pH-Induced Changes in the SERS Spectrum of Thiophenol at Gold Electrodes during Cyclic Voltammetry

Abstract

Thiophenol is a model compound used in the study of self-assembly of arylthiols on gold surfaces. In particular, changes in the surface-enhanced Raman scattering (SERS) spectra of these self-assembled monolayers (SAMs) with a change of conditions have been ascribed to, for example, differences in orientation with respect to the surface, protonation state, and electrode potential. Here, we show that potential-induced changes in the SERS spectra of SAMs of thiophenol on electrochemically roughened gold surfaces can be due to local pH changes at the electrode. The changes observed during the potential step and cyclic voltammetry experiments are identical to those induced by acid-base switching experiments in a protic solvent. The data indicate that the potential-dependent spectral changes, assigned earlier to changes in molecular orientation with respect to the surface, can be ascribed to changes in the pH locally at the electrode. The pH at the electrode can change as much as several pH units during electrochemical measurements that reach positive potentials where oxidation of adventitious water can occur. Furthermore, once perturbed by applying positive potentials, the pH at the electrode takes considerable time to recover to that of the bulk solution. It is noted that the changes in pH even during cyclic voltammetry in organic solvents can be equivalent to the addition of strong acids, such as CF₃SO₃H, and such effects should be considered in the study of the redox chemistry of pH-sensitive redox systems and potential-dependent SERS in particular.

Scheme 1. Compounds and Binding Modes Discussed in This Work

Figure 1

(Left) Intensity of several Raman bands of the AuSPh complex over time during heating in a microscope heating stage. The temperature increased monotonically from 160 to 180 °C between 700 and 825 s (black box) and was held at 180 °C until 875 s (solid red box) and thereafter allowed to cool rapidly (dashed red box). (Right) Raman spectra (λ_exc 785 nm) of the AuSPh complex before (black) and after (red) heating to 180 °C. The spectra are normalized and offset for clarity.

Figure 2

(Left) SERS spectra (λ_exc 785 nm) of PhSH on aggregated gold colloid in H₂O (black) at pH = 0 (red) and pH = 13 (blue). (Right) SERS spectra (λ_exc 785 nm) of PhS-Au on a roughened gold bead in CH₃CN before (black), after addition of TfOH (red), and after subsequent addition of Et₃N (blue). The spectra are normalized and offset for clarity. For full spectral range, see Figures S8 (colloids) and Figures S8 and S9 (SAM on gold beads).

Figure 3

Characteristic vibrational modes of computational model PhS-Au₄ (black) and the corresponding experimental SERS bands of PhS-Au SAMs (red). The calculated values (in cm^–1) are of Raman bands after Gaussian broadening, which produces a single band for the frequencies at 1090.98 and 1094.8 cm^–1. A scaling factor was not applied to the calculated frequencies.

Figure 4

(Left) Changes in ratio of intensities of the Raman bands at 1000 and 1025 cm^–1 (circle) during cyclic voltammetry of PhS-Au in CH₃CN with 0.1 M TBAPF₆. Potential is indicated by color from 0.0 V (black) to 0.9 V (red) versus Ag/AgCl. (Right) Corresponding SERS spectra (λ_exc 785 nm) at 0.0 V (black) and 0.9 V (red).

Figure 5

SERS spectra (λ_exc 785 nm) of PhS-Au at 0.2 V (black) and 1.2 V (red) versus Ag/AgCl during cyclic voltammetry (three cycles, top to bottom) in CH₃CN with 0.1 M TBAPF₆. Spectra are normalized on the band at 1575 cm^–1 and offset for a better comparison. Asterisk denotes CH₃CN Raman bands.

Figure 6

(Left) Changes in the ratio of intensities of the Raman bands at 1000 and 1025 cm^–1 (circle) and the changes in Raman shift of the bands at 1075 cm^–1 (square) and 1575 cm^–1 (triangle) of PhS-Au in CH₃CN with 0.1 M TBAPF₆ during a potential step experiment between 0.2 V (black) and 1.2 V (red) versus Ag/AgCl. (Right) Corresponding SERS spectra (λ_exc 785 nm) of PhS-Au at 0.2, 1.2, and at 0.2 V again (top to bottom).