Sodium alginate

Urease (E.C.3.5.1.5, Canavalia
ensiformis), 96-well robolid and polystyrene
microplate, chitosan (medium molecular weight; deacetylation degree,
75–85%) and poly-l-lysine (PLL; molecular weight,
150 000) were purchased from Sigma Chemical (St. Louis, MO).
Urea was obtained from Aladdin (Shanghai, China). Urea Assay Berthelot
kit, buffered chromogen (3% (w/v) phenol + 0.015% (w/v) sodium nitroprusside),
and alkaline hypochlorite solution (1.5% (w/v) sodium hydroxide +
0.12% (w/v) sodium hypochlorite) were purchased from Beijing Leagene
Biotechnology Co., Ltd. (Beijing, China). Sodium alginate (medium
viscosity; molecular weight, 80–120 kDa; mannuronate/guluronate
ratio, 0.6) was obtained from Tianjin Guangfu Fine Chemical Research
Institute (Tianjin, China). All other reagents were of analytical
grade and used without further purification. Ultrapure water was used
to prepare all solutions.
The human urine samples were collected
from healthy volunteers and centrifuged for 20 min at a speed of 1000
rpm to remove bacteria and protein aggregates. The supernatants were
filtered and diluted 100, 150, and 200 times with phosphate-buffered
saline (PBS) before analysis. The human serum samples were collected
from Beijing Friendship Hospital (Beijing, China) in accordance with
the rules of the local ethical committee. The serum samples were diluted
with PBS buffer three, six, and nine times before analysis.