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Cetyltrimethylammonium bromide (CTAB) has been used to enhance the selectivity of CO2 electrochemical reduction. Traditionally, this selectivity was attributed to repulsion of water molecules due to a CTAB self-assembled monolayer, which forms under negative potential and disassembles at positive voltage due to electrostatic repulsions. In this report, using in operando interface sensitivity sum frequency generation spectroscopy, we investigated the self-assembly behavior of CTAB across a broad electrochemical potential range. We observed that CTAB molecules form a stable monolayer at the Stern layer over the entire potential scan, even when the electrodes are positively charged. Rather than disassembling, the CTAB molecules reorient themselves to balance the electrostatic interactions and the non-covalent hydrophobic effects, the latter being the primary driving force maintaining the monolayer at a positive potential. This finding contrasts the traditional view that CTAB monolayers are absent when the electrodes are positively charged, indicating a stable and ordered monolayer with respect to the electrostatic repulsions at liquid/electrode interfaces. The balance between non-covalent and electrostatic interactions offers a facile and reversible electrochemical method to control the local environment and dominating interactions at the Stern layer of the electrode surface, thus providing a means for engineering a micro-electrochemical environment.
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This content will become publicly available on August 1, 2026
Ultra-thin inorganic oxide and nitride films as an alternative blocking layer for electrochemical sensors
The desire to translate biosensors for real time molecular monitoring has intensified due to the commercial success of 2-week continuous glucose monitors. However, a common limitation for emerging biosensors is that their lifetimes are often too short for commercially expected benchmarks of at least 3-day and ideally 2-week operation. Electrochemical sensors remain the preferred format of biochemical sensing thanks to their low cost, size, weight, and power requirements for mobile deployment. When exposed to biological fluid, all electrochemical sensors require a blocking layer to protect the electrode surface from fouling and redox interferents. Traditional blocking layer approaches rely on self-assembled monolayers which are often fragile to biological interferents like proteins and require specific electrode materials to improve their stability. Presented here is an evaluation of ultra-thin inorganic oxide and nitride films as an alternative to self-assembled monolayer blocking layers. Specifically, silicon oxide, silicon nitride, and aluminum oxide films were deposited by electron beam evaporation or atomic layer deposition at thicknesses of several nanometers to mimic the electrical capacitance of a conventional monolayer blocking layer. These oxide films were characterized over 7-days and demonstrated to provide poor protection against interfering redox currents from dissolved ferricyanide (150 - 300 µA/cm2) and oxygen reduction interference (30 - 60 µA/cm2). The oxide films were then used as a blocking layer in an electrochemical aptamer sensor using the previously published aptamer for phenylalanine. The phenylalanine sensor showed a binding affinity stronger than found in literature, but a reduced signal gain (∼ 20 % change in methylene blue redox current compared to the expected 50 % previously published on gold). It is speculated and supported by literature that these oxide and nitride films gradually dissolve over periods of days in an aqueous environment. Results further show that if lower quality oxide or nitride films are used, they may be more stable, but at the cost of initially higher in currents. While oxide and nitride films fail to improve upon the performance of thiol-blocking layers on gold electrodes, they may provide utility in some applications by allowing for alternate electrode materials and surfaces to be used instead of traditional self-assembled monolayers on gold electrodes.
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- Award ID(s):
- 2327102
- PAR ID:
- 10612480
- Publisher / Repository:
- Thin Solid Films, Elsevier
- Date Published:
- Journal Name:
- Thin Solid Films
- Volume:
- 824
- Issue:
- C
- ISSN:
- 0040-6090
- Page Range / eLocation ID:
- 140712
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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