Determination of Heavy Metal Inos Using Conductometric Biosensor Based on Sol-Gel Immobilized Urease

Heavy-metal ions are ubiquitous in nature, thus resulting in a serious environmental problem. Due to their high toxicity, there is an obvious need to determine them rapidly on site at trace levels. Although the typical detection methods such as atomic absorption spectrometry and inductively coupled plasma mass spectrometry are widely used for the determination of heavy-metal ions, 1 both methods require very sophisticated equipment and can not be used for field monitoring. Therefore, there is a need for simple and potable detection method. In general, electrochemical methods are able to selectively detect heavy-metal ions with less complex instrumentation. The techniques developed so far include ion-selective electrodes, polarography, and other voltammetric methods. 2 In recent years, electrochemical biosensors have received a great attention as promising alternatives for the determination of heavy-metal ions. A number of examples have been reported 3-9 and most of the biosensors are based on the use of urease enzyme immobilized by glutaraldehyde crosslinking with bovine serum albumin on electrode surface. 3-6 The determination of heavy-metal ions using the urease-immobilized biosensor is based on the measurement of the urease enzymatic activity which is inhibited by heavy-metal ions. It is well known that the inhibition of the urease by these ions results from the reaction with sulfhydryl groups of the active site of the enzyme. 10-12 The urease converts urea into ammonium and bicarbonate ions. urease NH2CONH2 + 3 H2O 2NH4 + + HCO3 - + OH