Controlled Movement of a Smart Miniature Submarine at Various

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Controlled Movement of a Smart Miniature Submarine at Various Interfaces Ying Chu, Liming Qin, Liang Zhen, and Qinmin Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06631 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018

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ACS Applied Materials & Interfaces

Controlled Movement of a Smart Miniature Submarine at Various Interfaces Ying Chu*, Liming Qin, Liang Zhen, and Qinmin Pan* (School of Chemistry and Chemical Engineering, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, P. R. China)

Corresponding authors: Ying Chu School of Chemistry and Chemical Engineering Harbin Institute of Technology, Harbin 150001, P. R. China E-mail: [email protected]

Qinmin Pan School of Chemistry and Chemical Engineering Harbin Institute of Technology, Harbin 150001, P. R. China E-mail: [email protected]

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ABSTRACT Smart miniaturized aquatic devices have many important applications, but their locomotion at different interfaces remains a challenge. Here we report a smart miniaturized submarine moving at various air/liquid or oil/water interfaces. The microsubmarine is fabricated by a CO2-responsive superhydrophobic copper mesh and is driven by the Marangoni effect. The microsubmarine can not only transfer among different interfaces reversibly, but also move horizontally at the interfaces freely. The unique locomotion of the device is attributed to CO2-triggered switch between superhydrophobicity and underwater superoleophobicity. Moreover, the microsubmarine exhibits good stability and excellent oil repellence at the oil/water interface. Our study provides a strategy for fabricating smart aquatic devices that have potential applications in environment monitoring, water purification or channel-free microfluidics, and so on.

Keywords: smart miniature aquatic device, CO2-triggered switchable wettability, various interfaces, controlled movements, Marangoni effect

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1. Introduction The construction of miniature smart aquatic devices, such as microrobots or microboats, has attracted enormous attention because of potential applications in surveillance,1 environment monitoring,2 microfluidics,3,4 targeted drug delivery,5 cell culture,6 and so on. Over the past decade, considerable studies had focused on designing miniaturized aquatic devices by using superhydrophobic materials with a water contact angle greater than 150°.7-10 Owing to their excellent water repellence, these devices could move on water surface freely without sinking. However, moving at different liquid/liquid interfaces, especially at various water/oil interfaces, is still a challenge for these devices because their surfaces are wettable for lipophilic liquids. In order to address the above-mentioned limitation, some researchers attempted to design devices moving at liquid/liquid interfaces. Among various strategies, underwater superoleophobicity (superhydrophilicity) was found to allow a miniaturized device to float at the water/oil interface freely.11,12 For example, Wang et al. reported an “oil-strider” floating at water/dichloroethane interface by using underwater superoleophobic copper wires.11 However, underwater superoleophobicity is only effective at the interfaces whose organic phases are heavier than water. Moreover, controlled movement among different interfaces remains a challenge for these devices. Another strategy to the limitation is the switch of wettability triggered by external stimuli like ultraviolet (UV)13 and heat treatment.14,15 By switch its wettability from superhydrophobicity to superhydrophilicity, a miniature device can move on a water surface as well as at various water/oil interfaces. However, such a switch usually involves a specific trigger or long-time stimulation. CO2 is recently emerging as a novel trigger to switch between hydrophobicity and hydrophilicity because of fast response and low toxicity.16-21 However, CO2-responsive aquatic devices have not been reported because hydrophobic (or hydrophilic) surfaces often show large drag in water. The drag limits fast movement of an aqueous device on the water surface. Therefore, it is necessary to fabricate a CO2-responive aquatic device that can move at different interfaces but also shows reduced drag. Herein, we report a smart miniature submarine that can move at 3



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different interfaces freely. The microsubmarine is constructed by a CO2-responsive superhydrophobic mesh and is driven by the Marangoni effect. By switching its wettability between superhydrophobicity and superhydrophilicity, the microsubmarine moves at not only water surface but also various oil/water interfaces. The present investigation provides a strategy to fabricate a smart miniaturized device with controllable locomotion at various interfaces.

(a) AgNO3/UV

NaOH/K2S2O8

PDEAEMA-CTA PDMS

Cu mesh Ag nanoparticle

PDMS

PDEAEMA–CTA

(b) Marangoni effect NIR

Air Superhydrophobicity

Bubbling of CO2

Release of Ethanol

Water Superhydrophilicity

CHCl3

Scheme 1. Illustration for (a) the fabrication of smart miniaturized submarine and (b) its controlled movement at different interfaces.

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2. Results and Discussion The fabrication of the smart miniaturized submarine is illustrated in Scheme 1. A copper mesh was firstly immersed in K2S2O8/NaOH solution and then in AgNO3 solution. The resulting mesh was treated with ultraviolet (UV) to form a Ag layer with hierarchical roughness. After immersion in a toluene solution of carboxyl-terminated poly(2-(diethylamino)ethyl methacrylate (PDEAEMA-CTA) and polydimethylsiloxane (PDMS), the copper mesh exhibited superhydrophobicity with a water contact larger than 150o. At last, the as-prepared superhydrophobic mesh was folded into a miniature submarine and a microactuator was installed in its end. (a)

(b)

(c)

(d)

(e)

N

(f)

Si

Figure 1. SEM images of the copper mesh (a-b) before and (c-d) after the modification of PDEAEMA-CTA and PDMS. EDS maps of the superhydrophobic copper mesh. (e) N and (f) Si.

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The surface morphology of the microsubmarine was observed by scanning electron microscopy (SEM). As shown in Figure 1a-b, the copper mesh is homogeneously coated with micro-sized clusters (7-8 µm in diameter) composed of nanoparticles, indicating the presence of hierarchical structures at both micrometer and nanometer scales. In contrast, the original copper mesh shows a smooth appearance (Figure S1, Supporting Information). The hierarchical micro/nano clusters efficiently increase the roughness of the copper mesh. After coating with the composite of PDEAEMA-CTA and PDMS, the copper mesh still keeps the hierarchical morphology (Figure 1c-f and Figure S2, Supporting Information). As a result, the hierarchical roughness and the low-surface-energy coating endowed the submarine with superhydrophobicity, unlike the hydrophobicity of the original copper mesh (Figure S3, Supporting Information). (a)

(b)

Air

Air

152.8 ± 2.0o (c)

0o 160

(d)

Water

150

C ontact angle / deg

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151.0 ± 1.5o

140 130

10 0 0

1

2

3 4 5 6 Number of cycles

7

8

Figure 2. Water contact angles of the CO2-responsive superhydrophobic microsubmarine in air (a) before and (b) after bubbling of CO2. (c) Underwater contact angle of the microsubmarine for a CHCl3 droplet. (d) Reversible wettability of the microsubmarine for water droplet after bubbling/removal of CO2. Owing to the existence of PDEAEMA-CTA, the as-prepared microsubmarine exhibited switchable wettability. Here the microsubmarine was first placed on the water surface and then CO2 was bubbled in the water. After 6


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bubbling for 5-8 min, the microsubmarine changed its wettability from superhydrophobicity (152.8 ± 2.0o) to superhydrophilicity with a water contact of 0o (Figure 2a-b). The submarine also showed underwater superoleophobicity because its contact angle for a CHCl3 droplet was 151.0 ± 1.5o (Figure 2c). After the removal of CO2 and then drying at 60 oC, the submarine recovered the superhydrophobicity with a water contact angle of 150.1 ± 1.0o. The switch between superhydrophobicity and superhydrophilicity finished in 5-15 min, and could be repeated for at least 5 cycles (Figure 2d). The superhydrophobic and CO2 responsive properties of the as-prepared microsubmarine are strongly dependent on the mass ratio of PDEAEMA-CTA to PDMS (Figure S5, Supporting Information). Moreover, the microsubmarine exhibited low water/oil adhesion. The adhesive forces for a water droplet in air and a CHCl3 droplet in water were measured to be 49.2 ± 17.5 and 40.4 ± 7.76 µN, respectively (Figure S4, Supporting Information). The CO2-triggered wettability switch is attributed to the protonation and deprotonation of PDEAEMA-CTA chains. In the absence of CO2, the tertiary amine was in the neutral state and PDEAEMA-CTA chains exhibited the collapsed state.18 Since the amount of PDEAEMA-CTA was much lower than that of PDMS, most of the tertiary amine was hided beneath hydrophobic PDMS chains in this case. The hierarchical roughness together with low-surface-energy PDMS coating gave rise to the superhydrophobicity of the microsubmarine. After bubbling of CO2 in water, carbonic acid produced via the reaction CO2 + H2 O ↔ H2CO3 ↔ HCO3 - + H+ .21,22 The resulting carbonic acid protonized the tertiary amine, and thus endowing the PDEAEMA-CTA chains with positive charge (Figure S6, Supporting Information).13 The repulsive interaction between the charged amines enabled the PDEAEMA-CTA/PDMS chains to expand into the “swollen” state.21 The expansion greatly increased the exposure of hydrophilic amine moiety in the PDEAEMA-CTA/PDMS coating, allowing the submarine surface to change from superhydrophobicity to superhydrophilicity. After stop of bubbling and then drying at 60 oC, the charged tertiary amine was deprotonated and the microsubmarine recovered its superhydrophobicity.

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The microsubmarine could move on various interfaces via the Marangoni effect.23 Here a hollow microball containing ethanol was used as a microactuator and installed at the end of the microsubmarine (Figure 3a). The microball consisted of Fe3O4@polydopamine (Fe3O4@PDA) nanoparticles and paraffin (Figure 3b), which had a diameter of ~0.40 ± 0.05 cm and a wall-thickness of ~0.4 mm (Figure S7a, Supporting Information). Owing to the photothermal property of Fe3O4@PDA nanoparticles,24,25 the microball rose its temperature to 63.1 oC after irradiating by near infrared (NIR) laser (808 nm, Figure 3c). Consequently, the paraffin surrounding the irradiated site was molten, resulting in the perforation of the ball and thereafter release of ethanol in a controlled way. Then we investigated the movements of the microsubmarine at different interfaces. As shown in Figure 4a, the submarine floated and remained motionless at the water surface due to its superhydrophobicity. After irradiating by NIR laser for 3-5 min, the shell of the microactuator was pierced (inset of Figure 4a) and ethanol released immediately. The diffusion of ethanol at the water surface caused the partial change of surface tension of water. Owing to the driving force produced by the Marangoni effect, the microsubmarine moved forward at a speed of 6.0 cm s−1 (Move 1, Supporting Information). (a)

(b)

(c) Fe3O4@PDA clusters

Irradiated by NIR

Figure 3. (a) Optical image of the CO2-responsive superhydrophobic microsubmarine, (b) SEM image of the microactuator shell consisted of Fe3O4@PDA nanoparticles and paraffin, (c) Infrared thermograph of the microactuator irradiated by NIR laser. Moreover, the microsubmarine could move from water surface to water/oil interface (Figure 4b). Here red dyed CHCl3 was used as the oil phase. In the absence of CO2, the microsubmarine was static on the water surface. After bubbling with CO2 for 5-8 min, the microsubmarine dove into the water. In this case, water filled into the 8


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hierarchical textures of the submarine and prevented its surface from contact with CHCl3. Therefore, the microsubmarine could float at the water/oil interface rather than sink into the oil. After removing CO2 and then drying in air, the microsubmarine floated on the water surface again. The transfer of the microsubmarine at different interfaces was attributed to the switch between superhydrophobicity and superhydrophilicity. The lifetime of the microsubmarine was assessed by transfer cycle at the interfaces. In this study, the microsubmarine reversibly transferred between the interfaces for 6 cycles (Figure 2d). Once the microactuator was irradiated by NIR laser, the microsubmarine moved at the water/CHCl3 interface at the speed of ~0.17 cm s−1 through the Marangoni effect (Move 2, Supporting Information). In addition, the microsubmarine could move at other oil/water interfaces (Figure S8 and Move 3, Supporting Information). These results indicate that the submarine can move on both water and oils by switching its wettability. One application of this submarine was demonstrated by the controlled transport and release of pH-indicator (e.g. litmus solution) at the different interfaces (Figure S9, Supporting Information). (a)

0s

2s

6s

(b) Air Water CHCl3

0s

330 s

320 s

Figure 4. Optical images show the movement of the miniature submarine at (a) water surface and (b) water/CHCl3 interface. The inset of Figure 4a is the microactuator after irradiating by NIR laser. Scale bar: 2 mm. The stability of the microsubmarine at the water/oil interface was also investigated. The microsubmarine stayed at the water/CHCl3 interface for at least 48 h while keeping its underwater oleophobic property. Its underwater

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contact angle and adhesion for CHCl3 were measured to be 146.1 ± 2.9o and 44.5 ± 13.4 µN, respectively (Figure S10-11, Supporting Information), indicating robust bonding of the PDEAEMA-CTA/PDMS coating on the microsubmarine. Additionally, the microsubmarine remained its CO2-triggered switch capability after the staying (Figure S10b, Supporting Information).

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3. Conclusion In summary, we demonstrated the controlled movement of a miniature submarine at various air/liquid or water/oil interfaces. The miniature submarine was constructed by a CO2-responsive superhydrophobic copper mesh and was propelled by the Marangoni effect. The transfer of the microsubmarine at different interfaces was attributed to CO2-triggered switch between superhydrophobicity and underwater superoleophobicity. Moreover, the microsubmarine showed good oil-repellent properties. We anticipate that our findings may provide a new method to address the controlled movement of an aquatic device at different interfaces, which has potential applications in environment monitoring, water purification, channel-free microfluidics, and so on.

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4. Experimental Section Materials Hydrochloric acid (HCl, 36.5%), ethanol (95%), acetone, toluene, silver nitrate (AgNO3), chloroform (CHCl3), dodecane, sodium hydroxide (NaOH), carbon disulfide (CS2), acetone, azobisisobutyronitrile (AIBN), N, N-diethylaminoethyl methacrylate (DEAEMA, 98%), tris(hydroxymethyl) aminomethane hydrochloride (Tris-HCl), dopamine hydrochloride (DA) and paraffin with ceresin were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Iron (III) chloride hexahydrate (FeCl3·6H2O), sodium acetate trihydrate (NaAc·3H2O), ethylene glycol, polyethylene glycol were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. (China). Copper mesh with a pore size of ~700 µm was provided by Wuzhou New Mater. Co., Ltd. (Guangxi, China). CO2 was offered by Harbin Liming gas Co., Ltd. (China).

Synthesis of CO2-responsive copolymer Dodecylsulfanylthiocarbonylsulfanyl-2-methyl propionic acid (DMP) was synthesized according to a described procedure.26 Typically, 8.076 g of 1-dodecanethiol, 19.24 g of acetone and 0.6 g of tricaprylylmethylammonium chloride were agitated at 10 oC in an argon atmosphere. Then 1.6 mL of aqueous NaOH solution (50 wt%), 3.04 g of CS2 and 4.04 g of acetone were successively added to the mixture dropwise. After stirring for 15 min, 7.13 g of CHCl3 was poured into the above mixture. Later, 1.6 mL of aqueous NaOH solution (50 wt%) was added dropwise to the resulting mixture. After stirring at room temperature for 8 h, 60 mL of water and 10 mL of concentrated HCl were successively added to the mixture. At last, the acetone was removed by reduced pressure distillation. The obtained solid was washed with 2-propanol and dried at room temperature. 1H NMR (CDCl3, 300 MHz): δH (ppm) = 3.98 (s, -COOCH2CH2-), 2.55 (s, -COOCH2CH2-), 2.68 (s, -NCH2CH3), 2.02–1.58 (m, -CH2C(CH3)2COOH) (Figure S12, Supporting Information).

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The carboxyl-terminated poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA-CTA) was synthesized by reversible addition-fragmentation transfer (RAFT) polymerization.27 364.64 mg of DMP, 16.4 mg of AIBN, 14.8 mg of DEAEMA was added to 12 mL of 1, 4-dioxane in a shlenk flask. The solution was degassed by three consecutive freeze-pump-thaw cycles. Then the shlenk flask was filled with argon. After stirring at 70 oC for 18 h, the resulting mixture was diluted with 1, 4-dioxane and then purified by dialysis for 72 h in water at pH 4.5. Finally, the yellow solution was dried at room temperature.

Preparation of the CO2-responsive superhydrophobic copper mesh A piece of copper mesh (30 mm × 20 mm) was successively cleaned by acetone, ethanol and distilled water. Then it was immersed into an aqueous solution of 1 mol L−1 NaOH and 0.05 mol L−1 K2S2O8 for 20 min.28 The resulting copper mesh was immersed into 1.0 mg ml−1 aqueous AgNO3 solution. This solution was irradiated by ultraviolet (UV, 254 nm, 3W) lamp for 20 min to deposit a layer of Ag nanoparticles on the copper mesh. After washing with distilled water and drying in air, the obtained copper mesh was immersed into a toluene solution containing carboxyl-terminated poly(2-(diethylamino)ethyl methacrylate), PDMS and curing agent for 5 min. The mass ratio of carboxyl-terminated poly(2-(diethylamino)ethyl methacrylate), PDMS and curing agent was 1:5.5:0.55. Finally, the resulting copper mesh was dried at 60 oC for 3 h.

Fabrication of the miniaturized submarine The miniaturized submarine was comprised of a hull and a microactuator. The hull was built by folding the superhydrophobic copper mesh into a size of 20 mm × 10 mm × 5 mm. The microactuator was fabricated by the following process. At first, 150 mg of Fe3O4@PDA nanoparticles29 was added to 1.0 g of molten paraffin under

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stirring at 80 oC. Then a granular CaCl2 (3.92 ± 0.5 mm in diameter) was dipped into the mixture to homogeneously coat the particle with a black layer. The obtained particle was immersed in water to remove the CaCl2 core, giving rise to a hollow microsphere. After filling with ethanol by a micro-syringe, the microsphere was fixed to the stern of the hull. The total mass of the microsubmarine was ~350 mg. The diving-surfacing and horizontal movement of the microsubmarine The microsubmarine was placed on a mixture of water and CHCl3. Owing to the superhydrophobic property, the microsubmarine floated on the water surface. After bubbling with CO2 (at a flow rate of 400 mL min−1) for 5-8 min, the microsubmarine changed its wettability from superhydrophobicity to superhydrophibicity. The microsubmarine dove into water and stayed at the interface of water/CHCl3. After bubbling with Ar and then drying at 60 oC for 5-15 min, the microsubmarine regained its superhydrophobicity. The movements of microsubmarine at either water surface or water/CHCl3 interface were realized by the controlled release of ethanol from the microactuator. At first, the microactuator was irradiated by NIR laser (808 nm, 800 mW) at room temperature. The distance between the laser source and the microactuator was approximately 3 cm. Once the shell of the microactuator was pierced by the irradiation, the microsubmarine started to move at a given rate by the release of ethanol. Characterizations Scanning electron microscope (SEM, FEI Quanta 200) and energy dispersive spectrometer (EDS, FEI Quanta 200) were used to measure the surface morphology and compositions of the superhydrophobic microsubmarine. Proton nuclear magnetic resonance (1H NMR) spectrum was recorded by a Bruker 400 device. Contact angles were measured on an OCA-20 (DataPhysics Instruments GmbH) instrument. DCAT-21 (DataPhysics) was employed to record the adhesive forces. Infrared images were recorded by a thermal infrared imager (T360, FLIR systems, Sweden).

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. SEM images, EDS maps and water contact angle of copper mesh; Adhesive force of the as-prepared microsubmarine in air and underwater; SEM image of the shell of the microactuator and NIR Infrared thermograph of the microactuator before the irradiation of NIR; Schematic illustration for the switch of chemical structure of the PDEAEMA-CTA chains; The underwater stability of the as-prepared microsubmarine; Controlled movement of the miniature submarine at the interface of n-dodecane/water; The controlled transport and release of pH indicator at the water/CHCl3 interface using the smart submarine. NMR spectra of carboxyl-terminated poly (2-(diethylamino)ethyl methacrylate). Acknowledgments This work was financially supported by China Postdoctoral Science Foundation (No. 2017M621262) and Fundamental Research Funds for the Central Unversities (No. HIT. NSRIF. 201831). The authors thank Prof. Xiangjun Zhang for the infrared thermal imager measurements. Conflict of Interest The authors declare no conflict of interest.

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TOC Air Water Water

Air Water 152.8 ± 2.0o

CHCl3

CHCl3 151.0 ± 1.5o

The reversible transfer of CO2-responsive miniature submarine at the interface of water/CHCl3.

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Controlled Movement of a Smart Miniature Submarine at Various

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