TABLE 2.

Comparison of reported Urea analysis methods.

Method	Analysis principle	Linear range	Detection limit	Analysis characteristics	RSD (%)	Ref.
FL	The urea-specific DNA aptamer was isolated by an exponential enrichment method. In terms of inherent fluorescence differences and color changes, the aptamer sensor used unmodified gold nanoparticles (AuNP) to transduce the signal of aptamer-urea binding, thereby showed high selectivity to urea.	20–150 mM	20 mM	Simple operation and low sensitivity	—	36
Electrochemical	A mixture of carbon nanotubes and platinum nanoparticle-reduced graphene oxide (rGO) was used to surface modify the glassy carbon electrode (GCE). The urea aptamer was then immobilized on the nanocomposite by covalent bonding. Thus, aptamers with high affinity and selectivity for urea were used to quantify urea.	0.0–0.1 nM, 1.0–150 nM	1.9 pM	Complex operation and expensive equipment	7.9%	37
Electrochemical	Molecularly imprinted polymers (MIPs) also contained DNA aptamers on gold nanoparticles containing carbon nanotube networks (AuNP/CNT). The material was placed on a glass-carbon electrode (GCE), and GCE showed double recognition ability after removing urea from the MIP cavity. After the modified electrode was exposed to urea, the interface charge transfer of the redox probe hexacyanoferrate was measured under certain conditions. The change of the charge transfer resistance depended on the urea concentration, so the urea can be detected with high specificity.	0.005–0.1 nM, 1–500 nM	900 fM	Complex operation and high sensitivity	5.5%	38
Electrochemical	The primary amine was functionalized GO by a one-pot solvothermal method using ethylene glycol as the solvent and ammonia as the nitrogen precursor. Based on the signal amplification of carbon nanotubes/amine-functionalized GO as a sensing platform, and Apt as a probe, the label-free electrochemical analysis of urea was performed.	1–30 nM, 100–2000 nM	370 pM	Complex operation and high sensitivity	6.7–11.5%	39
SERS	The substrate made from Au/Cu hybrid nanostructure arrays was used to detect urea. Adjusting the gap size between adjacent nanorods to a sub-10 nm range produced high-density hot spots, which enables the substrate to detect urea signals at low concentrations.	—	1 mM	Simple operation and low sensitivity	9.5%	34
SERS-FL	Apt mediated the CDFeN catalyzed H2O2 oxidation of TMB to form trifunctional probes of TMBox. The FL/SERS/Abs signals had a linear relationship with the concentration of Urea.	Flu:3.33–13.32 nmol/L; SERS:3.33–16.65 nnmol/L	1.12 nmol/L,1 nmol/L	Simple operation and high sensitivity	1.45–5.32%	This method