Modulation of ionic current behaviors based on a dual-channel micro/nano-pipette with ternary-form-charged model

Abstract

• Ternary-Formed-Charged model is proposed for simulation of ionic behaviors of a dual-channel pipette such as a resistor, diodes, and a bipolar junction transistor. • The uneven distribution and location of surface charge density are confirmed to cause asymmetric bipolar junction transistor behavior and pseudo-bipolar junction transistor behavior. • Scale ratio of radius of the pipette to liquid bridge and the effect of surface charge at the interface between liquid bridge and air is evaluated. Dual-channel micro- or nanopipettes can be used to perform various ionic current behaviors such as a resistor, diodes, and a bipolar junction transistor (BJT) via chemical modification. These ionic current devices could be modulated by charge pattern of modification, pH, size and scale ratio of pipette to the liquid bridge, whose intrinsic mechanism is indistinct. Herein, a universal Ternary-Form-Charged (TFC) model is proposed, in which the pipettes contain three individual tunable regions with different scale, location, surface charge density while ions can flux across the charged liquid bridge connected with them. Based on the TFC model, we could obtain three basic ionic behaviors mentioned above of dual-channel pipettes and demonstrate that these basic ionic behaviors of dual-channel pipette are dependent on the number of transition zone. Transformation from the BJT behavior to a pseudo-BJT behavior is attributed to a narrow middle charged region or a thick liquid bridge. Asymmetric BJT behavior is also observed under asymmetric distribution of location and quantity of surface charge density. Furthermore, we provide the approximate size of liquid bridge: 5–8 nm and 15–20 nm for dual-channel nanopipette and micropipette, respectively. In short, the TFC model and our simulation results shed light on the mechanism of dual-channel pipette based ionic circuits and provide a useful guide to design, modify and develop novel applications of dual-channel pipettes in nanofluidic sensors.