained their activity and were unstained.

Figure 12:

Micro/nanomotors actuated with NIR light. (A) Schematic of the NIR-driven Janus mesoporous silica nanoparticle motors with the size of hundreds nanometers [60]. Copyright 2016, American Chemical Society. (B) Schematic and snapshots of the NIR-controlled “on/off” motion for the (PSS/PAH)₂₀ AuNS micromotor [59]. Copyright 2015, Wiley-VCH. (C) Multilayer polymer micromotor motion controlled by the NIR laser [55]. Copyright 2014, American Chemical Society.

3.3.2 Visible light

As frequency increases into the Vis range, photons have enough energy to break the bond of some individual molecules according to the Planck-Einstein equation:

where h is the Planck constant, c is the light velocity and λ is the wavelength of light. As the wavelength of light decreases to a threshold value, electrons will be excited and then jump to the conduction bands; meanwhile, holes will be left behind in the valence bands [114], [115], [116]. Photo-excited free electrons and holes will take part into the redox reactions, generate charged ions around the motors and subsequently form an electric field, which will then drive the charged motors to move.

The most common photocatalytic materials are semiconductors and metal oxide with specific band gaps (E_g) that determine their light response regions, different materials have their own E_g and suitable wavelength. Table 1 presents some common materials with their E_g and suitable activation light wavelength. Materials with E_g lower than the photon energy of Vis light are expected to absorb Vis light and drive micro/nanomotors. As an example, Figure 13A presents the BiOI/Au micromotors, which are powered by blue and even green light [72].

Figure 13:

Light-driven micro/nanomotors actuated with UV and Vis light. (A) BiOI/Au micromotor actuated by Vis light. The “On/Off” control and the motion mean-square displacement for BiOI/Au micromotor are shown [72]. Copyright 2017, American Chemical Society. (B) Schematic illustrating the work mechanism of TiO₂/Au micromotor actuated by UV light [71]. Copyright 2015, American Chemical Society. (C) Visible light-driven black-TiO₂/Au micromotor. Mean-square displacement and schematic of the work mechanism of the micromotor [117]. Copyright 2017, American Chemical Society.

3.3.3 Ultraviolet

As frequency increases into the UV, photons now carry enough energy (about 3 eV or higher) to excite certain doubly bonded molecules into permanent chemical rearrangement. So, compared with Vis light, it may provide more energy to generate photo-excited electrons and thus form a stronger electric field.

In general, according to Table 2, there are more materials response to UV than Vis. Some researches shows that as for some photocatalytic materials such as AgCl, only under UV or very strong Vis can the motor be propelled, which presents a better applicability of UV light [118]. Also, as indicated in Figure 13B, such motor based on TiO₂/Au responds to the UV light with extremely low light intensity in pure water [71]. Furthermore, TiO₂/Au micromotor was demonstrated to respond to Vis light with TiO₂ replaced by black TiO₂, as shown in Figure 13C [117].

Table 2:

Comparison of motion velocity and estimated driving force for light-driven micro/nanomotors through bubble propulsion.

Materials	Actuation mechanism	Structure	Scale	Light	Velocity [μm/s]	Velocity [dbl/s]	Driving force [pN]	Appearance	Reference
ZVI/Pt	Bubble propulsion	Microsphere	Micro	UV-Vis	200	67	11.3		[68]
Ag-ZIF	Bubble propulsion	Microsphere	Micro	UV	310	3.4	263		[67]
TiO2/Pt	Bubble propulsion	Nanotubular	Submicro	UV-vis	35	233	0.66		[66]
AuNP/PANI/Pt	Bubble propulsion	Nanotubular	Micro	UV-Vis	88.7	4	16.7		[63]
Am-TiO2/Au	Bubble propulsion	Microsphere	Micro	UV	101	5	19		[65]
TiO2	Bubble propulsion	Nanotubular	Micro	UV	5.39	1.8	0.20		[61]
Carbon/Pt	Bubble propulsion	Microsphere	Micro	UV-Vis	500	50	471		[95]
TiO2	Bubble propulsion	Nanotubular	Micro	UV	324	4	244		[64]
TiO2/Au/Mg	Bubble propulsion	Microsphere	Micro	UV	80	5	12.1		[62]