Supporting Information
In the Supporting Information the reader may find detailed experimental information. In addition, the theory of cyclic voltammetry and electrochemical impedance spectroscopy is explained.
Acknowledgments
The author thanks the Vector Foundation, Germany, and the Fonds der Chemischen Industrie, Germany, for financial support.
Supporting Information
Cyclic Voltammetry
A CV is obtained by measuring the current between the working and the counter electrode as a function of the potential (normalized to the potential of the reference electrode). To do this, the experimenter uses a three-electrode setup and varies the potential of an electrode (the “working” electrode), which is immersed in an unstirred solution, and measures the resulting current.
A triangular potential sweeps the potential of the working electrode between the starting potential to the switching potential and back again. The scan rate v (in mV/s) is an important parameter, as will be shown below.
The current flows in or out of the working electrode to or from a counter electrode. The potential of the working electrode is controlled versus a reference electrode, e.g., a saturated calomel or a silver/silver chloride electrode. The reference electrode passes no current.
All these requirements can be fulfilled by a potentiostat.
Here are the main points to consider when starting out with CV:
• According to the IUPAC recommendation anodic peaks point upward, cathodic downward.
• The standard redox potential E0 can be calculated
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where EP are the peak potentials.
• If the current peaks appear to be sliding apart as a function of scan rate, the process is quasi-reversible (or, in an extreme case, irreversible).
Two qualities, parameterized by the standard electron rate constant k0 and the mass transport to or from the working electrode mtransport, determine the CV.
• If k0 >> mtrans then the electrode process is reversible and diffusion controlled.
• In the intermediate, the so-called quasi-reversible case, diffusion and electron transfer are in the same order (k0 ≈ mtrans).
• The process will be rate-determined if the mass transport is faster than the electron transfer (k0<<mtrans) and the electron transfer is irreversible.
There are three other clues that a cyclovoltammogram provides that indicate that the observed process is reversible.
The first is that the peak currents Ip for the forward and reverse reactions are the same,
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The second is that the peak current is proportional to the square root of the scan rate: Reversible processes show a v1/2-dependence of their current peaks according to the Randles-Sevcik equation:
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Third, the difference between the electrode potentials at which reduction and oxidation occur is constant at all scan rates: If the positions of the maximum current peaks with regard to their potential do not change as a function of the scan rate, and the heights of the anodic and cathodic peaks appear to be equal, then the process occurring is reversible. If the peaks are about 59 mV apart then the process is a reversible one-electron transfer (n = 1):
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With this eq. the experimenter can calculate the number of transferred electrons n.
In the quasi-reversible and the (electrochemical) irreversible case one can see that the current peaks are drifting apart with increasing scan rate up to hundreds of millivolts. Now the kinetic process, i.e. the electron transfer, is determining the electrochemical behavior.
In cyclic voltammetry the scan rate is a means that provides the experimenter with a tool to control the electrochemical process either by electron transfer process or by mass transport: By varying the scan rate we vary the diffusion layer thickness: at slow scan rates, the diffusion layer is thick, while at faster scan rates the diffusion layer is thinner. Since the electrochemical process reflects the competition between the electrode reaction and the diffusion, faster scan rates will favor electrochemical irreversibility (controlled by the electron transfer rate) and the peak potentials will drift apart.
STAT-ECL
The ECL method has the advantage of being sensitive, and the SPE used are cheap (€1-3 each), but the cost of [Ru(bpy)3]2+ is a disadvantage. With a commercial ECL system the measurements are very fast, the sensitivity is high, but the high price of the device (STAT- ECL) can be off-putting (€10,000: Metrohm/DropSens).
This device can measure different electrochemical procedures such as linear-sweep voltammetry, cyclic voltammetry, and square wave voltammetry, and can simultaneously detect the resulting chemiluminescence signal.
STAT-ECL is a portable potentiostat combined with a specific Electrochemiluminescence (ECL) cell that performs electrogenerated chemiluminescence studies with SPEs. A potentiostat and a Si-Photodiode integrated in the ECL cell (spectral response: 340 – 1100 nm) are combined.
All electrochemical features inclusive the amplification factor of the light intensity can be programmed with the DropView 8400 software from Metrohm/DropSens.
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Figure S1. User interface
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Figure S2. Potentiostat with ECL cell
Reaction Scheme of ECL
In the [Ru(bpy)3]2+ system the ECL-reaction can be expressed in the following way, here with tripropylamine (Pr3N) as a coreactant:
Scheme: Reaction mechanism ECL
(1) 
(2) 
(3) 
(4) 
(5) (Pr: propyl, Et: ethyl, h: energy of the emitted light).
Reaction (1) may be an electrode process or a direct oxidation with an oxidizing agent.
In contrast to the above scheme, there may be a direct oxidation of Pr3N (sometimes as a competition process).
Ru(bpy)32+ reacts with the Pr2NC·HEt radical to form Ru(bpy)32+*, a species in an excited state that undergoes radiative decay. In addition, Eq. (3) shows that the forming of Ru(bpy)32+* with the subsequent emission of light is pH-dependent.
Electrochemical Impedance Spectroscopy (EIS)Along with other electrochemical measurements such as cyclic voltammetry (CV), square wave voltammetry (SWV) or chronoamperometry (CA), electrochemical impedance spectroscopy (EIS) plays an important role in measuring the characteristics of electrode in electrolyte solutions.
If a potential is applied to an electrode-electrolyte interface, a flow of charge and matter occurs. Without going into details, in EIS the impedance (the resistance of the AC circuit) of the electrochemical system is measured as a function of the applied frequency. The processes at the electrode, immersed in the solution, can be described with different electrical compounds as resistors (solution resistor, Rs; charge transfer or polarization resistor, Rp), capacitor (Helmholtz double layer in front of the electrode, Cdl), and coil (inductance resulting from the current, which induces an electromotive force that opposes a change in current, L). One of the main purposes of EIS is to describe an electrochemical system through a combination of these passive electric compounds. In contrast to the ohmic resistor, capacitors and coils have a frequency-dependent resistance, which is inversely proportional to the frequency of the first and proportional to the frequency of the latter.
The impedance, Z, is represented as the complex number
Re: real part and Im: imaginary part of the impedance.
Therefore
The modulus of Z is
In electrochemistry, a Randles circuit is one of the simplest electrical circuits to describe an electrochemical system. It consists of an electrolyte resistance, RS, in series with the parallel combination of the double-layer capacitance, Cdl, and an impedance of a faradaic (i.e., electron-transfer) reaction, Rp. Sometimes a constant phase element CPE is used instead of an imperfect capacitor, which is in series to Rs or to Rp. The Warburg impedance (ZW) describes the diffusion. In real electrochemical systems, EIS is more complicated and the Randles circuit may not give appropriate results.
The Nyquist-plot in the following figure is measured with the STAT I 400 from Metrohm/DropSens.
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Figure S3. Nyquist diagram (measuring points and fitted curve) with Randles circuit
Figure shows a Nyquist-plot Nyquist diagram. Points: measuring points; solid line: fit with Randles circuit (Rs = 92Ω, Rp = 649Ω, Cdl = 2.2μF, Yo(Warburg) = 3.02 mMho(s)1/2), see insert. Note: “Mho” is Ohm written backwards. Y0 is the admittance (reciprocal of the impedance) multiplied by sqrω).
Raman SpectrometerThe AvaRaman spectrometer from Avantes is combined with 785 nm laser. The spectrometer is appropriately configured according to the wavelength of the laser.
The AvaRaman system is equipped with a cooling system. Cooling the detector down to -35°C cooling versus ambient, reduces the noise by a factor 2-3, enabling the usage of longer integration times to enhance the detection of small signals. The AvaRaman system used is delivered with the AvaSoft-Raman software.
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Figure S4. User interface of AvaRaman
Raman Probe
The Raman probe consists of a focusing probe with a 200 µm excitation fiber and 400 µm read fiber (all 1.5 m), focal length 7.5 mm. A manual shutter is included. Figure S5 shows the excitation and read fiber.
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Figure S5. Bifurcated excitation – read fiber. Middle: read fiber, surrounded by six excitation fibers (illuminated)
Raman Cell
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Figure S6. Raman cell from Metrohm/DropSens
Figure shows the used Raman cell for SPE (Metrohm/DropSens)
Synchronizing and Data Handling: RamanvoltammogramBy means of the following figure students are able to connect the different parts of the experiment. In detail, they connect the potentiostat with the three electrodes in the ECL- or Raman cell via a special cable (Metrohm/DropSens).
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Figure S7. Scheme of the Ramanvoltammogram
The following Figure S8 shows the user interface of the measuring programs (DropView and Ava Raman). After starting the DropView program, students have to set the scan rate, the start-, reverse- and end potential of the CV and the amplification factor of the ECL intensity. The Raman spectrometer software proposes integration time and averaging number of the spectra. With these proposed values no saturation is measured.
The Raman mode is set by the Raman button. Every new integration-time needs a new dark zero adjustment. After clicking the button the Raman intensity becomes zero.
After fixing the values of the cyclic voltammogram (scan rate and scan range, i.e. start, reversal and end values), the potentiostat can be started. After sending the set values to the potentiostat the software sends an equilibration command for about 3 seconds, and the potentiostat begins to run. Immediately, the spectrometer must be started manually.
The CV and spectrometer data must be synchronized. This means that if the potentiostat takes 200 current / voltage couples (+0.5 V -> -0.5 V -> +0.5V, scan rate 0.01 V/s) the spectrometer also must record 200 spectra – one per second. The spectrometer used has two options: single and continuous measurement. Here the last must be activated.
After the end of a measurement cycle, all data can be transferred into Excel and displayed separately: voltage against current and Raman intensity against wavenumber. One can separate those Raman data at a given wavenumber that show a maximum change as a function of the applied voltage. Another option may be plotting the whole spectrum against fixed voltages (i.e. the voltage where no reduction or oxidation occurs and the voltage of a current peak). Therefore, one can easily show the difference between the Raman spectra with and without an electrochemical reaction.
Published with license by Science and Education Publishing, Copyright © 2022 Achim Habekost
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Normal Style
Achim Habekost. In-situ Surface Enhanced Electrochemical Chemiluminescence and Raman Scattering with Screen-printed Gold- and Silver-Electrodes. World Journal of Chemical Education. Vol. 10, No. 1, 2022, pp 23-37. http://pubs.sciepub.com/wjce/10/1/4
MLA Style
Habekost, Achim. “In-situ Surface Enhanced Electrochemical Chemiluminescence and Raman Scattering with Screen-printed Gold- and Silver-Electrodes.” World Journal of Chemical Education 10.1 (2022): 23-37.
APA Style
Habekost, A. (2022). In-situ Surface Enhanced Electrochemical Chemiluminescence and Raman Scattering with Screen-printed Gold- and Silver-Electrodes. World Journal of Chemical Education, 10(1), 23-37.
Chicago Style
Habekost, Achim. “In-situ Surface Enhanced Electrochemical Chemiluminescence and Raman Scattering with Screen-printed Gold- and Silver-Electrodes.” World Journal of Chemical Education 10, no. 1 (2022): 23-37.
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