IR-Compatible PDMS microfluidic devices for monitoring of enzyme kinetics

Coupling infrared (IR) spectroscopy to microfluidic devices provides a powerful tool for characterizing complex chemical and biochemical reactions. Examples of microfluidic devices coupled with infrared spectroscopy have been limited, however, largely due to the difficulties associated with fabricating systems in common infrared transparent materials like CaF₂. Recent reports have shown that polydimethylsiloxane (PDMS) can be used as an IR transparent substrate when fabricated with thin layers. The use of soft lithography with PDMS expands the library of possible designs that can be achieved for IR measurements in microfluidics. In initial reports with thin PDMS, the target analytes were small molecules; however, IR spectroscopy offers a powerful tool to study protein structure and reactions. Here, a PDMS microfluidic device compatible with IR spectroscopy was fabricated by means of spin-coating of PDMS pre-polymer to obtain thin PDMS microfluidic features. The device was comprised of only PDMS and IR absorption of PDMS was significantly minimized due to the thickness (∼40 μm) of the PDMS layer. The use of thin PDMS allowed for measuring the amide I and II vibrational bands of proteins that have been difficult to measure in other microfluidic devices. To demonstrate the power of the system, the microfluidic device was successfully used to measure the enzyme kinetics as one class of important biochemical reactions with broad use in a variety of fields from medicine to biotechnology. As a model, the reaction of glucose oxidase with glucose was tracked by following the formation of gluconic acid. Michaelis-Menten kinetics from the device were compared with bulk solution measurements and found to be in good agreement.