Enzyme-Based Electrochemical Lactate Biosensors Capable of Operating in Low-Oxygen Environments
Authors: Uzunoglu Aytekin , Stanciu Lia 
Published: February 2020

The level of lactate in human blood is used to diagnose various pathological conditions including cardiogenic or endotoxic shocks, respiratory failure, liver disease, systematic disorders, renal failure, and tissue hypoxia. It is also used to determine the training status in sport medicine because the lactate level is associated with the anaerobic metabolic process in muscles. The detection of lactate using electrochemical biosensors has attracted great deal of interest due to their high sensitivity, selectivity, and simplicity. Lactate oxidase, an enzyme oxidizes lactate in the presence of molecular oxygen, is widely used to construct the electrochemical lactate biosensors in the literature. Although its high sensitivity and selectivity, the use of lactate oxidase enzyme makes the biosensors vulnerable to oxygen fluctuations in the medium, which results in errors in the measured lactate level. To reduce the oxygen dependency of lactate oxidase-based electrochemical biosensors, we modified the surface of the sensors with oxygen-rich CeO2-ZrO2 nanoparticles. In the first step of this work, CeO2-ZrO2 mixed metal oxide nanoparticles with large specific surface area and oxygen storage capacity were synthesized. Then, the surface of the biosensors was decorated with the nanoparticles which both acted as oxygen-buffers in O2-depleted medium and provided large surface area for the immobilization of the enzyme layer. The performance of the biosensors was evaluated in both O2-rich and -depleted environments. Our results indicated that high oxygen storage capacity of CeO2-ZrO2 nanoparticles can be exploited to reduce the oxygen dependency challenge. The constructed biosensors had a high sensitivity of 115.5 ± 1.4 µA*mM-1*cm-2 towards lactate with a wide linear range from 0.02 to 0.75 mM. It was observed that while unmodified biosensors lost their ability to detect lactate in O2-depleted solutions due to their high oxygen dependency, the CeO2-ZrO2-containing biosensors were not influenced by O2 concentration significantly, enabling lactate detection in low oxygen environments.	The level of lactate in human blood is used to diagnose various pathological conditions including cardiogenic or endotoxic shocks, respiratory failure, liver disease, systematic disorders, renal failure, and tissue hypoxia. It is also used to determine the training status in sport medicine because the lactate level is associated with the anaerobic metabolic process in muscles. The detection of lactate using electrochemical biosensors has attracted great deal of interest due to their high sensitivity, selectivity, and simplicity. Lactate oxidase, an enzyme oxidizes lactate in the presence of molecular oxygen, is widely used to construct the electrochemical lactate biosensors in the literature. Although its high sensitivity and selectivity, the use of lactate oxidase enzyme makes the biosensors vulnerable to oxygen fluctuations in the medium, which results in errors in the measured lactate level. To reduce the oxygen dependency of lactate oxidase-based electrochemical biosensors, we modified the surface of the sensors with oxygen-rich CeO2-ZrO2 nanoparticles. In the first step of this work, CeO2-ZrO2 mixed metal oxide nanoparticles with large specific surface area and oxygen storage capacity were synthesized. Then, the surface of the biosensors was decorated with the nanoparticles which both acted as oxygen-buffers in O2-depleted medium and provided large surface area for the immobilization the enzyme layer. The performance of the biosensors was evaluated in both O2-rich and -depleted environments. Our results indicated that high oxygen storage capacity of CeO2-ZrO2 nanoparticles can be exploited to reduce the oxygen dependency challenge. The constructed biosensors had a high sensitivity of 115.5 ± 1.4 µA*mM-1*cm-2 towards lactate with a wide linear range from 0.02 to 0.75 mM. It was observed that while unmodified biosensors lost their ability to detect lactate in O2-depleted solutions due to their high oxygen dependency, the CeO2-ZrO2-containing biosensors were not influenced by O2 concentration significantly, enabling lactate detection in low oxygen environments.