Chi 660a

We mostly adopted an RED stack configuration developed by M. C. Hatzell and B. E. Logan (Fig. S10)35 . Briefly, silicon gaskets separating the membranes and create a flow path between the membranes were cut to provide a cross section area (8 cm² for the commercial membranes, CMV and AMV, and 3.14 cm² for the PAMs) and the gasket thickness was ~1.3 mm. Power measurements were conducted based on chronopotentiometry (CHI660a, CH Instrument). Two platinum electrodes at the end of the RED stack introduced current into the system while two Ag/AgCl reference electrodes read out the potential of the membranes. All measurements were carried out without spacers and with 5 mL/min flow rate. The pumping energy for the solution supplies was disregarded in calculating power density.

The hPPy was prepared based on electrochemical polymerization using a potentiostat (CHI-660a, CH Instruments, Austin, TX, USA) controlled by Nova software. The electrochemical measurements were performed using a homemade three-electrode system consisting of a bare Au electrode as a working electrode, a platinum wire as the counter electrode, and an Ag/AgCl reference electrode at a scan rate of 100 mV/s. The morphologies of the hPPy films were studied using field emission scanning electron microscopy (FESEM). The FESEM images were recorded using the ISI DS-130C instrument (Akashi Co., Tokyo, Japan). For better capturing the SEM images of samples, the substrates were fixed on the SEM stage with carbon tapes. Pt films were deposited onto the surface of the substrate at room temperature. The sputtering deposition was performed for 15 s under a constant deposition rate. Then, the substrates were being placed into the FESEM chamber. For the cross-section image, a 45-degree stage was used.

Cyclic voltammetry (CV), square-wave voltammetry (SWV), and impedance spectroscopy (EIS) were performed using CHI 660A, 660D, and 1040A electrochemical workstations (CH Instruments, Bee Cave, TX, USA). The experiments were executed using a three-electrode system containing a working electrode printed with various carbon-based pastes, an Ag/AgCl/1.0 molL-1 KCL reference electrode (Mineral, Łomianki, Poland), and a gold wire that served as the auxiliary electrode (Sigma Aldrich, Darmstadt, Germany). All the electrochemical studies were performed at room temperature. Cyclic voltammetry was conducted with the potential window from -0.6 to 1 V at scan rates from 5 to 0.05 Vs⁻¹, or 0.1 Vs⁻¹. Square-wave voltammetry was executed at a pulse amplitude of 15 mV, an increment of 4 mV, and a frequency of 15 Hz. EIS measurements were performed at a DC potential of 0.2 V and an amplitude of 0.005 V in the frequency range from 1 to 100 kHz. The electrochemical measurements were conducted using a redox indicator −5 mM ferri/ferrocyanide in 0.1 M KCl solution.

Electrochemical experiments on the fabricated SNP modified electrode covered by graphene oxide were performed using a potentiostat (CHI-660A, CH Instruments, Inc., Austin, TX, USA) to confirm the electrochemical signal enhancement. CV, DPV and the amperometric i-t method were performed using a three-electrode system composed of the SNP-modified electrode covered by graphene oxide as the working electrode, a platinum (Pt) wire electrode as the counter electrode and a silver/silver chloride (Ag/AgCl) electrode as the reference electrode. The electrochemical buffer solution (PBS solution) was used for the electrochemical measurement. The parameters of this experiment were a sensitivity of 1.0 × 10⁻⁵ A/V, scan rate of 100 mV/s, quiet time of 2 s and sample interval of 1 mV. The range of the applied potential voltage for CV and DPV was from −0.2 V to 0.6 V. Considering the amperometric i-t technique, −0.3 V, 0.1 s and 1.0 × 10⁻⁵ A/V were used for the respective initial potential, sampling interval and sensitivity. Chemical neurotransmitters, namely dopamine, uric acid and ascorbic acid, were dissolved in PBS at various concentrations of dopamine and 50 μM of both uric acid and ascorbic acid.

Membrane potential was measured from the potential difference between a pair of Ag/AgCl reference electrodes that were in two compartments divided by the membrane of interest (Fig. S1). A potentiostat (CHI660a, CH Instrument) read an open circuit potential (OCP) from two reference electrodes immersed in 0.017 M and 0.510 M NaCl solutions. The volume of reservoirs was 10 mL and the diameter of membrane was 2 cm. OCP was measured once the membrane potential reached equilibrium.