Background and Purpose

We needed a potentiostat; a device capable of controlling the electrode potential and applying voltage for a long period of time, in order to carry out experiments under the regulation of genetic circuits by electrical stimulation. However, the commercial electrochemical analyzer was not affordable. Thus, we decided to make our own potentiostat, which was inexpensive but had the minimum functions described above . The potentiostat was used in the experiments and helped us obtain our data.

The aim of this page is to help iGEM teams consider electrogenetic experiments to obtain potentiostats affordably and to develop their work in electrogenetics. The principle of potentiostat is briefly explained, and the assembled electrical circuit diagram is shown. The function of the created device is presented and some suggested improvements are listed at the end.

Requirement and Design

The basic function of the potentiostat is shown below.

Regulates the potential difference between the working electrode (WE) and the reference electrode (RE) to be constant
Prevents current flow to the RE
Measures the current flowing to the WE

To achieve them, a circuit element called an operational amplifier (op-amp) is used. The op-amp is indicated by triangles in the circuit diagram, represented by two input terminals (+ and -) and one output terminal. The function of the op-amp is that there are no current flows into or out of the two input terminals and that the potential difference between the two input terminals is zero.

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Fig. 12.1 Circuit diagram of our potentiostat.

In Fig. 12.1, function 1 is implemented in part A, function 2 in part B, and function 3 in part C. Part C is called a non-resistive ammeter and can measure currents of ten several µA. The current value $I$ flowing in WE can be calculated by $I = E_\text{curr} / R_\text{C}$ using the resistance $R_\text{C}$ and the potential difference $E_\text{curr}$ from GND. In part D, the potential difference $E_\text{RE}$ between WE and RE is adjusted by changing the resistance value of the variable resistor. $E_\text{RE}$ can be measured by the potential difference from GND.

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Fig. 12.2 Appearance of the potentiostat created.

Fig. 12.2 shows the appearance of assembled potentiostat based on Fig. 12.1. The op-amp used is the Texas Instruments OP07 Precision Operational Amplifier. The device consists of two main parts: the circuit part, which has a three-layer structure, and the power supply part, which is composed of six AA alkaline batteries (9 V). The upper layer of the circuit section contains switches, variable resistors, and other components for operating the device. The middle and lower layers housen the potentiostat circuit. Three clips connect to the WE, RE, and counter electrode (CE), respectively. By appropriately connecting voltmeters to the exposed wires on the circuit board, the potentials of $E_\text{curr}$ and $E_\text{RE}$ can be measured.

Function

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Fig. 12.3 Applying voltage and measuring with the potentiostat.

Fig. 12.3 shows the connection of each electrode and potentiostat to a H-shaped cell (EC Frontier VB9 H-shaped cell). The following electrodes were used: the EC Frontier MPT Platinum mesh electrode as the WE, the ALS Electrochemistry & Spectroelectrochemistry RE-1B Reference electrode (Ag/AgCl) as the RE, and the ALS Electrochemistry & Spectroelectrochemistry Platinum counter electrode (23 cm) as the CE. LB medium was used as the solution.

The results of potential control and current measurement using the constructed potentiostat are as follows. By adjusting the resistance of the variable resistor, $E_\text{RE}$ could be controlled within a range of 1.34 V to 1.88 V when the WE was the cathode, and from −0.60 V to −0.21 V when the WE was the anode. However, the value of $E_\text{curr}$ could not be measured due to the noise from the circuit itself or peripheral devices, causing instability. Although a capacitor was incorporated to reduce noise, measurements were not successful.

Future

Here are some suggestions for improving this potentiostat. One potential improvement is the automatic recording of $E_\text{curr}$ . In our case, we only displayed $E_\text{curr}$ directly, however by recording a time series, it might be possible to statistically process the data and remove noise. Another possible improvement is revising the circuit design. Adding circuits to eliminate noise at the position of the non-resistance ammeter or physically isolating the WE circuit could potentially reduce the noise observed in $E_\text{curr}$ .