## 2 Mechanism of Ionic Current Sensing  
The system mainly consists of a copper working electrode (for contacting a metal object) and an electrometer connected in series with a copper sensing electrode. Both the working electrode and the sensing electrode, as well as the metal sheet, are immersed in an electrolyte solution. The mechanism of the contact-induced ionic current sensing can be understood as follows. Initially, the working electrode, sensing electrode and the metal sheet are immersed in the electrolyte solution without contact. Generally, a metal electrode is negatively charged in an electrolyte solution. As a result, the cations are attracted onto the electrode surface to form a so-called EDL. Since the materials of the two electrodes are the same, there is a neglected potential difference between them. As a result, there will be no electron flow between them before the working electrode and the metal sheet come into contact. When the copper working electrode and the metal sheet with a higher work function (e.g., gold sheet) are brought into electrical contact (Figure 1), some electrons will be transferred from the copper working electrode to the gold sheet until their Fermi levels are equal [33]. This process of electron migration leads to the generation of a contact potential difference (CPD) at the interface of the copper working electrode and the gold sheet. Specifically, the electric potential of the gold sheet is higher than that of the copper working electrode. As electrons move from the copper working electrode to the gold sheet, some electrons will migrate accordingly from the ground to the copper working electrode. For the gold sheet after contact, it becomes the new working electrode (named the gold-composite working electrode) with the copper working electrode working as the function of a copper electric wire. Meanwhile, the electric potential of the sensing electrode is unchanged. Then, the potential difference between the gold-composite working electrode and the sensing electrode is increased to an electric potential difference \(\Delta V\). Accordingly, the cations and the anions in the electrolyte solution are attracted to the gold-composite working electrode/electrolyte solution interface and the sensing electrode/electrolyte solution interface, respectively, forming the EDL capacitors. The accumulated anions on the sensing electrode/electrolyte solution interface will be neutralized by the discharged electrons through the electrometer. According to the connection of the electrometer discussed above, a negative ionic current signal pulse is detected. Figure 1: Mechanism of contact-induced ionic current sensing. Similarly, when the copper working electrode contacts a metal sheet with a lower work function (e.g., aluminum sheet), the aluminum sheet works as a function of the new working electrode (named the aluminum-composite working electrode). The electrons will flow from the aluminum-composite working electrode to the copper working electrode, leading to a lower electric potential of the aluminum-composite working electrode. Then, the anions and the cations are attracted onto the surfaces of the aluminum-composite working electrode and the sensing electrode, respectively. The accumulated cations on the sensing electrode/electrolyte solution interface will attract the electrons to the sensing electrode through the electrometer. Therefore, a positive ionic current signal pulse will be detected. It should be noted that the above-mentioned contact-induced ionic current is a non-Faradaic current, where only ion absorption and detachment at the electrode/electrolyte solution interface are involved. The contact-induced ionic current sensing process can be electrically modeled with an equivalent circuit. Specifically, the composite working electrode and the sensing electrode can be modeled as the parallel of a resistor \(R\) (\(R_{1}\), \(R_{2}\)) and an EDL capacitor \(C\) (\(C_{1}\), \(C_{2}\)), respectively. There is also a water resistor \(R_{\rm s}\) between the two electrodes.