Monitoring potential

Monitoring offers yet another way to improve the economics of cellulosic ethanol, by helping to ensure that any production process is working as efficiently and productively as possible. Various analytical techniques could perform that monitoring, including several forms of vibrational spectroscopy, such as near-infrared (NIR) and Raman, high performance liquid chromatography (HPLC) and mass spectrometry.

Unfortunately, there are also various challenges to performing this monitoring. One is that there are a wide range of factors that need monitoring during the production of cellulosic ethanol, including fluctuating concentrations of ethanol, sugars such as glucose and xylose, and inhibiting compounds such as furfural and acetic acid from the breakdown of the biomass.

Furthermore, the monitoring ideally needs to be performed continuously and in real-time, which tends to be difficult for many analytical techniques. Either they can only analyze discrete samples, as with HPLC, or they require any interfering, light-scattering particles to be filtered out first, as with vibrational spectroscopy. On top of this, the instruments required for these techniques tend to be quite expensive.

This explains why cellulosic ethanol biorefineries tend merely to monitor basic factors such as the temperature and pH of reactors, thus forgoing the benefits of ensuring their production process is always operating at optimum conditions. But that could change thanks to a team of researchers from the University of Brescia in Italy, led by Saad Abdullah, who have now developed a cheap, fully automated analytical device that can continuously monitor multiple factors in real time.

This device is based on a conventional piece of electronic equipment known as a potentiostat, which is designed to control three electrodes acting as a sensor for performing electrochemical techniques such as cyclic voltammetry and amperometry. These detect organic compounds from the current they generate when oxidized or reduced at a working electrode (in conjunction with reference and counter electrodes), with larger currents meaning greater concentrations.

Several research groups have already tried using potentiostats to monitor the concentration of various compounds in cellulosic ethanol bioreactors, including ethanol and glucose. To promote the necessary redox reactions, the working electrode is usually coated in enzymes such as glucose oxidase or alcohol dehydrogenase. The same enzyme-mediated approach should allow potentiostats to monitor many other organic compounds in bioreactors, such as furfural.

But the potentiostat developed by Abdullah and his team offers several additional innovations. Rather than just a single sensor, their potentiostat has space for six, which means it can simultaneously conduct six different analyses. They also added a chip to allow the potentiostat to communicate wirelessly by Bluetooth, so that it could constantly transmit data to an external computer for real-time analysis, rather than having to store the data for later analysis. This offers the possibility of placing the potentiostat, which is powered by a battery, inside a bioreactor for an extended period of time, during which it can continuously monitor several compounds and transmit the data in real time.

To test its abilities, Abdullah and his team used their potentiostat to monitor the simple fermentation of glucose into ethanol by the yeast Saccharomyces cerevisiae in test tubes, by constantly measuring the concentration of glucose. Taking advantage of their six sensors, they analyzed four test tubes at the same time, three of which contained different concentrations of yeast with the same concentration of glucose while the fourth was a control that just contained a glucose solution.

By performing this analysis twice, they were able to monitor test tubes containing six different yeast concentrations, from 0.1mL to 4mL. They inserted the four sensors for each analysis, with the working electrode of each set coated in glucose oxidase, into the tops of four separate test tubes, with each set connected to the potentiostat by a cable.

As Abdullah and his team report in a paper in Review of Scientific Instruments, they succeeded in continuously measuring the changing glucose concentrations in these test tubes for 24 hours. They found that, as expected, the concentrations steadily fell as the yeast fermented the glucose into ethanol, with the greatest falls occurring during the first five hours. Furthermore, the glucose concentrations fell faster in test tubes with greater concentrations of yeast. All of which confirms the great potential of this potentiostat.

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.