Enzymatic sensors based on the ISFETs for determination of heavy metal ions in some vegetables

Enzymatic sensors based on the ion sensitive field effect transistor (ISFET) for detection of heavy metal ions were developed. It was shown that a urease sensor is more sensitive to the above mentioned ions than a cholinesterases (ChEs) sensor. Inhibition of ChEs is reversible and their activity may be restored by the removal of metal ions from the membranes. ChEs are more stable than urease at the immobilization in the sensor membranes and in time of enzymatic membrane storage. The sensitivity of the urease sensor to heavy metal ions metals was found as follows: Hg²⁺>Cu²⁺>Cd²⁺ >Co²⁺. The detection limit depends on the metal ion to be analysed. It was in the range of 10^-4-10^-6 mol/l. To discriminate both, heavy metal ions and phosphoorganic pesticides in samples, it was necessary to use a system which combined urease and ChEs sensors. The use of easily replaceable enzymatic membranes from alginate gel or nitrocellulose (NC) is more suitable for repeated analysis than reactivation of these membranes. The standard deviation of sensor responses for different membrane castings did not exceed 10%. To prevent non-specific response (as a result of the influence of medium conditions and the inhibition of enzyme activity by vegetable sap), the analysed samples should be diluted 2-40 times by 5 mmol/l buffer solution, containing 140 mmol/l sodium chloride