The reason why the quaternary ammonium cation was used in this study as positively charged material is because of its unchanging valence in response to pH changes. As other positively charged compounds, poly-L-lysine (PLL) may also be considered due to its strong positive charge at neutral pH conditions, and its widespread use in bioassay and biosensing. However, unlike the quaternary ammonium cation, the ζ-potential of PLL significantly varies with pH changes around pH=7, which may negatively impact FET sensitivity. This is because ψ0 was pinned by the pH-sensitive surface group, referred to as a negative pH interference mechanism. Consequently, in FET biosensors, biosensitivity is inversely related to pH sensitivity. As our devices were modified with the quaternary ammonium cation, it is expected that the pH interference mechanism is relatively suppressed, which also contributes to the signal enhancements.