Fabrication of silver ion selective electrodes and their applications in biosensors

Abstract

Six calix[4]arene derivatives containing various donor atoms and different topology (L1 – L6) have been synthesized and used as neutral ionophores to fabricate silver ion selective electrodes (Ag-ISEs). The characteristics of six synthesized ionophores in polymeric membranes were compared in order to find the best one in terms of potentiometric selectivities and complex formation constants. The electrode based on L2 containing two benzothiazole groups showed the highest selectivity coefficient towards Ag+ which resulted from two nitrogen and two sulfur donor atoms of benzothiazole groups forming a stronger complex with Ag+ yielding a high complex formation constant. L2 was optimized in polymeric membrane to obtain the best characteristics of Ag-ISE using L2 (10 mmol kg⁻¹) and KTpClPB (50 mol% related to the ionophore) in the o-NPOE plasticized PVC membrane (1:2; PVC:o-NPOE by weight). The Ag-ISE fabricated from ionophore L2 exhibited good properties with a Nernstian response (59.7 ± 0.8 mV decade⁻¹ for macroelectrode and 59.8 ± 1.0 mV decade⁻¹ for microelectrode) with a low detection limit (ca. 5.0 x 10⁻⁷ M) and a wide linear working range (10⁻⁶ to 10⁻² M). The sensors could be used in a wide pH range (pH 2 – 8) with a response time less than 5 seconds. The electrodes were used as an indicator electrode for the argentometric titration of the mixture of Cl⁻ and Br⁻. The fabricated electrodes were also used for the first time in speciation analysis of silver nanoparticles (AgNPs) with good accuracy and precision. The macroelectrode fabricated from L2 was then applied in glucose biosensor by using AgNPs as cation marker. Basically, the enzyme-substrate reaction between β-D-glucose and glucose oxidase (GOx) produced hydrogen peroxide (H2O2) as a product. The generated H2O2 was able to oxidize AgNPs to free Ag+. The amount of Ag+ corresponded to the concentration of glucose could be directly monitored using the Ag-ISE. The working linear range was 0.1 – 3 mM in 10 mM magnesium acetate buffer pH 6.0. Parameters affected the reaction rate such as pH and the amount of GOx and AgNPs were explored. The lower detection limit was 1.0 x 10⁻⁵ M. The proposed sensor provided a double selective function and could be used to determine glucose in beverages with good accuracy and precision. The fabricated microelectrode from L2 was used to determine Ag+ in 1000 µL sample with the detection limit around 1 µM using sodium ion selective microelectrode as pseudo reference electrode. Such potentiometric measurement was then applied to detect the DNA hybridization on gold substrates immobilized with lipoic acid modified by pyrrolidinyl peptide nucleic acid carrying a 2-aminocyclopentanecarboxylic acid (Lip-acpcPNA) probe by dissolution of positively charged encapped AgNPs with H2O2. The detection limit of DNA was 2 µM (20 pmol) in 1000 µL sample. This Lip-acpcPNA probe could discriminate fully complementary DNA, non-complementary DNA and single base mismatched DNA using the simple potentiometric method.