Precise Placement of Microbubble Templates at Single Entity

Letter Cite This: ACS Macro Lett. 2018, 7, 1267−1271

pubs.acs.org/macroletters

Precise Placement of Microbubble Templates at Single Entity Resolution Ji Tae Kim,*,† Jaeyeon Pyo,‡ Seung Kwon Seol,‡,§ and Jung Ho Je*,∥ †

Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China Nano Hybrid Technology Research Center, Korea Electrotechnology Research Institute (KERI), Changwon-si, Gyeongsangnam-do 51543, Republic of Korea § Electro-Functionality Materials Engineering, Korea University of Science and Technology (UST), Changwon-si, Gyeongsangnam-do 51543, Republic of Korea ∥ X-ray Imaging Center, Department of Materials Science and Engineering, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang 37673, Republic of Korea

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‡

S Supporting Information *

ABSTRACT: Microbubbles have been used as a soft template to produce hollow structures for diverse applications in chemistry, materials science, and biomedicine. It is a challenge, however, to control their size and position at singleentity level. We report on an on-demand method to produce and place a single microbubble with programmed size and position. The method exploits scanning an electrolyte-filled micropipette to place a hydrogen (H2) bubble, generated by water electrolysis, on the desired position. The bubble growth is self-limited after the bubble size fits to the pipet aperture, yielding well-controlled bubble size. The bubble growth dynamics within the pipet is successfully investigated by a methodology that combines phase-contrast X-ray imaging and electric-current measurement. We show that the microbubbles, accurately controlled in size and position, can be used for the fabrication of various polypyrrole microcontainer arrays. We expect the scanning-pipet strategy could be generalized for manipulating various soft materials at will.

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icrobubbles1−4 have been widely investigated for their practical applications in soft condensed matter physics, chemistry, biomedicine, materials science, and mechanical engineering. The main examples include microfluidic components5,6 and diagnostic7,8/therapeutic9−11 agents. Recently, the utilization of microbubbles as a template has been emerging as an effective strategy to increase the productivity in threedimensional (3D) micro/nanofabrication.12−15 Compared to hard templates, such as anodic aluminum oxide (AAO) membranes16,17 and colloidal particles18,19 that require post template-removal, a microbubble can be used as a soft template that enables us to fabricate, without the postremoval process, hollow structures of diverse materials from metals,20−22 ceramics,23−25 to polymers.12,26,27 Conventional methods28 for microbubble soft templates, which are largely based on sonication, shear stirring, and electrolysis, offer high yield and low cost production but poor control over their size and position. Recently, the size uniformity has been significantly improved by employing microfluidic apparatuses for microbubble production.29−31 Specifically, microbubbles with polydispersity below 5% and a productivity up to 103 entities/s have been obtained by hydrodynamic flow control of gas/liquid fluids with the guidance of microchannels.30,31 However, precise positioning © XXXX American Chemical Society

of individual microbubbles on desired locations32 still remains a longstanding challenge. Here, we develop an on-demand open microfluidic method that produces and places microbubbles at single entity resolution. The method is based on using a micropipette, a laboratory apparatus allowing us to spatiotemporally control mass transfer or chemical reaction.33−40 By localized electrolysis of water in the pipet, we produce and place single hydrogen (H2) microbubbles on the desired positions. Visualization of growth dynamics of the microbubbles using real-time phase-contrast X-ray imaging41−44 enables us to reveal that the bubble growth is self-limited to the size of the pipet aperture. From this, a high degree of control over the bubble size is achieved by varying the pipet aperture. Finally, we demonstrate that microbubbles produced at the single entity level can be used as a soft template for fabricating polypyrrole (PPy) microcontainer arrays12,27,45 with programmed size and position. Received: August 26, 2018 Accepted: October 1, 2018

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Figure 1. Placement of a microbubble. (a) Schematic illustration of localized bubble formation by a micropipette. A hydrogen bubble is formed by electrolysis of water at the contact area of a water-filled micropipette on the Pt substrate negatively biased at −1.0 V (rb: the bubble radius, rp: the pipet aperture radius). (b−f) Sequential microradiographs show (b) absence of bubbles, (c) nucleation of 2−3 bubbles when the pipet contacts on the substrate (t = 0.0 s), (d) coalescence into one bubble (t = 0.5 s), (e) its growth with a spherical shape until the bubble fits to the pipet (rb ≈ rp) at t = 1.0 s, and (f) a decrease in the bubble (pill-like shape) growth rate (t = 2.0 s) (scale bar: 50 μm).

Figure 2. Dynamics of electrolytic bubble growth in a micropipette. (a,b) The bubble growth is self-limited due to electrical insulation of the bubble. (a) The rate of the bubble growth, dVb/dt, as a function of time, t (Vb: the volume of bubble, measured from Figure 1), with an applied voltage of −1.0 V to the substrate. (b) The measured electric current, I, as a function of t. When the bubble fits to the pipet, rb ≈ rp, a drastic decrease in I, corresponding to that in dVb/dt (a), is observed (red arrow). (c) The point of rb ≈ rp vs rp.

Movie S1 in Supporting Information) clearly exhibit how an electrolytic H2 bubble is grown inside the pipet aperture. No bubble is produced before the pipet−substrate contact, as shown in Figure 1b. As soon as the micropipette contacts the substrate that is negatively biased at −1.0 V, a couple of very small H2 bubbles are nucleated (Figure 1c) and merged into one bubble (Figure 1d−f). The merged spherical bubble, which has a contact angle of ∼145° on the Pt substrate, continuously grows until the bubble radius fits to the pipet radius, rb ≈ rp (Figure 1e). Afterward, the bubble shape transforms from a spherical to a pill-like shape, drastically reducing the bubble growth rate mostly due to electrical insulation of the bubble (Figure 1f). A single microbubble was produced in a second, which is slower than the conventional microfluidic approach.31 Figure 2a plots the growth rate of the bubble shown in Figure 1, dVb/dt, as a function of time t where Vb is the bubble volume, measured from the X-ray images of Figure 1. It is interesting to note that dVb/dt drastically decreases around rb ≈ rp. For rb < rp, the bubble growth rate is fast and governed by electrolysis of water. After rb ≈ rp, however, the bubble growth is self-limited as indicated in Figure 2a. Our phase-contrast Xray imaging provides the observation of a bubble with high spatiotemporal resolution, but its practical use still faces a longstanding claim that is the inapplicability of synchrotron facility to the manufacturing platform.

Figure 1a depicts our micropipette-assisted method to produce a microbubble. A micropipette (the pipet aperture radius, rp = 40 μm) filled with water (electrolyte) is initially placed at a separation of few tens of μm from a platinum (Pt)coated silicon (Si) substrate. Here, the Pt substrate, the working electrode, is negatively biased. (The counter electrode is immersed in the electrolyte inside the pipet.) As the pipet is moved down and contacts with the substrate, a H2 gas bubble is generated by electrolysis of the water confined in the pipet aperture as follows. 2H+ + 2e− → H 2(g)

We remark that one central requirement of this method is to control the bubble size (noted as the radius of the spherical bubble: rb). Thus, one important questionhow can one guarantee a high degree of control over the bubble size? motivated us to quantitatively investigate bubble growth dynamics. Conceivably, it is not easy to monitor the micronsized bubbles with optical microscopy, mostly due to substantial light scattering from the glass surface of the pipet. In this study, we employ real-time phase-contrast imaging with coherent synchrotron X-rays.41−44 This imaging method enables us to clearly visualize the boundaries of micron-sized bubbles even inside a micropipette and therefore makes it possible to investigate their growth dynamics with high accuracy. Sequential microradiographs in Figure 1b−f (and 1268

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Figure 3. Fabrication of a polypyrrole (PPy) microcontainer on a microbubble (a soft template). (a) A H2 microbubble is formed in the localized pyprrole (Py) monomer aqueous solution in a micropipette on the substrate with a negative bias of −1.0 V. (b) Electropolymerization along the bubble surface is implemented by switching the bias to a positive voltage of 1.5 V, forming a PPy microcontainer. (c−e) Sequential microradiographs showing the growth of the H2 bubble until the bubble fits to the pipet, rb ≈ rp. (f−h) Sequential microradiographs showing the electropolymerization. (f) PPy is grown along the bubble surface, upward from the substrate, and (g) the growth is continued with the guidance of the pipet (as magnified in the inset). (h) The PPy growth is terminated by the separation of the pipet from the substrate (scale bar: 50 μm).

Figure 4. Precise placement of 3D microcontainers. (a) A line array of PPy microcontainers with 80 μm diameter with 250 μm step (scale bar: 500 μm) and (inset) its magnified image (scale bar: 200 μm). (b) A 3 by 4 array of PPy microcontainers (diameter: 80 μm) fabricated with high uniformity (scale bar: 500 μm) using a micropipette of rp = 40 μm. (c) Histogram shows the average diameter of the microcontainers fabricated by a micropipette of rp = 40 μm is 77 μm (SD: 5.1 μm). (d) Controlled size of microcontainers by changing the micropipette aperture radius, rp (scale bar: 100 μm).

To alternatively find the point of rb ≈ rp, we measure electrical current, I, as a function of t, during the bubble growth under a constant voltage of −1.0 V, as plotted in Figure 2b. We point out that the current behavior in Figure 2b is very similar to that of dVb/dt (Figure 2a) except the initial I peak (gray circles in Figure 2b) that results from the consumption of H+ ions near the substrate. The current decline for rb < rp, which is toward a steady-state value (dashed line), is due to the increase in the depletion depth.46 Around rb ≈ rp, however, the current drastically drops (see the green window region in Figures 2a and b). The result suggests a practical way to find the point of rb ≈ rp as the current drop point (red arrow in Figure 2b) without the aid of synchrotron X-ray imaging. This gives us an insight how to control the bubble size with sufficient reliability. Figure 2c plots the point of rb ≈ rp as a function of the pipet

aperture radius rp, obtained from both the direct X-ray imaging and the electric current measurement in real time. This plot displays a linear tendency, which is conceivable from the initial growth stage in the Rayleigh model.47 To demonstrate the usefulness of the size-controlled bubble as a soft template, we fabricated PPy microcontainers that can be potentially used as miniaturized reactors for picoscale chemistry, a drug delivery system, and sensors.12,27 We employed an aqueous solution containing Py monomer and 2-naphthalenesulfonic acid (β-NSA) (surfactant) for fabricating the PPy microcontainers. The fabrication process consists of two steps as illustrated in Figure 3a and b: (1) Formation of a single H2 bubble on the substrate with a negative bias of −1.0 V (Figure 3a and microradiographs in Figure 3c−e) and (2) electropolymerization of PPy on the bubble by switching the 1269

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Pt-coated (100 nm thickness) Si wafers were used as an electrically conductive substrate. Fabrication of Microbubbles and Microcontainers. Positioning of the micropipette on the Pt substrate is precisely implemented by using 3-axis stepping motor stages (KOHZU Precision) with submicrometer precision. The control and monitoring of electric current were conducted using a potentiostat. To monitor the experimental process in real time, phase-contrast X-ray imaging was performed at the 7B2 beamline at the PLS 2.5 GeV, 150 mA storage ring in Pohang, Korea. The real-time microradiographs with a spatial resolution of 0.5 μm were obtained using a CdWO4 scintillator crystal and a CCD camera. Using home-built software, we synchronized the image acquisition and electric current measurement. The fabricated microcontainers were characterized using field emission scanning electron microscopy (FE-SEM, PHILIPS XL30SFEG).

bias to a positive value of 1.5 V (Figure 3b). The bias switching is performed at the point of rb ≈ rp (Figure 2c). Sequential microradiographs in Figure 3f−h clearly show that PPy is synthesized along the bubble surface, i.e., along the liquid/gas interface (see the inset of Figure 3g). We finally move up the pipet to terminate the PPy growth, forming a microcontainer with an opening mouth (Figure 3f). If the bias switching is performed after rb ≈ rp, the PPy growth becomes inactive due to electrical insulation of the bubble. The whole fabrication process of a PPy microcontainer is shown in Movie S2 in the Supporting Information. Note that the microcontainer shape can be controlled by adjusting the positive bias as reported previously.48 The result demonstrates a minimalist method to produce bubble templates and fabricate hollow (microcontainer) structures using a one-unit apparatus. Figure 4 demonstrates the fabrication of microcontainers with high size uniformity. By scanning a micropipette, we can fabricate microcontainers in programmed positions on the substrate. This capability was demonstrated by fabricating two representative pattern geometries: (1) a line array (Figure 4a) and (2) a rhombus-shape array (Figure 4b) of PPy microcontainers with a uniform size (diameter: 80 μm). The histogram in Figure 4c represents the size distribution of the microcontainers fabricated using a micropipette of rp = 40 μm. The average diameter is 77 μm with relative standard deviation of 6%, which is comparable to the value in the conventional microfluidic approach.30,31 Furthermore, the microcontainer size can be precisely controlled by changing the pipet aperture size. Figure 4d shows various sizes of microcontainers obtained with different apertures of rp = 25, 20, and 7.5 μm. The patterned PPy microcontainers with low density, high surface area, and short mass/charge transport length could potentially be used for integration of microdevices such as microreactors and sensors with programmed performance.12,27 In this work, we developed a micropipette-assisted ondemand method to produce and place microbubble soft templates at single entity resolution with a high degree of control over size and position. We revealed that the electrolytic bubble growth is self-limited inside the pipet, enabling uniformity and controllability in the bubble size. We expect the shapes of microbubble templates could be controlled by their wetting nature on various substrates. Furthermore, the use of a multibarrel micropipette system35 could provide the potential to improve the productivity in this method. Although in this work we restricted ourselves to electropolymerization of conducting polymers, the method can be generalized for various material growths and colloidal self-assemblies accompanying a decrease in the interfacial energy by materials adsorption on the bubble surface. This result could provide a minimalist, cost-effective, but high degree of control manner to manipulate diverse soft materials at single entity resolution. We believe that this “produce-and-place” approach could also be a protocol for employing soft templates to directly integrate hollow micro/nanostructures on various bionanodevices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00646.

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H2 microbubble formation in a micropipette (AVI) Electropolymerization of PPy on a bubble (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ji Tae Kim: 0000-0003-4662-0179 Seung Kwon Seol: 0000-0002-8733-4374 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Early Career Scheme (HKU 27207517) from the Research Grants Council of Hong Kong, the Seed Fund for Basic Research from the University of Hong Kong (201611159002), and National Research Foundation (NRF) grant funded by the Korea government (2017R1E1A1A01075274).

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EXPERIMENTAL SECTION

Sample Preparation. Glass micropipettes with controlled aperture diameters in the range of 10−50 μm were fabricated by a programmed heat-pulling process (P-97 Flaming/Brown Micropipette Puller, Sutter Instrument). For the microcontainer fabrication, the aqueous monomer solution containing 0.35 M pyrrole (Acros) and 0.2 M 2-naphthalenesulfonic acid (Aldrich) was used as an electrolyte. 1270

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