TRANSDUCTION OF SELECTIVE RECOGNITION OF HEAVY-METAL IONS BY CHEMICALLY MODIFIED FIELD-EFFECT TRANSISTORS (CHEMFETS)

Specifically substituted calix[4]arenes that complex selectively silver (2), copper (13), cadmium (23), and lead (29) cations were synthesized via di- and tetraalkylation of p-tert-butylcalix[4]arene. Calix[4]arenes derivatized with thioether, thioamide, and dithiocarbamoyl functionalities could be obtained in high overall yields. Integrated on a chemically modified field effect transistor (CHEMFET) the selective complexation of heavy metal ions by these calix[4]arenes is transduced directly into an electrical signal. An architecture for CHEMFETs is used in which a hydrophilic polyHEMA hydrogel is covalently attached between the gate oxide surface of a modified ISFET and the hydrophobic sensing membrane that contains the receptor molecule. CHEMFETs based on the calix[4]arene (2) with two diametrically substituted thioether functionalities responded selectively (60 mV decade-1) to a change in silver activity in the aqueous solution in the presence of potassium, calcium, cadmium, and copper ions (log K(ij) less-than-or-equal-to -4). The selectivity toward mercury (log K(ij) = -2.7) is comparable to the best neutral carrier based ISE so far. The calix[4]arene (13), with four dithiocarbamoyl groups, incorporated in a CHEMFET responsed Nernstian (30 mV decade-1) to a change in copper activity. The calix[4]arene (23), which has four dimethylthiocarbamoylmethoxyethoxy substituents, was selective in a CHEMFET (30 mV decade-1) toward cadmium in the presence of calcium and potassium ions. Two classes of ionophores can be distinguished for the selective detection of lead(II) cations by CHEMFETs, oxamide and thioamide ionophores, respectively. Tetrasubstitution of the calix[4]arene with thioamide groups (29) is a prerequisite for the selective detection of lead and leads to a highly selective CHEMFET (log K(ij) -3.4 to -5.2).