The normal level of urea in serum ranges from 15 mg/dL to 45 mg/dL (25 mM to 75 mM). The concentrations increase in the serum from 180 mg/dL to 480 mg/dL (300 mM to 800 mM) in patients suffering from renal insufficiency. The estimation of urea is likewise crucial in food science and environmental-monitoring. Urea has a strategic function in the marine nitrogen cycle as a source of excreted nitrogen by invertebrates and fish. Likewise, the bacterial decomposition of nitrogenous materials and terrestrial drainage are influenced by urea. Urea estimation is important during environmental monitoring. The annual worldwide production of urea exceeds 100 million metric tons, and the majority of which is used as fertilizer. Excessive nitrogen fertilizer application can lead to pest problems by increasing birth rate, longevity, and overall fitness of certain pests.

Urea may be responsible for reduction in soil pH [3].Various analytical methods for determining trace amounts of urea have been developed [3]. Optical [4], amperometric [5�C7], thermal [8,9] conductometric [10], and potentiometric [11�C15] methods are commonly used to measure urea in samples. Given the simple construction of potentiometric urea biosensors and the availability of the required instrumentation for their utilization, these biosensors are widely accepted [12�C14,16,17].The enzyme urease could be employed for urea determination, whereby the urease catalyzes the hydrolysis of urea to form alkaline reaction products. To construct a functional nanomaterial-based biosensor, the relationship between enzymes and nanomaterials such as fullerene and carbon nanotubes (CNTs) must be identified.

The interaction between the enzyme and Carfilzomib the biosensor could be a covalent or non-covalent bond. Several reports have identified the immobilization of biomolecules on CNTs via non-covalent interactions [18,19]. The improved stability, accessibility, and selectivity, as well as the reduced leaching, can be achieved through covalent bonding because the location of the biomolecule can be controlled [20].Several types of immobilization methods for biological molecules are available. These methods include entrapment, encapsulation, covalent binding, cross-linking, and adsorption. Fullerene is an allotrope of carbon with 60 �� electrons. Thus, fullerene resembles olefin molecules and can undergo nucleophilic attack by electron-releasing molecules such as amines.

Fullerene has low solubility in aqueous solutions [21�C24]. Fullerenes have been used in the fabrication of certain biosensors with enzymes such as lipase and urease. Lipase immobilized on fullerene was used to detect optical isomers of amino acid esters and urea at 10?1 to 10?4 M measured by the quartz crystal microbalance (QCM) method [21,23]. Urease possesses an amine (�CNH2) group that can directly react with 30 ��-bonds in fullerene.