2.1.3. Electrochemical circuit

The recording unit provides ports that allow connection to external signal generators via a LEMO-BNC cable. Electrodes were routed using the Python API provided by MaxWell Biosystems. Cathodic electrodes were connected to an internal common node available on the chip. This common node was routed to the device's external port, which was connected to the SMU. The ground potentials of the SMU and the recording unit were connected through the LEMO-BNC cable. Anodic electrodes were either undefined and hence left floating or routed around the cathodic electrodes. If defined, the electrodes surrounding the cathodic electrodes were selected as well for routing to form a directly neighboring anode. To achieve this configuration, the amplifiers routed to the anodic electrodes were connected to a system-internal stimulation DAC. Three 10-bit stimulation DACs are available on the chip. When the DAC connected to the anodic electrodes is set to a value of 512, these electrodes are set to the mid-potential of the chip at 1.65 V. The mid-potential of 1.65 V is also referred to as the floating potential of the chip²⁵ and results from the chip's internal power supply voltage of 3.3 V. This electrochemical setup resembles a two-electrode system, where the cathode acts as the working electrode and the anode as the counter electrode. No further reference electrode was placed in the bath. The equivalent electrical circuits are shown in the ESI^† in Fig. S2. Using the SMU, the voltage between the anodic and cathodic sets of electrodes can be varied, which alters the extent of electrochemical reactions (i.e., hydrolysis), which results in a local pH alteration. The concept of this confined pH alteration is visualized in Fig. 1. To determine the influence of applied voltage on the resulting pH on the chip, a voltage sweep was performed. For this, the voltage was gradually increased from 0–1.6 V in steps of 100 mV every 60 s. Here, the voltage is defined as the potential difference between the anode and the cathode.