Accepted Manuscript Fabrication of newspaper-based potentiometric platforms for flexible and disposable ion sensors Jo Hee Yoon, Kyung Hoon Kim, Nam Ho Bae, Gap Seop Sim, Yong-Jun Oh, Seok Jae Lee, Tae Jae Lee, Kyoung G. Lee, Bong Gill Choi PII: DOI: Reference:
S0021-9797(17)30942-6 http://dx.doi.org/10.1016/j.jcis.2017.08.036 YJCIS 22683
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
21 June 2017 9 August 2017 11 August 2017
Please cite this article as: J.H. Yoon, K.H. Kim, N.H. Bae, G.S. Sim, Y-J. Oh, S.J. Lee, T.J. Lee, K.G. Lee, B.G. Choi, Fabrication of newspaper-based potentiometric platforms for flexible and disposable ion sensors, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.08.036
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of newspaper-based potentiometric platforms for flexible and disposable ion sensors
Jo Hee Yoon,a,1 Kyung Hoon Kim, b,1 Nam Ho Bae, b,c Gap Seop Sim,d Yong-Jun Oh,c Seok Jae Lee,b Tae Jae Lee, b Kyoung G. Lee,b,* and Bong Gill Choia,*
a
Department of Chemical Engineering, Kangwon National University, Samcheok 25913,
Republic of Korea b
Nano-Bio Application Team, National Nanofab Center, Daejeon 34141, Republic of Korea
c
Department of Advanced Materials Science and Engineering, Hanbat National University,
34158, Daejeon Republic of Korea d
Nanostrucutre Process Team, National Nanofab Center, Daejeon 34141, Republic of Korea
*Corresponding author. E-mail: [email protected] (K.G. Lee), [email protected] (B.G. Choi) Fax: +82-33-570-6535 Tel: +82-33-570-6545 1
These authors contributed to the paper equally.
Abstract Paper-based materials have attracted a great deal of attention in sensor applications because they are readily available, biodegradable, inexpensive, and mechanically flexible. Although paper-based sensors have been developed, but important obstacles remian, which include the retention of chemical and mechanical stabilities when paper is wetted. Herein, we develop a simple and scalable process for fabrication of newspaper-based platforms by coating of parylene C and patterning of metal layers. As-prepared parylene C-coated newspaper (PC-paper) provides low-cost, disposable, and
mechanically
and chemically
stable
electrochemical
platforms for the
development of potentiometric ion sensors for the detection of electrolyte cations, such as, H+ and K+. The pH and K + sensors produced show near ideal Nernstian sensitivity, good repeatability, good ion selectivity, and low potential drift. These disposable, flexible ion sensors based on PC-paper platforms could provide new opportunities for the development of point-of-care testing sensors, for diagnostics, healthcare, and environment testing. Keywords: Ion sensor, platform, paper, potentiometry, polymer coating
1. Introduction The detection and monitoring of electrolyte ions are of considerable importance in many fields of industry, agriculture, biology, and environmental [1‒3]. Potentiometric sensors based on ion-selective electrode (ISE) materials are widely used because they readily and rapidly provide reliable measurements [4‒12]. Potentiometric ion sensors typically have a two-electrode configuration comprising an ion-sensing electrode and a reference electrode, and the potentials between these two electrodes can be quantitatively determined using the Nernst equation. However, increasing demands for healthcare monitoring, point-of-care testing, and on-site environmental detection systems require the development of flexible, lightweight, low cost, easy-to-fabricate, and disposable sensor devices [12‒17]. Hence, in addition to sensing electrode materials, flexible substrates, such as, flexible thermoplastic polymers, textiles, and paper, have attracted much attention [18‒22]. Over recent years, researchers have devised various flexible devices based on paper-based platforms for use in sensors, strain gauges, and in microfluidic and energy storage devices [23‒28]. Paper is attractive for sensor applications because it is readily available, biodegradable, inexpensive, and mechanically and chemically stable. Some paper-based sensors have been developed, but important obstacles remain, which include the retention of chemical and mechanical stabilities when paper is wetted. Herein, we demonstrate that newspaper-based platforms are useful for the fabrication of flexible and disposable potentiometric ion sensors for the detection of electrolyte cations, such as, H + and K+. The technique developed involved the chemical vapor deposition (CVD) of parylene C onto newspaper with uniform coating. The resulting parylene C-coated newspaper (PC-paper) could provide low-cost,
disposable, and mechanically and chemically stable electrochemical platforms for the development of potentiometric ion sensors. To demonstrate the feasibility of
such
platforms, PC-paper-based pH and K+ sensors were fabricated using a two-electrode configuration. The pH and K + sensors produced showed near ideal Nernstian sensitivity, good repeatability, good ion selectivity, and low potential drift.
2. Experimental 2.1. Reagents and Materials Aniline (99.5%), potassium hydrogen phthalate, potassium phosphate monobasic, borax, tris(hydroxymethyl)aminomethane, sodium hydroxide, hydrochloric acid, sodium chloride, potassium chloride, magnesium chloride, calcium chloride, ammonium chloride, valinomycin (potassium ionophore I), potassium tetrakis(4-chlorophenyl)borate (KTClPB), bis(2ethylhexyl) sebacate (DOS), poly(vinyl chloride) (PVC) with high molecular weight, and tetrahydrofuran (THF) were purchased from Sigma Aldrich. Sulfuric acid (Guaranteed Reagent) was purchased from Junsei chemical Co. Ltd. The dielectric ink ESL 242-SB was purchased from ESL ElectroScience.
2.2. Fabrication of PC-paper-based potentiometric ion sensor platforms The top and bottom sides of paper were completely coated with 5g of parylene C, via chemical vapor deposition (PDS 2010 Labcoater 2, Speedline Technology, Camdenton, MO). The parylene coating process contains three consequent steps: vaporization of solid dimer of parylene C at ~180C, the cleavage of dimer into two methylene-methlyene bonds at ~690C in order to yield the stable monomeric diradical, and the monomer of parylene C is absorbed and polymerized on the paper as soon as the monomers cooled down to the room temperature. The parylene-C coated paper was denoted as PC-paper. Gold and silver with thickness of 200
nm as electrode and Ti with thickness of 20 nm as an adhesion layer were evaporated through stencil mask using an E-beam evaporator (KVE T-C500200, South Korea). Gold and silver were utilized as working/counter and reference electrodes, respectively.
2.3. Preparation of pH and K + sensors Prior to prepare sensing and reference electrodes, the patterned gold and silver electrodes were electrochemically cleaned by cyclic voltammetry (CV) technique of 10 cycles using 1 M H2SO4 in a potential range of ‒0.3 ‒ +1.2 V at scan rate of 50 mV/s under a threeelectrode system. For pH sensing electrode materials, polyaniline (PANi) layer was electrochemically deposited onto the surface of a gold pattern by CV technique of 30 cycles using 0.5 M H 2SO4/0.25 M aniline monomer solution in a potential range of ‒0.1 ‒ +0.8 V at scan rate of 50 mV/s under a three-electrode system [29]. Pt wire and Ag/AgCl were used as counter and reference electrodes, respectively. The potassium ion selective electrode was prepared by dropcasting of potassium selective membrane solution (2 μL) five times. The potassium ion selective membrane contained 2 mg of valinomycin as ionophore, 0.5 mg of KTClPB as cation exchanger, 64.7 mg of DOS plasticizer, and 32.8 mg of PVC in 1 ml THF. The Ag/AgCl reference electrode was prepared by electroplating chloride on silver pattern using amperometric techniqe of 0.2 V (versus Ag/AgCl) in 0.1 M KCl solution. The resultant Ag/AgCl electrode was coated with dielectric ink ESL 242-SB mixed with KCl (30 wt%) and then cured in an oven at 120 oC for 10 minutes. The areas of sensing and reference electrodes were ? and 2 × 5 cm2, respectively.
2.4. Characterization and Measurements
Scanning electron microscope (SEM, Hitachi S-4800) was performed to collect morphological images of pristine newspaper and PC-paper. Electrochemical cleaning, deposition, and measurements were performed using a CHI 760 E (CH Instruments, USA). The pH values were measured using OrionTM Star A211 pH meter. The pH sensor used a hand made reference electrode and the potassium ion sensor was measured using a commercial Ag/AgCl reference electrode. All ion sensors were immersed in the solution for one minute for stabilization before measurement. To collect electromotive force (EMF) responses, pH solution was titrated with a pH meter by adding 1 M HCl and 1 M NaOH to the pH buffer solution. The pH buffer solution was prepared by mixing 5 mM potassium hydrogen phthalate, 5 mM potassium dihydrogen phosphate, 5 mM tris(hydroxymethyl)aminomethane, 2.5 mM borax, and 100 mM sodium chloride. For measuring K+ sensors, 1 M KCl solution was diluted to prepare a 1×10‒1 M to 1×10‒4 M KCl solutions. To measure bent pH and K+ sensors, ion sensor devices were fixed to a glass rod with respect to a bending radius of 7 mm. The slectivity coefficients of pH and K + sensors were calculated using the separatesolution method in the presence of different interfering ions at a concentration of 10‒2 M. Selectivity coefficients (K) was expressed as follows:
where K is the selectivity coefficient, I is primary ion, and J is interfering ion.
3. Results and Discussion Fig. 1a shows scheme of the CVD-based parylene C coating process, which reproducibly produces a continuous thin layer of parylene C on newspaper. Optical and SEM images of pristine newspaper and PC-paper confirmed the presence of a uniform, thin, transparent parylene C coating (Fig. 1b). SEM images showed the rough
networks of cellulose fibers and pores in pristine paper became smoother after parylene C coating due to the formation of polymer layers. In addition, PC-paper exhibited water resilience and mechanical strength (Fig. 2a ‒ 2d). In fact, parylene C coating caused the originally hydrophilic nature of pristine newspaper to become hydrophobic surface (Fig. 2a and 2b). We tested the ability of PC-paper to resist pH changes by immersion in acid and base solutions at pH values from 1 to 13 (Fig. 2c), for 1 hour. Not surprisingly, pristine newspaper absorbed a large amount of water at each pH condition (pH 1: 312 %, pH 4: 304 %, pH 7: 316 %, pH 10: 309 %, pH 13: 466 %, avg. 341.4 %) and at pH values of 10 and 13 discolored due to its poor chemical stability, which result in the leaching of impurities from newspaper that in turn result in buffering effects that adversely affect ion sensors. On the other hand, PCpapers showed excellent water resistance and low levels of water adsorption (pH 1: 34 %, pH 4: 36 %, pH 7: 35 %, pH 10: 35 %, pH 13: 76 %, and avg. 43.2 %), and acid/base immersion testing showed no detectable impurities leach into solution from PC-paper as determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES). This result showed that the parylene C coating process reproducibly produces pinhole-free coatings on cellulose fibers. Thus, it would appear parylene C completely blocks the leaching chemicals from newspaper, and water penetration, which are requirements of good ion-sensing platforms. The mechanical stability of PC-paper also evaluated under different pH conditions (Fig. 2d). Dry PC-paper had a Young’s modulus (E, 14.3 MPa) and tensile stress (σ, 28.1 MPa) much greater than dry pristine newspaper (E, 8.1 MPa; σ, 16.3 MPa). The increased mechanical properties of PC-paper must be attributed to the strongly interacted parylene C layers onto the surface cellulose fibers. Ion sensor platforms are exposed to aqueous solutions during used, and thus, it is important that mechanical strength i s maintained
in the wet state. When exposed to acidic and basic solutions, PC-papers showed slight decrease Young’s modulus (~10 MPa) and tensile stress (~25 MPa), which are much higher than wet newspaper (Fig. 2d). The excellent mechanical flexibility and chemical stability of PC-papers makes them attractive sensing platforms for potentiometric ions sensors. In order to use PCpaper as potentiometric sensor electrodes, a two-electrode configuration system was designed (Fig. 3a and 3b). We designed and fabricated PC-paper-based sensing platforms as user-friendly components, such as, perforated line and commercial USBtype sensing platforms (Fig. 3c). Because of their excellent mechanical flexibilities, as-prepared PC-paper platforms could be easily wrapped around circular tubes (Fig. 3c). Gold and silver layers were deposited onto the surfaces of PC-papers by stencil lithography and sputtering. As shown in SEM images, thin layers of gold and silver were successfully deposited (Fig. 3d). Using PC-paper, we fabricated potentiometric ion-selective sensors containing an ISE and a reference electrode. To demonstrate the pH sensor performance of these PC paper platform electrodes, a PANi layer was electrochemically deposited onto the surface of a gold pattern by cyclic voltammetry. PANi has been widely used for pH measurement because of its redox equilibrium, which involves phase-transition of PANi and H 3O+ ions [4,5]. The Ag/AgCl reference electrode was produced by electroplating chloride on pre-pattered Ag layers, and then coating this with dielectric ink mixed with KCl (30wt%). The resulting pH sensors were first tested by increasing pH levels from 2 to 12 (Fig. 4a). The pH levels were controlled by adding 1 M HCl or 1 M NaOH under buffered conditions, while pH sensors collected electromotive force (EMF) signals between PANi and Ag/AgCl electrodes. Obtained EMFs were plotted versus pH (Fig. 4b), and the sensitivity of the pH sensors was calculated to be 58.2
mV/pH with a R 2 of 0.99. The linearity of the plot obtained indicated that the pH sensors behaved in a Nernstian equation. To confirm their flexibilities, the same experiment was repeated with bent electrodes (Fig. 4a). As shown in Fig. 4b, the Nernstian response obtained confirmed a mechanically normal pH sensor state. This result indicates that the sensing stability of pH sensor has excellent mechanical resistance. Repeatability is one of the most important criteria of pH sensors. EMF signals were continuously collected according to the four-consecutive titration process. The pH sensor showed a very low hysteresis width of 2 mV after 2 cycles, and when measurements were performed with a bent sensor, cycling curves almost overlapped the original sensor curve of pre-normal pH sensor state. This result indicates that the pH sensors had good repeatability and excellent mechanical properties. Stability is also an important feature, as potential drifts can cause errors in long-term pH measurements that cannot be corrected by calibration. To measure potential drift, the pH sensor was placed in pH 8 buffer solution for 15 h (Fig. 4d). The sensor showed a low drift rate of 1.4 mV/h, which is much lower than those of other pH sensors. For practical applications, pH sensors need to provide an accurate measure of H + concentration in the presence of other cations. Sensor response time was measured by abruptly changing pH levels from 10.0 to 2.0 (Fig. 5). The pH sensor showed a response time of < 10 s, defined as the time needed to reach 90% of the equilibrium potential value. To investigate the ion selectivity of pH sensors, we calculated selectivity coefficients (K) using the separate-solution method (SSM) in the presence of K+, Na+, NH4+, Ca2+, and Mg2+ as interfering ions. The obtained K values are provided in Table 1. All K values were < 1, indicating that the sensor exhibited a preference for H + over the interfering cations [30].
For explore the versatility of PC-paper platform, PC-paper was also applied to K+ sensors. K+ ion selective electrodes were prepared by modifying the surface of PCpaper using a K+ selective cocktail. Sensor sensitivity, repeatability, stability, and ion selectivity were evaluated. Fig. 6a shows the EMF values measured at K+ concentrations ranging from 100 ‒ 0.1 mM, which covers the physiologic human range. The sensitivities of K+ sensors were evaluated with bent and unbent states. The sensors showed Nernstian responses of 56.1 and 55.4 mV/log [K+] for these two mechanical states, respectively (Fig. 6b). The repeatability mechanical stability of K+ sensors was also tested (Fig. 6c). The initial sensitivity (56.2 mV/log [K+]) of K+ sensors was maintained after repetitive testing. When K+ sensor outputs were continuously measured in 10 mM K+ solution for 15 h (Fig. 6d), a potential drift of 4.3 mV/h was observed, which indicated good durability. When measuring response time by changing K+ concentrations from 100 to 1 mM, K+ sensor exhibited a response time of < 10 s. Ion selective performance was conducted by SSM in the presence of H +, Na+, NH4+, Ca2+, and Mg2+. Measured K values were < 1, (Table 2), indicating the K+ sensors produced were able to selectively detect potassium ions in the presence of these different cations [30]. Table 3 shows the comparison sensor performances of PCpaper platform-based pH and K+ sensors. Our pH and K+ sensors exhibited superior or comparable sensing performances compared to other previously reported paper-based ion sensors.
4. Conclusions We developed a disposable, low-cost, flexible paper-based ion-sensing platforms for potentiometric ion sensors. Direct coating of parylene C on newspaper efficiently increased Young’s modulus and tensile stress and maintained mechanical properties
even after wetting and exhibited chemical resistance to strong acid and base solutions. Stencil lithography enabled the production of user-friendly USB-type sensing platforms. To demonstrate the potential of the PC-paper sensing platforms for ion sensors, pH and K+ sensors were fabricated using PC-papers. The PC-paper-based pH sensors and K+ sensors showed a high sensitivity and a good repeatability, even in a bent state. In addition, these sensors exhibited high ion selectivity and long-term stability. We believe these disposable, flexible ion sensors based on PC-paper platforms provide new opportunities for the development of point-of-care testing sensors, for diagnostics, healthcare, and environmental testing.
Acknowledgements This research was supported by the Bio & Medical Technology Development Program of the National Research Foundation (NRF)& funded by the Korean government (MSIP) (No. 2015M3A9D7067457) and Cooperative Research Program for Agriculture Science & Technology Development (Project No. PJ01252602) from the Rural Development Administration (RDA) of the Republic of Korea. This research was supported in part by the BioNano HealthGuard Research Center, funded by the Ministry of Science, ICT &Future Planning (MSIP) within the framework of a Global Frontier Project (H-GUARD 2014M3A6B2060302). References [1] Y. Qin, H.-J. Kwon, M. M. R. Howlader, M. J. Deen, RSC Adv. 5 (2015) 69086‒69109. [2]A. Lewenstam, Electroanalysis 26 (2014) 1171‒1181. [3]
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Table 1. Selective coefficients of pH sensors using SSM for primary ion (H+) against interfering ions.
Ions (J) K+
‒5.90
1.27×10-6
Na+
‒5.26
5.44×10-6
NH4+
‒3.32
4.77×10-4
Ca2+
‒5.14
7.23×10-6
Mg2+
‒3.90
1.26×10-4
Table 2. Selective coefficients of K+ sensors using SSM for primary ion (K+) against interfering ions.
Ions (J) H+
‒0.04
9.22×10-1
Na+
‒1.80
1.57×10-2
NH4+
‒1.41
3.86×10-2
Ca2+
‒5.04
9.22×10-6
Mg2+
‒5.77
1.71×10-6
Table 3. Comparison of the performance of paper-based ion sensors.
Substrate
Detection
material
ion
Linear range
Sensitivity Normal state
Ref.
Bent state
Paper
K+
10−3 ~ 10−7 M
56.7 ± 0.8
−
[31]
Paper
K+
10−1 ~ 10−3.1 M
53.3 ± 0.7
−
[23]
Paper
pH
pH 4 ~ 10
50
−
[32]
K+
10−1 ~ 10−4 M
56.1
55.4
In this
pH
pH 2 ~ 12
57.6
58.0
work
PC-paper
Fig. 1. (a) Scheme of fabrication of PC-paper by paralene C coating using CVD method. (b) Photographs and SEM images of pristine newsaper and PC-paper. All scale bars are 100 m.
Fig. 2. Photographs after dropping a water droplet on (a) pristine newspaper and (b) PC-paper. (c) Change of weight for pristine newspaper and PC-paper after immersing in solutions at different pH values. (d) Stress‒strain curves of dry pristine newspaper and PC-paper and wet newpaper and PC-papers after immersing in solutions at different pH values.
Fig. 3. (a) Scheme of PC-paper-based potentiometric ion-selective sensor dimensions (Units are mm). (b) Photograph of scalable production for potentiometric ion-selective sensors. (c) Photographs of application of USB-type sensing platforms and flexible state of ion-selective sensor. (d) Surface SEM images of Ag and Au coated electrodes on potentiometric ionselective sensors. Scale bar is 1 mm.
Fig. 4. (a) Response of pH sensors with increasing pH levels under mechanically normal and bent states. Inset is photographs of pH sensors at mechanically normal and bent states. (b) Plot of EMF signals versus pH for pH sensors under mechanically normal and bent states. (c) Repeatability test for pH sensors with varying pH levels in a range of 2.5‒11.5. (d) Longterm stability of pH sensors measured at pH 8 buffer solution.
Fig. 5. Temporal response in titration from pH 10.0 to pH 2.0 for pH sensor under mechanically normal and bent states.
Fig. 6. (a) Response of K+ sensors with increasing K+ levels under mechanically normal and bent states. (b) Plot of EMF signals versus Log [K+] for K+ sensors under mechanically normal and bent states. (c) Repeatability test for K+ sensors with varying K+ levels in a range of 10 ‒ 1 mM. (d) Long-term stability of K+ sensors measured at10 mM K+ solution.
Fig. 7. Temporal response by changing K+ concentrations from 100 to 1 mM for K+ sensor under mechanically normal and bent states.
Graphical abstract
Flexible and disposable USB-type ion sensors were fabricated using newspaper-based platforms by coating of parylene C and patterning of metal layers, showing a near-Nernstian reponse, a wide detection range, a rapid response time, repeatability, selectivity, and stability.
S0021-9797(17)30942-6 http://dx.doi.org/10.1016/j.jcis.2017.08.036 YJCIS 22683
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
21 June 2017 9 August 2017 11 August 2017
Please cite this article as: J.H. Yoon, K.H. Kim, N.H. Bae, G.S. Sim, Y-J. Oh, S.J. Lee, T.J. Lee, K.G. Lee, B.G. Choi, Fabrication of newspaper-based potentiometric platforms for flexible and disposable ion sensors, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.08.036
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of newspaper-based potentiometric platforms for flexible and disposable ion sensors
Jo Hee Yoon,a,1 Kyung Hoon Kim, b,1 Nam Ho Bae, b,c Gap Seop Sim,d Yong-Jun Oh,c Seok Jae Lee,b Tae Jae Lee, b Kyoung G. Lee,b,* and Bong Gill Choia,*
a
Department of Chemical Engineering, Kangwon National University, Samcheok 25913,
Republic of Korea b
Nano-Bio Application Team, National Nanofab Center, Daejeon 34141, Republic of Korea
c
Department of Advanced Materials Science and Engineering, Hanbat National University,
34158, Daejeon Republic of Korea d
Nanostrucutre Process Team, National Nanofab Center, Daejeon 34141, Republic of Korea
*Corresponding author. E-mail: [email protected] (K.G. Lee), [email protected] (B.G. Choi) Fax: +82-33-570-6535 Tel: +82-33-570-6545 1
These authors contributed to the paper equally.
Abstract Paper-based materials have attracted a great deal of attention in sensor applications because they are readily available, biodegradable, inexpensive, and mechanically flexible. Although paper-based sensors have been developed, but important obstacles remian, which include the retention of chemical and mechanical stabilities when paper is wetted. Herein, we develop a simple and scalable process for fabrication of newspaper-based platforms by coating of parylene C and patterning of metal layers. As-prepared parylene C-coated newspaper (PC-paper) provides low-cost, disposable, and
mechanically
and chemically
stable
electrochemical
platforms for the
development of potentiometric ion sensors for the detection of electrolyte cations, such as, H+ and K+. The pH and K + sensors produced show near ideal Nernstian sensitivity, good repeatability, good ion selectivity, and low potential drift. These disposable, flexible ion sensors based on PC-paper platforms could provide new opportunities for the development of point-of-care testing sensors, for diagnostics, healthcare, and environment testing. Keywords: Ion sensor, platform, paper, potentiometry, polymer coating
1. Introduction The detection and monitoring of electrolyte ions are of considerable importance in many fields of industry, agriculture, biology, and environmental [1‒3]. Potentiometric sensors based on ion-selective electrode (ISE) materials are widely used because they readily and rapidly provide reliable measurements [4‒12]. Potentiometric ion sensors typically have a two-electrode configuration comprising an ion-sensing electrode and a reference electrode, and the potentials between these two electrodes can be quantitatively determined using the Nernst equation. However, increasing demands for healthcare monitoring, point-of-care testing, and on-site environmental detection systems require the development of flexible, lightweight, low cost, easy-to-fabricate, and disposable sensor devices [12‒17]. Hence, in addition to sensing electrode materials, flexible substrates, such as, flexible thermoplastic polymers, textiles, and paper, have attracted much attention [18‒22]. Over recent years, researchers have devised various flexible devices based on paper-based platforms for use in sensors, strain gauges, and in microfluidic and energy storage devices [23‒28]. Paper is attractive for sensor applications because it is readily available, biodegradable, inexpensive, and mechanically and chemically stable. Some paper-based sensors have been developed, but important obstacles remain, which include the retention of chemical and mechanical stabilities when paper is wetted. Herein, we demonstrate that newspaper-based platforms are useful for the fabrication of flexible and disposable potentiometric ion sensors for the detection of electrolyte cations, such as, H + and K+. The technique developed involved the chemical vapor deposition (CVD) of parylene C onto newspaper with uniform coating. The resulting parylene C-coated newspaper (PC-paper) could provide low-cost,
disposable, and mechanically and chemically stable electrochemical platforms for the development of potentiometric ion sensors. To demonstrate the feasibility of
such
platforms, PC-paper-based pH and K+ sensors were fabricated using a two-electrode configuration. The pH and K + sensors produced showed near ideal Nernstian sensitivity, good repeatability, good ion selectivity, and low potential drift.
2. Experimental 2.1. Reagents and Materials Aniline (99.5%), potassium hydrogen phthalate, potassium phosphate monobasic, borax, tris(hydroxymethyl)aminomethane, sodium hydroxide, hydrochloric acid, sodium chloride, potassium chloride, magnesium chloride, calcium chloride, ammonium chloride, valinomycin (potassium ionophore I), potassium tetrakis(4-chlorophenyl)borate (KTClPB), bis(2ethylhexyl) sebacate (DOS), poly(vinyl chloride) (PVC) with high molecular weight, and tetrahydrofuran (THF) were purchased from Sigma Aldrich. Sulfuric acid (Guaranteed Reagent) was purchased from Junsei chemical Co. Ltd. The dielectric ink ESL 242-SB was purchased from ESL ElectroScience.
2.2. Fabrication of PC-paper-based potentiometric ion sensor platforms The top and bottom sides of paper were completely coated with 5g of parylene C, via chemical vapor deposition (PDS 2010 Labcoater 2, Speedline Technology, Camdenton, MO). The parylene coating process contains three consequent steps: vaporization of solid dimer of parylene C at ~180C, the cleavage of dimer into two methylene-methlyene bonds at ~690C in order to yield the stable monomeric diradical, and the monomer of parylene C is absorbed and polymerized on the paper as soon as the monomers cooled down to the room temperature. The parylene-C coated paper was denoted as PC-paper. Gold and silver with thickness of 200
nm as electrode and Ti with thickness of 20 nm as an adhesion layer were evaporated through stencil mask using an E-beam evaporator (KVE T-C500200, South Korea). Gold and silver were utilized as working/counter and reference electrodes, respectively.
2.3. Preparation of pH and K + sensors Prior to prepare sensing and reference electrodes, the patterned gold and silver electrodes were electrochemically cleaned by cyclic voltammetry (CV) technique of 10 cycles using 1 M H2SO4 in a potential range of ‒0.3 ‒ +1.2 V at scan rate of 50 mV/s under a threeelectrode system. For pH sensing electrode materials, polyaniline (PANi) layer was electrochemically deposited onto the surface of a gold pattern by CV technique of 30 cycles using 0.5 M H 2SO4/0.25 M aniline monomer solution in a potential range of ‒0.1 ‒ +0.8 V at scan rate of 50 mV/s under a three-electrode system [29]. Pt wire and Ag/AgCl were used as counter and reference electrodes, respectively. The potassium ion selective electrode was prepared by dropcasting of potassium selective membrane solution (2 μL) five times. The potassium ion selective membrane contained 2 mg of valinomycin as ionophore, 0.5 mg of KTClPB as cation exchanger, 64.7 mg of DOS plasticizer, and 32.8 mg of PVC in 1 ml THF. The Ag/AgCl reference electrode was prepared by electroplating chloride on silver pattern using amperometric techniqe of 0.2 V (versus Ag/AgCl) in 0.1 M KCl solution. The resultant Ag/AgCl electrode was coated with dielectric ink ESL 242-SB mixed with KCl (30 wt%) and then cured in an oven at 120 oC for 10 minutes. The areas of sensing and reference electrodes were ? and 2 × 5 cm2, respectively.
2.4. Characterization and Measurements
Scanning electron microscope (SEM, Hitachi S-4800) was performed to collect morphological images of pristine newspaper and PC-paper. Electrochemical cleaning, deposition, and measurements were performed using a CHI 760 E (CH Instruments, USA). The pH values were measured using OrionTM Star A211 pH meter. The pH sensor used a hand made reference electrode and the potassium ion sensor was measured using a commercial Ag/AgCl reference electrode. All ion sensors were immersed in the solution for one minute for stabilization before measurement. To collect electromotive force (EMF) responses, pH solution was titrated with a pH meter by adding 1 M HCl and 1 M NaOH to the pH buffer solution. The pH buffer solution was prepared by mixing 5 mM potassium hydrogen phthalate, 5 mM potassium dihydrogen phosphate, 5 mM tris(hydroxymethyl)aminomethane, 2.5 mM borax, and 100 mM sodium chloride. For measuring K+ sensors, 1 M KCl solution was diluted to prepare a 1×10‒1 M to 1×10‒4 M KCl solutions. To measure bent pH and K+ sensors, ion sensor devices were fixed to a glass rod with respect to a bending radius of 7 mm. The slectivity coefficients of pH and K + sensors were calculated using the separatesolution method in the presence of different interfering ions at a concentration of 10‒2 M. Selectivity coefficients (K) was expressed as follows:
where K is the selectivity coefficient, I is primary ion, and J is interfering ion.
3. Results and Discussion Fig. 1a shows scheme of the CVD-based parylene C coating process, which reproducibly produces a continuous thin layer of parylene C on newspaper. Optical and SEM images of pristine newspaper and PC-paper confirmed the presence of a uniform, thin, transparent parylene C coating (Fig. 1b). SEM images showed the rough
networks of cellulose fibers and pores in pristine paper became smoother after parylene C coating due to the formation of polymer layers. In addition, PC-paper exhibited water resilience and mechanical strength (Fig. 2a ‒ 2d). In fact, parylene C coating caused the originally hydrophilic nature of pristine newspaper to become hydrophobic surface (Fig. 2a and 2b). We tested the ability of PC-paper to resist pH changes by immersion in acid and base solutions at pH values from 1 to 13 (Fig. 2c), for 1 hour. Not surprisingly, pristine newspaper absorbed a large amount of water at each pH condition (pH 1: 312 %, pH 4: 304 %, pH 7: 316 %, pH 10: 309 %, pH 13: 466 %, avg. 341.4 %) and at pH values of 10 and 13 discolored due to its poor chemical stability, which result in the leaching of impurities from newspaper that in turn result in buffering effects that adversely affect ion sensors. On the other hand, PCpapers showed excellent water resistance and low levels of water adsorption (pH 1: 34 %, pH 4: 36 %, pH 7: 35 %, pH 10: 35 %, pH 13: 76 %, and avg. 43.2 %), and acid/base immersion testing showed no detectable impurities leach into solution from PC-paper as determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES). This result showed that the parylene C coating process reproducibly produces pinhole-free coatings on cellulose fibers. Thus, it would appear parylene C completely blocks the leaching chemicals from newspaper, and water penetration, which are requirements of good ion-sensing platforms. The mechanical stability of PC-paper also evaluated under different pH conditions (Fig. 2d). Dry PC-paper had a Young’s modulus (E, 14.3 MPa) and tensile stress (σ, 28.1 MPa) much greater than dry pristine newspaper (E, 8.1 MPa; σ, 16.3 MPa). The increased mechanical properties of PC-paper must be attributed to the strongly interacted parylene C layers onto the surface cellulose fibers. Ion sensor platforms are exposed to aqueous solutions during used, and thus, it is important that mechanical strength i s maintained
in the wet state. When exposed to acidic and basic solutions, PC-papers showed slight decrease Young’s modulus (~10 MPa) and tensile stress (~25 MPa), which are much higher than wet newspaper (Fig. 2d). The excellent mechanical flexibility and chemical stability of PC-papers makes them attractive sensing platforms for potentiometric ions sensors. In order to use PCpaper as potentiometric sensor electrodes, a two-electrode configuration system was designed (Fig. 3a and 3b). We designed and fabricated PC-paper-based sensing platforms as user-friendly components, such as, perforated line and commercial USBtype sensing platforms (Fig. 3c). Because of their excellent mechanical flexibilities, as-prepared PC-paper platforms could be easily wrapped around circular tubes (Fig. 3c). Gold and silver layers were deposited onto the surfaces of PC-papers by stencil lithography and sputtering. As shown in SEM images, thin layers of gold and silver were successfully deposited (Fig. 3d). Using PC-paper, we fabricated potentiometric ion-selective sensors containing an ISE and a reference electrode. To demonstrate the pH sensor performance of these PC paper platform electrodes, a PANi layer was electrochemically deposited onto the surface of a gold pattern by cyclic voltammetry. PANi has been widely used for pH measurement because of its redox equilibrium, which involves phase-transition of PANi and H 3O+ ions [4,5]. The Ag/AgCl reference electrode was produced by electroplating chloride on pre-pattered Ag layers, and then coating this with dielectric ink mixed with KCl (30wt%). The resulting pH sensors were first tested by increasing pH levels from 2 to 12 (Fig. 4a). The pH levels were controlled by adding 1 M HCl or 1 M NaOH under buffered conditions, while pH sensors collected electromotive force (EMF) signals between PANi and Ag/AgCl electrodes. Obtained EMFs were plotted versus pH (Fig. 4b), and the sensitivity of the pH sensors was calculated to be 58.2
mV/pH with a R 2 of 0.99. The linearity of the plot obtained indicated that the pH sensors behaved in a Nernstian equation. To confirm their flexibilities, the same experiment was repeated with bent electrodes (Fig. 4a). As shown in Fig. 4b, the Nernstian response obtained confirmed a mechanically normal pH sensor state. This result indicates that the sensing stability of pH sensor has excellent mechanical resistance. Repeatability is one of the most important criteria of pH sensors. EMF signals were continuously collected according to the four-consecutive titration process. The pH sensor showed a very low hysteresis width of 2 mV after 2 cycles, and when measurements were performed with a bent sensor, cycling curves almost overlapped the original sensor curve of pre-normal pH sensor state. This result indicates that the pH sensors had good repeatability and excellent mechanical properties. Stability is also an important feature, as potential drifts can cause errors in long-term pH measurements that cannot be corrected by calibration. To measure potential drift, the pH sensor was placed in pH 8 buffer solution for 15 h (Fig. 4d). The sensor showed a low drift rate of 1.4 mV/h, which is much lower than those of other pH sensors. For practical applications, pH sensors need to provide an accurate measure of H + concentration in the presence of other cations. Sensor response time was measured by abruptly changing pH levels from 10.0 to 2.0 (Fig. 5). The pH sensor showed a response time of < 10 s, defined as the time needed to reach 90% of the equilibrium potential value. To investigate the ion selectivity of pH sensors, we calculated selectivity coefficients (K) using the separate-solution method (SSM) in the presence of K+, Na+, NH4+, Ca2+, and Mg2+ as interfering ions. The obtained K values are provided in Table 1. All K values were < 1, indicating that the sensor exhibited a preference for H + over the interfering cations [30].
For explore the versatility of PC-paper platform, PC-paper was also applied to K+ sensors. K+ ion selective electrodes were prepared by modifying the surface of PCpaper using a K+ selective cocktail. Sensor sensitivity, repeatability, stability, and ion selectivity were evaluated. Fig. 6a shows the EMF values measured at K+ concentrations ranging from 100 ‒ 0.1 mM, which covers the physiologic human range. The sensitivities of K+ sensors were evaluated with bent and unbent states. The sensors showed Nernstian responses of 56.1 and 55.4 mV/log [K+] for these two mechanical states, respectively (Fig. 6b). The repeatability mechanical stability of K+ sensors was also tested (Fig. 6c). The initial sensitivity (56.2 mV/log [K+]) of K+ sensors was maintained after repetitive testing. When K+ sensor outputs were continuously measured in 10 mM K+ solution for 15 h (Fig. 6d), a potential drift of 4.3 mV/h was observed, which indicated good durability. When measuring response time by changing K+ concentrations from 100 to 1 mM, K+ sensor exhibited a response time of < 10 s. Ion selective performance was conducted by SSM in the presence of H +, Na+, NH4+, Ca2+, and Mg2+. Measured K values were < 1, (Table 2), indicating the K+ sensors produced were able to selectively detect potassium ions in the presence of these different cations [30]. Table 3 shows the comparison sensor performances of PCpaper platform-based pH and K+ sensors. Our pH and K+ sensors exhibited superior or comparable sensing performances compared to other previously reported paper-based ion sensors.
4. Conclusions We developed a disposable, low-cost, flexible paper-based ion-sensing platforms for potentiometric ion sensors. Direct coating of parylene C on newspaper efficiently increased Young’s modulus and tensile stress and maintained mechanical properties
even after wetting and exhibited chemical resistance to strong acid and base solutions. Stencil lithography enabled the production of user-friendly USB-type sensing platforms. To demonstrate the potential of the PC-paper sensing platforms for ion sensors, pH and K+ sensors were fabricated using PC-papers. The PC-paper-based pH sensors and K+ sensors showed a high sensitivity and a good repeatability, even in a bent state. In addition, these sensors exhibited high ion selectivity and long-term stability. We believe these disposable, flexible ion sensors based on PC-paper platforms provide new opportunities for the development of point-of-care testing sensors, for diagnostics, healthcare, and environmental testing.
Acknowledgements This research was supported by the Bio & Medical Technology Development Program of the National Research Foundation (NRF)& funded by the Korean government (MSIP) (No. 2015M3A9D7067457) and Cooperative Research Program for Agriculture Science & Technology Development (Project No. PJ01252602) from the Rural Development Administration (RDA) of the Republic of Korea. This research was supported in part by the BioNano HealthGuard Research Center, funded by the Ministry of Science, ICT &Future Planning (MSIP) within the framework of a Global Frontier Project (H-GUARD 2014M3A6B2060302). References [1] Y. Qin, H.-J. Kwon, M. M. R. Howlader, M. J. Deen, RSC Adv. 5 (2015) 69086‒69109. [2]A. Lewenstam, Electroanalysis 26 (2014) 1171‒1181. [3]
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Table 1. Selective coefficients of pH sensors using SSM for primary ion (H+) against interfering ions.
Ions (J) K+
‒5.90
1.27×10-6
Na+
‒5.26
5.44×10-6
NH4+
‒3.32
4.77×10-4
Ca2+
‒5.14
7.23×10-6
Mg2+
‒3.90
1.26×10-4
Table 2. Selective coefficients of K+ sensors using SSM for primary ion (K+) against interfering ions.
Ions (J) H+
‒0.04
9.22×10-1
Na+
‒1.80
1.57×10-2
NH4+
‒1.41
3.86×10-2
Ca2+
‒5.04
9.22×10-6
Mg2+
‒5.77
1.71×10-6
Table 3. Comparison of the performance of paper-based ion sensors.
Substrate
Detection
material
ion
Linear range
Sensitivity Normal state
Ref.
Bent state
Paper
K+
10−3 ~ 10−7 M
56.7 ± 0.8
−
[31]
Paper
K+
10−1 ~ 10−3.1 M
53.3 ± 0.7
−
[23]
Paper
pH
pH 4 ~ 10
50
−
[32]
K+
10−1 ~ 10−4 M
56.1
55.4
In this
pH
pH 2 ~ 12
57.6
58.0
work
PC-paper
Fig. 1. (a) Scheme of fabrication of PC-paper by paralene C coating using CVD method. (b) Photographs and SEM images of pristine newsaper and PC-paper. All scale bars are 100 m.
Fig. 2. Photographs after dropping a water droplet on (a) pristine newspaper and (b) PC-paper. (c) Change of weight for pristine newspaper and PC-paper after immersing in solutions at different pH values. (d) Stress‒strain curves of dry pristine newspaper and PC-paper and wet newpaper and PC-papers after immersing in solutions at different pH values.
Fig. 3. (a) Scheme of PC-paper-based potentiometric ion-selective sensor dimensions (Units are mm). (b) Photograph of scalable production for potentiometric ion-selective sensors. (c) Photographs of application of USB-type sensing platforms and flexible state of ion-selective sensor. (d) Surface SEM images of Ag and Au coated electrodes on potentiometric ionselective sensors. Scale bar is 1 mm.
Fig. 4. (a) Response of pH sensors with increasing pH levels under mechanically normal and bent states. Inset is photographs of pH sensors at mechanically normal and bent states. (b) Plot of EMF signals versus pH for pH sensors under mechanically normal and bent states. (c) Repeatability test for pH sensors with varying pH levels in a range of 2.5‒11.5. (d) Longterm stability of pH sensors measured at pH 8 buffer solution.
Fig. 5. Temporal response in titration from pH 10.0 to pH 2.0 for pH sensor under mechanically normal and bent states.
Fig. 6. (a) Response of K+ sensors with increasing K+ levels under mechanically normal and bent states. (b) Plot of EMF signals versus Log [K+] for K+ sensors under mechanically normal and bent states. (c) Repeatability test for K+ sensors with varying K+ levels in a range of 10 ‒ 1 mM. (d) Long-term stability of K+ sensors measured at10 mM K+ solution.
Fig. 7. Temporal response by changing K+ concentrations from 100 to 1 mM for K+ sensor under mechanically normal and bent states.
Graphical abstract
Flexible and disposable USB-type ion sensors were fabricated using newspaper-based platforms by coating of parylene C and patterning of metal layers, showing a near-Nernstian reponse, a wide detection range, a rapid response time, repeatability, selectivity, and stability.
