Electrochemical Impedance Spectroscopy in the Characterisation and Application of Modified Electrodes for Electrochemical Sensors and Biosensors

Abstract

Electrochemical impedance spectroscopy is finding increasing use in electrochemical sensors and biosensors, both in their characterisation, including during successive phases of sensor construction, and in application as a quantitative determination technique. Much of the published work continues to make little use of all the information that can be furnished by full physical modelling and analysis of the impedance spectra, and thus does not throw more than a superficial light on the processes occurring. Analysis is often restricted to estimating values of charge transfer resistances without interpretation and ignoring other electrical equivalent circuit components. In this article, the important basics of electrochemical impedance for electrochemical sensors and biosensors are presented, focussing on the necessary electrical circuit elements. This is followed by examples of its use in characterisation and in electroanalytical applications, at the same time demonstrating how fuller use can be made of the information obtained from complete modelling and analysis of the data in the spectra, the values of the circuit components and their physical meaning. The future outlook for electrochemical impedance in the sensing field is discussed.

Keywords: Warburg impedance; charge transfer resistance; constant phase element; electrochemical impedance spectroscopy; modified electrodes; sensors and biosensors.

Figure 1

Complex plane impedance plots for selected electrical equivalent circuits: (a) capacitive interfacial response, (b) faradaic electron transfer reaction controlled by kinetics over the whole frequency range and (c) electron transfer reaction with mass transfer control at low frequencies. R_Ω: cell resistance, R_ct: electron transfer resistance, C_dl: double-layer capacitance, Z_W: Warburg impedance.

Figure 2

Complex plane impedance plots for selected electrical equivalent circuits with ideal capacitor response (dotted lines) replaced by non-ideal capacitor constant phase elements (CPE) (solid lines). (a) Faradaic electron transfer reaction controlled by kinetics over the whole frequency range, and (b) capacitive interfacial response.

Figure 3

(a) Complex plane impedance plots recorded at gold quartz crystal microbalance (AuQCM)-modified electrodes in 0.05 M acetate +0.1 M KBr buffer solution, pH 5.0, at the OCP. (a) AuQCMMPS(−)/PDDA(+)/{HA/Mb}_2–6, lines show equivalent circuit fitting to the circuit in Figure 2b. (b) Capacitance values. Dark squares, from RCPE equivalent circuit fitting to the experimental spectra; light squares, from RC semicircle modelling; (0) theoretical-I C_i = 34 μF cm⁻² and (O) theoretical-II, C_i = 59 μF cm⁻² obtained by summing contributions from successive bilayers. From [10], reprinted with the permission of the American Chemical Society, Washington, DC, USA.

Figure 4

(a) Complex plane impedance plots at different electrodes in BR buffer (pH = 6) containing 9.0 mM BPA. Inset is the magnified plot of AuNP (5)/MWCNT (0.2)/GCE. Lines show equivalent circuit fitting. (b) Electrical equivalent circuit used to fit the spectra. From [11], reprinted with the permission of Elsevier, Amsterdam, The Netherlands.

Figure 5

Complex plane impedance plots recorded in 0.1 M BR (pH 3.0) buffer solution at −0.3, −0.2, 0.0 and 0.1 V vs. Ag/AgCl at (A) PNR_Ethaline/Fe₂O₃NP/GCE, (B) PMG_Ethaline/Fe₂O₃NP/GCE and (C) PNB_Ethaline/Fe₂O₃NP/GCE. (D) Electrical equivalent circuit used to fit the spectra. From [12], reprinted with the permission of Elsevier.

Figure 6

(a) Complex plane plots of screen-printed biosensors with MUA/IgG-Ab/BSA functionalisation after 30 min of incubation in (I) PBS, (II) 11.3 ng/mL IgG, (III) 113 ng/mL IgG, (IV) 1.13 μg/mL IgG, (V) 11.3 μg/mL IgG and (VI) 113 μg/mL IgG. (b) Calibration curve of buffered IgG solutions with relative R_CT vs. IgG concentration in half-logarithmic form, number of measurements = 3. MUA: 11-mercapto undecanoic acid, IgG: immunoglobulin G antibody, BSA: bovine serum albumin, PBS: phosphate-buffered saline. From [15], reprinted with the permission of Elsevier.

Figure 7

Biosensor response to Aβ oligomers as measured by EIS. The response of the biosensors to AβO was analysed by EIS. Following equilibration in vector solution alone (sensor), cumulative additions of AβO (10⁻¹²−10⁻⁶ M total Aβ peptide concentration) were performed for 20 min each prior to rinsing and EIS measurement in a solution of PBS containing 10 mM Fe(CN)₆³⁻/⁴⁻ (aq). Inset shows the Randles equivalent circuit model for this system, where R_s = solution resistance, R_ct = charge transfer resistance and CPE = constant phase element, a model of an imperfect double-layer capacitor. From [22], reprinted with the permission of Elsevier.

Figure 8

Schematic of the systematic protocol for SPCE surface modification and immunosensing. (PANI—polyaniline, AuNCs—gold nanocrystals, HSA—human serum albumin, Ab-HSA—anti-human serum albumin antibody, BSA—bovine serum albumin, EIS—electrochemical impedance spectroscopy). From [24], reprinted with the permission of MDPI.

Figure 9

Application areas for non-faradaic impedimetric biosensors. BSA: bovine serum albumin, HSA: human serum albumin. From [29], reprinted with the permission of Elsevier.

Figure 10

Heterogeneous, label-free, reagent-less sensing principle. Cubed laboratories use interdigitated electrodes and functionalised CNT for nucleic acid (NA) detection. Initially, a baseline impedance measurement was performed (i). Then, the single-stranded product of an asymmetric PCR mixed with hybridisation buffer was hybridised to the CNT-bound capture probes (ii). After washing with measurement buffer, another impedance measurement was performed (iii). All steps were performed while applying an AC field for di-electrophoresis, which supports specific hybridisation. From [34], reprinted with the permission of Elsevier.