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Inner surface modification of poly(dimethylsiloxane) microchannel with chitin for electrophoretic analysis of proteins

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Abstract

Microchip electrophoresis in a poly(dimethylsiloxane) microfluidic device is one of the versatile separation techniques in a micro total analysis system, while an unstable electroosmotic flow depending on an inner surface of a microchannel and a nonspecific adsorption of biogenic compounds onto the inner surface are often problematic in microchip electrophoresis. To overcome these drawbacks, a chitin-coated poly(dimethylsiloxane) microchannel was newly developed by a relatively simple experimental procedure based on vacuum drying. The obtained chitin-coating showed high durability during 10-times experiments with a stable electroosmotic flow. It was also confirmed that the chitin-coated poly(dimethylsiloxane) microchannel provided the pH-dependent electroosmotic flow related to the dissociations of amino and silanol groups of chitin and poly(dimethylsiloxane), respectively. The nonspecific adsorption of fluorescently labeled proteins onto the inner surface of the channel was well suppressed by the coating, resulting in the sharp and symmetric peak without tailing in the microchip electrophoretic analysis of proteins. The separation of the proteins was also demonstrated in the chitin-coated microchannel, so that the resolution and reproducibility of the migration time were improved as compared to those in the untreated microchannel.

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Acknowledgments

K.S. is grateful to the Grant-in-Aid for Young Scientists (B) (22750070) from the Japan Society for the Promotion of Science (JSPS), Grant-in-Aid for the Global COE Program from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and the Shimadzu Science Foundation. A part of this development was made by the SENTAN from the Japan Science and Technology Agency (JST).

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Authors and Affiliations

  1. Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto, 615-8510, Japan

    Kenji Sueyoshi, Yusuke Hori & Koji Otsuka

Authors
  1. Kenji Sueyoshi
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  2. Yusuke Hori
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  3. Koji Otsuka
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Corresponding author

Correspondence to Kenji Sueyoshi.

Appendices

Appendix: Normalization of the values of electroosmotic mobility

In the experiment, the values of electroosmotic mobility (μeo) were estimated by employing the untreated/chitin-coated microchannels filled with the various background solutions (BGSs). It is well-known that a value of μeo depends on both the pH and ionic strength (I) of the BGS. Thus, the experimentally obtained values of μeo under the different BGS conditions should be normalized by the ionic strength to evaluate the dependence of the μeo on the pH of the BGS. Generally, the value of μeo is linear to the value of (I)−1/2. Hence, the values of I in the different BGS was calculated by using Simul 5 software [Hruška et al. 2006]. The compositions and pH of the BGS, values of I, (I)−1/2, μeo and standardized μeo (μeo,st) are summarized in Tables A1 and A2.

Contact angle measurement

To evaluate the effect of the chitin-coating on the hydrophobicity/hydrophilicity of the surface of a poly(dimethylsiloxane) (PDMS) substrate, contact angles of a water droplet on the untreated, O2 plasma-treated, Ca solvent-treated, and chitin-coated PDMS planer plates were measured. Preparation procedures were described as follows:

A large planar PDMS plate (circular shape; diameter, 4 inch) was made by the conventional procedure. It was then cut as 1-inch square pieces. Some of them were dipped into a Ca solvent (CaCl2 2H2O-saturated methanol, see main manuscript) or chitin solution prepared with the Ca solvent and then dried under a vacuum condition. Some of untreated PDMS plates were treated with the O2 plasma for activation of the surface.

As a result of the contact angle measurement, the obtained values were summarized in Table A3. Regarding the hydrophobicity of the surface, chitin-coating reduced the contact angle from 108º on the untreated one to 95º, whereas the Ca solvent-treatment did not affect. In the case of O2 plasma treatment which is need for bonding of the PDMS plates, contact angle was drastically decreased by the activation of silanol group as well-known. However, it was also well-known that the activated silanol groups were gradually deactivated, so that the contact angle on O2 plasma-treated surface was increased from 38º to 61º and 94 º after 30 min and 1 day, respectively. This variation of the surface characteristic directly causes the unstable electroosmotic flow in the PDMS microchannel, which is inferior to microchip electrophoresis.

Table A1 Standardization of the values of μeo obtained in the chitin-coated PDMS microchannel
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Table A2 Standardization of the values of μeo obtained in the untreated PDMS microchannel
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Table A3 Hydrophobicity of the modified and untreated PDMS surfaces
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Sueyoshi, K., Hori, Y. & Otsuka, K. Inner surface modification of poly(dimethylsiloxane) microchannel with chitin for electrophoretic analysis of proteins. Microfluid Nanofluid 14, 933–941 (2013). https://doi.org/10.1007/s10404-012-1100-x

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