Real-Time SERS Tracking of Plasmon-Mediated Azo Coupling of 4-Aminothiophenol on Gold Nanoelectrode Platforms

We report a plasmon-mediated route to drive the azo coupling of 4-aminothiophenol to 4,4′-dimercaptoazobenzene on a gold nanostructure on a nanoelectrode (NSoNE) platform. Gold nanostructures were grown on electrochemically etched gold nanoelectrodes to form NSoNE substrates. The curved tip showed a larger enhancement factor than the planar rear surface, with values of 1.31× 10 4 and 3.08 × 10 3 . Surface-enhanced Raman spectra tracked the azo coupling reaction in real time. At 88 μW μm –2, the curved tip reached a steady state in 37.5 s, while the planar region required 108.4 s. Reducing the power to 44 μW μm –2 slowed the process. Phenyl isocyanide (PIC) served as a molecular thermometer. The C≡N peak of PIC showed a linear red shift with temperature, with a slope of −0.246 cm –1 °C –1 . Calibrated maps revealed a nonuniform thermal field that peaked at the tip. Potassium bromate was used to consume hot electrons. The reaction still proceeded in 0.8 mmol L –1 of KBrO 3 but with longer steady-state times of 56.4 s at the curved tip and 109.3 s at the planar region. Dark-field heating experiments indicate that thermal effects can effectively activate the 4-ATP coupling reaction, yet they are insufficient to achieve the maximum reaction state. Overall, the results demonstrate that thermal effects can activate and accelerate the reaction process, with the most pronounced influence near the tip; moreover, electron-related reaction pathways also play a certain role.