Measurement of a Bubble-Free Chemical Oscillator Using QCMs Treated with Self-Assembled Monolayers

The oscillation of a 1,4-cyclohexanedione-bromate (CHD-bromate) system was investigated by using a quartz crystal microbalance (QCM) without or with a self-assembled monolayer (SAM), where gold and platinum were used for QCM electrodes and SAMs were composed of HS­(CH₂)₁₁CH₃, HS­(CH₂CH₂O)₅H, and HS­(CH₂CH₂O)₅CH₃. The CHD-bromate system is well-known as a bubble-free oscillator and oscillates without or with a catalyst. In the CHD-bromate system without a catalyst, the oscillation of a resonant frequency shift (ΔF) of the QCM was observed in the Au-electrode QCMs without a SAM or with SAMs formed from HS­(CH₂)₁₁CH₃ and HS­(CH₂CH₂O)₅H. On the other hand, the HS­(CH₂CH₂O)₅CH₃ SAM suppressed the ΔF oscillation. The results revealed that in the CHD-bromate system without a catalyst, hydrophobic CH₃ terminal and helical conformation were important to prevent nonspecific adsorption of substances on a gold surface and its dissolution. In the CHD-bromate system with a catalyst (ferroin), the ΔF oscillation was observed in the Au-electrode QCM with the HS­(CH₂CH₂O)₅CH₃ SAM. The results suggest evidence that the change of the solution viscosity and density led to the ΔF oscillation. These results for Au-electrode QCMs were also corroborated by those for Pt-electrode QCMs.