Pt wire

A miniaturized EC-SERS flow cell was designed and fabricated to accommodate a standard three-electrode electrochemical system: a leakless Ag/AgCl reference electrode (LF-1-45 from Innovative Instruments Ltd), a Pt wire (Sigma-Aldrich) counter electrode, and a removable MLagg SERS substrate on FTO-coated glass as the working electrode (Fig. 1c). The internal volume of the flow cell was 26 µL. The EC-SERS flow cell and a three-inlet/one-outlet mixer module were fabricated with PDMS using 3D-printed moulds. The EC-SERS flow cell was sealed and mounted onto the stage of an inverted Raman set-up using custom 3D-printed holders and bases.
Custom-built syringe pumps were used to control the flow of solutions (buffer, CB[5] in buffer, and analyte in buffer) through the EC-SERS flow cell (Supplementary Fig. 4). Electrochemical measurements were conducted using a portable potentiostat (CompactStat) from Ivium Technologies. All potentials were referenced to the Ag/AgCl reference electrode. The syringe pumps, electrochemical measurements, and SERS spectra collection were all controlled and synchronized with Python scripts.

For the bipolar electrode experiments, the driving electrodes consisted of Pt wires (99.9%, Sigma-Aldrich, Taufkirchen, Germany) located at the inlet and outlet of the chip. They were connected to a power supply (2401 Keithley Source Meter) via external contacts to apply a potential difference between the electrodes. The voltage/current settings used for the deposition of bipolar electrodes are V = 1.3 kV and I = 10 mA, respectively, speed of printing of 10 µm/s, and nozzle-substrate distance of 500 µm. Before the alignment with the control layer, the resistance of the Pt electrodes was measured using a multimeter (Fluke 179), providing a conductivity value ≈ 5.6·10⁶ S/m.
The fluorescent experiments were carried out with a mixture of 100 µM Fluorescein sodium salt (FL) (Sigma-Aldrich) and 1.0 mM sodium phosphate buffer at pH 7.2. The solutions were prepared using deionized water (<18.2 MΩ cm, PURELAB flex). All solutions were maintained away from light by covering the vials with aluminum foil when not in use. For the image acquisition, a microscope Olympus IX51 and a Grasshopper^®3 (FLIR, US) color camera were used with a pE300^ultra LED illumination system (CoolLED, Andover, UK).

A microfluidic device comprised of a single microchannel in a poly(methyl methacrylate), PMMA, chip with a pair of vertical platinum (Pt) side-wall electrodes (Figure 1A) was fabricated using methods adapted from Adams et al. [32 (link)]. The microfabrication procedure involved micromilling (KERN MMP 522, Kern Microtechnik GmbH, Murnau, Germany) holes in a 3.2 mm thick PMMA sheet (GoodFellow Corp, Pittsburgh, PA, USA). Pt wires (76 μm, Sigma Aldrich, St. Louis, MO, USA) were threaded into the holes in the PMMA and hot-embossed at 160 °C for 4 min to embed the wire into the polymer. A single 50 μm-wide microchannel was then milled through the PMMA and wire, orthogonally to the wire orientation, to create access for fluid samples to be passed directly between the two cut edges of the wire that comprise sidewall electrodes (Figure 1C).