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Ferri/Ferrocyanide Redox Couple Electrolytes Hamper the Stability of Gold Electrodes during Electrochemical Impedance Spectroscopy Jaroslav Lazar, Christoph Schnelting, Evelina Slavcheva, and Uwe Schnakenberg Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b02367 • Publication Date (Web): 30 Nov 2015 Downloaded from http://pubs.acs.org on December 1, 2015

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Analytical Chemistry

Ferri/Ferrocyanide Redox Couple Electrolytes Hamper the Stability of Gold Electrodes during Electrochemical Impedance Spectroscopy Jaroslav Lazar1, Christoph Schnelting1, Evelina Slavcheva2, Uwe Schnakenberg1*

1

Institute of Materials in Electrical Engineering 1, RWTH Aachen University, Sommerfeldstr. 24, 52074 Aachen, Germany Institute of Electrochemistry and Energy Systems, Bulgarian Academy of Sciences, Akad. G. Bonchev 10, 1113 Sofia, Bulgaria

2

ABSTRACT: In the past decades, numerous measurements have applied Electrochemical Impedance Spectroscopy (EIS) in an electrode-electrolyte system consisting of gold electrodes and the redox couple potassium ferrocyanide / potassium ferricyanide (HCF). Yet these measurements are often hampered by false positive and negative results: Electrochemical impedance signals often display a non-linear drift in electrolyte systems containing the HCF redox couple which can mask the accuracy of the analysis. Thus, this paper aims to elucidate the stability and reliability of this particular electrode-electrolyte system. Here, different gold electrode cleaning treatments were compared with respect to adsorption and roughness of the surface of gold electrodes. They show substantial nonlinear long-term drifts of the charge-transfer resistance RD. In particular, the use of HCF-containing electrolytes causes adsorption and corrosion on the gold electrode surface, resulting in a non-linear impedance behavior that depends on the incubation period as well as on electrolyte composition. Consequently, it is strongly recommended not to use HCF containing electrolytes in combination with gold electrodes.

INTRODUCTION In past decades, Electrochemical Impedance Spectroscopy (EIS) has been widely used to investigate absorption processes at electrode surfaces. Some traditional applications of EIS involve immunosensing protocols1, DNA hybridization interactions2,3, virus detection4,5 as well as studies of microbial cell layers6 and single cell experiments7–9. The EIS technique has also been applied for the analysis of coatings, corrosion, and metal oxide films 10. Impedimetric studies in solutions can be categorized into two main groups: faradaic and non-faradaic electrode-electrolyte systems. Whereas faradaic systems provide information about the charge transfer resistance RD between electrolyte and electrode, non-faradaic systems mainly focus on the surface capacity change Cdl measured at low frequencies for characterizing the electrodeelectrolyte interface. Faradaic systems are often favored, however, because the measurements at low frequencies are problematic due to high electrochemical impedance and resulting leakage currents. Gold is frequently used as the electrode material, especially in biologically driven applications. This is attributed to the fact that the interaction of biomolecules with the gold electrode surface can easily be induced because of the deposition of an intermediate gold-affine alkanthiol-based layer which forms a self-assembling monolayer (SAM). Stability and reliability of the electrode-electrolyte system are crucial for every EIS measurement. Yet Bogomolova et al. reported about problems in using the standard hexacyanoferrate(III) / hexacyanoferrate(II) ([Fe(CN)6]3−/[Fe(CN)6]4−) (HCF) redox couple for EIS measurements, leading to changes in the electrode surface. The authors attributed these changes to four possible

main factors: (i) initial electrode contamination; (ii) repetitive measurements; (iii) additional cyclic voltammetry or differential pulse voltammetry; and (iv) additional incubations in the buffer between measurements. All these perturbations resulting from using HCF, in turn, lead to an increase in RD in a Randles model equivalent circuit that can cause a false positive or negative binding event11. Several studies describe the possible chemical processes on the electrode surface in the presence of HCF ions. Pharr et al. analyzed the HCF couple species in solution and on platinum electrode surfaces by infrared spectroscopy12,13 and showed formation of polymer HCF complexes. Iwasaki et al. investigated the chemical reactions of HCF on gold surfaces by surface plasmon resonance in combination with cyclic voltammetry potential measurements and confirmed the formation of a HCF complex film14. Electrochemical processes near the electrode surface critically affect the drift of the impedance signal, especially at low frequencies, and hinder the proper interpretation of the actually analyzed interaction. Thus, by using continuous EIS, this present study aims to determine the effect of electrode cleaning procedures on drift behavior and on time-dependent interferences between HCF ions and the electrode surface.

MATERIALS AND METHODS Thin-film electrodes were fabricated by sputtering a 30 nm titanium adhesion layer and a 150 nm gold layer or a 30 nm titanium and a 150 nm platinum layer on 100 mm oxidized silicon wafers (Au - Nordiko NS 2550, dc power of 250 W, pressure of 4.2 Pa, argon flow of 55 sccm; Pt - Nordiko NS 2550, dc power of 100

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W, pressure of 1.98 Pa, argon flow of 75 sccm). The wafers were diced into chips to form macroscopic electrodes (1.5 cm x 4 cm). The area of the working electrode was set at AWE = 0.283 cm2. In the electrochemical cell, a platinum mesh served as a counter electrode and a saturated Ag/AgCl electrode as a reference electrode, respectively (see Figure 1).

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RESULTS AND DISCUSSION INFLUENCE OF THE CLEANING METHODS A clean electrode surface is crucial for obtaining reproducible results with EIS. In this study, five cleaning methods similar to those published in15–17 and shown in Table 1 were tested and compared.

Table 1. Cleaning procedures of electrodes before measurements Method

Figure 1: Macroscopic electrode in a three point measurement setup The electrodes were connected to an impedance analyzer (Impedance / Gain Analyzer SI 1260, Ametek, Farnborough, United Kingdom) in combination with a potentiostat (Potentiostat / Galvanostat Model 263A, Princeton Applied Research, Oak Ridge, TN, USA) in order to carry out impedance and cyclic voltammetry measurements. CorrWare and ZPlot software (Scribner Software, Farnborough, Great Britain) were used to control the measurements. The frequency range of the EIS measurements was set from finitial = 100 kHz to ffinal = 1 Hz so that both the charge transfer-controlled region and the beginning of the diffusion-controlled region were covered. To stay within the linear region of the Butler-Volmer equation, the AC-voltage was set at VAC = 10 mV and the DC-voltage at the open-circuit potential (OCP). Between the two recorded spectra, the OCP was updated by measuring the open-circuit potential for 30 s so as to determine the electrochemical stability. The measurements were performed in a measurement buffer containing 5 mM K4Fe(CN)6 and 5 mM K3Fe(CN)6 (Sigma-Aldrich, Schnelldorf, Germany) in phosphate-buffered saline (PBS, Sigma-Aldrich) as a conducting electrolyte unless otherwise specified. The PBS had a pH value of 7.4 and consisted of 0.01 M phosphate buffer, 0.0027 M potassium chloride and 0.137 M sodium chloride. The phosphate buffer was diluted to the same molarity and pH as PBS, but with no additional salts. Deionized (DI) water was used for all dilutions. Moreover, the temperature was kept at 22 °C during all measurements. A UV lamp with a spotlight (Panacol-Elosol GmbH,Germany, UV-P 400/2, 1.4W/cm2) was utilized for pretreating the HCF solutions. All EIS measurement data were fitted using the standard Randles equivalent circuit model with ZView software. The fitted data were further transferred into a custom-made Matlab program (MathWorks, Inc, Natick, MA, USA) in order to display the following elements: the electrolyte resistance RE, the charge transfer resistance RD, the Warburg element ZW and the double-layer capacitance ZCPE, which was modeled as a constant-phase-element (CPE) with its parameters TCPE and ΦCPE to take account of the non-ideal behavior of the capacitance. This program allows generating Nyquist and Bode plots over time as well as calculations such as a derivative and margin of the impedance over time.

Description

NaOH

Incubated in 0.2 M sodium hydroxide solution for 20 min

APM

Incubated in ammonia and hydrogen peroxide mixture for 15 min (mixing ratio: NH3/H2O2/H2O = 1/1/8).

KOH

Incubated in 50 mM potassium hydroxide solution. The electrode potential was swept from E = -0.2 V down to E = -1.2 V once. The scan rate was 50 mV/s.

KOHcv

Incubated in 50 mM potassium hydroxide. The potential was cycled from E = -1.2 V to E = 0.1 V for five times at a rate of 50 mV/s..

Plasma

Cleaned in oxygen plasma (30s, HF power P = 100 W, flow rate Q = 10 sccm).

After the cleaning treatments, the chips were rinsed with DI water and the electrochemical measurements conducted in the presence of the HCF redox couple. Cleaning and measurement routines were repeated thrice. Figure 2 depicts the arithmetic averages of the charge transfer resistances RD in absolute and relative values as a function of time for the five cleaning methods. Hereby, a low RD value implies a clean surface that facilitates the charge transfer between electrode and electrolyte.

Figure 2: Mean values of charge-transfer resistances RD as a function of time after different cleaning processes; electrolyte: PBS with 5 mM HCF. Top: absolute values. Bottom: relative values The normalized values of RD show non-linear drifts of 25 to 50% within the first 30 minutes, which is critical for any potentiometric


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and amperometric and sensing application. In serious experiments, the time-depending sensor signals have to be corrected by the drift signal. In any case, there are differences among the various cleaning treatments. The plasma and the APM treatment showed the lowest RD absolute values. At the beginning of the measurements, charge transfer resistances of RD,Plasma = 10.92 ± 4.67 Ωcm2 and RD,APM = 18.00 ± 4.23 Ωcm2 were obtained. The decrease in RD over time for all other tested cleaning procedures indicates a poorly cleaned surface. The NaOH cleaning shows the worst results. This is underlined by the fact that at the beginning the charge transfer resistance is 20 times higher than the value obtained for plasma pretreated surfaces. As far as the electrochemical cleaning methods are concerned, potential cycling yields a cleaner surface than a simple potential sweep. None of the cleaning methods suffice to fulfill the conditions for reliable sensing experiments. Drifts are always observable. Even so, the plasma treatment provides the cleanest electrode surface. Thus, in the following experiments, all electrodes were cleaned via the plasma treatment.

STABILITY ANALYSIS As previously mentioned, repetitive measurements or incubation in certain electrolytes can lead to adverse effects which were analyzed in this study. The possible causes were grouped into the following three categories: composition of the electrolyte, electrode material, and microscopic roughness of the electrode depending on the incubation time. The parameters of all three categories were successively varied in order to elucidate the processes occurring at the electrode surface during the EIS measurements.

INFLUENCE OF THE ELECTROLYTE For characterizing the stability of the gold electrode chips were treated with 5 mM HCF in PBS buffer. As illustrated in Figure 3, time-resolved Nyquist plots were obtained from EIS measurements. In these measurements an unstable behavior of the gold electrode can clearly be observed, making it difficult to further analyze absorption processes usually detected by EIS.

Figure 3: Nyquist plots over time; electrolyte: PBS with 5 mM HCF. In order to clarify the unstable behavior, some facts are described in the following. Based on the Nernst’s equation, the open-circuit-potential (OCP) is linked to the concentration ratio of the redox species. RD, according to the Butler-Volmer equation, is also dependent on the concentration of the redox species (see Equation 1). The opencircuit-potential measurement shows stable OCP which corresponds to a constant concentration ratio of the oxidized and reduced species at the surface. At OCP, the charge transfer resistance RD can be determined as:

RD =

dη D RT RT , = = α (1−α ) dj D n ⋅ F ⋅ j0 n 2 ⋅ F 2 ⋅ k 0 ⋅ cox ⋅ cred

(1)

where η denotes the overvoltage, j the current density, R the gas constant, T the temperature, n the number of electrons involved in the reaction, F the Faraday’s constant, j0 the exchange current density, k0 the standard rate constant of the reaction and cox/red are the concentrations of the redox species18. Depending on the cleanliness, material of the electrode, potential limits, and the composition of the electrolyte, adsorption processes slow down the reaction kinetics. This behavior can be attributed to a decrease in the standard rate constant k0, which was also observed particularly during CV-measurements12. The question about what exactly adsorbs onto the electrode surface has not yet been fully clarified in earlier works: C. Pharr et al. analyzed the influence of ferrocyanide/ferricyanide redox couple processes on platinum electrodes using CVmeasurements and Fourier transform infrared spectroscopy (FTIR); they concluded that as long as the potential limits are small, the redox species themselves and not some intermediate species adsorb onto the electrode12,13. Therefore, potential cycling between E = -0,33 V and E = 0,88 V (vs. SCE) was performed. Upon using large potential limits during CV measurements, the peaks of both redox species in the FTIR spectrum decreased while a new peak occurred between them. The new peak was correlated to formation of a layer of a polymeric adsorbate, constituting a soluble Prussian Blue (KFeIII[FeII(CN)6]). This HCF complex can adsorb and grow on the electrode surface and reversibly desorb and decompose to reform solution-phase ferricyanide and ferrocyanide as a function of potential. It was concluded that at higher potentials, increasingly more Fe2+-ions were oxidized to Fe3+ions. The formation of polymeric HCF complexes on gold electrodes was also found at potentials above -0.45 V vs. Hg/Hg2SO4 under the open-circuit-potential3. The authors suggest that, depending on the potential, these complexes lead to the formation of Prussian Blue or Prussian White. Furthermore, a layer formation caused by potential cycling was confirmed by surface plasmon resonance spectroscopy (SPR)14. Based on these results, the time-dependent behavior of the double layer capacitance and the charge-transfer resistance in Figure 3 imply an adsorption process that takes place within the first 120 minutes of the measurement. A broadened electrical double layer caused by an adsorption leads to a lower capacitance and to an increase in the charge-transfer resistance19,20. Furthermore, five electrolyte compositions (Table 2) were studied in order to reliably evaluate the exact electrochemical processes occurring on the gold electrode surface. Particular compounds were excluded from the buffer so as to determine their influence on the uncontrolled impedance change.

Table 2: Electrolytes applied. Name

Particularity

PBS; 5 mM-HCF

Standard

PB; 5 mM-HCF

No saline

PBS; 1 mM-HCF

Lower HCF concentration

DI; 5 mM-HCF

No saline, not buffered

PBS, 5mM-HCF UV

Standard electrolyte pretreated with UV light



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Figure 4: RD and CPE parameters of the double layer as a function of time for different electrolytes.

Figure 5: Normalized values of RD and CPE parameters of the double layer with respect to time for different electrolytes.

Impedance measurements in the presence of the different electrolytes were analyzed with respect to time-dependent chargetransfer resistance RD, double-layer capacitance TCPE and phase ФCPE behavior. First, we focused on the capacitive part of the double layer. As shown in Figure 4 and 5, TCPE in 1 mM and 5 mM HCF in PBS decreases significantly over time. By contrast, TCPE remains relatively stable in the case of DI-water but with the exception of the first minutes of the measurement. It is known that the adsorption of anions in low concentration affects the kinetics of the HCF redox system21. Thus, chloride ions present in PBS may adsorb onto the gold surface, which, according to Pajkossy et al., leads to a compressed Randlessemicircle22,23. This compression corresponds to a ΦCPE < 1. All measurements in PBS solutions show exactly this behavior (see Figure 4 and 5). The rate constant of the chloride adsorption is known to be too high to be measured especially by EIS22. Therefore, it can be assumed that even at the beginning of the measurement, there are chloride ions on the gold surface as long as they are part of the electrolyte. Measurements without chloride ions show a ΦCPE close to 1. Having a strong adsorption, the HCF redox couple could gradually displace the chloride ions.. The result is the formation of a changing double layer at the beginning of the measurements, which would correspond to the observed decrease in TCPE in PBS solutions in Figure 4 and 5. These explanations can also be correlated to different behaviors of the time-dependent charge-transfer resistances RD in different electrolytes, as shown in Figure 4 and 5. As mentioned above, RD reaches a maximum after a certain time, which depends on the composition of the electrolyte.

By considering that pure phosphate buffer has no added salts, only phosphate ions and ions of the redox couple can be found on the gold surface. From SPR-measurements it is known that the surface coverage of phosphate ions decreases by adding [Fe(CN)6] 3/4, so that phosphate cannot clearly be identified in the SPRspectrum anymore14. The time constant for reaching the maximum RD is, with ∆tPB/5mM HCF = 47 min compared to ∆tPBS/5mM HCF = 132 min, lower in pure phosphate buffer than in phosphate buffered saline solution, because in the latter the chloride layer first has to be displaced by HCF. Containing less redox ions for this process, the time constant of the 1 mM HCF in PBS solution is even higher with ∆tPBS/1mM HCF = 183 min. Accordingly, the capacitive behavior in Figure 4 also indicates slower processes in the presence of lower HCF-concentrations. The DI water-based electrolyte contains no additional ions. Here, the capacitive behavior is only determined by the adsorption of the redox couple. The TCPE of this solution (see Figure 4 and 5) shows that the formation of the double layer takes place mainly within the first ten minutes. However, the time constant of ∆tDI/5mM HCF = 332 min for reaching the maximum of RD is quite large. Since the pH-value of this electrolyte is 6 and, thus, lower than that of the other solutions (buffered to pH 7.4), it is assumed that the formation of polymeric HCF-complexes depends on the pH-value. The decrease in RD after reaching the maxima is caused by evaporation of the electrolyte taking place during the measurements. According to equation (1), RD decreases if the concentration of the species increases. This was also measured by a linear decrease in the electrolyte resistance RE over time, since the conductivity increases with increasing concentration. The evaporation leads to higher redox couple concentrations and, therefore, according to equation (1) to a decrease in the RD. Another reason for the decrease in RD is the HCF decomposition and formation of CN- caused by UV-irradiation12. These unbound cyanide ions can lead to the formation of AuCN-ads or Au(CN)-2 24 and, thus, to a significant corrosion combined with an increase in a roughness of the gold electrode surface. To confirm this hypothesis, macroscopic electrodes in the standard HCF electrolyte were irradiated with UV-light for 10 hours to increase the concentration of free CN- ions. The RD values were recorded over time. As shown in Figure 6, RD decreases much faster after 10 minutes than in the case of non-pretreated standard HCF electrolyte. This decrease and the fact of a strongly etched and rougher electrode surface (see Figure 8 C) both support the idea of a CN- based etching and corrosion process at the gold surface.


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In contrast to the measurements including the redox couple, the measurements using pure PBS result in a comparatively constant impedance behavior. Minor changes can be attributed to evaporation and a small signal-to-noise ratio. However, a disadvantage of measuring in PBS is that without any charge transfer, the information content is much lower.

Figure 6: Relative and absolute change in RD as a function of time for standard HCF electrolyte with and without UV light irradiation. To increase the electrode stability over time during EIS measurements, platinum, as another favorable inert electrode material, can be used (Figure 7). Indeed, we observe significant advantages using platinum instead of gold: no adsorption of HCF complexes and no etching or corrosion processes take place at platinum electrode surfaces25. Pharr and Griffiths12,13 showed that only at high potentials far away from OCP is a thin layer of HCF complexes formed at platinum electrodes. A drawback for using platinum electrodes in biosensing and biochemical applications is their 50 % lower affinity to thiol groups compared to gold26.

43 % and, thus, is in the same order of magnitude as the passive one. It can be concluded that adsorption at the gold electrode in HCF-containing electrolytes persists even in the absence of electrochemical measurements. Even after deducting the standard deviation from the calculated average value, i.e. in the best case scenario, the charge-transfer resistance still increases three-fold within two hours. The fact that an inhibition of the charge-transfer occurs without additional electrical stimulation supports the assumption that even at OCP, a formation of HCF complexes is taking place at the gold surface, as reported by in Lee et al.3. To more precisely determine the roughness of the gold surface of the electrode after contact with the electrolyte, cleaned electrodes were incubated in pure PBS and PBS with 5 mM HCF for 20 hours and 14 days each. Surfaces were characterized by atomic force microscopy (AFM). The average of roughness parameter ഥ(Rrms) and its standard deviation s(Rrms ) are shown in Table 3. ࢞ The differences of the ഥ ࢞(Rrms) values of the samples only treated in PBS are in the range of measurement inaccuracies. By contrast, the roughness after incubation of the electrode in HCF-containing buffer differs. Here, an increase of the roughness by a factor of 1.76 was observed after 20 hours. After a two-week treatment, the factor amounts to 1.36. The lower factor compared to the 20 hours of treatment is caused mainly by the higher roughness at the starting point of this measurement. Due to a very inhomogeneous and impure surface, standard deviations are rather high after incubation in HCF. Figure 8 shows SEM images of both samples after an incubation time of 20 hours. On PBS treated samples the homogeneous grain structure of the sputtered gold particles is still visible. In contrast, surfaces of the HCF-treated samples are significantly rougher and fissured. The white dots in Figure 8C may indicate the enhanced etching at tips caused by UV-radiation. Surfaces exposed to the HCF-buffer shimmered more yellowish than the rest of the chip. This may imply possible residues of the yellowish electrolyte or a combination with the colorless “Prussian White” on the gold electrode. The color could be interpreted also as a presence of “Berlin Green”. In this case, all Fe2+ ions would be oxidized to Fe3+, which occurs only at potentials higher than 0.8 V (vs. SCE) and, thus, seems to be improbable in this case27.

Table 3: Roughness before and after incubation for 20 h or 14 days of different samples in a) PBS and b) 5 mM HCF in PBS 20 hours treatment

Figure 7: A comparison of the absolute and normalized RD with respect to time for gold and platinum electrodes in 5 mM HCF in PBS.

INFLUENCE OF INCUBATION IN CONTAINING FERRO-/FERRICYANIDE

a) PBS

0h

20 h

∆Rrms

0h

14 d

∆Rrms

ഥ(Rrms ) [nm] ࢞

1.39

1.38

-0.01

1.68

1.66

-0.02

s(Rrms ) [nm]

0.13

0.15

0.13

0.19

BUFFER

To analyze the influence of the impedance measurement cycles on the electrode behavior, EIS measurements were carried out only at the beginning and at the end of a two-hour treatment with HCFcontaining buffer. Again, the measurements were performed three times each. The difference in the normalized charge-transfer resistances amounts to ∆RD,passivenorm = 290% ± 90 %. The index “passive” indicates that no EIS measurements were performed during the incubation time. The electrodes remained clamped in the measurement setup to ensure permanent contact with the electrolyte and at the same position on the gold chip. The increase in RD during an active (with EIS measurements during the incubation time) electrode treatment amounts to ∆RD,activenorm = 304 % ±

14 days treatment

20 hours treatment

14 days treatment

b) PBS; 5mM HCF

0h

20 h

∆Rrms

0h

14 d

∆Rrms

ഥ(Rrms ) [nm] ࢞

1.40

2.47

1.07

1.81

2.47

0.66

s(Rrms ) [nm]

0.06

0.18

0.20

0.23

After 20 hours of incubation, additional EIS measurements were performed. Here, the RD of the HCF-treated sample is 19.76 times higher than that of the PBS-treated sample. This points to a covered electrode surface and, hence, is consistent with the surface analysis described before. In order to verify whether the adsorbed layer, which is formed at a gold electrode during a two-hour measurement period, can be



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dissolved by electrochemical cleaning again, 30 cyclovoltammetry (CV) cycles were performed after the EIS measurements (potential limits: -0.25 V to 0.7 V; scan rate: 100 mV/s). After cycling, EIS measurements were carried out again. During the first two hours, the averaged and normalized charge-transfer resistance increased from RD,1norm = 100 % ± 29 % to RD,2norm = 417 % ± 56 %. Due to CV, it decreased to RD,3norm = 241 % ± 44 %. Therefore, the surface can be recovered partially by CV, which corresponds to desorption of the previously adsorbed species. However, the initially clean state has not been regained.

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The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was gratefully supported by INTERREG IV-A 2007– 2013 EUREGIO MAAS-RIJN under grant INTERREG.EMR.INT4-1.2.-2009-11/058-MICROBIOMED. JL and US acknowledge Ilia Valov for valuable discussions.

REFERENCES

Figure 8: SEM-images of samples after 20 h in: A: PBS with standard HCF; B: PBS; C: PBS with standard HCF pretreated with 10 hour UV-light irradiation.

CONCLUSIONS Electrochemical impedance signals exhibit a non-linear drift in electrolytes containing HCF redox couples and PBS buffer on gold electrodes. The displacements of previously adsorbed chloride ions by HCF complexes on the electrode surface leads to an increase in RD, whereas etching of the gold surface by cyanide ions leads to a decrease in RD. This long-term drift associated with electrode surface modifications uncovered a very significant problem of EIS measurements with gold electrodes in combination with HCF redox couple. This must be carefully considered when carrying out characterization experiments of adsorption processes on gold surfaces. It is therefore strongly recommended not to use gold electrodes in combination with HCF containing electrolytes, especially for EIS. When using chemically stable electrode materials, such as platinum, the adsorption of HCF complexes and the corrosion of the electrode can be prevented. The understanding of the drift phenomenon is essential for the interpretation of all EIS experiments carried out in HCF-based electrolytes on gold surfaces.

AUTHOR INFORMATION Corresponding Author *Uwe Schnakenberg ([email protected]), phone: +49 241 80 27810

Notes The authors declare no competing financial interest.

Author Contributions

(1) van Gerwen, P.; Laureyn, W.; Laureys, W.; Huyberechts, G.; Beeck, M. op de; Baert, K.; Suls, J.; Sansen, W.; Jacobs, P.; Hermans, L.; Mertens, R.. Sensors and Actuators B: Chemical. 1998, 49, 73–80. (2) Fu, Y.; Yuan, R.; Xu, L.; Chai, Y.; Zhong, X.; Tang, D. Biochemical Engineering Journal. 2005, 23, 37–44. (3) Li, A.; Yang, F.; Ma, Y.; Yang, X., Biosens Bioelectron. 2007, 22, 1716–1722. (4) Lum, J.; Wang, R.; Lassiter, K.; Srinivasan, B.; Abi-Ghanem, D.; Berghman, L.; Hargis, B.; Tung, S.; Lu, H.; Li, Y. Biosens Bioelectron. 2012, 38, 67–73. (5) Wang, R.; Wang, Y.; Lassiter, K.; Li, Y.; Hargis, B.; Tung, S.; Berghman, L.; Bottje, W. Talanta. 2009, 79, 159–164. (6) Heiskanen, A. R.; Spégel, C. F.; Kostesha, N.; Ruzgas, T.; Emnéus, J., Langmuir. 2008, 24, 9066–9073. (7) Holmes, D.; Pettigrew, D.; Reccius, C. H.; Gwyer, J. D.; van Berkel, C.; Holloway, J.; Davies, D. E.; Morgan, H. Lab Chip. 2009, 9, 2881–2889. (8) Morgan, H.; Sun, T.; Holmes, D.; Gawad, S.; Green, N. G., J. Phys. D: Appl. Phys. 2007, 40, 61–70. (9) Sun, T.; Gawad, S.; Bernabini, C.; Green, N. G.; Morgan, H., Meas. Sci. Technol. 2007, 18, 2859–2868. (10) Lvovich, V. F. Impedance spectroscopy. Applications to electrochemical and dielectric phenomena; Wiley: Hoboken, N.J, 2012. (11) Bogomolova, A.; Komarova, E.; Reber, K.; Gerasimov, T.; Yavuz, O.; Bhatt, S.; Aldissi, M. Anal. Chem. 2009, 81, 3944–3949. (12) Pharr, C. M.; Griffiths, P. R., Anal. Chem. 1997, 69, 4673– 4679. (13) Pharr, C. M.; Griffiths, P. R., Anal. Chem. 1997, 69, 4665– 4672. (14) Iwasaki, Y.; Horiuchi, T.; Morita, M.; Niwa, O., Surface Science. 1999, 427-428, 195–198. (15) Carvalhal, R. F.; Sanches anches Freire, R.; Kubota, Electroanalysis. 2005, 17, 1251–1259. (16) Fischer, L. M.; Tenje, M.; Heiskanen, A. R.; Masuda, N.; Castillo, J.; Bentien, A.; Émneus, J.; Jakobsen, M. H.; Boisen, A. Microelectronic Engineering. 2009, 86, 1282–1285. (17) Yamamoto, K.; Nakamura, A.; Hase, U., IEEE Trans. Semicond. Manufact. 1999, 12, 288–294. (18) Hamann, C. H.; Hamnett, A.; Vielstich, W. Electrochemistry, 2nd ed.; Wiley-VCH: Weinheim, 2007. (19) Barsoukov, E.; Macdonald, J. R., Eds. Impedance spectroscopy. Theory, experiment, and applications; Wiley-Interscience a John Wiley & Sons Inc. publication: Hoboken, New Jersey, 2005. (20) Franks, W.; Schenker, I.; Schmutz, P.; Hierlemann, A., IEEE Trans Biomed Eng. 2005, 52, 1295–1302. (21) Kerner, Z.; Pajkossy, T., Electrochimica Acta. 2000, 46, 207– 211. (22) Kerner, Z.; Pajkossy, T., Electrochimica Acta. 2002, 47, 2055–2063. (23) Pajkossy, T., International Workshop on Impedance Spectroscopy for Characterisation of Materials and Structures International Workshop on Impedance Spectroscopy for Characterisation of Materials and Structures. 2005, 176, 1997–2003. (24) Dijksma, M.; Boukamp, B. A.; Kamp, B.; van Bennekom, W. P., Langmuir. 2002, 18, 3105–3112. (25) Huang, W.; McCreery, R., Journal of Electroanalytical Chemistry. 1992, 326, 1–12.


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(26) Petrovykh, D. Y.; Kimura-Suda, H.; Opdahl, A.; Richter, L. J.; Tar-lov, M. J.; Whitman, L. J. Langmuir. 2006, 22, 2578–2587. (27) Karyakin, A. A., Electroanalysis. 2001, 13, 813–819.

TOC graphic



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Macroscopic electrode in a three point measurement setup 203x152mm (96 x 96 DPI)


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250 200 2

RD [Ω cm ]

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KOH

15 time [min]

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KOHcv

APM


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RD [%]

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KOH

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KOHcv

APM


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30

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−Im{Z} [Ω cm2]

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Nyquist−Plot 10kHz 100Hz 10Hz

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time [min]

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RD [Ω cm ]

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1.05 1 0.95 0.9 0.85 0

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2

TCPE [F/cm ]

−4

φCPE

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500

2

x 10

1

PBS; 1mM HCF

PBS; 5mM HCF

PB; 5mM HCF


DI; 5mM HCF

1000

RD [%]

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T

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110 100

φ

CPE

[%]

0 0

90 0

PBS; 1mM HCF

PBS; 5mM HCF


PB; 5mM HCF

DI; 5mM HCF

RD [Ω cm2] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

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PBS; 5mM HCF; UV


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RD [%]

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PBS; 5mM HCF

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PBS; 5mM HCF; UV


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RD [Ω cm2] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57


40

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70

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1000

RD [%]

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PBS; 5mM HCF; Platinum


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264x211mm (96 x 96 DPI)


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Ferrocyanide

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