Integrated Electrochemical Microfluidic Platform

Voltammetry experiments were performed with an Autolab
PGSTAT204
potentiostat running on Nova 2.1 (Metrohm Nordic AB, Sweden). We used
the following electrodes purchased from Metrohm Nordic AB (Sweden):
screen-printed carbon electrode (DRP-150), glassy carbon working electrode
(GCE; diameter of 5 mm, RDE.GC50.S), Pt electrode (6.0331.010), and
a Ag/AgCl single-junction reference electrode (EQCM. refEL.S). A Mo
plate was also used as an electrode (Sigma-Aldrich, 357200-25.6G).
Electrochemical measurements in “beaker configuration”
were performed using a three-electrode system with the GCE as the
working electrode (WE), the Ag/AgCl single-junction reference electrode
(RE), and the Pt rod as the counter electrode (CE). For the experiments
conducted in the microfluidic cell (Figure 2a), three types of electrodes were prepared
and/or utilized. Electrode 1 was the DRP-150 with a carbon-based WE
modified with a film of electropolymerized PANI (150 CV scans in 0.1
M aniline/0.5 M H₂SO₄). Electrode 2 was the
Mo plate modified with a custom-made screen-printed carbon electrode
(SPCE). Electrode 3 was an unmodified DRP-150. More details about
the preparation of the electrodes are presented in the Supporting
Information. Figure S1 presents a real
picture of the device.
The fluidic cell was designed in AutoCAD
2020 and printed with
an Ultimaker 3 3D printer (Ultimaker B.V., the Netherlands) using
PLA filament (Ultimaker, the Netherlands). The rubber spacers (thicknesses
of 0.5 and 1 mm) were purchased from (Ecoflex, USA) and cut to an
appropriate size to create the microfluids for the cell. The cell
is composed of two internal compartments: the first compartment is
situated between Electrode 1 and Electrode 2, while the second is
situated between Electrode 2 and Electrode 3. The WEs of actuator
1 and actuator 2, the sensor, and (a common) RE are placed in the
first compartment, while the common CE (the carbon part of the DRP-150;
Electrode 3) is placed in the second compartment. The Ag element in
the DRP-150 in Electrode 1 acts as the common RE. The PANI-C element
of Electrode 1 acts as the WE of actuator 1 and acidifies the sample
upon the application of 0.4 V for 300 s. The Mo element in Electrode
2 acts as the WE of actuator 2 and delivers MoO₄^2– and H⁺ by
applying a current of 0.15 mA for 300 s. Finally, the C path in Electrode
2 acts as the WE of a DIP sensor that uses either CV or SWV. The potential
was switched in the cathodic direction to ensure the partial reduction
of Mo(VI) centers in the phosphomolybdate complex to Mo(V).