Urea has attracted attention because of its various potential applications such as hydrogen production, fuel cells, fertilizers, and electrochemical sensors. [1] Their long-term usages can lead to soil acidification and eutrophication, disturbing the ecosystem.[2] As an end-product of human metabolism, urea is a crucial biomarker that can access various human disorders such as kidney and renal function. Thus, the rapid sensing of the urea level in urine can play an important role in diagnostic areas, especially point-of-care testing devices. Most electrochemical biosensors rely on the enzymatic method. However, the utilization of the enzyme for the sensors appears to be limited due to complex processes of enzyme immobilization, high cost, short shelf life from the denaturation of the enzyme. [3] To overcome the limitations from using enzyme, non-enzymatic catalyst, especially nickel oxide, has been attracted with the advantage of good stability re-usability, high sensitivity, simplicity, low cost, and an excellent catalytic activity on detection of urea by the formation of the redox couple of Ni(II) and Ni(III).[4]

Despite many efforts to achieve a higher catalytic effect with bimetallic oxides with Co[5], Mo[6], and Mn[7], the improvement of electrochemical response of oxidation of urea is still challenging due to the low exposure of active sites. Therefore, the formation of hollow structured hierarchical catalysts can be considered to improve the sensitivity of urea detection besides exploration of highly performing compositions. Such a hierarchical structure will provide structural stability and facile transport channels for electrolytes by exploiting its inner and outer surface as active sites. For a flexible and disposable sensor platform, the paper has merit due to a porous cellulose matrix. The paper naturally allows a liquid sample to infiltrate the paper matrix by capillary force. Furthermore, the capillary force-driven transport can be utilized in the catalyst loading process to distribute the catalyst and conductive network uniformly within the paper, which made the fabrication process simpler and enhanced performance.

In this study, the combination of the hierarchical structure of nickel oxide and the paper matrix has demonstrated an increase in the sensitivity toward electrochemical sensing of urea. A filter paper and CNTs were used for the porous matrix and the conductive network, respectively. The hierarchical nickel cobalt oxide was synthesized with a one-pot hydrothermal method with the variation of Ni:Co atomic ratio, and then applied to the paper substrate. The structure and morphology of paper-based electrodes were characterized by XRD, SEM, and EDS, and the electrochemical response was measured by a potentiostat. A detailed description of the fabrication of paper-based sensors and the effect of hierarchical structure and bimetallic composition will be presented.

[1] B. K. Boggs, R. L. King, and G. G. Botte, “Urea electrolysis: direct hydrogen production from urine,” Chem. Commun., no. 32, pp. 4859–4861, Aug. 2009, doi: 10.1039/B905974A.

[2] L. Liu, H. Mo, S. Wei, and D. Raftery, “Quantitative analysis of urea in human urine and serum by 1 H nuclear magnetic resonance,” Analyst, vol. 137, no. 3, pp. 595–600, 2012, doi: 10.1039/C2AN15780B.

[3] K. Kim et al., “Fabrication of a Urea Biosensor for Real-Time Dynamic Fluid Measurement,” Sensors, vol. 18, no. 8, Art. no. 8, Aug. 2018, doi: 10.3390/s18082607.

[4] Nie, Huagui, et al. "Non-enzymatic electrochemical detection of glucose using well-distributed nickel nanoparticles on straight multi-walled carbon nanotubes." Biosensors and Bioelectronics 30.1 (2011): 28-34.

[5] Ding, Rui, et al. "Facile synthesis of mesoporous spinel NiCo2O4 nanostructures as highly efficient electrocatalysts for urea electro-oxidation." Nanoscale 6.3 (2014): 1369-1376.

[6] Liang, Yanhui, et al. "Enhanced electrooxidation of urea using NiMoO4· xH2O nanosheet arrays on Ni foam as anode." Electrochimica Acta 153 (2015): 456-460.

[7] Periyasamy, Sivakumar, et al. "Exceptionally active and stable spinel nickel manganese oxide electrocatalysts for urea oxidation reaction." ACS applied materials & interfaces 8.19 (2016): 12176-12185.