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Development of biofuels hinges on the ability to convert lignocellulose, the “woody” cell membrane that comprises most plant material, into something that’s useable as fuel. One method is to use enzymes to break down the cellulose into sugars, which can then be converted into ethanol through fermentation and distillation.

Because different plants have different cell-wall structures and compositions, scientists need to understand those differences and how efficiently enzymes are able to break the lignocellulose into simple sugars. To do this, Ames Laboratory scientist Emily Smith uses a characterization tool called Raman spectroscopy, which is ideally suited to working with such materials.

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“This method has several advantages over alternative analytical techniques,” Smith explains. “First, analysis requires very little material so you can take small samples from a growing plant over time with little damage to  the plant.”   The technique is also high-throughput. Because only very small samples are needed and little time is required to prepare samples, multiple samples can be analyzed quickly.

The basic Raman technique used by Smith, who is also an Iowa State University assistant professor of chemistry, employs an optical microscope, and specimens are illuminated with a laser beam.  As the laser light hits the sample, some of the light is scattered.  By analyzing the scattered light with a spectrometer, Smith can easily and quickly determine the chemical makeup of the plant material.

“Raman spectroscopy provides chemical signatures of  what’s in your sample,” Smith says. “We can focus on the signature of the particular species we’re interested in, for example lignin or cellulose.”

Because lignin binds the cellulosic polymers together, the higher the lignin content of the cell wall, the harder it is to access the cellulose. For some biofuel applications, plants with a higher cellulose-to-lignin content are more ideal.

Smith’s group is also using Raman spectroscopy to measure the enzymatic reactions used to break down the cellulosic polymers into sugars.

“Because the enzymes used can be expensive, it’s important to determine what conditions are optimum to produce the highest yield,” Smith says.

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A schematic for the scanning angle total internal reflection microscope that
Emily Smith displays in the photo above.

Besides the type of plant material used, variables include the maturity of the plant material, the size of the pieces being processed, pretreatment of the material, and the type and amount of enzyme used.

Also being studied is a hydrolysis process that uses enzymes attached to silica nanoparticles. This allows the nanoparticles and their attached enzymes to be recycled.  It also increases the yield of ethanol when saccharification and fermentation reactions are performed simultaneously.

Smith’s work has also focused on developing new instrumentation and uses for it. She is currently developing a different type of Raman technique known as scanning angle, total internal reflection Raman microscopy.

“Basically, it allows us to control the angle that the laser is directed on the sample,” Smith says. “By changing the incident angle, we hope to be able to develop a 3D image of the sample’s chemical content.”

Motorized controls allow the incident angle of the laser to range from 25.5 degrees to 75.5 degrees with a .05 degree angle resolution, giving researchers the capability to study the materials with 5 nanometer spatial resolution.

At an even more fundamental level, Smith’s group is looking at how the spectral signal is generated and how to enhance the signal strength.

“When I first arrived, I wasn’t expecting to do anything related to biofuels, but there is such an interest in them and a wealth of expertise here and at Iowa State that it’s really taken off,” Smith said. “As we’ve worked on that, our understanding of these techniques has grown and it’s brought up all kinds of new research projects for us to work on.”

~ by Kerry Gibson