Ultrasensitive and Disposable Electrochemical Sensor Electrodes

By Megan Craig, M.Sc.May 16 2022

For electrochemical trace analysis, improved electrochemical sensors with enhanced sensitivity and low detection limits are in great demand. Electrochemical sensing performance is primarily determined by the characteristics of electrode materials and the general electrode shape.

Image Credit: Visual Generation/Shutterstock.com

High electrochemical sensitivity has been achieved by using graphene, a 2-dimensional (2D) nanocarbon structure, as the main electrode material or as an addition to increase electron transport between a target analyte and the electrode surface. Screen-printed graphene electrodes (SPGEs), in particular, have been shown to have a significantly greater electrochemical response than screen-printed carbon electrodes (SPCEs).

Graphene-based electrodes have also been modified using diverse materials, including metal nanoparticles, conductive polymers, and ionic liquids (ILs), to improve their electrochemical performance. The wide electrochemical potential window, strong solvation ability, low volatility, nontoxicity, and great chemical stability of ILs make them particularly intriguing.

Furthermore, because ILs are liquid-state salt compounds generated by coupling positive and negative ions, they have a high electrical conductivity. SPGEs and SPCEs surface-modified with imidazolium-type and pyridinium-type ILs, in particular, have demonstrated dramatically better electrochemical responses to a variety of analytes. Developing excellent electrochemical electrodes based on ILs and graphene is so promising.

As a result, screen-printed electrodes (SPEs) have seen much use in electrochemical studies. However, no electrodes directly manufactured by screen printing inks comprising ILs and GP have been reported. In terms of electrode homogeneity, reusability, and controllability, electrodes made by screen printing from mixes of ILs and GP should outperform IL-modified SPEs.

Disposable screen-printed IL/graphene electrodes (SPIL-GEs) were created in a new study published in the journal Electrochemistry Communications utilizing pastes made by mixing ILs with electrolytically exfoliated graphene and carbon paste (CP) in a ball mill.

Furthermore, one pyridinium-type and five imidazolium-type ILs were tested, and the IL concentration was modified to enhance the electrochemical sensing performance for the detection of three typical electroactive analytes: Ferri/ferro cyanide (Fe(CN)₆)^3-/4- redox pair, dopamine (DA), and hydroquinone (HQ). The shapes and sizes of the developed electrodes were also modified to provide the optimum electrochemical response.

Methodology

All compounds employed in this study were analytical grade and were not purified further. The electrolyte for electrolytic exfoliation was a commercial poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT/PSS) solution (Clevios P Jet N from HC Starck, USA).

To integrate and homogenize screen-printing inks, a commercial ball mill (Retsch model Emax) was employed. The conductive paste was printed on PET substrates using a screen printer (DEK model 03ix). Clicking voltammetry (CV) measurements were performed using a potentiostat (µ-Autolab Type III, Methrom) with the GPES software to investigate electrochemical performance.

As shown in Figure 1A, five geometric designs of electrodes were created using Adobe Illustrator, with varied total electrode lengths and active regions for the working electrode (WE) and counter electrode (CE).

As shown in Figure 1C, an electrical connection was used to connect the printed electrodes to the potentiostat and electrochemically scrubbed to remove impurities on electrode surfaces in 0.1 M PBS solution (pH 7.4) by a voltage applied between - 0.3 and + 0.6 V in cyclic voltammetry (CV) mode for five cycles at a scan rate of 50 mVs^-1.

Figure 1. (A) Five designs of screen printed electrodes and typical photographs of (B) fabricated SPIL-GEs on a single PET substrate (190 × 210 mm²) and (C) a single SPIL-GE with an electrical connector. Image Credit: Kamsong, et al., 2022

Results

Figures 2A, 2B, and 2C show the impacts of three geometric factors on the CV response of SPGEs to 2.5 mM K₃Fe(CN)₆ in 0.1 M PBS solution (pH 7.4) at 50 mVs^-1: total electrode length, WE, and CE active areas, respectively.

The electrochemical characteristics of all designed electrodes are also compared to theoretical ones by determining their electrochemical surface areas (ESAs) from CV currents utilizing the Randles–Sevcik equation (Ip = KACn^3/2(Dv)^1/2, where K = 2.69 10⁵C V^-1/2 mol^-1, A = ESA (cm²), C = concentration (mol.cm^-3), n = number of electron per reaction (=1 for Fe^3+/4+), v = scan rate (Vs⁻¹) and D = diffusion constant (=7.67 × 10^-6 cm²s⁻¹ for K₄Fe(CN)₆)) as combined with J_p are shown in Figure 2D.

Figure 2. Cyclic voltammograms of SPGEs with different (A) overall electrode lengths (Designs (a)-(b)), (B) CE active area (Designs (c)-(d)) and (C) WE active area (Designs (b)-(c) and (e)) towards 2.5 mM K₃Fe(CN)₆ at 50 mVs⁻¹. (D) Electrochemical and geometric surface areas (ESA & GSA) of SPGEs with different designs. Image Credit: Kamsong, et al., 2022

The electrochemical performance of SPIL-GEs with six IL materials, PMPlm, BMIMBF₄, HMIMPF₆, C₉H₁₅N₃S, EMIMBF₄, and BMIMPF₆, was assessed by CV towards 5 mM K₃Fe(CN)₆ in 0.1 M PBS solution (pH 7.4) and compared to SPCE and SPGE, as shown in Figure 3A.

As shown in Figure 3B, the corresponding anodic peak amplitudes, as well as ESAs estimated using Randles–Sevcik, are displayed individually to examine the differences across all electrodes. The computed ESA values of SPIL-GEs are significantly greater than the GSA value, as seen in Figure 3C.

Figures 3C and 3D show the influence of PMPlm content varying from 0.5–2.0% (w/w) on CV response and anodic signals, as well as predicted ESAs of SPPMPlm-GEs towards 2.5 mM K₃Fe(CN)₆.

Figure 3. Cyclic voltammograms of (A) SPCE, SPGE, and SPIL-GEs with different IL materials towards 5 mM K₃Fe(CN)⁶ at 50 mVs⁻¹ and (B) corresponding J_p and ESAs. (C) Cyclic voltammograms of SPPMPlm-GEs with different PMPlm contents towards 2.5 mM K₃Fe(CN)₆ at 50 mVs⁻¹ and (d) corresponding J_p and ESAs. The dotted lines in (B) and (D) correspond to the geometric surface area (GSA) of WE. Image Credit: Kamsong, et al., 2022

CV measurements in 2.5 mM K₃Fe(CN)₆, 1 mM DA, and 1 mM HQ solutions were used to analyze and compare the electrochemical performance of the optimum SPIL-GE with 1% PMPlm with that of SPGEs and SPCEs, as shown in Figure 4A-4C, respectively.

Figure 4. Cyclic voltammograms of SPIL-GE, SPGE, and SPCE towards (a) 2.5 mM K₃Fe(CN)₆, (b) 1 mM dopamine (DA) and (c) 1 mM hydroquinone (HQ) at 50 mVs⁻¹. Image Credit: Kamsong, et al., 2022

Furthermore, as shown in Figure 5, the ideal SPIL-GE produces greater CV peak currents towards 2.5 mM K₃Fe(CN)₆ than two commercial electrodes (E1: graphene-modified SCPE and E2: SCPE).

Figure 5. Cyclic voltammograms of SPIL-GE and two commercial SPGE E1 and SPCE E2 towards 2.5 mM K₃Fe(CN)₆ at 50 mVs⁻¹. Image Credit: Kamsong, et al., 2022

FTIR spectroscopy was used to verify the chemical contents of SPIL-GE with the ideal IL (PMPlm) by comparing them to the spectra of its components, including PMPlm, GP/PEDOT/PSS, and CP, which is seen in Figure 6A. Figure 6B shows the surface morphologies of SPCE, SPGE, and SPIL-GE as determined by SEM.

Figure 6. (A) FTIR spectra of PMPlm, GP/PEDOT/PSS, CP and PMPlm/GP/PEDOT/PSS/CP. (B) SEM micrographs of SPCE, SPGE and SPIL-GE. Image Credit: Kamsong, et al., 2022

Conclusion

SPIL-GEs were created by mixing graphene and ILs with commercial CP to create conductive inks. For SPGEs with different overall electrode lengths, as well as WE and CE active areas, CV measurements were used to improve electrode geometries towards (Fe(CN)₆)^3-/4-. According to the CV results, the best design had a total length of 25 mm and active areas of 12.57 and 15.7 mm², respectively.

The electrochemical reaction of SPIL-GEs made of six IL materials, including PMPlm, HMIMPF₆, BMIMBF₄, C₉H₁₅N₃S, EMIMBF₄, and BMIMPF₆, to K₃Fe(CN)₆ was investigated. At the ideal PMPlm level of 1.0% (w/w), PMPlm produced the greatest CV peak currents when compared to other ILs.

The electrochemical response of the optimum SPIL-GE was compared to SPCEs and SPGEs for three common electroactive analytes: (Fe(CN)₆)^3-/4- redox couple, DA, and HQ. For the three analytes, the results showed that SPIL-GE had higher oxidation currents and lower anodic potentials than SPGE and SPCE. As a result, the SPIL-GE may be a viable option for advanced electrochemical sensing applications.

Journal Reference:

Kamsong, W., Primpray, V., Pasakon, P., Sriprachuabwong, C., Pakapongpan, S., Mensing, J.P., Wisitsoraat, A., Tuantranont, A. and Karuwan, C., 2022. Highly sensitive and disposable screen-printed ionic liquid/graphene based electrochemical sensors. Electrochemistry Communications, p.107209. Available Online: https://www.sciencedirect.com/science/article/pii/S138824812200011X.

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