The combination of electrochemistry (EC) and surface-enhanced Raman scattering (SERS) has received growing attention over the past several years due to the powerful features associated with the EC-SERS effect [1,2]. In particular, the electrochemical activation of metal electrodes allows the generation of SERS substrates which in turn enhance the Raman intensity in a single experiment.
Many electrochemical methods have been developed but are traditionally limited to aqueous media. Raman spectroelectrochemistry in organic solutions is an interesting alternative, but developing new EC-SERS procedures is still required.
This Application Note demonstrates that the electrochemical activation of gold and silver electrodes enables the detection of dyes and pesticides in organic media.
Measurements were performed using a SPELEC RAMAN instrument (785 nm laser), a Raman probe corresponding to the laser wavelength, and a Raman spectroelectrochemical cell for conventional electrodes (Figure 1).
Gold and silver working electrodes were used in combination with steel and Ag/AgCl counter and reference electrodes, respectively. The SPELEC RAMAN instrument was controlled with DropView SPELEC, a dedicated spectroelectrochemistry software that simultaneously acquires electrochemical and optical information. All hardware and software used for this study is compiled in Table 1.
Table 1. Hardware and software equipment overview
| Equipment | Metrohm number |
|---|---|
| SPELEC RAMAN Instrument | SPELECRAMAN |
| Raman Probe | RAMANPROBE |
| Raman spectroelectrochemical cell for conventional electrodes | RAMANCELL-C |
| Gold electrode tip | 6.09395.034 |
| Silver electrode tip | 6.09395.044 |
| Separate steel electrode | 6.0343.110 |
| Ag/AgCl reference electrode | 6.0728.120 |
| DropView SPELEC Software | DropView SPELEC |
Organic solvents typically exhibit many Raman bands that could impair the analysis of the characteristic bands of the molecule of interest. Figure 2 shows the Raman spectra from 12 different organic solvents. Medium selection is an important condition that must be considered for each application.
Figure 2. Characteristic Raman spectra of a) acetone, b) acetonitrile, c) cyclohexanone, d) chloroform, e) dichloromethane, f) dimethylformamide (DMF), g) dimethyl sulfoxide (DMSO), h) ethanol, i) glycerol, j) isopropanol, k) methanol, and l) tetrahydrofuran (THF).
As a proof of concept, the detection of crystal violet (a dye widely used in Raman spectroscopy) is carried out in organic media with the Au electrode (Table 1). The electrochemical activation of this electrode consists of the initial oxidation of the gold surface followed by its subsequent reduction to generate metallic nanostructures with SERS properties during the cathodic scan.
In order to achieve the highest enhancement factor, spectroelectrochemical detection of crystal violet was evaluated in different organic media. The best results were found by working in acetonitrile media with TBA (tetrabutylammonium hexafluorophosphate) as the supporting electrolyte. The potential was scanned from +0.70 V to +2.00 V and back to -0.60 V in the cathodic scan, returning to the initial potential (+0.70 V) at the end of the experiment. The cyclic voltammogram shown in Figure 3a was obtained in 0.1 mmol/L crystal violet and 0.1 mol/L TBA in acetonitrile using the described experimental conditions. Figure 3a shows the oxidation and reduction processes of the gold electrode at +1.80 V and +1.03 V, respectively.
Raman spectra were monitored concurrently during the electrochemical procedure. Although more than 50 spectra were recorded during the experiment, only a selection of them are plotted in Figure 3b to better visualize the optical signal evolution. Raman intensity increases during the cathodic scan, reaching its maximum value at +0.20 V. The characteristic Raman bands associated with the vibrational modes are perfectly well-defined.
The same experiment was carried out with varying concentrations of crystal violet. Analysis of the intensity associated with the Raman band at 1175 cm-1 enables the detection of 1 μmol/L crystal violet, demonstrating the level of sensitivity achievable with this method.
Figure 3. a) Cyclic voltammogram and b) evolution of Raman spectra at different potentials obtained in 0.1 mmol/L crystal violet and 0.1 mol/L TBA in acetonitrile while scanning the potential from +0.70 V to +2.00 V and back to +0.60 V. The integration time was 2000 ms.
Like the previous example with gold, the electrochemical activation of silver electrode surfaces also produces an enhancement of Raman intensity. The spectroelectrochemical detection of mancozeb in an organic solvent was performed with the Ag electrode to prove this concept. Mancozeb is a fungicide which is insoluble in water. Previous solubility tests concluded that DMSO is one of the best solvents to ensure the solubility of this pesticide. Experimental conditions were optimized by working in 0.5 mmol/L mancozeb and 0.1 mol/L TBA in DMSO. In this case, the potential was scanned between +0.60 V and -0.60 V for five cycles (Figure 4a) since activation of the silver surface in this organic medium requires an initial cycle for pretreatment purposes.
Figure 4b shows that although several Raman bands are related to the DMSO solvent, characteristic signals of mancozeb centered at 240, 422, 463, 516, 560, 660, 912, 990, 1187, 1272, 1522 and 1615 cm-1 are also detected at -0.60 V (second cycle).
Figure 4. a) Cyclic voltammogram and b) Raman spectra obtained in 0.5 mmol/L mancozeb and 0.1 mol/L TBA in DMSO while scanning the potential from +0.60 V to -0.60 V for five cycles. The integration time was 2000 ms.
Raman intensity of mancozeb was evaluated during the whole experiment and it remains constant from cycles 2 to 5. Therefore, only two cycles are required—the first to pretreat the surface, and the second to produce its SERS activation.
Some EC-SERS protocols are tedious, complex, and limited to aqueous solutions. Instrumental requirements can also complicate the improvement of electrochemical activation of metal electrodes. This study covered the detection of two different molecules (crystal violet and mancozeb) performed by Raman spectroelectrochemistry in organic solvents (acetonitrile and DMSO). Electrochemical activation, which consists of an initial oxidation followed by a reduction scan, allows the detection of 1 μmol/L crystal violet. Of particular interest is the detection of mancozeb, a fungicide insoluble in water which requires organic media as, for instance, DMSO.
- González-Hernández, J.; Ott, C. E.; Arcos-Martínez, M. J.; et al. Rapid Determination of the ‘Legal Highs’ 4-MMC and 4-MEC by Spectroelectrochemistry: Simultaneous Cyclic Voltammetry and In Situ Surface-Enhanced Raman Spectroscopy. Sensors 2022, 22 (1), 295. https://doi.org/10.3390/s22010295.
- Ibáñez, D.; González-García, M. B.; Hernández-Santos, D.; Fanjul-Bolado, P. Detection of Dithiocarbamate, Chloronicotinyl and Organophosphate Pesticides by Electrochemical Activation of SERS Features of Screen-Printed Electrodes. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2021, 248, 119174. https://doi.org/10.1016/j.saa.2020.119174.
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