Mixed ionophore ion-selective electrode for the improved dectection of urea in blood

A method of operating a multiple-use sensor array, said method comprising:

• i. providing the multiple-use sensor array, said multiple-use sensor array comprising a first ion-selective electrode and a second ion-selective electrode, said first ion-selective electrode including a membrane comprising (a) a first ionophore and (b) at least a second ionophore, said second ion-selective electrode including a membrane comprising the second ionophore, said first ionophore not being present in said second ion-selective electrode, wherein:
• the first ion-selective electrode is an ammonium-selective electrode;
• ii. in sequence contacting the multiple-use sensor array with one or more rinse solutions and optionally one or more calibration solutions, each of said rinse solutions being substantially devoid of the ion for which said first ionophore is selective;
• and
• iii. subsequently contacting the multiple-use sensor array with a sample of biological origin,
• wherein a sample cycle time required to complete one cycle of ii. and iii. is from 5 to 120 seconds.

A urea sensor comprising:

• an ammonium-selective electrode comprising a membrane, said membrane comprising a polymer and two ionophores, the two ionophores being (a) an ammonium-selective ionophore and (b) an ion-selective ionophore selected from a calcium-selective ionophore, a potassium-selective ionophore, and a sodium selective ionophore;
• an enzyme layer covering the ammonium-selective electrode, said enzyme layer comprising a polymer and urease;
• and
• an outer layer covering the enzyme layer.