## Materials and Methods
### Charge Measurements  
To investigate the influence of materials of chips and communications on the charge generation in the injector of the AFM-based biosensor (which is described in detail in Reference [6]), the corresponding chips or communications were placed near the injector used to supply the liquid into the measuring cell. The experimental setup is shown in Figure 1. The measurements of the electric charge were carried out with an electrometer coupled to the flow-based system for feeding the sample into the measuring cell (4) (Figure 1). The main elements of the sample supply system were: the peristaltic pump, the pipe with a tip (the injector of part of the AFM-fishing biosensor system for supplying a solution), and the measuring cell. During measurements, water or protein solution from a 50-mL polypropylene tube (1) was continuously pumped through the tip (6) into the cell (4) with an Ismatec ISM 597D peristaltic pump (IDEX Corp., Lake Forest, IL, USA) (3). A sterile silicone pipe (2,5) (length 40 cm, inner diameter 2 mm) with a tip (6) was used to supply the analyzed liquid. The tip used was a standard disposable tip for an automatic pipette (inner diameter 0.4 mm, 10 \(\upmu\) nominal capacity, epT.I.PS, Eppendorf, Germany). The flow rate (~15 \(\upmu\)l/sec) was selected so that droplets would form on the tip nozzle. The calculated drop volume was ~15 \(\upmu\)l. To maintain the potential of the initial solution at a constant ground level, a ground electrode was inserted into the analyzed liquid in polypropylene tube (1). The stainless-steel cell (4) served as an internal cylinder in a system coupled to an electrometer (7). The dimensions of this cell were as follows: height 90 mm, inner diameter 75 mm, outer diameter 77 mm, and thickness of the polypropylene insert between the outer and the inner plates was 0.2 mm. In general, the system for charge measurements was similar to the one used in Reference [5]. The charge in the cell was registered using an electrometer (7) developed in Institute of Biomedical Chemistry (IBMC, Moscow, Russia). The charge of the studied materials (mica and polypropylene) was registered upon the introduction of these materials in the measuring cell of the electrometer. The charge registration accuracy was 0.1 nC. The influence of materials of AFM chips (mica and HOPG) and of material commonly used in fluidic communications (polpropylene) on the charge generation was investigated by comparative analysis of the charge generated in the injector (6) in the presence and in the absence of these materials. As a fragment of polypropylene tube, a 50-mL polypropylene tube, modeling a polypropylene communication, was used. The studied materials were placed at a certain distance \(l\) (4 mm or 1 cm) from the injector nozzle in position (8) (Figure 1). In Figure 1, (8) indicates the position of the chips (from mica or graphite) or the polypropylene tube relative to the injector nozzle. To study the influence of the external low-frequency electric field on the charge generation in the injector, the injector was placed between two parallel metal plates, and AC voltage (negative half sine wave, 100 V amplitude, 50 Hz frequency) was applied to the plates (similar to Reference [11]). The distance between the plates was 1 cm. A generator developed in IMBC was used to supply the AC voltage. The temperature of water and protein solution was maintained using a thermostat, where a tube (1) containing the analyzed liquid was placed. The experiments were performed at 35 \({}^{\circ}\)C. Prior to the measurements, the system was equilibrated under the experimental conditions for at least 2 h. The duration of one measurement was 5 to 7 min. The air humidity was 45%. The experimental series for each set of parameters included at least three repetitions. The sequence of the experiments was as follows. The system was filled with water from a test tube (1) and washed with \(\sim\)10 ml of analyzed liquid (water or protein solution).