Trace Material Capture by Controlled Liquid Droplets on a

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Trace material capture by controlled liquid droplets on a superhydrophobic/hydrophilic surface Kenta Fukada, Naoya Kawamura, and Seimei Shiratori Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b02369 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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Analytical Chemistry

Trace material capture by controlled liquid droplets on a superhydrophobic/hydrophilic surface Kenta Fukada†, Naoya Kawamura‡ and Seimei Shiratori*†‡ †

Center for Material Design Science, School of Integrated Design Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan

‡

Department of Applied Physics and Physico-Informatics, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan

ABSTRACT A liquid droplet in contact with a superhydrophobic surface can be used to collect dissolved trace materials after evaporating the solvent. This process effect enhances detection limits, but a liquid droplet easily rolls off a superhydrophobic surface. Keeping it at a specific collecting spot area is challenging. Here the means for controlling and capturing a liquid droplet on a superhydrophobic surface is demonstrated. To induce a liquid droplet to a collecting spot, its rolling direction was controlled by two superhydrophobic fabric guides. The liquid droplet was then captured by hydrophilic polymer and hydrophilic nanoparticles at the measuring spot. After removing the solvent, the trace compounds were evaluated with a colorimetric analysis visible to the naked eye. KEYWORDS: Condensing enrichment, Trace material, Caffeine, Superhydrophobic surface, Water absorbent-polymer

INTRODUCTION Monitoring trace substances to make sure that they do not exceed toxic levels is important in human healthcare. For example, concentrations of antigens1, DNA2,3, and drugs4 in human fluids are inspected for disease diagnosis or for assessing effects of treatments. Surveys of harmful materials in drinking water5 or the environment6 are also important. Caffeine is a purine ring compound that is widely consumed. However, amounts exceeding 400 mg/day or 15 µg/ml in the plasma have serious toxicities; 5 g/day or 80 µg/ml in the plasma is considered lethal7-9. The anti-HIV drug Abacavir also has a purine ring and its concentration in the plasma or saliva is monitored for therapy effects at levels ranging from 1 ng/ml to 10µ g/ml10. Generally, high-performance instrumentation or specific reactions with target materials are utilized for a trace substance analysis. Liquid chromatography–tandem mass spectrometry

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is commonly used; however, its cost and required expertise are often prohibitive11,12. Antibody−antigen recognition and synthesized dyes are often preferred for detection of target materials. However, antibodies are easily denatured by temperature changes13 and their synthesis usually requires multiple steps and considerable time14,15. The collection of target materials to a specific area by changing surface wettability has been identified as an easy way to enhance detection limits. In the “coffee ring effect,” hydrophilic or hydrophobic surfaces accumulate nonvolatile materials at the boundary of a liquid droplet. This method uses only the limited area of the circle, leaving much of the surface area unused

16,17

. A superhydrophobic surface with a small hydrophilic patterned

area to interface with an aqueous liquid droplet has been utilized for enrichment of trace materials. For example, gradient hydrophobicity for centering a liquid droplet has been fabricated by photolithography18. Superhydrophilic micro-wells was fabricated with UV irradiation

of

octadecytrichlorosilane

19

photomasks . Inkjet printing

20

(OTS)-modified 21

or laser printing

nanodendritic

coatings

with

technology enabled the deposition of

hydrophilic materials directly without photomask. The condensing enrichment effect of a hydrophilic area on a superhydrophobic surface is suitable for enhancing detection limits. However, it is not ideal for collecting valuable or toxic materials because the liquid droplets can easily roll off the superhydrophobic areas. In this study, we examined condensing enrichment detection of trace materials by means of controlling and capturing a liquid droplet on a superhydrophobic surface that had hydrophilic collection areas. Aligned fiber or fabrics were used to control a liquid droplet22-24, and the flow path was formed by superhydrophobic fabric guides25. Normally, a flow path was directly deposited on the substrate, which made it difficult to later change direction or width26-28. Here, the superhydrophobic fabric guides were easily applied in a triangle structure with an interval that gradually decreased downstream towards a hydrophilic spot area formed by water-absorbent polymer and SiO2 nanoparticles having a 8-11 nm average diameter to avoid optical scattering29. Therefore, a liquid dropped on the superhydrophobic surface was automatically guided to and captured by the hydrophilic spot area. The enhanced adhesion strength was also investigated by changing the mixing ratio of these materials. Caffeine was the target material and the murexide reaction30 was utilized for caffeine sensing. After a droplet containing caffeine was captured and thermally heated at the hydrophilic spot, H2O2 and HCl were safely added via superhydrophobic fabric guides for a color reaction. The concentration of trace caffeine was estimated by the color intensity and calibration curves.


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EXPERIMENTAL SECTION Materials For device fabrication, hydrophobic fumed silica (hydrophobic SiO2, AEROSIL® RX200, EVONIK, Essen, Germany), hydrophilic colloidal silica (hydrophilic SiO2, ST-OS, Nissan Chemical Industries. Ltd., Tokyo, Japan), poly(vinyl alcohol) (PVA, Polymerization Degree about 1500, Wako Pure Chemical Industries, Ltd., Osaka, Japan), and polyester fabrics obtained by polyester mesh (clever, Aichi, Japan) were used. For sensing and target materials, anhydrous caffeine (Wako Pure Chemical Industries, Ltd., Osaka, Japan), (+)-catechin hydrate (Tokyo Chemical Industry Co., Ltd, Tokyo, Japan), abacavir (Tokyo Chemical Industry Co., Ltd, Tokyo, Japan), indole (Wako Pure Chemical Industries, Ltd., Osaka, Japan), and urea (Wako Pure Chemical Industries, Ltd., Osaka, Japan) were acquired without further purification. Device fabrication The fabrication of the detector is schematically depicted in Figure 1. A superhydrophobic surface was fabricated by dip coating. Specifically, a poly(ethylene terephthalate) film (PET, 2.5 cm × 6.0 cm) was dipped in hydrophobic SiO2 (4.0 wt% in ethanol) that had been stirred for 24 hours, and was then removed at a speed of 7.5 mm/s. Then, various mixing ratios of aqueous PVA/hydrophilic SiO2 were dropped (10 µl) on the superhydrophobic surface and dried to form superhydrophobic/hydrophilic patterned films. Superhydrophobic fabric guides were combined with the patterned film. Spacers (height=1 mm) were attached to both edges of the film and the superhydrophobic fabrics were coated with hydrophobic SiO2 to make a flow path. Sensing procedure A 10µl droplets containing target materials were safely guided by the flow hydrophobic path and were pinned at hydrophilic collection spots. The solvent was removed by heating to 60°C for 30 min. Then a 10µl mixture of H2O2 (33wt%, 5 µl) and HCl (1 M, 5 µl) was safely guided along the flow path. After heating to 100°C for 30 min, a colorimetric change was observed by color difference meter (Color Reader CR-13, KONICA MINOLTA, INC., Tokyo, Japan). The measurement area was masked with black tape for 1mm2.

RESULTS AND DISCUSSION Detection of trace materials A detector for trace materials in aqueous solutions would be widely used for safety and



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environmental monitoring. A liquid droplet containing a small amount of target material (caffeine) was analyzed by using the superhydrophobic/hydrophilic patterned film along with superhydrophobic fabric guides. The detection scheme in Figure 1 was performed in three steps as follows. 1) A small liquid droplet was pinned at the partially hydrophilic area on the superhydrophobic flexible substrate. 2) The excess solvent was removed by heating. 3) The collected target material was oxidized with H2O2 and HCl and the color change was measured to estimate the target quantity. A white-to-red color change for caffeine detection resulted from the oxidized purine ring (see Figure S1). Superhydrophobic/hydrophilic patterned film The water absorption property of PVA and the hydrophilicity of SiO2 were utilized for capturing liquid droplets on the superhydrophobic/hydrophilic surface. Various PVA/ SiO2 mixing ratios were used to optimize this activity: (a) SiO2 (0.2 wt%), (b) SiO2 (0.15 wt%)/PVA (0.05 wt%), (c) SiO2 (0.1 wt%)/PVA (0.1 wt%), (d) SiO2 (0.05 wt%)/PVA (0.15 wt%), and (e) PVA (0.2 wt%). The 10µl aqueous solutions were dropped on the superhydrophobic surface and heated at 120°C for 30 min, as shown in Figure 2. The (a) and (b) mixtures with mostly nanoparticles exhibited coffee ring structures, and (a) also easily peeled off because of weak bonding with the substrate. In contrast, the (d) and (e) mixtures with mostly polymer exhibited balloon structures and were not suitable for precise material analysis. Mixture (c) SiO2 (0.1 wt%)/PVA (0.1 wt%) exhibited a flatter surface relative to the coffee ring or balloon structures and was the best candidate for the hydrophilic spot. The optimum annealing temperature (20°C, 60°C, and 120°C) for fabricating the hydrophilic spot formed with SiO2 (0.1 wt%)/PVA (0.1 wt%) was determined, as shown in Figure 3(a). The spot diameter was enlarged for temperatures over the boiling point, and were smaller for temperatures under the boiling point. This difference was attributed to the remaining materials left by rapid evaporation at the edge of the solid/liquid interface, as shown in Figure 3(b). The 20µl PVA / SiO2 droplet exhibited a large spot area, whereas, the 5µl or 10µl droplets had almost the same smaller diameter. Because of the ease in handling the viscous liquid, 10µl droplets were chosen to fabricate hydrophilic spot areas. Comparison of hydrophilic spot areas for caffeine sensing Various hydrophilic spot areas for caffeine detection were investigated. The mixtures SiO2 (0.2 wt%), SiO2 (0.15 wt%)/PVA (0.05 wt%), SiO2 (0.1 wt%)/PVA (0.1 wt%), SiO2 (0.05 wt%)/PVA (0.15 wt%) and PVA (0.2 wt%) were used. After the color reaction, coffee ring or balloon structures were observed as shown in Figure 4(a), except for the SiO2 (0.1 wt%)/PVA (0.1 wt%) mixture. Oxidized caffeine is shown in the Figure 4(b), where the Fourier transform infrared (FT-IR) peaks at 1436 cm-1, 1742 cm-1 and 3322 cm-1 were used


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to determine the oxidization ratio. From the comparison of a non-optimized hydrophilic spot area (SiO2/PVA = 0.1 wt%/0.1 wt% and 120°C annealing temperature ) and an optimized hydrophilic spot area (SiO2/PVA = 0.1 wt%/0.1 wt% and 60°C annealing temperature) it was found that the intensity and concentration had a high correlation, as shown in Figure 4(c,d). Liquid flow path via superhydrophobic fabric guides In generally it was difficult to drop a liquid droplet on a specific hydrophilic spot area due to liquid swaying at the pipette tips by hand tremors (see Figure S6). Therefore, we fabricated two superhydrophobic fabric guides for precisely transferring liquid droplets to the hydrophilic measuring spots. As shown in Figure 5(a), the flow path resolved the difficulty of dropping a liquid at a specific area that is surrounded by superhydrophobic materials. The sliding angle of liquid droplets was controlled by changing the intervals of the fabrics (see Figure S7). For fabrics intervals equal or greater than the 10µl droplet diameters, the sliding behavior was not hindered by the ultra-wettability of the fabrics guides. In contrast, narrow intervals led to high sliding angles, possibly because of pressure from the fabrics in the narrow path. This flow path can be changed by the interval depending on the liquid droplet size; the ease of flow path changes is an advantage of using two superhydrophobic fabric guides. Therefore, large intervals were fabricated upstream for the ease of applying drops while narrow intervals were fabricated downstream for guiding the droplets to the enrichment site. The superhydrophobic fabric guides and the hydrophilic spot areas enabled a liquid droplet initially upstream to be transferred to the PVA/SiO2 complex, as shown in Figure 5(b). With an increasing PVA ratio in the hydrophilic spot or water absorption, the sliding angle of a liquid droplet increased and the retention was enhanced [Figure 5(c)]. Because of the PVA water absorbency, even a rolled liquid droplet was captured by the hydrophilic spot area containing SiO2 (0.1 wt%) / PVA (0.1 wt%). Selectivity of caffeine detection The detection selectivity of caffeine was demonstrated by comparisons with abacavir (ABC), catechin, indole, and urea. ABC is an anti-HIV drug that also has a purine ring. The color change from white to yellow due to oxidization is shown in Figure S1, where differences with caffeine were observed because of the less-oxidized purine ring of ABC. Results using the same detection procedure on the superhydrophobic/hydrophilic patterned film are shown in Figure 6. The ABC color change from white to yellow was observed, while the other materials had different color changes. The original color of catechin was yellow and thus a color change was not observed. Indole was evaporated during the heating step, while urea has a totally different chemical structure and a color change was not observed. Hence, this method can selectively detect caffeine by the color change reaction.



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Calibration curves by optical observation FT-IR peaks of oxidized caffeine on the superhydrophobic/hydrophilic patterned film and the colorimetric analysis with L* a* b* color space were shown in Figure 7(a). The reproducibility of caffeine sensing was confirmed by the color differences shown in Figure 7(b). The color value (a*) was related to the redness and calibration curves was calculated. Above 100 µg/ml, (a*) changed linearly. The difference in color was easily observed visually, which is an advantage of this method. In summary, the concentration of the trace material in a small liquid droplet was estimated by the condensing enrichment effect, FT-IR spectra, and colorimetric analysis (see Figure S11). Compared with commercialized one such as High Performance Liquid Chromatography(HPLC), this work also showed high reliability with easy and convenient methods (See Figure S12). This sensing method could be used to detect other trace materials that have specific color reactions. It will be particularly useful for outdoor inspection assays or point-of-care devices.

CONCLUSIONS A liquid droplet was guided by superhydrophobic fabrics and captured by a hydrophilic spot formed by a PVA/SiO2 complex. The organic/inorganic spot was a flat surface and was fabricated with 10 µl of SiO2 (0.1 wt%)/PVA (0.1 wt%) and annealed at 60°C. It was a suitable hydrophilic area compared with coffee ring structures formed by predominantly inorganic materials and balloon structures formed by mostly organic materials. Large fabric intervals upstream made it easy for drop application, while narrow intervals downstream enabled droplet control. The small interface between the superhydrophobic surface and a liquid droplet allows for collecting trace levels of caffeine. The concentration of the caffeine can be estimated visually via a colorimetric analysis. This device will detect materials safely without waste and will be suitable for in situ inspection of trace substances in the environment.


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Figure 1. Procedure for a trace substance detection (a) Device fabrication: A PVA/SiO2 mixture was enriched for forming a hydrophilic spot on a superhydrophobic surface. (b) Sensing: The quantity of caffeine can be estimated by concentration on the patterned film and observing a specific color reaction.



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Figure 2. Optimization of materials for fabrication of a hydrophilic spot area. (a) SiO2 (0.2 wt%), (b) SiO2 (0.15 wt%)/PVA (0.05 wt%), (c) SiO2 (0.1 wt%)/PVA (0.1 wt%), (d) SiO2 (0.05 wt%)/PVA (0.15 wt%), and (e) PVA (0.2 wt%). 10µl aqueous solutions were dropped on the superhydrophobic surface and heated at 120°C for 30 min. The organic/inorganic composition of the spot produced a flat surface. The mixture (c) was the most suitable for the hydrophilic spot area.

Figure 3. Optimization of fabrication temperature for the hydrophilic spot area using PVA (0.1 wt%) / SiO2 (0.1 wt%). The diameter of the hydrophilic spot area was smaller at temperatures under the boiling point and for quantities under 10 µl.


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Figure 4. Determination of a suitable hydrophilic spot area for caffeine sensing (a) Caffeine detection on hydrophilic spot areas having various mixtures of SiO2 (wt%)/PVA (wt%) noted in the images. (b) Oxidation of caffeine (c) FT-IR peaks of oxidized caffeine on a non-optimized hydrophilic spot area (SiO2/PVA = 0.1 wt%/0.1 wt% and an annealing temperature of 120°C) (d) FT-IR peaks of oxidized caffeine on an optimized hydrophilic spot area (SiO2 /PVA=0.1 wt%/0.1 wt% and an annealing temperature of 60°C).

Figure 5. Overview of a trace substance detector with a flow path. (a) The entire device was composed of superhydrophobic fabric guides on a superhydrophobic/hydrophilic patterned surface. (b) The direction of a falling droplet was controlled by the fabric guides until it was captured by a hydrophilic area for observation. (c) With an increasing PVA ratio in the hydrophilic spot, the sliding angle of the droplet increased, enhancing the water retention ability.



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Figure 6. Color reaction of enriched trace substances 10µl aqueous solutions with concentrations of 100 µg/mL, 250 µg/mL, 500 µg/mL and 750 µg/mL were enriched by condensation and reacted with H2O2 and HCl. Only caffeine exhibited redness from the reaction.

Figure 7. Calibration curves by colorimetric analysis (a) Specific FT-IR peaks of caffeine after the H2O2/HCl reaction on the patterned film and the colorimetric analysis with L* a* b* are shown. (b) Reproduction of color changes by caffeine sensing. (c) Calibration curve of optical value and caffeine concentration was shown. Concentrations of trace materials in a small liquid droplet can be estimated by colorimetric analysis.


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■ ASSOCIATED CONTENT Supporting Information Murexide reaction of caffeine and ABC, Contact angles, Sliding angles, FT-IR spectra, Color samples, Multivariate analysis, HPLC. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author *Seimei Shiratori. E-mail: [email protected]. Tel.: +81-45-566-1602. Fax: +81-45-566-1602. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS We are deeply grateful to Prof. Daniel Citterio, Dr. Yuki Hiruta, Dr. Yoshio Hotta, Dr. Kouji Fujimoto, Dr. Kyu-Hong Kyung, Mr. Taichi Nakashima, Mr. Yuki Tokura and Ms. Yukari Moriyama whose insightful comments and suggestions were of inestimable value for our study. HPLC measurement was supported by Dr. Yuki Hiruta. A part of this research was supported by SENTAN, from the Japan Science and Technology Agency (JST).

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Trace Material Capture by Controlled Liquid Droplets on a

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