An electrochemical sensor using Poly(3,4-ethylenedioxythiophene) (PEDOT) modified with iron nanoparticles (FeNPs), gold nanoparticles (AuNPs) and carbon ...
An impedance spectroscopy model consistent with nanoparticles properties in a Poly (3,4-ethylenedioxythiophene) matrix F. Alvarado-Hidalgo1, H. Phillips-Brenes2, G. Sáenz-Arce3, P. Hernández-Suárez1, B. Zúñiga-Sancho1, J. Sandoval-Sandoval1 and R. Starbird4 1Escuela
de Ciencia e Ingeniería en Materiales, Instituto Tecnológico de Costa Rica, Cartago, Costa Rica 2Escuela de Ingeniería Electrónica, , Instituto Tecnológico de Costa Rica, Cartago, Costa Rica 3Laboratorio de Materiales Industriales (LAMI), Departamento de Física, Universidad Nacional de Costa Rica, Heredia, Costa Rica 4Escuela de Química, Instituto Tecnológico de Costa Rica, Cartago, Costa Rica An electrochemical sensor using Poly(3,4-ethylenedioxythiophene) (PEDOT) modified with iron nanoparticles (FeNPs), gold nanoparticles (AuNPs) and carbon nanotubes (CNTs) was studied by measuring its impedance behavior in the frequency range (1–106 Hz) at 50 mV applied AC potential. The proposed model is based on an equivalent circuit containing a PEDOT layer and the corresponding nanoparticles. Diffusion phenomena is considered and measured in the proposed model, showing different performances according to the solution where the measurements were carried out. An electrolyte (KCl) and K2Fe(CN)6 were used as the electrolytic solution for the experiment. Specific diffusion phenomena were identified for each system, the PEDOT-FeNPs system has shown the higher admittance magnitude (Y0) in the K2Fe(CN)6 solution, with a value of 114,00 µS.s0,5, it implicates that the system with FeNPs has the best diffusion behavior, along with the PEDOT-AuNPs (70,47 µS.s0,5). Therefore, our model explains the physical behavior in the electrode interphase.
Introduction
Methodology
Electrochemical Impedance Spectroscopy (EIS) is a powerful tool for the interpretation of performance of electrochemical systems [1]. It allows to follow the variation of the internal resistance in a system and also to observe the material transport phenomena and transfer charge of the electrodes [2,3]. The performance of the electrochemical system is represented by an equivalent circuit that has the same behavior as the real system under given excitation [3]. The Warburg impedance occurs when current is controlled by diffusion of the charge carrier [4], besides, Warburg impedance describes mass transfer and adsorption phenomena [5]. The reference redox couple must exhibit a reversible potential in the system [6], a iron (Fe3+|Fe2+) couple is used as it is known for being a good reference redox substance at concentrations between 0.5-10.0 mM.
The gold electrodes (Au) were prepared with 100 nm gold layer thickness by electron beam evaporation of a high purity metal on polyimide films. The metal evaporation was performed in a background of 5x10-6 Torr at a deposition rate of 3 Å/s. After the deposition, a positive photoresist (S-1805) was used to pattern specific electrode area (0.0113 cm2). The Gold electrodes (Au) were coated using Poly(3,4ethylenedioxythiophene) (PEDOT), gold nanoparticles in a PEDOT matrix (AuNP), iron nanoparticles in a PEDOT matrix (FeNP) and carbon nanotubes in a PEDOT matrix (PEDOT/CNT).
Results and Discussion a)
Electrochemical Impedance Spectroscopy: The Impedance spectroscopy measurements were executed by a PGSTAT302N (FRA32M) unit with excitation amplitudes of 50 mV from 0.1 Hz to 1 MHz, in a KCl (1M) saline solution and Potassium Ferricyanide solution (K2Fe(CN)6) (REDOX standard) in the following concentrations: 2 mM, 5mM and 10 mM. The data from Table 1 belong to the proposed equivalent circuit by employing an electrolyte media (KCl), and a redox substance (K2Fe(CN)6). EIS analysis show that the solution resistance (Rsln) has almost a constant behavior regardless of the employed system, however, there is a slightly difference in components like Cdl, which shows an increment in the K2Fe(CN)6 system, this increment is due to the interaction of the redox system and the nanoparticles in the PEDOT matrix. The main difference is found by analyzing the Warburg element (W), which shows the value for Admittance at 1 rad/s known as Y0 and clearly states an increment in the K2Fe(CN)6 system when compared with the KCl couple. The highest Y0 value is obtained in the FeNP electrode, 114 μS.s0,5 , meanwhile the AuNP electrode showed an admittance value of 70,47 μS.s0,5. Equation 1 shows the relationship between Y0 and σ (Warburg Coefficient), and clearly sets that for higher Y0 values, the lower σ will be, which implicates that the resistance for the diffusion and mass transfer phenomena will not be as high as for those electrodes with the lower admittance values. Warburg impedance (Zw) is related to the Warburg coefficient by the Equation 2, where ω is the frequency in rad/s.
c)
*
d)
b)
𝜎= Fig 1. (a) Obtained impedance variation for the PEDOT and Au electrodes and (b) its corresponding phase angle variation. (c) Randless circuit and proposed equivalent circuit for the diffusion behavior. (d) Electron transfer scheme for the electrified electrode’s surface, modified from [7].
1 𝑌0 2
Component
R1 (Ω) Cdl (μF) R2(kΩ) C1(μF)
Conclusion The presence of Iron and Gold nanoparticles as well as the Carbon Nanotubes is confirmed by EIS, yielding different electrochemical behaviors for each one of the electrodes configuration. The Warburg element confirms that the FeNP electrode has the greater diffusion behavior due to the interaction among the redox system and the nanoparticles in the PEDOT matrix, obtaining an admittance of 114 μS.s0,5 at 1 rad/s.
Acknowledgment Fondos concursables FEES, project “Desarrollo de electrodos para la identificación de pesticidas y/o herbicidas en medio acuoso”. To the Instituto Tecnológico de Costa Rica (TEC), Centro de Investigación y de Servicios Químicos y Microbiológicos (CEQIATEC), Centro de Investigación y Protección Ambiental (CIPA) y al Centro de Investigación y Extensión en Materiales (Ciemtec), Centro de Investigación en Ciencia e Ingeniería de Materiales (CICIMA) de la UCR.
𝑍𝑤 = 𝜎 ×
Equation 2
Table 1. Obtained values for all the components in the proposed equivalent circuit.
Rsln (Ω)
Impedance plots in Fig 1.a, show a capacitive behavior at low frequencies, however, for frequencies beyond 100 Hz and 1000 Hz respectively, PEDOT and Au electrodes show a resistive behavior. Even though both of the electrodes follow a common path (Capacitive-Resistive), they clearly show different electrochemical performances, as can be seen from the Bode plots in Fig 1.b.
Equation 1
2 0,5 𝜔
Y0
(μS.s0,5)
Solution
Au*
PEDOT
FeNP
AuNP
PEDOT/CNT's
KCl
174,00
139,67
137,33
132,00
139,33
K2Fe(CN)6
174,00
137,33
136,70
133,33
142,33
KCl
509000,00
74,50
59,20
47,37
71,03
K2Fe(CN)6
180000,00
230,33
207,00
218,07
211,33
KCl
0,87
16,40
16,27
14,00
21,50
K2Fe(CN)6
0,90
22,90
25,40
24,33
28,03
KCl
-
65,30
60,50
58,20
45,50
K2Fe(CN)6
-
14,50
22,70
22,50
12,80
KCl
-
13,37
13,50
13,33
14,57
K2Fe(CN)6
-
17,80
16,90
16,67
18,47
KCl
-
14,63
14,10
13,67
16,03
K2Fe(CN)6
-
65,60
114,00
70,47
58,53
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