Spurr epoxy

Single carbon fibers (7-μm diameter, T650; Cytec Carbon Fibers LLC, Piedmont, SC, USA) were aspirated into borosilicate glass tubes (0.4 mm ID, 0.6 mm OD; A-M systems Inc., Sequim, WA, USA). The tubes were pulled to a fine tip around the fiber (Narishige puller, Los Angeles, CA, USA) and sealed with low viscosity epoxy (Spurr Epoxy; Polysciences Inc., Warrington, PA, USA). The exposed fibers were trimmed to a length of 200 μm. A droplet of mercury connected the fiber to a hookup wire (annealed nichrome; Goodfellow, Oakdale, PA, USA). The electrodes were soaked in isopropanol for at least 15 minutes prior to use (Bath et al., 2000 (link)).

CFEs were fabricated as previously described in ref. 51 (link) and 52 (link). Briefly, borosilicate capillaries (0.4 mm ID, 0.6 mm OD; A-M systems Inc., Sequim, WA, USA), each containing a single carbon fiber (7 μm diameter, T650; Cytec Carbon Fibers LLC., Piedmont, SC, USA), were pulled to a fine tip using a vertical puller (Narishige, Los Angeles, CA, USA). The tip was sealed with epoxy (Spurr Epoxy, Polysciences Inc., Warrington, PA, USA) and the exposed fiber was cut 400 μm from the glass seal using a scalpel under an optical microscope (Szx12, Olympus). A mercury drop was placed in the barrel for electrical contact to a hookup wire (Nichrome; Goodfellow, Oakdale, PA, USA). CFEs were soaked in isopropyl alcohol^{53 (link)} (Fisher Chemical, USA) for 20 minutes prior to use.
The electrochemical properties of the CFE were evaluated in vitro in artificial cerebral spinal fluid (aCSF, 142 mM NaCl, 1.2 mM CaCl₂, 2.7 mM KCl, 1.0 mM MgCl₂, 2.0 mM NaH₂PO₄, pH 7.4) by electrochemical impedance spectroscopy (EIS). During the EIS measurements, a sine wave (10 mV RMS amplitude) was superimposed onto the open circuit potential while varying the frequency from 1 to 105 Hz. EIS were carried out using a potentiostat/galvanostat (Autolab, Metrohm, USA) connected to a three-electrode electrochemical cell with a platinum counter electrode and an Ag/AgCl reference electrode.

Single carbon fibers (7 μm diameter, T650; Cytec Carbon Fibers LLC, Piedmont, SC, USA) were thread into borosilicate glass capillaries (0.4 mm ID, 0.6 mm OD; A-M systems Inc., Sequim, WA, USA) and pulled to a fine tip (8 μm OD) using a vertical electrode puller (Narishige puller, Los Angeles, CA, USA). Electrode tips were sealed using low viscosity epoxy (Spurr Epoxy; Polysciences Inc., Warrington, PA, USA). The exposed carbon fibers were trimmed to 400 μm length and electrical connection was established via a drop of mercury and a single nichrome wire (annealed nichrome; Goodfellow, Oakdale, PA, USA). Fully assembled electrodes were soaked in isopropanol (Bath et al. 2000 (link)) for 15 minutes prior to bare electrode calibration, prior to PEDOT/GO coating and prior to calibration post PEDOT/GO coating.