Low-Cost Portable Potentiostat for Biosensing Applications Using Atmega644

Introduction

This project involves the design and construction of a low-cost portable potentiostat capable of performing cyclic voltammetry on three-electrode electrochemical systems.

A potentiostat is an instrument used in chemical and biological tests that controls the voltage between two electrodes, working and reference, at a constant value. Chemical and biological tests are typically run in a three-electrode system, which includes the aforementioned working and reference electrode as well as a counter electrode. These systems are used to test the electrical activity of certain compounds or microbes, where the electrode acts as either the electron acceptor or electron donor. By monitoring the current and plotting the data against either time (chronoamperometry) or potential (voltammetry), information can be obtained as to the electrochemical activity of chemical compounds and/or microbes. For cyclic voltammetry, the potential is scanned in both the positive and negative directions at a predefined sweep rate (mV/sec), allowing the user to view both the oxidation and reduction reactions occurring. Commercially available potentiostats, while capable of performing additional techniques, often cost upwards of $5000 per channel. In addition, these commercial instruments are typically impractical for use in field research. For bio-sensing and other applications it is important for a low-cost, portable, field ready potentiostat to be available.
In this project, we will present a design for this potentiostat using the ATMega644, an external digital-to-analog converter (DAC), and a series of operational amplifiers (op-amps). We will utilize the serial peripheral interface (SPI) to communicate between the microcontroller and DAC, the op-amps to process the signal from the DAC and apply a potential to the electrochemical cell, and the internal analog-to-digital converter (ADC) to record the current at the working electrode.

High Level Design

Potentiostats are the main analytical instrument used in the field of electrochemistry as well as the emerging area of bioelectrochemical systems (BES). One area of BES is bio-sensing, which uses biological activity as a benchmark, has applications in environmental monitoring, medical diagnostics, and food safety assurance, among others. The idea of a microcontroller based potentiostat is presented by Gopinath and Russell (2006) using a PIC18F452 microcontroller. Our goal was to design a similar system using the ATMega644, while making some modifications to increase functionality.
Parameters including start and end potential, scan rate, and number of scans will be entered onto an alphanumerical keypad via an interface with the hyperterminal. Once all parameters are input the user signals the potentiostat to begin the scans. The applied voltage and current will be recorded and printed on the hyperterminal. Upon completion the system will remain idle scanning the keypad for user input.
The following is a brief description of some of major components that we used in our hardware design of the system.

1. AD7303 – Digital to Analog Converter (DAC)

The purpose of having a DAC in this project is mainly to create a triangular waveform. The DAC will start its operation once the user has inputted the parameters required to run the sweeping voltage. One problem that we face using this DAC is that we do not have enough resolution since it is only an 8-bit DAC (even though it has two channels).
The tricky parts of our project is to get the SPI working the way we wanted it to be. We started the project working with MAX5354 from MAXIM-IC, which has a much better resolution than AD7303, but were not able to get it working. Despite of the fact that we were able to get the clock signal (CLK), chip select (CS), and data in (DIN) to work, we did not get anything out of the MAX5354.
After working with MAX5354 for a while with no solution, we finally decided to use the two channel, 8-bit DAC (AD7303) from the Analog Devices, and we were able to produce the triangular waveform that we had been trying to get.

2. LM148 – Quad op-amp

The LM148 operational amplifier from National Semiconductor features low bias and offset current, as well as a high gain. The main advantage is that there are four isolated op amps in each package LM148. This simplifies circuitry, reduces overall price, and only requires the LM148 to be powered with ± 12V rather than each individual op amp. In our potentiostat it is used to process the signal from the AD7303 and feedback from the reference electrode to apply a potential on the counter electrode. The LM148 also processes the signal from the working electrode so that it can be read by the ATMega644’s internal analog to digital converter (ADC).
The AD7303 output gives a voltage between zero and three volts, which is then shifted down one volt by the first and second op amps (OA-1 and OA-2). This shift is variable by changing the potentiometers, but any change must be recalibrated in the code to ensure proper voltages. The third op amp (OA-3) sums the output of OA-2 with the reference voltage from OA-5. In this summation the sign is inverted, and the final op amp, op amp four (OA-4), inverts the signal from OA-3 back to the correct sign. The output pin of OA-4 is connected to the counter electrode.
Op amp six (OA-6) converts the current at the working electrode into a voltage. A precision resistor (1 kΩ, 1%) is used as feedback between the output and inverting input. In this conversion, the voltage is inverted, so a second op amp (OA-7) is used to revert the sign. The output of OA-7 is hooked to pin A.0, which is assigned as the input for the ATMega644’s ADC.

Parts List:

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