Fig 1.
(a) Schematic overview of key DStat components, including the computer (PC), the microcontroller (XMEGA), the analogue-to-digital converter (ADC), the digital-to-analogue converter (DAC), the potentiostatic circuit, and transimpedance amplifier. The DStat is interfaced to a three-electrode electrochemical cell, including a working electrode (WE), a counter electrode (CE), and a reference electrode (RE). Modules integral to DStat are coloured in green. Solid lines represent analogue connections. Dotted arrows represent digital connections. (b) Top-view picture of the DStat circuit board with labels corresponding to schematic components.
Fig 2.
Electrochemical cells and potentiostatic circuits.
RE: Reference Electrode, CE: Counter Electrode, WE: Working Electrode, R: Summing resistors, U1: Control amplifier, U2: Reference buffer amplifier, Rc: Compensated cell resistance, Ru: Uncompensated cell resistance. (a) Simplified three electrode cell model. (b) Basic potentiostatic circuit. (c) DStat potentiostatic circuit.
Fig 3.
Cell current conversion to voltage for ADC.
(a) Current measurement by shunt resistor. The measurement resistor RM causes a voltage drop proportional to the cell current i by Ohm’s Law. The voltage drop is measured across the resistor but the counter electrode voltage VCE (present on both sides of the resistor) complicates measurement. (b) Current measurement using a transimpedance amplifier. The measurement resistor RM is placed in a negative feedback loop of an op amp (U3) whose inverting input is connected to the working electrode. U3’s non-inverting input is tied to ground, producing a virtual ground at the inverting input. When current i flows through the working electrode, it induces a voltage drop VR across RM, which is balanced by U3 output VO to maintain the virtual ground at its inverting input.
Fig 4.
Open circuit DStat input-referred measurement noise standard deviations (SD) as a function of gain (set by the different DStat RMs) at a sample rate of 60 Hz measured over 60 s.
Blue diamonds are experimental measurements and green circles are TINA-TI simulation results. The equivalent input currents of a least significant bit (LSB) for three different ADC resolutions are shown as dashed red lines.
Fig 5.
Comparison of voltammetric measurements between instruments.
Arrows indicate scan directions. (a) Cyclic voltammetry (left) and square wave voltammetry (right) of 10 mM potassium hexacyanoferrate (III) collected using (commercial) screen printed electrodes. DStat (red traces), EmStat (green traces), and CheapStat (blue traces). Inset: picture of a screen printed electrode. (b) Differential Pulse Voltammetry of 4-aminophenol (10–100 μM) with DStat (left) and EmStat (right) collected using nanostructured microelectrodes. Curves were shifted vertically to align the current at -150 mV to 0 A to correct for changes in background current. Inset: picture of a nanostructured microelectrode.
Fig 6.
Solid arrows represent communication between the control computer and instruments over USB and dotted arrows represent communication between programs within the control computer over ZeroMQ. (a) When Dropbot’s control software μDrop reaches a programmed electrochemical measurement step, it pauses droplet actuation (in the figure, represented by a droplet parked at a circular electrochemical cell similar to the one described by Dryden et al. [31]) and requests an electrochemical measurement from the DStat software. The DStat software processes the request by initiating an experiment on the DStat hardware. (b) As the DStat hardware performs the experiment, the DStat software records the data. When the experiment is complete, the DStat software reports to μDrop that the measurement is complete and μDrop resumes its programmed droplet movement. A movie depicting the full process can be found in S1 Video.