Performance of Conducting Polymers Electropolymerized Under Various Conditions for Redox–Magnetohydrodynamics (R-MHD) Pumping
Authors: Khan Foysal Z , Fritsch Ingrid 
Published: February 2020

Poly(3,4-ethylene dioxythiophene), PEDOT, was electropolymerized under different electrochemical conditions onto electrodes from two different electrolytes, tetrabutylammonium hexafluorophosphate (TBAPF6) and lithium perchlorate (LiClO4) in propylene carbonate (PC). Films were characterized by cyclic voltammetry, chronoamperometry, chronopotentiometry, and electrochemical impedance to determine performance suitable for redox-magnetohydrodynamics (MHD) microfluidics. Scanning electrochemical microscopy revealed information about morphological features. Maximizing charge capacity and current density of such modified electrodes provides longer MHD pumping times and faster fluid speeds, respectively, which in turn allows for a wider range of possible lab-on-a-chip applications for this microfluidics method.

MHD is a non-mechanical pumping approach where fluid flows between electrodes by a net body force (
FB

), generated by the equation FB = j × B, where j is ionic current density and B is magnetic flux density.
1,2
It offers unique advantages that complement other microfluidics methods, such as flat flow profiles, tunability, portability, circular flow, and low voltage requirements.
3
The electronic current or potential applied at the electrodes generates ionic current by oxidizing and reducing redox species in the cell. Contamination to analyte in the surrounding solution can be avoided by immobilizing redox species on the electrode surfaces, which also offers fast access of electrodes to high concentrations of charge and therefore high current and high fluid velocity. PEDOT has been used successfully by us for this purpose because it generates MHD flow in the presence of a magnetic field while maintaining good chemical stability, reversible doping states, and a low redox potential.
4,5 
Its monomer, EDOT, is also commercially available. PEDOT performance can vary widely, depending on (1) the conditions under which it is electropolymerized and (2) electrolyte environments in which it is used for MHD pumping. Results will be presented that address both of these points.

Electrochemical deposition was chosen because it requires a small amount of monomer, forms well adherent films, incorporates dopant ions as needed, and allows programmability of deposition time.
6
Film efficiency can be tuned by optimizing deposition factors, such as electrodeposition conditions (cycled or stepped potential or current, duration, and number of cycles), solvents, electrolytes, monomer types, and concentrations.
7,8
In this study, primarily cyclic voltammetry was investigated for deposition in organic solvents that dissolve monomers more easily than aqueous solutions, and which generate thick, well adherent, and highly conductive films.
9 
PEDOT was deposited on gold microband electrodes (1.5 cm × 650 μm × 0.250 μm) for MHD experiments and electrochemical characterization. Additional studies involving potentiostatic and galvanostatic electrodeposition of other viable thiophene based monomers (e.g. ProDOT (3,4-Propylenedioxythiophene) and hydroxymethyl-EDOT) will also be described for charge and current maximization.

Propylene carbonate (PC) has been shown to be a suitable solvent for EDOT electrodeposition. Its high boiling point, compared to other organic solvents (e.g acetonitrile), avoids evaporation, and therefore maintains a stable monomer concentration in small volumes for long periods of time. Maximum current densities (acquired in the same aqueous electrolyte solution) for films electrodeposited from different electrolytes (LiClO4 and TBAPF6) in PC were within error of each other, but the charge densities depended on the potential range of electrodeposition. In MHD experiments, this performance of the polymers translated into similar fluid speeds and different pumping durations. The maximum achievable current density depended greatly on the type and ionic strength of the electrolyte solution in which the films were characterized having the following increasing order: 0.1 M NaCl (x) < 0.01 M PBS (1.36x) < 0.1 M PBS (5x). Poly-ProDOT films deposited from TBAPF6in PC exhibited lower current density and lower charge density in aqueous electrolytes than their PEDOT counterparts by factors of 1.65 and 2.55 times, respectively.

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