Non-Enzymatic Electrooxidation of Urea with a Disposable Paper-Based Sensor Using Hierarchical NiO Hollow Sphere
Authors: Lee Doohee , Yoon Jaesik , Lee Eunji , Yoon Young Soo , Kim Dong-Joo 
Published: February 2020

Urea in the urine, protein metabolism, is present in the final product of human. The level of urea is a crucial biomarker that can assess the metabolic activity of humans such as the liver and renal function. [1] A high level of urea in the urine can result in kidney failure, dehydration, and gastrointestinal bleeding, while a low level can lead to nephritic syndrome, hepatic failure, etc. [2] Therefore, accurate and rapid sensing of urea is essential in a diagnostic clinic.
Recently, paper-based analytical devices (PAD) are gaining its popularity as a manageable device for point-of-care equipment due to its inexpensive, portable, disposable, and environmentally-friendly qualities.[3] The majority of electrochemical biosensors rely on enzymes, such as urease; yet, utilization of enzyme is limited due to its high cost, poor reproducibility, and most of all, thermal and chemical instability which can influence the duration of sensor that depends on storage conditions such as temperature, pH, and humidity.[4] In addition, non-enzymatic biosensors has been attracted with the advantage of simplicity, redox flexibility, stability, and low cost.
As a non-enzymatic metal-based material, Ni-based nanomaterials exhibited excellent catalytic activity over urea originated from catalytic effect for the formation of the redox couple of Ni(II) and Ni(III) on the surface. [5] When compared to enzymatic-based sensors, non-enzymatic catalysts tend to present lower sensitivity and higher limit of detection corresponding to lower performances of urea biosensor. To address this limit, there have been efforts to modify nanostructures with various shapes such as nanofiber, nanoparticle, and nanoflake arrays. Here we investigated a hollow sphere structure with nanosheet building block by using a hydrothermal method. The hollow sphere structure can increase the selectivity and sensitivity for urea biosensor by providing facile transport channels through electrolyte that exploits its inner and outer surfaces as the active site with maintaining the structural stability of the catalyst. Integration of non-enzymatic hierarchical catalyst on disposable PAD will lead to a high potential for the development of point-of-care testing devices.
In this study, a non-enzymatic electrochemical paper-based sensor using hollow structure NiO was developed and demonstrated for the determination of urea. PADs were fabricated by sputtering on cotton paper, and hollow NiO was synthesized by hydrothermal method. Structure and morphologies of hierarchical NiO on PAD were characterized by XRD and SEM. Electrochemical properties such as selectivity and LOD were conducted by using a potentiostat with urea solution. A detailed description of the fabrication process and discussion of structure formation and sensing performances of hollow nickel oxide catalyst will be presented.

References

[1] Boggs, Bryan K., Rebecca L. King, and Gerardine G. Botte. "Urea electrolysis: direct hydrogen production from urine." Chemical Communications 32 (2009): 4859-4861.
[2] Srivastava, Saurabh, et al. "A self assembled monolayer based microfluidic sensor for urea detection." Nanoscale 3.7 (2011): 2971-2977.
[3] Martinez, Andres W., et al. "Patterned paper as a platform for inexpensive, low‐volume, portable bioassays." Angewandte Chemie International Edition 46.8 (2007): 1318-1320.
[4] Riklin, Azalla, et al. "Improving enzyme–electrode contacts by redox modification of cofactors." Nature 376.6542 (1995): 672.
[5] Nie, Huagui, et al. "Nonenzymatic electrochemical detection of glucose using well-distributed nickel nanoparticles on straight multi-walled carbon nanotubes." Biosensors and Bioelectronics 30.1 (2011): 28-34.

Acknowledgement

This work was partially supported by Agency for Defense Development (ADD) as global cooperative research for high performance and light weight bio-urine based fuel cell (UD160050BD), and the Ocean University of China-Auburn University (OUC-AU) Grants program.