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Article

Group Control of Photo-Responsive Colloidal Motors with a Structured Light Field

by
Dianyang Li
1,†,
Huan Wei
1,†,
Hui Fang
1,* and
Yongxiang Gao
2,*
1
Nanophotonics Research Center, Shenzhen Key Laboratory of Micro-Scale Optical Information Technology, Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen 518060, China
2
Institute for Advanced Study, Shenzhen University, Shenzhen 518060, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Photonics 2024, 11(5), 421; https://0-doi-org.brum.beds.ac.uk/10.3390/photonics11050421
Submission received: 30 March 2024 / Revised: 25 April 2024 / Accepted: 29 April 2024 / Published: 1 May 2024
(This article belongs to the Special Issue Emerging Topics in Structured Light)
Browse Figures
Versions Notes

Abstract

:
Using structured light to drive colloidal motors, due to its advantages of remote manipulation, energy tunability, programmability, and the controllability of spatiotemporal distribution, has been attracting much attention in the fields of targeted drug delivery, environmental control, chemical agent detection, and smart device design. Here, we focus on studying the group control of colloidal motors made from a photo-responsive organic polymer molecule NO-COP (N,O-Covalent organic polymer). These colloidal motors mainly respond to light intensity patterns. Considering its merits of fast refreshing speed, good programmability, and high-power threshold, we chose a digital micromirror device (DMD) to modulate the structured light field shining on the sample. It was found that under ultraviolet or green light modulation, such colloidal motors exhibit various group behaviors including group spreading, group patterning, and group migration. A qualitative interpretation is also provided for these observations.

1. Introduction

Structured light fields have increasingly contributed to the investigation of light–matter interactions, particularly with respect to active matter [1,2,3,4], which is a crucial and expanding field of research spanning disciplines such as physics, chemistry, mechanics, materials, and biology. Active matter is a category of non-equilibrium systems that consists of self-propelled units commonly found in artificial colloids and microorganisms such as bacteria, motor proteins, and green algae, which are capable of exhibiting a number of interesting group behaviors in their interactions with the environment [5], covering the macroscopic to microscopic scales.
Colloidal motors [6,7] are a class of particles capable of converting external energy into their own energy and performing some kind of motion, which are driven by chemical reactions [8] and various physical fields, such as light fields [9], electric fields [10], magnetic fields [11], and acoustic fields [12], among others. Chemical propulsion systems typically require a high concentration of fuel that can be quickly exhausted. Additionally, commonly used fuels such as hydrogen peroxide are toxic. Magnetic field and ultrasonic propulsion systems require high energy input and complicated experimental setups. Light-driven colloidal motors refers to colloidal motors powered by an optical field [13,14] and they have received worldwide attention from researchers due to their high programmability, remote manipulation, tunable energy, and ease of operation.
Light-driven colloidal motors work due to the formation of a gradient light field around the colloidal motors, which is mainly due to either the asymmetric morphology of the colloidal motors themselves or the uneven distribution of the external light field. Common asymmetric morphologies include Janus-like [15], rod-like [16], tube-like [17], gear-like [18], etc., which undergo asymmetric chemical reactions under light to produce chemical products of non-uniform concentration, or convert light energy into non-uniform temperature fields, electric fields, etc., thus inducing non-counteracting forces to drive colloidal motions [19]. The non-uniform external light field can be created by modifying the incident light’s angle of illumination, personalizing a template, or utilizing a device such as a spatial light modulator, which consequently facilitates the motion of the colloid motor [20,21,22]. The combination of various parameters (wavelength, intensity, polarization, etc.) of the structured light field and the colloidal motors system can stimulate many novel phenomena, which show attractive prospects for device design [23], biomedicine [24], and environmental treatment [25].
In this study, we use a digital micromirror device (DMD) to modulate the structured light field, due to its fast-refreshing speed, good programmability, and high-power threshold, and rely on the photocatalytic reaction of the colloidal motors to drive their motion. Its manipulation mechanism is different from often-used methods such as optical tweezers and laser steering where the driving force mainly comes from the momentum transfer from the light to the colloidal motors. In optical tweezers, in order to achieve the capture of micrometer particles, a large optical gradient force is required to suppress the Brownian motion and optical scattering force, which is usually satisfied only by the tight focusing of the light and may cause damage to the particles [26]. Laser steering is based on the transfer of optical angular momentum to the trapped object and is usually applied for studying the rotation behavior of the object [27]. The advantage of the approach we took here includes the low power of the light and its multiscale group control capability.
We applied two DMD-based optical systems (one works in ultraviolet LED light and the other works in green laser light) to investigate the group behaviors of specific colloidal motors made of NO-COP molecules, such as group spreading, group patterning, and group migration. These observations and analyses are potentially valuable for further studies of interactions between structured light fields and colloidal motors.

2. Materials and Experimental System

2.1. Materials

The samples we used were photo-responsive organic polymer colloidal motors made from polymer NO-COP, which can be prepared from the polycondensation reaction of cyanuric trichloride and barbituric acid at 180 °C for 48 h [28].
The NO-COP colloidal motor is spherical and can emit fluorescence at multi-wavelength excitation, appearing green when lit by UV or blue light and orange when lit by green light. It has a forbidden band width of 1.8 eV, coming from a valence band with a maximum energy of −0.16 eV, and a conduction band with a minimum energy of 1.64 eV. Under external illumination, the electron–hole accumulation occurs within the colloidal motor, which catalyzes the decomposition of water into protons (H+) and hydroxyl radicals (-OH). Since the diffusion of H+ in the water is faster than that of -OH, an ionic distribution around the surface of the colloidal motors will be formed. This process will lead to an electric field directed towards the colloidal motor, which will be responsible for controlling the motion of the colloidal motors.

2.2. Experimental System

In the current study, in order to investigate the group behavior of the NO-COP colloidal motors under the influence of illumination light intensity distribution, we used a DMD as the main device to modulate the structured light field. The DMD is docked with the Olympus inverted microscope, in our experimental system.
As shown in Figure 1, we set up two experimental systems where one takes an LED as the light source and the other takes a 532 nm laser as the light source, respectively. As shown in Figure 1a, the LED light source (DC4100 from Thorlabs, New Jersey, NJ, USA) is set at 365 nm and its output end is already pre-installed with a collimating lens. The LED light beam directly illuminates the DMD (V-7000 UV from ViALUX, Chemnitz, Germany), which is loaded with a spatial pattern to modulate the light beam. Subsequently, in the light path, a telescopic optic consisting of a lens L1 (f = 400 mm) and a microscope objective shrinks the DMD-loaded pattern onto the sample. The sample is also illuminated by the microscope light source and imaged onto the CCD.
As shown in Figure 1b, a second experimental system used the 532 nm single-longitudinal-mode solid-state laser (Torus532 from QUANTUM). Here a dichroic mirror (DM) instead of a beam-splitter (BS) is used. In the optical path, the 532 nm laser beam is reflected by mirrors M1 and M2, and then enters the beam expander. The expanded laser beam passes a liner polarizer LP and becomes vertically polarized. The DMD (V-9001 VIS from ViALUX) modulates the beam intensity distribution, and the loaded pattern is subsequently passed through the first 4f optical system composed of the L1 lens and the L2 lens and the second 4f optical system, consisting of the L3 lens and the microscope objective and is imaged onto the sample. The purpose of setting up the first 4f optical system is to create a plane where a spatial filter (APT) can be applied. The sample is also imaged onto a CCD for observation.

3. Experiment Results and Discussion

3.1. Group Spreading Behaviors of Colloidal Motors

With the experimental system as shown in Figure 1a, a circular pattern was loaded with DMD and projected onto the sample, and the colloidal motors’ distributions at different moments before, during, and after the DMD pattern loading were recorded as shown in Figure 2. It can be seen that once the DMD pattern loading is on, the colloidal motors within the circular light spot started to exclude each other and spread outward. Up to 10 s, the total number of motors within the light spot reduced significantly compared to the start at the 1.5 s. After the 10 s time point, the decrease in the number of colloidal motors slowed down. It was observed that the colloidal motors far away from the light spot behave as under Brownian motion. After 50 s, when the DMD loading is switched off, the colloidal motors started diffusing back into the empty space that had been created by illumination of the light spot. In order to see this behavior more quantitatively, we applied ImageJ2023 software to count the colloidal motors inside the central circular area of Figure 2 (with a diameter of approximately 52.72 μm) and plotted the curve of the total number changing over time (Refer to Appendix A, Figure A1). From the plot, the number change caused by the light can clearly be visualized and the response time is shown to be about 5 s.
We performed the same experiment in the experimental system as in Figure 1b with a 532 nm laser. The situation when the circular light spot was turned on and off is shown in Figure 3, and is similar to the observation shown in Figure 2, except that higher magnification images were obtained. At 51 s, the circular pattern is reprojected onto the sample and we saw that the phenomenon of light extruding the colloidal motors was repeated. Because these colloidal motors had a weak response to the green light, they require a higher laser power (1.96 mw) to drive their motion, resulting in a brighter spot in the wide field images.
We attribute this special spreading behavior to the self-diffusion swimming of the colloidal motors [29]. The light will catalyze the decomposition of water to H+ and -OH and form the ion distribution layers where the H+ is further away from the light illumination region, thus creating an electric field directed towards to the region interior. If the surface of the individual colloidal motors is negatively charged, they will tend to move outwards from the light illumination region. To confirm this hypothesis, we measured the zeta potential of the colloidal motors, which is −45.66 mV. Based on the Gouy–Chapman–Stern bilayer model [30], this negative zeta potential indicates the colloid has negative charge inside its compact layer. One way to verify such an electrophoretic mechanism is to add a salt solution to deactivate the zeta potential [31,32]. We added an appropriate amount of sodium chloride solution to the samples and found that the motion speed of the colloidal motors obviously decreased. In addition, the concentration of ionic products in the illuminated area is much larger than that in the unilluminated area, and the concentration difference between the inside and outside of the illuminated area will also form a concentration gradient force that affects the movement of the colloidal motors. These two mechanisms add together and thus lead to the group spreading behavior of the colloidal motors.
We observed that when initially turning on the green light with a relative high power, the colloidal motors first aggregated towards the light spot and then engaged in the group dispersion. We attributed the aggregation to the thermal buoyancy effect: because of the photothermal effect, the temperature gradient will be formed around the light spot, and the solution density will be changed accordingly, which will induce the circulation flow of the solution thus driving the colloidal motors to move close to the light spot. We actually observed that some colloidal motors moved upwards off the imaging surface, indicating the formation of some type of circulation flow.
We further examined the motion trajectory of the colloid motors by applying ImageJ2023 software on several representative colloid motors as shown in Figure 2 of the experiment with the UV light. The analysis results are displayed in Figure 4 where the trajectories up to 20 s are plotted. It is clearly shown that these colloid motors moved radially outwards from the center of the light spot.
We also investigated the optical power dependence of this diffusion behavior (the results are presented in Appendix A, Figure A2 and Figure A3). For the UV light experiment, at low power the colloidal motors did not show significant spreading, and the diffusion rate was slow. However, as the optical power increased, a thermal effect occurs and more photons were irradiated on the surface of the motor, stimulating more electrons and holes. This increased the photocatalytic reaction rate, enhancing the formation of the electric field for the motor group and improving the driving force, resulting in an increase in the speed of the group spreading. For the green light experiment, similar to the UV light experiment, the distance between the colloidal motors in the illuminated area increased, and the number of colloidal motors decreased with increasing light intensity, suggesting that the diffusion rate of the colloidal motors increased.

3.2. Group Patterning Behaviors of Colloidal Motors

Based on the observation of the light-extruding effect of the NO-COP colloidal motors, we designed the experiment to drive the colloidal motor group to form a specific distribution pattern. As shown in Figure 5 for the UV-LED illumination, the distribution patterns of the circle ring (Figure 5a), the stripes (Figure 5b), and the two neighboring squares (Figure 5c) can be clearly seen.
We also applied the trajectory analysis using ImageJ2023 software on the formation of these patterns up to 25 s. For the circle ring pattern, as shown in Figure 6a, most of the colloid motors originally sitting inside the light spot area moved radially outwards while most of the colloid motors originally sitting inside the center dark area underwent Brownian motion. For the stripe pattern, as shown in Figure 6b, it is interesting to see that the colloidal motors originally located at the outer two bright stripes tend to move perpendicular to the stripes while the colloidal motors originally located at the center bright stripes tend to move parallel to the stripes. For the two neighboring squares, as shown in Figure 6c, the colloidal motors originally near the internal two edges tend to move parallel to the edges. Currently, we do not have a clear explanation regarding all of these observations and it is certainly worth a further simulation study (such as those demonstrated in other colloid patterning behavior studies [33,34]).
Furthermore, it will be interesting to examine how the colloidal motors respond to light patterns with different resolutions. As displayed in Figure 7, when shining with the strip patterns with different spatial frequency, the distribution of colloidal motors cannot follow the light pattern accurately when the spatial frequency is higher than 100 DMD pixels per cycle (there is a graphical reduction in magnification to be considered on the imaging plane), but the response is still clear.
Similar pattern forming can also be observed in the green laser experiment system. For example, Figure 8 shows the stripe pattern forming process with a higher microscopic imaging resolution. We can see the empty space forms stripes with a width of only one or two colloids. This also demonstrated that the colloidal motors can response to high resolution light illumination patterns.

3.3. Group Migration Behaviors of Colloidal Motors

To demonstrate the potential application of the light response group behavior of the NO-COP colloidal motors, we performed a further experiment with the UV LED to actively migrate a group of colloid motors that can be trapped in the center dark region of the circular ring light pattern. As shown in Figure 9, from 20 s up to 100 s, the DMD pattern of the ring shape was horizontally shifted by 10 pixels to the right side every 20 s. During this slow light pattern switching, the colloidal motors trapped in the center dark region moved along with the moving dark region. At 100 s, we switched the DMD pattern back to the original place. As can be seen, after 20 s the previously trapped group of colloidal motors gradually moved back to the new dark center region.

4. Conclusions

With two DMD-based optical microscopy setups, respectively, using a UV LED and a green laser as the illumination light source, we successful recorded various group control motions of the NO-COP colloidal motors. The circular light spot experiments revealed the light-extruding property of the colloidal motors, while the other three more complex light spots (circular ring, stripes, and two neighboring squares) showed more interesting details of the motion behavior of the colloidal motors. The slow-moving circular ring experiment demonstrated the capability of actively migrating a group of colloidal motors. Although the light-extruding property can be understood by the electrophoretic property of the colloidal motors, further theoretical study is needed to explain the motion trajectories observed for the complex light spot illumination and also the spatial resolution of the response to the light. The method of colloidal motor multiscale group control we showed here may be useful for micro-device design and SERS detection.

Author Contributions

Conceptualization and methodology, Y.G. and H.F.; experiments, H.W. and D.L.; software, H.W. and D.L.; data analysis, H.W.; writing—original draft preparation, H.W. and D.L.; writing—review and editing, H.W., Y.G. and H.F.; visualization, H.W. and D.L.; project administration and funding acquisition, Y.G. and H.F. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the National Natural Science Foundation of China (12074268), the Key Project of Guangdong Provincial Department of Education (2023ZDZX3021), and the Scientific Instrument Developing Project of Shenzhen University (2023YQ024).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

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Figure A1. The number of colloidal motors counted inside the central circular region of Figure 2 (with a diameter of approximately 52.72 μm) as a function of time.
Figure A1. The number of colloidal motors counted inside the central circular region of Figure 2 (with a diameter of approximately 52.72 μm) as a function of time.
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Figure A2. Effects of different optical powers on the diffusion behavior of NO-COP colloidal motor populations under UV light, scale bar 10 μm. (a) 0.479 mw; (b) 0.617 mw; (c) 0.734 mw.
Figure A2. Effects of different optical powers on the diffusion behavior of NO-COP colloidal motor populations under UV light, scale bar 10 μm. (a) 0.479 mw; (b) 0.617 mw; (c) 0.734 mw.
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Figure A3. Effect of different optical powers on NO-COP colloidal motor motion under green light, scale bar 5 μm. (a) 0.148 mw; (b) 1.03 mw; (c) 3.12 mw; (d) 5.62 mw.
Figure A3. Effect of different optical powers on NO-COP colloidal motor motion under green light, scale bar 5 μm. (a) 0.148 mw; (b) 1.03 mw; (c) 3.12 mw; (d) 5.62 mw.
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Photonics 11 00421 g001
Figure 1. (a) Experimental system for LED light source (M1, M2, and M3 are mirrors; L2, tube lens; Objective, PlanApo60X from Olympus microscopic objective, NA = 1.42). (b) Experimental system for 532 nm laser light source. (L1 (f = 400 mm), L2 (f = 200 mm) and L3 (f = 400 mm) are lenses; L4, tube lens; M1, M2, M3, M4, and M5 are mirrors; BE, 20× beam expander (GBE20-A from Thorlabs); Objective, PL L50X microscopic objective in inverted microscope LM200 from LAITE).
Figure 1. (a) Experimental system for LED light source (M1, M2, and M3 are mirrors; L2, tube lens; Objective, PlanApo60X from Olympus microscopic objective, NA = 1.42). (b) Experimental system for 532 nm laser light source. (L1 (f = 400 mm), L2 (f = 200 mm) and L3 (f = 400 mm) are lenses; L4, tube lens; M1, M2, M3, M4, and M5 are mirrors; BE, 20× beam expander (GBE20-A from Thorlabs); Objective, PL L50X microscopic objective in inverted microscope LM200 from LAITE).
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Figure 2. Group diffusion behavior of NO-COP colloidal motors under UV light (0.734 mw), image magnification: 15×; scale bar, 20 μm.
Figure 2. Group diffusion behavior of NO-COP colloidal motors under UV light (0.734 mw), image magnification: 15×; scale bar, 20 μm.
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Figure 3. Group diffusion behavior of NO-COP colloidal motors under green light (1.96 mw), image magnification: 50×, scale bar 5 μm.
Figure 3. Group diffusion behavior of NO-COP colloidal motors under green light (1.96 mw), image magnification: 50×, scale bar 5 μm.
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Figure 4. Motion trajectory tracking of NO-COP colloidal motors under UV light in a circular light spot at 0, 5, and 20 s (Different lines and circles represent different colloidal motors), scale bar 10 μm.
Figure 4. Motion trajectory tracking of NO-COP colloidal motors under UV light in a circular light spot at 0, 5, and 20 s (Different lines and circles represent different colloidal motors), scale bar 10 μm.
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Figure 5. Experimental results of population patterning of NO-COP colloidal motors when DMD is loaded with (a) circles, (b) stripes, and (c) two square patterns of the same size under UV light, scale bar 20 μm.
Figure 5. Experimental results of population patterning of NO-COP colloidal motors when DMD is loaded with (a) circles, (b) stripes, and (c) two square patterns of the same size under UV light, scale bar 20 μm.
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Figure 6. Trajectory tracking of NO-COP colloidal motors during patterning (Different lines and circles represent different colloidal motors), (a) circle; (b) stripe; (c) square, scale bar 20 μm.
Figure 6. Trajectory tracking of NO-COP colloidal motors during patterning (Different lines and circles represent different colloidal motors), (a) circle; (b) stripe; (c) square, scale bar 20 μm.
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Figure 7. Experimental results of colloidal motor population patterning when DMD is loaded with different spatial frequency stripes. (a1–a4) The results when the light is on for the spatial frequencies of 100, 160, 240, and 400 DMD pixel per cycle. (b1–b4) The corresponding results when the light is turned off, scale bar 20 μm.
Figure 7. Experimental results of colloidal motor population patterning when DMD is loaded with different spatial frequency stripes. (a1–a4) The results when the light is on for the spatial frequencies of 100, 160, 240, and 400 DMD pixel per cycle. (b1–b4) The corresponding results when the light is turned off, scale bar 20 μm.
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Figure 8. Results of population patterning experiments of NO-COP colloidal motors after loading stripes under green light, scale bar 5 μm.
Figure 8. Results of population patterning experiments of NO-COP colloidal motors after loading stripes under green light, scale bar 5 μm.
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Figure 9. Group migration experiments of NO-COP colloidal motors under UV light, scale bar 10 μm.
Figure 9. Group migration experiments of NO-COP colloidal motors under UV light, scale bar 10 μm.
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MDPI and ACS Style

Li, D.; Wei, H.; Fang, H.; Gao, Y. Group Control of Photo-Responsive Colloidal Motors with a Structured Light Field. Photonics 2024, 11, 421. https://0-doi-org.brum.beds.ac.uk/10.3390/photonics11050421

AMA Style

Li D, Wei H, Fang H, Gao Y. Group Control of Photo-Responsive Colloidal Motors with a Structured Light Field. Photonics. 2024; 11(5):421. https://0-doi-org.brum.beds.ac.uk/10.3390/photonics11050421

Chicago/Turabian Style

Li, Dianyang, Huan Wei, Hui Fang, and Yongxiang Gao. 2024. "Group Control of Photo-Responsive Colloidal Motors with a Structured Light Field" Photonics 11, no. 5: 421. https://0-doi-org.brum.beds.ac.uk/10.3390/photonics11050421

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