# Determination of the Electrochemical Area of Screen-Printed Electrochemical Sensing Platforms

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

_{2}[7] or CO [8]), and metals (such as Sb and Sn [9]); with particular success within the food industry, a key indication of their quality and versatility, with respect to capsaicin [10] and garlic [11] which have been successfully analysed with screen-printed electrodes (SPEs). An important parameter to consider when utilising electrochemical sensors is the real electroactive area, especially within fundamental calculations of electrochemical processes, as well as providing a methodology for their benchmarking with respect to the quality control of SPEs.

_{4}and LiClO

_{4}). While Czervinski et al. [13] have provided a thorough overview of the various approaches to determine the electroactive area of noble metal electrodes, reporting that each method implemented to measure the electroactive area of an electrode is based upon very specific theory and assumptions, thus not providing a standardised method for the determination of these metal based electrodes.

_{3})

_{6}Cl

_{3}

^{3+/2+}used as the redox probe [15]. Other groups have also used CC to measure the area of gold nanostructures [16], CNT/NiO microfluidic electrode [17], and Ag@Pt nanorods [18] using ferrocyanide/ferricyanide redox probes. Additionally, alternative electrochemically irreversible probes such as NAD

^{+}and ascorbic acid have been used to measure the surface area of commercial SPEs using CV [19]. Moreover, Terranova et al. have utilised dilute H

_{2}SO

_{4}for determining the area of a MWCNT-PtNP modified electrode using a both CV and CC methods [20].

## 2. Materials and Methods

_{3})

_{6}Cl

_{3}

^{3+/2+}(RuHex) in 0.1 M KCl, 1 mM Dopamine in pH 7 phosphate buffer solution/0.1 M KCl (PBS), 1 mM β-Nicotinamide adenine dinucleotide (NADH) in pH 7 PBS/0.1 M KCl, 1 mM Capsaicin in 0.1 M HPO

_{4}, and 1 mM Ascorbic acid in pH 7 PBS/0.1 M KCl. The following diffusion coefficients were used in this work (cm

^{2}s

^{−1}): 6.74 × 10

^{−6}for dopamine [21], 7.40 × 10

^{−6}for NADH [22], 9.1 × 10

^{−6}for RuHex [23], 6.32 × 10

^{−6}for TMPD [24], 7.03 × 10

^{−6}for capsaicin, and 1.42 × 10

^{−6}for ascorbic acid [25].

^{−1}). For the area calculated using the Anson equation and chronocoulometric experiments, CC, two potentials were applied. The first potential was applied at a low voltage where no electrochemical Faradaic reaction occurred, and the second potential was applied in order detect the corresponding Faradaic process; total charge passed versus time was recorded for 6 s.

## 3. Results and Discussion

_{geo}, is determined through its physical dimensions, but there is no resemblance to the true electroactive area and there is no way of knowing which parts of the electrode surface are electrochemically active or indeed inactive. The most appropriate way is to use an interfacial technique such as electrochemistry; it is this approach that we consider herein.

#### 3.1. Determining the Electroactive Area Using Cyclic Voltammetry

_{p,f}is the voltammetric current (analytical signal) using the forward peak of the electrochemical process, F is the Faraday constant (C mol

^{−1}), v is the applied voltammetric scan rate (V s

^{−1}), R is the universal gas constant, T is the temperature in Kelvin, ${A}_{real}$ is the electroactive area of the electrode (cm

^{2}) and D is the diffusion coefficient (cm

^{2}s

^{−1}), α is the transfer coefficient (usually assumed to be close to 0.5), and n’ is the number of electrons transferred before the rate determining step. Equations (1)–(3) can be used to determine the electroactive area (A

_{real}) through a simple cyclic voltammetry experiment. In this approach, typically, a reliable redox probe within an aqueous electrolyte is used to determine a plot of the forward peak current, I

_{p,f}, as a function of applied voltammetric scan rate (v

^{1/2}). This is since the Randles–Ševćik equation is derived from assuming that the concentration of the electroactive species (in the bulk solution) is the same as that at the electrode surface, due to the development of the diffusion layer [32].

- (1)
- Which equation should be used for each redox probe utilised? i.e., which equation from (1)–(3) is the most suitable to use? Analysis of the peak-to-peak separation (ΔEp) of the recorded voltammogram is useful, where in the reversible limit the ΔEp is ~57mV and is independent of scan rate. In the case of quasi- and irreversible conditions, the ΔEp is larger and is dependent upon the voltage scan rate. The wave-shape of the forward peak allows one to determine between reversible and irreversible conditions; a full analysis is given in reference [32].
- (2)
- The R–S equations should only be used for the forward scan [32], this is due to the fact that on the forward wave, the product is electrochemically produced and diffusion occurs, giving, as a result, a concentration of zero product within the bulk solution compared to that at the electrode surface. Consequently, on the return scan, returning the electrochemically formed product back to its starting material, a decrease in the concentration of the product has occurred, resulting in a less intense backward peak than the forward one. The Randles–Ševćik equations are only an approximation, and therefore do not represent an exact value, unlike, as for example, the case of chronocoulometry.
- (3)
- The Randles–Ševćik equations are more suitable for macroelectrodes, therefore, which size of electrode can be utilized to satisfy the Randles–Ševćik equation? i.e., how big does the electrode need to be in order to give rise to the mass transport dominated by planar diffusion? Compton has undertaken experiments inferring that working electrodes of no less than 4 mm radius should be employed for investigations in aqueous solutions [33]. Their work demonstrates that for a simple electron transfer process, the ΔEp is reduced from 60.6 mV using a radius of 0.5 mm to 57.5 mV in the case of a radius of 4 mm and larger; the quantitative change is due to the geometric electrode size increasing such radial diffusion [33].
- (4)
- One must consider, is the electrode relatively flat and non-porous? In order for Equations (1)–(3) to hold, this should be the case. In the case of a SPE, the electrode is heterogeneous, comprising a range of different carbons (graphite, carbon black) and binder(s). It should be noted that the surface roughness of a SPE is typically 0.078 µm (see Figure S1). Over the timescale of the voltammetric experiment, as determined by Compton [32,34], the diffusion layer is larger than the SPE micro-features such that the electrode kinetics are heterogeneous and dominated by the faster electrode material, i.e., the edge plane features of the graphite(s)/carbon black(s). In this case, Equations (1)–(3) hold; see references [32,34] for the categorisation of electrochemically heterogeneous surfaces that may be encountered.
- (5)
- The potential window is not reversed too early, and the analysis of the forward peak is used on the first scan [32].
- (6)
- The scan rate is not too fast to make the cyclic voltammetric response become non-reversible. This is since the Randles–Ševćik equations are derived from assuming the concentration of the electroactive species in the bulk is the same as that at the surface of the electrode, which, as highlighted above, is due to a diffusion layer developing [32].
- (7)
- In the case of determining the electrode area, a reliable diffusion coefficient (D) value needs to be utilized. A useful approach is the Wilke–Chang [35] equation to determine the diffusion coefficient:$$D=\text{}7.4\text{}\times \text{}{10}^{-8}\text{}\frac{T\sqrt{xM\mathrm{s}}}{\eta {V}^{0.6}}$$
^{−1}), η is the viscosity of the solution (g cm^{−1}s^{−1}), and V is the molar volume of solute at normal boiling point (cm^{3}g^{−1}mol^{−1}). This equation predicts the D value with an exponential error of ±13%. As highlighted by Sitaraman et al. [36], finding the association parameter (x) becomes an issue for unknown systems, therefore the following correction has been proposed:$$D=5.4\text{}\times \text{}{10}^{-8}(\frac{TL{s}^{\frac{1}{3}}\sqrt{xMs}}{\eta {V}^{0.6}\sqrt{Vm}L{s}^{0.3}}{)}^{0.93}$$^{−1}). This methodology still has an error of ±13%, but is simpler when used by experimentalists. Clearly, temperature is critical in determining the electrochemical area. Changes and fluctuations in temperature will affect the information obtained from Equations (1)–(5). Consequently, the temperature at the time of measuring the electrochemical area should be measured and factored into these equations. - (8)
- The Randles–Ševćik equations are useful for single electron transfer processes that feature a 1:1 reaction stoichiometry, inversely however, for example, the reduction of protons to hydrogen (hydrogen evolution reaction, HER) has a 2:1 stoichiometry and experimental results deviate from theory [37]. The diffusion coefficients used here are either from the academic literature or deduced using Equation (5).

_{real}). These results are depicted in Figure 1, and their calculated A

_{real}and its percentage compared to the A

_{geo}(%Real = (A

_{real}/A

_{geo}) × 100) is reported in Table 1. (Note: in case of capsaicin, the second electrochemical process of the reaction was utilised and in case of the TMPD, the first oxidation peak was chosen).

_{real}).

#### 3.2. Determining the Electroactive Area Using Chronocoulometry

_{real}is the electroactive area, F is Faraday’s constant, C is the concentration, D is the diffusion coefficient, ${Q}_{dl}$ is the double layer capacitance, and ${\Gamma}_{0}$ is the coverage of adsorbed reactant (mol cm

^{−2}). The electrolysis of the diffusion species shows a dependence upon ${\mathrm{time}}^{1/2}$. Since the adsorbed materials are electrolyzed instantaneously, the charge is not time-dependant and the charging of the double layer is instantaneous and independent of time. Thus, as shown in Figure 3, the difference in the charge due to the ${Q}_{dl}+\text{}nF{A}_{real}{\Gamma}_{0}$ is readily evident but the gradient is the same, due to the reaction cited above. Therefore, CC is most accurate when considering near-ideal outer-sphere probes or highly adsorbed analytes.

_{real}and %Real reported in Table 2.

## 4. Conclusions

_{real}values using RuHex, with CC being the more accurate. The significance here is not only the utilised methodology itself, but the selection of the redox probe, showing that an ideal outer-sphere probe would be the best to use to obtain the real electroactive area of SPEs and indeed other electrodes. This work is an important and fundamental contribution to those experimentalists who use and benchmark the real electroactive area of screen-printed electrodes since it provides the first comparison of inner- and outer-sphere redox probes, highlighting the various parameters that need to be considered in order to obtain useful estimations of the real electroactive area.

## Supplementary Materials

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 1.**Cyclic voltammetry of 1 mM RuHex (

**A**); 1 mM NADH (

**B**); 1 mM dopamine (

**C**); 1 mM capsaicin (

**D**); 1 mM TMPD (

**E**) and 1 mM ascorbic acid (

**F**) at 0.05 V s

^{−1}(vs. SCE) and 19.6 °C. The arrow indicates the forward peak selected to calculate the A

_{geo}with Equations (2) and (3).

**Figure 2.**Classification of redox systems by Mcreery et al. [39] according to their kinetic sensitivity to particular surface modifications upon carbon electrodes. The figure has been adapted by the authors of this paper to clearly show outer- and inner-sphere probes. Fc: ferrocene, MV: methyl viologen, CPZ: chlorpromazine, and MB: methylene blue.

**Figure 3.**Representation of CC (charge vs. time

^{1/2}) for a non-adsorbed outer-sphere probe (

**A**) and for an adsorbed inner-sphere probe (

**B**). Note that, in (

**A**), the intercept is not truly zero due to the contribution of ${Q}_{dl}$ (see Equations (6) and (7)).

**Figure 4.**Anson plots resulting from using the following redox probes: 1 mM RuHex (

**A**); 1 mM NADH (

**B**); 1 mM dopamine (

**C**); 1 mM capsaicin (

**D**); 1 mM TMPD (

**E**) and 1 mM ascorbic acid; (

**F**) at 19.6 °C. The results of three different SPEs are presented.

**Table 1.**Calculated electrode areas (A

_{real}) for SPEs using the Randles–Ševćik equations (Equation (2) for quasi-reversible processes of RuHex, TMPD, and capsaicin and Equation (3) for irreversible processes such as NADH, dopamine, and ascorbic acid; note that dopamine has anodic and cathodic processes but is termed irreversible because of the relative rates of electron transfer and mass transport [31]; recorded at 19.6 °C for each redox probe, with the diffusion coefficient used in the calculation. The %Real, which is the percentage of A

_{real}divided by the A

_{geo}, is also presented (N = 3).

Electroactive Probe | Electrode Area Randles–Ševćik/cm^{2} | D/cm^{2} s^{−1} | %Real |
---|---|---|---|

RuHex | 0.062 | 9.10 × 10^{−6} | 83.25 |

NADH | 0.049 | 7.40 × 10^{−6} | 65.47 |

Dopamine | 0.090 | 6.74 × 10^{−6} | 120.18 |

Capsaicin | 0.093 | 7.03 × 10^{−6} | 123.74 |

TMPD | 0.057 | 6.32 × 10^{−6} | 75.64 |

Ascorbic acid | 0.109 | 1.42 × 10^{−6} | 145.65 |

**Table 2.**Calculated electrode areas (A

_{real}) for SPEs using the Anson plot equations (Equations (6) and (7)) recorded at 19.6 °C for each redox probe, with the diffusion coefficient used in the calculation. The %Real, which is the percentage of A

_{real}divided by the A

_{geo}, is also presented (N = 3).

Electroactive Probe | Electrode Area Anson/cm^{2} | D/cm^{2} s^{−1} | %Real |
---|---|---|---|

RuHex | 0.055 | 9.40 × 10^{−6} | 73.34 |

NADH | 0.077 | 7.40 × 10^{−6} | 103.27 |

Dopamine | 0.077 | 6.74 × 10^{−6} | 102.51 |

Capsaicin | 0.057 | 7.03 × 10^{−6} | 75.91 |

TMPD | 0.053 | 6.32 × 10^{−6} | 70.53 |

Ascorbic acid | 0.121 | 1.42 × 10^{−6} | 161.21 |

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**MDPI and ACS Style**

García-Miranda Ferrari, A.; Foster, C.W.; Kelly, P.J.; Brownson, D.A.C.; Banks, C.E.
Determination of the Electrochemical Area of Screen-Printed Electrochemical Sensing Platforms. *Biosensors* **2018**, *8*, 53.
https://doi.org/10.3390/bios8020053

**AMA Style**

García-Miranda Ferrari A, Foster CW, Kelly PJ, Brownson DAC, Banks CE.
Determination of the Electrochemical Area of Screen-Printed Electrochemical Sensing Platforms. *Biosensors*. 2018; 8(2):53.
https://doi.org/10.3390/bios8020053

**Chicago/Turabian Style**

García-Miranda Ferrari, Alejandro, Christopher W. Foster, Peter J. Kelly, Dale A. C. Brownson, and Craig E. Banks.
2018. "Determination of the Electrochemical Area of Screen-Printed Electrochemical Sensing Platforms" *Biosensors* 8, no. 2: 53.
https://doi.org/10.3390/bios8020053