Urea biosensor based on ammonia gas-sensitive Pt/GaAs Schottky diode

Metabolite-Sensitive Holographic Biosensors

“…An abnormally high level of urea in blood or a low level in urine is a strong indication of kidney dysfunction, and although the normal range in human serum is between 1 and 10 mM, levels can increase up to 100 mM under pathophysiological conditions (NH 2 ) 2 CO + 2H 2 O + H + → 2NH 4 + + HCO 3 - , and has been monitored for sensing urea by measuring changes in pH, , NH 4 + concentration, NH 3 gas, or CO 2 gas 31 using electrochemical or optical techniques.…”

“…26 An abnormally high level of urea in blood or a low level in urine is a strong indication of kidney dysfunction, 27 and although the normal range in human serum is between 1 and 10 mM, levels can increase up to 100 mM under pathophysiological conditions. 28 Urease (EC 3.5.1.5) catalyzes the hydrolysis of urea into ammonium and bicarbonate ions, 24 (NH 2 ) 2 CO + 2H 2 O + H + f 2NH 4 + + HCO 3 -, and has been monitored for sensing urea by measuring changes in pH, 8,11 NH 4 + concentration, 29 NH 3 gas, 30 or CO 2 gas 31 using electrochemical or optical techniques. The immobilization of urease onto a pH-sensitive hologram was investigated to establish the potential for an integrated holographic urea biosensor.…”

Performance Characteristics of a Urea Microsensor Employing a Micromachined Carbon Dioxide Electrode

“…The optimum pH of urease itself has been reported to be around 7.0 and the dependence of the rate constant on pH is weak between pH 6 and 8 [30]. In some urea sensors based on an ammonia gas sensor, the optimum pH of the sensor output has been reported to be around 8.5 [21,31,32]. On the other hand, an ISFET-based sensor and a metal oxide-based sensor showed a maximum response at 6±7 [7,16].…”

“…Although no explicit dependence was observed between pH 6.5 and pH 9.0 with the 90 % response time distributed between approximately 15 and 20 min, the response became sluggish below pH 6.0 and increased to 40 min at pH 5.5 (urea concentration: 2 mM). The response time of these urea microsensors along with the other conventional types of electrodes with a gas-permeable membrane is of the order of several minutes [18,21,31,32].…”

“…Complete hermetical sealing of a preformed membrane like poly(tetra¯uoroethylene) (PTFE) on the sensitive area has been found technically dif®cult and casting an appropriate precursor solution seems to be the only realistic method. When such a preformed gas-permeable membrane like porous PTFE is used, it must be af®xed to a separate holder or a conventional analysis system [31,32]. Although the thickness of the membrane could be decreased to several micrometers as far as its mechanical strength is ensured, the realization of an air-gap membrane which can be formed on microsensors is desired.…”