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A single-walled carbon nanotube thin film-based pH-sensing microfluidic chip Cheng Ai Li, Kwi Nam Han, Xuan-Hung Pham and Gi Hun Seong* A novel microfluidic pH-sensing chip was developed based on pH-sensitive single-walled carbon nanotubes (SWCNTs). In this study, the SWCNT thin film acted both as an electrode and a pH-sensitive membrane. The potentiometric pH response was observed by electronic structure changes in the semiconducting SWCNTs in response to the pH level. In a microfluidic chip consisting of a SWCNT pHsensing working electrode and an Ag/AgCl reference electrode, the calibration plot exhibited promising pH-sensing performance with an ideal Nernstian response of 59.71 mV pH1 between pH 3 and 11
Received 5th December 2013 Accepted 24th January 2014
(standard deviation of the sensitivity is 1.5 mV pH1, R2 ¼ 0.985). Moreover, the SWCNT electrode in the
DOI: 10.1039/c3an02195e
0.1 and 15 ml min1. The selectivity coefficients of the SWCNT electrode revealed good selectivity against common interfering ions.
www.rsc.org/analyst
microfluidic device showed no significant variation at any pH value in the range of the flow rate between
1. Introduction pH sensors are of great importance in real-time biomedical, clinical, in situ environmental monitoring and industrial process control applications.1–7 Among various detection methods, potentiometric detection using small electrodes is an excellent technique as it allows rapid, simple, and reliable pH measurement. Solid-state pH electrodes employing metal/metal oxides or polymer membranes as sensing layers have attracted much attention because of several advantages over conventional glass pH electrodes, i.e., robustness, cost-effective fabrication, no need for maintenance, exibility, possibility of miniaturization and compatibility with microuidic systems known as lab-on-a-chip and micro-total analytical systems.8–13 However, metal oxide-based pH electrodes require multiple fabrication steps to deposit several layers of metal on the substrate. In polymer membrane-based pH electrodes, solidcontact ion-to-electron transducers, such as conducting polymers or carbon nanostructured materials, are used between pH-sensitive polymer membranes and electrodes in order to achieve a stable potentiometric sensor response. Single-walled carbon nanotubes (SWCNTs) greatly improve potentiometric stability, which make them more attractive as transducers than conducting polymers.14 SWCNTs are well known for their outstanding electrical, chemical, thermal and mechanical properties and have been widely used as a promising electrode material in electroanalytical sensors.15 However, employing SWCNTs as pH-sensing materials in potentiometric pH sensors has not yet been explored.
Department of Bionano Engineering, Hanyang University, Ansan 425-791, South Korea. E-mail: [email protected]; Fax: +82-31-436-8148; Tel: +82-31-400-5202
This journal is © The Royal Society of Chemistry 2014
In this article, microuidic pH-sensing chips were developed based on SWCNT lms. The SWCNT thin lms acted both as a pH-sensing membrane and a working electrode to receive the potentiometric signal. The microuidic device integrated with a SWCNT pH-sensing electrode and an Ag/AgCl reference electrode can provide a rapid, highly sensitive, and continuous pH determination with small sample consumption. The potentiometric pH response from the strip-type SWCNT electrodes fabricated on a exible substrate was also examined in a beaker system.
2.
Experimental
2.1. Materials and instruments A solution of SWCNTs (0.3 mg ml1) was purchased from Top Nanosys Co. (Sung Nam, South Korea). Ag/AgCl paste (C61003P7) was obtained from Gwent Electronic Materials Ltd (UK). Buffer solutions with various pH values (pH 2–11) were purchased from Samchun Pure Chemical Co. (Seoul, South Korea). Poly(dimethylsiloxane) (PDMS) and prepolymer components (Sylgard 184 silicone Elastomer Kit) were purchased from Dow Corning (Midland, MI, USA) and AZ4620 photoresist polymer was obtained from Clarivant Corporation (Somerville, NJ, USA). All other chemicals were of analytical grade, and aqueous solutions were prepared with deionized (DI) water. Potentiometric measurements were carried out with an electrochemical analyzer (CHI 660C, CH Instruments Inc., USA). The morphologies of the electrodes were characterized by microscopy (Olympus IX71) and eld emission-scanning electron microscopy (FE-SEM) (Hitachi S-4800, operating at 15 kV). Sample loading was performed by using a syringe pump (PHD2000, Harvard Apparatus, Holliston, MA) which was connected to the inlet port of the microuidic chip.
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Fig. 1
Paper
Schematic fabrication process of the microfluidic pH-sensing chip.
2.2. Fabrication of the microuidic pH-sensing chip A schematic of the fabrication procedure is illustrated in Fig. 1. Homogeneous SWCNT lms were prepared on glass substrates using a vacuum ltration method. A standard photolithography method was then used to generate photoresist polymer patterns on the SWCNT lms. The SWCNT lms pre-patterned with the photoresist polymer were treated with O2 plasma in the CCP system.16 Aer treatment, the remaining photoresist polymer on
the SWCNT lms was removed with an ethanol solution. For pH determination, potentiometric measurements were used to estimate the potential shis and were performed with a twoelectrode system in which the SWCNT electrode was the working electrode (WE), and the Ag/AgCl electrode was the pseudo reference electrode (RE). The reference electrode was fabricated by painting Ag/AgCl paste onto one of the SWCNT electrodes. A PDMS mold fabricated by so lithography was
Fig. 2 (A) FE-SEM image of SWCNT films. The scale bar is 200 nm. (B) Photo of the developed flexible strip-type pH sensor on a PET substrate. WE is the SWCNT working electrode and RE is the Ag/AgCl reference electrode. (C) Potential responses from a strip-type pH sensor at varied pH values. (D) Calibration plot of the SWCNT-based, strip-type pH sensor in different pH buffer solutions (n ¼ 3).
2012 | Analyst, 2014, 139, 2011–2015
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bonded to the electrode substrate using low power O2 plasma treatment.17 The microuidic chips contained a sensing chamber with a 2 mm width and 15 mm height, whereas the electrodes had 1 mm width and spacing.
Published on 24 January 2014. Downloaded by University of Illinois at Chicago on 25/10/2014 00:29:05.
3.
Results and discussion
Homogeneous SWCNT lms were fabricated on the substrate with a thickness of 100 nm, which provided the most suitable conductivity and transparency. The average resistivity and transparency of the fabricated SWCNT lms were 400 ohm sq1 and 80%, respectively. In addition, the FE-SEM image of the SWCNT lms revealed that the ordered SWCNTs were wellconnected and formed a random and homogeneous lm on the substrate (Fig. 2A). Prior to integration into the microuidic device, the potentiometric response of the SWCNT sensing lm was examined using a exible strip-type pH sensor fabricated on a exible poly(ethylene terephthalate) (PET) substrate. As shown in Fig. 2B, the SWCNT lms exhibited a high degree of exibility with a negligible change in resistivity upon hard bending. The diameters of the working and reference electrode were 4 mm with 30 mm 2 mm connecting paths protected by a PET tape. Performance of a pH sensor is typically characterized by measuring the open circuit potential of the electrodes in solutions with various pH values. Fig. 2C illustrates the open circuit potential signal recorded aer immersing the SWCNT-based strip-type pH sensor in solutions with different pH values. The corresponding calibration plot (Fig. 2D) exhibits promising pHsensing performance with an ideal Nernstian response of 59.24 mV pH1 between pH 3 and 11 at 25 3 C (standard deviation of the sensitivity was 1.8 mV pH1, R2 ¼ 0.977). The SWCNT pH-sensitive electrode exhibited a nearly instantaneous response to varying pH solutions, yielding a steady-state signal within 5 s while a completely stabilized signal was observed within 30 s. The selectivity coefficients for the exible pH sensor on the SWCNT electrode were calculated in the presence of several interferents using the xed interference method (Table 1).18,19 For all cases, their selectivity coefficients revealed good selectivity against common interfering ions. The selectivity coefficients for K+, Na+, Li+ and Cl were acceptable compared to the literature results, which were less than 102 for an ion selective electrode.8 In previous reports, several carbon nanotube (CNT)-based pH sensors were demonstrated. CNT-modied glassy carbon electrodes were typically used, which required additional steps
for depositing CNTs on the electrodes. In potentiometric pH sensors, additional polymeric pH-sensitive membranes were prepared on the CNT-modied electrodes.12,20 In voltammetric pH sensors, the pH values were measured by the redox peak potentials of electrodes based on the electrochemical reaction of the electroactive groups on the SWCNTs.21,22 Moreover, the voltammetric pH sensors needed another electrode to make a three-electrode system. The open circuit potential between the SWCNT electrode and the Ag/AgCl reference electrode in different pH buffer solutions can be reasonably ascribed to the doping of the nanotube walls by H+ and OH ions that behave as electron acceptor and donor species, respectively. The change in the concentration of H+ and OH ions causes an electronic structure (rst transition S11) change in semiconducting SWCNTs, causing a respective downshi and up-shi of the Fermi level of SWCNTs in acidic and basic solutions.23,24 The Fermi level can be converted to the electrode potential relative to the standard hydrogen electrode reference with the following equation: E(electrochemical potential) ¼ F(work function)/e 4.44 V.25 More signicantly, the intensity of the rst transition S11 of semiconducting SWCNTs is reversibly tunable by changing the pH,23 which
Table 1 Selectivity coefficients of the flexible pH sensor based on the SWCNT electrode. The selectivity of the SWCNT pH-sensitive membrane was calculated using the fixed interference method
Interference ion (j)
Concentration (mol l1)
Selectivity coefficient (KH+–j)
K+ Na+ Li+ Cl
0.1 0.1 0.1 0.1
8.1 107 6.4 108 3.0 107 9.8 104
This journal is © The Royal Society of Chemistry 2014
Performances of the pH-sensitive SWCNT electrode in the microfluidic device. (A) Potential responses of the SWCNT electrode in the microfluidic device from pH 11 to pH 2 at a flow rate of 5 ml min1. (B) Calibration plot of the microfluidic pH-sensing chip in different pH buffer solutions (n ¼ 3). Fig. 3
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offers the possibility for use as a pH-sensitive membrane. In addition, SWCNTs may behave as semiconductors or metals depending on helicity and diameter, but the physical separation of semiconducting and metallic SWCNTs has proven to be one of the most difficult challenges to overcome.26 The characteristics of SWCNTs are advantageous for the pH sensor in our study, as the simply patterned thin layer of SWCNTs can act both as a pH-sensitive membrane and a working electrode. Since an ion selective electrode response is size-independent, the pH sensor based on SWCNT electrodes successfully created in a beaker system can be integrated into the microuidic system using microfabrication technology without inuencing performance. Fig. 3A illustrates the potentiometric responses as the pH level of the sample decreased from 11 to 2 at a ow rate of 5 ml min1. The results show that the developed microuidic pH-sensing chip responds well to changes in the sample pH value. In addition, the microuidic device can continuously detect the pH with the performance of the SWCNT lm. Moreover, the open circuit potentials of the pH-sensitive SWCNT electrode with an Ag/AgCl reference electrode in the microuidic device greatly resemble the measurement in beaker solutions. The corresponding calibration plot (Fig. 3B) exhibits promising pH-sensing performance with an ideal
Paper
Nernstian response of 59.71 mV pH1 between pH 3 and 11 at 25 3 C (standard deviation of the sensitivity is 1.5 mV pH1, R2 ¼ 0.985). In addition, inuence of the ow rate on the performance of the microuidic pH-sensing chips was also investigated, and the results are shown in Fig. 4A. The range of the ow rate was examined between 0.1 and 15 ml min1. The SWCNT electrode in the microuidic device shows no signicant variation at any pH value. An average sensitivity of 58.53 mV pH1 was observed with a variation less than 2% over the ow rate range. Therefore, the SWCNT electrode can be operated in a ow analysis system without degrading the performance. Fig. 4B shows the open circuit potential of the SWCNT electrode in KNO3, NaNO3, NaCl solutions successively diluted from 10 mM to 0.1 M at pH 7 with a ow rate of 5 ml min1. The open circuit potentials of the pH-sensitive SWCNT electrode with an Ag/AgCl pseudo reference electrode in the microuidic device show small variations in the KNO3 and NaNO3 solutions over the wide concentration range of 10 mM to 0.1 M, which is less than 7 mV. In addition, the potential variation is also less than 7 mV at low NaCl concentration (10 mM to 0.1 mM), while it is slightly affected at high NaCl concentration (10 mM to 0.1 M) because of the Ag/AgCl pseudo reference electrode used in the microuidic device, for which the variation is lower than 26.5 mV.
4. Conclusions In the present study, a novel SWCNT lm-based pH sensor was developed based on a exible PET substrate or integrated into a microuidic device, and the performance was investigated. The SWCNT thin lm acted both as an electrode and a pH-sensitive membrane. Therefore, the fabrication process provides advantages of lower costs and simpler processes than other all-solidstate pH electrodes. A pair of miniature SWCNT working and Ag/AgCl reference electrodes generated electrical potentials in solutions through the electronic structure change in the semiconducting SWCNTs in response to the pH level. Moreover, the sensitivity of the miniaturized SWCNT-based pH sensors was in agreement with the Nernstian response. As a result, the proposed pH-sensitive SWCNT electrodes in the microuidic device are suitable for practical uses and will enable simultaneous multi-analyte detection of metabolic processes in biological cells when several electrodes are combined.
Acknowledgements This study was supported by National Research Foundation (NRF) grants funded by the Korean government (MSIP) (no. 2011-0015545, 2008-0061891, and 2013R1A1A2054887). We also acknowledge the nancial support of the Ministry of Knowledge Economy (MKE) through the industrial infrastructure program for fundamental technologies (N000600001). Fig. 4 (A) Relationship between the potential responses and flow rates at pH 4, 7, and 10. (B) Effects of interferents on the potential responses at pH 7 with a flow rate of 5 ml min1 ranging from 105 to 101 M.
2014 | Analyst, 2014, 139, 2011–2015
Notes and references 1 E. Lindner and R. P. Buck, Anal. Chem., 2000, 72, 336A.
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2 E. Bitziou, D. O'Hare and B. A. Patel, Anal. Chem., 2008, 80, 8733. 3 C. E. Reimers, Chem. Rev., 2007, 107, 590. 4 A. J. Bandodker, V. W. S. Hung, W. Jia, G. Vald´ es-Ram´ırez, J. R. Windmiller, A. G. Martinez, J. Ram´ırez, G. Chan, K. Kerman and J. Wang, Analyst, 2013, 138, 123. 5 L. Li, A. L. Desouza and G. M. Swain, Analyst, 2013, 138, 4398. 6 S. Kim, T. Rim, K. Kim, U. Lee, E. Baek, H. Lee, C. K. Baek, M. Meyyappan, M. J. Deen and J. S. Lee, Analyst, 2011, 136, 5012. 7 J. Phair, L. Newton, C. McCormac, M. F. Cardosi, R. Leslie and J. Davis, Analyst, 2011, 136, 4692. 8 W. D. Huang, H. Cao, S. Deb, M. Chiao and J. C. Chiao, Sens. Actuators, A, 2011, 169, 1. 9 W. Y. Liao, C. H. Weng, G. B. Lee and T. C. Chou, Lab Chip, 2006, 6, 1362. 10 C. F. Lin, G. B. Lee, C. H. Wang, H. H. Lee, W. Y. Liao and T. C. Chou, Biosens. Bioelectron., 2006, 21, 1468. 11 I. A. Ges, B. L. Ivanov, D. K. Schaffer, E. A. Lima, A. A. Werdich and F. J. Baudenbacher, Biosens. Bioelectron., 2005, 21, 248. 12 G. A. Crespo, D. Gugsa, S. Macho and F. X. Rius, Anal. Bioanal. Chem., 2009, 39, 2371. 13 J. Noh, S. Park, H. boo, H. C. Kim and T. D. Chung, Lab Chip, 2011, 11, 664.
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14 M. Novell, M. Parrilla, G. A. Crespo, F. X. Rius and F. J. Andrade, Anal. Chem., 2012, 84, 4695. 15 L. Hu, D. S. Hecht and G. Gr¨ une, Chem. Rev., 2010, 110, 5790. 16 K. N. Han, C. A. Li, N. M. P. Bui, X. H. Pham, Y. H. Choa, E. K. Lee and G. H. Seong, Sens. Actuators, B, 2013, 177, 472. 17 K. N. Han, C. A. Li, N. M. P. Bui and G. H. Seong, Langmuir, 2009, 26, 598. 18 C. Macc` a, Electroanalysis, 2003, 15, 997. 19 Y. Umezawa, P. B¨ uhlmann, K. Umezawa, K. Tohda and S. Amemiya, Pure Appl. Chem., 2000, 72, 1851. 20 M. Kaempgen and S. Roth, J. Electroanal. Chem., 2006, 586, 72. 21 Z. Xu, X. Chen, X. Qu, J. Jia and S. Dong, Biosens. Bioelectron., 2004, 20, 579. 22 J. Weber, A. Kumar, A. Kumar and S. Bhansali, Sens. Actuators, B, 2006, 117, 308. 23 W. Zhao, C. Song and P. E. Pehrsson, J. Am. Chem. Soc., 2002, 124, 12418. 24 M. E. Itkis, S. Niyogi, M. E. Meng, M. A. Hamon, H. Hu and R. C. Haddon, Nano Lett., 2002, 2, 155. 25 S. Trasatti, Pure Appl. Chem., 1986, 58, 955. 26 C. B. Jacobs, M. J. Peairs and B. J. Venton, Anal. Chim. Acta, 2010, 662, 105.
Analyst, 2014, 139, 2011–2015 | 2015
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Cite this: Analyst, 2014, 139, 2011
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A single-walled carbon nanotube thin film-based pH-sensing microfluidic chip Cheng Ai Li, Kwi Nam Han, Xuan-Hung Pham and Gi Hun Seong* A novel microfluidic pH-sensing chip was developed based on pH-sensitive single-walled carbon nanotubes (SWCNTs). In this study, the SWCNT thin film acted both as an electrode and a pH-sensitive membrane. The potentiometric pH response was observed by electronic structure changes in the semiconducting SWCNTs in response to the pH level. In a microfluidic chip consisting of a SWCNT pHsensing working electrode and an Ag/AgCl reference electrode, the calibration plot exhibited promising pH-sensing performance with an ideal Nernstian response of 59.71 mV pH1 between pH 3 and 11
Received 5th December 2013 Accepted 24th January 2014
(standard deviation of the sensitivity is 1.5 mV pH1, R2 ¼ 0.985). Moreover, the SWCNT electrode in the
DOI: 10.1039/c3an02195e
0.1 and 15 ml min1. The selectivity coefficients of the SWCNT electrode revealed good selectivity against common interfering ions.
www.rsc.org/analyst
microfluidic device showed no significant variation at any pH value in the range of the flow rate between
1. Introduction pH sensors are of great importance in real-time biomedical, clinical, in situ environmental monitoring and industrial process control applications.1–7 Among various detection methods, potentiometric detection using small electrodes is an excellent technique as it allows rapid, simple, and reliable pH measurement. Solid-state pH electrodes employing metal/metal oxides or polymer membranes as sensing layers have attracted much attention because of several advantages over conventional glass pH electrodes, i.e., robustness, cost-effective fabrication, no need for maintenance, exibility, possibility of miniaturization and compatibility with microuidic systems known as lab-on-a-chip and micro-total analytical systems.8–13 However, metal oxide-based pH electrodes require multiple fabrication steps to deposit several layers of metal on the substrate. In polymer membrane-based pH electrodes, solidcontact ion-to-electron transducers, such as conducting polymers or carbon nanostructured materials, are used between pH-sensitive polymer membranes and electrodes in order to achieve a stable potentiometric sensor response. Single-walled carbon nanotubes (SWCNTs) greatly improve potentiometric stability, which make them more attractive as transducers than conducting polymers.14 SWCNTs are well known for their outstanding electrical, chemical, thermal and mechanical properties and have been widely used as a promising electrode material in electroanalytical sensors.15 However, employing SWCNTs as pH-sensing materials in potentiometric pH sensors has not yet been explored.
Department of Bionano Engineering, Hanyang University, Ansan 425-791, South Korea. E-mail: [email protected]; Fax: +82-31-436-8148; Tel: +82-31-400-5202
This journal is © The Royal Society of Chemistry 2014
In this article, microuidic pH-sensing chips were developed based on SWCNT lms. The SWCNT thin lms acted both as a pH-sensing membrane and a working electrode to receive the potentiometric signal. The microuidic device integrated with a SWCNT pH-sensing electrode and an Ag/AgCl reference electrode can provide a rapid, highly sensitive, and continuous pH determination with small sample consumption. The potentiometric pH response from the strip-type SWCNT electrodes fabricated on a exible substrate was also examined in a beaker system.
2.
Experimental
2.1. Materials and instruments A solution of SWCNTs (0.3 mg ml1) was purchased from Top Nanosys Co. (Sung Nam, South Korea). Ag/AgCl paste (C61003P7) was obtained from Gwent Electronic Materials Ltd (UK). Buffer solutions with various pH values (pH 2–11) were purchased from Samchun Pure Chemical Co. (Seoul, South Korea). Poly(dimethylsiloxane) (PDMS) and prepolymer components (Sylgard 184 silicone Elastomer Kit) were purchased from Dow Corning (Midland, MI, USA) and AZ4620 photoresist polymer was obtained from Clarivant Corporation (Somerville, NJ, USA). All other chemicals were of analytical grade, and aqueous solutions were prepared with deionized (DI) water. Potentiometric measurements were carried out with an electrochemical analyzer (CHI 660C, CH Instruments Inc., USA). The morphologies of the electrodes were characterized by microscopy (Olympus IX71) and eld emission-scanning electron microscopy (FE-SEM) (Hitachi S-4800, operating at 15 kV). Sample loading was performed by using a syringe pump (PHD2000, Harvard Apparatus, Holliston, MA) which was connected to the inlet port of the microuidic chip.
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Fig. 1
Paper
Schematic fabrication process of the microfluidic pH-sensing chip.
2.2. Fabrication of the microuidic pH-sensing chip A schematic of the fabrication procedure is illustrated in Fig. 1. Homogeneous SWCNT lms were prepared on glass substrates using a vacuum ltration method. A standard photolithography method was then used to generate photoresist polymer patterns on the SWCNT lms. The SWCNT lms pre-patterned with the photoresist polymer were treated with O2 plasma in the CCP system.16 Aer treatment, the remaining photoresist polymer on
the SWCNT lms was removed with an ethanol solution. For pH determination, potentiometric measurements were used to estimate the potential shis and were performed with a twoelectrode system in which the SWCNT electrode was the working electrode (WE), and the Ag/AgCl electrode was the pseudo reference electrode (RE). The reference electrode was fabricated by painting Ag/AgCl paste onto one of the SWCNT electrodes. A PDMS mold fabricated by so lithography was
Fig. 2 (A) FE-SEM image of SWCNT films. The scale bar is 200 nm. (B) Photo of the developed flexible strip-type pH sensor on a PET substrate. WE is the SWCNT working electrode and RE is the Ag/AgCl reference electrode. (C) Potential responses from a strip-type pH sensor at varied pH values. (D) Calibration plot of the SWCNT-based, strip-type pH sensor in different pH buffer solutions (n ¼ 3).
2012 | Analyst, 2014, 139, 2011–2015
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bonded to the electrode substrate using low power O2 plasma treatment.17 The microuidic chips contained a sensing chamber with a 2 mm width and 15 mm height, whereas the electrodes had 1 mm width and spacing.
Published on 24 January 2014. Downloaded by University of Illinois at Chicago on 25/10/2014 00:29:05.
3.
Results and discussion
Homogeneous SWCNT lms were fabricated on the substrate with a thickness of 100 nm, which provided the most suitable conductivity and transparency. The average resistivity and transparency of the fabricated SWCNT lms were 400 ohm sq1 and 80%, respectively. In addition, the FE-SEM image of the SWCNT lms revealed that the ordered SWCNTs were wellconnected and formed a random and homogeneous lm on the substrate (Fig. 2A). Prior to integration into the microuidic device, the potentiometric response of the SWCNT sensing lm was examined using a exible strip-type pH sensor fabricated on a exible poly(ethylene terephthalate) (PET) substrate. As shown in Fig. 2B, the SWCNT lms exhibited a high degree of exibility with a negligible change in resistivity upon hard bending. The diameters of the working and reference electrode were 4 mm with 30 mm 2 mm connecting paths protected by a PET tape. Performance of a pH sensor is typically characterized by measuring the open circuit potential of the electrodes in solutions with various pH values. Fig. 2C illustrates the open circuit potential signal recorded aer immersing the SWCNT-based strip-type pH sensor in solutions with different pH values. The corresponding calibration plot (Fig. 2D) exhibits promising pHsensing performance with an ideal Nernstian response of 59.24 mV pH1 between pH 3 and 11 at 25 3 C (standard deviation of the sensitivity was 1.8 mV pH1, R2 ¼ 0.977). The SWCNT pH-sensitive electrode exhibited a nearly instantaneous response to varying pH solutions, yielding a steady-state signal within 5 s while a completely stabilized signal was observed within 30 s. The selectivity coefficients for the exible pH sensor on the SWCNT electrode were calculated in the presence of several interferents using the xed interference method (Table 1).18,19 For all cases, their selectivity coefficients revealed good selectivity against common interfering ions. The selectivity coefficients for K+, Na+, Li+ and Cl were acceptable compared to the literature results, which were less than 102 for an ion selective electrode.8 In previous reports, several carbon nanotube (CNT)-based pH sensors were demonstrated. CNT-modied glassy carbon electrodes were typically used, which required additional steps
for depositing CNTs on the electrodes. In potentiometric pH sensors, additional polymeric pH-sensitive membranes were prepared on the CNT-modied electrodes.12,20 In voltammetric pH sensors, the pH values were measured by the redox peak potentials of electrodes based on the electrochemical reaction of the electroactive groups on the SWCNTs.21,22 Moreover, the voltammetric pH sensors needed another electrode to make a three-electrode system. The open circuit potential between the SWCNT electrode and the Ag/AgCl reference electrode in different pH buffer solutions can be reasonably ascribed to the doping of the nanotube walls by H+ and OH ions that behave as electron acceptor and donor species, respectively. The change in the concentration of H+ and OH ions causes an electronic structure (rst transition S11) change in semiconducting SWCNTs, causing a respective downshi and up-shi of the Fermi level of SWCNTs in acidic and basic solutions.23,24 The Fermi level can be converted to the electrode potential relative to the standard hydrogen electrode reference with the following equation: E(electrochemical potential) ¼ F(work function)/e 4.44 V.25 More signicantly, the intensity of the rst transition S11 of semiconducting SWCNTs is reversibly tunable by changing the pH,23 which
Table 1 Selectivity coefficients of the flexible pH sensor based on the SWCNT electrode. The selectivity of the SWCNT pH-sensitive membrane was calculated using the fixed interference method
Interference ion (j)
Concentration (mol l1)
Selectivity coefficient (KH+–j)
K+ Na+ Li+ Cl
0.1 0.1 0.1 0.1
8.1 107 6.4 108 3.0 107 9.8 104
This journal is © The Royal Society of Chemistry 2014
Performances of the pH-sensitive SWCNT electrode in the microfluidic device. (A) Potential responses of the SWCNT electrode in the microfluidic device from pH 11 to pH 2 at a flow rate of 5 ml min1. (B) Calibration plot of the microfluidic pH-sensing chip in different pH buffer solutions (n ¼ 3). Fig. 3
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offers the possibility for use as a pH-sensitive membrane. In addition, SWCNTs may behave as semiconductors or metals depending on helicity and diameter, but the physical separation of semiconducting and metallic SWCNTs has proven to be one of the most difficult challenges to overcome.26 The characteristics of SWCNTs are advantageous for the pH sensor in our study, as the simply patterned thin layer of SWCNTs can act both as a pH-sensitive membrane and a working electrode. Since an ion selective electrode response is size-independent, the pH sensor based on SWCNT electrodes successfully created in a beaker system can be integrated into the microuidic system using microfabrication technology without inuencing performance. Fig. 3A illustrates the potentiometric responses as the pH level of the sample decreased from 11 to 2 at a ow rate of 5 ml min1. The results show that the developed microuidic pH-sensing chip responds well to changes in the sample pH value. In addition, the microuidic device can continuously detect the pH with the performance of the SWCNT lm. Moreover, the open circuit potentials of the pH-sensitive SWCNT electrode with an Ag/AgCl reference electrode in the microuidic device greatly resemble the measurement in beaker solutions. The corresponding calibration plot (Fig. 3B) exhibits promising pH-sensing performance with an ideal
Paper
Nernstian response of 59.71 mV pH1 between pH 3 and 11 at 25 3 C (standard deviation of the sensitivity is 1.5 mV pH1, R2 ¼ 0.985). In addition, inuence of the ow rate on the performance of the microuidic pH-sensing chips was also investigated, and the results are shown in Fig. 4A. The range of the ow rate was examined between 0.1 and 15 ml min1. The SWCNT electrode in the microuidic device shows no signicant variation at any pH value. An average sensitivity of 58.53 mV pH1 was observed with a variation less than 2% over the ow rate range. Therefore, the SWCNT electrode can be operated in a ow analysis system without degrading the performance. Fig. 4B shows the open circuit potential of the SWCNT electrode in KNO3, NaNO3, NaCl solutions successively diluted from 10 mM to 0.1 M at pH 7 with a ow rate of 5 ml min1. The open circuit potentials of the pH-sensitive SWCNT electrode with an Ag/AgCl pseudo reference electrode in the microuidic device show small variations in the KNO3 and NaNO3 solutions over the wide concentration range of 10 mM to 0.1 M, which is less than 7 mV. In addition, the potential variation is also less than 7 mV at low NaCl concentration (10 mM to 0.1 mM), while it is slightly affected at high NaCl concentration (10 mM to 0.1 M) because of the Ag/AgCl pseudo reference electrode used in the microuidic device, for which the variation is lower than 26.5 mV.
4. Conclusions In the present study, a novel SWCNT lm-based pH sensor was developed based on a exible PET substrate or integrated into a microuidic device, and the performance was investigated. The SWCNT thin lm acted both as an electrode and a pH-sensitive membrane. Therefore, the fabrication process provides advantages of lower costs and simpler processes than other all-solidstate pH electrodes. A pair of miniature SWCNT working and Ag/AgCl reference electrodes generated electrical potentials in solutions through the electronic structure change in the semiconducting SWCNTs in response to the pH level. Moreover, the sensitivity of the miniaturized SWCNT-based pH sensors was in agreement with the Nernstian response. As a result, the proposed pH-sensitive SWCNT electrodes in the microuidic device are suitable for practical uses and will enable simultaneous multi-analyte detection of metabolic processes in biological cells when several electrodes are combined.
Acknowledgements This study was supported by National Research Foundation (NRF) grants funded by the Korean government (MSIP) (no. 2011-0015545, 2008-0061891, and 2013R1A1A2054887). We also acknowledge the nancial support of the Ministry of Knowledge Economy (MKE) through the industrial infrastructure program for fundamental technologies (N000600001). Fig. 4 (A) Relationship between the potential responses and flow rates at pH 4, 7, and 10. (B) Effects of interferents on the potential responses at pH 7 with a flow rate of 5 ml min1 ranging from 105 to 101 M.
2014 | Analyst, 2014, 139, 2011–2015
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