<title>Ag+-sensitive membranes for electrochemical and optical sensors</title>

Abstract

ABSTRACT The paper is focused on lipophilic acyclic thioethers as neutral ionophores for heavy-metals cations. Membranes based onplasticized PVC containing these carriers exhibited high selectivity toward silver cations over transition metal ions (logKAg,M <4.5) except mercury ions. Membrane containing the ionophore II (with 1-naphtyl pendant group) was used todesign silver-selective ISE which shows linear response in the range 3-6 pAg with a slope of 51.5 mV/pAg. The addition ofthe pH chromoionophore to the membrane composition allowed to obtain a silver-sensitive optrode membrane, respondingin the concentration range 0.01-1 mM Ag.Keywords: lipophilic thioethers, silver ionophores, PVC membranes, silver ISE, silver-sensitive optomembranes 1. INTRODUCTION The need of the silver determination and monitoring increases due to the environmental pollution caused mainly by its greatannual loss from industrial wastes. Many sophisticated methods including instrumental analytical techniques are utilized todetermine the silver ions concentration in measuring media. Another approach involves the monitoring of the silver cationsusing potentiometric chemical sensors. These devices give results in real-time with good accuracy and precision, withoutnecessity of sample preparation. Comprehensive solid-state silver-selective electrodes based on silver sulphide exhibit highselectivity over alkali-, alkaline-earth and some transition-metals ions, however strong mercury interference is observed [1].Polymeric membrane ion-selective electrodes (ISE) based on neutral ionophores can be an alternative to the solid-statesensors. Sulphur ligands are especially suitable because of their ability to coordinate selectively soft metal ions such asAg(I). Thiacrown ethers were commonly used as ionophores in plasticized PVC membranes of silver-selective ISE [2,3].Mono- and di-thiacrown ethers derivatives incorporated in PVC/DOP membranes showed the selectivity coefficients forsilver with respect to heavy and transition metals lower than -3 and with respect to alkali- and alkaline-earth-metals lowerthan -4. Interference from mercury ions was serious log KAgHg 2 to 4 .5 [3]. Other silver selective electrodes based onmembranes containing different thiacrown ethers (although of amount and position of sulphur atoms) exhibit similarselectivities. High selectivity toward silver (log KAg,M <4 and log KAg,Hg <2) with Nemstian responses in wide linear range2-6 pAg was found for thiabenzocrown derivatives [4]. Similar results were obtained for membranes containing acyclicthioethers [5]. Calixarene derivatives with sulphur and/or nitrogen donor atoms as silver ionophores in PVC membraneswere also investigated [6-8]. For dithioether functionalized calix[4]arene the selectivity coefficients were lower than -2.2 forHg(II) and lower than -4.6 for other cations tested [7]. Theoretical slopes in the activity range 1OlO1 M of silver ions wereobtained in this case.The monitoring of silver ions in aqueous media can be also performed using optical sensors (optrodes). The optical sensormembrane requires the presence of the silver sensitive chromoionophore i.e. an ionophore containing chromophoric groupwhich changes its optical properties upon the complexation of the analyte. The reaction of the analyte complexation by thechromoionophore should be selective, reversible and kinetically fast and the chromoionophore should be lipophilic enoughto ensure the durability ofthe sensing layer.Acridinium dyes were used as silver-sensitive chromoionophore in plasticized PVC membranes, which gave reversibleresponse in the range 0.05-10 mM [9]. Conventional ionophores suitable for ion-selective electrodes can be also used insensing layer of optical sensor. In this case a lipophilic pH chromoionophore is additionally incorporated to the membranecomposition. The principle of the signal processing is based on the equilibrium of the complexation reaction and on the