Dye-Enhanced Self-Electrophoretic Propulsion of Light-Driven TiO2–Au Janus Micromotors
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Light-driven synthetic micro-/nanomotors have attracted considerable attention in recent years due to their unique performances and potential applications. We herein demonstrate the dye-enhanced self-electrophoretic propulsion of light-driven TiO2–Au Janus micromotors in aqueous dye solutions. Compared to the velocities of these micromotors in pure water, 1.7, 1.5, and 1.4 times accelerated motions were observed for them in aqueous solutions of methyl blue (10−5 g L−1), cresol red (10−4 g L−1), and methyl orange (10−4 g L−1), respectively. We determined that the micromotor speed changes depending on the type of dyes, due to variations in their photodegradation rates. In addition, following the deposition of a paramagnetic Ni layer between the Au and TiO2 layers, the micromotor can be precisely navigated under an external magnetic field. Such magnetic micromotors not only facilitate the recycling of micromotors, but also allow reusability in the context of dye detection and degradation. In general, such photocatalytic micro-/nanomotors provide considerable potential for the rapid detection and “on-the-fly” degradation of dye pollutants in aqueous environments.
KeywordsTiO2–Au Janus micromotor Self-electrophoresis Light-driven Motion control Dye pollution Environmental remediation
TiO2–Au Janus micromotors can obtain energy from photocatalytic degradation of dyes in aqueous solution and exhibit light-induced dye-enhanced motion through self-electrophoretic effects without additional reagents.
Micromotors are faster in aqueous dye solutions than in pure water under the same UV light intensity.
The prepared micromotors are easily synthesized and exhibit excellent reusability in the degradation and detection of dye pollutants.
Nano-/micromotors have received growing attention in recent years because of their potential for application in biomedical and environmental fields [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14], such as in drug delivery and release [15, 16, 17, 18], isolation of biological targets , DNA hybridization , bacterial detection [21, 22], ion sensing , and water treatment [24, 25, 26, 27, 28, 29]. Developments in nanotechnology can pave new routes to future practical applications of nano-/micromotors in microscale environments. Among the various nano-/micromotors reported to date, the light-driven micro-/nanomotor based on photocatalytic reactions is particularly attractive due to its unique characteristics, including its remote control, cycling stop-and-go motion, and simple speed control through variation in the light intensity [30, 31, 32, 33]. In terms of potential materials for such applications, TiO2 is a common, low-cost, and highly efficient photocatalyst capable of decomposing a wide variety of organic and inorganic compounds in both liquid and gaseous states under UV irradiation [34, 35, 36, 37, 38, 39]. For example, light-driven micromotors based on TiO2 that have been reported to date include plain TiO2 particles, TiO2 rockets, and TiO2-based Janus micromotors. These micromotors are particularly unique in that they can be wirelessly controlled by light, in addition to being able to degrade organic pollutants because of the high photocatalytic activity of TiO2. For example, Guan et al. reported that Pt-TiO2 Janus micromotors can effectively degrade rhodamine B in water . However, it is currently unclear whether the speed of TiO2-based micromotors can be influenced by the different photocatalytic activities of TiO2 toward various organic pollutants. In other words, we wish to find out whether the photocatalytic degradation of organic pollutants enhances the self-electrophoretic propulsion of TiO2-based Janus micromotors.
3.1 Preparation of the TiO2–Au Janus Micromotors
TiO2 microspheres were prepared using a previously reported solvent extraction/evaporation method . Firstly, titanium butoxide (1.0 mL, Sigma #244112) was dissolved in ethanol (40 mL), and the resulting solution stirred for 3 min. After this time, the solution was incubated at room temperature for 3 h. The resulting TiO2 microspheres were then collected by centrifugation at 8000 rpm for 5 min and washed three times with ethanol (Guangzhou Chemical Reagent Co.) and ultrapure water (18.2 MΩ cm), prior to drying in air at room temperature. The TiO2 microspheres were then annealed at 400 °C for 2 h to obtain anatase TiO2 microspheres (1.0 μm mean diameter), which were then employed as base particles for the TiO2–Au light-driven Janus micromotors. After dispersion of the TiO2 particles (1.0 mg) in ethanol (2.0 mL), the resulting suspension was dropped onto glass slides and dried uniformly at ambient temperature to give particle monolayers. These particles were then partially covered with a thin gold layer by 3 cycles of 60 s ion sputtering (Q150T Turbo-pumped ES sputter coater/carbon coater, Quorum). The resulting metal layer thickness is 40 nm, as measured by a Dektak 150 Surface Profiler (Veeco). Finally, the desired TiO2–Au Janus micromotors were obtained following sonication of the glass slide in deionized water for 5 s.
3.2 Preparation of the Au–Ni–TiO2 Janus Micromotors
For the Au–Ni–TiO2 magnetic Janus micromotors, the TiO2 particle monolayers were prepared according to the above method. The particles were then sputter-coated with a thin nickel layer over 60 s using a Q150T Turbo-pumped ES sputter coater/carbon coater (Quorum). The nickel layer thickness is 10 nm, as measured by a Dektak 150 Surface Profiler (Veeco). The particles were subsequently sputter-coated with a layer of gold (40 nm) over 3 cycles of 60 s.
3.3 Characterization of the TiO2/Au Micromotors
Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were carried out using an EVO 18 Field Emission SEM (Zeiss, Germany). The X-ray diffraction (XRD) patterns of the samples were recorded on an X’Pert Pro X-ray diffractometer (PANalytical, Inc.).
3.4 Velocity Calibration Experiments
Velocity calibration experiments were carried out as follows. A sample (2 μL) of the aqueous suspension containing the TiO2–Au Janus micromotors was dropped onto a glass slide. The aqueous dye solution (2 μL) was then dropped onto the slide to allow direct mixing with the micromotor droplets. The samples were then subjected to UV irradiation (intensity = 40 × 10−3 W cm−2), which was generated using Mercury lamp sockets and a dichroic mirror (DM-400). The motion of the TiO2–Au Janus micromotors under UV radiation was observed and recorded at room temperature using an optical microscope (Eclipse Ti–S, Nikon Instrument, Inc.), equipped with 40× objectives, and a Zyla sCMOS digital camera (Andor) using the NIS-Elements AR software (version 4.3). The velocity of the nanoparticles was obtained using the Video Spot Tracker program, which calculates the velocity of the nanoparticles from videos recorded by the microscope system. The velocity of the micromotors was calculated from >50 objects from which an average was taken, and the errors were calculated using Microsoft Excel.
3.5 Electrochemical Potential Measurements
A Tafel plot was used to obtain the potentials established at different segments of the various Janus micromotors (Au and TiO2) under UV illumination (Intensity = 0.5 × 10−3 W cm−2, λ = 330–380 nm) in pure water and in aqueous solutions of MB (10−5 g L−1), CR (10−4 g L−1), and MO (10−4 g L−1). Gold and TiO2 films (all thicknesses = 100 nm) on ITO glass disks (diameter = 1.0 cm) were used as the working electrodes in the electrochemical potential measurements. A CHI600C electrochemical analyzer/workstation (CH Instruments, Inc.) was employed to measure the potentials at a scan rate of 5 mV s−1 over a potential range of −0.2 to 0.3 V.
3.6 Photocatalytic Degradation of the Dyes
To examine the photocatalytic degradation of various dye compounds by the micromotors under UV light irradiation, aqueous suspensions containing 2 × 107 TiO2–Au micromotors (200 μL) were added to 20 mL glass bottles containing the different dye compounds (15 mL, 25 μM). Prior to each photoreaction, the various mixtures were stirred in the dark for 30 min to achieve an adsorption–desorption equilibrium for the dye and the dissolved oxygen species on the TiO2 surface. After this time, the various mixtures were irradiated by UV light (40 W cm−2). The glass bottles were then divided into several groups. At the desired time intervals, samples (3.5 mL) of the suspensions were extracted from the glass bottles and were subjected to centrifugation and filtration. The absorbance of any residual dye in the solution was measured by UV–Vis spectrophotometry (Hitachi U-3010). The degradation rate of the dye was then calculated based on the determined absorbance.
3.7 Calibration of Micromotor Quantities
The number of micromotors present in any given sample was estimated following a process similar to that employed for cell counting using the optical microscope . Firstly, an image of an aqueous sample droplet (0.5 μL) was captured from the 1 mL micromotor solution and was placed on the surface of a glass slide. The total area of the circular drop was estimated by measuring its diameter, and the number of micromotors in a 1/80 portion of the drop was counted. This number was then extrapolated to the whole drop and to the full 200 μL sample to give a total of 2 × 107 micromotors (200 μL of the micromotor solution was added to 15 mL of the contaminated sample).
3.8 Reusability Experiments
For the cycling velocity test, the glass slides containing the Au–Ni–TiO2 micromotors were sonicated in deionized water for 5 s. The resulting aqueous solution containing the micromotors was then collected, and the micromotors were separated using an external magnet. The transparent upper solution was discarded, and the micromotors were washed three times with ethanol (Guangzhou Chemical Reagent Co.) and ultrapure water (18.2 MΩ cm). The velocity of the micromotors was then measured repeatedly over three cycles.
For the repeated photodegradation experiments, the Au–Ni–TiO2 micromotors were first separated from the above aqueous solution (obtained following sonication) using an external magnet. The transparent upper solution was discarded and replaced with solutions of the three dyes of interest (15 mL, 25 μM). The collected micromotors were then redispersed into the solution by sonication and reused for the photodegradation experiments.
3.9 Quantitative Study on Micromotor Performance
The effect of micromotor quantity (or number) on the photodegradation efficiency was performed by the addition of different quantities (i.e., 50–200 μL, corresponding to 0.5 × 107–2 × 107 micromotors) of the TiO2–Au micromotors to 20 mL glass bottles containing 15 mL of each dye solution (25 μM). The following procedure employed the steps previously described for the photocatalytic degradation of dyes.
The influence of UV light intensity on the velocity of the Janus micromotors in aqueous dye solutions was examined using light intensities of 5–40 mW cm−2 (due to equipment limits, only ND filters (8×, 16×) could be used to control the intensity). Subsequent steps were as described above for the velocity calibration experiments.
4 Results and Discussion
4.1 Janus Structure of the TiO2/Au Micromotors
4.2 Motion of the TiO2–Au Janus Micromotors in Aqueous Dye Solutions
As per the above equations, H+ is highly concentrated on the TiO2 side, and a local electric field pointing from the TiO2 end to the Au end is formed . The TiO2–Au Janus micromotors could therefore be driven by the local electric field with the TiO2 side pointing forward through electrophoresis. Indeed, we clearly observe that the TiO2–Au Janus micromotors move toward the TiO2 side in a 10−5 g L−1 aqueous solution of MB under UV irradiation (Video S2).
To further confirm this interesting phenomenon, we systematically investigated micromotor motion in aqueous solutions of CR and MO, and the relationship between the TiO2–Au Janus micromotor velocity and the dye concentration is shown in Fig. 3 (see also Video S3-5). As expected, micromotor motion is significantly enhanced in the presence of both dye molecules. Indeed, Fig. 3d shows that under the same UV light intensity, the maximum speeds of the micromotors in 10−4 g L−1 CR and 10−4 g L−1 MO solutions are approximately 1.5 and 1.4 times faster, respectively, than that recorded in pure water (Video S1). More specifically, the trajectories of the TiO2–Au micromotors in the MB (Fig. 3d1), CR (Fig. 3d2), and MO (Fig. 3d3) solutions in addition to that obtained in pure water (Fig. 3d0) under UV irradiation for 5 s reflect the corresponding maximum velocities. We also investigated the motion of pure TiO2 spheres in the aqueous dye solutions (Fig. S1) and found that the spheres remained essentially motionless. In contrast, the Au–TiO2 micromotors moved more rapidly than the TiO2 spheres, although a decrease in Au–TiO2 speed was observed upon increasing the dye concentration over a critical concentration. This can be attributed to Ohm’s law. More specifically, the presence of additional ions upon the introduction of the dye species results in an increase in the solution conductivity with increasing dye concentration, which in turn results in the ions weakening self-electrophoresis through a decrease in the inner electric field with increasing solution conductivity . Indeed, it is clear from Fig. 3a–c that the self-electrophoresis mechanism plays a key role in micromotor motion at low dye concentrations and where self-electrophoresis is essentially not affected by the presence of low ion concentrations. As the dye concentration increases, ion effects will become dominant, thus leading to a decrease in micromotor velocity. This can be summarized by the micromotors exhibiting a peak velocity through a balance of the positive self-electrophoresis effect and the negative ion effect in dye-containing solutions.
4.3 Electrochemical Potential Measurements
According to previous reports, self-electrophoretic propulsion can be attributed to the mixed potential difference between the two electrodes . For enhanced propulsion in such light-driven micromotors, we examined the mixed potentials between the Au and TiO2 electrodes in solutions of MB (10−5 g L−1), CR (10−4 g L−1), and MO (10−4 g L−1) and in pure water under UV light irradiation (Fig. 3e–h). As shown in Fig. 3h, the lowest potential difference (ΔE = 309 mV) between the Au and TiO2 electrodes is observed in pure water, while the potential differences between the two electrodes in the MB, CR, and MO solutions are 417, 396, and 384 mV, respectively. The larger potential difference between the Au and TiO2 electrodes in all three dye solutions compared to that in pure water clearly reflects the enhanced propulsion of the TiO2–Au Janus micromotors. In addition, the decrease in potential differences in the order ΔE MB > ΔE CR > ΔE MO is consistent with the observed speeds of the enhanced motion in individual dye solutions (i.e., V MB > V CR > V MO). We also examined the mixed potentials between the Au and TiO2 electrodes in 10−1–10−8 g L−1 aqueous solutions of MB (Fig. S2), and Tafel plots indicate that the variation in the mixed potentials between the electrodes is consistent with the velocities of the TiO2–Au Janus micromotors in different dye concentrations (Fig. 3). This further confirms that the different velocities observed in solution can be attributed to differences in electrochemical potentials. Analogous results were observed for aqueous CR and MO solutions.
4.4 Photocatalytic Degradation of Dyes
4.5 Direction Control and Reusability of the Micromotors
In conclusion, we observed that TiO2–Au Janus micromotors can obtain energy from the photocatalytic degradation of dyes in aqueous solutions without the requirement for any additional reagents. These micromotors also exhibit light-induced dye-enhanced motion through self-electrophoretic effects in dye solutions under UV irradiation. The velocities of the motors in 10−5 g L−1 MB, 10−4 g L−1 CR, and 10−4 g L−1 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. In addition, we found that the micromotor velocity strongly depends on the type of dyes employed, due to their different photodegradation rates. Furthermore, these micromotors exhibit excellent reusability in the degradation and detection of dye pollutants. These findings indicate the potential for tuning the motion of photocatalytic micro-/nanomotors in addition to “on-the-fly” degradation of dye pollutants in aqueous environments.
We gratefully acknowledge the financial support from the NSFC (21674039).
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