Cross Channel Thread-Based Microfluidic Device for Separation of

Demonstration Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

pubs.acs.org/jchemeduc

Cross Channel Thread-Based Microfluidic Device for Separation of Food Dyes Chunxiu Xu,* Danli Jiang, Jiayi Lin, and Longfei Cai* School of Chemistry and Environmental Engineering, Hanshan Normal University, Chaozhou, Guangdong 521041, China S Supporting Information *

ABSTRACT: We described a method for electrophoretic separation of food dyes on a thread-based microfluidic analytical device. A cross channel thread-based microfluidic analytical device was fabricated and used to separate a mixture of food dyes. By controlling and adjusting the potentials applied onto four reservoirs, carmine and sunset yellow could be separated within 2 min. This method has the advantages of low cost, fast separation, and easy fabrication and operation, which make this very suitable for performing as a demonstration in universities. The students enjoyed this demonstration and presented some interesting and constructive questions after the demonstration, which indicated that the students were interested in the demonstration. This demonstration could also be modified to be a laboratory experiment which could be carried out within 3 h. KEYWORDS: Upper-Division Undergraduate, Analytical Chemistry, Demonstrations, Electrophoresis, Microscale Lab

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In the past few years, thread has been used as another cheap alternative for fabricating microfluidic analytical devices.22 Reches et al.23 have described the characteristics of thread as follows: (1) They have a low cost and broad availability. (2) They are flexible, lightweight, and difficult to break. (3) The fluids flow along the threads by capillary action owing to the hydrophilicity of threads, and thus no external power is needed. (4) They are easy to functionalize, and the devices could be disposed by burning. In this work, we described a demonstration of electrophoretic separation of food dyes on a thread-based device. A cross channel thread-based microfluidic device was fabricated and used to separate food dyes. By controlling and adjusting the potentials applied onto four reservoirs, the mixed food dyes were first loaded and then injected into a separation channel for electrophoretic separation. Only 3.5 min was required for the sample loading and electrophoretic separation, indicating its suitability as a demonstration in classroom or teaching lab. This demonstration was designed and used for a selective experiment of the Comprehensive Chemistry Experiment course for the undergraduates majoring in chemistry at the sixth semester.

n recent years, microfluidic chips (lab-on-a-chip) have emerged as a robust platform for single cell analysis,1−3 point-of-care testing,4,5 high throughput screening,6−8 and synthesis of materials and organic compounds,9,10 owing to the advantages of microfluidic chips over the traditional platforms, including low sample and reagent consumption, rapid analysis, and high degree of automation. A new trend in the microfluidic chips field is the introduction of microfluidic chips into chemical education in universities and colleges.11−17 The unique features, advantages, and phenomena observed in microfluidic experiments are unique and could motivate students to learn chemistry. A typical laboratory experiment in teaching lab is the observation of laminar flow and the calcite precipitation that occurred on the fluid−fluid interface.12 The bottleneck for introduction of microfluidics into teaching laboratory, however, is the sophisticated methods, including photolithography,18 chemical etching,19 and laser microfabrication,20 for fabricating microfluidic devices on silicon, polymers, and glass. Paper-based microfluidic analytical devices (μPADs) have advantages of low cost, easy and fast fabrication, and ease of use over those microfluidic devices fabricated on the substrates of glass, silicon, and polymers.21 These features make the μPADs a suitable platform to introduce microfluidic chips into a common teaching laboratory. In 2013, we described a laboratory experiment performed on μPADs: the students fabricated μPADs using a wax pen and then used these to determine amino acids in a tea leaf extract.14 Later, by demonstrating electrophoretic separation on a straight paper channel, we introduced two analytical techniques, electrophoresis and μPADs, to the classroom.16 In 2015, Wang et al.17 described a laboratory experiment performed on μPADs. The students fabricated the reaction and detection zones by simply patterning filter paper using a permanent marker pen, and then used this to measure nitrite in a colorimetric assay. © XXXX American Chemical Society and Division of Chemical Education, Inc.

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MATERIALS All reagents used were of analytical grade, and distilled water was used throughout. A mixed food dye solution composed of 1.0 g L−1 carmine and 1.0 g L−1 sunset yellow was prepared by dissolving 100 mg of carmine (Shanghai Maikun Chemical Co., LTD) and 100 mg of sunset yellow (Shanghai Maikun Chemical Co., LTD) in 60 mL of H2O and then diluted to 100 mL with H2O. A buffer solution (pH 9.3) composed of 10 mM NaH2PO4 and 10 mM Na2B4O7 was prepared by mixing Received: October 12, 2017 Revised: April 10, 2018

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DOI: 10.1021/acs.jchemed.7b00784 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Demonstration

156 mg of NaH2PO4·2H2O and 381 mg of Na2B4O7·10H2O in 60 mL of H2O and then diluted to 100 mL with H2O. A high voltage power supply with four platinum electrodes was provided by Dongwen High Voltage Power Supply (Tianjin) Co., Ltd.

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CROSS CHANNEL THREAD-BASED MICROFLUIDIC DEVICE The materials used for fabrication of thread-based microfluidic device were shown as Figure 1. Two cotton threads with

Figure 3. (a) Image showing four electrodes fixed into the plastic posts. (b) Image of a thread-based separation device with electrodes inserted into their respective reservoirs, and the posts fixed on a benchtop surface.

would flow along the threads by capillary action. After the threads were completely wetted with buffer, 120 μL of mixed food dye solution was added into the sample reservoir. A high voltage power supply was used to adjust the potentials applied onto four reservoirs. In the sample loading step, 50, 600, 200, and 200 V were applied onto S, SW, B and BW, respectively, for 90 s. In the subsequent sample injection and separation step, 200, 200, 50, and 600 V were applied onto S, SW, B, and BW, respectively, for 120 s. The images during sample loading, injection, and separation were obtained using a smart mobile phone. The separation resolution, R, was calculated as the following equation: 2D R= W1 + W2 (1)

Figure 1. Materials required for making a cross channel thread-based microfluidic device, including a ruler, a glass slide, cotton thread, knife, epoxy resin, and micropipette tips.

lengths of 7.5 and 2.5 cm were cut with a scissor, and the threads were then washed with soap and water sequentially. Four 5 mm inner diameter and 8 mm tall micropipette tips, serving as reservoirs, were cut from plastic micropipette tips of 300 μL. A slit of 2-mm-tall and 1-mm-wide was then made on the top of tips by cutting with a knife. The slits were used to support the threads by sticking tails of these threads into slits, with ends of threads reaching the bottom of reservoirs. The distances from the cross junction to the sample reservoir (S), sample waste reservoir (SW), buffer reservoir (B), and buffer waste reservoir (BW) were 6, 6, 6, and 55 mm, respectively. The reservoirs were then fixed onto a glass slide using epoxy resin surrounding the reservoirs (Figure 2). Details of fabricating the thread-based device were also available in Supporting Information.

where D is the distance between two separated dyes in the separation channel, and (W1 + W2)/2 is the averaged width of the two separated dyes. (The effects of various parameters on separation resolution were provided in the Supporting Information.)

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HAZARDS The apparatus, especially the high voltage power supply and electrodes, should be handled with caution. The instructor should wear nonconductive gloves and keep away from the electrodes and power supply during sample loading, sample injection, and electrophoretic separation when the high voltage was applied onto threads. The high voltage power supply must be electrically grounded before use, and the power of the high voltage power supply should be switched off before sample loading and after electrophoretic separation was completed. Additionally, the chemicals used in this experiment, including carmine, sunset yellow, NaH2PO4, and Na2B4O7 may be harmful to the skin and respiration system. The operators should wear protective gloves and a long-sleeve lab coat when dealing with these chemicals.

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DEMONSTRATION On the cross channel thread-based microfluidic device, the sample was injected for electrophoretic separation by pinched injection. In the sample loading step, 50, 600, 200, and 200 V were applied onto S, SW, B, and BW, respectively. Carmine and sunset yellow which are negatively charged would flow from S to SW, while buffer solution in B and BW flow to SW, generating a pinched flow of sample solution from S to SW (Figure 4). In the subsequent sample injection and electrophoretic separation step, 200, 200, 50, and 600 V were applied onto S, SW, B, and BW, respectively; the sample at the cross intersection was thus injected into the separation channel for electrophoretic separation. Meanwhile, the sample remaining in the sampling channel would flow back to S and SW,

Figure 2. Image of a cross channel thread-based device fabricated with the materials shown in Figure 1.

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PROCEDURE FOR SEPARATION OF FOOD DYES Four platinum electrodes were welded onto the terminals of electric wire, and then fixed to the plastic posts (cut from the plastic tube) using adhesive tape (Figure 3a). After these electrodes were inserted into the reservoirs, the plastic posts were fixed onto a benchtop surface using double-sided tape (Figure 3b). A 120 μL portion of buffer solution was added into reservoirs B, BW, and SW, respectively; the buffer solution B

DOI: 10.1021/acs.jchemed.7b00784 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Demonstration

has advantages of rapid separation, easy fabrication, low cost, and easy operation. These features make it very suitable to perform as a demonstration in universities. It takes less than 5 min to perform this demonstration with the aid of a high voltage power supply because the sample could be loaded, injected, and separated within 3.5 min. Thus, when this demonstration was modified to a laboratory experiment, a couple of high voltage power supplies in the teaching laboratory should be sufficient for all the students to take turns to run the experiments. Questions and feedback from the students include these examples listed below: Could this pinched injection and electrophoretic separation be performed on a paper-based analytical device? Does electro-osmotic flow exist on threads? How can we quantify these separated food dyes? Answering these questions could improve and strengthen the understanding of students in microfluidic analysis, electrophoretic separation on paper-based and thread-based microfluidic analytical device.

Figure 4. Time-sequence images illustrating the sample flow in the sampling channel. (a−d) Images captured at time intervals of 0 (a), 30 (b), 60 (c), and 90 (d) s after potentials were applied onto reservoirs during the sample loading step. The potentials applied onto reservoirs S, SW, B, and BW were 50, 600, 200, and 200 V, respectively.

respectively (Figure 5). The total time needed for sample loading, sample injection, and electrophoretic separation was

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00784. Detailed procedure for preparations and step-by-step instructions (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Authors

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

Figure 5. Time-sequence images illustrating sample injection and separation on a thread-based device. (a−f) Images captured at time intervals of 15 (a), 30 (b), 60 (c), 90 (d), 105 (e), and 120 (f) s after potentials were applied onto reservoirs during the sample injection and separation step. The potentials applied onto reservoirs S, SW, B, and BW were 200, 200, 50, and 600 V, respectively.

ORCID

Longfei Cai: 0000-0001-7025-8048 Notes

The authors declare no competing financial interest.

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3.5 min, demonstrating the fast speed of this thread-based microfluidic separation system for electrophoretic separation of food dyes. This demonstration was designed for the students majoring in chemistry at the sixth semester, when the students have studied two important courses, Analytical Chemistry and Instrumental Analysis. Owing to the great potentials of microfluidic analysis in analytical applications, student exposure to microfluidic analysis is important. This demonstration is a cheap and suitable alternative for introducing microfluidic analysis to undergraduate students. The main objectives of this demonstration are (1) Instructors can introduce the microfluidic concept to undergraduates by a relative cheap and easyto-operate way. (2) Through exposure to this demonstration, the students realize that materials in daily life could also be used for analytical applications, which may motivate their passion and interests in learning analytical chemistry. (3) Electrophoretic separation could be easily observed on thread through this demonstration, which may be helpful for the students to understand electrophoretic separation.

ACKNOWLEDGMENTS The authors thank Dr. Meng Sun at the University of Michigan for checking and polishing the writing of the paper. Financial support from the Research Start-up Fund of Hanshan Normal University (Grant QD20120521 and QD20110616) is gratefully acknowledged.

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CONCLUSIONS We designed a cross channel thread-based microfluidic device for electrophoretic separation of mixed food dyes. This method C

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DOI: 10.1021/acs.jchemed.7b00784 J. Chem. Educ. XXXX, XXX, XXX−XXX