1. ≠ –
ˆÎ Á∏°≠ æ≠¬ –˙˚ ¨∏–fl”∏ ΔœÛ, ͘ ˆÂ˙
«– –fl Ó°≠ ŸÁœ‘ ÃÎ«Ì ÷Ÿ. ؘpH æ≠¬ ˆ˙
¯§, Œº ˜◊ ¯§, º˜ ËÁ, ≠’∞ ¶∂ Ó° ≈Ï fl‰œ‘
ÁÎ «Ì ÷Ÿ. ◊° ˚Û §–˚ ¯§[1], ì ¿‰∫ ¸Ë ø˙
Æ£ˆ∫Õ(Ion-Selective Field Effect Transistor; ISFET) ʃ [2]Ù Ì–⁄[3, 4], ´‚∞[5] Ó« ¯§ ∞˙ª ÁÎœ¬ Ó« Ÿ Á— æ˘«pHæ≠Èà ¶¤«Ì ÷Ÿ. ◊Ø™ ÃØ— Ê˝ª ÃÎœ¬pHæ≠¬ ¶¤° Ù∫ ¯§ π‚μ° Œ°«Ì, fl°˚Œ
∏Œ™ Œ° ° Óà ‰œ© ¸¶˚Œ Œ« “° Ó¡Úª °
¯Ÿ. ˚Û≠, œ› °§Ã™ ¿fi «· ˆÂ°≠ £ÌœÌ ¸£‘
ˆÂ ÀÁ(Point-of-Care Testing; POCT)¶ ¯‡œ‚ ßÿ≠¬
“¸≠Õ ÁÎ ÎÃ∫ª Æ· æ≠« ≥flà ›Â√ ‰œŸ. ◊°
˚Û ™Î ‚˙Ȫ ¢Ò— ŸÁ—pH æ≠° ¨∏ «Ì ÷∏Á,
ؘ ¯§ ˆ ∞˙Œ ™Î ·§[6]Ù ∫“™Î∞˙[7]Óª ÃÎ
œ© ∫… ‚Ûª √μ œÌ ÷Ÿ. ◊Ø™ ÃØ— Ê˝Èμ ™Î ∞
˙ ¶∂ ⁄º° Ó¡Úà ÷Ÿ¬ Õ˙ —§» ¯§ ¸ßÕ ∑∫ ÿ Ûμ¶ °¯Ÿ¬ —Ë° ÷Ÿ.
ª ¨∏°≠¬ ’°≠ ¶√— ‚∏«pH æ≠Èà °ˆ¬ ƶ
°Èª ÿ·œÌ, «¶ ˆÂ ÀÁ° ˚Î °…∫à ÷¬ æ≠¶ ≥ flœ¥Ÿ. œ›˚Œ “¸‚¸¯–(MEMS) ¯§Èª ∞Îœ©
ŒÁ¸ ¸ÿ(suspended type electrodes)ª ¶¤œÌ, ◊ Áð
∫“™Î©Í(Carbon Nanotube; CNT)¶ ™Î√Æ ¸¬Œ ¸∫
√—, »§∫˙ ÿÛμ° Ù∫pH æ≠¶ ¶¤œ¥Ÿ. «—, ∏§
˝ª ÃÎœ© ¶¤» æ≠È £« ›¿ ˜Ã¶ ŸÃÌ, «¶ ÁÎ
°…∫ª ÆŒœ¥Ÿ.
2. ¶¤ Ê˝
1˜◊¯˙Ζ≥ ‚˯–˙(Department of Mechanical Engineering, POSTECH)
Hyoja-dong, Nam-gu, Pohang, Gyungbuk 790-784, Korea
2»øΖ≥ ‚Ë≥˯–˙(Department of Mechanical Design Engineering, Andong National University)
Gyeongdong-ro, Andong 760-749, Korea
3˜◊¯˙Ζ≥ ∂’˝Ì¯–Œ(Division of Integrative Bioscience and Biotechnology, POSTECH)
Hyoja-dong, Nam-gu, Pohang, Gyungbuk 790-784, Korea
+Corresponding author : [email protected] (Received : Apr. 22, 2013, Accepted : May. 18, 2013)
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by- nc/3.0)which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
pISSN 1225-5475/eISSN 2093-7563
ŒÁ¸ ∫“™Î©Í ™Î√ƶ ÃΗ pHæ≠«
¶¤˙ ∏§
Øø¿1• ÷ÏÆ1• »¬¢2• „ÿ∫1• ”ŸË1,3,+
Fabrication and Calibration of pH Sensor Using Suspended CNT Nanosheet
Hyobong Ryu1, WooSeok Choi1, Taechang An2, Joonseong Heo1, and Geunbae Lim1,3,+
Abstract
In this research, the pH sensor was developed using CNT nanosheet with Nafion coating for the advanced medical sensor such as a blood gas analyzer. The CNT nanosheet was formed by dielectrophoresis and water-meniscus between cantilever-type electrodes. Then, the process of the heat annealing and the Nafion coating was conducted for reducing contact resistance and giving proton selectivity respectively. We measured the response of the pH sensor as the electrolyte-gated CNT-nanosheet field effect transistor. The sensor showed a linear current ratio in a similar range of the normal blood pH. A calibration method for decreasing of the response variation among sensors has also been introduced. Coefficient of variance of the pH sensor was decreased by applying the calibration method. A linear relation between the calibrated response of the sensors and pH variance was also obtained. Finally, the pH sensor with a high resolution was fabricated and we verify the feasibility of the sensor by applying the calibration method.
Keywords : pH sensor, Carbon nanotube, Nafion, Calibration
2.1 æ≠« ¶¤
ª ¨∏°≠ ÁΗ æ≠« ¶¤Ê˝∫Fig. 1« ¯§μ° ™∏
™ ÷Ÿ. ’˙ Si ‚«° SiN¶ ˙– ≠– ‚Û ı¯(Low- Pressure Chemical Vapor Deposition; LPCVD) Ê˝ª ÃÎœ
©1 ≠¶ √∞Ÿ. ◊ ß°Cr/Au¸ÿª20/200 ¨ « ÙÃŒ ¸∫
—Ÿ. ◊ ƒ° ›¿∫ ì ƒ¢(Reactive Ion Etching; RIE) Ê˝
ª ÃÎÿSiN¶ –œ ƒ¢— ƒ°, RSE200Î◊ª ÃÎÿSiª ƒ
¢œ© ‹»∏(cantilever)¶ ∏ÁŸ.
≠ͪ ÃÎœ© ›μº¸CNT Î◊« •È ≥ÆÕ Î◊ ”
CNT£« –ͪ ø√° ¯‡œ¥Ÿ[8]. ≤Í˙ ˙Í(H2SO4/ HNO3)ª Œ«Ò3:1Œ •’— Î◊° ›μº¸CNT 1∑ª ÷Ì, 6 √£ ø» Ωƒ ¯ø ≥ÆflŸ. ◊ ð ªÃ¬ˆ(Deionized water; DI water)Œ fl≠√∞Ì, ¯…–Æ‚¶ ÃÎœ© ߸√≤
Ÿ. ߸»CNT¶ Ÿ√ ª ªÃ¬ˆŒ Î◊«pH° fl∫à …
ßÓˆ ©Ø ˜ fl≠√∞‚¶ ›π—Ÿ.
ÿÒ» CNT Î◊ª ‹»∏ ¸ÿ Áð √ÆÌ, ‘ˆfl˝‚
(function generator)¶ ÃÎœ©1 MHz« ≥˘¶6-10 VŒ ¸
ÿ Á‹° …Ó ÿŸ. CNT° ¸ÿ Á𠧃«È, ≤∫ Î◊ª
¶≈œ© ∞« fiœ∫ø∫(water meniscus)¶ ¸∫√≤ Û¬Œ
ŒÁ¸ ¸ÿ Áð ≤∫CNT¶ ≠≠˜ –¯ √∞‘ »Ÿ[8, 9].
◊ ƒ° ¸≠‚¶ ÃÎœ©200…°≠10-15–«CNT ≠≥Æ ˙
§ª ≈£Ÿ. PDMS (Polydimethylsiloxane)¶ ÃÎœ© ¶¤—
∂éŒ §Œª ŸÃ∫ ß° ·’œÌ, ◊ §Œª Îœ©
Nafion Î◊ª ÷‘√≤Ÿ. ◊∏‘ ‘∏Œ· §Œ ©‚∏≠« ’œ
— “Æ« Á∏ ÷‘«ÓCNT° ⁄√… ˆ ÷μœ œ¥Ÿ. ◊ ƒ
°PDMS¶ Ÿ√ ¶≈œÌ, Nafion ⁄√ª ßœ© ¸≠‚°≠95
… °≠5–£ ≠≥ƶ ¯‡—Ÿ.
2.2 ¯§ ÂÒ
ظ¸‚μø˚ª fl˝√∞‚ ßœ©, ¸ÿ Á‹° ‘ˆfl˝‚
(Tektronix AFG3101)¶ ÃÎœ¥Ÿ. «—, pH æ≠« ¸‚˚ Ø
∫∫ ›μº –Æ‚(HP 4156A) ÂÒ¶ ÃÎœ© ¯§œ¥Ÿ. ŒÁ
¸ ¸ÿ Áð ¸∫» CNTÕ ⁄√» Nafion∫ ¸⁄ˆÃÊ (Hitachi SU6600)ª ÃÎœ© ◊ ¸Ûª ÆŒœ¥Ì, Energy Dispersive Spectrometry (EDS) –ƪ ÎÿNafion ⁄√ ©Œ
¶ ÆŒœ¥Ÿ.
3. ·˙ ◊ Ì˚
3.1 æ≠« ¸Û
Fig. 2(a)-(d) °≠ ∏©ˆÌÃpHæ≠ ¶¤ ‹Ë« ¢ ˙§ª
¸⁄ˆÃʪ Îœ© ¸˚œ¥Ÿ. Fig. 2(a)°≠ ŒÁ¸ ¸ÿ ÁÃ
°CNT° ’œœ‘ ¨·» Õª ÆŒ “ ˆ ÷Ÿ. Fig. 2(b)°≠¬
¸ÿ Á𠸂 ʂ∏ŒCNT° fl §ƒ«Ó ÷¬ Õª º ˆ
÷Ÿ. ◊ ð ¸ÿ˙CNT£« ¢À ˙◊ª ∑flÌ, Î◊° ˜‘
… ˆ ÷¬ Ø‚ “¯∞Ȫ ¶≈œ‚ ßœ© ≠≥Æ ˙§ª ≈°Ì
™ÈFig. 2(e)°≠≥≥ ¸μμ(conductance)° ©‘ ı° œ¬ Õ ª À ˆ ÷Ÿ.
Fig. 1. Schematic process flow of making pH sensor with CNT nanosheet coated with proton permeable membrane.
Fig. 2. SEM images of (a) the suspended CNT nanosheet, (b) the magnified center of the CNT nanosheet, (c) after heat annealing and (d) Nafion coating. (e) I-V graph of CNT nanosheet and after heat annealing. (f) EDS analysis of CNT coated with Nafion.
◊ ƒ°Fig. 2(d)°≠≥≥Nafion ⁄√ª ≈°‘ «¬•, Fig.
2(f)°≠Nafion° ˜‘«Ó ÷¬ ““(F) ∫–à À‚ ∏Œ·
Nafion ⁄√à fl ÃÁÓ ≥Ÿ¬ Õª ÆŒ “ ˆ ÷˙Ÿ. ø ¨
∏° Òœ©CNT ™Î√Æ ¸∫ ƒ° ¯˜˚∏ŒNafionª ⁄√‘
∏Œ·CNT-¸ÿ, CNT-CNT£« ¸‚˚ ¢’ª Êÿœˆ Ì,
¯§ ÎÛ Î◊ ”°≠pH ¯§° 삪 ÷¬ Á∫⁄∏ª ¯§œ
¬ Õà °…ÿ¯Ÿ. à ˙§ª Îœ© ≈£Î ‚ΩÒ Ò≤ª ı°
√≥ ˆ ÷Ì, ¯§ μ¶ ‚Û √≥ ˆ ÷˙Ÿ[10].
3.2 pH ¯§
ª ¨∏°≠ ¶¤—pH æ≠¶ ø« ¨∏°≠ ≥flœ¥¯ Ê
˝ª ‚ª∏ŒpH Ø≠° ˚• æ≠« ›¿ª ¯§œ¥Ÿ[10].
CNT° ¨·» ¸ÿ Á‹°0.6 V« ¸–ª œ§œ‘ …Ó÷Ì, ‘
ÃÆ(gate)Î ¸ÿª ÃÎœ© ‘ÃÆ ¸–ª0°≠1 VŒ Ø≠ √
∞Á ÂπŒ-“∫(drain-source)¸˘¶ ¯§—Ÿ. ÂπŒ-“∫
¸˘ ™°≠ ‘ÃÆ ¸–ÃVg= 0 V œ ß« ‚Ô‚¶, ¢¢«pH
™°≠ ∏—Ÿ. ◊ ™∫Fig. 3« ·˙≥≥ §Û ˜◊«pH ŒŸ ¸
ß(‡pH 7.4)°≠, pH° ˚Û ±¸˚∏Œ ™Ã Ø≠œ¥Ÿ. ø
¨∏° Òœ© ›μº¸CNT¶ ÃÎœ¥Ì, ¸ÿ Áð ™Î√Æ
¸¬ŒCNT¶ ¨·œ© ¯§ •È˚ª ı°√◊∏Á, fl°˚∏Œ CNT≠≥Æ ˙§˙ ∂éŒ §Œª Η œ§— “Æ«Nafion
⁄√ Ê˝ª μ‘‘∏Œ·, pH æ≠« ÿÛμ ‚Ûª Ã¯Ó ª˙
Ÿ.
3.3 ∏§ Ê˝ μ‘
’°≠«pH¯§ª Îœ©pH Ø≠° ˚Û æ≠« ±¸˚Œ ›
¿ ·˙¶ Úª ˆ ÷˙Ÿ. ◊Ø™ ©Ø ¯§ «¬ ¯§ Û« ÃØŒ
¶¤» æ≠∂Ÿ Ÿ“ ̘° fl˝ “ ˆ° ÷‘ »Ÿ. ◊° ˚Û
∏§˝(calibration method)ª μ‘‘∏Œ·, ◊ ˜Ã¶ Ÿœ ˆ
÷‘ ∏œ“ ˆ ÷Ÿ. ª ¨∏°≠ ¶¤—pH æ≠¬ ¸Ëø˙ Æ
£ˆ∫Õ Ø∫ª ∏ÃÁ[8], ◊° ˚Û ŸΩ˙ ∞à •ˆ»Ÿ[11].
©‚≠Ids¬ ÂπŒ-“∫ ¸˘ÃÌ, ÷¬ ¸‚“Æ, Ϭ ŸÃ
∫°≠« Ãøμ, ¬ ¯¯« ظ≤, Âr∫ ˝¨˛« ظ≤, A¬
§Œ« È˚, d¬ ≥–√Õ« £›, Vds¬ ÂπŒ-“∫ ¸–, L∫
§Œ« ÊÃ, Vg¬ ‘ÃÆ ¸–, VT¬ ÆŒ ¸–(threshold voltage) ™ÃŸ. ©‚≠ ¯§ Î◊ ”« Á∫⁄°CNT ™Î√Æ
° ‘ÃÆ ¸–∏Œ 삪 ð¬ ÕªƒVÛÌ ŒÌ, ƒ(1)ª §
ÆœÈ Ÿ√ ŸΩ˙ ∞∫ ƒª Úª ˆ ÷Ÿ.
©‚≠ƒI¬pH Î◊∏Œ Œ—Ids« Ø≠Æß. ·π «Ë˚
∏Œ ¿• ◊ª ∏œÌ ◊ ™Èª ΑœÈ ƒ(2)«ƒV™ª ∏
“ ˆ ÷Ÿ.
Fig. 3. Response of the pH sensor.
Fig. 4. Response distribution of (a) using previous method and (b) calibrated method at pH 7.2. Dotted line is the mean value of each result.
(1)
(2)
◊ ™ÃpH° «— æ≠« ≈£ Ø≠ §μÛÌ ˝¢œ©, ıŒÓ
∏§ ›¿ ™ª ˝¢“ ˆ ÷Ÿ.
Ã∏‘ μ‘— ∏§™(calibrated response)˙ ø ¯§ Ê˝ª ÃÎœ© ∏— ©Ø æ≠ £« ›¿ ™ ˜Ã –˜¶Fig. 4°≠ Ò
≥œ¥Ÿ. ¢¢« Ê˝∏Œ pH 7.2°≠« Øø ˈ ™
(Coefficient of Variance; CV)ª ∏œÈ, ø ¯§ Ê˝°≠¬
CV= ‡33% Ã˙∏™ ∏§˝ª ˚ÎœÈ ◊ ™Ã ‡9%Œ “—
·˙¶ Úª ˆ ÷˙Ÿ. ö Îœ© æ≠£« ̘¶ ∫¯˚∏Œ Ÿœ ˆ ÷¬ Õª ÆŒœ¥Ÿ.
ÃØ— Ê˝∏Œ §Û ˜◊pH« ŒŸ°≠, ¶¤—pH æ≠«
•ÿÓ±(standard curve)ª ∏œ¥Ÿ. Fig. 5°≠ ™∏≠ Õ≥≥
pHØ≠° ˚Û ±¸˚∏Œ ◊ ™Ã Øœ¬ Õ∏Œ ™∏μŸ.
4. ·–
ª ¨∏°≠¬ ŒÁ¸ ¸ÿ ÁðCNT ™Î√ƶ ¸∫œÌ Nafionª ⁄√‘∏Œ·, ÿÛμÕ »§∫à Ù∫pH æ≠¶ ≥fl
œ¥Ÿ. «Æ‹ ‚« ß° ›” ı¯˙ ˙© ƒ¢ Ê˝ª ÃÎœ©
ŒÁ¸ ¸ÿª ¶¤œÌ, ظ¸‚μø˙ Î◊ ¶≈ √ ˝‚¬ fiœ
∫ø∫¶ ÃÎœ©CNT ™Î√ƶ ∏ÁŸ. ¶¤» ™Î√Æ° ≠
≥Æ ˙§ª ≈ƒ “¯∞ª ¶≈œÌCNTÕ ¸ÿ Áë ¨·ª
‚Û√◊Ÿ. ◊ÆÌ æ≠« ›¿ ±√∫ ‚Ûª ßœ©Nafion ª
⁄√œ¬• ÷Ó, PDMS∂éŒ §Œª ¶¤œÌ ◊Õª ÃÎœ
© ’œ— “Æ« Áà ⁄√° ÁÎ… ˆ ÷μœ œ¥Ÿ. Ã∏‘ ¶
¤» æ≠¬ §Û ˜◊«pH ¸ßŒpH 7.4 ŒŸ°≠ ±¸˚Œ ›
¿ª ∏¥∏Á, μ‘— ∏§˝ª Îœ© æ≠£« ̘μ Ê–˜
“ √≥ ˆ ÷˙Ÿ. ◊° ˚Û ¶¤ Áˆ∫˙ ÁÎ °…∫ª Œ Æ Œ— “¸≠»pH æ≠¶ ¶¤ œ¥Ÿ.
Á« ¤
à ÌÆ∫2013‚μ §Œ(≥∞˙–‚˙Œ)« Á¯∏Œ —π¨∏
Á‹« ˆ¯ª fiΔ ˆ‡» ¨∏”(No. 2012R1A1A2007580).
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