New calix[4]arene derivatives as ionophores in polymeric membrane electrodes for Ag(I): comparative selectivity studies and detection of DNA hybridization.

Four calix[4]arene derivatives containing various donor atoms and different topology (L1-L4) have been synthesized and used as neutral ionophores to fabricate silver ion selective electrodes (Ag-ISEs) which were characterized in terms of their potentiometric selectivities and complex formation constants. The ionophore L2 having two nitrogen and two sulfur donors showed stronger interactions with Ag(+) and the highest selectivity coefficient towards Ag(+). The best membrane electrode was prepared from L2 and used to fabricate silver ion selective microelectrodes (Ag-ISμEs) which could detect silver ions in 1000 μL samples with detection limit around 1 μM using sodium ion microelectrodes as a pseudo reference electrode. Such potentiometric measurement was then applied to detect DNA hybridization on a gold substrate, employing immobilized lipoic acid-modified pyrrolidinyl PNA (Lip-acpcPNA) as a probe. The hybridization between the neutral Lip-acpcPNA probe and DNA target led to a negatively charged surface that could bind positively charged silver nanoparticles (AgNPs(+)) via electrostatic interactions. The hybridization signal was observed by dissolution of the electrostatically adsorbed AgNPs(+) with hydrogen peroxide. Excellent discrimination of complementary from single mismatched and non-complementary DNA targets was achieved under non-stringent conditions. The detection limit of DNA was 0.2 μM in 1000 μL samples.